Lattice Oxygen Exchange in Rutile IrO2 during the Oxygen Evolution Reaction

Toward Realistic Models of the Electrocatalytic Oxygen Evolution Reaction

The electrocatalytic oxygen evolution reaction (OER) supplies the protons and electrons needed to transform renewable electricity into chemicals and fuels. However, the OER is kinetically sluggish; it operates at significant rates only when the applied potential far exceeds the reversible voltage. The origin of this overpotential is hidden in a complex mechanism involving multiple electron transfers and chemical bond making/breaking steps. Our desire to improve catalytic performance has then made mechanistic studies of the OER an area of major scientific inquiry, though the complexity of the reaction has made understanding difficult. While historically, mechanistic studies have relied solely on experiment and phenomenological models, over the past twenty years ab initio simulation has been playing an increasingly important role in developing our understanding of the electrocatalytic OER and its reaction mechanisms. In this Review we cover advances in our mechanistic understanding of the OER, organized by increasing complexity in the way through which the OER is modeled. We begin with phenomenological models built using experimental data before reviewing early efforts to incorporate ab initio methods into mechanistic studies. We go on to cover how the assumptions in these early ab initio simulations─no electric field, electrolyte, or explicit kinetics─have been relaxed. Through comparison with experimental literature, we explore the veracity of these different assumptions. We summarize by discussing the most critical open challenges in developing models to understand the mechanisms of the OER.

Atomic-Level Observation of Potential-Dependent Variations at the Surface of an Oxide Catalyst during Oxygen Evolution Reaction

Understanding the intricate details of the surface atomic structure and composition of catalysts during the oxygen evolution reaction (OER) is crucial for developing catalysts with high stability in water electrolyzers. While many notable studies highlight surface amorphization and reconstruction, systematic analytical tracing of the catalyst surface as a function of overpotential remains elusive. Heteroepitaxial (001) films of chemically stable and lattice-oxygen-inactive LaCoO3 are thus utilized as a model catalyst to demonstrate a series of atomic-resolution observations of the film surface at different anodic potentials. The first key finding is that atoms at the surface are fairly dynamic even at lower overpotentials. Angstrom-scale atomic displacements within the perovskite framework are identified below a certain potential level. Another noteworthy feature is that amorphization (or paracrystallization) with no long-range order is finally induced at higher overpotentials. In particular, surface analyses consistently support that the oxidation of lattice oxygen is coupled with amorphous phase formation at the high potentials. Theoretical calculations also reveal an upward shift of oxygen 2p states toward the Fermi level, indicating enhanced lattice oxygen activation when atom displacement occurs more extensively. This study emphasizes that the degradation behavior of OER catalysts can distinctively vary depending on the overpotential level.

One-Dimensional La0.2Sr0.8Cu0.4Co0.6O3-δ Nanostructures for Efficient Oxygen Evolution Reaction

Producing oxygen and hydrogen via the electrolysis of water has the advantages of a simple operation, high efficiency, and environmental friendliness, making it the most promising hydrogen production method. In this study, La0.2Sr0.8Cu0.4Co0.6O3-δ (LSCC) nanofibers were prepared by electrospinning to utilize non-noble perovskite oxides instead of noble metal catalysts for the oxygen evolution reaction, and the performance and electrochemical properties of LSCC nanofibers synthesized at different firing temperatures were evaluated. In an alkaline environment (pH = 14, 6 M KOH), the nanofibers calcined at 650 °C showed an overpotential of 209 mV at a current density of 10 mA cm-2 as well as good long-term stability. Therefore, the prepared LSCC-650 NF catalyst shows excellent potential for electrocatalytic oxygen evolution.

