Solid electrolytes which comprise lithium and magnesium triflate ionic salts in polyacrylonitrile (PAN)-based gels are used to fabricate electrochemical double-layer capacitors in conjunction with ethylene carbonate (EC) and propylene carbonate (PC) as plasticizers and high-density graphite (HDG) as polarizable electrodes. The conductivity of the solid electrolytes is around 10(-3) S cm(-1) at ambient temperatures. Composites which consist of PAN + EC + PC + electrolyte are electrochemically stable over a wide potential range. Cells of the type: HDG/PAN-EC-PC-Li and Mg-triflate/HDG exhibit single-electrode discharge capacitance values of 480 and 383 muF cm(-2), respectively, with attractive charge-discharge characteristics.