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Electrocatalysts for the oxygen evolution reaction (OER) are an important component for the transition from fossil to sustainable energy. Commercialization of cost-effective earth-abundant electrocatalysts is in large parts hindered by their degradation. In this short review, I identify common processes leading to a decrease in electrocatalyst activity, followed by an introduction of staple methods to determine degradation electrochemically and by additional physical characterization, which has the potential to remove ambiguities of purely electrochemical studies. I conclude by a summary of the key challenges for an accurate determination of degradation processes and highlight interesting directions to advance the understanding of degradation processes on electrocatalysts.

Improving the stability of electrocatalysts for the oxygen evolution reaction OER through materials design has received less attention than improving their catalytic activity. We explored the effects of Mn addition to a cobalt oxide for stabilizing the catalyst by comparing single phase CoOx and Co0.7Mn0.3 Ox films electrodeposited in alkaline solution. The obtained disordered films were classified as layered oxides using X ray absorption spectroscopy XAS . The CoOx films showed a constant decrease in the catalytic activity during cycling, confirmed by oxygen detection, while that of Co0.7Mn0.3 Ox remained constant within error as measured by electrochemical metrics. These trends were rationalized based on XAS analysis of the metal oxidation states, which were Co2.7 and Mn3.7 in the bulk and similar near the surface of Co0.7Mn0.3 Ox, before and after cycling. Thus, Mn in Co0.7Mn0.3 Ox successfully stabilized the bulk catalyst material and its surface activity during OER cycling. The development of stabilization approaches is essential to extend the durability of OER catalysts

Understanding the process of water oxidation, especially intermediate species, represents an important step toward gaining a mechanistic understanding of new emerging catalysts. The aim of this study is exploring the process of water oxidation and electrolyte orientation under external potential when using an emerging water oxidation catalyst, CoBi, in sodium borate (NaBi) buffer using in situ attenuated–total-reflection Fourier transform infrared spectroscopy (ATR-FTIR) spectroscopy. CoBi is generated via electrodeposition from aqueous solutions containing borate and Co2+. IR spectra were obtained for CoBi films under applied potentials supporting water oxidation catalysis. The spectra of water and CoBi on ZnSe/Cr/Au electrode surfaces change in intensity and their slope depends on the potential, which is rarely reported. The appearance of new bands at certain potentials is interpreted in terms of the potential-dependent re-alignment of water and borate molecules both from the film and electrolyte. A superoxide surface intermediate at 1027 cm-1 was observed in both thin and thick films. It is proposed to be Co (III)OO*H bridging and relates to a fast water oxidation process. The chemical structure of the intermediate species is proposed finally.

Improving the stability of electrocatalysts for the oxygen evolution reaction (OER) through materials design has received less attention than improving their catalytic activity. We explored the effect of Mn addition to a cobalt oxide for stabilizing the catalyst by comparing Na-containing CoOx and (Co0.7Mn0.3)Ox films electrodeposited in alkaline solution. The obtained disordered films were classified as layered oxides using X-ray absorption spectroscopy (XAS). The CoOx films showed a constant decrease in the catalytic activity during cycling, confirmed by oxygen detection, while that of (Co0.7Mn0.3)Ox slightly increased as measured by electrochemical metrics. These trends were rationalized based on XAS analysis of the metal oxidation states, which were Co2.8+ and Mn3.7+ near the surface after cycling. Thus, adding Mn to CoOx successfully stabilized the catalyst material and its activity during OER cycling. The development of stabilization approaches is essential to extend the durability of OER catalysts., arXiv admin note: text overlap with arXiv:2010.06957

ACS applied materials & interfaces 14(12), 14129-14136 (2022). doi:10.1021/acsami.1c20337, Published by American Chemical Society, Washington, DC

