Thermally reduced rGO-wrapped CoP/Co₂P hybrid microflower as an electrocatalyst for hydrogen evolution reaction

Cobalt phosphides (CoP_x) are potential candidates for use as high-efficiency hydrogen evolution reaction electrocatalysts that can replace noble metals, such as Pt. Typically, CoP_x can be synthesized by phosphidation with Co-based precursors such as oxides or hydroxides. In this study, we propose a new strategy for synthesizing CoP_x through the thermal reduction in cobalt phosphate (Co₃(PO₄)₂). A reduced graphene oxide-wrapped CoP/Co₂P hybrid microflower was successfully synthesized by a facile coprecipitation method in a Co₃(PO₄)₂ matrix, followed by a thermal reduction process. Co₃(PO₄)₂ can be transformed to CoP/Co₂P by treatment at 700°C for 1 hour, maintaining the original particle morphology with the assistance of reduced graphene oxide (rGO). In a 0.5 mol/L H₂SO₄ solution, the rGO-CoP/Co₂P microflower catalyzes the hydrogen evolution reaction with an overpotential of 156 mV at a current density of 10 mA cm⁻², a Tafel slope of 53.8 mV dec⁻¹, and good stability as observed through long-term CV and chronoamperometry tests.