Symposium EN10—Transformation, Reaction and Organization at Functional Interfaces for Sustainable Energy Systems and Environmental Managements

This tutorial provides a colloquial introduction to the interface science in electrochemistry, and the central role played by interfaces and interphases in the advanced batteries. It will also cover the recent advances in the field of high energy battery chemistries and materials.

Interfaces between materials and water or aqueous fluids are central to the performance of almost all water-energy systems. In this tutorial, Dr. Darling will first introduce global-scale challenges related to the water-energy nexus and then reveal how the properties of water/solid interfaces are interconnected with these challenges. He will present various strategies that are used by researchers to manipulate interfacial properties, ranging from mussel-inspired dopamine chemistry to atomic layer deposition and related techniques. He will provide examples of applications implementing such approaches, including anti-fouling surfaces, catalytically active materials, membranes with unusual transport properties, selective sorbents, and photothermal systems for solar steam generation.

Revealing the chemical reactions that occur at catalytic and electrochemical interfaces under realistic conditions is critical to selecting and designing improved materials for chemical synthesis, energy storage, and corrosion prevention. Obtaining chemical information with nm-scale interface sensitivity is however a significant challenge given these interfaces are typically buried between a bulk support and dense reaction environment. ^1,2

Here we introduce and apply several complementary methods based on soft X-ray spectroscopies and neutron reflectometry for studying electrochemical and catalyst interfaces under realistic liquid- and gas-phase environments. This includes membrane-based approaches which we have been involved in developing over recent years that enable operando x-ray photoelectron and absorption spectroscopy (XPS/XAS) of solid-liquid and solid-gas interfaces at atmospheric pressures and above.^2-5 These rely on reaction cells sealed with X-ray/electron-transparent membranes such as thin (<100 nm) silicon nitride or graphene membranes that remain impermeable to gases and liquids.

We demonstrate how these approaches can track the chemical evolution of solid-gas interfaces such as copper surfaces during methanol oxidation,⁶ and supported nanoparticle catalysts in atmospheric pressure environments.⁴ We also show how total electron yield (TEY) mode XAS can monitor the evolution of solid-liquid interfaces under electrochemical control, including the oxidation/reduction of Ni electrodes in aqueous media, and solid-electrolyte interphase (SEI) formation on Li-ion battery anodes.⁵ This is complemented by neutron reflectometry which reveals the precise thickness of the layers formed on electrode surfaces with sub-nm depth resolution, even for light elements such as Li, C, O. We are thus able to follow the chemical and structural evolution of the solid-electrolyte interphase during its formation under realistic electrochemical conditions. We further introduce some promising new methods for accessing the chemistry of electrode-electrolyte interfaces in solid-state batteries,⁷ using Hard X-ray Photoelectron Spectroscopy.

We expect these methods to be valuable in understanding a wide range of interfacial reactions across the electrochemical and catalytic sciences, and will summarize outstanding challenges in this area that are still to be overcome.

References
1. Wu et al. Phys. Chem. Chem. Phys. 2015, 17, 30229.
2. Weatherup et al. Top. Catal. 2018, 61, 2085.
3. Velasco-Velez et al. Angew. Chemie 2015, 54, 14554.
4. Weatherup et al. J. Phys. Chem. Lett. 2016, 7, 1622.
5. Weatherup et al. J. Phys. Chem. B 2018, 122, 737.
6. Eren et al. Phys. Chem. Chem. Phys. 2020, 22, 18806
7. Brugge et al. J. Mater. Chem. A 2020, 8, 14265

Polymer electrolyte fuel cell performance is limited mainly by the cathode, where poor oxygen reduction reaction (ORR) kinetics and slow transport of O₂ to active sites lead to large overpotentials. State-of-the-art high surface area Pt alloy catalysts also suffer from poor durability due to loss of surface area (particle growth) and leaching of base metals. We have recently demonstrated that the use of intermetallic nanoparticle catalysts such as L1₀-PtCo can enable significant improvements in long-term performance due to improved stabilization of base metal in the ordered lattice. We have also demonstrated a strong role of the catalyst support in determining catalyst performance and durability, providing additional opportunities to control and tune catalyst properties through control of support morphology, porosity, and graphitization. New catalysts based on L1₀-CoPt combined with advanced support structures have shown excellent durability in fuel cell membrane electrode assemblies (MEAs), meeting DOE targets for accelerated stress tests (30K voltage cycles in MEA) with minimal performance degradation. Recent results and future directions for this catalyst research will be discussed.

Photothermal catalysis represents a promising strategy to utilize the renewable energy source (e.g., solar energy) to drive chemical reactions more efficiently. Successful and efficient photothermal catalysis relies on the availability of ideal photothermal catalysts, which can provide both large areas of catalytically active surface and strong light absorption power simultaneously. Such duplex requirements of a photothermal catalyst exhibit opposing dependence on the size of the catalyst nanoparticles, i.e., smaller size is benefcial for achieving higher surface area and more active surface, whereas larger size favors the light absorption in the nanoparticles. In this article, we report the synthesis of ultrafine RuOOH nanoparticles with a size of 2–3 nm uniformly dispersed on the surfaces of silica (SiOx) nanospheres of hundreds of nanometers in size to tackle this challenge of forming an ideal photothermal catalyst. The ultrasmall RuOOH nanoparticles exhibit a large surface area as well as the ability to activate adsorbed molecular oxygen. The SiOx nanospheres exhibit strong surface light scattering resonances to enhance the light absorption power of the small RuOOH nanoparticles anchored on the SiOx surface. Therefore, the RuOOH/SiOx composite particles represent a new class of efficient photothermal catalysts with a photothermal energy conversion efficiency of 92.5% for selective aerobic oxidation of benzyl alcohol to benzylaldehyde under ambient conditions.

