Hofmeister‐Type Anion Regulation Suppresses Electrochemical Acidification for Long‐Life TEMPO‐Based Aqueous Flow Batteries

ABSTRACT 2,2,6,6‐Tetramethylpiperidine 1‐oxyl (TEMPO)‐based aqueous redox flow batteries are attractive for grid‐scale energy storage owing to their high redox potentials, fast kinetics, and high solubilities. However, their practical application is limited by progressive catholyte acidification, which accelerates disproportionation and ring‐opening degradation of TEMPO species, particularly at high concentrations. Herein, representative TEMPO derivatives are systematically investigated to elucidate the origin of acidification and its impact on cycling stability. Operando electrochemical and spectroscopic analyses reveal a common two‐stage acidification process that is largely independent of molecular substituents. The initial irreversible acidification originates from parasitic chloride oxidation and subsequent hydrolysis of reactive chlorine species, whereas the second stage arises from reversible proton release associated with nitroxyl radical/ N ‐oxoammonium interconversion. More importantly, Hofmeister‐type anion regulation is identified as a dominant factor governing proton activity and catholyte stability. Guided by this mechanism, replacing chaotropic chloride anions with kosmotropic sulfate anions effectively suppresses acidification and substantially improves cycling durability. Consequently, the optimized 1.0 M catholyte delivers 97.6% capacity retention after 300 cycles (413 h), while several‐fold lifetime enhancements are achieved across structurally distinct TEMPO systems. This study establishes a Hofmeister‐regulated electrolyte engineering strategy for stable aqueous flow batteries.