Tuning Electrode and Separator Sizes For Enhanced Performance of Electrical Double-Layer Capacitors

An electrical double-layer capacitor (EDLC) comprises two porous electrodes sandwiching an electrolyte-permeable separator, which prevents the electrodes from short-circuiting. While previous studies have mainly focused on electrolyte and electrode properties of EDLCs, the device configuration in terms of electrode and separator sizes received less attention, with separators often simplistically modelled as infinitely large reservoirs of ions. Herein, we investigate how the relationship between electrode and separator thicknesses impacts EDLC charging. We find that the assumption of bulk reservoir holds only under specific conditions. Moreover, we identify a tradeoff between stored energy density and pressure variations within the separator, potentially jeopardizing the EDLC durability. We also explore the influence of ionic liquid additives on EDLC charging. While prior research has shown that trace amounts of uncharged additives with strong electrode affinity can significantly enhance energy storage, we observe this effect as negligible for electrodes and separators of comparable sizes. Instead, we show how to optimize EDLC performance by fine-tuning the concentration of additives and separator-to-electrode size ratio to maximize stored energy density.