Fabrication and characterization of heteropolyacid (H₃PW₁₂O₄₀)/directly polymerized sulfonated poly(arylene ether sulfone) copolymer composite membranes for higher temperature fuel cell applications

The feasibility of heteropolyacid (HPA)/sulfonated poly(arylene ether sulfone) composite membranes for use in proton exchange membrane (PEM) fuel cells was investigated. Partially disulfonated poly(arylene ether sulfone)s (BPSH) copolymers were prepared by direct aromatic nucleophilic copolymerization and solution-blended with a commercial HPA, phosphotungstic acid. Fourier transform infrared (FTIR) spectroscopy band shifts showed that sulfonic acid groups on the polymer backbone interact with both bridging tungstic oxide and terminal tungstic oxide in the phosphotungstic acid molecule, indicative of an intermolecular hydrogen bonding interaction between the copolymer and the HPA additive. The composite membranes generally exhibited a low HPA extraction after water vapor treatment, except for the 60mol% disulfonated BPSH where significant HPA extraction from the composite membrane occurred because of excessive matrix swelling. The composite membrane not only had good thermal stability (decomposition temperature in nitrogen >300°C), but also showed improved mechanical strength and lower water uptake than the unfilled membranes possibly due to the specific interaction. The composite membranes displayed good proton conductivity especially at elevated temperatures (e.g. 130°C). For example, fully hydrated membranes consisting of 30wt.% HPA and 70wt.% BPSH with 40mol% disulfonation had a conductivity of 0.08S/cm at room temperature which linearly increased up to 0.15S/cm at 130°C. In contrast, the pure copolymer had a proton conductivity of 0.07S/cm at room temperature only reached a maximum conductivity of 0.09S/cm, most probably due to dehydration at elevated temperatures. The dehydration process was monitored by dynamic infrared spectra by observing the intensity reduction of the sulfonate group and distinctive changes of shape in the hydroxyl vibrations as the sample was heated. Combining infrared results with dynamic thermogravimetric data showed that the composite membrane had much higher water retention from 100 to 280°C than the pure sulfonated copolymer. Those results suggested that the incorporation of HPA into these proton conducting copolymers should be good candidates for elevated temperature operation of proton exchange membrane fuel cells. Application to operating fuel cells at high temperatures is now being investigated.