Fe2+ -Doped Layered Double (Ni, Fe) Hydroxides as Efficient Electrocatalysts for Water Splitting and Self-Powered Electrochemical Systems

Magnesium-promoted rapid self-reconstruction of NiFe-based electrocatalysts toward efficient oxygen evolution

The acceleration of active sites formation through surface reconstruction is widely acknowledged as the crucial factor in developing high-performance oxygen evolution reaction (OER) catalysts for water splitting. Herein, a simple one-step corrosion method and magnesium (Mg)-promoted strategy are reported to develop the NiFe-based catalyst with enhanced OER performance. The Mg is introduced in NiFe materials to preparate a "pre-catalyst" Mg-Ni/Fe2O3. In-situ Raman shows that Mg doping would accelerate the self-reconstruction of Ni/Fe2O3 to form active NiOOH species during OER. In-situ infrared indicates that Mg doping benefits the formation of *OOH intermediate. Theoretical analysis further confirms that Mg doping can optimize the adsorption of oxygen intermediates, accelerating the OER kinetics. Accordingly, the Mg-Ni/Fe2O3 catalyst exhibits excellent OER performance with overpotential of 168 mV at 10 mA cm-2. The anion exchange membrane water electrolyzer achieved 200 mA cm-2 at voltage of 1.53 V, showing excellent stability over 500 h as well. This work demonstrates the potential of Mg-promoted strategy in regulating the activity of transition metal-based OER electrocatalysts.

Boosted electrocatalytic activity and durability of CuFe/NC by modulating the interfacial composition and electronic structure for efficient oxygen reduction reaction

Oxygen reduction reaction (ORR) serves as the foundation for various electrochemical energy storage devices. Fe/NC catalysts are expected to replace commercial Pt/C as oxygen electrode catalysts based on the structural tunability at the atomic level, abundant iron ore reserves and excellent activity. Nevertheless, the lack of durability and low active site density impede its advancement. In this work, a durable catalyst, CuFe/NC, for ORR was prepared by modulating the interfacial composition and electronic structure. The introduction of Cu nanoclusters partially eliminates the Fenton effect from Fe and optimizes the electron structure of FeNx, thereby effectively enhancing the long-term durability and activity. The prepared CuFe/NC exhibits a half-wave potential (E1/2) of 0.90 V and superior stability with a decrease in E1/2 of only 20 mV after 10,000 cycles. The assembled alkaline Zinc-Air batteries (ZABs) with CuFe/NC exhibit an open-circuit potential of 1.458 V. At a current density of 5 mA cm-2, the batteries are capable of operation for 600 h with a stable polarization. This CuFe/NC may promote the practical application of novel and renewable electrochemical energy storage devices.

Designed functions of oxide/hydroxide nanosheets via elemental replacement/doping

Partial replacement of one structural element in a solid with another of a similar size was conducted to impart functionality to the solids and modify their properties. This phenomenon is found in nature in coloured gemstones and clay minerals and is used in materials chemistry and physics, endowing materials with useful properties that can be controlled by incorporated heteroelements and their amounts. Depending on the area of research (or expected functions), the replacement is referred to as "isomorphous substitution", "doping", etc. Herein, elemental replacement in two-dimensional (2D) oxides and hydroxides (nanosheets or layered materials) is summarised with emphasis on the uniqueness of their preparation, characterisation and application compared with those of the corresponding bulk materials. Among the 2D materials (graphene, metallenes, transition metal chalcogenides, metal phosphate/phosphonates, MXenes, etc.), 2D oxides and hydroxides are characterised by their presence in nature, facile synthesis and storage under ambient conditions, and possible structural variation from atomic-level nanosheets to thicker nanosheets composed of multilayered structures. The heteroelements to be doped were selected depending on the target application objectively; however, there are structural and synthetic limitations in the doping of heteroelements. In the case of layered double hydroxides (single layer) and layered alkali silicates (from single layer to multiple layers), including layered clay minerals (2 : 1 layer), the replacement (commonly called isomorphous substitution) is discussed to understand/design characteristics such as catalytic, adsorptive (including ion exchange), and swelling properties. Due to the variation in their main components, the design of layered transition metal oxide/hydroxide materials via isomorphous substitution is more versatile; in this case, tuning their band structure, doping both holes and electrons, and creating impurity levels are examined by the elemental replacement of the main components. As typical examples, material design for the photocatalytic function of an ion-exchangeable layered titanate (lepidocrocite-type titanate) and a perovskite niobate (KCa2Nb3O10) is discussed, where elemental replacement is effective in designing their multiple functions.

