Materials For Oxygen Evolution Reaction Research Articles

Bimetallic two-dimensional materials for electrocatalytic oxygen evolution

Electrocatalytic oxygen evolution reaction (OER) is one of the important half reactions of electrocatalytic water splitting. However, the slow kinetic process involving four-electron transfer severely limits its reaction efficiency, which in turn limits the overall electrocatalytic hydrolysis efficiency. In order to improve the activity of the electrocatalytic OER, researchers mainly update the catalyst from three aspects, that is, increase the conductivity of the electrocatalyst, and the quantity and quality of active sites. Two-dimensional (2D) engineering can effectively reduce the resistance of the materials and greatly increase the number of electrochemically active sites, while heterometal doping, or the bimetal strategy, can improve the quality of active sites via changing the electronic structure of the material. Thus, the combination of the two can enhance the activity of electrocatalytic OER in all three aspects: conductivity, number and quality of active sites. However, there is currently no review on this topic. Therefore, in this review, we summarize the application of bimetallic 2D materials in electrocatalytic OER from four aspects: the structure, synthesis strategy, catalytic efficiency, and reaction mechanism.

Reconstruction of FeNi layered dihydroxides by cobalt doping to improve the electrocatalytic activity of oxygen evolution reaction

The increasing demand of clean energy in modern society calls for the development of efficient and renewable energy sources to reduce the consumption of fossil fuels. Although FeNi layered dihydroxide (FeNi-LDH) is a promising candidate material for oxygen evolution reaction (OER) catalysis, its catalytic performance can be further improved by the increased amounts of active sites, and the enhancement of electrical conductivity and durability. In this case, FeNiCo-LDH with the introduction of cobalt ions to FeNi-LDH can obtain a large active specific surface area by adjusting the electronic structure, and act as the main active substance in the reconstruction process to promote the catalytic performance of OER. Taking advantage of the low cost and easy availability of 304 stainless steel plates, the FeNiCo-LDH nanoplates were uniformly loaded onto the treated 304 plates by a simple two-step method. The as-prepared electrodes show superior OER in 1 M KOH, with 221 and 252 mV overpotentials to attain 10 and 350 mA cm−2 current densities, respectively with an ultra-low Tafel slope of 30.9 mV dec−1. Besides, the electrode also showed good stability in a chronopotential electrolysis at 10 mA cm−2 for 100 h.

Vanadium Copper Hydroxy Carbonate Hydrate Towards the Electrocatalytic Oxygen Evolution Reaction

Hydrogen with the highest energy density of 142 kJ/g has been considered as the green fuel to replace the non-renewable sources of energy in order to fulfil the increasing global energy demand. Electrocatalytic water splitting is one of the well-known processes to generate hydrogen gas, where oxygen evolution reaction (OER) occurring at the anode is the most sluggish part due to the involvement of multi-step electron transfer process. RuO2/IrO2 based electrodes are the best-known anode for such electrocatalytic cell. Due to the scarcity and high cost of such noble metals, it is urgent requirement to design non-noble metal based electrocatalytic material for OER. Recent advancements have demonstrated the utilization of vanadium-based and copper-containing materials as the most promising electrocatalyst for OER due to the strong synergic electronic and structural effects. In the present work, different ratios (1:2, 1:1 and 2:1) of vanadium copper hydroxy carbonate hydrates (VCHCH) were synthesized using hydrothermal and wet-chemical methods. The obtained materials were then deposited onto carbon-cloth substrate electrodes by drop casting and used as working electrode in a three-electrode cell setup consisting of a platinum-mesh as the counter electrode and Hg/HgO (1.0 M KOH) as the reference electrode in 1.0 M KOH, alkaline electrolytic solution. The electrochemical studies revealed that the inactive vanadium hydroxide became highly active towards the OER upon the addition of copper into the material. A current density of 10 mA/cm2 was achieved at the potential of 0.747 V, 0.768V and 0.760 V vs. Hg/HgO for the copper to vanadium ratios of 1:2, 1:1, and 2:1, respectively. The material also demonstrated a high stability with a consistent current density over 20 hours of experimental time. Figure 1

(Invited) Advancements in Alkaline Water Electrolysis: The Future of Clean Hydrogen Production

