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Direct methanol fuel cells (DMFCs) using alkaline electrolytes are of interest due to the applicability of nonprecious metal-based materials for electrocatalysts. However, the lack of understanding of the methanol oxidation reaction (MOR) mechanism in alkaline media hinders the development of active catalysts for the MOR. In this work, ambient-pressure XPS and in situ surface-enhanced infrared spectroscopy were performed on the Pt surface in order to gain experimental insights into the reaction pathway for the MOR. We present a comprehensive reaction mechanism for the MOR in alkaline media and show that the MOR proceeds via two different pathways depending on the electrode potential. We confirmed the formation of partially hydrogenated CO adsorbates [HxCOad···(OH) (1 < x < 3)] via water and/or hydroxide ion-mediated dissociation of methanol. The HxCOad···(OH) species were further dehydrogenated to COad in the potential range of 0.40-0.60 VRHE and subsequently oxidized to CO2 by reactive OHad on the Pt surface at 0.65 VRHE (pathway I). Furthermore, H3C-Oad intermediates were observed at potentials higher than 0.9 VRHE, at which the MOR proceeds mainly via H3C-Oad instead of COad intermediates (pathway II). The oxidation current related to this conversion from H3C-Oad to CO2 (pathway II) dominates the overall MOR current, suggesting that the H3C-Oad pathway could be one of the keys to enhancing the MOR activity in an alkaline environment. Our findings pave the way toward a design strategy for MOR electrocatalysts with improved activity based on the experimental reaction mechanisms that have been identified.

Proton activity at the electrified interface is central to the kinetics of proton-coupled electron transfer (PCET) reactions for making chemicals and fuels. Here we employ a library of protic ionic liquids in an interfacial layer on platinum and gold to alter local proton activity, where the intrinsic oxygen-reduction reaction (ORR) activity is enhanced up to fivefold, exhibiting a volcano-shaped dependence on the pKa of the ionic liquid. The enhanced ORR activity is attributed to strengthened hydrogen bonds between ORR products and ionic liquids with comparable pKas, resulting in favourable PCET kinetics. This proposed mechanism is supported by in situ surface-enhanced Fourier-transform infrared spectroscopy and our simulation of PCET kinetics based on computed proton vibrational wavefunctions at the hydrogen-bonding interface. These findings highlight opportunities for using non-covalent interactions between hydrogen-bonded structures and solvation environments at the electrified interface to tune the kinetics of ORR and beyond. Understanding the role of hydrogen bonds at the electrode interface is important for controlling the kinetics of the oxygen-reduction reaction. Here the authors modify gold and platinum surfaces with a series of protic ionic liquids to show that pKa can be used to optimize proton-coupled electron transfer through hydrogen bonding.

Generating charge carriers with lifetimes long enough to drive catalysis is a critical aspect for photoelectrochemical and photocatalytic systems, and a key determinant of their efficiency. Metal oxides are widely explored as photoanodes for photoelectrochemical water oxidation. However, their application is limited by the disparity between the picosecond–nanosecond lifetimes of electrons and holes photoexcited in bulk metal oxides versus the millisecond–second timescale of water oxidation catalysis. This Review addresses the charge-carrier dynamics underlying the performance of metal oxide photoanodes and their ability to drive photoelectrochemical water oxidation, alongside comparison with metal oxide function in photocatalytic and electrocatalytic systems. We assess the dominant kinetic processes determining photoanode performance, namely, charge generation, polaron formation and charge trapping, bulk and surface recombination, charge separation and extraction, and, finally, the kinetics of water oxidation catalysis. We examine approaches to enhance performance, including material selection, doping, nanostructuring, junction formation and/or co-catalyst deposition. Crucially, we examine how such performance enhancements can be understood from analyses of carrier dynamics and propose design guidelines for further material or device optimization. Metal oxides are widely used in photoelectrochemical and photocatalytic systems for fuel synthesis and environmental remediation. In this Review, we examine the kinetic challenges, from charge generation to water oxidation catalysis, that determine the performance of metal oxide photo(electro)catalysts.

Noncovalent interactions at electrified interfaces are key to improving activity for the oxygen evolution reaction (OER). Here, we showed that on RuO2(110) in alkaline solutions, OER activity is ca...

