OER activity manipulated by IrO6 coordination geometry: an insight from pyrochlore iridates

Investigation of Structural, Optical, Electrical, and Biological Properties of a Porous Platinum Electrode for Neurostimulation Devices

The structural and optical properties, as well as the electrical and biological characteristics of a porous platinum (Pt) structure for neurostimulation applications, are investigated. Critical factors such as biocompatibility, electrical performance, and structural and optical differences, which can adversely affect the functionality of implantable devices, are systematically analyzed and compared with general electrodes. By employing an integration of three-dimensional simulations and implantation experiments, we demonstrate that the remarkably extensive surface area, low reflectance, and outstanding peak current values inherent in porous Pt facilitate effective stimulation while simultaneously ensuring a high degree of biological safety. Our findings suggest that these beneficial characteristics collectively position porous Pt as a notably promising candidate for implantable electrodes in biomedical devices.

Beyond conventional structures: emerging complex metal oxides for efficient oxygen and hydrogen electrocatalysis

The core of clean energy technologies such as fuel cells, water electrolyzers, and metal-air batteries depends on a series of oxygen and hydrogen-based electrocatalysis reactions, including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), which necessitate cost-effective electrocatalysts to improve their energy efficiency. In the recent decade, complex metal oxides (beyond simple transition metal oxides, spinel oxides and ABO3 perovskite oxides) have emerged as promising candidate materials with unexpected electrocatalytic activities for oxygen and hydrogen electrocatalysis owing to their special crystal structures and unique physicochemical properties. In this review, the current progress in complex metal oxides for ORR, OER, and HER electrocatalysis is comprehensively presented. Initially, we present a brief description of some fundamental concepts of the ORR, OER, and HER and a detailed description of complex metal oxides, including their physicochemical characteristics, synthesis methods, and structural characterization. Subsequently, we present a thorough overview of various complex metal oxides reported for ORR, OER, and HER electrocatalysis thus far, such as double/triple/quadruple perovskites, perovskite hydroxides, brownmillerites, Ruddlesden-Popper oxides, Aurivillius oxides, lithium/sodium transition metal oxides, pyrochlores, metal phosphates, polyoxometalates and other specially structured oxides, with emphasis on the designed strategies for promoting their performance and structure-property-performance relationships. Moreover, the practical device applications of complex metal oxides in fuel cells, water electrolyzers, and metal-air batteries are discussed. Finally, some concluding remarks summarizing the challenges, perspectives, and research trends of this topic are presented. We hope that this review provides a clear overview of the current status of this emerging field and stimulate future efforts to design more advanced electrocatalysts.

Microbial-induced Synthesis of nano NiFe LDH for High-efficiency Oxygen Evolution

NiFe layered double hydroxide (LDH) currently are the most efficient catalysts for the oxygen evolution reaction (OER) in alkaline environments. However, the development of high-performance low cost OER electrocatalysts using straightforward strategies remains a significant challenge. In this study, we describe an innovative microbial mineralization-based method for in situ-induced preparation of NiFe LDH nanosheets loaded on nickel foam and demonstrate that this material serves as an efficient oxygen evolution electrocatalyst. In the microbial mineralization process, bacteria adhere to electrode materials and promote the surface nucleation of nanomaterials when metal ions are present. Specifically, our findings indicate that biomineralization accelerates the formation and regulation of NiFe LDH. The new electrocatalyst displays excellent OER performance, with a small overpotential of 220 mV at 10 mA cm-2 and a Tafel slope down to 38.6 mV dec-1 in alkaline solution. The remarkable OER performance of the microbial mineralization-derived electrocatalyst is attributed to the synergistic effect of NiFe LDH and a bacterial-specific surface area that contains multiple active sites. This study has uncovered a new approach for the assembly of NiFe LDH that relies on biomineralization to bring about morphological and structural modification of LDH nanosheets.

