An enhanced stability and efficiency of SPEEK-based composite membrane influenced by amphoteric side-chain polymer for vanadium redox flow battery

Sulfonated poly(ether ether ketone) (SPEEK) based composite membranes for vanadium redox flow battery (VRFB) are fabricated through blending various contents of sulfonated poly (aryl ether sulfone) (SPAES) containing sulfoalkylamine chains. Influenced by the pendant sulfoalkyl groups of SPAES, an enhanced synergistic proton transport channel is formed, showing a trade-off balance between proton conductivity and permeability. The SPEEK/SPAES-15 membrane exhibits the highest ion selectivity (28.8 × 103 S min cm−3) against pristine SPEEK (3.9 × 103 S min cm−3), Nafion 117 (4.0 × 103 S min cm−3), and Nafion 212 (3.0 × 103 S min cm−3). And, the SPEEK/SPAES-15 membrane gives much higher coulombic efficiencies (CE: 96.0–99.0%) and energy efficiencies (EE: 87.2-70.3%) at the current density from 60 to 200 mA cm−2, which is attributed to the increased proton conductivity and ion selectivity, and the decreased permeability. Even at a high current density of 150 mA cm−2, a stable 75.0% EE is realized, and it maintains the good structural stability and durability after 600-time charge-discharge cycles. This proves that it is a facile technique to approach the high-performance proton exchange membrane by regulating the synergistic sulfonic acid proton network.