Selenide-Based Electrocatalysts and Scaffolds for Water Oxidation Applications

In situ growth of well-ordered NiFe-MOF-74 on Ni foam by Fe2+ induction as an efficient and stable electrocatalyst for water oxidation

Uniform and well-ordered NiFe-MOF-74 was in situ grown on Ni foam by Fe²⁺ induction, exhibiting enhanced activity toward the OER with a very low overpotential of 223 mV at a current density of 10 mA cm⁻².

Construction of a hierarchical NiFe layered double hydroxide with a 3D mesoporous structure as an advanced electrocatalyst for water oxidation

A facile and cost-effective one-step hydrothermal method is used to synthesize NiFe LDH microclusters with a 3D hierarchically mesoporous architecture. This superior electrocatalyst can achieve a current density of 10 mA cm⁻² with an ultralow overpotential of 211 mV toward the oxygen evolution reaction.

Coupling FeSe2 with CoSe: an effective strategy to create stable and efficient electrocatalysts for water oxidation

A novel mutually beneficial CoSe/FeSe₂ nanohybrid electrocatalyst was prepared by a simple one-step hydrothermal method, exhibiting enhanced OER performances.

One-pot synthesis of iron–nickel–selenide nanorods for efficient and durable electrochemical oxygen evolution

Fe–Ni–Se nanorods have been synthesized by a facile one-pot method and they acted as efficient and durable OER catalysts.

A review of anion-regulated multi-anion transition metal compounds for oxygen evolution electrocatalysis

Recent advances in the anion regulation on multi-anion transition metal compounds as electrocatalysts for oxygen evolution reaction are reviewed.

HMTA-assisted formation of hierarchical Co-based materials built by low-dimensional substructures as water oxidation electrocatalysts

We report a 3D Co-based micro-flower constructed from 2D nanosheets, which exhibits excellent OER performance.

General Oriented Synthesis of Precise Carbon-Confined Nanostructures by Low-Pressure Vapor Superassembly and Controlled Pyrolysis

Earth-abundant metal-based nanostructured materials have been widely studied for potential energy conversion and storage. However, controlled synthesis of functional nanostructures with high electron conductivity, high reaction activity, and structural stability is still a formidable challenge for further practical applications. Herein, for the first time, we develop a facile, efficient, and general method for the oriented synthesis of precise carbon-confined nanostructures by low-pressure vapor superassembly of a thin metal-organic framework (MOF) shell and subsequent controlled pyrolysis. The selected nanostructured metal oxide precursors not only act as metal ion sources but also orient the superassembly of gaseous organic ligands through the coordination reactions under the low-pressure condition, resulting in the formation of a tunable MOF shell on their surfaces. This strategy is further successfully extended to obtain various precise carbon-confined nanostructures with diverse compositions and delicate morphologies. Notably, these as-prepared carbon-confined architectures exhibit outstanding electrochemical performances in water splitting and lithium storage. The remarkable performances are mainly attributed to the synergistic effect from appropriate chemical compositions and stable carbon-confined structures. This synthetic approach and proposed mechanism open new avenues for the development of functional nanostructured materials in many frontier fields.

Carbon cloth supported cobalt phosphide as multifunctional catalysts for efficient overall water splitting and zinc-air batteries

Excellent performance and multifunctional electrocatalysts are of great importance for the development of renewable energy. Here, a multifunctional carbon cloth supported cobalt phosphide (CoP@CC) electrocatalyst was synthesized via a simple electrochemical deposition followed by subsequent phosphorization. This self-standing CoP@CC presents superior HER performance in both acid and alkaline media and excellent OER activity in an alkaline environment. When applied as both anode and cathode catalyst to overall water splitting, CoP@CC only required a low cell voltage of 1.68 V to drive a current density of 10 mA cm^-2, making this catalyst a potential candidate in industrial application. Moreover, inspired by its satisfying ORR activity, CoP@CC was first used as the air-electrode in zinc-air batteries, which outputted an open-circuit potential of 0.6 V and a power density of ∼30 mW cm^-2.

