Research Progress of Oxygen Evolution Reaction Catalysts for Electrochemical Water Splitting

Synergistic Fe-Mn-Cu ternary alloys enhance bifunctional activity and stability for alkaline water splitting

Developing cost-effective, high-performance electrocatalysts for water splitting remains a critical challenge for advancing renewable energy technologies. Herein, we present a novel ternary alloy catalyst, 20Fe-80Mn-20Cu, designed and optimized for hydrogen evolution (HER) and oxygen evolution reactions (OER). The catalyst, synthesized via electrodeposition, demonstrates exceptional bifunctional activity and stability, outperforming binary (20Fe-80Mn) and benchmark electrodes, such as Pt and DSA. Linear sweep voltammetry (LSV) revealed that 20Fe-80Mn-20Cu requires a remarkably low overpotential (without iR drop correction) of 172 mV for HER and 147 mV for OER to achieve a current density of 10 mA cm- 2, significantly surpassing the performance of binary alloys and bare substrates. Tafel slope analysis further confirmed the catalytic efficiency, with values of 53 mV dec- 1 for HER and 56 mV dec- 1 for OER. Electrochemical impedance spectroscopy (EIS) revealed low charge transfer resistance, highlighting the alloy's excellent electron transport properties. Raman and XRD investigations revealed the catalyst's unique structural and compositional features, including extra crystallographic reflections indicating increased surface activity. Stability tests conducted at ± 250 mA cm- 2 over 4 days demonstrated excellent durability, with only 7% (HER) and 5% (OER) performance drops. Post-stability characterizations, including XRD and EDX, revealed Mn and Fe redistribution and Cu enrichment on the surface, as well as the formation of stable copper oxides under OER conditions. These findings establish 20Fe-80Mn-20Cu as a promising candidate for scalable water splitting, offering an energy-saving potential of up to 5.5 V per cm2 of the electrode surface. This study increases our understanding of alloy-based catalysts and demonstrates a feasible approach for efficient and sustainable hydrogen production.

Challenges and Emerging Trends in Hydrogen Energy Industrialization: From Hydrogen Evolution Reaction to Storage, Transportation, and Utilization

Green hydrogen (H2) emerges as a sustainable alternative to fossil fuels, offering a clean method to store renewable energy through water electrolysis with high energy content and zero carbon emissions. While research largely focuses on specific aspects such as hydrogen evolution reaction (HER), seawater HER electrocatalysts, and electrolyzer development, these studies often overlook the broader hydrogen economy from an integrated industry chain perspective. This review bridges that gap by providing a comprehensive analysis of hydrogen energy industrialization, covering advancements in HER, seawater HER, and electrolyzers, all aim at enabling industrial-scale H2 production. It further explores innovations and challenges in hydrogen storage and transportation, as well as real-world projects spanning the green hydrogen supply chain. Additionally, life cycle assessment studies validate the environmental benefits of using renewable energy sources for green H2 production. Furthermore, this review highlights advancements in counter-oxygen evolution reactions and organic oxidation reactions, alongside strategies to mitigate competing chlorine evolution reactions. Through this comprehensive examination, this review aims to inform readers of the latest developments in hydrogen energy industrialization, explore its growth potential, and provide new insights to propel the hydrogen economy forward.

FeHf Binary Hydroxide/Oxide Nanostructures as Catalysts for Oxygen Evolution

We present pulsed electrodeposition (PED) of FeHf binary hydroxide/oxide (FeHf-BH) nanocomposites from aqueous electrolyte baths containing NO3 - ions. The deposition was carried out on a graphite foil at room temperature. This study, for the first time, demonstrated a controlled variation of Fe (5.9-49.9 avg. at. %) and Hf (2.4-58.7 avg. at. %) in the deposited materials. We showed the high scalability of FeHf-BH deposition by tuning the PED parameters. The morphology, composition, chemical structure, and oxidation states of metals in the materials were investigated by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The deposited materials consist of agglomerated nanoparticles sized 50-150 nm. Thermal annealing studies revealed improved crystallinity, with the appearance of thermodynamically stable oxide phases of Fe3O4, Fe2O3, and HfO2 in the composites. The oxygen evolution activity of the materials was analyzed in an alkaline medium based on the Hf content. The optimized material containing 11.9 avg. at. % Hf demonstrated an OER onset potential of 1.63 V vs RHE, Tafel slope of 47 mV dec-1, and required only 470 mV overpotential to reach a 50 mA cm-2 OER current. These PED strategies of designing FeHf-BH materials may open an avenue for designing other catalytically active and stable multimetallic hydroxides/oxides composites.

Fluorine as a Key Element in Solid-State Chemistry of Mixed Anions 3d Transition Metal-Based Materials for Electronic Properties and Energy

Mixed anion compounds containing fluorine and based on 3d transition elements represent a class of materials with significant interest in solid-state chemistry. Indeed, their highly varied chemical composition, structural diversity, and the resulting electronic properties provide a rich playground for imagining new applications in the field of energy. The anions and the chemical bonds they form with the 3d transition elements are at the heart of this review. Key parameters such as electronegativity, hardness, and polarizability are introduced and discussed to better understand the charge capacity of the anion and the bonds formed in the solid. Oxyfluorides represent the most studied family due to the size similarity of the two anions, and part of the review is dedicated to the specific synthesis of these materials by systematically adjusting the fluorine content within various structures and analyzing the electronic and electrochemical properties of these compositions. The final sections focus on materials with structures often exhibiting a two-dimensional character, where ionic blocks coexist with covalent layers, such as fluorochalcogenides, fluoropnictides, and fluorotetrelides. The compositions and structures are systematically correlated with the electronic properties.

