Sulfonated poly(ether ether ketone) (S-PEEK) was synthesized by sulfonation of commercial PEEK. The S-PEEK membrane was prepared by casting from organic solution dissolving the polymer, and the polymer electrolyte was obtained by soaking the S-PEEK membrane in water or sulfuric acid solution. The effect of soaking solvent on the liquid uptake and the ionic conductivity of the polymer electrolyte has been investigated. The quasi-solid-state electric double layer capacitors (EDLCs) which consisted of activated carbon electrodes and polymer electrolyte were assembled, and their electrochemical characteristics were studied by cyclic voltammetry and charge–discharge cycle tests.