Inorganic-organic hybrid polymers with pendent sulfonated cyclic phosphazene side groups as potential proton conductive materials for direct methanol fuel cells

A synthetic method is described to produce a proton conductive polymer membrane with a polynorbornane backbone and inorganic-organic cyclic phosphazene pendent groups that bear sulfonic acid units. This hybrid polymer combines the inherent hydrophobicity and flexibility of the organic polymer with the tuning advantages of the cyclic phosphazene to produce a membrane with high proton conductivity and low methanol crossover at room temperature. The ion exchange capacity (IEC), the water swelling behavior of the polymer, and the effect of gamma radiation crosslinking were studied, together with the proton conductivity and methanol permeability of these materials. A typical membrane had an IEC of 0.329 mmol g^-1 and had water swelling of 50 wt%. The maximum proton conductivity of 1.13 × 10^-4 S cm^-1 at room temperature is less than values reported for some commercially available materials such as Nafion. However the average methanol permeability was around 10^-9 cm s^-1, which is one hundred times smaller than the value for Nafion. Thus, the new polymers are candidates for low-temperature direct methanol fuel cell membranes.