High-Selectivity Proton Exchange Membranes with Low Ion Exchange Capacity and Hydrophobic Side Chain-Induced...

The proton (H⁺) and vanadium ion (Vⁿ⁺) selectivity of proton-conductive membrane is one of the key components for vanadium redox flow batteries (VRFBs). In this work, a hydrophobic side chain was designed to accelerate proton conduction with high selectivity of H⁺ and Vⁿ⁺ for the VRFB membrane. The grafting of hydrophobic butyl side chains into the membrane (PBIOSO₃-But) induced the formation of a high microphase separation capacity to form large and connected ion conductive channels with low ion exchange capacity (IEC). As a result, the PBIOSO₃-But membrane with low IEC of 1.26 mmol g^-1 shows area resistance of 0.19 Ω cm² as well as vanadium permeability of 3.2 × 10^-9 cm² s^-1, leading to a high H⁺/Vⁿ⁺ selectivity of 2.51 × 10¹⁰ mS s cm^-3 (higher than Nafion 212, 4.62 × 10⁸ mS s cm^-3). Notwithstanding its low ion-exchange capacity, this membrane demonstrates H⁺/Vⁿ⁺ selectivity surpassing that of recently reported microphase separation membranes. Compared to the Nafion 212 membrane (74.3% EE; 0.81% per cycle), the PBIOSO₃-But membrane exhibited superior VRFB performance, achieving an energy efficiency of 83.2% at 200 mA cm^-2 and a low retention rate of 0.22% per cycle. These values compare favorably with those of recently reported membranes.