Earth-abundant Fe-Ni-Co oxide are recognized electrocatalysts for the oxygen evolution reaction (OER) in water splitting. In the present work, Fe-Ni-Co alloy thin films are electrodeposited onto copper substrates and surface oxides generated by voltammetry in 1 M KOH. The electrocatalysts are aged until changes in the OER are observed at different applied current densities. Regenerated catalysts occur via a cathodic post-treatment. Stability of the electrocatalysts are examined with voltammetry, chronopotentiometry and XANES analyses. Fresh, unaged samples show predominately Fe(III), Ni(II) and metallic Co states present in the oxide. Changes of the electron density distribution were observed upon aging of the electrocatalysts that could be reversed through the regeneration step. The Fe-Ni-Co thin films were fabricated by electrodeposition from a boric acid-containing electrolyte onto rotating cylinder electrodes. The OER activity of the electrodeposits in 1 M KOH was a function of deposit composition, as expected although alloys with a small amount of Co maintained a constant Tafel slope over a larger range than a comparable Fe-Ni electrocatalyst. Chronopotentiometric aging during an anodic current density of 10 mA/cm² for 30 hours, was followed by a cathodic post treatment current density, that fully recovered the OER activity, and the limits of the cathodic post treatment with more extreme aging conditions will be discussed.

The interconversion between chemical energy and electricity relies critically on electrode-electrolyte interfaces, which consists of the electrode surface regions and the solvation layers, also called the electric double layers (EDLs). While the electrode structures have been extensively studied using in-situ characterization techniques, to date the structure of EDLs remains largely unexplored. This is not surprising, considering the weakly ordered, partially mobile, and vacuum-incompatible nature of the liquid molecules in the EDL. To address this challenge, we have developed a novel technique, electrochemical three-dimensional atomic force microscopy (EC-3D-AFM), to directly image both the electrode surface and EDLs under operando conditions. Utilizing ultrasensitive force detection techniques, we are able to achieve atomic and molecular resolution. I will discuss our recent findings on the potential-dependent EDLs of graphitic and other van der Waals electrodes using the EC-3D-AFM method. In addition, I will also discuss our efforts on operando spectroscopy of electrode-electrolyte interfaces, as well as joint experiment-theory approaches to understand the hidden mechanisms of interfacial electrochemical energy conversion processes.

Separators in Lithium Ion Batteries (LIBs) are electronically isolating membranes that prevent physical contact between the two electrodes while allowing ionic transport. Therefore, they are considered to be a crucial part in battery safety.¹ In commercially available LIBs, separators are predominately made from porous polyolefin films such as polypropylene (PP) or polyethylene (PE). They can be coated with thin layers of ceramics (e.g., Al₂O₃, MgO₂, …), which improves their stability against shrinking under thermal stress and their resistance to electrochemical oxidation by high voltage positive electrodes.³
Traditionally, the effective transport coefficient (ratio between porosity and tortuosity) is used for the characterization of LIB separator performance. However, to fully capture all aspects of battery performance, additional structural parameters like pore space connectivity are necessary.^{4 5} Here, we show how the 3D structure of the separator pore space directly influences separator wetting and thereby battery performance.
Our research group developed an approach to visualize and quantify LIB separators with focused ion beam scanning electron microscopy (FIB-SEM).² Using FIB-SEM we acquired the 3D tomographic data of commercial coated and uncoated PE separators. Comparing the effective transport coefficients, pore connectivity and mechanical stability of the two different separators, we quantify the influence of the ceramic coating on structure and performance.
The 3D data sets also allow us to simulate the wetting of the separator membranes with liquid electrolyte. We see that during this so-called imbibition-process up to 30% gas is entrapped in the separator. Using partial wetting theory, we show that the specific 3D structure of the separator is responsible for this incomplete wetting. Comparing the wetting simulations to separator performance measurements, we demonstrate, that incomplete wetting can explain the discrepancy between theoretically predicted and experimentally measured transport coefficients. We also show that quasi-static wetting models overestimate the amount of residual gas in the membranes and that realistic wetting models have to consider both, the physico-chemical properties of the liquid electrolyte and the 3D structure of the separator pore space.⁶

1. Zhang, S. S. A review on the separators of liquid electrolyte Li-ion batteries. J. Power Sources 164, 351–364 (2007).
2. Lagadec, M. F., Ebner, M., Zahn, R. & Wood, V. Communication—Technique for Visualization and Quantification of Lithium-Ion Battery Separator Microstructure. J. Electrochem. Soc. 163, 992–994 (2016).
3. Lee, H., Yanilmaz, M., Toprakci, O., Fu, K. & Zhang, X. A review of recent developments in membrane separators for rechargeable lithium-ion batteries. Energy Environ. Sci. 7, 3857–3886 (2014).
4. Lagadec, M. F., Zahn, R., Müller, S. & Wood, V. Topological and network analysis of lithium ion battery components: The importance of pore space connectivity for cell operation. Energy Environ. Sci. 11, 3194–3200 (2018).
5. Lagadec, M. F., Zahn, R. & Wood, V. Characterization and performance evaluation of lithium-ion battery separators. Nat. Energy 4, 16–25 (2018).
6. Sauter, C., Zahn, R. & Wood, V. Understanding Electrolyte Infilling of Lithium Ion Batteries. J. Electrochem. Soc. 167, (2020).