The kinetics of aqueous outer-sphere electron-transfer (ET) reactions are determined in large part by noncovalent electrostatic interactions that originate from the surrounding electrolyte solution...

Metal oxides and oxyhydroxides exhibit state-of-the-art activity for the oxygen evolution reaction (OER); however, their reaction mechanism, particularly the relationship between charging of the oxide and OER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides using operando UV-vis spectroscopy coupled with time-resolved stepped potential spectroelectrochemistry. The Ni2+/Ni3+redox peak potential is found to shift anodically from Mn- < Co- < Fe- < Zn-doped samples, suggesting a decrease in oxygen binding energetics from Mn- to Zn-doped samples. At OER-relevant potentials, using optical absorption spectroscopy, we quantitatively detect the subsequent oxidation of these redox centers. The OER kinetics was found to have a second-order dependence on the density of these oxidized species, suggesting a chemical rate-determining step involving coupling of two oxo species. The intrinsic turnover frequency per oxidized species exhibits a volcano trend with the binding energy of oxygen on the Ni site, having a maximum activity of ∼0.05 s-1at 300 mV overpotential for the Fe-doped sample. Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited by O-O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limited by the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energy and intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OER kinetics.

Understanding the nature of active sites is central to controlling (electro)catalytic activity. Here we employed surface X-ray scattering coupled with density functional theory and surface-enhanced infrared absorption spectroscopy to examine the oxygen evolution reaction on RuO2 surfaces as a function of voltage. At 1.5 VRHE, our results suggest that there is an –OO group on the coordinatively unsaturated ruthenium (RuCUS) site of the (100) surface (and similarly for (110)), but adsorbed oxygen on the RuCUS site of (101). Density functional theory results indicate that the removal of –OO from the RuCUS site, which is stabilized by a hydrogen bond to a neighbouring –OH (–OO–H), could be the rate-determining step for (100) (similarly for (110)), where its reduced binding on (100) increased activity. A further reduction in binding energy on the RuCUS site of (101) resulted in a different rate-determining step (–O + H2O – (H+ + e−) → –OO–H) and decreased activity. Our study provides molecular details on the active sites, and the influence of their local coordination environment on activity. Understanding the nature of active sites is central to controlling the activity of a given catalyst. This work combines operando characterization and computational techniques to examine the oxygen evolution reaction mechanism on RuO2 surfaces.

Complex oxides, such as ABO3 perovskites, are an important class of functional materials that exhibit a wide range of physical, chemical, and electrochemical properties, including high oxygen electrocatalytic activity, tunable electronic/ionic conductivity, and ferroelectricity. When complex oxides are engineered as thin films, their chemical and physical properties can be modified to be markedly different from their bulk form, providing additional degrees of freedom in materials design. In this review, we survey the landscape of strain-induced design of complex oxides in the context of oxygen electrocatalysis and ferroelectricity. First, we identify the role of strain in influencing oxide electronic properties, driven by the combination of modification of Bsingle bondO bond length and octahedral distortion in perovskites. We describe electronic structure parameters, such as the O 2p-band center, that quantitatively capture these electronic changes, highlighting the broad influence of the O 2p-band center on surface reactivity (oxygen adsorption and dissociation energy) and bulk defect energetics (oxygen vacancy formation and migration energy). Motivated by the promise of the influence of strain on material properties relevant for oxygen electrocatalysis and ferroelectricity, we describe the advances in state-of-the-art thin-film fabrication and characterization that have enabled a high degree of experimental control in realizing strain effects in oxide thin-film systems. In oxygen electrocatalysis, leveraging strain has not only resulted in activity enhancements relative to bulk unstrained material systems but also revealed mechanistic influences of oxide phenomena, such as bulk defect chemistry and transfer kinetics, on electrochemical processes. Similarly for ferroelectric properties, strain engineering can both enhance polarization in known ferroelectrics and induce ferroelectricity in material systems that would be otherwise non-ferroelectric in bulk. Based on understanding of a diverse range of perovskite functionalities, we offer perspectives on how further coupling of strain, oxygen electrocatalysis, and ferroelectricity opens up pathways toward the emergence of novel device design features with dynamic control of increasing complex chemical and high-performance electronic processes. Ministry of Education (MOE) Nanyang Technological University Accepted version This work was supported in part by the Skoltech-MIT Center for Electrochemical Energy. Z. F. acknowledges startup funding from Oregon State University. X.R.W. acknowledges supports from the Nanyang Assistant Professorship grant from Nanyang Technological University and Academic Research Fund Tier 1 (RG108/17 and RG177/18) from Singapore Ministry of Education, Singapore. K.A.S. was supported in part by the Linus Pauling Distinguished Post-Doctoral Fellowship Pacific Northwest National Laboratory (PNNL, Laboratory Directed Research and Development Program 69319). PNNL is a multiprogram national laboratory operated for DOE by Battelle. D. L. was supported by Advanced Support Program for Innovative Research ExcellenceI funding (#15540-18-47811) provided by the Office of the Vice President for Research at the University of South Carolina. R.J. and D.M. were supported by the National Science Foundation (NSF) Software Infrastructure for Sustained Innovation (SI2) award No. 1148011.

