ical hydrogen evolution reaction (HER) is an effective method to turn intermittent electric energy into renewable hydrogen energy. [2] However, the HER performance is dramatically limited by high overpotential energy barriers and kinetic sluggishness. [3] To address this problem, lots of effective and low-cost non-noble metalbased materials have been explored as efficient electrocatalysts for HER to decline the overpotential and accelerate reaction kinetics. [4] Among them, molybdenum diselenide (MoSe 2 ) is extensively investigated as a cathodic material for HER due to suitable hydrogen adsorption energy and electrical conductivity. [5] Note that these important research advances are basing on an assumption that MoSe 2 is stable in HER. However, currently, some researchers have found the dissolution of Mo in Mo-based electrocatalysts during HER process, [6][7][8][9][10] but the influence of Mo dissolution on chalcogenide under reduction condition and the promotion activity mechanism and leaching of chalcogen are still unclear. Exploring the surface structural transformation and the origin of active species are beneficial to the rational design of highly efficient electrocatalysts. The activation process of many catalysts under their operating conditions directly indicated a negative catalytic stability of themselves, [11][12][13] due to

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Due to their excellent properties and unique structures, transition metal sulfides play an important role in the development of efficient and stable photoelectric catalysts. In recent years, their potential applications have expanded from photoelectric catalysis to energy storage, especially as materials for key components of electrochemical energy storage. As a typical multifunctional metal sulfide catalyst, Co9S8 is highly attractive due to its high conductivity, better stability, suitable band structure, enhanced performance and wide applications. A large number of studies have shown that strategically modified Co9S8-based materials have greater advantages in various applications compared with pure Co9S8. Therefore, this review will evaluate the physicochemical properties and the preparation of different dimensions of Co9S8-based materials, and the influence of different structures on the photoelectrochemical energy of materials will be described. In addition, the research progress regarding the evolution of hydrogen photocatalytic, electrocatalytic water splitting and various electrochemical energy storage materials will be emphasized. Finally, the challenges faced by Co9S8-based materials and the research directions for their future applications will be discussed.