Hydrogel‐Derived Honeycomb Ni 3 S 4 /N,P‐C as an Efficient Oxygen Evolution Catalyst

Effect of Cooperative Redox Property and Oxygen Vacancies on Bifunctional OER and HER Activities of Solvothermally Synthesized CeO2/CuO Composites

Herein, we report the synthesis of the CeO₂/CuO composite as a bifunctional oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) electrocatalyst in a basic medium. The electrocatalyst with an optimum 1:1 CeO₂/CuO shows low OER and HER overpotentials of 410 and 245 mV, respectively. The Tafel slopes of 60.2 and 108.4 mV/dec are measured for OER and HER, respectively. More importantly, the 1:1 CeO₂/CuO composite electrocatalyst requires only a 1.61 V cell voltage to split water to achieve 10 mA/cm² in a two-electrode cell. The role of oxygen vacancies and the cooperative redox activity at the interface of the CeO₂ and CuO phases is explained in the light of Raman and XPS studies, which play the determining factor for the enhanced bifunctional activity of the 1:1 CeO₂/CuO composite. This work provides guidance for the optimization and design of a low-cost alternative electrocatalyst to replace the expensive noble-metal-based electrocatalyst for overall water splitting.

Bimetallic ruthenium–nickel alloy nanostructure supported on nickel foam for efficient alkaline hydrogen evolution at large current density

A ruthenium–nickel alloy nanostructure exhibits a low overpotential and excellent activity stability for the hydrogen evolution reaction in 1 M KOH.

Boosting Electrocatalytic Oxygen Evolution over Ce-Co9 S8 Core-Shell Nanoneedle Arrays by Electronic and Architectural Dual Engineering

An dual electronic and architectural engineering strategy is a good way to rationally design earth-abundant and highly efficient electrocatalysts of the oxygen evolution reaction (OER) for sustainable hydrogen-based energy devices. Here, a Ce-doped Co₉ S₈ core-shell nanoneedle array (Ce-Co₉ S₈ @CC) supported on a carbon cloth has been designed and developed to accelerate the sluggish kinetics of the OER. Profiting from valance alternative Ce doping, a fine core-shell structure and vertically aligned nanoneedle arrayed architecture, Ce-Co₉ S₈ @CC integrates modulated electronic structure, highly exposed active sites, and multidimensional mass diffusion channels; together, these afford a favorable catalyzed OER. Ce-Co₉ S₈ @CC exhibits remarkable performance in the OER in an alkaline medium, where the overpotential requires only 242 mV to deliver a current density of 10 mA cm^-2 for the OER; this is 70 mV superior to that of Ce-free Co₉ S₈ catalyst and other counterparts. Good stability and impressive selectivity (nearly 100 % Faradic efficiency) are also demonstrated. When integrated into a two-electrode OER//HER electrolyzer, the as-prepared Ce-Co₉ S₈ @CC displays a low operation potential of 1.54 V at 10 mA cm^-2 and long-term stability, thus demonstrating great potential for economical water electrolysis.

Sulfur Vacancies-Engineered Ni3 S4-x Hollow Microspheres with Optimized Anionic Adsorption Energy for High-Performance Supercapacitor

Nickel sulfides with high theoretical capacitance have aroused tremendous attention for next-generation supercapacitors. Unfortunately, the structural durability of nickel sulfides is insufficient to support the long-term working situation. Herein, Ni₃ S_4-x hollow microspheres with sulfur vacancies (Ni₃ S_4-x HMs) are constructed by a liquid-phase anion exchange process using the Ni-MOF as the precursor. Both experimental investigation and theoretical analysis suggest that the deliberately introduced sulfur vacancies effectively improve the anionic adsorptive ability of nickel sulfides in the KOH electrolyte, significantly enhancing the reversible capacitance and structural durability (1884 F g^-1 at 2 A g^-1 , capacity retention of 97.9% after 10 000 cycles). In addition, an asymmetrical solid-state supercapacitor consisting of Ni₃ S_4-x HMs cathode and activated carbon anode shows infusive energy/power density (33.05 Wh kg^-1 /1.68 kW kg^-1 ) and remains 82.4% over 10 000 repeated charging/discharging processes in the KOH-PVA gel electrolyte. The strategies can be developed to enlighten the structural design of various metal sulfides materials adopted in electrochemical energy storage devices including alkali ion batteries, supercapacitors, and electrocatalysts.

