Synthetic methods greatly control the structural and functional characteristics of the materials. In this article, porous NiO samples were prepared in conventional-reflux and microwave assisted heating method under homogeneous precipitation conditions. The NiO samples synthesized in conventional reflux method showed flakelike morphology, whereas the sample synthesized in microwave methods showed hierarchical porous ball like surface morphology with uniform ripple-shaped pores. The NiO samples characterized using BET method were found to bear characteristic meso- and macroporosity due to differently crystallized Ni(OH)(2) precursors under various heating conditions. Thermogravimety analysis showed morphology dependent decomposition of Ni(OH)(2) precursors. The microwave synthesized porous NiO sample with unique morphology and pore size distribution showed significantly improved charge storage and electrochemical stability than the flaky NiO sample synthesized by employing conventional reflux method. The cyclic voltammetry measurements on microwave synthesized NiO sample showed considerably high capacitance and better electrochemical reversibility. The charge-discharge measurements made at a discharge current of 2 A/g showed higher rate specific capacitance (370 F/g) for the NiO sample synthesized by microwave method than the sample synthesized by reflux method (101 F/g). The impedance study illustrates lower electronic and ionic resistance of rippled-shaped porous NiO due to its superior surface properties for enhanced electrode-electrolyte contact during the Faradaic redox reactions. It has been further established from the Ragone plot that the microwave synthesized NiO sample shows higher energy and power densities than the reflux synthesized NiO sample. Broadly, this study reveals that microwave-mediated synthesis approach is significantly a better strategy for the synthesis of porous NiO suitable to electrochemical supercapacitor applications.

Three nano-porous NiO samples with high specific surface area were prepared by a simple hydrothermal method under homogeneous precipitation conditions using CTAB as a template and urea as the hydrolysis controlling agent. This study was done to determine the effect of different anions (acetate, nitrate and chloride) present in the precursor salts on the morphology and pseudocapacitance behavior of NiO. The samples were characterized by thermogravimetry (TG), differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD), Brunauer-Emmet-Teller (BET) isotherm and field emission scanning electron microscopy (FESEM). The final NiO samples showed different hierarchical surface morphologies and their effect on the electrochemical pseudocapacitance behavior was carefully studied by cyclic voltammetry, galvanostatic charge-discharge cycles (chronopotentiometry) and impedance spectroscopic techniques. The specific capacitance of NiO sample synthesized by NO3- ion intercalation showed higher surface area, intermediate porosity and a novel pine-cone morphology with nano-wire surface attachments. This sample exhibits the highest pseudocapacitance of 279 F g(-1) at a scan rate of 5 mV s(-1), calculated from the cyclic voltammetry measurements. The sample synthesized by Cl- intercalation shows a nano-flower morphology with lower surface area, porosity and pseudocapacitance behaviour. The NiO sample prepared in the presence of CH3COO- ions showed a honeycomb type surface morphology with an intermediate pseudocapacitance value but higher reversibility. The galvanostatic charge-discharge and impedance spectroscopic measurements on these NiO electrodes were consistent with CV results. The Coulombic efficiency of all the three NiO samples was found to be high (∼85 to ∼99%) after 100 galvanostatic charge-discharge cycles. This study shows that the surface morphology and porosity of NiO are strongly influenced by the anions in the precursor salts, and in turn affect significantly the pseudocapacitance behavior and the power performance of NiO powders.

In this work, NiO powders with a spherical morphology were synthesized by a simple hydrothermal technique using organic surfactants as templates and urea as the hydrolysis controlling agent. The effect of cationic (cetyl trimethyl ammonium bromide), anionic (sodium dodecyl sulfate), and nonionic (Triton X-100) surfactants for tuning the surface area, pore size, pore volume, and electrochemical properties of NiO powders was investigated. The NiO powders were characterized by X-ray diffraction, scanning electron microscopy, the Brunauer−Emmett−Teller method, cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy. We observed that the charge-storage mechanism in our NiO-based electrodes is significantly Faradic in nature rather than capacitive type. The ionic nature of the surfactant used in the preparation of NiO powders shows a considerable effect on their capacitance behavior. The specific capacitance values were found to increase in the order of NiO-T (144 F g−1) < NiO-C (239 F g−1) < NiO-S (411 F g−1) at a current density of 200 mA g−1 in 2 M KOH aqueous electrolyte solution. The NiO-S sample exhibits the highest surface redox reactivity and shows the specific capacitance of 235 F g−1 over 100 cycles at a current density of 500 mA g−1 in a life cycle test.

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Spinel NiCo 2 O 4 material has received considerable attention as an excellent supercapacitor material. In this study, we report facile and cost-effective solvothermal method for the synthesis of mesoporous NiCo 2 O 4 anchored on reduced graphene oxide (rGO). The electrochemical activity of the NiCo 2 O 4 -rGO and pristine NiCo 2 O 4 materials were evaluated by cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS). The NiCo 2 O 4 -rGO composite electrode shows high specific capacitance value of 870 F g -1 at current density of 2 A g -1 and it retains 600 F g -1 capacitance even at high current density of 20 A g -1 . Pristine NiCo 2 O 4 shows 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 . Further, NiCo 2 O 4 -rGO nanocomposite shows excellent cyclic performance with 90% capacitance retention even after 5000 charge-discharge cycles at high current density of 4 A g -1 , whereas pristine NiCo 2 O 4 electrode shows only 45% capacitance retention. The high specific capacitance, remarkable rate capability and excellent cycling performance offered by NiCo 2 O 4 -rGO composite is attributed to the high surface area and high conductivity. In addition, rGO Recently, different NiCo 2 O 4 -graphene hybrid nanocomposites have been designed by different approaches and their performance tested for supercapacitors. 26, 34-39 For example, Wang et al. synthesised NiCo 2 O 4 -rGO composite using self-assembly method by exfoliatingNi-Co hydroxides and assembling with GO followed by heat treatment. These self-assembled 2D nanosheets of NiCo 2 O 4 -rGO composite exhibits higher specific capacitance of 835 F g -1

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