Optimal Electrocatalyst Design Strategies for Acidic Oxygen Evolution

Hydrogen, a clean resource with high energy density, is one of the most promising alternatives to fossil. Proton exchange membrane water electrolyzers are beneficial for hydrogen production because of their high current density, facile operation, and high gas purity. However, the large-scale application of electrochemical water splitting to acidic electrolytes is severely limited by the sluggish kinetics of the anodic reaction and the inadequate development of corrosion- and highly oxidation-resistant anode catalysts. Therefore, anode catalysts with excellent performance and long-term durability must be developed for anodic oxygen evolution reactions (OER) in acidic media. This review comprehensively outlines three commonly employed strategies, namely, defect, phase, and structure engineering, to address the challenges within the acidic OER, while also identifying their existing limitations. Accordingly, the correlation between material design strategies and catalytic performance is discussed in terms of their contribution to high activity and long-term stability. In addition, various nanostructures that can effectively enhance the catalyst performance at the mesoscale are summarized from the perspective of engineering technology, thus providing suitable strategies for catalyst design that satisfy industrial requirements. Finally, the challenges and future outlook in the area of acidic OER are presented.

Recent Advances in Iridium-based Electrocatalysts for Acidic Electrolyte Oxidation

Ongoing research to develop advanced electrocatalysts for the oxygen evolution reaction (OER) is needed to address demand for efficient energy conversion and carbon-free energy sources. In the OER process, acidic electrolytes have higher proton concentration and faster response than alkaline ones, but their harsh strongly acidic environment requires catalysts with greater corrosion and oxidation resistance. At present, iridium oxide (IrO2) with its strong stability and excellent catalytic performance is the catalyst of choice for the anode side of commercial PEM electrolysis cells. However, the scarcity and high cost of iridium (Ir) and the unsatisfactory activity of IrO2 hinder industrial scale application and the sustainable development of acidic OER catalytic technology. This highlights the importance of further research on acidic Ir-based OER catalysts. In this review, recent advances in Ir-based acidic OER electrocatalysts are summarized, including fundamental understanding of the acidic OER mechanism, recent insights into the stability of acidic OER catalysts, highly efficient Ir-based electrocatalysts, and common strategies for optimizing Ir-based catalysts. The future challenges and prospects of developing highly effective Ir-based catalysts are also discussed.

Novel sulfate solid supported binary Ru-Ir oxides for superior electrocatalytic activity towards OER and CER

Electrolysis for producing hydrogen powered by renewable electricity can be dramatically expanded by adapting different electrolytes (brine, seawater or pure water), which means the anode materials must stand up to complex electrolyte conditions. Here, a novel catalyst/support hybrid of binary Ru3.5Ir1Ox supported by barium strontium sulfate (BaSrSO4) was synthesized (RuIrOx/BSS) by exchanging the anion ligands of support. The as-synthesized RuIrOx/BSS exhibits compelling oxygen evolution (OER) and chlorine evolution (CER) performances, which affords to 10 mA cm-2 with only overpotential of 244 mV and 38 mV, respectively. The performed X-ray adsorption spectra clearly indicate the presence of an interface charge transfer effect, which results in the assignment of more electrons to the d orbitals of the Ru and Ir sites. The theoretical calculations demonstrated that the electronic structures of the catalytic active sites were modulated to give a lower overpotential, confirming the intrinsically high OER and CER catalytic activity.

Electrocatalytic on-site oxygenation for transplanted cell-based-therapies

Implantable cell therapies and tissue transplants require sufficient oxygen supply to function and are limited by a delay or lack of vascularization from the transplant host. Previous exogenous oxygenation strategies have been bulky and had limited oxygen production or regulation. Here, we show an electrocatalytic approach that enables bioelectronic control of oxygen generation in complex cellular environments to sustain engineered cell viability and therapy under hypoxic stress and at high cell densities. We find that nanostructured sputtered iridium oxide serves as an ideal catalyst for oxygen evolution reaction at neutral pH. We demonstrate that this approach exhibits a lower oxygenation onset and selective oxygen production without evolution of toxic byproducts. We show that this electrocatalytic on site oxygenator can sustain high cell loadings (>60k cells/mm3) in hypoxic conditions in vitro and in vivo. Our results showcase that exogenous oxygen production devices can be readily integrated into bioelectronic platforms, enabling high cell loadings in smaller devices with broad applicability.