Ultrasonic-assisted Fenton reaction inducing surface reconstruction endows nickel/iron-layered double hydroxide with efficient water and organics electrooxidation

Nickel/iron-layered double hydroxide (NiFe-LDH) tends to undergo an electrochemically induced surface reconstruction during the water oxidation in alkaline, which will consume excess electric energy to overcome the reconstruction thermodynamic barrier. In the present work, a novel ultrasonic wave-assisted Fenton reaction strategy is employed to synthesize the surface reconstructed NiFe-LDH nanosheets cultivated directly on Ni foam (NiFe-LDH/NF-W). Morphological and structural characterizations reveal that the low-spin states of Ni2+ (t2g6eg2) and Fe2+ (t2g4eg2) on the NiFe-LDH surface partially transform into high-spin states of Ni3+ (t2g6eg1) and Fe3+ (t2g3eg2) and formation of the highly active species of NiFeOOH. A lower surface reconstruction thermodynamic barrier advantages the electrochemical process and enables the NiFe-LDH/NF-W electrode to exhibit superior electrocatalytic water oxidation activity, which delivers 10 mA cm-2 merely needing an overpotential of 235 mV. Besides, surface reconstruction endows NiFe-LDH/NF-W with outstanding electrooxidation activities for organic molecules of methanol, ethanol, glycerol, ethylene glycol, glucose, and urea. Ultrasonic-assisted Fenton reaction inducing surface reconstruction strategy will further advance the utilization of NiFe-LDH catalyst in water and organics electrooxidation.

Metal Chalcogenide-Hydroxide Hybrids as an Emerging Family of Two-Dimensional Heterolayered Materials: An Early Review

Two-dimensional (2D) materials and phenomena attract huge attention in modern science. Herein, we introduce a family of layered materials inspired by the minerals valleriite and tochilinite, which are composed of alternating "incompatible", and often incommensurate, quasi-atomic sheets of transition metal chalcogenide (sulfides and selenides of Fe, Fe-Cu and other metals) and hydroxide of Mg, Al, Fe, Li, etc., stacked via electrostatic interaction rather than van der Waals forces. We survey the data available on the composition and structure of the layered minerals, laboratory syntheses of such materials and the effect of reaction conditions on the phase purity, morphology and composition of the products. The spectroscopic results (Mössbauer, X-ray photoelectron, X-ray absorption, Raman, UV-vis, etc.), physical (electron, magnetic, optical and some others) characteristics, a specificity of thermal behavior of the materials are discussed. The family of superconductors (FeSe)·(Li,Fe)(OH) having a similar layered structure is briefly considered too. Finally, promising research directions and applications of the valleriite-type substances as a new class of prospective multifunctional 2D materials are outlined.

Analysis of the oxygen evolution activity of layered double hydroxides (LDHs) using machine learning guidance

Layered double hydroxides (LDHs) are excellent catalysts for the oxygen evolution reaction (OER) because of their tunable properties, including chemical composition and structural morphology. An interplay between these adjustable properties and other (including external) factors might not always benefit the OER catalytic activity of LDHs. Therefore, we applied machine learning algorithms to simulate the double-layer capacitance to understand how to design/tune LDHs with targeted catalytic properties. The key factors of solving this task were identified using the Shapley Additive explanation and cerium was identified as an effective element to modify the double-layer capacitance. We also compared different modelling methods to identify the most promising one and the results revealed that binary representation is better than directly applying atom numbers as inputs for chemical compositions. Overpotentials of LDH-based materials as predicted targets were also carefully examined and evaluated, and it turns out that overpotentials can be predicted when measurement conditions about overpotentials are added as features. Finally, to confirm our findings, we reviewed additional experimental literature data and used them to test our machine algorithms to predict LDH properties. This analysis confirmed the very credible and robust generalization ability of our final model capable of achieving accurate results even with a relatively small dataset.

