Nanograin-Boundary-Abundant Cu 2 O-Cu Nanocubes with High C 2+ Selectivity and Good Stability during Electrochemical CO 2 Reduction at a Current Density of 500 mA/cm 2 .

Surface and interface engineering, especially the creation of abundant Cu/Cu interfaces and nanograin boundaries, is known to facilitate C production during electrochemical CO reductions over copper-based catalysts. However, precisely controlling the favorable nanograin boundaries with surface structures (e.g., Cu(100) facets and Cu[(100)×(110)] step sites) and simultaneously stabilizing Cu/Cu interfaces is challenging, since Cu species are highly susceptible to be reduced into bulk metallic Cu at high current densities. Thus, an in-depth understanding of the structure evolution of the Cu-based catalysts under realistic CORR conditions is imperative, including the formation and stabilization of nanograin boundaries and Cu/Cu interfaces. Herein we demonstrate that the well-controlled thermal reduction of CuO nanocubes under a CO atmosphere yields a remarkably stable CuO-Cu nanocube hybrid catalyst (CuO(CO)) possessing a high density of Cu/Cu interfaces, abundant nanograin boundaries with Cu(100) facets, and Cu[(100)×(110)] step sites. The CuO(CO) electrocatalyst delivered a high C Faradaic efficiency of 77.4% (56.6% for ethylene) during the CORR under an industrial current density of 500 mA/cm. Spectroscopic characterizations and morphological evolution studies, together with time-resolved attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) studies, established that the morphology and Cu/Cu interfacial sites in the as-prepared CuO(CO) catalyst were preserved under high polarization and high current densities due to the nanograin-boundary-abundant structure. Furthermore, the abundant Cu/Cu interfacial sites on the CuO(CO) catalyst acted to increase the *CO adsorption density, thereby increasing the opportunity for C-C coupling reactions, leading to a high C selectivity.