The feasibility of zinc-iron flow batteries using mixed metal ions in mildly acidic chloride electrolytes was investigated. Iron electrodeposition is strongly inhibited in the presence of Zn 2+ and so the deposition and stripping processes at the negative electrode approximate those of normal zinc electrodes. In addition, the zinc ions have no significant effect on the Fe(II/III) couple at the positive electrode. This enables the use of mixed Zn-Fe electrolytes and microporous separators in place of expensive ion-exchange membranes. Considering the low-cost materials and simple design, zinc-iron chloride flow batteries represent a promising new approach in grid-scale energy storage. The preferential deposition of zinc occurs with similar behavior on titanium, graphite and glassy carbon substrates. A proof-of-concept zinc-iron chloride battery starting with mixed electrolytes was demonstrated and maintains a consistent open-circuit voltage of about 1.5 V and stable performance during over 10 days and 100 cycles of continuous charge-discharge cycling. Zinc-based hybrid flow batteries are being widely-developed due to the desirable electrochemical properties of zinc such as its fast kinetics, negative potential (E 0 = −0.76 V SHE ) and high overpotential for the hydrogen evolution reaction (HER). Many groups are developing zinc-bromine batteries, and they address challenges associated with bromine toxicity and the organic complexing agents used to reduce its vapor pressure.1,2 Other positive electrode couples using cerium, vanadium, nickel and iron are also being investigated.
3-5Zinc-ferricyanide flow batteries using alkaline electrolytes were developed in the late 1970 s, but progress was reportedly hindered by high membrane costs and challenges with handling solid zinc oxide precipitates.6 Currently, zinc-ferricyanide flow batteries are also being developed by ViZn, Inc.7 Some of the challenges of using the Fe(CN) 3−/4− 6 couple include its low solubility on the order of 0.2-0.5 mol L −1 , and the possible generation of toxic gas if it mixes with acid. [8][9][10] More recently, a zinc-iron flow battery based on deep eutectic solvents (DES) with an open-circuit potential of 1.02 V was described, but it operated at a low current density of 0.5 mA cm −2 and so it was concluded that high-temperature operation would be required in order to obtain useful power densities.
11Of the possible reactions to use for a positive electrode, the aqueous Fe(II/III) redox couple is among the safest and cheapest, and it has high solubility and fast kinetics even on uncatalyzed carbon materials.12 However, most studies of zinc-iron batteries have focused on the alkaline chemistry (using Fe(CN) 3−/4− 6 at the positive electrode), and there are only a few reports of zinc-iron flow batteries based on the acidic chemistry. A recent study combined an alkaline (2.4 M NaOH) negative electrode with an acidic (1 M HCl) positive electrode to achieve high performance, but this required the use of two ion-exchange membranes as well as a third e...