The Oxygen Evolution Reaction Drives Passivity Breakdown for Ni-Cr-Mo Alloys

Corrosion is the main factor limiting the lifetime of metallic materials, and a fundamental understanding of the governing mechanism and surface processes is difficult to achieve since the thin oxide films at the metal-liquid interface governing passivity are notoriously challenging to study. In this work, a combination of synchrotron-based techniques and electrochemical methods is used to investigate the passive film breakdown of a Ni-Cr-Mo alloy, which is used in many industrial applications. This alloy is found to be active toward oxygen evolution reaction (OER), and the OER onset coincides with the loss of passivity and severe metal dissolution. The OER mechanism involves the oxidation of Mo4+ sites in the oxide film to Mo6+ that can be dissolved, which results in passivity breakdown. This is fundamentally different from typical transpassive breakdown of Cr-containing alloys where Cr6+ is postulated to be dissolved at high anodic potentials, which is not observed here. At high current densities, OER also leads to acidification of the solution near the surface, further triggering metal dissolution. The OER plays an important role in the mechanism of passivity breakdown of Ni-Cr-Mo alloys due to their catalytic activity, and this effect needs to be considered when studying the corrosion of catalytically active alloys.

Lithium-Directed Transformation of Amorphous Iridium (Oxy)hydroxides To Produce Active Water Oxidation Catalysts

The oxygen evolution reaction (OER) is crucial to future energy systems based on water electrolysis. Iridium oxides are promising catalysts due to their resistance to corrosion under acidic and oxidizing conditions. Highly active iridium (oxy)hydroxides prepared using alkali metal bases transform into low activity rutile IrO₂ at elevated temperatures (>350 °C) during catalyst/electrode preparation. Depending on the residual amount of alkali metals, we now show that this transformation can result in either rutile IrO₂ or nano-crystalline Li-intercalated IrO_x. While the transition to rutile results in poor activity, the Li-intercalated IrO_x has comparative activity and improved stability when compared to the highly active amorphous material despite being treated at 500 °C. This highly active nanocrystalline form of lithium iridate could be more resistant to industrial procedures to produce PEM membranes and provide a route to stabilize the high populations of redox active sites of amorphous iridium (oxy)hydroxides.

Bioinspired and Bioderived Aqueous Electrocatalysis

The development of efficient and sustainable electrochemical systems able to provide clean-energy fuels and chemicals is one of the main current challenges of materials science and engineering. Over the last decades, significant advances have been made in the development of robust electrocatalysts for different reactions, with fundamental insights from both computational and experimental work. Some of the most promising systems in the literature are based on expensive and scarce platinum-group metals; however, natural enzymes show the highest per-site catalytic activities, while their active sites are based exclusively on earth-abundant metals. Additionally, natural biomass provides a valuable feedstock for producing advanced carbonaceous materials with porous hierarchical structures. Utilizing resources and design inspiration from nature can help create more sustainable and cost-effective strategies for manufacturing cost-effective, sustainable, and robust electrochemical materials and devices. This review spans from materials to device engineering; we initially discuss the design of carbon-based materials with bioinspired features (such as enzyme active sites), the utilization of biomass resources to construct tailored carbon materials, and their activity in aqueous electrocatalysis for water splitting, oxygen reduction, and CO₂ reduction. We then delve in the applicability of bioinspired features in electrochemical devices, such as the engineering of bioinspired mass transport and electrode interfaces. Finally, we address remaining challenges, such as the stability of bioinspired active sites or the activity of metal-free carbon materials, and discuss new potential research directions that can open the gates to the implementation of bioinspired sustainable materials in electrochemical devices.

Long-Term Stability Challenges and Opportunities in Acidic Oxygen Evolution Electrocatalysis

Polymer electrolyte membrane water electrolysis (PEMWE) has been regarded as a promising technology for renewable hydrogen production. However, acidic oxygen evolution reaction (OER) catalysts with long-term stability impose a grand challenge in its large-scale industrialization. In this review, critical factors that may lead to catalyst's instability in couple with potential solutions are comprehensively discussed, including mechanical peeling, substrate corrosion, active-site over-oxidation/dissolution, reconstruction, oxide crystal structure collapse through the lattice oxygen-participated reaction pathway, etc. Last but not least, personal prospects are provided in terms of rigorous stability evaluation criteria, in situ/operando characterizations, economic feasibility and practical electrolyzer consideration, highlighting the ternary relationship of structure evolution, industrial-relevant activity and stability to serve as a roadmap towards the ultimate application of PEMWE.

