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 is no obvious organic molecules mainly because of too low stability and to a lesser extend a low standard potential. Currently, a stable performance was 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-benzene disulfonate, or water-soluble derivations of 2,2,6,6-Tetramethyl-1-piperidinyloxy, results in unstable battery performance. Based on 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 symmetric RFB setup. It has four fully protonated morpholino groups and a solubility of 2 M (107 Ah/L) in water and a formal redox potential of 0.89 VNHE.