Electrochemical capacitance study on Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanowires for super capacitors application

One-dimensional Co₃O₄ nanowires have been prepared by utilizing the ordered mesoporous silica material SBA-15 as template. The results of transmission electron microscope (TEM) and N₂ adsorption–desorption characterizations show that the Co₃O₄ nanowires possess a uniform size and a large Brunauer-Emmett-Teller (BET) surface area. Its electrochemical performance was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques in various concentration of KOH solution. This nanomaterial shows a small resistance, a high specific capacitance (SC), and a strong cyclic stability. The maximal SC value of 373 F·g⁻¹ was obtained in 6 M electrolyte under the scan speed of 3 mV·s⁻¹ at the first CV cycle. After 500 CV cycles, the SC value is about 90% of the original value. It is considered that the short path of ion transfer given by nanomaterial brought on the great pseudo capacitance performance.     

Electrochemical performance of potentio-dynamically deposited Co<Subscript>3</Subscript>O<Subscript>4</Subscript> electrodes: influence of annealing temperature

Co₃O₄ thin films were deposited potentiodynamically on to the stainless steel substrate. Prepared samples were annealed within the temperature range 473 K to 873 K by the interval of 100 K. XRD study reveals cubic crystal structure of Co₃O₄. FE-SEM showed compact agglomerated granular type morphology. Electrochemical characterization of electrodes showed pseudo capacitive behavior. Maximum value of specific capacitance (441.17 F/g) was achieved at the scan rate 2 mV/s in 1 M KOH with 87.88% stability. Charge–discharge curves showed nonlinear behavior and used to calculate the specific energy, specific power and columbic efficiency which were 20.98 W/kg, 15.96 kW/kg and 86.63% respectively. EIS of complex impedance spectra showed internal resistance ~0.9435 Ω.     

A facile method to improve the high rate capability of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanowire array electrodes

The capability of fast charge and fast discharge is highly desirable for the electrode materials used in supercapacitors and lithium ion batteries. In this article, we report a simple strategy to considerably improve the high rate capability of Co₃O₄ nanowire array electrodes by uniformly loading Ag nanoparticles onto the surfaces of the Co₃O₄ nanowires via the silver-mirror reaction. The highly electrically conductive silver nanoparticles function as a network for the facile transport of electrons between the current collectors (Ti substrates) and the Co₃O₄ active materials. High capacity as well as remarkable rate capability has been achieved through this simple approach. Such novel Co₃O₄-Ag composite nanowire array electrodes have great potential for practical applications in pseudo-type supercapacitors as well as in lithium ion batteries.     

Synthesis and characterization of Cd-doped ZnMn<Subscript>2</Subscript>O<Subscript>4</Subscript> microspheres as supercapacitor electrodes

In this paper, we demonstrate the effects of Cd-doping ZnMn₂O₄ on structural and electrochemical performance. Cd-doped ZnMn₂O₄ spheres with diameters of about 2 μm were successfully synthesized by a facile hydrothermal method at 200 °C for 18 h. The fabricated Cd-doped ZnMn₂O₄ samples were characterized by X-ray diffraction, scanning electron microscopy, Brunauer Emmett Teller surface area analyzer and X-ray photoelectron spectroscopy. The electrochemical performance was investigated by cyclic voltammetry and electrochemical impedance spectrometry. The experimental results show that the synthesized spherical Cd-doped ZnMn₂O₄ exhibit far better rate capability and cyclic stability than that of pure spinel porous ZnMn₂O₄ microspheres. The result of cyclic voltammetry measurement indicates that the obtained Cd-doped ZnMn₂O₄ microspheres exhibited the high specific capacitance of 364 Fg^?1 at 2 mV/s.     

