Comb-shaped anion exchange membranes: Hydrophobic side chains grafted onto backbones or linked to cations?

Abstract

In order to investigate the relationship between the side chain topology of comb-shaped anion exchange membranes (AEMs) and membranes properties, two types of hydrophobic side chains attached AEMs that based on QAPPO are synthesized, namely side chains grafted onto backbones and side chains tethered to cations, resulting in AxSB-QAPPO and AxSC-QAPPO AEMs, respectively. Compared with AxSC-QAPPO, the side chains in AxSB-QAPPO AEMs are more inclined to facilitate the membranes to form broad and interconnected ion channels, promoting ion conduction and providing more free volume for the external water to enter the membranes, which are benefit to improving the ionic conductivity and chemical stability of the AxSB-QAPPO samples. At 80 °C, the OH− conductivity of A8SB-QAPPO is 101.2 mS cm−1, while the values for A8SC-QAPPO and QAPPO are only 56.1 and 48.5 mS cm−1 respectively. After the stability test, the weight and IC retentions of A8SB-QAPPO are 83.0% and 77.0%, respectively, while the values for A8SC-QAPPO are 76.5% and 68.0%, respectively, and the values for QAPPO are 56.0% and 42.0%, respectively. Besides, the influence of side chain length on the membranes properties is studied. The chemical stability for both AxSB-QAPPO and AxSC-QAPPO AEMs is enhanced by increasing the side chain length from 5 to 14. After the stability test, the IEC retentions of A5SB-QAPPO, A8SB-QAPPO, A14SB-QAPPO, A5SC-QAPPO, A8SC-QAPPO and A14SC-QAPPO are 60.0%, 78.0%, 90.0%, 49.0%, 70.0% and 75.0%, respectively. While for ionic conductivity, the AEMs with the side chain length of 8 exhibit the highest OH− conductivity. Hence, we speculate that the comb-shaped AEMs with hydrophobic side chains attached to backbones (AxSB-QAPPO) are superior to the membranes with the side chains linked to cations (AxSC-QAPPO), and the membranes performance can be further optimized by adjusting the side chain length.