Maytal Caspary Toroker, Technion-Israel Institute of Technology
Francesc Illas, University of Barcelona
Michele Pavone, University of Napoli Federico II
Guofeng Wang, University of Pittsburgh
EN07.01: Nanocatalysts for Water Splitting
Maytal Caspary Toroker
Monday AM, December 02, 2019
Sheraton, 2nd Floor, Liberty BC
8:30 AM - EN07.01.01
HydroGEN—A Consortium Working on Efficient and Advance Water Splitting Materials (AWSM)
Huyen Dinh1,Katie Randolph2,Adam Weber3,Tadashi Ogitsu4,Anthony McDaniel5,Richard Boardman6,Elise Fox7,James Vickers2,David Peterson2,Eric Miller2,Ned Stetson2
National Renewable Energy Laboratory1,Department of Energy2,Lawrence Berkeley National Laboratory3,Lawrence Livermore National Laboratory4,Sandia National Laboratories5,Idaho National Laboratory6,Savanah River National Laboratory7Show Abstract
HydroGEN (https://www.h2awsm.org/) Energy Materials Network (EMN) is an U.S. Department of Energy (DOE) EERE Fuel Cell Technologies Office (FCTO)-funded consortium that aims to accelerate the discovery and development of advanced water splitting materials (AWSM) for sustainable, large-scale hydrogen production, and to more effectively enable the widespread commercialization of hydrogen and fuel cell technologies. This is in line with the H2@Scale initiative (https://www.energy.gov/eere/fuelcells/h2-scale), with the goal to meet U.S. DOE’s ultimate production cost target of $2/kg H2. HydroGEN EMN is a six national laboratories consortium comprises National Renewable Energy Laboratory (NREL) - lead, Lawrence Berkeley National Laboratory (LBNL), Sandia National Laboratory (SNL), Lawrence Livermore National Laboratory (LLNL), Idaho National Laboratory (INL), and Savannah River National Laboratory (SRNL).
The HydroGEN Consortium offers more than 80 materials capabilities nodes to help address RD&D challenges in efficiency, durability and cost. The capabilities span computational tools and modeling, materials synthesis, characterization, process manufacturing and scale-up, and analysis. Detailed descriptions of all the HydroGEN nodes are available in a searchable format on the HydrogGEN website (https://www.h2awsm.org/capabilities), including information such as the host National Lab, the capability experts, and a synopsis of the node’s unique aspects and capability bounds. By design, the nodes are cross-cutting, and any given node may be useful for one or several advanced water splitting (AWS) technologies. Leveraging the HydroGEN Consortium’s leading technical experts and extensive collection of unique capabilities is expected to help advance all the advance water splitting technologies, including advanced electrolysis (low and high temperature), photoelectrochecmical (PEC) and solar thermochemical (STCH) routes which includes hybridized thermochemical and electrolysis approaches to water splitting.
This presentation will provide an overview of the HydroGEN EMN consortium and highlight some water splitting materials projects. These highlights will include theory and experimental approaches for understanding and characterizing water splitting materials. HydroGEN looks forward to growing its community of industry, university and laboratory collaborators that can partner with member-laboratory experts by way of CRADAs and potential future FOAs.
9:00 AM - EN07.01.02
Monolayer MoS2 with Better Hydrogen Evolution Catalysis Than Pt
North Carolina State University1Show Abstract
The development of low-cost high-performance catalysts for hydrogen production via water splitting stands as a key step towards addressing modern energy and environment challenges. We demonstrate a strategy to enable earth-abundant MoS2 a better catalyst than Pt, the best catalyst to date but too precious to be practically useful, for the hydrogen evolution reaction (HER) of water. The strategy is developed by leveraging on new fundamental understanding for the HER catalysis of MoS2. We find that substrates can affect the catalytic activity of MoS2 by forming an interfacial tunneling barrier with MoS2 and also modifying the chemical nature of MoS2 through charge transfer. As a result, we achieve excellent catalytic performance at the monolayer MoS2 films with optimal densities (7-10%) of sulfur vacancy by using substrates that are able to form low interfacial tunneling barriers and transfer electrons to MoS2, such as Ti. The catalytic performance of monolayer MoS2 films may be further improved by crumpling the films on Ti-coated flexible substrates because the crumpling gives rise to compressive strain in MoS2 that may lower the Tafel slope although generates no obvious effect on the exchange current density. The better-than-Pt performance of MoS2 is remarkably stable with no degradation after continuous reaction for more than two months. This work provides a viable solution for the challenge of hydrogen evolution from water and paves the way towards the utilization of hydrogen energy.
9:15 AM - EN07.01.03
Highly Efficient Overall Water Splitting in Acid with Metal Nanosheets
Dongshuang Wu1,Kohei Kusada1,Hiroshi Kitagawa1
Kyoto University1Show Abstract
Water is the only available fossil-free source of hydrogen. Splitting water electrochemically is among the most commonly used techniques, however, it accounts for only 4% of global hydrogen production, predominately because of the high cost and low performance of catalysts promoting the oxygen evolution reaction (OER) . Currently, only Ir oxides show moderate stability for OER in acid but still require high overpotentials. In contrast, Ru is the most active OER catalyst and is nearly 5~16 times cheaper than Ir these five years, however, Ru has a serious degradation problem . Here, we report a highly efficient catalyst in acid, that is, solid-solution RuIr nanosized-coral (RuIr-NC) consisting of 3 nm-thick sheets with only 6 at% Ir. Among OER catalysts, RuIr-NC shows the highest mass/intrinsic activity and unprecedented stability (Figure 1). An electrolyzer using RuIr-NC as both electrodes can reach 10 mA cm−2geo at 1.485 V for 120 h without noticeable degradation, which outperforms known systems. This electrolyzer is also significantly less expensive than the commercial IrOx and Pt system. Operando spectroscopy and atomic-resolution electron spectroscopy indicate the high-performance results from the ability of the unique facets of RuIr-NC to resist the formation of dissolvable metal oxides and to transform ephemeral Ru into a long-lived catalyst.
9:30 AM - EN07.01.04
Importance of Interfacial Band Structure between the Substrate and Mn3O4 Nanocatalysts During Electrochemical Water Oxidation
Moo Young Lee1,Heonjin Ha1,Ki Tae Nam1
Seoul National University1Show Abstract
Inspired from the Mn4CaO5 cluster in natural Photosystem II, attempts to replace conventional precious metal based electrocatalysts for oxygen evolution reaction (OER) with Mn-based ones have been intensively conducted. Recently, we successfully synthesized uniform and assembled Mn3O4 nanoparticles (NPs) with superior OER activity to conventional bulk Mn oxides, and revealed highly stabilized Mn(III) species on the surface and different OER mechanism from the conventional ones. Using our unique NPs as a structural platform, we have been now conducting the detailed mechanistic investigations such as how the surface of our NPs changes and how the electrons transfer throughout our NPs during OER.
In this regard, we strongly believe that understanding and controlling the charge transfer during OER is not only helpful in comprehending the reaction mechanism but also important in establishing rational strategies of catalysts design. From the previous literature, we think that the overall charge transfer processes in the film-type OER catalysts can be categorized into three transfer pathways: the electrons should transfer through i) electrolyte-catalysts interface, ii) inner catalysts, and iii) catalysts-substrate interface. Among the three pathways, the importance of the charge transfer at the catalysts-substrate interface has not been investigated, that is, there is a lack of interpretation on the interfacial effects between the catalysts and the substrate.
In this study, we discovered the importance of interfacial band structure between the substrate and Mn3O4 NPs for OER, which has not considered in electrocatalytic systems so far. We designed the band structure model of the catalysts-substrate interface and explained the substrate dependent OER activity of Mn3O4 NPs. Moreover, we improved the OER activity of Mn3O4 NPs by introducing the metal interlayer with a proper band structure. We further investigated substrate dependent impedance analysis, resulting in the same tendency to the expectation from our interfacial band structure model.
10:15 AM - EN07.01.05
Quantum-Mechanical Embedding Methods for Surface Catalysis
Technische Universität Wien1Show Abstract
The charge transfer reactions upon dissociative absorption of O2 on metal surfaces is critical for many catalytic processes, yet poses a challenging problem for state-of-the-art ab-initio theoretical modeling. For example, the experimentally observed activation barrier for O2 dissociation on Al (111) is not captured by conventional density functional theory. However, accurate wavefunction-based methods that are well suited to handle the multireference character of the charge transfer state are ill suited to describe extended metal surfaces. Embedding methods offer a way forward, by combining highly accurate wave-function based approaches at the absorption site with an environment described by density functional theory. In the case of O2 on Al(111), such an approach naturally yields an adiabatic barrier . I review our recent advances and challenges in solving catalytic problems using embedding methods, including the build-up of six-dimensional potential energy surface (PES) suited for quasi-classical trajectory calculations , projection-based approaches that enable freeze-thaw cycles, and the extension to high-level methods such as Quantum Monte Carlo or Coupled Cluster that can treat periodic boundary conditions.
 F. Libisch, C. Huang, and E. A. Carter Accounts of Chemical Research, 47, 2768 (2014)
 R. Yin, Y. Zhang, F. Libisch, E. A. Carter, H. Guo, and B. Jiang J. Chem. Phys. Lett. 9, 3271 (2018)
10:45 AM - EN07.01.06
Stacks of Highly-Ordered Nanowire Arrays Achieve Ultra-High Mass Activity for Oxygen Evolution Reaction and Efficient Transportation of Evolved Oxygen
Ye Ji Kim1,Yeon Sik Jung1
Although hydrogen is the most abundant element in the universe and can be used as a fuel with zero carbon emissions, its economic and stable production still remain as an important issue. One of the promising routes of H2 production is polymer electrolyte membrane water electrolysis (PEMWE), which require noble metal electrocatalysts (e.g., Pt, Ir, Ru) for efficient oxygen evolution reaction (OER). However, to reduce the high capital expense resulting from the high loading of noble metal electrocatalysts, their electrochemically active surface area (ECSA) and specific activity need to be maximized.
Here, we suggest woodpile-structured Ir electrocatalysts, consisting of 3-dimensional (3D) stacks of highly-ordered nanowire building blocks, as a novel catalyst structure to markedly improve mass activity compared to conventional nanoparticle-based catalysts. We show that the 3D Ir nanowire stack can be controllably fabricated via solvent-assisted nanotransfer printing (S-nTP). The woodpile-structured Ir electrocatalysts fulfill simultaneously (1) a large electrochemically active surface area, (2) long-range connectivity of building blocks for high electronic conductivity, (3) well-defined pore structures for highly efficient transport of evolved gas species, and (4) extensive geometric controllability for maximization of catalytic performances. With these aspects, Woodpile-structured Ir thin film achieves a high mass activity of 3.7 A/mg (at 1.5 V vs RHE) in half cell and 140 A/mg (at 1.8 V) in single cell, which is ~36 times higher than that of the state-of-the-art commercial Ir nanoparticle catalyst. Based on the advantages from structural engineering, OER mass activity over 80% of its initial values was achieved during more than 500 repeating chronoamterometry cycles. Furthermore, in order to clarify the substantially improved surface specific activity and mass activity of OER electrode, we performed systematic analysis on the effect of a facile escape of evolved O2 gas bubbles with a systematic control of the 3D geometry. Although there is no experimental demonstration, previous studies regarding the nanostructured catalysts suggested possibility for increase in the specific activity due to the facilitated mass transport. Our S-nTP technology enables extensive control of the 3D geometry (e.g. parallel vs. perpendicular stacks) without changing other factors related to catalytic activities such as physiochemical characteristics and ECSA. Systematic tuning of 3D geometry and nanowire building blocks revealed that O2 bubble transport is the key limiting step in the OER catalysts, providing practical design rules for more efficient OER electrode in PEMWE.
11:00 AM - EN07.01.07
Zipping Up Ni-Fe Hydroxide-Coated Hematite Nanowires—A Citrate-Chelating Strategy Promotes Photoelectrochemical Water Splitting
Tianyu Liu1,Mingyang Li2,Yexiang Tong2,Yat Li1
University of California, Santa Cruz1,Sun Yat-sen University2Show Abstract
Oxygen evolution reaction (OER) catalyst coatings improve the photoelectrochemical (PEC) water splitting performance of photoanodes via facilitating charge transfer across electrode/electrolyte interfaces and passivating surface hole-trap states. Unfortunately, the occurrence of interface segregation between photoanodes and their OER coatings often impedes fast charge transport and therefore, hinders the performance enhancement. This presentation will introduce a citrate-assisted strategy to mitigate the challenge on the interface quality. We discover that sodium citrate, a molecular linker chelating both hematite (α-Fe2O3) nanowires and Ni-Fe hydroxide [NiFe(OH)x] OER catalyst overlayers, results in seamless, conformal, and pinhole-free coatings. Such the high-quality photoanode/OER catalyst interfaces enable the hematite nanowire photoanode to achieve an early turn-on potential of 0.53 V vs. reversible hydrogen electrode, corresponding to an ultralow OER overpotential of 0.13 V. The turn-on potential is approximately 50% lower than those of the state-of-the-art hematite photoanodes encapsulated in transition-metal-oxide OER catalysts. The overall characterization results on morphologies and photoelectrochemical activities presented in this presentation will not only provide guidelines to experimentally design and prepare high-performance photoelectrodes, but also inspire theoretical multiscale models to fully reveal the benefits of molecule-zipped electrode/catalyst interfaces towards efficient and cost-effective water splitting.
11:15 AM - EN07.01.08
Solution-Based Growth of ZnO@ZIF-8 Core-Shell Nanowires-MOF for Efficient Photoelectrochemical Water Splitting
Alejandro Galan1,Andrew Gallant1,Del Atkinson1,Dagou Zeze1
Durham University1Show Abstract
The search for new materials functionalities has led to development of many different types of composite materials that bring enhanced benefits from the synthetic combination of different materials properties arising from their chemical and structural characteristics.
The evolution of nanomaterials over the last few decades has prompted widespread research on their integration into a range of applications resulting from their outstanding physical, optical and electric properties. Nanowires (NWs) have been at forefront of these developments due to the variety of NW materials, especially semiconductors, highly controllable growth coupled with the large surface to volume ratio and the high aspect ratio. ZnO in particular is highly researched due to its wide band gap (3.37 eV) and high exciton binding energy (60 meV) that confers interesting piezoelectric and optoelectronic properties. In recent years, research interest in metal-organic frameworks (MOFs) has developed rapidly. MOFs are a zeolite-like nanomaterial that have been extensively studied due to their remarkably high surface area, enabling catalytic and gas storage properties. MOFs are reticular nanomaterials formed by transition metal cations coordinated by multidentate organic linkers. This combination results in a crystalline material with a flexible structure that can be tuned accordingly to the organic linker chosen. Among the many existent MOF families, the zeolitic imidazolate frameworks (ZIF) are one of the most researched because of their exceptional thermal and chemical stability. ZIF-8 is a standout MOF comprised of zinc as transition metal nuclei and imidazolate linkers as the coordinating agent.
Here, these different forms of nanoscale materials prompted the idea for synergetic combination to create novel nanocomposite materials that exploit the individual outstanding properties, in this case for ultimate application in hydrogen production. In this work, the idea is the realisation of the integration of ZnO NWs and ZIF-8 to form a core-shell structure in which ZnO is the core and ZIF-8 is the shell around it (ZnO@ZIF-8) via a low-cost and fast production processes. The ZnO NWs are grown through a typically used chemical bath deposition method while the ZIF-8 shell is grown by surface conversion of the NWs using spin coating of a methanol solution containing the imidazole linker. The cheap and easy route to grow the core-shell structure through this process is remarkable, especially considering that the most commonly used alternative is a hydrothermal growth that requires the use of a pressure vessel, water/dimethylformamide mixtures and long reaction times. In our case, only few droplets of a methanol solution are required to obtain comparable results.
This work presents the structural analysis and the optoelectronic characterisation of these nanocomposite materials and aims to show the first results using ZnO@ZIF-8 structures as a photoanode for the photoelectrocatalytic (PEC) water splitting of water. Electron microscopy and x-ray analysis are used to study the structure and voltammetry and optical methods are sued to study the electronic properties. The effect of different coating conditions are compared to demonstrate the control over the growth of the MOF shell in relation to thickness and the long-range homogeneity. It is shown that the shell thickness is a critical factor that can directly influence the performance of the photoanode by controlling the electrolyte diffusion time to the ZnO NW core. The effectiveness of the core-shell structure as photoanode is compared to as-grown ZnO NWs. The PEC tests also show that the presence of the shell in the core-shell structure passivates the surface states of the ZnO core, improving its stability over time, and protecting it from photocorrosion. This in turn results in an improved photocurrent density and better durability, meaning that the shell improves both the performance and lifetime of the photoanode.
11:30 AM - EN07.01.09
Coupling Interface Constructions of 2D Nanosheet Arrays/FeNi Heterostructures for Efficient Electrochemical Water Splitting
Qian Xiang1,Yi Wu1,Wenlong Chen1,Fan Li1,Tao Deng1,Jianbo Wu1
Shanghai Jiao Tong University1Show Abstract
Water splitting is considered as a pollution-free and efficient solution to produce hydrogen energy. Low-cost and efficient electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are needed. Recently, chemical vapor deposition is used as an effective approach to gain high-quality MoS2 nanosheets (NSs), which possess excellent performance for water splitting comparable to platinum. Herein, MoS2 NSs grown vertically on FeNi substrates are obtained with in situ growth of Fe5Ni4S8 (FNS) at the interface during the synthesis of MoS2. The synthesized MoS2/FNS/FeNi foam exhibits only 120 mV at 10 mA cm−2 for HER and exceptionally low overpotential of 204 mV to attain the same current density for OER. Density functional theory calculations further reveal that the constructed coupling interface between MoS2 and FNS facilitates the absorption of H atoms and OH groups, consequently enhancing the performances of HER and OER. Layered double hydroxide (LDH) has also been widely applied to electrocatalysis for water spiliting, especially toward the OER, owing to its flexible layered structure and multifunctionality. Herein, FeNi LDH nanosheet arrays are directly synthesized on various metal foils by a facile hydrothermal method. Compared with single Fe or Ni substrates, the obtained FeNi LDH/ FeNi foil exhibited an ultrasmall onset overpotential of ∼90 mV, high catalytic activity (overpotential of 130 mV @ 10 mA/cm2), and durable stability in 0.1 M KOH electrolyte. We also demonstrate, by utilizing density functional theory calculations, that the growth of the hydroxide interfacial layer between LDH and FeNi foil makes the LDH possess more favorable adsorption to the OH intermediate during OER than the pure LDH. This reveals that the vertical FeNi LDH arrays on the FeNi alloy substrate are prone to be an efficient catalyst toward water splitting.
These newly developed synthesis of vertical grown nanosheets arrays on the FeNi substrates with low overpotentials and high performance strongly demonstrated the potential applications of 2D nanosheet arrays electrode for electrocatalysis, especially for high access of ion or gas molecule as well as excellent electron and ion conductivity. The findings in this work could impact the design and fabrication of 2D nanosheet arrays heterostructures electrodes for the application of catalysis, environmental science, and surface reaction.
 ACS Energy Letters 3 (10), 2357-2365.
 Advanced materials 30 (38), 1803151.
EN07.02: Photo- and Electro-Catalysis I
Monday PM, December 02, 2019
Sheraton, 2nd Floor, Liberty BC
1:30 PM - EN07.02.01
Identifying the Limiting Processes at Electrochemical Interfaces—From Experimental Data to Multiscale Modeling
Anja Bieberle-Hütter1,Kiran George1,Matthijs van Berkel1,Xueqing Zhang1,Vivek Sinha1,Rochan Sinha1,Yihui Zhao1,Aafke Bronneberg1
Electrochemical interfaces are the heart of many alternative, sustainable energy applications. They determine the efficiency and the overall performance. However, in most cases we do not know which processes take place at the interface and which processes are limiting the performance. Experimental electrochemical measurement methods cannot measure mechanisms directly; measuring intermediate species is rather challenging [1,2]. Analysis methods, such as equivalent circuit fitting for electrochemical impedance data, do not allow to link the electrochemistry directly to the measured data. Modeling of these interfaces is a multiscale challenge as discussed in  and is still in its infancy.
In this talk, we introduce the audience to this dilemma with a case study on water oxidation in photo-electrochemical water splitting. We will discuss our recent experimental results on metal oxide (Fe2O3, WO3) – electrolyte interfaces [4-7]; in particular, we will discuss electrochemical and operando infrared spectroscopy data in the field of water splitting. Based on these studies, we will show the limitations of experimental studies in analyzing the limiting processes at the interface. We then introduce our new multiscale modeling approach that allows simulating electrochemical data directly from an electrochemical model [8,9]. We use a combination of atomistic and microkinetic modeling. We will show our approach step by step and will present simulated, electrochemical data, such as current-voltage curves and electrochemical impedance spectra. In addition, we will show surface coverage plots which are experimentally not available and will compare the simulated, electrochemical data to the experimental data. Furthermore, we will introduce kinetic Monte Carlo simulations to complement and extend the current multi-scale modeling approach. Our new multiscale modeling approach is generic and can be used for other electrochemical interfaces, such as those in fuel cells, electrolysers, or batteries.
 O. Zandi and T.W. Hamann, Nat. Chem. 8 (2016) 778.
 Y. Zhang, H. Zhang, A. Liu, C. Chen, W. Song, J. Zhao, J. Am. Chem. Soc. 140 (2018) 3264.
 X. Zhang and A. Bieberle-Hütter, ChemSusChem 9 (2016) 1223.
 R. Sinha, R. Lavrijsen, M.A. Verheijen, E. Zoethout, H. Genuit, M.C.M. van de Sanden, A. Bieberle-Hütter, ACS Omega (2019) accepted.
 R. Sinha, I. Taneyli, R. Lavrijsen, M.C.M. van de Sanden, A. Bieberle-Hütter, Electrochim. Acta 258 (2017) 709.
 Y. Zhao, S. Balasubramanyam, R. Sinha, R. Lavrijsen, M.A. Verheijen, A.A. Bol, A. Bieberle-Hütter, ACS App. Energy Mater. 1 (11) (2018) 5887.
 Y. Zhao, G. Brocks, H. Genuit, R. Lavrijsen, M.A. Verheijen, A. Bieberle-Hütter, Adv. Energy Mater. (2019) submitted.
 K. George, M. van Berkel, X. Zhang, A. Bieberle-Hütter, J. Phys. Chem. C 123 (15) (2019) 9981.
 X.Q. Zhang, P. Klaver, R. van Santen, M.C.M. van de Sanden, A. Bieberle-Hütter, J. Phys. Chem. C 120 (2016) 18201.
2:00 PM - EN07.02.02
The Role of Overlayers on Water Splitting Catalysis
Maytal Caspary Toroker1
Technion-Israel Institute of Technology1Show Abstract
Understanding the role of an overlayer material on a catalyst is crucial for improving catalytic activity. hematite (α-Fe2O3) is a widely studied catalyst commonly used for solar water splitting. It was found experimentally that the water splitting efficiency with α-Fe2O3 was enhanced by deposition of an α-Al2O3 overlayer. In order to understand the origin of this improvement, we perform first-principles calculations with density functional theory + U on the α-Fe2O3(0001) surface with an α-Al2O3surface overlayer. In agreement with experiment, we find that α-Al2O3 coverage decreases the overpotential required for water oxidation on α-Fe2O3. We explain this improvement through the decrease in the work function of α-Fe2O3 upon α-Al2O3 coverage that aids in extracting electrons during the water oxidation reaction. We suggest that selecting an overlayer with a smaller work function than that of the catalyst as a strategy for future development of better catalysts.
2:15 PM - EN07.02.03
Anodic Instability of the SrIrO3 Water Splitting Catalyst—In Situ Microscopy and Electrolyte Variation Studies
Andrew Akbashev1,2,William C. Chueh1,2
Stanford University1,SLAC National Accelerator Laboratory2Show Abstract
Structural instability of highly active oxide catalysts during electrochemical water splitting poses a significant challenge on the way to their implementation. The structural, morphological and electronic changes in materials under oxidizing conditions largely determine the resulting catalyst’s integrityat the solid-electrolyte interface. In some cases, the structural evolution of materials can be driven or accompanied by the dissolution and/or precipitation processes that are controlled by the electrolyte formulation and pH. Providing insight into the degradation mechanisms is crucial for distinguishing and classifying possible pathways and factors affecting the degradation kinetics.
In our work we study strontium iridate (SrIrO3) that, while being among the most active catalysts reported so far, exhibits structural instability during the oxygen evolution reaction. For quantitative analysis of degradation, we use SrIrO3 epitaxial thin films that have an easily characterizable surface with well-defined step terraces. We perform microscopic studies of the degradation process of SrIrO3 via a combination of in situatomic force microscopy (AFM), ex situcharacterization techniques (XPS, XRD, TEM) and variation in the electrolyte composition. In our in situAFM experiments we built a three-electrode flow cell that enables a high flux and efficient replenishing of the electrolyte, which allow us to reach high anodic potentials and reaction currents above those typically used to assess materials stability (over 10 mA/cm2). By enabling a reference height during in situelectrochemical AFM, we track morphological evolution and quantify the dissolution rate of SrIrO3. We demonstrate that the degradation of SrIrO3 catalyst follows different pathways in acidic and basic electrolytes. Furthermore, we found specific electrolyte formulations that can suppress the degradation of SrIrO3 and impede the kinetics of its anodic dissolution.
2:30 PM - EN07.02.04
Structure Predictions in Wet Environments for Ethanol and Water Adsorption on Anatase TiO2 (101) Surfaces
Giuseppe Fisicaro1,Simona Filice1,Silvia Scalese1,Giuseppe Compagnini2,Riccardo Reitano3,Ioannis Deretzis1,Luigi Genovese4,Stefan Goedecker5,Antonino La Magna1
CNR Institute for Microelectronics and Microsystems1,Dipartimento di Scienze Chimiche, Università di Catania2,Dipartimento di Fisica e Astronomia, Università di Catania3,Laboratoire de simulation atomistique (L_Sim)4,Department of Physics, University of Basel5Show Abstract
Titanium dioxide exhibits superior photocatalytic properties, mainly occurring in liquid environments through molecular adsorptions and dissociations at the solid/liquid interface. The presence of these wet environments is often neglected when performing ab-initio calculations for the interaction between the adsorbed molecules and the TiO2 surface. In this study we consider two solvents, i.e. water and ethanol, and show that the proper inclusion of the wet environment in the methodological scheme is fundamental for obtaining reliable results. Our calculations are based on structure predictions at a density functional theory level for molecules interacting with the anatase TiO2 (101) surface under both vacuum and wet conditions. A soft-sphere implicit solvation model is used to describe the polar character of the two solvents. The integration of ab-initio structure predictions and contiuum modelling for the solvent allows to tackle the varius scales of a solid/liquid interface with a reduced computational cost. As a result, we find that surface oxygen vacancies become energetically favorable with respect to subsurface vacancies at the solid/liquid interface. Ethanol molecules are able to strongly passivate these vacancies, whereas water molecules only weakly interact with the (101) surface, allowing the coexistence of surface vacancy defects and adsorbed species. Infrared and photoluminescence spectra of anatase nanoparticles exposing predominantly (101) surfaces dispersed in water and ethanol support the predicted molecular-surface interactions, validating the whole computational paradigm. The combined analysis allows for a better interpretation of TiO2 processes in wet environments based on improved computational models with implicit solvation features.
2:45 PM - EN07.02.05
Formation of Polarons—Their Electronic Structures and Effects in Metal-Oxide-Photoelectrocatalysts: α-Fe2O3 and BiVO4
Muhammad Huda1,Hori Pada Sarker1,Wolfram Jaegermann2
The University of Texas at Arlington1,TU Darmstadt2Show Abstract
Production of H2 from water splitting via photo-electrochemical process is one of the most talked about potential green technologies. The key material for this technology is photo-electro-catalyst. These materials are required to absorb sunlight, excite and transport charge carriers efficiently, and thereby split water molecule at the liquid-semiconductor interface. Metal-oxides are thought to be the most stable materials under the intense interfacial reactive conditions. Despite some of the metal-oxides have near-suitable bandgap, the overall solar to hydrogen generation efficiencies by water splitting are not as high as expected. One of the reasons for this deficient efficiency is attributed to the poor transport properties in metal-oxides, especially in the 3d transition metal-oxides. In this presentation, two representative metal-oxides photo-catalysts will be considered, α-Fe2O3 and BiVO4. Despite being efficient solar absorber materials, these have poor charge transport properties. From detail electronic structure calculations, polaronic states of these oxides will be depicted. In addition, it will be shown that how these polaronic states affect not only the transport properties but also the open circuit voltages, Voc. We will also discuss how well these computational results compares with the recent experimental outcome. The work of MNH is supported by National Science Foundation, grant #1609811. Computations were performed on Texas Advanced Computing Center.
3:30 PM - EN07.02.06
Recent Method and Application Advances in Multiscale Characterization of Carrier Transport in Materials for Water Splitting, Such as Cation- and Anion-Doped Bismuth Vanadate
Michel Dupuis1,Viswanath Pasumarthi1,Pavan Kumar Behara1,Taifeng Liu2,3,Can Li3
University at Buffalo, The State University of New York1,Henan University2,Dalian Institute of Chemical Physics3Show Abstract
The holy-grail in efficient and cost-effective conversion of solar energy into electrical and chemical energy is solar energy-driven water splitting using semi-conductor-based photo-catalysts. Overall conversion efficiencies of best systems so far are however far from the level needed for practical applications. Viable materials must exhibit good visible light absorption and carrier generation, good carrier transport, and good carrier redox reactivity. This presentation will focus on multiscale modeling of carrier transport in semi-conductors, combining quantum chemical calculations of polaron hopping by Marcus/Holstein theory and kinetic Monte Carlo (KMC) modeling of mesoscale transport. We will highlight recent method developments for cation- and anion-doped BiVO4.
