Carbon- and energy-efficient ethanol electrosynthesis via interfacial cation enrichment

The use of acidic electrolytes in CO2 reduction avoids costly carbonate loss. However, the energy efficiency of acid-fed electrolysers has been limited by high hydrogen production and operating potentials. We find that these stem from the lack of alkali cations at the catalyst surface, limiting CO2 and CO adsorption. In acid-fed membrane electrode assembly systems, the incorporation of these cations is challenging as there is no flowing catholyte. Here an interfacial cation matrix (ICM)–catalyst heterojunction is designed that directly attaches to the catalyst layer. The negatively charged nature of the ICM enriches the alkali cation concentration near the cathode surface, trapping generated hydroxide ions. This increases the local electric field and pH, increasing multi-carbon production. Integrating the ICM strategy with a tailored copper–silver catalyst enables selective ethanol production through a proton-spillover mechanism. We report a 45% CO2-to-ethanol Faradaic efficiency at 200 mA cm−2, carbon efficiency of 63%, full-cell ethanol energy efficiency of 15% (3-fold improvement over the best previous acidic CO2 reduction value) and energy cost of 260 GJ per tonne ethanol, the lowest among reported ethanol-producing CO2 electrolysers. Acidic CO2 electroreduction is carbon efficient but suffers from low energy efficiency and selectivity. Here an interfacial cation matrix is developed to enrich alkali cations and increase the local pH at a Cu–Ag catalyst surface, improving efficiency. A 45% CO2-to-ethanol Faradaic efficiency and 15% energy efficiency for ethanol production are achieved.