Unveiling the roles of phase and nanostructure in the electrocatalytic hydrogen evolution activity of cobalt phosphide nanoparticles

Abstract

Phase- and nanostructure-controlled synthesis of cobalt phosphides and the exploration of their electrocatalytic activity for HER are of great importance for advancing their practical applicability. Herein, we present a facile method for phase- and nanostructure-controlled synthesis of cobalt phosphides – namely, Co₂P, CoP, and CoP/Co₂P nanocomposite – via the thermal decomposition of cobalt(II) acetate in the presence of oleylamine (as a reducing agent) and trioctylphosphine (as a phosphorus source). The desired cobalt phosphide phase and nanostructure were obtained by controlling the P/Co molar ratio and the decomposition temperature. The mechanism of cobalt phosphide formation is best described by the Kirkendall effect. Initially, cobalt metal nanoparticles (NPs) are formed; then, due to different diffusion rates – outward for cobalt atoms and inward for phosphorus atoms – cobalt phosphides with different phases, compositions, and morphologies are obtained. Specifically, pseudospherical hollow Co₂P NPs with a hexagonal crystal structure, solid CoP NPs with an orthorhombic crystal structure, and a dendritic CoP/Co₂P nanostructure were obtained. These materials were evaluated as electrocatalysts for HER in 0.5 M H₂SO₄ solution to unveil the roles of phase and nanostructure. The results demonstrated that Co₂P NPs achieved a geometric cathodic current density of 10 mA cm⁻² at an overpotential of 242 mV, while CoP NPs required only 192 mV to reach the same current density. The CoP/Co₂P nanocomposite exhibited outstanding geometric performance, achieving 10 mA cm⁻² at a significantly lower overpotential of 152 mV. Comparison of the intrinsic HER activities revealed the following trend: CoP > Co₂P > CoP/Co₂P. The high geometric activity of the CoP/Co₂P composite – despite exhibiting the lowest intrinsic activity – is attributed to its unique dendritic nanostructure, which substantially enhances the electrochemically active surface area. Moreover, the potential electron transfer between the CoP and Co₂P phases facilitates the generation of additional active sites, further contributing to its improved overall HER performance.