Poly(terphenyl pyridine) based amphoteric and anion exchange membranes with high ionic selectivity for vanadium redox flow batteries

Abstract

A membrane with excellent ionic selectivity is crucial for achieving high performance in vanadium redox flow batteries (VFBs). In this study, ether-free poly(terphenyl acetylpyridine) (PTAP) with abundant pyridine groups is synthesized and utilized as the matrix. Through the Menshutkin reaction, ethanesulfonic acid side chains and quaternary ammonium side chains are separately grafted onto PTAP to form amphoteric ion exchange membranes (AIEMs, PTAP-x%BSA) and anion exchange membranes (AEMs, PTAP-y%QA), using 2-bromoethanesulfonic acid and 3-bromo-N,N,N-trimethylpropan-1-aminium bromide as grafting reagents. Simultaneously, the quaternization process facilitates the formation of pyridinium cations within the polymer framework. The introduction of hydrophilic and flexible side chains enables membranes with microphase separation structure. The chemical structure of side chain end group significantly impacts the properties of membranes. The amphoteric ion structure endows PTAP-50 %BSA with enhanced intermolecular interaction and limited volume swelling. Compared with the PTAP-50 %QA AEM with bicationic groups, the PTAP-50 %BSA AIEM with amphoteric ions exhibits much higher ion selectivity (1.13 × 10⁶ S min cm⁻²) due to its ultra-low vanadium ion permeability (2.31 × 10⁻⁸ cm² min⁻¹). Additionally, the PTAP-50 %BSA AIEM demonstrates low area resistance (0.22 Ω cm²), high tensile strength (39.7 MPa) and excellent chemical stability. In terms of VRFB performance, the PTAP-50 %BSA AIEM demonstrates an excellent capacity retention rate at a current density of 100 mA cm⁻² within 300 cycles, maintaining coulombic efficiencies above 99 %, with energy efficiencies around 81 % and voltage efficiencies. Therefore, the PTAP-50 %BSA AIEM exhibits great potential for application in VRFBs.