A highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction.

The slow kinetics of the oxygen evolution reaction (OER) and high cost of OER electrocatalysts are among the main barriers for proton exchange membrane water electrolysis (PEMWE) technology [1]. Improving the design of active and low-loaded OER catalysts requires a fundamental understanding of catalyst degradation mechanisms.Current state-of-the-art OER catalysts for PEM electrolyzers are iridium-based, such as pure iridium oxide or mixed oxides of iridium and ruthenium. The dissolution of iridium catalyst in acidic environment during OER has been reported extensively in both liquid electrolyte [2] and membrane electrode assemblies (MEAs) [3]. Current understanding of the iridium catalyst degradation reveals a trade-off between stability and activity. Hydrous iridium oxide (IrOx) shows higher activity, but less stability compared to rutile iridium oxide (IrO2) [4, 5]. Mechanistic studies reveal Ir (IV) oxide being the stable oxide and Ir (III) oxide being the intermediate species during OER and dissolving rapidly [4, 5]. Furthermore, density functional theory (DFT) and experimental work show that oxygen binding energy is affected by surface facets and sub-surface structures for iridium-based catalysts [6]. Surface oxide skins can exhibit unique structural features, including different coordination, compression or expansion related to lattice mismatch, and weaker or stronger binding relative to the pure metal or metal oxide, which govern the OER kinetics at end-of-life (EOL). [6]Obtaining an electrocatalyst structure that provides both high activity and high stability, particularly at high current densities, remains a challenge. Stabilizing the weak iridium intermediate caused by oxygen vacancies will be of great importance [5] and requires a fundamental understanding of iridium surface oxides to address this challenge. It is unclear how the surface oxides are developed as degradation progresses and what roles of other accompanied phenomena, such as surface reconstruction and segregation [6], are playing during degradation. Therefore, the formation and structural change of iridium surface oxide and subsequent impact on dissolution rates during accelerated stress test (AST) is the focus of this work.Identical-location scanning transmission electron microscopy (IL-STEM) is a powerful approach for tracking the evolving morphology and chemistry of a catalyst particle during degradation [7, 8]. In this work, commercially available iridium oxide catalyst will be studied using IL-STEM. Aberration-corrected STEM coupled with energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) will be used to study the catalyst surface structure and composition at the atomic scale. The effect of different upper potential limits and potential scanning profiles will also be investigated. The dissolution rates of iridium under these different AST protocols will be determined using time-resolved inductively coupled plasma-mass spectrometry (ICP-MS). Changes in the composition and structure of iridium surface oxide before and after ASTs will be analyzed with EDS and EELS and corroborated with results from X-ray photoelectron spectroscopy (XPS) and, further down the road, operando X-ray absorption spectroscopy (XAS). The outcome of this work can shed light on the strategies to maintain high activity for precious metal catalysts.[9]References[1] Buttler, A., Spliethoff, H., Renewable and Sustainable Energy Reviews, 2018, 82, 2440-2454.[2] Cherevko S., Geiger S., Kasian O., et al., J. Electroanal. Chem. 2016, 774, 102–110.[3] Yu, H., Bonville, L., Jankovic, J., et al. Appl. Catal. B: Environ., 2020, 260, 118194.[4] Pfeifer V., Jones T.E., Velasco Velez J.J., et al., Phys. Chem. Chem. Phys. 2016, 18, 2292–2296.[5] Geiger, S., Kasian, O., Ledendecker, M. et al. Nat. Catal. 2018, 1, 508–515.[6] Alia, S.M., Ha, M-A., Anderson, G.C., et al. J. Electrochem. Soc. 2019, 166, F1243-F1252.[7] Hartl, K., Hanzlik, M, Arenz, M., Energy Environ. Sci., 2011, 4, 234-238.[8] Rasouli, S., Myers, D., Kariuki N., et al., Nano Lett. 2019, 19, 46−53.[9] This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the H2NEW Consortium. Electron microscopy was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

4 - Oxygen evolution reaction (OER) at nanostructured metal oxide electrocatalysts in water electrolyzers

