Programmable Geometric Core–Shell In₂O₃@Cu₂O Catalysts for Near-Unity CO Selectivity in Electrocatalytic CO₂ Reduction

The electrocatalytic CO2 reduction reaction (CO2RR) to carbon monoxide (CO) represents a promising strategy for carbon recycling; however, achieving high selectivity under industrially relevant current densities remains a significant challenge. In this study, we report the development of a geometrically programmable core-shell catalyst (In2O3@Cu2O) fabricated via spray pyrolysis, in which the In2O3 core size precisely controls strain effects, interfacial electronic properties, and spatial confinement. The optimized In2O3@Cu2O catalyst exhibits near-unity Faradaic efficiency for CO (99%) across a broad current density range of 50-200 mA cm-2, while effectively suppressing both the hydrogen evolution reaction (HER) and C-C coupling. In situ spectroscopic analysis confirms the absence of C2 reaction intermediates (*OCCOH) and reveals a strain-induced redshift in the *CO vibrational frequency (from 2090 to 2052 cm-1), indicating weakened adsorption strength. Core-size-dependent performance evaluations further illustrate that a balanced geometric configuration effectively blocks In2O3-mediated formate generation pathways while optimizing active site exposure. This synergistic integration of spatial confinement, electronic modulation, and strain engineering establishes a robust design principle for selective CO2-to-CO conversion, offering a scalable and rational strategy for catalyst development in industrial CO2RR applications.