Skeleton-coated CoCu-Based bimetal hollow nanoprisms as High-Performance electrocatalysts for oxygen evolution reaction

CoSx materials with high catalytic activity are considered as promising HER electrocatalysts, but their inherent low electrical conductivity and easy loss of active sites have greatly limited their applications in OER electrocatalysis. Herein, we present a convenient method to synthesize Co-Cu hollow nanoprisms after wrapping and calcining with trithiocyanuric acid (C3H3N3S3) (denoted N-Co-Cu-S-x HNs). The results showed that Cu doping modified the charge density of Co center, leading to the enhancement of the intrinsic activity of the Co3S4 active center, meanwhile wrapping trithiocyanuric acid on the surfaces and calcinating to form N-containing C skeleton as a flexible substrate to encapsulate the catalysts, which effectively protected the active sites inside the catalysts. Notably, the OER catalyst that was optimized by adjusting the metal ratio and controlling the trithiocyanuric acid incorporation exhibited a low overpotential of 306 mV under a current density of 10 mA cm-2 and showed a superior durability of more than 27 h. This work may provide some insights into the preparation of oxygen evolution reaction catalysts with excellent performance through doping transition metals and protecting the internal active sites strategies.

Progress of Heterogeneous Iridium-Based Water Oxidation Catalysts

The water oxidation reaction (or oxygen evolution reaction, OER) plays a critical role in green hydrogen production via water splitting, electrochemical CO₂ reduction, and nitrogen fixation. The four-electron and four-proton transfer OER process involves multiple reaction intermediates and elementary steps that lead to sluggish kinetics; therefore, a high overpotential is necessary to drive the reaction. Among the different water-splitting electrolyzers, the proton exchange membrane type electrolyzer has greater advantages, but its anode catalysts are limited to iridium-based materials. The iridium catalyst has been extensively studied in recent years due to its balanced activity and stability for acidic OER, and many exciting signs of progress have been made. In this review, the surface and bulk Pourbaix diagrams of iridium species in an aqueous solution are introduced. The iridium-based catalysts, including metallic or oxides, amorphous or crystalline, single crystals, atomically dispersed or nanostructured, and iridium compounds for OER, are then elaborated. The latest progress of active sites, reaction intermediates, reaction kinetics, and elementary steps is summarized. Finally, future research directions regarding iridium catalysts for acidic OER are discussed.

Green synthesis of water splitting electrocatalysts: IrO2 nanocages via Pearson's chemistry

Highly porous iridium oxide structures are particularly well-suited for the preparation of porous catalyst layers needed in proton exchange membrane water electrolyzers. Herein, we report the formation of iridium oxide nanostructured cages, via a water-based process performed at room temperature, using cheap Cu2O cubes as the template. In this synthetic approach, based on Pearson's hard and soft acid-base theory, the replacement of the Cu2O core by an iridium shell is permitted by the difference in hardness/softness of cations and anions of the two reactants Cu2O and IrCl3. Calcination followed by acid leaching allow the removal of residual copper oxide cores and leave IrO2 hierarchical porous structures with outstanding activity toward the oxygen evolution reaction. Fundamental understanding of the reaction steps and identification of the intermediates are permitted by coupling a set of ex situ and in situ techniques including operando time-resolved X-ray absorption spectroscopy during the synthesis.

The Ir-OOOO-Ir transition state and the mechanism of the oxygen evolution reaction on IrO2(110)

