A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Morphology and composition regulation of Prussian blue analogues to boost electrocatalytic oxygen evolution reaction

Prussian blue analogues (PBAs) have well-dispersed active sites and porous nanostructure. Reasonable design and construction of the nanostructure of PBAs is a promising option to obtain cost-effective electrocatalysts for high-efficiency oxygen evolution reaction (OER). Nevertheless, current structural engineering is costly and complex due to the inevitable involvement of additional etching agents or procedures. In this paper, a simple temperature-control strategy without additional etchant, is presented for the preparation of hollow CoFe-PBA precursors with porous nanobox structure, and then the hollow CoFe-PBA@NiFeRu-LDH nanoboxes (named as CoFe-PBA@NiFeRu-LDH NBs) heterogeneous catalyst is obtained. The preferable composition and structure provide more accessible active sites and better electronic structure for OER. As a result, the optimized CoFe-PBA@NiFeRu-LDH NBs demonstrates an impressive ability to accelerate OER in alkaline electrolyte with a minimal overpotential (219 mV at 10 mA cm-2) and excellent stability. More importantly, by combining CoFe-PBA@NiFeRu-LDH NBs with Pt/C for overall water electrolysis, an ultra-low voltage (1.52 V at 10 mA cm-2) is required. This study offers a facile and effective idea for chemical and morphological control in the manufacture of efficient electrocatalysts.

Surface Reconstruction of Amorphous Ni─Co─S─O Material with a Functional Gradient Layer for Highly Efficient and Stable Alkaline Hydrogen Evolution

Alkaline water electrolysis holds potential for large-scale, high-purity hydrogen production. However, the slow kinetics of water dissociation and challenging conditions in alkaline environments complicate the search for an electrocatalyst with both high activity and durability. In this study, a highly active and stable amorphous electrocatalyst is introduced, 3Ni─Co─S─O, developed via a straightforward electrodeposition method for alkaline hydrogen evolution reaction (HER). Notably, the surface of the 3Ni─Co─S─O catalyst undergoes a compositional reconstruction during alkaline HER, yet maintains its amorphous structure. This reconstruction spans roughly 6 µm, leading to a gradual decrease in Ni and S content and a corresponding increase in O concentration toward the surface, thereby forming a stable, gradient active layer. Benefitting from this layer and its amorphous nature, the 3Ni─Co─S─O catalyst demonstrates superior alkaline HER activity-requiring only 170 mV to achieve an industrial HER current density of 1000 mA cm-2. It also showcases remarkable stability, with a mere 15 mV increase in overpotential during continuous HER at 300 mA cm-2 for 24 h, outperforming the commercial Pt/C catalyst. The research provides a novel approach to designing high-performance amorphous alkaline HER electrocatalysts cost-effectively and contributes insights for understanding and developing advanced amorphous catalysts across various applications.

Oxygen vacancy induced construction of NiCo2O4/NiSe2 heterostructure electrocatalyst for high-efficiency oxygen evolution reaction

Spinel-type transition metal oxide (AB2O4) are promising non-precious metal catalysts for oxygen evolution reaction (OER), addressing the challenges of high cost and poor stability in electrocatalytic water splitting. However, their widespread application is hindered by low electrical conductivity and suboptimal performance. In this study, we developed an oxygen vacancy anchoring strategy to construct a highly efficient and durable NiCo2O4-NiSe2 heterojunction catalyst on a nickel foam substrate. The three-dimensional cross-linked porous nanorod array facilitates mass and charge transportation. The abundant interconnected interfaces enhance electron transfer kinetics and adsorption of OH intermediates, as confirmed by theoretical simulations. As a result, the as-synthesized NiCo2O4@NiSe2/NF exhibits an overpotential of only 299 mV at a current density of 60 mA cm-2, significantly outperforming the single-phase NiCo2O4 and demonstrating exceptional durability.

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.

Ultrafast Synthesis of Nanoscale Metal Borides for Efficient Hydrogen Evolution

Nanoscale metal borides, with exceptional physicochemical properties, have been attracted widespread attention. However, traditional synthesis methods of metal borides often lead to surface coking and large particle sizes. Herein, we have employed a flash Joule heating (FJH) technique to enable the ultrafast synthesis of metal boride nanomaterials. The synthesized materials encompass a wide range of diverse categories, including alkaline-earth metal borides (CaB6), transition metal borides (TiB2, VB2, CrB2, MoB, MoB2, MnB2, MnB4, FeB, CoB, NiB), noble-metal borides (RuB2, RuB1.1), and rare-earth metal borides (LaB6, CeB6). As an example, the RuB2 demonstrates highly desirable electrocatalytic performance for all-pH hydrogen evolution reaction (HER). Especially, under the acidic condition, it exhibits an overpotential as low as 15 mV at a current density of 10 mA cm-2, with a nearly 100 % faradic efficiency. Additionally, in situ Raman spectra confirm that both Ru and B sites serve as active sites for the HER. Moreover, the stability of RuB2 can be further enhanced by optimizing the microenvironments of the anolyte composition (H+, K+). More importantly, the experimental and density functional theory (DFT) calculations reveal that the co-existence of H+ and K+ localized around the RuB2 plays a crucial role in further enhancing the stability.

