Iridium single atoms incorporated in Co3O4 efficiently catalyze the oxygen evolution in acidic conditions

Engineering the regulation strategy of active sites to explore the intrinsic mechanism over single‑atom catalysts in electrocatalysis

The development of efficient and sustainable energy sources is a crucial strategy for addressing energy and environmental crises, with a particular focus on high-performance catalysts. Single-atom catalysts (SACs) have attracted significant attention because of their exceptionally high atom utilization efficiency and outstanding selectivity, offering broad application prospects in energy development and chemical production. This review systematically summarizes the latest research progress on SACs in five key electrochemical reactions: hydrogen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, nitrogen reduction reaction, and oxygen evolution reaction. Initially, a brief overview of the current understanding of electrocatalytic active sites in SACs is provided. Subsequently, the electrocatalytic mechanisms of these reactions are discussed. Emphasis is placed on various modification strategies for SAC surface-active sites, including coordination environment regulation, electronic structure modulation, support structure regulation, the introduction of structural defects, and multifunctional site design, all aimed at enhancing electrocatalytic performance. This review comprehensively examines SAC deactivation and poisoning mechanisms, highlighting the importance of stability enhancement for practical applications. It also explores the integration of density functional theory calculations and machine learning to elucidate the fundamental principles of catalyst design and performance optimization. Furthermore, various synthesis strategies for industrial-scale production are summarized, providing insights into commercialization. Finally, perspectives on future research directions for SACs are highlighted, including synthesis strategies, deeper insights into active sites, the application of artificial intelligence tools, and standardized testing and performance requirements.

Management of Platinum Electronic States through Metal Host-Guest Interactions for Enhanced Oxygen Reduction

Controlling the electronic states of Pt-based catalysts holds great promise for enhancing the intrinsic activity of the oxygen reduction reaction (ORR). Herein, inspired by first-principles simulations, we propose a strategy using metal host-guest interactions to tune Pt 5d electronic characteristics to optimize the adsorption strength of the key *OH intermediate. The hybrid electrocatalyst of Pt nanoparticles on a single-atom Co-N-C support (Pt@CoL SAs) exhibits a half-wave potential of 0.92 V and a mass activity of 3.2 A·mgPt -1 at 0.9 V in 0.1 M HClO4, which is a 20-fold enhancement compared with commercial Pt/C. Impressively, the Pt loading in the catalyst is as low as 1.70 wt %, which represents the lowest value reported in the relevant literature on Pt-based acidic ORR catalysts. Comprehensive spectroscopy investigations and theoretical simulations revealed that the precise regulatory effect of Co in various dispersion states effectively weakens the intermediate adsorption and reduces the energy barrier for the water decomposition step. Our finding provides valuable insights for the development of advanced ultralow-Pt ORR catalysts via the integration engineering of multiple metal sites.

Monolayer Covalent C60 Networks Anchored with Uniform Ultrasmall Iridium Nanoparticles for Boosting Electrocatalytic Hydrogen Evolution

The design of highly active and durable acidic hydrogen evolution reaction (HER) electrocatalysts remains a critical challenge for advancing hydrogen production technologies. Monolayer graphullerene, a two-dimensional (2D) carbon network derived from C60 fullerenes, exhibits exceptional properties such as structural stability, high specific surface area, superior in-plane electron conductivity, and distinctive electron-accepting behavior, positioning it as an ideal catalyst support. In this work, we report the synthesis of ultrafine iridium nanoparticles (∼1.7 nm) anchored on monolayer graphullerene (Ir NP@MLG) and demonstrate its excellent HER performance in acidic media. Comprehensive morphological and structural analyses confirm the atomic-scale dispersion of Ir nanoparticles on the monolayer graphullerene framework. The Ir NP@MLG hybrid catalyst achieves excellent HER activity with an ultralow overpotential of η10 = 18 mV (vs RHE) and a Tafel slope of 16.54 mV dec-1, surpassing most reported Ir-based catalysts. Notably, it exhibits a mass activity of 3.48 A mg-1 at an overpotential of -50 mV (vs RHE), representing one of the highest values among state-of-the-art Ir catalysts. Stability tests reveal exceptional durability, with negligible activity loss after 260 h of continuous operation. The superior performance originates from (1) the monolayer graphullerene's 2D conductive network facilitating rapid charge transfer and (2) strong metal-support interactions optimizing electronic structure and nanoparticle stabilization. This study establishes monolayer graphullerene as an interesting carbon support for developing electrocatalysts, providing deep insights into the design of efficient hydrogen energy systems.

Engineering Twins within Lattice-Matched Co/CoO Heterostructure Enables Efficient Hydrogen Evolution Reactions

Twinning, as an effective strain engineering strategy, has demonstrated significant potential in modifying cost-effective transition metal electrocatalysts. However, controllable construction and structure-activity relationships of twinning in electrocatalysts remain formidable challenges. Here, we engineered a lattice-matched Co/CoO heterostructure with enriched twin boundaries through flash Joule heating, where the twins form via lattice matching within homogeneous space groups. XAFS analysis reveals significantly reduced Co coordination numbers in the heterostructure, indicating substantial atomic displacement from the equilibrium positions. The coherent twinning interfaces induce trapped strain, downshifting the d-band center by 0.4 eV and flattening bands near the Fermi level, optimizing the electronic structure for the hydrogen evolution reaction. Consequently, the engineered heterostructure exhibits exceptional performance with an ultralow overpotential of 49 mV at 10 mA cm-2 in alkaline media and remarkable stability over 500 h. Notably, the water splitting can be driven with an ultralow cell voltage of 2.05 V at 1 A cm-2.

Reduced Graphene Oxide-Coated Iridium Oxide as a Catalyst for the Oxygen Evolution Reaction in Alkaline Water Electrolysis

Producing hydrogen by water electrolysis has attracted significant attention as a potential renewable energy solution. In this work, a catalyst with reduced graphene oxide (rGO) loaded on IrO2/TiO2 (called rGO/IrO2/TiO2) was designed for the catalytic oxygen evolution reaction (OER). The catalyst was synthesized by coating graphene oxide onto a pretreated IrO2/TiO2 precursor, followed by thermal treatment at 450 °C to achieve reduction and the adhesion of graphene to the substrate. The graphene support retained its intact sp2 carbon framework with minor oxygen-containing functional groups, which enhanced electrical conductivity and hydrophilicity. Benefiting from the synergistic effect of an rGO, IrO2, and TiO2 matrix, the rGO/IrO2/TiO2 catalyst only needed overpotentials of 240 mV and 320 mV to reach 10 mA cm-2 and 100 mA cm-2 in the OER, along with excellent stability over 50 h. Its morphology and crystalline structure were characterized by SEM and XRD spectroscopy, and its electrochemical performance was tested by LSV analysis, EIS impedance spectrum, and double-layer capacitance (Cdl) measurements. This work introduces an innovative and eco-friendly strategy for constructing a high-performance, functionalized Ir-based catalyst.

Ferroelectric Polarization Effects of Single-Atom Catalysts on Water Oxidation

The oxygen evolution reaction (OER) performance of single-atom catalysts (SACs) heavily depends on their substrates. However, heterojunctions with traditional substrate materials often fail to provide the desired dynamic interface effects. Here, through a systematic study of the ferroelectric heterostructure In2Se3/C-N-M, the feasibility of using ferroelectric materials to achieve dynamic optimization of the OER activity on SACs is demonstrated. The ferroelectric In2Se3 is confirmed to be an effective substrate for improving the stability of various SACs, supported by theoretical results of their negative formation energy and positive dissolution potential. Activity analysis indicates that among these In2Se3/C-N-M systems, the In2Se3/C-N-Ir can achieve near-ideal catalytic activities through polarization switching. It can unprecedentedly catalyze OER via a hybrid pathway of adsorbate evolution mechanism and O-O coupling mechanism under different pH conditions (from pH = 1 to pH = 13). Machine learning models have been developed to conduct feature analysis and make ultrafast predictions of OER activity, which identify that the interfacial charge transfer triggered by ferroelectric polarization is the key to fine-tuning the OER performance of SACs. This work provides a theoretical framework that utilizes ferroelectric polarization as a powerful approach to navigate the design of efficient SACs.

