Solid-polymer electrolytes comprised of silicotungstic acid (SiWA) in a polyvinyl alcohol (PVA) matrix were developed for electrochemical capacitors (ECs). Addition of phosphoric acid has further enhanced the ionic conductivity of a SiWA-PVA electrolyte. The proton conductivity of the phosphoric acid modified electrolyte reached 0.01 Scm(-1) with good stability. Both SiWA-PVA and H(3)PO(4)-modified SiWA-PVA electrolytes conducted protons via a hopping mechanism. All-solid ECs relying on the modified polymer electrolyte using graphite electrodes demonstrated high rate performance. The devices were able to charge and discharge up to 100 V/s in cyclic voltammetry, exceeding their liquid-based counterpart. Thermal and structural characterizations revealed that the addition of phosphoric acid enhanced the thermal stability of the polymer electrolyte and promoted the retention of crystallized water without altering the bonding structure of the original SiWA-PVA network. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.064112jes] All rights reserved.