Fundamental Properties of Organic Redox Couples for Aqueous 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 (ORFB) are a promising solution, as they meet the demanding requirements for large-scale energy storage.¹ They are environmentally friendly, easily scalable, highly efficient, and have low cost. Quinones are an excellent candidate for ORFB as they have a system cost of <$100/KWh and a charge capacity of 200-490 Ah/kg.

Organic molecules such as quinones are ideal for an aqueous redox flow battery system based not only on their fast kinetics, but also on their ability to 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. Electrochemical studies were undertaken to understand these effects. This understanding will be beneficial for the deployment of these organic reactions in an ORFB.

Electrochemical properties of various quinones in acidic media were studied in an electrochemical half-cell at a rotating disk electrode (Figure 1). Diffusion coefficients were found to be in the range of 10^-6 – 10^-7 cm²/sec, while rate constants were found to be in the range of 10^-3-10^-4 cm/sec (Figure 2,3). We have explored a range of quinones with different substituent groups to achieve the desired solubility and electrode potentials suitable for use in the full redox flow cell. We will present understanding of the relationship between the values of kinetic parameters and substituent group placement. Additionally, we have screened other water-soluble redox molecules such as quinoxalines and hydrazides. We will also present our current understanding of the electrochemistry of these compounds and their suitablity for redox flow batteries.

Acknowledgements

The work presented here was funded by ARPA-E and the Loker Hydrocarbon Institute of the University of Southern California.

Reference

1)  Yang, B.; Hoober-Burkhardt, L.E. Prakash, G.K.; Narayanan, S.R. ECS Meeting, San Francisco, CA, USA, Oct 27-Nov 1, 2013