Summary and Perspectives

The flexible design of VRFB, with decoupled configuration of power and capacity, makes it a promising technique for large-scale energy storage to integrate with the grid. As a core component, the IEM requires a comprehensive property, such as high proton conductivity, low vanadium ion permeability, good chemical and mechanical stability, and low cost. However, it is still a major challenge to synthesize high-performance and cost-effective membranes toward the VRFB application. In this chapter, the state-of-the-art of the membranes is introduced, including the per- fluorinated, partially fluorinated, non-fluorinated, and porous membranes and their derivatives. The large vanadium ion permeability and high cost are the major focus for the development of the commercial Nation membranes, a typical perfluorinated counterpart. It is effective to modify the Nation membranes by incorporating inorganic and organic additives to tailor the intrinsic channels and reduce the use of raw- materials. As an alternative to the Nation membrane, the non-fluorinated hydrocarbon membranes are developed, with low vanadium ion permeability and competitive cost. Nonetheless, proton conductivity and chemical stability are the challenging issues. The same strategy has been widely used to enhance the membranes with the inorganic and organic species. The porous membranes are further explored based on the size sieving effect. The ion selectivity, a key factor affecting the membrane performance, is improved by adjusting the pore or channel size. This includes filling the matrix, modifying the surface, turning the component of coagulating bath (for porous membrane), and so on.

The hydrocarbon polymer is a promising candidate for VRFB application, in view' of its high ion selectivity and low cost. However, its chemical stability is unsatisfactory. Further development of the membranes should be focused on tailoring the polymers toward high ion selectivity, long-term chemical and mechanical stability, and low cost. First, the hydrocarbon polymers can be improved by introducing the fluorine-containing groups or stable species to enhance the stability and proton conductivity and meanwhile maintaining the low cost. Second, a judicious combination of different polymers, various ion exchange groups, and modification or preparation methods could also be a promising way for membranes of high ion selectivity, good chemical stability, and low cost. Finally, the degradation mechanism in the cell operation process should be further investigated for better understanding.

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