Confined Mixed-Valence CuO/Cu ₂ O Hollow Nanoreactors for Synergistic CO ₂ Electroreduction to C ₂₊ Products

Electrochemical reduction of CO 2 to multicarbon (C 2+ ) 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/Cu 2 O 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/Cu 2 O interface acts as an electronic buffer to stabilize Cu + -rich active species under reduction conditions. This synergy delivers a C 2+ 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 + /Cu 2+ interface sustains Cu + activity. This work establishes a generalizable confinement electronic codesign strategy for efficient CO 2 to C 2+ conversion under industrially relevant conditions.