Symposium EN07 : Materials Science for Efficient Water Splitting

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.

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.

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) [1]. 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 [2]. 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⁻²_geo at 1.485 V for 120 h without noticeable degradation, which outperforms known systems. This electrolyzer is also significantly less expensive than the commercial IrO_x 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.

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.

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 [1]. 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 [2], 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.

[1] F. Libisch, C. Huang, and E. A. Carter Accounts of Chemical Research, 47, 2768 (2014)
[2] R. Yin, Y. Zhang, F. Libisch, E. A. Carter, H. Guo, and B. Jiang J. Chem. Phys. Lett. 9, 3271 (2018)

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.

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.

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.

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.
References:
[1] ACS Energy Letters 3 (10), 2357-2365.
[2] Advanced materials 30 (38), 1803151.

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 [3] 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.
[1] O. Zandi and T.W. Hamann, Nat. Chem. 8 (2016) 778.
[2] Y. Zhang, H. Zhang, A. Liu, C. Chen, W. Song, J. Zhao, J. Am. Chem. Soc. 140 (2018) 3264.
[3] X. Zhang and A. Bieberle-Hütter, ChemSusChem 9 (2016) 1223.
[4] R. Sinha, R. Lavrijsen, M.A. Verheijen, E. Zoethout, H. Genuit, M.C.M. van de Sanden, A. Bieberle-Hütter, ACS Omega (2019) accepted.
[5] R. Sinha, I. Taneyli, R. Lavrijsen, M.C.M. van de Sanden, A. Bieberle-Hütter, Electrochim. Acta 258 (2017) 709.
[6] 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.
[7] Y. Zhao, G. Brocks, H. Genuit, R. Lavrijsen, M.A. Verheijen, A. Bieberle-Hütter, Adv. Energy Mater. (2019) submitted.
[8] K. George, M. van Berkel, X. Zhang, A. Bieberle-Hütter, J. Phys. Chem. C 123 (15) (2019) 9981.
[9] X.Q. Zhang, P. Klaver, R. van Santen, M.C.M. van de Sanden, A. Bieberle-Hütter, J. Phys. Chem. C 120 (2016) 18201.

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.

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.

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.

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, V_oc. 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.

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.

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.

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.

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 [1]. In this direction, we have developed highly flexible and light-weight heterostructures with an overall thickness of ~50 µm by using low-temperature techniques [2]. Surface passivated vertically aligned zinc oxide (ZnO, ~1 µm length) nanostructures with amorphous cobalt-oxide (Co_xO_y, < 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.

Acknowledgments:
We acknowledge the financial support of the National Research Foundation of Korea (NRF-2017R1D1A1B03035194, 2018R1A5A1025511, and 2017R1A2B4012119).

[1] Fan et al. Adv. Mater. 28 (2016) 4283.
[2] Koteeswara Reddy et al. ACS Appl. Mater. Interfaces 8 (2016) 3226.

Efficient dye-sensitized photocathodes offer new opportunities for converting sunlight into storable energy cheaply and sustainably.[1] 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.[2] 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.[3] 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.

REFERENCES
[1] E. A. Gibson, Chem. Soc. Rev., 2017, 46, 6194 – 6209,
[2] 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.
[3] 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.

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

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.

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 NiO_x to date, and the studies on relevant energetic are lacking due to the properties of the materials of NiO_x 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/SiO_x/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.

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.

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 Sr₂Fe_1.5Mo_0.5O₆ perovskite with promising bifunctional catalytic activity towards oxygen reduction and evolution reactions [1-3] (ii) Fe-doped ZrO₂for low temperature SOFCs [4] and (iii) CuFeO₂delafossite for CO₂photorreduction [5]. In all cases, we will discuss how surface oxygen vacancies -boosted by aliovalent doping- enable better performaces and change product selectivity.


REFERENCES
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[2] ABMG, M. Pavone, Chem. Mater 28, 490 (2016)
[3] ABMG, M. Pavone, J. Mater. Chem. A 5, 12735 (2017)
[4] 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
[5] 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)

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.

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.

[1] M. Bonchio et al. Nature Chemistry 11,146 (2019).

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 IrO_x 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 IrO_x 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).

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.

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.

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.

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.
[1] 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.
[2] 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.

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 A_1g and five E_g 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.

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.

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.

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.

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.

Hydrogen, with a global production rate of about 50 billion kg/year, is one of the world’s most important chemicals [1]. 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. [2] 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. [3] 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 [4].
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 mg_Pt cm^-2, anode: IrO₂/Nafion^® with loading of 0.2-0.3 mg_Ir 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.

References
[1] K.E. Ayers, J.N. Renner, N. Danilovic, J.X. Wang, Y. Zhang, R. Maric, H. Yu, Catalysis Today 262 (2016) 121-132.
[2] K.A. Lewinski, D. van der Vilet, S.M. Luopa, ECS Transactions 69 (2015) 893-917.
[3] C. Rozain, E. Mayousse, N. Guillet, P. Millet, Applied Catalysis B: Environmental 182 (2016) 123-131.
[4] C. Klose, P. Trinke, T. Bohm, B. Bensmann, S. Vierrath, R. Hanke-Rauschenbach, S. Thiele, Journal of Electrochemical Society 165(16) (2018) F1271-F1277.