Investigation for the synthesis of hierarchical Co<Subscript>3</Subscript>O<Subscript>4</Subscript>@MnO<Subscript>2</Subscript> nanoarrays materials and their application for supercapacitor

We designed and fabricated hierarchical Co₃O₄@MnO₂ nanoarrays directly grown on nickel foam by hydrothermal and calcination methods. After the investigation of growth mechanism, we found that the deposition of MnO₂ was based on the self-decomposition of KMnO₄ and the reducibility of Co₃O₄ during the hydrothermal process. Thanks to the hierarchical structure, the obtained electrode exhibited excellent capacitive performance in supercapacitor. It delivered 21.72 F cm^?2 at a current density of 5 mA cm^?2 and retained ~94 % capacitance of initial value after 5000 cycles.     

Preparation and characterization of mesoporous Co<Subscript>3</Subscript>O<Subscript>4</Subscript> electrode material

Mesoporous electrode materials of spinel Co₃O₄ were synthesized by hydrothermal method using polyethylene glycol-6000 (PEG-6000) as dispersant and subsequent calcination at different temperatures in air. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), nitrogen adsorption and electrochemical measurements. The results showed that the dispersant PEG-6000 had a distinct effect to control the porosity, particle size and homogeneity of Co₃O₄; the calcination temperature had a significant influence on the crystal structure, surface area, porosity and morphology, and indeed electrochemical performance. The resultant Co₃O₄ sample calcinated at 350° C possessed a narrow mesopore distribution around 4 nm and exhibited excellent electrochemical performance. It had the specific capacitance as high as 348.7 F/g, increased by 21.5% over that of pure Co₃O₄ and showed good cyclical charge–discharge stability.     

A simple approach for the synthesis of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocrystals

By using Co²⁺ and Co³⁺ salts, and freshly extracted ovalbumin, Co₃O₄ nanocrystals have been synthesized successfully. The pH of the solution was self-regulated for the hydrolysis of metal ions as the ovalbumin-water mixture was highly basic. Water soluble ovalbumin proteins served as a perfect matrix for entrapment of Co²⁺ and Co³⁺ ions thus forming a gel. Upon heat treatment, the dried gel precursor decomposed into nanocrystalline Co₃O₄. The crystallite size obtained by XRD line profile fitting was 45 ± 8 nm and particle size estimated from the SEM was in the range 20 nm-2 μm. EPR results show a very good fit to literature reports for nanocrystals in the size range of 8–17 nm. Even though the overall particle size is quite large and its distribution is quite wide EPR results confirm nanocrystalline nature of the particles obtained. Presented route is simple, cost effective, and environmentally friendly.     

A Simple Microwave-Assisted Route to Prepare Black Cobalt,Co<Subscript>3</Subscript>O<Subscript>4</Subscript>

In this communication, we report a novel microwave-assisted decomposition reaction of cobalt nitrate hexahydrate, Co(NO₃)₂ · 6H₂O. Using microwave processing (10 min, 2.45 GHz), phase-pure tricobalt tetroxide (black cobalt, Co₃O₄) was obtained. The compound was characterized by x-ray powder diffraction and infrared spectroscopy.__________From Neorganicheskie Materialy, Vol. 41, No. 5, 2005, pp. 564–565.Original English Text Copyright © 2005 by Kurtulus, Guler.This article was submitted by the authors in English.     

Enhanced electrochemical reduction of hydrogen peroxide by Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanowire electrode

Crystalline Co₃O₄ nanowire arrays with different morphologies grown on Ni foam were investigated by varying the reaction temperature, the concentration of precursors, and reaction time. The Co₃O₄ nanowires synthesized under typical reaction condition had a diameter range of approximately 500–900 nm with a length of 17 µm. Electrochemical reduction of hydrogen peroxide (H₂O₂) of the optimized Co₃O₄ nanowire electrode was studied by cyclic voltammetry. A high current density of 101.8 mA cm^?2 was obtained at ?0.4 V in a solution of 0.4 M H₂O₂ and 3.0 M NaOH at room temperature compared to 85.8 mA cm^?2 at ?0.35 V of the Co₃O₄ nanoparticle electrode. Results clearly indicated that the Ni foam supported Co₃O₄ nanowire electrode exhibited superior catalytic activity and mass transport kinetics for H₂O₂ electrochemical reduction.     

