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Understanding how materials that catalyse the oxygen evolution reaction (OER) function is essential for the development of efficient energy-storage technologies. The traditional understanding of the OER mechanism on metal oxides involves four concerted proton-electron transfer steps on metal-ion centres at their surface and product oxygen molecules derived from water. Here, using in situ 18 O isotope labelling mass spectrometry, we provide direct experimental evidence that the O 2 generated during the OER on some highly active oxides can come from lattice oxygen. The oxides capable of lattice-oxygen oxidation also exhibit pH-dependent OER activity on the reversible hydrogen electrode scale, indicating non-concerted proton-electron transfers in the OER mechanism. Based on our experimental data and density functional theory calculations, we discuss mechanisms that are fundamentally different from the conventional scheme and show that increasing the covalency of metal-oxygen bonds is critical to trigger lattice-oxygen oxidation and enable non-concerted proton-electron transfers during OER.

Numerous studies have reported electronic activity descriptors of oxygen evolution reaction (OER) for oxide catalysts under a single reaction mechanism. However, recent works have revealed that a single mechanism is not at play across oxide chemistries. These works underscore a need to deeply investigate the electronic structure details of active oxide catalysts and how they align with the OER potential, which is critical to understanding the interfacial charge-transfer kinetics that dictate catalytic mechanisms. In this work, we use soft X-ray emission and absorption spectroscopy of perovskites to analyze the partial density of states on an absolute energy scale, from which energetic barriers for electron transfer and surface deprotonation were estimated and correlated with OER activity. Through this lens, we identify that decreasing the solid-state charge-transfer energy of perovskites can change the mechanisms of the OER from electron-transfer-limited to proton-electron-coupled, to proton-transfer-limited reactions. This concept is supported by the analysis of potential energy surfaces for sequential and concerted proton-electron transfer pathways using a Marcus model. Our work highlights the importance of understanding the physical origin of experimental OER activity trends with electronic descriptors and the need to promote surface deprotonation from oxides to discover new catalysts with enhanced activity.

Late transition metal phosphides have been reported to have high activity for catalyzing hydrogen evolution reaction (HER), yet their active site and stability are not well-understood. Here we report systematic activity and stability study of CoP for HER by combining electrochemical measurements for CoP nanoparticles (NPs) with ex situ and in situ synchrotron X-ray absorption (XAS) spectroscopy at phosphorus and cobalt K edges, as well as density functional theory (DFT) calculations. Colloidally synthesized CoP NPs showed high HER activity in both acid and base electrolytes, comparable to previous work, where no significant pH dependence was observed. Transmission electron microscopy-energy dispersive spectroscopy study of CoP NPs before and after exposure to potentials in the range from 0 to 1.4 V vs. the reversible hydrogen electrode (RHE) revealed that the P/Co ratio reduced with increasing potential in the potentiostatic measurements prior to HER measurements. The reduced P/Co ratio was accompanied with the emergence of (oxy)phosphate(s) as revealed by XAS, and reduced specific HER activity, suggesting the important role of P in catalyzing HER. This hypothesis was further supported by DFT calculations of HER on the most stable (011) surface of CoP and voltage dependent intensities of both phosphide and phosphate components from P-K edge X-ray spectroscopy. This work highlights the need of stabilizing metal phosphides and optimizing their surface P sites in order to realize the practical use of metal phosphides to catalyze HER in electrochemical and photoelectrochemical devices.

Understanding the structural, physical and chemical properties of the surface and interfaces of different metal-oxides and their possible applications in photo-catalysis and biology is a very important emerging research field. Motivated in this direction, this article would enable understanding of how different fluids, particularly water, interact with oxide surfaces. We have studied the water contact angle of 3d transition metal oxide thin films of SrTiO3, and of 4f rare-earth oxide thin films of Lu2O3. These metal oxides were grown using pulsed laser deposition and they are atomically flat and with known orientation and explicitly characterized for their structure and composition. Further study was done on the effects of oxygen vacancies on the water contact angle of the 3d and 4f oxides. For 3d SrTiO3 oxide with oxygen vacancies, we have observed an increase in hydroxylation with consequent increase of wettability which is in line with the previous reports whereas an interesting opposite trend was seen in the case of rare-earth Lu2O3 oxide. Density functional theory simulations of water interaction on the above mentioned systems have also been presented to further substantiate our experimental findings.

