Subnanometer Cu Clusters on Porous Ag Enhancing Ethanol Production in Electrochemical CO₂ Reduction

Controlling the electrochemical CO2 reduction process for multicarbon production is challenging. Ethanol is typically produced with lower selectivity compared to ethylene. In addition, ill-defined catalytic active sites and elusive mechanisms of C–C coupling further hinder the enhancement of ethanol generation. Here, we carefully regulated the quantity of the Cu atoms and deposited them onto a Ag inverse-opal structure (AgIOs) using the pulse-electrodeposition method. Subnanometer Cu clusters demonstrated a 2.5 times higher Faradaic efficiency for ethanol production compared to that for ethylene at −1.05 V vs RHE. Conversely, as the size of Cu increased to nanometers, ethylene became the dominant product. Excessive adsorption of CO on Cu clusters, which migrates from the Ag surface, is attributed to the improved ethanol production. Abundant Ag/Cu boundaries and adjacent spacing between Ag and Cu clusters may enhance the surface migration of CO. In contrast, the preferential site-selective CO adsorption on large Cu nanoparticles is associated with solution-mediated CO migration. Operando shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) revealed a high coverage of the CO on the Cu clusters. The initial intermediate *OCCOH by C–C coupling appeared for both Cu clusters and nanoparticles. However, Cu clusters accommodated more carbonaceous intermediates, highlighting the critical role of CO and intermediate coverages on Cu in ethanol production