Investigation of Tetramorpholinohydroquinone as a Potential Catholyte in a Flow Battery

Redox flow batteries (RFB) based on organic redox active species have low-cost potential for large-scale electrical energy storage. Currently, there are a range of organic molecules as candidates for the negative side of the RFB: e.g., anthraquinones and alloxazines. However, for the positive side there are no obvious organic molecules mainly because of stability that is too low and to a lesser extent a low standard potential. Currently, a stable performance has been achieved only with inorganic or organo-inorganic species at the positive side: e.g., bromine, ferricyanide, and water-soluble derivatives of ferrocene. Previous attempts to utilize hydroquinones at the positive side, such as 4,5-dibenzoquinone-1,3-benzenedisulfonate, or water-soluble derivations of 2,2,6,6-tetramethyl-1-piperidinyloxy resulted in unstable battery performance. On the basis of the available literature it appears as if unsubstituted and partially unsubstituted hydroquinones are not stable in water. For this reason a new fully substituted hydroquinone was specifically designed and examined for the positive side of the organic RFB in a symmetric RFB setup. It has four fully protonated morpholino groups, a solubility of 2 M (107 Ah/L) in water, and a formal redox potential of 0.89 VNHE; however, its performance is highly dependent on the pH and the electrolyte composition.