The generation of electrical voltage through the flow of an electrolyte over a charged surface may be used for energy transduction. Here, it is shown that enhanced electrical potential differences/streaming potential (V_s) may be obtained through the flow of salt water on LFS (liquid filled surfaces) infiltrated with lower dielectric constant liquid, such as oil, harnessing electrolyte slip and associated surface charge. A record large figure of merit, in terms of the voltage generated per unit applied pressure, of 0.043 mV/Pa, was obtained through the use of the LFS, greater by a factor of 1.4, when compared to air filled surfaces (AFS). These results lay the basis for innovative surface charge engineering methodology for the study of electrokinetic phenomena at the microscale with applications to new electrical power sources.

We will also discuss a methodology for substantially indicating the magnitude of the electrokinetic streaming potential (V_s) from ~ 0.02 V to ~ 1.6 V, based on experimental results. The underlying idea is to use textured LFS, filled with low viscosity oils, for electrolyte flow. The charge density at the electrolyte-oil interface as well as the enhanced slip is thought to be responsible for the enhancement of the V_s. The specific influence of the filling oil viscosity on the V_s was probed with respect to the viscosity. It was found, through experimental analysis as well as computational simulations, that the fluid slip length was inversely proportional to the filling oil viscosity, and influences the V_s. The obtained results provide new perspectives related to complex electrolyte flow conditions as may be relevant to chemical and biological separations.

Novel low cost/electrically conductive/corrosion resistant nitrogen-doped ultrananocrystalline diamond (N-UNCD) coating provides excellent chemically robust encapsulation of commercial natural graphite (NG)/copper (Cu) anodes and new textured Si-based anodes, currently under development for Li-ion batteries (LIB), providing a solution to the problem of LIBs’ anode materials degradation due to chemical corrosion induced by Li ions. The N-UNCD encapsulating coating allows for good conductivity of both electrons and Li-ions while exhibiting outstanding chemical and electrochemical inertness to Li ions-induced corrosion, eliminating de Li-induced chemical corrosion of graphite powder in current commercial graphite/copper composite anodes, which result in degradation of the capacity energy after undesirable relatively short numbers of charge/discharge cycles of LIBs. In addition, the electrically conductive N-UNCD coating produce a substantial increase in the mechanical strength of the anode’s graphite powder and will also produce the same results for structured Si anodes, resulting in the formation of robust Solid-Electrolyte Interface (SEI) films, which effectively eliminate unceasingly cracking of SEI, observed in graphite/copper anodes, which expose fresh graphite anode surface, thus, repairing of the SEI by electrochemical reduction of the electrolyte and the formation of additional SEI compounds, which increase the impedance for the lithium ion diffusion and causes the electrolyte to be prematurely depleted in anodes without the N-UNCD coating. In addition to the excellent protection to chemical attack of anodes, new preliminary R&D will be presented indicating that electrically conductive N-UNCD coatings can also be used to coat LIB’s oxide cathodes to protect them from Li-induced corrosion, as well, and that insulating/corrosion resistant UNCD coating can be used to coat the inner walls of novel metallic aluminum LIB’s cases and membranes to protect them from corrosion induced by the Li-based battery environment, and thus enabling to use aluminum as a LIB’s case, thus to replace current more expensive partially corrosion resistant metals. The pathway to commercialization of the new UNCD coating technology for LIBs and other LI-based batteries, like thermal Li-sulfur batteries will be discussed.

Synchrotron-based characterization methods can provide valuable information on the nature of catalyst/adsorbate interactions and a number of soft X-ray based techniques will be reviewed in this presentation. We will focus our discussion on systems involving catalyst clusters supported on single-crystalline oxides, particularly TiO₂, ZnO, and Fe₂O₃. The examples that we will present include studies related to electrochemistry as well as environmental chemistry.

With synchrotron ultra-violet light one can provide information on occupied valence electronic structure and charge-transfer mechanisms occurring during reaction and adsorbate bonding using ultra-violet photoemission (UPS) and by tuning the photon energy it’s possible to use excitation resonances to enhance sensitivity to defect states. Synchrotron-based X-ray photoelectron spectroscopy (XPS) is another valuable tool for probing surface reactions and cross-sections for core-level excitations can also be tuned with photon energy. Since XPS involves photoexcitation from a core level to a plane-wave final excited state far above E_F, the measured binding energies reflect potential shifts in the occupied core levels reflecting the chemistry of the atomic bonding. Another valuable measurement is to probe the empty electronic states just above the Fermi level by monitoring the X-ray absorption near edge structure (XANES). This can provide further information on the changes induced by chemical bonding and the hybridization of empty electronic states and we will show that it provides valuable information that is complementary to the UPS/XPS probes of the occupied states. In the case of the L edges (2p states), the dipole-allowed transition to empty 3d states are very sensitive to details of the changes in the empty-state electronic structure, crystal-field splitting, and other effects induced in the interfaces on chemical adsorption and reaction.

It is helpful to combine this data with models to better understand the local atomic and electronic states and density-functional theory (DFT) calculations allow one to model surface structures and compute electronic band structures, including empty states. A Bader analysis can provide information on charge-transfers that are involved, and methods have been developed to compute the XANES spectra directly, including spectra considering the presence of the (2p) core hole. We will contrast data with predictions to show how progress is being made in understanding the underlying chemistry involved. We will conclude with a summary of how synchrotron studies, combined with DFT calculations, will provide valuable information on the atomic and electronic origins of interface structures.

