Oligo (ethylene glycol)-grafted poly(terphenyl indole piperidinium) with high water diffusivity for anion exchange membrane fuel cells

Abstract

The water diffusivity of polymeric anion exchange membranes (AEMs) plays a critical role in determining the AEM fuel cells (AEMFCs) performance because it generates an imbalance in the amount of water at the anode and cathode. However, the correlation between water diffusivity and the chemical structure of AEMs remains unclear. This study focuses on a series of oligo(ethylene glycol) (OEG) functionalized poly(arylene indole piperidinium) AEMs, in which OEG grafts are directly attached onto the polymer backbones. The polar and flexible OEG side chains afford a microphase-separated morphology, close polymer-chain packing, and extra sites for water–ion transport, resulting in high ion-exchange-capacity (IEC)-normalized conductivity, improved water diffusivity, and dimensional stability. Moreover, these side chains provide excellent ex-situ stability in a 1 M NaOH solution (80 °C) for 1080 h, surpassing the stability of its alkyl side chain counterpart (PITP-C10Q85) by a significant margin. The resultant PITP-OEG-85 with the highest water diffusivity exhibits a highest peak power density of 1.23 W cm−2 at 80 °C despite its low IEC value and conductivity. In-situ durability tests of this membrane at 0.2 A cm−2 for 24 h revealed the degradation of piperidinium under harsh conditions (70 % anode relative humidity at 60 °C). This work opens a new avenue for designing highly conductive and alkali-stable AEM materials with high water diffusivity to realize high-performance AEMFCs.