Crystal Phase Engineering of In2O3 Electrocatalysts for Enhanced CO2-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.