Sulfonated poly(ether ether ketone) (S-PEEK) with different degree of sulfonation (DS) has been prepared and evaluated as a proton conducting membrane for electric double layer capacitor (EDLC). The polymer electrolytes prepared with S-PEEK membrane exhibited ionic conductivities about 1.2 x 10(-3)-4.5 x 10(-3) S cm(-1) at room temperature, which depended oil both soaking solvent and degree of sulfonation. The quasi-solid-state EDLCs consisted of activated carbon electrodes and S-PEEK membrane were assembled, and their electrochemical characteristics were Studied by cyclic voltammetry and charge-discharge cycle tests. The effect of DS oil the electrochemical performances of EDLCs has been investigated. (c) 2008 Elsevier Ltd. All rights reserved.