Solvent Effect on the
Pore-Size Dependence of an Organic
Electrolyte Supercapacitor
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Abstract
Organic electrolytes such as tetraethylammonium tetrafluoroborate
dissolved in acetonitrile (TEA-BF<sub>4</sub>/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-BF<sub>4</sub>/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