Porous NiFe-Oxide Nanocubes as Bifunctional Electrocatalysts for Efficient Water-Splitting

Electrocatalytic water-splitting, a combination of oxygen and hydrogen evolution reactions (OER and HER), is highly attractive in clean energy technologies, especially for high-purity hydrogen production, whereas developing stable, earth-abundant, bifunctional catalysts has continued to pose major challenges. Herein, a mesoporous NiFe-oxide nanocube (NiFe-NC) system is developed from a NiFe Prussian blue analog metal-organic framework as an efficient bifunctional catalyst for overall water-splitting. The NiFe-NCs with ∼200 nm side length have a Ni/Fe molar ratio of 3:2 and is a composite of NiO and α/γ-Fe₂O₃. The NCs demonstrate overpotentials of 271 and 197 mV for OER and HER, respectively, in 1 M KOH at 10 mA cm^-2, which outperform those of 339 and 347 mV for the spherical NiFe-oxide nanoparticles having a similar composition. The electrolyzer constructed using NiFe-NCs requires an impressive cell voltage of 1.67 V to deliver a current density of 10 mA cm^-2. Along with a mesoporous structure with a broad pore size distribution, the NiFe-NCs demonstrate the qualities of a desired corrosion-resistant water-splitting catalyst with long-term stability. The exposure of active sites at the edges and vertices of the NCs was validated to play a crucial role in their overall catalytic performance.

Defect Chemistry of Nonprecious-Metal Electrocatalysts for Oxygen Reactions

Oxygen electrocatalysis, including the oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER), is a critical process for metal-air batteries. Therefore, the development of electrocatalysts for the OER and the ORR is of essential importance. Indeed, various advanced electrocatalysts have been designed for the ORR or the OER; however, the origin of the advanced activity of oxygen electrocatalysts is still somewhat controversial. The enhanced activity is usually attributed to the high surface areas, the unique facet structures, the enhanced conductivities, or even to unclear synergistic effects, but the importance of the defects, especially the intrinsic defects, is often neglected. More recently, the important role of defects in oxygen electrocatalysis has been demonstrated by several groups. To make the defect effect clearer, the recent development of this concept is reviewed here and a novel principle for the design of oxygen electrocatalysts is proposed. An overview of the defects in carbon-based, metal-free electrocatalysts for ORR and various defects in metal oxides/selenides for OER is also provided. The types of defects and controllable strategies to generate defects in electrocatalysts are presented, along with techniques to identify the defects. The defect-activity relationship is also explored by theoretical methods.

"Cuju"-Structured Iron Diselenide-Derived Oxide: A Highly Efficient Electrocatalyst for Water Oxidation

Electrocatalysts with outstanding performance have been highly desired toward exploration of new energy storage and conversion devices/systems as well as making an efficient and eco-friendly utilization of green energy. In this study, we composed an iron-based binary diselenide-derived oxide (Fe-SDO) with a facile one-step hydrothermal method to utilize the earth-abundant iron and the probably prosperous catalytic performance of metal-selenides compounds. The catalyst exhibits an overpotential of 226 mV at a current density of 10 mA/cm², a Tafel slope of 41 mV dec^-1, and robust durability after catalyzing vigorous OER for 36 h constantly. Through several analytical methods conducted before and after the oxygen evolution reaction activation on FeSe₂ it was discovered that such catalyst possessed a morphology as "Cuju"-like balls with porosity inside in which we explored the vacancy defects and lattice distortion that play significant roles in generating the high electrocatalytic performance of our proposed catalyst by inducing remarkable electron conductivity in the porous Cuju balls (a Chinese traditional football). Throughout our work the superb electrocatalyst performance of the iron-based compounds was demonstrated, and subsequently the underlying reason for such electrocatalyst performance was addressed, which may push boundaries for the exploration of iron-based compounds as OER catalyst and large-scale commercial application of such compounds in the future.

