Novel Hydrocarbon-Based Electrolytes for Fuel Cells

Novel sulfonated polyarylenethioethersulfone (SPTES) polymers have been developed at the Air Force Research Laboratory (AFRL) which are capable of significantly improving the performance and affordability of polymer electrolyte fuel cells. These high molecular weight polymers have an aromatic backbone with high sulfonic acid content along with bulky phenyl-based end-capping agents. These polymers are strong, chemically inert and thermally stable. SPTES polymer system obtained by solvent casting with 50% degree of sulfonation (SPTES-50) exhibited high proton conductivity (> 100mS/cm) at 65 °C and 85% RH. Single cell analysis using a membrane electrode assembly (MEA) has shown high proton mobility. One of the drawbacks of this system is the high water uptake at operating conditions. High swelling is potentially detrimental and could weaken its structural integrity during thermal cycling by making it prone to developing microcracks or pores and reducing its durability. In order to overcome this problem, AFRL has developed the next generation SPTES polymer system (6F-SPTES) incorporating hydrophobic moieties onto the thermoplastic backbone to enhance its durability by maintaining high water stability and retaining high ionic conductivity. These polymers exhibit good mechanical, chemical and thermal properties. Electrochemical evaluation was performed using non-linear regression analysis to obtain Tafel parameters. Tafel parameters obtained indicate electrochemical performance comparable to Nafion. Catalyst utilization for 6F-SPTES MEAs using conventional electrode inks with perfluorinated binders was similar to that exhibited by Nafion. Overall fuel cell performance was on par with Nafion at both low and high temperatures. The high thermal stability and high intrinsic proton conductivities of 6F-SPTES-50 qualify these polymers to be potential alternatives to Nafion as electrolyte separators for fuel cells.