Redox flow batteries: Mitigating cross-contamination via bipolar redox-active materials and bipolar membranes

The development of post-vanadium electrolytes using abundant materials with versatile redox chemistries will enable cost-effective energy storage and widespread implementation of redox flow batteries (RFBs). However, performance-loss due to cross-contamination of catholyte/anolyte in the membrane-based RFBs becomes a technological hurdle toward long-term cyclability. To tackle such challenge, new materials chemistry and cell chemistry have been demonstrated. A promising class of bipolar redox-active materials emerges, which permits the use of the same electrolyte in both half-cells and can ultimately solve the detrimental cross-contamination. In addition, new cell configuration, by using bipolar membranes with stacked anion- and cation-selective layers, is developed to suppress the cross-contamination through the Donnan-exclusion effect. Here, an overview of the most recent advances in bipolar redox-active materials and bipolar membranes for RFBs is provided.