Building ion selective channels through bulky camphorsulfonic acid side chains in sulfonated polybenzimidazole membranes for vanadium redox flow batteries

Abstract

Chemical structure design of the functional side chains is essential to ion selective channels in ion conductive membranes, significantly affecting the efficiency of vanadium redox flow batteries (VRFBs). Herein, a novel camphorsulfonic acid group has been proposed as a bulky, non-coplanar proton conductive functional side chain, and grafted onto sulfonated polybenzimidazole (SPBICa). It helps to weaken the tight packing of the polymer chains, not only improving the self-aggregation ability of the sulfonic acid groups to form a larger ionic cluster (8.97 nm) than that of the pristine SPBI (7.85 nm), but also creating free volume (sizes concentrated in 1–3 Å) with proton/vanadium ion sieving capabilities. The interconnected microphase-separated channels couple with free volume sieving pathways, demonstrating excellent properties. The SPBICa-1.10 membrane, with a camphorsulfonic acid grafting degree of 110%, achieves high proton conductivity of 53.3 mS cm⁻¹, extremely low area resistance of 0.14 Ω cm² and vanadium ion permeability of 1.97 × 10⁻⁹ cm² s⁻¹. Consequently, the membrane exhibits a remarkable ion selectivity of 27.1 × 10⁹ mS s cm⁻³, approximately 49.5 fold that of the commercial Nafion 212 membrane. The VRFB assembled with the SPBICa-1.10 membrane achieves an energy efficiency of 81.3% at a high current density of 200 mA cm⁻². After 650 charge–discharge cycles, the battery demonstrated a very low capacity decay rate of only 0.15% per cycle, significantly outperforming batteries using the SPBI and Nafion 212 membranes. This work provides an efficient molecular design strategy for highly selective and stable ion conductive membranes in VRFB applications.