Hybrid Aqueous Alkaline Zinc/TEMPO Flow Battery: A Sustainable High Voltage Green Energy Storage Device

Abstract

Aqueous organic redox flow batteries (AORFBs) have number of distinct benefits for large scale energy storage, including synthetically configurable high oxidation/reduction redox potentials and excellent solubility. AORFBs still have poor energy density due to the low cell voltage, which is limited by either catholyte or anolyte. The hybrid RFB inherits the benefits of both aqueous and non-aqueous systems, demonstrating promising characteristics for next generation RFBs such as high potential window and maximum energy density. The hybrid neutral Zinc/TEMPO polymer hybrid flow battery exhibits the limiting cell voltage of 1.4 V, which resulting the low energy density. Herein, we freshly coupled alkaline Zn(OH) 4 2− /4-Hydroxy 2,2,6,6-Tetramethylpiperidine 1-oxyl (4HT) as a hybrid RFB system which exhibits the high cell voltage of 2.1 V with 25.32 Wh l −1 energy density. The galvanostatic charge-discharge shows stable performance at 5 to 40 mA cm −2 with the maximum 90% coulombic efficiency. The high rate GCD profile of alkaline Zn(OH) 4 2- /4HT at 40 mA cm −2 current density have shown 90% coulombic efficiency and 60% energy efficiency. More importantly, the alkaline-based Zn(OH) 4 2− /4HT exhibits remarkable capacity retention of 99.99% over 100 cycles with 25.32 Wh l −1 energy density. Thus, the freshly developed alkaline Zn(OH) 4 2− /4HT system would be a potential high voltage RFB for bulk energy storage.