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

Abstract

Substituting the kinetically sluggish oxygen evolution reaction with the thermodynamically favorable benzyl alcohol oxidation reaction is a compelling strategy for producing high-value chemicals and hydrogen. Herein, phosphorus (P)-doped hollow spherical shell structure NiS₂ with abundant sulfur (S) vacancy, denoted as S_vac–P–NiS₂, was synthesized and investigated as a bifunctional electrocatalyst for benzyl alcohol (BA) oxidation and hydrogen evolution reaction (HER). Two important processes occured during P doping; (1) the formation of high valence nickel (Ni³⁺), wherein the electrons in the Ni e_g orbit flowed to the foreign P and (2) the Ni–S antibonding orbit became more susceptible to accepting electrons and facilitating the formation of S vacancy. The above results were confirmed using X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and crystal orbital Hamilton population (COHP) analysis. High-valence Ni, as a high-energy catalytic active site, lowered the energy barrier of the reaction rate-determining step and accelerated the reaction kinetics. Meanwhile, the S vacancy contributed to the activation of C–H bonds in benzyl alcohol, as demonstrated by differential charge density calculations and quantified by pCOHP calculations. Owing to these advantages, the dopant and vacancy exhibited indispensable synergistic effects in the electrocatalytic process, which greatly promoted the electrocatalytic performance of S_vac–P–NiS₂. This work provides insights into the formation mechanisms of vacancies in doped materials and elucidates the nature of the improved catalyst performance.