The iron–chromium (FeCr) redox flow battery (RFB) was among the first flow batteries to be investigated because of the low cost of the electrolyte and the 1.2 V cell potential. We report the effects of chelation on the solubility and electrochemical properties of the Fe3+/2+ redox couple. An Fe electrolyte utilizing diethylenetriaminepentaacetic acid (DTPA) exhibits efficient and high-performance flow battery cycling at pH 9 versus a Cr-chelate complex utilizing 1,3-diaminopropanetetraacetic acid (PDTA). The FeDTPA electrolyte can be cycled at concentrations up to 1.35 M, equating to a storage capacity of 36.2 Ah L–1, with near-quantitative efficiency. When paired with a CrPDTA electrolyte, the equilibrium cell potential of the all-chelated FeCr RBF is 1.2 V with a maximum discharge power of 216 mW cm–2. Key aspects of the coordination chemistry of FeDTPA are compared with CrPDTA and highlight the importance of molecular-level understanding for driving flow battery system performance.

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Here, we outline some basic pitfalls in the electrochemical investigation of aqueous metal complexes and advocate for the use of bulk electrolysis in redox flow cells for electrolyte analysis. We...