Naphthalene Diimide As a Two-Electron Anolyte for Aqueous and Neutral pH Redox Flow Batteries
Developing organic materials that show stable and highly reversible redox-activity is a key challenge for achieving cost-effective and sustainable aqueous redox flow batteries (RFBs) for practical energy storage applications. We report a water-soluble naphthalene diimide (NDI) derivative using glycinyl groups, which was tested and established as a new class of versatile negolytes capable of storing two electrons in a single molecule in aqueous and neutral pH RFBs. The potassium salt of N,N’-bis(glycinyl)naphthalene diimide [K2-BNDI] showed reversible and stable two-electron reductions accompanied by ion pairing. A prototype aqueous RFB consisting of [K2-BNDI] and 4-OH-TEMPO was constructed and showed excellent cyclability, high energy and voltage efficiencies. This study suggests a versatile way of designing new negolytes inspired by organic semiconductors that operate in a multi-electron redox mode.