Entropy Drives Accelerated Ion Diffusion upon Carbon Dioxide Expansion of Electrolytes

Carbon dioxide-expanded liquids, organic solvents with high concentrations of soluble carbon dioxide (CO₂) at mild pressures, have gained attention as green catalytic media due to their improved properties over traditional solvents. More recently, carbon dioxide-expanded electrolytes (CXEs) have demonstrated improved reaction rates in the electrochemical reduction of CO₂, by increasing the rate of delivery of CO₂ to the electrode while maintaining facile charge transport. However, recent studies indicate that the limiting behavior of CXEs at higher CO₂ pressures is a decline in solution conductivity due to reduced polarity, leading to poorer charge screening and greater ion pairing. In this article, we employ molecular dynamics simulations to investigate the energetic driving forces behind the diffusive properties of an acetonitrile and tetrapropylammonium hexafluorophosphate (TPrAPF₆) CXE with increasing CO₂ concentration. Our results indicate that entropy drives solvent and electrolyte diffusion with increasing CO₂ pressure. The activation energy of ion diffusion increases with higher concentrations of CO₂, indicating that increasing the temperature may improve solution conductivity in these systems. This trend in the activation energies is traced to stronger cation-anion Coulombic interactions due to weaker solvent screening at high CO₂ concentrations, suggesting that the choice of ion may provide a route to diminish this effect.