CoSex nanocrystalline-dotted CoCo layered double hydroxide nanosheets: a synergetic engineering process for enhanced electrocatalytic water oxidation

Manipulating the electrical conductivity and morphology of Co-based (hydr)oxides is significant for optimizing energy conversion in the oxygen evolution reaction (OER). Herein, 2D CoSe_x nanocrystalline-dotted porous CoCo layered double hydroxide nanosheets (Co-Se NSs) were designed and synthesized via a modified in situ reduction and interface-directed assembly in an inert atmosphere. During the synchronous reduction/precipitation reaction between Co²⁺-oleylamine and NaHSe at the toluene-water interface, the hydrated Co-O and Co-Se clusters are generated and sequentially assemble under strong extrusion driven by the interfacial tension. Owing to the enriched vacancies on the lateral surfaces, the obtained loose and porous Co-Se NS presents low crystallinity. Moreover, electrons could spontaneously transfer from the CoCo LDH to the neighboring CoSe_x nanocrystallites due to the stronger electron-withdrawing capability of metallic CoSe_x, and thus more Co atoms in the CoCo layered double hydroxide (LDH) present a high oxidation state. This synergistic manipulation in the structure, component, and electron configuration of the Co-Se NS can increase the density of the OER active-sites, improve the electrical conductivity, and also offer a large accessible surface area and permeable channels for ion adsorption and transport. As a result, the resulting Co-Se NSs feature high catalytic activity towards OER, in particular a low onset potential of 1.48 V and an overpotential of only 290 mV at a current density of 10 mA cm^-2 for the Co-Se-2 NS, as well as good stability in an accelerated durability test. The strategy developed here provides a reliable and valid way to synthesize multicomponent NSs, and is able to be extended to other areas of application.

Formation of Ni-Fe Mixed Diselenide Nanocages as a Superior Oxygen Evolution Electrocatalyst

Exploring effective electrocatalysts is a crucial requirement for boosting the efficiency of water splitting to obtain clean fuels. Here, a self-templating strategy is reported to synthesize Ni-Fe mixed diselenide cubic nanocages for the electrocatalytic oxygen evolution reaction (OER). The diselenide nanocages are derived from corresponding Prussian-blue analog nanocages, which are first obtained by treating the nanocube precursor with a site-selective ammonia etchant. The resulting Ni-Fe mixed diselenide nanocages perform as a superior OER electrocatalyst, which affords a current density of 10 mA cm^-2 at a small overpotential of 240 mV; a high current density, mass activity, and turnover frequency of 100 mA cm^-2 , 1000 A g^-1 , and 0.58 s^-1 , respectively, at the overpotential of 270 mV; a Tafel slope as small as 24 mV dec^-1 ; and excellent stability in alkaline medium.

Architecting a Mesoporous N-Doped Graphitic Carbon Framework Encapsulating CoTe2 as an Efficient Oxygen Evolution Electrocatalyst

To improve the efficiency of cobalt-based catalysts for water electrolysis, tremendous efforts have been dedicated to tuning the composition, morphology, size, and structure of the materials. We report here a facile preparation of orthorhombic CoTe₂ nanocrystals embedded in an N-doped graphitic carbon matrix to form a 3D architecture with a size of ∼500 nm and abundant mesopores of ∼4 nm for the oxygen evolution reaction (OER). The hybrid electrocatalyst delivers a small overpotential of 300 mV at 10 mA cm^-2, which is much lower than that for pristine CoTe₂ powder. After cycling for 2000 cycles or driving continual OER for 20 h, only a slight loss is observed. The mesoporous 3D architecture and the strong interaction between N-doped graphitic carbon and CoTe₂ are responsible for the enhancement of the electrocatalytic performance.

Metal-Organic Framework Template Derived Porous CoSe2 Nanosheet Arrays for Energy Conversion and Storage

Porous CoSe₂ on carbon cloth is prepared from a cobalt-based metal organic framework template with etching and selenization reaction, which has both a larger specific surface area and outstanding electrical conductivity. As the catalyst for oxygen evolution reaction, the porous CoSe₂ achieves a lower onset potential of 1.48 V versus the reversible hydrogen electrode (RHE) and a small potential of 1.52 V (vs RHE) at an anodic current density of 10 mA cm^-2. Especially, the linear sweep voltammogram curve of the porous CoSe₂ is in consist with the initial curve after durability test for 24 h. When tested as an electrode for supercapacitor, it can deliver a specific capacitance of 713.9 F g^-1 at current density of 1 mA cm^-2 and exhibit excellent cycling stability in that a capacitance retention of 92.4% can be maintained after 5000 charge-discharge cycles at 5 mA cm^-2. Our work presents a novel strategy for construction of electrochemical electrode.

