High proton conduction in zirconium silicate/lignin/ionic liquids based- membranes for high temperature PEM fuel cells

Abstract

In this work, novel Nafion-free membranes composed of zirconium silicate, lignin, and ionic liquids supported on polytetrafluoroethylene have been synthesized for high-temperature applications. Incorporating lignin into zirconium silicate at room temperature has significantly increased the zirconium silicate’s proton conductivity by two orders of magnitude, from 10-4 to 10-2S/cm. Furthermore, the addition of ionic liquids caused more enhancement in proton conductivity. Two ionic liquids were studied in this work, 1-Ethyl-3-methylimidazolium bis (trifluoromethyl sulfonyl) imide and 1-Hexyl-3-methylimidazolium tricyanomethanide). The incorporation of 1-Hexyl-3-methylimidazolium tricyanomethanide demonstrated the highest proton conduction at room temperature, reaching 0.106S/cm with an ion exchange capacity of 1.67meq/g. This most conductive membrane has also demonstrated a slight decrease in proton conductivity (10-3S/cm) at 150°C showing a high anhydrous conductivity. The membranes’ conductivity was shown to be stable over time as well. The water uptake and swelling ratio results showed improved water retention and dimensional stability. Additional characterization techniques demonstrated positive effect of lignin and ionic liquids inclusion into zirconium silicate on morphology and mechanical properties. Overall, the reported membranes appear to be suitable for high-temperature proton exchange membrane fuel cell applications.