Fluorinated TEMPO: a new redox-active catholyte material for aqueous Zn-anode hybrid flow batteries

Abstract

Organic redox-active materials for aqueous redox flow batteries (ARFBs) have received extensive attention due to their abundant resources and high tunability. However, organic catholyte materials are often limited by chemical/electrochemical instability and low water solubility, making it challenging to meet the requirements of a long lifetime and high energy density for practical applications. Here, we disclose a previously unknown strategy through the tactical introduction of the fluorine functionality into 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) to create a new organic catholyte material sodium 2-(4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl)-2,2-difluoroacetate (DFTEMPO). It exhibits higher cell voltage, rate capability, and cycle stability in ARFBs than its non-fluorinated analogue and 4-HO–TEMPO. The Zn/DFTEMPO hybrid flow battery shows a cell voltage of 1.64 V and a peak power density of up to 244.9 mW cm⁻². Notably, symmetric cells of DFTEMPO realize a capacity decay rate as low as 0.022% h⁻¹ over 1000 cycles lasting 250 hours, demonstrating the excellent stability of DFTEMPO. A series of characterization studies and DFT calculations are conducted to understand the unique fluorine effect in enhancing cell performance, paving a new way to design organic redox-active materials for ARFBs.