Hydrogen is a leading clean energy among the global initiatives to combat climate change. Currently, the primary source of hydrogen is produced from steam methane reforming (SMR), will produced 5.5 tons of carbon dioxide for every ton of hydrogen. Elimination of carbon dioxide in hydrogen production can be accomplished via water electrolysis.One commercially available water electrolysis system currently relies on proton exchange membrane (PEM) technology. Such electrolyzers require the use of expensive platinum group metal (PGM) catalysts and perfluorinated membranes. With respect to the PGM catalysts, iridium is used for the oxygen evolution reaction (OER) and only 7-8 tons of Ir is produced globally per year. Iridium’s scarcity and increased demand from PEM water electrolyzer (PEMWE) has led to its cost rising to > $6,400 per ounce. The reliance on PGM catalysts alone will significantly hinder the green hydrogen future. The solution to eliminating or dramatically reducing the need for PGM materials is alkaline exchange membrane water electrolysis.Alkaline exchange membrane electrolysis (AEMWE) is a burgeoning technology that combines a solid-state electrolyte (alkaline exchange membrane) with the ability to use PGM-free earth abundant catalyst materials. Currently AEMWE is in its adolescence. Many advancements have been made in membrane technology producing materials which are more chemically stable and operationally robust. Such advances have enabled demonstration of AEMWE without the use of supporting electrolyte thus further decreasing the technology’s cost witthout requiring corrosion resistant materials. Furthermore, advances in PGM-free catalysts, mainly NiFeM OER materials, have demonstrated comparable to improved performance with respect to iridium oxide. Therefore, the previous decade of research has indeed breathed increased interest and research effort into AEMWE.Here in, we report high performance and durability demonstrations of state of the art AEMWE technology. In this work, it is shown that the AEMWE with novel alkaline exchange membranes can operate for hundreds of hours on pure water only at 1000 mA/cm2. Such achievement is however not only limited to novel membrane development, but also due to membrane electrode assembly (MEA) design. The MEA fabrication is a critical step in determining performance and durability owing to the severely limited processing options in current alkaline exchange membrane technology. Furthermore, it is also demonstrated that in collaboration with the State University of New York Buffalo and University of Delaware, that NiFeM OER catalysts are indeed capable of replacing iridium oxide, achieving a similar potential of 1.8 V at 1000 mA/cm2.

Encapsulating Fe2 O3 Nanotubes into Carbon-Coated Co9 S8 Nanocages Derived from a MOFs-Directed Strategy for Efficient Oxygen Evolution Reactions and Li-Ions Storage.

The development of high-efficiency, robust, and available electrode materials for oxygen evolution reaction (OER) and lithium-ion batteries (LIBs) is critical for clean and sustainable energy system but remains challenging. Herein, a unique yolk-shell structure of Fe2 O3 nanotube@hollow Co9 S8 nanocage@C is rationally prepared. In a prearranged sequence, the fabrication of Fe2 O3 nanotubes is followed by coating of zeolitic imidazolate framework (ZIF-67) layer, chemical etching of ZIF-67 by thioacetamide, and eventual annealing treatment. Benefiting from the hollow structures of Fe2 O3 nanotubes and Co9 S8 nanocages, the conductivity of carbon coating and the synergy effects between different components, the titled sample possesses abundant accessible active sites, favorable electron transfer rate, and exceptional reaction kinetics in the electrocatalysis. As a result, excellent electrocatalytic activity for alkaline OER is achieved, which delivers a low overpotential of 205mV at the current density of 10mA cm-2 along with the Tafel slope of 55mV dec-1 . Moreover, this material exhibits excellent high-rate capability and excellent cycle life when employed as anode material of LIBs. This work provides a novel approach for the design and the construction of multifunctional electrode materials for energy conversion and storage.

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

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

Tuning Dynamically Formed Active Phases and Catalytic Mechanisms of In Situ Electrochemically Activated Layered Double Hydroxide for Oxygen Evolution Reaction.

