A Cost‐Efficient Bifunctional Ultrathin Nanosheets Array for Electrochemical Overall Water Splitting

The design of cost‐efficient earth‐abundant catalysts with superior performance for the electrochemical water splitting is highly desirable. Herein, a general strategy for fabricating superior bifunctional water splitting electrodes is reported, where cost‐efficient earth‐abundant ultrathin Ni‐based nanosheets arrays are directly grown on nickel foam (NF). The newly created Ni‐based nanosheets@NF exhibit unique features of ultrathin building block, 3D hierarchical structure, and alloy effect with the optimized Ni₅Fe layered double hydroxide@NF (Ni₅Fe LDH@NF) exhibiting low overpotentials of 210 and 133 mV toward both oxygen evolution reaction and hydrogen evolution reaction at 10 mA cm^?2 in alkaline condition, respectively. More significantly, when applying as the bifunctional overall water splitting electrocatalyst, the Ni₅Fe LDH@NF shows an appealing potential of 1.59 V at 10 mA cm^?2 and also superior durability at the very high current density of 50 mA cm^?2.     

Synergistic Coupling of Ni Nanoparticles with Ni3C Nanosheets for Highly Efficient Overall Water Splitting

Exploring earth‐abundant bifunctional electrocatalysts with high efficiency for water electrolysis is extremely demanding and challenging. Herein, density functional theory (DFT) predictions reveal that coupling Ni with Ni₃C can not only facilitate the oxygen evolution reaction (OER) kinetics, but also optimize the hydrogen adsorption and water adsorption energies. Experimentally, a facile strategy is designed to in situ fabricate Ni₃C nanosheets on carbon cloth (CC), and simultaneously couple with Ni nanoparticles, resulting in the formation of an integrated heterostructure catalyst (Ni–Ni₃C/CC). Benefiting from the superior intrinsic activity as well as the abundant active sites, the Ni–Ni₃C/CC electrode demonstrates excellent bifunctional electrocatalytic activities toward the OER and hydrogen evolution reaction (HER), which are superior to all the documented Ni₃C‐based electrocatalysts in alkaline electrolytes. Specifically, the Ni–Ni₃C/CC catalyst exhibits the low overpotentials of only 299 mV at the current density of 20 mA cm^?2 for the OER and 98 mV at 10 mA cm^?2 for the HER in 1 m KOH. Furthermore, the bifunctional Ni–Ni₃C/CC catalyst can propel water electrolysis with excellent activity and nearly 100% faradic efficiency. This work highlights an easy approach for designing and constructing advanced nickel carbide‐based catalysts with high activity based on the theoretical predictions.     

Transition‐Metal‐Doped RuIr Bifunctional Nanocrystals for Overall Water Splitting in Acidic Environments

The establishment of electrocatalysts with bifunctionality for efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in acidic environments is necessary for the development of proton exchange membrane (PEM) water electrolyzers for the production of clean hydrogen fuel. RuIr alloy is considered to be a promising electrocatalyst because of its favorable OER performance and potential for HER. Here, the design of a bifunctional electrocatalyst with greatly boosted water‐splitting performance from doping RuIr alloy nanocrystals with transition metals that modify electronic structure and binding strength of reaction intermediates is reported. Significantly, Co‐RuIr results in small overpotentials of 235 mV for OER and 14 mV for HER (@ 10 mA cm^?2 current density) in 0.1 m HClO₄ media. Therefore a cell voltage of just 1.52 V is needed for overall water splitting to produce hydrogen and oxygen. More importantly, for a series of M‐RuIr (M = Co, Ni, Fe), the catalytic activity dependence at fundamental level on the chemical/valence states is used to establish a novel composition‐activity relationship. This permits new design principles for bifunctional electrocatalysts.     

