Influence of addition of cobalt oxide on microstructure and electrochemical capacitive performance of nickel oxide

Ni–Co oxide nanocomposite was prepared by thermal decomposition of the precursor obtained via a new method—coordination homogeneous coprecipitation method. The synthesized sample was characterized physically by X-ray diffraction, scanning electron microcopy, energy dispersive spectrum, transmission electron microscope, and Brunauer–Emmett–Teller surface area measurement, respectively. Electrochemical characterization of Ni–Co oxide electrode was examined by cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance measurements in 6-mol L⁻¹ KOH aqueous solution electrolyte. The results indicated that the addition of cobalt oxide not only changed the morphology of NiO but also enhance its electrochemical capacitance value. A specific capacitance value of 306 F g⁻¹ of Ni–Co oxide nanocomposite with n _Co = 0.5 (n _Co is the mole fraction of Co with respect to the sum of Co and Ni) was measured at the current density of 0.2 A g⁻¹, nearly 1.5 times greater than that of pure NiO electrode. Lower resistance and better rate capability can also be observed.     

Electrocatalytic oxidation of aspirin and acetaminophen on a cobalt hydroxide nanoparticles modified glassy carbon electrode

The electrocatalytic oxidation of aspirin and acetaminophen on nanoparticles of cobalt hydroxide electrodeposited on the surface of a glassy carbon electrode in alkaline solution was investigated. The process of oxidation and the kinetics have been investigated using cyclic voltammetry, chronoamperometry, and steady-state polarization measurements. Voltammetric studies have indicated that in the presence of drugs, the anodic peak current of low valence cobalt species increases, followed by a decrease in the corresponding cathodic current. This indicates that drugs are oxidized on the redox mediator which is immobilized on the electrode surface via an electrocatalytic mechanism. With the use of Laviron’s equation, the values of anodic and cathodic electron-transfer coefficients and charge-transfer rate constant for the immobilized redox species were determined as α _s,a = 0.72, α _s,c = 0.30, and k _s = 0.22 s⁻¹. The rate constant, the electron transfer coefficient, and the diffusion coefficient involved in the electrocatalytic oxidation of drugs were reported. It was shown that by using the modified electrode, aspirin and acetaminophen can be determined by amperometric technique with detection limits of 1.88 × 10⁻⁶ and 1.83 × 10⁻⁶ M, respectively. By analyzing the content of acetaminophen and aspirin in bulk forms using chronoamperometric and amperometric techniques, the analytical utility of the modified electrode was achieved. The method was also proven to be valid for analyzing these drugs in urine samples.     

Carbon paste electrode modified by cobalt ions dispersed into poly (N-methylaniline) preparing in the presence of SDS: application in electrocatalytic oxidation of hydrogen peroxide

Conducting and stable poly (N-methylaniline) film was prepared by using the repeated potential cycling technique in aqueous solution containing N-methylaniline, sulfuric acid, and sodium dodecyl sulfate (SDS) at the surface of carbon paste electrode (CPE). The transition metal ions of Co(ІІ) were incorporated to the polymer by immersion of the modified electrode in 0.1 M cobalt chloride solution for 10 min. The electrochemical characterization of this modified electrode exhibits stable redox behavior of Co(ІІ)→Co(ІІІ) and formation of insoluble oxide/hydroxide cobalt species on the CPE surface. The modified electrode showed well-defined and stable redox couples in alkaline aqueous solution. The modified electrode showed excellent electrocatalytic activity for oxidation of hydrogen peroxide. The response of modified electrode toward the H₂O₂ oxidation was examined using cyclic voltammetry, differential pulse voltammetry, square wave voltammetry, and chronoamperometry. This modified electrode has many advantages such as simple preparation procedure, good reproducibility, and high catalytic activity toward the hydrogen peroxide oxidation. Such characteristics were explored for the specific determination of hydrogen peroxide in cosmetics product sample, giving results in excellent agreement with those obtained by standard method.     

