Sulfur defect rich Mo-Ni3S2 QDs assisted by O-C[double bond, length as m-dash]O chemical bonding for an efficient electrocatalytic overall water splitting

A review on defect modulated electrocatalysts for the oxygen evolution reaction

The oxygen evolution reaction (OER) is crucial for applications such as water splitting and rechargeable metal-air batteries. Recent research has focused on improving the activity and stability of OER electrocatalysts through various strategies including structural innovation, heteroatom doping, and conductivity enhancement. Among these, defect engineering has proved particularly effective, allowing precise modulation of the materials' electronic structure at the atomic level. This review addresses defect-rich materials that exhibit superior electrochemical properties for OER applications, with a particular focus on developments from the past five years. The discussion starts with an overview of the OER catalytic mechanism and then delves into the types of defects, synthesis methods, and their impact on electrochemical performance. This review concludes with insights into the rational design and synthesis of advanced electrocatalysts, aiming to improve efficiency and extend operational longevity. The objective is to highlight approaches for creating high-performance OER electrocatalysts that outperform noble-metal based systems in both activity and stability.

Sulfur Vacancy-Engineered Co3S4/MoS2-Interfaced Nanosheet Array for Enhanced Alkaline Overall Water Splitting

Electrochemical water splitting, a crucial reaction for renewable energy storage, demands highly efficient and stable catalysts. Defect and interface engineering has been widely acknowledged to play a pivotal role in improving electrocatalytic performance. Herein, we demonstrate a facile strategy to construct sulfur vacancy (Sv)-engineered Co3S4/MoS2-interfaced nanosheet arrays to modulate the interface electronic structure in situ reduction with NaBH4. The abundant sulfur vacancies and well-arranged nanosheet arrays in Sv-Co3S4/MoS2 lead to pronounced electrocatalytic properties for hydrogen and oxygen evolution reactions (HER/OER) in an alkaline medium, with observed overpotentials of 156 and 209 mV at 10 mA cm-2, respectively. Additionally, as a bifunctional electrocatalyst, Sv-Co3S4/MoS2 requires a cell voltage of 1.67 V at 10 mA cm-2 for overall water splitting and exhibits long-term stability with activity sustained for more than 20 h. This study provides a novel approach to producing transition metal compound-interfaced electrocatalysts with rich vacancies under mild conditions, showcasing their potential for efficient water splitting applications.

High-Valence Mo Doping and Oxygen Vacancy Engineering to Promote Morphological Evolution and Oxygen Evolution Reaction Activity

The rational design of high-efficiency, low-cost electrocatalysts for electrochemical water oxidation in alkaline media remains a huge challenge. Herein, combined strategies of metal doping and vacancy engineering are employed to develop unique Mo-doped cobalt oxide nanosheet arrays. The Mo dopants exist in the form of high-valence Mo6+, and the doping amount has a significant effect on the structure morphology, which transforms from 1D nanowires/nanobelts to 2D nanosheets and finally 3D nanoflowers. In addition, the introduction of vast oxygen vacancies helps to modulate the electronic states and increase the electronic conductivity. The optimal catalyst MoCoO-3 exhibits greatly increased active sites and enhanced reaction kinetics. It gives a dramatically lower overpotential at 50 mA cm-2 (288 mV), much smaller than that of the undoped counterpart (418 mV) and comparable to those of the recently reported electrocatalysts. Density functional theory results further verify that the increased electronic conductivity and optimized adsorption energy toward oxygen evolution reaction intermediates are mainly responsible for the enhanced catalytic activity. Moreover, the assembled two-electrode electrolyzer (MoCoO-3||Pt/C) exhibits superior performance with the cell potential decreased by 233 mV to reach a current density of 50 mA cm-2 with respect to the benchmark counterpart catalysts (RuO2||Pt/C). This work might contribute to the rational design of effective, low-cost electrocatalyst materials by combining multiple strategies.

