Regulating Phase Conversion from Ni3 Se2 into NiSe in a Bifunctional Electrocatalyst for Overall Water-Splitting Enhancement

One-step Sintering Synthesis of Ni3Se2-Ni Electrode with Robust Interfacial Bonding for Ultra-stable Hydrogen Evolution Reaction

Exploring efficient and robust self-supporting hydrogen evolution reaction (HER) electrodes using simple, accessible, and low-cost synthetic processes is crucial for the commercial application of water electrolysis at high current densities. Ni-based self-supporting electrodes are widely studied owing to their low cost and good catalytic performance. However, to date, the preparation of Ni-based electrodes requires multistep and complex preparation processes. In this study, a novel one-step in situ sintering method to synthesize mechanically stable and highly active Ni3Se2-Ni electrodes with well-controlled morphologies and structures is developed. Their excellent performance and durability can be attributed to the numerous highly active nano-Ni3Se2 catalysts embedded on the surface of the Ni skeleton, the excellent conductivity of the interconnected conductive network, and the strong interfacial bonding between Ni3Se2 and Ni. As a result, the Ni3Se2-Ni600 electrode can operate stably at 85 and 400 mA cm-2 for more than 800 and 300 h, respectively. Moreover, the Ni3Se2-Ni600 electrode displays outstanding stability for over 500 h in a commercial two-electrode system. This study provides a feasible one-step synthesis method for low-cost, high-efficiency metal selenide-metal self-supporting electrodes for water electrolysis.

Modulating Electronic Structure by Etching Strategy to Construct NiSe2 /Ni0.85 Se Heterostructure for Urea-Assisted Hydrogen Evolution Reaction

Exploring and developing novel strategies for constructing heterostructure electrocatalysts is still challenging for water electrolysis. Herein, a creative etching treatment strategy is adopted to construct NiSe2 /Ni0.85 Se heterostructure. The rich heterointerfaces between NiSe2 and Ni0.85 Se emerge strong electronic interaction, which easily induces the electron transfer from NiSe2 to Ni0.85 Se, and tunes the charge-state of NiSe2 and Ni0.85 Se. In the NiSe2 /Ni0.85 Se heterojunction nanomaterial, the higher charge-state Ni0.85 Se is capable of affording partial electrons to combine with hydrogen protons, inducing the rapid formation of H2 molecule. Accordingly, the lower charge-state NiSe2 in the NiSe2 /Ni0.85 Se heterojunction nanomaterial is more easily oxidized into high valence state Ni3+ during the oxygen evolution reaction (OER) process, which is beneficial to accelerate the mass/charge transfer and enhance the electrocatalytic activities towards OER. Theoretical calculations indicate that the heterointerfaces are conducive to modulating the electronic structure and optimizing the adsorption energy toward intermediate H* during the hydrogen evolution reaction (HER) process, leading to superior electrocatalytic activities. To expand the application of the NiSe2 /Ni0.85 Se-2h electrocatalyst, urea is served as the adjuvant to proceed with the energy-saving hydrogen production and pollutant degradation, and it is proven to be a brilliant strategy.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Cuboid-like phosphorus-doped metal-organic framework-derived CoSe2 on carbon cloth as an advanced bifunctional oxygen electrocatalyst for rechargeable zinc-air batteries

Zinc-air batteries (ZABs) are regarded as attractive devices for electrochemical energy storage and conversion due to their outstanding electrochemical performance, low price, and high safety. However, it remains a challenge to design a stable and efficient bifunctional oxygen catalyst that can accelerate the reaction kinetics and improve the performance of ZABs. Herein, a phosphorus-doped transition metal selenide/carbon composite catalyst derived from metal-organic frameworks (P-CoSe₂/C@CC) is constructed by a self-supporting carbon cloth structure through a simple solvothermal process with subsequent selenization and phosphatization. The P-CoSe₂/C@CC exhibits a low overpotential of 303.1 mV at 10 mA cm^-2 toward the oxygen evolution reaction and an obvious reduction peak for the oxygen reduction reaction. The abovementioned electrochemical performances for the P-CoSe₂/C@CC are attributed to the specific architecture, the super-hydrophilic surface, and the P-doping effect. Remarkably, the homemade zinc-air battery based on our P-CoSe₂/C@CC catalyst shows an expected peak power density of 124.4 mW cm^-2 along with excellent cycling stability, confirming its great potential application in ZABs for advanced bifunctional electrocatalysis.