The detrimental impacts of climate change coupled with increasing global energy demand has resulted in a significant research effort to develop clean technologies for energy generation, conversion, storage, distribution as well as the removal of CO2 from various industrial sectors. Undoubtedly, electrocatalysis will play a major role in each of these aspirations, which is reflected in the topics covered in this Special Issue. The contributions included here range from the more mature areas of fuel-cell-relevant reactions and electrochemical water splitting to rapidly emerging reactions such as CO2 reduction and nitrogen conversion to ammonia with further mechanistic insights provided by new experimental techniques and computational studies. Electrocatalytic reactions are at the heart of fuel-cell technology and therefore understanding and improving the efficiency of these reactions remains a highly active area of research. This is reflected in this Special Issue by the work that encompasses many aspects of fuel cells including reactions at the anode and cathode, dissolution of the catalyst, the role of the catalyst support, and understanding the dynamics between the electrodes in a fuel cell. Özaslan and co-workers investigate the role of the capping agent on Pt nanocubes and how it influences both the structural stability of the catalyst and ORR performance. Sandbeck, Cherevko et al. also investigate Pt and determine that dissolution occurs to a different extent on different Pt single-crystal basal planes and polycrystalline Pt. However, not only is corrosion of the catalyst an issue but Maillard and co-workers demonstrate that corrosion of the carbon catalyst support used in proton exchange membrane fuel cells (PEMFCs) is also problematic and involves a Pt-catalyzed decarboxylation mechanism which leads to CO and CO2 evolution. Kunze-Liebhäuser and co-workers demonstrate that the high stability of zirconium oxycarbide lends it well to anodic reactions such as alcohol or CO oxidation. Tremiliosi-Filho et al. show that the electrocatalytic oxidation of ethanol on disordered Pt(111) surfaces is highly influenced by the presence of defects on the surface and provides insights into the operation of real catalysts. Varela and co-workers gain new insights into fuel cell operation by inserting an external reference electrode in a direct formic acid fuel cell (DFAFC) and direct methanol fuel cell (DMFC) under stationary and oscillatory conditions. Electrolysis of water coupled to renewable energy sources is a promising method to produce green hydrogen with zero emissions. The past decade has witnessed remarkable progress in the understanding of both the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER) for water electrolyzers. This Special Issue reflects the ongoing efforts in: 1) elucidation of structure–activity–stability relations, 2) investigations of the role of the interface structure and the support, 3) combining electrochemical methods with in situ advanced characterization, and 4) studying the electrocatalytic HER and OER using both model extended surfaces and nanoparticles. Model studies on single-crystalline surfaces are essential to gain detailed knowledge on the relations between the electrode structure and the electrocatalytic properties. Feliu and co-workers combine cyclic voltammetry, in situ spectroscopy, and laser-induced temperature jump technique to investigate the interfacial properties of Ni-modified Pt(111) surfaces in phosphate electrolyte for the HER. Arenz, Aschauer and co-workers also use Pt single-crystalline surfaces to establish the structure-sensitivity of the OER in acidic electrolyte by combining experimental work and theoretical calculations. The slow kinetics of the OER causes significant overpotentials in water electrolyzers. To improve the efficiency of water electrolysis, it is necessary to design and develop more active and stable OER electrocatalysts. Zhao and co-workers review the field of OER electrocatalysis with a special focus on multimetallic-based catalysts to improve the OER performance in both acidic and alkaline electrolytes. In acidic media, where polymer electrolyte membrane electrolyzers operate, catalysts based on Ir are required. Escudero-Escribano and co-workers show that both the composition and concentration of the acidic electrolyte play an important role in the performance of Ir nanoparticles for the OER. In alkaline electrolyzers, earth-abundant materials are typically used as OER catalysts. Cao, Zheng, and co-workers present highly active and stable hollow nanocubes based on Co–Fe hydroxides for OER in alkaline media. Abu Sayeed and O'Mullane present an electrodeposition method to fabricate bimetallic Co–Fe oxides for both the HER and OER. Boettcher and co-workers report an electrochemical study on the structure–activity relations for Fe (oxy)hydroxides on different metallic supports, showing how the activity can be significantly tuned by the substrate–electrocatalyst interactions. Schuhmann, Andronescu, and co-workers report a method to improve the OER activity of Ni–Fe (oxy)hydroxides by enhancing their electrical conductivity. Arenz, Delgado, and co-workers report an accelerated durability test for high-surface-area Ni-based (oxy)hydroxides for OER in alkaline media. Katayama and co-workers present a method to fabricate Cu-deposited catalysts for the OER in alkaline media. Finally, the OER is also a very relevant reaction in batteries. Risch and co-workers combine electrochemical methods with X-ray absorption to investigate the electrocatalytic OER on LiMn2O4 in LiOH electrolytes. Electrocatalysis encompasses a wide variety of chemical transformations that are not solely limited to fuel-cell-relevant or water-splitting reactions, which are currently of intense interest. Kortlever and co-workers have reviewed another reaction gaining significant attention, namely electrochemical CO2 conversion into fuels and valuable chemicals. In particular, they focus on the effect of the electrolyte employed in the electrocatalytic reaction and its influence on efficiency and selectivity. Herranz et al. also study the electrochemical CO2 reduction reaction using a thin-film Cu2O electrode; they investigate the oxidation state of the catalyst via post mortem analysis with XPS, where complete reduction of the surface to Cu was found; their work has implications for previous work on rough Cu2O electrodes. Scherson and co-workers investigate hydroxylamine oxidation on polycrystalline electrodes and determine that N2 is formed which is dependent on the pH and applied potential. Symes et al. investigate the effect of ultrasound on the electro-oxidation of sulfate solutions to generate useful and powerful oxidants like persulfate and find that at low sulfate concentration and low current density, the use of ultrasound results in a lower concentration of this oxidant. Stimming and co-workers investigate the V(II)/V(III) and V(IV)/V(V) redox reactions employed in redox flow batteries; by developing a method to accurately assess the electrochemically active surface area of the working electrodes they conclude that in disagreement to the received wisdom that porous carbon does not catalyze vanadium redox chemistry. The development of in situ or operando techniques is a key research area that is continuously being developed to gain a better understanding of the mechanisms of electrocatalytic reactions. This is particularly important when considering the validation of theoretical predictions, which is described below, and covered in this Special Issue. Kibler et al. have used in situ scanning tunneling microscopy (STM) to study the adsorption of unreactive acetate on Au(111) surfaces where a phase transition within an adsorbed adlayer is observed, providing key information on the role of reactive adsorbates such as formate on electrocatalytic reactions. Cuesta and co-workers review the area of in situ infrared spectroscopy identifying key theoretical aspects of the technique and highlight recent uses in studying the electrochemical CO2 reduction reaction for the detection of reaction intermediates. Horch et al. demonstrate the integration of ultra-high-vacuum equipment with an electrochemical cell as a way of producing complex surface structures not attainable by regular electrochemical methods. They study increased step density on Pt(111), Cu(111), and Pt/Cu(111) electrodes by STM and the effect on their electrochemical behavior in acidic and alkaline electrolytes. Computational electrocatalysis is a rapidly emerging field that is employed to gain a more fundamental mechanistic understanding of quite complex reactions such as those covered in this Special Issue. This field not only provides support for experimental observation but is being used to predict the activity of electrocatalysts not yet synthesized in the laboratory. Malek and co-workers provide a perspective where they critically assess the use of artificial-intelligence-driven modelling and computational approaches for such a task and take CO2 conversion as a test case. In addition, Tang and Jiang used first-principles density functional theory (DFT) to predict that Ti, Sc, and Fe dimer clusters supported on phosphorene constitute promising electrocatalysts for N2 reduction to NH3. Baletto et al. developed a multi-scale approach to study the catalytic properties of MgO(100) supported Pt nanoparticles for the ORR, where reconstruction of the interface layer is predicted to increase activity. A major challenge, however, in the application of theoretical models is the incorporation of the electrochemical interface and the electrolyte into the simulation while keeping computational times manageable. Rossmeisl and co-workers have used ab initio methods to construct a thermodynamically realistic interface to present simulated cyclic voltammograms of Cu basal plane electrodes that are validated by comparison to experimental data over a large pH range, which therefore provides an atomistic understanding of the interfacial structure of Cu electrodes. Calle-Vallejo et al. tackle the challenge of modelling the role of solvation and its influence on the adsorption energy of species at surfaces. They evaluate the influence of van der Waal interactions on the solvation of *OH adsorbed on alloys of Pt. Chan and co-workers present a hybrid continuum/ab initio method where they introduce a capacitor model for the relationship between the reaction energetics and the potential and charge. This results in an order of magnitude reduction in computational costs to determine electrochemical reaction energetics. To conclude, the topics outlined in this Special Issue highlight the beneficial impact that electrocatalysis can play in developing a cleaner and more sustainable society. The future of this field is indeed bright and brings together not only the expertise of electrochemists but material scientists, theoreticians, engineers, and surface scientists. The outcome is the continuous development of new materials with enhanced performance underpinned by the greater understanding of reaction mechanisms via integration of electrochemical systems with sophisticated in situ techniques and validation with increasingly realistic simulation environments. Professor Anthony O'Mullane received his PhD degree (2001) from University College Cork (Ireland) and completed postdoctoral fellowships at Technische Universitat Darmstadt (Germany), the University of Warwick (UK), and Monash University (Australia). He previously held a position (2008) at RMIT University (Australia) until moving to Queensland University of Technology (QUT) in 2013. He is a Fellow of the Royal Society of Chemistry and Fellow of the Royal Australian Chemical Institute (FRACI). He is the immediate past-Chair of the Electrochemistry Division of the RACI and served as vice chair of the Physical Electrochemistry Division of the International Society of Electrochemistry. His research interests are the electrochemical synthesis and characterization of nanostructured materials; electrocatalysis (water splitting, fuel-cell-relevant reactions); catalysis (water remediation); room-temperature liquid metals; Li-metal-based batteries; and the application of electrochemical methods to various aspects of physical, chemical, and biological science. He has published over 170 journal articles in these areas. María Escudero-Escribano is an assistant professor at the University of Copenhagen (Denmark) since 2017. She received her PhD in Chemistry from the Autonomous University of Madrid (Spain) in 2011. She completed postdoctoral fellowships at the Technical University of Denmark and Stanford University (US). At the University of Copenhagen, María leads the Nanoelectrocatalysis Group, which investigates tailored electrochemical interfaces for sustainable energy conversion and production of renewable fuels and chemicals. She is the Chair of the Danish Electrochemical Society since 2018 and holds a Villum Young Investigator Grant from the Villum Foundation. María has received numerous awards in recognition of her early-career achievements, including the European Young Chemist Award 2016 (Gold Medal, 35-year-old level), the Energy Technology Division Young Investigator Award 2018 from the Electrochemical Society, the Princess of Girona Scientific Research Award 2018, the Young Researchers Award 2019 from the Spanish Royal Society of Chemistry, and the Clara Immerwahr Award 2019. Ifan Stephens is Senior Lecturer at the Department of Materials at Imperial College London. Prior to his appointment to Imperial in 2017, he was at the Department of Physics at the Technical University of Denmark (DTU); he was first employed as a postdoctoral researcher, then as assistant professor, and finally as associate professor and leader of the Electrocatalysis Group there. In 2015, Massachusetts Institute of Technology (MIT) appointed Ifan as the Peabody Visiting Associate Professor. He taught and conducted research at the Department of Mechanical Engineering at MIT for a whole semester. Ifan′s research aims to enable the large-scale electrochemical conversion of renewable energy to fuels and valuable chemicals and vice versa. Such processes will be critical in order to allow the increased uptake of renewable energy. Ifan has published 66 papers on topics including oxygen reduction, oxygen evolution, CO2 reduction and N2 reduction. Ifan′s research on H2O2 electrosynthesis led to the establishment of the spinout HPNow, which he co-founded. Katharina Krischer is a Professor of Physics at the Technical University of Munich (TUM), Germany. She is also a member of the Catalysis Research Center of TUM and serves on editorial boards of several journals on electrochemistry or nonlinear sciences. She did her Ph.D. at the Fritz-Haber-Institut, Berlin, in the group of Prof. Ertl. After postdoctoral training at Princeton University, USA, she returned as a group leader to the Fritz-Haber-Institut, and completed her habilitation in 1998. In 2002 she moved to Munich to take on her current position. Her research interests cover two broad topics, electrochemistry and nonlinear dynamics. She works on photoelectrochemistry, solar fuels, and semiconductor electrochemistry as well as on nonlinear phenomena during electrochemical reactions. Furthermore, she has a strong interest in theory, bridging the gap between physico-chemical continuum models describing self-organization phenomena at the solid-liquid interface and normal form approaches and abstract mathematical models. She has coauthored about 130 publications in peer-reviewed journals and a text book on "Physics of Energy Conversion". She was elected a fellow of the International Society of Electrochemistry and is a member of the German Physical Society (DPG) and the Society of German Chemists (GDCh).