Fe and Cu dual-doped Ni3S4 nanoarrays with less low-valence Ni species for boosting water oxidation reaction

Transition metal materials with high efficiency and durable electrocatalytic water splitting activity have attracted widespread attention among scientists. In this work, two cation co-doped Ni₃S₄ nanoarrays grown on a Ni foam support were firstly synthesized through a typical two step hydrothermal process. Cu and Fe co-doping can regulate the internal electron configuration of the material, thus reducing the activation energy of the active species. Moreover, density functional theory calculations demonstrate that a low Ni²⁺ amount improves the adsorption energy of H₂O, which facilitates the formation and reaction of intermediate species in the water splitting process. The experimental results indicate that the Cu and Fe co-doped Ni₃S₄ material has superior electrochemical activity for water oxidation reaction to pure Ni₃S₄, Fe doped Ni₃S₄ and Cu doped Ni₃S₄. The Fe-Cu-Ni₃S₄ material displays a significantly enhanced electrocatalytic performance with low overpotentials of 230 mV at 50 mA cm^-2 and 260 mV at 100 mA cm^-2 for the oxygen evolution reaction under alkaline conditions. It's worth noting that when Fe-Cu-Ni₃S₄ was used as the anode and cathode, a small cell voltage of 1.59 V at 10 mA cm^-2 was obtained to achieve stable overall water splitting. Our work will afford a novel view and guidance for the preparation and application of efficient and environmentally friendly water splitting catalysts.

Heterostructure Ni3S4-MoS2 with interfacial electron redistribution used for enhancing hydrogen evolution

Developing highly effective and inexpensive electrocatalysts for hydrogen evolution reaction (HER), particularly in a water-alkaline electrolyzer, are crucial to large-scale industrialization. The earth-abundant molybdenum disulfide (MoS₂) is an ideal electrocatalyst in acidic media but suffers from a high overpotential in alkaline solution. Herein, nanospherical heterostructure Ni₃S₄-MoS₂ was obtained via a one-pot synthesis method, in which Ni₃S₄ was uniformly integrated with MoS₂ ultrathin nanosheets. There were abundant heterojunctions in the as-synthesized catalyst, which were verified by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The structure features with interfacial electron redistribution was proved by XPS and density functional theory (DFT) calculations, which offered several advantages to promote the HER activity of MoS₂, including increased specific surface area, exposed abundant active edge sites and improved electron transfer. Ni₃S₄-MoS₂ exhibited a low overpotential of 116 mV at 10 mA cm^-2 in an alkaline solution with a corresponding Tafel slope of 81 mV dec^-1 and long-term stability of over 20 h. DFT simulations indicated that the synergistic effects in the system with the chemisorption of H on the (002) plane of MoS₂ and OH on the (311) plane of Ni₃S₄ accelerated the rate-determining water dissociation steps of HER. This study provides a valuable route for the design and synthesis of inexpensive and efficient HER electrocatalyst, heterostructure Ni₃S₄-MoS₂.

The in situ derivation of a NiFe-LDH ultra-thin layer on Ni-BDC nanosheets as a boosted electrocatalyst for the oxygen evolution reaction

A Ni-based metal organic framework (Ni-BDC) and subsequently derived NiFe-LDH were studied to overcome the defect of the low availability of active sites for the oxygen evolution reaction (OER) during the water splitting process.

Dual Single-Atomic Ni-N4 and Fe-N4 Sites Constructing Janus Hollow Graphene for Selective Oxygen Electrocatalysis

Nitrogen-coordinated metal single atoms in carbon have aroused extensive interest recently and have been growing as an active research frontier in a wide range of key renewable energy reactions and devices. Herein, a step-by-step self-assembly strategy is developed to allocate nickel (Ni) and iron (Fe) single atoms respectively on the inner and outer walls of graphene hollow nanospheres (GHSs), realizing separate-sided different single-atom functionalization of hollow graphene. The Ni or Fe single atom is demonstrated to be coordinated with four N atoms via the formation of a Ni-N₄ or Fe-N₄ planar configuration. The developed Ni-N₄ /GHSs/Fe-N₄ Janus material exhibits excellent bifunctional electrocatalytic performance, in which the outer Fe-N₄ clusters dominantly contribute to high activity toward the oxygen reduction reaction (ORR), while the inner Ni-N₄ clusters are responsible for excellent activity toward the oxygen evolution reaction (OER). Density functional theory calculations demonstrate the structures and reactivities of Fe-N₄ and Ni-N₄ for the ORR and OER. The Ni-N₄ /GHSs/Fe-N₄ endows a rechargeable Zn-air battery with excellent energy efficiency and cycling stability as an air-cathode, outperforming that of the benchmark Pt/C+RuO₂ air-cathode. The current work paves a new avenue for precise control of single-atom sites on carbon surface for the high-performance and selective electrocatalysts.

