Regulated Density of Atomic Cu Sites Dictates Switching from C1 to C2+ Products in CO Electroreduction

Abstract Single-atom catalysts (SACs) are generally considered C1-selective in CO2/CO electroreduction, as spatially isolated sites cannot efficiently sustain adjacent CO-derived intermediates required for C─C coupling. Here we show that this selectivity bias is not intrinsic to atomically dispersed Cu, but depends on single-atom site density. Across a series of Cu–N3O catalysts with different Cu site densities, increasing site density drives a clear selectivity crossover from H2 and CH4 formation to C2+ products. The total C2+ Faradaic efficiency (FE) rises from 9.6% on the low-density to 78.1% on the high-density at 1 bar CO, and further reaches 87.7% at 7 bar CO, whereas the low-density remains C1-selective. In situ attenuated-total-reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) reveals that dense Cu single atoms sustain a larger *COatop population and simultaneously shift interfacial water from strongly hydrogen-bonded species towards free-water-rich configurations. Density functional theory (DFT) calculations show that higher site density stabilizes CO-containing surface states and lowers the barrier of the acetate-forming pathway. These findings identify a CO-rich and proton-accessible interfacial state that favors C─C coupling over C1 hydrogenation, establishing site density as a structural descriptor for multicarbon electrosynthesis on single-atom catalysts.