Tuning oxygen vacancy concentration in Indium oxide via sulfur doping for enhancing CO2 electroreduction to formate

Abstract

Electrocatalytic reduction of carbon dioxide (CO₂) to high-value chemicals represents a desirable approach to address the escalating CO₂ emissions and energy shortage challenges. Optimally designing oxygen vacancies in electrocatalysts is a powerful strategy to improve electrocatalytic CO₂ reduction to formate through modulation of their electronic structure, thus resulting in accelerating CO₂ reduction kinetics. Nevertheless, achieving precise control over the concentration and distribution of surface oxygen vacancies in electrocatalysts has not been thoroughly investigated or clarified. In this work, a series of sulfur-doped In₂O₃ electrocatalysts (S-In₂O₃) with varying concentrations of oxygen vacancies were synthesized through a rational design to achieve highly efficient electrocatalytic CO₂ reduction to formate. Experimental results and density functional theory (DFT) calculations demonstrate that the synergistic effect of sulfur doping and oxygen vacancies can significantly enhance CO₂ activation, causing the overall density of states of S-In₂O₃ to shift toward lower energy levels, thereby facilitating the formation of *OOCH intermediates. The optimized S-In₂O₃ electrocatalyst achieved a maximum formate Faradaic efficiency (FE) of 97.1 % at −1.1 V vs RHE, while maintaining a FE of over 90 % throughout 50-h stability test. Further investigation revealed the intrinsic relationship among sulfur doping, oxygen vacancy concentration, and CO₂RR performance. This work provides a viable elemental doping strategy to fine-tune the oxygen vacancy concentration in In-based catalysts for efficient electrocatalytic CO₂RR to formate.