Ni3S2 nanostrips@FeNi-NiFe2O4 nanoparticles embedded in N-doped carbon microsphere: An improved electrocatalyst for oxygen evolution reaction

A bionic carbon framework activates the oxygen lattice sites of NiFe2O4 to enhance electrochemical water splitting

A well-designed carbon-supported material enables the promotion of efficient oxygen evolution. Herein, a NiFe2O4 catalyst supported by a bionic carbon framework (BCF) was synthesized, and the elements doped into the BCF facilitate electron transfer between oxygen and active metal centers. The as-prepared catalyst exhibits a low overpotential and Tafel slope, and the design approach presented here represents a promising method to design electrocatalysts for water splitting.

Laser-Induced Co-Doped FePS3 with Massively Phosphorus Sulfur Vacancies Nanosheet for Efficient and Highly Stable Electrocatalytic Oxygen Reaction

Purposely optimizing material structure to reduce the energy change of the rate-determining step (RDS) for promoting oxygen evolution reaction (OER) catalytic performance is a major strategy to enhance the energy efficiency of electrocatalytic water splitting. Density functional theory (DFT) simulations indicate that creating a large number of defects on or inside the 2D FePS3 is very beneficial for its catalytic reaction of OER, especially when there are more defects, the structural diversity of the surface is more conducive to the adsorption and reaction of intermediates. In particular, when Co-doped FePS3 surfaces produce a large number of S and P defects and expose metallic Fe as active sites, its catalytic performance, especially the catalytic stability, is significantly enhanced. A facile and efficient laser-ablation-in-liquid method is then designed to combine Co with 2D layered crystal FePS3. Amazingly, the laser-induced (Fe0.53Co0.46)PS3 sample exhibits excellent OER performance, with an overpotential at 288 mV and a small Tafel slope of 58.3 mV dec-1. Moreover, (Fe0.53Co0.46)PS3 operates stably for 138 h at 10 mA cm-2 and 27 h at 100 mA cm-2, which shows that the stability of (Fe0.53Co0.46)PS3 far exceeds that of most of OER catalysts of Fe─Co system so far, and the comprehensive OER performance is in the first echelon of transition metal catalyst systems. This work proposes an in-depth understanding of the structural mechanism design of massive phosphorus sulfur vacancies by laser-induced manufacturing and will shed new light on promoting the stability of transition metal-based OER catalysts without any precious alternatives.

Grain-Boundary-Rich Pt/Co3O4 Nanosheets for Solar-Driven Overall Water Splitting

Interfacial engineering is considered an effective strategy to improve the electrochemical water-splitting activity of catalysts by modulating the local electronic structure to expose more active sites. Therefore, we report a platinum-cobaltic oxide nanosheets (Pt/Co3O4 NSs) with plentiful grain boundary as the efficient bifunctional electrocatalyst for water splitting. The Pt/Co3O4 NSs exhibit a low overpotential of 55 and 201 mV at a current density of 10 mA cm-2 for the hydrogen evolution reaction and oxygen evolution reaction in 1.0 M potassium hydroxide, respectively. A negligible degradation of 1.52 V at a current density of 10 mA cm-2 after continuous operation for 100 h, demonstrates the long-term stability of the catalyst. Furthermore, the overall water-splitting performance of the Pt/Co3O4 NSs surpasses that of the commercial Pt/C||RuO2. The density functional theory calculation results explain that the improvement of catalyst activity is attributed to the moderate adsorption/desorption energy of *H and the low reaction energy barrier of the rate-determining step. This work presents a novel vision to design bifunctional catalysts for the storage and conversion of hydrogen energy.

Anchoring Ni(OH)2-CeOx Heterostructure on FeOOH-Modified Nickel-Mesh for Efficient Alkaline Water-Splitting Performance with Improved Stability under Quasi-Industrial Conditions

Developing low-cost and industrially viable electrode materials for efficient water-splitting performance and constructing intrinsically active materials with abundant active sites is still challenging. In this study, a self-supported porous network Ni(OH)2-CeOx heterostructure layer on a FeOOH-modified Ni-mesh (NiCe/Fe@NM) electrode is successfully prepared by a facile, scalable two-electrode electrodeposition strategy for overall alkaline water splitting. The optimized NiCe0.05/Fe@NM catalyst reaches a current density of 100 mA cm-2 at an overpotential of 163 and 262 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, in 1.0 m KOH with excellent stability. Additionally, NiCe0.05/Fe@NM demonstrates exceptional HER performance in alkaline seawater, requiring only 148 mV overpotential at 100 mA cm-2. Under real water splitting conditions, NiCe0.05/Fe@NM requires only 1.701 V to achieve 100 mA cm-2 with robust stability over 1000 h in an alkaline medium. The remarkable water-splitting performance and stability of the NiCe0.05/Fe@NM catalyst result from a synergistic combination of factors, including well-optimized surface and electronic structures facilitated by an optimal Ce ratio, rapid reaction kinetics, a superhydrophilic/superaerophobic interface, and enhanced intrinsic catalytic activity. This study presents a simple two-electrode electrodeposition method for the scalable production of self-supported electrocatalysts, paving the way for their practical application in industrial water-splitting processes.

