Aqueous hybrid flow batteries are being developed because of their high performance and low cost relative to other energy storage technologies. Zinc-bromine flow batteries, which use an acidic chemistry, have demonstrated high performance by many groups. However, their application has been limited by the safety concerns regarding bromine toxicity. Zinc-ferricyanide flow batteries, which operates in alkaline environment, have also been widely developed, but there have been reported challenges with the high membrane costs, precipitation of zinc oxides and the possibility of generating toxic gas in the event of electrolyte mixing with acid. In our lab, we are investigating a new type of zinc-iron chloride flow battery operating with the reactions given by:

negative Zn²⁺ + 2e^- → Zn -0.76 V

positive Fe³⁺ + e^- → Fe²⁺ +0.77 V

cell Zn²⁺ + 2Fe²⁺ ↔ Zn + 2Fe³⁺ 1.53 V

The key to making this work is to use the anomalous codeposition (ACD) of zinc from mixed ZnCl₂-FeCl₂ electrolytes. This effect prevents iron from electroplating during battery charging, so the negative electrode behaves similar to a normal zinc electrode. As with all-iron flow batteries, this type of flow battery also has an inherent crossover tolerance, which is important for long-term stability as well as operability using inexpensive microporous separators. Here, we report results from three-electrode studies of the anomalous codeposition as well as preliminary flow battery experiments.