Electrocatalytic activity and surface oxide reconstruction of bimetallic iron-cobalt nanocarbide electrocatalysts for the oxygen evolution reaction

For renewable energy technology to become ubiquitous, it is imperative to develop efficient oxygen evolution reaction (OER) electrocatalysts, which is challenging due to the kinetically and thermodynamically unfavorable OER mechanism. Transition metal carbides (TMCs) have recently been investigated as desirable OER pre-catalysts, but the ability to tune electrocatalytic performance of bimetallic catalysts and understand their transformation under electrochemical oxidation requires further study. In an effort to understand the tunable TMC material properties for enhancing electrocatalytic activity, we synthesized bimetallic FeCo nanocarbides with a complex mixture of FeCo carbide crystal phases. The synthesized FeCo nanocarbides were tuned by percent proportion Fe (i.e. % Fe), and analysis revealed a non-linear dependence of OER electrocatalytic activity on % Fe, with a minimum overpotential of 0.42 V (15-20% Fe) in alkaline conditions. In an effort to understand the effects of Fe composition on electrocatalytic performance of FeCo nanocarbides, we assessed the structural phase and electronic state of the carbides. Although we did not identify a single activity descriptor for tuning activity for FeCo nanocarbides, we found that surface reconstruction of the carbide surface to oxide during water oxidation plays a pivotal role in defining electrocatalytic activity over time. We observed that a rapid increase of the (FexCo1-x)2O4 phase on the carbide surface correlated with lower electrocatalytic activity (i.e. higher overpotential). We have demonstrated that the electrochemical performance of carbides under harsh alkaline conditions has the potential to be fine-tuned via Fe incorporation and with control, or suppression, of the growth of the oxide phase.

Breaking the Activity and Stability Bottlenecks of Electrocatalysts for Oxygen Evolution Reactions in Acids

Oxygen evolution reaction (OER) is a cornerstone reaction for a variety of electrochemical energy conversion and storage systems such as water splitting, CO2 /N2 reduction, reversible fuel cells, and metal-air batteries. However, OER catalysis in acids suffers from extra sluggish kinetics due to the additional step of water dissociation along with its multiple electron transfer processes. Furthermore, OER catalysts often suffer from poor stability in harsh acidic electrolytes due to the severe dissolution/corrosion processes. The development of active and stable OER catalysts in acids is highly demanded. Here,  the recent advances in OER electrocatalysis in acids are reviewed and the key strategies are summarized to overcome the bottlenecks of activity and stability for both noble-metal-based and noble metal-free catalysts, including i) morphology engineering, ii) composition engineering, and iii) defect engineering. Recent achievements in operando characterization and theoretical calculations are summarized which provide an unprecedented understanding of the OER mechanisms regarding active site identification, surface reconstruction, and degradation/dissolution pathways. Finally, views are offered on the current challenges and opportunities to break the activity-stability relationships for acidic OER in mechanism understanding, catalyst design, as well as standardized stability and activity evaluation for industrial applications such as proton exchange membrane water electrolyzers and beyond.

Active and durable R2MnRuO7 pyrochlores with low Ru content for acidic oxygen evolution

The production of green hydrogen in water electrolyzers is limited by the oxygen evolution reaction (OER). State-of-the-art electrocatalysts are based on Ir. Ru electrocatalysts are a suitable alternative provided their performance is improved. Here we show that low-Ru-content pyrochlores (R₂MnRuO₇, R = Y, Tb and Dy) display high activity and durability for the OER in acidic media. Y₂MnRuO₇ is the most stable catalyst, displaying 1.5 V at 10 mA cm^-2 for 40 h, or 5000 cycles up to 1.7 V. Computational and experimental results show that the high performance is owed to Ru sites embedded in RuMnO_x surface layers. A water electrolyser with Y₂MnRuO₇ (with only 0.2 mg_Ru cm^-2) reaches 1 A cm^-2 at 1.75 V, remaining stable at 200 mA cm^-2 for more than 24 h. These results encourage further investigation on Ru catalysts in which a partial replacement of Ru by inexpensive cations can enhance the OER performance.

Role of MoOx Surficial Modification in Enhancing the OER Performance of Ru-Pyrochlore

