Near Neutral Aqueous Fe-Cr Complex Flow Battery
Authors: LI Liyu , Luo Qingtao 
Published: July 2022

A redox-flow battery (RFB), as schematically shown is a unique type of rechargeable battery in which the electrochemical energy is stored in soluble redox couples contained in electrolyte tanks, and the electrical energy and the chemical energy are converted back and forth inside a device called “stack”. This unique structure successfully separates the power (stack) and the energy (electrolyte) of the battery system and can be economically designed for long-duration energy storage applications. While vanadium flow batteries have achieved initial commercial deployments, the cost of a four-hour system at deployment-scale is still more than $300/kWh, more than 50% of which comes from the cost of the energy storage media – Vanadium element. DOE’s multiple programs clearly indicates that future capital cost reductions will require replacing vanadium with lower cost raw materials to approach the $100/kWh targets required for wider-scale deployment of energy storage systems. Since 2018, attracted by its low electrolyte cost, our team have been working on the legendary Fe-Cr redox flow battery system, which was first invented by Dr. Lawrence Thaller of US NASA in 1975, to develop a low[1]cost flow battery product. The energy storage capacity decay caused by H2 generation, which comes from the negative electrode due to the low standard potential of Cr 2+ /Cr 3+ , makes it not practical for long-term energy[1]storage operations. After two years’ research, we have successfully developed an advanced Fe-Cr redox flow battery system. In this system, no capacity decay over continuous charge and discharge operation has been successfully achieved by an extremely low H2 generation design and a unique capacity recovery technology using a reductant and a homogeneous catalyst. Continuous operation of a test stand using a kW-scale stack and 100L electrolyte for a total 1000 cycles has been successfully achieved. Related information and results were published in the granted US patents US 10,826,102 and US 10,777,836, and the US patent applications 20200373594, 20200373595, 20200373600, 20200373601. During our previous development of the advance Fe-Cr redox flow battery, we discovered that the stability and electrochemical activity of Fe- and Cr-containing species in the aqueous solution can be remarkably improved by complexing them with some ligands. The acidity of this novel electrolyte solution is much weaker than that of the traditional Fe-Cr flow battery system, i.e., ~0.1M vs. >2M, which largely reduces the corrosivity of the electrolyte and allows for wider applications of this system with minimal environmental impact, especially for residential users and small-scale commercial and industrial users. Several different complexing ligands were identified. All showed very stable performance and low side reactions. Up to now, the test has been continuously carried out for more than three months and more than 500 cycles, using a kW-scale stack and 100 L electrolyte solution. Much-improved electrochemical reactivity of the Fe- and Cr-complex also eliminates the need for metal catalysts for the Cr 2+ +e ↔ Cr 3+ reaction, significantly improved the stability of the system for long-term operation.