Chemically stable anion exchange membranes based on C2-Protected imidazolium cations for vanadium flow battery

Abstract Imidazolium-based anion exchange membranes (AEMs) are attractive as the separator for vanadium redox flow battery (VFB) application. However, the lack of fundamental understanding of the correlations between imidazolium chemical structure and their physical properties as well as cell performance constrains the design of advanced AEMs in VFBs. In this work, by designing the “clickable” imidazolium compounds from C2-methyl or C2-phenyl substituted imidazolium to C2-phenyl substituted benzimidazolium, a series of PSf-based AEMs having pendant C2-protected imidazolium derivatives were prepared by efficient CuAAC reaction to explore how the nature of C2 substitution affected the electrochemical property of AEMs and the resulting VFB performance. Interestingly, PSf-MIm with C2-methyl protected imidazolium group showed lowest area resistance (0.3 Ω cm2, IEC = 1.66 meq./g) in 3 M H2SO4 aqueous solution but unfavorable vanadium permeability due to its highest swelling ratio. The introduction of bulky C2-phenyl substituted benzimidazolium led to the distinct microphase separation in PSf-PhBIm membrane, and thus reduced water uptake, high vanadium selectivity, and comparable conductivity were observed. As a result, the single VFB with PSf-MIm-1.2 membrane exhibited better electrochemical performance with a coulombic efficiency (CE) of 97.0%, and an energy efficiency (EE) of 82.4% at a current density of 120 mA/cm2, higher than those of Nafion N115 membrane (EE = 75.5%) and unsubstituted imidazolium-based AEMs (EE = 80.6%). More importantly, both ex situ stability testing in 1.5 M (VO2)2SO4/3 M H2SO4 solution for 90 days and in situ cycling performance at a current density of 120 mA/cm2 demonstrated that the chemical structure of C2-substituted imidazolium based AEMs remained intact after stability tests as confirmed by NMR analysis, while significant degradation was found for unsubstituted PSf-Im membrane via possible nucleophilic addition mechanism. Therefore, the VFB with PSf-MIm membrane showed the best long-term durability with only 34.1% loss in EE for 3638 h of operating (4800 cycles). This work not only fills the knowledge gap on the structure-property relationship of imidazolium-based AEMs for VFB application, but also gives us new directions to design stable AEMs for durable VFBs.