A divalent electrolyte salt based on 1,1,4,4-tetramethyl piperazine has been synthesized and applied in electric double-layer capacitors (EDLCs). Traits of the divalent salts have been accessed as well as monovalent quaternary alkyl ammonium salts by the means of galvanostatic charge–discharge tests and ionic conductivity measurements. Compared with monovalent salts, the divalent salts do enlarge the charge storage ability of EDLCs remarkably. However, highly concentrated charge density on the divalent cation has a strong interaction with the organic solvent of propylene carbonate. The adverse effect of this heavy solvation on the performance of EDLCs has been investigated. Moreover, the influence of pore size distribution on the storage ability of these cations at the porous carbon electrode has been addressed.