Despite numerous experimental and theoretical studies devoted to the oxygen evolution reaction OER , the mechanism of the OER on transition metal oxides remains controversial. This is in part owing to the ambiguity of electrochemical parameters of the mechanism such as the Tafel slope and reaction orders. We took the most commonly assumed adsorbate mechanism and calculated the Tafel slopes and reaction orders with respect to pH based on microkinetic analysis using the steady state approximation. The analysis was performed for an ideal electrocatalyst without scaling of the intermediates as well as for one on the top of a volcano relation and one on each leg of the volcano relation which exhibits scaling of the intermediates. For these four cases, the number of possible Tafel slopes strongly depends on surface coverage. Furthermore, the Tafel slope becomes pH dependent when the coverage of intermediates changes with pH. These insights complicate the identification of a rate limiting step by a single Tafel slope at a single pH. Yet, simulations of reaction orders complementary to Tafel slopes can solve some ambiguities to distinguish between possible rate limiting steps. The most insightful information can be obtained from the low overpotential region of the Tafel plot. The simulations in this work provide clear guidelines to experimentalists for the identification of the limiting steps in the adsorbate mechanism using the observed values of the Tafel slope and reaction order in pH dependent studies

A new implementation for XAS measurements based on zone plates is presented. With a two-color scheme, several fluorescence yields can be probed simultaneously., X-ray absorption spectroscopy (XAS) is a powerful element-specific technique that allows the study of structural and chemical properties of matter. Often an indirect method is used to access the X-ray absorption (XA). This work demonstrates a new XAS implementation that is based on off-axis transmission Fresnel zone plates to obtain the XA spectrum of La0.6Sr0.4MnO3 by analysis of three emission lines simultaneously at the detector, namely the O 2p–1s, Mn 3s–2p and Mn 3d–2p transitions. This scheme allows the simultaneous measurement of an integrated total fluorescence yield and the partial fluorescence yields (PFY) of the Mn 3s–2p and Mn 3d–2p transitions when scanning the Mn L-edge. In addition to this, the reduction in O fluorescence provides another measure for absorption often referred to as the inverse partial fluorescence yield (IPFY). Among these different methods to measure XA, the Mn 3s PFY and IPFY deviate the least from the true XA spectra due to the negligible influence of selection rules on the decay channel. Other advantages of this new scheme are the potential to strongly increase the efficiency and throughput compared with similar measurements using conventional gratings and to increase the signal-to-noise of the XA spectra as compared with a photodiode. The ability to record undistorted bulk XA spectra at high flux is crucial for future in situ spectroscopy experiments on complex materials.

The requirements for beneficial materials restructuring into a higher performance oxygen evolution reaction (OER) electrocatalyst are still a largely open question. Here erythrite (Co3(AsO4)2·8H2O) is used as a Co-based OER electrocatalyst to evaluate its catalytic properties during in situ restructuring into an amorphous Co-based catalyst in four different electrolytes at pH 7. Using diffraction, microscopy, and spectroscopy, a strong effect in the restructuring behavior is observed depending of the anions in the electrolyte. Only carbonate electrolyte can activate the catalyst material, which is related to its slow restructuring process. While the catalyst turnover frequency (TOF) undesirably reduces by a factor of 28, the number of redox active sites continuously increases to a factor of 56, which results in an overall twofold increase in current of the restructured catalyst after 800 cycles. The activation is attributed to an adequate local order, a high Co oxidation state close to 3+, and a high number of redox-active Co ions. These three requirements for beneficial restructuring provide new insights into the rational design of high-performance OER catalysts by electrochemical restructuring. Bundesministerium für Bildung und Forschung/[05K19KE1]/BMBF/Alemania Deutsche Forschungsgemeinschaft/[EXC 2008-390540038]/DFG/Alemania European Research Council/[No. 804092]/ERC/Unión Europea UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Electroquímica y Energía Química (CELEQ) UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Ciencia e Ingeniería de Materiales (CICIMA)

Electrocatalysts for bifunctional oxygen reduction (ORR) and oxygen evolution reaction (OER) are commonly studied under hydrodynamic conditions, rendering the use of binders necessary to ensure the mechanical stability of the electrode films. The presence of a binder, however, may influence the properties of the materials under examination to an unknown extent. Herein, we investigate the impact of Nafion on a highly active ORR/OER catalyst consisting of MnFeNi oxide nanoparticles supported on multi-walled carbon nanotubes. Electrochemical studies revealed that, in addition to enhancing the mechanical stability and particle connectivity, Nafion poses a major impact on the ORR selectivity, which correlates with a decrease in the valence state of Mn according to X-ray absorption spectroscopy. These findings call for awareness regarding the use of electrode additives, since in some cases the extent of their impact on the properties of electrode films cannot be regarded as negligible.

