Crystal Phase Engineering of In ₂ O ₃ Electrocatalysts for Enhanced CO ₂ ‐to‐Formate

The catalytic performance of metal oxides is profoundly influenced by their crystal phase, yet the underlying mechanism often remains elusive. Herein, we systematically investigate the crystal‐phase‐dependent activity of indium oxide (In ₂ O ₃ ) for the electrocatalytic CO ₂ reduction reaction (CO ₂ RR) to formate. Through a solvent‐controlled hydrothermal synthesis, we prepared phase‐pure cubic (c‐In ₂ O ₃ ) and hexagonal (h‐In ₂ O ₃ ) polymorphs. Electrochemical evaluations reveal that h‐In ₂ O ₃ significantly outperforms c‐In ₂ O ₃ , achieving a superior formate Faradaic efficiency of 86.3% at −0.8 V versus reversible hydrogen electrode and demonstrating a notably higher partial current density across a wide potential window. In situ attenuated total reflection surface‐enhanced infrared absorption spectroscopy identifies a more intense signal for the *OCHO intermediate on h‐In ₂ O ₃ , indicating facilitated reaction kinetics. Density functional theory calculations reveal the origin of this enhancement: the predominant (104) facet of h‐In ₂ O ₃ not only strengthens CO ₂ adsorption but also significantly lowers the energy barrier for the formation of the *OCHO intermediate, the rate‐determining step. Furthermore, this facet concurrently suppresses the competing hydrogen evolution reaction and CO pathway. This work elucidates the intrinsic advantages of the hexagonal phase in In ₂ O ₃ ‐based CO ₂ RR electrocatalysts, providing a fundamental principle for catalyst design via crystal‐phase engineering.