Controlled Cu Loading on ITO Shifts C1-to-C2 Oxygenate Pathways via Ensemble–Interface Cooperativity in CO2 Electroreduction

Electrochemical CO2 reduction provides a promising route to convert CO2 and renewable electricity into liquid fuels and chemicals, but selectively directing Cu-based catalysts toward specific C1 or C2 oxygenates remains challenging due to competing reaction pathways. A central challenge is to control not only the identity of Cu sites, but also their spatial connectivity and interfacial electronic environment that governs the fate of *CO-derived intermediates. In this work, we report that Cu loading on indium tin oxide (ITO) can serve as a systematic control parameter to tune Cu ensemble–oxide interface cooperativity in CO2 electroreduction and alter CO2RR pathways. By increasing Cu loading from 1 wt %, 5 wt % to 10 wt %, the dominant liquid product evolves from methanol (MeOH) to mixed methanol/ethanol (EtOH) and finally to EtOH. Importantly, ethylene (C2H4) was observed across the Cu/ITO series, while the EtOH/C2H4 ratio increases progressively with Cu loading, indicating that the loading-dependent change involves the branching of C2 intermediates after C–C bond formation as well. Structural and surface analyses reveal a concurrent evolution from highly dispersed Cu species toward larger Cu-rich domains, together with systematic changes in the Cu–ITO interfacial electronic environment. Together, these observations support an ensemble–interface cooperative picture in which increasing Cu loading promotes the development of Cu-rich ensembles while maintaining an electronically modified Cu–ITO interfacial environment, thereby shifting the liquid oxygenate pathway from MeOH toward EtOH. The concurrent increase in the EtOH/C2H4 ratio further suggests that these structural and interfacial changes also influence the fate of oxygen-containing C2 intermediates and the post C–C-coupling branching between C2H4 and EtOH. This work therefore provides system-specific insight into how Cu-domain structure and oxide-support interactions can redirect oxygenate pathways in Cu-catalyzed CO2 electroreduction.