G. Rajeshkhanna

Synthesis of mesoporous NiCo2O4–rGO by a solvothermal method for charge storage applications

The spinel NiCo2O4 material has received considerable attention as an excellent supercapacitor material. In this study, we report a facile and cost-effective solvothermal method for the synthesis of mesoporous NiCo2O4 anchored on reduced graphene oxide (rGO). The electrochemical activity of the NiCo2O4–rGO and pristine NiCo2O4 materials were evaluated by cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS). The NiCo2O4–rGO composite electrode shows a high specific capacitance value of 870 F g−1 at a current density of 2 A g−1 and it retains 600 F g−1 capacitance even at a high current density of 20 A g−1. Pristine NiCo2O4 shows a poor capacitance value of 315 F g−1 at 2 A g−1 and it retains only 191 F g−1 at 10 A g−1. Furthermore, the NiCo2O4–rGO nanocomposite shows an excellent cyclic performance with 90% capacitance retention even after 5000 charge–discharge cycles at a high current density of 4 A g−1, whereas a pristine NiCo2O4 electrode shows only 45% capacitance retention. The high specific capacitance, remarkable rate capability and excellent cycling performance offered by the NiCo2O4–rGO composite is attributed to the high surface area and high conductivity. In addition, rGO is believed to shorten the diffusion, migration paths for electrolyte ions and an easy access for electrolyte ions into redox centers.

In situ fabrication of graphene decorated microstructured globe artichokes of partial molar nickel cobaltite anchored on a Ni foam as a high-performance supercapacitor electrode

By taking advantage of the splendid properties of graphene (electrical conductivity) and transition metal oxides (pseudocapacitance nature), we have in situ fabricated novel microstructured globe artichokes of a rGO/Ni0.3Co2.7O4 composite on a nickel foam through a simple surfactant free hydrothermal method followed by calcination process. The globe artichoke flower-like morphology is constructed by hundreds of self-assembled micropetals interconnected with several layers and circles at the base to form microspheres of uniform dimension. The as-obtained morphology of the microstructured globe artichokes enhanced the stability and electrochemical performance of the hybrid electrode due to of its unique structure. Therefore, the synergetic effects and interconnected structure of the thus made binder free rGO/Ni0.3Co2.7O4 hybrid electrode allows better charge transport and exhibits superb specific capacitance and areal capacitance of 1624 F g−1 and 2.37 F cm−2 at a current density of 2 A g−1. Moreover, the specific capacitance increases from 1088 F g−1 to 1728 F g−1 at the end of 7000 cycles, which indicates that the material becomes active with cycling. Furthermore, when the power density increased by 16 times i.e. from 0.5 to 8 kW kg−1 the energy density sinks to 40 from 56.39 W h kg−1 (i.e., 29% reduction only), suggesting a remarkable electrochemical performance for supercapacitor applications.

Charge storage, electrocatalytic and sensing activities of nest-like nanostructured Co3O4

We synthesized nanostructured Co3O4 samples using anionic (SDS), cationic (CTAB) and nonionic (Triton X-100) surfactant molecules in hydrothermal conditions and subsequent calcination. This approach facilitates the synthesis of porous Co3O4 material with bundle-like-sheet, nest-like and flake-like morphologies with specific surface areas in the range of 50-77m2g-1. Among these materials, the nest-like nanostructured Co3O4 material has unique pore architecture, larger pore volume, low solution and charge transfer resistance, and found to be an active material for charge storage, electrocatalytic and sensing applications. The specific capacitance value of the nest-like Co3O4 is 404Fg-1 at a current density of 2Ag-1 with 80% specific capacitance retention. The electrocatalytic oxidation of methanol occurs at lower onset potential on this material with good electrochemical stability. It has good sensing ability for glucose with high sensitivity of 929μAcm-2mM-1, fast response time of ∼0.5s and detection limit as low as ∼1μM. These results show that the nest-like nanostructured Co3O4 material is a versatile candidate for various applications.

In situ grown nano-architectures of Co3O4 on Ni-foam for charge storage application

AbstractNanostructured Co3O4 on Ni-foam has been synthesized with diverse morphologies, high surface area and porosity by employing different surfactants under hydrothermal conditions and subsequent calcination. The surfactants strongly influence the physicochemical properties of cobalt oxide samples. The cobalt oxide grown on Ni-foam without surfactant had flower-like morphology. However, cobalt oxides synthesized by using cationic (CTAB) and non-ionic (Triton X-100) surfactants showed flake-like morphology, but the spatial arrangement of flakes was found to be different in both the samples. The surfactant-assisted cobalt oxide showed average crystallite size of ∼6.6–9.8 nm, surface area of 60–80 m2g−1 and porosity (pore diameter ∼3.8 nm). These samples were found to perform better as charge storage electrode materials. The specific capacitance values of cationic and non-ionic surfactant-assisted cobalt oxide materials, at a current density of 1.0 A g−1, were 1820 and 806 F g−1, respectively, compared to 288 F g−1 of cobalt oxide prepared without surfactant. They also showed excellent capacity retention for over 3000 charge-discharge cycles at higher current densities. The difference in the capacitance values of cationic and non-ionic surfactant-assisted cobalt oxide is due to the difference in the flake arrangement. Graphical AbstractThe surfactant assisted synthesized flake-like morphologies of cobalt oxide shows high charge storage performance.