Defective Metal Oxides: Lessons from CO₂ RR and Applications in NO_x RR

Sluggish reaction kinetics and the undesired side reactions (hydrogen evolution reaction and self-reduction) are the main bottlenecks of electrochemical conversion reactions, such as the carbon dioxide and nitrate reduction reactions (CO₂ RR and NO₃ RR). To date, conventional strategies to overcome these challenges involve electronic structure modification and modulation of the charge-transfer behavior. Nonetheless, key aspects of surface modification, focused on boosting the intrinsic activity of active sites on the catalyst surface, are yet to be fully understood. Engingeering of oxygen vacancies (OVs) can tune surface/bulk electronic structure and improve surface active sites of electrocatalysts. The continuous breakthroughs and significant progress in the last decade position engineering of OVs as a potential technique for advancing electrocatalysis. Motivated by this, the state-of-the-art findings of the roles of OVs in both the CO₂ RR and the NO₃ RR are presented. The review starts with a description of approaches to constructing and techniques for characterizing OVs. This is followed by an overview of the mechanistic understanding of the CO₂ RR and a detailed discussion on the roles of OVs in the CO₂ RR. Then, insights into the NO₃ RR mechanism and the potential of OVs on NO₃ RR based on early findings are highlighted. Finally, the challenges in designing CO₂ RR/NO₃ RR electrocatalysts and perspectives in studying OV engineering are provided.