In Situ Reconstructed Mo-doped Amorphous FeOOH Boosts the Oxygen Evolution Reaction

Developing a fast and highly active oxygen evolution reaction (OER) catalyst to change energy kinetics technology is essential for making clean energy. Herein, we prepare three-dimensional (3D) hollow Mo-doped amorphous FeOOH (Mo-FeOOH) based on the precatalyst MoS₂ /FeC₂ O₄ via in situ reconstruction strategy. Mo-FeOOH exhibits promising OER performance. Specifically, it has an overpotential of 285 mV and a durability of 15 h at 10 mA cm^-2 . Characterizations indicate that Mo was included inside the FeOOH lattice, and it not only modifies the electronic energy levels of FeOOH but also effectively raises the inherent activity of FeOOH for OER. Additionally, in situ Raman analysis indicates that FeC₂ O₄ gradually transforms into the FeOOH active site throughout the OER process. This study provides ideas for designing in situ reconstruction strategies to prepare heteroatom doping catalysts for high electrochemical activity.

High-density electron transfer in Ni-metal-organic framework@FeNi-layered double hydroxide for efficient electrocatalytic oxygen evolution

The electrochemical oxygen evolution reaction is a bottleneck reaction in hydrolysis and electrolysis because the four-step electron transfer leads to slow reaction kinetics and large overpotentials. This situation can be improved by fast charge transfer by optimizing the interfacial electronic structure and enhancing polarization. Herein, a unique metal (Ni) organic (diphenylalanine, DPA) framework Ni(DPA)₂ (Ni-MOF) with tunable polarization is designed to bond with FeNi-LDH (layered double hydroxides) nanoflakes. The Ni-MOF@FeNi-LDH heterostructure delivers excellent oxygen evolution performance exemplified by an ultralow overpotential of 198 mV at 100 mA cm^-2 compared to other (FeNi-LDH)-based catalysts. Experiments and theoretical calculations show that FeNi-LDH exists in an electron-rich state in Ni-MOF@FeNi-LDH due to polarization enhancement caused by interfacial bonding with Ni-MOF. This effectively changes the local electronic structure of the metal Fe/Ni active sites and optimizes adsorption of the oxygen-containing intermediates. Polarization and electron transfer of Ni-MOF are further enhanced by magnetoelectric coupling consequently giving rise to better electrocatalytic properties as a result of high-density electron transfer to active sites. These findings reveal a promising interface and polarization modulation strategy to improve electrocatalysis.

Marigold Flower-Shaped Metal-Organic Framework Supported Manganese Vanadium Oxide Electrocatalyst for Efficient Oxygen Evolution Reactions in an Alkaline Medium

The electrochemical oxygen evolution reaction (OER) is a key process in many renewable energy systems. The development of low-cost, long-lasting alternatives to precious-metal catalysts, particularly functional electrocatalysts with high activity for OER processes, is crucial for reducing the operating expense and complexity of renewable energy generating systems. This work describes a concise method for generating marigold flower-like metal-organic frameworks (MOFs) aided manganese vanadium oxide via a hydrothermal procedure for increased OER activity. As synthesized MOF MnV oxide has a higher surface area due to the 3D flower-like structure, which is reinvented with enhanced electrocatalytic active sites. These distinctive structural features result in remarkable catalytic activity for MOF MnV oxide microflowers towards OER with a low overpotential of 310 mV at 50 mA cm^-2 and a Tafel slope with only 51.4 mV dec^-1 in alkaline conditions. This study provides a concise method for developing an optimized catalytic material with greater morphology and beneficial features for potential energy and environmental applications.

Pearson’s Principle-Inspired Robust 2D Amorphous Ni-Fe-Co Ternary Hydroxides on Carbon Textile for High-Performance Electrocatalytic Water Splitting

Layered double hydroxide (LDH) is widely used in electrocatalytic water splitting due to its good structural tunability, high intrinsic activity, and mild synthesis conditions, especially for flexible fiber-based catalysts. However, the poor stability of the interface between LDH and flexible carbon textile prepared by hydrothermal and electrodeposition methods greatly affects its active area and cyclic stability during deformation. Here, we report a salt-template-assisted method for the growth of two-dimensional (2D) amorphous ternary LDH based on dip-rolling technology. The robust and high-dimensional structure constructed by salt-template and fiber could achieve a carbon textile-based water splitting catalyst with high loading, strong catalytic activity, and good stability. The prepared 2D NiFeCo-LDH/CF electrode showed overpotentials of 220 mV and 151 mV in oxygen evolution and hydrogen evolution reactions, respectively, and simultaneously had no significant performance decrease after 100 consecutive bendings. This work provides a new strategy for efficiently designing robust, high-performance LDH on flexible fibers, which may have great potential in commercial applications.

