CeO₂-supported In₂O₃ for enhanced CO₂ electroreduction to formate: the role of support defects

Simultaneous activation of CO₂ and H₂O remains a key challenge in the electrochemical carbon dioxide reduction reaction (CO₂RR) to formate. Beyond providing structural support, the catalyst support can serve as a secondary active component that facilitates reactant adsorption and activation. Herein, an In₂O₃/CeO₂ catalyst was constructed by tuning the oxygen vacancy (O_V) concentration in the CeO₂ support. The kinetic analysis, in situ spectroscopic studies, and density functional theory calculations collectively confirm that the abundant O_V-associated Lewis acid sites facilitate the adsorption and dissociation of H₂O, thereby supplying reactive hydrogen species for CO₂-derived intermediates. This process enhances the hydrogenation steps in CO₂RR and selectively promotes the formation of formate. In addition, defective support simultaneously stabilizes highly dispersed In₂O₃ species with enhanced interfacial electron transfer, promoting the adsorption of the *OCHO intermediate. Remarkably, the optimal catalyst (20%In₂O₃/CeO₂-O_VR) delivers a high current density of 238.0 mA cm^-2 and a high Faradaic efficiency for formate (FE_formate%) of 96.0% at -1.37 V vs. RHE, while maintaining FE_formate% above 90% over a broad potential range (-0.77 to -1.37 V vs. RHE). Furthermore, it demonstrates high stability with no significant activity decay over 50 h of operation. This study highlights support-defect engineering as a strategy for promoting efficient and selective CO₂ electroreduction to formate.