Sulfonated Polyphosphazene-Polybenzimidazole Membranes for DMFCs

Proton-conducting membranes were prepared from blends of sulfonated poly[bis(phenoxy)phosphazene] (SPOP) and polybenzimidazole (PBI), where the latter was used as a cross-linking component. The membranes were evaluated in terms of room-temperature water swelling and proton conductivity, methanol permeability at 60°C, and direct methanol fuel cell (DMFC) performance at 60°C with a 1.0 M methanol and air at atmospheric pressure. Initial screening of membranes was carried out using a "sandwich" membrane electrode assembly (MEA) design, where a polyphosphazene film was inserted between two half-MEAs made with Nafion 112. MEAs were also prepared by hot-pressing catalyst electrodes directly onto SPOP-PBI blended films. Comparable current-voltage data were obtained with the two MEA configurations. Membrane performance in a DMFC was dependent on the blend composition (ion-exchange capacity, IEC, of the SPOP and wt % of added PBI). For an 82 μm thick membrane composed of IEC SPOP with 3 wt % PBI, the maximum power density was ( vs. with Nafion 117), while the methanol crossover was 2.6 times lower than that with Nafion 117.