Nanograin-Boundary-Abundant Cu2O-Cu Nanocubes with High C2+ Selectivity and Good Stability during Electrochemical CO2 Reduction at a Current Density of 500 mA/cm2

Surface and interface engineering, especially the creation of abundant Cu⁰/Cu⁺ interfaces and nanograin boundaries, is known to facilitate C₂₊ production during electrochemical CO₂ reductions over copper-based catalysts. However, precisely controlling the favorable nanograin boundaries with surface structures (e.g., Cu(100) facets and Cu[n(100)×(110)] step sites) and simultaneously stabilizing Cu⁰/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 CO₂RR conditions is imperative, including the formation and stabilization of nanograin boundaries and Cu⁰/Cu⁺ interfaces. Herein we demonstrate that the well-controlled thermal reduction of Cu₂O nanocubes under a CO atmosphere yields a remarkably stable Cu₂O-Cu nanocube hybrid catalyst (Cu₂O(CO)) possessing a high density of Cu⁰/Cu⁺ interfaces, abundant nanograin boundaries with Cu(100) facets, and Cu[n(100)×(110)] step sites. The Cu₂O(CO) electrocatalyst delivered a high C₂₊ Faradaic efficiency of 77.4% (56.6% for ethylene) during the CO₂RR under an industrial current density of 500 mA/cm². Spectroscopic characterizations and morphological evolution studies, together with in situ time-resolved attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) studies, established that the morphology and Cu⁰/Cu⁺ interfacial sites in the as-prepared Cu₂O(CO) catalyst were preserved under high polarization and high current densities due to the nanograin-boundary-abundant structure. Furthermore, the abundant Cu⁰/Cu⁺ interfacial sites on the Cu₂O(CO) catalyst acted to increase the *CO adsorption density, thereby increasing the opportunity for C-C coupling reactions, leading to a high C₂₊ selectivity.