4:00 PM - EN07.02.07
Relationship between the Photocatalytic Hydrogen Ion Reduction and Charge Carrier Dynamics of Pt/Cd1-xNixS catalysts
Tayirjan Isimjan1,Partha Maity2,Omar Mohammed2,Hicham Idriss3
SABIC-CRD at KAUST1,KAUST2,SABIC-CRD-KAUST/University College London3Show Abstract
The doping induced local impurity brings a significant modification to photocatalytic performance of the semiconductor. Finding the correlation between charge carrier dynamics and catalytic performance of the semiconductors offers useful information both for the fundamental understanding of the catalytic mechanism and the catalyst optimization. Here, the electron transfer during the hydrogen ion reduction reaction in the presence of Pt/CdS doped with Ni2+ (Cd0.98Ni0.02S) was studied using fs-pump probe transient absorption spectroscopy (TAS) complemented by photocatalytic tests. Cd0.98Ni0.02S is composed of both hexagonal and cubic phases with average particles of 7 nm in size; determined from TEM. TAS of Cd0.98Ni0.02S in the presence and absence of the hole scavenger (benzyl alcohol) and in the presence and absence of Pt helped to further probe into the origin of the two most pronounced signals in the 400-800 nm range: the ground state bleaching (GSB) at ca. 480 nm and the photo-induced absorption (PIA) at ca. 600 nm. The first is largely linked to electron de-excitation lifetime and the latter to hole lifetime. From the decay kinetics, it was possible to compute for the charge transfer yields (f) from the semiconductor to the Pt metal particles (electron sink) and for hole trapping lifetime in the presence of benzyl alcohol. The rate of the photocatalytic hydrogen production shows a positive relationship with the decay kinetics obtained by fs-pump probe measurements.
4:15 PM - EN07.02.08
Highly Efficient Membrane Electrode Assembly with Less than 10 nm NiFe-Layered Double Hydroxides for Anion Exchange Membrane Water Electrolysis
Hiroyuki Koshikawa1,Hideaki Murase1,Takao Hayashi1,Kosuke Nakajima1,Hisanori Mashiko1,Seigo Shiraishi1,Yoichiro Tsuji1
Panasonic Corporation1Show Abstract
Anion exchange membrane (AEM) water electrolysis is expected to offer a way of converting and storing electrical power generated from renewable energy into hydrogen. However, the development of a membrane electrode assembly (MEA) that are free of noble metals in the catalyst layer and capable of outperforming conventional Pt and Ir-based catalysts, remains a major challenge. Layered double hydroxides (LDHs) are known to exhibit high activity toward oxygen evolution reaction (OER) proceeding at the anode, and increasing the effective surface area by the miniaturization of LDH is a useful strategy to improve OER activity. In the present study, we report a one-pot synthesis of nanometer-sized NiFe-LDH through the application of a spontaneous gelation-deflocculation method and evaluated its electrochemical properties.
Nanometer-sized NiFe-LDH was synthesized by liquid phase reaction in the presence of a chelating agent. The distribution of the LDH particle size was measured by small angle x-ray scattering and the average diameter was calculated to be 7.2 nm, which is less than one tenth of the diameter of conventional LDHs. The chelating agent introduced into the media was thought to increase the concentration of metal hydroxide nuclei and suppress excessive growth of the LDH crystal, resulting in the synthesis of nanometer-sized LDH. NiFe-LDH showed superior OER activity compared to conventional iridium dioxide (IrOx) catalyst in terms of the overpotential required for flowing 10 mA cm-2 (254 mV for NiFe-LDH and 261 mV for IrOx). This overpotential for NiFe-LDH is one of the lowest reported for non-noble metal-based OER electrocatalysts. Notably, an MEA for AEM water electrolysis using NiFe-LDH as an anode catalyst exhibited an energy conversion efficiency of 74.7% in 1 M KOH for flowing 1 A cm-2 at 80 °C. This efficiency is the highest among MEAs implemented with LDH reported to date, and offers a viable replacement for IrOx anode catalyst. Further optimization of the synthesis conditions of our NiFe-LDH and the formation processes of the catalyst layer of the MEA will substantially increase the efficiency.
4:30 PM - EN07.02.09
Semiconductor Photocathodes Modified with Transparent Reduced Graphene Oxide Films Loaded with Molecular Sensitizers and Electrocatalysts for Hydrogen Evolution
Molly MacInnes1,Nicolai Lehnert1,Stephen Maldonado1
University of Michigan1Show Abstract
This presentation will describe thin, smooth reduced graphene oxide (RGO) films that also contain either a dissolved hydrogen evolution reaction (HER) molecular catalyst (cobalt (III) bis[benzenedithiolate]) and/or a molecular chromophore on p-type gallium phosphide (GaP) photocathodes. Molecular electrocatalysts and dyes can be employed heterogeneously by immobilization onto an electrode surface but the mode of attachment is typically specific to the particular electrode type. We have developed a method of depositing thin (≤ 10 nm), adherent graphene oxide films that can physisorb molecular species through π-π interactions. These thin films are deposited on a variety of electrode materials by spin coating graphene oxide suspensions. Immersion of these films in non-aqueous solutions of cobaltocene followed by drying produces flat, highly conductive reduced graphene oxide films that are platforms for the physisorption of molecular species. Electrochemical, photoelectrochemical, and spectroscopic data will be presented that demonstrates the robustness of these films and their ability to be modified with a molecular HER electrocatalyst or with a molecular dye. Additionally, finite-element modelling data will be presented that investigates activities of molecular catalysts immobilized on the outer-most surface as well as intercalated within the RGO film. The utility of these films for sensitization and protection against corrosion processes will be featured.
4:45 PM - EN07.02.10
Highly Flexible and Efficient Photoanodes Using Surface Functionalized Metal-Oxide Nanostructures for PEC Applications
Koteeswara Reddy Nandanapalli1,Devika Mudusu1,Jinkyu Song1,Kyung-In Jang1,Sungwon Lee1
Daegu Gyeongbuk Institute of Science & Technology (DGIST)1Show Abstract
Recent years, the development of flexible devices for various day-to-day applications including energy harvesting devices, medical devices, wearable electronics, energy storage, communications, sensors, has received great attentiveness owing to their affordability, applicability & wearability, light-weight, and conformality . In this direction, we have developed highly flexible and light-weight heterostructures with an overall thickness of ~50 µm by using low-temperature techniques . Surface passivated vertically aligned zinc oxide (ZnO, ~1 µm length) nanostructures with amorphous cobalt-oxide (CoxOy, < 2 nm thickness) have been developed on different electron-collecting layers deposited flexible sheets. The impact of the electron-collecting layer on the morphology and structure of ZnO nanostructures along with their electrical properties has been investigated. Then, the photoelectrochemical water-oxidation performance of the heterostructures along with their chemical stability and durability were studied. Besides these, the influence of multiple bendings on the device performance of the structures was explored.
We acknowledge the financial support of the National Research Foundation of Korea (NRF-2017R1D1A1B03035194, 2018R1A5A1025511, and 2017R1A2B4012119).
 Fan et al. Adv. Mater. 28 (2016) 4283.
 Koteeswara Reddy et al. ACS Appl. Mater. Interfaces 8 (2016) 3226.
Maytal Caspary Toroker, Technion-Israel Institute of Technology
Francesc Illas, University of Barcelona
Michele Pavone, University of Napoli Federico II
Guofeng Wang, University of Pittsburgh
EN07.03: Photo- and Electro-Catalysts II
Tuesday AM, December 03, 2019
Sheraton, 2nd Floor, Liberty BC
8:30 AM - EN07.03.01
Integrating Supramolecular Photocatalysts into Photoelectrocatalytic Devices
Newcastle Univ1Show Abstract
Efficient dye-sensitized photocathodes offer new opportunities for converting sunlight into storable energy cheaply and sustainably. We are developing dye-sensitized NiO cathodes for the photo-reduction of carbon dioxide or water to high energy products (solar fuels) using the lessons we have learnt from solar cells. The potential advantage of this strategy is it exploits the selectivity of a molecular catalyst in a robust device. Assembling two photoelectrodes in a tandem configuration (see figure) enables water oxidation at the photoanode to supply electrons to the photocathode to be consumed in the reduction of e.g. H+ to H2. Generating hydrogen on one electrode and oxygen on another enables the two gasses to be collected separately. Additionally, by separating the functions of light absorption, charge transport and catalysis between the colloidal semiconductor and molecular components, the activity of each can be optimised, rather than relying on one material to have all the necessary credentials. The electron-transfer dynamics are key to the performance and a major challenge is slowing down charge recombination between the photoreduced dye and the oxidised NiO so that chemistry can take place. Highlights from recent work examining charge-transfer at the interface between NiO and new supramolecular photocatalysts using transient absorption spectroscopy and time-resolved infrared spectroscopy will be presented. The effect of the environment on the kinetics will be discussed. The relationship between the structure, dynamics and performance of the photoelectrocatalytic devices will be summarized.
 E. A. Gibson, Chem. Soc. Rev., 2017, 46, 6194 – 6209,
 N. Põldme, L. O’Reilly, I. Fletcher, I. Sazanovich, M. Towrie, C. Long, J. G. Vos, M. T. Pryce, E. A. Gibson Chem. Sci., 2019,10, 99-112.
 F. A. Black, A. Jacquart, G. Toupalas, S. Alves, A. Proust, I.P. Clark, E.A. Gibson, G. Izzet Chem. Sci. 2018, 9, 5578-5584.
9:00 AM - EN07.03.02
Artificial Photosynthesis Using Perovskite Materials: A SrSnO3 Case Study
Conor Price1,Ned Taylor1,Francis Davies1,Shane Davies1,Steven Hepplestone1
University of Exeter1Show Abstract
Photoelectrolysis offers a mechanism for long term, clean energy storage via hydrogen production. Recently, several perovskites have shown promise for this application [1,2]. Perovskites, ABX3, are highly customisable, with several options for the A, B and X ions. This results in a large range of tailorable properties, such as the band gap and the relative stability, making them highly suitable for photoelectrolysis. The bulk properties of many perovskites have been explored [2,3]. However, their potential for photoelectrolysis is mainly governed by surfaces, making their study critical.
Using techniques such as density functional theory, we develop a large scale theoretical screening process to tackle the large range of A, B and X combinations. We can further tailor the surface band structure with aggregates/surface coatings. Here we present the results of this method for a group of stannate perovskites, highlighting a case study perovskite, SrSnO3. We examine the effect of forming surfaces and the introduction of adsorbants (hydrogen, oxygen and OH groups) onto the surface, taking note of the band alignment with respect to the evolution potentials. These adsorptions allow us to investigate the intermediate energetics of the hydrogen evolution and oxygen evolution, and the overpotentials associated with these reactions. We then conclude with a discussion of how aggregates/surface coatings can be used to tailor the surface properties for more efficient water splitting.
1. Int. J. Hydrogen En., 2002, 27, 991-1022
2. Energy Environ. Sci., 2012, 5, 9034-9043
3. J. Mater. Res., 2017, 22(7), 1859-1871
9:15 AM - EN07.03.03
Understanding the Complex of 3d Transition Metal Dopants and Sulfur Vacancies in the Activation of MoS2 Basal Plane for HER—From Structures to Intrinsic Descriptors
Mingjie Liu1,Qin Wu1
Brookhaven National Laboratory1Show Abstract
MoS2 based materials as promising alternative electrocatalysts for hydrogen evolution reaction (HER) have been studied extensively. The activation of MoS2 basal plane for HER can be achieved by introducing transition metal dopants or sulfur vacancies. The activation is known as strengthening the hydrogen atom binding on “active” sites compared with on pristine MoS2 basal plane. The intrinsic descriptors for hydrogen binding, however, are still ambiguous in contrast to the d-band center for transition metals. In this work, we systematically studied 3d metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) doped in MoS2 from their local structures to the intrinsic descriptors for hydrogen binding. We found that the metal dopants prefer to form clusters, and the sulfur vacancy formation is strongly affected by the local dopants configuration. Two mechanisms for hydrogen adsorption on MoS2 based materials with dopant/vacancy complex are clarified. The intrinsic descriptors for both mechanisms are proposed respectively. Our results illustrate the dopant and vacancy can synergistically form and catalyze HER effectively. The intrinsic descriptors would be beneficial to understand this system and provide guidelines for other transition metal dichalcogenides HER catalysts design.
9:30 AM - EN07.03.04
Fundamental Energetics of n-Si/SiOx/p-NiO Photoanode for Water Oxidation and Sulfur Doping Effect on PEC Performance
Jemee Joe1,Thi Anh Ho1,Changdeuck Bae1,Hyunjung Shin1
Sungkyunkwan University1Show Abstract
n-Si/metal oxide heterojunction structures have been intensively investigated for application in photoelectrochemical (PEC) water splitting obtaining high voltages and preventing surface photocorrosion. Thanks to the relatively high conduction band offset compared to the valence band offset with Si, p-NiO has been studied as both hole-selective and photocatalytic layers on Si surfaces. However, most of the Si/NiO heterojunctions focused on NiOx to date, and the studies on relevant energetic are lacking due to the properties of the materials of NiOx with unknown phases and the related surface reactions. Here, we describe the energetic study of simple stoichiometric NiO on n-Si and its relation to the water oxidation reactions. Atomic layer deposition (ALD) technique was used for fabricating NiO films for it could offer pin-hole-free, and compact layers in an ultraprecise manner, and systematic study. Moreover, doping sulfur in NiO further tuned the surface energetics by modifying the resulting conductivity. Our optimized n-Si/SiOx/p-NiO(S) photoanodes showed well over 24 h of stability under 1 sun illumination with 27 mAcm-2, suggesting a great potential for efficient PEC water splitting.
9:45 AM - EN07.03.05
Concurrent Photocatalytic Hydrogen Generation and Dye Degradation Using MIL-125-NH2 under Visible Light Irradiation
Stavroula Kampouri1,Tú Nguyên1,Mariana Spodaryk1,2,Robert Palgrave3,Andreas Züttel1,2,Berend Smit1,Kyriakos Stylianou1
Ecole Polytechnique Federale de Lausanne1,EMPA Materials Science and Technology2,University College London3Show Abstract
Solar to hydrogen (H2) energy conversion represents the Holy Grail of energy science and technology. For decades, numerous materials have been developed and used as photocatalysts for this purpose. However, their inadequate visible light absorbance, poor stability and fast charge recombination have prevented their wide, industrial-scale deployment. Additionally, the use of noble metal-based co-catalysts and the toxicity of the majority of electron donors employed in these photocatalytic systems limit the profitability of this technology. This study aims to tackle these issues using a two-fold strategy. Firstly, we systematically studied the impact of different transition metal-based co-catalysts towards the photocatalytic water reduction, when they are physically mixed with the visible-light active MIL-125-NH2. All co-catalyst/MIL-125-NH2 photocatalytic systems were found to be highly stable after photocatalysis, with the NiO/MIL-125-NH2 and Ni2P/MIL-125-NH2 systems exhibiting high hydrogen (H2) evolution rates of 1084 and 1230 μmol h-1 g-1, respectively. Secondly, we investigated how different electron donors affected the stability and H2 generation rate of the best Ni2P/MIL-125-NH2 system and found that triethylamine fulfils both requirements. By understanding the impact of these components, we then replaced the electron donor with rhodamine B (RhB), a dye that is commonly used as a simulant organic pollutant, with the aim of integrating the visible-light driven H2 evolution with water remediation in a single process. This is the first study where a metal-organic framework (MOF) system is used for this dual-photocatalytic activity under visible light illumination and our proof-of-concept approach envisions a sustainable wastewater remediation process driven by the abundant solar energy, while H2 is produced, captured and further utilized. The discovery of such photocatalytic systems paves the way for the development of processes using wastewater to produce clean H2 and our approach could be extended to other applications where the concurrent oxidation and reduction of species could be of great importance.
10:30 AM - EN07.03.06
Defective But Effective—The Role of Defects in Oxide-Based Electrocatalysis
Univ of Naples Federico II1Show Abstract
Ab initio simulations play an increasingly important role in materials sciences. In the energy conversion scenario, computational modeling can elucidate, with atomic resolution, the subtle composition-structure-property relationships behind the performance of functional materials and complex interfaces. In heterogenous electrocatalysis, the underlying bulk and surface charge/mass transport events are often stronlgy dependent on the presence/absence of structural defects. Thus, a deep knowledge of defect chemistry is key for understanding, tuning and optimizing the properties of different catalysts.
In particular, this contribution will report recent advances in the DFT-based characterization of the role of defects in strongly correlated oxides as electrodes in (photo)electrocatalytic devices. Three case studies will be presented: (i) a new triple-conducting oxide based on Sr2Fe1.5Mo0.5O6 perovskite with promising bifunctional catalytic activity towards oxygen reduction and evolution reactions [1-3] (ii) Fe-doped ZrO2for low temperature SOFCs  and (iii) CuFeO2delafossite for CO2photorreduction . In all cases, we will discuss how surface oxygen vacancies -boosted by aliovalent doping- enable better performaces and change product selectivity.
 ABMG, D. Bugaris, M. Pavone, J. P. Hodges, A. Huq, F. Chen, H-C. zur Loye, E. A. Carter. J. Am. Chem. Soc 134, 6826 (2012)
 ABMG, M. Pavone, Chem. Mater 28, 490 (2016)
 ABMG, M. Pavone, J. Mater. Chem. A 5, 12735 (2017)
 P. Madkikar, D. Menga, G. Harzer, T. Mittermeier, A. Siebel, F. E. Wagner, M. Merz, S. Schuppler, P. Nagel, ABMG, M. Pavone, H. A. Gasteiger, M. Piana. J. Electrochem. Soc. 2019 166, F3032-F3043
 J. Gu, A. Wuttig, J. W. Krizan, Y. Hu, Z. M. Detweller, R. J. Cava, A. B. Bocarsly J. Phys. Chem. C 117, 12415 (2013)
11:00 AM - EN07.03.07
Design and High-Throughput Discovery of Conjugated Polymer Photocatalysts for Photocatalytic Hydrogen Evolution from Water
Reiner Sprick1,Yang Bai1,Christian Meier1,Liam Wilbraham2,Martijn Zwijnenburg2,Andrew Cooper1
University of Liverpool1,University College London2Show Abstract
Photocatalytic hydrogen production from water is a research area of immense interest as hydrogen has been identified as a potential energy carrier of the future. We have previously shown that conjugated polymers can be active for photocatalytic hydrogen evolution from water in the presence of a sacrificial electron donor. However, a major challenge remains to find materials with higher efficiencies as often the efficiency of thee conjugated polymer photocatalysts is limited by their poor wettability, lack of visible light absorption or unsuitable band positions.
In this contribution we show that these factors can be controlled by structural design giving materials with higher activities. Even more active materials were found by using an integrated computational and experimental high throughput approach of large polymer libraries. These libraries of conjugated polymers were obtained by high throughput synthesis methods and then analyzed using high throughput well plate systems, such as powder XRD, UV-vis, photoluminescence lifetime, and gas sorption. Furthermore, the performance as photocatalysts was tested using an in-house build testing system. Using this approach allowed us to make and test more examples of conjugated polymer photocatalysts than the total number of published structures in the literature combined. As a result, we were able to find materials with much higher activities compared to our previously best materials. In fact, the best material has one of the highest rates for hydrogen production reported in the literature under visible light and a high external quantum efficiency of 20.7% at 420 nm, which is one of the highest reported to date for organic polymer photocatalysts. Furthermore, we were able to correlate the performance of these materials with their properties by using machine learning. We found a good overall correlation with a set of factors highlighting the interconnectivity of various factors responsible for the photocatalytic activity of the materials.
11:15 AM - EN07.03.08
Exciton Delocalization vs Charge Separation in Biomimetic PBI-Based Supramolecules for Artificial Photosynthesis
Margherita Maiuri1,Mattia Russo1,Luca Moretti1,Francesco Rigodanza2,Andrea Sartorel2,Maurizio Prato3,Marcella Bonchio2,Giulio Cerrulo1
Politecnico di Milano1,University of Padova and CNR-ITM2,University of Trieste3Show Abstract
Converting solar energy into chemical energy with the aid of a photocatalyst is a promising strategy to solve long-term energy problems. Specifically, engineering a synthetic analogue of a photosynthetic unit which absorbs sunlight and uses it to split water, is an ambitious challenge towards the achievement of an artificial photosynthesis.
Towards this goal, major synthetic efforts have been focused on the design of dyads based on a light-absorber (usually a Ru–based complex) and a water oxidation catalyst (WOC). Upon dyad photoexcitation an electron transfer event occurs, however it is usually strongly quenched by possible ultrafast intra-dyad recombination, thus limiting the dyad efficiency.
Recently, some of us proposed a new synthetic strategy inspired by the concept of the ‘quantasome’, defined as the minimal photosynthetic unit responsible for the solar-energy conversion. A quantasome uses a self-assembled light-harvesting antenna in combination with one catalytic cofactor. The artificial quantasome reported here is specifically designed for oxygen evolution1. It exploits the self-assembly of multiple perylene bisimides (PBI) chromophores which interact with a Ru-polyoxometalate WOC (Ru4POM). The resulting [PBI]5Ru4POM complex shows an amphiphilic structure and dynamic aggregation into large two-dimensional domains, forming cylindrical units where five PBIs surround one Ru4POM. The PBI self-assembly induces a clear red-shift of the optical absorption, suggesting strong excitonic delocalization. The interplay between the creation of excitonic states, the formation of charge transfer states and the deactivation of the excitons dictates the efficiency of the quantasome.
Femtosecond transient absorption shows that upon excitation of the PBI moieties, the [PBI]5Ru4POM singlet excited state undergoes an ultrafast (ca. 1ps) decay to a charge transfer (CT) state, followed by a formation of a stable charge-separated state1. The ultrafast CT formation is taken as signature of a good quantum efficiency. Nevertheless, a clear understanding of the formation of a favorable PBI-based excitonic manifold is still lacking.
Here we apply two-dimensional electronic spectroscopy (2DES) to study the interplay between exciton delocalization and CT formation in the [PBI]5Ru4POM quantasome. 2DES represents a suitable tool to study exciton delocalization in multi-chromopores systems, providing a series of excitation/detection correlation energy maps at different probe delays T. Exploiting sub 20-fs broadband visible pulses, we report a 2DES measurement obtained on the quantasome and we compare it with the ones obtained from an isolated PBI in solution and a self-assembled PBI aggregate.
In 2DES maps, we distinguish two contributions: the signals on the diagonal and the ones observed out of the diagonal (cross-peaks). For the quantasome, at early times (T<20fs), we observe instantaneous positive diagonal peaks appearing in the blue/green spectral region (500-550 nm), reflecting the bleaching of the PBIs excitonic transitions, as well as one negative cross-peaks in the red-shifted detection window at 550/700nm excitation/detection wavelength, assigned to a photoinduced absorption from individual PBI excited states (as confirmed by a reference 2DES map on isolated PBI). At later times (T=100fs) we notice the formation of a second negative cross peak at the 550/600nm wavelengths, completed in about 1ps. Interestingly, similar signal is also observed in the PBI aggregate and we ascribe it to an excited state absorption band from the PBI exciton manifold. By comparing the spectral shapes and the timescales of the two crosspeaks in the PBI aggregate and in the quantasome we distinguish excitonic effects from electron transfer events.
Our results will contribute in understanding the design principles underlying the PBI-based supramolecular systems for efficient artificial photosynthesis.
 M. Bonchio et al. Nature Chemistry 11,146 (2019).
11:30 AM - EN07.03.09
Oxygen Evolution Reaction of Epitaxially Stabilized Columbite IrO2
Kyuho Lee1,Raul Flores1,2,Yunzhi Liu1,Michal Bajdich2,Yasuyuki Hikita2,Robert Sinclair1,Harold Hwang2,1
Stanford University1,SLAC National Accelerator Laboratory2Show Abstract
The conversion of electrical energy into hydrogen gas via water electrolysis is widely considered to be a promising energy storage mechanism to compensate the intermittent nature of the leading sources of renewable energy.1,2 The key to enhancing the overall efficiency of polymer electrolyte membrane (PEM) electrolyzers lies in the development of acid-stable catalysts for the oxygen evolution reaction (OER), the rate-limiting half-reaction of water electrolysis.2 Recently, ternary iridium oxides have been reported to show high catalytic activity and stability in acid, often associated with the formation of distinct IrOx structures at the surface by the leaching of the second cation.3,4 Various Ir-O structures, including polymorphs of IrO2, are identified as possible candidates for these highly active IrOx structures.3 In parallel, theoretical calculations predict that some polymorphs of IrO2, including columbite IrO2, could be more active OER catalysts than the conventional rutile IrO2.5 These studies strongly motivate the synthesis of new polymorphs of IrO2, but their stabilization has been limited due to the outstanding stability of rutile IrO2.5-7
An effective approach to overcome this challenge is to employ heteroepitaxy, in which the metastable polymorph is preferentially stabilized by growing on a well lattice-matched substrate due to enhanced interfacial energy.8 We have successfully stabilized columbite IrO2 (100) in epitaxial thin films for the first time using pulsed laser deposition. X-ray diffraction and transmission electron microscopy measurements are used to identify and characterize the structure of these films. Measurements of the OER catalytic activity of columbite IrO2 (100) thin films reveal clear differences from their rutile counterpart. Details of the activity and comparison with theoretical calculations will be discussed.
1 Turner, J. A., Science 305, 972 (2004).
2 Fabbri, E. et al., Catal. Sci. Technol. 4, 3800 (2014).
3 Seitz, L. et al., Science 353, 1011 (2016).
4 Lebedev, D. et al., Chem. Mater. 29, 5182 (2017).
5 Xu, Z. and Kitchin, J. R., Phys. Chem. Chem. Phys. 17, 28943 (2015).
6 Mehta, P. et al., ACS Appl. Mater. Interfaces 6, 3630 (2014).
7 Manjón, F. J. et al., Phys. Status Solidi B 246, 9 (2009).
8 O. Y. Gorbenko et al., Chem. Mater. 14, 4026 (2002).
11:45 AM - EN07.03.10
Effect of Dissolved O2 on Band Alignment at n-GaN/NaOH Interface
Yuki Imazeki1,Supawan Ngamprapawat1,Masahiro Sato1,Katsushi Fujii2,Tsutomu Minegishi1,Masakazu Sugiyama1,Yoshiaki Nakano1
The University of Tokyo1,RIKEN Center for Advanced Photonics2Show Abstract
For efficient solar-to-hydrogen conversion via photoelectrochemical water splitting, semiconductor electrodes such as Fe2O3 and BiVO4 have been developed. On such n-type semiconductors, O2 gas is generated by photoelectrochemical water oxidation. Therefore, amount of dissolved O2 increases with the progress of water splitting in the vicinity of semiconductor/electrolyte interface. Band alignment around the interface has an essential impact on the carrier transport which dominates the progress of photoelectrochemical reactions, and the dissolved O2 will certainly affects the band alignment. We therefore investigated the band alignment under illumination at the interface with and without dissolved O2 using n-GaN/NaOH. as a model system.
The band alignment at the semiconductor/electrolyte interface is characterized by the flat band potential and the (quasi-) Fermi level of a semiconductor. The former can be obtained via Mott-Schottky analysis and the latter can be measured as open-circuit-potential (OCP). These measurements were performed with the three electrodes system which consists of a single crystalline n-GaN (0001) photoanode, a platinum wire as a counter electrode and a Hg/HgO/1M NaOH (+0.113 V vs. SHE) as a reference electrode. Before a measurement, the surface of the n-GaN electrode was conditioned by electrochemical reduction using cyclic voltammetry from -0.25 to +0.45 V vs. RHE in a 1M NaOH. degassed by evacuation. To characterize the band alignment under illumination, a He-Cd laser (325 nm) was used for excitation of carriers in n-GaN. To evaluate the drift of Fermi level under illumination, OCP was measured as a function of photon flux ranging from 1.1×109 to 7.5×1016 s-1cm-2 either in degassed 1M NaOH or in the same electrolyte bubbled with O2 at 1 atm.
Without illumination, the flat band potential was -0.53 and -0.60 V vs. RHE for the degassed and the O2-bubbled NaOH, respectively. The negative shift in the flat band potential with dissolved O2 would be due to the potential difference at the electric double layer, since adsorbed oxygen on the surface of n-GaN may alter the structure of the double layer. Fermi level was evaluated as +0.70 and +0.73 V for the degassed and the O2-bubbled NaOH, respectively. As a result, band bending was increased from 1.23 to 1.33 V with O2 bubbling. OCP, i.e., Fermi level, is the potential at which anodic current is balanced with cathodic current. Therefore, its positive shift would be due to the increased cathodic current by oxygen reduction reaction.
Under illumination, OCP approached the flat band potential with increasing photon flux, assuming that the flat band potential was never changed by illumination. Between photon flux of 1.1×109 and 1.2×1011 s-1cm-2, O2 bubbling resulted in larger band bending by 0.09 - 0.14 V compared to the values at each photon flux for the degassed NaOH. In contrast, between photon flux of 1.2×1012 and 1.9×1014 s-1cm-2, band bending was 0.01 – 0.03 V smaller with O2 bubbling. When OCP exceeded the redox potential of H2 evolution, the behavior of OCP versus photon flux was affected by the existence of dissolved O2.
The observation above seems to indicate that O2 dissolved in an electrolyte adjacent to the semiconductor surface induces at least two phenomena: (1) a structural change in the electric double layer due to the adsorption of oxygen on the semiconductor surface, (2) removal of electrons from the semiconductor surface due to oxygen reduction current, resulting in the increase of band bending in the semiconductor. The relative impact of these phenomena on the band alignment may depend on several factors such as illumination intensity, O2 concentration in an electrolyte and the atomistic structure of the surface. The combination of Mott-Schottky analysis and OCP measurement as a function of light intensity will provide quantitative clues for the fundamental clarification of such impact of dissolved O2 in an electrolyte.
