Advanced Membranes Boost the Industrialization of Flow Battery

ConspectusFlow battery (FB) is nowadays one of the most suited energy storage technologies for large-scale stationary energy storage, which plays a vital role in accelerating the wide deployment of renewable energies. FBs achieve the energy conversion by reversible redox reactions of flowing active species at the positive and negative sides. An ion conducting membrane (ICM) is necessary to separate the anolyte and catholyte, while conducting charge-balanced ions to form a complete electric circuit simultaneously. However, the commonly used commercial perfluorinated sulfonated ion exchange membranes suffer from low selectivity and high cost. The widely studied nonfluorinated ion exchange membranes have poor chemical stability. Most importantly, these membranes are confronted with a trade-off between selectivity and conductivity. That has motivated researchers to explore novel membrane materials with innovative design. Among them, porous membranes based on the "ion sieving conducting" mechanism instead of the "ion exchange conducting" mechanism from traditional ion exchange membranes were put forward, upon which very impressive progress has been achieved in recent years. Different from ion exchange membranes, the porous ICMs can separate active species from charge-balanced ions by pore size exclusion. As a result, by controlling the pore structures, porous membranes can break up the selectivity and conductivity trade-off.However, the initial performance of the first reported porous ICM is not high enough to afford the industrialization of FBs. As a group striving for the integration of basic, applied and translational research, we have devoted substantial efforts to optimizing the current membrane fabrication methods, developing novel fabrication techniques, exploring novel membrane materials and increasing the membrane performance for over a decade. Based on these efforts, the preliminary industrial applications of ICMs in FBs have been realized. In this Account, we summarize and discuss the development process of ICMs in various FB systems, mainly including vanadium FBs and zinc-based FBs. We begin the discussion from the brief description of FBs and their ICMs. Then we will illustrate the "ion sieving conducting" mechanism of ICMs deeply and elaborately. This account will focus on our contributions to accelerating the industrial applications of ICMs in FB energy storage systems, upon which the modification strategies on ICMs will be illustrated thoroughly, including the regulation of formation parameters, the introduction of functional materials or functional separation layers and the implementation of post-treatment steps. We also discuss the current challenges of ICMs and propose prospects on their future development.