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

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.