EN07.04: Collaborative Modelling and Experiments for Water Splitting
Tuesday PM, December 03, 2019
Sheraton, 2nd Floor, Liberty BC
1:30 PM - EN07.04.01
Oxygen Electro-Adsorption Kinetics on Well-Defined Oxide Surfaces
Jin Suntivich1,Ding-Yuan Kuo1,Hanjong Paik1,Darrell Schlom1
Cornell University1Show Abstract
We present our measurements of the oxygen electro-adsorption kinetics on IrO2 and RuO2 surfaces and their implications for our oxygen evolution reaction (OER) understanding. The slow kinetics of the OER is currently the largest source of inefficiency in water-splitting devices. The common picture is that the slow OER kinetics is a reflection of the OER’s multi-electron nature; as the OER takes place, the catalyst must stabilize a series of proton and electron transfer events via surface electro-adsorption. We present our measurements of the kinetics of these electro-adsorption events, specifically the deprotonation of H2O* to OH* and OH* to O*, two of the elementary steps of the OER. Our approach uses rate-dependent cyclic voltammetry to isolate the electro-adsorption rate constants from the electrochemical driving force. We further vary the electrolytes to examine how the rate constant varies with pH to reveal the nature of the proton and electron transfers on IrO2 and RuO2. Our measurement provide insights into the electro-adsorption process on oxides and information on the kinetics of the elementary steps within the OER.
2:00 PM - EN07.04.02
BSE Calculations for Hematite Water Splitting
Nadav Snir1,Maytal Caspary Toroker1
Technion-Israel Institute of Technology1Show Abstract
Hematite is a widely-studied photo-anode in photoelectrochemical cells (PEC) due to its visible-light band-gap (~2.2 eV) and chemical stability. There are many DFT+U studies for the electronic structure of hematite. In current work we go into more accurate calculations of hematite's absorption spectrum using the one-shot Green's functions (G0W0) and Bethe-Salpeter equations (BSE) methods which take excited states into account. We compared the calculated absorption spectra to experimental spectra and found a match in peak energy position. Furthermore, we found anisotropy of the absorption spectrum that is concurrent with the crystallographic structure of hematite. We also calculated the absorption spectra of hematite intermediates during the oxygen evolution reaction using the G0W0-BSE method and found that the *O intermediate is the dominant species under bias.
2:15 PM - EN07.04.03
Sn-Addition on Polycrystalline Hematite Reduces the Grain Boundary Blocking Effect Enhancing the Electronic Conductivity
Flavio De Souza2,Mario Soares1,Edson Leite1
UFSCar1,University Federal-ABC2Show Abstract
The poor electronic properties of polycrystalline hematite have been prevent its potential application as photoanode for light induced water splitting. Despite some progress have been made overcome this limitations remains as important challenge in the solar fuel production field [1-2]. The present work showed that the Sn-addition enhances the electronic transport of polycrystalline hematite (α-Fe2O3) by reduces the grain boundary block effect. A controlled sintering process allowed us freezing the state of electronic defects, in which the electrical properties of hematite are governed by the grain boundary and Sn segregation. Electrical measurements showed that the current flows preferentially through the grain boundary with presence of Sn-segregated. Sn-addition probably leads to decrease the grain boundary resistance. Atomic force microscopy and electric force microscopy (AFM/EFM) measurements confirm the results of the impedance analysis. The identification of preferential grain boundaries for electrical conductivity may have a direct influence on the light-induced water splitting performance of the hematite photoanode.
 Soares, M. R. S.; Costa, C. A. R.; Lanzoni, E. M.; Bettini, J.; Ramirez, C. A.; Souza, Flavio L.; Leite, E. R. “Unraveling the role of Sn-segregation in the electronic of polycrystalline hematite: raising the electronic conductivity by lowering the grain boundary blocking effect”, Advanced Electronic Materials, 6, 1900065, 2019.
 Jian Wang, Nicola H Perry, Liejin Guo, Lionel Vayssieres, Harry L Tuller. “On the Theoretical and Experimental Control of Defect Chemistry and Electrical and Photoelectrochemical Properties of Hematite Nanostructures”ACS applied materials & interfaces, 11, 2031, 2019.
2:30 PM - EN07.04.04
Optimization of Photoelectrochemical Performance of Hematite for Solar Water Splitting
University of Pretoria1Show Abstract
Hematite has attracted much attention due its superior properties for applications as a photoanode in a solar water splitting. Thin films and nanostructures of hematite were synthesized on fluorine-doped tin oxide by spray pyrolysis, dip coating and spin coating. X-ray diffraction results revealed (104) and (110) planes for all films, describing the rhombohedral structure of hematite. Raman spectra of the films confirmed two A1g and five Eg vibrational phonon modes of hematite. Scanning electron microscopy films showed nanoparticles of grain sizes varying from 30 to 40 nm. Cross-sectional images revealed film thickness of 430, 452 and 622 nm 521 nm for films prepared by spray pyrolysis, dip and spin coating techniques respectively. The films had an indirect band gap ranging from 1.96 to 2.30 eV. We carried out pump probe spectroscopic measurements on spin-coated samples to determine the electron-hole recombination rates in photo-excited samples. We obtained three distinct relaxation decay and recombination lifetimes in the ranges of sub-picosecond, hundreds of picoseconds and single digit nanoseconds. Furthermore, ab-anitio studies of surface doped hematite films using Cu, Zn and Cu/Zn co-doping showed thermodynamic stability.
2:45 PM - EN07.04.05
Engineering 1T-MoS2 with Transition Metal Doping as Efficient Electrocatalysts for Alkaline Hydrogen Evolution Reaction
Nuwan Attanayake1,Lakshay Dheer2,Sasitha Abeyweera1,Umesh Waghmare2,Daniel Strongin1
Temple University1,Jawaharlal Nehru Centre for Advanced Scientific Research2Show Abstract
Molecular hydrogen is considered as one of the most promising energy carriers due to their high energy density and environmental friendliness. The emission of unprecedented amounts of greenhouse gases during the combustion of fossil fuels is believed to be a major cause in climate change. Thus, greener fuels are sought after more than ever. The production of hydrogen gas by electrolysis of water or hydrogen evolution reaction (HER) using energy from renewable sources is one of the most evaluated areas as a viable solution. These extensive studies have identified many earth abundant materials to perform with high energy efficiency in the reduction of protons to hydrogen gas in acidic media. Nevertheless, there are few studies to accommodate materials as catalysts to reduce water to hydrogen gas in alkaline conditions. This would be a vital factor since the more challenging oxygen evolution reaction (OER) catalysts show good efficiencies in alkaline conditions (i.e. pH 14). Thus, materials which would split water to generate hydrogen in alkaline conditions are sought after as it would allow the water splitting reaction to be carried out in one electrolyte. MoS2 gained a lot of attention as an efficient catalyst in reducing protons to hydrogen in acidic conditions. Nevertheless, they suffer in stability and activity in alkaline conditions. In this study we report the synthesis of doped 1T phase of MoS2 to gain stability and provide improved activity than the pure 1T phase of MoS2. The metallic 1T phase of MoS2 was successfully doped with with varying concentrations of Co and Ni atoms in the basal planes of these MoS2 sheets. This doping reduced the overpotential (@10 mA/cm2) for the HER to 110 mV from 220 mV, relative to pure 1T phase in alkaline conditions with 10% of Co dopants in them. 10% Ni doping gives 120 mV overpotential for the HER at the same current density. These doped materials show low tafel slopes (~125 mV/dec) and good stability for the HER for more than 24 hours. We use density functional theory (DFT) calculations to explain the improvement in the catalytic activity and stability in these doped materials. These calculations suggest that the introduction of the impurities in these sheets lowers the energy required for the water dissociation step and reduces the binding energy towards the hydroxyl ions that wouldn’t leave the surface on pure MoS2. Furthermore, the calculations show that the introduction of the dopants reduce the H binding energy on the active sites which is a crucial factor in efficient HER catalysis.
3:30 PM - EN07.04.06
Multiscale Modelling Strategies for Alloy Design
IMDEA Materials Institute1,Technical University of Madrid2Show Abstract
Materials are transversal to all technologies and they are always at the core of the major disruptive discoveries. Either the synthesis of a material with novel properties leads to a technological breakthrough or current engineering materials are introduced and slowly improved for novel applications following a trial-and-error strategy. Both circumstances act as limiting factors of technological progress but advances in computer power and modeling tools have changed this scenario. In particular, a large number of modeling tools are nowadays available for particular ranges of length and time scales (density functional theory, molecular mechanics, computational thermodynamics, finite elements and finite differences, etc.). They have already proven their potential to predict the formation, structure and properties of materials and have been used to design materials with improved properties or unexpected structures, such as new catalysts or Li-based materials for batteries.
These success stories were possible because the critical structure or properties depended on phenomena with a particular time and length scale, which could be simulated using only one of the techniques mentioned above. This is not always the case and, in fact, it is unlikely to occur in materials for engineering applications. For instance, balanced mechanical properties (stiffness, strength, toughness) depend on many different processes which take place along nine or more orders of magnitude in length scales (from nm to m) and this dependence on different length scales is more evident in multifunctional (smart) materials.
In this talk, a roadmap for multiscale modelling of engineering materials for structural applications is presented around two pillars of virtual processing and virtual testing. In the first pillar, the microstructural development during processing is simulated through a cascade a simulation tools including ab initio calculations, cluster expansion and kinetic Monte Carlo to determine the phase diagram and the interface mobility. This information is used in combination with classical nucleation theory and phase-field modelling to obtain the microstructure (grain size, size, shape and spatial distribution of second phases). Virtual testing is accomplished by means of computational homogenization of a representative volume element of the polycrystalline microstructure, in which the properties of the single crystals and grain boundaries were obtained from ab initio, molecular dynamics and dislocation dynamics in combination with transition state theory. The roadmap is demonstrated in the development of Al-Cu alloys for aerospace applications. Future applications of this strategy to design new materials for enhanced electrocatalytic water splitting by elastic strain engineering is outlined.
4:00 PM - EN07.04.07
Carbon-Based Heterojunction with Optimizing Oxygen Concentration for Photoelectrochemical Water Splitting
Minyeong Je1,Yelyn Sim2,Uk Sim2,Heechae Choi1
University of Cologne1,Chonnam National University2Show Abstract
Recently, two-dimensional (2D) carbon-based heterojunction materials have exhibited high photocatalytic activities for water splitting and hydrogen evolution reaction (HER). Reduced graphene oxide (rGO) and graphitic carbon nitrides (C3N4) have attracted strong research interests in junctions because of excellent compatibility with other 2D materials and versatile functionalities. However, the functionality of rGO-C3N4 heterojunction is not very reliable, which means that the water splitting photocatalytic activity largely fluctuates with fabrication conditions. In this study, the effect of oxygen/carbon (O/C) ratio of rGO and C-O binding chemistry on the physicochemical properties and possibility to fine tune the photocatalysis of rGO-C3N4 composite was systematically and theoretically investigated using density function theory (DFT) calculations. Interestingly, from DFT calculations results, the work functions of pristine rGO (PrGO) and defective (DrGO) were found to be linearly increase with O/C ratio.
Especially, it indicated that PrGO and DrGO with O/C ratio range from 4% to 13% have a suitable work function value for the photocatalyst in junctions with C3N4, considering feasibilities of charge separations and water redox levels. It revealed that the charge transfer only occurs in PrGO with 4% O/C ratio and DrGO with 8 % and 13 % O/C ratio. Free energy diagram and volcano plot of H adsorbed on PrGO and C3N4/DrGO heterojunction was investigated to further understand the HER mechanisms. Furthermore, the experimental results were well verified that the rGO-C3N4 heterojunction having the proper O/C ratio shows the enhanced water splitting photocatalytic activity. Thus, it was demonstrated through DFT calculations and experimental results that controlling the proper O/C ratio is very important in the rGO-C3N4 heterojunction.
4:15 PM - EN07.04.08
Zn:BiVO4/Mo:BiVO4 Homojunction as Efficient Photoanode for Photoelectrochemical Water Splitting
Jae Myeong Lee1,Ji Hyun Baek1,Xinjian Shi2,Gill Sang Han1,Dong Hoe Kim3,Hyun Suk Jung1,Xiaolin Zheng2
Sungkyunkwan University1,Stanford University2,Sejong University3Show Abstract
Photoelectrochemical (PEC) water splitting has been extensively studied as a method to convert sunlight and water to hydrogen. Among the many obstacles facing PEC water splitting, a critical challenge is the lack of efficient photoanodes for the water oxidation reaction. Here, we demonstrate a n-n+ type-II homojunction of Zn:BiVO4/Mo:BiVO4, which enhance the bulk transport and surface charge transfer processes of the well-studied BiVO4 photoanode. In this homojunction, the Zinc and Molybdenum dopants move the BiVO4 Fermi level closer to valence band maximum and conduction band minimum, respectively, establishing a n-n+ type-II homojunction. The staggered band alignment between Zn:BiVO4 and Mo:BiVO4 facilitates electron-hole separation between the interface of two layers. The Molybdenum is a well-known donor dopant for BiVO4 and increases the donor concentration, leading to higher electrical conductivity. The Zinc dopant increases the number of oxygen chemisorption sites at the surface layer, reducing surface recombination rate. In summary, we demonstrate that a Zn:BiVO4/Mo:BiVO4 homojunction structure improves both charge transport and transfer efficiency.
4:30 PM - EN07.04.09
Investigation of Highly Stable Proton Exchange Membrane Water Electrolyzers with Low Catalyst Loading and Reduced Hydrogen Crossover
Ryan Ouimet1,Reza Mirshekari1,Haoran Yu1,Zhiqiao Zeng1,Leonard Bonville1,Prasanna Mani2,Allison Niedzwiecki2,Chris Capuano2,Katherine Ayers2,Radenka Maric1
University of Connecticut1,Nel Hydrogen2Show Abstract
Hydrogen, with a global production rate of about 50 billion kg/year, is one of the world’s most important chemicals . Today, hydrogen is mainly produced from natural gas and coal, generating CO2 as a byproduct. Recently, Proton Exchange Membrane Water Electrolyzers (PEMWEs) have been gaining in interest worldwide as a renewable energy source for hydrogen production. However, the product costs have not yet reached the market targets, mainly due to the immaturity of the manufacturing processes and materials cost. The catalyst coated membrane (CCM) is the largest cost contributor to the cell, due to the high catalyst loadings of platinum group metals (PGMs) and the current labor-intensive methods for CCM fabrication. In order to address the catalyst cost issue, there is a need to reduce the catalyst loading and develop the manufacturing process. Furthermore, stability and long-term operation at high current density has been challenging for electrodes with low PGM loading (less than 1 mg cm-2). Recent literature has shown longer-term catalyst stability from hundreds to several thousand hours of cell operation. Lewinski et al.  have demonstrated a nano-structured thin film (NSTF) cell with Ir anode loading of 0.25 mg cm-2 which achieved stability of 5000 h at 2 A cm-2. Rozain et al.  have also studied a low-loaded Ti supported Ir oxide anode catalyst (0.12 mg cm-2) which showed more than 1000 h of operation at 1 A cm-2. On the other hand, one of the principal technical issues hampering the extensive use of PEMWEs is hydrogen crossover which causes a serious safety hazard in the anodic compartment where oxygen is produced as the lower explosion limit of hydrogen in oxygen is about 4 vol%. It has been shown that the integration of a recombination layer incorporated directly into the membrane has significantly improved the hydrogen crossover with the ability to decrease this safety hazard .
In this work, a novel single-step electrode fabrication process, Reactive Spray Deposition Technique (RSDT), was employed to reduce the electrode manufacturing cost and develop 86 cm2 full CCMs with low catalyst loadings (cathode: Pt/Vulcan XC-72R with loading of 0.3 mgPt cm-2, anode: IrO2/Nafion® with loading of 0.2-0.3 mgIr cm-2) which meet the desired stability of >1000 h at 1.8 A cm-2, 50 °C, 400 psi hydrogen pressure. A Pt recombination layer directly introduced into the membrane was also used to reduce hydrogen crossover to the commercial threshold. The microstructural characteristics of the RSDT-derived CCMs were studied by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Stability and cell performance of the RSDT-derived CCMs were investigated using diagnostic tests including polarization analysis, cyclic voltammetry and electrochemical impedance spectroscopy.
 K.E. Ayers, J.N. Renner, N. Danilovic, J.X. Wang, Y. Zhang, R. Maric, H. Yu, Catalysis Today 262 (2016) 121-132.
 K.A. Lewinski, D. van der Vilet, S.M. Luopa, ECS Transactions 69 (2015) 893-917.
 C. Rozain, E. Mayousse, N. Guillet, P. Millet, Applied Catalysis B: Environmental 182 (2016) 123-131.
 C. Klose, P. Trinke, T. Bohm, B. Bensmann, S. Vierrath, R. Hanke-Rauschenbach, S. Thiele, Journal of Electrochemical Society 165(16) (2018) F1271-F1277.
4:45 PM - EN07.04.10
From Rusting to Solar Power Plants—A Successful Nano-Patterning of Stainless Steel 316L for Visible Light-Induced Photoelectrocatalytic Water Splitting
American University in Cairo1Show Abstract
A novel propitious nanoporous anodized stainless steel 316L (NASS316L) photoanode was developed for water splitting. The anodization could successfully produce a uniform nanoporous (∼ 90 nm in pore diameter) array (∼ 2.0 m thick) of NASS316L with a high pore density. Several techniques, including FESEM, EDX, XRD, XPS, ICP-OES, and UV−vis-NIR spectrophotometry, were employed to characterize the catalyst and to assess and interpret its activity toward water splitting. Surprisingly, the NASS316L retained almost the same composition of
the bare stainless steel 316L, which recommended a symmetric dealloying mechanism during anodization. It also possessed a narrow band gap energy (1.77 eV) and a unique photoelectrocatalytic activity (∼ 4.1 mA cm−2 at 0.65 V versus Ag/AgCl, 4-fold to that of α-Fe2O3) toward water splitting. The onset potential (−0.85 V) in the photocurrent-voltage curve of the NASS316L catalyst demonstrated a negative shift in its Fermi level when compared to α-Fe2O3. The high (23% at 0.2 V vs Ag/AgCl) incident-photon-to-current conversion efficiency and the robust durability revealed from the in situ analysis of the produced H2 gas continued recommending the peerless inexpensive and abundant NASS316L catalyst for potential visible-induced solar applications.
EN07.05: Poster Session
Maytal Caspary Toroker
Tuesday PM, December 03, 2019
Hynes, Level 1, Hall B
8:00 PM - EN07.05.01
Understanding the Reasons for the Absence of Photoactivity in β Manganese Vanadate (Mn2V2O7)
University of California, Irvine1Show Abstract
With depleting fossil fuels reserves and their possible role in accelerating climate change, solar energy stands out as one promising source for providing clean energy at a multi-terawatt scale to meet ever-increasing global energy demand. The challenge of its intermittent nature can be tackled by storing it in an energy dense material like chemical fuel. Water can be the abundant raw material for this conversion process. Thus, photoelectrochemical water splitting can be one possible route to solve the energy crisis.
Till date, no semiconductor deployed for photoelectrochemical water splitting has met the stringent criteria of the right bandgap of 1.6-2 eV, optimal band edge positions (CBM, few mV negative of HER and VBM, 300-500 mV positive of OER), electrochemical stability (at alkaline pH for OER) and good electron transport properties. However, manganese vanadate (Mn2V2O7), has been reported to have 1.8 eV bandgap, near optimal band edge positions, electrochemical stability in the alkaline pH and Mn 3d orbital based valence band leading to curvature in the valence band light holes.
In this work, we have developed a simple spin coating method to fabricate tunable, nanoporous thin films of manganese vanadate. The grain size can be tuned from 20nm to 200nm whereas the film thickness can be tuned from 100nm to 1.5 microns. The direct bandgap has been found to be 2 eV. Contrary to previous reports, the material is found to suffer from serious stability issues. It dissolves in neutral, acidic and mildly acidic pH within minutes, and undergoes vanadium leaching and amorphousization at alkaline pH. It is relatively stable in mildly alkaline pH, but, use of common buffer ions like phosphate accelerate the film dissolution even in this pH due to low Ksp of manganese phosphate.
Also, unlike previous reports, the material shows no photoactivity in pristine form or when stabilized by atomic layer deposited TiO2. Photoluminescence spectrum suggests the possibility of very small charge carrier localizing domains resulting in an uncharacteristically broad PL peak. Further studies have been done using transient absorption spectroscopy to study the carrier lifetime to probe long-lived traps and bulk charge carrier recombination. XPS measurements have been conducted to study the chemical environment of the film surface and exchange current density and impedance has been measured to probe the presence of any resistive surface layer impeding the charge transfer across the interface.
8:00 PM - EN07.05.02
Developing Stable and Efficient Water Splitting Devices for Solar H2 Production under Concentrated Light
Mohd Khan1,Purushothaman Varadhan2,Hicham Idriss1,Jr-Hau He2
SABIC1,King Abdullah University of Science and Technology2Show Abstract
Photoelectrochemical (PEC) water splitting into hydrogen and oxygen molecules has been touted as the most promising route for converting solar energy into a storable form. After decades of research solar-to-hydrogen (STH) efficiencies are now approaching 20% using III-V semiconductor solar cells, making the process close to implementation. Yet, the high cost and instability of these cells prohibit their practical use. Stable water splitting reactions should yield stoichiometric hydrogen and oxygen and these are seldom reported. Herein this work we demonstrate the importance of measurement of gaseous products (H2 and O2) , and show cases where total inactivity was observed due to corrosion while current measurements indicated stable and high performances. At the same time, we demonstrate integrated water splitting devices that use nickel for stoichiometric (2:1) hydrogen and oxygen production. Specifically we demonstrate over 400 hours of stability @ 12 % STH under 1 sun condition. In parallel we also demonstrate the use of concentrated light for as means for lowering the cost of H2 production. Specifically we report demonstrate devices that work under 25 suns @ 10% STH with stability of more than 250 hours, paving the way to developing practical solar H2 generators.
8:00 PM - EN07.05.03
Activation of Reaction Sites in Multinary Phyllosilicate Catalysts for Electrochemical Water Oxidation
Byunghoon Kim1,Ju Seong Kim1,Kisuk Kang1
Seoul National University1Show Abstract
The practical realization of a water-splitting system necessitates the development of high-performance oxygen evolution reaction (OER) catalysts. Despite tremendous research efforts aimed at identifying earth-abundant 3d transition-metal-based catalysts, their insufficient catalytic efficiencies continue to jeopardize their real-world application. Herein, we introduce amorphous cobalt–iron phyllosilicates (ACFPs) as highly efficient OER catalysts. The ACFPs were designed by tailoring the metal chemistry of the phyllosilicate framework and prepared using a facile room-temperature precipitation method. Structural characterization using X-ray diffraction, Fourier-transform infrared spectroscopy, and X-ray absorption spectroscopy revealed that the ACFP structure consists of laminations of silicate (SiO4) layers and layered Co–Fe (oxy)hydroxide motifs. The OER properties of the ACFP series were also sensitively affected by the Co/Fe ratio, with an exceptionally low overpotential (η ~ 329 mV for a current density of 10 mA cm−2) delivered at the optimized composition of 40 at.% Fe. This catalytic efficiency is greater than that of the structurally analogous Co–Fe (oxy)hydroxide as well as those of pure Co phyllosilicate and pure Fe phyllosilicate, suggesting the beneficial role of the phyllosilicate framework along with the synergistic interplay of Co and Fe ions in the framework. Density functional theory calculations revealed that the introduction of Fe at the surface of Co phyllosilicate perturbs the local structural environment of oxygen sites, providing additional active sites. This work enables more rapid optimization of novel phyllosilicate-based OER catalysts and suggests a valid strategy for the design of high-performance catalysts by chemically tuning both the redox-active and redox-inert elements concomitantly.
8:00 PM - EN07.05.04
Quantification of Surface Reactivity and Step-Selective Etching on Single-Crystal BiOI(001)
Julia Martin1,Roy Stoflet1,Katarina Himmelberger1,Alexander Carl1,Ronald Grimm1
Worcester Polytechnic Institute1Show Abstract
We synthesized single-crystal BiOI to characterize the interplay between etching chemistry and interfacial chemical and electronic structure at the (001) face of this emerging solar-relevant, 2-D-layered material. Experiments utilized BiOI single-crystals grown from BiOI powder via physical vapor transport (PVT) and via chemical vapor transport (CVT) with elemental I2 as a transport agent. Optical images of nascent crystals reveal 1–10 mm2 (001) facets but microscopic defects and pitting. Etching studies alternatively included ten-second exposures to 6 M aqueous HF, 6 M HCl, and a stepwise treatment of HF with an acetone rinse. Optical images of HF- or HCl-etched samples demonstrate clean terrace-rich regions and increased scattering at step-rich regions. X-ray photoelectron spectra (XPS) of HF- or HCl-etched, step-dense regions reveal increased iodine as well as fluoride or chloride features, while XRD presents BiI3 features. We attribute optical, XPS, and XRD results to the dissolution of interfacial bismuth oxides and the concomitant formation of BiI3 at the steps of the BiOI(001) face. Surface analyses of samples subjected to sequential HF and acetone rinsing demonstrate no detectable BiI3 by XRD, and Bi:O:I photoelectron area ratios demonstrative of stoichiometric BiOI. In concert with separate studies demonstrating high solubility of BiI3 in polar organic solvents, the sequential HF and acetone treatments appear to yield chemically pristine BiOI(001). Ultraviolet photoelectron spectra (UPS) of these surfaces demonstrate significant differences in work-function and Fermi-edge values for nascent, for HF-etch, and for HCl-etched BiOI(001) as compared to tape-cleaved, pristine BiOI(001). Importantly, UP spectra of BiOI(001) subjected to a sequential HF and acetone treatment resemble spectra of tape-cleaved, pristine BiOI(001). In concert, the sequential HF and acetone treatment yields BiOI(001) that best mimic the chemical and electronic structure of tape-cleaved surfaces. We discuss these results in the context of polycrystalline BiOI samples for tandem junction photovoltaics (PV).
8:00 PM - EN07.05.05
Doped Diamond-Like Carbon Films for Electronic Applications
Philip Schneider1,Marie Francoise Millares1,Iulian Gherasoiu2,Harry Efstathiadis1
SUNY Polytechnic Institute College of Nanoscale Science and Engineering1,State University of New York Polytechnic Institute2Show Abstract
Diamond is rapidly emerging as an outstanding semiconductor material for the 21stcentury. New device applications are envisioned based on its superior properties such as an ultrawide bandgap, the highest electron and hole mobilities, high electric field breakdown strength, all combined with unmatched thermal conductivity and radiation hardness. Toward the realization of its promises as a material for electronic devices, the successful doping of the diamond with boron and phosphorus are important stepping stones. Various techniques are used for the synthesis of diamond including detonation that results in the formation of micron-sized aggregates of nanometer scale diamonds, high-pressure, high-temperature chemical vapor deposition (CVD) or plasma-enhanced CVD. This work focuses on the fabrication of Diamond-Like Carbon (DLC) through Plasma-Enhanced Chemical Vapor Deposition, and on the measurement of the properties of the material doped with boron and phosphorus from surface a source. The drive-in and activation of the dopants is done through thermal annealing. By testing different annealing temperatures, we are able to observe how the structural, optical and electrical properties of the carbon films were affected. It was found that synthesizing DLC at low temperatures (~100oC) in conjunction with annealing enhances the material crystallinity, its conductivity and allows higher transparency of the film.
8:00 PM - EN07.05.06
Sodium Flame Synthesis of Nano-Structured Powders for Catalytic Applications
Nick Yin1,Mary Krause1,Geoff Smith1,Larry Wang1,Grodon Smith1
Global Advanced Metals USA, Inc.1Show Abstract
Nano-structured, high surface area metal or ceramic powders have a growing interest within many markets including energy conversion, catalysts, and electronic materials. Manufacturing high purity metal powders at an industrial scale is a challenge due to trace impurities impacting powder performance. In particular, oxygen, nitrogen, and carbon impurities can adversely affect the physical and chemical properties of refractory metals such as titanium, tantalum, tungsten, and niobium. Here we present a nano-structured powder produced using a scalable sodium synthesis technology. Primary particle size and aggregate morphology of the resulting powder can be optimized for the given application by adjusting process parameters including optional metal or gas doping. In one such application, we adapt the powder for photocatalytic applications with promising performance.
8:00 PM - EN07.05.07
Synthesis, Characterization and Application of MoO3 by Hydrothermal Route Assisted by Microwaves as an Adsorbent of Organic Dyes
Larissa Garcia1,Fenelon Pontes1,Rafaela Silva1,Regina Capeli1,Elson Longo2
UNESP São Paulo State University1,UFSCAR- Federal University of São Carlos2Show Abstract
Dyes and pigments, released from several industries such as textile, food, paper and cellulose, cosmetic and plastic industries, are the main organic pollutant compounds in wastewaters. The presence of dyes in water sources, even at low concentrations is very harmful to human beings and microorganisms. A dye can generally be described as a colored substance that has an affinity to the substrate to which it is being applied and is used to impart color to materials of which it becomes an integral part. Most of the current treatment methods work through biological treatment processes, coagulation, oxidation, filtration, membrane separation, and adsorption. Among these methods, Adsorption presents advantages due to its simplicity, effectiveness, and cost-benefit. As adsorbent material, we have the MoO3 an example of transition metal oxides has been widely used because of their different properties such as electrochromism, thermochromism, and photochromism, as well as having three polymorphs, which are called α - MoO3, β-MoO3, h-MoO3. [1-4] In this work were synthesized MoO3 nanostructures characterized by the techniques of XRD and BET where it was possible to observe that the prepared samples had the hexagonal, h-MoO3 phases with a surface area of 6.5 m2/g and a mean crystallite size of 50,56 nm, and orthorhombic, α -MoO3 with surface area of 14 m2/g mean crystal size of 25.80 nm for the sample using 20 mL of 6M HNO3. MEV and TEM analyzes showed hexagonal prisms and nanobelts with a diameter of 200 nm and a length of 2μm. The spectra in the Infrared Region allowed identifying the presence of residual organic matter in the synthesis process. Diffuse reflectance spectra in the ultraviolet and visible region allowed to calculate the Band- Gap of h-MoO3, in the value of 2.65 eV and 2.90 eV for the α -MoO3 phase. For the adsorption study using different concentrations of Rhodamine B and Methylene Blue, was obtained spontaneous, endothermic, chemical nature adsorption.
