Mechanistic Basis of Conductivity in Carbon Dioxide-Expanded Electrolytes: A Joint Experimental-Theoretical Study.

Electrolyte conductivity contributes to the efficiency of devices for electrochemical conversion of carbon dioxide (CO 2 ) into useful chemicals, but the effect of the dissolution of CO 2 gas on conductivity has received little attention. Here, we report a joint experimental-theoretical study of the properties of acetonitrile-based CO 2 -expanded electrolytes (CXEs) that contain high concentrations of CO 2 (up to 12 M), achieved by CO 2 pressurization. Cyclic voltammetry data and paired simulations show that high concentrations of dissolved CO 2 do not impede the kinetics of outer-sphere electron transfer but decrease the solution conductivity at higher pressures. In contrast with conventional behaviors, Jones reactor-based measurements of conductivity show a nonmonotonic dependence on CO 2 pressure: a plateau region of constant conductivity up to ca. 4 M CO 2 and a region showing reduced conductivity at higher [CO 2 ]. Molecular dynamics simulations reveal that while the intrinsic ionic strength decreases as [CO 2 ] increases, there is a concomitant increase in ionic mobility upon CO 2 addition that contributes to stable solution conductivities up to 4 M CO 2 . Taken together, these results shed light on the mechanisms underpinning electrolyte conductivity in the presence of CO 2 and reveal that the dissolution of CO 2 , although nonpolar by nature, can be leveraged to improve mass transport rates, a result of fundamental and practical significance that could impact the design of next-generation systems for CO 2 conversion. Additionally, these results show that conditions in which ample CO 2 is available at the electrode surface are achievable without sacrificing the conductivity needed to reach high electrocatalytic currents.