Pyrochlore ruthenate (Y₂ Ru₂ O_7-δ ) is highlighted as a promising oxygen evolution reaction (OER) catalyst for water splitting in polymer electrolyte membrane electrolyzers. However, an efficient electronic modulation strategy for Y₂ Ru₂ O_7-δ is required to overcome its electrochemical inertness. Herein, a surface manipulation strategy involving implanting MoO_x moieties on nano Y₂ Ru₂ O_7-δ (Mo-YRO) using wet chemical peroxone method is demonstrated. In contrast to electronic structure regulation by intramolecular charge transfer (i.e., substitutional strategies), the heterogeneous Mo-O-Ru micro-interfaces facilitate efficient intermolecular electron transfer from [RuO₆ ] to MoO_x . This eliminates the bandgap by inducing Ru 4d delocalization and band alignment rearrangement. The MoO_x modifiers also alleviate distortion of [RuO₆ ] by shortening Ru-O bond and enlarging Ru-O-Ru bond angle. This electronic and geometric structure tailoring enhances the OER performance, showing a small overpotential of 240 mV at 10 mA cm^-2 . Moreover, the electron-accepting MoO_x moieties provide more electronegative surfaces, which serve as a protective "fence" to inhibit the dissolution of metal ions, thereby stabilizing the electrochemical activity. This study offers fresh insights into the design of new-based pyrochlore electrocatalysts, and also highlights the versatility of surface engineering as a way of optimizing electronic structure and catalytic performance of other related materials.

Supported Iridium-based Oxygen Evolution Reaction Electrocatalysts - Recent Developments

The commercialization of acidic proton exchange membrane water electrolyzers (PEMWE) is heavily hindered by the price and scarcity of oxygen evolution reaction (OER) catalyst, i. e. iridium and its oxides. One of the solutions to enhance the utilization of this precious metal is to use a support to distribute well dispersed Ir nanoparticles. In addition, adequately chosen support can also impact the activity and stability of the catalyst. However, not many materials can sustain the oxidative and acidic conditions of OER in PEMWE. Hereby, we critically and extensively review the different materials proposed as possible supports for OER in acidic media and the effect they have on iridium performances.

X-ray photoemission and absorption study of the pyrochlore iridates (Eu1-xBix)2Ir2O7, 0 ⩽x ⩽ 1

The pyrochlore iridates (Eu1-xBi_x)₂Ir₂O₇(0⩽x⩽1) undergo an anomalous negative lattice expansion for small Bi-doping (x⩽0.035) (region I) and a normal lattice expansion forx⩾0.1(region II); this is accompanied by a transition from an insulating (and magnetically ordered) to a metallic (and with no magnetic ordering) ground state. Here, we investigate (Eu1-xBi_x)₂Ir₂O₇(0⩽x⩽1) using hard x-ray photoemission spectroscopy and x-ray absorption fine structure (XAFS) spectroscopy. By analyzing the Eu-L₃, Ir-L₃and Bi-L₂&L₃edges x-ray absorption near edge structure spectra and Eu-3dcore-level XPS spectra, we show that the metal cations retain their nominal valence, namely, Ir⁴⁺, Bi³⁺and Eu³⁺, respectively, throughout the series. The Ir-4fand Bi-4fcore-level XPS spectra consist of screened and unscreened doublets. The unscreened component is dominant In the insulating range (x⩽0.035), and in the metallic region (x⩾0.1), the screened component dominates the spectra. The Eu-3dcore-level spectra remain invariant under Bi doping. The extended XAFS data show that the coordination around the Ir remains well preserved throughout the series. The evolution of the valence band spectra near the Fermi energy with increasing Bi doping indicates the presence of strong Ir(5d)-Bi(6p) hybridization which drives the metal-to-insulator transition.

Recent Progress in Advanced Electrocatalyst Design for Acidic Oxygen Evolution Reaction

Proton exchange membrane (PEM) water electrolyzers hold great significance for renewable energy storage and conversion. The acidic oxygen evolution reaction (OER) is one of the main roadblocks that hinder the practical application of PEM water electrolyzers. Highly active, cost-effective, and durable electrocatalysts are indispensable for lowering the high kinetic barrier of OER to achieve boosted reaction kinetics. To date, a wide spectrum of advanced electrocatalysts has been designed and synthesized for enhanced acidic OER performance, though Ir and Ru based nanostructures still represent the state-of-the-art catalysts. In this Progress Report, recent research progress in advanced electrocatalysts for improved acidic OER performance is summarized. First, fundamental understanding about acidic OER including reaction mechanisms and atomic understanding to acidic OER for rational design of efficient electrocatalysts are discussed. Thereafter, an overview of the progress in the design and synthesis of advanced acidic OER electrocatalysts is provided in terms of catalyst category, i.e., metallic nanostructures (Ir and Ru based), precious metal oxides, nonprecious metal oxides, and carbon based nanomaterials. Finally, perspectives to the future development of acidic OER are provided from the aspects of reaction mechanism investigation and more efficient electrocatalyst design.