Electrochemical capacitance of nickel oxide nanotubes synthesized in anodic aluminum oxide templates

Nickel oxide (NiO) nanotubes for supercapacitors were synthesized by chemically depositing nickel hydroxide in anodic aluminum oxide templates and thermally annealing at 360 °C. The synthesized nanotubes have been characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The capacitive behavior of the NiO nanotubes was investigated by cyclic voltammetry, galvanostatic charge–discharge experiment, and electrochemical impedance spectroscopy in 6 M KOH. The electrochemical data demonstrate that the NiO nanotubes display good capacitive behavior with a specific capacitance of 266 F g⁻¹ at a current density of 0.1 A g⁻¹ and excellent specific capacitance retention of ca. 93% after 1,000 continuous charge–discharge cycles, indicating that the NiO nanotubes can become promising electroactive materials for supercapacitor.     

An EQCM study on the influence of saccharin on the corrosion properties of nanostructured cobalt and cobalt-iron alloy coatings

Nanostructured cobalt (Co) and cobalt-iron (CoFe) alloy coatings were electrodeposited from sulfate solutions in the presence and absence of saccharin. The effects of saccharin on the corrosion behavior of Co and CoFe alloy coatings were investigated using the electrochemical quartz crystal microbalance (EQCM) technique coupled with cyclic voltammetry (CV) measurements. Saccharin was added to the electrolyte as a grain refiner and brightener. Interestingly, opposite corrosion behaviors were found for all nanostructured coatings in 0.1 M H₂SO₄ and 0.1 M NaOH. The use of saccharin as an additive in the plating solution accelerated the anodic reaction for all deposits in acidic medium. The mass decreases while dissolution rate increased with higher saccharin concentration. Meanwhile, formation of a thick passive film on the Co electrode surface were enhanced while a hindering effect was observed for CoFe alloy coatings deposited in the presence of saccharin in alkaline solution. The anodic and cathodic curves obtained from potentiodynamic polarization experiments were also in agreement with the EQCM results.     

Electrochemical preparation, characterization, and electrocatalytic studies of Nafion–ruthenium oxide modified glassy carbon electrode

Electrochemical synthesis of ruthenium oxide (RuOx) onto Nafion-coated glassy carbon (GC) electrode and naked GC electrode were carried out by using cyclic voltammetry. Electrochemical deposition of RuOx onto Nafion-coated electrode was monitored by in situ electrochemical quartz crystal microbalance (EQCM). Surface characterizations were performed by scanning electron microscope (SEM) and atomic force microscope (AFM). SEM and AFM images revealed that ruthenium oxide particles incorporated onto the Nafion polymer film. In addition, a GC electrode modified with ruthenium oxide–Nafion film (RuOx–Nf–GC) was shown excellent electrocatalytic activity towards dopamine (DA) and ascorbic acid (AA). The anodic peak current increases linearly over the concentration range of 50 μM–1.1 mM for DA with the correlation coefficient of 0.999, and the detection limit was found to be (S/N = 3) 5 μM. Owing to the catalytic effect of the modified film towards DA, the modified electrode resolved the overlapped voltammetric responses of AA and DA into two well-defined voltammetric peaks with peak-to-peak separation about 300 mV. Here, RuOx–Nf–GC electrode employed for determination of DA in the presence of AA. This modified electrode showed good stability and antifouling properties.     

The electrochemically controlled sorption of d - metal cations by ion exchangers based on titanium phosphate

A study of the sorption of d-metal ions by titanium phosphate on a platinum electrode shows that the potential of the platinum electrode significantly influences the uptake of cations by the ion-exchanger. Cathodic polarization leads to a significant increase of the sorption whereas anodic polarization is accompanied by desorption. Therefore it is possible to perform cycles of sorption and desorption simply by switching the potential. The results obtained can be explained by the fact that the layer of titanium phosphate acts as a phase which extracts protons or hydroxide ions. The electrochemical generation of hydroxide ions at negative potentials can, especially at less acidic pH, lead to the formation of metal hydroxides in the ion-exchanger phase. This interference is less important at a lower solution pH. Received: 5 March 1997 / Accepted: 3 April 1998     

