Until very recently, the choice of electroactive materials in aqueous RFBs has been limited mostly to transition metal redox species, such as all-vanadium RFBs, which however are limited by the high cost of vanadium. Aqueous soluble organic (ASO) redox-active materials have recently shown promise as alternatives to transition metal ions to be employed in RFBs because of structural tunability, cost-effectiveness, availability, and safety features. To date, reported research on ASO species is rather limited and has been focused mostly on quinone, viologen, TEMPO, and ferrocene compounds.
In this presentation, we describe development of a new high energy density organic redox material as a promising ASO anolyte.1In our research, we focused on investigating the primary ASO properties (i.e., solubility and redox potential) through a framework of combined nuclear magnetic resonance (NMR), density functional theory (DFT), organic synthesis, and electrochemical studies. Rational introduction of functional moieties in an asymmetrical configuration initiates preferential solvation2that significantly enhances the solubility from near-zero to up to 1.8 M in potassium based supporting electrolyte. The electrochemical performance of the new organic redox couple based anolytes was evaluated in a RFB using the well-established ferro/ferricyanide catholyte that leads to a high cell voltage of 1.4 V. Cycled at a high ASO concentration of 1.4 M (96 % of its maximum solubility of 1.45 M in 1 M NaOH ), the flow battery produced an exceptional reversible volumetric capacity of 67.4 Ah L-1demonstrating for the first time of a ASO redox active material with reversible capacity equivalent to 2.8 M electron concentration.
1 Hollas, A.et al.A biomimetic high-capacity phenazine-based anolyte for aqueous organic redox flow batteries. Nat Energy3, 508-514, doi:10.1038/s41560-018-0167-3 (2018).
2 Han, K. S.et al.Preferential Solvation of an Asymmetric Redox Molecule. J Phys Chem C120, 27834-27839, doi:10.1021/acs.jpcc.6b09114 (2016).