A review on synthesis and modification of cobalt-molybdenum based sulfides as hydrogen and oxygen evolution catalysts for water electrolysis

Hydrogen energy as the alternative to fossil fuel received wide attention. Among different methods of hydrogen production, water electrolysis is the cleanest and most promising method. However, its development is greatly limited by hydrogen evolution reaction (HER) reaction and oxygen evolution reaction (OER) reaction. Therefore, it is very necessary to develop low-cost, high activity, and good stability of non-precious metal catalyst to replace the precious metal catalyst (Pt, RuO2), and to further reduce overpotential and power consumption. To date, great efforts have been made to develop transition metal compounds with electrocatalytic performance, such as sulfides, oxides, and carbides materials. Especially, transition metal sulfides play a very important role as catalysts and have attracted wide attention for their structural adjustability, high abundance, and mild preparation process. In this introduction, we focus on the role of transition metal sulfides in catalysts for water electrolysis, especially in a few most studied sulfides such as cobalt/molybdenum based sulfides and the recently reported homogeneous cobalt-molybdenum based sulfides, in which cobalt acts as a key element. Composition, morphology, and size are the most important factors that influence the intrinsic activity, electrical conductivity, and electronic structure of these transition metal sulfides. Besides, the effect of carbon nanomaterials as carrier materials is also discussed. The aim of this review is to emphasize the great potentials of cobalt and molybdenum based sulfides with high performance, and to help improve the performance of existing sulfides and explore new modification methods.