Quaternized poly (2, 6-dimethyl-1, 4-phenylene oxide) anion exchange membranes based on isomeric benzyltrimethylammonium cations for alkaline fuel cells

Abstract

Benzyltrimethylammonium (BTMA) is most frequently-used organic cations in anion exchange membrane (AEM) materials. However, BTMA-based AEMs always suffer from low ionic conductivity and insufficient alkaline stability for practical alkaline fuel cells. Here, we present a systematic investigation of a series of side-chain-type poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) AEMs with constitutional isomerism in BTMA cations. Three isomeric BTMA cations, e.g. meta-BTMA, ortho-BTMA, and para-BTMA, were tethered onto PPO backbones via a flexible spacer using CuAAC reaction, producing side-chain-type AEMs, namely m-QPPO, o-QPPO, and p-QPPO membranes, respectively. As expected, side-chain-type PPO AEMs displayed higher hydroxide conductivity as compared to a control PPO-QA membrane where BTMA cations were directly linked on PPO backbones, due to the microphase-separated morphologies as confirmed by small-angle X-ray scattering (SAXS) results. Although these isomeric quaternized PPO copolymers had identical chemical composition and polymer architectures, they did not share similar properties. Specifically, among three side-chain-type AEMs, the highest hydroxide conductivity of 42.8 mS/cm was observed for m-QPPO membrane having meta-BTMA cations with an ion exchange capacity of 1.93 meq./g at 20 °C, as a result of its high water uptake. In addition to high conductivity, m-QPPO membrane showed superior alkaline stability with respect to o-QPPO and p-QPPO membranes. After 200 h of aging in 1 M NaOH at 60 °C, 85% of the hydroxide conductivity was retained for m-QPPO AEMs, while more than 30% conductivity loss was observed for o-QPPO and p-QPPO membranes. NMR analysis of the aged membrane suggested that SN2 nucleophilic substitution at benzyl groups is the dominant degradation mechanisms. Furthermore, the AEM fuel cells using these PPO AEMs with isomeric BTMA cations were investigated, and the cell with highly conductive and durable m-QPPO membranes exhibited the best performance with a peak power density of 333 mW/cm2 at a current density of 700 mA/cm2 at 60 °C, comparable to other AEMFCs with PPO-based AEMs. Consequently, this work not only provides a facile and effective strategy to precisely synthesize isomeric AEMs, but also contributes to fundamental insights into the structure-property relationship as well as alkaline fuel cell performance for these isomeric BTMA-based AEMs, which are not explored before.