Nanoscale morphology dependent pseudocapacitance of NiO: Influence of intercalating anions during synthesis.

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.