Cyclic voltammetric, a.c. and d.c. polarization behavior of boron-doped CVD diamond

Boron-doped polycrystalline diamond films have been deposited over a molybdenum substrate by the microwave plasma CVD process using a methane and hydrogen gas mixture at a pressure of 35.7 Torr. Boron doping of diamond has been achieved in situ by using a solid boron source while growing diamond in the CVD process. The a.c. impedance of boron-doped diamond films in 0.5 M NaCl solution has been determined and compared with the results obtained with a molybdenum substrate. Capacitance, solution resistance, and polarization resistance (corrosion rate) have been determined using the experimental data plotted in Nyquist and Bode formats. D.C. polarization techniques such as linear and Tafel polarization have been used to evaluate the doped diamond coated molybdenum and molybdenum for corrosion resistance characteristics in terms of charge-transfer coefficients and corrosion rate. Cyclic voltammetry has been used to evaluate the molybdenum, platinum, and doped and undoped diamond coated molybdenum materials in 0.5 M NaCl solution. We have observed two time constants with a doped diamond electrode / solution interface. Solution resistance was found to be constant irrespective of the electrode in the same electrolyte solution.     

CVD diamond layers for electrochemistry

Diamond electrodes of different morphologies and qualities were manufactured by hot filament chemical deposition (HF CVD) techniques by changing the parameters of diamond growth process. The estimation of diamond quality and identification of different carbon phases was performed by Raman spectroscopy measurements. The effect of diamond quality and amorphous carbon phase content on the electrochemical response of an obtained diamond electrode in 0.5 M H₂SO₄ as supporting electrolyte was investigated by cyclic voltammetry with [Fe(CN)₆]^4?/3? as a redox probe. The kinetic parameters such as catalytic reaction rate constant k₀ and electron transfer coefficient α were determined. The obtained results show that the analytical performance of undoped diamond electrodes can be implemented just by the change of diamond layers quality.     

Electrochemical properties and dissolution of U-5 wt % Zr Alloy in HNO3 solutions

The main parameters of dissolution of U-5 wt % Zr alloy in HNO₃ solutions were determined by linear voltammetry. Without applying external potential, the electrode potential E ⁰ in 0.5–6.0 M HNO₃ solutions increases. Its value is determined by the formation of a film containing hydrated Zr(IV) and U(IV) oxides and by the cathodic reduction of the acid. The reduction of NO₃ — ions on the surface of U-5 wt % Zr alloy in 0.5–6.0 M HNO₃ occurs with high overvoltage. In experiments without applying external potential, an increase in the HNO₂ yield in accordance with this reaction leads to an increase in the alloy dissolution rate with an increase in the HNO₃ concentration in the electrolyte from 0.5 to 6.0 M. At potentials higher than 450 mV vs. Ag/AgCl electrode, the U-5 wt % Zr electrode undergoes transpassivation. Data on the electrolysis at controlled potential in the region close to the transpassivation potential showed that an increase in the electrode potential at the given HNO₃ concentration leads to an increase in the alloy dissolution rate.     

Electrochemical properties and solubility of URh<Subscript>3</Subscript> in nitric acid solutions

Electrochemical properties of the intermetallic compound URh₃ in 0.5–8 M HNO₃ solutions were studied by linear voltammetry. The electrochemical characteristics of URh₃ in nitric acid solution were determined for the first time using the Tafel equation. URh₃ is highly resistant to both chemical and anodic dissolution, which is due to formation of passive films on the electrode surface. All the anodic oxidation processes observed on the electrode led to secondary passivation of the alloy and not to its dissolution. The conclusions based on the electrochemical data were confirmed by experiments in 8 M HNO₃.     

