Aqueous organic redox flow batteries (AORFBs) have been developed as safe and economical energy storage systems for renewable energy applications. Herein, the solubility of two-electron-transfer organic molecules, naphthalene diimides (NDIs), was improved by adding two propyl-spaced ammonium functionalities. Under neutral conditions, the di-ammonium-functionalized NDIs exhibited a solubility of similar to 0.7 M (1.4 M/e-) in water. Using 0.3 M NDIs as negolyte (negative electrolyte) and iodide/triiodide as posolyte (positive electrolyte), AORFBs achieved 300 galvanostatic cycles with approximately 100 % capacity retention. The post-mortem analyses revealed negligible chemical decomposition with no crossover while using a Nafion membrane. This study presents a promising NDI negolyte that can achieve stable two-electron transfer in AORFBs.