Thiosulfate species promoting hydrogen evolution reaction at the heterointerface of Ir clusters-loaded WS2 nanosheets

We synthesize Ir cluster-loaded monolayer WS2 nanosheets for water electrolysis and fabricate unique functional heterointerfaces with thiosulfate species formed from sulfide ions of WS2. In situ XAFS measurements reveal that water dissociation is promoted between positively charged thiosulfates and negatively charged Ir clusters at the interfacial active sites for hydrogen evolution reaction.

Rationally-Constructed Porous Nanosheet Ni2P-Ni5P4 Heterostructures for Robust Oxygen Evolution Electrode

The development of superior non-noble-metal-based oxygen evolution reaction (OER) electrocatalysts is essential for large-scale hydrogen production. In this study, an integrated porous nanosheet Ni2P-Ni5P4 heterostructures were designed as an excellent OER electrocatalyst. The synthesized heterostructures demonstrated notable activity, achieving a small overpotential of 260 mV to sustain a typical 10 mA cm-2 current density, along with exceptional durability over 2000 CV cycles. The distinctive porous nanosheet structure enhances the exposure of active sites and improves mass transport efficiency. Density functional theory (DFT) calculations revealed that the d-band center of active Ni sites was shifted downward, reducing the adsorption strength of critical oxygen-containing intermediates (*O, *OH, and *OOH) in the Ni2P-Ni5P4 heterostructures. This modification lowered the reaction barrier for the rate-determining step (RDS) involving the transformation from *O to *OOH, thereby boosting inherent OER activity. Additionally, partial electron localization in combination with the RDS *O intermediate was observed by electron localization functions (ELFs) in Ni2P-Ni5P4, weakening the overall interaction. Further integrated crystal orbital Hamiltonian population calculations confirmed a reduced Ni-O net bonding energy of 0.69 eV for the adsorbed *O compared to that of Ni2P (1.49 eV) and Ni5P4 (1.12 eV) aligning with the DFT and ELF findings. These results provide a promising approach and valuable guidance for the design of cost-effective OER electrocatalysts suitable for energy storage applications, including metal-air cells and water oxidation processes.

Leveraging Iron in the Electrolyte to Improve Oxygen Evolution Reaction Performance: Fundamentals, Strategies, and Perspectives

Electrochemical water splitting is a pivotal technology for storing intermittent electricity from renewable sources into hydrogen fuel. However, its overall energy efficiency is impeded by the sluggish oxygen evolution reaction (OER) at the anode. In the quest to design high-performance anode catalysts for driving the OER under non-acidic conditions, iron (Fe) has emerged as a crucial element. Although the profound impact of adventitious electrolyte Fen+ species on OER catalysis had been reported forty years ago, recent interest in tailoring the electrode-electrolyte interface has spurred studies on the controlled introduction of Fe ions into the electrolyte to improve OER performance. During the catalytic process, scenarios where the rate of Fen+ deposition on a specific host material outruns that of dissolution pave the way for establishing highly efficient and dynamically stable electrochemical interfaces for long-term steady operation. This review systematically summarizes recent endeavors devoted to elucidating the behaviors of in situ Fe(aq.) incorporation, the role of incorporated Fe sites in the OER, and critical factors influencing the interplay between the electrode surface and Fe ions in the electrolyte environment. Finally, unexplored issues related to comprehensively understanding and leveraging the dynamic exchange of Fen+ at the interface for improved OER catalysis are summarized.

Nearly Barrierless Four-Hole Water Oxidation Catalysis on Semiconductor Photoanodes with High Density of Accumulated Surface Holes

The sluggish water oxidation reaction (WOR) is considered the kinetic bottleneck of artificial photosynthesis due to the complicated four-electron and four-proton transfer process. Herein, we find that the WOR can be kinetically nearly barrierless on four representative photoanodes (i.e., α-Fe2O3, TiO2, WO3, and BiVO4) under concentrated light irradiation, wherein the rate-limiting O-O bond formation step is driven by accumulated surface photogenerated holes that exhibit a superior fourth-order kinetics. The activation energy is about 0.03 eV for the fourth-order kinetic pathway, which is quantitatively estimated by combining the Population model and Butler-Volmer model with the Eyring-like equation and is further confirmed by density functional theory calculations. The WOR rate under this condition shows more than 1 order of magnitude enhancement compared with that of first-, second-, or third-order kinetics. Focusing on α-Fe2O3, the accumulated high-density surface holes form adjacent FeV═O intermediates that effectively activate surface-adsorbed H2O molecules via the hydrogen-bonding effect, as revealed by operando Raman measurements and ab initio molecular dynamics simulations. This work discloses a systematic understanding of the internal relations between activation energy and reaction orders of surface holes for future WOR study.

