Mitigating Ring‐Opening to Develop Stable TEMPO Catholytes for pH‐Neutral All‐Organic Redox Flow Batteries

Abstract Redox‐active organics are highly attractive in aqueous organic redox flow batteries (AORFBs). However, the lack of capacity dense, stable organic catholytes remains a challenge to develop energy‐dense, long cycle‐life AORFBs. Herein, a stable organic catholyte, 4‐[3‐(trimethylammonium)acetylamino]‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl chloride (TMAAcNHTEMPO) is developed through rational molecular engineering using connective acetamido and trimethylammonium groups. Paired with bis‐(trimethylammonium) propyl viologen tetrachloride anolyte, stable AORFBs (up to 1500 cycles) with a low capacity fade rate of ca. 0.0144% h⁻¹ are achieved. Experimental characterizations and theoretical simulations revealed that TMAAcNH‐TEMPO is largely stabilized by the reduced reactivity of the nitroxyl radical moiety that mitigates a ring‐opening side reaction.