Aprotic Solvent Exposes an Altered Mechanism for Copper-Catalyzed Ethylene Electrosynthesis

The selectivity and efficiency of Cu-catalyzed CO₂ or CO electroreduction are known to be sensitive to the electrolyte composition. However, in aqueous media, changes to pH and ionic composition do not alter the electrokinetic profile of C₂ product formation, commonly invoked to proceed via a rate-limiting pH-independent C-C coupling step to form an oxyanionic *CO dimer. We hypothesize that new mechanistic pathways can be exposed in an aprotic solvent-based electrolyte, where inhibited interfacial charge stabilization can favor pathways with electroneutral intermediates resulting from proton-coupled electron-transfer (PCET) steps from an exogenous donor. We herein report CO electroreduction to higher-order products on a polycrystalline Cu catalyst with dimethyl sulfoxide as the solvent and phenol as the proton donor. CO is reduced principally to C₂ products including ethylene, acetate, ethylene glycol, and ethane with negligible methane production. In stark contrast to aqueous electrolytes, we observe a low Tafel slope (27 ± 1 mV dec^-1) and Nernstian dependence on proton activity for ethylene formation, suggesting a dramatically different mechanism involving quasi-equilibrated PCET steps. This work highlights the critical role of the solvent environment and proton donor in dictating the mechanistic landscape of CO electroreduction, exposing new strategies for tuning product selectivity in hydrocarbon electrosynthesis.