Vacancy formation mechanism and synergy with doping in NiS2-based electrocatalyst for benzyl alcohol oxidation and hydrogen evolution

Abstract

Substituting the kinetic-sluggish oxygen evolution reaction with the thermodynamically favorable benzyl alcohol oxidation reaction is a compelling strategy to produce high-value chemicals and hydrogen. Herein, phosphorus (P) doped hollow spherical shell structure NiS2 with abundant sulfur (S) vacancy, denoted as Svac-P-NiS2 is synthesized and investigated as a bifunctional electrocatalyst for benzyl alcohol (BA) oxidation and hydrogen evolution reaction (HER). Two important processes occur during P doping: (1) promoting the formation of high valence nickel (Ni3+), where electron filling in the Ni eg orbit flows to the foreign P, and (2) directing the Ni-S antibonding orbit more susceptible to accept electrons and facilitating the formation of S vacancy. The above results are proved by X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and the crystal orbital Hamilton population (COHP) analysis, respectively. The high valence Ni as the high-energy catalytic active site lowers the energy barrier of the reaction rate-determining step and accelerates reaction kinetics. Meanwhile, the S vacancy contributes to the activation of C-H bonds in benzyl alcohol which is demonstrated by differential charge density calculation and quantified by pCOHP calculation. Benefitting from these advantages, dopant and vacancy exhibit the indispensable synergistic effect in the electrocatalytic process, which greatly promotes the electrocatalytic performance of Svac-P-NiS2. This work provided insights into the formation mechanisms of vacancy in doped materials and elucidated the nature of the improved catalyst performance.