Nanoflake-like cobalt hydroxide/ordered mesoporous carbon composite for electrochemical capacitors

Nanocomposites consisting of mesoporous carbon CMK-3 and cobalt hydroxide nanoflakes are synthesized by a chemical precipitation method. The successful growth of nanometer-sized Co(OH)₂ flakes on the surface of CMK-3 is confirmed by scanning electron microscopy. The Co(OH)₂/CMK-3 composite electrodes are investigated for its use in the electrochemical capacitors with cyclic voltammograms, chronopotentiometric measurements, and electrochemical impedance spectroscopy. Experimental studies reveal that the Co(OH)₂/CMK-3 composite electrode with the 20 wt.% CMK-3 presents excellent electrochemical performance with specific capacitance of 750 F/g (or 910 F/g after being corrected for the weight percentage of the Co(OH)₂ phase). The overall improved electrochemical behavior accounts for the unique structure design in the Co(OH)₂/CMK-3 composite in terms of porous nanostructure, large specific surface area, and good electrical conductance. The Co(OH)₂/CMK-3 composite electrode also shows better rate capability and cyclic stability, suggesting its potential applications as the electrode materials for electrochemical capacitors.     

镍钴氢氧化物的制备及其电化学性能

采用化学共沉淀法成功制备了片状镍钴氢氧化物,并探究了不同镍钴物质的量比对样品形貌及电化学性能的影响。通过X射线衍射(XRD)、扫描电子显微镜(SEM)、X射线光电子能谱仪(XPS)及比表面积孔径分析仪(BET)对样品的结构、形貌进行了表征,并利用循环伏安法、恒电流充放电法等对其电化学性能进行了分析。结果表明,n(Ni)∶n(Co)=4∶1的样品直接用作电极材料时,具有最好的电化学性能:在0.5 A/g的电流密度下拥有1852 F/g的高比容量;电流密度增大20倍时,仍拥有1330 F/g的高比容量。以镍钴氢氧化物为正极,活性炭为负极组装的非对称式超级电容器在346 W/kg的功率密度下,能量密度达52 Wh/kg,在循环10000圈之后电容保持率为92%。优异的电化学性能表明,片状镍钴氢氧化物是很有应用潜力的电极材料之一。     

Fabrication of TiO2/Ti nanotube electrode and the photoelectrochemical behaviors in NaCl solutions

Titanium oxide nanotube electrodes were successfully prepared by anodic oxidation on pure Ti sheets in 0.5 wt.% NH₄F + 1 wt.% (NH₄)₂SO₄ + 90 wt.% glycerol mixed solutions. Nanotubes with diameter 40–60 nm and length 7.4 μm were observed by field emission scanning electron microscope. The electrochemical and photoelectrochemical characteristics of TiO₂ nanotube electrode were investigated using linear polarization and electrochemical impedance spectroscopy techniques. The open-circuit potential dropped markedly under irradiation and with the increase of Cl⁻ concentrations. A saturated photocurrent of approximately 1.3 mA cm⁻² was observed under 10-W low-mercury lamp irradiation in 0.1 M NaCl solution, which was much higher than film electrode. Meanwhile, the highest photocurrent in NaCl solution implied that the photogenerated holes preferred to combine with Cl⁻. Thus, a significant synergetic effect on active chlorine production was observed in photoelectrocatalytic processes. Furthermore, the generation efficiency for active chlorine was about two times that using TiO₂/Ti film electrode by sol–gel method. Finally, the effects of initial pH and Cl⁻ concentration were also discussed.     