Probing Multiscale Disorder in Pyrochlore and Related Complex Oxides in the Transmission Electron Microscope: A Review

Transmission electron microscopy (TEM), and its counterpart, scanning TEM (STEM), are powerful materials characterization tools capable of probing crystal structure, composition, charge distribution, electronic structure, and bonding down to the atomic scale. Recent (S)TEM instrumentation developments such as electron beam aberration-correction as well as faster and more efficient signal detection systems have given rise to new and more powerful experimental methods, some of which (e.g., 4D-STEM, spectrum-imaging, in situ/operando (S)TEM)) facilitate the capture of high-dimensional datasets that contain spatially-resolved structural, spectroscopic, time- and/or stimulus-dependent information across the sub-angstrom to several micrometer length scale. Thus, through the variety of analysis methods available in the modern (S)TEM and its continual development towards high-dimensional data capture, it is well-suited to the challenge of characterizing isometric mixed-metal oxides such as pyrochlores, fluorites, and other complex oxides that reside on a continuum of chemical and spatial ordering. In this review, we present a suite of imaging and diffraction (S)TEM techniques that are uniquely suited to probe the many types, length-scales, and degrees of disorder in complex oxides, with a focus on disorder common to pyrochlores, fluorites and the expansive library of intermediate structures they may adopt. The application of these techniques to various complex oxides will be reviewed to demonstrate their capabilities and limitations in resolving the continuum of structural and chemical ordering in these systems.

Electrocatalytic Oxygen Evolution Reaction in Acidic Conditions: Recent Progress and Perspectives

The electrochemical oxygen evolution reaction (OER) is an important half-cell reaction in many renewable energy conversion and storage technologies, including electrolyzers, nitrogen fixation, CO₂ reduction, metal-air batteries, and regenerative fuel cells. Among them, proton exchange membrane (PEM)-based devices exhibit a series of advantages, such as excellent proton conductivity, high durability, and good mechanical strength, and have attracted global interest as a green energy device for transport and stationary sectors. Nevertheless, with a view to rapid commercialization, it is urgent to develop highly active and acid-stable OER catalysts for PEM-based devices. In this Review, based on the recent advances in theoretical calculation and in situ/operando characterization, the OER mechanism in acidic conditions is first discussed in detail. Subsequently, recent advances in the development of several types of acid-stable OER catalysts, including noble metals, non-noble metals, and even metal-free OER materials, are systematically summarized. Finally, the current key issues and future challenges for materials used as acidic OER catalysis are identified and potential future directions are proposed.

Surface Reconstruction for Forming the [IrO6]-[IrO6] Framework: Key Structure for Stable and Activated OER Performance in Acidic Media

The surface reconstruction of iridium-based derivatives (A_xIr_yO_z) was extensively demonstrated to have an excellent oxygen evolution reaction (OER) performance in an acidic medium. It is urgent to use various spectroscopy and computational methods to explore the electronic state changes in the surface reconstruction process. Herein, the underestimated Lu₂Ir₂O₇ was synthesized and investigated. Four typical forms of electrochemistry impedance spectra involved in the reconstruction process revealed three dominating forms of reconstructed pyrochlore in the OER stage, including the inner intact pyrochlore, mid metastable [IrO₆]-[IrO₆] framework, and the outer collapse amorphous layer. The enhancing electron transport efficiency of the corner-shared [IrO₆]-[IrO₆] framework was revealed as a critical role in acidic systems. The density of state (DOS) for the constructed [IrO₆]-[IrO₆] framework corroborated the enhancement of Ir-O hybridization and the downshift of the d-band center. Additionally, we contrast the pristine and reconstruction properties of the Pr₂Ir₂O₇, Eu₂Ir₂O₇, and Lu₂Ir₂O₇ in alkaline and acidic media. The DOS and the XANES results reveal the scale relationship between the O 2p band center and the intrinsic activity for bulk pyrochlore in alkaline media. The highest O 2p center and the highest Ir-O hybridization of Lu₂Ir₂O₇ exhibited the best OER performance among the Ir-based pyrochlore, up to a ninefold improvement in Ir-mass activity compared to IrO₂. Our findings emphasize the electrochemical behavior of the reconstruction process for activated water-splitting performance.

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.

