Self assembled 12-tungstophosphoric acid-silica mesoporous nanocomposites as proton exchange membranes for direct alcohol fuel cells

Abstract

A highly ordered inorganic electrolyte based on 12-tungstophosphoric acid (H3PW12O40, abbreviated as HPW or PWA)–silica mesoporous nanocomposite was synthesized through a facile one-step self-assembly between the positively charged silica precursor and negatively charged PW12O40 3- species. The self-assembled HPW–silica nanocomposites were characterized by small-angle XRD, TEM, nitrogen adsorption–desorption isotherms, ion exchange capacity, proton conductivity and solid-state 31P NMR. The results show that highly ordered and uniform nanoarrays with long-range order are formed when the HPW content in the nanocomposites is equal to or lower than 25 wt%. The mesoporous structures/textures were clearly presented, with nanochannels of 3.2–3.5 nm in diameter. The 31P NMR results indicates that there are ( SiOH2 +)(H2PW12O40 -) species in the HPW–silica nanocomposites. A HPW–silica (25/75 w/o) nanocomposite gave an activation energy of 13.0 kJ mol-1 and proton conductivity of 0.076 S cm-1 at 100 ◦C and 100 RH%, and an activation energy of 26.1 kJ mol-1 and proton conductivity of 0.05 S cm-1 at 200 ◦C with no external humidification. A fuel cell based on a 165 mm thick HPW–silica nanocomposite membrane achieved a maximum power output of 128.5 and 112.0 mW cm-2 for methanol and ethanol fuels, respectively, at 200 ◦C. The high proton conductivity and good performance demonstrate the excellent water retention capability and great potential of the highly ordered HPW–silica mesoporous nanocomposites as high-temperature proton exchange membranes for direct alcohol fuel cells (DAFCs).