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.

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.

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).

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.

[1] 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
[2] E. Forgacs et al. Removal of synthetic dyes from wastewaters: a review. Environment International 30 (2004) 953–971.
[3] E. Khosla et al., Ionic dye adsorption by zinc oxide nanoparticles. Chemistry and Ecology 31 (2015), 173–185.
[4] A. Manivel et al. Synthesis of MoO3 nanoparticles for azo dye degradation by catalytic ozonation. Materials Research Bulletin 1 (2014) 184-194.

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.

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.

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 ΔG_H 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.

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.

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.

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-L_2,3 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.

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 RuO_x or IrO_x,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 group^{6, 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.

References

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).

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/Marcus^1-2 in which a key quantity is the electron-transfer coupling matrix element, V_AB, 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.

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.

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 [1]. The solar based energy can be converted and stored in the form of fuels, for example, hydrogen (H₂) by water splitting [2]. 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.

References
[1] I. H. Gentle and K. Hellgardt, Sci.Rep. 8, 12807 (2018).
[2] S. Sharma, S. Singh and N. Khare, Int. J. Hydrogen Energy. 41, 21088 (2016).
[3] W. Wang, M. Xu, X. Xu, W. Zhou, and Z. Shao, Angew. Chem. Int. Ed. doi.org/10.1002/anie.201900292 (2019).
[4] D. Kumar, S. Singh and N. Khare, Int. J. Hydrogen Energy. 43, 8198 (2018).

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.

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 (V_S, V_Se, V_SSe, V_Mo, V_MoS3), and antisite defects (Mo_SSe, Se_Mo, S_Mo) 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.

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).

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.

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.

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.

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

References
[1] A. G. Tamirat, J. Rick, A. A. Dubale, W. N Su and B. J Hwang, Nanoscale Horiz., 2016, 1, 243–267.
[2] K. Takanabe, ACS Catal., 2017, 7 (11), 8006–8022.

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.

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.

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.

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 [1]. 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 [2].
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.

[1] D. Friebel et al., J. Am. Chem. Soc., 137 (2015) 1305.
[2] A. Li, et al., Angew. Chem. Int. Ed., 58 (2019) 1.
[3] N. Mironova-Ulmane et al., Cent.Eur. J. Phys. 9 (2011) 1096.
[4] Y. Li et al., J. Phys. Phys. Chem. C 120 (2016) 4511.

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.

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.

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[3]. 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 [4]. 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 [5]. 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/(CdSe_xS_1-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/(CdSe_xS_1-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 V_RHE) 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.

References
[1] Y. Tachibana, L. Vayssieres, and J. R. Durrant, Nat. Photonics, 2012, 6, 511–518.
[2] 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.
[3] H.-J. Ahn, M.-J. Kim, K. Kim, M.-J. Kwak and J.-H. Jang, Small, 2014, 10, 2325–2330.
[4] F. Navarro-Pardo, H. Zhao, Z.M. Wang and F. Rosei, Acc. Chem. Res., 2018, 51, 609–618.
[5] 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.

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.

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 [1], 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 [2]. 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 [3], 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.

[1] J. Liu et al., Science, 347, 970, 2015.
[2] M. Lichterman et al., Energy Environ. Sci., 8, 2409, 2015.
[3] X. Zhang and S. Ptasinska, Sci. Rep., 6, 24848, 2016.

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.

References
Nature Chemistry 2010, 2, 826-831
Nature Chemistry 2019, 11, 146-153

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.

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.

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 E_g of 1.5-3.2 eV with amorphous a-Si:H (E_g = 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.

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.

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.

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.

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 k_BT) 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.

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 La_0.85Sr_0.15CoO₃ 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.

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.

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.

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.

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.

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.

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 [1]. 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 Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF), a highly active OER catalyst in alkaline electrolytes [2], 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 [3]. 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.

References
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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.

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.

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.

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 M_x-P_y (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.

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.

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.

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.

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.

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 21^st 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 Sr_yIrO_x form which exhibits stronger disorder than conventional amorphous IrO_x, partially explaining its champion OER activity.

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 Pr_xBa_1-xCoO_3-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 BaCo_0.98Ti_0.02O_3-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.

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.

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^[1], 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)^[2]. 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^[3] (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^[4].
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.

[1] 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.
[2] Song F.; X. Hu, Nat. Comm, 2014 Volume 5, 447.
[3] Tahir M.; Pan L.; Idrees F.; Zhang X.; Wang L.; Zou J.; Lin Wang Z. Nano Energy, Volume 37, 2017, Pages 136-157.
[4] Eftekhari A. Materials Energy Today, Volume 5, 2017, Pages 37-57.

A novel TiN_xO_y (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.

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 [3]. 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.

[1] Dekel et al., Journal of Power Sources, 375, 158 (2018)
[2] Gottesfeld et al., Journal of Power Sources 375, 170 (2018)
[3] Davydova et al., ACS Catal. 8, 6665 (2018)
[4] Miller et al., Angew. Chem. 128, 6108 (2016)

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.

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 [1], and their influence on PEC performance.

[1] H. Seo, Y. Ping, G. Galli. Chem. Mater. 30, 7793 (2018).

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.

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.

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.

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, In_xGa_1-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.

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.