Hydrous iridium oxides (IrOx) are the best oxygen evolution electrocatalysts available for operation in acidic environments. In this study, we employ time-resolved operando spectroelectrochemistry to investigate the redox-state kinetics of IrOx electrocatalyst films for both water and hydrogen peroxide oxidation. Three different redox species involving Ir3+, Ir3.x+, Ir4+, and Ir4.y+ are identified spectroscopically, and their concentrations are quantified as a function of applied potential. The generation of Ir4.y+ states is found to be the potential-determining step for catalytic water oxidation, while H2O2 oxidation is observed to be driven by the generation of Ir4+ states. The reaction kinetics for water oxidation, determined from the optical signal decays at open circuit, accelerates from ∼20 to

Obtaining molecular scale information about the water oxidation mechanism is challenging. In Nature, using experimental and theoretical tools, Nong et al. determine that the rate-determining step on state-of-the-art iridium dioxide involves the chemical O-O bond formation between a water molecule and adsorbed oxygen; the accumulation of oxidized states reduces the barrier for this step.

Hydrous iridium oxides (IrOx) are the best oxygen evolution electrocatalysts available for operation in acidic environments. In this study, we employ time-resolved operando spectroelectrochemistry to investigate the redox states kinetics of IrOx electrocatalyst films for both water and hydrogen peroxide oxidation. Three different redox species involving Ir3+, Ir4+ and Ir4.x are identified spectroscopically and their concentrations are quantified as a function of applied potential. The generation of Ir4.x+ states is found to be the potential determining step for catalytic water oxidation, whilst H2O2 oxidation is observed to be driven by the generation of Ir4+ states. The reaction kinetics for water oxidation, determined from the optical signal decays at open circuit, accelerate from ~ 20 s to < 0.5 s with increasing applied potential above 1.3V vs. RHE (i.e. TOFs per active Ir state increasing from 0.05 to 2 s-1). In contrast, the reaction kinetics for H2O2 are found to be almost independent of the applied potential (increasing from 0.1-0.3 s-1 over a wider potential window), indicative of a first order reaction mechanism. These spectroelectrochemical data quantify the increase of both the density of active Ir4.x+ states and the TOFs of these states with applied positive potential, resulting in the observed sharp turn on of catalytic water oxidation current. We reconcile these data with the broader literature while providing a new kinetic insight into IrOx electrocatalytic reaction mechanisms, indicating a first order reaction mechanism for H2O2 oxidation driven by Ir4+ states, and a higher order reaction mechanism involving the co-operative interaction of multiple Ir4.x+ states for water oxidation.