Recent Development of Oxygen Evolution Electrocatalysts in Acidic Environment

The proton exchange membrane (PEM) water electrolysis is one of the most promising hydrogen production techniques. The oxygen evolution reaction (OER) occurring at the anode dominates the overall efficiency. Developing active and robust electrocatalysts for OER in acid is a longstanding challenge for PEM water electrolyzers. Most catalysts show unsatisfied stability under strong acidic and oxidative conditions. Such a stability challenge also leads to difficulties for a better understanding of mechanisms. This review aims to provide the current progress on understanding of OER mechanisms in acid, analyze the promising strategies to enhance both activity and stability, and summarize the state-of-the-art catalysts for OER in acid. First, the prevailing OER mechanisms are reviewed to establish the physicochemical structure-activity relationships for guiding the design of highly efficient OER electrocatalysts in acid with stable performance. The reported approaches to improve the activity, from macroview to microview, are then discussed. To analyze the problem of instability, the key factors affecting catalyst stability are summarized and the surface reconstruction is discussed. Various noble-metal-based OER catalysts and the current progress of non-noble-metal-based catalysts are reviewed. Finally, the challenges and perspectives for the development of active and robust OER catalysts in acid are discussed.

Electronic structure, cohesive and magnetic properties of iridium oxide clusters adsorbed on graphene

In this study, we investigated and revealed the electronic properties, geometric structures and binding behavior of small (IrO)_n and [Formula: see text] (n = 1-5) clusters within first principles calculations based on the density functional theory. The electronic and magnetic properties of small nanoclusters displayed significant size dependency due to strong quantum confinement effect. Moreover we considered the binding and structural modification of the clusters on graphene surface as a substrate. The cohesive energy per atom of isolated clusters increased with size of the cluster n. This shows that the increase in coordination number results in a more stable nanocluster with increased number of saturated bonds. Pristine (IrO)_n and [Formula: see text] clusters presented different structural motives at equilibrium. The ground states of (IrO)_n and [Formula: see text] clusters considered in this study were all magnetic except for (IrO)₄, [Formula: see text] , and [Formula: see text] . HOMO-LUMO gap E_HLG values displayed large variations due to size of the cluster, hence bond saturation. The structural configurations of free standing nanoclusters are slightly modified, when adsorbed on graphene. The adsorption behavior of a cluster on graphene was improved by an applied electric field yielding larger binding energy and larger charger transfer. We observed that electronic and magnetic ground state of the clusters strongly depend on optimized structural configuration for both bare and adsorbed on graphene monolayer.

Fundamental understanding of the acidic oxygen evolution reaction: mechanism study and state-of-the-art catalysts

The oxygen evolution reaction (OER), as the anodic reaction of water electrolysis (WE), suffers greatly from low reaction kinetics and thereby hampers the large-scale application of WE. Seeking active, stable, and cost-effective OER catalysts in acidic media is therefore of great significance. In this perspective, studying the reaction mechanism and exploiting advanced anode catalysts are of equal importance, where the former provides guidance for material structural engineering towards a better catalytic activity. In this review, we first summarize the currently proposed OER catalytic mechanisms, i.e., the adsorbate evolution mechanism (AEM) and lattice oxygen evolution reaction (LOER). Subsequently, we critically review several acidic OER electrocatalysts reported recently, with focus on structure-performance correlation. Finally, a few suggestions on exploring future OER catalysts are proposed.

Assembly of a Highly Active Iridium-Based Oxide Oxygen Evolution Reaction Catalyst by Using Metal-Organic Framework Self-Dissolution

The proton exchange membrane (PEM) electrolyzer for hydrogen production has multiple advantages but is greatly restricted by expensive iridium and sluggish oxygen evolution reaction (OER) kinetics. The most promising way to reduce the precious metal loading is to design and develop highly active Ir-based catalysts. In this study, a versatile approach is reported to prepare a hybrid in the form of a catalyst-support structure (Fe-IrO_x@α-Fe₂O₃, abbreviated Ir@Fe-MF) by utilizing the self-dissolving properties of Fe-MIL-101 under aqueous conditions. The formation of this hybrid is mainly due to the Ir⁴⁺ and released Fe³⁺ ions coprecipitated to assemble into Fe-IrO_x nanoparticles, and the Fe³⁺ released from the inward collapse of the metal-organic framework (MOF) spontaneously forms α-Fe₂O₃. The prepared Ir@Fe-MF-2 hybrid exhibits enhanced catalytic activity toward OER with a lower onset potential and Tafel slop, and only 260 mV overpotential is required to drive the current density to reach 10 mA cm^-2. The performed characterizations clearly indicate that the IrO₆ coordination structure is changed significantly by Fe incorporated into the IrO₂ lattice. The performed X-ray adsorption spectra (XAS) provides evidence that Ir 5d orbital degeneracy is eliminated because of multiple orbitals being semi-occupied in the presence of Fe, which is mainly responsible for the enhancement of OER activity. These findings open an opportunity for the design and preparation of more efficient OER catalysts of transition metal oxides by utilization of the MOF materials. It should be highlighted that a long-term stability of this catalyst run at a high current density in acidic conditions still faces great challenges.

