With the recent great increase in utilization of renewable energy technologies such as wind turbines and solar cells, the need for environmentally friendly large-scale energy storage has also increased. Aqueous Organic Redox Flow Batteries (AqORFBS) are considered a promising technology for a new generation of grid-connected storage devices due to the possibility to decouple power output and capacity, inexpensive active material utilization, cheap aqueous electrolyte and ease of maintenance. One of the top-performing molecules for the negative electrolyte for AqORFBS is 9,10-Anthraquinone-2,7-Disulfonic Acid (AQDS) owing to its chemical stability, high solubility, low reduction potential and fast kinetics.

Building off of previous work[1], where AQDS was found to dimerize in aqueous solution and that the electrochemical reaction was impeded at higher concentrations, this study employed cyclic voltammetry and rotating disk electrode voltammetry to evaluate the electron transfer kinetics over a range of AQDS concentrations in both a 1 M sulfuric acid electrolyte and a 1 M pH 10 sodium carbonate buffered electrolyte. It was found that both the diffusion coefficient as well as the kinetic parameters of the electron transfer reaction changed with concentration of AQDS in the investigated range of 1-50 mM, with the strength of the effect depending on the choice of electrolyte. AQDS, along with a multitude of other organic molecules have been characterized electrochemically in regards of their suitability for redox flow batteries, but almost always at concentrations of no more than a few mM, which is likely not characteristic for the final application which often employs concentrations of upwards of 2 M[2]. Thus, this work points out the importance of taking concentration effects into account when considering the performance of a molecule for full-scale redox flow batteries.

1. Carney, T. J., et al. (2017). "Concentration-Dependent Dimerization of Anthraquinone Disulfonic Acid and Its Impact on Charge Storage." Chemistry of Materials 29(11): 4801-4810.
2. Pan, F. and Q. Wang (2015). "Redox Species of Redox Flow Batteries: A Review." Molecules 20(11): 20499-20517.