Promises and challenges of polyoxometalates (POMs) as an alternative to conventional electrolytes in redox flow batteries (RFBs)

A comprehensive review of redox flow batteries (RFBs) based on multi-electron redox reactions is provided in relation to that of the conventional single-electron reaction-based RFBs. Performance optimization, cross-over analysis, and modifications in the cell assembly of vanadium redox flow batteries (VRFBs) are available in the literature, because of their simple and reversible single-electron redox reactions. However, with the introduction of electrolytes capable of undergoing multi-electron reactions within a single molecule, such as that with polyoxometalates (POMs), an understanding of redox reactions during charge-discharge processes and their correlation with the cell performance has become paramount to achieve their maximum potential in RFBs. Based on the rate coefficient, number of electrons, and solubility of the electrolytes, optimizing operating parameters is crucial to alleviate species crossover, irreversibility, or precipitation leading to charge imbalance and capacity loss; consequently, these systems require specific considerations. The review emphasizes on the development of POM-based RFBs and the challenges associated with them. Performance characteristics of various symmetric and asymmetric POM-based RFBs are discussed along with possible methods to avoid irreversibility and precipitation for further improvement.