Quinhydrone Formation and Its Impact on the Cell Voltage of the Quinone-Bromide Flow Battery

Understanding complexation of redox-active species in flow battery electrolytes is important for engineering cell voltage. Complexation between reactants and products of a chemical reaction can significantly shift its thermodynamics. This effect is more pronounced in concentrated solutions. In the quinone-bromide flow battery (QBFB) [1], such complexation, which exists on both sides, is suspected of being responsible for discrepancies of order 100 mV between the measured open circuit potential and that expected from simple Nernstian behavior. Here we focus our attention on the negative electrolyte, where quinone complexes with its reduced counterpart, hydroquinone, to form the quinhydrone dimer. Density functional theory and UV-Vis spectroscopy combine to provide insights into the quinhydrone forming mechanism and evaluate the equilibrium constant. This quinhydrone equilibrium permits the accurate prediction of the measured voltage of a quinone half-cell vs. nominal quinone and hydroquinone concentrations. By further considering tribromide and pentabromide formation on the bromide side, the voltage model explains the linear and sharply ascending cell voltage vs. state of charge behavior observed in the QBFB.

[1] B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon and M.J. Aziz, “A metal-free organic-inorganic aqueous flow battery”, Nature 505, 195 (2014), http://dx.doi.org/10.1038/nature12909