Functionalized 3D Mo2N Current Collectors Drive Multi-Phase Ni-based Synergy and Mitigate Surface Reconstruction for Enhanced Oxygen Evolution Catalysis

Electrochemical water splitting is a promising approach for sustainable hydrogen production, but the oxygen evolution reaction (OER) remains a bottleneck due to sluggish kinetics, poor activity, and limited stability and scalability. Here, a Mo2N-functionalized nickel is designed foam (NF@Mo2N) and subsequently transform into a Mo2N/NiSe/Ni2P multi-phase heterostructure through selenization and phosphorization, to address these challenges. The optimized NF@Mo2N/NiSe/Ni2P catalyst integrates three key strategies: (I) functionalizing NF with Mo2N to enhance conductivity and charge transfer, (II) engineering a collaborative multi-interface heterostructure to optimize active sites and reaction kinetics, and (III) precisely controlling phase formation through selenization and phosphorization to mitigate surface reconstruction and ensure long-term stability. The catalyst not only achieves an overpotential of 242 mV@10 mA cm-2 and remarkable stability over 350 h, but also achieves a low overpotential of 395 mV at a high current density of 800 mA cm-2, outperforming the pristine other control samples. Theoretical analysis reveals that the Mo2N-stabilized NiSe/Ni2P heterostructure on NF enhances conductivity and optimizes adsorption energies of OER intermediates, leading to improved catalytic performance and stability. This work provides a new strategy for designing high-performance, non-precious metal OER catalysts for industrial applications and advancing sustainable hydrogen production.

Diiron(IV)-Oxo Species and Water Oxidation: How Crucial is Electronic Cooperativity?

Water splitting, crucial for generating oxygen and hydrogen, remains a central challenge in chemistry due to its importance in developing sustainable energy sources and addressing environmental concerns. Consequently, numerous complexes have been developed to split water and release oxygen and hydrogen, albeit typically requiring external sources such as thermal, photo, or electrochemical methods. In this context, the discovery of a (μ-oxo)bis(μ-carboxamido) diiron(IV) complex, [FeIV₂O(L)₂]2+ (L=N,N-bis-(3',5'-dimethyl-4'-methoxypyridyl-2'-methyl)-N'-acetyl-1,2-diaminoethane), which activates both C-H and O-H bonds without external stimuli, has attracted significant attention. Notably, this complex generates hydroxyl radicals (⋅OH) without O₂ evolution and displays termolecular kinetics, presenting a rare and intriguing mechanistic puzzle. In this work, we explore the catalytic mechanism of water oxidation by this diiron(IV) complex using DFT methods. Our computational findings validate experimental observations regarding the necessity of a second water molecule in the reaction, revealing a bifurcated electron-proton transfer (BEPT) pathway driven by termolecular reactivity. Moreover, we highlight the crucial role of excess water molecules in stabilising the reaction intermediates, particularly via interaction with the -OMe groups to form a water cluster model. The inclusion of explicit water molecules was found to reduce the activation barrier to 23.5 kJ/mol from the reactant and 62.7 kJ/mol from the reactant complex, whereas, with only one water molecule present, the barrier was 344.3 kJ/mol, highlighting the critical role of the adventitious water molecule at the active site. Our study underscores the importance of metal-metal cooperativity, ligand design, spin-state modulation, and second-sphere effects in shaping the catalytic behaviour. These insights provide a detailed understanding of the electronic structure and reactivity, offering valuable guidelines for future catalyst design in water oxidation and beyond.

High-Entropy Selenides with Tunable Lattice Distortion as Efficient Electrocatalysts for Oxygen Evolution Reaction

Developing stable and active electrocatalysts is crucial for enhancing the oxygen evolution reaction (OER) efficiency, which sluggish kinetics hinder sustainable hydrogen production. High entropy selenides (HESes) feature with random distribution of multiple metals cations and unique electronic and size effect of Se anion, allowing for precious regulation of their catalytic properties towards high OER activity. In this work, we report a series of high-entropy selenides catalysts with tunable lattice strain for electrocatalytic oxygen evolution. Electrochemical measurements show that the quinary (NiCoMnMoFe)Sex requires only 291 mV to reach 10 mA cm-2 and exhibits a superior stability with negligible current decay during 100 h's continuous operation. By combining experimental measurements and theoretical calculation, the study reveals that the lattice distortion, reflected by the local microstrain near the active site, plays a vital role in boosting the OER activity of HESes.

Ag-Bi2O3-Nanostructured Composite Electrodes toward Catalyzing Oxygen Evolution Reaction: Exploring Oxygen Evolution Reaction Kinetics in Composites from Doping to Establishing a Heterojunction