Adjusting the valence band center of Co-Ni-bimetallic sulfides through lattice expansion and stacking faults triggered by strain engineering to boost oxygen evolution reaction

Stress engineering can improve catalytic performance by straining the catalyst lattice. An electrocatalyst, Co₃S₄/Ni₃S₂-10%Mo@NC, was prepared with abundant lattice distortion to boost oxygen evolution reaction (OER). With the assistance of the intramolecular steric hindrance effect of metal-organic frameworks, slow dissolution by MoO₄^2- of the Ni substrate and recrystallization of Ni²⁺ was observed in the process of Co(OH)F crystal growth with mild temperature and short time reaction. The lattice expansion and stacking faults created defects inside the Co₃S₄ crystal, improved the material conductivity, optimized the valence band electron distribution of the material, and promoted the rapid conversion of the reaction intermediates. The presence of reactive intermediates of the OER under catalytic conditions was investigated using operando Raman spectroscopy. The electrocatalysts exhibited super high performance, a current density of 10 mA cm^-2 at an overpotential of 164 mV and 100 mA cm^-2 at 223 mV, which were comparable to those of integrated RuO₂. Our work for the first time demonstrates that the dissolution-recrystallization triggered by strain engineering is a good modulation approach to adjust the structure and surface activity of catalyst, suggesting promising industrial application.

Synergistic catalysis of graphitic carbon nitride supported bimetallic sulfide nanostructures for efficient oxygen generation

Herein, a series of g-C₃N₄ supported bimetallic sulfide nanostructures (Ni₃S₂/MoS₂/ng-C₃N₄, n = 10, 20 and 30) was prepared by a hydrothermal method and subsequently a thermal annealing approach. Ni₃S₂/MoS₂/20g-C₃N₄ with controlled composition exhibits efficient OER activity with a low overpotential of 183 mV at 10 mA cm^-2, which outperforms the vast majority of sulfide OER electrocatalysts reported previously.

Janus bimetallic materials as efficient electrocatalysts for hydrogen oxidation and evolution reactions

The development of hydrogen energy is limited by the high cost of platinum group metals (PGM). There is an urgent need to design efficient PGM-free electrocatalysts in the hydrogen electrode. Herein, Janus Ni/W bimetallic materials are proposed as an effective PGM-free bifunctional hydrogen electrocatalyst. By constructing the bimetallic materials, a synergistic effect is realized to enhance the reaction kinetics and improve the catalytic performance. In general, Ni can provide excellent H_ad sites, and W serves as OH_ad sites. Therefore, the synergistic effect of Ni and W can improve the kinetics of hydrogen evolution reaction and the hydroxide oxidation reaction. Ni/W@NF can obtain the hydrogen evolution reaction current density of 10 mA cm^-2 with an overpotential of only 62.6 mV, and the exchange current density of hydroxide oxidation reaction can reach 1.83 mA cm^-2. This work provides a new idea for the design of high-efficiency and low-cost PGM-free bifunctional hydrogen electrocatalysts.

Lattice distortion of crystalline-amorphous nickel molybdenum sulfide nanosheets for high-efficiency overall water splitting: libraries of lone pairs of electrons and in situ surface reconstitution