Synthesis of Fe2O3 Nanorod and NiFe2O4 Nanoparticle Composites on Expired Cotton Fiber Cloth for Enhanced Hydrogen Evolution Reaction

The design of cheap, noble-metal-free, and efficient electrocatalysts for an enhanced hydrogen evolution reaction (HER) to produce hydrogen gas as an energy source from water splitting is an ideal approach. Herein, we report the synthesis of Fe2O3 nanorods-NiFe2O4 nanoparticles on cotton fiber cloth (Fe2O3-NiFe2O4/CF) at a low temperature as an efficient electrocatalyst for HERs. Among the as-prepared samples, the optimal Fe2O3-NiFe2O4/CF-3 electrocatalyst exhibits good HER performance with an overpotential of 127 mV at a current density of 10 mA cm-2, small Tafel slope of 44.9 mV dec-1, and good stability in 1 M KOH alkaline solution. The synergistic effect between Fe2O3 nanorods and NiFe2O4 nanoparticles of the heterojunction composite at the heterointerface is mainly responsible for improved HER performance. The CF is an effective substrate for the growth of the Fe2O3-NiFe2O4 nanocomposite and provides conductive channels for the active materials' HER process.

Synergistic engineering of heteroatom doping and heterointerface construction in V-doped Ni(OH)2/FeOOH to boost both oxygen evolution and urea oxidation reactions

The exploitation of cost-effective and abundant non-noble metal electrocatalysts holds great significances in enhancing the efficiency of oxygen evolution reaction (OER) and/or urea oxidation reaction (UOR). Herein, we report an electrocatalyst with co-existing V-dopants and Ni(OH)2/FeOOH interfaces (referred to as A-NiFeV/NF, with "A" indicating "activated"). The electron coupling between Ni, Fe and V, analyzed through X-ray photoelectron spectroscopy, indicates that Ni and Fe both receive electrons from the V. Additionally, the Fe can also lead to a bias toward a lower valence of the Ni centers in Ni(OH)2. Further in situ Raman spectroscopy reveals that Ni2+(OH)2 inevitably undergoes transformation into amorphous Ni3+OOH during the activation process, however, the synergistic effects of V-dopants and Ni(OH)2/FeOOH interfaces keep the Ni centers mostly in a lower oxidation state of +2 even at high potential ranges. These low-valence Ni centers are proposed to be positively correlated with the optimized OER activity of the Ni-based electrocatalysts. As a result, the designed A-NiFeV/NF electrocatalyst exhibits low overpotentials of 234 and 313 mV to propel current densities of 10 and 100 mA/cm2, and a small Tafel slope of 37.8 mV/dec for OER in 1.0 M KOH. The catalyst demonstrates a stable OER activity for over 100 h at 100 mA/cm2. Additionally, it can be integrated with a solar cell to construct a solar-driven electrolytic OER device without additional electric input. Similarly, for the small molecule oxidation, UOR, only ∼1.33 and ∼1.39 V vs. RHE (RHE: reversible hydrogen electrode) are required to achieve 10 and 100 mA/cm2, respectively, in an electrolyte composed of 1.0 M KOH with 0.33 M urea.

Directed electron regulation promoted sandwich-like CoO@FeBTC/NF with p-n heterojunctions by gel electrodeposition for oxygen evolution reaction

Metal organic framework (MOF) is currently-one of the key catalysts for oxygen evolution reaction (OER), but its catalytic performance is severely limited by electronic configuration. In this study, cobalt oxide (CoO) on nickel foam (NF) was first prepared, which then wrapped it with FeBTC synthesized by ligating isophthalic acid (BTC) with iron ions by electrodeposition to obtain CoO@FeBTC/NF p-n heterojunction structure. The catalyst requires only 255 mV overpotential to reach a current density of 100 mA cm^-2, and can maintain 100 h long time stability at 500 mA cm^-2 high current density. The catalytic properties are mainly related to the strong induced modulation of electrons in FeBTC by holes in the p-type CoO, which results in stronger bonding and faster electron transfer between FeBTC and hydroxide. At the same time, the uncoordinated BTC at the solid-liquid interface ionizes acidic radicals which form hydrogen bonds with the hydroxyl radicals in solution, capturing them onto the catalyst surface for the catalytic reaction. In addition, CoO@FeBTC/NF also has strong application prospects in alkaline electrolyzers, which only needs 1.78 V to reach a current density of 1 A cm^-2, and it can maintain long-term stability for 12 h at this current. This study provides a new convenient and efficient approach for the control design of the electronic structure of MOF, leading to a more efficient electrocatalytic process.

Unveiling the role of defects in iron oxyhydroxide for oxygen evolution

Development of earth-abundant and robust oxygen evolution reaction (OER) catalysts is imperative for cost-effective hydrogen production via water electrolysis. Herein, we report ultrafine iron (oxy)hydroxide nanoparticles with average particle size of 2.6 nm and abundant surface defects homogeneously supported on oleum-treated graphite (FeO_x(n)@HG-T), providing abundant active sites for the OER. The optimal FeO_x(0.03)@HG-110 exhibits high electrocatalytic OER activity and excellent stability. Electrochemical testing results and theoretical calculations reveal that the outstanding OER activity of FeO_x(0.03)@HG-110 is due to its stronger charge transfer ability and lower OER energy barrier than defect-free FeO_x nanoparticles. This work demonstrates that the OER performance of oxyhydroxide-based electrocatalysts can be improved by surface defect engineering.