Cobalt oxide nanopowders are synthesized by the pyrolysis of aerosol particles of water solution of cobalt acetate. Cobalt nanopowder is obtained by subsequent reduction of obtained cobalt oxide by annealing under a hydrogen atmosphere. The average crystallite size of the synthesized porous particles ranged from 7 to 30 nm, depending on the synthesis temperature. The electrochemical characteristics of electrodes based on synthesized cobalt oxide and reduced cobalt oxide are investigated in an electrochemical cell using a 3.5 M KOH solution as the electrolyte. The results of electrochemical measurements show that the electrode based on reduced cobalt oxide (Re-Co3O4) exhibits significantly higher capacity, and lower Faradaic charge–transfer and ion diffusion resistances when compared to the electrodes based on the initial cobalt oxide Co3O4. This observed effect is mainly due to a wide range of reversible redox transitions such as Co(II) ↔ Co(III) and Co(III) ↔ Co(IV) associated with different cobalt oxide/hydroxide species formed on the surface of metal particles during the cell operation; the small thickness of the oxide/hydroxide layer providing a high reaction rate, and also the presence of a metal skeleton leading to a low series resistance of the electrode.
Two different approaches were developed to fabricate core–shell tungsten and tungsten oxide (W@WO3) nanostructures in combination with a hydrogen reduction technique. One is a combination of aerosol and pyrolysis approaches, which produce spherical tungsten oxide nanoparticles with a hexagonal crystal structure. The other is a combination of templating and impregnating approaches, which lead to tungsten oxide with a monoclinic crystalline structure. Subsequent hydrogen reduction lead to the formation of core–shell W@WO3 nanostructures with two different tungsten metallic phases. Structural and morphological characterizations were performed using X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM). The electrochemical performance of W@WO3 electrodes synthesized by two different approaches was studied using cyclic voltammetry, impedance spectroscopy, and galvanostatic charge–discharge methods. The capacity of core–shell W@WO3 nanostructures can reach 148 F g–1 compared with 21 F g–1 at 0.43 A g–1 for the WO3 sample, which is also higher than the reported literature value. It is shown that the W@WO3 nanostructures have significantly better electrochemical performance than WO3 alone.
A simple chemical bath deposition method has been developed to study the formation of nanoplate morphology of tungsten oxide. The obtained materials were characterized by field emission scanning electron microscopy, transmission electron microscopy, x-ray diffractometry, Raman spectroscopy, and UV–vis diffuse reflectance spectroscopy. The photocatalytic activity of the resulting samples was further evaluated by degradation of Rhodamine B under light irradiation. It was found that both synthesis parameters and morphology affected the tungsten oxide photocatalytic activity. Tungsten oxide nanoplates obtained by a simple chemical bath deposition method have demonstrated a higher specific area and higher photocatalytic activity compared to the nanopowders obtained by the hydrothermal method.
Electrochemical pseudocapacitors, along with batteries, are the essential components of today’s highly efficient energy storage systems. Cobalt oxide is widely developing for hybrid supercapacitor pseudocapacitance electrode applications due to its wide range of redox reactions, high theoretical capacitance, low cost, and presence of electrical conductivity. In this work, a recovery annealing approach is proposed to modify the electrochemical properties of Co3O4 pseudocapacitive electrodes. Cyclic voltammetry measurements indicate a predominance of surface-controlled redox reactions as a result of recovery annealing. X-ray diffraction, Raman spectra, and XPES results showed that due to the small size of cobalt oxide particles, low-temperature recovery causes the transformation of the Co3O4 nanocrystalline phase into the CoO phase. For the same reason, a rapid reverse transformation of CoO into Co3O4 occurs during in situ oxidation. This recrystallization enhances the electrochemical activity of the surface of nanoparticles, where a high concentration of oxygen vacancies is observed in the resulting Co3O4 phase. Thus, a simple method of modifying nanocrystalline Co3O4 electrodes provides much-improved pseudocapacitance characteristics.
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