Alkyl Spacer Grafted ABPBI Membranes with Enhanced Acid-Absorption Capabilities for Use in Vanadium Redox Flow Batteries
Jung-Kyu Jang, Sang-Woo Jo, Jun Woo Jeon, Byoung Gak Kim, Sang Jun Yoon, Duk Man Yu, Young Taik Hong, Hee‐Tak Kim, Tae‐Ho Kim
Abstract
Achieving high proton conductivity, low vanadium ion permeability, and high chemical stability using a single material remains a key challenge for hydrocarbon-based membranes for use in vanadium redox flow batteries (VRFBs). Herein, we report amorphous poly(2,5-benzimidazole) (ABPBI) membranes with alkyl spacers which can meet these requirements. Spacer-grafted poly(2,5-benzimidazole)s (ABPBIs) with different grafting ratios were synthesized via an N-substitution reaction and denoted as ABPBI-0, -10, -25, -40, and -50, respectively. The structure to battery performance relationship was investigated through spectroscopic and electrochemical analysis. As the grafting ratio increased, the microstructure of the ABPBI derivative membranes was transformed from a semicrystalline to an amorphous structure because of reduced chain packing, increasing H2SO4-absorption capability and consequently reducing area-specific resistances. Also, they exhibited lower vanadium permeabilities compared with Nafion 115 owing to Donnan exclusion effect. As a combined effect of these characteristics, the membranes outperformed Nafion 115 membrane with notably high coulomb efficiencies and energy efficiencies. Furthermore, a grafted ABPBI membrane showed stable battery cycling performance more than 500 cycles at 200 mAcm–2, and 1000 h continuous operation at 1C-rate, demonstrating excellent chemical stability against highly oxidizing VO2+ solution.