Abstract
Electric double-layer capacitors (EDLCs) are electrical devices that store
energy by adsorption of ionic species at the inner surface of porous electrodes. Compared
with aqueous electrolytes, ionic liquid and organic electrolytes have the advantage of larger
potential windows, making them attractive for the next generation of EDLCs with superior
energy and power densities. The performance of both ionic liquid and organic electrolyte
EDLCs hinges on the judicious selection of the electrode pore size and the electrolyte
composition, which requires a comprehension of the charging behavior from a microscopic
view. In this Perspective, we discuss predictions from the classical density functional theory
(CDFT) on the dependence of the capacitance on the pore size for ionic liquid and
organic electrolyte EDLCs. CDFT is applicable to electrodes with the pore size ranging
from that below the ionic dimensionality to mesoscopic scales, thus unique for
investigating the electrochemical behavior of the confined electrolytes for EDLC
applications.