International audience; The reactivity of water with Li-rich layered Li2RuO3 and partial exchange of Li2O with H2O within the structure is studied under aqueous (electro)chemical conditions. Upon slow delithiation in water over long time periods, micron-sized Li2RuO3 particles structurally transform from an O3 structure to an O1 structure with a corresponding loss of 1.25 Li ions per formula unit. The O1 stacking of the honeycomb Ru layers is imaged using high-resolution high-angle annular dark-field scanning transmission electron microscopy, and the resulting structure is solved by X-ray powder diffraction and electron diffraction. In situ X-ray absorption spectroscopy suggests that reversible oxidation/reduction of bulk Ru sites is realized on potential cycling between 0.4 and 1.25 VRHE in basic solutions. In addition to surface redox pseudocapacitance, the partially delithiated phase of Li2RuO3 shows high capacity, which can be attributed to bulk Ru redox in the structure. This work demonstrates that the interaction of aqueous electrolytes with Li-rich layered oxides can result in the formation of new phases with (electro)chemical properties that are distinct from the parent material. This understanding is important for the design of aqueous batteries, electrochemical capacitors, and chemically stable cathode materials for Li-ion batteries.

Nitrogen-doped carbons are among the fastest-growing class of materials used for oxygen electrocatalysis, namely, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), thanks to their low cost, environmental friendliness, excellent electrical conductivity, and scalable synthesis. The perspective of replacing precious metal-based electrocatalysts with nitrogen-doped carbon is highly desirable for reducing costs in energy conversion and storage systems. In this review, the role of nitrogen and N-induced structural defects on the enhanced performance of N-doped carbon electrocatalysts toward the OER and the ORR as well as their applications for energy conversion and storage technologies is summarized. The synthesis of N-doped carbon electrocatalysts and the characterization of their nitrogen functional groups and active sites for the conversion of oxygen are also reviewed. The electrocatalytic performance of the main types of N-doped carbon materials for OER/ORR electrocatalysis are then discussed. Finally, major challenges and future opportunities of N-doped carbons as advanced oxygen electrocatalysts are highlighted. Published version B.W., H.M., and D.M.M. contributed equally to this work. This work was supported by the Volkswagen Foundation (Freigeist Fellowship No. 89592). B.W. also acknowledges the funding by the CSC Fellowship. Open access funding enabled and organized by Projekt DEAL.

Generator-collector experiments offer insights into the mechanisms of electrochemical reactions by correlating the product and generator currents. Most commonly, these experiments are performed using a rotating ring-disk electrode (RRDE). We developed a double electrode flow cell (DEFC) with exchangeable generator and detector electrodes where the electrode width equals the channel width. Commonalities and differences between the RRDE and DEFC are discussed based on analytical solutions, numerical simulations and measurements of the ferri-/ferrocyanide redox couple on Pt electrodes in a potassium chloride electrolyte. The analytical solutions agree with the measurements using electrode widths of 5 and 2 mm. Yet, we find an unexpected dependence on the exponent of the width so that wider electrodes cannot be analysed using the conventional analytical solution. In contrast, all the investigated electrodes show a collection efficiency of close to 35.4% above a minimum rotation speed or flow rate, where the narrowest electrode is most accurate at the cost of precision and the widest electrode the least accurate but most precise. Our DEFC with exchangeable electrodes is an attractive alternative to commercial RRDEs due to the flexibility to optimize the electrode materials and geometry for the desired reaction.

Many positive electrode materials in lithium ion batteries include transition metals which are difficult to describe by electronic structure methods like density functional theory (DFT) due to the presence of multiple oxidation states. A prominent example is the lithium manganese oxide spinel Li$_x$Mn$_2$O$_4$ with $0\leq x\leq2$. While DFT, employing the local hybrid functional PBE0r, provides a reliable description, the need for extended computer simulations of large structural models remains a significant challenge. Here, we close this gap by constructing a DFT-based high-dimensional neural network potential (HDNNP) providing accurate energies and forces at a fraction of the computational costs. As different oxidation states and the resulting Jahn-Teller distortions represent a new level of complexity for HDNNPs, the potential is carefully validated by performing X-ray diffraction experiments. We demonstrate that the HDNNP provides atomic level details and is able to predict a series of properties like the lattice parameters and expansion with increasing Li content or temperature, the orthorhombic to cubic transition, the lithium diffusion barrier, and the phonon frequencies. We show that for understanding these properties access to large time and length scales as enabled by the HDNNP is essential to close the gap between theory and experiment., 20 pages, 13 figures

