The electrocatalytic reduction of carbon dioxide, powered by renewable electricity,to produce valuable fuels and feedstocks provides a sustainable and carbon-neutralapproach to the storage of energy produced by intermittent renewable sources1.However, the highly selective generation of economically desirable products such asethylene from the carbon dioxide reduction reaction (CO2RR) remains a challenge2.Tuning the stabilities of intermediates to favour a desired reaction pathway canimprove selectivity3–5, and this has recently been explored for the reaction on copperby controlling morphology6, grain boundaries7, facets8, oxidation state9 anddopants10. Unfortunately, the Faradaic efciency for ethylene is still low in neutralmedia (60 per cent at a partial current density of 7 milliamperes per square centimetrein the best catalyst reported so far9), resulting in a low energy efciency. Here wepresent a molecular tuning strategy—the functionalization of the surface ofelectrocatalysts with organic molecules—that stabilizes intermediates for moreselective CO2RR to ethylene. Using electrochemical, operando/in situ spectroscopicand computational studies, we investigate the infuence of a library of molecules,derived by electro-dimerization of arylpyridiniums11, adsorbed on copper. We fndthat the adhered molecules improve the stabilization of an ‘atop-bound’ COintermediate (that is, an intermediate bound to a single copper atom), therebyfavouring further reduction to ethylene. As a result of this strategy, we report theCO2RR to ethylene with a Faradaic efciency of 72 per cent at a partial current densityof 230 milliamperes per square centimetre in a liquid-electrolyte fow cell in a neutralmedium. We report stable ethylene electrosynthesis for 190 hours in a system basedon a membrane-electrode assembly that provides a full-cell energy efciency of 20per cent. We anticipate that this may be generalized to enable molecular strategies tocomplement heterogeneous catalysts by stabilizing intermediates through localmolecular tuning.