Gas Transport in Proton Exchange Membranes for use in Fuel Cell Applications
The objectives of this research were to study the gas transport properties of proton exchange membranes (PEM), namely disulfonated poly(arylene ether sulfone) (BPSH-35), post sulfonated diels-alder poly(phenylene) (SDAPP), and poly(perfluoro sulfonic acid) (Nafion). The O2 gas permeabilities were found to be lower in BPSH and SDAPP as compared to poly(perfluoro sulfonic acid) because of difference in Tg (TgBSPH= 250 oC, TgSDAPP= 330 oC versus TgNafion=150 oC). Higher Tg polymers have a more rigid, inflexible polymer segments causing a reduction in gas permeability. In comparison to SDAPP, BPSH has a lower O2 gas permeability because of the bulky side groups in the SDAPP backbone.
O2 sorption measurements were carried out both under non-humidified and humidified conditions as a function of relative humidity and temperature at a normal PEM operating pressure of 1 atm. Under non-humidified conditions, BPSH, SDAPP, and Nafion 112 exhibited Henry's Law sorption, consistent with dilute dissolution of O2 into the polymer matrix. The enthalpies of sorption were calculated to determine the interaction of O2 with each membrane. The sorption enthalpies in BPSH and SDAPP increased with increasing pressure indicating the formation of more O2-O2 interactions. The enthalpies in Nafion 112 were relatively constant with increasing pressure. In the presence of moisture, the sorption behavior changed from Henry's Law to Type IV sorption behavior, which is common in hydrophilic polymers. The SDAPP membrane was found to have the highest percent wet O2 mass uptake because of a higher number of sulfonic acid groups interacting with the water/O2 system.
Finally the O2 sorption for various porous catalyst powders, consisting of platinum supported on carbon was measured in the non-humidified and humidified state. The catalysts were found to have Knudsen diffusion in the non-humidified state with 20 wt% Pt-C having the largest O2 sorption. In the humidified state, the highest O2 mass uptake was achieved with 40 wt% Pt-C. These results are explained in terms of the trade-off between catalyst dispersion and catalyst size. Furthermore, O2 sorption measurements were utilized for membrane electrode assemblies containing 40 wt% Pt-C and hot pressed at 210 oC for BPSH-35 (25 and 80K) and Nafion 112 membranes. The same sorption behavior occurred in the MEAs as in the neat membrane, but at a lower capacity. This is because the electrode introduces a more tortuous path to the gas molecules permeating across the membrane.