Synthesis, Characterization and Application of Thioindigosulfonic Acids as Electrolytes in an Aqueous Organic Redox Flow Battery

Abstract

The potential of the indigo chromophore as a long-time stable, redox-active unit in an organic redox flow battery (ORFB) has rarely been considered so far. The present work demonstrates a first synthetic access to thioindigosulfonic acids by sulfonation of the parent dye. To evaluate the electrolytic properties of the thioindigosulfonic acids, the solubility in sulfuric acid, the Nernst potentials, the diffusion coefficients, the charge transfer coefficients, the exchange current densities and the rate constants of the electron exchange reaction have been determined. Furthermore, thioindigo-based electrolytes could be operated successfully over a period of 200 charge/discharge cycles in a flow cell. The electrochemical stability of the electrolytes as well as the absence of crossover phenomena was proven by comparison of their ¹H NMR spectra before and after the charge/discharge study. Therefore, thioindigosulfonic acids could offer an opportunity to develop a stable organic electrolyte for the application in an aqueous ORFB.