The active phase and catalytic mechanisms of Ni-based layered double hydroxide (LDH) materials for oxygen evolution reaction (OER) have no common consensus and remain controversial. Moreover, engineering the site activity and the number of active sites of LDHs is an efficient approach to advance the OER activity, as the thickness and stacking structure of the LDHs layer limit the catalytic activity. This work presents an interesting in situ approach of tuning the site activity and number of active sites of NiMn-LDHs, which exhibit the superior OER performance (onset overpotential of 0.17 V and overpotential of 0.24 V at 10 mA cm-2). The fundamental mechanistic insights and active phases during the OER process are characterized by in operando techniques along with the computational density functional theory calculations, revealing that the Ni site constitutes the OER activity and the dynamically generated NiOOH moiety is the active phase. We also prove that Ni sites undergo a reversible oxidation state under the working conditions to create active NiOOH species which catalyze the water to generate oxygen. These findings suggest that the Ni(III) phase in NiMn-LDHs is the OER active site and Mn promotes the electronic properties of Ni sites. Utilizing in situ/in operando techniques and theoretical calculation, we find that the in situ intercalation of guest anions allows the expansion of the LDH layers and keeps the active NiOOH species under the oxidation state of +3 via electron coupling, which ultimately tunes the site populations and site activity toward the superior OER activity, respectively. This work thus targets to provide insight into strategies to design the next generation of highly active catalysts for water electrolysis and fuel cell technologies.

Why the optimum thermodynamic free-energy landscape of the oxygen evolution reaction reveals an asymmetric shape

The development of oxygen evolution reaction (OER) electrocatalysts has been spurred by thermodynamic considerations on the free-energy landscape. It is a common paradigm that the optimum thermodynamic free-energy landscape reveals a symmetric shape in that all reaction intermediates are stabilized at the equilibrium potential of the reaction. However, so far, no OER electrocatalyst has been reported that corresponds to the thermodynamic ideal because of the presence of a linear scaling relationship. Therefore, the common approach builds on the breaking of the scaling relations to establish a catalytic material that is close to the symmetric picture, yet, with minor successes. Relating to the simple two-electron hydrogen evolution reaction (HER), it was recently reported that the optimum thermodynamic free-energy landscape reveals an asymmetric shape rather than a symmetric form as soon as overpotential and kinetic effects are factored in the analysis. This finding motivates scrutinizing whether the symmetric free-energy landscape as the thermodynamic ideal in the OER is justified. Transferring the knowledge from the HER to the OER results in the introduction of the electrochemical-step asymmetry index (ESAI), representing the concept of the asymmetric thermodynamic free-energy diagram. By comparing the ESAI to the symmetric picture in terms of the electrochemical-step symmetry index (ESSI), it is demonstrated herein that the asymmetric rather than the symmetric free-energy landscape corresponds to the thermodynamic ideal. This outcome suggests changing the mindset when applying the concept of free-energy diagrams for the discovery of OER materials by heuristic material-screening techniques.

Single-Atom Doping and High-Valence State for Synergistic Enhancement of NiO Electrocatalytic Water Oxidation.

The NiO-based electrocatalytic oxygen evolution reaction (OER) of water splitting is recognized as a promising approach to produce clean H2 fuel. However, the OER performance is still low, and especially, the overpotential is larger than 200mV at the current density of 10mA cm-2 . Herein, an Ir@IrNiO sample is prepared with single-atom (SA) Ir4+ doping and surface metallic Ir nanoparticles loaded onto the NiO. Owing to the bonding of the loaded Ir with surface-exposed Ni2+ , the nearby Ni atoms exist in the +3valence state, that is, the surface-loaded Ir particles behave like a stabilizer for the Ni3+ sites. Under the synergistic effect of SA Ir4+ and high-valance-state Ni3+ , the Ir@IrNiO nanostructure effectively reduces the overpotential to 195mV at acurrent density of 10mA cm-2 . Moreover, it gives an Ir-content-normalized current density of 0.0457 A mgIr -1 , 72.1 times higher than that of the best commercialized IrO2 (6.33 × 10-4 A mgIr -1 ), under the condition of 1.5V versus reversible hydrogen electrode. Operando Raman and X-ray absorption fine-structure (XAFS) measurements reveal that there are more surface-active species of Ni3+ , which adsorb and activate water molecules to form Ni3+ -*OH at low voltage, the intermediate of Ni4+ -•O is then formed at a relatively high bias voltage, and then the •O is transferred to the SA Ir4+ sites to generate Ir4+ -O-O with OH at increased voltage. This work can help design more SA-based highly active OER materials.