 A. Santos-Beltrán et al Heat treatment effect of MoO3 on the MB removal and its reuse. Journal of Physics and Chemistry of Solids 121 (2018) 266–275
 E. Forgacs et al. Removal of synthetic dyes from wastewaters: a review. Environment International 30 (2004) 953–971.
 E. Khosla et al., Ionic dye adsorption by zinc oxide nanoparticles. Chemistry and Ecology 31 (2015), 173–185.
 A. Manivel et al. Synthesis of MoO3 nanoparticles for azo dye degradation by catalytic ozonation. Materials Research Bulletin 1 (2014) 184-194.
8:00 PM - EN07.05.08
The Importance of the Interfacial Contact—Is Reduced Graphene Oxide Always an Enhancer in Photo(Electro)Catalytic Water Oxidation?
Zhirun Xie1,Hui Ling Tan2,Yun Hau Ng3
University of New South Wales1,Kyushu University2,City University of Hong Kong3Show Abstract
Optimization of the interfacial contact between graphene and semiconductor has been proposed to be essential to improve their charge interactions. Herein, we fabricate bismuth vanadate-reduced graphene oxide (BiVO4/rGO) composites with tailored interfacial contacting extent and reveal their opposite behaviors in the photoelectrochemical (PEC) and powder suspension (PS) water oxidation systems. The BiVO4/rGO with high rGO coverage (BiVO4/rGO HC) features a remarkable superiority in PEC photocurrent enhancement to its low rGO coverage analogue composite (BiVO4/rGO LC). On the contrary, BiVO4/rGO HC shows detrimental effect while BiVO4/rGO LC exhibits an enhanced performance for oxygen evolution in the PS system. This phenomenon is attributed to the changes in the hydrophobicity of BiVO4/rGO composite in conjunction with the interfacial contact configuration. Enriched BiVO4/rGO interfacial contact is found to improve the charge separation efficiency and charge transfer ability as well as the energetics of the composite material, explaining the superior PEC performance of BiVO4/rGO HC. However, the highly hydrophobicity of BiVO4/rGO HC ensuing from the higher rGO reduction degree triggers poor water miscibility, reducing the surface wettability and therefore hampering the photocatalytic O2 evolution activity of the sample. This work also highlights water miscibility as the governing issue in the PS system.
8:00 PM - EN07.05.09
The Remarkable Structure of Surface-Terminated Grain Boundary in Perovskite Oxides and Implications for Oxygen Electrocatalysis
Yoon Heo1,Hyung Bin Bae1,Seungkyu Choi1,Sung-Yoon Chung1
A grain boundary forms as an internal interface when two crystalline grains with mutually different crystallographic orientations are in direct contact with each other. As a result, atomic arrangement at grain boundaries differs from that of the bulk, showing serious displacements deviating from the original symmetric positions. As these symmetry-broken configurations are difficult to achieve in the bulk crystals, grain boundaries are considered distinctive platforms that can exhibit new physical properties. By using both sintered polycrystals with various grain sizes and thin films on bicrystal substrates, it is directly verified that surface-terminating grain boundaries in LaCoO3 and LaMnO3 are exceptional in oxygen evolution electrocatalysis, showing more than an order of magnitude higher activity. A combination of atomic-scale structure observation and density functional theory calculations demonstrates that the displacement of atoms in metal–oxygen octahedra correlates with significant splitting of the degenerate transition-metal 3d orbitals, and subsequently much easier charge transfer between metals and oxygen is attained. In addition to identifying the grain boundaries as strikingly active sites, the findings suggest that symmetry breaking by atom displacements in metal–oxygen octahedra is an efficient approach to remarkably enhance the oxygen electrocatalytic efficiency in perovskite oxides.
8:00 PM - EN07.05.10
Tailoring the d-Band Center and Electronic Structure of Co-N4 Electrocatalyst for Hydrogen Evolution Reaction
Sabhapathy Palani1,2,3,Indrajit Shown1,Wei-Fu Chen2,Kuei-Hsien Chen1,2,Li-Chyong Chen2
Academia Sinica1,National Taiwan University2,National Tsing Hua University3Show Abstract
Electrochemical hydrogen generation via the hydrogen evolution reaction (HER) offers a promising solution for a green renewable-energy generation. Platinum (Pt) and Pt-based nanomaterials are mostly served as an efficient electrocatalyst for HER; however, due to its high cost and low abundance limits its large scale commercial application. Therefore, developing nonprecious metal based electrocatalyst for HER is highly necessary for large scale hydrogen production. Over the past decades, tremendous efforts have been made for Pt free electrocatalysts for HER. Recent advances in carbon nanomaterials (RGO, CNT, etc.,) have shown their promising future in energy-related electrocatalytic reactions (ORR, OER, and HER) especially, after heteroatom (such as N, B, P, and S) doping, the catalytic activity enhanced. Among all the reported electrocatalysts, the co-doping of trace transition metals on heteroatoms doped carbon materials leads to form metal complexes (Ex. Co-Nx), showing promising HER activity in the aqueous electrolyte. In addition, the reported density functional theory (DFT) simulation shows that when combining both coordinations into one complex, the optimized charge distribution results in an ideal value of ΔGH in Metal-Carbon-Nitrogen (M-C-N) hybrid system and which is much better than the single or mixture system of M-C and M-N.
In this presentation, we will demonstrate a new HER electrocatalyst based on Co-N4 system. The cobalt-based catalysts are prepared by a one-step pyrolysis process at the high temperature in which vitamin-B12 (metal precursor) and thiourea (sulfur precursor) together with reduced graphene oxide (RGO). We observed that the electrocatalytic activities of synthesized catalysts are strongly related to the pyrolysis temperature, metal loading, and acid leaching. The as-synthesized catalyst was characterized by XRD, XPS, and XAS. The results indicate that Co-corrin complexes (Vitamin B12) together with RGO have been decomposed at the high temperature to form new structure (ex. N-Co-C and N-Co-S). Furthermore, the density functional theory (DFT) calculation reveals, this conjugation induces downshift of the d-band center of cobalt, which decreases its hydrogen binding energy. The downshift of d-band center favors the electrochemical desorption of adsorbed hydrogen and leads to a relatively moderate Co−H binding strength, which helps for enhanced hydrogen evolution reaction. The comparison of HER activity and stability of cobalt electrocatalyst in alkaline and all pH electrolytes will be discussed at the meeting.
8:00 PM - EN07.05.11
Ni(OH)2-WP Hybrid Nanorod Arrays for Superior Electrocatalyst toward Hydrogen Evolution Reaction in Alkaline Media
Dokyoung Kim1,Kijung Yong1
Pohang University of Science and Technology1Show Abstract
The development of efficient non-noble hydrogen evolution electrocatalysts in alkaline media is crucial for sustainable production of H2 through water electrolysis. Generally, the mechanism of hydrogen evolution reaction (HER) consists of a generation of hydrogen intermediates (Volmer reaction) and followed electrochemical desorption (Heyrovsky reaction) or H2 recombination (Tafel reaction). Compared with the process in acidic media, HER in alkaline solution requires extensive energy barrier owing to slow kinetics of water dissociation step caused by different reacting species in Volmer and Heyrovsky reaction in acidic (H3O+) and alkaline (H2O/OH-) media. To overcome this problem, an alkaline HER catalyst composed of Ni(OH)2-WP nanorod arrays on carbon paper was synthesized via thermal evaporation and electrodeposition. This hybrid catalyst exhibited outstanding HER activity and required a low overpotential of only 77 mV to drive current density of 10 mA/cm2 and a Tafel slope of 71 mV/dec. The hybrid catalyst also showed long-term electrochemical stability, maintaining its activity for 24 h. This improved HER efficiency was attributed to the synergetic effect of WP and Ni(OH)2: Ni(OH)2 effectively lowers the energy barrier during water dissociation and also provides active sites for hydroxyl adsorption, whereas WP adsorbs hydrogen intermediates and efficiently produces H2 gas. This interfacial cooperation offers not only excellent HER catalytic activity but also new strategies for the fabrication of effective non-noble-metal-based electrocatalysts in alkaline media.
8:00 PM - EN07.05.12
NiCoP Double Transition Metal Phosphide/Ti3C2 MXene 3D Architecture as Efficient Bifunctional Photocatalyst for Solar Water Splitting
Jiyeon Kang1,Seulgi Kim1,Sungho Song2,Dongju Lee1
Chungbuk National University1,Kongju National University2Show Abstract
Hydrogen energy is the ultimate challenge for achieving clean and sustainable energy in the current situation of environmental destruction caused by fossil fuels. Photocatalytic and photoelectrochemical water splitting under irradiation by sunlight has received much attention for production of renewable hydrogen from water on a large scale. Among various photocatalytic materials, transition metal phosphides have been studied as promising catalysts for hydrogen evolution reaction (HER) in solar water splitting, but have issues due to their low conductivity and stability.
Herein, three-dimensional (3D) porous architecture was fabricated with Double transition metal phosphide and conductive two-dimensional MXene hybrids. Ti3C2 MXene was used as a co-catalyst for exciton separation and charge transfer acceleration as well as a support material for the stability of NiCoP catalyst, due to its high conductivity and mechanical strength.
Not only the 3D interconnected architecture with large electrochemically active surface area and structural stability but also the synergistic effect between the electrochemical activities of NiCoP and MXene Ti3C2 leads to excellent electrochemical performance.
8:00 PM - EN07.05.13
Engineering Facet-Heterojunction Rutile TiO2 Photocatalyst for Highly Efficient Overall Water Splitting
Chaomin Gao1,Lina Zhang1,Shenguang Ge1,Xin Cheng1,Kang Cui1
University of Jinan1Show Abstract
Rutile TiO2 in principle is a more potential photocatalyst owing to its narrower bandgap, higher thermodynamically stability and less intragrain defects when compared with anatase TiO2. Nevertheless, rutile TiO2 still hasn’t achieved the comparable photocatalytic activity with anatase-based TiO2 on account of its high recombination rate of electron-hole pairs. Electrons in rutile are deep trapped, which shows shorter lifetime in picosecond region, however, the amount of electrons in microsecond to millisecond region is larger. In order to effectively separate the long-life electron, a novel rutile facet heterojunction TiO2 with long-distance electronic pathway was firstly proposed to match the transportation of long-life electron, aiming to suppressing the carrier recombination and improving the photocatalytic activity. Here, a structural rutile TiO2 with facet heterojunction mechanism, which could promote the separation of charge carriers efficiently, was designed for photocatalytic overall water splitting with the two-step hydrothermal strategy. In addition, compared with rutile TiO2 nanorod, more than 60 and 25 times higher of generated photocurrent and H2 productive rates are obtained for rutile facet heterojunction TiO2, respectively.
8:00 PM - EN07.05.14
Realizing Multiple Heterojunction within Single Titanium Dioxide Nanoparticles for Highly Efficient Solar Hydrogen Production
Yoonjun Cho1,Jong Hyeok Park1
Yonsei University1Show Abstract
The surface disorder-engineering of TiO2 has been drawing intensive interests in tuning surface energy states and thus boosting photocatalytic activity, but yet remains on application as in surface disorder layer. Selective localization of this disorder layer has not been reported so far, of which it can provide unpredictable strategy within the heterojunction interfaces to achieve novel metal-free photocatalysis. Conventional TiO2 polymorph with mixed-phase heterojunction (anatase and rutile, Degussa P25) possessing energetic type-II band alignment at the interface, outperforming single-phase TiO2 is employed in this work for single material photocatalyst.
Here we report conceptually different synthetic process to selectively localizing the cystal disorder layer between the anatase and rutile phase of a single TiO2 nanoparticle, for highly efficient photocatalytic hydrogen (H2) generation. Phase selective disorder-engineering of rutile TiO2 followed by thermal re-oxidation below the phase transformation temperature could realize multiple heterojunction of anatase/disorder layer/rutile within a single P25 nanoparticle (denoted as DE-P25). The multiple heterojunction was thoroughly confirmed and analyzed by electron energy-loss spectroscopy (EELS) and electron holography mapping for the first time. EELS Ti-L2,3 spectrum geometry of DE-P25 showed distinctive oxygen-deficient variation between the two crystals which corresponds with the drastic decrease of potential and negative charge density within the disordered layer due to localized unpaired electrons. Also, PL decay profiles exhibiting dramatically enhanced charge separation efficiencies of DE-P25 indicate that the rutile phase incorporated with the disordered layer in the TiO2 nanoparticle induces dominant direct excition formation and reduces the self-trap of charge carriers, thus suppressing the electron/hole recombination.
The novel designing of multiple heterojunction in single TiO2 nanoparticles could not only demonstrate efficient charge separation through interfacial charge polarization, but also novel metal-free H2 generation rate of 3.994 μmol/cm-2 h-1, which is ~11 times higher than that of Pt-decorated P25. This rational approach can provide breakthrough in stagnated efficiencies of metal oxide-based materials for commercial-scale photocatalytic H2 production.
8:00 PM - EN07.05.15
Effect of the Potential Deposition on the Photoelectroactivity of the SnSx/Sb2S3 Thin Films
Moisés de Araújo1,Francisco Willian Lucas2,Lucia Helena Mascaro1
Universidade Federal de São Carlos1,Universidade de São Paulo2Show Abstract
Semiconductor-based materials containing earth-abundant and low-toxicity elements are preferably desired to be applied either in photoelectrochemical or photovoltaic cells. Fitting these conditions, there are tin sulphide and antimony sulphide. Both materials have a narrow band gap and a high absorption coefficient. Most of the methodologies developed so far for the production of these materials are based on vacuum technique, which is a high cost method. Very little attention has been given for other methods known to be low-cost, such as, electrodeposition. In light of all these, the aim of this work is to evaluate the electrodeposition potential of SbSn followed by sulphurisation on the photoelectroactivity of this material. Additionally, the microstructure, morphology and optical properties of the unsulphured and sulphurised films were also studied. The electrodeposition of the films were carried out potentiostatically at -1.07, -1.14 and -1.18 V vs Ag/AgCl/Cl-(sat. KCl) on FTO substrate with a deposition charge density of -332 mC cm-2. The bath composition was 2 mM SnCl2 and 4 mM K2Sb2(C4H2O6)2 dissolved in 0.1 M KNa(C2H2O3)2 pH 6.0. The electrodeposited films were subsequently sulphurised at 270 °C per 3 h under sulphur vapour and argon gas flux. The XRD data for all unsulphured films were indexed to the SbSn and Sb phases, which was additionally confirmed by Raman spectra. Once sulphurised, the XRD and Raman data indicated the Sb2S3 and SnS phases for all the electrodes. The SnS2 phase was detected for the sulphurised films electrodeposited at -1.07 and -1.14 V. Regarding the optical properties, the estimated optical band gap for a direct (allowed) electronic transition of all the sulphurised films were around 1.8 eV. This value can be attributed to both Sb2S3 and SnS phases. Concerning the morphology of the films, the SEM images showed dendritic like morphology for the unsulphured films electrodeposited at -1.14 and -1.18 V, whilst the film obtained at -1.07 V presented cube like morphology. The sulphurised films obtained at -1.07 and -1.14 V showed rode like morphology and the sulphurised one which was electrodeposited at -1.18 V had needle and small rods like morphologies. The photoelectrochemical assessments were performed in 0.5 M Na2SO4 + 1 mM 4-nitrophenol at pH 2.0 and under a solar simulator (100 mW cm-2, AM 1.0 G). The results revealed that the electrodeposited film at -1.14 V resulted in the highest cathodic photocurrent density once sulphurised which was -113 μA cm-2 at -0.315 V. We believe that the SnSx (SnS + SnS2) and Sb2S3 phases formed a heterostructure SnSx/Sb2S3 in the film which facilitates carriers separation and transportation and improves photocurrent density values. In order to confirm it, Sn and Sb films were prepared from the optimal electrodeposition condition and then sulphurised at the same condition as mentioned previously. The characterisation analysis showed that the sulphurised Sn film was made up of SnS and SnS2, whilst Sb2S3 phase was identified for the sulphurised Sb film. In terms of photocurrent density values, sulphurised Sn and Sb films had cathodic photocurrent densities of -2.4 and -12.2 μA cm-2, respectively. These results confirm the existence of the heterostructure SnSx/Sb2S3 as its photocurrent density value was higher than the photocurrent density for the individual sulphurised phases SnSx and Sb2S3. Band diagrams for the sulphurised Sn and Sb films were constructed from Mott-Schottky analysis and band gap data. The results showed that the valence band maximum (VBM) and conduction band minimum (CBM) of the Sb2S3 film are shifted to lower energy vs. vacuum compared to the SnSx film. This band energy position favours the transportation of the photogenerated minority carriers from SnSx-CBM to Sb2S3-CBM, increasing the charge separation and, consequently, the photocurrent.
8:00 PM - EN07.05.16
The Effect of Electrochemical Pre-Treatment in Sulfide Solutions on the Performance of Iron-Based Oxygen Evolution Catalysts
Billal Zayat1,Debanjan Mitra1,Sri Narayanan1
University of Southern California1Show Abstract
Electrochemical splitting of water is a very attractive technique for the production of clean hydrogen. Water electrolysis under alkaline conditions is particularly attractive compared to acidic water electrolysis because of the potential for a lower cost system. The alkaline system avoids the need for precious metals and perfluorinated membranes. However, the oxygen evolution reaction (OER) has sluggish kinetics leading to increased voltage losses and hence, reduced energy efficiency.1 Therefore, the development of a robust and low-cost alkaline water electrolyzer relies on developing inexpensive, durable, and efficient electrocatalysts for OER.
Due to the instability and high cost of noble metal oxides such as RuOx or IrOx,2 transition metal oxides anchored on a nickel substrate are commonly used for OER in alkaline conditions.3,4 The overpotential for these electrocatalysts is approximately 300 mV at 10 mA/cm2.5 This relatively high overpotential coupled with the still expensive cost of a nickel substrate shows that there is a need to improve the catalyst performance and also reduce the cost of nickel-based substrates.
We report here on the performance characteristics of an inexpensive, durable, and efficient OER electrocatalyst with a high-surface area low-carbon steel mesh substrate. The first step in the electrode fabrication involved pre-treating the steel mesh by polarizing the electrode under alkaline conditions in a solution of sodium sulfide. This pre-treatment step increases the surface area of the steel mesh substrate by forming iron hydroxides. The modified steel substrate is then treated with nickel nitrate and annealed at 200°C to form a catalytically active nickel hydroxide layer. The electrode is rendered exceptionally robust in 30% potassium hydroxide and highly active towards OER in alkaline conditions by surface modification with nickel.6, 7
This electrode has the dual advantage of having a low overpotential of 227 mV at 10 mA/cm2 while being significantly less expensive than common electrodes with nickel-based substrates. In addition, it shows excellent stability over 100 hours of continuous oxygen evolution with no loss in performance. This enhanced activity compared to what has been previously reported by our group6, 7 is attributed to the increased catalytic surface area due to the sodium sulfide pre-treatment. Furthermore, we have examined the effect of changing the pre-treatment conditions, especially the number of pre-treatment cycles, and studied its effect on the surface area and catalyst performance. We have also studied the electrochemical and morphological characteristics using electrochemical impedance spectroscopy, SEM, and XPS.
Finally, we demonstrated a fully-functioning “all-iron” alkaline electrolyzer that uses the surface-modified steel for the OER electrode and nickel-molybdenum co-sputtered steel for the hydrogen evolution reaction electrode. The cell achieves a voltage of 1.7 V at 100 mA/cm2 at 70°C. The cell also shows outstanding stability during a 100-hour test at 1000 mA/cm2 at room temperature with a cell voltage of 2.1 V.
1. S. R. Narayan, A. Manohar, and S. Mukerjee, Interface Mag., 24, 65–69 (2015).
2. C. C. L. McCrory, S. Jung, J. C. Peters, and T. F. Jaramillo, J. Am. Chem. Soc., 135, 16977–16987 (2013).
3. M. Gong et al., J. Am. Chem. Soc., 135, 8452-8455 (2013).
4. L. Trotochaud et al., J. Am. Chem. Soc., 136: 6744-6753 (2014).
5. C. Tang et al., Angew. Chem. Int. Ed., 54, 9351-9355 (2015).
6. D. Mitra and S. R. Narayanan, Top. Catal., 61, 591–600 (2018).
7. D. Mitra et al., J. Electrochem. Soc., 165, F392–F400 (2018).
8:00 PM - EN07.05.17
Electron Transfer in the Solid State—Extension of the Corresponding Orbitals Transformation for Calculating Electron Transfer Parameter VAB to Periodic Systems
Pavan Kumar Behara1,Michel Dupuis1
University at Buffalo, The State University of New York1Show Abstract
We are interested in the fundamental characterization of solar-to-fuels conversion systems in the three phases of ‘light absorption’, ‘carrier transport’, and ‘carrier reactivity’, all contributing to the overall photo-electro-conversion efficiency. Our major focus is on ‘carrier transport’, specifically the modeling of transport of photo-generated electrons and holes in complex crystalline materials.
Our approach is based on the two-state model of Emin/Holstein/Marcus1-2 in which a key quantity is the electron-transfer coupling matrix element, VAB, a measure of the strength of overlap between the initial and final electron-localized states. We report on the extension of the molecular method of Farazdel et al.3 to solid state periodic systems. The implementation handles any periodic HF- or DFT-based spin-constrained initial and final states and is carried out in the framework of the CP2K code.4 Test cases will be presented.
This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award # DE-SC0019086.
1. Emin, D.; Holstein, T., Adiabatic Theory of Hall Mobility of Small Polaron in Hopping Regime. Bulletin of the American Physical Society 1968, 13, 464.
2. Marcus, R. A., Chemical + Electrochemical Electron-Transfer Theory. Annual Review of Physical Chemistry 1964, 15, 155-196.
3. Farazdel, A.; Dupuis, M.; Clementi, E.; Aviram, A., Electric Field Induced Intramolecular Electron Transfer in Spiro Pi-Electron Systems and Their Suitability as Molecular Electronic Devices - a Theoretical Study. Journal of the American Chemical Society 1990, 112, 4206-4214.
4. Hutter, J.; Iannuzzi, M.; Schiffmann, F.; VandeVondele, J., Cp2k: Atomistic Simulations of Condensed Matter Systems. Wiley Interdisciplinary Reviews-Computational Molecular Science 2014, 4, 15-25.
8:00 PM - EN07.05.18
Cobalt Molybdate-Based Electrocatalyst for Efficient Hydrogen Evolution in Alkaline Media
University College London1Show Abstract
Nowadays, much attention has been put on green energy production. Hydrogen is one of the most promising energy sources in the future. Electrolysis of water to generate hydrogen is one of the important strategies to produce hydrogen and much effort has been paid to develop costless, efficient and long lifetime electrocatalysts. Recently, transition metal phosphides (TMP) are largely explored due to its excellent hydrogen evolution reaction (HER) performance and low cost. However, the stability of TMP for long-time use is restricted due to its intrinsic structure and therefore HER performance is restricted. Moreover, although the HER mechanism in acid media is well documented, there is still large to explore in alkaline media for HER reaction. In this work, cobalt molybdate phosphides was successfully prepared to make a self-standing electrode. The optimised electrodes showed a small overpotential to reach a current density of 10 mA cm-2 in alkaline media. The electrocatalysts also showed an excellent stability in a 48h test. Furthermore, DFT calculation combined with XPS tests revealed the whole process to induce P in the reaction and could give some insights to explain the role of P in HER. Results of in situ techniques showed some interesting phenomenon happened at the interface and gave some insights in explaining the HER mechanism in alkaline media.
8:00 PM - EN07.05.19
Enhanced Photoelectrochemical Performance of Ternary Ag/PaNi/NaNbO3 Nanocomposite Photoanode for PEC Water Splitting
Dheeraj Kumar1,Neeraj Khare1
Indian Institute of Technology Delhi1Show Abstract
The consumption of petroleum derivatives, for example, coal, oil and gaseous petrol have turned into a worldwide issue of environmental concern. The energy utilization is quickly increasing with rising population because of which there is a necessity of using sustainable green renewable energy sources to satisfy the needs of energy supply . The solar based energy can be converted and stored in the form of fuels, for example, hydrogen (H2) by water splitting . Hydrogen is an ideal green fuel and can replace fossil fuels in the future Photoelectrochemical (PEC) water splitting is one of the most promising methods for H2 generation on a large scale using a semiconductor nanocomposite materials. [3, 4].
In the present work, a ternary Ag/PaNi/NaNbO3 nanocomposite photoanode has been successfully fabricated by chemically grafting of Ag nanoparticles (NPs) over binary PaNi/NaNbO3 nanocomposite for PEC water splitting applications. The Ag NPs on the surface of binary PaNi/NaNbO3 nanocomposite, act as a photosensitizer under visible-light illumination due to the surface plasmonic effect. The ternary Ag/PaNi/NaNbO3 nanocomposite photoanode exhibit an enhancement in the PEC activity as compared to binary PaNi/NaNbO3 and NaNbO3 photoanodes. Under light illumination, the current density of the ternary Ag/PaNi/NaNbO3 nanocomposite has been achieved to be ~ 5.93 mA/cm2 (at 1V vs. Ag/AgCl), which is ~5 fold and ~2 fold higher than pristine NaNbO3 and binary PaNi/NaNbO3, respectively. The enhanced PEC activity of ternary Ag/PaNi/NaNbO3 composites is attributed to the improved light absorption in the visible part of the solar spectrum due to (1) the surface plasmonic effect (as Ag NPs act as photosensitizer for visible-light harvesting), because of coupling with PaNi intermediate material which is used as a remarkable electron transporter which easily effectively separate the e- - h+ pairs and decrease the recombination rate, and due to type-II heterojunction formation between PaNi and pristine NaNbO3. The results show the potentiality of fabricated ternary Ag/PaNi/NaNbO3 photoanode materials towards improved PEC water splitting application.
 I. H. Gentle and K. Hellgardt, Sci.Rep. 8, 12807 (2018).
 S. Sharma, S. Singh and N. Khare, Int. J. Hydrogen Energy. 41, 21088 (2016).
 W. Wang, M. Xu, X. Xu, W. Zhou, and Z. Shao, Angew. Chem. Int. Ed. doi.org/10.1002/anie.201900292 (2019).
 D. Kumar, S. Singh and N. Khare, Int. J. Hydrogen Energy. 43, 8198 (2018).
8:00 PM - EN07.05.20
Exfoliated NiPS3 Nanosheets as an Oxygen Evolution Reaction Electrocatalyst
Raksha Dangol1,Zhengfei Dai1,2,Apoorva Chaturvedi1,Qingyu Yan1
Nanyang Technological University1,State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University2Show Abstract
Nanostructuring is an important aspect of synthesis of nanomaterials. 2D materials, considered as promising group of materials for electrocatalysis, can be easily exfoliated into nanosheets by ultrasonic-sum-solvent assisted methods. Nanosheet structures, thus formed, have a large surface area which, expose higher active sites and give rise to fast electron transport during water splitting process. Hence, this process is used to exfoliate bulk NiPS3, a representative material from MPX3 family synthesized using Chemical Vapour Transport method, by the application of ultrasonic bath for a good yield of NiPS3 nanosheets. Various solvents were systematically investigated for obtaining high yield of the nanosheets, whereby Formamide had the highest yield among the ten commonly used solvents. Thus-obtained nanosheets are employed as an oxygen evolution reaction (OER) catalyst. The NiPS3 nanosheets demonstrate an excellent electrocatalytic OER properties, like a low overpotential of 301 mV at the current density of 10 mA cm-2 and a small Tafel slope of 43 mV dec-1 and a remarkable long-term stable performance. Upon further investigation on the performance, it was observed that the improved catalytic activity was due to a large electrochemically active surface area and high intrinsic activity. It was also found that the in-situ formation of NiOOH and its interface enhances the OH− adsorption and reduces Gibbs free energy of the reaction intermediates.
8:00 PM - EN07.05.21
In Situ Raman Spectro-Electrochemical Studies of Support-Catalyst Interface of Nanocarbon Florets Supported Co3O4 with an Energy Efficient Electrocatalysis under a Magnet
Indian Institute of Technology Bombay1Show Abstract
The demand for worldwide energy calls for the need to build up an effective technology for the manufacture of a clean and sustainable energy source as a fuel cell. Hydrogen is considered as cleanest and high specific energy density fuel source. The object is to provide catalysts with a catalyst-support which can produce maximum hydrogen with a minimum applied energy, ie high current density with low overpotential. For this purpose, comprehending the mechanistic involvement of support-catalyst interface is critical for effective designing of industrially relevant electrocatalytic processes such as alkaline hydrogen evolution reaction (alHER). The understanding of kinetically sluggish alHER exhibited by both Pt and Pt-group-metal-free catalysts is primarily derived from indirect electrochemical parameters such as Tafel slope. Addressing these lacunae, we establish the critical role of nanocarbon florets (NCF) based electrochemical support in generating key cobalt-oxohydroxo (OH-Co=O) intermediate during alHER through operando Raman spectro-electrochemistry. Specifically, interfacial nano-engineering of a newly designed carbon support (NCF) with a spinel Co3O4 nanocube catalyst is demonstrated to achieve facile alHER (-0.46 V @10 mA/cm2). Such efficient alHER is mainly attributed to the unique lamellar morphology with high mesoporous surface area (936 m2/g) of NCF that catalyzes the rate-determining water dissociation step and facilitates rapid ion diffusion. The dissociated water drives the formation of OH-Co=O intermediate, spectroscopically captured for the first time through the emergence of ��OH-Co=O Raman peak (1074 cm-1). Subsequent alHER proceeds through Volmer-Heyrovsky route (119 mV/dec) via Td Co2+↔Co3+↔Co4+ oxidative pathway. Concomitant graphitization of NCF through the disappearance of ��sp3C-H (2946 cm-1) supports the co-operative dynamics at the Co3O4-NCF interface. Thus, NCF positively contributes to the lowering of overpotential with low charge-transfer resistance (Rct=35.8 Ω) and high double layer capacitance (Cdl=410 mF/cm2). Thus, the participation of both catalyst and its support to enhance HER activity is thereby confirmed in contrast to conventionally established understanding of passive support.
