A bromide-ligand ferrocene derivative redox species with high reversibility and electrochemical stability for aqueous redox flow batteries

An important performance aspect of aqueous redox flow batteries (RFBs) using organic redox couples is the ability to obtain the highest concentration and reversibility of charge storage species possible. This paper introduces a new approach of using a dimethylethylferrocenylmethylammonium bromide (Fc1N112-Br, C15H22FeNBr) as a positive organic redox moiety, resulting in exceptionally high stability and reversibility as well as excellent solubility in aqueous solution without any supporting electrolytes. The Fc1N112-Br is prepared via a nucleophilic substitution of dimethylaminomethyl ferrocene (C13H17FeN) with bromoethane (C2H5Br) in acetonitrile (C2H3CN). It is experimentally confirmed that the Fc1N112-Br exhibits the significantly excellent electrochemical performance such as high solubility of more than 2.9 M (72 times higher than that of an unsubstituted ferrocene), nearly equal diffusion coefficients of Fe2+ and Fe3+ species, charge transfer coefficients of close to 0.5, and significantly improved reaction kinetics. In addition, the Fc1N112-Br in the cell operation shows a discharge retention capacity ratio (vs. the first discharge capacity) of 99% or more at cycle 20. Consequently, the Fc1N112-Br leads to the apparent redox reaction that is not only electrochemically stable and reversible but also quasi-reversible and symmetric. This paper provides a new direction for improving the performance of organic redox couple-based aqueous RFBs and the Fc1N112-Br can be considered as an active species for aqueous RFBs. [Display omitted] •A dimethylethylferrocenylmethylammonium-bromide is suggested, C15H22FeNBr.•This is very effective as an aqueous-soluble organic positive redox-active species.•High reversibility and stability of the Fe2+/Fe3+ redox reaction are indicated.•Its solubility is greater than 2.9 M, 72 times higher than a pristine ferrocene.•Discharge capacity retention ratio keeps significantly stable at about 99%.