An investigation of proton conduction in select PEM’s and reaction layer interfaces-designed for elevated temperature operation

The proton conductivity of several alternative proton exchange membranes, i.e. SPES-40 (a sulfonated polyarylene ether sulfone), SPSS-40 (sulfonated polysulfide sulfone) and SPES-PS (a polyether sulfone post-sulfonated) were studied using a four-probe ac-impedance method as a function of temperature. 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. The results under 100% relative humidity, showed that SPES-40 has similar proton conductivity as Nafion® in the membrane within our experimental conditions. The values for the other membranes investigated were lower. Attempts to correlate these observed differences with parameters such as equivalent weight (EW), water uptake (λ), acidity (pKa), etc. showed that the prime contributor was the difference in microstructure of the membranes. 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®. Single cell PEM polarization curves were measured for both Nafion® 117 and SPES-40 membrane keeping the ionomer in the reaction layer same as the membrane. Comparison of the performance showed similar ohmic polarization characteristics. However, their performance in the low current density activation polarization region indicated poorer oxygen reduction reaction kinetics with SPES-40 material as compared to Nafion®.