Underpotential deposition of cobalt onto polycrystalline platinum

An electrochemical study of cobalt electrodeposition onto a polycrystalline platinum electrode from an aqueous solution (10⁻² M CoCl₂ + 1 M NH₄Cl (pH 9.5)) was carried out through cyclic voltammetry and potential step techniques. Analysis of the voltammetric data clearly showed that a cobalt adlayer is formed during the application of potential in the underpotential deposition (upd) region. Formation of this cobalt adlayer involved the simultaneous presence of both adsorption and 2D nucleation processes. Cobalt adlayers obtained by linear voltammetry in upd region were analyzed employing diffuse reflectance spectroscopy (DRS). By using theoretical quantum studies at PM6//HF/LANL1MB level, it was possible to assign the peaks obtained by DRS at 328 and 337 nm to the cobalt adsorption on Pt(111) and Pt(100), respectively, while the signals recorded at 355 and 362 nm were related with the clean platinum surfaces Pt(100) and Pt(111). Also, quantum calculations at the PM6 level indicated that the energy formation order is Co-Pt(100) > Co-Pt(111) > Co-Pt(110) > Co-Co(surface).     

Sensitive determination of adenine on poly(amidosulfonic acid)-modified glassy carbon electrode

A novel and reliable direct electrochemical method was established for the detection of adenine, based on the differential pulse anodic stripping response at a poly(amidosulfonic acid) (poly-ASA)-modified glassy carbon electrode (GCE) fabricated by electropolymerization. The characterization of electrochemically synthesized poly-ASA film was investigated by atomic force microscopy, electrochemical impedance spectroscopy, and voltammetric methods. This poly-ASA-modified GCE could greatly enhance the detection sensitivity of adenine. At optimum conditions, the anodic peak exhibits a good linear concentration dependence in the range from 3.0 × 10⁻⁸ to 1.0 × 10⁻⁶ M (r = 0.9994). The detection limit is 8.0 × 10⁻⁹ M (S/N = 3). The proposed method could be used to determinate the adenine in tablets of vitamin B₄ with satisfactory results.     

Low-temperature co-fired ceramic microchannels with individually addressable screen-printed gold electrodes on four walls for self-contained electrochemical immunoassays

Microchannel devices were constructed from low-temperature co-fired ceramic (LTCC) materials with screen-printed gold (SPG) electrodes in three dimensions—on all four walls—for self-contained enzyme-linked immunosorbant assays with electrochemical detection. The microchannel confines the solution to a small volume, allowing concentration of electroactive enzymatically generated product and nearby electrodes provide high-speed and high-sensitivity detection: it also facilitates future integration with microfluidics. LTCC materials allow easy construction of three-dimensional structures compared with more traditional materials such as glass and polymer materials. Parallel processing of LTCC layers is more amenable to mass production and fast prototyping, compared with sequential processing for integrating multiple features into a single device. LTCC and SPG have not been reported previously as the basis for microchannel immunoassays, nor with integrated, individually addressable electrodes in three dimensions. A demonstration assay for mouse IgG at 5.0 ng/mL (3.3 × 10^-11 M) with electrochemical detection was achieved within a 1.8 cm long × 290 μm high × 130 μm wide microchannel (approximately 680 nL). Two of four SPG electrodes span the top and bottom walls and serve as the auxiliary electrode and the assay site, respectively. The other two (0.7 cm long × 97 μm wide) are centered lengthwise on the sidewalls of the channel. One serves as the working and the other as the pseudoreference electrode. The immunoassay components were immobilized at the bottom SPG region. Enzymatically generated p-aminophenol was detected at the internal working electrode within 15 s of introducing the enzyme substrate p-aminophenyl phosphate. A series of buffer rinses avoided nonspecific adsorption and false-positive signals.     