In this project, we have employed a systematic approach to develop active, selective, and stable catalyst materials for important electrochemical reactions involving energy conversion. In particular, we have focused our attention on developing active catalyst materials for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). HER: We have synthesized and investigated several highly active and acid stable non-precious metal HER catalysts, including: [Mo3S13]2- nanoclusters (Nature Chemistry, 2014) and molybdenum phosphosulfide (MoP|S) (Angewandte Chemie, 2014). We have also aimed to engineer these catalyst formulations in a membrane electrode assembly (MEA) for fundamental studies of water electrolysis at high current densities, approximately 1 A/cm2 (ChemSusChem, 2015). We furthermore investigated transition metal phosphide (TMP) catalysts for HER by a combined experimental–theoretical approach (Energy & Environmental Science, 2015). By synthesizing different TMPs and comparing experimentally determined HER activities with the hydrogen adsorption free energies, ΔGH, calculated by density functional theory, we showed that the TMPs follow a volcano relationship for the HER. Using our combined experimental–theoretical model, we predicted that the mixed metal TMP, Fe0.5Co0.5P, should have a near-optimal ΔGH. We synthesized several mixtures of Co and Fe phosphides alloys and confirmed that Fe0.5Co0.5P exhibits the highest HER activity of the investigated TMPs (Energy & Environmental Science, 2015). The understanding gained as to how to improve catalytic activity for the HER, particularly for non-precious metal materials, is important to DOE targets for sustainable H2 production. OER: We have developed a SrIrO3/IrOx catalyst for acidic conditions (submitted, 2016). The SrIrO3/IrOx catalyst significantly outperforms rutile IrO2 and RuO2, the only other OER catalysts to have reasonable stability and activity in acidic electrolyte, and in fact demonstrates the best activity for any known OER catalyst measured in either acidic or in alkaline electrolyte. For alkaline conditions we have demonstrated that the combined effect of cerium as a dopant and gold as a metal support, significantly enhances the OER activity of electrodeposited NiOx films. This NiCeOx-Au catalyst delivers high OER activity in alkaline media, and is among the most active OER electrocatalysts reported to date (Nature Energy, accepted 2016). These studies of new catalysts for the OER, both in acid and in base, are fundamental to enabling new technologies of interest for the DOE, including the production of sustainable fuels and chemicals. ORR: One method to significantly reduce the Pt loading in fuel cell devices is to increase the ORR activity of Pt based systems. To this end we have synthesized a high surface area supported meso-structured PtxNi alloy thin film with a double gyroid morphology that both exhibits high activity and stability for the ORR (submitted, 2016). We have furthermore developed a Ru-core, Pt-shell system that improves the per Pt site activity by more than a factor of 2 (ChemElectroChem, 2014). Further refinement, optimizing Pt-shell thickness and reducing particle sintering during processing, enabled us to obtain a mass activity that is 2 times higher than commercial Pt/C from TKK. These are important contributions to the DOE goal of reducing Pt loading since an improved understanding of how to increase mass activity and stability helps enable low Pt content fuel cells.

