Quaternary ammonium functionalized Diels–Alder poly(phenylene)s (QDAPPs) with different ion exchange capacities (IECs) are examined as membranes in all-vanadium redox flow batteries. QDAPP membrane behavior is compared to a standard, Nafion 212, in measurements of cycling efficiencies, areal specific resistance (ASR), vanadium permeation and durability. The IEC of the QDAPPs clearly shows an impact on the cell ASR and vanadium crossover. The results imply a trade-off between performance, indicated by cell voltage loss at a given current density, and rate of cross-over driven capacity loss in the system. Among the membranes studied, QDAPP with moderate IEC represents the best trade-off of these factors and exhibits higher performance and lower capacity loss compared to Nafion 212. All QDAPP membranes are found to be more durable than the analogous cation exchange membrane, sulfonated DAPP (SDAPP), in V5+ solution. •Quaternary ammonium functionalized poly(phenylene)s are studied for flow batteries•Membrane Ion Exchange Capacity controls conductivity, vanadium cross-over•Optimized QDAPP membrane exhibits high selectivity for use in VRFB•High energy efficiency and low capacity decay observed relative to Nafion