Exploring the acid enrichment application of piperidinium-functionalized cross-linked poly(2,6-dimethyl-1,4-phenylene oxide) anion exchange membranes in electrodialysis

Abstract

The use of electrodialysis (ED) fabricated with proton blocking anion exchange membranes (AEMs) is a promising method to realize waste acid reclamation. To develop AEMs with low proton leakage, in this work, we designed a series of N-methylpiperidine (MPRD)-functionalized poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) cross-linked by weak base 1,3-di-4-piperidylpropane (DiPRD). Our investigations demonstrate the weak base cross-linker could enhance the density of membrane matrix and reduce the hydrophilicity of AEM matrix, which have been verified by thermogravimetric analysis and water absorption with low swelling ratio of 2.43%. Benefit from the unique structure with denser membrane matrix and additional protonated weak base piperidine structure, the acid blocking performance of the optimized crosslinked AEM was enhanced with the H+ concentration of 1.98 M in concentrate cell during 24 h-ED process at current density of 20 mA cm−2 (initial H+ concentration: 1.0 M), outperforming the AEM without cross-linking structure (1.73 M). Nevertheless, it was found that the small-sized proton diffusion in ion-nanochannels of AEM and the aged membrane matrix in ED process with high sulfuric acid concentration resulted in proton leakage to some degree. As a result, a trade-off between the proton blocking and sulfate ion transport of the cross-linked AEMs is achieved. It is believed that this study can provide a guideline for designing advanced proton blocking AEMs.