Holey Assembly of Two-Dimensional Iron-Doped Nickel-Cobalt Layered Double Hydroxide Nanosheets for Energy Conversion Application

Layered double hydroxides (LDHs) containing first-row transition metals such as Fe, Co, and Ni have attracted significant interest for electrocatalysis owing to their abundance and excellent performance for the oxygen evolution reaction (OER) in alkaline media. Herein, the assembly of holey iron-doped nickel-cobalt layered double hydroxide (NiCo-LDH) nanosheets ('holey nanosheets') is demonstrated by employing uniform Ni-Co glycerate spheres as self-templates. Iron doping was found to increase the rate of hydrolysis of Ni-Co glycerate spheres and induce the formation of a holey interconnected sheet-like structure with small pores (1-10 nm) and a high specific surface area (279 m²  g^-1 ). The optimum Fe-doped NiCo-LDH OER catalyst showed a low overpotential of 285 mV at a current density of 10 mA cm^-2 and a low Tafel slope of 62 mV dec^-1 . The enhanced OER activity was attributed to (i) the high specific surface area of the holey nanosheets, which increases the number of active sites, and (ii) the improved kinetics and enhanced ion transport arising from the iron doping and synergistic effects.

Design and Synthesis of Ir/Ru Pyrochlore Catalysts for the Oxygen Evolution Reaction Based on Their Bulk Thermodynamic Properties

Density functional theory (DFT) has proven to be an invaluable and effective tool for identifying highly active electrocatalysts for the oxygen evolution reaction (OER). Herein, we take a computational approach to first identify a series of rare-earth pyrochlore oxides based on Ir and Ru as potential OER catalysts. The DFT-based phase diagrams, Pourbaix diagrams (E vs pH), projected density of states, and band energy diagrams were used to identify prospective OER catalysts based on rare-earth Ir and Ru pyrochlores. The predicted materials were synthesized using the spray-freeze freeze-drying approach to afford nanoparticulate oxides conforming to the pyrochlore structural type A₂B₂O₇ where A = Nd, Gd, or Yb and B = Ir or Ru. In agreement with the computed Pourbaix diagrams, the materials were found to be moderately stable under OER conditions. All prepared materials show higher stability as compared to the benchmark IrO₂ catalyst, and the OER mass activity of Yb₂Ir₂O₇ and the ruthenate pyrochlores (Nd₂Ru₂O₇, Gd₂Ru₂O₇, and Yb₂Ru₂O₇) were also found to exceed those of the benchmark IrO₂ catalyst. We find that the OER activity of each pyrochlore series (i.e., iridate or ruthenate) generally improves as the size of the A-site cation decreases, indicating that maintaining control over the structure can be used to influence the electrocatalytic properties.

Self-Supported Hierarchical IrO2@NiO Nanoflake Arrays as an Efficient and Durable Catalyst for Electrochemical Oxygen Evolution

Although traditional IrO₂ nanoparticles loaded on a carbon support (IrO₂@C) have been taken as a benchmark catalyst for the oxygen evolution reaction (OER), their catalytic efficiency, operation stability, and IrO₂ utilization are far from satisfactory due to the inferior powdery structure and inevitable corrosion of both IrO₂ and C under the oxidizing potentials. Here, a rational design of a self-supported hierarchical nanocomposite, composed of IrO₂@NiO nanoparticle-built porous nanoflake arrays vertically growing on nickel foam, is proposed, which is demonstrated as a versatile strategy to achieve improved OER activity, remarkable long-term stability, and significantly reduced loading of IrO₂ (0.62 atom %). Impressively, the resultant catalyst drives a steady OER current density of 10 mA cm^-2, requiring 278 mV overpotential in 1.0 M KOH electrolyte for 25 h and outmaneuvring commercial IrO₂@C with much higher mass loading. Further electrochemical investigation and mechanism analysis disclose that the greatly improved electrocatalytic activity stems from the advantageous hierarchical structure and the synergistic effect between IrO₂ and underlying potential-induced NiOOH, whereas the outstanding durability is attributed to the unique role of NiO in preventing IrO₂ dissolution.

