MOF- & COF-integrated composite separators/membranes: innovations for sustainable and high-performance redox flow batteries

Advancements in membrane engineering are perceived as the primary concern for leading redox flow batteries (RFBs) to large-scale energy storage technology. Separators/Membranes that offer balanced trade-offs between permeability and selectivity, with controlled wettability, solvent resistance, and robustness, have emerged as promising materials for separation in RFBs. Traditional ion exchange membranes (IEMs), like Nafion, are prone to high permeability to active species and high costs, which limit their commercial viability and the overall performance of RFBs. Over the past decade, the development of non-ionic/porous membranes has garnered tremendous research attention due to their industrial-level properties, including porosity and flexibility, as well as advancements in fabrication methods and the principle of operation, such as size-based sieving effects. In this context, recent research has focused on intrinsic porous materials, such as covalent organic frameworks (COFs) and metal–organic frameworks (MOFs), for membrane fabrication due to their modifiability in pore channels, ion selectivity, and design flexibility. However, this technology is still in its early stages. This review summarizes recent advances in MOF/COF separators for RFBs, providing insights into existing challenges (e.g., desired pore size, processability, or scaling) and fundamental strategies (such as functional side chain modification) to achieve targeted functionality. Apart from the intrinsic features of frameworks, optimized structural properties (e.g., pore size) and performance evaluation are crucial to improve the quality of the separator, which are discussed in detail. This review provides valuable insights and can guide future research directions for designing next-generation MOF/COF separators in RFBs.