Cycling Analysis of a Quinone-Bromide Redox Flow Battery

We report the dependence upon current density of voltage polarization; charge capacity; and current, voltage, and energy efficiency for a flow battery comprising 2,7-anthraquinone disulfonic acid and hydrobromic acid as redox-active species in the electrolytes. We develop relationships predicting several of these figures of merit from the polarization curves. The decrease in capacity with increasing current density is shown to be a direct consequence of the interplay of the polarization curves and the voltage limits imposed during cycling. The linearity of the polarization curves results in an inverse linear relationship between instantaneous voltage efficiency and current density. The average voltage efficiency over a complete cycle is shown to follow this same relationship when the open-circuit voltage and polarization resistance are evaluated at 50% state of charge. Current efficiency loss mechanisms are classified according to whether they lead directly to cycle capacity loss. The current efficiency increases with current density due to constant-rate loss mechanisms at a rate of 1.08 mA/cm(2), which is consistent with the rate of bromine crossover. Quinone crossover is negligible at 140 pA/cm(2). The effective differential capacity retention rate is 99.90% per cycle over 40 cycles. Mechanistic interpretations for these results are offered and interrelationships derived. (C) The Author(s) 2015. Published by ECS. All rights reserved.