Prediction of Pourbaix diagrams of quinones for redox flow battery by COSMO-RS

Abstract

Redox-flow batteries are relevant to store energy from intermittent sources such as solar panels or wind turbines, thereby smoothing their energy supply. Up to now, most redox-flow batteries are based on vanadium. Vanadium is a rare and expensive material, thus recent research has focused on redox-flow batteries based on organic compounds and, in particular, anthraquinones as electroactive materials. However, the tunability of organic chemistry poses a needle-in-haystack challenge as the structures exhibiting the most desirable electrochemical properties may be hard to pinpoint. Moreover, the low water solubility of the most readily available anthraquinones may hinder their use as battery electrolytes. To aid in such endeavor, a theoretical approach is proposed to predict Pourbaix diagrams of redox-active organic compounds, allowing in silico anticipation of their electrochemical behavior in the entire pH range. DFT/COSMO-RS predicted pKa and reduction potentials are in good agreement with experimental data, and the resulting calculated Pourbaix diagrams are also in agreement with 4 experimental ones from literature, proving the reliability of the method. Finally, the effect of nature and position of some functional groups on the anthraquinone backbone is discussed, illustrating the power of the method to both understand and quantify the electrochemical activity of redox active organic materials.