Highly Anion Conductive Polymers: How Do Hexafluoroisopropylidene Groups Affect Membrane Properties and Alkaline Fuel Cell Performance?

Abstract

Novel anion conductive aromatic copolymers containing hexafluoroisopropylidene groups as the hydrophobic component and fluorenyl groups substituted with pendant hexyltrimethylammonium groups as the hydrophilic component were synthesized and characterized. Precursor copolymers, BAF-AF, were synthesized by a nickel(0) promoted polycondensation reaction and had a high molecular weight (Mn = 10–12 kDa, Mw = 77–115 kDa). Quaternization of BAF-AF using dimethyl sulfate gave tough and bendable thin BAF-QAF membranes having the ion exchange capacity (IEC) from 1.3 to 2.4 mequiv g–1 by solution casting. The morphology of BAF-QAFs was investigated by TEM images and SAXS profiles, and a nanoscale fine phase-separated structure was confirmed. The BAF-QAF membrane with IEC of 2.4 mequiv g–1 showed a superior OH– conductivity (134 mS cm–1 at 80 °C) in water. The membranes retained high conductivity under strongly alkaline conditions (∼4 M KOH at 80 °C) for 1000 h. An H2/O2 anion alkaline fuel cell using the BAF-QAF membrane and binder achieved the maximum power density of 319 mW cm–2 at 702 mA cm–2 at 60 °C and 100% RH. Hexafluoroisopropylidene groups contributed to improving membrane properties as anion exchange membranes for alkaline fuel cells.