Self assembled 12-tungstophosphoric acid–silica mesoporous nanocomposites as proton exchange membranes for direct alcoholfuel 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 PW₁₂O₄₀³⁻ 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 μm 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).