Effect of different acid oxidation on morphology,dispersion and optical band-gap of multi-walled carbon nanotubes

In this paper the effect of different acid oxidation on morphology, dispersion and optical band gap of multi-walled carbon nanotubes (CNTs) is reported. Oxidation of CVD synthesized MWCNTs were carried out in 8M HNO₃, 8M H₂SO₄, 8M HNO₃/H₂O₂ and 8M H₂SO₄/HNO₃. Oxidized nanotubes sample were characterized by XRD, SEM, FT-IR, TGA, UV-Vis spectroscopy and Raman analysis. Oxidized-nanotubes show a lower shift in XRD peak and an increase in d-spacing, which implies deterioration of MWNTs. New peaks in FT-IR spectra of oxidized-nanotube samples at around 1742 cm^?1 confirms the presence of carbonyl groups. Optical bandgap, molar absorptivity coefficient is estimated for pristine and oxidized nanotubes sample using UV-Vis spectroscopy data and Tauc plot. It is observed that optical bandgap decreases on oxidation and lowest bandgap 2.92 eV is observed for H₂SO₄ treated MWNTs. The information on the optical bandgap of the MWNTs is of great importance for the development of optoelectronic devices.     

Preparation of electroplated Ni-P-ultrafine diamond, Ni-P-carbon nanotubes composite coatings and their corrosion properties

Ni-P-ultrafine diamond (UFD) and Ni-P-carbon nanotubes (CNTs) composite coatings were deposited by electroplating at 76°C. The relation between the content of the incorporation and the amount of the UFD and CNTs in the electroplating solution was investigated. The corrosion behavior of the composite coatings was evaluated by polarization curves and electrochemical impedance spectroscopy in 0.1 M NaCl and 0.5 M H₂SO₄ solutions. It was found that increasing the UFD content in the coatings displays better corrosion performance, while a contrary result for the CNTs incorporated alloys was found due to the special structural state. The results show the incorporation of UFD and CNTs in Ni-P coatings is advantageous for forming better passive films.     

Binder-free polyaniline interconnected metal hexacyanoferrates nanocomposites (Metal = Ni,Co) on carbon fibers for flexible supercapacitors

Improvement of the electrical conductivity, specific capacitance and binder-free polyaniline (PANI) interconnected with metal(II) hexacyanoferrate(III) (MHCF) nanocomposites (M?=?Ni, Co) on flexible carbon fibers (CF) were designed in our present research goal. PANI/MHCF/CF nanocomposites were prepared by one-step co-polymerization method. Electrochemical studies like cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy were analyzed. Under the optimized conditions, the nanocomposites demonstrated remarkable electrochemical performances as supercapacitor electrode with outstanding specific capacitances of ~725 F g^?1 at a current density of 1 A g^?1, and retained ~325 F g^?1 even at a high current density of 20 A g^?1 in 0.5 M H₂SO₄?+?0.5 M Na₂SO₄ solution. The excellent cycling stability with capacitance retention of 80% after 1000 cycles may be a potential electrode material for future supercapacitor when its cycling stability and rate performance are addressed.     

Preparation and characterization of Pt-polypyrrole nanocomposite for electrochemical reduction of oxygen

An easy and simple method of one-step reaction was employed to synthesize the platinum-adsorbed polypyrrole nanocomposite (Pt-PPy). The prepared nanocomposite materials were characterized using UV–vis absorption spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and cyclic voltammetry. Polypyrrole within nanocomposite could crosslink to improve its stability on the Au substrates. O₂ reduction was performed at Au electrodes modified Pt-PPy in O₂-saturated 0.5 M H₂SO₄ solution. The results clearly show that modification of Pt-PPy nanocomposite results in the enhancement of the electrocatalytic reduction of oxygen. The nanocomposite may provide a novel electrode material for application in fuel cells and oxygen sensors.     

