A Highly Hydroxide Conductive, Chemically Stable Anion Exchange Membrane, Poly(2,6 dimethyl 1,4 phenylene oxide)-b-Poly(vinyl benzyl trimethyl ammonium), for Electrochemical Applications

Abstract

A chemically stable copolymer [poly(2,6 dimethyl 1,4 phenylene oxide)-b-poly(vinyl benzyl trimethyl ammonium)] with two ion exchange capacities, 3.2 and 2.9 meq g−1, was prepared as anion exchange membranes (AEM-3.2 and AEM-2.9). These materials showed high OH− conductivities of 138 mS.cm−1 and 106 mS.cm−1, for AEM-3.2 and AEM-2.9 respectively, at 60°C, and 95% RH. The OH− conductivity = 45 mS.cm−1 for AEM-3.2 at 60% RH and 60°C in the absence of CO2. Amongst the ions studied, only OH− is fully dissociated at high RH. The lower Ea = 10–13 kJ.mol−1 for OH− compared to F− ∼ 20 kJ.mol−1 in conductivity measurements, and of H2O from self-diffusion coefficients suggests the presence of a Grotthuss hopping transport mechanism in OH− transport. PGSE-NMR of H2O and F− show that the membranes have low tortuosity, 1.8 and 1.2, and high water self-diffusion coefficients, 0.66 and 0.26 × 10−5 cm2.s−1, for AEM-3.2 and AEM-2.9 respectively. SAXS and TEM show that the membrane has several different sized water environments, ca. 62 nm, 20 nm, and 3.5 nm. The low water uptake, λ = 9–12, reduced swelling, and high OH− conductivity, with no chemical degradation over two weeks, suggests that the membrane is a strong candidate for electrochemical applications.