Ultralayered Co 3 O 4 structures with high porosity have been synthesized by a facile homogeneous precipitation process under hydrothermal conditions. The superstructures consist of well-arranged micrometer length rectangular 2D flakes with high specific surface area, pore volume, and uniform pore size distribution. The electrochemical measurements demonstrate that charge storage occurs in ultralayered Co 3 O 4 due to reversible redox reactions. The chargeÀdischarge study shows that the material is capable of delivering very high specific capacitance of 548 F g À1 at a current density of 8 A g À1 and retains 66% of capacitance at 32 A g À1 . The chargeÀdischarge stability measurements show excellent specific capacitance retention capability, ca. 98.5% after 2000 continuous chargeÀdischarge cycles at high current density of 16 A g À1 . The exceptional cyclic, structural, and electrochemical stability at higher current rate with ∼100% Coulombic efficiency, and very low ESR value from impedance measurements promise good utility value of ultralayered Co 3 O 4 material in fabricating a wide range of high-performance electrochemical supercapacitors.

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.

An all-solid-state hybrid supercapacitor based on hierarchical CoS2/CuCo2S4 offers excellent rate capacitance, low charge-transfer resistance, high-rate energy density, and extended service stability.

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 the context of immense control of synthesis methods on the structural and functional characteristics of the materials, nanowire morphologies of Co 3 O 4 are synthesized in conventional reflux and microwave-assisted methods, under homogeneous precipitation conditions. The Co 3 O 4 sample synthesized by the conventional reflux method consists of randomly distributed thin nanowires while the microwave reflux method generates higher-dimensional and arranged Co 3 O 4 nanowires. The surface area and pore structural analysis of the Co 3 O 4 samples show significant difference in their meso-and macroporosity as well as specific surface area, due to differently crystallized products. The UVÀVis-DRS study shows crystallite size dependent optical transitions and band gaps. The magnetic study illustrates finite size effect and low temperature ferromagnetism in both samples; the lower-dimensional nanowires being more ferromagnetic than the higher-dimensional Co 3 O 4 nanowires. Due to smaller crystallite size and more accessible surface sites, the Co 3 O 4 sample synthesized by the conventional reflux method shows better charge storage, high Coulombic efficiency, and enhanced rate response during the pseudocapacitance studies. However the Co 3 O 4 sample synthesized using the microwave-assisted method shows better high rate cyclic stability due to its more rigid orientated nanowire structure. Further, the Ragone plot exhibits considerably higher energy and power densities of lower-dimensional Co 3 O 4 nanowires. Broadly, this study reveals that, under nonhydrothermal homogeneous precipitation conditions, the conventional reflux synthesized lower-dimensional Co 3 O 4 nanowires bear superior surface properties than the microwave synthesized higher-dimensional Co 3 O 4 nanowires, for electrochemical supercapacitor applications.