Synthesis and supercapacitance of flower-like Co(OH)<Subscript>2</Subscript> hierarchical superstructures self-assembled by mesoporous nanobelts

A facile hydrothermal strategy was first proposed to synthesize flower-like Co(OH)₂ hierarchical microspheres. Further physical characterizations revealed that the flower-like Co(OH)₂ microspherical superstructures were self-assembled by one-dimension nanobelts with rich mesopores. Electrochemical performance of the flower-like Co(OH)₂ hierarchical superstructures were investigated by cyclic voltammgoram, galvanostatic charge–discharge and electrochemical impedance spectroscopy in 3 M KOH aqueous electrolyte. Electrochemical data indicated that the flower-like Co(OH)₂ superstructures delivered a specific capacitance of 434 F g⁻¹ at 10 mA cm⁻² (about 1.33 A g⁻¹), and even kept it as high as 365 F g⁻¹ at about 5.33 A g⁻¹. Furthermore, the SC degradation of about 8% after 1,500 continuous charge–discharge cycles at 5.33 A g⁻¹ demonstrates their good electrochemical stability at large current densities.     

Electrochemical preparation of oxide materials for electrochemistry and electronics

Electrochemical formation of barium tungstate (BaWO₄) was studied as a model case of electrochemical formation of an advanced oxide material for electronics. BaWO₄ is formed on the surface of tungsten electrode during oxidation in alkaline media (pH > 12) containing a corresponding cation. The analysis of electrochemical as well as electrochemical quartz crystal microbalance (EQCM) data taken during these experiments identifies at least three qualitatively different steps composing the electrode process. Effects of the potential, applied current density and alkaline earth metal cation concentration are demonstrated using cyclic voltammetry and galvanostatic experiments. Specific constraints of the ECC formalism for the electrochemical oxide deposition following from the galvanostatic data are discussed. Received: 2 October 1997 / Accepted: 4 December 1997     

Electrocatalytic oxidation of quinine sulfate at a multiwall carbon nanotubes-ionic liquid modified glassy carbon electrode and its electrochemical determination

The electrocatalytic oxidation of quinine sulfate (QS) was investigated at a glassy carbon electrode, modified by a gel containing multiwall carbon nanotubes (MWCNTs) and room-temperature ionic liquid of 1-Butyl-3-methylimidazolium hexafluorophate (BMIMPF₆) in 0.10 M of phosphate buffer solution (PBS, pH 6.8). It was found that an irreversible anodic oxidation peak of QS with E _pa as 0.99 V appeared at MWCNTs-RTIL/glassy carbon electrode (GCE). The electrode reaction process was a diffusion-controlled one and the electrochemical oxidation involved two electrons transferring and two protons participation. Furthermore, the charge-transfer coefficient (α), diffusion coefficient (D), and electrode reaction rate constant (k _f) of QS were found to be 0.87, 7.89 × 10⁻³ cm²⋅s⁻¹ and 3.43 × 10⁻² s⁻¹, respectively. Under optimized conditions, linear calibration curves were obtained over the QS concentration range 3.0 × 10⁻⁶ to 1.0 × 10⁻⁴ M by square wave voltammetry, and the detection limit was found to be 0.44 μM based on the signal-to-noise ratio of 3. In addition, the novel MWCNTs-RTIL/GCE was characterized by the electrochemical impedance spectroscopy and the proposed method has been successfully applied in the electrochemical quantitative determination of quinine content in commercial injection samples and the determination results could meet the requirement.     

The surface characteristics of polycrystalline cobalt electrooxidised in sulfuric acid solutions

The oxidation of cobalt electrodes has been carried out by means of cyclic voltammetry and coulometry under controlled potential in sulfuric acid solutions of different concentrations. The electrochemical scanning tunneling microscope/scanning tunneling microscope (ECSTM/STM) systems constructed by the authors and scanning electron microscopy (SEM) with the SEM-EDX system of surface analysis of the elements have been used. The procedure applied in this work made it possible to observe the fragments of the same surface by means of SEM and ECSTM/STM. The most typical images for a polycrystalline Co electrode with a ±10% accuracy at the scales of 4800 nm × 4800 nm and 100 nm × 100 nm are presented and the results are discussed. In a diluted electrolyte (0.1 M), irregular forms of a stable cobalt oxide with Co:O ratio ∼1:1 appear. Unreproducible results have been obtained in a 1.0 M H₂SO₄ solution. Compact and relatively regular layers of cobalt oxide of the same ratio have been obtained in 0.1 M H₂SO₄, as well as in 10.0 M sulfuric acid solution, under controlled oxidation potential at the passivation range. Received: 6 January 1999 / Accepted: 5 May 1999     

