Effect of Chelation on Iron–Chromium Redox Flow Batteries

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.