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Electrochemical hydrogen production is considered the most reliable approach to transfer the renewable energies to the chemical energynamely, the hydrogenfor storage, and intensive attention has been directed to the nonprecious catalyst development for water splitting reactions. Among the catalyst candidates, metal sulfides have been extensively explored as an emerging electrocatalyst material for oxygen evolution reaction (OER) in water splitting reaction, because of their abundant active centers, good electrical conductivity, and high intrinsic activity. By optimizing the structure and chemical states, some advanced catalysts have been reported recently, which was instructive and inspiring for novel catalyst development. Herein, the recent advances in electrocatalytic performance and optimization strategies of transition-metal sulfide for OER are reviewed systematically and comprehensively. The fundamental catalytic mechanism and key parameters of OER are first presented and then followed by the physicochemical properties of metal sulfides, which could be helpful in understanding the correlation between the structure and catalytic performance. Importantly, the intrinsic activity of metal sulfides boosted by the general strategies, in terms of the defect/vacancy effect, lattice mismatch, phase engineering, heterostructure, and the doping effect, is mainly discussed in this work. The challenges and opportunities related to the further development of metal sulfide materials with high activity and long-term durability are finally proposed. It can be concluded that these regulatory strategies could largely improve the electrocatalytic performance by increasing the active site exposure and reducing the energy barrier of catalytic reactions. In addition, the problems and future challenges in improving the catalytic performance of metal sulfide materials are presented, which provides beneficial enlightenment and guidance for the development of efficient and low-cost electrocatalysts in the future. Hopefully, this effort would be helpful to the design and preparation of metal sulfides catalyst for OER.

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The microenvironment tuning of Ni species, a promising non-precious catalyst, is significant in the energy and environmentally relevant urea electro-oxidation reaction (UOR). Herein, we found that the high-valent Ni species induced by the inactive MoO2 in mixed nanocrystals of NiO/MoO2 were effective for urea oxidation. The redox interaction of MoO2 and NiO revealed by the spectroscopic analysis well supported the formation of high-valent Ni species and the changes in the surface chemical state. High catalytic activity and stability for urea oxidation were observed by a series of electrochemical measures compared to the counterpart catalysts of MoO2 and NiO. The optimal NiO/MoO2 hybrid catalyst showed a UOR activity of 73.1 mA cm–2 at 1.50 V, which was about 12-fold that of the NiO catalyst. In addition, largely improved catalytic kinetics and catalytic stability for UOR were also demonstrated. Because of the inactive activity of MoO2 and the low performance of NiO, the largely improved preference can be affirmatively attributed to the efficient catalytic synergism of NiO/MoO2 in the mixed nanocrystals. The current finding clarifies the catalytic promotion effect of the inactive Mo species on Ni-based catalysts for urea oxidation, which would be instructive for Ni/Mo-relevant catalyst development.

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