Thermoelectric properties of WO<Subscript>3</Subscript>-based ceramics doped with Co<Subscript>2</Subscript>O<Subscript>3</Subscript>

Tungsten trioxide (WO₃) doped with cobalt sesquioxide (Co₂O₃) was prepared by a conventional mixed oxide processing route and the thermoelectric properties were studied from 300 up to 1,000 K. The addition of Co₂O₃ to WO₃ resulted in an increase in both the grain size and porosity, indicating that Co₂O₃ promotes the grain grown of WO₃. The magnitude of the electrical conductivity (σ) and the absolute value of the Seebeck coefficient (|S|) depended strongly on the Co₂O₃ content. As for the power factor (σS ² ), the 5.0 mol% sample has the maximum value of the power factor which is 0.12 μWm⁻¹K⁻² at 873 K.     

Facile synthesis and characterization of polypyrrole/Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocomposites with adjustable intrinsic electroconductivity

Polypyrrole (PPY)/Co₃O₄ nanocomposites (NCs) were synthesized by a facile in situ polymerization of pyrrole in the presence of Co₃O₄ nanoparticles which were obtained by a rheological phase reaction method. The structure and morphology of the as-prepared PPY/Co₃O₄ NCs were investigated by X-ray diffraction, Fourier transform infrared spectra, Raman spectra, scanning electron microscopy and transmission electron microscopy, which confirmed the formation of the nanocomposites and indicated some interactions between PPY chains and Co₃O₄ nanoparticles. Different PPY/Co₃O₄ ratios were selected in order to study conductive properties. The electrical conductivity measurements indicated that ac conductivity tended to remain constant up to about 10⁷ Hz for all samples, and thereafter increased with frequency. The desired electrical properties of PPY/Co₃O₄ NCs can be modulated simply by controlling the contents of Co₃O₄ nanoparticles.     

Synthesis,transport and dielectric properties of polyaniline/Co<Subscript>3</Subscript>O<Subscript>4</Subscript> composites

Conducting polyaniline/cobaltous oxide composites have been synthesized using in situ deposition technique by placing fine graded/cobaltous oxide in polymerization mixture of aniline. The a.c. conductivity and dielectric properties are studied by sandwiching the pellets of these composites between the silver electrodes. It is observed that the values of conductivities increase up to 30 wt% of cobaltous oxide in polyaniline and decrease thereafter. Initial increment in conductivity is due to extended chain length of polyaniline where polarons possess sufficient energy to hop between favourable sites. Beyond 30 wt% of cobaltous oxide in polyaniline, blocking of charge carriers takes place reducing the conductivity values. It can be noted that the value of dielectric constant increases up to 10 wt% of cobaltous oxide. Thereafter, it decreases up to 30 wt% of cobaltous oxide and again increases up to 40 wt% of cobaltous oxide and decreases thereafter. The observed behaviour is attributed to the variation of a.c. conductivity. And it is observed that the dielectric loss increases up to 10 wt% of cobaltous oxide in polyaniline, decreases to a lower value of 20 wt% of cobaltous oxide and increases to 35 wt% and thereafter decreases. These values go in accordance with the values of dielectric constant. The results obtained for these composites are of greater scientific and technological importance.     

Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocrystals on single-walled carbon nanotubes as a highly efficient oxygen-evolving catalyst

The efficient catalytic oxidation of water to dioxygen is envisioned to play an important role in solar fuel production and artificial photosynthetic systems. Despite tremendous efforts, the development of oxygen evolution reaction (OER) catalysts with high activity and low cost under mild conditions remains a great challenge. In this work, we develop a hybrid consisting of Co₃O₄ nanocrystals supported on single-walled carbon nanotubes (SWNTs) via a simple self-assembly approach. A Co₃O₄/SWNTs hybrid electrode for the OER exhibits much enhanced catalytic activity as well as superior stability under neutral and alkaline conditions compared with bare Co₃O₄, which only performs well in alkaline solution. Moreover, the turnover frequency for the OER exhibited by Co₃O₄/SWNTs in neutral water is higher than for bare Co₃O₄ catalysts. Synergetic chemical coupling effects between Co₃O₄ nanocrystals and SWNTs, revealed by the synchrotron X-ray absorption near edge structure (XANES) technique, can be regarded as contributing to the activity, cycling stability and stable operation under neutral conditions. Use of the SWNTs as an immobilization matrix substantially increases the active electrode surface area, enhances the durability of catalysts under neutral conditions and improves the electronic coupling between Co redox-active sites of Co₃O₄ and the electrode surface.      

