High-Selectivity Proton Exchange Membranes with Low Ion Exchange Capacity and Hydrophobic Side Chain-Induced Micro-Phase Separation for Vanadium Redox Flow Batteries

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.