Confined Mixed-Valence CuO/Cu2O Hollow Nanoreactors for Synergistic CO2 Electroreduction to C2+ Products.

Electrochemical reduction of CO2 to multicarbon (C2+) products requires both high local *CO intermediate coverage and stable Cu+ species, two requirements that are often contradictory under high current densities. Here we report a hollow porous CuO/Cu2O nanoreactor that integrates spatial confinement with interfacial electronic modulation via scalable spray pyrolysis. The hollow cavity creates a confined microenvironment that extends *CO diffusion pathways and enriches *CO intermediates, while the mixed-valence CuO/Cu2O interface acts as an electronic buffer to stabilize Cu+-rich active species under reduction conditions. This synergy delivers a C2+ Faradaic efficiency of 82.4 ± 1.8% at 900 mA cm-2 and a partial current density of 741.3 mA cm-2. Operando spectroscopy reveals that the confined structure promotes *CO retention and C-C coupling, while the dynamic Cu+/Cu2+ interface sustains Cu+ activity. This work establishes a generalizable confinement electronic codesign strategy for efficient CO2 to C2+ conversion under industrially relevant conditions.