Rational Design of Cobalt-Iron Selenides for Highly Efficient Electrochemical Water Oxidation

Exploring active, stable, earth-abundant, low-cost, and high-efficiency electrocatalysts is highly desired for large-scale industrial applications toward the low-carbon economy. In this study, we apply a versatile selenizing technology to synthesize Se-enriched Co_1-xFe_xSe₂ catalysts on nickel foams for oxygen evolution reactions (OERs) and disclose the relationship between the electronic structures of Co_1-xFe_xSe₂ (via regulating the atom ratio of Co/Fe) and their OER performance. Owing to the fact that the electron configuration of the Co_1-xFe_xSe₂ compounds can be tuned by the incorporated Fe species (electron transfer and lattice distortion), the catalytic activity can be adjusted according to the Co/Fe ratios in the catalyst. Moreover, the morphology of Co_1-xFe_xSe₂ is also verified to strongly depend on the Co/Fe ratios, and the thinner Co_0.4Fe_0.6Se₂ nanosheets are obtained upon selenization treatment, in which it allows more active sites to be exposed to the electrolyte, in turn promoting the OER performance. The Co_0.4Fe_0.6Se₂ nanosheets not only exhibit superior OER performance with a low overpotential of 217 mV at 10 mA cm^-2 and a small Tafel slope of 41 mV dec^-1 but also possess ultrahigh durability with a dinky degeneration of 4.4% even after 72 h fierce water oxidation test in alkaline solution, which outperforms the commercial RuO₂ catalyst. As expected, the Co_0.4Fe_0.6Se₂ nanosheets have shown great prospects for practical applications toward water oxidation.

Ternary Porous Cobalt Phosphoselenide Nanosheets: An Efficient Electrocatalyst for Electrocatalytic and Photoelectrochemical Water Splitting

Exploring efficient and earth-abundant electrocatalysts is of great importance for electrocatalytic and photoelectrochemical hydrogen production. This study demonstrates a novel ternary electrocatalyst of porous cobalt phosphoselenide nanosheets prepared by a combined hydrogenation and phosphation strategy. Benefiting from the enhanced electric conductivity and large surface area, the ternary nanosheets supported on electrochemically exfoliated graphene electrodes exhibit excellent catalytic activity and durability toward hydrogen evolution in alkali, achieving current densities of 10 and 20 mA cm^-2 at overpotentials of 150 and 180 mV, respectively, outperforming those reported for transition metal dichalcogenides and first-row transition metal pyrites catalysts. Theoretical calculations reveal that the synergistic effects of Se vacancies and subsequent P displacements of Se atoms around the vacancies in the resulting cobalt phosphoselenide favorably change the electronic structure of cobalt selenide, assuring a rapid charge transfer and optimal energy barrier of hydrogen desorption, and thus promoting the proton kinetics. The overall-water-splitting with 10 mA cm^-2 at a low voltage of 1.64 V is achieved using the ternary electrode as both the anode and cathode, and the performance surpasses that of the Ir/C-Pt/C couple for sufficiently high overpotentials. Moreover, the integration of ternary nanosheets with macroporous silicon enables highly efficient solar-driven photoelectrochemical hydrogen production.

In situ cathodic activation of V-incorporated NixSy nanowires for enhanced hydrogen evolution

In situ cathodic activation (ISCA) of V-incorporated Ni_xS_y nanowires supported on nickel foam (VS/Ni_xS_y/NF) can be realized in an alkaline hydrogen evolution reaction (HER) process, which provides not only clearly enhanced activity but also ultrahigh stability for HER. The ISCA process is continuous linear sweep voltammetry (LSV) on VS/Ni_xS_y/NF as a cathodic electrode with gradually enhanced HER activity. The activated VS/Ni_xS_y/NF (A-VS/Ni_xS_y/NF) demonstrates enhanced HER activity with an overpotential of 125 mV to drive 10 mA cm^-2, which is much lower than that of other samples. It may be predicted that the ISCA-derived amorphous VOOH film covering on A-VS/Ni_xS_y/NF accelerates the HER process, and NiOOH may protect active sites from decaying, leading to excellent activity and structural stability. However, for single metal sulfides, the ISCA process of nickel or vanadium sulfides is not available, implying that the synergistic effect between Ni and V of VS/Ni_xS_y/NF may be the key to drive ISCA in alkaline HER. In addition, its ultra-high stability confirms that the stable active sites and nanostructures of A-VS/Ni_xS_y/NF are derived from ISCA. Therefore, the ISCA of V-incorporated transition metal sulfides in the alkaline HER process may be a facile and promising method to obtain efficient electrocatalysts.