Grain-boundary-rich layered double hydroxides via a boron-assisted strategy for the oxygen evolution reaction

This study reports a facile boron-assisted strategy to prepare NiFe LDHs with rich grain boundaries. The formation of these grain boundaries originates from the imperfect oriented attachment between primary LDH particles transformed from amorphous borides/borates. The obtained grain-boundary-rich NiFe LDHs exhibit excellent oxygen evolution reaction activity.

Manipulating the Electronic Structure of Graphite Intercalation Compounds for Boosting the Bifunctional Oxygen Catalytic Performance

Developing highly efficient bifunctional catalysts for the oxygen reduction and oxygen evolution reaction (ORR/OER) can open possibilities for future zinc air batteries (ZABs). Herein, cost-effective and highly conductive few-layer ferric and nickel chloride co-intercalated graphite intercalation compounds (FeCl₃ -NiCl₂ -GIC) are designed as bifunctional oxygen catalysts for ZAB. The optimized few-layer FeCl₃ -NiCl₂ -GIC catalyst exhibits a small overpotential of 276 mV at 10 mA cm^-2 for the OER and achieves a high onset potential of 0.89 V for the ORR. The theoretical analysis demonstrates the electron-rich state on the carbon layers of FeCl₃ -NiCl₂ -GIC during the catalytic process favors the kinetics of electron transfer and lowers the absorption energy barriers for intermediates. Impressively, the ZAB assembled with few-layer FeCl₃ -NiCl₂ -GIC catalyst displays a 160 h cycling stability and a high energy efficiency of 72.6%. This work also suggests the possibility of utilizing layer electronic structure regulation on graphite intercalation compounds as effective bifunctional catalysts for ZABs.

Quasi-Parallel NiFe Layered Double Hydroxide Nanosheet Arrays for Large-Current-Density Oxygen Evolution Electrocatalysis

Designing advanced electrocatalysts for oxygen evolution at large current density (>500 mA cm^-2 ) is critical to practical water splitting applications. Herein, a novel quasi-parallel NiFe layered double hydroxide (NiFe LDH) nanosheet arrays with pattern alignment on Ni foam was developed. The initial α-Ni(OH)₂ layer induced effective coprecipitation between Ni²⁺ and Fe³⁺ for the formation of LDH phase, guaranteeing the electronic pulling effect among metal cations and enhancing the interaction between active materials and substrate for excellent adhesion and electrical conductivity. Quasi-parallel NiFe LDH nanoarrays exhibited outstanding oxygen evolution activity with a small Tafel slope of 30.1 mV dec^-1 and overpotentials of 196, 255, and 284 mV at a current density of 10, 500, and 1000 mA cm^-2 in 1.0 m KOH solution, respectively, and high stability over 40 h at 750 mA cm^-2 . This work presents a new strategy towards fabricating electrode materials with exceptional performance.

Three-dimensional hierarchical conductive metal–organic frameworks/NiFe layered double hydroxide/carbon nanofibers: an efficient oxygen evolution reaction catalyst for Zn–air batteries

The structural and compositional modulation of inexpensive hydroxides is not only important, but also an ongoing challenge for the preparation of efficient oxygen evolution reaction (OER) electrocatalysts.

FeCo Nanoparticles Encapsulated in N-Doped Carbon Nanotubes Coupled with Layered Double (Co, Fe) Hydroxide as an Efficient Bifunctional Catalyst for Rechargeable Zinc-Air Batteries

Low-cost bifunctional nonprecious metal catalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are critical for the commercialization of rechargeable zinc-air batteries (ZABs). However, the preparation of highly active and durable bifunctional catalysts is still challenging. Herein, an efficient catalyst is reported consisting of FeCo nanoparticles embedded in N-doped carbon nanotubes (FeCo NPs-N-CNTs) by an in situ catalytic strategy. Due to the encapsulation and porous structure of N-doped carbon nanotubes, the catalyst shows high activity toward ORR and excellent durability. Furthermore, to enhance the OER activity, CoFe-layer double hydroxide (CoFe-LDH) is coupled with FeCo NPs-N-CNTs by in situ reaction approach. As the air electrode for rechargeable ZABs, the cell with CoFe-LDH@FeCo NPs-N-CNTs catalyst exhibits high open-circuit potential (OCP) of 1.51 V, high power density of 116 mW cm^-2 , and remarkable durability up to 100 h, demonstrating its great promise for the practical application of the rechargeable ZABs.

