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

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 (H-1-NMR) spectroscopic studies reveal its exceptional stability and ultra-low permeability (1.49 x 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).