Designing a bimetallic transition metal oxide/hydroxide composite for effective electrocatalytic oxygen evolution reaction

The oxygen evolution reaction (OER), owing to its sluggish nature, demands efficient and stable catalysts with low production cost, during the electrocatalytic water splitting. In this respect, a series of bimetallic ZnO-Ni(OH)2 composite materials were prepared as electrode materials for OER. The synthetic procedure followed the polyvinylpyrrolidone (PVP) and dopamine (DA) assisted hydrothermal process. The growth modification ability of PVP and adhesive property of polydopamine (from DA) were utilized to attain the 3D interconnected structures. The precursor concentrations were varied to obtain most efficient catalyst, ZNDP-1. In alkaline medium, ZNDP-1 showed a low over potential of 170 mV (@10 mAcm−2 current density) and a Tafel slope of 61 mV dec−1 with significant stability. The surface defects, porosity and synergism within metal centers might cause efficient electrocatalytic activity. Furthermore, DFT studies were performed to understand the mechanistic pathway.

Three-Dimensional Flower-like Fe, C-Doped-MoS2/Ni3S2 Heterostructures Spheres for Accelerating Electrocatalytic Oxygen and Hydrogen Evolution

The exploration of high-efficiency bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has long been challenging. The rational design of a catalyst by constructing heterostructures and a doping element are possibly expected to achieve it. Herein, the utilization of flower-like Fe/C-doped-MoS2/Ni3S2-450 spherical structural materials for electrocatalytic HER and OER is introduced in this study. The carboxyferrocene-incorporated molybdenum sulfide/nickel sulfide (MoySx/NiS) nanostructures were prepared by solvothermal method. After annealing, the iron and carbon elements derived from ferrocenecarboxylic acid enhanced the electrical transport performance and provided rich electronic sites for HER and OER in alkaline media. Specifically, the optimized flower-like Fe/C-doped-MoS2/Ni3S2-450 exhibited efficient bifunctional performance in alkaline electrolyte, with low overpotentials of 188 and 270 mV required to deliver a current density of 10 mA cm−2 for HER and OER, respectively. This work provides valuable insights for the rational design of energy storage and conversion materials by the incorporation of transition metal and carbon elements into metal sulfide structures utilizing metallocene.

Template-Free Hydrothermal Growth of Nickel Sulfide Nanorods as High-Performance Electroactive Materials for Oxygen Evolution Reaction and Supercapacitors

Transition-metal sulfides with nanostructured features grown on a conductive substrate have been suggested as a promising alternative to precious metal-based electrocatalysts for energy conversion ...

Improved oxygen evolution reaction performance with addition of Fe to form FeyCux-yCo3-xO4 and FeyNix-yCo3-xO4 (x = 0.5, 1 and y = 0.1, 0.15) spinel oxides

Transition metal spinel oxides have received renewed attention with their demonstrated usefulness as anodes for oxygen evolution reaction. The careful choice of the synthesis method and experimental conditions allow for the tailoring of the film nanostructure and surface area. We have prepared various spinel oxides, FeyCux-yCo3-xO4 and FeyNix-yCo3-xO4 (for y = 0.1 or 0.15 and x = 1 or 0.5 for both Fe-Cu and Fe-Ni containing oxides), using the thermal decomposition method. The films were analyzed using several structural, chemical and electrochemical methods such as X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS). The SEM analysis of FeyCux-yCo3-xO4 electrodes shows that surface is covered with grains, while the surface of FeyNix-yCo3-xO4 appears smoother and more compact. For all our compositions, the X-ray data support the existence of the spinel structure. XPS measurements were performed to study the cation distribution and surface chemical composition of the electrodes, both survey spectra and core level spectra were recorded. The high-resolution spectra for the Co, Cu and Ni 2p peaks indicate the existence of different Co2+/Co3+, Cu+/Cu2+ and Ni2+/Ni3+ ratio for these materials (note that the Fe signal was often too small in the XPS spectra to yield meaningful results). The analysis of O 1s spectra indicate that the main peak is composed of three components that can be assigned to lattice oxygen, adsorbed oxygen containing species such as hydroxides and surface bonded water. Electrochemical measurements show that when the current is corrected for the real surface area, FeyCux-yCo3-xO4 and FeyNix-yCo3-xO4 show significant enhancement with almost double the current density for the oxygen evolution reaction at 1 V under the same conditions compared to Co3O4. Fe has some synergic effect with Ni, Cu and Co, which promote the catalytic activity. A suitable combination of metal cations in the spinel structure could markedly enhance current density and thus, chemical composition plays a crucial role in the electrocatalytic activity of the materials for oxygen evolution reaction.