Oxygen Evolution Reaction Activity in IrO x/SrIrO3 Catalysts: Correlations between Structural Parameters and the Catalytic Activity

Understanding how structural properties affect the oxygen evolution reaction (OER) of a catalyst can reveal important information not only on the catalytic mechanism but also on the general design strategy of OER catalysts. We report a variation of ∼0.15 V in the overpotential of the recently discovered IrO _x/SrIrO₃ OER catalysts, which directly correlates with the structural parameters of the as-synthesized SrIrO₃ epitaxial films. This variation is caused by both extrinsic area enhancement and intrinsic electronic structure modification driven by defect formation. These correlations not only indicate that microscopic film defects play an important role in the activity of the IrO _x/SrIrO₃ catalyst but also provide readily accessible parameters predictive of the activity post-transformation to IrO _x/SrIrO₃. Establishing strong associations between the catalytic activity and key structural and electronic parameters, rather than synthetic variables, provides important guidance to control and study these complex catalysts independent of the synthetic technique.

A mesoporous C,N-co doped Co-based phosphate ultrathin nanosheet derived from a phosphonate-based-MOF as an efficient electrocatalyst for water oxidation

A mesoporous C,N-co doped Co-based phosphate ultrathin nanosheet derived from 2D phosphate MOFs has been explored and exhibits highly efficient OER performance.

Efficient oxygen evolution electrocatalysis in acid by a perovskite with face-sharing IrO6 octahedral dimers

The widespread use of proton exchange membrane water electrolysis requires the development of more efficient electrocatalysts containing reduced amounts of expensive iridium for the oxygen evolution reaction (OER). Here we present the identification of 6H-phase SrIrO₃ perovskite (6H-SrIrO₃) as a highly active electrocatalyst with good structural and catalytic stability for OER in acid. 6H-SrIrO₃ contains 27.1 wt% less iridium than IrO₂, but its iridium mass activity is about 7 times higher than IrO₂, a benchmark electrocatalyst for the acidic OER. 6H-SrIrO₃ is the most active catalytic material for OER among the iridium-based oxides reported recently, based on its highest iridium mass activity. Theoretical calculations indicate that the existence of face-sharing octahedral dimers is mainly responsible for the superior activity of 6H-SrIrO₃ thanks to the weakened surface Ir-O binding that facilitates the potential-determining step involved in the OER (i.e., O* + H₂O → HOO* + H⁺ + e_¯).

A Porous Pyrochlore Y2 [Ru1.6 Y0.4 ]O7-δ Electrocatalyst for Enhanced Performance towards the Oxygen Evolution Reaction in Acidic Media

A robust porous structure is often needed for practical applications in electrochemical devices, such as fuel cells, batteries, and electrolyzers. While templating approach is useful for the preparation of porous materials in general, it is not effective for the synthesis of oxide-based electrocatalysts owing to the chemical instability of disordered porous materials thus created. Now the synthesis of phase-pure porous yttrium ruthenate pyrochlore oxide using an unconventional porogen of perchloric acid is presented. The lattice oxygen defects are formed by the mixed-valence state of Ru^4+/5+ through the partial substitution of Ru⁴⁺ with Y³⁺ cations, leading to the formation of mixed B-site Y₂ [Ru_1.6 Y_0.4 ]O_7-δ . This porous Y₂ [Ru_1.6 Y_0.4 ]O_7-δ electrocatalyst exhibits a turnover frequency (TOF) of 560 s^-1 (at 1.5 V versus RHE) for the oxygen evolution reaction, which is two orders of magnitude higher than that of the RuO₂ reference catalyst (5.41 s^-1 ).

Effect of lattice strain on the electro-catalytic activity of IrO2 for water splitting

Lattice strain control of the OER activity of IrO₂.

Cultivating crystal lattice distortion in IrO2via coupling with MnO2 to boost the oxygen evolution reaction with high intrinsic activity

Enhancing the OER performance by cultivating Jahn–Teller distortion in IrO₂ with Mn oxide being the substrate material.