Near to neutral pH all-iron redox flow battery based on environmentally compatible coordination compounds

We demonstrate a redox flow battery at a near to neutral of pH 8.6 using nontoxic iron-coordination compounds as redox carriers in both negative and positive electrolytes. The negative electrolyte contains a compound commercially in use as fertilizer, the racemic form of iron(III)-N,N’-ethylene-bis-(o-hydroxyphenylglycine), with a reduction potential of -613 mV vs Ag/AgCl, 1 M NaCl. The positive electrolyte contains sodium ferrocyanide, with a reduction potential of +235 mV vs Ag/AgCl, 3 M KCl. The all-iron aqueous redox flow cell exhibits a cell voltage of 848 mV at 50% state-of-charge (SOC). The solubility for both reactants in this work was up to ∼0.2 M. Permeation experiments were performed with three different cation exchange membranes: the Nafion membranes NR-212 and N-324, and the low-cost sulfonated poly(ether ketone) SPEEK membranes. A capacity retention of nearly 100% over 75 charge discharge cycles was achieved with the N-324 membrane. No significant decay in capacity or voltage efficiency was observed during two days of cycling at low 14% and high 93% SoC limits. Using a low-cost sulfonated poly(ether ketone) (SPEEK) membrane, a good capacity utilization of 86.5% was demonstrated at a voltage efficiency of 81.5% at 25 mA cm–2 current density, and a peak power density of 44.5 mW cm–2 was achieved at 95 mA cm–2 current density. However, crossover was detected. Reactant crossover at N-324 and SPEEK membranes was 5·10–12 cm2 s–1 and 6.2·10–11 cm2 s–1, respectively. The new combination of nontoxic iron complexes represents a promising option for stationary environmentally friendly energy storage applications.