Branching-Induced Intermolecular Repulsion Effects Drive Stable and Sustainable Flow Batteries on Condensed Nitroxyl Radicals.

Aqueous organic redox flow batteries (AORFBs) play a critical role in scalable energy storage applications where safety, cost, and lifetime matter most. However, harnessing the organics with redox chemistry is plagued by major molecular engineering principles in reversible processes and transformations. Herein, breaking from the conventional linear substituent structures, we report a chain-branched dual-ammonium nitroxyl radicals derivative as a stable and flowable catholyte for AORFBs. Paired with a viologen anolyte, the AORFBs with condensed electrolytes deliver a high-capacity retention rate of 99.992%/cycle (99.85%/day) and a peak power density of 140.3 mW cm. In situ ultraviolet-visible characterization and theoretical simulation elucidate that the branched dual-ammonium structure accelerates ∼40% of the binding energy barrier, thereby enhancing intermolecular electrostatic repulsion. This effect effectively inhibits side reactions triggered by nucleophilic attacks, particularly in condensed nitroxyl radicals, maintaining the structural stability of both radical and oxoammonium states as well as their reversible transformations. Our redox organic formulation offers a direction towards stable and high-energy density AORFBs that seamlessly integrate eco-friendliness, durability, and sustainability.