FeS2/CoS2 Interface Nanosheets as Efficient Bifunctional Electrocatalyst for Overall Water Splitting

Electrochemical water splitting to produce hydrogen and oxygen, as an important reaction for renewable energy storage, needs highly efficient and stable catalysts. Herein, FeS₂/CoS₂ interface nanosheets (NSs) as efficient bifunctional electrocatalysts for overall water splitting are reported. The thickness and interface disordered structure with rich defects of FeS₂/CoS₂ NSs are confirmed by atomic force microscopy and high‐resolution transmission electron microscopy. Furthermore, extended X‐ray absorption fine structure spectroscopy clarifies that FeS₂/CoS₂ NSs with sulfur vacancies, which can further increase electrocatalytic performance. Benefiting from the interface nanosheets' structure with abundant defects, the FeS₂/CoS₂ NSs show remarkable hydrogen evolution reaction (HER) performance with a low overpotential of 78.2 mV at 10 mA cm⁻² and a superior stability for 80 h in 1.0 m KOH, and an overpotential of 302 mV at 100 mA cm⁻² for the oxygen evolution reaction (OER). More importantly, the FeS₂/CoS₂ NSs display excellent performance for overall water splitting with a voltage of 1.47 V to achieve current density of 10 mA cm⁻² and maintain the activity for at least 21 h. The present work highlights the importance of engineering interface nanosheets with rich defects based on transition metal dichalcogenides for boosting the HER and OER performance.     

Regulating Electronic Structure of Iron Nitride by Tungsten Nitride Nanosheets for Accelerated Overall Water Splitting

Two essential characteristics that are required for hybrid electrocatalysts to exhibit higher oxygen and hydrogen evolution reaction (OER and HER, respectively) activity are a favorable electronic configuration and a sufficient density of active sites at the interface between the two materials within the hybrid. In the present study, a hybrid electrocatalyst is introduced with a novel architecture consisting of coral-like iron nitride (Fe₂N) arrays and tungsten nitride (W₂N₃) nanosheets that satisfies these requirements. The resulting W₂N₃/Fe₂N catalyst achieves high OER activity (268.5 mV at 50 mA cm⁻²) and HER activity (85.2 mV at 10 mA cm⁻²) with excellent long-term durability in an alkaline medium. In addition, density functional theory calculations reveal that the individual band centers experience an upshift in the hybrid W₂N₃/Fe₂N structure, thus improving the OER and HER activity. The strategy adopted here thus provides a valuable guide for the fabrication of cost-effective multi-metallic crystalline hybrids for use as multifunctional electrocatalysts.     

Ruthenium Nanoparticles on Cobalt‐Doped 1T′ Phase MoS2 Nanosheets for Overall Water Splitting

2D MoS₂ nanostructures have recently attracted considerable attention because of their outstanding electrocatalytic properties. The synthesis of unique Co–Ru–MoS₂ hybrid nanosheets with excellent catalytic activity toward overall water splitting in alkaline solution is reported. 1T′ phase MoS₂ nanosheets are doped homogeneously with Co atoms and decorated with Ru nanoparticles. The catalytic performance of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is characterized by low overpotentials of 52 and 308 mV at 10 mA cm^?2 and Tafel slopes of 55 and 50 mV decade^?1 in 1.0 m KOH, respectively. Analysis of X‐ray photoelectron and absorption spectra of the catalysts show that the MoS₂ well retained its metallic 1T′ phase, which guarantees good electrical conductivity during the reaction. The Gibbs free energy calculation for the reaction pathway in alkaline electrolyte confirms that the Ru nanoparticles on the Co‐doped MoS₂ greatly enhance the HER activity. Water adsorption and dissociation take place favorably on the Ru, and the doped Co further catalyzes HER by making the reaction intermediates more favorable. The high OER performance is attributed to the catalytically active RuO₂ nanoparticles that are produced via oxidation of Ru nanoparticles.     

Highly N-Doped Fe/Co Phosphide Superstructures for Efficient Water Splitting

Developing an inexpensive bifunctional electrocatalyst for overall water splitting is critical for acquiring scalable green hydrogen and thereby realizing carbon neutralization. Herein, an “all-in-one” method is developed for the fabrication of highly N-doped binary FeCo-phosphides (N-FeCoP) with hierarchical superstructure, this delicately designed synthesis route allows the following merits for benefiting water splitting electrocatalysis in alkaline, including high N/defect-doping for mediating the surface property of the as-made N-FeCoP, binary Fe and Co components exhibiting strong coupling interaction, and 3D hierarchical superstructure for shortening diffusion length and thereby improving reaction kinetics. Electrochemical measurements reveal that the N-FeCoP sample exhibits very low overpotentials for initiating the hydrogen and oxygen evolution reactions. Remarkably, overall water splitting can be promoted on N-FeCoP using a commercial primary Zn-MnO₂ battery. The developed synthesis strategy may potentially inspire the preparation of other N-doped metal-based nanostructures for broad electrocatalysis.     