To reduce damage to carbon containing components of fuel cell stacks during start-up and shutdown, extensive research has been carried out to produce reversal tolerant anodes (RTAs) using oxygen evolution reaction (OER) catalysts [1-4]. However, most of these results were obtained using resource intensive in-situ testing that suffers from long experimental times. To address this, a series of ex-situ experiments was devised to characterize the activity and durability of OER catalysts in a simulated polymer electrolyte fuel cell (PEMFC) environment. The dissolution/re-deposition mechanism of the OER catalysts were investigated using a combination of linear sweep voltammetry and potential stepping experiments within the normal operating range of PEMFCs (0 V to 1.2 V vs RHE). During normal fuel cell operation, IrO2 based catalysts form soluble Ir3+ species as an intermediate between metallic Ir and IrO2. Ir3+ ions can be washed out of the cell which diminishes the reversal tolerance of the anode. After our electrochemical testing, dissolved Ir3+ concentrations in the electrolyte were determined using the inductively coupled plasma mass spectrometry (ICP-MS) method. An in-line ICP-MS technique was previously used by Cherevko et al. to determine the potential resolved dissolution in real time [5]. To validate the ex-situ accelerated testing protocol, experimental reversal tolerance tests were carried out at four different temperatures (20, 40, 60 and 80 °C) and the results showed an increase in the concentration of Ir detected in the electrolyte solutions (Fig. 1). Additionally, different OER catalysts were tested and the results were correlated with in-situ reversal tolerance tests. The effect of OER catalyst structure and support interactions on catalyst stability was investigated by a combination of surface analytical and electrochemical techniques. The surface analysis revealed that there was dissolution and re-deposition occurring during accelerated degradation testing. SEM-EDX imaging of the catalyst layer after testing, showed relocation of the OER catalyst to the cracks of the gas diffusion layer supporting a microporous layer (GDL/MPL) substrate (Fig. 1). Taniguchi A, Akita T, Yasuda K, Miyazaki Y (2004) Analysis of electrocatalyst degradation in PEMFC caused by cell reversal during fuel starvation. J Power Sources 130:42–49. doi: 10.1016/j.jpowsour.2003.12.035Ralph TR, Hudson S, Wilkinson DP (2006) Electrocatalyst stability in PEMFCs and the role of fuel starvation and cell reversal tolerant anodes. ECS Trans 1:67–84. doi: 10.1149/1.2214545Mandal P, Litster S (2016) Investigation and mitigation of degradation in polymer electrolyte fuel cell due to cell reversal using oxygen evolution catalyst. Meet Abstr MA2016-01:1419–1419.Jung J, Park B, Kim J (2012) Durability test with fuel starvation using a Pt/CNF catalyst in PEMFC. Nanoscale Res Lett 7:34. doi: 10.1186/1556-276X-7-34Cherevko S, Geiger S, Kasian O, Kulyk N, Grote J-P, Savan A, Shrestha BR, Merzlikin S, Breitbach B, Ludwig A, Mayrhofer KJJ (2016) Oxygen and hydrogen evolution reactions on Ru, RuO2, Ir, and IrO2 thin film electrodes in acidic and alkaline electrolytes: A comparative study on activity and stability. Catalysis Today 262:170–180. doi: 10.1016/j.cattod.2015.08.014 Figure 1. Images: backscattered electron (BSE) detector images at 500x magnification of OER catalysts deposited on a GDL/MPL (a) before ex-situ testing and (b) after ex-situ testing. The bright dots were confirmed by EDX to contain Ir atoms. Bar graph: Electrolyte Ir concentration after accelerated degradation protocol. Performed in a flooded, N2 purged, 0.09 M H2SO4, three electrode cell for 30,000 cycles from 0.05 V to 1.2 V vs. RHE with a 1 s hold at each potential. OER catalysts were prepared by sonication in 2-propanol overnight and then 400 μg deposited dropwise onto the GDL/MPL substrate. Three trials were carried at each temperature and the error bars are the standard error of the trials. Figure 1

