Surface engineering of ZnO electrocatalyst by N doping towards electrochemical CO2 reduction

The discovery of efficient, selective, and stable electrocatalysts can be a key point to produce the large-scale chemical fuels via electrochemical CO₂ reduction (ECR). In this study, an earth-abundant and nontoxic ZnO-based electrocatalyst was developed for use in gas-diffusion electrodes (GDE), and the effect of nitrogen (N) doping on the ECR activity of ZnO electrocatalysts was investigated. Initially, a ZnO nanosheet was prepared via the hydrothermal method, and nitridation was performed at different times to control the N-doping content. With an increase in the N-doping content, the morphological properties of the nanosheet changed significantly, namely, the 2D nanosheets transformed into irregularly shaped nanoparticles. Furthermore, the ECR performance of ZnO electrocatalysts with different N-doping content was assessed in 1.0 M KHCO₃ electrolyte using a gas-diffusion electrode-based ECR cell. While the ECR activity increased after a small amount of N doping, it decreased for higher N doping content. Among them, the N:ZnO-1 h electrocatalysts showed the best CO selectivity, with a faradaic efficiency (FE_CO) of 92.7% at −0.73 V vs. reversible hydrogen electrode (RHE), which was greater than that of an undoped ZnO electrocatalyst (FE_CO of 63.4% at −0.78 V_RHE). Also, the N:ZnO-1 h electrocatalyst exhibited outstanding durability for 16 h, with a partial current density of −92.1 mA cm⁻². This improvement of N:ZnO-1 h electrocatalyst can be explained by density functional theory calculations, demonstrating that this improvement of N:ZnO-1 h electrocatalyst comes from (i) the optimized active sites lowering the free energy barrier for the rate-determining step (RDS), and (ii) the modification of electronic structure enhancing the electron transfer rate by N doping.