Highly Ion Selective Proton Exchange Membrane Based on Sulfonated Polybenzimidazoles for Iron–Chromium Redox Flow Battery

The iron–chromium redox flow battery (ICRFB) has great potential for large-scale energy storage, due to its low capital cost of redox-active materials. However, the trade-off between conductivity and selectivity in the membranes limits its applications. Herein, a series of sulfonated polybenzimidazoles with exactly controlled sulfonation degree (SD) (S-PBI-x, x refers to SD) are designed and synthesized via direct copolymerization from the sulfonated monomer. Combined with the electrostatic repulsion of the formed imidazoliums, the S-PBIx membranes facilitate the proton transport and repel the redox-active ion crossover efficiently. Especially, when compared with Nafion 212 membrane, the S-PBI-100 membrane displays a comparable conductivity and more than an order of magnitude lower Fe3+ and Cr3+ permeabilities. Thus, a higher columbic efficiency (CE) of 98.2% and energy efficiency (EE) of 83.17% are achieved at 80 mA cm–2 for the corresponding ICRFB. Most importantly, no chemical degradation is observed for the S-PBI-100 membrane after in situ and ex situ stability tests.