Polystyrene-Block-Poly(ethylene-ran-butylene)-Block-Polystyrene Triblock Copolymer Separators for a Vanadium-Cerium Redox Flow Battery

Abstract

A series of polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS)-based anion exchange membranes (AEMs) were synthesized via chloromethylation of SEBS followed by quaternization with trimethylamine (TMA). AEMs functionalized with TMA+ cations exhibited high chloride ion conductivity of 33.6 mS/cm at 70°C. A V-Ce RFB employing the SEBS-based AEM as the separator yielded an energy efficiency of 86% at a current density of 50 mA/cm2 with a 10% drop in capacity over 20 charge/discharge cycles. In contrast, a V-Ce RFB using Nafion212 as the separator had an energy efficiency of 80% and a 40% drop in capacity over 20 charge/discharge cycles. The observed capacity fade was primarily due to cation intermixing between the anodic and cathodic compartments – much better permselectivity was obtained with the AEM separator. After 60 charge-discharge cycles (350 hours of operation), the ion exchange capacity and ionic conductivity of the AEM dropped by about 20%. There was no observed change in mechanical properties. The oxidative stability of the AEM was evaluated ex situ by immersion in 1.5 M VO2+ + 3 M H2SO4 for 500 hours - the ionic conductivity remained constant over this timeframe. The chemical and mechanical stability and high conductivity of SEBS-based AEMs make them promising separator candidates for electrode-decoupled RFBs.