The hybrid engineering of crystalline NiSex nanorod arrays with amorphous Ni–P film towards promoted overall water electrocatalysis

A superhydrophilic hetero-nanostructured material composed of amorphous Ni–P film and crystalline NiSex nanorod arrays on Ni foam (denoted as Ni–P/NiSex/NF) was synthesized through a facile hydrothermal reaction combined with an electrodeposition process, and it possessed outstanding electrocatalytic performance toward OWS and outstanding durability.

Grass-like NixSey nanowire arrays shelled with NiFe LDH nanosheets as a 3D hierarchical core–shell electrocatalyst for efficient upgrading of biomass-derived 5-hydroxymethylfurfural and furfural

In situ synthesis of NixSey–NiFe LDH core–shell nanoarrays on nickel foam for electrochemical oxidation of 5-hydroxymethylfurfural and furfural.

NiSe/Ni3Se2 on nickel foam as an ultra-high-rate HER electrocatalyst: common anion heterostructure with built-in electric field and efficient interfacial charge transfer

One grand challenge in green hydrogen production is to design efficient HER electrocatalysts for high-rate alkaline water electrolysis. Nickel chalcogenide coatings on nickel foam (NF) are promising HER electrocatalysts, but their high-rate performances are yet to be improved. The current work reports a NiSe/Ni₃Se₂@NF for alkaline HER, which requires an overpotential of only 336 mV to achieve an ultra-high current density of 1250 mA cm⁻², outperforming commercial Pt/C. The low onset potential of NiSe/Ni₃Se₂@NF is attributed to its morphology, and high surface area, as well as multiple active sites and electronic structure modulation because of the heterostructure. While these features are well-known within the current knowledge framework, new understandings are proposed on its superior high-rate performance. The common-anion feature offers abundant interfacial Ni–Se bonding and low resistance for efficient interfacial charge transfer, whereas the heterovalent-Ni-cation in the heterostructure results in a built-in electric field that further enhances the high-rate performance. This work provides new insights on both the mechanistic and methodological aspects of designing high-performance electrocatalysts operating at high current densities.

The NiSe/Ni₃Se₂ common anion heterostructure is a superior electrocatalyst for ultra-high-rate HER owing to its built-in electric field and efficient interfacial charge transfer.

Porous Ultrathin NiSe Nanosheet Networks on Nickel Foam for High-Performance Hybrid Supercapacitors

Transition metal selenides (TMSs) with excellent electrochemical activity and high intrinsic electrical conductivity have attracted considerable attention owing to their potential use in energy storage devices. However, the low energy densities of the reported TMSs, which originate from the small active surface area and poor electrolyte ion mobility, substantially restrict their application potential. In this work, porous ultrathin nickel selenide nanosheet networks (NiSe NNs) on nickel foam are fabricated by using a novel, facile method, that is, selenylation/pickling of the pre-formed manganese-doped α-Ni(OH)₂ . Removal of Mn resulted in NNs with a highly porous structure. The 3D framework of the NNs and the inherent nature of the NiSe affords high ion mobility, abundant accessible activated sites, vigorous electrochemical activity, and low resistance. One of the highest specific capacities of TMSs ever reported, that is, 443 mA h g^-1 (807 μAh cm^-2 ) at 3.0 A g^-1 , is achieved with the NNs as electrodes. The assembled NiSe NNs//porous carbon hybrid supercapacitor delivers a high energy density of 66.6 Wh kg^-1 at a power density of 425 W kg^-1 , with excellent cycling stability. This work provides a new strategy for the production of novel electrode materials that can be applied in high-performance hybrid supercapacitors, and a fresh pathway towards commercial applications of hybrid supercapacitors based on TMS electrodes.

Rational Design of Ruthenium and Cobalt-Based Composites with Rich Metal-Insulator Interfaces for Efficient and Stable Overall Water Splitting in Acidic Electrolyte

