Recent Advances in Inexpensive Aqueous Batteries for Large Scale Electrical Energy Storage
Authors: Narayanan S. R. , Manohar Aswin K , Malkhandi Souradip , Yang Bo , Yang Chenguang , Trinh Phong , Hoober-Burkhardt Lena , Kim Kyu Min , Prakash G. K. Surya 
Published: February 2020

The integration of electrical energy generated from renewable sources such as solar and wind power into the electricity grid faces the challenge of intermittent electricity output from these renewable sources. Storing the electricity during times of excess production and releasing the electrical energy to the grid during times of peak demand is an obvious solution. Rechargeable batteries are very attractive for energy storage because of their high energy efficiency and scalability. [1-3]
Since grid-scale electrical energy storage at a global scale requires hundreds of gigawatt-hours to be stored, the batteries for this application must be inexpensive, robust, safe and sustainable. None of today’s mature battery technologies meet all of these requirements. In this presentation, we will summarize the recent research advancements in three aqueous battery systems that have the potential to meet the demanding requirements of grid-scale energy storage: (1) alkaline iron-air battery, (2) the iron-chloride redox flow battery and (3) a new aqueous organic redox flow battery. These three battery systems satisfy the primary criterion of using of inexpensive or abundantly-available and sustainable materials for energy storage. The use of toxic heavy metals is completely avoided.
By careful selection of additives, the iron electrode of the alkaline iron-air battery can now be charged at high as C-rate with no more than 5% loss in faradaic efficiency to parasitic hydrogen evolution. The iron electrode also has a utilization 0.3 Ah/g and can be discharged continuously at rates as high as 3C. Such a high-performance iron electrode has also been cycled over 500 times without the loss of capacity or change in faradaic efficiency. Separated air electrodes based on carbon and spinel oxides can be charged and discharged with a significant reduction with overpotential losses not exceeding 250 mV at 10 mA/cm2. [4,5]
The electro-deposition efficiency of the iron-chloride flow battery has been improved to as high as 95% with use of additives for complexing the iron (II) in solution and by controlling the pH to be higher than 2. An iron-chloride flow cell that uses an anion-exchange membrane for the transport of chloride ions has been operated successfully over multiple charge discharge cycles.
We have advanced a new type of Organic Redox Flow Battery (ORBAT) that employs two different water-soluble organic redox couples on the positive and negative side of a flow battery. Compounds such as quinones are particularly suitable as redox couples. No precious metal catalyst is needed because of the fast proton-coupled electron transfer processes. The ORBAT cell uses a membrane-electrode assembly configuration similar to that used in polymer electrolyte fuel cells and can be charged and discharged multiple times at high faradaic efficiency without any noticeable degradation of performance. The ORBAT configuration presents a unique opportunity for developing an inexpensive and sustainable metal-free rechargeable battery for large-scale electrical energy storage.
Acknowledgements
The authors thank ARPA-E, US Army RDECOM, the University of Southern California, and the Loker Hydrocarbon Research Institute for funding the research.
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