Redox Flow Batteries (RFBs) are among the most promising and innovative solutions to face the intermittency issues come from the renewable energy. Specifically, their peculiar design physically separates the power-generating stack from the energy capacity stored in the electrolyte. This feature allows for flexibility, modularity, and easy upgrades according with the necessity/application. Indeed, the RFBs can be designed from kW to MW with sufficient extended duration (> 10 hours), assessing superior safety in comparison with Li-ion batteries.

Nowadays, Vanadium Redox Flow Batteries (VRFB) dominates the market, thought their widespread application is hindered for economic, low energy/power density values and sustainability drawbacks. In this fashion, an emerging strategy from scientific community is the replacement of the vanadium by water-soluble molecules. Herein, polyoxometalates (POMs) offer exceptional advantages, enabling unusual capability to accept/donor a large number of electrons under reversible and stable manner. Additionally, their solubility in water is remarkable and the synthesis process is quite straightforward and suitable for large production.

Herein, the POM based on K₆[Fe₄(H₂O)₂(PW₉O₃₄)₂]·20H₂O (hereafter Fe4(PW9)2) has been demonstrated for the first time in RFB as redox active specie for the negative half-cell. The synthesis of Fe4(PW9)2 has been carried out following the steps published previously [1]. After that, the POM was stabilized in a buffer electrolyte at pH 5, using 1 M concentration of (lithium acetate and acetic acid) as supporting electrolyte. This POM is able to transfer up to 10 e-, operating at low potentials (ca. to -1 V vs Ag/AgCl) with excellent reversibility. Particularly, the Fe³⁺/Fe²⁺ redox processes appeared between 0 to 0.2V vs Ag/AgCl, involving 4 electrons; and the W⁵⁺/W⁶⁺ redox processes started below -0.5 V vs Ag/AgCl, comprising two plateaus of 3 electrons each one, the second one limited by competing H2 evolution, and possibly involving structural rearrangements affecting the corresponding oxidation potential. The long-term stability of the Fe4(PW9)2 operating in the protentional range between -0.5 to -1 vs. Ag/AgCl has been demonstrated, showing the feasibility of the Fe4(PW9)2 electrolyte operates up to 44 h achieving excellent reversibility. Finally, in operando measurements using X-ray absorption/fluorescence radiation with the individual monitoring of the Fe K-edge and W L-edge energies confirm the dynamic structure, reversibility and oxidation states at several current densities applied.