Low-Ir-Content Ir0.10Mn0.90O2 Solid Solution for Highly Active Oxygen Evolution in Acid Media

Iridium (Ir)-based materials are the most widely used oxygen evolution reaction (OER) electrocatalysts in proton exchange membrane water electrolysis (PEMWE). However, their commercial application suffers from high cost and insufficient activity. To optimize the atom utilization efficiency of Ir, the aim is to engineer and develop a rutile-structured solid solution catalyst with minimal Ir content, which is identified through a phase boundary. Here, Ir0.10Mn0.90O2 represents the lowest Ir content in the desired IrO2-MnO2 solid solution. The Ir0.10Mn0.90O2 catalyst exhibits outstanding OER performance in acidic electrolytes, reaching a remarkable mass activity of 1135 A g-1 Ir at an overpotential of 300 mV, which is ≈50 times higher than that of a commercial IrO2 catalyst. Additionally, it demonstrates excellent stability at a current density of 200 mA cm-2 over 120 h during PEMWE operations. Density functional theory (DFT) calculations indicate that the hydroxylation process can be efficiently promoted by the electron-withdrawing on Ir sites in Ir0.10Mn0.90O2, contributing to the enhancement of OER activity.

Manipulating Electron Structure through Dual-Interface Engineering of 3C-SiC Photoanode for Enhanced Solar Water Splitting

Interface engineering is crucial for enhancing the efficiency of semiconductor-based solar energy devices. In this work, we report a novel dual-interface engineering strategy by designing a Ni(OH)2/Co3O4/3C-SiC photoanode that achieves remarkable enhancements in photoelectrochemical (PEC) water splitting performance. The optimized photoanode delivers a photocurrent density of 1.68 mA cm-2 at 1.23 V vs the reversible hydrogen electrode (RHE), representing an 8-fold increase compared to pristine 3C-SiC, along with excellent operational stability. In this architecture, Co3O4 serves as a highly efficient hole-extraction layer and forms a p-n junction with 3C-SiC, enhancing the separation of photogenerated electron-hole pairs. At the Ni(OH)2/Co3O4 interface, the formation of Ni-O-Co bonds facilitates rapid charge transfer and accelerates oxygen evolution reaction (OER) kinetics. The microwave photoconductivity decay (μ-PCD) measurements confirm a prolonged minority carrier lifetime, demonstrating the critical role of electronic structure modulation in improving charge separation and reducing recombination. Using advanced synchrotron radiation and X-ray absorption spectroscopy, we unveil critical modifications to the interfacial electronic structure induced by the dual-interface engineering and their roles in enhancing PEC performance. These findings establish a clear relationship between electronic structure modulation, charge carrier dynamics, and PEC performance, providing new insights into interface design strategies for highly efficient solar-driven water splitting systems.

Mesoporous Single-Crystalline Particles as Robust and Efficient Acidic Oxygen Evolution Catalysts

The scarcity of iridium (Ir) limits its widespread use in acidic oxygen evolution reaction (OER). Herein, mesoporous single-crystalline spinel Co3O4 with atomically dispersed low-valence-state Ir has been developed to enable Ir's efficient and stable utilization. The surface Pourbaix diagram suggests that under acidic OER conditions, O* fully covers both Co3O4(111) and (110) surfaces, passivating Co sites but enhancing Co3O4's structural stability, a benefit further improved by Ir doping. Mesopores offer numerous loading sites for Ir single atoms (13.8 wt %), which activate the originally O*-passivated Co3O4(111) surface by creating high-intrinsic-activity Co-Ir bridge sites; meanwhile, Ir and Co leaching rates are reduced to about 1/4 and 1/5, respectively, compared to conventional Ir/Co3O4 catalysts. Our catalyst exhibits a low η10 of 248 mV for over 100 h, showcasing its potential in water electrolysis.

CoOx clusters-decorated IrO2 electrocatalyst activates NO3- mediator for benzylic C-H activation

Electrochemical conversion of petrochemical-derived hydrocarbons to high-value oxygenates can utilize renewable energy and reduce carbon emissions. However, this involves the challenging activation of inert C(sp3)-H bonds at room temperature. Here, we introduce an electrocatalyst:mediator assembly in which CoOx clusters-decorated IrO2 electrocatalyst activates NO3- mediator to a highly reactive radical capable of abstracting a hydrogen atom from benzylic C-H. The interface between CoOx and IrO2 promotes NO3- activation by facilitating the desorption of NO3● radical for subsequent reaction. Our strategy is demonstrated through the selective oxidation of toluene to benzaldehyde with high Faradaic efficiency of 86( ±1)% at 25 mA/cm2, a factor of >3 times higher than the bare electrocatalyst. The electrocatalyst:mediator assembly is operated stably for 100 h, with minimal decline in performance. When translated into a flow system, a Faradaic efficiency of 60( ±4)% at 200 mA/cm2 was achieved.

Recent Development of Ir- and Ru-Based Electrocatalysts for Acidic Oxygen Evolution Reaction

Proton exchange membrane (PEM) water electrolyzers are one type of the most promising technologies for efficient, nonpolluting and sustainable production of high-purity hydrogen. The anode catalysts account for a very large fraction of cost in PEM water electrolyzer and also determine the lifetime of the electrolyzer. To date, Ir- and Ru-based materials are types of promising catalysts for the acidic oxygen evolution reaction (OER), but they still face challenges of high cost or low stability. Hence, exploring low Ir and stable Ru-based electrocatalysts for acidic OER attracts extensive research interest in recent years. Owing to these great research efforts, significant developments have been achieved in this field. In this review, the developments in the field of Ir- and Ru-based electrocatalysts for acidic OER are comprehensively described. The possible OER mechanisms are first presented, followed by the introduction of the criteria for evaluation of the OER electrocatalysts. The development of Ir- and Ru-based OER electrocatalysts are then elucidated according to the strategies utilized to tune the catalytic performances. Lastly, possible future research in this burgeoning field is discussed.

Local Electronic Regulation by Oxygen Coordination with Single- Atomic Iridium on Ultrathin Cobalt Hydroxide Nanosheets for Electrocatalytic Oxygen Evolution

Rationally optimizing the atomic and electronic structure of electrocatalysts is an effective strategy to improve the activity of the electrocatalytic oxygen evolution reaction (OER), yet it remains challenging. In this work, atomic heterointerface engineering is developed to accelerate OER by decorating iridium atoms on low-crystalline cobalt hydroxide nanosheets (Ir-Co(OH)x) via oxygen-coordinated bonds to modulate the local electronic structure. Leveraging detailed spectroscopic characterizations, the Ir species were proved to promote charge transfer through Ir-O-Co coordination between the Ir atom and the Co(OH)x support. As a result, the optimized Ir-Co(OH)x exhibits excellent electrocatalytic OER activity with a low overpotential of 251 mV to drive 10 mA cm-2, which is 63 mV lower than that of pristine Co(OH)x. The experimental results and density functional theory calculations reveal that the isolated Ir atoms can regulate the local coordination environment and electronic configuration of Co(OH)x, thus accelerating the catalytic OER kinetics. This work provides an atomistic strategy for the electronic modulation of metal active sites in the design of high-performance electrocatalysts.

