Hexagonal-Phase Cobalt Monophosphosulfide for Highly Efficient Overall Water Splitting

Abstract

The rational design and synthesis of the non-precious, efficient, and stable electrocatalysts to replace the precious noble metals are crucial to the future of hydrogen economy. Herein, a partial-sulfurization/phosphorization strategy is proposed to synthesize a non-stoichiometric pyrrhotite-type cobalt monophosphosulphide material (Co0.9S0.58P0.42) with a hexagonal close packed (hcp) phase for electrocatalytic water splitting. By regulating the degree of sulfurization, the P/S atomic ratio in the cobalt monophosphosulphide can be tuned to activate the Co3+/Co2+ couples. The synergy between the non-stoichiometric nature and the tunable P/S ratio results in the strengthened Co3+/Co2+ couples and tunable electronic structure, and thus efficiently promotes the oxygen/hydrogen-evolution reaction (OER/HER) processes towards overall water splitting. Especially for OER, the Co0.9S0.58P0.42 material, featured with a uniform yolk-shell spherical morphology, shows a low overpotential of 266 mV at 10 mA cm-2 (η10) with a low Tafel slope of 48 mV dec-1 as well as high stability, which is comparable with the reported promising OER electrocatalysts. Coupled with the high HER activity of Co0.9S0.58P0.42, the overall water splitting is demonstrated with a low η10 at 1.59 V and good stability. This study shows that phase engineering and composition control can be the elegant strategy to realize the Co3+/Co2+ couple activation and electronic structure tuning to promote the electrocatalytic process. The proposed strategy and approaches allow the rational design and synthesis of transition metal monophosphosulphides towards advanced electrochemical applications.