Synergism of Geometric Construction and Electronic Regulation: 3D Se‐(NiCo)Sx/(OH)x Nanosheets for Highly Efficient Overall Water Splitting

The exploration of highly efficient electrocatalysts for both oxygen and hydrogen generation via water splitting is receiving considerable attention in recent decades. Up till now, Pt‐based catalysts still exhibit the best hydrogen evolution reaction (HER) performance and Ir/Ru‐based oxides are identified as the benchmark for oxygen evolution reaction (OER). However, the high cost and rarity of these materials extremely hinder their large‐scale applications. This paper describes the construction of the ultrathin defect‐enriched 3D Se‐(NiCo)S_x/(OH)_x nanosheets for overall water splitting through a facile Se‐induced hydrothermal treatment. Via Se‐induced fabrication, highly efficient Se‐(NiCo)S_x/(OH)_x nanosheets are successfully fabricated through morphology optimization, defect engineering, and electronic structure tailoring. The as‐prepared hybrids exhibit relatively low overpotentials of 155 and 103 mV at the current density of 10 mA cm^?2 for OER and HER, respectively. Moreover, an overall water‐splitting device delivers a current density of 10 mA cm^?2 for ≈66 h without obvious degradation.     

Cr‐Doped FeNi–P Nanoparticles Encapsulated into N‐Doped Carbon Nanotube as a Robust Bifunctional Catalyst for Efficient Overall Water Splitting

Exploring high‐efficiency, stable, and cost‐effective bifunctional electrocatalysts for overall water splitting is greatly desirable and challenging. Herein, a newly designed hybrid catalyst with Cr‐doped FeNi–P nanoparticles encapsulated into N‐doped carbon nanotubes (Cr‐doped FeNi–P/NCN) with unprecedented electrocatalytic activity is developed by a simple one‐step heating treatment. The as‐synthesized Cr‐doped FeNi–P/NCN with moderate Cr doping exhibits admirable oxygen evolution reaction and hydrogen evolution reaction activities with overpotentials of 240 and 190 mV to reach a current density of 10 mA cm^?2 in 1 m KOH solution. When used in overall water splitting as a bifunctional catalyst, it needs only 1.50 V to give a current density of 10 mA cm^?2, which is superior to its typically integrated Pt/C and RuO₂ counterparts (1.54 V @ 10 mA cm^?2). Density functional theory calculation confirms that Cr doping into a FeNi‐host can effectively alter the relative Gibbs adsorption energy and reduces the theoretical overpotential. Additionally, the synergetic effects between Cr‐doped FeNi–P nanoparticles and NCNs are regarded as significant contributors to accelerate charge transfer and promote electrocatalytic activity in hybrid catalysts.     

Cation‐Modulated HER and OER Activities of Hierarchical VOOH Hollow Architectures for High‐Efficiency and Stable Overall Water Splitting

Atom‐scale modulation of electronic regulation in nonprecious‐based electrocatalysts is promising for efficient catalytic activities. Here, hierarchically hollow VOOH nanostructures are rationally constructed by partial iron substitution and systematically investigated for electrocatalytic water splitting. Benefiting from the hierarchically stable scaffold configuration, highly electrochemically active surface area, the synergistic effect of the active metal atoms, and optimal adsorption energies, the 3% Fe (mole ratio) substituted electrocatalyst (VOOH‐3Fe) exhibits a low overpotential of 90 and 195 mV at 10 mA cm^?2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline media, respectively, superior than the other samples with a different substituted ratio. To the best of current knowledge, 195 mV overpotential at 10 mA cm^?2 is the best value reported for V or Fe (oxy)hydroxide‐based OER catalysts. Moreover, the electrolytic cell employing the VOOH‐3Fe electrode as both the cathode and anode exhibits a cell voltage of 0.30 V at 10 mA cm^?2 with a remarkable stability over 60 h. This work heralds a new pathway to design efficient bifunctional catalysts toward overall water splitting.     

