An Advanced Iron-Chromium Redox Flow Battery

Nowadays, the electricity generation cost using renewable resources has been reduced to such a low level that it is possible to replace most fossil fuel-based energy with renewables in the near future. However, the intermittent property with most renewables, such as solar and wind, requires vast amount, flexible and affordable energy storage products. None of the current widely used energy storage technologies can meet these requirements. An aqueous-based true redox flow battery has many unique advantages, such as long lifetime, safe, non-capacity decay, minimal disposal requirement, and flexible power and energy design. All these make it a great candidate for the vast renewable energy storage market. The key issue with this technology is the cost and availability of the energy-storage media. Due to the limited vanadium resources, it is very difficult for the vanadium-based redox flow battery to be widely used for fast-growing renewable energy storage market. Iron-chromium redox flow battery was invented by Dr. Larry Thaller’s group in NASA more than 45 years ago. The unique advantages for this system are the abundance of Fe and Cr resources on earth and its low energy storage cost. Even for a mixed Fe/Cr system, the electrolyte raw material cost can still be less than 10$/kWh. The major issue with this system is the continuous capacity decay due to hydrogen generation, which makes this system impractical for long-term operations. Here we report our recent results on this system. By using a special system design, the hydrogen generation level was largely reduced, down to less than 0.25% per full cycle. A capacity recovery system was successfully developed, which can compensate the system capacity loss due to hydrogen generation. More than 1000 cycles of operation have been carried out using a system with a kW-scale stack and 100 L electrolyte over a few months. Stable system capacity and stable continuous operation were successfully achieved. With these results, a containerized product is proposed. For a 20’ ISO container-sized product, the deliverable energy is 250 kWh, and the normal and maximum discharge powers are 25 and 50 kW respectively. The cost for such a product is lower than 80$/kWh, and the energy storage cost using this product is less than $0.02/kWh (assuming the product has 15 yr lifetime, and it operates 300 full cycle per year). With this energy storage cost, it is possible to achieve our ambitious 100% renewable energy goal in the near future.