Side-chain-type anion exchange membranes for vanadium flow battery: properties and degradation mechanism

Abstract

Several side-chain-type AEMs have been synthesized for exploring the effect of the chemical architecture on the performance and degradation mechanism in the vanadium flow battery (VFB). The side-chain-type AEMs having PTA groups (PSf-c-PTA) showed higher area resistance (0.6 Ω cm²) but lower vanadium permeability than those with QA moieties (PSf-c-QA), probably due to the lower water uptake and low swelling ratio. This trade-off further led to the better performance of the single VFB with the PSf-c-PTA membrane (coulombic efficiency > 98%, energy efficiency = 84.3%) at a current density of 120 mA cm⁻², while the energy efficiency was 75.5% for the cell with the Nafion 115 membrane under the same testing conditions. After the ex situ chemical stability testing of the prepared AEMs in 1.5 M VO₂⁺/3 M H₂SO₄ at 40 °C for 30 days, the side-chain-type AEMs exhibited no obvious chemical degradation of polymer backbones as confirmed by NMR results. However, the control PSf-PTA membrane with the benzylethylethanaminium group underwent severe chemical degradation of both the polymer backbone and the cationic groups. These results suggest that separating the cationic groups from the polymer backbone could improve the chemical stability of AEMs in VFB. Moreover, in contrast to the ex situ stability tests, the chemically unstable PSf-PTA membrane displayed better cell performance with 60% of EE remaining after 1000 cycles, and the performance of the VFB with side-chain-type AEMs faded after only hundreds of cycles. Further characterization of the aged AEMs in the VFBs showed that mechanical degradation resulted from excessive swelling in the electrolyte solution, rather than chemical degradation, and was the dominant factor that influenced the durability of VFBs when using AEMs as the separator. This finding gives us new fundamental insights into the ex situ and in situ degradation mechanisms of AEMs for VFB application.