PWA-doped PEG/SiO2 proton-conducting hybrid membranes for fuel cell applications

A series of hybrid membranes were prepared through acid-catalyzed sol-gel process from alkoxysilane-endcapped poly(ethylene glycol) (PEG) precursor, which was further modified with monophenyl trimethoxysilane (MPh) during the course of sol-gel reaction. Phosphotungstic acid (PWA) was doped into hybrid matrix at different levels to provide the resulting membrane with proton conducting behavior. Water uptake, proton conducting property, methanol permeability and microstructure of these hybrid membranes were investigated. The 29Si solid-state NMR analysis of hybrid membranes showed that doping of PWA into hybrid matrix led to different stereo-chemical structures. PWA doping indirectly promotes formation of TP3 (PhSi(OSi)3) structure during sol-gel reaction and the relative contribution of each structure was calculated quantitatively. The water uptake was decreased as both MPh and PWA content increased. The proton conductivity of water-swollen membranes was in the order of 10−3 to 10−5 S cm−1. The methanol permeability of hybrid membranes reduced markedly beyond 50 phr PWA content and which is about more than one order lower than that of Nafion 115. Finally, the feasibility of these hybrid membranes as electrolyte in proton conducting membrane fuel cells (PEMFC) was investigated and the preliminary results were made into comparison with similar hybrid membranes and Nafion 115. A maximum current density of 32 mA cm−2 obtained with PWA-doped PEG/SiO2 hybrid membrane in hydrogen/oxygen (H2/O2) mode at ambient conditions is comparable with that of similar hybrid membrane which was evaluated at high temperature and pressure.