Caveat of High Temperature Accelerated Stability Test for Anion Exchange Membranes

Abstract

Alkaline stability of hydroxide-conducting anion exchange membranes (AEMs) is a significant requirement for practical applications of alkaline membrane fuel cells. Recently, quaternized aryl ether-free polyaromatics with alkaline stable polymer backbone structure have been receiving much attention as promising materials for AEMs.1 Since a prolonged stability test (≥1000 h) is time-consuming and costly, a harsher test condition at more elevated temperature and higher alkaline concentration to push the limit of AEM stability was advised for degradation and durability studies of such membranes. In this study, we investigate alkaline stability of aryl ether-free Diels-Alder polyphenylene (DAPP) with a trimethylammonium cation attached to an electron-donating hexamethylene spacer2 under low and high temperature (80, 120 and 160 °C) alkaline stability test conditions. Although high temperature stability testing may seem to be a useful tool to measure alkaline stability in a short time period, we observed different phenomena when DAPP AEM was subjected to the extreme test conditions in a pressure vessel. Even though DAPP AEM did not show any structural change or significant decrease in ion exchange capacity or hydroxide conductivity in 4 M NaOH at 80 °C after 3000 h, at the elevated temperature of 160 °C, the membrane underwent instant crosslinking resulted in insolubility of the membrane and sharp decrease in water uptake and swelling degree. The source of crosslinking is the low concentration of residual alkyl bromide from the incomplete quaternization process, which often causes initial conductivity decrease of AEMs. The crosslinked membrane did not show any further change in their properties, exhibiting consistent ion exchange capacity and hydroxide conductivity during the follow-up alkaline stability test (4 M NaOH, 80 °C, 1000 h). In this presentation, structural analysis and membrane properties including ion exchange capacity, water uptake, swelling degree, solubility, mechanical properties and hydroxide conductivity of DAPP AEMs after different temperature alkaline stability testing will be discussed to elucidate the unexpected consequence of high temperature accelerated stability test. References E. J. Park, Y. S. Kim. J. Mater. Chem. A, 2018, 6, 15456–15477.M. R. Hibbs. J. Polym. Sci., Part B: Polym. Phys., 2013, 51, 1736–1742.