Vacancy and Dopant Co-Constructed Active Microregion in Ru-MoO3- x/Mo2AlB2 for Enhanced Acidic Hydrogen Evolution

Accurate identification of catalytic active regions is crucial for the rational design and construction of hydrogen evolution catalysts as well as the targeted regulation of their catalytic performance. Herein, the low crystalline-crystalline hybrid MoO3- x/Mo2AlB2 with unsaturated coordination and rich defects is taken as the precursor. Through the Joule heating reaction, the Ru-doped MoO3- x/Mo2AlB2 catalyst is successfully constructed. Building on the traditional view that individual atoms or vacancies act as active sites, this article innovatively proposes the theory that vacancies and doped atoms synergistically construct active microregions, and multiple electron-rich O atoms within the active microregions jointly serve as hydrogen evolution active sites. Based on X-ray absorption fine structure analysis and first-principles calculations, there is a strong electron transfer among Ru atoms, Mo atoms, and O atoms, leading to extensive O atoms with optimized electronic structure in the active microregions. These O atoms exhibit an H* adsorption free energy close to zero, thereby enhancing the catalytic activity for hydrogen evolution. This work provides a brand-new strategy for the design and preparation of electrocatalytic materials and the systematic regulation of the local electronic structure of catalysts.

Phase Engineering Modulates the Electronic Structure of the IrO2/MoS2 Heterojunction for Efficient and Stable Water Splitting

The engineering of dual-functional catalytic systems capable of driving complete water dissociation in acidic environments represents a critical requirement for advancing proton exchange membrane electrolyzer technology, yet significant challenges remain. In this work, we investigate an IrO2/MoS2/CNT heterostructure catalyst demonstrating enhanced bifunctional performance for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) under acidic conditions. Strategic incorporation of IrO2 into the MoS2/CNT heterojunction induces a partial phase transformation from 2H to the metastable 1T configuration in MoS2, thereby modulating the electronic structure of IrO2 and improving the catalytic performance for overall water splitting. The optimized IrO2/MoS2/CNT catalyst exhibited exceptional overpotentials of 9 mV (HER) and 182 mV (OER) at a current density of 10 mA cm-2 in acidic media. Full-cell evaluations further confirmed its practical potential, showing a 1.47 V operation voltage that outperforms standard Pt/C||IrO2 counterparts by 120 mV. The experimental results revealed that the n-n heterojunction between IrO2/CNT and MoS2/CNT generates a built-in electric field, enhancing charge redistribution and electron transport. Moreover, density functional theory simulations further identify iridium centers as dominant catalytic loci, with a metastable 1T-MoS2 phase mediating charge equilibration at atomic interfaces. This modification facilitates *OH adsorption and *OOH deprotonation and lowers the kinetic barrier during the water-splitting process.

Anode Alchemy on Multiscale: Engineering from Intrinsic Activity to Impedance Optimization for Efficient Water Electrolysis

The past decade has seen significant progress in proton exchange membrane water electrolyzers (PEMWE), but the growing demand for cost-effective electrolytic hydrogen pushes for higher efficiency at lower costs. As a complex system, the performance of PEMWE is governed by a combination of multiscale factors. This review summarizes the latest progress from quantum to macroscopic scales. At the quantum level, electron spin configurations can be optimized to enhance catalytic activity. At the nano and meso scales, advancements in atomic structure optimization, crystal phase engineering, and heterostructure design improve catalytic performance and mass transport. At the macro scale, innovative techniques in gas bubble management and internal resistance reduction drive further efficiency gains under ampere-level operating conditions. These modifications at the quantum level cascade through meso- and macro-scales, affecting charge transfer, reaction kinetics, and gas evolution management. Unlike conventional approaches that focus solely on one scale-either at the catalyst level (e.g., atomic, or crystal modifications) or at the device level (e.g., porous transport layers design)-combining multiscale optimizations unlocks greater performance improvements. Finally, a perspective on future opportunities for multiscale engineering in PEMWE anode design toward commercial viability is offered.

Proton Exchange Membrane Water Splitting: Advances in Electrode Structure and Mass-Charge Transport Optimization

Proton exchange membrane water electrolysis (PEMWE) represents a promising technology for renewable hydrogen production. However, the large-scale commercialization of PEMWE faces challenges due to the need for acid oxygen evolution reaction (OER) catalysts with long-term stability and corrosion-resistant membrane electrode assemblies (MEA). This review thoroughly examines the deactivation mechanisms of acidic OER and crucial factors affecting assembly instability in complex reaction environments, including catalyst degradation, dynamic behavior at the MEA triple-phase boundary, and equipment failures. Targeted solutions are proposed, including catalyst improvements, optimized MEA designs, and operational strategies. Finally, the review highlights perspectives on strict activity/stability evaluation standards, in situ/operando characteristics, and practical electrolyzer optimization. These insights emphasize the interrelationship between catalysts, MEAs, activity, and stability, offering new guidance for accelerating the commercialization of PEMWE catalysts and systems.

Isolated Metal Centers Activate Small Molecule Electrooxidation: Mechanisms and Applications

Electrochemical oxidation of small molecules shows great promise to substitute oxygen evolution reaction (OER) or hydrogen oxidation reaction (HOR) to enhance reaction kinetics and reduce energy consumption, as well as produce high-valued chemicals or serve as fuels. For these oxidation reactions, high-valence metal sites generated at oxidative potentials are typically considered as active sites to trigger the oxidation process of small molecules. Isolated atom site catalysts (IASCs) have been developed as an ideal system to precisely regulate the oxidation state and coordination environment of single-metal centers, and thus optimize their catalytic property. The isolated metal sites in IASCs inherently possess a positive oxidation state, and can be more readily produce homogeneous high-valence active sites under oxidative potentials than their nanoparticle counterparts. Meanwhile, IASCs merely possess the isolated metal centers but lack ensemble metal sites, which can alter the adsorption configurations of small molecules as compared with nanoparticle counterparts, and thus induce various reaction pathways and mechanisms to change product selectivity. More importantly, the construction of isolated metal centers is discovered to limit metal d-electron back donation to CO 2p* orbital and reduce the overly strong adsorption of CO on ensemble metal sites, which resolve the CO poisoning problems in most small molecules electro-oxidation reactions and thus improve catalytic stability. Based on these advantages of IASCs in the fields of electrochemical oxidation of small molecules, this review summarizes recent developments and advancements in IASCs in small molecules electro-oxidation reactions, focusing on anodic HOR in fuel cells and OER in electrolytic cells as well as their alternative reactions, such as formic acid/methanol/ethanol/glycerol/urea/5-hydroxymethylfurfural (HMF) oxidation reactions as key reactions. The catalytic merits of different oxidation reactions and the decoding of structure-activity relationships are specifically discussed to guide the precise design and structural regulation of IASCs from the perspective of a comprehensive reaction mechanism. Finally, future prospects and challenges are put forward, aiming to motivate more application possibilities for diverse functional IASCs.

Tailoring Octahedron-Tetrahedron Synergism in Spinel Catalysts for Acidic Water Electrolysis

The instability issues of oxide-based electrocatalysts during the oxygen evolution reaction (OER) under acidic conditions, caused by the oxidation and dissolution of the catalysts along with the current-capacitance effect, constrain their application in proton exchange membrane water electrolysis (PEMWE). To address these challenges, we tailored the spinel structure of Co3O4 and exploited the synergism between the tetrahedron and octahedron sites by partially substituting Co with Ni and Ru (denoted as NiRuCoOx), respectively. Such a catalyst design creates a Ru-O-Ni electronic coupling effect, facilitating a direct dioxygen radical-coupled OER pathway. Density-functional theory (DFT) calculations and in situ Raman spectroscopy results confirm that Ru is the active site in the diatomic oxygen mechanism while Ni stabilizes lattice oxygen and the Ru-O bonding. The designed NiRuCoOx catalyst exhibits an exceptionally low overpotential of 166 mV to achieve a current density of 10 mA cm-2. Moreover, when serving as the anode in PEMWE, the NiRuCoOx requires 1.72 V to reach a current density of 3A cm-2, meeting the 2026 target set by the U.S. Department of Energy (DOE: 1.8 V@3A cm-2). The PEMWE device can operate stably for more than 1500 h with a significantly reduced performance decay rate of 0.025 mV h-1 compared to commercial RuO2 (2.13 mV h-1). This work provides an efficient method for tailoring the octahedron-tetrahedron sites of spinel Co3O4, which significantly improves the activity and stability of PEMWE.

