Large-scale energy storage has attracted increasing interests due to its urgent need in grid management (load leveling and peak shaving), the grid reliability and utilization and the integration of renewable energy sources. Among a wide range of energy storage technologies, vanadium flow battery (VFB) has a unique combination of high efficiency, high reliability, flexible design and long cycle life, which makes it an ideal candidate for large scale energy storage. Currently, urgent needs are still in the fabrication of membrane separators for these batteries under the recognition that high-quality membranes with low cost are vital to achieve large-scale acceptable VFB systems. The membranes traditionally used in VFB are perfluoro sulfonic acid polymers such as Dupont Nafion®. Although they show both high proton conductivity and chemical stability, the extremely high cost and low ion selectivity of these membranes (high vanadium crossover) have limited their further commercialization.  Although the non-perfluorinated hydrocarbon polymers like sulfonated or quaternary ammoniated aromatic polymers are widely investigated, these membranes often suffer from poor chemical instability which is proved to be induced by the introduction of ion exchange groups.  Recently, we first reported the application of porous membranes in VFB ¹  (Figure 1). The  idea is based on separating vanadium ions from protons via pore size exclusion. The totally new design and concept can potentially overcome the traditional restriction from ion exchange membranes and provide more materials option. Followed our idea, different kinds of porous membranes were explored and investigated in VFB. Quite impressive progress was achieved via optimizing the membrane materials and morphology. In this presentation, the progress and challenge of porous membranes in the application of VFB will be demonstrated.