Hexagonal-Phase Cobalt Monophosphosulfide for Highly Efficient Overall Water Splitting

The rational design and synthesis of nonprecious, efficient, and stable electrocatalysts to replace precious noble metals are crucial to the future of hydrogen economy. Herein, a partial sulfurization/phosphorization strategy is proposed to synthesize a nonstoichiometric pyrrhotite-type cobalt monophosphosulfide material (Co_0.9S_0.58P_0.42) with a hexagonal close-packed phase for electrocatalytic water splitting. By regulating the degree of sulfurization, the P/S atomic ratio in the cobalt monophosphosulfide can be tuned to activate the Co³⁺/Co²⁺ couples. The synergy between the nonstoichiometric 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 toward overall water splitting. Especially for OER, the Co_0.9S_0.58P_0.42 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 to that of the reported promising OER electrocatalysts. Coupled with the high HER activity of Co_0.9S_0.58P_0.42, 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 monophosphosulfides toward advanced electrochemical applications.