Engineering Self-Reconstruction via Flexible Components in Layered Double Hydroxides for Superior-Evolving Performance

Most transition metal-based catalysts for electrocatalytic oxygen evolution reaction (OER) undergo surface reconstruction to generate real active sites favorable for high OER performance. Herein, how to use self-reconstruction as an efficient strategy to develop novel and robust OER catalysts by designing pre-catalysts with flexible components susceptible to OER conditions is proposed. The NiFe-based layered double hydroxides (LDHs) intercalated with resoluble molybdate (MoO₄ ^2- ) anions in interlayers are constructed and then demonstrated to achieve complete electrochemical self-reconstruction (ECSR) into active NiFe-oxyhydroxides (NiFeOOH) beneficial to alkaline OER. Various ex situ and in situ techniques are used to capture structural evolution process including fast dissolution of MoO₄ ^2- and deep reconstruction to NiFeOOH upon simultaneous hydroxyl invasion and electro-oxidation. The obtained NiFeOOH exhibits an excellent OER performance with an overpotential of only 268 mV at 50 mA cm^-1 and robust durability over 45 h, much superior to NiFe-LDH and commercial IrO₂ benchmark. This work suggests that the ECSR engineering in component-flexible precursors is a promising strategy to develop highly active OER catalysts for energy conversion.

Sub-Nanometer Pt Clusters on Defective NiFe LDH Nanosheets as Trifunctional Electrocatalysts for Water Splitting and Rechargeable Hybrid Sodium-Air Batteries

It is challenging to develop highly efficient and stable multifunctional electrocatalysts for improving the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR) for sustainable energy conversion and storage systems such as water-alkali electrolyzers (WAEs) and hybrid sodium-air batteries (HSABs). In this work, sub-nm Pt nanoclusters (NCs) on defective NiFe layered double hydroxide nanosheets (Ni_xFe LDHs) are synthesized by a facile electrodeposition method. Due to the synergistic effect between Pt NCs and abundant atomic M(II) defects, along with hierarchical porous nanostructures, the Pt/Ni_xFe LDHs catalysts exhibit superior trifunctional electrocatalytic activity and durability toward the HER/OER/ORR. A WAE fabricated with Pt/Ni_xFe LDHs electrodes needs 1.47 V to reach a current density of 10 mA cm^-2, much lower than that of the mixed 20% Pt/C and 20% Ir/C catalysts. An HSAB assembled by Pt/Ni_xFe LDHs as a binder-free air cathode displays a high open-circuit voltage, a narrow overpotential gap, and remarkable rechargeability. This work provides a feasible strategy for constructing freestanding efficient trifunctional electrocatalysts for sustainable energy conversion and storage systems.

Vertically-interlaced NiFeP/MXene electrocatalyst with tunable electronic structure for high-efficiency oxygen evolution reaction

Layered double hydroxides (LDHs) with decent oxygen evolution reaction (OER) activity have been extensively studied in the fields of energy storage and conversion. However, their poor conductivity, ease of agglomeration, and low intrinsic activity limit their practical application. To date, improvement of the intrinsic activity and stability of NiFe-LDHs through the introduction of heteroatoms or its combination with other conductive substrates to enhance their water-splitting performance has drawn increasing attention. In this study, vertically interlaced ternary phosphatised nickel/iron hybrids grown on the surface of titanium carbide flakes (NiFeP/MXene) were successfully synthesised through a hydrothermal reaction and phosphating calcination process. The optimised NiFeP/MXene exhibited a low overpotential of 286 mV at 10 mA cm^-2 and a Tafel slope of 35 mV dec^-1 for the OER, which exceeded the performance of several existing NiFe-based catalysts. NiFeP/MXene was further used as a water-splitting anode in an alkaline electrolyte, exhibiting a cell voltage of only 1.61 V to achieve a current density of 10 mA cm^-2. Density functional theory (DFT) calculations revealed that the combination of MXene acting as a conductive substrate and the phosphating process can effectively tune the electronic structure and density of the electrocatalyst surface to promote the energy level of the d-band centre, resulting in an enhanced OER performance. This study provides a valuable guideline for designing high-performance MXene-supported NiFe-based OER catalysts.

