Building water molecule chains in polybenzimidazole membrane toward superior vanadium redox flow battery

Optimizing the proton conductivity of polybenzimidazole (PBI) membrane is of great significance to realize vanadium redox flow battery (VRFB) with the superior performance and low cost for commercialization. However, the fundamental reason of the poor proton conductivity of PBI, which severely hinders its application in VRFB, is not addressed yet. To tackle this issue, hydrogen bonds between PBI chains are scissored through a deprotonation strategy by using KOH/ethanol as the casting solvent in this study, weakening the interaction between PBI chains and enlarging the free volume to build water molecule chains inside. The water molecule chains effectively promote the proton transfer under Grotthuss mechanism, increasing the proton conductivity by over 1400 %. The energy efficiency of the VRFB assembled with the engineered membrane is improved from 71.5 % to 83.3 % under 80 mA cm-2, in addition to preserving the membrane's ability to inhibit the permeability of vanadium ions as well as maintaining the structural stability. This study innovates a facile strategy for membrane optimization, and the profound analysis of structure-property relationship of PBI membrane is highly critical for guiding the development of membrane.