MXene-Assisted NiFe sulfides for high-performance anion exchange membrane seawater electrolysis

Anion exchange membrane seawater electrolysis is vital for future large-scale green hydrogen production, however enduring a huge challenge that lacks high-stable oxygen evolution reaction electrocatalysts. Herein, we report a robust OER electrocatalyst for AEMSE by integrating MXene (Ti3C2) with NiFe sulfides ((Ni,Fe)S2@Ti3C2). The strong interaction between (Ni,Fe)S2 and Ti3C2 induces electron distribution to trigger lattice oxygen mechanism, improving the intrinsic activity, and particularly prohibits the dissolution of Fe species during OER process via the Ti-O-Fe bonding effectively, achieving notable stability. Furthermore, the good retention of sulfates and the abundant groups of Ti3C2 provide effective Cl- resistance. Accordingly, (Ni,Fe)S2@Ti3C2 achieves high OER activity (1.598 V@2 A cm-2) and long-term durability (1000 h) in seawater system. Furthermore, AEMSE with industrial current density (0.5 A cm-2) and durability (500 h) is achieved by (Ni,Fe)S2@Ti3C2 anode and Raney Ni cathode with electrolysis efficiency of 70% and energy consumption of 48.4 kWh kg-1 H2.

A facile synthesis of N-doped carbon encapsulated multimetallic carbonitride as a robust electrocatalyst for oxygen evolution reaction

Electrocatalytic water splitting is a promising solution for generating clean hydrogen. Transition metal compounds are among the most extensively investigated catalysts developed to date for water oxidation in alkaline media, a process also known as the oxygen evolution reaction (OER). However, the application of these catalysts was constrained by insufficient stability arising from surface oxidation and metal dissolution under high OER potential. In this work, we developed a facile approach using urea-based gel as the precursor of preparing a series of multimetallic carbonitride particles which were encapsulated by N-doped carbon (NC). In particular, (MoCoFeNiZr)CN@NC core-shell structure delivered a low overpotential of 246 mV at a current density of 10 mA cm-2 in 1 M KOH during OER. Importantly, operando differential electrochemical mass spectrometry (DEMS), together with multiple microscopic and spectroscopic analyses, indicated that the NC shells effectively maintained the crystalline stability of carbonitride via suppressing the surface reconstruction during catalysis. The highly graphitic NC also demonstrates excellent stability against oxidation. This work shows a promising strategy of stabilizing electrocatalyst at high anodic potential, paving the way for the development of robust electrode materials for energy conversion.

Ultrafast Synthesis of IrB1.15 Nanocrystals for Efficient Chlorine and Hydrogen Evolution Reactions in Saline Water

The production of storable hydrogen fuel through water electrolysis powered by renewable energy sources such as solar, marine, geothermal, and wind energy presents a promising pathway toward achieving energy sustainability. Nevertheless, state-of-the-art electrolysis requires support from ancillary processes which often incur financial and energy costs. Developing electrolysers capable of directly operating with water that contains impurities can circumvent these processes. Herein, we demonstrate the efficient and durable electrolysis of saline water to produce chlorine gas (Cl2) and hydrogen using structurally ordered IrB1.15, synthesized through ultrafast joule heating. IrB1.15 exhibits remarkable performance, achieving overpotentials of 75 mV for the chlorine evolution reaction (CER) and 12 mV for hydrogen evolution reactions (HER) at current densities of 10 mA cm-2. Moreover, IrB1.15 displays a durability of over 90 h towards both CER and HER. Density functional theory reveals that IrB1.15 has adsorption energies significantly closer to 0 eV for Cl and H, compared to IrO2 and Pt/C. Furthermore, in situ Raman investigations reveal that Ir in IrB1.15 serves as the active center for CER, while the introduction of B atoms to Ir lattices mitigates the formation of absorbed hydrogen species on the Ir surface, thereby enhancing the performance of IrB1.15 in HER.

In-situ synthesis to promote surface reconstruction of metal-organic frameworks for high-performance water/seawater oxidation

Metal-organic frameworks (MOFs) have gained tremendous notice for the application in alkaline water/seawater oxidation due to their tunable structures and abundant accessible metal sites. However, exploring cost-effective oxygen evolution reaction (OER) electrocatalysts with high catalytic activity and excellent stability remains a great challenge. In this work, a promising strategy is proposed to regulate the crystalline structures and electronic properties of NiFe-metal-organic frameworks (NiFe-MOFs) by altering the organic ligands. As a representative sample, NiFe-BDC (BDC: C8H6O4) synthesized on nickel foam (NF) shows extraordinary OER activity in alkaline condition, delivering ultralow overpotentials of 204, 234 and 273 mV at 10, 100, and 300 mA cm-2, respectively, with a small Tafel slope of 21.6 mV dec-1. Only a slight decrease is observed when operating in alkaline seawater. The potential attenuation is barely identified at 200 mA cm-2 over 200 h continuous test, indicating the remarkable stability and corrosion resistance. In-situ measurements indicate that initial Ni2+/Fe2+ goes through oxidation process into Ni3+/Fe3+ during OER, and eventually presents in the form of NiFeOOH/NiFe-BDC heterojunction. The unique self-reconstructed surface is responsible for the low reaction barrier and fast reaction kinetics. This work provides an effective strategy to develop efficient MOF-based electrocatalysts and an insightful view on the dynamic structural evolution during OER.