Nanostructured materials on 3D nickel foam as electrocatalysts for water splitting

Highly efficient and low-cost electrocatalysts are essential for water spitting via electrolysis in an economically viable fashion. However, the best catalytic performance is found with noble metal-based electrocatalysts, which presents a formidable obstacle for the commercial success of electrolytic water splitting-based H₂ production due to their relatively high cost and scarcity. Therefore, the development of alternative inexpensive earth-abundant electrode materials with excellent electrocatalytic properties is of great urgency. In general, efficient electrocatalysts must possess several key characteristics such as low overpotential, good electrocatalytic activity, high stability, and low production costs. Direct synthesis of nanostructured catalysts on a conducting substrate may potentially improve the performance of the resultant electrocatalysts because of their high catalytic surface areas and the synergistic effect between the electrocatalyst and the conductive substrate. In this regard, three dimensional (3D) nickel foams have been advantageously utilized as electrode substrates as they offer a large active surface area and a highly conductive continuous porous 3D network. In this review, we discuss the most recent developments in nanostructured materials directly synthesized on 3D nickel foam as potential electrode candidates for electrochemical water electrolysis, namely, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). We also provide perspectives and outlooks for catalysts grown directly on 3D conducting substrates for future sustainable energy technologies.

Ultrathin Two-Dimensional Nanostructured Materials for Highly Efficient Water Oxidation

Water oxidation, also known as the oxygen evolution reaction (OER), is a crucial process in energy conversion and storage, especially in water electrolysis. The critical challenge of the electrochemical water splitting technology is to explore alternative precious-metal-free catalysts for the promotion of the kinetically sluggish OER. Recently, emerging two-dimensional (2D) ultrathin materials with abundant accessible active sites and improved electrical conductivity provide an ideal platform for the synthesis of promising OER catalysts. This Review focuses on the most recent advances in ultrathin 2D nanostructured materials for enhanced electrochemical activity of the OER. The design, synthesis and performance of such ultrathin 2D nanomaterials-based OER catalysts and their property-structure relationships are discussed, providing valuable insights to the exploration of novel OER catalysts with high efficiency and low overpotential. The potential research directions are also proposed in the research field.

Hierarchical Co(OH)F Superstructure Built by Low-Dimensional Substructures for Electrocatalytic Water Oxidation

The development of new materials/structures for efficient electrocatalytic water oxidation, which is a key reaction in realizing artificial photosynthesis, is an ongoing challenge. Herein, a Co(OH)F material as a new electrocatalyst for the oxygen evolution reaction (OER) is reported. The as-prepared 3D Co(OH)F microspheres are built by 2D nanoflake building blocks, which are further woven by 1D nanorod foundations. Weaving and building the substructures (1D nanorods and 2D nanoflakes) provides high structural void porosity with sufficient interior space in the resulting 3D material. The hierarchical structure of this Co(OH)F material combines the merits of all material dimensions in heterogeneous catalysis. The anisotropic low-dimensional (1D and 2D) substructures possess the advantages of a high surface-to-volume ratio and fast charge transport. The interconnectivity of the nanorods is also beneficial for charge transport. The high-dimensional (3D) architecture results in sufficient active sites per the projected electrode surface area and is favorable for efficient mass diffusion during catalysis. A low overpotential of 313 mV is required to drive an OER current density of 10 mA cm^-2 on a simple glassy carbon (GC) working electrode in a 1.0 m KOH aqueous solution.