Lattice distortion is an important way to improve the electrocatalytic performance and stability of two-dimensional transition metal materials (2d-TMSs). Herein, a lattice distortion nickel-molybdenum sulfide electrocatalyst on foam nickel (NiMoS₄-12/NF) has been synthesized through a novel, simple, and effective crystalline-amorphous strategy. The electrocatalyst only requires 1.47 V to obtain 10 mA cm^-2 for overall water splitting (OWS) and can function stably for 100 h at a current density of 100 mA cm^-2, demonstrating an excellent electrocatalytic performance and stability. From the results of the transmission electron microscopy (TEM) and electron paramagnetic resonance spectroscopy (EPR), it can be seen that the (104) crystal lattice of NiMoS₄-12 undergoes interface strain under the crystalline-amorphous state, resulting in rich sulfur defects caused by lattice distortion, which could improve the intrinsic catalytic activity of NiMoS₄-12. According to the differential charge density analysis, around the sulfur defects, the Mo and Ni atoms with abundant lone pairs of electrons acted as libraries of lone pairs of electrons to enable an efficient hydrogen evolution reaction (HER). From the total density of states (TDOS) and the Gibbs free energy of hydrogen adsorption (ΔG_H*), the libraries of lone pairs of electrons not only effectively optimized the distribution of the surface electron density of states at the Fermi level, but also reduced the ΔG_H*, thereby improving the intrinsic HER electrocatalytic performance. The in situ Raman test results demonstrate that during the oxygen evolution reaction (OER), the surface of the nickel molybdenum sulfide was reconstructed, and highly active Ni-OOH was generated. From the calculated free energy diagrams, the Ni-OOH could optimize the reaction barrier of the rate-determining step (RDS) for the OER to enhance the slow oxygen evolution reaction kinetics. This work will contribute to the rational design of a 2d-TMSs electrocatalyst, as well as investigation of the catalytic mechanism.

Double MOF gradually activated S bond induced S defect rich MILN-based Co(z)-NiMoS for efficient electrocatalytic overall water splitting

Herein, cactus like nanorods with rich S defects and functional group MILN-based Co(z)-NiMoS are synthesized through a facile method. First, we prepared MIL-88B precursor to give a fairly dispersed frame structure, and then Coⁿ⁺ was doped into disulfides, which are rich in sulfur bonds, and the imidazole group was cleverly connected into graphitized carbon via self-etching of ZIF-67. The doping of Coⁿ⁺ and functional groups makes tiny changes in the sulfide lattice, which promotes the unsaturation degree of the S bond and activates it gradually. The prepared semi frame sulfide with a unique structure, on the one hand, ensures the hydrophilicity and multiple active specific surface area, which lays superior functions in morphology. On the other hand, coupling metals that have strong valence change ability and functional groups by active S bonds play an important role in the process of electrocatalytic reaction. Amazingly, disintegration and self-reconstruction of MILN-based Co(z)-NiMoS occurs during oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) due to the activation of the S bond, which provides a new perspective for the overall water splitting mechanism. The electrochemical results show that the MILN-based Co(z)-NiMoS electrocatalyst exhibits overpotentials of HER, OER, and overall water splitting (OWS) to be 169 mV, 170 mV, and 1.466 V, respectively, making it a promising electrode material for OWS applications.

Self-supported Ni3S2@Ni2P/MoS2 heterostructures on nickel foam for an outstanding oxygen evolution reaction and efficient overall water splitting

Hydrogen production by electrocatalytic water splitting is a pollution-free, energy-saving, and efficient method. The low efficiency of hydrogen production, high overpotentials and expensive noble-metal catalysts have limited the development of hydrogen production from electrocatalytic water splitting. Therefore, the exploration of bifunctional electrocatalysts for water overall splitting to produce hydrogen is of profound significance. Herein, Ni₃S₂@Ni₂P/MoS₂ heterostructure electrocatalysts were synthesized on Ni foam through an environmentally friendly hydrothermal method and low-temperature phosphating method. The synergistic effects between different components and the mutual substitution principle between sulfur atoms and phosphorus atoms greatly improve the OER performance of the electrocatalyst. It is also an effective strategy to optimize the adsorption energies of intermediates by the design of heterostructured catalysts composed of multiple substances. Ni₃S₂@Ni₂P/MoS₂ only requires a low overpotential (η₁₀) of 175 mV at a current density of 10 mA cm^-2 in 1.0 M KOH solution and the stable duration exceeds 40 h. In addition, this heterogeneous structure is assembled into an electrolytic cell for overall water splitting, which exhibits a low cell voltage of 1.61 volts and retains the robust stability over 30 h at 10 mA cm^-2. The Ni₃S₂@Ni₂P/MoS₂ heterostructure prepared in this research provides a strategy for exploring other heterostructured electrocatalysts with different components.