European Patent Application Number EP20382294, a patent application by A. B. and J. L.-F. was previously filed for the intellectual property described in this article The oxygen evolution reaction (OER) is a fundamental process to develop a technology that can drive energy transition towards renewable and sustainable fuels. Nevertheless, efficient and straightforward methodologies to obtain superior and stable electrodes need to be implemented to approach this technology to real applications. Recently, self-supported catalysis emerged as a promising solution. However, catalyst design is still limited by the low chemical tunability and elevated preparation times and costs. Herein, a solution combustion (SC) methodology is described to produce designed self-supported electrocatalysts that excel in the OER and mitigate previous limitations. M-doped NiO-based electrocatalysts (with M = Fe, Co, Mn, and Zn) were self-supported by the SC method on nickel foam, and overperformed analogous benchmarked catalysts prepared by other methods. Notably, in Fe-doped NiO, the overpotential required to drive the OER at 10 mA cm−2 was found to be 190 mV, the lowest reported so far for metal oxide electrocatalysts at pH 13. By the combination of spectroelectrochemical (SEC) and electrochemical impedance spectroscopy (EIS), we studied the role of the metal dopant cation, showing that dopant metals assist the formation of the active species responsible for the high (electro)catalytic activity. We envision that the presented simple, cost-time efficient methodology would stimulate the preparation and study of effective self-supported metal-oxide catalysts for a broad range of applications.

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.

Understanding the adsorption and oxidation of NO on metal oxides is of immense interest to environmental and atmospheric (bio)chemistry. Here, we show that the surface oxygen activity, defined as the oxygen 2p-band centre relative to the Fermi level, dictates the adsorption and surface coverage of NOx and the kinetics of NO oxidation for La1−xSrxCoO3 perovskites. Density functional theory and ambient-pressure X-ray photoelectron spectroscopy revealed favourable NO adsorption on surface oxygen sites. Increasing the surface oxygen activity by increasing the strontium substitution led to stronger adsorption and greater storage of NO2, which resulted in more adsorbed nitrogen-like species and molecular nitrogen formed upon exposure to CO. The NO oxidation kinetics exhibited a volcano trend with surface oxygen activity, centred at La0.8Sr0.2CoO3 and with an intrinsic activity comparable to state-of-the-art catalysts. We rationalize the volcano trend by showing that increasing the NO adsorption enhances the oxidation kinetics, although NO adsorption that is too strong poisons the surface oxygen sites with adsorbed NO2 to impede the kinetics. Understanding the mechanism for the catalytic conversion of NOx is crucial to develop superior greenhouse gas abatement schemes, although it remains challenging. Here, the authors reveal important aspects of the redox properties of NOx on a La1–xSrxCoO3 perovskite by a combination of density functional theory calculations and ambient-pressure X-ray photoelectron spectroscopy.

Cobalt-containing perovskite oxides are promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolyzers. However, a lack of fundamental understanding of oxide surfaces impedes rational catalyst design for improved activity and stability. We couple electrochemical studies of epitaxial La1−xSrxCoO3−δ films with in situ and operando ambient pressure X-ray photoelectron spectroscopy to investigate the surface stoichiometry, adsorbates, and electronic structure. In situ investigations spanning electrode compositions in a humid environment indicate that hydroxyl and carbonate affinity increase with Sr content, leading to an increase in binding energy of metal core levels and the valence band edge from the formation of a surface dipole. The maximum in hydroxylation at 40% Sr is commensurate with the highest OER activity, where activity scales with greater hole carrier concentration and mobility. Operando measurements of the 20% Sr-doped oxide in alkaline electrolyte indicate that the surface stoichiometry remains constant during OER, supporting the idea that the oxide electrocatalyst is stable and behaves as a metal, with the voltage drop confined to the electrolyte. Furthermore, hydroxyl and carbonate species are present on the electrode surface even under oxidizing conditions, and may impact the availability of active sites or the binding strength of adsorbed intermediates via adsorbate–adsorbate interactions. For covalent oxides with facile charge transfer kinetics, the accumulation of hydroxyl species with oxidative potentials suggests the rate of reaction could be limited by proton transfer kinetics. This operando insight will help guide modeling of self-consistent oxide electrocatalysts, and highlights the potential importance of carbonates in oxygen electrocatalysis.

Understanding the interaction of CO2 with perovskite metal oxide surfaces is crucial for the design of various perovskite (electro)chemical functionalities, such as solid oxide fuel cells, catalyti...