Electrochemical water splitting involving two-half chemical cell reactions is a promising approach to generate hydrogen and oxygen. Although this method is sustainable, the sluggish kinetics of the oxygen evolution reaction (OER) occurring at the anode due to a high overpotential is an issue to be addressed. Recently, various chemical and structural engineering approaches have been explored to improve the efficiency of the OER by reducing the overpotential. Among them, incorporating noble metals into the electrodes by doping or creating heterojunctions is an appealing approach to develop efficient OER electrocatalysts. Based on this principle, herein, we synthesized a bismuth-oxide (Bi2O3) electrocatalyst incorporated with silver nanoparticles (Ag NPs) by a facile one-step hydrothermal method to take advantage of the high conductivity of Ag NPs and the low band gap along with fast redox reaction of Bi2O3. With the Ag+ concentration in the hydrothermal precursor solution, the thickness of hydrothermally formed Bi2O3 nanoplates decreases, resulting in the increased electrochemical surface area (ECSA) from 71 to 300 cm2. The optimal electrode, heterojunction-formed Ag-Bi2O3 (denoted H-Ag1.00-Bi2O3), exhibits the lowest overpotential of 260 mV for the OER at a current density of 10 mA cm-2 with an excellent durability of 77.5% after stability tests for 240 h due to the number of active sites produced by Ag doping (manifesting defects), and heterojunction established between Ag nanoparticles and Bi2O3 nanoplates. The approach explored in this work could be further utilized to produce other effective electrocatalysts for accelerating OER performances.

Advances in Catalysts for Hydrogen Production: A Comprehensive Review of Materials and Mechanisms

This review explores the recent advancements in catalyst technology for hydrogen production, emphasizing the role of catalysts in efficient and sustainable hydrogen generation. This involves a comprehensive analysis of various catalyst materials, including noble metals, transition metals, carbon-based nanomaterials, and metal-organic frameworks, along with their mechanisms and performance outcomes. Major findings reveal that while noble metal catalysts, such as platinum and iridium, exhibit exceptional activity, their high cost and scarcity necessitate the exploration of alternative materials. Transition metal catalysts and single-atom catalysts have emerged as promising substitutes, demonstrating their potential for enhancing catalytic efficiency and stability. These findings underscore the importance of interdisciplinary approaches to catalyst design, which can lead to scalable and economically viable hydrogen production systems. The review concludes that ongoing research should focus on addressing challenges related to catalyst stability, scalability, and the integration of renewable energy sources, paving the way for a sustainable hydrogen economy. By fostering innovation in catalyst development, this work aims to contribute to the transition towards cleaner energy solutions and a more resilient energy future.

Can NiFe-Layered-Double-Hydroxide Catalysts Suppress Carbon Corrosion in Electrochemical Oxygen Evolution?

Sustainable energy societies demand rechargeable batteries using ubiquitous-material electrodes of geopolitical-risk-free elements. We aim to develop low-overpotential oxygen-evolution-reaction (OER) catalysts that suppress carbon corrosion of gas-diffusion electrodes (GDEs) to realize two-electrode rechargeable Zn-air batteries (r-ZABs). Herein, single-walled-carbon-nanotube (SWNT) thin films are used as a scaffold for a benchmark OER catalyst, doping-free NiFe-layered double hydroxide (NiFeLDHs), operating in r-ZABs using alkali aqueous electrolytes. Metal compositions of NiFeLDHs are controlled with an atomic-level quality using Prussian-blue-analog nanoparticles of NixFe1-x[Fe(CN)6]0.67 (x = 0-1). The nanoparticles with dimensions of ∼8 nm adhere to SWNTs on carbon paper as a GDE model by a drop-casting method using their aqueous dispersion solutions. Ni0.6Fe0.4[Fe(CN)6]0.67 shows OER activity by hydrolysis for generating NiFeLDH nanodots of metal compositions between Ni0.5Fe0.5 and Ni0.6Fe0.4 with a size distribution of 1.75 ± 0.26 nm and exposing OER-active (018) and (015) planes on SWNTs. The activity is investigated by regulating the loading amounts of the NPs to avoid aggregating the nanodots. An optimal low-loading amount of 270 nmol cm-2 minimizes iR-corrected overpotential to 156 mV at 10 mA cm-2. The iR-uncorrected overpotential is 260 mV and suppresses carbon corrosion of SWNTs and carbon black. Using an r-ZAB half-cell with a Zn foil, OER-driven charging stably proceeds at 10 mA cm-2 over 3 h with an average voltage of 1.99 V vs Zn/Zn2+. Limited metal electrodes have further improved OER overpotentials by third-element doping, while carbon electrodes still offer room for discovering intrinsically high OER activities of NiFeLDHs without doping.

Graphitic Carbon Nitride Structures on Carbon Cloth Containing Ultra- and Nano-Dispersed NiO for Photoactivated Oxygen Evolution

The development of low-cost and high-efficiency oxygen evolution reaction (OER) photoelectrocatalysts is a key requirement for H2 generation via solar-assisted water splitting. In this study, we report on an amenable fabrication route to carbon cloth-supported graphitic carbon nitride (gCN) nanoarchitectures, featuring a modular dispersion of NiO as co-catalyst. The synergistic interaction between gCN and NiO, along with the tailoring of their size and spatial distribution, yield very attractive OER performances and durability in freshwater splitting, of great significance for practical end-uses. The potential of gCN electrocatalysts containing ultra-dispersed, i. e. "quasi-atomic" NiO, exhibiting a higher activity than the ones containing nickel oxide nanoaggregates, is further highlighted by their activity even in real seawater. This work suggests that efficient OER catalysts can be designed through the construction of optimized interfaces between transition metal oxides and carbon nitride, yielding inexpensive and promising noble metal-free systems for real-world applications.

