Electric Double‐Layer Capacitor Composed of Activated Carbon Fiber Cloth Electrodes and Solid Polymer Electrolytes Containing Alkylammonium Salts

Solid polymer electrolytes consisting of complexes of poly(ethylene oxide)‐grafted poly(methyl)‐methacrylate (PEO‐PMMA) and tetraalkylammonium salts [tetrabutylammonium perchlorate, tetraethylammonium perchlorate, and tetraethylammonium tetrafluoroborate (TEABF 4 )] have been investigated for electric double‐layer capacitors with activated carbon fiber cloth electrodes. The PEO‐PMMA and tetraalkylammonium composites obtained showed high ionic conductivity (>10 −4 S cm −1 at 298 K). The ionic conductivity depended on both the concentration and the size of each ion. The composites had good stability over a wide potential range ( ca. 5.0 V). When the PEO‐PMMA and TEABF 4 composites were used in solid‐state electric double‐layer capacitors with activated carbon fiber cloths as polarizable electrodes, the capacitors showed charge/discharge behavior with large values of capacitance and high coulombic efficiency. The long voltage retention was observed in the self‐discharge test of the capacitor with TEABF 4 .