Well-Defined Phase-Controlled Cobalt Phosphide Nanoparticles Encapsulated in Nitrogen-Doped Graphitized Carbon Shell with Enhanced Electrocatalytic Activity for Hydrogen Evolution Reaction at All-pH

Rational design and development of highly active, low-cost, and stable nonprecious metal electrocatalysts for hydrogen evolution reaction (HER) is extremely critical for making the water splitting process more economical and energy-saving. Herein, phase-controlled cobalt phosphide nanoparticles encapsulated in a nitrogen-doped graphitized carbon shell (termed CoxP@NC) were prepared by hydrogen reduction of organic–inorganic cobalt phosphonate hybrid materials with different Co/P molar ratios. Compared with the synthesized pure-phase CoP@NC and Co2P@NC, the hybrid-phase CoP/Co2P@NC exhibits enhanced HER activity at all-pH values, affording low overpotentials of 126 mV (0.5 M H2SO4, pH = 0), 198 mV (1.0 M KOH, pH = 14), and 459 mV (1.0 M PBS, pH = 7) to achieve a current density of 10 mA cm–2. The high activity is ascribed to the doping of the N atom, enough accessible electrocatalytic active boundary sites, and synergistic interaction among the components, especially the electronic effect of CoP and Co2P. ...