NiSe‐Ni0.85Se Heterostructure Nanoflake Arrays on Carbon Paper as Efficient Electrocatalysts for Overall Water Splitting

Fabricating cost‐effective, bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in basic media is critical for renewable energy generation. Here, NiSe/CP, Ni_0.85Se/CP, and NiSe‐Ni_0.85Se/CP heterostructure catalysts with different phase constitutions are successfully prepared through in situ selenylation of a NiO nanoflake array oriented on carbon paper (CP) by tuning the original Ni/Se molar ratio of the raw materials. The relationship between the crystal phase component and electrocatalytic activity is systematically studied. Benefiting from the synergetic effect of the intrinsic metallic state, facile charge transport, abundant catalytic active sites, and multiple electrolyte transmission paths, the optimized NiSe‐Ni_0.85Se/CP exhibits a remarkably higher catalytic activity for both the HER and OER than single‐phase NiSe/CP and Ni_0.85Se/CP. A current density of 10 mA cm⁻² at 1.62 V and a high stability can be obtained by using NiSe‐Ni_0.85Se/CP as both the cathode and anode for overall water splitting under alkaline conditions. Density functional theory calculations confirm that H and OH⁻ can be more easily adsorbed on NiSe‐Ni_0.85Se than on NiSe and Ni_0.85Se. This study paves the way for enhancing the overall water splitting performance of nickel selenides by fabricating heterophase junctions using nickel selenides with different phases.     

3D Nitrogen‐Anion‐Decorated Nickel Sulfides for Highly Efficient Overall Water Splitting

Developing non‐noble‐metal electrocatalysts with high activity and low cost for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of paramount importance for improving the generation of H₂ fuel by electrocatalytic water‐splitting. This study puts forward a new N‐anion‐decorated Ni₃S₂ material synthesized by a simple one‐step calcination route, acting as a superior bifunctional electrocatalyst for the OER/HER for the first time. The introduction of N anions significantly modifies the morphology and electronic structure of Ni₃S₂, bringing high surface active sites exposure, enhanced electrical conductivity, optimal HER Gibbs free‐energy (ΔG_H*), and water adsorption energy change (ΔG_H2O*). Remarkably, the obtained N‐Ni₃S₂/NF 3D electrode exhibits extremely low overpotentials of 330 and 110 mV to reach a current density of 100 and 10 mA cm^?2 for the OER and HER in 1.0 m KOH, respectively. Moreover, an overall water‐splitting device comprising this electrode delivers a current density of 10 mA cm^?2 at a very low cell voltage of 1.48 V. Our finding introduces a new way to design advanced bifunctional catalysts for water splitting.     

Multimetal Borides Nanochains as Efficient Electrocatalysts for Overall Water Splitting

The development of cost‐efficient, active, and stable electrode materials as bifunctional catalysts for electrochemical water splitting is crucial to high‐performance renewable energy storage and conversion devices. In this work, the synthesis of Co‐based multi‐metal borides nanochains with amorphous structure is reported for boosting the oxygen evolution (OER) and hydrogen evolution reactions (HER) by one‐pot NaBH₄ reduction of Co²⁺, Ni²⁺, and Fe²⁺ under ambient temperature. In all the investigated Co‐based metal borides, NiCoFeB nanochains show the excellent OER performance with a low overpotential of 284 mV at 10 mA cm^‐2 and Tafel slope of 46 mV dec^‐1, respectively, together with excellent catalytic stability, and robust HER performance with an overpotential of 345 mV at 10 mA cm^‐2. The density functional theory (DFT) calculations reveal that the excellent electrocatalytic performance is mainly attributed to optimal electronic structure by tuning the Co‐3d band activities by the incorporation of Ni and Fe for enhanced water splitting via the potentially existed Co⁰ state. Moreover, the electrolyzer using NiCoFeB nanochains as anode and cathode offers 10 mA cm^‐2 at a cell voltage of 1.81 V, comparable to commercial Pt/C // Ir/C, providing a simple method to design and explore highly efficient and cheap bifunctional electrocatalysts for overall water splitting.     

