Converting surface-oxidized cobalt phosphides into Co 2 (P 2 O 7 )-CoP heterostructures for efficient electrocatalytic hydrogen evolution

Abstract Exploring noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) is a key issue in a hydrogen economy blueprint. As one of the promising candidates, transition metal phosphides unfortunately suffer from inevitable surface oxidation which obstructs active-site exposure. Herein, a facile reduction followed by a surface phosphorization is introduced to convert surface-oxidized cobalt phosphides to a Co 2 (P 2 O 7 )-CoP heterostructure embedded in N-doped carbon (Co 2 (P 2 O 7 )-CoP/NC), accomplishing an efficient HER in both acidic and alkaline electrolytes. It affords low overpotentials ( η 10 ) of 88 and 97 mV to reach a current density of −10 mA cm −2 , and small Tafel slopes of 51 and 61 mV dec −1 in 0.5 M H 2 SO 4 and 1.0 M KOH, respectively, outperforming the parent surface-oxidized Co 2 P and most previously-reported Pt-free electrocatalysts. The remarkably improved electrocatalysis should be ascribed to the strong surface acidity of the Co 2 (P 2 O 7 ) component and thereby the promoted HER kinetics on Co 2 (P 2 O 7 )-CoP interfaces. This work will encourage the development of cost-efficient electrocatalysts via surface engineering.