Two Co(II) Isostructural Bifunctional MOFs via Mixed-Ligand Strategy: Syntheses, Crystal Structure, Photocatalytic Degradation of Dyes, and Electrocatalytic Water Oxidation

The solvothermal reactions involving cobalt ions with 5-methylisophthalic acid (H2MIP) and 1,3-bis(2-methylimidazol)propane (BMIP) yielded two cobalt(II) organic frameworks: {[Co4(MIP)4(BMIP)3]·1/2DMA}n (SNUT-31) and {[Co4(MIP)4(BMIP)3]·(EtOH)2·H2O]}n (SNUT-32) where DMA represents N,N-dimethylacetamide and EtOH signifies ethyl alcohol. Single-crystal X-ray diffraction analyses reveal that SNUT-31 and SNUT-32 possess an isomorphic structure, featuring a unique 2-fold interpenetration of 3D frameworks in a parallel manner. Notably, both SNUT-31 and SNUT-32 demonstrate remarkable performance in electrocatalytic oxygen evolution reactions and exhibit exceptional photocatalytic degradation capabilities against a model comprising three distinct dyes: rhodamine B, methyl orange, and methyl blue.

Scale-Up, Continuous and Low-Temperature Production of Multimetal Based Electrocatalysts toward Water Electrolysis

Electrocatalytic water splitting is a crucial strategy for advancing hydrogen energy and addressing the global energy crisis. Despite its significance, the need for a straightforward and swift method to synthesize electrocatalysts with exceptional performance remains pressing. In this study, we demonstrate a novel approach for the preparation of multimetal-based electrocatalysts in a continuous flow reactor, enabling the quick synthesis of a large number of products through a streamlined process. The resultant NiFe-LDH comprises nanoflakes with a high specific surface area and requires only 255.4 mV overpotential to achieve a current density of 10 mA·cm-2 in 1 M KOH, surpassing samples fabricated by conventional hydrothermal methods. Our method can also be applied to craft a spectrum of other multimetal-based electrocatalysts, including CoFe-LDH, CoAl-LDH, NiMn-LDH, and NiCoFe-LDH. Additionally, the NiFe-LDH electrocatalyst is further applied to anodic methanol electrooxidation coupled with cathodic hydrogen evolution. Moreover, the simplicity and generality of our fabrication method render it applicable for the facile preparation of various multimetal-based electrocatalysts, offering a scalable solution to the quest for high-performance catalysts in advancing sustainable energy technologies.

Spin Crossover and Exchange Effects on Oxygen Evolution Reaction Catalyzed by Bimetallic Metal Organic Frameworks

Bimetallic metal-organic frameworks (BMOFs) have shown a superior oxygen evolution reaction (OER) performance, attributed to the synergistic effects of dual metal sites. However, the significant role of these dual-metal synergies in the OER is not yet fully understood. In this study, we employed density functional theory to systematically investigate the OER performance of NiAl- and NiFe-based BMOFs by examining all possible spin states of each intermediate across diverse external potentials and pH environments. We found that the spin state featuring a shallow hole trap state and Ni ions with a higher oxidation state serve as strong oxidizing agents, promoting the OER. An external potential-induced spin crossover was observed in each intermediate, resulting in significant changes in the overall reaction and activation energies due to altered energy levels. Combining the constant potential method and the electrochemical nudged elastic band method, we mapped the minimum free energy barriers of the OER under varied external potential and pH by considering the spin crossover effect for both NiAl and NiFe BMOFs. The results showed that NiFe exhibits better OER thermodynamics and kinetics, which is in good agreement with experimentally measured OER polarization curves and Tafel plots. Moreover, we found that the improved OER kinetics of NiFe not only is attributed to lower barriers but also is a result of improved electrical conductivity arising from the synergistic effects of Ni-Fe dual-metal sites. Specifically, replacing the second metal Al with Fe leads to two significant outcomes: a reduction in both the band gap and the effective hole mass compared to NiAl, and the initiation of super- and double-exchange interactions within the Ni-F-Fe chain, thereby enhancing electron transfer and hopping and leading to the improved OER kinetics.

The mechanism of water oxidation using transition metal-based heterogeneous electrocatalysts

The water oxidation reaction, a crucial process for solar energy conversion, has garnered significant research attention. Achieving efficient energy conversion requires the development of cost-effective and durable water oxidation catalysts. To design effective catalysts, it is essential to have a fundamental understanding of the reaction mechanisms. This review presents a comprehensive overview of recent advancements in the understanding of the mechanisms of water oxidation using transition metal-based heterogeneous electrocatalysts, including Mn, Fe, Co, Ni, and Cu-based catalysts. It highlights the catalytic mechanisms of different transition metals and emphasizes the importance of monitoring of key intermediates to explore the reaction pathway. In addition, advanced techniques for physical characterization of water oxidation intermediates are also introduced, for the purpose of providing information for establishing reliable methodologies in water oxidation research. The study of transition metal-based water oxidation electrocatalysts is instrumental in providing novel insights into understanding both natural and artificial energy conversion processes.

Na Substitution Steering RuO6 Unit in Ruthenium Pyrochlores for Enhanced Oxygen Evolution in Acid

Although Ruthenium-based pyrochlore oxides can function as promising catalysts for acidic water oxidation, their limitations in terms of stability and activity still need to be addressed for further application in practical conditions. In this work, the possibility to enhance both oxygen evolution reaction activity and durability of Gd2Ru2O7- δ through partial replacement with Na+ in Gd3+ sites is first offered, leading to the electronic and geometric regulation of active center RuO6. Na+ triggers the emergence of Ru<4+ and the electron rearrangement of active-centered RuO6. Specifically, Ru ions with a negative d-band center after Na+ doping exhibit weaker adsorption energies of *O and result in the conversion of the rate-limiting step from *O/*OOH to *OH/O*, reducing energy barriers for boosting activities. Therefore, the NaxGd2- xRu2O7- δ requires a low overpotential of 260 mV at 10 mA cm-2 in 0.1 m HClO4 electrolyte. Moreover, the higher formation energy of Ru vacancy and less distorted RuO6 enable the as-prepared NaxGd2- xRu2O7- δ to operate steadily at 10 mA cm-2 for 300 h and multi-current chronopotentiometry with current densities from 20 to 100 mA cm-2 for 60 h in acidic proton exchange membrane electrolyzer, respectively.