ABSTRACT:(Pb, La)TiO3 (PLT) ferroelectric thin films are one of the promising candidates for applications in dynamic random memory and pyroelectric detectors, etc. However, the chemical composition in the surface layer of thin films is usually different from that in the “bulk” region of the thin films. For instance, it has been reported that there is Pb, Ti, or La enrichment in the surface layer of the (Pb, La)TiO3 thin films. According to a numerical modeling using effective-medium theory, which was developed in our group, we discussed the effect of the surface layer on the dielectric constant of (Pb, La)TiO3 thin films. The effect of Pb, Ti, or La enrichment in the surface layer on the dielectric constant of PLT thin films was discussed. And a case of the “upgraded” and the “down-graded” thin films was expatiated and discussed.

The intermittency of renewable energy poses significant challenges to developing a grid system that utilizes primarily renewable-based sources. Proton exchange membrane water electrolyzers (PEMWEs) can convert surplus energy from the grid into chemical energy in the form of green hydrogen, which can then be used to generate electricity when the renewable supply is low. Despite much technological advancement over the last two decades, PEMWEs still face many barriers to widespread commercialization. Around a quarter of the cost of the PEMWE system is attributed to iridium, a highly scarce material used as the oxygen evolution reaction (OER) catalyst on the anode side. To enable lower loadings/higher utilization of these catalysts, fundamental studies on iridium activity and degradation are necessary. As it stands in the literature, studies on how physical properties of the catalyst (i.e. crystallinities, valance states, etc.) affect PEMWE performance are scarce. This work aims to elucidate the relationship between structural properties and efficiency/performance for iridium oxide (IrOx ) catalysts in PEMWEs. Five commercially available iridium oxide catalysts were characterized (47491 Alfa Aesar, TEC77100, TEC77110, FIO-01, FIO-11) using transmission electron microscopy (TEM), scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Brunauer–Emmett–Teller (BET) analysis, and x-ray diffraction (XRD) to obtain crystallinities, particle size distributions, specific surface areas, thermal stabilities, elemental compositions, and valance states. Afterwards, kinetic performance was baselined by taking cyclic voltammograms (CVs) via rotating disk electrode (RDE) experiments and electrochemical surface areas (ECSA) were approximated by using a novel analysis developed in this work that can relate the hysteresis in OER potentials to a double layer capacity. Results show that greater amorphous character of IrOx was correlated with increased kinetic performance at the expense of electrochemical stability across redox potentials between 0-1.6V. From the cyclic voltammograms, the catalyst with the most amorphous character underwent multiple (two, possible three) redox transitions, while the one with the most crystalline character showed no evidence of any redox transitions. It was also found that increased ECSA was correlated with greater physical surface areas obtained from BET analysis and that greater amorphous character resulted in a higher Ir III :Ir IV ratio. For future work, we aim to incorporate these fundamental structure-function insights to create standardized accelerated stress tests for PEMWEs, as standards for durability testing have yet to be established.

The performance of water electrolyzers hinges on the availability of effective catalyst materials for the oxygen evolution reaction (OER). The electrocatalysis of the OER is particularly challenging in proton exchange membrane water electrolysis (PEMWE). Only few materials can withstand the corrosive conditions at the PEMWE anode whilst being active towards the OER. Iridium oxide, the state-of-the-art OER catalyst in PEMWE, represents an exception, providing both high activity and stability under oxidizing potentials in acidic electrolyte. The scarcity of iridium, however, makes it necessary to develop next-generation catalysts with improved iridium utilization that enable significantly reduced iridium loadings at PEMWE anodes.This contribution will discuss the atom-level principles underlying the unique properties of iridium oxide in defying the often-observed correlation between OER activity and corrosion of metal-oxide catalysts [1–3]. Commonly considered OER mechanisms require the cleavage of at least one metal–oxygen bond, which therefore must not be too strong for achieving optimal activity—a consequence of the Sabatier principle. On the other hand, a high strength of metal–oxygen bonds is required for (bulk) stability of the oxide lattice, resulting in a common tradeoff between OER activity and stability of metal-oxide catalysts [4]. The exceptional performance of iridium dioxide is explained based on a recently proposed OER mechanism that proceeds without requiring the splitting of metal–oxygen bonds [5]. Instead, the oxygen molecule evolves via a peculiar Ir–OOOO–Ir transition state, leaving all Ir–O bonds intact. This explains why iridium dioxide can present strong Ir–O bonds whilst still being highly active towards the OER, emphasizing the unique role of crystalline IrO2 as anode catalyst in PEMWE.To achieve improvements in iridium utilization in practice, we recently developed a synthesis strategy leveraging the outstanding properties of crystalline iridium dioxide. Common synthesis routes require high-temperature steps to form the crystalline phase, which concomitantly leads to particle growth and a decreased active surface area. On the contrary, the new synthesis method proceeds under mild temperatures in the presence of a strong oxidizing agent to afford small (about 2 nm) IrO2 nanoparticles with a high degree of crystallinity [6]. The obtained catalyst demonstrated high mass-specific OER activity with very good stability in ex situ glass-cell experiments. The synthesis method was scaled up to produce catalyst quantities that enabled the fabrication of membrane–electrode assemblies (MEAs). The results from in situ PEMWE testing confirmed the outstanding performance of the nano-crystalline IrO2 catalyst.

