Organic Non-Nucleophilic Electrolyte Resists Carbonation during Selective CO 2 Electroreduction.

The spontaneous reaction of CO 2 with water and hydroxide to form (bi)carbonates in alkaline aqueous electrolytes compromises the energy and carbon efficiency of CO 2 electrolyzers. We hypothesized that electrolyte carbonation could be mitigated by operating the reaction in an aprotic solvent with low water content, while also employing an exogenous non-nucleophilic acid as the proton donor to prevent parasitic capture of CO 2 by its conjugate base. However, it is unclear whether such an electrolyte design could simultaneously engender high CO 2 reduction selectivity and low electrolyte carbonation. We herein report selective CO 2 electroreduction with low carbonate formation on a polycrystalline Au catalyst using dimethyl sulfoxide as the solvent and acetic acid/acetate as the proton-donating medium. CO 2 is reduced to CO with over 90% faradaic efficiency at potentials relative to the reversible hydrogen electrode that are comparable to those in neutral aqueous electrolytes. 1 H and 13 C NMR studies demonstrate that only millimolar concentrations of bicarbonates are reversibly formed, that the proton activity of the medium is largely unaffected by exposure to CO 2 , and that low carbonation is maintained upon addition of 1 M water. This work demonstrates that electrolyte carbonation can be attenuated and decoupled from efficient CO 2 reduction in an aprotic solvent, offering new electrolyte design principles for low-temperature CO 2 electroreduction systems.