Further, for the mass production of hydrogen, the term “magneto-electrolysis” plays a crucial role, where the magnetic fields influence the performance of electrocatalysis processes. Thus we explore the interactions of magnetic fields with electrocatalysts and the control to the formation of the diffusion layer over the electrode-electrolyte interface. This effect in the reaction interface directs the mass transport of reactive components at the electrode surface and control the reaction rate at the electrode-electrolyte interface. This magnetization behavior is present after removal of the small disc magnets, indicating the remanent magnetization of the para/ferromagnetic catalyst, which plays a crucial role in the electrocatalytic enhancement process. Therefore, by using a 0.05 T magnetic field the overpotential of Co3O4-NCF was reduced by 12%. The total energy saved by using the 0.05 T magnetic field is 13% of its normal usage. Thus, magneto-electrocatalysis can be the next energy efficient technique for hydrogen evolution and subsequently magneto-catalysis become the alternative of electrocatalysis for water splitting.
8:00 PM - EN07.05.22
Triggering Catalytic Active Sites for Hydrogen Evolution Reaction by Intrinsic Defects in Janus Monolayer MoSSe
Wenwu Shi1,Zhiguo Wang1
University of Electronic Science and Technology of China1Show Abstract
Janus transition-metal dichalcogenides have been predicted to be promising candidates for hydrogen evolution reaction (HER) due to their inherent structural asymmetry. However, the effect of intrinsic defects, including vacancies, antisites, and grain boundaries, on their catalytic activity is still unknown. MoSSe provides an ideal platform for studying such defects, since theoretical calculation has indicated that the formation energies of point defects and grain boundaries on MoSSe were lower than that of pristine MoS2 monolayer. In this work, density functional theory is utilized to study all of the possible intrinsic defects on the MoSSe monolayer for HER. The MoSSe monolayer with 4|4, 4|8a, 5|7b, 8|10a GBs, vacancies (VS, VSe, VSSe, VMo, VMoS3), and antisite defects (MoSSe, SeMo, SMo) shows enhanced HER performance. The adsorption behaviors of hydrogen on defects were explained by using a “states-filling” model. The adsorption energy of hydrogen during catalysis changes linearly with the work required to fill unoccupied electronic states within the catalysts. The work required to fill the unoccupied electronic states of MoSSe monolayer can be described via an integral formulation of the DOS of catalyst.
The electronic structures of Janus MoSSe were further analyzed to reveal the deep mechanism of enhanced catalytic activity for HER by introducing defects. The pristine Janus MoSSe shows a direct band gap of 1.55 eV. It is difficult for H to donate its electron to Janus MoSSe by overcoming large barrier of 1.55 eV. Therefore, unstable adsorption states were formed when the H atom adsorbed on the MoSSe surface. However, for MoSSe with Vs and 8|10a GBs, defective states appear below and above the Fermi energy. The gap states would provide an unoccupied state near the Fermi energy. The Fermi energy level shifts up after H adsorption, which is due to the transfer of charge from H atom to MoSSe. Our calculation reveal that the introduction of gap states in hydrogen adsorbed systems is the origin of enhanced HER activity.
8:00 PM - EN07.05.23
Microkinetic Modeling of Water Oxidation in Photoelectrochemical Cells—The Impact of Surface States on the Electrochemistry
Anja Bieberle-Hütter1,Kiran George1,Tigran Khachatrjan2,Matthijs van Berkel1,2,3
DIFFER1,Technische Universiteit Eindhoven2,Vrije Universiteit Brussel3Show Abstract
Water splitting in photoelectrochemical (PEC) cells involves two set of reactions: the hydrogen evolution reaction and the oxygen evolution reaction (OER). We focus on the OER at the semiconductor-water interface to identify limiting processes at the interface. Recently, we have developed a general approach to relate electrochemical measurements to the kinetics of multistep reactions at the semiconductor-electrolyte interface1. The hematite (Fe2O3) – water interface is used as model system in this study. A microkinetic model of OER is developed and is formulated as a state-space model (SSM); the applied potential is the input and the current density is the output. By solving the model, j-V plots and impedance spectra can be simulated similar to experimental measurements.1
According to the literature, charge transfer under illumination happen directly via the valence band and indirectly via surface states. In our model, we use Gerischer theory and hence the electron transfer rates are defined in terms of empty and occupied energy states on either side of the interface. For any given surface state the charge transfer rate can be calculated similar to the calculation of charge transfer rates via valence band or conduction band.2 By using these rates in the state space model, indirect charge transfer via surface states is simulated. Charge transfer via both valence band and surface states are modeled and compared. The impact of surface state properties on the electrochemical measurements is investigated. Steady state j-V plots, impedance spectra, and linear sweep voltammetry curves simulated from the model will be presented for different energy states within the band gap. The sensitivity of these measurements to the intermediate reaction rates, voltage-scan-rate, semiconductor parameters, and other interface parameters is discussed.
1 K. George, M. van Berkel, X. Zhang, R. Sinha, and A. Bieberle-Hütter, J. Phys. Chem. C 123, 9981 (2019).
2 R. Memming, Semiconductor Electrochemistry, 2nd ed. (Wiley-vch, Weinheim, Germany, 2015).
8:00 PM - EN07.05.24
Bimetallic Phosphide Encapsulated on Carbon Matrix Derived from Metal-Phenolic Network as Stable and Efficient Electrocatalysts for Oxygen Evolution Reaction
Gwan Hyun Choi1,Clament Sagaya Selvam Neethinathan1,Piljin Yoo1
Sungkyunkwan University1Show Abstract
Rational design of electrocatalyst with high activity and stability is imperative for developing sustainable water electrolysis. In this work, we have developed bimetallic phosphide embedded in P-doped carbon layers as a stable electrocatalyst for oxygen evolution reaction (OER). Here, distinct from other metal/carbon sources, tannic acid (TA)-based metal-phenolic networks (MPNs) are utilized to design stable and efficient hybrid structures. Especially, thanks to the strong coordination between the phenolic ligands and metal ions in atomic/molecular scale, which facilitates the nanometer-sized hybrids formation with evenly distributed/embedded metal nanoparticles in few layers of graphitic carbons without any aggregations. It has been observed that every single nanoparticle (transition metals=Fe, Co and Ni) in the hybrid can serve as active site while carbon layers can prevent massive surface oxidation and poisoning under OER conditions. Precisely defined metal/carbon interface structure was achieved via simple carbonization step. The subsequent phosphidation results in metal phosphides that remarkably increases the number of active sites. Meanwhile, controlling the appropriate atomic ratio of metal and electronegative P atoms enhances the conductivity of the hybrid electrocatalyst. Since phosphidation process is diffusion-driven reaction from outmost carbon layer to inner metallic phases, the final product has the unique structural characteristics that consist of core metallic phase and peripheral metal phosphide phase with P-doped carbon layers. We have demonstrated that the bimetallic phosphide shows enhanced activity (~270mV at 10mA cm-2, under 1M KOH alkaline condition) compared with monometallic phosphides, state-of-the-art IrO2 and other benchmark electrocatalysts. The structural contributions to the electrocatalyst activity and long-term stability (~100 hours) has been systematically investigated by using synchrotron-based X-ray absorption spectroscopy (XAS) and photoelectron spectroscopy (XPS). Combining with structural investigations and electrochemical studies, it can be concluded that its enhanced activity stems from synergistic function of each of metal phosphide species with its structural merits. Our findings suggest that the metal phosphide electrocatalysts derived from metal-phenolic networks with controlled structural features are promising strategy to develop various metal phosphide electrocatalysts for widespread electrochemical applications.
8:00 PM - EN07.05.25
Visible/Near-Infrared Driven Photocatalyst Based on Upconversion Nanoparticles and Graphitic Carbon Nitride
Yong Il Park1
Chonnam National University1Show Abstract
Graphitic carbon nitride (g-C3N4) is a promising visible-light-driven photocatalyst. g-C3N4 is inexpensive, air-stable, and non-toxic material and they have been proven to exhibit high performance in photocatalysis. To use solar energy more efficiently, it is necessary to broaden the light absorption of g-C3N4 is required. To take advantage of the near-infrared (NIR) region of solar energy, the g-C3N4 was combined with upconverting nanoparticles (UCNPs). The UCNPs convert NIR photons to visible light which can activate the g-C3N4 photocatalyst. The use of visible and NIR light source improves the photocatalytic activity of the UCNP@g-C3N4 nanocomposites. In order to optimize the photocatalytic efficiency of the nanocomposite, the absorption and emission wavelengths of the UCNPs were tuned by controlling the lanthanide dopant composition. The nanocomposite exhibited excellent photocatalytic activity under simulated solar light illumination.
8:00 PM - EN07.05.26
Effect of Electronic and Structural Properties of Transparent Conductive Colloid Mediator on Z-Scheme Water Splitting over Printable Photocatalyst Sheets
Hiromasa Tokudome1,2,Sayuri Okunaka1,2,Shingo Oozu1,2,Takeshi Ikeda1,2,Kazunari Domen3,4
TOTO Ltd1,Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem)2,The University of Tokyo3,Shinshu University4Show Abstract
Photocatalytic water splitting is attracted a significant attention as a low-cost production process for renewable hydrogen using abundant sunlight. In the reaction system using photocatalyst particles, Z-scheme system via two-step excitation can be a practical candidate instead of one-step reaction on visible-light-responsive single photocatalyst particle with difficult development. Z-scheme water splitting can drive by employing two kinds of visible light responsive photocatalysts selectable from various ones in the presence/absence of appropriate mediators such as redox ions or conductive materials. We previously reported that a Z-scheme-type printable photocatalyst sheet exhibited the solar-to-hydrogen energy conversion efficiency (STH) of 0.1%. This sheet is fabricated by screen printing and consisted of mixed films of two kinds of visible-light-active photocatalysts and a conductive mediator nanocolloid on a glass substrate. 1) Recently, we designed a new photocatayst sheet based on transparent colloidal indium-tin-oxide mediator, attaining the STH of 0.4%. 2) In this study, we fabricated a Z-scheme-type photocatalyst sheet composed of a new transparent and low-cost mediator, antimony (Sb) -doped tin oxide (ATO), and examined the water splitting activity.
Z-scheme-type sheets were prepared via screen-printing on borosilicate glass substrates using a viscous paste including Rh-doped SrTiO3, BiVO4 and ATO particles. The sheets produced hydrogen and oxygen from pure water in the stoichiometric ratio under visible light irradiation. This result showed that the ATO colloid acted as an electron mediator between the two photocatalysts. Moreover, it is found that the water splitting activity varied with Sb doping concentration and particle morphology of ATO colloid.
1) Q. Wang, T. Hisatomi, Q. Jia, H. Tokudome, M. Zhong, C. Wang, Z. Pan, T. Takata, M. Nakabayashi, N. Shibata, Y. Li, I-D. Sharp, A. Kudo, T. Yamada, K. Domen, Nat. Mater., 2016, 15, 611–615.
2) Q. Wang, S. Okunaka, H. Tokudome, T. Hisatomi, M. Nakabayashi, N. Shibata, T. Yamada, K. Domen, Joule, 2018, 2, 2667-2680.
8:00 PM - EN07.05.27
Aerosol-Assisted Chemical Vapour Deposition of alpha-Fe2O3 Thin Films for Water-Splitting
Matthew Surman1,Andrew Johnson1,Michael Hill1,Salvador Eslava1
University of Bath1Show Abstract
The global interest in developing renewable fuels as an alternative to fossil-based fuels has supported a resurgence in the use of hydrogen; at present, hydrogen is produced predominantly through the steam reformation of methane, and so research has been focused on producing hydrogen more cost-effectively from sustainable sources with a smaller carbon footprint.
Semi-conducting materials with an appropriate band-gap are able to absorb photons emitted by the Sun in order to drive the conversion of water into hydrogen and oxygen. In order for this photoelectrochemical water-splitting to take place, the band-gap of the semi-conducting material must bridge the redox potentials of the two relevant water-splitting half-reactions.
Hematite (alpha-Fe2O3) is a well-studied material for photoelectrochemical water-splitting; it has reported optical band-gaps between 1.9–2.2 eV, giving it a theoretical solar-to-hydrogen efficiency of around 15 %.1 Iron is also an earth-abundant metal, making hematite an inherently cheap-to-produce material, which raises few concerns about supply longevity.
This work focuses on the designing and synthesis of new iron(II) complexes as single-source precursors for hematite deposition using AA-CVD, which can affect the structure and morphology of the resultant films, and thus alter the optical and electronic properties of the film for more favourable photoelectrochemical water-splitting.2
 A. G. Tamirat, J. Rick, A. A. Dubale, W. N Su and B. J Hwang, Nanoscale Horiz., 2016, 1, 243–267.
 K. Takanabe, ACS Catal., 2017, 7 (11), 8006–8022.
8:00 PM - EN07.05.28
Growth and Characterization of Tantalum Oxynitrate Thin Film for Solar Water Splitting
Roberto Masso-Ferret1,Javier Wu1,Ratnakar Palai1
University of Puerto Rico at Río Piedras1Show Abstract
In the view of the current global energy crisis and environmental problems, it is a pressing need to explore and exploit some sustainable energy systems. Oxynitrides are considered as important materials for photocatalytic water splitting applications. In this paper, we report on growth, optimization, and characterization of TaON thin films by sputtering on Si, glass, and sapphire substrates. Structural, electrical, and optoelectronic properties of TaON thin films were investigated using various characterization techniques. The crystalline structure of the films was analyzed using X-ray Diffraction XRD. In order to understand the conduction mechanisms of the samples, we performed current-voltage characteristics. Hall Effect measurements were done on the samples in order to determine band gap and charge carrier’s mobility. The electronic and photocatalytic properties of a prototype for water splitting will be discussed in detail.
8:00 PM - EN07.05.29
Enhancement of Photocatalytic Oxygen Evolution Reaction of Nanoscale BiFeO3 Using IrO2 Nanoparticles
Wegdan Ramadan1,2,Detlef Bahnemann1,3
Leibniz Universität Hannover1,Alexandria University-Faculty of Science2,St Petersburg State University, Laboratory ‘‘Photoactive Nanocomposite Materials3Show Abstract
BiFeO3 (BFO) is a multiferriocs that combines antiferromagnetic and ferroelectric order well above room temperature also it is narrow band gap semiconductor (~ 2.2-2.7 eV) hence, it can harvest significant amount of visible light. Combining such desired properties on simple ternary compound makes it easier to utilize in many different folds. However, the performance of BFO in overall water splitting is still poor due to the fast recombination of the photogenerated charges and the position of its conduction band with potential less negative than that for water reduction meanwhile its valence band potential is more positive than that required for water oxidation. Here we report on the enhancement of the photocatalytic oxygen evolution reaction, OER, of BiFeO3 nanoparticles using IrO2 nanoparticles loaded on the BFO surface using impregnation method. OER showed two folds enhancement upon loading with 2wt% IrO2. IrO2 is one of the best catalyst for OER, unfortunately it is also one of the most expensive rare elements hence their applicability is limited by the high cost. Reducing IrO2 content onto the system should be an option to make its application feasible and cost effective and loading on IrO2 nanoparticles on the surface could be one feasible solution. Scanning the loaded IrO2 content from 0.5 wt% to 4 wt% showed a maximum of OER at 2wt % followed by a decrease in activity. XPS showed the formation of Ir (IV) and TEM indicated non uniform distribution of it on the surface. Charge carrier life time and dynamics for pure BFO and IrO2 loaded BFO have been studied by means of diffuse reflectance laser flash photolysis spectroscopy. Transient absorption of the charge carriers indicated a significant increase in the charge life time in the case of 2wt% IrO2 loading compared to the other IrO2 contents. Band positions between BFO and IrO2 favors the formation of heterojunction at the interface between IrO2 and BiFeO3 that enhances the separation of the photogenerated charges and the photocatalytic OER performance. The position of the conduction band could be manipulated by forming solid solution with SrTiO3 towards formation of Sr1−xBixTi1−xFexO3 having different x values and/or doping BFO to achieve overall water splitting which is our future work.
8:00 PM - EN07.05.30
Effect of Hf Doping on ZnO Photo-Anode
Boulos Alfakes1,Corrado Garlisi1,Nitul Rajput1,ChunYu Lu1,Ibraheem Almansouri1,Giovanni Palmisano1,Matteo Chiesa1,2
Khalifa University of Science and Technology1,UiT The Arctic University of Norway2Show Abstract
The urgent need for a sustainable storable alternative to oil, boosts the research on hydrogen generation using water splitting. ZnO is a promising material in this context, and has been widely studied as an anode in a water splitting solar cell. Different dopants, such as Al, Cu, Ag and N, were utilized to enhance the ZnO performance for this type of applications. In this work, ALD grown Hf doped ZnO is presented as a valid material for utilization as an anode in PEC cell. The effect of Hf doping on the photoelectrochemical performance is studied optically, structurally and electrically. A decrease in the anode resistivity, as well as an increase in the carrier concentration was observed. As a result, an impressive 250% increase in the photocurrent was obtained in the Hf doped sample in comparison to the undoped ZnO. Furthermore, DFT simulations are presented to compliment the detailed experimental characterization, to support the interpretation for this enhanced performance. Those results open the research for utilizing this material into more common water splitting structures, such as nanorods and nanowires, which can be efficiently and conformably coated through atomic layer deposition.
8:00 PM - EN07.05.32
A Band Diagram Based Device Modeling Approach for Designing Photoelectrochemcial Anodes
Botong Miao1,Asif Iqbal1,Kirk Bevan1
McGill University1Show Abstract
Photoelectrochemcial (PEC) cells offer a promising route towards combating climate change through the production of hydrogen fuel. However, PEC cells have remained inefficient due to the sluggish oxygen evolution reaction present at the photoanode. This key inefficiency bottleneck remains a major obstacle to the large-scale commercialization of PEC cells. In this regard, operational photovoltage and photocurrent trends provide key metrics for characterizing and improving the performance PEC photoanodes. However, a quantitative band diagram approach for systematically engineering these properties is still lacking. Here, we discuss how self-consistent solutions to the semiclassical equations governing PEC cells can provide fundamental band diagram based insights into improving their operation. Our work focuses on modeling the ultimate factors determining the photovoltage and photocurrent under active operation and how these quantities can be directly correlated with electron and hole densities in illuminated PECs. In general, this study underscores the engineering opportunities provided by a rigorous device modeling based design approach.
8:00 PM - EN07.05.33
Fabrication Process Analysis of Non-Precious Metal Electrochemical Water Oxidation Catalysts of MOx (M = Mn, Fe, Co, Ni) by Raman Spectra
Katsushi Fujii1,Kayo Koike1,Kei Morishita1,Satoshi Wada1
RIKEN, RAP1Show Abstract
Non-precious metal oxides are often used materials for electrochemical catalysts and photoelectrochemical co-catalysts of water oxidation. The mechanisms of water oxidations on these catalysts are complicated. Not only -O but also -OOH is believed to play an important role in the water oxidation. Especially, the Fe oxide site for the case in the Fe oxide mixed in the other non-precious metal oxides is said to be the active site . Unfortunately, non-precious metal oxides are usually stable in basic solution but not stable in neutral and acidic solutions even the abilities are comparable to precious metal oxide (IrOx for example) electrochemical water oxidation catalysts. The MnOx is, however, reported to be relatively stable even in the acidic solutions when the applied potential is selected .
The water oxidation properties of non-precious metal oxides are still obscure and discussed. The reasons for the complexity of the non-precious oxides electrocatalytic properties are probably the effects that the oxides have multi oxidation numbers, multi-crystal polymorphism, and wide nonstoichiometric compositions. Therefore, the chemical states and bonds of the metal atoms are important characteristics to analyze the properties. One of the non-precious metal oxide formations is thermal oxidation of metal nitrate aqueous solution. In this report, the chemical state changes of the MOx (M = Mn, Fe, Co, Ni) during the catalyst formation process were observed by Raman spectra as the first step of the analysis of the metal oxide catalysts.
The Raman spectra for Mn and Ni nitrate aqueous solution are relatively strong compared to those for Fe and Co nitrates. The spectra show the difference between the different metals. Comparison from their (Ni and Co) reported solid oxide Raman spectra [3,4], some of the peaks are close to its TO related modes and 2LO. The peak around 1000 to 1100 cm-1, which is close to the peak assigned as 2LO in the solids, are most strong. It should be noted that the metal oxides Raman spectra were different with changes in its concentration of metal nitrate in water. The Raman spectra after dropping the nitrates on carbon paper were similar to the form in aqueous solution but changed. The Raman spectra were weakened after the anneal at 250°C for 2.5 hrs in the air. These results probably show that nonprecious metal can change its chemical form easily.
 D. Friebel et al., J. Am. Chem. Soc., 137 (2015) 1305.
 A. Li, et al., Angew. Chem. Int. Ed., 58 (2019) 1.
 N. Mironova-Ulmane et al., Cent.Eur. J. Phys. 9 (2011) 1096.
 Y. Li et al., J. Phys. Phys. Chem. C 120 (2016) 4511.
8:00 PM - EN07.05.34
Method for Determining Bond Energy in Nanostructured Water
Vitaly Bondarenko1,Svetlana Volchek1,Vladimir Petrovich1,Valentina Yakovtseva1,Sergey Redko1,Alexander Grigoriev1
Belarusian State University of Informatics and Radioelectronics1Show Abstract
Today, there are the following basic ideas about the structure of water as a partially self-organizing system with a dipole as an elementary structural element: (1) in addition to neutral dipole water molecules, water also contains the OH- and H+ ions that are products of the water dissociation; (2) water contains water molecules that are linearly associated with hydrogen bonds, and the closed (by the ring principle) structural organization of water is not excluded at that; (3) water forms three-dimensional nanoclusters. Each nanocluster can contain from 100 to 500, and even up to 910 water molecules. The nanocluster size is estimated to be equal to 1.5 nm. The basis for the cluster construction is a tetragonal cell, at the vertices of which water molecules linked by the hydrogen bonds are located. This paper discusses the study of the change in the loss tangent for deionized water in the frequency range from 25 to 106 Hz. The water temperature during the loss tangent registration was 293 and 323 K. The acidity value of deionized water was regulated by hydrochloric acid. Using water as an example, it is shown that monitoring the change in the loss tangent is a high sensitive method that allows determining the activation energy of relaxation processes in nanostructured water with high accuracy. The paper presents an analysis of the use of open- and closed-type sensors. The use of closed-type sensors, the electrodes of which are not in direct contact with the liquid under study, is justified for monitoring the properties of aqueous solutions. When studying the properties of water using a closed-type sensor, the sensor electrodes together with the fluoroplastic coating serve essentially to create an electric polarizing field in the volume of the liquid studied. When registering the dispersion of the loss tangent and other immittance characteristics, electrode reactions associated with the electric current passage through the metal-dielectric-liquid interface (Faraday currents) do not affect the results – they simply do not exist. At the same time, in the bulk of water and its solutions, not only polarization processes, but ion transfer processes take place as well. These processes occur simultaneously. Thus, a closed-type sensor registers mainly the volume properties of water and allows, the acidity values of the solution to be unambiguously determined at any frequency not exceeding 105 Hz.
Maytal Caspary Toroker, Technion-Israel Institute of Technology
Francesc Illas, University of Barcelona
Michele Pavone, University of Napoli Federico II
Guofeng Wang, University of Pittsburgh
EN07.06: Novel Water-Splitting Catalysts I
Wednesday AM, December 04, 2019
Sheraton, 2nd Floor, Liberty BC
8:30 AM - EN07.06.01
Nanometal for Hydrogen Evolution
University of California, Riverside1Show Abstract
Hydrogen in metals underpins many key technologies in energy storage and conversion such as hydrogen storage and metal-hydride batteries, but little is known about hydrogen in many nanosystems such as gold nanoclusters. In this talk, I will discuss our recent computational efforts to elucidate the role of hydrogen in metal nanoclusters and the relevant catalytic impact. Especially, we focus on the energetics of H-metal interaction and how it changes the electronic structure of the nanosystems and impacts hydrogen evolution.
9:00 AM - EN07.06.02
Interfacial Engineered Colloidal “giant” Core/Shell Quantum Dots Sensitized Carbon Nanotubes-TiO2 Hybrid Photoanode for High-Efficiency Hydrogen Generation
Gurpreet Selopal1,2,Mahyar Mohammadnezhad2,Omar Abdelkarim2,Haiguang Zhao3,François Vidal2,Zhiming Wang1,Federico Rosei2,1
IFFS-UESTC1,INRS-EMT2,Qingdao University3Show Abstract
Solar-driven photoelectrochemical (PEC) hydrogen (H2) generation is an attractive approach for the sustainable production of clean and renewable fuels, to address future global energy demands [1-2]. However, the low photon-to-fuel conversion efficiency and long-term stability of PEC devices are major challenges to be addressed to enable large-scale commercialization. The sensitization of the wide band gap semiconductors with colloidal chalcogenide quantum dots (QDs) as light harvesters is an effective approach to extend the absorption spectrum toward the visible and near infrared region (NIR) region, leading to significant improvement of the PEC performance. A specially designed “giant” core/shell QDs exhibit superior optoelectronic properties such as better photophysical/chemical stability, suppressed non-radiative Auger recombination, improved quantum yield (QY), and improved exciton lifetime and the formation of quasi-type-II core/shell by tailoring the shell thickness/composition as well as the core size . Also, the carbon nanomaterials such as carbon nanotubes (CNTs), graphene, graphene nanoribbons and graphene oxide are widely used in various optoelectronic devices due to their unique structural and optoelectronic properties such as excellent conductivity, high mechanical strength and optical transparency . Herein, for the first time, we explore a simple, fast and cost-effective approach to fabricate high efficiency and stable PEC devices for H2 generation, by using a specially designed colloidal “giant” CdSe/(CdSexS1-x)5/(CdS)2 core/shell QDs as sensitizer and a hybrid photoanode with small amounts of CNTs into a TiO2 mesoporous film.
We will discuss the synergistic effect of promising optoelectronic properties of specially designed colloidal giant” CdSe/(CdSexS1-x)5/(CdS)2 core/shell QDs and improved electron transport (reduced charge transfer resistance) within the TiO2-CNTs hybrid anodes enabled by the directional path of CNTs to the photo-injected electrons towards FTO. Resulting, PEC devices based on TiO2/QDs-CNTs (T/Q-C) hybrid photoanode with optimized amount of CNTs (0.015 wt%), yield a saturated photocurrent density of 15.90 mA.cm-2 (at 1.0 VRHE) under one sun illumination (AM 1.5 G, 100 mW×cm-2), which is 40% higher than the reference device based on TiO2/QDs (T/Q) photoanodes. In addition, enhanced stability of the PEC device based on T/Q-C hybrid photoanodes (~19% loss of its initial photocurrent density) as compared with the T/Q photoanode (~35% loss) after two hours of continuous one sun illumination will be also presented and discussed in details. These results provide fundamental insights and a different approach to improve the efficiency and long-term stability of PEC devices and represent an essential step towards the commercialization of this emerging solar-to-fuel conversion technology.
 Y. Tachibana, L. Vayssieres, and J. R. Durrant, Nat. Photonics, 2012, 6, 511–518.
 S. C. Warren, K. Voitchovsky, H. Dotan, C. M. Leroy, M. Cornuz, F. Stellacci, C. Hebert, A. Rothschild and M. Graetzel, Nat. Mater, 2013, 12, 842–849.
 H.-J. Ahn, M.-J. Kim, K. Kim, M.-J. Kwak and J.-H. Jang, Small, 2014, 10, 2325–2330.
 F. Navarro-Pardo, H. Zhao, Z.M. Wang and F. Rosei, Acc. Chem. Res., 2018, 51, 609–618.
 K. T. Dembele, G. S. Selopal, C. Soldano, R. Nechache, J. C. Rimada, I. Concina, G. Sberveglieri, F. Rosei and A. Vomiero, J. Phys. Chem. C, 2013, 117, 14510–14517.
9:15 AM - EN07.06.03
Molecular Insight into the Osmolality of Ionic Liquid with Lower Critical Solution Temperature Transition Behavior
Hyungmook Kang1,2,Akanksha Menon1,Robert Kostecki1,Chris Dames2,Jeffrey Urban1
Lawrence Berkeley National Laboratory1,University of California, Berkeley2Show Abstract
A subclass of ionic liquids (ILs) undergoes a thermoresponsive liquid-liquid phase transition of lower critical solution temperature (LCST). In liquid-liquid mixtures with an LCST transition, a single and miscible phase appears at lower temperatures, whereas the single-phase mixture separates into two immiscible phases upon heating above a critical temperature. The IL-based mixtures as draw solutes have opened up new water purification potentials such as forward-osmosis desalination.
This study tracks the changes in long-range order and local-molecular environment of an IL-water mixture with LCST transition, experimentally and theoretically. In the mixture, the IL forms loosely hold aggregate structures that grow in size leading up to a critical temperature, whereas the aggregation of a fully miscible aqueous mixture by minor chemical modification of the anion shrinks versus increasing temperature. Radial distribution functions from Molecular Dynamics simulations support the observation of aggregation phenomena in the IL-water mixtures.
For the usage as the osmosis draw solutes for water purification, the osmolality of the IL in water is measured as a function of concentration. The geometric nature of ILs introduce steric hindrance and strong attractive interaction between cation and anion causes a non-monotone trend of the osmolality. Molecular approaches considering the interaction between ions and the number of water molecules near each ion from Molecular Dynamics simulation suggest a method to define free ions in the mixture, which can improve the osmotic pressure.
9:30 AM - EN07.06.04
A Combined Spectroscopic and First-Principles Approach for the Atomistic-Level Description of Semiconductor/Electrolyte Interface
Masahiro Sato1,Yuki Imazeki1,Takahito Takeda1,Masaki Kobayashi1,Susumu Yamamoto1,Iwao Matsuda1,Jun Yoshinobu1,Yoshiaki Nakano1,Masakazu Sugiyama1
The University of Tokyo1Show Abstract
Photocatalytic water splitting is one of the most attractive technologies for storing sunlight. However, today’s photocatalysts (photoelectrodes) still suffer from low efficiency or poor stability , and to overcome these problems, the physical basis of photoelectrochemical reactions has to be understood in more detail. Band alignment (, i.e., the relative position between the conduction band minimum (CBM) or the valence band maximum (VBM) of the semiconductor and the redox potentials in the electrolyte at the semiconductor/electrolyte interface,) determines whether the reaction of interest proceeds or not, and therefore is the key factor for photoelectrochemical reactions. Nevertheless, from an atomistic point of view, little is known about the photocatalyst(semiconductor)/electrolyte interface, and the relationship between the band alignment and the geometric structure at the interface remains unrevealed.