Sulfur oxidation mediated controllable reconstruction on LiCo1.9Fe0.1O4 for boosted electrochemical water oxidation

Appropriate contact between catalysts and reactants calls for optimized exposure of active sites in the near-surface region, which can be accomplished by tuning the surface reconstruction degree. Understanding and conducting the controllable surface reconstruction of oxygen evolution reaction (OER) catalysts lays the foundation to finetune their OER activity. Herein, we explore the construction of a tunable amorphous oxyhydroxide shell on LiCo1.9Fe0.1O4via heat-sulfurization, followed by electrochemical treatment. The 8-electron sulfide oxidation reaction (SOR) transforms the sulfide shell to amorphous oxyhydroxide and generates surface-anchored SO42-, which act together to boost the OER. The electrocatalyst with optimal sulfurization exhibits 2.57 times higher than the current density at 1.6 V vs. RHE compared to the original LCFO. This work is dedicated to understanding controllable reconstruction and designing efficient OER electrocatalysts.

Investigation of Electron Transfer Properties on Silicalite-1 Zeolite for Potential Electrocatalytic Applications

To develop high-performance electrocatalysts is critical to sustainable conversion and storage of renewable energy. Silicalite-1 (S-1) zeolite is considered promising for constructing electrocatalysts featuring uniform and precise porosity and a stable structural skeleton even at extreme potentials. However, its electrochemical properties remain poorly understood, particularly regarding the roles of internal pore channels. Herein, inner- and outer-sphere electron transfer (ISET/OSET) routes on the S-1 zeolite were investigated by classical redox probes. The results for the first time revealed that the ISET kinetics inside the pores of S-1 zeolite is more rapid than that on external surfaces, optimized by microporous scale channels and terminated hydroxyl groups. Conversely, the kinetics of the OSET did not closely depend on the porosity and surface properties of the S-1 zeolite. These electrochemical insights further initiated a lithium-ion-incorporated S-1 zeolite with rapid ISET kinetics for electrocatalysis of oxygen reduction. It demonstrated a high performance of 85% selectivity for H2O2 production in a neutral solution and a yield of 9.2 mol gcat-1 h-1 when configured in a flow cell.

Assembly and Valence Modulation of Ordered Bimetallic MOFs for Highly Efficient Electrocatalytic Water Oxidation

Metal synergy can enhance the catalytic performance, and a prefabricated solid precursor can guide the ordered embedding, of secondary metal source ions for the rapid synthesis of bimetallic organic frameworks (MM'-MOFs) with a stoichiometric ratio of 1:1. In this paper, Co-MOF-1D containing well-defined binding sites was synthesized by mechanical ball milling, which was used as a template for the induced introduction of Fe ions to successfully assemble the ordered bimetallic Co1Fe1-MOF-74@2 (where @2 denotes template-directed synthesis of MOF-74). Its electrocatalytic performance is superior to that of the conventional one-step-synthesized Co1Fe1-MOF-74@1 (where @1 denotes one-step synthesis of MOF-74), and the ratio of the two metal sources, Co and Fe, is close to 1:1. Meanwhile, the iron valence states (FeII and FeIII) in Co1Fe1-MOF-74@2 were further regulated to obtain the electrocatalytic materials Co1Fe1(II)-MOF-74@2 and Co1Fe1(III)-MOF-74@2. The electrochemical performance test results confirm that Co1Fe1(II)-MOF-74@2 regulated by valence state has a better catalytic performance than Co1Fe1(III)-MOF-74@2 in the oxygen evolution reaction (OER) process. This phenomenon is related to the gradual increase in the valence state of Fe ions in Co1Fe1(II)-MOF-74@2, which promotes the continuous improvement in the performance of the MOF before reaching the optimal steady state and makes the OER performance reach the optimum when the FeII/FeIII mixed-valence state reaches a certain proportion. This provides a new idea for the directed synthesis and optimization of highly efficient catalysts.

Construction of "Metal Defect/Oxygen Defect Junction" in ZnFe2O4-NiCo2O4 Heterostructures for Enhancing Electrocatalytic Oxygen Evolution

Defect engineering is a promising approach to improve the conductivity and increase the active sites of transition metal oxides used as catalysts for the oxygen evolution reaction (OER). However, when metal defects and oxygen defects coexist closely within the same crystal, their compensating charges can diminish the benefits of both defect structures on the catalyst's local electronic structure. To address this limitation, a novel strategy that employs the heterostructure interface of ZnFe2O4-NiCo2O4 to spatially separate the metal defects from the oxygen defects is proposed. This configuration positions the two types of defects on opposite sides of the heterojunction interface, creating a unique structure termed the "metal-defect/oxygen-defect junction". Physical characterization and simulations reveal that this configuration enhances electron transfer at the heterostructure interface, increases the oxidation state of Fe on the catalyst surface, and boosts bulk charge carrier concentration. These improvements enhance active site performance, facilitating hydroxyl adsorption and deprotonation, thereby reducing the overpotential required for the OER.