The reduction and oxidation (redox) of transition metals allow storing charge/energy in Li-ion batteries and electrochemical capacitors and play an important role in catalysis of electrochemical reactions, such as oxygen reduction reaction (ORR) in fuel cells and metal-air batteries and oxygen evolution reaction (OER) in electrolytic cells. In this review, we present and discuss a universal origin of inductive effect associated with metal substitution in Ni, Co, Fe, Mn-based complexes, (hydr-)oxides, and lithium intercalation compounds by alignment of the electron levels of metal complex redox with partially filled metal d-state and oxygen p-state of oxides on the absolute energy scale. Increased redox potentials of metal complexes and oxides are shown to correlate with the increased electronegativity of the substituting metal, which results in the enhancement of the ORR/OER activity. Such observations provide new insights into potential strategies to optimize ORR/OER catalytic activity by tuning the redox properties of metal sites.

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.

Summary Rutile RuO 2 is known to exhibit high catalytic activity for the oxygen evolution reaction (OER) and large pseudocapacitance associated with redox of surface Ru; however, the mechanistic link between these properties and the role of pH is yet to be understood. Here, we report that the OER activities of the (101), (001), and (111) RuO 2 surfaces increased, whereas the potential of a pseudocapacitive feature just before OER shifted to lower potentials ("super-Nernstian" shift) as pH increased on the reversible hydrogen electrode scale. This behavior contrasts with that of the (100) and (110) surfaces, which showed pH-independent Ru redox and OER activity. The link between catalytic and pseudocapacitive behavior illustrates the importance of this redox feature in generating active sites, thus building new mechanistic understanding of the OER.

The CO2 electroreduction reaction (CO2RR) is a promising avenue to convert greenhouse gases into high-value fuels and chemicals, in addition to being an attractive method for storing intermittent renewable energy. Although polycrystalline Cu surfaces have long been known to be unique in their capabilities of catalyzing the conversion of CO2 to higher-order C1 and C2 fuels, such as hydrocarbons (CH4, C2H4, etc.) and alcohols (CH3OH, C2H5OH), product selectivity remains a challenge. Rational design of more selective catalysts would greatly benefit from a mechanistic understanding of the complex, multiproton, and multielectron conversion of CO2. In this study, we select three metal catalysts (Pt, Au, Cu) and apply in situ surface enhanced infrared absorption spectroscopy (SEIRAS) and ambient-pressure X-ray photoelectron spectroscopy (APXPS), coupled to density-functional theory (DFT) calculations, to get insight into the reaction pathway for the CO2RR. We present a comprehensive reaction mechanism for the CO2RR and show that the preferential reaction pathway can be rationalized in terms of metal-carbon (M-C) and metal-oxygen (M-O) affinity. We show that the final products are determined by the configuration of the initial intermediates, C-bound and O-bound, which can be obtained from CO2 and (H)CO3, respectively. C1 hydrocarbons are produced via OCH3,ad intermediates obtained from O-bound CO3,ad and require a catalyst with relatively high affinity for O-bound intermediates. Additionally, C2 hydrocarbon formation is suggested to result from the C-C coupling between C-bound COad and (H)COad, which requires an optimal affinity for the C-bound species, so that (H)COad can be further reduced without poisoning the catalyst surface. It is suggested that the formation of C1 alcohols (CH3OH) is the most challenging process to optimize, as stabilization of the O-bound species would both accelerate the formation of key intermediates (OCH3,ad) but also simultaneously inhibit their desorption from the catalyst surface. Our findings pave the way toward a design strategy for CO2RR catalysts with improved selectivity, based on the experimental/theoretical reaction mechanisms that have been identified. These results also suggest that designing the electronic structure of the catalyst is not the sole determining factor to achieve highly selective CO2RR catalysis; rather, tuning additional experimental reaction conditions such as electrolyte-intermediate interactions also become critical.

