Carbon nanotubes (CNTs) have been employed as electrode materials in rechargeable zinc bromine redox flow batteries (ZBB) owing to their high electrocatalytic activity, remarkable electrical conductivity, and excellent mechanical strength with high Young’s modulus. The electrocatalytic effect of single-walled carbon nanotube (SWCNT) and multiwalled carbon nanotube (MWCNT) electrodes for the 2Br–/Br2 redox couple has been investigated for zinc bromine redox flow battery application. The anodic peak current density of SWCNT electrode is found to be about 16 mA cm–2, which is almost 50% higher than that of MWCNT, indicating the enhanced electrocatalytic effect of SWCNT perhaps due to a large amount of basal planes. The peak separation between the anodic and cathodic process at SWCNT and MWCNT electrodes is 201 and 126 mV, respectively, demonstrating the quasireversible nature of the 2Br–/Br2 redox reaction. Moreover, the peak separation for the MWCNT electrode is 37% less compared to that on the SWCNT electrode, revealing better reversibility. FTIR, Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) have been used to further investigate the composition and morphological changes of CNT before and after cycling. Zinc bromine redox flow cell made with CNT-anchored carbon felt (CF) as bromine electrode exhibits improved electrochemical performance in terms of efficiency and durability. Particularly, SWCNT-modified electrode possesses 98% energy efficiency retention even after 200 cycles of charge–discharge process, offering great promise as high-performance electrodes for zinc bromine redox flow battery.

The effect of zinc utilization and the performance characteristics in a zinc–bromine redox flow cell is reported. Simultaneously changing the quality of the electrolyte and the electrode substrate material resulted in an increase in the surface coverage with randomly oriented crystallographic (101) planes and, as a result, the cell exhibited higher Faradaic efficiency.

Herein for the first time, we have reported the performance and characteristics of new high-voltage zinc–vanadium (Zn–V) metal hybrid redox flow battery using a zinc bromide (ZnBr2)-based electrolyte. The Zn–V system showed an open-circuit voltage of 1.85 V, which is very close to that of zinc–bromine flow cell. The obtained results exhibited a voltaic, Coulombic, and energy efficiencies of 88, 82, and 72% at 20 mA.cm–2, respectively, in which low-cost microporous membrane was used as a separator. However, the cell tested using Nafion-117 membrane showed voltaic, Coulombic, and energy efficiencies of 84, 83, and 71%, respectively at a current density of 20 mA.cm–2. Furthermore, the Zn–V cell performance is also compared with the Zn–Br2 flow system to highlight the advancement of the new Zn–V system. The cell also showed stable performance up to 50 cycles at a current density of 20 mA.cm-2.

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