Organic Non-Nucleophilic Electrolyte Resists Carbonation During Selective CO2 Electroreduction

The spontaneous reaction of CO2 with water and hydroxide to form (bi)carbonates in alkaline aqueous electrolytes com-promises the energy and carbon-efficiency of CO2 electrolyzers. We hypothesized that electrolyte carbonation could be mitigated in an aprotic solvent with low water content, by employing an exogenous non-nucleophilic acid capable of driving proton transfer without parasitic capture of CO2 by its conjugate base. However, it is unclear whether such an electrolyte design could engender high CO2 reduction selectivity and low electrolyte carbonation. We herein report selective CO2 electro-reduction on polycrystalline Au catalyst using dimethyl sulfoxide as the solvent and acetic acid / acetate as the proton donating medium with low carbonate formation. CO2 is reduced to CO with over 90% faradaic efficiency at potentials relative to the reversible hydrogen couple that are comparable to those in neutral aqueous electrolytes. 1H and 13C 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 CO2, and that low carbonation is maintained upon addition of 1 M water. This work demonstrates that electrolyte carbonation and efficient CO2 reduction can be decoupled from each other in an aprotic solvent, offering new electrolyte design principles for low-temperature CO2 electroreduction systems.