Pseudocapacitive behaviors of nanostructured manganese dioxide/carbon nanotubes composite electrodes in mild aqueous electrolytes: effects of electrolytes and current collectors

Nanostructured MnO₂/carbon nanotubes composite electrode material was prepared using the liquid-phase deposition reaction starting with potassium permanganate (KMnO₄) and manganese acetate (Mn(Ac)₂·4H₂O) as the reactants and carbon nanotubes (CNTs) as the substrates. The structure and morphology of the material was characterized by X-ray diffraction, infrared spectroscopy, and transmission electron microscope techniques. The electrochemical properties of the nano-MnO₂/CNTs composite electrode in 1 M LiAc and 1 M MgSO₄ solutions and in 1 M RAc (R = Li, Na, and K)–1 M MgSO₄ mixed solutions, respectively, were studied. Experimental results demonstrated that the specific capacitance and rate discharge ability of the nano-MnO₂/CNTs composite electrode in 1 M LiAc–1 M MgSO₄ mixed solution is superior to that in 1 M LiAc or 1 M MgSO₄ solution. For the 1 M RAc (R = Li, Na, and K)–1 M MgSO₄ mixed electrolytes, the specific capacitance of the composite electrode was found to be in the following order: LiAc > NaAc > KAc.     

The effect of ‘crystallinity’ and structural disorder on the electrochemical performance of substituted nickel hydroxide electrodes

β_bc-Nickel hydroxide exhibit non-uniform broadening reflections in their PXRD pattern due to the presence of structural disorder. β_bc-Nickel hydroxide electrodes with smaller crystallite size and structural disorder reversibly exchanges 0.9e/Ni. Co/Zn/Ca/Cd-substituted β_bc-nickel hydroxide samples also display non-uniform broadening of reflections in their powder X-ray diffraction patterns with smaller crystallite size and exchanges 0.7–0.8e/Ni. Hydrothermal treatment of β_bc-nickel hydroxide slurry at 170 °C results in an ordering of reflections in their powder X-ray diffraction pattern with an increased crystallite size. Crystalline β-nickel hydroxide electrode reversibly exchanges 0.3–0.4e/Ni. Hydrothermal-treated Co/Zn/Ca/Cd-substituted β_bc-nickel hydroxide slurries at 170 °C display sharp reflections with similar crystallite size and electrochemical activities as that of crystalline β-nickel hydroxide. This clearly demonstrates that partial substitution of Co/Zn/Ca/Cd in the nickel hydroxide matrix does not show any dramatic improvement in their electrochemical activity at 25–30 °C. Structural disordered material with smaller crystallite size delivers electrochemical activity close to theoretical capacity.     

Electrochemical determinations of 6-mercaptopurine on the surface of a carbon nanotube-paste electrode modified with a cobalt salophen complex

A mixture of multi-walled carbon nanotube/graphite paste electrode modified with a salophen complex of cobalt was prepared and was applied for the study of the electrochemical behavior of 6-mercaptopurine (MP) using cyclic and differential pulse voltammetry (DPV). An excellent electrocatalytic activity toward the oxidation of MP was achieved, which led to a considerable lowering in the anodic overpotential and remarkable increase in the response sensitivity in comparison with unmodified electrode. Utilizing DPV method, a linear dynamic range of 1–100 μM with detection limit of 0.1 μM was obtained in phosphate buffer of pH 3.0. The electrochemical detection system was very stable, and the reproducibility of the electrode response, based on the six measurements during 1 month, was less than 3.0% for the slope of the calibration curves of MP. The electrochemical method as a simple, sensitive, and selective method was developed for the determination of MP in pharmaceutical dosage form and human plasma without any treatments.