An iron-vanadium redox flow battery utilizing 15 μm thick HCl doped meta-polybenzimidazole (m-PBI) membranes is used. Ex-situ tests for m-PBI membranes show a much lower permeability for Fe2+ and V3+ ions than when using Nafion 212. Specifically, cells utilizing 50 μm thick Nafion 212 show a strong electrolyte imbalance (catholyte moving to anolyte), a low charge efficiency (CE) of 90%, and a high capacity loss rate (CLR) of 0.63 Ahr∙L−1 per cycle, indicating low energy efficiency and stability. In contrast to this, cells utilizing m-PBI reveal a CE of 99% and a CLR of just 0.11 Ahr∙L−1 per cycle. After 20 cycles, the discharge capacity is three times higher than for the cell with Nafion 212. Since the polymer needed for a 15 μm thick m-PBI membrane costs 97% less than for a 50 μm thick Nafion membrane, the utilization of m-PBI membranes is also economically advantageous.