Plasma-Induced Oxygen Defect Engineering in Perovskite Oxide for Boosting Oxygen Evolution Reaction

Perovskite oxides are considered highly promising candidates for oxygen evolution reaction (OER) catalysts due to their low cost and adaptable electronic structure. However, modulating the electronic structure of catalysts without altering their nanomorphology is crucial for understanding the structure-property relationship. In this study, a simple plasma bombardment strategy is developed to optimize the catalytic activity of perovskite oxides. Experimental characterization of plasma-treated LaCo0.9Fe0.1O3 (P-LCFO) reveals abundant oxygen vacancies, which expose numerous active sites. Additionally, X-ray photoelectron spectroscopy and X-ray absorption fine structure analyses indicate a low Co valence state in P-LCFO, likely due to the presence of these oxygen vacancies, which contributes to an optimized electronic structure that enhances OER performance. Consequently, P-LCFO exhibits significantly improved OER catalytic activity, with a low overpotential of 294 mV at a current density of 10 mA cm-2, outperforming commercial RuO2. This work underscores the benefits of plasma engineering for studying structure-property relationships and developing highly active perovskite oxide catalysts for water splitting.

2D NiFe2O4/Ni(OH)2 Heterostructure-Based Self-Supporting Electrode With Synergistic Surface/Interfacial Engineering for Efficient Water Electrooxidation

To meet the industrial demand for overall water splitting, oxygen evolution reaction (OER) electrocatalysts with low-cost, highly effective, and durable properties are urgently required. Herein, a facile confined strategy is utilized to construct 2D NiFe2O4/Ni(OH)2 heterostructures-based self-supporting electrode with surface-interfacial coengineering, in which abundant and ultrastable interfaces are developed. Under the high molar ratio of Ni/Fe, both spinel oxide and hydroxides phases are formed simultaneously to obtain 2D NiFe2O4/Ni(OH)2 heterostructure. The in-depth analysis indicates that the NiFe2O4/Ni(OH)2 interface displays strong electronic interactions and triggers the formation of crystalline-amorphous coexisting catalytic active NiOOH. Meanwhile, the stable catalyst-collector interface favors the electron transfer and oxygen molecules transport. The resultant 2D NiFe2O4/Ni(OH)2@CP electrode exhibits superior OER performance, including a low overpotential of 389 mV and a long operating time of 12 h at 1 A cm-2. This work paves a novel method for fabricating efficient and low-cost electrocatalysts for electrochemical conversation devices.

Non-Noble-Metal-Based Electrocatalysts for Acidic Oxygen Evolution Reaction: Recent Progress, Challenges, and Perspectives

The oxygen evolution reaction (OER) plays a pivotal role in diverse renewable energy storage and conversion technologies, including water electrolysis, electrochemical CO2 reduction, nitrogen fixation, and metal-air batteries. Among various water electrolysis techniques, proton exchange membrane (PEM)-based water electrolysis devices offer numerous advantages, including high current densities, exceptional chemical stability, excellent proton conductivity, and high-purity H2. Nevertheless, the prohibitive cost associated with Ir/Ru-based OER electrocatalysts poses a significant barrier to the broad-scale application of PEM-based water splitting. Consequently, it is crucial to advance the development of non-noble metal OER catalysis substance with high acid-activity and stability, thereby fostering their widespread integration into PEM water electrolyzers (PEMWEs). In this review, a comprehensive analysis of the acidic OER mechanism, encompassing the adsorbate evolution mechanism (AEM), lattice oxygen mechanism (LOM) and oxide path mechanism (OPM) is offered. Subsequently, a systematic summary of recently reported noble-metal-free catalysts including transition metal-based, carbon-based and other types of catalysts is provided. Additionally, a comprehensive compilation of in situ/operando characterization techniques is provided, serving as invaluable tools for furnishing experimental evidence to comprehend the catalytic mechanism. Finally, the present challenges and future research directions concerning precious-metal-free acidic OER are comprehensively summarized and discussed in this review.

Vanadate-Mediated Mismatch Configuration over the Reconstructed Nickel-Iron Electrocatalyst for Boosting Alkaline Oxygen Evolution

During the oxygen evolution reaction (OER), catalyst candidates that can fully trigger self-reconstruction to derive active species with favorable configurations are expected to overcome the sluggish reaction kinetics. Herein, we innovatively propose the introduction of heterogeneous vanadate dopants into nickel-iron alloy precatalysts, where the crystal mismatch structure induces local electron delocalization in the hexagonal close packed alloy phase, thereby facilitating adequate electrochemical reconstruction to form (oxy)hydroxides as the real catalytic species. Simultaneously, the participation of vanadate in the reconstruction also triggers mismatch in the derived (oxy)hydroxides, reinforcing the metal-oxygen covalence, so that lattice oxygen activation is kinetically favorable and facilitates the OER via the lattice oxygen pathway. Optimized reconstructed catalyst r-NiFeVOx-NF exhibits a low overpotential of 220 mV at current densities of 10 mA cm-2 and considerably stable operation. Our study opens up opportunities for achieving robust OER performance through the design and fabrication of the mismatch catalytic configuration.