Crystallinity-Modulated Electrocatalytic Activity of a Nickel(II) Borate Thin Layer on Ni3 B for Efficient Water Oxidation

The exploration of new efficient OER electrocatalysts based on nonprecious metals and the understanding of the relationship between activity and structure of electrocatalysts are important to advance electrochemical water oxidation. Herein, we developed an efficient OER electrocatalyst with nickel boride (Ni₃ B) nanoparticles as cores and nickel(II) borate (Ni-B_i ) as shells (Ni-B_i @NB) via a very simple and facile aqueous reaction. This electrocatalyst exhibited a small overpotential of 302 mV at 10 mA cm^-2 and Tafel slope of 52 mV dec^-1 . More interestingly, it was found that the OER activity of Ni-B_i @NB was closely dependent on the crystallinity of the Ni-B_i shells. The partially crystalline Ni-B_i catalyst exhibited much higher activity than the amorphous or crystalline analogues; this higher activity originated from the enhanced intrinsic activity of the catalytic sites. These findings open up opportunities to explore nickel(II) borates as a new class of efficient nonprecious metal OER electrocatalysts, and to improve the electrocatalyst performance by modulating their crystallinity.

Highly Active Three-Dimensional NiFe/Cu2 O Nanowires/Cu Foam Electrode for Water Oxidation

Water splitting is of paramount importance for exploiting renewable energy-conversion and -storage systems, but is greatly hindered by the kinetically sluggish oxygen evolution reaction (OER). In this work, a three-dimensional, highly efficient, and durable NiFe/Cu₂ O nanowires/Cu foam anode (NiFe/Cu₂ O NWs/CF) for water oxidation in 1.0 m KOH was developed. The obtained electrode exhibited a current density of 10 mA cm^-2 at a uniquely low overpotential of η=215 mV. The average specific current density (j_s ) was estimated, on the basis of the electrocatalytically active surface area, to be 0.163 mA cm^-2 at η=310 mV. The electrode also displayed a low Tafel slope of 42 mV decade^-1 . Moreover, the NiFe/Cu₂ O NWs/CF electrode could maintain a steady current density of 100 mA cm^-2 for 50 h at an overpotential of η=260 mV. The outstanding electrochemical performance of the electrode for the OER was attributed to the high conductivity of the Cu foam and the specific structure of the electrode with a large interfacial area.

Ternary NiCo2 Px Nanowires as pH-Universal Electrocatalysts for Highly Efficient Hydrogen Evolution Reaction

A bimetallic-structured ternary phosphide (NiCo₂ P_x ) as a novel pH-universal electrocatalyst for hydrogen evolution reaction is presented. It exhibits both high activity and long-term stability in all the tested alkaline, neutral, and acidic media. The excellent catalytic performance endows it with a bright future in the large-scale electrochemical water splitting industry.

NiO/CoN Porous Nanowires as Efficient Bifunctional Catalysts for Zn-Air Batteries

The development of highly efficient bifunctional catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is crucial for improving the efficiency of the Zn-air battery. Herein, we report porous NiO/CoN interface nanowire arrays (PINWs) with both oxygen vacancies and a strongly interconnected nanointerface between NiO and CoN domains for promoting the electrocatalytic performance and stability for OER and ORR. Extended X-ray absorption fine structure spectroscopy, electron spin resonance, and high-resolution transmission electron microscopy investigations demonstrate that the decrease of the coordination number for cobalt, the enhanced oxygen vacancies on the NiO/CoN nanointerface, and strongly coupled nanointerface between NiO and CoN domains are responsible for the good bifunctional electrocatalytic performance of NiO/CoN PINWs. The primary Zn-air batteries, using NiO/CoN PINWs as an air-cathode, display an open-circuit potential of 1.46 V, a high power density of 79.6 mW cm^-2, and an energy density of 945 Wh kg^-1. The three-series solid batteries fabricated by NiO/CoN PINWs can support a timer to work for more than 12 h. This work demonstrates the importance of interface coupling and oxygen vacancies in the development of high-performance Zn-air batteries.

Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives

We review the fundamental aspects of metal oxides, metal chalcogenides and metal pnictides as effective electrocatalysts for the oxygen evolution reaction.