Spinel-type oxygen-incorporated Ni3+ self-doped Ni3S4 ultrathin nanosheets for highly efficient and stable oxygen evolution electrocatalysis

Spinel-type structured materials have attracted considerable attention and been regarded as promising alternative catalysts for oxygen evolution reaction (OER). However, the regulation of catalytically active octahedral sites in spinel structure to realize high activity and good stability for OER electrocatalysis is still a great challenge. Herein, we propose a self-doping strategy to boost OER performance of spinel-type Ni₃S₄ enriched high valence Ni³⁺ as active sites. By sacrificing Ni-based metal-organic framework, the ultrathin Ni₃S₄ manosheets are topologically grown on conductive Ni foam substrate and realize the simultaneous Ni³⁺ self-doping and surface oxygen incorporation during in situ sulfidation conversion process. These compositional and structural characteristics endow it with enhanced adsorption binding strength, enabling the highly efficient OER. As a result, the Ni₃S₄/NF exhibits excellent activity and outstanding stability toward OER electrocatalysis in alkaline medium, which only demands an ultralow overpotential of 266 mV to deliver a current density of 10 mA cm^-2 and manifests the stable OER process for at least 75 h. Moreover, when used as an effective overall water splitting electrolyzer, the Ni₃S₄/NF achieves a current density of 10 mA cm^-2 at only 1.638 V with good long-term stability.

Agar-induced hollow porous carbon nanospheres anchored platinum for high-performance hydrogenation

Hollow porous carbon has attracted a great deal of interest as catalyst-support because of its high surface area, low density and large pore volume. Herein, we develop a layer-by-layer assembly method to effectively load Pt nanoparticles on hollow porous carbon nanospheres (Pt/HPC) through using modified-SiO₂ nanospheres as the template and agar as the carbon resource. The gel properties of agar (e.g., sensitivity to temperature and high mechanical strength) makes the Pt nanoparticles well crosslink with carbon, as well as endows the carbon nanospheres with robust stability. The synthesized Pt/HPC was employed as a catalyst in the hydrogenation reduction of rhodamine B (RhB). The catalytic results demonstrate that Pt/HPC is very promising for RhB hydrogenation as compared to commercial Pt/C catalyst. It is proven that such excellent activity of Pt/HPC can be attributed to the combined merits of hollow porous architecture and well combination between HPC and Pt nanoparticles.

Facile synthesis of self-supported amorphous phosphorus-doped Ni(OH)2 composite anodes for efficient water oxidation

Self-supporting phosphorus-doped Ni(OH)₂ anodes were synthesized via a facile one-pot hydrothermal method. They are promising for real applications with low fabrication cost, high activity, long stability, and fast responses to current changes.

Interface engineering of oxygen-vacancy-rich NiCo2O4/NiCoP heterostructure as an efficient bifunctional electrocatalyst for overall water splitting

Inexpensive bifunctional electrocatalysts towards oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is highly desirable from the perspective of energy conversion.

Oxygen Vacancy-Rich In-Doped CoO/CoP Heterostructure as an Effective Air Cathode for Rechargeable Zn-Air Batteries

An efficient and low-cost electrocatalyst for reversible oxygen electrocatalysis is crucial for improving the performance of rechargeable metal-air batteries. Herein, a novel oxygen vacancy-rich 2D porous In-doped CoO/CoP heterostructure (In-CoO/CoP FNS) is designed and developed by a facile free radicals-induced strategy as an effective bifunctional electrocatalyst for rechargeable Zn-air batteries. The electron spin resonance and X-ray absorption near edge spectroscopy provide clear evidence that abundant oxygen vacancies are formed in the interface of In-CoO/CoP FNS. Owing to abundant oxygen vacancies, porous heterostructure, and multiple components, In-CoO/CoP FNS exhibits excellent oxygen reduction reaction activity with a positive half-wave potential of 0.81 V and superior oxygen evolution reaction activity with a low overpotential of 365 mV at 10 mA cm^-2 . Moreover, a home-made Zn-air battery with In-CoO/CoP FNS as an air cathode delivers a large power density of 139.4 mW cm^-2 , a high energy density of 938 Wh kg_Zn ^-1 , and can be steadily cycled over 130 h at 10 mA cm^-2 , demonstrating great application potential in rechargeable metal-air batteries.

Tuning the morphological and electronic structure of amorphous nickel-based electrocatalysts by anion regulation for water oxidation in neutral media

Self-supported water oxidation catalysts with high activity under neutral conditions were fabricated with tunable morphology and electronic structure by anion regulation.