Structural Reconstruction of a Cobalt- and Ferrocene-Based Metal-Organic Framework during the Electrochemical Oxygen Evolution Reaction

Metal-organic frameworks (MOFs) are increasingly being investigated as electrocatalysts for the oxygen evolution reaction (OER) due to their unique modular structures that present a hybrid between molecular and heterogeneous catalysts, featuring well-defined active sites. However, many fundamental questions remain open regarding the electrochemical stability of MOFs, structural reconstruction of coordination sites, and the role of in situ-formed species. Here, we report the structural transformation of a surface-grown MOF containing cobalt nodes and 1,1'-ferrocenedicarboxylic acid linkers (denoted as CoFc-MOF) during the OER in alkaline electrolyte. Ex situ and in situ investigations of CoFc-MOF film suggest that the MOF acts as a precatalyst and undergoes a two-step restructuring process under operating conditions to generate a metal oxyhydroxide phase. The MOF-derived metal oxyhydroxide catalyst, supported on nickel foam electrodes, displays high activity toward the OER with an overpotential of 190 mV at a current density of 10 mA cm-2. While this study demonstrates the necessity of investigating structural evolution of MOFs during electrocatalysis, it also shows the potential of using MOFs as precursors in catalyst design.

Inhibiting Dissolution of Active Sites in 80 °C Alkaline Water Electrolysis by Oxyanion Engineering

Commercial alkaline water electrolysers typically operate at 80 °C to minimize energy consumption. However, NiFe-based catalysts, considered as one of the most promising candidates for anode, encounter the bottleneck of high solubility at such temperatures. Herein, we discover that the dissolution of NiFe layered double hydroxides (NiFe-LDH) during operation not only leads to degradation of anode itself, but also deactivates cathode for water splitting, resulting in decay of overall electrocatalytic performance. Aiming to suppress the dissolution, we employed oxyanions as inhibitors in electrolyte. The added phosphates to the electrolyte inhibit the loss of NiFe-LDH active sites at 400 mA cm-2 to 1/3 of the original amount, thus reducing the rate of performance decay by 25-fold. Furthermore, the usage of borates, sulfates, and carbonates yields similar results, demonstrating the reliability and universality of the active site dissolution inhibitor, and its role in elevated water electrolysis.

Confined growth of Ultrathin, nanometer-sized FeOOH/CoP heterojunction nanosheet arrays as efficient self-supported electrode for oxygen evolution reaction

Self-supported electrodes, featuring abundant active species and rapid mass transfer, are promising for practical applications in water electrolysis. However, constructing efficient self-supported electrodes with a strong affinity between the catalytic components and the substrate is of great challenge. In this study, by combining the ideas of in-situ construction and space-confined growth, we designed a novel self-supported FeOOH/cobalt phosphide (CoP) heterojunctions grown on a carefully modified commercial Ni foam (NF) with three-dimensional (3D) hierarchically porous Ni skeleton (FeOOH/CoP/3D NF). The specific porous structure of 3D NF directs the confined growth of FeOOH/CoP catalyst into ultra-thin and small-sized nanosheet arrays with abundant edge active sites. The active FeOOH/CoP component is stably anchored on the rough pore wall of 3D NF support, leading to superior stability and improved conductivity. These structural advantages contributed to a highly facilitated oxygen evolution reaction (OER) activity and enhanced durability of the FeOOH/CoP/3D NF electrode. Herein, the FeOOH/CoP/3D NF electrode afforded a low overpotential of 234 mV at 10 mA cm-2 (41 mV smaller than FeOOH/CoP grown on unmodified Ni foam) and high stability for over 90 h, which is among the top reported OER catalysts. Our study provides an effective idea and technique for the construction of active and robust self-supported electrodes for water electrolysis.

Nickel Hydroxide-Based Electrocatalysts for Promising Electrochemical Oxidation Reactions: Beyond Water Oxidation

Transition metal hydroxides have attracted significant research interest for their energy storage and conversion technique applications. In particular, nickel hydroxide (Ni(OH)2), with increasing significance, is extensively used in material science and engineering. The past decades have witnessed the flourishing of Ni(OH)2-based materials as efficient electrocatalysts for water oxidation, which is a critical catalytic reaction for sustainable technologies, such as water electrolysis, fuel cells, CO2 reduction, and metal-air batteries. Coupling the electrochemical oxidation of small molecules to replace water oxidation at the anode is confirmed as an effective and promising strategy for realizing the energy-saving production. The physicochemical properties of Ni(OH)2 related to conventional water oxidation are first presented in this review. Then, recent progress based on Ni(OH)2 materials for these promising electrochemical reactions is symmetrically categorized and reviewed. Significant emphasis is placed on establishing the structure-activity relationship and disclosing the reaction mechanism. Emerging material design strategies for novel electrocatalysts are also highlighted. Finally, the existing challenges and future research directions are presented.

