Poly (aryl piperidinium) anion exchange membranes for acid recovery: The effect of backbone structure

The anion exchange membrane (AEM)-based diffusion dialysis is considered as an efficient strategy for the recovery of acid. Recently, the novel ether-free poly (aryl piperidinium) (PAP) AEMs modified by side chain grafting has been applied in the area of acid separation, showing excellent acid flux. However, there is little study focusing on the influence of polymer backbone structures of AEMs in acid separation. Herein, a series of PAP AEMs with different backbone arrangements based on diphenyl, m-terphenyl, and p-terphenyl were prepared for acid separation via diffusion dialysis. Compared with diphenyl and p-terphenyl, the m-terphenyl-based AEM exhibits a fantastic comprehensive performance, with a dialysis coefficient of 20.1 × 10-3 m h−1 and H+/Fe2+ selectivity of 196. The acid flux and selectivity can be kept after 7 days of acidic stability test in 1 mol/L HCl and 0.25 mol/L FeCl2. This can be attributed to its unique spatial configuration that induces a microphase-separated structure, which is beneficial to H+ conduction. Meanwhile, its high rigidity and low solubility could restrain the water uptake and swelling, and thus reduce the Fe2+ flux and enhance the selectivity. Furthermore, the stability of all PAP AEMs was tested in 2 mol/L HCl at 60 °C for 60 days, and their ionic conductivities still remained above 90 %. This study highlights a simple and reliable strategy to enhance the acid separation performance of AEMs via the facile backbone design.