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A quaternary amine‐functionalized naphthalene diimide (NDI) moiety is synthesized and considered as a redox‐active species for application in aqueous organic redox flow batteries. For the first time, this NDI is characterized electrochemically in aqueous solutions, using cyclic and rotating disk electrode voltammetry, bulk electrolysis, as well as 1 H‐nuclear magnetic resonance ( 1 H‐NMR) spectroscopy. The molecule reaches a solubility of 0.68 m in water and reversibly delivers two electrons at attractive potentials for flow battery applications. Further exploration with 1 H‐NMR reveals a strong dimerization of the NDI species with an equilibrium constant of 146 m −1 . Using diffusion NMR coupled with rotating disk electrode voltammetry, it is shown that the dimer retains limited redox‐activity, yielding two electrons per dimer unit. However, using galvanostatic bulk electrolysis, close to the theoretical capacity is obtained, indicating a fast dissociation reaction of the reduced dimer. Finally, the NDI species shows excellent stability; after constant cycling for 1 week, no degradation is detected. In conclusion, NDI is demonstrated to be a highly attractive candidate for aqueous redox flow batteries.
Wiberg et al. (2019) studied this question.