Proton transport characteristics for selected PEM's, designed for elevated temperature operation: Effect of relative humidity

Abstract

The proton conductivity of several new alternative proton exchange membranes i.e SPES-40 (a sulfonated poly arylene ether sulfone), SPSS-40 (sulfonated poly sulfide sulfone) and SPES-PS (a polyether sulfone post sulfonated) were studied using a four-probe ac-impedance method as a function of temperature and relative humidity (RH). Further, proton conductivity was also investigated for the same ionomers in the form of micro-aggregates such as those typically encountered in the reaction layer (the interfacial layer of the electrode containing the catalyst). For this a new configuration of the conventional reaction layer in a membrane electrode assembly (MEA) was used, which enabled the isolation of proton conductivity to be the principle contributor to the ac impedance. Conductivity of all membranes showed strong dependence on temperature and RH. The results under 100% relative humidity, showed that SPES-40 has similar proton conductivity as Nafion in the membrane within the range our experimental conditions. The values for the other membranes investigated were lower. Conductivity values increase with the increase of temperature and RH for all membranes investigated. However at lower relative humidity, all of the alternative membranes have lower conductivity performance comparing to Nafion 117. Attempts to correlate these observed differences with parameters such as EW (equivalent weight), λ, acidity (pK a) etc., showed that the prime contributor was the difference in microstructure of the membranes. Activation energy for proton conduction in these different polymer membranes based on Arrhenius plots showed similar values. Conductivity of these polymeric ionomers when present as micro-aggregates in the reaction layer showed very different values as compared to the bulk membranes. There was a great divergence in conduction as a function of increase in temperature with Nafion® showed a far greater rate of increase of conductivity than SPES-50 and SPES-PS. Blends of these ionomers with Nafion showed intermediate values, albeit lower with characteristics closer to Nafion.