Substrate roughness,deposit thickness and the corrosion of electroless nickel coatings

The affect of substrate roughness and coating thickness on the corrosion resistance of electroless nickel coatings on mild steel in a 0.99M Na₂SO₄ + 0.01 M H₂SO₄ + 0.05M NaCl solution has been investigated using electrochemical techniques. The coating was electrochemically more active than pure nickel. The anodic polarization, corrosion potential, and corrosion current depend on the substrate roughness and coating thickness. The substrate roughness decreased for finishes in the order, as-ground, 240 grit, 600 grit, and 1 m diamond polish, but the corrosion current on relatively thin coatings decreased in the order 240 grit, as-ground, 600 grit, and 1 m diamond finish. The corrosion potential and the corrosion current of coatings more than about 10m thick were independent of the surface roughness and similar to those observed with pure nickel. The fraction porosity was estimated to be about 0.005 in a coating about 5 m thick on a 600 SiC grit substrate.     

Extraction of Mo from solutions in HNO<Subscript>3</Subscript> and other mineral acids with solutions of dibutyl hydrogen phosphate in a diluent

Extraction of Mo from HNO₃ solutions with solutions of HDBP in xylene and CCl₄ in a wide range of Mo concentrations was studied. The Mo distribution ratios are considerably higher with CCl₄ diluent compared to xylene, but the extractant capacity in both cases is the same and corresponds to the ratio HDBP: Mo = 2. The active species in the Mo extraction is the HDBP dimer. In the first step, an acidic molybdenyl salt with HDBP of the composition MoO₂(DBP)₂(HDBP)₂, exhibiting certain secondary extraction properties toward rare-earth elements, is formed in all the cases. The dependence of the Mo extraction on the aqueous solution acidity passes through a minimum at 3–4 M HNO₃. The subsequent increase in the Mo distribution ratios is associated with the simultaneous extraction of HNO₃ (or nitrate ion) whose concentration in the extract is considerably lower than the Mo concentration. With an increase in the loading of the extract with molybdenum, the acidic molybdenum salt of HDBP undergoes restructurization, probably associated with additional coordination of H₂MoO₄ to it. The dependence of the Mo distribution ratio on the acid concentration in the extraction from sulfuric acid solutions passes through a minimum at 3–4 M H₂SO₄, which correlates with the first step of the acid dissociation, and in the extraction from HClO₄ the dependence passes through a minimum and a maximum. In the extraction from hydrochloric acid solutions, the Mo extractability decreases with an increase in the acid concentration, owing to complexation in the aqueous phase. The nature of processes occurring at various loadings of the extract with molybdenum are discussed.     

Effect of O2, H2O2, and HNO2 on the stability of Np(III–VII) in aqueous solutions

Published data on reactions of Np ions with O₂, H₂O₂, HNO₂, and HNO₃ in solutions of various compositions in a wide pH range are considered. O₂ oxidizes Np(III) in acid solution and Np(IV) and Np(V) in alkaline solutions. H₂O₂ exhibits dual behavior. In weakly acidic solutions, it converts Np(III) and (IV) to Np(V), in 0.75?C1 M NaHCO₃ it oxidizes Np(V) to Np(VI), whereas in dilute HClO₄ and HNO₃ and in carbonate and alkali solutions it reduces Np(VI), and in alkali solutions it reduces Np(VII). The first step of reduction in most cases is the formation of the Np(VI) peroxide complex, and the next step is the intramolecular charge transfer. In concentrated HNO₃ solutions, H₂O₂ converts Np(V) to Np(IV) and Np(VI) and then reduces Np(VI). Some radiation-, photo-, and sonochemical reactions occur via formation of excimers, i.e., of dimers arising from excited and unexcited Np ions. The excimer decomposes into two ions with higher and lower oxidation states. In reduction reactions, the excimer eliminates H₂O₂ (in addition to the H₂O₂ arising as primary product of water radiolysis). In HNO₃ solutions, oxidation of Np ions occurs only in the presence of HNO₂ arising as reaction product or upon radiolysis, photolysis, or sonolysis. The active species are NO ₂ ^? , NO₂, and NO⁺ present in equilibrium with HNO₂.     

