Bi-functional side chain architecture tuned amphoteric ion exchange membranes for high-performance vanadium redox flow batteries

The blended amphoteric membranes possessed a reducing cation exchange capacity with increasing the anion exchange capacity. Bi-functional side-chain-type polymers are conducive to have both high cation exchange capacity and high anion exchange capacity at the same time, and further enhance ion conductivity of amphoteric ion exchange membranes (AIEMs). Herein, novel side-chain type AIEMs are prepared via ring-opening metathesis polymerization of norbornene possessing the advantages of yielding easily modified molecular architectures, simple and controllable polymerization, and producing the membranes with high chemical stability. To avoid the occurrence of excess water swelling arising from the increased zwitterionic groups, hydrophobic spacers (alkyl chain-(CH₂)₆ -) were introduced to be situated between the aromatic main chain and ionic groups. This unique structure favors ion clusters to be aggregated and thus forms highly ordered water channels so that the protons can be efficiently transported along the center of channels and possess excellent proton conductivity. In addition, the ion crosslinking interactions enhance the mechanical stability of AIEMs. The resultant AIEM (1/2) exhibits a higher ion conductivity of 7.58 × 10^-2 S cm^-1 and vanadium ion permeability of 0.21 × 10^-9 cm² s^-1 at room temperature. Based on these good properties, the vanadium redox flow batteries (VRBs) with prepared membranes and Nafion 212 were tested. The energy efficiency (EE) of VRBs with AIEM (1/2) still reaches 84.76% at the current density of 160 mA cm^-2, which is better than that with Nafion 212 (76.26%). Moreover, the EE of VRBs with AIEM (1/2) is 82.11% after 200 cycles, indicating that the prepared membranes possess a good stability.