Nearly X-ray amorphous hollow sphere manganese oxides (hollow sphere MnO 2 ) have been synthesized by a carboxylic acid-mediated system containing KMnO 4 and Na 2 S 2 O 4 under ambient conditions for supercapacitor applications. The product was characterized by powder XRD, Raman spectroscopy and thermal analysis. SEM and TEM were used to investigate the morphology of MnO 2 . The as-prepared MnO 2 was X-ray amorphous and had particles in the size range 0.1-1 mm. A mechanism has been proposed for the formation of hollow sphere structures in the micro-emulsion medium. Upon annealing the sample at temperatures greater than 500 C, the amorphous MnO 2 transforms into Mn 2 O 3 . Cyclic voltammetry and galvanostatic charge-discharge cycling were used to evaluate the electrochemical performance. The initial discharge capacities were found to be 283 and 188 F g À1 in 0.1 M Ca(NO 3 ) 2 and 0.1 M Na 2 SO 4 , respectively, at a current density of 0.5 mA cm À2 . The higher specific capacitance in the electrolyte with a bivalent cation is attributed to the reduction of two Mn 4+ to Mn 3+ by each of the bivalent cations present in the electrolyte.

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.

We have demonstrated a single step synthesis of halogen functionalized graphenes (HGs) through electrochemical exfoliation of graphite in aqueous potassium halide solutions. Few layers of as-synthesized HGs are characterized using powder X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Electron paramagnetic spectroscopy (EPR), Brunauer-Emmett-Teller (BET) surface area analysis, Field emission scanning electron microscopy (FESEM), and Transmission electron microscopy (TEM). The degree of halogenation is found to vary between 2.32 and 0.26 atomic % in fluorinated graphene (FG) and iodinated graphene (IG) respectively, which could be attributed to the difference in reactivity of the halogen species generated during the exfoliation process. A suitable mechanism is proposed in accordance with the exfoliation phenomena and functionalization of halogens on the graphene sheets based on the experimental observation. As synthesized HGs are utilized as electrode materials for zinc bromine flow batteries (ZBB) and the electrocatalytic effect of HGs electrodes for the 2Br−/Br2 redox couple has been investigated in detail. Cyclic voltammetry (CV), and charge-discharge studies were carried out to study the electrochemical performance of the HGs. Among all HGs, FG has shown the superior electrocatalytic behavior for 2Br−/Br2 redox reaction. The anodic (11.2 mA cm−2) and cathodic (10.7 mA cm−2) peak current densities are higher for FG than that of other halogenated graphenes. ZBB flow cell fabricated with FG as bromine electrode exhibits enhanced electrochemical performance in terms of efficiency (81% of voltaic efficiency and 72% energy efficiency) and durability up to 350 cycles.

Carbon nanotubes (CNTs) have been utilized as positive electrodes in rechargeable zinc bromine redox flow battery (ZBB) due to their high electrocatalytic activity, enhanced electrical conductivity and unusual mechanical strength with high Young's modulus. The electrochemical behavior of 2Br−/Br2 at pristine glassy carbon electrode (GCE) and CNT modified GCE studied by cyclic voltammetry (CV) in 0.05 M aqueous zinc bromide solution shows clear evidence for enhanced electrocatalytic activity for CNT based electrodes. The voltammetric peak current density at CNT is 16 mA cm−2 (2.5 times higher than that at GCE) presumably due to more active sites, demonstrating high electrocatalytic effect of CNT toward 2Br−/Br2 redox reaction. Composition and morphological changes have been further investigated using FTIR, Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Zinc bromine flow cell with CNT coated carbon felt (CF) as bromine electrode reveals improved the electrochemical performance in terms of voltage (VE), columbic (CE) and energy efficiencies (EE). The VE, CE and EE of a zinc bromine flow cell with CNT (90%) modified bromine electrode are 87%, 77% and 67% respectively.