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 (CO₂) into useful chemicals, but the effect of the dissolution of CO₂ gas on conductivity has received little attention. Here, we report a joint experimental-theoretical study of the properties of acetonitrile-based CO₂-expanded electrolytes (CXEs) that contain high concentrations of CO₂ (up to 12 M), achieved by CO₂ pressurization. Cyclic voltammetry data and paired simulations show that high concentrations of dissolved CO₂ 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 CO₂ pressure: a plateau region of constant conductivity up to ca. 4 M CO₂ and a region showing reduced conductivity at higher [CO₂]. Molecular dynamics simulations reveal that while the intrinsic ionic strength decreases as [CO₂] increases, there is a concomitant increase in ionic mobility upon CO₂ addition that contributes to stable solution conductivities up to 4 M CO₂. Taken together, these results shed light on the mechanisms underpinning electrolyte conductivity in the presence of CO₂ and reveal that the dissolution of CO₂, 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 CO₂ conversion. Additionally, these results show that conditions in which ample CO₂ is available at the electrode surface are achievable without sacrificing the conductivity needed to reach high electrocatalytic currents.