We investigated the charge storage ability of electrolytes containing polyaniline and polyaniline-like molecules, and indicate the possibility of increasing the energy density of electric double layer capacitors (EDLC) by applying this electrolyte. Cyclic voltammetry and galvanostatic charge/discharge measurements show that polyaniline and most of the analogous compounds can be used as reversible charge storage materials in propylenecarbonate (PC) and/or γ-butyrolactone (GBL) which are widely used as electrolytes of EDLCs and batteries. Especially N,N′-diphenylphenylenediamine (NNDP) is highly soluble in GBL and PC, and is able to store large amount of electric charge by its two-step redox reaction. With an NNDP/GBL electrolyte solution, about 30% redox capacity is added to the electric double layer capacity of an activated carbon electrode. The shuttle effect with the NNDP cation is effectively suppressed by using an ion-exchange membrane as separator. Although further improvement in solubility and n-type charge storage materials are needed to effectively increase the energy density, we expect that our new system can double the amount of energy density of current EDLCs.