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, I 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. In particular, I will introduce the high energy surface X-ray diffraction technique which facilitates fast capturing of structural evolution of less stable surface under electrocatalytic conditions. Following the survey, I would like 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 high-efficiency and low-cost catalysts for oxygen evolution reaction (OER) are critical for electrochemical water splitting to generate hydrogen as the clean fuel for sustainable energy conversion and storage. The development of those catalysts relies on discovering their active sites and understanding of their catalytical states so that rational design strategies can be applied. Transition metal sulfides are an emerging type of OER catalyst that exhibits superior activity even better than commercial standards such as RuO2. However, they undergo structural and compositional change during the reaction, which adds difficulties in studying catalytic active sites. Here, we use cobalt sulfide, Co9S8, as a representative example. Utilizing multimodal operando characterizations including Raman spectroscopy, X-ray absorption spectroscopy, and X-ray reflectivity, we find that Co9S8 ultimately converts to oxide cluster (CoOx) containing six oxygen coordinated Co as the basic unit which is the true catalytic center to promote high OER activity. The density functional theory (DFT) calculations verify the in-situ generated CoOx consisting of di-µ-oxo Co-Co motifs in CoO6 octahedral clusters as the actual active sites. Our results also provide insights to design other transition metal X-ides (X: C, P, N, S, etc.) as efficient electrocatalysts that experience a similar restructuring in OER.

A central theme in renewable energy technologies today is to design nanostructured catalysts at the solid/gas and solid/liquid interfaces towards desired reactions. A recent advance in materials for electrochemical energy and fuels conversion is to synthesize oxide supported metal nanoparticles in a process termed “exsolution”. Exsolution generates stable and catalytically active metal nanoparticles via phase precipitation out of a host oxide. Unlike traditional nanoparticle infiltration techniques, the nanoparticle catalysts from exsolution are anchored in the parent oxide. This anchored structure makes the exsolved nanoparticles more resistant against particle agglomeration as compared to the infiltrated ones. As a promising technique, exsolution has now been successfully implemented to produce stable supported nanoparticle catalysts in a wide range of applications, including H₂O and CO₂ splitting, three-way catalysts, solid oxide fuel cells, and ceramic membrane reactors.

The current exsolution studies typically report metal nanoparticles of 10s of nm size and separation of 10s-100s of nm. While the obtained improvement in reaction kinetics is already promising, there is plenty of room at the bottom to increase the density and dispersion of exsolved nanoparticles to achieve higher catalytic activities. In this work, we propose that point defect formation in the oxide lattice to be the fundamental knob to control exsolution and demonstrate this approach in epitaxial La_0.6Sr_0.4FeO₃ (LSF) thin films. By quantifying surface defect states with ambient pressure X-ray spectroscopy (APXPS), we identify the initial oxygen vacancy formation and the following Schottky defect formation as the primary defect reactions in exsolution. Then, by tuning the formation energy of these two defects in LSF with biaxial lattice strain, we examined their impacts on metallic iron (Fe⁰) exsolution. Lattice strain tunes the formation energy, and thus, the abundance of these defects, and alters the amount and size of the resulting exsolution particles. As a result, the LSF surface that has more of these point defects, also has the highest Fe⁰ concentration, the largest particle density, as well as the finest particle size. Finally, with the aid of density functional theory (DFT), and Monte Carlo (MC) calculations, we suggest surface oxygen vacancy clusters to be the nucleation sites for the exsolved particles. These observations highlight the critical role of surface point defects in controlling the size and density of the exsolved nanoparticles on the perovskite surfaces.

Iron-based materials, prevalent in steel infrastructures as well as soils and minerals, participate in chemical reactions in the water cycle. Iron surfaces undergo corrosion, an electrochemical redox process, with oxidation of the iron anode and the cathodic reaction driven by O₂ and water. The surface corrosion occurs in ambient conditions and in complex environments, where ions are known to catalyze the rate of oxidation. Metallic iron readily transforms in the presence of water and O₂ to form a complex mixture of iron oxides, hydroxides and oxyhydroxides, known as rust. Therefore, to understand the surface processes and changes in oxidation states, decreasing the reaction rate is required to measure factors on the surface oxidation mechanism.
Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to measure surface oxidation of iron. An iron substrate with and without NaCl, was exposed to varying partial pressures of O₂ and H₂O vapor. The O1s, C1s, Fe2p, Na2s, and Cl2p regions were analyzed in comparison to the pristine iron surface. The evolution of oxide species in the O1s and C1s regions were observed with increasing O₂ pressure, which we interpreted as an irreversible formation of carbonates and bicarbonates at the surface. The results were corroborated by the lab-based ex situ XPS following the air exposure. Combination of the experiments helped to understand the fundamental steps of oxidation and the role of O₂ for changing surface composition in the corrosion reaction. The ions from NaCl play a significant role in surface oxidation, during the initial stages of surface corrosion, impacting surface chemistry in energy processes, mineral cycling and material degradation.

Iron oxide surfaces play a significant role in catalytic reactions such as water splitting, the Haber process, and in mineral cycling in naturally-occurring environmental processes. These processes rely on the change of the oxidation state of the iron oxide surface by oxidation-reduction reactions. Many of these transformations occur in the high-pressure regime, which makes these reactions difficult to investigate at the atomic level due to extraneous factors affecting the surface at the equilibrium/reactive conditions. However, it is still important to identify the phase transformation of iron oxide materials in situ during these processes.
In this study, iron oxide surface transformations are investigated using ambient pressure - X-ray photoelectron spectroscopy (AP-XPS). The Fe₃O₄(001) surface was exposed to reducing and oxidizing conditions by dosing hydrogen and oxygen gases, respectively at various temperatures and pressures. AP-XPS was used to measure changes in the surface electronic structure and oxidation states of the Fe₃O₄(001) surface near ambient pressure conditions in the Fe 2p, O 1s, and valence band regions. The shape of the valence band spectra was used to track changes in the surface electronic structure during the reactions. These studies suggest that Fe₃O₄(001) reduces when annealed in ultra-high vacuum and exposed to H₂ and upon oxidation, Fe₂O₃ is produced. These experiments are compared with α-Fe₂O₃(0001) that reduces to Fe₃O₄ when annealed in ultra-high vacuum. This study gives an important insight to the in situ surface redox chemistry and phase transformations of iron oxide surfaces relevant for catalytic and environmental processes.