Unraveling the Role of the Stoichiometry of Atomic Layer Deposited Nickel Cobalt Oxides on the Oxygen Evolution Reaction

Nickel cobalt oxides (NCOs) are promising, non-precious oxygen evolution reaction (OER) electrocatalysts. However, the stoichiometry-dependent electrochemical behavior makes it crucial to understand the structure-OER relationship. In this work, NCO thin film model systems are prepared using atomic layer deposition. In-depth film characterization shows the phase transition from Ni-rich rock-salt films to Co-rich spinel films. Electrochemical analysis in 1 m KOH reveals a synergistic effect between Co and Ni with optimal performance for the 30 at.% Co film after 500 CV cycles. Electrochemical activation correlates with film composition, specifically increasing activation is observed for more Ni-rich films as its bulk transitions to the active (oxy)hydroxide phase. In parallel to this transition, the electrochemical surface area (ECSA) increases up to a factor 8. Using an original approach, the changes in ECSA are decoupled from intrinsic OER activity, leading to the conclusion that 70 at.% Co spinel phase NCO films are intrinsically the most active. The studies point to a chemical composition dependent OER mechanism: Co-rich spinel films show instantly high activities, while the more sustainable Ni-rich rock-salt films require extended activation to increase the ECSA and OER performance. The results highlight the added value of working with model systems to disclose structure-performance mechanisms.

Manipulating electron redistribution between iridium and Co6Mo6C bridging with a carbon layer leads to a significantly enhanced overall water splitting performance at industrial-level current density

Nowadays, alkaline water electrocatalysis is regarded as an economical and highly effective approach for large-scale hydrogen production. Highly active electrocatalysts functioning under large current density are urgently required for practical industrial applications. In this work, we present a meticulously designed methodology to anchor Ir nanoparticles on Co6Mo6C nanofibers (Co6Mo6C-Ir NFs) bridging with nitrogen-doped carbon as efficient bifunctional electrocatalysts with both excellent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activity and stability in alkaline media. With a low Ir content of 5.9 wt%, Co6Mo6C-Ir NFs require the overpotentials of only 348 and 316 mV at 1 A cm-2 for the HER and OER, respectively, and both maintain stability for at least 500 h at ampere-level current density. Consequently, an alkaline electrolyzer based on Co6Mo6C-Ir NFs only needs a voltage of 1.5 V to drive 10 mA cm-2 and possesses excellent durability for 500 h at 1 A cm-2. Density functional theory calculations reveal that the introduction of Ir nanoparticles is pivotal for the enhanced electrocatalytic activity of Co6Mo6C-Ir NFs. The induced interfacial electron redistribution between Ir and Co6Mo6C bridging with nitrogen-doped carbon dramatically modulates the electron structure and activates inert atoms to generate more highly active sites for electrocatalysis. Moreover, the optimized electronic structure is more conducive to the balance of the adsorption and desorption energies of reaction intermediates, thus significantly promoting the HER, OER and overall water splitting performance.

Optimizing the intermediates adsorbability and revealing the dynamic reconstruction of Co6Fe3S8 solid solution for bifunctional water splitting

Co9S8 has been extensively studied as a promising catalyst for water electrolysis. Doping Co9S8 with Fe improves its oxygen evolution reaction (OER) performance by regulating the catalyst self-reconfigurability and enhancing the absorption capacity of OER intermediates. However, the poor alkaline hydrogen evolution reaction (HER) properties of Co9S8 limit its application in bifunctional water splitting. Herein, we combined Fe doping and sulfur vacancy engineering to synergistically enhance the bifunctional water-splitting performance of Co9S8. The as-synthesized Co6Fe3S8 catalyst exhibited excellent OER and HER characteristics with low overpotentials of 250 and 84 mV, respectively. It also resulted in the low Tafel slopes of 135 mV dec-1 for the OER and 114 mV dec-1 for the HER. A two-electrode electrolytic cell with Co6Fe3S8 used as both the cathode and anode produced a current density of 10 mA cm-2 at a low voltage of only 1.48 V, maintaining high stability for 100 h. The results of in/ex-situ experiments indicated that the OER process induced electrochemical reconfiguration, forming CoOOH/FeOOH active species on the catalyst surface to enhance its OER performance. Density functional theory (DFT) simulations revealed that Fe doping and the presence of unsaturated coordination metal sites in Co6Fe3S8 promoted H2O and H* adsorption for the HER. The findings of this study can help develop a strategy for designing highly efficient bifunctional water splitting electrocatalysts.