Carefully assessing the energetics along the pathway of the oxygen evolution reaction (OER), our computational study reveals that the "classical" OER mechanism on the (110) surface of iridium dioxide (IrO₂) must be reconsidered. We find that the OER follows a bi-nuclear mechanism with adjacent top surface oxygen atoms as fixed adsorption sites, whereas the iridium atoms underneath play an indirect role and maintain their saturated 6-fold oxygen coordination at all stages of the reaction. The oxygen molecule is formed, via an Ir-OOOO-Ir transition state, by association of the outer oxygen atoms of two adjacent Ir-OO surface entities, leaving two intact Ir-O entities at the surface behind. This is drastically different from the commonly considered mono-nuclear mechanism where the O₂ molecule evolves by splitting of the Ir-O bond in an Ir-OO entity. We regard the rather weak reducibility of crystalline IrO₂ as the reason for favoring the novel pathway, which allows the Ir-O bonds to remain stable and explains the outstanding stability of IrO₂ under OER conditions. The establishment of surface oxygen atoms as fixed electrocatalytically active sites on a transition-metal oxide represents a paradigm shift for the understanding of water oxidation electrocatalysis, and it reconciles the theoretical understanding of the OER mechanism on iridium oxide with recently reported experimental results from operando X-ray spectroscopy. The novel mechanism provides an efficient OER pathway on a weakly reducible oxide, defining a new strategy towards the design of advanced OER catalysts with combined activity and stability.

Activated chemical bonds in nanoporous and amorphous iridium oxides favor low overpotential for oxygen evolution reaction

To date, the search for active, selective, and stable electrocatalysts for the oxygen evolution reaction (OER) has not ceased and a detailed atomic-level design of the OER catalyst remains an outstanding (if not, compelling) problem. Considerable studies on different surfaces and polymorphs of iridium oxides (with varying stoichiometries and dopants) have emerged over the years, showing much higher OER activity than the conventionally reported rutile-type IrO₂. Here, we have considered different metastable nanoporous and amorphous iridium oxides of different chemical stoichiometries. Using first-principles electronic structure calculations, we investigate the (electro)chemical stability, intercalation properties, and electronic structure of these iridium oxides. Using an empirical regression model between the Ir-O bond characteristics and the measured OER overpotentials, we demonstrate how activated Ir-O bonds (and the presence of more electrophilic oxygens) in these less understood polymorphs of iridium oxides can explain their superior OER performance observed in experiments.

Here the authors use density-functional theory calculations to examine structure-property relations of nanoporous and amorphous iridium oxides and reconcile the superior oxygen evolution reaction catalytic performance reported in previous experiments.

Coupling structural evolution and oxygen-redox electrochemistry in layered transition metal oxides

Lattice oxygen redox offers an unexplored way to access superior electrochemical properties of transition metal oxides (TMOs) for rechargeable batteries. However, the reaction is often accompanied by unfavourable structural transformations and persistent electrochemical degradation, thereby precluding the practical application of this strategy. Here we explore the close interplay between the local structural change and oxygen electrochemistry during short- and long-term battery operation for layered TMOs. The substantially distinct evolution of the oxygen-redox activity and reversibility are demonstrated to stem from the different cation-migration mechanisms during the dynamic de/intercalation process. We show that the π stabilization on the oxygen oxidation initially aids in the reversibility of the oxygen redox and is predominant in the absence of cation migrations; however, the π-interacting oxygen is gradually replaced by σ-interacting oxygen that triggers the formation of O-O dimers and structural destabilization as cycling progresses. More importantly, it is revealed that the distinct cation-migration paths available in the layered TMOs govern the conversion kinetics from π to σ interactions. These findings constitute a step forward in unravelling the correlation between the local structural evolution and the reversibility of oxygen electrochemistry and provide guidance for further development of oxygen-redox layered electrode materials.

Epitaxial Core-Shell Oxide Nanoparticles: First-Principles Evidence for Increased Activity and Stability of Rutile Catalysts for Acidic Oxygen Evolution

Due to their high activity and favorable stability in acidic electrolytes, Ir and Ru oxides are primary catalysts for the oxygen evolution reaction (OER) in proton-exchange membrane (PEM) electrolyzers. For a future large-scale application, core-shell nanoparticles are an appealing route to minimize the demand for these precious oxides. Here, we employ first-principles density-functional theory (DFT) and ab initio thermodynamics to assess the feasibility of encapsulating a cheap rutile-structured TiO₂ core with coherent, monolayer-thin IrO₂ or RuO₂ films. Resulting from a strong directional dependence of adhesion and strain, a wetting tendency is only obtained for some low-index facets under typical gas-phase synthesis conditions. Thermodynamic stability in particular of lattice-matched RuO₂ films is instead indicated for more oxidizing conditions. Intriguingly, the calculations also predict an enhanced activity and stability of such epitaxial RuO₂ /TiO₂ core-shell particles under OER operation.

