New strategy to synthesize optimal cobalt diselenide@hollow mesoporous carbon nanospheres for highly efficient hydrogen evolution reaction

Cobalt diselenide (CoSe₂) has been considered as a prospective hydrogen evolution reaction (HER) catalyst due to its good electrocatalytic activity and chemical stability under acidic conditions. In this regard, various strategies for effectively utilizing CoSe₂ as an electrocatalyst have been introduced. However, to the best of our knowledge, the development of new strategies for the formation of cobalt diselenide with small crystal sizes within carbon substrates, as well as facilitating phase-controlling method to improve HER properties, has been rarely reported. Herein, we propose a new method that can control the location and crystal size of cobalt diselenide nanocrystals in hollow mesoporous carbon nanospheres (HC) by a facile impregnation method and one-step selenization process. In particular, the co-addition of cobalt nitrate and SeO₂ into HC plays an important role in preventing cobalt diselenide particles from protruding to the outside of the HC by an immediate chemical reaction in the inner void of HC. Interestingly, by controlling the amount of impregnated SeO₂ in HC, phase-controlling from the orthorhombic to cubic phase at the same heat treatment temperature is achieved. The optimized polymorphic CoSe₂ loaded in HC exhibited a much higher HER performance, with a Tafel slope of 45.7 mV per decade (dec⁻¹), than orthorhombic CoSe₂ loaded in HC (53.9 mV dec⁻¹) and cubic CoSe₂ loaded in HC (50.8 mV dec⁻¹). The synergistic effects of the crystal size and location of CoSe₂ in the HC and its phase properties contribute to the outstanding HER performance.