Functionalised carbazole as cathode for high voltage non-aqueous organic redox flow battery

Prospective high reduction potential cathode material has been proposed that can be used in non-aqueous redox flow battery application. New class of material, 3,6-dibromo-9-(p-tolyl)-9H-carbazole (3) incorporating carbazole core, showing a very good reversibility is employed as the cathode material and dissolved oxygen in the solvent mixture is used as the anolyte material. Labile positions of the carbazole have been substituted with the electron withdrawing groups, which increases the reduction potential of the redox couple. Apart from substituting the labile positions, we have also explored the possible structural modification responsible for stabilizing the cation radical of carbazole moiety and obtained the reversible behavior thereafter. From this, it is evident that free radical is stabilized upon delocalization of the charge in the molecule. The mass-transport and redox parameters, diffusion coefficient and heterogeneous electron transfer rate coefficient values are high enough to realize good battery performance. A solvent mixture of acetonitrile and dichloromethane (4:1) has been used in this work in order to increase the solubility of electroactive material in the medium.