Author(s): Bell, Alexis; Deretzis, Ioannis; Donat, Felix; Gallant, Betar; Lee, Jung-Ho; Morante, Joan Ramon; Ogale, Satishchandra; Risch, Marcel; Simonov, Alexandr; Warren, Emily; Neil, Simon; Darby, Claire

Selectively exposing active surfaces of Pt based nanoframes NFs can promote electrocatalysis of small organic molecules, especially regarding improved diffusion and anti poisoning properties. However, the systematic investigation on the synthesis, as well as structure property relationship, of Pt based NFs with tunable external and internal surface structures is still at its early stage. Herein, we report a facile, environmental and one pot approach to fabricate PtCuNi NFs with tunable external and internal surface structures by flexibly adjusting coordination and reducing agents. Interestingly, electrocatalytic results reveal that the PtCuNi NFs with variable external structures possess higher performance activity and anti CO poisoning capability than those with tunable internal structures as well as commercial Pt C. Especially, the PtCuNi eb NFs external branch NFs exhibit the excellent specific activities of methanol and formic acid oxidation reactions MOR and FAOR , 10.7 and 7.9 times higher than those of commercial Pt C, respectively. The PtCuNi eb NFs also possess a superior diffusion ability for methanol electrooxidation 0.0276 and formic acid electrooxidation 0.0153 compared to other PtCuNi NFs with plentiful internal surface. The enhanced MOR and FAOR activities of PtCuNi eb NFs are ascribed to its abundant external surface area and high defect density e.g. vacancy, subtle lattice distortion and high index facets , which results in an optimal anti CO poisoning capability due to the diffusion and ligand effects. This work opens up a new pathway for enhancing the electrooxidation performance anti poisoning property and diffusion rate of liquid fuels by tuning the surface structures of nanoframe catalysts

The oxygen evolution reaction (OER) is considered a key reaction for electrochemical energy conversion but slow kinetics hamper application in electrolyzers, metal-air batteries and other applications that rely on sustainable protons from water oxidation. In this review, the prospect of epitaxial perovskite oxides for the OER at room temperature in alkaline media is reviewed with respect to fundamental insight into systematic trends of the activity. First, we thoroughly define the perovskite structure and its parameter space. Then, the synthesis methods used to make electrocatalytic epitaxial perovskite oxide are surveyed, and we classify the different kinds of electrodes that can be assembled for electrocatalytic investigations. We discuss the semiconductor physics of epitaxial perovskite electrodes and their consequences for the interpretation of catalytic results. Prototypical mechanisms of the OER are introduced and comparatively discussed. OER investigations on epitaxial perovskite oxides are comprehensively surveyed and selected trends are graphically highlighted. The review concludes with a short perspective on opportunities for future electrocatalytic research on epitaxial perovskite oxide systems., Author Version

Resonant inelastic X ray scattering RIXS is a promising method for elucidating detailed electronic structure of materials in a broad range of chemical and physical applications. Here, we use the fine fluorescence energy resolution of a RIXS spectrometer to obtain various proxies of the Mn L edge X ray absorption spectra XAS of the perovskite La0.6Sr0.4MnO3 LSMO as a model catalyst for the oxygen evolution reaction OER and evaluate the suitability for in situ surface studies of this electrocatalyst. We conclude that the inverse partial fluorescence yield IPFY of the O 2p 1s transition and the partial fluorescence yield of the 3s 2p transition 3s PFY are most suitable for determining changes at the surface of the perovskite, because distortions at grazing incidence measurements are low. In particular, the negligible angular dependence of the 3s PFY spectra can be perfectly simulated using a fluorescence model in the thin sample limit which is justified by low reabsorption of 3s photons. Remarkably, the 3s PFY reveals an influence of water vapor on the electronic reconstruction of the LSMO surface. Thus, our work paves the road for quantitative distortion free X ray spectroscopy of transition metal oxide surfaces under in situ conditions, which is needed to understand fundamental chemical processes such as corrosion and catalysis

