Proton conductivity of mesoporous sol–gel zirconium phosphates for fuel cell applications

Abstract

Zirconium phosphate has been extensively studied as a proton conductor for proton exchange membrane (PEM) fuel cell applications. Here we report the synthesis of mesoporous, templated sol–gel zirconium phosphate for use in PEM applications in an effort to determine its suitability for use as a surface functionalised, solid acid proton conductor in the future. Mesoporous zirconium phosphates were synthesised using an acid–base pair mechanism with surface areas between 78 and 177 m² g⁻¹ and controlled pore sizes in the range of 2–4 nm. TEM characterisation confirmed the presence of a wormhole like pore structure. The conductivity of such materials was up to 4.1 × 10⁻⁶ S cm⁻¹ at 22 °C and 84% relative humidity (RH), while humidity reduction resulted in a conductivity decrease by more than an order of magnitude. High temperature testing on the samples confirmed their dependence on hydration for proton conduction and low hydroscopic nature. It was concluded that while the conductivity of these materials is low compared to Nafion, they may be a good candidate as a surface functionalised solid acid proton conductor due to their high surface area, porous structure and inherent ability to conduct protons.