The low overpotential regime of acidic water oxidation part II: trends in metal and oxygen stability numbers

The operating conditions of low pH and high potential at the anodes of polymer electrolyte membrane electrolysers restrict the choice of catalysts for the oxygen evolution reaction (OER) to oxides based on the rare metals iridium or ruthenium. In this work, we investigate the stability of both the metal atoms and, by quantitative and highly sensitive ¹⁸O isotope labelling experiments, the oxygen atoms in a series of RuO _x and IrO _x electrocatalysts during the OER in the mechanistically interesting low overpotential regime. We show that materials based on RuO _x have a higher dissolution rate than the rate of incorporation of labelled oxygen from the catalyst into the O₂ evolved ("labelled OER"), while for IrO _x -based catalysts the two rates are comparable. On amorphous RuO _x , metal dissolution and labelled OER are found to have distinct Tafel slopes. These observations together lead us to a full mechanistic picture in which dissolution and labelled OER are side processes to the main electrocatalytic cycle. We emphasize the importance of quantitative analysis and point out that since less than 0.2% of evolved oxygen contains an oxygen atom originating from the catalyst itself, lattice oxygen evolution is at most a negligible contribution to overall OER activity for RuO _x and IrO _x in acidic electrolyte.

Interrelations of Oxygen Evolution and Iridium Dissolution Mechanisms

The widespread utilization of proton exchange membrane (PEM) electrolyzers currently remains uncertain, as they rely on the use of highly scarce iridium as the only viable catalyst for the oxygen evolution reaction (OER), which is known to present the major energy losses of the process. Understanding the mechanistic origin of the different activities and stabilities of Ir‐based catalysts is, therefore, crucial for a scale‐up of green hydrogen production. It is known that structure influences the dissolution, which is the main degradation mechanism and shares common intermediates with the OER. In this Minireview, the state‐of‐the‐art understanding of dissolution and its relationship with the structure of different iridium catalysts is gathered and correlated to different mechanisms of the OER. A perspective on future directions of investigation is also given.

Synergetic Cobalt-Copper-Based Bimetal-Organic Framework Nanoboxes toward Efficient Electrochemical Oxygen Evolution

The development of efficient oxygen electrocatalysts and understanding their underlying catalytic mechanism are of significant importance for the high-performance energy conversion and storage technologies. Herein, we report novel CoCu-based bimetallic metal-organic framework nanoboxes (CoCu-MOF NBs) as promising catalysts toward efficient electrochemical oxygen evolution reaction (OER), fabricated via a successive cation and ligand exchange strategy. With the highly exposed bimetal centers and the well-designed architecture, the CoCu-MOF NBs show excellent OER activity and stability, with a small overpotential of 271 mV at 10 mA cm^-2 and a high turnover frequency value of 0.326 s^-1 at an overpotential of 300 mV. In combination of quasi in situ X-ray absorption fine structure spectroscopy and density-functional theory calculations, the post-formed CoCu-based oxyhydroxide analogue during OER is believed to account for the high OER activity of CoCu-MOF NBs, where the electronic synergy between Co and neighbouring Cu atoms promotes the O-O bond coupling toward fast OER kinetics.

Rapid oxygen exchange between hematite and water vapor

Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in technology and environmental studies, as it is closely linked to both catalytic activity and material degradation. The atomic-scale details are mostly unknown, however, and are often ascribed to poorly defined defects in the crystal lattice. Here we show that even thermodynamically stable, well-ordered surfaces can be surprisingly reactive. Specifically, we show that all the 3-fold coordinated lattice oxygen atoms on a defect-free single-crystalline "r-cut" ([Formula: see text]) surface of hematite (α-Fe2O3) are exchanged with oxygen from surrounding water vapor within minutes at temperatures below 70 °C, while the atomic-scale surface structure is unperturbed by the process. A similar behavior is observed after liquid-water exposure, but the experimental data clearly show most of the exchange happens during desorption of the final monolayer, not during immersion. Density functional theory computations show that the exchange can happen during on-surface diffusion, where the cost of the lattice oxygen extraction is compensated by the stability of an HO-HOH-OH complex. Such insights into lattice oxygen stability are highly relevant for many research fields ranging from catalysis and hydrogen production to geochemistry and paleoclimatology.