Boosting Alkaline Hydrogen Evolution by Creating Atomic-Scale Pair Cocatalytic Sites in Single-Phase Single-Atom-Ruthenium-Incorporated Cobalt Oxide

Compared with acidic environments, promoting the water dissociation process is crucial for speeding up hydrogen evolution reaction (HER) kinetics in alkaline electrolyte. Although the construction of heterostructured electrocatalysts by hybridizing noble metals with metal (hydr)oxides has been reported as a feasible approach to achieve high performance, the high cost, complicated fabrication process, and unsatisfactory mass activity limit their large-scale applications. Herein, we report a single-phase HER electrocatalyst composed of single-atom ruthenium (Ru) incorporated into a cobalt oxide spine structure (denoted as Ru SA/Co3O4), which possesses exceptional HER performance in alkaline media via unusual atomic-scale Ru-Co pair sites. In particular, Ru SA/Co3O4 exhibits a very low overpotential of 44 mV at 10 mA cm-2 and an outstanding mass activity of 4700 mA mg-1 at 50 mV overpotential, superior to those of commercial Pt/C, Ru nanoparticles supported on Co3O4 (denoted as Ru NP/Co3O4) and other reported Ru-based electrocatalysts. With insights from theoretical calculations, the synergistic interactions between Ru and Co pair active sites in Ru SA/Co3O4 are revealed to catalyze diverse fundamental steps of the alkaline HER; i.e., the Ru sites can effectively accelerate water adsorption/dissociation and OH- desorption, whereas the Co sites are favorable for H* adsorption and H2 evolution.

Atomic-scale self-rearrangement of hetero-metastable phases into high-density single-atom catalysts for the oxygen evolution reaction

Maximizing metal-substrate interactions by self-reconstruction of coadjutant metastable phases can be a delicate strategy to obtain robust and efficient high-density single-atom catalysts. Here, we prepare high-density iridium atoms embedded ultrathin CoCeOOH nanosheets (CoCe-O-IrSA) by the electrochemistry-initiated synchronous evolution between metastable iridium intermediates and symmetry-breaking CoCe(OH)2 substrates. The CoCe-O-IrSA delivers an overpotential of 187 mV at 100 mA cm-2 and a steady lifespan of 1000 h at 500 mA cm-2 for oxygen evolution reaction. Furthermore, the CoCe-O-IrSA is applied as a robust anode in an anion-exchange-membrane water electrolysis cell for seawater splitting at 500 mA cm-2 for 150 h. Operando experimental and theoretical calculation results demonstrate that the reconstructed thermodynamically stable iridium single atoms act as highly active sites by regulating charge redistribution with strongly p-d-f orbital couplings, enabling electron transfer facilitated, the adsorption energies of intermediates optimized, and the surface reactivity of Co/Ce sites activated, leading to high oxygen evolution performance. These results open up an approach for engineering metastable phases to realize stable single-atom systems under ambient conditions toward efficient energy-conversion applications.

Optimizing Acidic Oxygen Evolution Reaction via Modulation Doping in Van der Waals Layered Iridium Oxide

Anodic oxygen evolution reaction (OER) exhibits a sluggish four-electron transfer process, necessitating catalysts with exceptional catalytic activity to enhance its kinetic rate. Van der Waals layered oxides are ideal materials for catalyst design, yet its stability for acidic OER remains large obstacle. Doping provides a crucial way to improve the activity and stability simultaneously. However, doping in Van der Waals layered oxides remains a great challenge since it easily leads to lattice distortion or even the crystal structure damage. In this work, we successfully doping acid-resistant niobium (Nb) into Van der Waals layered edge-shared 1T phase iridium oxide (1 T-IrO2) via alkali-assisted thermal method. 1 T-IrO2 with a 5 % Nb doping (Nb0.05Ir0.95O2) only required an overpotential of 191 mV to achieve a current density of 10 mA cm-2 in 0.5 M H2SO4, 56 mV lower than that of 1T-IrO2. When applied in proton exchange membrane water electrolyzer, Nb0.05Ir0.95O2 show stable operation at a high current density of 1.2 A cm-2 for over 50 days. Density functional theory calculation reveals that doping Nb changes the potential-determining step from the *OOH deprotonation process in 1 T-IrO2 to the *O-OH coupling process in Nb0.05Ir0.95O2.

Recent achievements in noble metal-based oxide electrocatalysts for water splitting

The search for sustainable energy sources has accelerated the exploration of water decomposition as a clean H2 production method. Among the methods proposed, H2 production via water electrolysis has garnered considerable attention. However, the process of H2 production from water electrolysis is severely limited by the slow kinetics of the anodic oxygen evolution reaction and large intrinsic overpotentials at the anode; therefore, suitable catalysts need to be found to accelerate the reaction rate. Noble metal-based oxide electrocatalysts retain the advantages of abundant active sites, high electrical conductivity of noble metals, and low cost, which make them promising electrocatalysts; however, they suffer from the challenge of an imbalance between catalytic activity and stability. This review presents recent research progress in noble metals and their oxides as electrocatalysts. In this review, two half-reactions (the hydrogen evolution reaction and the oxygen evolution reaction) of water electrolysis are described. Recently reported methods for the synthesis of noble metal-based oxide electrocatalysts, improvement strategies, and sources of enhanced activity and stability for these types of catalysts are presented. Finally, the challenges and future perspectives in the field are summarised. This review is expected to help improve the understanding of noble metal-based oxide electrocatalysts.

Interface Engineering of RuO2/Ni-Co3O4 Heterostructures for enhanced acidic oxygen evolution reaction

RuO2 has been recognized as a standard electrocatalyst for acidic oxygen evolution reaction (OER). Nonetheless, its high cost and limited durability are still ongoing challenges. Herein, a RuO2/Ni-Co3O4 heterostructure confining a heterointerface (between RuO2 and Ni-doped Co3O4) is constructed to realize enhanced OER performance. Specifically, RuO2/Ni-Co3O4 containing a low Ru content (2.7 ± 0.3 wt%) achieves an overpotential of 186 mV at a current density of 10 mA cm-2 with a long-run stability (≥1300 h). Also, it exhibits a mass activity of 1202.29 mA mgRu-1 at an overpotential of 250 mV, exceeding commercial RuO2. The results disclose an optimum electron transfer at the heterointerface, wherein Ni doping improves the adsorption energy of oxygen-containing intermediates, thereby facilitating OER. This study presents an effective approach for designing highly active and stable OER electrocatalysts.

Site-specific synergy in heterogeneous single atoms for efficient oxygen evolution

Heterogeneous single-atom systems demonstrate potential to break performance limitations of single-atom catalysts through synergy interactions. The synergy in heterogeneous single atoms strongly dependes on their anchoring sites. Herein, we reveal the site-specific synergy in heterogeneous single atoms for oxygen evolution. The RuTIrV/CoOOH is fabricated by anchoring Ru single atoms onto three-fold facial center cubic hollow sites and Ir single atoms onto oxygen vacancy sites on CoOOH. Moreover, IrTRuV/CoOOH is also prepared by switching the anchoring sites of single atoms. Electrochemical measurements demonstrate the RuTIrV/CoOOH exhibits enhanced OER performance compared to IrTRuV/CoOOH. In-situ spectroscopic and mechanistic studies indicate that Ru single atoms at three-fold facial center cubic hollow sites serve as adsorption sites for key reaction intermediates, while Ir single atoms at oxygen vacancy sites stabilize the *OOH intermediates via hydrogen bonding interactions. This work discloses the correlation between the synergy in heterogeneous single atoms and their anchoring sites.

