The environmental stability of a solid silicotungstic acid (SiWA)-H3PO4-poly(vinyl alcohol) (PVA) electrolyte in electrochemical supercapacitor (EC) applications was improved by the incorporation of nano-SiO2. Under ambient conditions, enhanced water retention and improved stability were observed for the electrolyte with SiO2 compared to the electrolyte without SiO2. Due to the strengthening in bond interaction between SiO2 and water molecules, the electrolyte with SiO2 exhibited a more consistent proton conductivity than the one without SiO2. In addition, a cross-linking effect of SiO2 on the PVA polymer was observed as a secondary function. The performance of the SiO2-containing electrolyte was evaluated in a planar double layer EC. The thin solid EC demonstrated high rate capability of 5,000 Vs(-1) in cyclic voltammograms with good environmental stability and excellent cycle life (up to 100,000 cycles demonstrated). (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.053303jes] All rights reserved.