Solvent Effect on the Pore-Size Dependence of an Organic Electrolyte Supercapacitor

Organic electrolytes such as tetraethylammonium tetrafluroborate dissolved in acetonitrile (TEA-BF4/ACN) are widely used in commercial supercapacitors and academic research, but conflicting experimental results have been reported regarding the dependence of surface-area-normalized capacitance on the pore size. Here we show from a classical density functional theory the dependence of capacitance on the pore size from 0.5 to 3.0 nm for a model TEA-BF4/ACN electrolyte. We find that the capacitance-pore size curve becomes roughly flat after the first peak around the ion diameter, and the peak capacitance is not significantly higher than the large pore average. We attribute the invariance of capacitance with the pore size to the formation of an electric double layer structure that consists of counterions and highly organized solvent molecules. This work highlights the role of the solvent molecules in modulating the capacitance and reconciles apparently conflicting experimental reports.