Mn0.75Ru0.25O2 with Low Ru Concentration for Active and Durable Acidic Oxygen Evolution

Ruthenium has emerged as a promising alternative to iridium in water-splitting anodes. However, it becomes overoxidized and dissolves at industry-relevant working conditions. To enhance the activity and stability of electrocatalysts for oxygen evolution reaction, an isostructural rutile MnRu oxide with low Ru concentration (Mn0.75Ru0.25O2) is synthesized and an asymmetric Mn-O-Ru dual-site active center is developed. It exhibits 154 mV overpotential at 10 mA cm-2 and can operate stably at 200 mA cm-2 for 670 h with a degradation rate of 29 uV/h-1. A proton exchange membrane water electrolyzer achieves stable operation at 1 A cm-2 for 700 h with a degradation rate of 53 uV h-1. Structural analysis and isotopic labeling correlate the asymmetric nature of the Mn-O-Ru dual-site active center, which facilitates the oxygen evolution reaction along the radical coupling pathway, with the stabilization of the cations and the lattice oxygen in isostructural rutile Mn0.75Ru0.25O2.

Achieving Higher Activity of Acidic Oxygen Evolution Reaction Using an Atomically Thin Layer of IrOx over Co3O4

The development of electrocatalysts with reduced iridium (Ir) loading for the oxygen evolution reaction (OER) is essential to produce low-cost green hydrogen from water electrolysis under acidic conditions. Herein, an atomically thin layer of iridium oxide (IrOx) has been uniformly dispersed onto cobalt oxide (Co3O4) nanocrystals to improve the efficient use of Ir for acidic OER. In situ characterization and theoretical calculations reveal that compared to the conventional IrOx cluster, the atomically thin layer of IrOx shows stronger interaction with the Co3O4 and consequently higher OER activity due to the Ir-O-Co bond formation at the interface. Equally important, the facile synthetic method and the promising activity in the proton exchange membrane water electrolyzer, reaching 1 A cm-2 at 1.7 V with remarkable durability, enable potential scale-up applications. These findings provide a mechanistic understanding for designing active, stable and lower-cost electrocatalysts with well-defined structures for acidic OER.

Exogenous Coreactant-Free Electrocatalytic Reactive Oxygen Species-Driven Dual-Signal Molecularly Imprinted Electrochemiluminescence Sensor for the Detection of Trenbolone

Conventional dual-signal electrochemiluminescence (ECL) sensors feature high sensitivity and reliability, but the involvement of coreactants inevitably results in a complex configuration and shows reproducibility risk. Here, we propose an exogenous coreactant-free dual-signal platform, comprising luminol (anodic luminophore), CdSe quantum dots (cathodic luminophore), and Co3O4/Ti3C2 electrocatalyst (coreaction promoter). At different redox potentials, Co3O4/Ti3C2 induces water oxidation and oxygen reduction to produce •OH and O2•- radicals, which subsequently drive cathodic and anodic ECL emission, respectively. The dual-signal ECL pathways are confirmed by reactive oxygen species (ROS) scavenging, and the effect of dissolved oxygen is excluded. By integrating with molecular imprinting technology, this dual-signal ECL sensor is employed for the quantitative detection of trenbolone. It exhibits high sensitivity, broad linear range (1.0 × 101 to 1.0 × 108 fg mL-1), low detection limit, and detection capability of real samples. This work presents a new endogenous ROS-driven dual-signal ECL sensor that excludes the use of exogenous coreactants and offers new insights into ROS generation in a coreactant-free sensor design.

Efficient Nitrate to Ammonia Conversion on Bifunctional IrCu4 Alloy Nanoparticles

Electrochemical nitrate reduction (NO3RR) to ammonia presents a promising alternative strategy to the traditional Haber-Bosch process. However, the competitive hydrogen evolution reaction (HER) reduces the Faradaic efficiency toward ammonia, while the oxygen evolution reaction (OER) increases the energy consumption. This study designs IrCu4 alloy nanoparticles as a bifunctional catalyst to achieve efficient NO3RR and OER while suppressing the unwanted HER. This is achieved by operating the NO3RR at positive potentials using the IrCu4 catalyst, which allows a Faradaic efficiency of 93.6% for NO3RR. When applied to OER catalysis, the IrCu4 alloy also shows excellent results, with a relatively low overpotential of 260 mV at 10 mA cm-2. Stable ammonia production can be achieved for 50 h in a 16 cm2 flow electrolyzer in simulated working conditions. Our research provides a pathway for optimizing NO3RR through bifunctional catalysts in a tandem approach.

Oxidative reconstructed Ru-based nanoclusters forming heterostructures with lanthanide oxides for acidic water oxidation

Achieving rapid anodic oxygen evolution reaction (OER) kinetics and improving the stability of the corresponding ruthenium (Ru)-based catalysts is a current priority for the realisation of industrial water splitting. However, the activity and stability of O2 evolution in electrocatalysis are largely inhibited by the insufficient adsorption of the reactant H2O and too strong adsorption of the intermediate OOH*, as well as by the dissolution of the active site due to excessive oxidation. To solve this challenge, herein, we developed a regulatory strategy combining lanthanide oxides and metal oxidative reconfiguration. The introduction of Eu2O3 effectively promotes the adsorption of H2O, optimizes the adsorption energy of OOH*, and reduces the reaction energy barrier of acidic OER process. And the metal oxidation remodeling process exposed more active sites and prevented the peroxidation process. The optimized Ru/Eu2O3@CNT catalyst showed the highest catalytic activity and stability in acidic OER. Its mass activity was 1219.1 A gRu-1 and the TOF value reached 4.4 s-1 at 1.48 V. Additionally, Ru/Eu2O3@CNT after oxidative reconstruction demonstrates the industrially needed current density of 1.0 A cm-2 at 1.71 V in PEM electrolyser, achieving stability in excess of 200 h.

Engineering p-Orbital States via Molecular Modules in All-Organic Electrocatalysts toward Direct Water Oxidation

Oxygen evolution reaction (OER) is an indispensable anode reaction for sustainable hydrogen production from water electrolysis, yet overreliance on metal-based catalysts featured with vibrant d-electrons. It still has notable gap between metal-free and metal-based electrocatalysts, due to lacking accurate and efficient p-band regulation methods on non-metal atoms. Herein, a molecular modularization strategy is proposed for fine-tuning the p-orbital states of series metal-free covalent organic frameworks (COFs) for realizing OER performance beyond benchmark precious metal catalysts. Optimized combination of benzodioxazole/benzodiimide-based building blocks achieves an impressive applied potential of 1.670 ± 0.004 V versus reversible hydrogen electrode (RHE) and 1.735 ± 0.006 V versus RHE to deliver enhanced current densities of 0.5 and 1.0 A cm-2, respectively. Moreover, it holds a notable charge transfer amount (stands for a long service life) within operation period that outperforms all reported metal-free electrocatalysts. Operando differential electrochemical mass spectrometry (DEMS) with isotope labeling identifies the adsorbate evolution mechanism (AEM). A variety of spectroscopic techniques and density functional theory (DFT) calculations reveal that the p-band center of these catalysts can be shifted stepwise to optimize the oxygen intermediate adsorption and lower the reaction energy barrier. This work provides a novel perspective for enhancing the electrocatalytic performance of metal-free COFs.

Atomically Asymmetrical Ir-O-Co Sites Enable Efficient Chloride-Mediated Ethylene Electrooxidation in Neutral Seawater

The chloride-mediated ethylene oxidation reaction (EOR) of ethylene chlorohydrin (ECH) via electrocatalysis is practically attractive because of its sustainability and mild reaction conditions. However, the chlorine oxidation reaction (COR), which is essential for the above process, is commonly catalyzed by dimensionally stable anodes (DSAs) with high contents of precious Ru and/or Ir. The development of highly efficient COR electrocatalysts composed of nonprecious metals or decreased amounts of precious metals is highly desirable. Herein, we report a modified Co3O4 with a single-atom Ir substitution (Ir1/Co3O4) as a highly efficient COR electrocatalyst for chloride-mediated EOR to ECH in neutral seawater. Ir1/Co3O4 achieves a Faradaic efficiency (FE) of up to 94.8 % for ECH generation and remarkable stability. Combining experimental results and density functional theory (DFT) calculations, the unique atomically asymmetrical Ir-O-Co configuration with a strong electron coupling effect in Ir1/Co3O4 can accelerate electron transfer to increase the reaction kinetics and maintain the structural stability of Co3O4 during COR. Moreover, a coupling reaction system integrating the anodic chloride-mediated and cathodic H2O2-mediated EOR show a total FE of ~170 % for paired electrosynthesis of ECH and ethylene glycol (EG) using ethylene as the raw material. The technoeconomic analysis highlights the promising application prospects of this system.

