Robust Ion‐Selective Membrane for Redox Flow Batteries Based on Ultralow Sulfonation Degree Poly(Ether Sulfone)

Low‐cost and high‐performance ion‐conducting membranes are of urgent need for the large‐scale launch of vanadium redox flow batteries. In this work, sulfonated poly(ether sulfone) (SPES) with extremely low sulfonation degree of 5% is utilized to prepare ion‐selective membranes, which possess significantly enhanced chemical stability as compared with the high sulfonation degree SPES. High ion‐selectivity of the membrane is achieved via a novel solvent‐controlled swelling method invented by the group, with which the proton conductivity and vanadium permeability of the membrane can be precisely adjusted at molecular level. The influence of sulfonation degree on the chemical stability of SPES is revealed. The characteristics of the membrane, including water uptake, proton conductivity, VO2+ permeability, and tensile strength are comprehensively studied. At 100 mA cm–2, the energy efficiency of the vanadium redox flow battery (VRFB) cell equipped with the low sulfonation degree SPES membrane reaches 81.1% and shows excellent cycling stability over 150 charge–discharge cycles. Ultralow sulfonation degree sulfonated poly(ether sulfone) (SPES) ion‐selective membrane prepared via solvent‐controlled swelling method possesses excellent chemical stability, tunable proton conductivity, and ion selectivity. In vanadium redox flow battery (VRFB) single cells, the SPES membrane shows promising energy efficiency higher than 81% at 100 mA cm2 and stable cycling stability.