A bifunctional oxygen reduction reaction (ORR)/ oxygen evolution reaction (OER) catalyst is essential for rechargeable metal-air batteries and regenerative fuel cells. Platinum (Pt) and iridium oxide (IrO2) are the state-of-the-art ORR and OER catalysts, respectively. However the high price and scarcity of these platinum group metals (PGMs) has been an obstacle for wide spread application of these catalysts. Recently, in alkaline media, carbon based ORR and perovskite OER catalysts have demonstrated similar or even better catalytic activities compared to the counterpart PGM catalysts [1, 2]. Therefore, if we combine these two non-PGM catalysts, a non-PGM bifunctional ORR/OER catalyst can be obtained. A hindrance in this approach is the vulnerability of carbon-based ORR catalysts to oxidation in the OER potential range, i.e., potentials > 1.5 V vs. RHE. Thus development of robust ORR catalysts under practical OER conditions is a key to realize this kind of bifunctional catalysts. The carbon support used in our ORR catalysts was black pearl (BP) 2000 [1]. In preliminary tests, however, we found that BP 2000 undergoes oxidization at potentials around ca. 1.2 V vs. RHE and above (data not shown). In this work, we used reduced graphene oxide (rGO) as an alternative support to synthesize oxidation resistant ORR catalysts. The OER catalyst we chose was a perovskite (La1-xSrx)CoO3-δ (LSC). Pre-synthesized LSC was added into the initial solution of the rGO based ORR catalyst synthesis process, and after drying and heat-treatment, bifunctional (LSC + rGO) catalysts were obtained. In measuring the OER activity of the LSC catalyst, acetylene black (AB) carbon was added to the LSC (LSC + AB) to increase the electrical conductivity. Fig. 1 shows the comparison of ORR/OER activities between (LSC + AB) and (LSC + rGO). As expected, the ORR activity of (LSC + rGO) is greatly improved by ca. 200 mV in terms of E½ , in comparison to that of (LSC + AB). Interestingly even the OER activity of (LSC + rGO) becomes higher than that of (LSC + AB). Thanks to the enhancement of both ORR and OER activities with (LSC + rGO), highly active bifunctional catalysts are obtained. In this talk, material analysis results and diverse electrochemical performances of the (LSC + rGO) catalysts will be presented. Acknowledgements Support from the Directed Research of the Los Alamos National Laboratory’s Laboratory Directed Research & Development (LDRD-DR) is greatly acknowledged. References Chung et al., Nat. Commun. 4, 1922 (2013).Suntivich et al., Science, 334, 1383 (2011). Figure 1

Hydrogen is regarded as the most promising energy carrier. The pure hydrogen produced from electrochemical water splitting by using renewable energy is the most attractive and effective approach.[1] In comparison to alkaline water electrolysis, proton exchange membrane water electrolyzers (PEMWEs) could operate at higher current densities, lower ohmic loss, more compact system design, faster system response and more stable towards load-cycling and shutdowns. [2] The sluggish kinetics of oxygen evolution reaction (OER) at the anode of PEMWEs has restricted its practical applications. To date, iridium (Ir) metal and its oxide are usually selected as the state-of-the-art catalyst for OER due to their robust and long-term durability under the harsh acidic conditions. Nevertheless, Ir is an extremely scarce element, and its price has been doubled in the past few years. The development of low cost and highly effective electrocatalyst for OER in an acidic medium is urgently needed but remains a great challenge.In this work, we have prepared an ultra-thin RuO2 nanosheets by a simple molten salt method. High resolution STEM results show that the as-prepared RuO2 nanosheets is about 1-2 nm in thickness and hold a large number of defects, like Ru vacancy, grain boundary and amorphous. Toward OER, the as-prepared RuO2 nanosheets demonstrate an outstanding OER activity in acidic electrolyte, with an overpotential of only 199 mV for reach the current density of 10 mA cm-2 geo at a catalyst loading of 125 μg cm-2 geo. At the overpotential of 230 mV, the specific and mass activities of RuO2 nanosheets electrode is up to 0.89 mA cm-2 oxide and 0.516 A mg-1 Ru, which is 14.9 and 80.5 times higher than that of commercial RuO2 catalyst, respectively. Density functional theory (DFT) calculations (Figure 2e and 2f) indicated that the Ru vacancy defect on RuO2 surface could servers as the highly active site for remarkably weaken the binding energies of *O as compared to that of *OOH, which decrease the energy gap between ΔGO and ΔGOOH and thus dramatic enhanced OER performance. We also applied the catalysts in a homemade PEMWE device, at the applied cell voltage of 1.65 V, the current density of the RuO2 nanosheets catalyst cell reaches 0.93 A cm-2, which is almost 3 times larger than that of the cell with commercial RuO2 catalyst (0.31 A cm-2) at the same conditions, demonstrating that the RuO2 nanosheets possess great potential toward developing high performance PEMWEs.This work is under progress. Meanwhile, further characterizations, including XRD, XPS and more detailed morphology and electrochemical performance characterization of the RuO2 nanosheets catalyst will be presented at the meeting.The work described in this paper was financially supported by the Shenzhen Clean Energy Research Institute (No. CERI-KY-2019-003), Shenzhen Peacock Plan (KQTD2016022620054656), Shenzhen Key Laboratory project (ZDSYS201603311013489), The authors acknowledge the assistance of SUSTech Core Research Facilities.

The use of Single Atom Catalysts (SACs) for acidic Oxygen Evolution Reaction (OER) is an emerging field of research with prospects to maximise the dispersion of active sites and the metal utilisation. Therefore, it is promising for reducing the amount of noble metal needed to efficiently electrocatalyse the OER. The objective is to achieve comparable activity to conventional unsupported and supported iridium and ruthenium oxide catalysts but with significantly lower loading of precious metal. The present review summarises the recent progress in this field, discussing the preparation of such materials, the structural characterisation techniques suited to probe single metal atoms, as well as the hitherto achieved activity and stability in acidic OER conditions. We conclude the short review with a summary of the main observations and perspectives for this class of materials.

