The iron-chromium (Fe-Cr) redox flow battery is one of the earliest flow battery chemistries to be investigated and developed, first studied systematically by NASA in the 1970s and subsequently by Mitsui Engineering and Shipbuilding in Japan. It uses earth-abundant, low-cost materials and remains of active research interest and commercial development interest for large-scale stationary energy storage.
Overview
Like all flow batteries, the Fe-Cr system stores energy in liquid electrolytes held in external tanks and pumped through an electrochemical cell stack during charge and discharge. The positive (catholyte) and negative (anolyte) electrolytes are both aqueous solutions of iron and chromium chlorides in hydrochloric acid. Unlike the vanadium redox flow battery, the two half-cells use different active elements, which introduces cross-contamination as a long-term operational challenge.
Electrochemistry
The cell reactions at nominal open-circuit voltage of approximately 1.18–1.2 V are:
- Positive electrode (charge): Fe²⁺ → Fe³⁺ + e⁻
- Negative electrode (charge): Cr³⁺ + e⁻ → Cr²⁺
Discharge reverses both reactions. The electrolyte is an aqueous solution of FeCl₂/FeCl₃ and CrCl₂/CrCl₃ in hydrochloric acid (typically 1–3 M HCl), with the same base solvent used in both half-cells to minimise osmotic imbalance across the membrane.
Key Performance Characteristics
- Open-circuit voltage: ~1.18–1.2 V
- Energy density: 15–30 Wh L⁻¹ (electrolyte level)
- Round-trip efficiency: 70–80% (system level)
- Electrolyte cost: Low — iron and chromium chlorides are commodity chemicals
- Operating temperature: 10–40°C (standard); can be extended with additives
- Calendar life: >10 years reported for well-managed systems
Advantages
- Low electrolyte material cost: iron and chromium chlorides are inexpensive and globally available
- Aqueous electrolyte: non-flammable, environmentally benign relative to organic alternatives
- Scalable capacity: independent of power rating (scale tanks for energy, stack for power)
- Long cycle life: no solid-state electrode degradation; active material is in solution
- Established chemistry: decades of fundamental research provide a solid knowledge base
Limitations
- Hydrogen evolution reaction (HER) at the chromium electrode reduces coulombic efficiency, particularly at high charge rates and elevated temperatures
- Slow Cr³⁺/Cr²⁺ kinetics at carbon electrodes require catalytic activation (e.g., Bi or Pb additives) to achieve acceptable performance
- Cross-contamination through the membrane over time leads to capacity imbalance between the two half-cells, requiring periodic electrolyte rebalancing
- Lower energy density than vanadium redox flow batteries requires larger tank volumes for equivalent storage
- HCl electrolyte is corrosive; system materials must be selected accordingly
Applications
Grid-scale stationary energy storage is the primary target application, particularly in regions where low capital cost is prioritised over energy density. Fe-Cr flow batteries are being commercialised in China for utility-scale storage projects. They are not suited to mobile or space-constrained applications due to their low volumetric energy density.
References
- Sun, C. et al. “Review of the Development of First-Generation Redox Flow Batteries: Iron-Chromium System.” ChemSusChem, 2022. doi:10.1002/cssc.202101798
- Zeng, Y.K. et al. “Effect of Chelation on Iron–Chromium Redox Flow Batteries.” ACS Energy Letters, 2020. doi:10.1021/acsenergylett.0c00761
- Ke, X. et al. “Hydrogen evolution mitigation in iron-chromium redox flow batteries via electrochemical purification of the electrolyte.” Journal of Power Sources, 2022. doi:10.1016/j.jpowsour.2022.232012
- Thaller, L.H. “Electrically rechargeable redox flow cells.” NASA Technical Memorandum, 1974
- Wang, W. et al. “Aqueous iron-based redox flow batteries for large-scale energy storage.” National Science Review, 2025. doi:10.1093/nsr/nwaf218