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 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.     

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.     

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.     

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.     

Hierarchical Co<Subscript>3</Subscript>O<Subscript>4</Subscript>@Ni-Co-O supercapacitor electrodes with ultrahigh specific capacitance per area

High specific capacitance per area is a critical requirement for a practical supercapacitor electrode, and needs a combination of high mass-loading of the electrochemically active material per area, and high utilization efficiency of this material. However, pursuing high mass-loading on conventional electrodes usually leads to an increase in “dead” material which is not accessible to the electrolyte in the supercapacitor, and thus prevents high utilization efficiencies of the material being realized. Here we show that this antagonism can be overcome by incorporating the electrochemically active material in a mesoporous hierarchical architecture. Fabrication of ternary ordered hierarchical Co₃O₄@Ni-Co-O nanosheet-nanorod arrays—involving the growth of densely aligned slim Ni-Co-O nanorods (diameter <20 nm) on Co₃O₄ microsheets which had been previously loaded on macroporous nickel foam—gives a material with excellent electrochemical performance as a supercapacitor electrode. At a current density of 5 mA/cm², the electrodes have both high mass loading per area (12 mg/cm²) and high efficiency of 2098 F/g, giving specific capacitances per area as high as ～25 F/cm². When the current density was increased from 5 to 30 mA/cm², 72% of the specific capacitance was retained and, furthermore, no significant decrease in capacitance was observed over 1000 charge/discharge cycles. The combination of these merits makes the composite material an excellent candidate for practical application as a supercapacitor electrode and, more generally, highlights the increased efficacies of materials which can result from fabricating mesoporous hierarchical structures at the nanoscale.

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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.     

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.     

Fabrication and magnetic properties of Fe<Subscript>3</Subscript>Co<Subscript>7</Subscript> alloy nanowire arrays

Fe₃Co₇ alloy nanowire arrays have been fabricated by direct current electrodeposition of Fe²⁺ and Co²⁺ into anodic aluminum oxide (AAO) templates. The phase structure and magnetic properties of the nanowires were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM). Magnetic measurements show that the coercivity and remanence of the as-deposited Fe₃Co₇ Alloy nanowires increase dramatically after heat-treatment at 773 K for 2 h, and the nanowire arrays exhibit uniaxial magnetic anisotropy with easy magnetization direction along the nanowire axes owing to the large shape anisotropy. The great difference between practical coercivity and ideal coercivity was also discussed in detail.     

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.      

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.     

Thermal dissociation and hydrogen reduction of DyMn<Subscript>2</Subscript>O<Subscript>5</Subscript>

The dissociation and hydrogen reduction of DyMn₂O₅ have been studied in the temperature range 973–1173 K. The results were used to construct partial p-T-x phase diagrams of the Dy-Mn-O system. Our experimental data demonstrate that the thermal dissociation of DyMn₂O₅ is a three-step process, accompanied by the formation of distinct compounds in the following sequence: DyMn₂O₅ → DyMnO₃ + Mn₃O₄ + O₂ → DyMnO₃ + MnO + O₂ → Dy₂O₃ + MnO + O₂. For both DyMn₂O₅ and DyMnO₃, we have evaluated the standard enthalpy and entropy of formation from elements.     

Thermoelectric properties and texture evaluation of Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9</Subscript> prepared by a cost-effective multisheet cofiring technique

Highly textured Ca₃Co₄O₉ thermoelectrics were fabricated by a so-called multisheet cofiring (MSC) technique in combination with the commonly practiced spark plasma sintering (SPS) technique. X-ray diffraction analysis revealed that the MSC technique significantly enhances the degree of texture compared with the conventional SPS technique. This was confirmed by microstructure analysis performed on polished and chemically etched specimens. The power factor and the thermoelectric figure of merit for the specimen produced by the MSC technique were estimated to be ~5.2 × 10⁻⁴ W/mK² and ~0.4, respectively, both of which are ~20% larger than those of the normally processed specimens. A comparative study of the thermoelectric properties, such as electrical resistivity and Seebeck coefficient, clearly demonstrated that texture mainly influences the electrical resistivity and consequently enhances the thermoelectric properties.     

Enhanced magnetoelectric coupling in La-modified Bi<Subscript>5</Subscript>Co<Subscript>0.5</Subscript>Fe<Subscript>0.5</Subscript>Ti<Subscript>3</Subscript>O<Subscript>15</Subscript> multiferroic ceramics

Multiferroic properties of La-modified four-layered perovskite Bi_5?x La _x Fe_0.5Co_0.5Ti₃O₁₅ (0 ≤ x ≤ 1) ceramics were investigated, by analyzing the magnetodielectric effect, magneto-polarization response and magnetoelectric conversion. X-ray diffraction indicated the formation of pure Aurivillius ceramics, and Raman spectroscopy revealed the Bi ions displacement and the crystal structure variation. The enhancement of ferromagnetic and ferroelectric properties was observed in Bi_5?x La _x Fe_0.5Co_0.5Ti₃O₁₅ after La modification. The evidence for enhanced ME coupling was determined by magnetic field-induced marked variations in the dielectric constant and polarization. A maximum ME coefficient of 1.15 mV/cm·Oe was achieved in Bi_4.25La_0.75Fe_0.5Co_0.5Ti₃O₁₅ ceramic, which provides the possible promise for novel magnetoelectric device application.     

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.      

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.     

Electrochemical fabrication and magnetic properties of Fe<Subscript>7</Subscript>Co<Subscript>3</Subscript> alloy nanowire array

A new type of magnetic material, Fe₇Co₃ nanowires, was successfully synthesized for the first time via a simple electrodeposition method. Highly uniform, self-ordered porous anodic aluminum oxide (AAO) membranes were prepared by the way of electrochemical. Fe₇Co₃ alloy nanowire arrays were fabricated in the porous alumina template in an aqueous solution of FeCl₂ and CoCl₂ by direct current electrodepositing. The microstructures of nanowires and AAO template were characterized by XRD, SEM, and TEM. The results show that a single Fe₇Co₃ nanowire is 40 nm in width and 2.5 μm in length with a preferred crystal face (110) during growing. The Fe₇Co₃ nanowire arrays have uniaxial magnetic anisotropy with easy magnetization direction along the nanowire axis due to the large shape anisotropy. It also shows that Fe₇Co₃ nanowire is a well-soft magnetic phase compared with Fe nanowires. It illustrates that Fe₇Co₃ possess higher saturation magnetization.     

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.     

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.