Asymmetric Rh-O-Co bridge sites enable superior bifunctional catalysis for hydrazine-assisted hydrogen production

Hydrazine-assisted water splitting is a promising strategy for energy-efficient hydrogen production, yet challenges remain in developing effective catalysts that can concurrently catalyze both the hydrogen evolution reaction (HER) and hydrazine oxidation reaction (HzOR) in acidic media. Herein, we report an effective bifunctional catalyst consisting of Rh clusters anchored on Co3O4 branched nanosheets (Rh-Co3O4 BNSs) synthesized via an innovative arginine-induced strategy. The Rh-Co3O4 BNSs exhibit unique Rh-O-Co interfacial sites that facilitate charge redistribution between Rh clusters and the Co3O4 substrate, thereby optimizing their valence electronic structures. When the current density reaches 10 mA cm-2, the Rh-Co3O4 BNSs require working potentials of only 32 mV for the HER and 0.26 V for the HzOR, far surpassing commercial Pt/C. Furthermore, the Rh-Co3O4 BNSs can work efficiently for hydrazine-assisted water electrolysis with a low voltage of 0.34 V at 10 mA cm-2 and excellent stability. Theoretical calculations reveal that the optimized valence electronic structure within interfacial Rh-O-Co sites not only reduces the adsorption energy barrier of Co3O4 for H* in the HER; but also optimizes the hydrazine adsorption in the HzOR and lowers the free energy change in the potential-determining step, where the facilitated dehydrogenation is observed in in situ Raman spectra. This work provides a viable approach for designing efficient bifunctional catalysts for future hydrazine-assisted hydrogen production.

Controlled Structural Activation of Iridium Single Atom Catalyst for High-Performance Proton Exchange Membrane Water Electrolysis

Iridium single atom catalysts are promising oxygen evolution reaction (OER) electrocatalysts for proton exchange membrane water electrolysis (PEMWE), as they can reduce the reliance on costly Ir in the OER catalysts. However, their practical application is hindered by their limited stability during PEMWE operation. Herein, we report on the activation of Ir-doped CoMn2O4 in acidic electrolyte that leads to enhanced activity and stability in acidic OER for long-term PEMWE operation. In-depth material characterization combined with electrochemical analysis and theoretical calculations reveal that activating Ir-doped CoMn2O4 induces controlled restructuring of Ir single atoms to IrOx nanoclusters, resulting in an optimized Ir configuration with outstanding mass activity of 3562 A gIr-1 at 1.53 V (vs RHE) and enhanced OER stability. The PEMWE using activated Ir-doped CoMn2O4 exhibited a stable operation for >1000 h at 250 mA cm-2 with a low degradation rate of 0.013 mV h-1, demonstrating its practical applicability. Furthermore, it remained stable for more than 400 h at a high current density of 1000 mA cm-2, demonstrating long-term durability under practical operation conditions.

Tunning valence state of cobalt centers in Cu/Co-CoO1-x for significantly boosting water-gas shift reaction

Dual active sites with synergistic valence state regulation under oxidizing and reducing conditions are essential for catalytic reactions with step-wise mechanisms to modulate the complex adsorption sites of reactant molecules on the surfaces of heterogeneous catalysts with maximized catalytic performances, but it has been rarely explored. In this work, uniformly dispersed CuCo alloy and CoO nanosheet composite catalysts with dual active sites are constructed, which shows huge boost in activity for catalyzing water-gas shift reaction (WGSR), with a record high reaction rate reaching 204.2 μmolCO gcat.-1 s-1 at 300 °C for Cu1Co9Ox amongst the reported Cu-based and Co-based catalysts. A synergistic mechanism is proposed that Coδ+ species can be easily reduced by CO adsorbed on Cu and Co0 can be oxidized by H2O. Systematic in situ characterization results reveal that the addition of Cu can regulate the redox properties of Co species and thus modulate the adsorption properties of catalysts. Particularly, doping of Cu0 sites weakens the affinity of the surface to CO or CO2 to a moderate level. Moreover, it also promotes the oxidation of *CO to *COOH and the desorption of the product CO2, reducing the carbon poisoning of the catalyst and thus increasing the reactivity. The results would provide guidance for the construction of novel heterogeneous catalyst with dual active sites and clarify its underlying reactivity enhancement mechanism induced by the tunning of valence state of metal centers for heterogeneous catalytic reactions.

Enhanced Acidic Oxygen Evolution Reaction Performance by Anchoring Iridium Oxide Nanoparticles on Co3O4

The sluggish kinetics of the anodic process, known as the oxygen evolution reaction (OER), has posed a significant challenge for the practical application of proton exchange membrane water electrolyzers in industrial settings. This study introduces a high-performance OER catalyst by anchoring iridium oxide nanoparticles (IrO2) onto a cobalt oxide (Co3O4) substrate via a two-step combustion method. The resulting IrO2@Co3O4 catalyst demonstrates a significant enhancement in both catalytic activity and stability in acidic environments. Notably, the overpotential required to attain a current density of 10 mA cm-2, a commonly used benchmark for comparison, is merely 301 mV. Furthermore, stability is maintained over a duration of 80 h, as confirmed by the minimal rise in overpotential. Energy spectrum characterizations and experimental results reveal that the generation of OER-active Ir3+ species on the IrO2@Co3O4 surface is induced by the strong interaction between IrO2 and Co3O4. Theoretical calculations further indicate that IrO2 sites loaded onto Co3O4 have a lower energy barrier for *OOH deprotonation to form desorbed O2. Moreover, this interaction also stabilizes the iridium active sites by maintaining their chemical state, leading to superior long-term stability. These insights could significantly impact the strategies for designing and synthesizing more efficient OER electrocatalysts for broader industrial application.

Ir-O-Mn embedded in porous nanosheets enhances charge transfer in low-iridium PEM electrolyzers

Using metal oxides to disperse iridium (Ir) in the anode layer proves effective for lowering Ir loading in proton exchange membrane water electrolyzers (PEMWE). However, the reported low-Ir-based catalysts still suffer from unsatisfying electrolytic efficiency and durability under practical industrial working conditions, mainly due to insufficient catalytic activity and mass transport in the catalyst layer. Herein we report a class of porous heterogeneous nanosheet catalyst with abundant Ir-O-Mn bonds, achieving a notable mass activity of 4 A mgIr-1 for oxygen evolution reaction at an overpotential of 300 mV, which is 150.6 times higher than that of commercial IrO2. Ir-O-Mn bonds are unraveled to serve as efficient charge-transfer channels between in-situ electrochemically-formed IrOx clusters and MnOx matrix, fostering the generation and stabilization of highly active Ir3+ species. Notably, Ir/MnOx-based PEMWE demonstrates comparable performance under 10-fold lower Ir loading (0.2 mgIr cm-2), taking a low cell voltage of 1.63 V to deliver 1 A cm-2 for over 300 h, which positions it among the elite of low Ir-based PEMWEs.