Hierarchically porous FeNi3@FeNi layered double hydroxide nanostructures: one-step fast electrodeposition and highly efficient electrocatalytic performances for overall water splitting

FeNi-layered double hydroxide (LDH) is thought to be an excellent electrocatalyst for oxygen evolution reaction (OER) but it always shows extremely poor electrocatalytic activity toward hydrogen evolution reaction (HER) in alkaline media. Hence, it is significant to improve its HER activity to make it a bifunctional electrocatalyst for the decomposition of water. Here, a simple galvanostatic electrodeposition method was designed for the successful construction of the bifunctional FeNi₃@FeNi LDH electrocatalyst. The as-prepared catalyst displayed excellent electrocatalytic activity for HER/OER in 1.0 M KOH. To drive the current density of 10 mA cm^-2 for HER/OER, an overpotential of 106/199 mV was needed, respectively. In a two-electrode system with the FeNi₃@FeNi LDH/NF as the anode and the cathode simultaneously, the overpotential hardly changed after continuously working for 168 h at 10 mA cm^-2. Compared with other FeNi-based catalysts, the present catalyst possessed close or better electrocatalytic activity.

Interfacial engineering of CeO2 on NiCoP nanoarrays for efficient electrocatalytic oxygen evolution

Transition metal phosphides (TMP)-based oxygen evolution reaction (OER) catalysts constructed by interface engineering strategy have a broad prospect due to their low cost and good performance. Herein, a novel CeO₂/NiCoP nanoarray with intimate phosphide (NiCoP)-oxide (CeO₂) interface was developed via in situ generation on nickel foam (NF). This structure is conducive to increasing active sites and accelerating charge transfer, and may be conducive to regulating electronic structure and adsorption energy. As expected, optimal 1.4-CeO₂/NiCoP/NF delivers a low overpotential of 249 mV at the current density of 10 mA cm^-2 with a Tafel slope of 77.2 mV dec^-1. CeO₂/NiCoP/NF boasts one of the best OER catalytic materials among recently reported phosphides (TMP)-based OER catalysts and composite catalysts involving CeO₂. This work provides an effective strategy for the construction of hetero-structure with CeO₂ with oxygen vacancies to improve the OER performance of phosphides.

Co, Fe codoped holey carbon nanosheets as bifunctional oxygen electrocatalysts for rechargeable Zn-air batteries

Unique Co, Fe codoped holey carbon nanosheets with high surface area and abundant bimetal single atoms (CoFe@HNSs) exhibited remarkable bifunctional oxygen electrocatalytic activity (0.704 V) with very positive half-wave potential (0.897 V) for the ORR and small potential (1.601 V) to drive 10 mA cm^-2 for the OER, outperforming commercial Pt/C and IrO₂, respectively. Furthermore, as the air-cathode for rechargeable Zn-air batteries, the CoFe@HNS based device exhibits a high-power density of 131.3 mW cm^-2 and long-term stability over 140 h, indicating the attractive potential of CoFe@HNSs applied in energy storage and conversion.

Sacrificial ZnO nanorods drive N and O dual-doped carbon towards trifunctional electrocatalysts for Zn–air batteries and self-powered water splitting devices

Integrated energy systems (IES) have attracted increasing attention in recent years.

Current trends and perspectives on emerging Fe-derived noble-metal-free oxygen electrocatalysts

This article discusses recent progress in the development of Fe-derived noble metal-free electrocatalysts, including the strategies used for design, synthesis, and assessment of their performance in alkaline conditions.

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.

A hybrid transition metal nanocrystal-embedded graphitic carbon nitride nanosheet system as a superior oxygen electrocatalyst for rechargeable Zn-air batteries

