Overcoming the conductivity-selectivity trade-off in flow battery membranes via weak supramolecular interaction mediated pseudo-nanophase separation

Escalating demand for energy-related applications, e.g., redox flow batteries (RFBs) and water electrolysis, has spurred extensive research on high-performance and cost-effective ion-conducting membranes. Nonetheless, the conductivity-selectivity trade-off dilemma in ion-conducting membranes have long been a ubiquitous and pernicious issue. In this work, we report an intriguing finding that such trade-off can be overcome via supramolecular interaction mediated pseudo-nanophase separation enabled by weak supramolecular interaction-grafted sidechains. Specifically, triethanolamine (TEA) is noncovalently tethered onto polybenzimidazole (PBI) polymers, leading to their preference for forming discrete large-sized hydrophilic TEA nanophases within hydrophobic PBI matrix as inferred from the integrating experimental and theoretical research. Consequently, vanadium ions are effectively prevented by the continuous hydrophobic PBI phase due to the tightly packed PBI chains and the Donnan effect from protonated benzimidazole motifs. Meanwhile, proton transfer is appreciably enhanced by the presence of large hydrophilic domains and proceeds continuously within the hydrophobic PBI matrix through an extensive hydrogen bonding network. The resulting membrane-based vanadium RFB achieves an outstanding energy efficiency of >80 % at high current densities up to 240 mA cm−2, and demonstrates exceptional cycling stability with a capacity decay rate as low as 0.04 % per cycle over 500 cycles, the slowest value reported in recent years.