High-voltage and durable pH-neutral aqueous redox flow batteries based on quaternary ammonium cations functionalized naphthalene diimide and nitroxyl radical systems

Aqueous organic redox flow batteries (AORFBs) are regarded as one of the most promising battery systems for grid-scale and sustainable energy storage, but the longstanding problems, typically, low energy density and inferior cycling stability, must be resolved. Herein, highly water-soluble naphthalene diimide (NDI) derivatives modified with two positively charged and hydrophilic ammonium terminals are synthesized and used as two-electron anolytes for AORFBs. The redox chemistry of NDI molecules is understood by cyclic voltammetry and the corresponding Pourbaix diagram, indicating the two single-electron processes without being protons under neutral pH conditions. Interestingly, the crossover of 4-ammonium-(2,2,6,6-tetramethylpiperidin-1-yl)oxyl (NMe-TEMPO) species occurs, but the hydrogenation reaction of crossed NMe-TEMPO in the anolyte is significantly inhibited, thus alleviating the capacity degradation of AORFBs. As a result, the NDI/NMe-TEMPO AORFBs deliver no distinct capacity decay over 1300 cycles and a stable cycling life over 3500 cycles with 0.00394% capacity fade rate per cycle, which is one of the representative examples among the existing pH-neutral AORFBs. This work clarifies the capacity decay and two single-electron reduction mechanisms and presents a significant advancement for constructing high-voltage and durable pH-neutral AORFBs based on the two-electron naphthalene diimide and nitroxyl radical systems.