The origin of the high electrochemical activity of pseudo-amorphous iridium oxides

Combining high activity and stability, iridium oxide remains the gold standard material for the oxygen evolution reaction in acidic medium for green hydrogen production. The reasons for the higher electroactivity of amorphous iridium oxides compared to their crystalline counterpart is still the matter of an intense debate in the literature and, a comprehensive understanding is needed to optimize its use and allow for the development of water electrolysis. By producing iridium-based mixed oxides using aerosol, we are able to decouple the electronic processes from the structural transformation, i.e. Ir oxidation from IrO₂ crystallization, occurring upon calcination. Full characterization using in situ and ex situ X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron microscopy allows to unambiguously attribute their high electrochemical activity to structural features and rules out the iridium oxidation state as a critical parameter. This study indicates that short-range ordering, corresponding to sub-2nm crystal size for our samples, drives the activity independently of the initial oxidation state and composition of the calcined iridium oxides.

The origins of the superior catalytic activity of poorly crystallized Ir-based oxide material for the OER in acid is still under debate. Here, authors synthesize porous IrMo oxides to deconvolute the effect of Ir oxidation state from short-range ordering and show the latter to be a key factor.

Stabilizing Highly Active Ru Sites by Suppressing Lattice Oxygen Participation in Acidic Water Oxidation

In hydrogen production, the anodic oxygen evolution reaction (OER) limits the energy conversion efficiency and also impacts stability in proton-exchange membrane water electrolyzers. Widely used Ir-based catalysts suffer from insufficient activity, while more active Ru-based catalysts tend to dissolve under OER conditions. This has been associated with the participation of lattice oxygen (lattice oxygen oxidation mechanism (LOM)), which may lead to the collapse of the crystal structure and accelerate the leaching of active Ru species, leading to low operating stability. Here we develop Sr-Ru-Ir ternary oxide electrocatalysts that achieve high OER activity and stability in acidic electrolyte. The catalysts achieve an overpotential of 190 mV at 10 mA cm^-2 and the overpotential remains below 225 mV following 1,500 h of operation. X-ray absorption spectroscopy and ¹⁸O isotope-labeled online mass spectroscopy studies reveal that the participation of lattice oxygen during OER was suppressed by interactions in the Ru-O-Ir local structure, offering a picture of how stability was improved. The electronic structure of active Ru sites was modulated by Sr and Ir, optimizing the binding energetics of OER oxo-intermediates.

Spatial Confinement of a Carbon Nanocone for an Efficient Oxygen Evolution Reaction

A major bottleneck of large-scale water splitting for hydrogen production is the lack of catalysts for the oxygen evolution reaction (OER) with low cost and high efficiency. In this work, we proposed an electrocatalyst of a curved carbon nanocone embedded with two TMN₄ active sites (TM = transition metal) and used first-principles calculations to investigate their OER mechanisms and catalytic activities. In the particular spatial confinement of a curved nanocone, we found that the distance between intermediates adsorbed on two active sites is shorter than the distance between these two active sites. This finding can be used to enhance OER activity by distance-dependent interaction between intermediates through two different mechanisms. The first mechanism in which an O₂ molecule is generated from two neighboring *O intermediates exhibits a linear activity trend, and the lowest overpotential is 0.27 V for the FeN₄ system. In the second mechanism, selective stabilization of the *OOH intermediate is realized, leading to a new scaling relationship (ΔG_*OOH = ΔG_*OH + 3.04 eV) associated with a modified OER activity volcano (theoretical volcano apex at 0.29 V). The studied mechanisms of the spatial confinement of a carbon nanocone provide a new perspective for designing efficient OER catalysts.