A stable N-doped NiMoO4/NiO2 electrocatalyst for efficient oxygen evolution reaction

Recently, there has been a significant interest in the study of highly active and stable transition metal-based electrocatalysts for the oxygen evolution reaction (OER). Non-noble metal nanocatalysts with excellent inherent activity, many exposed active centers, rapid electron transfer, and excellent structural stability are especially promising for the displacement of precious-metal catalysts for the production of sustainable and "clean" hydrogen gas through water-splitting. Herein, efficient electrocatalyst N-doped nickel molybdate nanorods were synthesized on Ni foam by a hydrothermal process and effortless chemical vapor deposition. The heterostructure interface of N-NiMoO4/NiO2 led to strong electronic interactions, which were beneficial for enhancing the OER activity of the catalyst. Excellent OER catalytic activity in 1.0 M KOH was shown, which offered a small overpotential of 185.6 mV to acquire a current density of 10 mA cm-2 (superior to the commercial benchmark material RuO2 under the same condition). This excellent electrocatalyst was stable for 90 h at a constant current density of 10 mA cm-2. We created an extremely reliable and effective OER electrocatalyst without the use of noble metals by doping a nonmetal element with nanostructured heterojunctions of various active components.

Tetranuclear CoII4O4 Cubane Complex: Effective Catalyst Toward Electrochemical Water Oxidation

The reaction of Co(OAc)2·6H2O with 2,2'-[{(1E,1'E)-pyridine-2,6-diyl-bis(methaneylylidene)bis(azaneylylidene)}diphenol](LH2) a multisite coordination ligand and Et3N in a 1:2:3 stoichiometric ratio forms a tetranuclear complex Co4(L)2(μ-η1:η1-OAc)2(η2-OAc)2]· 1.5 CH3OH· 1.5 CHCl3 (1). Based on X-ray diffraction investigations, complex 1 comprises a distorted Co4O4 cubane core consisting of two completely deprotonated ligands [L]2- and four acetate ligands. Two distinct types of CoII centers exist in the complex, where the Co(2) center has a distorted octahedral geometry; alternatively, Co(1) has a distorted pentagonal-bipyramidal geometry. Analysis of magnetic data in 1 shows predominant antiferromagnetic coupling (J = -2.1 cm-1), while the magnetic anisotropy is the easy-plane type (D1 = 8.8, D2 = 0.76 cm-1). Furthermore, complex 1 demonstrates an electrochemical oxygen evolution reaction (OER) with an overpotential of 325 mV and Tafel slope of 85 mV dec-1, required to attain a current density of 10 mA cm-2 and moderate stability under alkaline conditions (pH = 14). Electrochemical impedance spectroscopy studies reveal that compound 1 has a charge transfer resistance (Rct) of 2.927 Ω, which is comparatively lower than standard Co3O4 (5.242 Ω), indicating rapid charge transfer kinetics between electrode and electrolyte solution that enhances higher catalytic activity toward OER kinetics.

Enhancing the electrocatalytic activities of metal organic frameworks for the oxygen evolution reaction with bimetallic groups

Controlling the ratio of metals in bimetallic organic frameworks (MOFs) can not only alter the structures but also tailor the properties of MOFs. Herein, we report a series of electrocatalytically active CoxNiy-based bimetallic MOFs that are synthesized with the 3,5-pyridinedicarboxylic acid (3,5-H2pdc) ligand (where x : y = 20 : 1, 15 : 1, 10 : 1, 5 : 1, 1 : 1, and 1 : 20) and a facile, scalable, low temperature synthetic route. The materials have one-dimensional (1D), rod-like microstructures with different aspect ratios. While they all electrocatalyze the oxygen evolution reaction (OER) in alkaline solution (1 M KOH), their electrocatalytic performances vary substantially depending on their compositions. The CoxNiy-MOF with an optimal ratio of x : y = 15 : 1 (Co15Ni1-MOF) electrocatalyzes the OER with the highest maximum current density (92.2 mA cm-2 at 1.75 V vs. RHE) and the smallest overpotential (384 mV vs. RHE at 10 mA cm-2) in a 1 M KOH solution. It is also stable under constant current application during the electrocatalytic OER. This work demonstrates the application of bimetallic MOFs that are synthesized following a simple, low temperature synthetic route for the OER and their tailorable electrocatalytic properties for the OER by varying the ratio of two metals and the synthetic conditions used to produce them.

In situ synthesis of rosette-like Co-doped FeNiOOH/NF for seawater oxidation

The development of high activity and strong resistance to seawater corrosion oxygen evolution reaction (OER) electrocatalysts for seawater electrolysis has broad application prospects. Herein, we prepare Co-doped FeNiOOH rosette-like nanoflowers on nickel foam (NF) with different Co dosages by one-step solvothermal method. The Co0.2-FeNiOOH/NF exhibits a low overpotential (η10) of 185 mV and Tafel slope of 30 mV dec-1 in 1 M KOH. Moreover, it shows a low η10 of 244 mV in alkaline seawater electrolyte. The remarkable OER performance of Co0.2-FeNiOOH/NF is ascribed to the fact that the introduction of Co regulates the morphology and electron structure of the material, which provides abundant active sites for the reaction and promotes charge transfer. In situ Raman results demonstrate that NiOOH and γ-FeOOH are the key active species for the OER. This study provides a feasible basis for seawater electrolysis over transition metal (oxy)hydroxides.

