Executive Summary

Iron-chromium flow batteries were pioneered and studied extensively by NASA in the 1970’s – 1980’s and by Mitsui in Japan. The iron-chromium flow battery is a redox flow battery (RFB). Energy is stored by employing the Fe2+ – Fe3+ and Cr2+ – Cr3+ redox couples. The active chemical species are fully dissolved in the aqueous electrolyte at all times. Like other true RFBs, the power and energy ratings of the iron-chromium system are independent of each other, and each may be optimized separately for each application. All the other benefits and distinctions of true RFBs compared to other energy storage systems are realized by iron-chromium RFBs.

Discussion

During the discharge cycle, Cr2+ is oxidized to Cr3+ in the negative half-cell and an electron is released to do work in the external circuit through the negative and positive terminals of the AC/DC converter. In the positive half-cell during discharge, Fe3+ accepts an electron from the external circuit and is reduced to Fe2+. These reactions are reversed during charge, when current is supplied from the external circuit through the AC/DC converter. Hydrogen (H+) ions are exchanged between the two half-cells to maintain charge neutrality as electrons leave one side of the cell and return to the other side. The hydrogen ions diffuse through the separator, which electronically separates the half cells. In early implementations of the iron-chromium RFB, diffusion of the iron and chrome ions across the separator created an imbalance between the positive and negative electrolytes, resulting in an irreversible system capacity loss. Modern electrolyte formulations using mixed iron and chromium on both sides of the separator have eliminated the irreversible loss and enabled the use of low cost, porous separator materials. These porous separators have also eliminated the “membrane fouling” failure mode that occurs with ion exchange membranes used in early iron-chrome and some other current RFB technologies.

The standard cell voltage is 1.18 volts and cell power densities are typically 70-100 mW/cm2. The comparatively low cell voltage results in a low energy density, but developers can still meet the EPRI footprint target of 500 ft2 per MWh of storage. The DC/DC efficiency of this battery has been reported in the range of 70-80%. Efficiency of this system is enhanced at higher operating temperatures in the range of 40-60 oC (105-140 oF), making this RFB very suitable for warm climates and practical in all climates where electrochemical energy storage is feasible. The iron and chromium chemistry is environmentally benign compared to other electrochemical systems, in that the iron and chromium species present have very low toxicity and the dilute, water-based electrolyte has a very low vapor pressure. These factors combine to make the iron-chromium RFB one of the safest systems for energy storage in personnel and environmental terms.

Practical Challenges

The standard potential of the Cr2+ – Cr3+ couple is near the hydrogen evolution potential. Care must be taken in the design of iron-chrome RFBs to minimize parasitic side reactions and then to reverse the associated capacity loss and electrolyte imbalance. Current developers of iron-chromium RFBs appear to have mitigated this side reaction and implemented effective re-balancing subsystems with minimal system efficiency loss.

Conclusion

Iron-chromium flow batteries are available for telecom back-up at the 5 kW – 3 hour scale and have been demonstrated at utility scale. Current developers are working on reducing cost and enhancing reliability. This system has the potential to be very cost effective at the MW – MWh scale, according to a 2010 EPRI report on energy storage.