Boosting electrocatalytic hydrogen evolution via partial oxidation of rhenium through cobalt modification in nanoalloy structure

Although the theoretical electrocatalytic activity of rhenium (Re) for the hydrogen evolution reaction is comparable to that of platinum, the experimental performance of reported rhenium-based electrocatalysts remains unsatisfactory. Herein, we report a highly efficient and stable electrocatalyst composed of rhenium and cobalt (Co) nanoalloy embedded in nitrogen-doped carbon film (Re3Co2@NCF). The Re3Co2@NCF electrocatalyst exhibited remarkable hydrogen evolution performance, with an overpotential as low as 30 ± 3 mV to reach a current density of 10 mA cm-2. In addition, the Re3Co2@NCF demonstrated exceptional stability over several days at a current density of 150 mA cm-2. Theoretical calculations revealed that alloying cobalt with rhenium altered the electronic structure of the metals, causing partial oxidation of the superficial metal atoms. This modification provided a balance for various intermediates' adsorption and desorption, thereby boosting the intrinsic activity of rhenium for hydrogen evolution reaction. This work improves the electrocatalytic performance of rhenium to its theoretical activity, suggesting a promising future for rhenium-based electrocatalysts.

Constructing Highly Porous Low Iridium Anode Catalysts Via Dealloying for Proton Exchange Membrane Water Electrolyzers

Iridium (Ir) is the most active and durable anode catalyst for the oxygen evolution reaction (OER) for proton exchange membrane water electrolyzers (PEMWEs). However, their large-scale applications are hindered by high costs and scarcity of Ir. Lowering Ir loadings below 1.0 mgcm-2 causes significantly reduced PEMWE performance and durability. Therefore, developing efficient low Ir-based catalysts is critical to widely commercializing PEMWEs. Herein, an approach is presented for designing porous Ir metal aerogel (MA) catalysts via chemically dealloying IrCu alloys. The unique hierarchical pore structures and multiple channels of the Ir MA catalyst significantly increase electrochemical surface area (ECSA) and enhance OER activity compared to conventional Ir black catalysts, providing an effective solution to design low-Ir catalysts with improved Ir utilization and enhanced stability. An optimized membrane electrode assembly (MEA) with an Ir loading of 0.5 mgIr cm-2 generated 2.0 A cm-2 at 1.79 V, higher than the Ir black at a loading of 2.0 mgIr cm-2 (1.63 A cm-2). The low-Ir MEA demonstrated an acceptable decay rate of ≈40 µV h-1 during durability tests at 0.5 (>1200 h) and 2.0 A cm-2 (400 h), outperforming the commercial Ir-based MEA (175 µV h-1 at 2.0 mgIr cm-2).

Built-in electric field control of electron redistribution (NiFe-based electrocatalyst) with efficient overall water splitting at industrial temperature

Nickel-iron layered double hydroxide (NiFe-LDH) is hindered in its further development in water splitting due to its slow kinetics of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). In this study, the synthesis of OER (FeO(OH)/NiFe-LDH) and HER (Fe7S8(NiS)/NiFe-LDH) catalysts endowed with inherent electric fields exhibited exceptional electrocatalytic properties. The presence of the built-in electric field modulated the redistribution of electrons within the catalyst, while the formation of a heterostructure preserved the intrinsic characteristics of the catalyst. Moreover, this electron redistribution optimized the catalyst's adsorption of reaction intermediates (O*, OH*, OOH*, and H*) during the catalytic process, thereby enhancing the performance of both OER and HER. The electrolytic cell, equipped with these catalysts, achieved the current density of 10 mA cm-2 at a remarkably low potential of 1.409 V under industrial temperature conditions and demonstrated an ultra-long-term stability of 200 h.

Integrating Interactive Ir Atoms into Titanium Oxide Lattice for Proton Exchange Membrane Electrolysis

Iridium (Ir)-based oxide is the state-of-the-art electrocatalyst for acidic water oxidation, yet it is restricted to a few Ir-O octahedral packing modes with limited structural flexibility. Herein, the geometric structure diversification of Ir is achieved by integrating spatially correlated Ir atoms into the surface lattice of TiO2 and its booting effect on oxygen evolution reaction (OER) is investigated. Notably, the resultant i-Ir/TiO2 catalyst exhibits much higher electrocatalytic activity, with an overpotential of 240 mV at 10 mA cm-2 and excellent stability of 315 h at 100 mA cm-2 in acidic electrolyte. Both experimental and theoretical findings reveal that flexible Ir─O─Ir coordination with varied geometric structure plays a crucial role in enhancing OER activity, which optimize the intermediate adsorption by adjusting the d-band center of active Ir sites. Operando characterizations demonstrate that the interactive Ir─O─Ir units can suppress over-oxidation of Ir, effectively widening the stable region of Ir species during the catalytic process. The proton exchange membrane (PEM) electrolyzer, equipped with i-Ir/TiO2 as an anode, gives a low driving voltage of 1.63 V at 2 A cm-2 and maintains stable performance for over 440 h. This work presents a general strategy to eliminate the inherent geometric limitations of IrOx species, thereby inspiring further development of advanced catalyst designs.

Out-of-plane coordination of iridium single atoms with organic molecules and cobalt-iron hydroxides to boost oxygen evolution reaction

Advancements in single-atom-based catalysts are crucial for enhancing oxygen evolution reaction (OER) performance while reducing precious metal usage. A comprehensive understanding of underlying mechanisms will expedite this progress further. Here we report Ir single atoms coordinated out-of-plane with dimethylimidazole (MI) on CoFe hydroxide (Ir1/(Co,Fe)-OH/MI). This Ir1/(Co,Fe)-OH/MI catalyst, which was prepared using a simple immersion method, delivers ultralow overpotentials of 179 mV at a current density of 10 mA cm-2 and 257 mV at 600 mA cm-2 as well as an ultra-small Tafel slope of 24 mV dec-1. Furthermore, Ir1/(Co,Fe)-OH/MI has a total mass activity exceeding that of commercial IrO2 by a factor of 58.4. Ab initio simulations indicate that the coordination of MI leads to electron redistribution around the Ir sites. This causes a positive shift in the d-band centre at adjacent Ir and Co sites, facilitating an optimal energy pathway for OER.

Breaking the Mutual-Constraint of Bifunctional Oxygen Electrocatalysis via Direct O─O Coupling on High-Valence Ir Single-Atom on MnOx

Insufficient bifunctional activity of electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is the major obstruction to the application of rechargeable metal-air batteries. The primary reason is the mutual constraint of ORR and OER mechanism, involving the same oxygen-containing intermediates and demonstrating the scaling limitations of the adsorption energies. Herein, it is reported a high-valence Ir single atom anchored on manganese oxide (IrSA-MnOx) bifunctional catalyst showing independent pathways for ORR and OER, i.e., associated 4e- pathway on high-valence Ir site for ORR and a novel chemical-activated concerted mechanism for OER, where a distinct spontaneous chemical activation process triggers direct O─O coupling. The IrSA-MnOx therefore delivers outstanding bifunctional activities with remarkably low potential difference (0.635 V) between OER potential at 10 mA cm-2 and ORR half-wave potential in alkaline solution. This work breaks the scaling limitations and provides a new avenue to design efficient and multifunctional electrocatalysts.

Regulating Local Coordination Sphere of Ir Single Atoms at the Atomic Interface for Efficient Oxygen Evolution Reaction

Single-atom catalysts dispersed on an oxide support are essential for overcoming the sluggishness of the oxygen evolution reaction (OER). However, the durability of most metal single-atoms is compromised under harsh OER conditions due to their low coordination (weak metal-support interactions) and excessive disruption of metal-Olattice bonds to enable lattice oxygen participation, leading to metal dissolution and hindering their practical applicability. Herein, we systematically regulate the local coordination of Irsingle-atoms at the atomic level to enhance the performance of the OER by precisely modulating their steric localization on the NiO surface. Compared to conventional Irsingle-atoms adsorbed on NiO surface, the atomic Ir atoms partially embedded within the NiO surface (Iremb-NiO) exhibit a 2-fold increase in Ir-Ni second-shell interaction revealed by X-ray absorption spectroscopy (XAS), suggesting stronger metal-support interactions. Remarkably, Iremb-NiO with tailored coordination sphere exhibits excellent alkaline OER mass activity and long-term durability (degradation rate: ∼1 mV/h), outperforming commercial IrO2 (∼26 mV/h) and conventional Irsingle-atoms on NiO (∼7 mV/h). Comprehensive operando X-ray absorption and Raman spectroscopies, along with pH-dependence activity tests, identified high-valence atomic Ir sites embedded on the NiOOH surface during the OER followed the lattice oxygen mechanism, thereby circumventing the traditional linear scaling relationships. Moreover, the enhanced Ir-Ni second-shell interaction in Iremb-NiO plays a crucial role in imparting structural rigidity to Ir single-atoms, thereby mitigating Ir-dissolution and ensuring superior OER kinetics alongside sustained durability.