It is particularly challenging to find a bifunctional catalyst capable of accelerating both the O2 evolution reaction (OER) and O2 reduction reaction (ORR) in aqueous solutions. The discovery of such a material would bring reversible fuel cells and metal air batteries much closer to technological fruition. However, in a real device, such a catalyst will need to retain its activity across an enormous potential window of at least 1 V over several years. To the best of our knowledge, little is known about the factors controlling the stability of bifunctional catalysts. In this contribution, we investigate the electrochemical activity and stability of a NiCo2O4 catalyst, synthesized at Johnson Matthey as part of EU funded project FLOWCAMP. Using post-mortem techniques, such as inductively coupled plasma mass spectrometry (ICP-MS), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), we provide experimental evidence about catalyst deactivation mechanisms. Results will also include a spectroelectrochemical in-situ UV-Vis study, that provides insights to the reaction mechanism for the OER. Lastly, the activity and stability of NiCo2O4 catalysts is also evaluated in oxygen-electrode prototypes and characterized in a half-cell configuration. In figure 1 is shown the ORR and OER activity of NiCo2O4 before and after accelerated degradation tests (ADT), in rotating disk electrode (RDE) configuration, employing three different electrochemical potential windows: i) 0.6 VRHE - 1 VRHE; ii) 0.6 VRHE - 1.7 VRHE; and iii) VRHE - 1.7 VRHE. As seen by the blue lines, significant ORR deactivation is observed after cycling across both ORR and OER potentials, whereas a 6% decrease in ORR current at 0.7 VRHE is observed after cycling for 1 hour within the ORR potential window. At the same time, degradation is accompanied by significant redox peak changes, as observed in the inset of Figure 1, which could indicate structural changes occurring during the OER. Post-mortem ICP-MS results indicate that catalyst dissolution accompanies this transformation during the OER. Alongside, in-situ UV-vis experiments identified changes in redox species as a function of applied potential, which are further correlated with the electrochemical current to determine the reaction order with respect to the amount of surface oxidized states. In this presentation, I will clarify the factors that control the activity and stability of NiCo2O4 catalysts, by coupling electrochemical measurements with in-situ UV-vis, and post mortem characterization techniques. In my talk, I will focus on understanding the causes for deactivation of the ORR current, which will aid further design of stable ORR and OER catalysts in alkaline media. A cknowledgements: This work is supported by the project ’’FLOWCAMP’’ (www.flowcamp-project.eu) under the European union’s innovation training network (grant agreement 765289). [1] L. Francàs, S. Corby, S. Selim, L. Dongho, C. A. Mesa, R. Godin, E. Pastor, I. E. L. Stephens, K. Choi, J. R. Durrant “Spectroelectrochemical study of water oxidation on nickel and iron oxyhydroxide electrocatalysts,” Nat. Commun., vol. 10, no. 5208, 2019. Figure 1

© 2020 Elsevier B.V. Electrochemical Stern layers are observed on the surfaces of RuO2 single crystals in 0.1 M CsF electrolyte. The Stern layers formed at the interfaces of RuO2 (110) and (100) are compared to the previously reported Stern layer on Pt(111) [Liu et al., J. Phys. Chem. Lett., 9 (2018) 1265]. While the Cs+ density profiles at the potentials close to hydrogen evolution reactions are similar, the hydration layers intervening the surface and the Cs+ layer are significantly denser on RuO2 surfaces than that on Pt(111) surface, reflecting the oxygen termination of RuO2 surfaces. The overall similarities between Stern layers on ruthenium surfaces and platinum surface suggest the universal presence of Stern layers in all well-defined solid-electrolyte interfaces.

While the surface atomic structure of RuO2 has been well studied in ultra high vacuum, much less is known about the interaction between water and RuO2 in aqueous solution. In this work, in situ surface X-ray scattering measurements combined with density functional theory (DFT) were used to determine the surface structural changes on single-crystal RuO2(110) as a function of potential in acidic electrolyte. The redox peaks at 0.7, 1.1 and 1.4 V vs. reversible hydrogen electrode (RHE) could be attributed to surface transitions associated with the successive deprotonation of -H2O on the coordinatively unsaturated Ru sites (CUS) and hydrogen adsorbed to the bridging oxygen sites. At potentials relevant to the oxygen evolution reaction (OER), an -OO species on the Ru CUS sites was detected, which was stabilized by a neighboring -OH group on the Ru CUS or bridge site. Combining potential-dependent surface structures with their energetics from DFT led to a new OER pathway, where the deprotonation of the -OH group used to stabilize -OO was found to be rate-limiting., Skoltech-MIT Center for Electrochemical Energy (Agreement 02/MI/MIT/CP/11/07633/GEN/G/00), National Science Foundation (U.S.) (Grant ACI-1548562)

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.