|Schematic of ORBAT. Click to enlarge.|
Scientists at USC have developed a novel water-based Organic Redox Flow Battery (ORBAT) for lower cost, long lasting large-scale energy storage. An open access paper on their work is published in the Journal of the Electrochemical Society.
ORBAT employs two different water-soluble organic redox couples on the positive and negative side of a flow battery. Redox couples such as quinones are particularly attractive for this application, the researchers said. (Quinones are oxidized derivatives of aromatic compounds.) No precious metal catalyst is needed because of the fast proton-coupled electron transfer processes. Furthermore, in acid media, the quinones exhibit good chemical stability. These properties render quinone-based redox couples very attractive for high-efficiency metal-free rechargeable batteries, they found.
Since grid-scale electrical energy storage 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. The vanadium redox flow battery is one such battery technology that has reached an advanced level of development for grid-scale applications. However, the limited resources of vanadium, the high expense associated with the cell materials, and the toxicity hazard of using large quantities of soluble vanadium, have been the major challenges to the widespread adoption of the vanadium redox flow battery.
Aiming to overcome these disadvantages, we have demonstrated for the first time an aqueous redox flow battery that uses water-soluble organic redox couples at both electrodes that are metal-free. Such a battery has the potential to meet the demanding cost, durability, eco-friendliness, and sustainability requirements for grid-scale electrical energy storage. We have termed this battery an Organic Redox Flow Battery (ORBAT).—Yang et al.
In ORBAT, two different aqueous solutions of water-soluble organic redox substances such as quinones are circulated past electrodes. The positive and negative electrodes are separated by a proton-conducting polymer electrolyte membrane. In the paper, the researchers a solution of 1,2-benzoquinone-3,5-disulfonic acid (BQDS) for the positive electrode; the negative electrode uses a solution of anthraquinone-2-sulfonic acid (AQS). By choosing the appropriate organic redox couples for the positive and negative electrodes, they projected that a cell voltage as high as 1.0 V is possible. The quinones have a charge capacity in the range of 200–490 Ah/kg, and cost about $5–10/kg or $10–20/kWh, leaving ample scope for achieving the US Department of Energy’s target of $100/kWh for the entire battery system.
The batteries last for about 5,000 recharge cycles, giving them an estimated 15-year lifespan. Lithium-ion batteries degrade after around 1,000 cycles, and cost 10 times more to manufacture.—Professor Sri Narayan, corresponding author
Narayan collaborated with Surya Prakash, Prakash, professor of chemistry and director of the USC Loker Hydrocarbon Research Institute, as well as USC’s Bo Yang, Lena Hoober-Burkhardt, and Fang Wang.
The tanks of electroactive materials can be made as large as needed—increasing total amount of energy the system can store—or the central cell can be tweaked to release that energy faster or slower, altering the amount of power (energy released over time) that the system can generate.
While previous battery designs have used metals or toxic chemicals, Narayan and Prakash wanted to find an organic compound that could be dissolved in water. Such a system would create a minimal impact on the environment, and would likely be cheap, they figured. Currently, the quinones needed for the batteries are manufactured from naturally occurring hydrocarbons. In the future, the potential exists to derive them from carbon dioxide, Narayan said.
The team is currently testing testing and analyzing the behavior of other redox couples in the quinone family, assess the impact of solubility on full cell performance, and optimize the structure of the membrane-electrode assemblies. Solubility is still a challenge for this type of redox flow battery, they noted.
Choosing a substituent such as sulfonic acid to modify both positive and negative electrode materials appears to be the most promising approach at this time to meet the challenge of solubility in water. However, understanding the effect of substituent group type and placement on the standard reduction potential and kinetic reversibility are also important areas for further study.—Yang et al.
The team has filed several patents in regards to design of the battery, and next plans to build a larger scale version.
This research was funded by the ARPA-E Open-FOA program (DE-AR0000337), the University of Southern California, and the Loker Hydrocarbon Research Institute.
Bo Yang, Lena Hoober-Burkhardt, Fang Wanga, G. K. Surya Prakash and S. R. Narayanan (2014) “An Inexpensive Aqueous Flow Battery for Large-Scale Electrical Energy Storage Based on Water-Soluble Organic Redox Couples” J. Electrochem. Soc. volume 161, issue 9, A1371-A1380 doi: 10.1149/2.1001409jes