Construction of a ruthenium-doped CoFe-layered double hydroxide as a bifunctional electrocatalyst for overall water splitting

In this study, ruthenium-doped CoFe-based layered double hydroxides on Ni foam (CoFe-ZLDH/Ru@NF) were fabricated via an etching-precipitation strategy. The resultant CoFe-ZLDH/Ru@NF exhibited excellent activity, showing low overpotentials of 219.8 mV and 60.9 mV to reach the current density of 10 mA cm-2 for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. As a bifunctional electrocatalyst, it was assembled in an anion exchange membrane water electrolyser (AEMWE) unit, performing as an anode and cathode simultaneously, which only required a cell voltage of 2.33 V to accomplish the industrial level current density of 1 A cm-2 and operated steadily for over 12 h, making it promising for utilization in hydrogen production.

Ruthenium-Manganese Solid Solution Oxide with Enhanced Performance for Acidic and Alkaline Oxygen Evolution Reaction

Proton exchange membrane water electrolysers and alkaline exchange membrane water electrolysers for hydrogen production suffer from sluggish kinetics and the limited durability of the electrocatalyst toward oxygen evolution reaction (OER). Herein, a rutile Ru0.75 Mn0.25 O2-δ solid solution oxide featured with a hierarchical porous structure has been developed as an efficient OER electrocatalyst in both acidic and alkaline electrolyte. Specifically, compared with commercial RuO2 , the catalyst displays a superior reaction kinetics with small Tafel slope of 54.6 mV dec-1 in 0.5 M H2 SO4 , thus allowing a low overpotential of 237 and 327 mV to achieve the current density of 10 and 100 mA cm-2 , respectively, which is attributed to the enhanced electrochemically active surface area from the porous structure and the increased intrinsic activity owing to the regulated Ru>4+ proportion with Mn incorporation. Additionally, the sacrificial dissolution of Mn relieves the leaching of active Ru species, leading to the extended OER durability. Besides, the Ru0.75 Mn0.25 O2-δ catalyst also shows a highly improved OER performance in alkaline electrolyte, rendering it a versatile catalyst for water splitting.

Metal-Oxides- and Metal-Oxyhydroxides-Based Nanocomposites for Water Splitting: An Overview

Water electrolysis is an important alternative technology for large-scale hydrogen production to facilitate the development of green energy technology. As such, many efforts have been devoted over the past three decades to producing novel electrocatalysis with strong electrochemical (EC) performance using inexpensive electrocatalysts. Transition metal oxyhydroxide (OxH)-based electrocatalysts have received substantial interest, and prominent results have been achieved for the hydrogen evolution reaction (HER) under alkaline conditions. Herein, the extensive research focusing on the discussion of OxH-based electrocatalysts is comprehensively highlighted. The general forms of the water-splitting mechanism are described to provide a profound understanding of the mechanism, and their scaling relation activities for OxH electrode materials are given. This paper summarizes the current developments on the EC performance of transition metal OxHs, rare metal OxHs, polymers, and MXene-supported OxH-based electrocatalysts. Additionally, an outline of the suggested HER, OER, and water-splitting processes on transition metal OxH-based electrocatalysts, their primary applications, existing problems, and their EC performance prospects are discussed. Furthermore, this review article discusses the production of energy sources from the proton and electron transfer processes. The highlighted electrocatalysts have received substantial interest to boost the synergetic electrochemical effects to improve the economy of the use of hydrogen, which is one of best ways to fulfill the global energy requirements and address environmental crises. This article also provides useful information regarding the development of OxH electrodes with a hierarchical nanostructure for the water-splitting reaction. Finally, the challenges with the reaction and perspectives for the future development of OxH are elaborated.

Rational Design of Molybdenum-Doped Cobalt Nitride Nanowire Arrays for Robust Overall Water Splitting

Rational design of efficient electrocatalysts is highly imperative but still a challenge for overall water splitting. Herein, we construct self-supported Co₃ N nanowire arrays with different Mo doping contents by hydrothermal and nitridation processes that serve as robust electrocatalysts for overall water splitting. The optimal Co₃ N-Mo_0.2 /Ni foam (NF) electrode delivers a low overpotential of 97 mV at a current density of 50 mA cm^-2 as well as a highly stable hydrogen evolution reaction (HER). Density functional theory (DFT) calculations prove that Mo doping can effectively modulate the electronic structure and surface adsorption energies of H₂ O and hydrogen intermediates on Co₃ N, leading to improved reaction kinetics with high catalytic activity. Further modification with FeOOH species on the surface of Co₃ N-Mo_0.2 /NF improves the oxygen evolution reaction (OER) performance benefiting from the synergistic effect of dual Co-Fe catalytic centers. As a result, the Co₃ N-Mo_0.2 @FeOOH/NF catalysts display outstanding OER catalytic performance with a low overpotential of 250 mV at 50 1 mA cm^-2 . The constructed Co₃ N-Mo_0.2 /NF||Co₃ N-Mo_0.2 @FeOOH/NF water electrolyzer exhibits a small voltage of 1.48 V to achieve a high current density of 50 mA cm^-2 at 80 °C, which is superior to most of the reported electrocatalysts. This work provides a new approach to developing robust electrode materials for electrocatalytic water splitting.