Electrochemical reduction of CO₂ provides a promising route for sustainable production of fuels and chemicals such as multi-carbon hydrocarbons and oxygenates. While most efforts are focused on developing catalytic materials for CO₂ electroreduction, it is also critical to understand other factors beyond catalytic materials such as the local environment of the catalysts, which can mediate the transport and local concentration of reaction species and influence reaction pathways. Here we present our recent study of catalyst microenvironment for CO₂ electroreduction in flow cells with gas-diffusion electrodes (GDEs). For proof-of-concept, we use commercial Cu nanoparticles and homogeneously disperse polytetrafluoroethylene (PTFE) nanoparticles in the catalyst layer, which created a hydrophobic microenvironment of the Cu catalysts. The Cu/C/PTFE electrode showed a significant improvement in the activity, Faradaic efficiency, and C₂₊ product selectivity for CO₂ reduction compared to a regular Cu/C electrode without added PTFE. Moreover, the CO₂ reduction activity on the Cu/C/PTFE electrode increased with the CO₂ gas flow rate, suggesting a gas-phase transport of CO₂ inside the catalyst layer. The improved performance is attributed to the reduced diffusion layer thickness that accelerates CO₂ mass transport, increases the local concentration of CO₂ near the catalyst surface, and enhances CO₂ adsorption for the reaction. Compared to regular GDEs, the electrode with added PTFE creates a balanced gas/liquid microenvironment and solid-liquid-gas interfaces inside the catalyst layer, which can enhance the mass transport and kinetics of CO₂ electrolysis, providing a general approach to improve gas-involving electrocatalysis.

Fuel cells and electrolysis based on a polymer electrolyte membrane (PEM) are key technologies for a hydrogen society to produce green hydrogen, store large amounts of energy or electrify heavy-duty vehicles. However, durability and efficiency of the membrane electrode assembly is still a major focus of research. This talk gives two examples on investigating ageing phenomena in a fuel cell electrode. The first examines carbon corrosion in the electrode via Focused-Ion-Beam (FIB) tomography, showing that the major mechanism of this degradation phenomenon is loss of active catalyst ^[1]. In the second investigation, the polymer degradation in the membrane is studied via Raman, showing that the polymer degradation is favored on the anode side ^[2].
The second part of the talk deals with reducing the iridium catalyst loading in the anode of PEM water electrolysis. In the effort of reducing the catalyst, a major problem arises in the in-plane conductivity of the electrode, which leads to poor efficiencies ^[3]. Two alternatives are presented, which counter this problem by increasing the electrical conductivity. Employing a nanofiber interlayer enabled an 80% reduction of catalyst material without compromising efficiency or durability^[4]. In the second more simplistic approach, blending the polymeric binder in the electrode with PEDOT:PSS, an electronically conductive polymer, yielded a similar effect ^[5].

[1] Hegge, F., Sharman, J., Moroni, R., Thiele, S., Zengerle, R., Breitwieser, M., & Vierrath, S. (2019). Impact of Carbon Support Corrosion on Performance Losses in Polymer Electrolyte Membrane Fuel Cells. Journal of The Electrochemical Society, 166(13), F956.
[2] Böhm, T., Moroni, R., Breitwieser, M., Thiele, S., & Vierrath, S. (2019). Spatially resolved quantification of ionomer degradation in fuel cells by confocal Raman microscopy. Journal of The Electrochemical Society, 166(7), F3044.
[3] Bernt, M., Siebel, A., & Gasteiger, H. A. (2018). Analysis of voltage losses in PEM water electrolyzers with low platinum group metal loadings. Journal of the Electrochemical Society, 165(5), F305.
[4] Hegge, F., Lombeck, F., Cruz Ortiz, E., Bohn, L., von Holst, M., Kroschel, M., ... & Vierrath, S. (2020). Efficient and Stable Low Iridium Loaded Anodes for PEM Water Electrolysis Made Possible by Nanofiber Interlayers. ACS Applied Energy Materials.
[5] Ortiz, E. C., Hegge, F., Breitwieser, M., & Vierrath, S. (2020). Improving the performance of proton exchange membrane water electrolyzers with low Ir-loaded anodes by adding PEDOT: PSS as electrically conductive binder. RSC Advances, 10(62), 37923-37927.

Semiconducting photoelectrodes may possess numerous types of structural defects, with each exerting influence over material properties and photoelectrochemical performance. We previously demonstrated that Raman spectroscopy can identify distortions within the crystal lattice of hematite electrodes that were synthesized by the annealing of lepidocrocite. The distortion inhibited photoelectrocatalysis and was tentatively attributed to protohematite, a form of hematite where hydroxyl ligands are trapped within the crystal lattice. Here, the mechanism of hematite formation while annealing lepidocrocite is probed through in-situ variable temperature Raman spectroscopy and conventional thermal analysis. Results show a branching pathway, where protohematite forms between 200 and 400 °C in a reaction environment-dependent fashion. Once formed, the protohematite is remarkably persistent and harmful to photoelectrocatalysis. These results demonstrate a powerful method to study solid state phase transitions and provide insight into the formation of a harmful defect in hematite photoanodes that was previously not considered.

