Fundamental Properties of Organic Redox Couples for Aqueous Flow Batteries
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 suitablefor 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.
The work presented here was funded by ARPA-E and the Loker Hydrocarbon Institute of the University of Southern California.