Microwave solvothermal synthesis of Component-Tunable High-Entropy oxides as High-Efficient and stable electrocatalysts for oxygen evolution reaction

Transition-metal-based high-entropy oxides (HEOs) are appealing electrocatalysts for oxygen evolution reaction (OER) due to their unique structure, variable composition and electronic structure, outstanding electrocatalytic activity and stability. Herein, we propose a scalable high-efficiency microwave solvothermal strategy to fabricate HEO nano-catalysts with five earth-abundant metal elements (Fe, Co, Ni, Cr, and Mn) and tailor the component ratio to enhance the catalytic performance. (FeCoNi₂CrMn)₃O₄ with a double Ni content exhibits the best electrocatalytic performance for OER, namely low overpotential (260 mV@10 mA cm^-2), small Tafel slope and superb long-term durability without obvious potential change after 95 h in 1 M KOH. The extraordinary performance of (FeCoNi₂CrMn)₃O₄ can be attributed to the large active surface area profiting from the nano structure, the optimized surface electronic state with high conductivity and suitable adsorption to intermediate benefitting from ingenious multiple-element synergistic effects, and the inherent structural stability of the high-entropy system. In addition, the obvious pH value dependable character and TMA⁺ inhibition phenomenon reveal that the lattice oxygen mediated mechanism (LOM) work together with adsorbate evolution mechanism (AEM) in the catalytic process of OER with the HEO catalyst. This strategy provides a new approach for the rapid synthesis of high-entropy oxide and inspires more rational designs of high-efficient electrocatalysts.

Engineering cobalt molybdate nanosheet arrays with phosphorus-modified nickel as heterogeneous electrodes for highly-active energy-saving water splitting

Electrochemical urea electrolysis has been regarded as a promising strategy to replace traditional water-splitting technology to achieve hydrogen fuel due to its cost savings and high energy efficiency. Designing efficient bifunctional electrocatalysts easily is important but still faces significant challenges. Herein, an interface engineering strategy is used to construct a hybrid material by coupling cobalt molybdate (CoMoO₄) nanosheet arrays with phosphorus-modified nickel (P-Ni) particles on copper foam (P-Ni@CoMoO₄/CF) through the hydrothermal and in-situ electrodeposition process. Benefiting from the abundant catalytic active sites, low charge transfer resistance, and synergistic coupling effect, the optimal P-Ni@CoMoO₄/CF electrocatalyst presents a superior bifunctional activity for urea oxidation reaction (UOR) and hydrogen evolution reaction (HER). In detail, a small overpotential of 125 mV and a low potential of 1.36 V is required to attain the current density of 100 mA cm^-2 for HER and UOR, respectively. In the process of urea electrolysis, the P-Ni@CoMoO₄/CF-based electrolyzer provides a current density of 100 mA cm^-2 with an overall voltage of 1.50 V, about 170 mV less than that in a traditional water electrolyzer. The high performance of P-Ni@CoMoO₄/CF outperforms many recently reported electrodes, suggesting its promising application in energy-saving hydrogen production. Our work proposes a novel idea for the rational design and exploitation of low-cost and robust bifunctional electrodes for electrocatalysis.

Evaluation of electrosynthesized reduced graphene oxide-Ni/Fe/Co-based (oxy)hydroxide catalysts towards the oxygen evolution reaction

In this work, the specific role of the addition of graphene oxide (GO) to state-of-the-art nickel-iron (NiFe) and cobalt-nickel-iron (CoNiFe) mixed oxides/hydroxides towards the oxygen evolution reaction (OER) is investigated. Morphology, structure, and OER catalytic activity of the catalysts with and without GO were studied. The catalysts were fabricated via a two-step electrodeposition. The first step included the deposition of GO flakes, which, in the second step, were reduced during the simultaneous deposition of NiFe or CoNiFe. As a result, NiFe-GO and CoNiFe-GO were fabricated without any additives directly on the nickel foam substrate. A significant improvement of the OER activity was observed after combining NiFe with GO (OER overpotential η(10 mA·cm-2): 210 mV) compared to NiFe (η: 235 mV) and GO (η: 320 mV) alone. A different OER activity was observed for CoNiFe-GO. Here, the overall catalytic activity (η: 230 mV) increased compared to GO alone. However, it was reduced in comparison to CoNiFe (η: 224 mV). The latter was associated with the change in the morphology and structure of the catalysts. Further OER studies showed that each of the catalysts specifically influenced the process. The improvement in the OER by NiFe-GO results mainly from the structure of NiFe and the electroactive surface area of GO.

Operando Scanning Electrochemical Probe Microscopy during Electrocatalysis

Scanning electrochemical probe microscopy (SEPM) techniques can disclose the local electrochemical reactivity of interfaces in single-entity and sub-entity studies. Operando SEPM measurements consist of using a SEPM tip to investigate the performance of electrocatalysts, while the reactivity of the interface is simultaneously modulated. This powerful combination can correlate electrochemical activity with changes in surface properties, e.g., topography and structure, as well as provide insight into reaction mechanisms. The focus of this review is to reveal the recent progress in local SEPM measurements of the catalytic activity of a surface toward the reduction and evolution of O₂ and H₂ and electrochemical conversion of CO₂. The capabilities of SEPMs are showcased, and the possibility of coupling other techniques to SEPMs is presented. Emphasis is given to scanning electrochemical microscopy (SECM), scanning ion conductance microscopy (SICM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical cell microscopy (SECCM).