Traditionally, catalysts are limited by adsorbate-surface interactions that lead to a “volcano” behavior of activity as a function of binding energy, qualitatively described by the Sabatier principle. Thus one of the grand challenges in modern catalysis is the rational design of systems that can surpass this optimum. Ferroelectric perovskites are an interesting class of nanomaterials since they can present two distinct chemical surfaces depending on the polarization direction. This dual presentation potentially frees catalyst design from the constraints of a simple Sabatier framework, where on any single catalytic surface the adsorbates involved in the reaction must bind neither too weakly nor too strongly. Here we demonstrate a polarization-dependent electrochemical performance of ferroelectric BaTiO₃ (BTO) thin films for the hydrogen evolution reaction (HER).

A combination of detailed surface sensitive, core-level spectroscopy and first-principles computation are used to gain insight into the catalytic effects of a change in the polarization state of the BTO. The model BTO catalyst is grown by molecular beam epitaxy and exhibits an excellent ferroelectric response in Piezo-force microscopy (PFM) and polarization-filed (P-E) hysteresis measurements. We use electrochemical poling to control the polarization direction of the single-crystal thin film, enabling sensitive measurements of the catalytic surface in both up and down polarization states. Angle-resolved X-ray photoelectron spectroscopy and Ultra-violet Photoelectron spectroscopy reveal a lower work function and high valence band density of states at the surface of BTO when poled up.
These measurements are complemented by density functional theory (DFT) calculations, employed to gain insights into the localized density of states in BTO slabs, as well as the effect of polarization switching on the HER reaction barriers. Our calculations demonstrate that upward polarized samples have a higher density of electrons near the solid-electrolyte interface and predict a lower barrier for proton adsorption as the rate-limiting step for HER reaction. Experimentally, we observe a higher exchange current density on the upward polarized BTO, consistent with the first-principles result.

By leveraging model single-crystal thin film catalysts in experiments with well-defined polarization, we gain insights into the effects of ferroelectric polarization at the catalytic surface, paving the way for further development of ferroelectric-enhanced electrocatalysis.

Microreactor-Assisted Nanomaterial Deposition (MAND) process combines the merits of microreaction technology and chemical synthesis of nanomaterials. This technique uses continuous flow microreactors for the synthesis, assembly, and deposition of nanomaterials. In synthesis, microreactor technology offers large surface-area-to-volume ratios within microchannel structures to accelerate heat and mass transport. This accelerated transport allows for rapid changes in reaction temperatures and concentrations, leading to more uniform heating and mixing in the deposition process. The possibility of synthesizing nanomaterials in the required volumes at the point-of-application eliminates the need to store and transport potentially hazardous materials while providing new opportunities for tailoring novel nanostructures. MAND processes control the heat transfer, mass transfer, and reaction kinetics using well-defined microstructures of the active unit reactor cell that can be replicated to produce higher chemical production volumes. This critical feature opens a promising avenue in developing scalable nanomanufacturing. Furthermore, the continuous flow microreactor opens up the opportunity to conveniently assemble unique nanostructures and nanostructured thin films. There are many innovative solution-based routes towards syntheses of nanomaterials. The majority of these syntheses, however, were carried out using small-batch reactors. Results-to-date demonstrates the possibility to control the reacting flux, including small intermediate-reaction molecules, nanoclusters, nanoparticles, and structured assembly of nanomaterials.
I will discuss recent progress in using continuous microreactors to control the chemical transformation, reaction, and organization of functional nanomaterials and the applications of these functional nanomaterials for sustainable energy systems (e.g., solar photovoltaics). There is a significant opportunity to improve all solar PVs to maximize annual energy yield and increase service lifetime by engineering the interface. First, a portion of the light is reflected when light encounters an abrupt change in the refractive index. Thus, it is essential to reduce reflection for improving efficiency. Besides, the accumulated mass (i.e., soils, dust, pollutants, and air particles) on a cover can significantly reduce the power output of a solar PV module ranges from 1% to as much as 70% in some areas without cleaning. One way to mitigate the soiling effect is to regularly clean the solar arrays, which increase the LCOE and requires water. We have investigated the use of MAND to control the interfaces on installed solar modules in the field to mitigate these issues.

Nowadays wearable electronics are playing a more and more important part in daily life. Therein, paper-based electronics stand out by virtue of various advantages like low-cost, lightweight, and high flexibility. Nevertheless, it is still challenging to achieve long-term use of paper-based electronics due to their poor electromechanical durability and washability. This work adopts a novel high-performance polyimide paper as substrate and compact parylene encapsulation as a protective and reinforced layer, simultaneously enhancing electromechanical durability and machine-washability for the first time. The result shows parylene-encapsulated PI paper-based electronics show greatly electromechanical performance, abrasion resistance, bending durability, and outstanding machine washability. Moreover, such paper-based electronics demonstrates outstanding stability over different harsh environments. With 1D circuit board assemblies design and fabrication, the resultant electronic yarns are easily integrated into fabrics for wide wearable applications. Such water-resistant paper-based electronics show great feasibility for wearable applications other than dry disposable electronics.