Herein we investigate the reaction intermediates formed during CO oxidation on copper-substituted ceria nanoparticles (Cu0.1Ce0.9O2-x) by means of in situ spectroscopic techniques and identify an activity descriptor that rationalizes a trend with other metal substitutes (M0.1Ce0.9O2-x, M = Mn, Fe, Co, Ni). In situ X-ray absorption spectroscopy (XAS) performed under catalytic conditions demonstrates that O2-transfer occurs at dispersed copper centers, which are redox active during catalysis. In situ XAS reveals a dramatic reduction at the copper centers that is fully reversible under catalytic conditions, which rationalizes the high catalytic activity of Cu0.1Ce0.9O2-x. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) show that CO can be oxidized to CO32- in the absence of O2. We find that CO32- desorbs as CO2 only under oxygen-rich conditions when the oxygen vacancy is filled by the dissociative adsorption of O2. These data, along with kinetic analyses, lend support to a mechanism in which the breaking of copper-oxygen bonds is rate-determining under oxygen-rich conditions, while refilling the resulting oxygen vacancy is ratedetermining under oxygen-lean conditions. On the basis of these observations and density functional calculations, we introduce the computed oxygen vacancy formation energy (Evac) as an activity descriptor for substituted ceria materials and demonstrate that Evac successfully rationalizes the trend in the activities of M0.1Ce0.9O2-x catalysts that spans three orders of magnitude. The applicability of Evac as a useful design descriptor is demonstrated by the catalytic performance of the ternary oxide Cu0.1La0.1Ce0.8O2-x, which has an apparent activation energy rivaling those of state-of-the-art Au/TiO2 materials. Thus, we suggest that cost-effective catalysts for CO oxidation can be rationally designed by judicious choice of substituting metal through the computational screening of Evac.

The development of hybrid devices for photo-driven water oxidation with dinuclear molecular ruthenium catalysts on solid supports aims both at high efficiencies of the catalyst and at high stability of the linking to the surface. We herein report a systematic study regarding the stability of catalysts anchored on an indium tin oxide (ITO) support via different acid functional groups at the ligand backbone: viz., a single carboxylate anchor, two carboxylate anchors, and a phosphonate anchor. The integrity of the hybrid electrodes ITO|mesoITO|catalyst was evaluated under acidic aqueous conditions as a function of the applied electric potential below and above the onset of the oxygen evolution reaction (OER). Rotating ring disk electrode (RRDE) experiments allowed us to distinguish between the water oxidation and the desorption processes. X-ray photoemission (XPS) and X-ray absorption spectroscopy (XAS) after electrochemical treatment showed high chemical stability of the catalyst core structure but pronounc...

Manganese oxides with rich redox chemistry have been widely used in (electro)catalysis in applications of energy and environmental consequence. While they are ubiquitous in catalyzing the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), redox processes occurring on the surface of manganese oxides are poorly understood. We report valence changes at OER- and ORR-relevant voltages of a layered manganese oxide film prepared by electrodeposition. X-ray absorption spectra were collected in situ in O2-saturated 0.1 M KOH using inverse partial fluorescence yield (IPFY) at the Mn L3,2-edges and partial fluorescence yield (PFY) at the O K-edge. Overall, we found reversible yet hysteretic Mn redox and qualitatively reproducible spectral changes by Mn L3,2 IPFY XAS. Oxidation to a mixed Mn3+/4+ valence preceded the oxygen evolution at 1.65 V vs RHE, while manganese reduced below Mn3+ and contained tetrahedral Mn2+ during oxygen reduction at 0.5 V vs RHE. Analysis of the pre-edge in O K-edge XAS pr...

Oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) electrocatalysts including carbon-, non-precious metal-, metal alloy-, metal oxide-, and carbide/nitride-based materials are of great importance for energy conversion and storage technologies. Among them, metal oxides (e.g., perovskite and pyrochlore) are known to be promising candidates as electrocatalysts. Nevertheless, the intrinsic catalytic activities of pyrochlore oxides are still poorly understood because of the formation of undesirable phases derived from the synthesis processes. Herein, we present highly pure single crystalline pyrochlore nanoparticles with metallic conduction (Pb2Ru2O6.5) as an efficient bi-functional oxygen electrocatalyst. Notably, it has been experimentally shown that the covalency of Ru–O bonds affects the ORR and OER activities by comparing the X-ray absorption near edge structure (XANES) spectra of the metallic Pb2Ru2O6.5 and insulating Sm2Ru2O7 for the first time. Moreover, we followed the interatomic distance changes of Ru–O bonds using in situ X-ray absorption spectroscopy (XAS) to investigate the structural stabilities of the pyrochlore catalysts during electrocatalysis. The highly efficient metallic Pb2Ru2O6.5 exhibited outstanding bi-functional catalytic activities and stabilities for both ORR and OER in aqueous Zn–air batteries.

Chemical and structural changes preceding electrocatalysis obfuscate the nature of the active state of electrocatalysts for the oxygen evolution reaction (OER), which calls for model systems to gain systematic insight. We investigated the effect of bulk oxidation on the overpotential of ink‐casted LiMn2O4 electrodes by a rotating ring‐disk electrode (RRDE) setup and X‐ray absorption spectroscopy (XAS) at the K shell core level of manganese ions (Mn−K edge). The cyclic voltammogram of the RRDE disk shows pronounced redox peaks in lithium hydroxide electrolytes with pH between 12 and 13.5, which we assign to bulk manganese redox based on XAS. The onset of the OER is pH‐dependent on the scale of the reversible hydrogen electrode (RHE) with a Nernst slope of −40(4) mV/pH at −5 μA monitored at the RRDE ring. To connect this trend to catalyst changes, we develop a simple model for delithiation of LiMn2O4 in LiOH electrolytes, which gives the same Nernst slope of delithiation as our experimental data, i. e., 116(25) mV/pH. From this data, we construct an ERHE‐pH diagram that illustrates robustness of LiMn2O4 against oxidation above pH 13.5 as also verified by XAS. We conclude that manganese oxidation is the origin of the increase of the OER overpotential at pH lower than 14 and also of the pH dependence on the RHE scale. Our work highlights that vulnerability to transition metal redox may lead to increased overpotentials, which is important for the design of stable electrocatalysts., LiMn2O4 oxidizes by delithiation in LiOH at pH values below 14, as confirmed by X‐ray absorption spectroscopy. This chemical modification reduces the catalytic activity for oxygen evolution and induces a pH dependence on the reversible hydrogen electrode scale. The results indicate that vulnerability to transition‐metal redox reactions may lead to increased overpotentials, which is important for the design of stable electrocatalysts.

Chemical and structural changes preceding electrocatalysis obfuscate the nature of the active state of electrocatalysts for the oxygen evolution reaction OER , which calls for model systems to gain systematic insight. We investigated the effect of bulk oxidation on the overpotential of ink casted LiMn2O4 electrodes by a rotating ring disk electrode RRDE setup and X ray absorption spectroscopy XAS at the K shell core level of manganese ions Mn K edge . The cyclic voltammogram of the RRDE disk shows pronounced redox peaks in lithium hydroxide electrolytes with pH between 12 and 13.5, which we assign to bulk manganese redox based on XAS. The onset of the OER is pHdependent on the scale of the reversible hydrogen electrode RHE with a Nernst slope of 40 4 mV pH at 5 amp; 956;A monitored at the RRDE ring. To connect this trend to catalyst changes, we develop a simple model for delithiation of LiMn2O4 in LiOH electrolytes, which gives the same Nernst slope of delithiation as our experimental data, i.e., 116 25 mV pH. From this data, we construct an ERHE pH diagram that illustrates robustness of LiMn2O4 against oxidation above pH 13.5 as also verified by XAS. We conclude that manganese oxidation is the origin of the increase of the OER overpotential at pH lower than 14 and also of the pH dependence on the RHE scale. Our work highlights that vulnerability to transition metal redox may lead to increased overpotentials, which is important for the design of stable electrocatalysts

