A Co2 P/WC Nano-Heterojunction Covered with N-Doped Carbon as Highly Efficient Electrocatalyst for Hydrogen Evolution Reaction

Abstract

The hydrogen evolution reaction (HER) produces clean hydrogen through an electrochemical process. However, new nonprecious-metal electrocatalysts for the HER are required to reduce the consumption of energy. Herein, we report a new Co₂ P/WC nano-heterojunction that consists of Co₂ P and WC composite phases coated with a few-layer N-doped graphitic carbon shells (Co₂ P/WC@NC). The composite was prepared by a one-step annealing of the polyoxometalate Na₉ (NH₄ )₅ [{(B-α-PW₉ O₃₄ )Co₃ (OH)(H₂ O)₂ (Ale)}₂ Co]⋅35 H₂ O (Co₇ P₆ W₁₈ ) and dicyandiamide (DCA). The preparation method consisted of the simultaneous phosphorization of Co and carbonization of W in a confined space to isolate a Co₂ P/WC nano-heterojunction phase for the first time. Co₂ P/WC@NC facilitated the generation of hydrogen in the electrolysis process, which had an overpotential of only 91 mV at a current density of 10 mA cm^-2 in the acid solution; an excellent HER performance (2 H⁺ +2 e^- →H₂ ) and Tafel slope (40 mV dec^-1 ) as well as durability over a period of 50 h were achieved. Theoretical calculations showed that the Co₂ P, WC, and N_pyridinic C dopants in the material synergistically promoted the HER activity rather than the individual constituents. Furthermore, Co₂ P/WC@NC nano-heterojunctions showed good HER performance in the whole pH range of electrolytes and considerable durability in acidic media containing transition metal ions, which may attract more attention for the exploration and optimization of nano-heterojunction catalysts for the HER.

Keywords: cobalt phosphide; hydrogen evolution reaction; nano-heterojunction; polyoxometalate; tungsten carbide.