Aggregation and Capacity Limiting Effects in Anthraquinone-Based Flow Battery Negolytes

Abstract

Anthraquinone-based molecules constitute promising electroactive materials for use in aqueous organic flow batteries. It has been observed that in high concentration in aqueous solutions these molecules tend to aggregate. It has been speculated that the aggregation causes a decrease in capacity as the number of electrons partaking in the electrochemical processes gets smaller. Three water-soluble anthraquinones were studied in different electrolytes in order to have a more meaningful insight into the possible causes, aggregation phenomena and electrochemical performance. 1H-NMR and PGSTE measurements were performed in non-deuterated solvents. Single cell testing was performed to assess the number of electrons. The aggregation constants have been determined using data from both 1H-NMR and PGSTE at different concentrations. Those data were fit into the Isodesmic model wherein the diffusion coefficient of the monomers and aggregation constants were extracted and compared. In all cases NMR signals showed aggregation taking place but it did not translate to reduced number of electrons. Capacity retention was very profound in the case of disodium anthraquinone-2,7-disuflonate (AQDS) in carbonate solution, wherein a reduced amount of electrons from 2 to 1 was observed. 2,6 dihydroxyanthraquinone (DHAQ), to a less extent, also showcased a reduced amount of electrons in carbonate solution.