Research progress on optimized membranes for vanadium redox flow batteries

Energy storage systems are considered one of the key components for the large-scale utilization of renewable energy, which usually has an intermittent nature for production. In this case, vanadium redox flow batteries (VRFBs) have emerged as one of the most promising electrochemical energy storage systems for large-scale application, attracting significant attention in recent years. To achieve a high efficiency in VRFBs, the polymer electrolyte membrane between the positive and negative electrodes is expected to effectively transfer protons for internal circuits, and also prevent cross-over of the catholyte and anolyte. However, the high cost of membrane materials is currently a crucial factor restricting the large-scale application of VRFBs. In this review, key aspects related to the polymer electrolyte membranes in VRFBs are summarized, including their functional requirements, characterization methods, transport mechanisms, and classification. According to its classification, the latest research progress on the polymer electrolyte membrane in VRFBs is discussed in each section. Finally, the research directions and development of next-generation membrane materials for VRFBs are proposed, aiming to present a future perspective of this component in full batteries and inspire the ongoing efforts for building high-efficiency VRFBs in the power grid.