Since the development of aerogels in the 1930s, they have attracted much attention due to their unique porous structure and lightweight properties. Such structures are useful for many applications including thermal and sound insulation, energy storage, waste-water treatment, and flame retardant materials. To date, silica-based aerogels have dominated the industry and research interests, although, while limited, some polymeric-based materials exists. More recently, polymer-based aerogels have drawn much attention, but to date have primarily been processed via freeze drying methods. While this method is more tunable for polymer-based aerogels, the resultant morphology and properties are inferior to aerogels processed using super critical drying (SCD). In this work, SCD process optimization and process-structure-property studies on polymer nano-composite based aerogels will be presented. The mechanisms of solvent exchange and diffusion during gel and aerogel processing will also be investigated and discussed. Boron nitride nanotubes (BNNTs) have been chosen as a filler candidate for these composite aerogels due to their lightweight, oxidative, structural, and radiation shielding properties. Current work has shown that the composite precursor gels exhibit enhanced mechanical and thermal performance. To this end, a fundamental study regarding these materials will be presented and include multi-scale characterization results and analyses.

Dissolved oxygen (DO) and hydrogen peroxide are important indicators of water quality. Hydrogen peroxide is also widely used in water and pollutant treatment as an excellent oxidizer. Thus, an aqueous oxygen and hydrogen sensor with high sensitivity, rapid response and good selectivity is in high demand. However, previously reported sensors, which are mainly based on enzymes, noble metals or metal oxides, have drawbacks of either high cost or weak response. Here, we introduce a novel electrochemical sensor based on van der Waals materials. Our sensors exhibit ultrahigh sensitivity and low detection limit for oxygen and hydrogen peroxide detection, outperforming typical noble metal-based electrochemical sensors. Through electrochemical kinetic analysis and simulations, we quantify the reaction kinetics and find that the high performance can be attributed to the facile charge transfer at the electrode-aqueous solution interface.

One path towards mitigating the global warming consequences of the rising global cooling demand is the search for alternative refrigeration technologies. We have identified one such technology that has electrochemical origins and remains to be experimentally demonstrated. The entropy of carefully chosen cathodic and anodic reactions may be utilized to induce refrigeration by employing them in either Brayton or Stirling type cycles.
In this work, the concept of the Brayton Electrochemical Refrigeration (BECR) cycle will be first introduced. This will include an overview of the working of the system, the governing material properties and operating parameters, and predictions of its performance. Then, the development and results of the experimental proof-of-concept of the BECR cycle will be discussed. Finally, the material level challenges to a commercial product will be discussed. Arguments will be made as to why the commonly leveraged solvation entropy is not a promising pathway to commercialize the BECR cycle and thermally regenerative electrochemical systems in general. A less explored alternative mechanism of entropy change is proposed.

Selective thermal emitters have attracted researchers due to their utility in applications such as gas sensing or thermophotovoltaic (TPV) energy harvesting. Depending on the application, the desired emission spectrum of an emitter can vary. For example, in TPV applications, a broadband emitter can increase the output power density of the diode at the cost of lower energy conversion efficiency due to excess energy in higher energy photons being lost to thermalization of the carriers. Meanwhile, a narrowband emitter can improve the conversion efficiency via reduced thermalization of the diode, resulting in lowered output power density. A narrowband selective emitter designed with its absorption energy spectrum placed just above the bandgap could also be paired with a TPV diode for cases where the diode heating needs to be minimized. For this reason, a balance between the output power density and conversion efficiency is needed for the optimal performance of a TPV system. As such, one of the approaches to optimizing a TPV system’s performance is utilizing a selective emitter with multiple narrowbands and stacking of a number of TPV diodes whose bandgaps correspond to the different peak emission wavelengths of the emitter. Photonic crystals (PhCs)/metamaterials (MMs) have been used to create selective emitters with different peak emission wavelengths and bandwidths. However, most PhC/MM emitters are limited to a single band emission spectrum, and only a few MMs have been reported with a multiband emission spectrum. Moreover, PhC/MM emitters that work at shorter wavelengths such as the visible or near-/mid-infrared are based on nanometer scale feature sizes where it is challenging to write the pattern over a large area and fabrication imperfections can lead to lowered performances. The thermal stability of a nano-patterned emitter could also decrease at high temperatures. In this work, we report multiband selective emitters that are built with a planar, thin layer of a dielectric material deposited on top of a metallic layer. The multiple emission bands of these emitters directly result from the inteference fringes in reflectance, as the transmittance of the structure is zero over the wavelength spectrum studied. The peak emission wavelengths of these emitters can be tuned by varying the thickness of the dielectric layer due to the dependence of reflectance on the film’s thickness. Compared to PhC/MM emitters, the thin-film emitters reported here do not need nano/micro patterning, which makes the fabrication process longer and more complicated, and can be fabricated over the entire area of the substrate with ease. Therefore, we believe that our work can provide a path forward for optimizing thermal applications such as TPV energy harvesting via selective emitters with a large working area, fabrication ease, and multiple emission bands.

Until recently the design and synthesis of heterogeneous catalysts have been dominated through enthalpic factors (e.g., charge-charge interactions, charge-transfer interactions). With emergence of high entropy materials (HEMs), another avenue to design and synthesize catalytic materials has opened up. The definition of HEMs is any material that consists of the solid solution of more than five components that allow great flexibility in tuning surface compositions and interfacial functionalities. Here we present the synthesis of high entropy electrocatalysts that potentially outperform the traditional catalysts in energy-related catalysis reactions. The synthesis strategies through entropy maximization will be discussed.

In this talk, I will discuss our recent progress on the development of heterometallic seed-mediated synthesis for monodisperse penta-twinned Cu nanorods using Au nanocrystals as seeds. The nanorod aspect ratio can be readily adjusted from 2.8 to 13.1 by varying the molar ratio between Au seeds and Cu precursor, resulting in narrow longitudinal plasmon resonances tunable from 762 to 2201 nm. The growth pathway features coevolving shape and composition whereby nanocrystals become progressively enriched with Cu concomitant with nanorod growth. The nanorods possess active surface sites for the high-rate electrochemical reduction of carbon dioxide into liquid fuels.