Oxygen-deficient Cu doped NiFeO nanosheets hydroxide as electrode material for efficient oxygen evolution reaction and supercapacitor

The development of renewable energy conversion and storage has triggered the development of electrode materials for oxygen evolution reaction (OER) and supercapacitors. Here we report a highly active Cu doped NiFe nanosheets hydroxide electrode with rich oxygen vacancies (OVs) (denoted as H-NiFeCuO/NF) prepared by in situ anodic electrodeposition on the three-dimensional macroporous nickel foam (NF) substrate followed by heat treatment with H2. The as-prepared H-NiFeCuO/NF electrode showed the initial potential of 1.44 V (versus RHE) for OER and 980 F g−1 specific capacity as supercapacitor in 1 M KOH. Further investigation suggested that the tuning of composition and structure by doping copper ions and creating OVs helped accelerate the electrochemical reactions. This practice provides an efficient approach for the fabrication of heteromultimetallic hydroxide monolithic electrode with high performance in OER or supercapacitor application.

Synergistic coupling of NiFe layered double hydroxides with Co-C nanofibers for high-efficiency oxygen evolution reaction

In recent years, the design of high-performance and low-cost electrocatalysts for oxygen evolution reaction (OER) has captured great interest in the field of energy conversion and storage. Layered double hydroxide (LDH) with special configuration usually shows a highly electrochemical activity and become a desirable functional material for OER. In this work, carbon nanofibers embedded with Co nanoparticles have been prepared via an electrospinning and carbonization process, which can be served as a conductive support for the growth of NiFe LDH nanosheets through an electrodeposition route to produce a hierarchical 3D structure (denoted as 3D core-shell Co-C@NiFe LDH nanofibers). Owing to the high conductivity, large surface area and strong electron transfer between NiFe LDH nanosheets and Co-C nanofibers, the as-prepared 3D core-shell Co-C@NiFe LDH nanofibrous catalyst shows an excellent OER performance with an overpotential of only 249 mV at the current density of 10 mA cm−2 and a small Tafel slope of 57.9 mV dec−1, demonstrating an outstanding electrocatalytic activity for water oxidation. Furthermore, the 3D core-shell Co-C@NiFe LDH nanofibers also exhibit a superior long-term stability, with no obvious decrease in current density after nearly 190 h testing. Hence, the low-cost 3D core-shell Co-C@NiFe LDH nanofibrous electrocatalyst with an outstanding OER performance offers a novel chance for practical electrochemical water splitting application.

Nitrogen-doped-carbon coated FeCo modified CoFe2O4 nanoflowers heterostructure with robust stability for oxygen evolution and urea oxidation

The exploitation of catalysts with low-cost and superior oxygen evolution reaction (OER) and urea oxidation reaction (UOR) performance is important to expand the application of water splitting. Herein, the FeCo/CoFe2O4 nanoflowers are successfully embedded in N-doped graphitic carbon (NGC) by hydrothermal method and subsequently annealing process (FeCo/CoFe2O4@NGC). It only needs 1.47 V and 1.32 V for OER and UOR to produce a current density of 10 mA cm−2. Besides, it can work continuously for 100 h at different current densities (10; ∼ 1000 mA cm−2), displays outstanding stability, which is mainly ascribed to the N-doped carbon encapsulated FeCo modified CoFe2O4 heterostructure and porous structure. This study thus provides a simple synthesis method to design and manufacture low-cost and high-performance catalytic materials for OER and UOR, and promotes the industrial application of water splitting.

Dynamically Stable Active Sites from Surface Evolution of Perovskite Materials during the Oxygen Evolution Reaction.

