Controlling the sub-nano ion channels by crosslinking in PVDF-based anion exchange membrane for enhanced mono/bivalent anion permselectivity and acid reclamation

Abstract

Tailoring hydrophilicity in poly (vinylidene fluoride) (PVDF) by ozone pre-treatment forms nanophase-separated ion-conducting membranes with the potential to separate mono/bivalent anions and recover acids with high purity. Herein, six PVDF-based anion exchange membranes (AEMs) with three different architectures with varying chemical structures are fabricated to evaluate their effect on Cl−/SO42− separation performance and the H+/Mn + selectivity. The methodologies used to prepare AEMs from the graft copolymer are: (i) pre-quaternization with an alkyl spacer forming a partially quaternized membrane, (ii) crosslinking in the partially quaternized membranes, (iii) insitu quaternization and crosslinking of the graft copolymer. These three different engineering aspects induce different effects of hydrophobicity, free volumes, hydration numbers, and sub-nanometer ion channels with varying diameters in the membrane matrix. The membrane QC-AEM-1 with sub-nano ion channels of a lower diameter, high surface resistance, and low hydration number shows the highest Cl−/SO42− selectivity of 48.7. Q-AEM-2 shows the maximum UH + value of 49 × 10−3 m h−1 whereas QC-AEM-2 shows the highest S value of 69 using HCl/FeCl2 mixture, whereas the crosslinked AEMs without any pre-quaternization show the lowest PSO42−Cl−, UH+, S values. Our results demonstrate that using a perfluorinated membrane backbone such as PVDF is enough to impart a regulated hydrophobicity for optimum selectivity. We believe that controlling the degree of grafting, pre-quaternization, and crosslinking constructs stable sub-nano ion channels with regulated diameters that selectively separate mono/bivalent anions.