The recent development of ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has made it possible to probe the electronic structure of the semiconductor/electrolyte interface under nearly in situ conditions . However, XPS results are not self-explanatory in the sense that it requires XPS signatures obtained from other experiments in order to identify the chemical elements and the nature of their chemical bonds.
Considering the above, in this contribution, we probe the relationship between the band alignment and the geometric structure at the semiconductor/electrolyte interface by combining AP-XPS measurements and first-principles calculations. As a starting point, we investigate water adsorption on crystalline GaN (0001) surface to avoid excessive complexity.
The band alignment between semiconductors and electrolytes are determined by both the intrinsic and non-intrinsic properties. The non-intrinsic properties, that is, ones that cannot be determined from the bulk properties of the materials alone, are: the (1) band bending and (2) the surface dipole layer.
AP-XPS analysis allowed us to evaluate the band bending from the energy difference between the VBM and the Fermi level of the n-GaN substrate. In line with previous studies , the results have showed that the water adsorption on the GaN substrate reduces the band bending. The first-principles calculations have showed that the density of mid-gap states are reduced upon O, OH, or H adsorption. Taking both experimental and computational results into account, one can conclude that the band bending is reduced by the dissociative adsorption of water molecules. In addition, the O 1s spectra, which are interpreted using the computed O 1s binding energies, indicates that some portion of water molecules dissociates into O atoms as well as OH groups.
The potential shift due to the surface dipole layer is roughly estimated from the AP-XPS experiments by detecting the change in the O 1s binding energy of the water molecules in the gas phase. By comparing the results with the computed surface dipoles of various interface models and the amount of surface species estimated from the AP-XPS O 1s spectra, we have shown that the surface dipole is determined not only by the dipole of the adsorbates but also by the charge transfer between the substrate and the adsorbates.
In conclusion, we have found that it is possible to obtain an atomistic-level description of the interface-specific properties that determine the band alignment at the semiconductor/electrolyte interface. The approach described above will help us decide how to realize the desired band alignment at the photocatalyst/electrolyte interfaces.
 J. Liu et al., Science, 347, 970, 2015.
 M. Lichterman et al., Energy Environ. Sci., 8, 2409, 2015.
 X. Zhang and S. Ptasinska, Sci. Rep., 6, 24848, 2016.
9:45 AM - EN07.06.05
Earth-Abundant Metal-Metalloid Materials as Highly-Efficient Oxygen-Evolving Electrocatalyst
Jean Marie Vianney Nsanzimana1,Vikas Reddu1
Nanyang Technological University1Show Abstract
Abstract: Electrochemical energy conversion and storage devices, including metal-air batteries, regenerative fuel cells, and water-splitting cells are critical to satisfy the future energy demand of human society. Though electrochemical water splitting technology has been well-established among other techniques as a clean and efficient technology for hydrogen production because of the possibility of coupling to other renewable sources, such as solar and wind energy, it is still limited by the sluggish anodic oxygen evolution reaction (OER). As this sluggish electrochemical reaction is also involved in many energy storage and conversion technologies, it has become a hot topic over the last decades. Precious metal-based catalysts such as Iridium- and Ruthenium-based oxides and their composites are used predominantly, but the scarcity and low stability limit their application at large scale. As a consequence, intensive efforts have been devoted to developing cost-effective catalysts with superior oxygen-evolving activity and stability. The earth-abundant metal-metalloid materials represent an emerging family of highly efficient oxygen-evolving catalysts due to their ability for charge transfer between different elements and modified electronic structures lowering the kinetic energy barriers of the electrochemical processes. Herein, we present a fast and simple method of synthesizing iron-doped amorphous nickel boride on graphene oxide sheets. The hybrid exhibits outstanding OER performance and stability in prolonged OER operation. In 1.0 M KOH, only an overpotential of 230 millivolts is required to afford a current density of 15 milliampere cm-2 and showed outstanding stability in alkaline solution. The superior OER activity of the as-prepared catalyst is attributed to (i) unique amorphous structure to allow abundant active sites, (ii) synergistic effect of constituents and, (iii) strong coupling of active material and reduced graphene oxide. This work not only provides new perspectives to design highly effective material for OER, but also opens a promising avenue to tailor the electrochemical properties of metal boride which could be extended to other materials for energy storage and conversion technologies.
 a) Nano Energy 2016, 29, 126; b) Journal Electroanalitical Chemistry 2011, 660, 254.
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 a) Chem. Soc. Rev. 2015, 44, 2060; b) Energy Environmental</gwmw> Science 2012, 5, 9331.
 Chem. Soc. Rev. 2017, 46, 337.
 a) Jean Marie Vianney Nsanzimana, et al. Advanced Energy Materials 2018, 8 (1), 1701475; b) Jean Marie Vianney Nsanzimana, et al. Chemistry European Journal 2018, 24 (69), 18502.
10:30 AM - EN07.06.06
A Supramolecular Approach to Water Oxidation
University of Trieste1,CICbiomagune2Show Abstract
Catalytic water oxidation is a necessary step towards splitting of water. We have achieved this important goal using two approaches:
1) A stable nano-structured anode, obtained by the deposition of an oxygen evolving polyoxometalate cluster (a totally inorganic ruthenium catalyst, Ru4POM) electrostatically self-assembled to a conducting substrate of multiwalled carbon nanotubes (MWCNTs) is able to electrochemically oxidize water at low overpotentials and high TOF and TON.1
2) A robust multi-perylenebisimide (PBI) chromophore assembled around a polyoxometalate water oxidation catalyst (Ru4POM) shows enhanced water oxidation properties when irradiated with visible light.2
During this talk, we will describe the most recent achievements of our labs on this topic.
Nature Chemistry 2010, 2, 826-831
Nature Chemistry 2019, 11, 146-153
11:00 AM - EN07.06.07
Hierarchical Sb2Se3 Light Absorber—Nanostructured Photocathode for Highly Efficient Solar to Hydrogen Conversion
Jaemin Park1,Wooseok Yang1,Jeiwan Tan1,Hyungsoo Lee1,Sang-gi Shim1,Jooho Moon1
Yonsei University1Show Abstract
For practical hydrogen production via photoelectrochemical (PEC) water splitting, proper nanostructuring enabling of sufficient light absorption and effective charge carrier transport to the electrolyte is of immense importance. In this study, we propose novel nanostructure for photocathode using Sb2Se3 which is nontoxic, low band gap and earth-abundant materials. The planar Sb2Se3 film is fabricated with molecular precursor ink to avoid direct contact of conductive substrate and electrolyte then the branch-like Sb2Se3 nanowires sequentially decorate the planar film by an additional facile spin coating. UV-vis spectroscopy shows that internal scattering at nanowires enables the reduced reflectance by bilayer structure. After the surface modification with TiO2 and co-catalyst Pt, impedance spectroscopy and intensity-modulated photovoltage spectroscopy are also conducted to examine the charge transport of the film in which the electrons are easily transported to the electrolyte due to higher surface area. Furthermore, incident photon to current efficiency at the long wavelength for hierarchically bilayer Sb2Se3 based photocathode reveals increases dramatically compared with monolayer film, indicating more efficient light harvesting in wide wavelength area. Consequently, improved optical and electrical characteristics allow us to demonstrate the record high photocurrent at 0 V versus reversible hydrogen electrode up to 28.5 mA cm–2. This implies the successful achievement of the highly efficient Sb2Se3 based photocathode with a hierarchically bilayer structure. Our findings clearly illustrate the impact of our Sb2Se3 device as a promising candidate for practical PEC water splitting.
11:15 AM - EN07.06.08
Development of a Method to Characterize Active Sites in Photocatalysis Using Operando Transmission Electron Microscopy
Eric Stach1,Vincent Verret1,Pawan Kumar1,Noah Glachman1,Khim Karki2,Daan Hein Alsem2,Deep Jariwala1
University of Pennsylvania1,Hummingbird Scientific2Show Abstract
Hydrogen gas has the potential to be a clean source of sustainable energy due to its high energy density. However, greenhouse gas emissions are still a major byproduct of current hydrogen production methods. Photoelectrochemistry provides a promising, environmentally friendly route to hydrogen production. However, the atomic scale mechanisms of the photocatalysts that facilitate the water splitting reaction are currently poorly understood. Further understanding of the chemical physics governing the active hydrogen evolution sites would allow for better design of photoelectrochemical devices and thus lead to improved reaction efficiencies. This will overcome one of the major barriers impeding this promising technology. We have developed a unique operando photoelectrochemistry transmission electron microscope liquid cell sample holder which can be used to characterize these reactions in real time at nanometer length scales. This system builds upon our prior developments of operando electrochemical liquid cell holders, by including the additional provision of an optical fiber directed at the sample to provide full solar spectrum illumination. In order to provide accurate, quantitative information, it is necessary to accurately deposit the photocatalyst of interest onto microfabricated electrodes. In this research, a precise sample deposition technique utilizing an inkjet printer has been developed along with stable suspensions of known photocatalysts, leading to site-specific deposition onto the electrode chips. Specifically, this experimental design allows for correlation between I-V characteristics and real time, high magnification imaging and spectroscopy, elucidating information about photocatalytic mechanisms at the nanoscale. The following photocatalysts used were chosen because the proposed mechanism for each exhibits a spatial dependence: plasmonically enhanced catalysis for Au nanoprisms and catalytically active edge sites for MoS2 flakes. These experiments will lay the groundwork for the use of this novel experimental design to investigate a wide variety of photoelectrochemical systems and will allow determination of the mechanisms by which selected photocatalysts induce water splitting and the identification of defect features that serve as the active sites.
11:30 AM - EN07.06.09
Hydrogenated TiO2-Silicon Tandem Cell Platelets for Direct Water Splitting
University of Augsburg1Show Abstract
Hydrogenated TiO2 (H:TiO2 or black titania) shows highly increased absorption in the visible frequency spectrum of light in parallel with an improved photoelectrochemically (PEC) water splitting activity. These findings dragged interest on the properties and in particular on the synthesis, electrical properties and stability of H:TiO2.
It became also apparent that there might be no single compound which simultaneously fulfills all requirements for unassisted (i.e. without externally applied electric fields) direct PEC water splitting, which are firstly band edge energy levels and quasi-Fermi energy level positions providing over potentials needed to enable hydrogen (HER) and oxygen evolution (OER) reactions, secondly a potential difference of more than 1.23 V, and thirdly chemical stability under highly photo corrosive working conditions.
Coupling of H:TiO2 with a mobility energy gap Eg of 1.5-3.2 eV with amorphous a-Si:H (Eg = 1.1-1.8 eV), which results in a theoretical maximal solar to hydrogen (STH) efficiency of 20% in thin film tandem cell structures, could be a route toward low cost PEC water splitting systems.
Nanoparticle suspensions might play a key-role in achieving efficient and cost effective PEC water splitting. The reason for this is, that nanostructured semiconductors possess a large surface to volume ratio and by choosing an appropriate particle size, particle geometry (e.g. platelet) and catalysts mutually dependent conversion steps, which crucially determine photovoltaic and PEC performance in their bulk counterparts, can be decoupled. This concerns in particular the generation and selective separation of photogenerated electron-hole (e-h) pairs which are mainly controlled in bulk materials by diffusion, gradients of the electrical potential and finally by gradients of the quasi-Fermi energies. In nanostructures localized surface states with extremely fast charge carrier transfer times and charge carrier transport times to the interfaces much smaller than e-h pair recombination times in the bulk determine the selectivity and efficiency.
A very important feature of the here proposed PEC tandem cell platelets is the very short charge carrier transport length to the facing large surfaces. The thickness of the tandem top- and bottom-electrode can be adjusted independently from their lateral dimension and adapted to optimize photogenerated electron-hole pair separation. In contrast to conventional solar tandem cell architectures, where thick absorbing semiconductor layers are applied and carrier diffusion is important, ballistic transport of photogenerated electrons and holes to the liquid/solid and solid/solid interfaces by utilizing very thin top- and bottom-layers with H:TiO2 and a-Si:H can be achieved.
In this work H:TiO2 thin films were grown in-situ by fully reactive sputtering of a metallic Ti target on silicon and silica substrates. We mapped the critical process parameters consisting of the sputter gas ratio and the RF-sputter power over a wide range for determining growth condition resulting in maximal hydrogen incorporation. Quantitative depth profiling of the H-content in the H:TiO2 thin films was performed by helium elastic recoil detection analysis (He-ERDA) which revealed an H-content of more than 2.7 at.%. The optical absorption coefficient and temperature dependent electrical conductivity will be discussed in context of the thermal stability and hydrogen content. The temperature dependent resistivity and Seebeck coefficient of H:TiO2 thin films were determined by consecutive temperature cycles up to 723K and show to be stable up to 673 K, above which a slight increase in the resistivity was observed. First H:TiO2-silicon-iridium tandem cell platelets will be presented an their properties discussed.
11:45 AM - EN07.06.10
Combinatorial Thin-Film Deposition for High-Throughput Electrocatalyst Testing
Larry Scipioni2,Anthony Thompson1,Hector Colon-Mercado1,Elise Fox1,James Greer2,Adam Shepard2
Savannah River National Laboratory1,PVD Products2Show Abstract
Catalytic processes can often be improved and optimized by changing the composition of the catalyst, but it is often not feasible to explore the entire compositional space because of the time and cost of performing multiple syntheses. High-throughput methods allow for the exploration of a wide range of variables in a relatively short period of time. Here we demonstrate the high-throughput testing of electrocatalysts for SO2 oxidation, the electrochemical step of the Hybrid Sulfur (HyS) process for water splitting, over a full range of compositions of Au, Pt, and V. Catalysts were synthesized by combinatorial sputter deposition of multiple thin film patches onto a glassy carbon substrate and compositions were confirmed by XPS. A scanning electrochemical microscope (SECM) with a scanning droplet system (SDS) attachment was used to obtain CV curves and map electrochemical activity across the entire glassy carbon plate, allowing rapid optimization of metal ratios for the reaction.
EN07.07: Computational Characterization and Design
Wednesday PM, December 04, 2019
Sheraton, 2nd Floor, Liberty BC
1:30 PM - EN07.07.01
Electrocatalysis with Graphitic Carbon Catalysts
Troy Van Voorhis1
Massachusetts Institute of Technology1Show Abstract
Pyrolytic graphitic carbon materials posess tantalizing activity toward a range of reactions - from hydrogen evolution, to oxygen reduction and oxygen evolution. However, the uncontrolled nature of their synthesis makes it virtually impossible to rationally improve them. Recently, graphitic carbon catalysts GCCs) - small molecule catalysts that are directly conjugated to the edges of graphistic carbon - have been shown to posess similar reactivity but with controllable synthesis. This presentation will give an overview of the mechanism of oxygen reduction (ORR) in GCCs and present the results of several large-scale computational screening experiments aimed at improving the activity and selectivity of the catalysts. In particular, we find that the GCCs do not obey the commonly accepted scaling relations that limit ORR performance in traditional heterogeneous catalsts. We hypothesize this is due to the fact that these are organic and organometallic complexes, which are empricially known to provide a greater degree of chemical specificiy than inorganic solids and surfaces. We conclude with an analysis of the transferrability of these ideas to the reverse reactionof ORR.
2:00 PM - EN07.07.02
First-Principles Predictions of Photoelectrode Materials—From Bulk to Complex Interfaces
Tuan Anh Pham1,Tyler Smart2,Yuan Ping2,Brandon Wood1,Tadashi Ogitsu1
Lawrence Livermore National Laboratory1,University of California, Santa Cruz2Show Abstract
The generation of hydrogen from water and sunlight through photoelectrochemical cells (PECs) offers a promising approach for producing scalable and sustainable carbon-free energy. The design of high-performance PECs requires a detailed understanding of physicochemical properties of not only the semiconductor photoelectrode, but also the interface between the material and liquid water. In this presentation, we discuss how first-principles simulations can be utilized to unravel the key chemical and electronic properties of photoelectrode materials in the bulk phase and at the interface with liquid water. Specific discussion focuses on how hybrid density functional theory and many-body perturbation theory can be used to provide high-fidelity description of the electronic properties of photoelectrode materials, including their band gap and band edge positions. In addition, we show how first-principles molecular dynamics simulations can be coupled with near-ambient-pressure XPS experiments for the identification of solid/liquid interfacial chemical composition and speciation, which is necessary for devising meaningful strategies to improve PEC performance and durability. Examples for different materials, from Co3O4 to III-V semiconductors will be discussed, based on which we suggest a more general roadmap for obtaining a realistic and reliable description of the chemistry of complex.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
2:15 PM - EN07.07.03
Estimation of Electronic Entropy Contributions to Oxygen Vacancy Formation Reaction in Nonstoichiometric Oxides via Analysis of Electronic Structure Properties
University of Florida1Show Abstract
The primary motivation of this work is to develop a method through which the entropy change of oxygen vacancy formation may be computed from first principles for strongly correlated metal oxides which exhibit intrinsic spin polarization. Namely, a methodology via which the electronic structure may be utilized to predict electronic entropy contributions to the overall free energy change. Leveraging electronic structure properties facilitates the process by circumventing the need for extensive combinatorics and statistical methods which are conventional approaches for entropy calculations within the realm of statistical physics. Nonstoichiometric cerium dioxide (CeO2) and the manganite perovskites (AMnO3) form a prototypical composition space meeting the criteria above and possessing a high degree of versatility with applications ranging from electrocatalysis and solar thermochemical energy storage to electronic and magnetic devices. The acquisition of a fundamental understanding of the mechanisms governing entropy in these materials would enhance the rate at which they may be optimized and tailored for current applications and potentially enable the discovery of novel applications for which they are ideal candidates.
It is commonly assumed that finite temperature electronic contributions to the free energy of insulators and semiconductors may be neglected due to the lack or absence of electron density near the Fermi level. This may be the case when treating ideal, pristine systems since there are no electronic levels available for population due to thermal excitation of electrons near the valence band maximum. This sizable bandgap in medium to wide gap materials limit the number of electronic configurations accessible to the highest occupied levels. The validity of this assumption is highly questionable however, especially when charged defect levels are present within the energy gap increasing the density of states near the Fermi level. Shallow defect levels with unpaired electrons near the VBM would allow for an increase in the number of microstates for thermal population of electrons contributing to the electronic configurational free energy.
The electronic density-of-states data from a structural relaxation at the HSE06 level of theory with a 12-atom Ce4O8 bulk supercell was implemented to calculate the Fermi level (electron chemical potential) as a function of temperature. The Fermi-Dirac distribution along with the normalized DOS were utilized to estimate the majority carrier concentration (assuming only electronic charge carriers) via the difference between the conduction band and valence band integrals.
A greedy algorithm is utilized to evaluate the Fermi level at which the error between the calculated carrier concentration (via above parameters) at a given temperature and the predicted/estimated carrier concentration is minimized.
The corresponding electronic entropy agrees relatively well with results from work by Naghavi et al. in magnitude and variation with respect to temperature. Further validating the significance of electronic entropy contributions to the energetics of oxygen vacancy formation in ceria and other similar mixed conducting oxides which may be comparable to if not greater than structural configurational and vibrational entropic terms. Recent work by Lany highlighted this relationship between temperature, Fermi level and charged defect formation energies specifying that electronic entropy is the physical or thermodynamic consequence of this phenomenon. Specifically, the energy level of the oxygen vacancy, the conduction band effective mass and the temperature dependence of the conduction band minimum were claimed to be the electronic structure properties that govern the electronic entropy contribution to vacancy formation free energies. The work presented herein provides numerical evidence to support the claims made by Lany in a purely analytical deduction.
3:30 PM - EN07.07.04
Multiscale Modeling of Charge Carrier Dynamics in Complex Metal Oxides for Solar Water Splitting—The Case of Anion Doping in BiVO4
Viswanath Pasumarthi1,Pavan Kumar Behara1,Michel Dupuis1
University at Buffalo, The State University of New York1Show Abstract
This presentation will deal with computational modeling dealing with doping strategies toward enhanced carrier transport and overall photo-electro-conversion (PEC) efficiency of semiconductor electrodes, in particular BiVO4 the best anode material to date. We carried out multiscale modeling, combining DFT+U calculations of e/h polaron hopping by Marcus/Holstein theory and Kinetic Monte Carlo (KMC) modeling of collective charge carrier transport at the mesoscale. We will highlight the approach first for a stoichiometric bulk BiVO4 where simulations revealed that hole transport is bimodal, featuring fast rattling but transport-inefficient hops and slower transport-efficient hops, while electron transport is slower yet. We will then discuss the structure and stability of electron and hole polarons in the presence of sulfur doping. DFT calculations reveal that sulfur atoms substitute oxygen and act as hard traps (~ 14 kBT) for electrons in their vicinity. We will describe how sulfur incorporation affects electron and hole transport dynamics resulting in enhanced transport efficiency. If time permits, we will highlight modeling of cation doping by W/Mo, including the design of homo-junctions for enhanced carrier separation.
Photoelectrochemical (PEC) water splitting is a renewable energy conversion process in which hydrogen and oxygen are generated from water using energy from sunlight that is attracting much interest in a sustainable energy economy. However, there exist several scientific and engineering challenges in implementing this technology at industrial scale. A broad critical challenge is to identify suitable semiconductor materials for photoelectrodes in PEC cells: they must exhibit good visible light absorption and carrier generation, good carrier transport, and good redox reactivity. Metal oxides have the promising advantages of stability in aqueous medium and wide and inexpensive availability of the constituent elements. Bismuth vanadate (BiVO4), a ternary metal oxide is one of the best performing photoelectrode material to date, with its most photoactive monoclinic phase having a wide band gap of ~2.4-2.5 eV. Systematic investigations of the performance limiting factors have led to significant progress over the past decade. Engineering the material using strategies such as nanostructuring, or metal-ion doping by tungsten (W) and molybdenum (Mo) have resulted in photocurrents to reach ~6.7 mA/cm2, ~90 % of theoretical maximum of ~7.5 mA/cm2. Recently, anion doping with nitrogen/sulfur was found to decrease the bandgap by up to ~300 meV for improved solar spectrum absorption.
This work is supported in part by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award # DE-SC0019086.
3:45 PM - EN07.07.05
Phenyl Oxidation at Oxygen Evolution Potentials—Impact on Alkaline Membrane Electrolyzer Durability
Dongguo Li1,Ivana Matanovic1,Albert S. Lee1,Hoon T. Chung1,Yu Seung Kim1
Los Alamos National Lab1Show Abstract
The durability of alkaline membrane/ionomer is a critical requirement for commercially viable alkaline membrane water electrolysis. In this presentation, we report the oxidation of the phenyl group in the ionomer adsorbed on the oxygen evolution catalysts, which may cause performance deterioration in AEM electrolyzers. The 1H-NMR analysis in combination with rotating disk electrode studies shows that the oxidation of phenyl group occurs at oxygen evolution potentials, ca. 1.6 V under both low and high pH conditions. Further study indicates that the phenyl oxidation also depends on the nature of catalysts. Commercial Pt/C and IrO2 noble metal catalysts exhibit much faster phenyl oxidation compared with La0.85Sr0.15CoO3 perovskite oxide. Density functional theory calculations also confirm that the phenyl adsorption is significantly less pronounced on the perovskite oxide catalyst, implying phenyl adsorption at the oxygen evolution potentials is a critical factor determining the phenyl oxidation. This research provides a path for the development of more durable AEM electrolyzers with components that can minimize the adverse impact induced by the phenyl group oxidation, such as the development of novel ionomers with fewer phenyl moieties or catalysts with less phenyl-adsorbing characteristics.
Maytal Caspary Toroker, Technion-Israel Institute of Technology
Francesc Illas, University of Barcelona
Michele Pavone, University of Napoli Federico II
Guofeng Wang, University of Pittsburgh
EN07.08: Fundamental Mechanisms for Water Splitting I
Thursday AM, December 05, 2019
Sheraton, 2nd Floor, Liberty BC
8:30 AM - EN07.08.01
Linking Surface Properties to Electrochemical Activity of Water Splitting Catalysts
Research and Technology Center, Robert Bosch LLC1Show Abstract
Improvements in water splitting technology requires detailed understanding of associated electrochemical reactions, the oxygen evolution reaction and the hydrogen evolution reaction, as well as catalyst surfaces on which these reactions occur. This talk will first focus on the oxygen evolution reaction and present examples of how X-ray absorption spectroscopy characterization can be combined with electrochemistry to study the oxidation state of the catalysts under reaction conditions. Then, it will transition to hydrogen evolution reaction and reexamine electrochemical activity of precious-metal catalysts in alkaline environment. In this example, detailed analysis of the micropolarization region of rotating disc electrode experiments will be combined with calculations of the relevant surface conditions, in order to interpret the characterized exchange current densities in the context of surface properties of the catalysts.
9:00 AM - EN07.08.02
Unravelling the Structure—Properties Relationship of MOCVD Co3O4/TiO2 Heterojunctions for Solar Water Splitting
Constantin Vahlas1,Adeline Miquelot1,Olivier Debieu2,Stephanie Roualdes3,Christina Villeneuve4,Nathalie Prud'homme5,Jeremy Cure6,Vincent Rouessac7,George Papavieros8,Vassilios Constantoudis8
CIRIMAT1,INPT2,Université de Montpellier3,Université Paul Sabatier4,Université Paris-Sud5,LAAS6,CNRS7,NCSR Demokritos8Show Abstract
Green H2 production by solar water splitting entirely relies on the intrinsic properties of the photocatalyst. Among the numerous photocatalytic materials investigated on purpose, heterojunctions based on TiO2, the quintessential component of state of the art photocatalytic systems, are expected to expand the photocatalytic activity to the visible domain of the solar spectrum. In this perspective, literature reports on the p/n Co3O4/TiO2(anatase) heterostructure are contradictory as they mention either improvement or degradation of their photocatalytic activity with regard to the single anatase photoanode. The aim of the present contribution is to contribute to the elucidation of this question and, subsequently to the illumination of the impact of the intrinsic properties of this material, explored at nanoscale, on its photocatalytic activity under visible light illumination.
We process the Co3O4/TiO2 heterostructure in the form of a two films stack by a sequential MOCVD process, starting from cobalt 2,2,6,6-tetramethyl-3,5-heptanedionato, Co(dmp)3, and titanium tetra-isopropoxide, TTIP. Deposition in the temperature range 350 to 550 °C provides Co3O4 and anatase films with variable characteristics. We implement a multiscale, holistic microstructural, optical and electrical characterization of the two types films alone and of various Co3O4/TiO2 bilayers, using state of the art techniques, microscopic (SEM-FEG including advanced image analysis, AFM, HRTEM, EBSD/TKD), spectrometric (FTIR, Raman, UV-Vis-NIR, ellipsometry, XPS, XRD, EPMA, EDX), and electrical/electronic (conductive AFM, Hall and Van der Pauw resistivity, Seebeck coefficient).
We show that, with increasing deposition temperature, anatase films evolve from dense to porous, nanotree-like columnar structures. This evolution results in a remarkable increase of the exchange surface, coinciding with a change of growth direction, from <110> to <001> and to the increase of the porosity, to a decrease of the optical absorption, and to an increase of both the electrical resistivity and the carriers density.
In the same conditions, the morphology of the Co3O4 films evolve in the opposite direction, from columnar to dense. These films show a minimum room temperature resistivity of 21.4 Ω.cm, and Hall resistivity measurements establish their p-type semiconducting behavior with a maximum holes density of 1.8 ± 0.8×1018 cm-3, and a maximum holes mobility of 0.16 cm2.V-1.s-1. We attribute the linear T–1/4 temperature dependence of the film conductivity to a 3-dimension variable range hopping conduction mechanism of holes. The surface topographies of the deposition temperature dependent morphologies perfectly match the current maps at the AFM scale, demonstrating that the variable range hopping conduction of holes occurs through the Co3O4 nanocrystallites.
The 66 h cumulative H2 photogeneration from the single anatase films increases from 4.4 to 78.9 mmol.m-2 with increasing the deposition temperature from 325 to 450 °C. This result points out the importance of the beneficial effect of the morphological nano-complexification, and of the crystallographic diversification of the exchange facets on the photocatalytic performance over detrimental aspects inherent in this evolution, namely, the decreases of the optical absorption and the increase of residual stress, also described here. We process heterojunctions by cross combining dense and columnar single layers of the two semiconductors. The resulting H2 photogeneration is strongly reduced with regard to the anatase films, despite a strong decrease of the thickness of the external Co3O4 layer to less than 50 nm, aiming at the exposure of both semiconductors and of their interfacial zones to the light. This result is in agreement with the strong decrease of the electrical conductivity perpendicular to the stack. Detailed analysis of the interface is actually in progress in order to reveal the reasons of this behavior.
9:15 AM - EN07.08.03
Unfolding the Effect of the O2 and N2 Plasma on Hydrogen Evolution Reaction of MoS2
Indian Institute of Technology1Show Abstract
Defect engineering is widely adopted technique to increase the density of exposed active sites. Plasma technique is proved as an effective technique to tune the surface properties and edge reactive sites for greatly improving the electrochemical activities. In this report, controlled oxygen (O) and nitrogen (N) plasma used to generate defects and introduce dopants N and O in MoS2 nanosheets to form defect-induced nanocomposite material MoS2-xXx (X= N, O) with sulphur vacancies. Defects in the MoS2 nanosheets help to improve the number of active sites but extra defects deteriorate electron mobility. So here mild plasma technique is used to (1) increase the hydrogen evolution reaction (HER) catalytic activity of MoS2 S-edge and (2) electron mobility of MoS2 nanosheets for fast electron transfer which are one of the responsible factors to increase the electrochemical activities. Electrochemical measurements prove that with the combination of both active sites along with the incorporation of O & N dopants enhance HER activities. The superior activity and stability for the hydrogen evolution reaction with low overpotential and fast electron transfer is derived from the coexistence of both doping effect and sulphur vacancies.