Perovskite oxides are an important class of oxygen evolution reaction (OER) catalysts in alkaline media, despite the elusive nature of their active sites. Here, we demonstrate that the origin of the OER activity in a La1-xSrxCoO3 model perovskite arises from a thin surface layer of Co hydr(oxy)oxide (CoOxHy) that interacts with trace-level Fe species present in the electrolyte, creating dynamically stable active sites. Generation of the hydr(oxy)oxide layer is a consequence of a surface evolution process driven by the A-site dissolution and O-vacancy creation. In turn, this imparts a 10-fold improvement in stability against Co dissolution and a 3-fold increase in the activity-stability factor for CoOxHy/LSCO when compared to nanoscale Co-hydr(oxy)oxides clusters. Our results suggest new design rules for active and stable perovskite oxide-based OER materials.

High-valence-state Ni–Fe bimetallic phosphonate nanoribbons catalyst for enhanced photocatalytic and electrocatalytic oxygen production

The design and synthesis of economy and efficiency materials for oxygen evolution reaction (OER) have been a continuous hot spot in the field of scientific study. Herein, a high-valence-state two-dimension (2D) NiFe phosphonate-based (NiFeP) nanoribbons catalyst has been constructed through a one-step solvothermal process. The NiFeP nanoribbons exhibit highly active in both photocatalytic and electrocatalytic water oxidation due to the 2D nanoribbons and high-valence Ni3+ sites. The 2D nanoribbons not only provide more reactive sites for OER but also shorten bulk diffusion distance with better photoexcited carrier transport from the interior to the surface. Meanwhile, the existence of high-valence Ni3+ could be acted as an efficient redox site to reduce the overpotential and facilitate the catalytic reaction. In consequence, the NiFeP nanoribbons catalyst demonstrates a superior O2 yield of 65.7% and O2 production rate of 25.97 umol s−1 g−1, which are comparable or even much higher than those other reported transition metal oxide photocatalysts. At last, the possible proton-coupled electron transfer mechanism is also proposed. This study not only demonstrates the potential of a low-cost metal phosphonate OER catalyst but also provides a referential system for the fabrication of high activity and stability catalysts toward replacing noble metals for energy storage and conversion. High-valence-state 2D NiFe phosphonate-based nanoribbons catalyst has been constructed for the first time through a one-step solvothermal process without any organic additives or template agent. The as-prepared catalyst of NiFeP nanoribbons exhibits highly active in both photocatalytic and electrocatalytic water oxidation.

Fabrication of Ni–MOF-derived composite material for efficient electrocatalytic OER

The poverty-stricken population cannot withstand the shocks of global warming and can hardly pay the soaring expenditure of fossil fuels whose reservoirs are already approaching the threshold limit. These adventures guided the researchers towards water oxidation as a source of energy. In this study, a metal-organic framework (MOF)-based material was fabricated and catalyzed the lethargic oxygen evolution reaction (OER). The synthesized material is characterized with different analytical techniques to confirm structural, morphological and textural properties. Its huge surface area (87 m2/g) rendered it a promising material to carry the OER. It exhibited shallow onset potential that is, 1.40 V vs. RHE and displayed an exceptionally low overpotential of 1.42 V vs. RHE to reach the benchmark current density (10 mA/cm2) with remarkably small Tafel slope that is, 34 mV/dec. It surpassed the state of the art electrocatalyst for OER, that is, IrO2 and RuO2 in efficiency as well as in stability.

Recent Progress on NiFe-Based Electrocatalysts for the Oxygen Evolution Reaction.

The seriousness of the energy crisis and the environmental impact of global anthropogenic activities have led to an urgent need to develop efficient and green fuels. Hydrogen, as a promising alternative resource that is produced in an environmentally friendly and sustainable manner by a water splitting reaction, has attracted extensive attention in recent years. However, the large-scale application of water splitting devices is hindered predominantly by the sluggish oxygen evolution reaction (OER) at the anode. Therefore, the design and exploration of high-performing OER electrocatalysts is a critical objective. Considering their low prices, abundant reserves, and intrinsic activities, NiFe-based bimetal compounds are widely studied as excellent OER electrocatalysts. Moreover, recent progress on NiFe-based OER electrocatalysts in alkaline environments is comprehensively and systematically introduced through various catalyst families including NiFe-layered hydroxides, metal-organic frameworks, NiFe-based (oxy)hydroxides, NiFe-based oxides, NiFe alloys, and NiFe-based nonoxides. This review briefly introduces the advanced NiFe-based OER materials and their corresponding reaction mechanisms. Finally, the challenges inherent to and possible strategies for producing extraordinary NiFe-based electrocatalysts are discussed.