Recent progress of transition metal sulfides as electrocatalysts for hydrogen/oxygen evolution reactions

With industrial and economic development, a large amount of fossil fuel consumption causes a series of problems for the sustainable development of our humanity, such as energy crisis and environmental problems. Therefore, it is highly desirable to develop alternative clean and renewable energies (such as solar, wind, and hydro energies) and related energy conversion technologies to overcome the above-mentioned problems. However, these energy sources suffer from intermittent availability because of regional or seasonal factors. Converting these energies into electrical energy and storing the excess electrical energy in the form of chemical bonds (like H-2) by electrochemical water splitting has been regarded as a promising strategy for the optimal use of these energies. Electrochemical water splitting is composed of two half-cell reactions, the hydrogen evolution reaction (HER) at the cathode to produce hydrogen and the oxygen evolution reaction (OER) at the anode to produce oxygen. However, both the HER and OER processes exhibit sluggish kinetics and thus need noble metal-based materials (Pt, IrO2, and RuO2) as electrocatalysts to improve the overall energy conversion efficiency. The scarcity and high cost of the noble metal limit their large-scale applications. Therefore, it is highly desirable and challenging to develop durable and efficient electrocatalysts for the HER/OER composed of earth-abundant elements. Transition metal sulfides with multi-valence states, low-cost and earth-abundance are promising electrocatalysts for the HER/OER in alkaline media. In the past years, substantial efforts have been devoted to developing a variety of transition metal sulfides to boost the HER/OER electrocatalytic activity from two aspects. One is from the aspect of material/ electrode structure (such as size, dimension and morphology), and most of the work was focused on the preparation of the material/electrode with a high specific surface area, open pore structure and high electrical conductivity to improve the number of active sites and their utilization, facilitate the charge/mass transfer. However, the prepared materials/electrodes usually exhibit great differences in specific surface area, pore structure and electrical conductivity, which is not an efficient strategy to peruse highly active electrocatalysts. The other one is from the aspect of intrinsic activity, which is closely related to the adsorption free energy of the reaction intermediate species (such as -H-ads and -OHads). The adsorption free energy of the reaction intermediates can be optimized by the tuning of the electronic structure of the active sites thus increasing the intrinsic activity of the HER/OER. Many strategies have been developed for tuning the electronic structure, such as valence regulation, heteroatom doping, defect construction and heterointerface construction. With a specific focus on transition metal sulfides for the alkaline electrolyzed water, this paper mainly reviews the recent progress of transition metal sulfides as electrocatalysts for the HER/OER in alkaline media. First, the HER/OER reaction mechanisms and performance evaluation parameters are briefly introduced. Second, the activity improvement strategies of the electrocatalysts are summarized extensively, including (1) increasing the number of active sites per unit area, promoting the exposure of active sites and facilitating the charge/mass transfer through the material/electrode structure design, and (2) increasing the intrinsic activity by tuning the electronic structure of the material. Finally, the future development direction of transition metal-based electrocatalysts for the HER/OER is proposed to provide a guideline for the rational design of high-performance electrocatalysts.