Recently, environmental issues such as global warming and depletion of fossil fuels have become important topic, and progress toward sustainable societies with renewable energies such as solar and wind energy is a critical issue to overcome the social problems. Conversion of surplus energies to hydrogen via water electrolysis has attracted attention as energy storage method to utilize renewable energies effectively. However, activity, durability, and cost of catalysts are insufficient for practical application in water electrolysis systems due to high over-potential and low kinetics of electrode reactions, especially, oxygen evolution reaction (OER) on the anode [1-2] for polymer electrolyte water electrolyzer (PEWE). Iridium oxide (IrO2) particles have been used as anode catalysts, because of its high activity and durability for OER. However, decreasing iridium loading amount is required to reduce catalyst cost. In a previous study, we have successfully synthesized novel IrO2 nanoparticle catalyst supported on carbon nanotubes with large surface area of IrO2 catalyst to improved OER activity [3]. For further improvement of OER activity, increasing the specific activity of the catalyst for OER is required. One of the approaches to improve catalytic activity is formation of alloy of iridium with other metals with high activity, such as ruthenium. In the present study, we have synthesized novel alloy nanoparticle catalysts, IrRuOx, supported on the reduced graphene oxide (rGO), as active catalysts for OER. The IrRuOx / rGO catalyst was characterized and evaluated in catalytic activity for OER in sulfuric acid solution.The IrRuOx / rGO catalyst was synthesized by hydrothermal method. Briefly, required amount of metal complexes, H2IrCl6 and RuCl3, and graphene oxide (GO) prepared by modified Hummers’ method were dispersed in ethanol/water mixture and the mixture was heated at 80˚C for 6 h. Then, the mixture was heated at 150˚C in hydrothermal autoclave for 4 h to form IrRuOx nanoparticles. The IrRuOx / rGO catalysts were characterized by transmission electron microscopy (TEM), energy dispersed X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical methods. The OER activity of the IrRuOx / rGO was examined in 0.5 M H2SO4 solution by linear sweep voltammetry (LSV) using a rotating disk electrode (RDE) system. A TEM image of the Ir0.7Ru0.3Ox / rGO catalyst is shown in Figure 1. The Ir0.7Ru0.3Ox nanoparticles with wide particle size distribution were dispersed on the surface of rGO substrate. An average size of the nanoparticles was 1.9 ± 0.8 nm. The loading amount of the Ir0.7Ru0.3Ox catalyst is estimated to be approximately 23.9 wt% by EDX measurements. In addition, the EDX analysis reveals that the ratio of Ir to Ru of the IrRuOx nanoparticles is proportional to the ratio of starting materials, H2IrCl6 and RuCl3. The XRD patterns of the IrRuOx / rGO catalysts with different Ir/Ru ratio are shown in Figure 2. The XRD data shows the (200) peak of IrRuOx is shifted to small angle and became broader with increasing Ir ratio of the catalysts. Furthermore, the XPS results of the IrRuOx / rGO catalysts showed binding energy of Ir 4f peak was shifted to a lower energy state with increasing the ratio of Ru. The XRD and XPS data suggest that nanoparticles of the catalyst form alloy of RuO2 and IrO2. In addition, the shift of the binding energy of Ir 4f peak reflects the modification of the electronic state of Ir, which affects the catalytic activity for OER, by alloy formation with Ru atom. Figure 3 shows linear sweep voltammogram of the IrRuOx / rGO catalyst obtained in 0.5 M H2SO4 solution. Electrochemical measurement reveals activity of the IrRuOx / rGO catalyst for OER is higher than that of the IrO2 / rGO catalyst, especially, onset potential of the reaction is 68 mV lower than that on the IrO2 / rGO catalyst. High activity of the IrRuOx / rGO catalyst indicates that alloying IrO2 with RuO2 improves the catalytic activity for OER. In summary, we successfully prepared the alloy catalysts of IrO2 and RuO2 supported on rGO substrate, and the IrRuOx / rGO catalysts can be a promising candidate as anode for water electrolysis. Acknowledgements This work was supported by JSPS KAKENHI Grant number 17K05969.