Electrochemical energy storage by making H2 an energy carrier from water splitting relies on four elementary reactions, i.e., the hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein, the central objective is to recommend systematic protocols for activity measurements of these four reactions and benchmark activities for comparison, which is critical to facilitate the research and development of catalysts with high activity and stability. Details for the electrochemical cell setup, measurements, and data analysis used to quantify the kinetics of the HER, HOR, OER, and ORR in acidic and basic solutions are provided, and examples of state-of-the-art specific and mass activity of catalysts to date are given. First, the experimental setup is discussed to provide common guidelines for these reactions, including the cell design, reference electrode selection, counter electrode concerns, and working electrode preparation. Second, experimental protocols, including data collection and processing such as ohmic- and background-correction and catalyst surface area estimation, and practice for testing and comparing different classes of catalysts are recommended. Lastly, the specific and mass activity activities of some state-of-the-art catalysts are benchmarked to facilitate the comparison of catalyst activity for these four reactions across different laboratories. Ministry of Education (MOE) National Research Foundation (NRF) C.W. and R.R.R. contributed equally to this work. The authors thank Jin Suntivich for the fruitful discussion. This work was supported in part by the Skoltech-MIT Center for Electrochemical Energy and the Cooperative Agreement between the Masdar Institute, Abu Dhabi, UAE and the MIT, Cambridge, MA, USA (02/MI/MIT/CP/11/07633/GEN/G/00). This work was supported in part by the Toyota Research Institute through the Accelerated Materials Design and Discovery program. C.W. and Z.J.X. thank the financial support by the Singapore Ministry of Education Tier 2 Grant (MOE2017-T2-1-009) and the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme through the Cambridge Centre for Advanced Research and Education in Singapore (CARES). I.E.L.S. gratefully acknowledges the Department of Mechanical Engineering at Massachusetts Institute of Technology for awarding him the Peabody Visiting Associate Professorship.

We investigate LiMn2O4 as a model catalyst for the oxygen evolution reaction (OER), because it shares the cubane structure with the active site of photosystem II. Specifically, we study the influence of different particle sizes of LiMn2O4 on the OER in a sodium hydroxide electrolyte. The product currents of manganese corrosion and oxygen evolution were obtained by rotating ring disk electrodes (RRDE). Physical characterization by various methods supports identical surface chemistry and microstructure of the pristine powders. We obtained similar oxygen current densities of 40(14) μA cmECSA−2 and 26(5) μA cmECSA−2 for micro- and nano-sized particles at 1.68 V vs. RHE. However, the total current densities differed drastically and while the micro-powder had a high disk current density of 205(2) μA cmECSA−2, its faradaic efficiency was only 25%. In contrast, the faradaic efficiency of the nanopowder was at least 75%. We hypothesize that a Mn redox process may occur in the bulk in parallel and possibly in combination with oxygen evolution on the surface based on the observed difference between the total and product current densities. Knowledge of the product currents is crucial for distinguishing the mechanisms of corrosion and catalysis and for designing better catalysts with high faradaic efficiency. peerReviewed

Discovery of electrocatalytic materials for high-performance energy conversion and storage applications relies on the adequate characterization of their intrinsic activity, which is currently hindered by the dearth of a protocol for consistent and precise determination of double layer capacitance (C DL). Herein, we propose a seven-step method that aims to determine C DL reliably by scan rate-dependent cyclic voltammetry considering aspects that strongly influence the outcome of the analysis, including (a) selection of a suitable measuring window, (b) the uncompensated resistance, (c) optimization of measuring settings, (d) data acquisition, (e) selection of data suitable for analysis, (f) extraction of the desired information, and (g) validation of the results. To illustrate the proposed method, two systems were studied: a resistor–capacitor electric circuit, and a glassy carbon disk in an electrochemical cell. With these studies, it is demonstrated that when any of the mentioned steps of the procedure are neglected, substantial deviations of the results are observed with misestimations as large as 61% in the case of the investigated electrochemical system. Moreover, we propose allometric regression as a more suitable model than linear regression for the determination of C DL for both the ideal and the non-ideal systems investigated. We stress the importance of assessing the accuracy of not only highly specialized electrochemical methods, but also of those that are well-known and commonly used as it is the case of the voltammetric methods. The procedure proposed herein is not limited to the determination of C DL, but can be effectively applied to any other analysis that aims to deliver quantitative results via voltammetric methods, which is crucial for the study of kinetic and diffusion phenomena in electrochemical systems.

