Hydroxide Conducting and Proton Conducting Poly(ionic liquids)

Abstract

Ion conducting polymers are key components in fuel cells, e.g., as solid-state electrolyte membrane separators and ionomers in electrodes. In this study, a series of novel poly(ionic liquid) (PIL) homopolymers and block copolymers were synthesized with desired customizable physicochemical properties (e.g., high ion conductivity, excellent alkaline chemical stability, high O2 solubility) to address current limitations in the advancement of long-lasting, low-cost fuel cells. For alkaline fuel cells, styrene-based PIL homopolymers with various covalently attached saturated N-heterocyclic cations (i.e., methylpyrrolidinium, methylpiperidinium, methylazepanium, methylazocanium, methylazonanium) were successfully synthesized. High alkaline chemical stability was observed for PILs with 5-, 6-, 7-, 8-membered cation rings, suggesting the potential of PILs containing saturated N-heterocyclic cations as building blocks for anion exchange membranes. Subsequently, commercially scalable PIL pentablock terpolymers containing styrene/saturated N-heterocyclic cations were synthesized and demonstrated high ion conductivity and high alkaline chemical stability. Long-lasting alkaline fuel cell performance was demonstrated with these polymers as robust membrane separators. For proton exchange membrane fuel cells, a series of proton-conducting sulfonated PIL block copolymers were successfully synthesized as a promising alternative to the widely used expensive perfluorosulfonic acid ionomers (i.e. Nafion). The resulting block polymers contain both mobile cations (H+) and mobile anions (TFSI-), and exhibit tunable physical and ion transport properties via different PIL block compositions. High proton conductivity (comparable to Nafion) and substantial improvement in hydrogen fuel cell performance were achieved by the incorporation of sulfonated PIL block copolymer as an ionomer in the electrodes. Overall, this work demonstrates the successful design of hydroxide conducting and proton conducting polymers based on a diverse set of chemistries with high ion transport and enhanced hydrogen fuel cell performance.