Development and characterization of crosslinked PPO-based anion exchange membranes for AEM fuel cells

Abstract

This work describes the synthesis and characterization of a series of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)-based anion exchange membranes (AEMs), crosslinked with bis-imidazolium cations, and investigated as potential ionic conductors for AEM fuel cells. PPO has excellent mechanical and thermal properties, and its backbone can be functionalized by means of several possible routes. Among them, bromination was carried out with a quantitative control of the amount of the linked halogens. PPO with bromination degrees of 14 mol% and 33 mol% were crosslinked by employing a new procedure with 1,6-bis(2-methylimidazol-1-yl)hexane (bimh). This crosslinking method granted a fast and highly efficient crosslinking reaction, resulting in an almost quantitative substitution of bromine atoms (higher than 94%). The obtained cast membranes were thoroughly characterized by thermogravimetry, 1H NMR, ATR-FTIR and EDX spectroscopies, and their properties were assessed through the evaluation of water uptake (WU) and swelling ratio (SR), as well as of alkaline stability and ex-situ ionic conductivity by means of electrochemical impedance spectroscopy (EIS). WU and EIS tests were conducted as a function of temperature in the 20–100 °C range and in conditions of complete hydration (95% relative humidity). The prepared AEMs appeared well-uniform, with water retention and ion-carrying abilities properly controlled and affected by the increment of the bromination degree of PPO. Water uptake and ionic conductivity rose while increasing the crosslinking degree, up to a maximum of 22.3% and 140.6 mS cm−1 at 100 °C, respectively, as the consequence of the increased number of sites that can anchor OH− ions.