Understanding what controls Li-O2 battery discharge product chemistry and morphology is key to enabling its practical deployment as a low-cost, high-specific-energy energy conversion technology. Several studies have recently shown that the addition of substantial quantities (hundreds to thousands ppm) of water and weak acids to dimethoxyethane (DME)-based electrolytes can significantly increase Li-O2 battery discharge capacity, without substantially changing the discharge product chemistry, which remains Li2O2. The exact mechanisms behind these device-level improvements, however, are not yet understood. In this study, we show that the presence of water in a DME-based electrolyte decreases the rate of Li2O2 nucleation on the electrode surface during Li-O2 battery discharge, using potentiostatic electrochemical measurements, and direct, ex situ observations of Li2O2 particles. We also show that adding water to an acetonitrile (MeCN)-based electrolyte results in LiOH upon discharge, as opposed to only Li2O2. Using first principles calculations, we propose that this change in discharge product chemistry is attributable to increased proton availability, as shown by a lower pKa for water in MeCN than in DME. This study combines kinetic and morphological analyses with first principles calculations, and elucidates relationships among electrolyte composition, discharge product chemistry and growth mechanisms for the rational design of efficient metal-air batteries.

Understanding the mechanism of the oxygen evolution reaction (OER) is essential to develop better electrocatalysts for solar fuel generation. Measuring the pH dependence of the OER activity can provide insights on the reaction path that are otherwise difficult to access experimentally, in particular on the coupling of protons and electrons during the reaction. We argue that the use of a pH-dependent reference electrode, such as the reversible hydrogen electrode, is more suitable for these studies as it assures that the overpotential is fixed while varying the pH. We provide criteria for pH dependence when this reference is used and validate the existing results with our measurements on RuO2 powders. A statistical analysis of the existing results allows us to sketch trends in the reaction order on pH with respect to the number of d electrons, oxidation states, and crystal families, providing the groundwork for future OER mechanistic studies on oxides.

The active phase responsible for low-temperature CO oxidation in nanoparticulate CuO/CeO2 catalysts was identified as surface-substituted CuyCe1-yO2-x. Contrary to previous studies, our measurements on a library of well-defined CuO/CeO2 catalysts have proven that the CuO phase is a spectator species, whereas the surface-substituted CuyCe1-yO2-x phase is active for CO oxidation. Using in situ X-ray absorption spectroscopy, we found that the copper ions in CuyCe1-yO2-x remain at high oxidation states (Cu3+ and Cu2+) under oxygen-rich catalytic conditions without any evidence for Cu+. Artificial neural network potential Monte Carlo simulations suggest that Cu3+ and Cu2+ preferentially segregate to the {100} surface of the CuyCe1-yO2-x nanoparticle, which is supported by aberration-corrected electron microscopy measurements. These results pave the way for understanding, at the atomic level, the mechanisms and descriptors pertinent for CO oxidation on these materials and hence the rational design of next-generation catalysts.

This Review is focused on ion-transport mechanisms and fundamental properties of solid-state electrolytes to be used in electrochemical energy-storage systems. Properties of the migrating species significantly affecting diffusion, including the valency and ionic radius, are discussed. The natures of the ligand and metal composing the skeleton of the host framework are analyzed and shown to have large impacts on the performance of solid-state electrolytes. A comprehensive identification of the candidate migrating species and structures is carried out. Not only the bulk properties of the conductors are explored, but the concept of tuning the conductivity through interfacial effects-specifically controlling grain boundaries and strain at the interfaces-is introduced. High-frequency dielectric constants and frequencies of low-energy optical phonons are shown as examples of properties that correlate with activation energy across many classes of ionic conductors. Experimental studies and theoretical results are discussed in parallel to give a pathway for further improvement of solid-state electrolytes. Through this discussion, the present Review aims to provide insight into the physical parameters affecting the diffusion process, to allow for more efficient and target-oriented research on improving solid-state ion conductors.

