Microstructure regulation of porous polybenzimidazole proton conductive membranes for high-performance vanadium redox flow battery

As one of the important parts of vanadium redox flow battery (VRFB), the proton conductive membrane (PCM) should have high proton selectivity (PS) of H⁺ to VO²⁺ in addition to the specific properties, such as outstanding oxidative stability, long lifetime and low price. However, polybenzimidazole PCMs usually have lower proton transference resulting in lower PS. In this study, three kinds of porous poly (oxyphenylene benzimidazole) (OPBI) membrane, such as OPBI-MSA-VIPS membrane, OPBI-DMAc-VIPS membrane and OPBI-DMAc-NIPS membrane, formed respectively by vapor induced phase separation (VIPS) and non-solvent induced phase separation (NIPS) method, using methanesulfonic acid (MSA) and N,N-dimethylacetamide (DMAc) as solvent separately, are investigated to prepare a kind of high performance PCM with high PS. The OPBI-DMAc-NIPS membrane has a fingerlike microstructure with two dense skin layers. The OPBI-MSA-VIPS membrane has a uniform cellular-like microstructure with single dense skin layer, which endows the membrane's excellent vanadium resistance and high PS. The PS of OPBI-MSA-VIPS membrane is one order higher than that of OPBI-DMAc-VIPS membrane which has a spongy-like structure without dense skin layer. Furthermore, we study the effect of the polymer concentration on the structure and the corresponding battery properties of OPBI-MSA-VIPS membrane in detail. When the casting solution concentration is 6 wt % (P6 membrane)，the PS reaches the highest, about 7.74 times higher than that of commercial Nafion 115. More importantly, P6 membrane displays relatively long cycling stability. The EE value of the VRFB equipped with P6 membrane maintains stability after 3000 cycles at the current density of 160 mA/cm². This work highlights an excellent porous OPBI membrane that has the potential to be used as high performance PCM for new energy battery.