Why Iron-Chromium (Fe-Cr) Redox Flow Batteries Now?

By Thomas Gebauer, Redox One CEO

Anyone following the progress of renewable energy knows that the modern electric grid is undergoing tremendous changes. There are various reasons for this - from decreasing solar PV and wind energy costs to positive policies to reduce GHG emissions, and increased electrification. A key part of integrating cleaner, emission free generation has been the tremendous growth of utility scale energy storage. 10 years ago, short duration systems (less than 30 minutes) were used to assure a reliable grid by providing ancillary support like frequency regulation and voltage support, which are often impacted by renewables. The demonstration of early energy storage systems led to broader use for capacity needed for a few hours, less than 100 days, a year. But today, four-hour systems are being used daily to match the intermittent generation of renewables to peak loads. This trend of growing capacity from kW to MW and now GW systems, along with longer durations from 30 minutes to four hours is expected to continue, with that functionality typical in new systems being installed. With grid researchers and regulators now looking at systems of 8-10 hours being needed over the next five years, and even longer duration storage beyond 2030.

storage installation

That’s where Redox Flow Batteries (RFBs) come into their own. 100’s of organisations will be coming together in Glasgow to attend the International Flow Battery Forum (IFBF). Engineers, project developers, scientists, investors, policy maker/regulators and others will all be attending IFBF 2024 Significant work is underway, and will be presented there, on the exploration of new chemistries and materials, tactics to drive performance up, and costs down, ways of maturing the technology, and optimising for new Long Duration Energy Storage (LDES) applications. Redox Flow Batteries have already proven themselves as capable and mature in utility and large-scale applications. Given the growing need for LDES technologies that are safe and cost effective, RFBs will likely play a critical role in energy storage. They’re likely to experience high growth over the next five years, as the need from power conditioning 10 years ago, migrates to Long Duration Energy Storage LDES over the next 10 years.

So, given the existence of capable RFBs today, why Iron-Chromium (Fe-Cr) RFBs now? The answer: Because Fe-Cr RFBs have one of the safest chemistries, and offer massive scalability, with low-cost potential. New innovations also enable more low-cost potentials.

Fe-Cr RFBs are the original flow battery. Developed by Larry Thaller et al at NASA in the 70’s and 80's. Since then, 100’s of systems has been produced. A Megawatt-hour scale system was successfully demonstrated in 2014. The improvements over time include:

  • Mixed species electrolytes (the same electrolyte is used on both sides, so elementally the same, so no diffusion, no permanent capacity losses from diffusion)
  • The use of micro-porous membranes taking advantage of mixed species to avoid reliability issues, allow greater operating range window without poisoning the membrane, and reduces the costs by over 95%
  • Purity controls on electrolytes to reduce H2 formation rate by 100X
  • Re-balance systems to reverse side reactions and maintain capacity

Minimise side reactions

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From EnerVault DOE ARRA Demonstration

Fe-Cr RFBs are particularly attractive for their massive in-place supply and low-cost minerals. Fe and Cr have large annual extraction/refining capacity, reserve base, and alternative usage. No exotic materials required! 


Global mining company Tharisa PLC has brought additional innovation to the space, utilising their expertise in mining and refining. Direct from chromite Fe-Cr electrolytes. Previous electrolytes were made from purified Fe and Cr in chlorides. This involves extensive purification steps for both the Fe and Cr. Tharisa has found a way to produce Fe-Cr in fewer purification steps by producing the electrolytes directly from chromite FeCr2O4, without going through the costly process of separating Cr from Fe, only to blend them back together. This process enables the potential for the marginal energy costs to be dramatically below current incumbent storage and other flow batteries.  

And what of all that learning, and supply chain developed for RFBs? Fe-Cr RFBs can be used in existing RFB designs, just with lower electrolyte costs, and lower stack costs, and by leveraging the expertise and supply chains already in place. Stack costs can be further reduced and made more reliable by incorporating micro‑porous membranes instead of ionic membranes. Safety is further enhanced by using minerals with low environmental hazard and simplified handling/containment using the high solubility of Fe-Cr and being able to use low pH solvents. 

There’s still lots of work to be done to scale the technology up, complete testing, and begin pilot level demonstrations. But stay tuned in 2025, for results from Redox One, as it brings enhanced safety and low costs, to proven technology and set to address the rapidly growing LDES market.