Polyoxometalate-Based Symmetric Redox Flow Batteries: Performance in Mild Aqueous Media

Polyoxometalate (POM)-based redox flow batteries (RFBs) are proposed to boost the implementation of large-scale energy storage systems in the electric grid. Even though vanadium chemistry in RFBs has been globally commercialized, the huge demand for stationary energy storage and the limited availability of the materials have expanded research toward other electroactive species. POMs have gained interest thanks to their compositional versatility as well as their highly reversible multielectron transfers. Herein, we report the performance of two POM–RFBs operating in a symmetric cell configuration under mild conditions. The POM-based electrolytes consist of two cobalt-containing Keggin-type POMs, either [α-CoW12O40]6– (CoW12) or [α-Co(H2O)SiW11O39]6– (CoSiW11), dissolved in aqueous 1 M acetic acid/lithium acetate buffer. Both POM–RFBs showed high efficiency (over 88% of energy efficiency), with the CoW12-RFB exhibiting very good cycling stability with a 98.9% capacity retention over 100 cycles at 20 mA cm–2. Conversely, CoSiW11 showed a capacity decay of up to 50% after 100 cycles at 5 mA cm–2. The stability of the electrolytes after cycling was confirmed by cyclic voltammetry. Furthermore, a simple colorimetric method to monitor the state of charge of the electrolyte was developed, which improves the prospects of exploitation of POM–RFBs.