The great promise of hydrogen energy and hydrogen production from water through proton exchange membrane (PEM) or membrane-free electrolysis drives the pursuit of highly active and acid-stable electrocatalysts with dual functionality and reduced cost for overall water splitting in acidic media. Here, we report a new Ru-modified cobalt-based electrocatalyst embedded in a nitrogen-doped carbon (NC) matrix with rationally designed Mott-Schottky heterostructure to realize high activity and stability toward overall water splitting in a strongly acidic environment. Such a composite was facilely prepared by carbonization of cobalt-based MOF, followed by galvanic exchange between cobalt and Ru, and then controlled partial oxidation. The partial oxidation of RuCo implanted inside the NC matrix led to the formation of a class of RuO₂/Co₃O₄-RuCo@NC composites with rich metal-semiconductor interfaces to facilitate the charge-transfer process. As a result, the composite displayed remarkable electrocatalytic activity toward both oxygen/hydrogen evolutions in acidic media. Significantly, they also afforded low overpotentials of 247 and 141 mV for OER and HER, respectively, and a cell voltage of 1.66 V for overall water splitting at 10 mA cm^-2. In addition, excellent operation stability in 0.5 M H₂SO₄ solutions, comparable to those of them working in alkaline conditions, is demonstrated due to the protection of a coated carbon thin film. The presented work opens a new opportunity toward designing bifunctional electrocatalysts for acidic water electrolysis.

Facile synthesis of Co-Fe-B-P nanochains as an efficient bifunctional electrocatalyst for overall water-splitting

Design of cost-effective bifunctional electrocatalysts for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is vital for developing hydrogen energy for the future. Herein, a cost-effective phosphorus-doped Co-Fe-B material with chain-like structure (denoted as Co1-Fe1-B-P) is reported as an efficient and novel bifunctional electrocatalyst for the OER and HER, and was produced via a facile water-bath synthesis and subsequent phosphorization. For the OER, the as-prepared Co1-Fe1-B-P nanochains require an extremely low overpotential of about 225 mV at 10 mA cm-2 and possess a small Tafel slope of 40 mV dec-1 in alkaline media. Impressively, the HER properties of Co1-Fe1-B-P nanochains are superior to those of P-free Co-Fe-B in terms of overpotential at 10 mA cm-2 (173 mV vs. 239 mV) and kinetic Tafel slope (96 mV dec-1vs. 105 mV dec-1). The synergetic effect between Co-Fe-B and doped-P is mainly responsible for the satisfactory bifunctional performance, while the one-dimensional (1D) chain-like structure endows Co1-Fe1-B-P with abundant catalytically active sites that enhance the atom utilization efficiency. Moreover, the developed Co1-Fe1-B-P nanochains can be simultaneously utilized as both the cathode and anode for overall water-splitting, which requires a cell voltage of only 1.68 V to deliver 10 mA cm-2. This work provides a feasible and promising protocol to realize metal borides as efficient electrocatalysts in energy-related applications.

Atomic Layer Deposition of NiOOH/Ni(OH)2 on PIM-1-Based N-Doped Carbon Nanofibers for Electrochemical Water Splitting in Alkaline Medium

Portable and flexible energy devices demand lightweight and highly efficient catalytic materials for use in energy devices. An efficient water splitting electrocatalyst is considered an ideal future energy source. Well-aligned high-surface-area electrospun polymers of intrinsic microporosity (PIM-1)-based nitrogen-doped carbon nanofibers were prepared as a free-standing flexible electrode. A non-noble-metal catalyst NiOOH/Ni(OH)₂ was precisely deposited over flexible free-standing carbon nanofibers by using atomic layer deposition (ALD). The morphology, high surface area, nitrogen doping, and Ni states synergistically showed a low onset potential (η_HER =-40 and η_OER =290 mV vs. reversible hydrogen electrode), small overpotential at η₁₀ [oxygen evolution reaction (OER)=390.5 mV and hydrogen evolution reaction (HER)=-147 mV], excellent kinetics (Tafel slopes for OER=50 mV dec^-1 and HER=41 mV dec^-1 ), and high stability (>16 h) towards water splitting in an alkaline medium (0.1 m KOH). The performance was comparable with that of state-of-the-art noble-metal catalysts (e.g., Ir/C, Ru/C for OER, and Pt/C for HER). Post-catalytic characterization with X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy further proved the durability of the electrode. This study provides insight into the design of 1D-aligned N-doped PIM-1 electrospun carbon nanofibers as a flexible and free-standing NiOOH/Ni(OH)₂ decorated electrode as a highly stable nanocatalyst for water splitting in an alkaline medium.

A universal and controllable strategy of constructing transition-metal nitride heterostructures for highly enhanced bifunctional electrocatalysis

A family of transition-metal nitride heterostructures were synthesized by a universal and controllable method to remedy the drawbacks of ordinary bifunctional electrocatalysts.