Sulfonic-functionalized heteropolyacid-SiO2 nanoparticles were synthesized by grafting and oxidizing of a thiol-silane compound onto the heteropolyacid-SiO2 nanoparticle surface. The surface functionalization was confirmed by solid-state NMR spectroscopy. The composite membrane containing the sulfonic-functionalized heteropolyacid-SiO2 nanoparticles was prepared by blending with Nafion (R) ionomer. TG-DTA analysis showed that the composite membrane was thermally stable up to 290 degrees C. The DMFC performance of the composite membrane increased the operating temperature from 80 to 200 degrees C. The function of the sulfonic-functionalized heteropolyacid-SiO2 nanoparticles was to provide a proton carrier and act as a water reservoir in the composite membrane at elevated temperature. The power density was 33 mW cm(-2) at 80 degrees C, 39 mW cm(-2) at 160 degrees C and 44 mW cm(-2) at 200 degrees C, respectively. (c) 2005 Elsevier B.V. All rights reserved.