Iridium- and Ruthenium based catalysts are the only feasible ones for the oxygen evolution reaction (OER) in acidic media. The high price and scarcity of Ir is a major hurdle to realize electrolysis in the GW-scale. Ru-based catalysts can be suitable candidates to replace Ir, since they are more active for the OER than Ir. However, Ru catalysts lack sufficient stability during the OER. This is because, Ru tends to form soluble upper oxides e.g. RuO4 at high voltages (> 1.4 V). Previous studies reveal that Na- or K-doping can increase Ru’s durability during the OER.1,2 However, recent studies demonstrate that Ru’s activity and stability depends on the structure and composition of the Ru-based catalyst.3 In this work, we report a series of Ru double perovskites, namely R2NiRuO6 (R = Pr, Nd, Tb, Dy, Y, Ho and Er)4 with low ruthenium content and high OER activity in acidic electrolyte. The oxides have been synthesized by wet-chemistry and thoroughly characterized by XRD (Rietveld refinement), TEM and XPS.The OER was studied in the rotating ring disk (RDE) configuration in 0.1 M HClO4. Due to the absence of Sr or alkali metals in their structure, in addition to displaying high initial OER activity, the perovskites are stable during OER cycles between 1.1 and 1.7 V, with Dy2NiRuO6 being stable at least during 500 consecutive cycles (see Figures 1a and 1b).Characterization results reveal that Ru-O distances in the perovskites follow the order Dy<Er<Y<Ho<Tb<Nd<Pr. As shown in Figure 1c, the OER activity, i.e., the potential recorded at 10 mA·cm-2, follows the same trend. Also, the durability of the catalysts follows the same trend.The smaller Ru-O distance indicates a higher oxidation state of Ru atoms in Dy2NiRuO6. In addition, the gives symmetry and order to the structure. perovskite, making it more stableXPS and TEM-EDS post-mortem studies of Dy2NiRuO6 reveal a loss of Ni after 500 OER cycles and the presence of Ru3+ cations, being the main cause of the loss of OER activity.The R2NiRuO6 double perovskites reported in this work are the first example of highly active and stable Ru-based perovskites for the OER in acidic electrolyte that can opens the way for the replacement of Ir from state of the art PEMWEs.(1) Retuerto, M.; Pascual, L.; Calle-Vallejo, F.; Ferrer, P.; Gianolio, D.; Pereira, A. G.; García, Á.; Torrero, J.; Fernández-Díaz, M. T.; Bencok, P.; Peña, M. A.; Fierro, J. L. G.; Rojas, S. Na-Doped Ruthenium Perovskite Electrocatalysts with Improved Oxygen Evolution Activity and Durability in Acidic Media. Nat. Commun. 2019, 10 (1), 2041. https://doi.org/10.1038/s41467-019-09791-w.(2) Rodríguez-García, I.; Galyamin, D.; Pascual, L.; Ferrer, P.; Peña, M. A.; Grinter, D.; Held, G.; Abdel Salam, M.; Mokhtar, M.; Narasimharao, K.; Retuerto, M.; Rojas, S. Enhanced Stability of SrRuO3 Mixed Oxide via Monovalent Doping in Sr1-XKxRuO3 for the Oxygen Evolution Reaction. J. Power Sources 2022, 521, 230950. https://doi.org/10.1016/j.jpowsour.2021.230950.(3) Paoli, E. A.; Masini, F.; Frydendal, R.; Deiana, D.; Malacrida, P.; Hansen, T. W.; Chorkendorff, I.; Stephens, I. E. L. Fine-Tuning the Activity of Oxygen Evolution Catalysts: The Effect of Oxidation Pre-Treatment on Size-Selected Ru Nanoparticles. Catal. Today 2016, 262, 57–64. https://doi.org/10.1016/j.cattod.2015.10.005.(4) Kayser, P.; Alonso, J. A.; Muñoz, A.; Fernández-Díaz, M. T. Structural and Magnetic Characterization of the Double Perovskites R2NiRuO6 (R = Pr-Er): A Neutron Diffraction Study. Acta Mater. 2017, 126, 114–123. https://doi.org/10.1016/j.actamat.2016.12.024. Figure 1

Anion exchange membrane (AEM) water electrolysis can afford cheaper hydrogen (H2) production compared to the current state-of-the-art of proton exchange membrane (PEM) electrolysis. In AEM electrolysis, i.e., in alkaline environment, platinum group metal (PGM)-free catalysts can be adopted as oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalysts, contrary to the PEM electrolysis in which PGM-catalysts should be used. Furthermore, expensive titanium flow-field that is indispensable in acidic PEM electrolyzer can be replaced by inexpensive stainless flow-field. These advantages of AEM electrolysis make AEM electrolysis attractive technology to produce H2 in low cost. However currently performance of AEM electrolysis is much lower than that of PEM electrolysis. Therefore to make the AEM electrolysis technology viable, significant advancement in AEM electrolysis technology is needed. Many PGM-free OER catalysts measured in aqueous alkaline electrolytes, such as 0.1 M KOH or NaOH, have demonstrated as high activities as that of PGM IrO2 OER catalysts measured in an aqueous acid electrolytes in an electrochemical cell test. This implies that the low AEM electrolysis performance is possibly caused by some detrimental effect of AEM/anion exchange ionomer (AEI) onto catalysts. In this work we exploited different types of catalysts and AEIs to investigate the effect of catalyst-AEI interaction on AEM electrolysis performance. We observed that catalyst-AEI interaction substantially affects the AEM water electrolysis performance. The higher activity and durability of perovskite oxide OER catalyst than IrO2 OER catalyst in AEM water electrolysis can be explained by this catalyst-AEI interaction. In this talk, we will discuss the causes for low AEM electrolysis and possible pathways to improve AEM water electrolysis performance. Acknowledgements The authors gratefully acknowledge research support from the HydroGEN Advanced Water Splitting Materials Consortium, established as part of the Energy Materials Network under the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office.

Efficient and earth abundant electrocatalysts for high-performance oxygen evolution reaction (OER) are essential for the development of sustainable energy conversion technologies. Here, the 3d transition-metal-doped TiO2 were synthesized and used the catalysts for alkaline oxygen evolution reaction. Commercial catalyst usually uses carbon as support such as Ir/C and Ru/C. But during oxygen evolution reaction, carbon will be corroded seriously. So, catalyst will detach from carbon support cause efficiency of catalyst to decay very quickly. In order to overcome these problems, we develop TiO2 as a support because TiO2 is a very durable material in high potential region and high concentration alkaline electrolyte. However, TiO2 is not a good catalyst for oxygen evolution reaction and suffer from its poor conductivity. To solve these drawbacks, we synthesized different 3d transition metal doping TiO2 in this work. After electrochemical tests, we choose (Ni, Fe) dual doped TiO2 was chosen because of its potential for oxygen evolution reaction. First of all, different different doping compositions of Ni and Fe were investigated, and it was found that Ni:Fe in molar ratio 3:1 showed the best oxygen evolution reaction performance. Next, different doping amounts of Ni to Fe, from 6:2, 3:1 to 1.5:0.5 were also evaluated, where Ni:Fe in 6:2 was the best composition for alkaline oxygen evoution. Furthermore, we modified catalyst by adding long chain anion, hydrogen treatment and loading NiFe LDH on (Ni, Fe) dual doped TiO2. Finally, NiFe/TiNiFe perform better oxygen evolution reaction than pure NiFe LDH. At the current density reach 10 mA/cm2, the required potential of NiFe/TiNiFe was 10 mV less than for pure NiFe LDH. After chronopotentiometric stability test for 48 hours, NiFe/TiNiFe showed almost no decay during reaction and is more superior than NiFe LDH. Our work successfully solves activity and stability issue at the same time by an excessive dual doping approach.