Properties of Redox Couples for Use in Organic Redox Flow Batteries

With penetration levels of renewable energy sources, such as wind and solar, nearing 33% in many places, large-scale energy storage is desperately needed. Organic redox flow batteries (ORBAT), based on water-soluble quinone and anthraquinone derivatives, are a promising solution, as they meet the demanding requirements for large-scale energy storage [1-3]. ORBAT is environmentally friendly, easily scalable, highly efficient, and have low cost. These systems have the potential to meet the DoE cost target of $100/kWh.  

Organic molecules, such as quinones,  are ideal for an aqueous redox flow battery system based not only on their fast kinetics and high capacity, but also because they can be tuned for solubility and electrode potential. Addition of substituent groups affects not only solubility and electrode potential but also the kinetics and mechanism of the redox  reactions. We will present the chemical and electrochemical properties of various molecules that were studied to  understand the effect of substituents.

We will also present our current understanding of any chemical transformations that these compounds undergo during the cycling process. We have followed these changes using electrochemical and spectroscopic techniques.

The results in Figure 1a suggest that as more charge was added, the oxidation current increased, indicating the transfer from oxidized to reduced species, which is as expected for anthraquinone 2,6-disulfonic acid.

However, the 4,5-dihydroxy-1,3-benzenedisulfonic acid molecule undergoes a chemical reaction, known as Michael addition, that changes the composition of the starting molecules, and thus changes the cycling characteristics of ORBAT. This transformation can be seen in the persistent reduced state of the positive side material in Figure 1b.

Understanding the kinetics of the reactions of the redox molecules in water is of utmost importance in ensuring chemical reversibility, high efficiency and high capacity necessary for a useful ORBAT system.

Acknowledgement

The work presented here was funded by ARPA-E Open FOA (DE-AR0000353) and the University of Southern California.

References:

1)Yang, B.; Hoober-Burkhardt, L.E.; Wang, F.; Prakash, G.K.S.; Narayanan, S.R. (2013) J. Electrochem. Soc. 2014, 161, A1371

2)Yang, B.; Hoober-Burkhardt, L.E.; Prakash, G.K.S.; Narayanan, S.R. 224^th ECS Conference, San Francisco, CA (2013)

3) Yang, B.; Hoober-Burkhardt, L.E.; Prakash, G.K.S.; Narayanan, S.R. 225^th ECS Conference, Orlando, Fl (2013)