9:30 AM - EN07.08.04
Comparison of Light-Intensity-Dependent Open-Circuit Potential among TiO2, SrTiO3 and GaN Single-Crystalline Photoanodes
Supawan Ngamprapawat1,Yuki Imazeki1,Tsutomu Minegishi1,Masahiro Sato1,Katsushi Fujii2,Masakazu Sugiyama1
The University of Tokyo1,RIKEN Center for Advanced Photonics2Show Abstract
Photoelectrochemical (PEC) water splitting is a promising technology that converts solar energy into a storable energy carrier, hydrogen. Many signs of progress have been being reported. Nevertheless, the efficiency and the stability of PEC water splitting is still unsatisfactory. Finding a photoelectrode material which has an appropriate alignment of quasi-Fermi energies under illumination with respect to redox potentials of water is an essential condition for the progress of PEC water splitting.
Semiconductor photoelectrodes are a key component. They are often selected based on the band-edge energies, assuming a flat-band situation. However, band bending generally exists at the semiconductor/electrolyte interface, which makes the flat-band condition unattainable under insufficient light intensity. When the light intensity exceeds a certain value, the band bending is reduced by an accumulation of photo-generated carriers. By comparing the light intensity dependence of band bending among a variety of materials, it would be possible to explore the fundamental processes governing the function of semiconductor photoelectrodes from the viewpoint of semiconductor physics, similarly to the analysis for photovoltaic devices.
In this study, light-intensity-dependent open-circuit potential (OCP) of three types of single crystalline photoelectrodes: (i) undoped and 0.05 wt% Nb-doped TiO2, (ii) 0.01 wt% and 0.05 wt% Nb-doped STO, and (iii) n-type GaN on Si was investigated with varied photon flux from 109 to 1017 s-1cm-2. A He-Cd laser (325 nm) was used as a light source. The light-intensity-dependent OCP of all materials showed a similar tendency. Plateau of OCP versus light intensity was observed at the photon flux less than 1011 s-1cm-2. The positions of the plateau for all materials range between -0.3 and 0 V vs SHE. These positions depend on several factors: surface condition of the photoelectrode, impurity in semiconductor, and dissolved gas in the electrolyte. Conversely, when the photon flux exceeded 1011 s-1cm-2, the linear relationship was clearly observed between the logarithm of photon flux and OCP, indicating the accumulation of charge carriers in the bulk of semiconductor and the reduction in band bending, in a similar manner to solid-state junctions of semiconductors.
Furthermore, the ideality factor of a diode was evaluated from the slope of the plots of OCP versus the logarithm of photon flux. The ideality factors of the doped photoelectrodes, TiO2 and STO, were in a range of 1.0 – 1.3, while that of the undoped TiO2 was 1.6. The ideality factor of GaN photoelectrode, prepared by the hetero-epitaxial growth on Si, was larger than 2.0. The distribution of interface states and defects, and the recombination of carriers in the depletion region could be the main reasons for the deviation from unity. These results also showed that the better crystal quality resulted in ideality factors close to unity, indicating the suppressed recombination of photo-generated carriers in the semiconductor bulk.
As the photon flux increased, the OCP shifted toward the flat-band potential. At the photon flux of 1017 s-1cm-2, OCP of all materials exceeded the hydrogen evolution potential, i.e. hydrogen evolution reaction (HER) became possible at the counter electrode. The difference between the OCP and the flat-band potential at high photon flux depends on materials and doping concentration; an ideal photoelectrode should show virtually no difference.
In summary, all three materials exhibited the linear relationship in the plot of OCP versus the logarithm of photon flux as it is observed in photovoltaic devices. Ideality factor of a diode at solid-liquid interfaces close to unity was obtained only for photoelectrodes fabricated from single-crystalline wafers with low defect density. The study of light-intensity-dependent OCP allows us to determine which material has a suitable photo-response for HER under certain light intensity.
9:45 AM - EN07.08.05
Fields Matter—Better Water Splitting Through Magnetic Field-Assisted Processing of Hematite Thin Films
Daniel Stadler1,Myeongwhun Pyeon1,Vanessa Rauch1,Mehmet Gursoy2,Meenal Deo1,Yakup Gönüllü1,Thomas Fischer1,Taejin Hwang3,Sanjay Mathur1
University of Cologne1,Selçuk Üniversitesi2,KITECH3Show Abstract
Even though the potential of hematite thin films for water splitting applications are widely accepted, researchers are still tackling the ‘rust challenge’. We report here on the influence of external magnetic fields applied parallel or perpendicular to the substrate during plasma enhanced chemical vapor deposition (PECVD) of hematite (α-Fe2O3) nanostructures. Hematite films grown from iron precursors showed pronounced changes in crystallographic textures depending upon whether PECVD was performed with or without the influence of external magnetic field. Static magnetic fields created by rod-type (RTMs) or disk-type magnets (DTMs) resulted in hematite films with anisotropic or equiaxed grains, respectively. Using RTMs, a superior photoelectrochemical (PEC) performance was obtained for hematite photoanodes synthesized under perpendicularly applied magnetic field (with respect to substrate), whereas parallel magnetic field resulted in the most efficient hematite photoanode in the case of DTM. Our experimental data on microstructure and functional properties of hematite films showed that application of magnetic fields parallel and perpendicular have a significant effect on the crystallite size and texture with preferred growth and/or suppression of grains with specific texture in α-Fe2O3films. Investigations on the water splitting properties of the hematite films in a photoelectrochemical reactor revealed that photocurrent values of hematite photoanodes were remarkably different for films deposited with (0.659 mA/cm2) or without (0.484 mA/cm2) external magnetic field.
10:30 AM - EN07.08.06
Adsorption Energetics and Surface Phase Transformations in Iridium and Ruthenium-Based Oxygen Evolution Reaction Catalysts
University Catholique de Louvain1Show Abstract
Important devices such as electrolyzers or fuel cells depend on the control and understanding of electrocatalytic processes. Ab initio techniques have been extremely useful in providing the needed insight in the catalytic mechanisms leading even to computationally-motivated design of new electrocatalysts. There remain, however, many open questions in the field and fundamental studies of the adsorption processes on model single-crystals are strongly called. In this talk, I will present recent results on combined theory-experiment approaches towards the understanding of important oxide catalysts for oxygen evolution reaction (OER) using molecular-beam epitaxy (MBE) grown single-crystals and ab initio techniques. I will especially focus on the comparison between computed and experimental adsorption energies in two model systems: RuO2 and IrO2, helping estimate the typical errors on the very common DFT adsorption computations performed in electrocatalysis. Finally, I will discuss more recent results on the understanding of complex phase transformations during hydrogen desorption and cyclic voltammetry especially on IrO2 and using a combination of DFT and statistical mechanics.
11:00 AM - EN07.08.07
Surface and Sub-Surface Structure of Ba0.5Sr0.5Co0.8Fe0.2O3-δ Perovskite Particles for Oxygen Evolution Reaction Probed by Electron Microscopy Techniques
Tzu-Hsien Shen1,Liam Spillane2,Yang Shao-Horn3,Vasiliki Tileli1
École polytechnique fédérale de Lausanne1,Gatan Inc.2,Massachusetts Institute of Technology3Show Abstract
Water splitting is seen as a promising way to store clean energy. However, the sluggish oxygen evolution reaction (OER, 4OH− → O2 + 2H2O + 4e−) at the anode is the bottleneck that limits the water electrolysis efficiency. Perovskite family with the ability of tunable electronic structure is considered as a strong candidate to replace precious metal catalysts that currently dominate the market for oxygen evolution reaction . Understanding the surface of perovskites is one of the keys to design protocols for next-generation OER electrocatalysts as most of catalytic processes occur at the surface. In this study, transmission electron microscopy (TEM) based techniques with exceptional spatial resolution are used to fulfill this purpose and perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF), a highly active OER catalyst in alkaline electrolytes , is taken as the target catalyst. Electron energy-loss spectroscopy (EELS) operated under scanning TEM (STEM) mode is performed by probing the surface of the as-synthesized BSCF particles dispersed in KOH. The EELS results show a profile of decreasing Ba concentration from bulk toward the surface and a reduced valence of Co ions at the very surface of BSCF. Quantitative analysis using simulated EEL data confirms the compositional alteration of the surface structure. Selected area fast Fourier transform patterns of high resolution TEM images of BSCF reveal a spinel structure of surface Co-rich phase as well as the lowering of the crystallinity as approaching the surface. STEM-EELS of the identical BSCF particle after chronoamperometric measurements in OER regime shows that the reduced oxidation state of surface Co-rich phase remains after OER.
In order to further understand how the BSCF surface structure evolves during electrochemical processes, in situ TEM methodologies are applied . Real-time structural monitoring during cyclic voltammetry shows surface dissolution which is likely A-site Ba2+/Sr2+ leaching at BSCF surface [4,5]. The expansion of the BSCF particle due to OER is also observed during cycling. In conclusion, the electron microscopic studies give information on complex surface structure of BSCF which might be related to the OER catalytic properties, and in situ TEM observation provides possible degradation mechanisms of BSCF perovskite catalysts.
 W. T. Hong, M. Risch, K. A. Stoerzinger, A. Grimaud, J. Suntivich, and Y. Shao-Horn, Energy Environ. Sci. 8, 1404 (2015).
 J. Suntivich, K. J. May, H. a Gasteiger, J. B. Goodenough, and Y. Shao-horn, Science (80-. ). 334, 1383 (2011).
 B. Han, K. A. Stoerzinger, V. Tileli, A. D. Gamalski, E. A. Stach, and Y. Shao-Horn, Nat. Mater. 16, 121 (2017).
 K. J. May, C. E. Carlton, K. A. Stoerzinger, M. Risch, J. Suntivich, Y. L. Lee, A. Grimaud, and Y. Shao-Horn, J. Phys. Chem. Lett. 3, 3264 (2012).
 E. Fabbri, M. Nachtegaal, T. Binninger, X. Cheng, B. J. Kim, J. Durst, F. Bozza, T. Graule, R. Schäublin, L. Wiles, M. Pertoso, N. Danilovic, K. E. Ayers, and T. J. Schmidt, Nat. Mater. 16, 925 (2017).
11:15 AM - EN07.08.08
Programmable NiFe/Au Barcode Nanomesh for Enhanced Oxygen Evolution
Can Cui1,Liaoyong Wen1
University of Connecticut1Show Abstract
The oxygen evolution reaction (OER) is the most important half-reaction in renewable electrochemical energy storage and conversion, including water splitting, CO2 reduction, and metal-air batteries. First-row transition metal oxides and hydroxides (e.g., Ni, Co, Fe and Mn) is one of the most promising alternatives to replace the precious metal oxides (e.g., Ir and Ru) in alkaline conditions. To address the large overpotential and poor stability of the low-cost transition metal oxides based OER catalysts, many strategies have been explored, including elemental doping, surface decoration by noble metal, nanostructuring et al.. However, these challenges still exist, which greatly hinder their large-scale applications.
Here, we report a novel three-dimensional NiFe/Au barcode nanomesh based on alumina nanoporous template to pursuit a desirable OER catalyst. The morphological feature and composition of the NiFe/Au barcode nanomesh can be easily tuned by using different templates and electrodeposition processes. The spatial confinement effect of the uniform porous structure in a range of hundreds of nanometers could efficiently facilitate the oxygen gas evolution with small bubbles. Most importantly, the synergistic interaction of the alternating layered NiFe alloy and Au structure could not only provide a large number of active sites, but also decrease the intrinsic resistance of the formed NiFe alloy oxides during the OER, which in turn significantly decreases the required overpotential. By choosing suitable thickness ratios of NiFe and Au layers, respectively, a very low overpotential below 300 mV was obtained under a current density of 10 mA/cm2, which is one of the lowest reported values from the NiFe based composites. A negligible decrease of the potential was observed even after a 3-day test. These programmable three-dimensional NiFe/Au nanomeshs may provide a new type of cost-effective and long-term stable electrocatalysts for scalable energy applications.
11:30 AM - EN07.08.09
Design and Preparation of CNTs Aerogel/Wood Bilayer-Structures for Solar Steam and Sewage Treatment
Lei Miao1,Xiaojiang Mu1,Jianhua Zhou1
Guilin University of Electronic Technology1Show Abstract
The shortage of clean water is one of the predominant causes of human mortality, especially in remote rural areas. Currently, solar steam generation is being adopted as an efficient, sustainable, and low-cost means for water desalination to produce clean water1. CNTs aerogel is a good solar absorber with huge surface area, high absorptance of solar irradiation, light weight and good insulation. However, the strong hydrophilicity of aerogel hinders heat localization in the pore and further decreasing water evaporation rate for pure CNTs aerogel solar evaporator. Here, in this work, we designed CNTs aerogel/wood bilayer-structures, i.e.; insertion of natural wood sheet between aerogel and evaporation interface to control the hydrophilicity of the whole evaporator and make the absorbed heat more compatible with the water to be evaporated, then the evaporation rate could be improved greatly.
Chitosan and CNTs composed aerogels were prepared by one-step freeze-drying treated Chitosan and CNTs mixture. The highest evaporation rate reached up to 1.64 kg m-2 h-1 in the evaporation system with pure CNT aerogels as absorber. While, for the bilayer structure with wood as interface layer, the evaporation rate reached up to 2.22 kg m-2 h-1 which increased by about 40% comparing to the pure CNT aerogels absorber. Infrared imager was used to record the temperature of evaporator in the process of evaporation. It is obvious that the thermal insulation and local heating properties of the composite material are much better than that of CNT aerogel. Desalination result shows that although the evaporation rate decreases slightly, it can still reach 1.86 kg m-2 h-1, which providing a new idea for efficient desalination. In addition, the two-stage filter effect can be achieved by wood sheet and aerogel composite. The macropore of the wood sheet is a primary filtration, which can be used to preliminarily remove the larger pollutants or harmful substances in the sewage. The mesoporous and microporous of aerogel are secondary filtration, which can further remove the smaller particles or harmful substances in sewage. Driven by solar energy, the whole filter system could purify waste liquid and domestic sewage effectively. Cost-effective bilayer structures could prepare solar absorber with good steam generation performance and sewage treatment capacity in large quantities, which holds great potential for practical applications, especially in remote rural areas and environmental polluted area.
1. Shuaiming He etal. Energy. Environ.Sci.2019 12 1558-1567.
11:45 AM - EN07.08.10
Li and P Co-Doped Heptazine Based g-C3N4 Monolayer
Deepak Gorai1,Tarun Kumar Kundu1
Indian Institute of Technology Kharagpur1Show Abstract
In this work, quantum chemical first principle density functional theory calculations are performed by using VASP and Gaussion09 software to investigate the geometric, electronic and optical properties of Li and P co-doped carbon nitride (g-C3N4) monolayer. The co-doping process results in significantly narrow the band gap of g-C3N4. The optical absorption shows better visible-light response. Highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) show strong delocalization and hence photo-generated e-/h+ pair separation abilities of them are better than pristine monolayer g-C3N4. The theoretical evidence for the possibility of using g-C3N4 with excellent photocatalytic properties has been provided.
keywords: Density Functional Theory, Co-doping, VASP, Gaussion09, g-C3N4 Monolayer, Photo-catalyst
EN07.09: Fundamental Mechanisms for Water Splitting II
Thursday PM, December 05, 2019
Sheraton, 2nd Floor, Liberty BC
1:30 PM - EN07.09.01
Halide Perovskites—From Understanding Fundamental Physics to Optoelectronic Applications
Los Alamos National Laboratory1Show Abstract
Hybrid organic−inorganic perovskites (HOPs) have demonstrated an extraordinary potential for clean sustainable energy technologies and low-cost optoelectronic devices. This talk overviews the main features of three dimensional (3D) and layered two-dimensional (2D) HOPs by combining solid-state physics concepts with simulation tools based on density functional theory. A comparison between layered and 3D HOPs highlights differences and similarities such as spin-orbit effects, quantum and dielectric confinements and excitonic properties. In 3D HOPs we study in depth the effects of electron-phonon coupling leading to polaron formation across the broad range of materials. Calculated electronic structure, charge density, changes the geometry, and reorganization energies are further related to experimentally measured specific vibrational modes, Huang-Rhys parameters and Jahn-Teller like distortions. These effects lead to formation of meta-stable deep-level charge states, which potentially responsible for photocurrent degradation in thin-film perovskite devices. The photophysics of 2D materials is defined by an interplay of strongly bound excitons and lower-energy states associated with the edges of the perovskite layers. The latter provide a direct pathway for dissociating excitons into longer-lived free carriers that substantially improve the performance of optoelectronic devices. Our theoretical simulations rationalize specifics of electronic structure of these materials, dynamics and a role of interfacial states. We also outline specific ways to rationally control geometry of edges in 2D HOP materials via external fields, contact interfaces and composition of organic compound. Overall, our results provide insights towards the material design for various applications.
2:00 PM - EN07.09.02
Mechanisms for Hydrogen Evolution on Transition Metal Phosphides and Pt
Chenyang Li1,Per Lindgren2,Georg Kastlunger2,Andrew Peterson2,Tim Mueller1
Johns Hopkins University1,Brown University2Show Abstract
Water splitting for renewable energy storage and conversion is limited by the high cost of Pt-based catalysts used for the hydrogen evolution reaction (HER). Earth-abundant transition metal phosphides have emerged as active HER catalysts with lower cost than Pt. However, the catalytically active sites and reaction mechanisms on these catalysts remain unclear. Here we present models of the HER on four different transition metal phosphide surfaces (Co2P, CoP, Fe2P, and FeP) using cluster expansions parameterized by density functional theory. By running Monte Carlo simulations, we predict the structures and energetics of adsorbed hydrogen as a function of temperature and applied potential, allowing us to determine the potential-dependent activities of different sites while fully accounting for interactions among adsorbed hydrogen atoms. For comparison, we have used the same approach to model the HER mechanisms on Pt surfaces – including Pt(111), Pt(110), and Pt(100). We demonstrate the effects of potential-dependent surface coverage on HER current density and propose mechanisms for the HER on these model surfaces. The present model provides a general and effective approach to study catalytic reaction mechanisms and probe catalytically active sites, which can facilitate the design of highly active catalysts.
2:15 PM - EN07.09.03
Investigation of the Influence of Electrolyte Temperature on the Performance of Cu2O towards Solar Water Splitting
Iqra Reyaz Hamdani1,Ashok Bhaskarwar1
Indian Institute of Technology Delhi1Show Abstract
Cuprous oxide (Cu2O) is a nontoxic and inexpensive semiconductor with the direct bandgap of 2.0 eV. It is an excellent material for solar driven water splitting, having a good theoretical solar to hydrogen conversion efficiency of 18%. Various deposition techniques have been explored for Cu2O thin film deposition, such as spray pyrolysis, sputtering, and electrodeposition, and chemical vapor deposition, thermal and chemical oxidation. However, electrodeposition is the most convenient method since it is economical and provides an easy control on the properties of the film, such as thickness, crystallinity, and roughness. Thus, Cu2O deposited by electrodeposition method is a low cost, sustainable and an efficient material. In this work, Cu2O thin films were electrodeposited on FTO substrates at different bath temperatures, at a constant bias potential of -0.4 V with respect to Ag/AgCl. A comprehensive study was carried out to investigate the effect of bath temperature on the morphological and optical properties of Cu2O. The performance of Cu2O as a photocathode in a photoelectrochemical device, and hence its application in photoelectrolysis of water towards hydrogen evolution, was determined and analyzed at three different temperatures i.e., at room temperature (25°C), 65°C and 85°C. The electrolyte used was 0.4 M copper sulfate hexahydrate and 3 M lactic acid. The pH of electrolyte was kept at 9 by the addition of 4 M aqueous NaOH. From the SEM images, the surface morphology was found to change from crystalline small grain sizes through polyhedral structures to amorphous structured fine grains as the temperature was increased from 25°C to 85°C. From the XRD studies, films deposited at higher temperature of 85°C showed the presence of CuO which was also visually detectable by the black color. All the films showed the presence of phase (111), however its intensity was highest at temperature of 65°C. The thickness of films increased from ~150 nm to 10 μm as the temperature was increased. However, in this range their uniformity decreased. The charge carrier density as evaluated from the Mott-Schottky equation, was highest for the films deposited at 65°C, and its magnitude was about 1.5 x 1029 cm-3. From the photoelectrochemical measurements, the films synthesized at 65°C exhibited the highest performance and generated a maximum photocurrent density of 1.6 mA/cm2.
2:30 PM - EN07.09.04
Highly Flexible and Porously Structured Metal Phosphide/MXene Arrays as Efficient Bifunctional Electrocatalysts for Overall Water Splitting
Clament Sagaya Selvam Neethinathan1,Gwan Hyun Choi1,Piljin Yoo1
Sungkyunkwan university1Show Abstract
The development of efficient and stable bifunctional electrocatalysts that outperform noble metal electrocatalysts is an important task and an ongoing challenge for sustainable hydrogen generation. To accomplish this goal, an ordered hierarchical structure with more exposed active sites for strong electrolyte contact should be designed. Herein we report a synthetic strategy of binder free, free-standing and flexibly structured Mx-Py (M=Fe, Co and Ni) decorated MXene arrays as efficient bifunctional electrocatalysts. Specifically, 2-dimensional (2D) MXene sheets prepared by improvised etching method render a flexible platform for the hierarchical decoration of highly mesoporous CoP, allowing them to exhibit remarkably high electrocatalytic performances. The phosphidation process provides enhanced conductivity and increased number of active sites for the hybrid. As a result, CoP/MXene electrocatalyst shows enhanced oxygen evolution reaction (OER) activity, with substantially lower overpotential (230 mV at 10 mA cm−2) compared to those of state-of-the-art IrO2 (270 mV). Furthermore, a hybrid bifunctional electrode (CoP/MXene//CoP/MXene) exhibits highly stable and efficient overall water splitting performance (1.56V at 10 mA cm−2) as compared to the benchmark electrode couple IrO2/C//Pt/C (1.62V at 10 mA cm−2) in alkaline solution. The hybrid electrocatalyst having Co atoms with exposed active sites accelerates the OER kinetics. Likewise, the negatively charged MXene sheets enriched with functional groups increase the proximal migration of H+ ions toward the cathode, thereby facilitating the HER kinetics as evidenced by DFT calculation. This study also provides a better understanding of the electronic effect induced by the MXene termination groups. Furthermore, under the OER conditions, the CoP/MXene is partially oxidized to Co-(oxy) hydroxide clusters, acting as a real active species for the efficient electrocatalytic performance. Furthermore, the post-stability analysis indicated that the massive oxidation of M-P bond (under OER experimental conditions) is suppressed by MXene layer. These promising results demonstrate the structural and compositional merits of CoP/MXene hybrid for scalable electrochemical applications. Our findings also reveal that rational tuning of Metal/MXene interface can remarkably promote the OER/HER electrochemistry. We conclude that the as-fabricated flexible and free-standing MXene-based films can be directly used as electrodes for various electrochemical applications.
2:45 PM - EN07.09.05
Wafer Scale Synthesis of MoS2-Graphene Heterostructure at Low Temperature and its HER Application
Kubra Aydin1,Hyeong-U Kim2,Vinit Kanade1,Mansu Kim3,Hyunho Seok1,Changmin Kim3,Dongmok Whang3,Jae-Hyun Lee4,Tae Sung Kim3,1
Sungkyunkwan University Advanced Institute of NanoTechnology1,Northwestern University2,Sungkyunkwan University3,Ajou University4Show Abstract
In this research, we successfully indicate the fabrication and characterization of MoS2-graphene heterostructure (MGH) using Plasma enhance chemical vapor deposition (PECVD) and its HER activity. Graphene is a material that has two-dimensional (2D) planar structure of carbon atoms with small overlap between the valence and conduction bands. 2D planar structure of graphene can agree to hybrid with different materials and this agreement can lead to enhanced catalytic activity and stability. Therefore, Transition-metal dichalcogenides (TMDs) is one of materials for enhancement performance with graphene. TMDs are atomically thin semiconductors of the type MX2, where M is a transition metal (Mo, W) and X is a chalcogen (S, Se, or Te). In addition, band gap of MoS2 is depending on the number of MoS2 layers. MoS2 can be applied wide application areas because of its tunable band gap like hydrogen storage, capacitors.
Heterostructure was distinguished as lateral and vertical heterostructure. Lateral heterostructure includes different 2D layers on one plane. Vertical heterostructure is that different 2D layers are stocked up onto one plane. When graphene forms vertical heterostructure with MoS2, the Dirac cone of graphene splits. Therefore, it can be tuned band gap of material using heterostructure method. When we consider regarding its tunable band gap and relatively high carrier mobility, MoS2 has been investigated for applications in future electronic devices. Especially, the combination of MoS2 with graphene opens up new possibilities in electronic applications and also shows great potential as a catalyst. Perfectly, this various band gap material finds many uses; for example, in hydrogen storage, capacitors, electrochemical devices.
First of all, Graphene was synthesized on Cu foils using chemical vapor deposition (CVD) process. Graphene was growth using CH4:H2 (30:50 sccm) at 1040 celsius degree for 1h and then furnace was rapidly cooled to room temperature under same atmosphere. The synthesized graphene was transferred on substrate (SiO2/Si or glassy carbon electrode). Mo was deposited by 1 nm of thickness on transferred graphene by E-beam evaporator. it was loaded into PECVD chamber for sulfurization at 150 celsius degree under H2S and Ar plasma for 1.5 h. Finally, MoS2 was directly growth on graphene layer as a vertical heterostructure layer. It was characterized by using Raman spectroscopy and high-resolution transmission electron microscopy. The MoS2 was confirmed as 5-6 layers using Raman and HR-TEM analysis. XPS analysis with depth profiling can confirm chemical states of both top MoS2 layer and bottom graphene layer in MGH. Furthermore, hydrogen evolution reaction (HER) of MGH and bare MoS2 was evaluated. MGH electrodes shows lower over potential and Tafel slope than bare MoS2 electrode, which is indicating their enhanced catalytic performance. This rising up can be attributed to the high density of defects (grain boundaries and sulfur vacancies) present in MoS2 synthesized on graphene, which is increasing the total number of active sites and then provide an effective charge transport pathway through the highly conducting graphene layer.
3:30 PM - EN07.09.06
Understanding Photoelectrode/Catalyst Interface for Solar Water Splitting
Boston College1Show Abstract
As a potentially low-cost, high-efficiency solar energy storage solution, solar water splitting faces great challenges. One of the issues is the poor catalytic activity and low stability. Applications of co-catalysts have been shown effective to correct the deficiency by promoting desired chemical reactions so as to minimize charge recombination at the surface and to reduce parasitic corrosion reactions. The detailed behaviors of the light absorber/catalyst interface, however, remain poorly understood. Here we present our recent research in this area. We show that the application of the co-catalysts may greatly influence the charge separation behaviors of the photoelectrode. Detailed thermodynamic and kinetic measurements support our understanding. Furthermore, we show that the photoelectrode substrate also exerts great influences on the co-catalyst behaviors. A strong interaction between the photoelectrode and the catalyst can be beneficial. The combined system may also serve as a new platform to understand heterogeneous catalysis such as water oxidation at a level previously inaccessible. The knowledge generated by our work will likely contribute significantly to the development of solar water splitting technology for a future powered by renewable energies.
4:00 PM - EN07.09.07
Water Oxidation by Manganese Oxide Systems—Molecular and Layered Catalysts Inspired by Nature
Michael Zdilla1,Daniel Strongin1,Eric Borguet1,John Perdew1,Michael Klein1,Ian McKendry2,Haowei Peng1,Richard Remsing1,Ran Ding1
Temple University1,Los Alamos National Laboratory2Show Abstract
An overview of our recent progress in two areas of biologically inspired water oxidation chemistry is presented. 1) The preparation of synthetic manganese clusters that mimic the active site of the oxygen evolving complex Photosystem II has given catalysts that are able to activate water for redox reactions at low overpotentials, and has offered insights into mechanistic chemistry at these systems of potential relevance to nature. 2) The synthetic modification and optimization of layered manganese oxide catalysts, guided by theory, has offered clues into the electronic, structural, defect, and geometric features that favor redox chemistry in the interlayer of the manganese oxide phase birnessite. By a combination of theory, simulation, and experiment, the optimization of birnessite to turn it from a poor catalyst into an excellent catlayst for electrochemical water oxidation will be presented.
4:15 PM - EN07.09.08
InGaAsP as a Promising Narrow Band Gap Semiconductor for Photoelectrochemical Water Splitting
Joshua Butson1,Parvathala Narangari1,Mykhaylo Lysevych1,Jennifer Wong-Leung1,Yimao Wan1,Siva Karuturi1,Hoe Tan1,Chennupati Jagadish1
The Australian National University1Show Abstract
While photoelectrochemical (PEC) water splitting is a very promising route towards zero-carbon energy, conversion efficiency remains limited. Semiconductors with narrower band gaps can absorb a much greater portion of the solar spectrum, thereby increasing efficiency. However, narrow band gap (~1 eV) III-V semiconductor photoelectrodes have not yet been thoroughly investigated. In this study, the narrow band gap quaternary III-V alloy InGaAsP is demonstrated for the first time to have great potential for PEC water splitting, with the long-term goal of developing high-efficiency tandem PEC devices, which require both wide and narrow band gap absorbers. TiO2-coated InGaAsP photocathodes generate a photocurrent density of over 30 mA/cm2 with an onset potential of 0.45 V versus RHE, yielding an applied bias efficiency of over 7%. This is an excellent performance, given that nearly all power losses can be attributed to reflection losses. X-ray photoelectron spectroscopy and photoluminescence spectroscopy show that InGaAsP and TiO2 form a type-II band alignment, greatly enhancing carrier separation and reducing recombination losses. TiO2 also greatly improves the stability of InGaAsP, which is susceptible to corrosion in acidic electrolyte. Future work will include the minimisation of reflection losses. This can be achieved by fabricating InGaAsP nanostructures, for which a large-area top-down random nano-mask technique has been developed. Beyond water splitting, the tunable band gap of InGaAsP could be of further interest in other areas of photocatalysis, such as CO2 reduction.
