Regulation of the electronic structure and surface wettability of a Co9S8 electrocatalyst by nitrogen and phosphorous co-doping for efficient overall water splitting

Abstract

Pentlandite cobalt sulfide (Co₉S₈) has gained significant attention as a water-splitting electrocatalyst on account of its intrinsic metallic conductivity and active sites. However, the robust sulfur–hydrogen (S–H_ads) bonds formed on Co₉S₈^’s surface significantly impede the atomic hydrogen desorption and suppress the overall HER performance. Herein, nitrogen and phosphorous anion co-doped Co₉S₈ (N,P-Co₉S₈) was in situ synthesized on cobalt foam, and the material can serve as an efficient precious metal-free bifunctional catalyst for electrochemical water splitting in basic media. Remarkably, N,P-Co₉S₈ exhibits much better HER activity than the pristine Co₉S₈ with smaller overpotentials of 80 and 189 mV to deliver current densities of 10 and 100 mA cm⁻², respectively, in 1.0 M KOH solution, as well as a lower Tafel slope of 104 mV dec⁻¹. The remarkable HER performance is mainly attributed to the electronic structure and surface wettability of Co₉S₈ regulated by anion doping. The N and P anion co-doping can synergistically increase the water dissociation, optimize atomic hydrogen desorption, and accelerate mass transfer, giving rise to the improvement of HER performance. Moreover, N,P-Co₉S₈ is found to bifunctionally electrolyze the OER in alkaline electrolytes with outstanding activity and super-catalytic stability. The assembled water electrolyzer using N,P-Co₉S₈ as both the anode and cathode only needs small cell voltages of 1.90 and 1.74 V to drive a current density of 100 mA cm⁻² at 25 and 65 °C, respectively.