Imidazolium Cation Based Anion-Conducting Electrolyte Membranes Prepared by Radiation Induced Grafting for Direct Hydrazine Hydrate Fuel Cells

Abstract

Graft-type anion-conducting electrolyte membranes (AEMs) with imidazolium cations on graft polymers were synthesized through radiation-induced graft polymerization of N-vinylimidazole (NVIm) on poly(ethylene-co-tetrafluoroethylene) (ETFE) films, followed by N-propylation and ion-exchange reactions. The N-propylation proceeded quantitatively, whereas the ion-exchange reactions in 1 M KOH at 60°C were accompanied by partial β-elimination of the imidazolium cations to yield copolymer grafts consisting of poly-(1-vinyl-3-propylimidazolium hydroxide) and double bonds (polyethyne) (AEM2), which exhibited an ion-exchange capacity (IEC) of 0.85 mmol g−1 and ionic conductivity of 10 mS cm−1. AEM2 showed alkaline stability at 60°C but it gradually degraded at 80°C for ca. 150 h. The copolymer-type AEM (AEM3) with an IEC of 1.20 mmol g−1 was prepared through the copolymerization of NVIm with styrene on ETFE films, followed by the same N-propylation and ion-exchange reactions. AEM3 was characterized to be a terpolymer, owing to partial β-elimination and showed higher alkaline durability in 1 M KOH at 80°C. As a result, it exhibited higher conductivity (>10 mS cm−1) for 250 h. Therefore, alkylimidazolium cations in copolymer grafts are a promising anion conducting group for alkaline-durable AEMs. A maximum power density of 75 mW cm−2 is obtained for AEM3 in a direct hydrazine hydrate fuel cell.