Plasma-Assisted Synthesis of NiCoP for Efficient Overall Water Splitting

Efficient water splitting requires highly active, earth-abundant, and robust catalysts. Monometallic phosphides such as Ni₂P have been shown to be active toward water splitting. Our theoretical analysis has suggested that their performance can be further enhanced by substitution with extrinsic metals, though very little work has been conducted in this area. Here we present for the first time a novel PH₃ plasma-assisted approach to convert NiCo hydroxides into ternary NiCoP. The obtained NiCoP nanostructure supported on Ni foam shows superior catalytic activity toward the hydrogen evolution reaction (HER) with a low overpotential of 32 mV at -10 mA cm^-2 in alkaline media. Moreover, it is also capable of catalyzing the oxygen evolution reaction (OER) with high efficiency though the real active sites are surface oxides in situ formed during the catalysis. Specifically, a current density of 10 mA cm^-2 is achieved at overpotential of 280 mV. These overpotentials are among the best reported values for non-noble metal catalysts. Most importantly, when used as both the cathode and anode for overall water splitting, a current density of 10 mA cm^-2 is achieved at a cell voltage as low as 1.58 V, making NiCoP among the most efficient earth-abundant catalysts for water splitting. Moreover, our new synthetic approach can serve as a versatile route to synthesize various bimetallic or even more complex phosphides for various applications.

Carbon-Coated Nickel Phosphide Nanosheets as Efficient Dual-Electrocatalyst for Overall Water Splitting

Low-cost and efficient electrocatalysts for overall water splitting are in high demand for a wide range of applications across renewable and clean energy. Here, we report a simple one-step synthesis of a three-dimensional (3D) carbon-coated Ni₈P₃ nanosheet array as bifunctional catalyst for both hydrogen evolution reactions (HER) and oxygen evolution reactions (OER). The nanosheet array possesses low overpotentials, high current densities, and small Tafel slopes in both HER and OER and shows high electrocatalytic activities and long-term stability. The carbon layer with high electric conductivity serves not only as a protective layer to prevent Ni₈P₃ dissolution but also as an active layer to decrease the electrocatalysis overpotential. The nanosheet array has HER outstanding activity in both acid and alkaline media. Its superior performance in OER can be due to the synergistic interaction at the Ni₈P₃/NiO_x heterojunction. Furthermore, cell voltage as low as 1.65 V can achieve 10 mA cm^-2 current density for full water splitting in an alkaline water electrolyzer, indicating potential application of C@ Ni₈P₃ as bifunctional catalyst for clean and renewable energy utilization.

MOF-Derived Zn-Doped CoSe2 as an Efficient and Stable Free-Standing Catalyst for Oxygen Evolution Reaction

Developing highly active electrocatalysts with low cost and high efficiency for oxygen evolution reactions (OER) is important for the practical implementations of hydrogen energy. Here, we report a Zn-doped CoSe₂ nanosheets grown on free-standing carbon fabric collector (CFC), which was synthesized by using a metal-organic framework (MOF) as precursor and followed by a selenylation process. Importantly, the Zn-doped CoSe₂/CFC electrode exhibited an obviously enhanced catalytic activity for OER in 1 M KOH aqueous solution compared with CoSe₂/CFC, showing a small overpotential of 356 mV for a current density of 10 mA cm^-2, a small Tafel slope of 88 mV dec^-1, and an excellent stability. The robust and free-standing electrode shows great potential as an economic catalyst for OER applications.

Porous Nickel-Iron Selenide Nanosheets as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction

Exploring non-noble and high-efficiency electrocatalysts is critical to large-scale industrial applications of electrochemical water splitting. Currently, nickel-based selenide materials are promising candidates for oxygen evolution reaction due to their low cost and excellent performance. In this work, we report the porous nickel-iron bimetallic selenide nanosheets ((Ni0.75Fe0.25)Se2) on carbon fiber cloth (CFC) by selenization of the ultrathin NiFe-based nanosheet precursor. The as-prepared three-dimensional oxygen evolution electrode exhibits a small overpotential of 255 mV at 35 mA cm(-2) and a low Tafel slope of 47.2 mV dec(-1) and keeps high stability during a 28 h measurement in alkaline solution. The outstanding catalytic performance and strong durability, in comparison to the advanced non-noble metal catalysts, are derived from the porous nanostructure fabrication, Fe incorporation, and selenization, which result in fast charge transportation and large electrochemically active surface area and enhance the release of oxygen bubbles from the electrode surface.