Surface active sites on Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanobelt and nanocube model catalysts for CO oxidation

CO oxidation has been performed on Co₃O₄ nanobelts and nanocubes as model catalysts. The Co₃O₄ nanobelts which have a predominance of exposed {011} planes are more active than Co₃O₄ nanocubes with exposed {001} planes. Temperature programmed reduction of CO shows that Co₃O₄ nanobelts have stronger reducing properties than Co₃O₄ nanocubes. The essence of shape and crystal plane effect is revealed by the fact that turnover frequency of Co³⁺ sites of {011} planes on Co₃O₄ nanobelts is far higher than that of {001} planes on Co₃O₄ nanocubes.      

Novel porous starfish-like Co<Subscript>3</Subscript>O<Subscript>4</Subscript>@nitrogen-doped carbon as an advanced anode for lithium-ion batteries

A Co-based metal-organic framework (Co-MOF) with a unique three-dimensional starfish-like nanostructure was successfully synthesized using a simple ultrasonic method.After subsequent carbonization and oxidation,a nanocomposite of nitrogen-doped carbon with a Co3O4 coating (Co3O4@N-C) with a porous starfish-like nanostructure was obtained.The final hybrid exhibited excellent lithium storage performance when evaluated as an anode material in a lithiumion battery.A remarkable and stable discharge capacity of 795 mAh·g-1 was maintained at 0.5 A·g-1 after 300 cycles.Excellent rate capability was also obtained.In addition,a full Co3O4@N-C/LiFePO4 battery displayed stable capacity retention of 95％ after 100 cycles.This excellent lithium storage performance is attributed to the unique porous starfish-like structure,which effectively buffers the volume expansion that occurs during Li+ insertion/deinsertion.Meanwhile,the nitrogendoped carbon coating enhances the electrical conductivity and provides a buffer layer to accommodate the volume change and accelerate the formation of a stable solid electrolyte interface layer.     

Synthesis and microwave absorption property of ternary composites: polyaniline/CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/Ba<Subscript>3</Subscript>Co<Subscript>2</Subscript>Fe<Subscript>24</Subscript>O<Subscript>41</Subscript>

Polyaniline (PANI)/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite was prepared by an in-situ polymerization method. The phase structure, morphology and magnetic properties of the as-prepared PANI/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite were characterized by XRD, FT-IR, SEM, TEM, and VSM, respectively. The microwave absorption properties of the composite were investigated by using a vector network analyzer in the 2–18 GHz frequency range. The results show that the maximum reflection loss value of the PANI/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite reaches ?30.5 dB at 10.5 GHz with a thickness of 3 mm and the bandwidth of reflection loss below ?10 dB reaches up to 1.2 GHz. The excellent microwave absorption properties of the as-prepared PANI/CoFe₂O₄/Ba₃Co₂Fe₂₄O₄₁ composite due to the enhanced impedance match between dielectric loss and magnetic loss.     

Electrical properties of Ni<Subscript>0.93</Subscript>Co<Subscript>0.02</Subscript>Mn<Subscript>0.05</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> + BaTiO<Subscript>3</Subscript> ME composites

Polycrystalline samples of mixed composites of Ni_0.93Co_0.02Mn_0.05Fe₂O₄ + BaTiO₃ were prepared by conventional double sintering ceramic method. The phase analysis was carried out by using X-ray diffraction technique. Variation of dc resistivity and thermo emf was studied as a function of temperature. AC conductivity (σ_ac) was investigated in the frequency range 100 Hz–1 MHz. The loss tangent (tan δ) measurements conclude that the conduction mechanism in these samples is due to small polaron hopping. The magnetoelectric conversion factor, i.e. dc(ME) _H was studied as a function of intensity of magnetic field and the maximum value 407 μV/cm/Oe was observed at a field of 0.8 kOe in a composite with 85% BaTiO₃ and 15% Ni_0.93Co_0.02Mn_0.05Fe₂O₄ phase.     

