Engineering Crystalline Structure of In ₂ O ₃ for Enhanced Electroreduction of CO ₂ to Formate

ABSTRACT The electrocatalytic CO 2 reduction reaction (CO 2 RR) to formate provides a sustainable pathway for CO 2 conversion. Indium oxide (In 2 O 3 )‐based catalysts have exceptional selectivity toward formate; however, the regulation and effects of crystalline structure on their performance need to be thoroughly investigated. Herein, we present the methodically controlled synthesis of In 2 O 3 catalysts with unique crystallographic structures: rhombohedral In 2 O 3 (h‐In 2 O 3 ), cubic In 2 O 3 (c‐In 2 O 3 ), and mixed‐phase In 2 O 3 (h/c‐In 2 O 3 ), and conduct a comprehensive evaluation of their performance in CO 2 RR to formate. Remarkably, the h‐In 2 O 3 catalyst demonstrates a Faradaic efficiency of formate (~95%) and current density surpassing both c‐In 2 O 3 and h/c‐In 2 O 3 . In addition, the h‐In 2 O 3 catalyst exhibits excellent comprehensive performance in terms of operating potential range (−0.87 ~ −1.27 V vs. RHE), catalyst stability (70 h), pH range of electrolyte (3.00 ~ 14.00), and CO 2 concentration (20% ~ 100%). Density functional theory studies reveal that among various phases and facets of In 2 O 3 , the (104) facet of the h‐In 2 O 3 most effectively stabilizes the critical reaction intermediate, a contribution that is key to its enhanced activity for formate generation from CO 2 RR. This investigation elucidates key insights into the engineering crystalline structure of In 2 O 3 catalysts pertinent to CO 2 RR, thereby presenting a methodical approach for developing highly efficient electrocatalysts.