Transition-metal oxides with the perovskite structure are promising catalysts to promote the kinetics of the oxygen evolution reaction (OER). To improve the activity and stability of these catalysts, a deeper understanding about the active site, the underlying reaction mechanism, and possible side reactions is necessary. We chose smooth epitaxial (100)-oriented La0.6Sr0.4MnO3 (LSMO) films grown on Nb:SrTiO3 (STNO) as a model electrode to investigate OER activity and stability using the rotating ring−disk electrode (RRDE) method. Careful electrochemical characterization of various films in the thickness range between 10 and 200 nm yields an OER activity of the epitaxial LSMO surface of 100 μA/cm2ox at 1.65 V vs RHE, which is among the highest reported for LSMO and close to (110)-oriented IrO2. Detailed post-mortem analysis using XPS, XRD, and AFM revealed the high structural and morphological stability of LSMO after OER. The observed correlation between activity and Mn vacancies on the surface suggested Mn...

O2 reduction in aprotic Na–O2 batteries results in the formation of NaO2, which can be oxidized at small overpotentials (

Highly efficient bifunctional oxygen electrocatalysts are indispensable for the development of highly efficient regenerative fuel cells and rechargeable metal-air batteries, which could power future electric vehicles. Although perovskite oxides are known to have high intrinsic activity, large particle sizes rendered from traditional synthesis routes limit their practical use due to low mass activity. We report the synthesis of nano-sized perovskite particles with a nominal composition of Lax(Ba[subscript 0.5]Sr[subscript 0.5])[subscript 1−x]Co[subscript 0.8]Fe[subscript 0.2]O[subscript 3−δ] (BSCF), where lanthanum concentration and calcination temperature were controlled to influence oxide defect chemistry and particle growth. This approach produced bifunctional perovskite electrocatalysts ∼50 nm in size with supreme activity and stability for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The electrocatalysts preferentially reduced oxygen to water (, MIT & Masdar Institute Cooperative Program (02/MI/MIT/CP/11/07633/GEN/G/00), MIT Skoltech Initiative

The sluggish kinetics of the oxygen reduction reaction (ORR) limit the efficiency of numerous oxygen-based energy conversion devices such as fuel cells and metal-air batteries. Among earth abundant catalysts, manganese-based oxides have the highest activities approaching that of precious metals. In this Review, we summarize and analyze literature findings to highlight key parameters that influence the catalysis of the ORR on manganese-based oxides, including the number of electrons transferred as well as specific and mass activities. These insights can help develop design guides for highly active ORR catalysts and shape future fundamental research to gain new knowledge regarding the molecular mechanism of ORR catalysis.

The ferri-/ferrocyanide redox couple is ubiquitous in many fields of physical chemistry. We studied its photochemical response to intense synchrotron radiation by in situ X-ray absorption spectroscopy (XAS). For photon flux densities equal to and above 2 × 1011 s–1 mm–2, precipitation of ferric (hydr)oxide from both ferricyanide and ferrocyanide solutions was clearly detectable, despite flowing fast enough to replace the solution in the flow cell every 0.4 s (flow rate 1.5 mL/min). During cyclic voltammetry, precipitation of ferric (hydr)oxide was promoted at reducing voltages and observed below 1011 s–1 mm–2. This was accompanied by inhibition of the ferri-/ferrocyanide redox, which we probed by time-resolved operando XAS. Our study highlights the importance of considering both electrochemical and spectroscopic conditions when designing in situ experiments.

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities of LiCoO2 nanorods with sizes in the range from 9 to 40 nm were studied in alkaline solution. The sides of these nanorods were terminated with low-index surfaces such as (003), while the tips were terminated largely with high-index surfaces such as (104), as revealed by high-resolution transmission electron microscopy. Electron energy loss spectroscopy demonstrated that low-spin Co(3+) prevailed on the sides, while the tips exhibited predominantly high- or intermediate-spin Co(3+). We correlated the electronic and atomic structure to higher specific ORR and OER activities at the tips as compared to the sides, which was accompanied by more facile redox of Co(2+/3+) and higher charge transferred per unit area. These findings highlight the critical role of surface terminations and electronic structures of transition-metal oxides on the ORR and OER activity.