A soluble highly-sulfonated polybenzimidazole with high molecular weight as membrane for vanadium flow battery with enhanced performance and long-term stability

A series of novel sulfonated polybenzimidazole (SPBI) membranes containing flexible double ether segments, having high ion exchange capacity (IEC), good solubility as well as high molecular weight, are designed for vanadium flow batteries (VFBs). The bis-ether structure of self-synthesized sulfonic acid monomer, 4,4'-((2-sulfo-1,4-phenylene)bis(oxy))dibenzoic acid, improves the solubility of produced SPBI and the microphase separation structure of prepared membrane by disrupting the regularity of the backbone through a non-planar structure and attenuating the interchain stacking effect of the polymer backbone. Such a highly reactive bis-ether monomer also leads to high molecular weights (2.37−2.82 dL g⁻¹). The copolymerization using another sulfonic acid monomer, Monosodium-5-Sulfoisophthalate monomer, leads to a high IEC of up to 2.08 mmol g⁻¹. As a result, the SPBI membrane shows a low area resistance of 0.21 Ω cm² and a low vanadium ion permeability of 3.36 × 10⁻⁹ cm² s⁻¹. The VFB cell employing SPBI membrane achieves a very high energy efficiency exceeding 82 % at 200 mA cm⁻². In addition, an ultra-long lifetime of over 3400 cycles is accomplished.