Ammonium‐Functionalized Naphthalene Diimide as Two‐Electron‐Transfer Negolyte for Aqueous Redox Flow Batteries

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.