In this study, we, for the first time, demonstrate a general solid-phase pyrolysis method to synthesize hybrid transition metal nanocrystal-embedded graphitic carbon nitride nanosheets, namely M-CNNs, as a highly efficient oxygen electrocatalyst for rechargeable Zn-air batteries (ZABs). The ratios between metallic acetylacetonates and the g-C₃N₄ precursor can be controlled where Fe-CNNs_-0.7, Ni-CNNs_-0.7 and Co-NNs_-0.7 composites have been optimized to exhibit superior ORR/OER bifunctional electrocatalytic activities. Specifically, Co-CNNs_-0.7 exhibited not only a comparable half-wave potential (0.803 V vs. RHE) to that of the commercial Pt/C catalyst (0.832 V) with a larger current density for the ORR but also a lower overpotential (440 mV) toward the OER compared with the commercial IrO₂ catalyst (460 mV), revealing impressive application in rechargeable ZABs. As a result, ZABs using Co-CNNs_-0.7 as the cathode exhibited an excellent peak power density of 85.3 mW cm^-2 with a specific capacity of 675.7 mA h g^-1 and remarkable cycling stability of 1000 cycles, outperforming the commercially available Pt/C + IrO₂ catalysts. This study highlights the synergy from heterointerfaces in oxygen electrocatalysis, thus providing a promising approach for advanced metal-air cathode materials.

A multi-interfacial FeOOH@NiCo2O4 heterojunction as a highly efficient bifunctional electrocatalyst for overall water splitting

Electrocatalytic water decomposition is the key to sustainable energy, and the design and synthesis of cost-effective electrocatalysts is the main objective of electrocatalytic water splitting. In this paper, multi-interfacial FeOOH@NiCo2O4 hybrid nanoflowers are prepared through a two-step hydrothermal reaction. In such heterostructures, NiCo2O4 nanoflowers are coated with a layer of FeOOH nanoparticles. In addition, the obtained electrocatalyst could provide abundant electroactive sites and the formation of FeOOH@NiCo2O4 nanointerfaces can also improve the charge transfer rate. As a result, under the HER and OER conditions, the prepared catalysts show an outstanding electrocatalytic performance. Moreover, in a two-electrode water splitting system, the FeOOH@NiCo2O4 heterostructure, as a dual-function electrocatalyst, needs a cell voltage of only 1.58 V at a current density of 10 mA cm-2. This study provides a facile and feasible method to construct different kinds of heterostructures as bifunctional electrocatalysts with multiple interfaces by a simple hydrothermal method.

Holey Cobalt-Iron Nitride Nanosheet Arrays as High-Performance Bifunctional Electrocatalysts for Overall Water Splitting

Designing efficient metal nitride electrocatalysts with advantageous nanostructures toward overall water splitting is of great significance for energy conversion. In this work, holey cobalt-iron nitride nanosheet arrays grown on Ni foam substrate (CoFeN_x HNAs/NF) are prepared via a facile hydrothermal and subsequent thermal nitridation method. This unique HNA architecture can not only expose abundant active sites but also facilitate the charge/mass transfer. Resulting from these merits, the CoFeN_x HNAs/NF exhibits high catalytic performance with overpotentials of 200 and 260 mV at 10 mA cm^-2 for the hydrogen evolution reaction (HER) and 50 mA cm^-2 for the oxygen evolution reaction (OER), respectively. Furthermore, when using CoFeN_x-500 HNAs/NF as both anode and cathode, the alkaline electrolyzer could afford a current density of 10 mA cm^-2 at 1.592 V, higher than many other metal nitride-based electrocatalysts. This work signifies a simple approach to prepare holey metal nitride nanosheet arrays, which can be applied in various fields of energy conversion and storage.

Water Splitting: From Electrode to Green Energy System

Hydrogen (H2) production is a latent feasibility of renewable clean energy. The industrial H2 production is obtained from reforming of natural gas, which consumes a large amount of nonrenewable energy and simultaneously produces greenhouse gas carbon dioxide. Electrochemical water splitting is a promising approach for the H2 production, which is sustainable and pollution-free. Therefore, developing efficient and economic technologies for electrochemical water splitting has been an important goal for researchers around the world. The utilization of green energy systems to reduce overall energy consumption is more important for H2 production. Harvesting and converting energy from the environment by different green energy systems for water splitting can efficiently decrease the external power consumption. A variety of green energy systems for efficient producing H2, such as two-electrode electrolysis of water, water splitting driven by photoelectrode devices, solar cells, thermoelectric devices, triboelectric nanogenerator, pyroelectric device or electrochemical water-gas shift device, have been developed recently. In this review, some notable progress made in the different green energy cells for water splitting is discussed in detail. We hoped this review can guide people to pay more attention to the development of green energy system to generate pollution-free H2 energy, which will realize the whole process of H2 production with low cost, pollution-free and energy sustainability conversion.

