Phase-controllable cobalt phosphide heterostructure for efficient electrocatalytic hydrogen evolution in water and seawater

Cobalt phosphides attract broad attention as alternatives to platinum-based materials towards hydrogen evolution reaction (HER). The catalytic performance of cobalt phosphides largely depends on the phase structure, but figuring out the optimal phase towards HER remains challenging due to their diverse stoichiometries. In our work, a series of cobalt phosphide nanoparticles with different phase structures but similar particle sizes (CoP-Co₂P, Co₂P-Co, Co₂P, and CoP) on a porous carbon network (PC) were accurately synthesized. The CoP-Co₂P/PC heterostructure demonstrates upgraded HER catalytic activity with a low overpotential of 96.7 and 162.1 mV at 10 mA cm⁻² in 1 M KOH and 1 M phosphate-buffered saline solution, respectively, with a long-term (120 h) durability. In addition, the CoP-Co₂P/PC exhibits good HER performance in alkaline seawater, with a small overpotential of 111.2 mV at 10 mA cm⁻² and a low Tafel slope of 64.2 mV dec⁻¹, as well as promising stability. Density functional theory results show that the Co₂P side of the CoP-Co₂P/PC heterostructure has the best Gibbs free energy of each step for HER, which contributes to the high HER activity. This study sets the stage for the advancement of high-performance HER electrocatalysts and the implementation of large-scale seawater electrolysis.