A Stable, Low Permeable TEMPO Catholyte for Aqueous Total Organic Redox Flow Batteries

Abstract Aqueous organic redox flow batteries (AORFBs) are highly attractive for large‐scale energy storage because of their nonflammability, low cost, and sustainability. (2,2,6,6‐Tetramethylpiperidin‐1‐yl)oxyl (TEMPO) derivatives, a class of redox active molecules bearing air‐stable free nitroxyl radicals and high redox potential (>0.8 V vs NHE), has been identified as promising catholytes for AORFBs. However, reported TEMPO based molecules are either permeable through ion exchange membranes or not chemically stable enough for long‐term energy storage. Herein, a new TEMPO derivative functionalized with a dual‐ammonium dicationic group, N 1 , N 1 , N 1 , N 3 , N 3 , 2, 2, 6, 6‐nonamethyl‐ N 3 ‐(piperidinyloxy)propane‐1,3‐bis(ammonium) dichloride ( N 2 ‐TEMPO ) as a stable, low permeable catholyte for AORFBs is reported. Ultraviolet–visible (UV–vis) and proton nuclear magnetic resonance ( 1 H‐NMR) spectroscopic studies reveal its exceptional stability and ultra‐low permeability (1.49 × 10 −12 cm 2 s −1 ). Coupled with 1,1′‐bis[3‐(trimethylammonio)propyl]‐4,4′‐bipyridinium tetrachloride ((NPr) 2 V) as an anolyte, a 1.35 V N 2 ‐TEMPO /(NPr) 2 V AORFB with 0.5 m electrolytes (9.05 Wh L −1 ) delivers a high power density of 114 mW cm −2 and 100% capacity retention for 400 cycles at 60 mA cm −2 . At 1.0 m electrolyte concentrations, the N 2 ‐TEMPO /(NPr) 2 V AORFB achieves an energy density of 18.1 Wh L −1 and capacity retention of 90% for 400 cycles at 60 mA cm −2 .