Improvement of Trisodium Citrate-Modified NiFe-Layered Double Hydroxide Nanosheets with Carbon Black for Oxygen Evolution Reaction

The pursuit of highly active and cost-effective catalysts toward oxygen evolution reaction (OER) is a crucial strategy to resolve the imminent energy crisis. NiFe layered double hydroxide (NiFe LDH) is acknowledged as one of the most promising OER electrocatalysts in alkaline electrolytes. Herein, we report a novel stepwise approach to synthesize NiFe LDHs materials merging with carbon black (CB) via trisodium citrate (TC), modifying toward OER. Benefiting from the inimitable wrapped structure, the decreased size of porous nanosheets and the superconductivity of CB substrate, NiFe LDHs/CB-TC presents excellent catalytic features with a comparative overpotential (236 mV at 10 mA cm−2) and an ultralow Tafel slope (31 mV dec−1), which are almost lower than those of advanced catalysts associated with expensive carbonaceous materials. Therefore, it is expected that such a high-activity and low-cost material can be a promising catalyst employed for the electrochemical energy storage and conversion systems.

Methanol-assisted synthesis of Ni3+-doped ultrathin NiZn-LDH nanomeshes for boosted alkaline water splitting

The construction of nanoporous structure combined with the optimization of electronic structure toward electrocatalysts could be a promising and effective approach to boosting their catalytic performance. Herein, we rationally synthesized a novel Ni³⁺-doped ultrathin NiZn layered double hydroxide nanomesh supported on nickel foam (Ni(ii,iii)Zn-LDH/NF-nm) by a facile one-step methanol-assisted hydrothermal method. Results show that methanol can not only trigger the generation of ultrathin nanomesh structure, but adjust portion of Ni²⁺ to Ni³⁺ and thus to result in the Ni³⁺-doped NiZn-LDH nanomesh material. The nanoporous feature endows Ni(ii,iii)Zn-LDH/NF-nm with abundant exposed catalytic active sites and fast mass transfer for alkaline water electrolysis. More importantly, the Ni³⁺ doping can facilitate the available formation of highly active NiOOH phase on the surface for the oxygen evolution reaction (OER), accompanied by increased oxygen vacancies that can greatly enhance the electronic conductivity, leading to the improved intrinsic activity and the accelerated electrocatalytic OER reaction kinetics. As expected, the as-prepared Ni(ii,iii)Zn-LDH/NF-nm has relatively low overpotentials of 320 and 370 mV to drive large current densities of 100 and 500 mA cm^-2, respectively, and a small Tafel slope of 63.9 mV dec^-1, extremely superior to RuO₂/NF and NiZn-LDH/NF-ns counterpart. Meanwhile, the electrolyzer assembled for overall water splitting by Ni(ii,iii)Zn-LDH/NF-nm yields the outstanding catalytic activity and stability. This work highlights a feasible strategy to design and develop high-efficiency water splitting electrocatalysts via engineering on composition and nanostructure.

NiFe2O4 hollow nanoparticles of small sizes on carbon nanotubes for oxygen evolution

CNT-supported Ni–Fe bimetallic oxide hollow nanoparticles with an ultra-small size based on Kirkendall effect are fabricated and this catalyst exhibits excellent OER performances and robust stability, superior to the benchmark IrO₂ catalyst.

Ultrathin sulfate-intercalated NiFe-layered double hydroxide nanosheets for efficient electrocatalytic oxygen evolution

As an important two-dimensional material, layered double hydroxides (LDHs) show considerable potential in electrocatalytic reactions. However, the great thickness of the bulk LDH materials significantly limits their catalytic activity. In this work, we report ultrathin NiFe-LDH nanosheets with sulfate interlayer anions (Ni₆Fe₂(SO₄)(OH)₁₆·7H₂O) (U-LDH(SO₄²⁻)), which can be synthesized in gram-scale by a simple solvothermal method. The U-LDH(SO₄²⁻) shows excellent stability and great electrocatalytic performance in OER with a current density of 10 mA cm⁻² at a low overpotential of 212 mV and a small Tafel slope of 65.2 mV dec⁻¹, exhibiting its great potential for a highly efficient OER electrocatalyst.

Ultrathin NiFe-LDH nanosheets with sulfate interlayer anions prepared in gram-scale via one-pot solvothermal method exhibited excellent OER activity.