Maytal Caspary Toroker, Technion-Israel Institute of Technology
Francesc Illas, University of Barcelona
Michele Pavone, University of Napoli Federico II
Guofeng Wang, University of Pittsburgh
EN07.10: Metal and Metal Oxide Catalysts
Maytal Caspary Toroker
Friday AM, December 06, 2019
Hynes, Level 2, Room 203
8:30 AM - EN07.10.01
Identifying Active Surface Entities on Metal Oxides for Oxygen Evolution by Multimodal Surface X-Ray Probes
Argonne National Laboratory1Show Abstract
Electrocatalysts are materials designed to provide a facilitating environment for electrochemical conversion and synthesis of materials and fuels from atmospheric molecules, which is one of the most important challenges facing societal need of energy in 21st century. One of the major hurdles developing electrocatalysts is the lack of holistic information of the evolving surface structure of materials during electrochemical operation. This is particularly formidable for oxygen evolution reaction (OER), where the oxidizing environment is corrosive and can significantly rearrange the electrocatalyst surface structure. Therefore, identifying how the surface structure of materials evolves during the OER is essential to the development of more active and stable electrocatalysts and broadly to the prospect of materials and energy sustainability. Surface-sensitive X-ray probes from modern synchrotron sources including surface X-ray scattering and grazing incidence X-ray spectroscopy provide a very powerful suite of toolkits to decipher the surface subtlety and evolution. If utilizing these techniques in a well-coordinated approach, one can deliver thorough and deep fundamental insights of surface transformations (e.g. structural, chemical and electronic) during the electrocatalytic process.
In this talk, we will firstly render a brief survey of various surface sensitive X-ray techniques to specifically probe structural and chemical aspects of electrocatalytic materials, in particular the combined approach to differentiate the contribution from surface and bulk layers. Following the survey, we would like to present a few prototypic studies of metal oxide model systems for surface OER processes. First example is the detailed understanding of the interaction between water and single crystal RuO2 (110) in acidic electrolytes under OER conditions, especially surface atomic structure rearrangements as a function of potential quantified by X-ray crystal truncation rods. Unique oxygen absorbent species on the Ru sites were detected at an OER relevant potential. A new OER pathway with the rate-limiting deprotonation of the –OH group can be suggested by integrating potential-dependent surface structures with DFT-calculated energetics. The second demonstration is to present a comprehensive study of the emergent surface transformation of SrIrO3, the most active OER electrocatalyst reported to date, especially the amorphous boundary layer that forms from the pristine crystalline structure on the surface with OER cycling. In virtue of multimodal X-ray probing, a step-by-step transformation mechanism of the amorphization process could be explicitly illuminated. Our X-ray results show that the amorphization is triggered by the lattice oxygen activation and the structural reorganization facilitating coupled cation and anion diffusions is key to the realization of the OER active structure in the final SryIrOx form which exhibits stronger disorder than conventional amorphous IrOx, partially explaining its champion OER activity.
9:00 AM - EN07.10.02
Self-Assembling Oxide Catalyst for Electrochemical Water Splitting
Ilia Valov1,Daniel Bick2,Deok-Yong Cho3,David Mueller1,Rainer Waser2
Research Center Juelich1,RWTH Aachen University2,Chonbuk University3Show Abstract
The demand on efficient and economically reasonable conversion and storage of energy from renewable power sources is an essential challenge and existential task for the modern society. Emerging technologies like metal-air batteries, fuel-cells and electrolysers, which are mostly limited by the oxygen electrocatalysis, depend on reliable catalyst materials suited for long term application in alkaline environments. Candidates for precious metal free catalyst materials range from metal alloys to oxides and nitrides. Several perovskites have been suggested as economically reasonable catalysts for the oxygen evolution reaction (OER) and showed eligible overpotentials. Among these, the perovskite system BaCoO3 (BCO), and especially the double perovskite PrxBa1-xCoO3-δ (PBCO) has shown superior properties (low overpotential) and has been identified as one of the most promising OER materials.
Application of oxides as OER electrocatalysts is limited by two general problems. The first one is their insufficient electronic conductivity. The second and main problem for all OER catalysts is the degradation of the structure (amorphisation) and electrocatalytic properties during long-term operation.
We report on a new perovskite self-assembling material system BaCo0.98Ti0.02O3-δ:Co3O4, which exhibits higher current densities for the OER and over 10-fold increased lifetime in comparison to the most reliable electrocatalyst (PBCO) reported up-to-date. Importantly, all electrolysis experiments were performed at temperatures typical for industrial application i.e. 353 K. By systematic modification of chemical composition/doping and defect chemistry in binder free BCO perovskite-based catalyst films, simultaneous aging and characterisation with electrochemical methods, XRD, XPS, XANES and EXAFS we were able to identify the degradation mechanism related to cation leaching and amorphization. We demonstrate that the initial crystalline structure rapidly and completely transforms to an amorphous electrochemically active material, which retains its electrochemical properties until its service life end.
Based on the combination of analysis, calculation and aging experiments, a model is derived, which is able to explain the connection between short-range order, defect chemistry and catalyst stability. This model is capable of making predictions to future OER catalyst material design.
9:15 AM - EN07.10.03
In Situ Probing of Ultrafast Electrochemical Reactions with High Catalyst Mass Activities for Efficient Water Splitting
University of Tennessee1Show Abstract
Water splitting into hydrogen/oxygen with Proton exchange membrane electrolyzer cells (PEMECs) has become more attractive due to their high efficiencies even at low-temperature operation. In this talk, the ultrafast and multiscale electrochemical reactions in an operating PEMEC, including oxygen evolution reactions (OERs) and hydrogen evolution reactions (HERs), will be revealed. The in-situ probing results shows the OERS and HERs mainly occur on catalyst layers at the rim of the pores of the thin/tunable liquid/gas diffusion layers (TT-LGDLs), and there is a large portion of catalysts being not effectively utilized. Based on these discoveries, a novel thin/tunable gas diffusion electrode (GDE) is developed by depositing the catalyst on a tunable pattern that is observed to be active for the OER/HER. The thin GDEs with a total thickness of about 25 µm significantly improve the catalyst mass activity and utilization and exhibit excellent PEMEC performance with a very simple fabrication process and low cost.
9:30 AM - EN07.10.04
Tailored Nickel-Iron Layered Double Hydroxide Platelet Size for Optimized Oxygen Evolution Reaction Catalysis
Daire Tyndall1,Valeria Nicolosi1,João Coelho1,Sonia Jaskaniec1
Trinity College Dublin1Show Abstract
The oxygen evolution reaction (OER) has drawn significant interest in the field of renewable and sustainable energy in recent years, with potential applications for hybrid electric vehicles (HEV) in the form of electrolyzer cells for hydrogen production, fuel cells or metal-air batteries, among others. Perhaps the most significant feature of OER is the fact that it is a necessary ‘step’ in the evolution of H2 gas by water electrolysis, bringing with it an associated potential (E ≈ 1.23 V). To overcome this potential barrier with minimum overpoetential (η), an effective electrocatalyst is required to facilitate the reaction. Nickel-Iron Layered Double Hydroxide (NiFe-LDH) has been shown to exhibit efficient catalysis of the OER, demonstrating overpotentials in composite systems which are competitive with previously studied electrocatalysts based on rare earth metals such as ruthenium and iridium, as well as being competitive in an economic perspective. NiFe-LDH has other advantages over rare earth catalysts such as earth abundance, cost and stability (in operating conditions). The nature of the material in question is that the vast majority of catalytic active sites are located at open coordination sites at the edge of the NiFe-LDH hexagonal platelets. This means that average particle dimensions will play an important role in determining the density of active sites within a NiFe-LDH electrode and hence, it’s catalytic ability.
Synthesis of high quality, planar NiFe-LDH platelets with regular hexagonal morphology was achieved using a wet chemistry method at a relatively low temperature (100 oC) using triethanolamine (TEA) as a ‘capping agent’ to allow homogeneous coprecipitation of both the nickel and iron metal centres within brucite-lake layers. Using platelets synthesized in this way, OER catalysis has been demonstrated with η = 0.36 V, a competitive value when compared to many state-of-the-art OER electrocatalysts in the same conditions (5 mVs-1, quoted at current density j = 10 mAcm-2). The work aims to develop methods of post-synthetic treatment of NiFe-LDH dispersions to reduce lateral platelet dimensions and further improve edge-site density for electrocatalytic optimization. This study comes in accordance with the growing number of high-performance electrocatalysts being studied for OER.
In this work, a combination of platelet size reduction and selection techniques are utilized to isolate particle dispersions with controlled mean platelet dimensions (<L>) with the aim of elucidating the relationship between <L> and η. To do so, methods of tip-sonication and centrifugation were employed. Tip-sonication (7 h) alone can reduce <L> from 0.78 ± 0.01 µm to 0.29 ± 0.01 µm while further centrifugation in the relatively narrow range of centrifuge rates 1000 – 5000 rpm can isolate useable dispersions with <L> as low as 0.20 ± 0.01 µm. When tested in electrolyzer cell working conditions (1 M KOH, 0 – 0.6 V) electrodes based on size-selected material exhibited a clear reliance of electrocatalytic performance on the mean platelet size <L>. Further, composite studies are carried out to establish ideal materials for optimizing the NiFe-LDH performance to achieve η values which are competitive in the field of OER catalysis.
 Gong M.; Li Y.; Wang H.; Liang Y.; Wu J. Z.; Zhou J.; Wang J.; Regier T.; Wei F.; Dai H. Journal of the American Chemical Society 2013 135 (23), 8452-8455.
 Song F.; X. Hu, Nat. Comm, 2014 Volume 5, 447.
 Tahir M.; Pan L.; Idrees F.; Zhang X.; Wang L.; Zou J.; Lin Wang Z. Nano Energy, Volume 37, 2017, Pages 136-157.
 Eftekhari A. Materials Energy Today, Volume 5, 2017, Pages 37-57.
9:45 AM - EN07.10.05
Synthesis of Titanium Oxynitride Thin Films with Tunable Bandgaps for Alternative Energy Applications in the Full Solar Spectrum Range
Dhananjay Kumar1,Nikhil Mucha1,Surabhi Shaji1,Manosi Roy1,Balamurugan Balasubramanian2,Prakash Apte1
North Carolina A&T State University1,University of Nebraska–Lincoln2Show Abstract
A novel TiNxOy (TiNO) material system with a range of x (0≤x≥1) and y (0≤y≤2) values has been synthesized in thin film form using the non-equilibrium nature of a pulsed laser deposition process. A proper control of x and y values has been found to result in the material system transformation from a metallic rock-salt structure to an insulating rutile structure (TiO2). The in-between x and y values yield compounds with a range of bandgap materials capable of absorbing radiation in the full solar spectrum. When N atoms in TiN are partially substituted by O atoms, the top of the valence band (valence band maxima) shifts down leaving the bottom of the conduction band (conduction band minima) unaffected. Since, the valence band maximum of the oxynitride compounds is located at higher potential energy than that for pure TiO2 due to the contribution of N 2p orbitals, the partial substitution of N by O result in the reduction of bandgap values with respect to those of the extreme compounds, i.e. TiN or TiO2.
X-ray diffraction and x-ray photoelectron spectroscopy measurements have been carried out that lend evidence in support of the realization of the aforementioned values of x and y. The optical measurements on TiNO films involving photoluminescence and ultra violet studies have established the existence of bandgaps in the range of 1.6 to 3.0 eV. The current voltage characteristics recorded from ITO/TiNO/Cu(Au) configurations have shown that the current (I) and corresponding voltage (V) lie in the same quadrant with a similar polarity in the dark conditions while the current and the corresponding voltage lie in quadrants with different polarity. The positive sign of power (I times V= positive) under dark conditions indicates dissipation of power in TiNO while the negative of sign of the power (I times V= negative) indicates the power generation capability of TiNO system. The fabrication of TiNO system via controlled oxidation has an advantage over conventional methods of bandgap manipulation of TiO2 via nitridation due to the higher activation energy of a nitridation process (672 kJ/mol) than that of an oxidation process (496 kJ/mol). As the number of photocatalysts/semiconductors that are active under visible light irradiation is very limited, our approach to develop a unique visible-light-driven TiNO photocatalytic and photovoltaic material system can open a new avenue for solar devices.
10:30 AM - EN07.10.06
Electrocatalytic Activity of Pd-CeO2 Catalysts for Anion Exchange Membrane Fuel Cells
Pamir Alpay1,Sanjubala Sahoo1,Hamish Miller2,Dario Dekel3
University of Connecticut1,Istituto di Chimica dei Composti Organometallici2,The Wolfson Department of Chemical Engineering, and the Nancy and Stephan Grand Technion Energy program (GTEP), Technion- Israel Institute of Technology3Show Abstract
There is significant interest in the development of platinum-free electrode materials for Anion Exchange Membrane Fuel Cells (AEMFCs) [1,2]. Here we present a combined experimental and theoretical study of the Hydrogen Oxidation Reaction (HOR) activity in alkaline medium for Pd-CeO2 anode catalysts. Specifically, we model Pd-CeO2 substrates where Pd atoms are either adsorbed or embedded on (111) CeO2 surfaces. The computations are performed using density functional theory using the generalized gradient approximation and the necessary Hubbard corrections. The HOR activity is explored using the Tafel-Volmer mechanism. The Tafel reaction is one of the rate-determining steps for HOR activity, which involves the dissociation of the H2 molecule. The activity descriptors for the HOR under alkaline conditions however, have been suggested to be the H bonding energy (HBE) and the OH bonding energy (OHBE), which provides a quantification of the H and OH bond strength onto the Pd and ceria. We determine dissociation pathways for H2 molecules on both Pd-CeO2 substrates. Our findings show that the Pd adsorbed (111) CeO2 becomes highly reactive for H2 dissociation, exhibiting a low dissociation barrier compared to that of a pure CeO2. Our computations are in agreement with experiments where a high HOR performance has been achieved for a bifunctional Pd-CeO2 system compared to CeO2 . Additional calculations are carried out to understand the HOR activity as a function of the concentration of adsorbed Pd atoms. Oxidized single Pd atoms or very small clusters coordinated to the CeO2 oxygens show improved HOR activity. Our research provides the underlying principles towards the design of highly active HOR catalysts for AEMFC without utilizing platinum.
 Dekel et al., Journal of Power Sources, 375, 158 (2018)
 Gottesfeld et al., Journal of Power Sources 375, 170 (2018)
 Davydova et al., ACS Catal. 8, 6665 (2018)
 Miller et al., Angew. Chem. 128, 6108 (2016)
11:00 AM - EN07.10.07
Enhancing the Activity of Molybdenum Diselenide for Electrocatalytic Hydrogen Evolution via Transition-Metal Doping
Akash Jain1,Vasu Kuraganti2,Maya Bar-Sadan2,Ashwin Ramasubramaniam1
University of Massachusetts Amherst1,Ben-Gurion University of the Negev2Show Abstract
Electrochemical water splitting is a promising fossil-fuel free route for hydrogen production. However, the hydrogen evolution reaction (HER) is most efficiently catalyzed by expensive platinum group metals and there is significant interest in finding alternative, earth-abundant replacements. Recently, there has been progress in employing relatively inexpensive transition-metal dichalcogenides (TMDCs) like MoSe2 for HER but these materials still require an appreciably larger overpotential than Pt, which reduces their efficiency. Here, we show that the catalytic activity of MoSe2 can be improved significantly by doping with earth-abundant transition-metal (TM) dopants such as Mn, Fe, Co and Ni. Using density functional theory (DFT) calculations, we study the influence of these substitutional dopants on the thermodynamics of H adsorption at edges, Se-vacancy, and basal plane sites, and explain trends in adsorption energies via electronic structure considerations and structural changes arising from the atomic-size mismatch between Mo and dopant atoms. A key finding from our studies is that selected TM-dopants promote the formation of highly active Se-vacancy sites—in some cases even making these vacancies thermodynamically favorable—thereby increasing the activity of the native MoSe2 catalyst. DFT calculations shed light on recent experiments on Mn-doped MoSe2 nanoflowers that display improved HER charge-transfer kinetics, smaller Tafel slope, and overpotential than undoped MoSe2. Overall, our work suggests that the electrocatalytic activity of MoSe2 and similar transition-metal dichalcogenides can be enhanced by employing substitutional dopants, not for their inherent activity, but as promoters of highly active chalcogen vacancies.
11:15 AM - EN07.10.08
Role of Defects and Polarons in Surface Morphology and Electronic Structure in BiVO4
Wennie Wang1,Giulia Galli1,2
The University of Chicago1,Argonne National Laboratory2Show Abstract
As one of the most widely-studied oxides for photocatalysis, bismuth vanadate (BiVO4) is a highly promising material candidate as a photoanode for water photocatalysis. In addition to having strong absorption across the visible spectrum and a conduction band edge near the hydrogen evolution potential, BiVO4 is stable under aqueous conditions and relatively facile and cheap to synthesize. It has been reported that charge transport, interfacial charge transfer, and recombination events are limiting factors for photoelectrocatalytic (PEC) performance. Thus, significant effort has gone into optimizing PEC devices based on BiVO4, including doping, nanostructuring, and pairing with other catalysts or semiconductors. However, a major challenge in the field has been to have well defined experimental samples and computational methodologies to disentangle the influence of surface/interface morphology, defects, and the presence of water.
Our objective is to holistically understand the connections between surface morphology and PEC performance, and bridge gaps between theoretical and experimental methods by making direct comparison between computed and measured electronic properties. Using first-principles calculations, we study the surface morphology and electronic structure of BiVO4 , and compare with single-crystalline and epitaxially-grown samples. We present our work on pristine and defective surfaces, and connect our calculations with measured band edges, work functions, and STM images. In particular, we discuss the role and formation of polarons in the bulk and at the surface , and their influence on PEC performance.
 H. Seo, Y. Ping, G. Galli. Chem. Mater. 30, 7793 (2018).
11:30 AM - EN07.10.09
Enhanced Catalytic Activity of Edge-Exposed 1T Phase WS2 Grown Directly on WO3 Nanohelical Array for Water Splitting
Noho Lee1,Il Yong Choi1,Kyung-Yeon Doh1,Jaewon Kim1,Hyeji Sim1,Donghwa Lee1,Si-Young Choi1,Jong Kyu Kim1
POSTECH, Korea1Show Abstract
Among various candidates for renewable energy sources, much attention has been given to hydrogen (H2) produced by hydrogen evolution reaction (HER) via electrochemical water splitting. As promising materials for electro-catalytic electrodes, layered 2 dimensional (2D) transition metal dichalcogenides (TMDCs) such as MoS2 and WS2 have drawn great attention as alternatives to precious metals such as platinum (Pt) and gold (Au).
On the other hand, it is highly desirable to control the morphology and phase of electrodes materials to optimize the catalytic activity. For example, edge planes of layered TMDCs are known to be catalytically more active than basal planes due to nearly thermoneutral properties in proton adsorption. Morphological controls of TMDCs were investigated to expose edge sites effectively by tuning nucleation and growth behaviors, for example, double gyroid MoS2 thin film using atomically controlled surface, vertically aligned TMDCs by rapid sulfurization and defects-mediated formation of in-plane edges of MoS2 nanosheets. Controlling crystallographic phase is also important since the atomic configuration of TMDCs affects the catalytic activity substantially. Greater catalytic performances of metallic 1T phase MoS2 and WS2 than semiconducting 2H phase ones due to improved intrinsic reactivity and charge transfer kinetics for HER process, and strain-induced preferential formation of 1T phase WS2, MoS2 and MoSe2 have been reported. Therefore, it would be very advantageous to develop an engineered nanostructure whose surface can be easily modified to facilitate the nucleation and strain-induced growth of edge-plane-exposed TMDCs with 1T phase preferentially.
Herein, hierarchical electrode consisting of edge-exposed 1T phase WS2 grown on the array of surface-modified WO3 nanohelixes (NHs) and its enhanced catalytic performance in hydrogen evolution reaction (HER) are presented. Oxygen-deficient WO3 NHs surface modified by a controlled annealing facilitates the formation of a rich edge-exposed WS2 during sulfurization. In addition, metallic 1T phase WS2 is preferentially grown on the WO3 NHs owing to the strain induced between WS2 and curved WO3 NHs. Such desirable structural properties, metallic 1T phase and a rich edge-exposed morphology, for a catalytic electrode directly resulted in an enhanced HER performance in water splitting.
11:45 AM - EN07.10.10
“All Electric” Ion Pumps
Gideon Segev1,2,Shane Ardo3,Rylan Kautz3,David Larson1,Joel Ager1,4,Francesca Maria Toma1
Lawrence Berkeley National Laboratory1,Tel Aviv University2,University of California, Irvine3,University of California, Berkeley4Show Abstract
Ion pumps are devices that use external power to introduce a net ionic flux. For example, in the photosynthetic process, light is used to pump protons up a concentration gradient which will be used in sequential steps to generate fuels. In living cell membranes, nano scale channels pump different types of ions against a concentration gradient consuming energy from the hydrolysis of ATP. Although widely used in nature, there are very little technologies that can unleash the vast potential of ion pumps. Unlike electrons in electronic devices where contacts serve as nearly ideal sources and sinks for charge carriers, ions are sourced and removed by chemical reactions. These reactions are in many cases energetically expensive and require tailoring specific catalysts to the specific ions to be pumped. In this contribution we report a first of its kind “all electric”, ion pump based on an electronic ratchet mechanism.
Electronic ratchets are devices that utilize modulation in a spatially varying electric field to drive steady state current. Similar to peristaltic pumps, where the pump mechanism is not in direct contact with the pumped fluid, electronic ratchets induce net current with no direct charge transport between the power source and the pumped charge carriers. Thus, electronic ratchets can be used to pump ions in steady state with no electrochemical reactions between the power source and the pumped ions resulting in an “all electric” ion pump.
Porous capacitor based ion pumps were fabricated by coating the two surfaces of nano-porous alumina wafers with gold. The electric field within the nano-pores is modulated by oscillating the capacitors voltage. Thus, when immersed in solution, ions within the pores experience a modulating electric field resulting in ratchet based ion pumping. The device pumping performance was studied for various input signals, geometries and solutions. The proposed ion pumps may be used as building blocks in a wide range of applications such as artificial photosynthesis, water desalination, chemical separations and many others.
EN07.11: Novel Water-Splitting Catalysts II
Friday PM, December 06, 2019
Hynes, Level 2, Room 203
1:30 PM - EN07.11.01
Implicit Solvation with Periodic Boundary Conditions—Implementation and Application to Reactions at Interfaces
Chimie Paristech-CNRS1,Institut Universitaire de France2Show Abstract
We present the implementation of an implicit solvation model in the periodic CRYSTAL code. The solvation energy is separated into two components: the electrostatic contribution arising from a self-consistent reaction field treatment obtained within a generalized finite-difference Poisson model, augmented by a nonelectrostatic contribution proportional to the solvent-accessible surface area of the solute. A discontinuous dielectric boundary is used, along with a solvent-excluded surface built from interlocking atom-centered spheres on which apparent surface point charges are mapped. The procedure is general and can be performed at both the Hartree–Fock and density functional theory levels, with pure or hybrid functionals, for systems periodic in 0, 1, and 2 directions, that is, for isolated molecules and extended polymers and surfaces .
As application of the implemented solvent model, we consider the reaction of CO and H2O to form CO2 and H2 on Pt(111), using periodic density functional theory (DFT) calculations. Since this reaction has already been investigated at the DFT level with different mechanisms proposed (see  for instance), it therefore offers a suitable playground to investigate the role of different effects on the proposed mechanisms. In particular, we compare GGA to hybrid DFT models, considering both dispersion and implicit solvation effects. We discuss the results obtained on the full reaction paths, in which transition states are fully characterized.
 F. Labat, B. Civalleri, R. Dovesi J. Chem. Theory Comput. 13, 5969 (2018)
 L. C. Grabow, A. A. Gokhale, S. T. Evans, J. A. Dumesic, M. Mavrikakis J. Phys. Chem. C, 112, 4608 (2008)
2:00 PM - EN07.11.02
Anchoring Ir Single Atom Sites on NiFe Oxyhydroxides for High-Efficient Water Oxidation Reaction
Xueli Zheng1,Yi Cui1
Stanford University1Show Abstract
The efficiency with which renewable fuels and feedstocks are synthesized from electrical sources is limited at present by the sluggish water oxidation reaction. Here, using density functional approach, we predict that the optimal energetics for water oxidation could be achieved by systematic increase of the oxidation of the Ir active sites within typical NiFe framework. We successfully anchored Ir single sites on NiFe oxyhydroxides (Ir0.1/Ni9Fe) via a unique cryogenic (cryo)-photochemical reduction method. Ir0.1/Ni9Fe catalyst exhibits the lowest overpotential (166 millivolts) at 10 milliamperes per square centimeter and retains its performance following 200 hours of operation in 1 M KOH electrolyte. In situ grazing incidence X-ray absorption spectroscopy measurements/simulations reveal that and Bader charge analysis indicate the charge transfer between Ir and Ni/Fe/O which reveals an average oxidation state of +5.3 on Ir sites under operating conditions. The favorable coordination environment of such single sites plays important roles for the enhanced water oxidation activity.
2:15 PM - EN07.11.03
Kinetics of Photogenerated Charge Carriers at Semiconductor-Electrolyte Interface in Photoelectrochemical Water-Splitting
Chang-Ming Jiang1,Jason Cooper2,Ian Sharp1
Technical University of Munich1,Lawrence Berkeley National Laboratory2Show Abstract
Understanding the identity of surface states, as well as their roles in catalyzing photoelectrochemical (PEC) reactions, is imperative for achieving efficient solar fuel production. Copper vanadate (CVO) is an emerging semiconductor photoanode material system that supports a variety of room temperature stable phases depending on the Cu:V stoichiometry. This feature of CVO provides a unique opportunity to study the role of composition, phase, and surface states on key PEC characteristics. In this work, thin films of four CVO phases: Cu5V2O10, Cu11V6O26, γ-Cu3V2O8, and β-Cu2V2O7, were prepared by reactive co-sputtering. A suite of complimentary experimental tools (including spectroscopic ellipsometry, transient absorption, and transient photocurent) were applied to determine trends in basic optical properties, photocarrier dynamics, and surface state-induced charge trapping. Analysis of these results reveals competing bulk and surface properties; increasing Cu content provides stronger light absorption but reduces oxygen evolution activity. The rate constants for water oxidation and surface recombination are quantified as a function of composition, applied electrochemical potential, and incident light intensity. Composition trends for interfacial charge transfer and recombination rate constants indicate that Cu cations near the surface serve as trap states for photogenerated holes. Surprisingly, it is found that the surface recombination rate remains nearly constant with illumination intensity over the range of 0.1 – 1.5 suns. At the same time, the heterogeneous charge transfer rate constant is enhanced by a factor of 2 to 6, depending on composition and applied potential, over the same intensity range. Based on these results, we propose modifications to the existing kinetic model and provide insights into the kinetic factors that underlie competitive recombination and charge transfer at transition metal oxide photoelectrode surfaces.
3:00 PM - EN07.11.04
InGaN-Si Tandem Photoelectrodes for Photoelectrochemical Water Splitting by MOCVD—Understanding System Design Considerations Necessary to Bridge Theory and Experiment
Andrew Wong1,2,Ye Sheng Yee1,Thomas Jaramillo1,2,James Harris1
Stanford University1,SLAC National Accelerator Laboratory2Show Abstract
For some time, it has been predicted that the development of tandem absorber photoelectrodes can allow for superior utilization of the solar spectrum for photoelectrochemical water splitting. Based on stoichiometry, InxGa1-xN has a variable band gap that can span from 3.4 to 0.7 eV, which makes it ideal for pairing with Si in tandem photoabsorber devices, in principle. Furthermore, MOCVD of InGaN directly onto a Si p-n junction can provide a path to reducing the cost of tandem absorbers towards commercialization. In this presentation, our work into the development of tandem InGaN-Si photoelectrodes via MOCVD will be discussed. Specifically, our successful growth of the designed InGaN-Si system and the performance of this system will be presented. Our efforts elucidate necessary considerations when pursuing design of photoelectrochemical systems that are relevant for discussions between experimentalists and theorists during system design. These inter-related considerations include:
- Controlling electronic trap and surface states within the absorber materials and defect tolerance
- Controlling crystallization, growth, and morphology of absorber materials
- Controlling carrier concentration and carrier type
- Reducing the density of structural defects in the absorber materials
- Managing lattice strain resulting from mismatches in lattice constant and thermal expansion coefficients
Overall, this work on tandem InGaN-Si photoelectrodes for photoelectrochemical water splitting can provide a useful case study that can inform the dialog for the coordination between experimentalists with theorists for the discovery of the next generation of materials for water splitting. This dialog continues to grow in importance as the interface between theory and experiment continues to become more robust.
3:15 PM - EN07.11.05
Novel Stable Three-Dimensional (3D) Stainless Steel-Based Electrodes for Efficient Water Splitting
Haojie Zhang1,Juliana Martins de Souza e Silva1,Xiaopeng Li2,A. Wouter Maijenburg1,Stefan Schweizer1,Ralf Wehrspohn1,3
Martin-Luther-Universität Halle-Wittenberg1,Shanghai Advanced Research Institute, Chinses Academic of Science (SARI-CAS)2,Fraunhofer-Institut für Mikrostruktur von Werkstoffen und Systemen IMWS3Show Abstract
Stainless steel (SS) is a promising material for the preparation of high-active electrode for efficient water splitting. However, the stability of coupled electrocatalysts on the surface of SS is a significant challenge for the construction of three-dimensional stable electrodes. In this work, we present an efficient and universal process to enhance the interfacial interaction between SS and highly-active electrocatalysts for the preparation of 3D electrodes through the formation of an interfacial network of carbon nanotubes (CNTs) on the surface of SS. The strongly-interconnected CNTs network increases the surface area of the SS support that benefits the modification of highly-active electrocatalysts and also serves as an electron/charge-conductive highway between the electrocatalysts and the support. The modified electrocatalysts on CNTs further improve the performance of the water splitting. The 3D structure of the prepared 3D electrode was visualized and analyzed by X-ray microscopy. Our strategy is an efficient approach to combine highly-active electrocatalysts with SS for the preparation of active and stable 3D electrodes, that can be used further explored in various areas.