Multifunctional Strategies of Advanced Electrocatalysts for Efficient Urea Synthesis

The electrochemical reduction of nitrogenous species (such as N2, NO, NO2 -, and NO3 -) for urea synthesis under ambient conditions has been extensively studied due to their potential to realize carbon/nitrogen neutrality and mitigate environmental pollution, as well as provide a means to store renewable electricity generated from intermittent sources such as wind and solar power. However, the sluggish reaction kinetics and the scarcity of active sites on electrocatalysts have significantly hindered the advancement of their practical applications. Multifunctional engineering of electrocatalysts has been rationally designed and investigated to adjust their electronic structures, increase the density of active sites, and optimize the binding energies to enhance electrocatalytic performance. Here, surface engineering, defect engineering, doping engineering, and heterostructure engineering strategies for efficient nitrogen electro-reduction are comprehensively summarized. The role of each element in engineered electrocatalysts is elucidated at the atomic level, revealing the intrinsic active site, and understanding the relationship between atomic structure and catalytic performance. This review highlights the state-of-the-art progress of electrocatalytic reactions of waste nitrogenous species into urea. Moreover, this review outlines the challenges and opportunities for urea synthesis and aims to facilitate further research into the development of advanced electrocatalysts for a sustainable future.

High Coverage Sub-Nano Iridium Cluster on Core-Shell Cobalt-Cerium Bimetallic Oxide for Highly Efficient Full-pH Water Splitting

The construction of sub-nanometer cluster catalysts (<1 nm) with almost complete exposure of active atoms serves as a promising avenue for the simultaneous enhancement of atom utilization efficiency and specific activity. Herein, a core-shell cobalt-cerium bimetallic oxide protected by high coverage sub-nanometer Ir clusters (denoted as Ir cluster@CoO/CeO2) is constructed by a confined in situ exsolution strategy. The distinctive core-shell structure endows Ir cluster@CoO/CeO2 with enhanced intrinsic activity and high conductivity, facilitating efficient charge transfer and full-pH water splitting. The Ir cluster@CoO/CeO2 achieves low overpotentials of 49/215, 52/390, and 54/243 mV at 10 mA cm-2 for hydrogen evolution reaction/oxygen evolution reaction (HER/OER) in 0.5 m H2SO4, 1.0 m PBS, and 1.0 m KOH, respectively. The small decline in performance after 300 h of operation renders it one of the most effective catalysts for full-pH water splitting. DFT calculations indicate that oriented electron transfer (along the path from Ce to Co and then to Ir) creates an electron-rich environment for surface Ir clusters. The reconstructed interface electronic environment provides optimized intermediates adsorption/desorption energy at the Ir site (for HER) and at the Ir-Co site (for OER), thus simultaneously speeding up the HER/OER kinetics.

Lattice-Confined Single-Atom Catalyst: Preparation, Application and Electron Regulation Mechanism

Lattice-confined single-atom catalyst (LC SAC), featuring exceptional activity, intriguing stability and prominent selectivity, has attracted extensive attention in the fields of various reactions (e.g., hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), etc.). To design a "smart" LC SAC for catalytic applications, one must systematically comprehend updated advances in the preparation, the application, and especially the peculiar electron regulation mechanism of LC SAC. In this review, the specific preparation methods of LC SAC based on general coordination strategy are updated, and its applications in HER, OER, ORR, N2 reduction reaction (NRR), advanced oxidation processes (AOPs) and so forth are summarized to display outstanding activity, stability and selectivity. Uniquely, the electron regulation mechanisms are first and deeply discussed and can be primarily categorized as electron transfer bridge with monometallic active sites, novel catalytic centers with polymetallic active sites, and positive influence by surrounding environments. In the end, the existing issues and future development directions are put forward with a view to further optimize the performance of LC SAC. This review is expected to contribute to the in-depth understanding and practical application of highly efficient LC SAC.

Research Progress in Structure Evolution and Durability Modulation of Ir- and Ru-Based OER Catalysts under Acidic Conditions

Green hydrogen energy, as one of the most promising energy carriers, plays a crucial role in addressing energy and environmental issues. Oxygen evolution reaction catalysts, as the key to water electrolysis hydrogen production technology, have been subject to durability constraints, preventing large-scale commercial development. Under the high current density and harsh acid-base electrolyte conditions of the water electrolysis reaction, the active metals in the catalysts are easily converted into high-valent soluble species to dissolve, leading to poor structural durability of the catalysts. There is an urgent need to overcome the durability challenges under acidic conditions and develop electrocatalysts with both high catalytic activity and high durability. In this review, the latest research results are analyzed in depth from both thermodynamic and kinetic perspectives. First, a comprehensive summary of the structural deactivation state process of noble metal oxide catalysts is presented. Second, the evolution of the structure of catalysts possessing high durability is discussed. Finally, four new strategies for the preparation of stable catalysts, "electron buffer (ECB) strategy", combination strength control, strain control, and surface coating, are summarized. The challenges and prospects are also elaborated for the future synthesis of more effective Ru/Ir-based catalysts and boost their future application.

Local compressive strain-induced anti-corrosion over isolated Ru-decorated Co3O4 for efficient acidic oxygen evolution

Enhancing corrosion resistance is essential for developing efficient electrocatalysts for acidic oxygen evolution reaction (OER). Herein, we report the strategic manipulation of the local compressive strain to reinforce the anti-corrosion properties of the non-precious Co3O4 support. The incorporation of Ru single atoms, larger in atomic size than Co, into the Co3O4 lattice (Ru-Co3O4), triggers localized strain compression and lattice distortion on the Co-O lattice. A comprehensive exploration of the correlation between this specific local compressive strain and electrocatalytic performance is conducted through experimental and theoretical analyses. The presence of the localized strain in Ru-Co3O4 is confirmed by operando X-ray absorption studies and supported by quantum calculations. This local strain, presented in a shortened Co-O bond length, enhances the anti-corrosion properties of Co3O4 by suppressing metal dissolutions. Consequently, Ru-Co3O4 shows satisfactory stability, maintaining OER for over 400 hours at 30 mA cm-2 with minimal decay. This study demonstrates the potential of the local strain effect in fortifying catalyst stability for acidic OER and beyond.

Hollow Structure Derived Phosphide Nanosheets for Water Oxidation

Avoiding the stacking of active sites in catalyst structural design is a promising route for realizing active oxygen evolution reaction (OER). Herein, using a CoFe Prussian blue analoge cube with hollow structure (C-CoFe PBA) as a derived support, a highly effective Ni2P-FeP4-Co2P catalyst with a larger specific surface area is reported. Benefiting from the abundant active sites and fast charge transfer capability of the phosphide nanosheets, the Ni2P-FeP4-Co2P catalyst in 1 m KOH requires only overpotentials of 248 and 277 mV to reach current density of 10 and 50 mA cm-2 and outperforms the commercial catalyst RuO2 and most reported non-noble metal OER catalysts. In addition, the two-electrode system consisting of Ni2P-FeP4-Co2P and Pt/C is able to achieve a current density of 10 and 50 mA cm-2 at 1.529 and 1.65 V. This work provides more ideas and directions for synthesizing transition metal catalysts for efficient OER performance.