POM@ZIF Derived Mixed Metal Oxide Catalysts for Sustained Electrocatalytic Oxygen Evolution

The design of efficient and stable oxygen evolution reaction (OER) catalysts based on noble-metal-free materials is crucial for energy conversion and storage. In this work, it was demonstrated how polyoxometalate (POM)-doped ZIF-67 can be converted into a stable oxygen evolution electrocatalyst by chemical etching, cation exchange, and thermal annealing steps. Characterization by X-ray photoelectron spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy and Raman spectroscopy indicate that POM-doped ZIF-67 derived carbon-supported metal oxides were synthesized. The resulting composite shows structural and compositional advantages which lead to low overpotential (306 mV at j=10 mA ⋅ cm^-2 ) and long-term stability under harsh OER conditions (1.0 M aqueous KOH).

Non-Kinetic Effects Convolute Activity and Tafel Analysis for the Alkaline Oxygen Evolution Reaction on NiFeOOH Electrocatalysts

A large variety of nickel-based catalysts has been investigated for the oxygen evolution reaction (OER) in alkaline media. However, their reported activity, as well as Tafel slope values, vary greatly. To understand this variation, we studied electrodeposited Ni₈₀ Fe₂₀ OOH catalysts with different loadings at varying rotation rates, hydroxide concentrations, with or without sonication. We show that, at low current density (<5 mA cm^-2 ), the Tafel slope value is ≈30 mV dec^-1 for Ni₈₀ Fe₂₀ OOH. At higher polarization, the Tafel slope continuously increases and is dependent on rotation rate, loading, hydroxide concentration and sonication. These Tafel slope values are convoluted by non-kinetic effects, such as bubbles, potential-dependent changes in ohmic resistance and (internal) OH^- gradients. As best practise, we suggest that Tafel slopes should be plotted vs. current or potential. In such a plot, it can be appreciated if there is a kinetic Tafel slope or if the observed Tafel slope is influenced by non-kinetic effects.

Dealloying fabrication of hierarchical porous Nickel–Iron foams for efficient oxygen evolution reaction

Designing and preparing highly active oxygen evolution reaction (OER) electrodes are essential for improving the overall efficiency of water splitting. Increasing the number of active sites is the simplest way to enhance OER performance. Herein, we present a dealloy-etched Ni–Fe foam with a hierarchical nanoporous structure as integrated electrodes with excellent performance for OER. Using the dealloying method on the Ni–Fe foam framework, a nanoporous structure is produced, which is named nanoporous Ni–Fe@Ni–Fe foam (NP-NF@NFF). Because of the peculiarities of the dealloying method, the NP-NF@NFF produced contains oxygen vacancies and heterojunctions. As a result, NP-NF@NFF electrode outperforms state-of-the-art noble metal catalysts with an extremely low overpotential of 210 and 285 mV at current densities of 10 and 100 mA cm−2, respectively. Additionally, the NP-NF@NFF electrode shows a 60-h stability period. Therefore, NP-NF@NFF provides new insights into the investigation of high-performance transition metal foam electrodes with effective active sites for efficient oxygen evolution at high current densities.

Development of Binder-Free Three-Dimensional Honeycomb-like Porous Ternary Layered Double Hydroxide-Embedded MXene Sheets for Bi-Functional Overall Water Splitting Reactions

In this study, a honeycomb-like porous-structured nickel-iron-cobalt layered double hydroxide/Ti3C2Tx (NiFeCo-LDH@MXene) composite was successfully fabricated on a three-dimensional nickel foam using a simple hydrothermal approach. Owing to their distinguishable characteristics, the fabricated honeycomb porous-structured NiFeCo-LDH@MXene composites exhibited outstanding bifunctional electrocatalytic activity for pair hydrogen and oxygen evolution reactions in alkaline medium. The developed NiFeCo-LDH@MXene electrocatalyst required low overpotentials of 130 and 34 mV to attain a current density of 10 mA cm-2 for OER and HER, respectively. Furthermore, an assembled NiFeCo-LDH@MXene‖NiFeCo-LDH@MXene device exhibited a cell voltage of 1.41 V for overall water splitting with a robust firmness for over 24 h to reach 10 mA cm-2 current density, signifying outstanding performance for water splitting reactions. These results demonstrated the promising potential of the designed 3D porous NiFeCo-LDH@MXene sheets as outstanding candidates to replace future green energy conversion devices.

An efficient hydrogen evolution photocatalyst of Rh@Cr2O3 loaded PbMoO4 twenty-six facets polyhedron

Shape anisotropic semiconductor is advanced catalyst for resolving energy crises. However, modification studies are still needed to overcome its intrinsic disadvantages, such as the lack of active sites. For this...

Hexagonal perovskite Sr6(Co0.8Fe0.2)5O15 as an efficient electrocatalyst towards the oxygen evolution reaction

The high overpotential required for the oxygen evolution reaction (OER)—due to the transfer of four protons and four electrons—has greatly hindered the commercial viability of water electrolysis. People have been...

Hierarchically structured nickel/molybdenum nitride heterojunctions as superior bifunctional electrodes for overall water splitting

A 3D hierarchical heterostructure of intermetallic compound heterojunctions is first rationally designed and presented as a highly-active bifunctional electrode for water splitting.