From Atomic-Level Synthesis to Device-Scale Reactors: A Multiscale Approach to Water Electrolysis

ConspectusThe development of an advanced energy conversion system for water electrolysis with high efficiency and durability is of great significance for a hydrogen-powered society. This progress relies on the fabrication of electrocatalysts with superior electrochemical performance. Despite decades of advancements in exploring high-performance noble and non-noble metal electrocatalysts, several challenges persist at both the micro- and macrolevels in the field of water electrolysis.At the microlevel, which encompasses electrocatalyst synthesis and characterization, design strategies for high-performance electrocatalysts have primarily focused on interface chemical engineering. However, comprehensive understanding and investigation of interface chemical engineering across various length scales, from micrometers to atomic scales, are still lacking. This deficiency hampers the rational design of catalysts with optimal performance. Under harsh reaction conditions, such as high bias potential and highly acidic or alkaline media, the surface of catalyst materials is susceptible to undergoing "reconstruction", deviating from what is observed through ex situ characterization techniques postsynthesis. Conventional ex situ characterization methods do not provide an accurate depiction of the catalyst's structural evolution during the electrocatalytic reaction, hindering the exploration of the catalytic mechanism.At the macrolevel, pertaining to catalysis-performance evaluation systems and devices, traditional laboratory settings employ a conventional three-electrode or two-electrode system to assess the catalytic performance of electrocatalysts. However, this approach does not accurately simulate hydrogen production under realistic industrial conditions, such as elevated temperatures (60-70 °C), high current densities exceeding 0.5 A cm-2, and flowing electrolytes. To address this limitation, it is crucial to develop testing equipment and methodologies that replicate the actual industrial conditions.In this Account, we propose a multiscale research framework for water electrolysis, spanning from microscale synthesis to macroscale scaled reactor design. Our approach focuses on the design and evaluation of high-performance HER/OER (hydrogen evolution reaction/oxygen evolution reaction) electrocatalysts, incorporating the following strategies: Leveraging principles of interface chemical engineering across various length scales (micrometers, nanometers, and atoms) enables the design of catalyst materials that enhance both activity and durability. This approach provides a comprehensive understanding of the intricate interplay between the catalyst structure and activity, implementing in situ/operando characterization techniques to monitor dynamic interfacial reactions and surface reconstruction processes. This facilitates a profound exploration of catalytic reaction mechanisms, offering insights into the catalyst's structural evolution during the electrocatalytic reaction. We construct a laboratory-scale membrane electrode assembly (MEA) electrochemical reactor capable of operating at high current densities (>1 A cm-2) to evaluate the electrocatalytic performance under simulated industrial conditions. This ensures objective and authentic assessments of the catalyst application potential. Throughout the following sections, we illustrate the application of interface chemical engineering on different length scales in designing diverse electrocatalyst materials. We rely on in situ characterization techniques to gain a profound understanding of the mechanisms behind the HER and OER. Additionally, we describe the development of both acidic and alkaline MEA electrochemical reactors to enhance the precision of electrocatalytic performance evaluation. Finally, we provide a concise overview of the challenges and opportunities in this field.

Reversible Hydrogen Electrode (RHE) Scale Dependent Surface Pourbaix Diagram at Different pH

In the analysis of electrocatalysis mechanisms and the design of catalysts, the effect of electrochemistry-induced surface coverage is a critical consideration that should not be overlooked. The surface Pourbaix diagram emerges as a fundamental tool in this context, providing essential insights into the surface coverage of adsorbates generated via electrochemical potential-driven water activation. A classic surface Pourbaix diagram considers the pH effects by correcting the free energy of H+ ions by the concentration-dependent term: -kBT ln(10) × pH, which is independent of the reversible hydrogen electrode (RHE) scale. However, this is sometimes inconsistent with the experimentally observed potential-dependent surface coverage at an RHE scale, especially under high-pH conditions. Here, we derived the pH-dependent surface Pourbaix diagram at an RHE scale by considering the energetics computed by density functional theory with the Bayesian Error Estimation Functional with van der Waals corrections (BEEF-vdW), the electric field effects, the derived adsorption-induced dipole moment and polarizability, and the potential of zero-charge. Using Pt(111) as the typical example, we found that the surface coverage predicted by the proposed RHE-dependent surface Pourbaix diagram can significantly minimize the discrepancy between theory and experimental observations, especially under neutral-alkaline, moderate-potential conditions. This work provides a new methodology and establishes guidelines for the precise analysis of the surface coverage prior to the evaluation of the activity of an electrocatalyst.

FeP-Fe3O4 nanospheres for electrocatalytic N2 reduction to NH3 under ambient conditions

The electrocatalytic nitrogen reduction reaction (eNRR) under ambient conditions is deemed a promising alternative for NH3 synthesis. In this paper, an FeP-Fe3O4 nanocomposite electrocatalyst was prepared by phosphating annealing using Fe2O3 as a precursor, and the resulting FeP-Fe3O4 exhibited excellent N2-to-NH3-producing activity over a wide potential window. The highest faradaic efficiency of FeP-Fe3O4 is 11.02% at -0.1 V vs. reversible hydrogen electrode (RHE), and the maximum NH3 yield reaches 12.73 μg h-1 mgcat-1, comparable to or exceeding the reported values in this field. Furthermore, the FeP-Fe3O4 nanocomposite electrocatalyst presents high electrochemical stability, selectivity, and durability.