Small alkali cations direct CO electroreduction to hydrocarbons rather than oxygenates.

Electrochemical CO reduction has the potential to enable low-carbon-intensity chemicals and fuels, but the reaction yields a mixture of multi-carbon products, and the underlying selectivity-driving mechanisms are unclear. Here we explore trends in alkali cations and find, in contradistinction to carbon dioxide electroreduction, that lithium promotes ethylene production. We study the electrolyte-catalyst interface using operando Raman spectroscopy and simulations and find that hydrated Li+ on the electrode surface has the greatest hydrogen bonding and the least cation-dipole interaction with the oxygen site on intermediates. These interactions suppress hydrogenation on carbon and promote the competing hydrodeoxygenation reaction that leads to hydrocarbons. We leverage this understanding and reduce the oxygen affinity of copper via antimony doping, suppressing the formation of the O-tethered CHCHO* intermediate on the surface that would otherwise lead to oxygenates. Combining these strategies, we achieve an ethylene faradaic efficiency of 79% at 150 mA cm-2 and an energy efficiency of 39% in a membrane electrode assembly electrolyser.