Oxides are instrumental to applications such as catalysis, sensing, and wetting, where the reactivity with water can greatly influence their functionalities. We find that the coverage of hydroxyls (∗OH) measured at fixed relative humidity trends with the electron-donor (basic) character of wetted perovskite oxide surfaces. Using ambient pressure X-ray photoelectron spectroscopy, we report that the affinity toward hydroxylation, coincident with strong adsorption energies calculated for dissociated water and hydroxyl groups, leads to strong H bonding that is favorable for wetting while detrimental to catalysis of the oxygen reduction reaction (ORR). Our findings provide novel insights into the coupling between wetting and catalytic activity and suggest that catalyst hydrophobicity should be considered in aqueous oxygen electrocatalysis.

Understanding reactions at the electrode/electrolyte interface (EEI) is essential to developing strategies to enhance cycle life and safety of lithium batteries. Despite research in the past four decades, there is still limited understanding by what means different components are formed at the EEI and how they influence EEI layer properties. We review findings used to establish the well-known mosaic structure model for the EEI (often referred to as solid electrolyte interphase or SEI) on negative electrodes including lithium, graphite, tin, and silicon. Much less understanding exists for EEI layers for positive electrodes. High-capacity Li-rich layered oxides yLi2-xMnO3·(1-y)Li1-xMO2, which can generate highly reactive species toward the electrolyte via oxygen anion redox, highlight the critical need to understand reactions with the electrolyte and EEI layers for advanced positive electrodes. Recent advances in in situ characterization of well-defined electrode surfaces can provide mechanistic insights and strategies to tailor EEI layer composition and properties.

We present a simple and generalizable synthetic route toward phase-pure, monodisperse transition-metal-substituted ceria nanoparticles (M0.1Ce0.9O2-x, M = Mn, Fe, Co, Ni, Cu). The solution-based pyrolysis of a series of heterobimetallic Schiff base complexes ensures a rigorous control of the size, morphology and composition of 3 nm M0.1Ce0.9O2-x crystallites for CO oxidation catalysis and other applications. X-ray absorption spectroscopy confirms the dispersion of aliovalent (M3+ and M2+) transition metal ions into the ceria matrix without the formation of any bulk transition metal oxide phases, while steady-state CO oxidation catalysis reveals an order of magnitude increase in catalytic activity with copper substitution. Density functional calculations of model slabs of these compounds confirm the stabilization of M3+ and M2+ in the lattice of CeO2. These results highlight the role of the host CeO2 lattice in stabilizing high oxidation states of aliovalent transition metal dopants that ordinarily would be intractable, such as Cu3+, as well as demonstrating a rational approach to catalyst design. The current work demonstrates, for the first time, a generalizable approach for the preparation of transition-metal-substituted CeO2 for a broad range of transition metals with unparalleled synthetic control and illustrates that Cu3+ is implicated in the mechanism for CO oxidation on CuO-CeO2 catalysts.

Understanding electrochemical and chemical reactions at the electrode-electrolyte interface is of fundamental importance for the safety and cycle life of Li-ion batteries. Positive electrode materials such as layered transition metal oxides exhibit different degrees of chemical reactivity with commonly used carbonate-based electrolytes. Here we employed density functional theory methods to compare the energetics of four different chemical reactions between ethylene carbonate (EC) and layered (LixMO2) and rocksalt (MO) oxide surfaces. EC dissociation on layered oxides was found energetically more favorable than nucleophilic attack, electrophilic attack, and EC dissociation with oxygen extraction from the oxide surface. In addition, EC dissociation became energetically more favorable on the oxide surfaces with transition metal ions from left to right on the periodic table or by increasing transition metal valence in the oxides, where higher degree of EC dissociation was found as the Fermi level was lowered into the oxide O 2p band.

Catalysts for chemical and electrochemical reactions underpin many aspects of modern technology and industry, from energy storage and conversion to toxic emissions abatement to chemical and materials synthesis. This role necessitates the design of highly active, stable, yet earth-abundant heterogeneous catalysts. In this Review, we present the perovskite oxide family as a basis for developing such catalysts for (electro)chemical conversions spanning carbon, nitrogen, and oxygen chemistries. A framework for rationalizing activity trends and guiding perovskite oxide catalyst design is described, followed by illustrations of how a robust understanding of perovskite electronic structure provides fundamental insights into activity, stability, and mechanism in oxygen electrocatalysis. We conclude by outlining how these insights open experimental and computational opportunities to expand the compositional and chemical reaction space for next-generation perovskite catalysts.