Unique and hierarchically structured novel Co<Subscript>3</Subscript>O<Subscript>4</Subscript>/NiO nanosponges with superior photocatalytic activity against organic contaminants

In the present study, novel Co₃O₄/NiO nanosponges designed for the photocatalytic degradation of organic contaminants were synthesized by a simple precipitation technique. The formation of sponge-like nanostructures was clearly evident through the TEM analysis. The photocatalytic efficiency was tested against rhodamine B (RhB) and congo red (CR) dye solutions. Co₃O₄/NiO nanosponges showed excellent and enhanced photocatalytic efficacy compared to those of Co₃O₄, NiO nanoparticles, and standards like TiO₂ and ZnO. The influence of paramount important operational parameters was explored and the conditions for the best photocatalytic efficiency were optimized. The trapping experiment revealed that the reactive oxygen species (ROS) identified was $OH radical. These findings certainly open up a new way for synthesizing a morphology dependent photocatalyst.     

Uniformly grown PtCo-modified Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanosheets as a highly efficient catalyst for sodium borohydride electrooxidation

A facile hydrothermal synthetic method, followed by in situ reduction and galvanic replacement processes, is used to prepare PtCo-modified Co₃O₄ nanosheets (PtCo/Co₃O₄ NSs) supported on Ni foam. The prepared nanomaterial is used as an electrocatalyst for NaBH₄ oxidation in alkaline solution. The morphology and phase composition of PtCo/Co₃O₄ NSs are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The catalytic performance of PtCo/Co₃O₄ NSs is investigated by cyclic voltammetry (CV) and chronoamperometry (CA) in a standard three-electrode system. Current densities of 70 and 850 mA·cm^–2 were obtained at–0.4 V for Co/Co₃O₄ and PtCo/Co₃O₄ NSs, respectively, in a solution containing 2 mol·L^–1 NaOH and 0.2 mol·L^–1 NaBH₄. The use of a noble metal (Pt) greatly enhances the catalytic activity of the transition metal (Co) and Co₃O₄. Besides, both Co and Co₃O₄ exhibit good B–H bond breaking ability (in NaBH₄), which leads to better electrocatalytic activity and stability of PtCo/Co₃O₄ NSs in NaBH₄ electrooxidation compared to pure Pt. The results demonstrate that the as-prepared PtCo/Co₃O₄ NSs can be a promising electrocatalyst for borohydride oxidation.

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Dielectric properties of B<Subscript>2</Subscript>O<Subscript>3</Subscript> doped Sm(Co<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> ceramics at microwave frequency

The microwave dielectric properties and the microstructures of Sm(Co_1/2Ti_1/2)O₃ ceramics with B₂O₃ additions (0.25 and 0.5 wt%) prepared by conventional solid-state route have been investigated. The prepared Sm(Co_1/2Ti_1/2)O₃ exhibited a mixture of Co and Ti showing 1:1 order in the B-site. Doping with B₂O₃ (up to 0.5 wt%) can effectively promote the densification of Sm(Co_1/2Ti_1/2)O₃ ceramics with low sintering temperature. It is found that Sm(Co_1/2Ti_1/2)O₃ ceramics can be sintered at 1,260 °C due to the grain boundary phase effect of B₂O₃ addition. At 1,290 °C, Sm(Co_1/2Ti_1/2)O₃ ceramics with 0.5 wt% B₂O₃ addition possess a dielectric constant (ε _r) of 27.7, a Q × f value of 33,600 (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of −11.4 ppm/ °C. The B₂O₃-doped Sm(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices requiring low sintering temperature.