Atomic-level Cu active sites enable energy-efficient CO₂ electroreduction to multicarbon products in strong acid

Electrochemical CO₂ reduction provides a promising strategy to synthesize C₂₊ compounds with reduced carbon intensity; however, high overall energy consumption restricts practical implementation. Using acidic media enables high CO₂ utilization and low liquid product crossover, but to date has suffered low C₂₊ product selectivity. Here we hypothesize that adjacent pairs of atomic-copper active sites may favour C–C coupling, thus facilitating C₂₊ product formation. We construct tandem electrocatalysts with two distinct classes of active sites, the first for CO₂ to CO, and the second, a dual-atomic-site catalyst, for CO to C₂₊. This leads to an ethanol Faradaic efficiency of 46% and a C₂₊ product Faradaic efficiency of 91% at 150 mA cm⁻² in an acidic CO₂ reduction reaction. We document a CO₂ single-pass utilization of 78% and an energy efficiency of 30% towards C₂₊ products; an ethanol crossover rate of 5%; and an ethanol product concentration of 4.5%, resulting in an exceptionally low projected energy cost of 249 GJ t⁻¹ for the electrosynthesis of ethanol via the CO₂ reduction reaction.