Cobalt Covalent Doping in MoS2 to Induce Bifunctionality of Overall Water Splitting

The layer‐structured MoS₂ is a typical hydrogen evolution reaction (HER) electrocatalyst but it possesses poor activity for the oxygen evolution reaction (OER). In this work, a cobalt covalent doping approach capable of inducing HER and OER bifunctionality into MoS₂ for efficient overall water splitting is reported. The results demonstrate that covalently doping cobalt into MoS₂ can lead to dramatically enhanced HER activity while simultaneously inducing remarkable OER activity. The catalyst with optimal cobalt doping density can readily achieve HER and OER onset potentials of ?0.02 and 1.45 V (vs reversible hydrogen electrode (RHE)) in 1.0 m KOH. Importantly, it can deliver high current densities of 10, 100, and 200 mA cm^?2 at low HER and OER overpotentials of 48, 132, 165 mV and 260, 350, 390 mV, respectively. The reported catalyst activation approach can be adapted for bifunctionalization of other transition metal dichalcogenides.     

Cobalt Iron Hydroxide as a Precious Metal‐Free Bifunctional Electrocatalyst for Efficient Overall Water Splitting

Highly efficient and stable electrocatalysts from inexpensive and earth‐abundant elements are emerging materials in the overall water splitting process. Herein, cobalt iron hydroxide nanosheets are directly deposited on nickel foam by a simple and rapid electrodeposition method. The cobalt iron hydroxide (CoFe/NF) nanosheets not only allow good exposure of the highly active surface area but also facilitate the mass and charge transport capability. As an anode, the CoFe/NF electrocatalyst displays excellent oxygen evolution reaction catalytic activity with an overpotential of 220 mV at a current density of 10 mA cm^?2. As a cathode, it exhibits good performance in the hydrogen evolution reaction with an overpotential of 110 mV, reaching a current density of 10 mA cm^?2. When CoFe/NF electrodes are used as the anode and the cathode for water splitting, a low cell voltage of 1.64 V at 10 mA cm^?2 and excellent stability for 50 h are observed. The present work demonstrates a possible pathway to develop a highly active and durable substitute for noble metal electrocatalysts for overall water splitting.     

Iron Vacancies Induced Bifunctionality in Ultrathin Feroxyhyte Nanosheets for Overall Water Splitting

Exploring of new catalyst activation principle holds a key to unlock catalytic powers of cheap and earth‐abundant materials for large‐scale applications. In this regard, the vacancy defects have been proven to be effective to initiate catalytic active sites and endow high electrocatalytic activities. However, such electrocatalytically active defects reported to date have been mostly formed by anion vacancies. Herein, it is demonstrated for the first time that iron cation vacancies induce superb water splitting bifunctionality in alkaline media. A simple wet‐chemistry method is developed to grow ultrathin feroxyhyte (δ‐FeOOH) nanosheets with rich Fe vacancies on Ni foam substrate. The theoretical and experimental results confirm that, in contrast to anion vacancies, the formation of rich second neighboring Fe to Fe vacancies in δ‐FeOOH nanosheets can create catalytic active centers for both hydrogen and oxygen evolution reactions. The atomic level insight into the new catalyst activation principle based on metal vacancies is adaptable for developing other transition metal electrocatalysts, including Fe‐based ones.     

Ultrathin Porous NiFeV Ternary Layer Hydroxide Nanosheets as a Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting

Herein, the hydrothermal synthesis of porous ultrathin ternary NiFeV layer double hydroxides (LDHs) nanosheets grown on Nickel foam (NF) substrate as a highly efficient electrode toward overall water splitting in alkaline media is reported. The lateral size of the nanosheets is about a few hundreds of nanometers with the thickness of ≈10 nm. Among all molar ratios investigated, the Ni_0.75Fe_0.125V_0.125‐LDHs/NF electrode depicts the optimized performance. It displays an excellent catalytic activity with a modest overpotential of 231 mV for the oxygen evolution reaction (OER) and 125 mV for the hydrogen evolution reaction (HER) in 1.0 m KOH electrolyte. Its exceptional activity is further shown in its small Tafel slope of 39.4 and 62.0 mV dec^?1 for OER and HER, respectively. More importantly, remarkable durability and stability are also observed. When used for overall water splitting, the Ni_0.75Fe_0.125V_0.125‐LDHs/NF electrodes require a voltage of only 1.591 V to reach 10 mA cm^?2 in alkaline solution. These outstanding performances are mainly attributed to the synergistic effect of the ternary metal system that boosts the intrinsic catalytic activity and active surface area. This work explores a promising way to achieve the optimal inexpensive Ni‐based hydroxide electrocatalyst for overall water splitting.