Surface Lattice Oxygen Activation on Sr2Sb2O7 Enhances the Photocatalytic Mineralization of Toluene: from Reactant Activation, Intermediate Conversion to Product Desorption

Transition-metal oxide photocatalysis has attracted increasing attention in environmental remediation and solar energy conversion. Surface lattice oxygen is the key active site on the metal oxide, but its role and activation mechanism in the photocatalytic VOC mineralization are still unclear. In this work, we have demonstrated that Sr₂Sb₂O₇ exhibits an excellent photocatalytic activity and stability compared to TiO₂ (P25) in gaseous toluene mineralization because the lattice oxygen on Sr₂Sb₂O₇ can be activated efficiently. The lattice oxygen of Sr₂Sb₂O₇ promotes the adsorption and activation of O₂ and H₂O molecules and enhances the production of ^•O₂^- and ^•OH radicals, as confirmed by the electron spin resonance and DFT calculations. The in situ diffuse reflectance infrared Fourier transform spectroscopy spectra are applied to dynamically monitor the intermediate activation and selective conversion. Combined with DFT calculation, the role and the mechanism of lattice oxygen in photocatalysis have been revealed. Owing to the promoted surface lattice oxygen, the selectivity for benzoic acid formation is enhanced and final product desorption is promoted, which could largely advance the ring opening and mineralization of toluene. This work reveals the origin of lattice oxygen activation and the role for efficient VOC degradation at the atomic scale.

Effect of the Morphology of the High-Surface-Area Support on the Performance of the Oxygen-Evolution Reaction for Iridium Nanoparticles

The development of affordable, low-iridium-loading, scalable, active, and stable catalysts for the oxygen-evolution reaction (OER) is a requirement for the commercialization of proton-exchange membrane water electrolyzers (PEMWEs). However, the synthesis of high-performance OER catalysts with minimal use of the rare and expensive element Ir is very challenging and requires the identification of electrically conductive and stable high-surface-area support materials. We developed a synthesis procedure for the production of large quantities of a nanocomposite powder containing titanium oxynitride (TiON x ) and Ir. The catalysts were synthesized with an anodic oxidation process followed by detachment, milling, thermal treatment, and the deposition of Ir nanoparticles. The anodization time was varied to grow three different types of nanotubular structures exhibiting different lengths and wall thicknesses and thus a variety of properties. A comparison of milled samples with different degrees of nanotubular clustering and morphology retention, but with identical chemical compositions and Ir nanoparticle size distributions and dispersions, revealed that the nanotubular support morphology is the determining factor governing the catalyst's OER activity and stability. Our study is supported by various state-of-the-art materials' characterization techniques, like X-ray photoelectron spectroscopy, scanning and transmission electron microscopies, X-ray powder diffraction and absorption spectroscopy, and electrochemical cyclic voltammetry. Anodic oxidation proved to be a very suitable way to produce high-surface-area powder-type catalysts as the produced material greatly outperformed the IrO2 benchmarks as well as the Ir-supported samples on morphologically different TiON x from previous studies. The highest activity was achieved for the sample prepared with 3 h of anodization, which had the most appropriate morphology for the effective removal of oxygen bubbles.

Recent advances in understanding oxygen evolution reaction mechanisms over iridium oxide

Recent spectroscopic and computational studies concerning the oxygen evolution reaction over iridium oxides are reviewed to provide the state-of-the-art understanding of its reaction mechanism.

Low-iridium electrocatalysts for acidic oxygen evolution

The widespread use of proton-exchange membrane water electrolysis is limited by the dynamically sluggish oxygen evolution reaction (OER), which is mediated by noble iridium-based materials as active and stable electrocatalysts. Significant efforts have been made to decrease the amount of iridium in OER catalysts without sacrificing their catalytic performances. In this frontier paper, we present the main common issues relevant to the iridium-catalyzed OER, including catalytically active species, catalytic mechanisms and activity-stability relation. We also take iridium-based perovskites as an example, and summarize the recent theoretical and experimental advances in available strategies that can lead to highly efficient, low-iridium oxygen evolution electrocatalysts under acidic conditions. Finally, we propose the remaining challenges and future directions for exploring acidic OER catalysts.