Water splitting reactions represent a promising future avenue for clean energy storage and fuels, and the development of inexpensive and commercially friendly electrocatalysts is essential for the widespread utilization of such processes. Oxygen evolution reaction (OER), also known as water oxidation reaction, is a key component of the overall water splitting reaction, and currently, the most effective catalysts for OER are RuO₂ and IrO₂. Due to the scarcity of noble metals like Ru and Ir, effective catalysts composed of more abundant transition metals are desired. Among these transition metal catalysts, (Ni, Co)S materials have shown significant promise as effective OER catalysts. In this project, we explore the enhanced OER activity through the use of (Ni, Co)S catalysts and also explore different means by which to optimize these catalysts through different NiCo compositions. Through the use of electrochemical tests, we have observed favorable reaction kinetics at higher concentrations of Co and lower overpotentials at higher concentrations of Ni, with good stability overall. We attribute these characteristics to the in-situ development of NiOOH active sites and the higher conductivity of Co. This work provides insights in the development of efficient and noble metal free OER electrocatalysts based on the optimization of transition metal compositions, and future physical characterization can advance our understanding of a catalyzed water splitting mechanism and the development of chemically stable electrocatalysts.

Limited fossil fuel reserves and green house emissions have driven intense research activities over the past few decades on the development of clean and renewable energy sources. Hydrogen is one such source with a high gravimetric energy density and the combustion of which does not generate carbon contained products. However, hydrogen needs to be generated from sources such as water or hydrocarbons, and such processes become viable only when input energy is supplied through renewable sources. Photoelectrochemical (PEC) water splitting is such a suitable pathway to generate hydrogen from water using solar energy. In this process, a semiconductor based material generates electron-hole pair by harvesting solar energy, which paves the way to produce hydrogen mediated by chemical reactions through decomposition of water. However, in spite of about 50 years since its discovery by Fujishima and Honda, PEC water splitting remains a challenging task because of a lack of suitable material. In general, metal oxide such as Titanium dioxide (TiO₂) acts as an excellent photocatalyst material¹, but suffers from limited light absorption window (in ultra violet region which is only 5% of full solar spectrum) and relatively higher recombination of the photogenerated charges. In this regard , CeO₂, a rare earth metal oxide with a more negative conduction band position and lower band gap is one of the most efficient companion of TiO₂ leading to suitable band alignment in a CeO₂-TiO₂ heterojunction². Moreover, CeO₂ shows visible light sensitivity and good electron transfer ability mediated by its variable Ce valences such as Ce³⁺ and Ce⁴⁺, high capacity for oxygen storage, capture and release and high thermal stability³. Here we demonstrate that by using TiO₂ nanorods functionalized with CeO₂ nanoparticles, photocurrent density enhances 3 times over that obtained from the bare TiO₂ nanorods. The TiO₂ nanorods are fabricated on FTO coated glass substrates by hydrothermal technique. The size and distribution of the CeO₂ nanoparticles grown over these nanorods can be precisely controlled by optimizing the deposition/growth parameters and at an optimum condition, an effective heterojunction forms enabling easy transfer of photogenerated charges. This is also confirmed by Electrochemical Impedance Spectroscopy (EIS), where lowest charge transfer resistance is observed for CeO₂-TiO₂ heterojunction photoanodes. Further, Applied Bias Photo-to-current Efficiency (ABPE %) shows a 2.52 times enhancement for the heterojunctions compared to that of bare TiO₂ nanorods. In addition to expected effect of heterojunction in assisting charge separation, visible light absorption by CeO₂ (marked by a 50 nm red-shift in the absorption spectra) also helps in achieving better performance in the PEC operation. We conclude that such heterojunctions made with all oxide components (CeO₂ and TiO₂) may serve better for PEC applications by virtue of their chemical stability under aqueous media. The results of this work with extensive details will be presented.
References
1. Li, C. et al. J. Am. Chem. Soc. 137, 1520–1529 (2015).2. Zhou, Q. et al. Electrochim. Acta 209, 379–388 (2016).3. Montini, T., Melchionna, M., Monai, M. & Fornasiero, PChem. Rev. 116, 5987−6041 (2016).

Ammonia is a promising hydrogen carrier as it reduces the cost of long–range transportation of hydrogen. Ammonia electrolysis provides a convenient method to extract hydrogen from ammonia at end users. However, the conventional platinum carbon electrode suffers from fast degradation and electrode fouling at electrode–electrolyte interface during the ammonia electrochemical oxidation〈AOR〉. Here, we report that nitriding electrochemical deposited nickel cobalt layered double hydroxide nanosheets can achieve high electrocatalytic activity for AOR in non–aqueous solution. The AOR onset overpotential of NiCo₂N nanosheets is 0.55 V which is about 0.25 V lower than that of the platinum carbon electrode. Ultraviolet–visible and Mass spectroscopy studies reveal NiCo₂N nanosheets can suppress the formation of metal amine complex and preferentially oxidize ammonia to N₂ with a faradic efficiency of over 90 percent. DFT calculation indicates that the downshift of the metal d–band center on NiCo₂N nanosheets surface accelerated the generation of NH⁺_X cations and desorption of final N₂*, boosting ammonia electrochemical oxidation toward N₂. Overall, this work introduces a practical strategy for synthesizing an active and stable electrocatalyst for AOR.