Effect of carbon on corrosion resistance of powder-processed Fe-0·35%P alloys

The corrosion behaviour of phosphoric irons containing 0.35 wt % P, 2% copper, 2% nickel, 1% silicon, 0.5% molybdenum, with/without 0.15% carbon prepared by powder forging route were studied in different environments. The various environments chosen were acidic (0.25 M H₂SO₄ solution of pH 0.6), neutral/marine (3.5% NaCl solution of pH 6.8) and alkaline (0.5 M Na₂CO₃ + 1.0 M NaHCO₃ solution of pH 9.4). The corrosion studies were conducted using Tafel extrapolation and linear polarization methods. The studies also compare Armco iron with phosphoric irons. It was observed that the addition of carbon improved the corrosion resistance of a Fe-0.35%P-2%Ni-2%Cu-1%Si-0.5%Mo alloy in all the environments. Corrosion rates were highest in acid medium, minimal in alkaline medium and low in neutral solution. SEM/EDAX was used to characterize the compositions.     

Activated carbons derived from sugarcane bagasse for high-capacitance electrical double layer capacitors

Activated carbon (AC) from sugarcane bagasse was prepared using a simple two-step method of carbonization and chemical activation with four different activating agents (HNO₃, H₂SO₄, NaOH, and KOH). Amorphous carbon structure as identified by X-ray diffraction was observed in all samples. Scanning electron microscopy revealed that the AC had more porosity than the non-activated carbon (non-AC). Specific capacitance of the non-AC electrode was 32.58 F g^?1 at the current density of 0.5 A g^?1, whereas the AC supercapacitor provided superior specific capacitances of 50.25, 69.59, 109.99, and 138.61 F g^?1 for the HNO₃ (AC-HNO₃), H₂SO₄ (AC-H₂SO₄), NaOH (AC-NaOH), and KOH (AC-KOH) activated carbon electrodes, respectively. The AC-KOH electrode delivered the highest specific capacitance (about 4 times of the non-AC electrode) because of its good surface wettability, the largest specific surface area (1058.53 m² g^?1), and the highest total specific pore volume (0.474 cm³ g^?1). The AC-KOH electrode also had a great capacitance retention of almost 100% after 1000 GCD cycles. These results demonstrate that our AC developed from sugarcane bagasse has a strong potential to be used as high stability supercapacitor electrode material.

     

Electrochemical properties and dissolution of UPd<Subscript>3</Subscript> in nitric acid solutions

Electrochemical properties of the intermetallic compound UPd₃ in 0.5–8 M HNO₃ solutions were studied by linear voltammetry. In 0.5–2 M HNO₃ solutions, the UPd₃ surface is in the passive state. At HNO₃ concentrations exceeding 4 M, the alloy passivation was not observed. The previously unknown electrochemical characteristics of UPd₃ in nitric acid solutions were obtained using the Tafel equation. The values of Е(i = 0) and v_corr increased from 39 mV and 38 μg cm^–2 h^–1 in 0.5 M HNO₃ to 821 mV and 11 mg cm^–2 h^–1 in 8 M HNO₃, respectively. Dissolution experiments have shown that UPd₃ can dissolve in HNO₃ solutions of concentration exceeding 4 M at room temperature. In 8 M HNO₃, the dissolution rate can reach 17 mg cm^–2 h^–1 at 25°С, with the dissolution being virtually equimolar and accelerating with time.     

Synthesis of polypyrrole film by pulse galvanostatic method and its application as supercapacitor electrode materials

Polypyrrole (PPy) film-coated stainless steel electrodes were prepared from aqueous solution containing 0.5 M p-toluene sulphonic acid and 0.1 M pyrrole using pulse galvanostatic method (PGM) and galvanostatic method (GM). The morphology was characterized by scanning electron microscopy. The electrochemical properties of PPy films were investigated with cyclic voltammetry, charge–discharge tests, and ac impedance spectroscopy. The results showed that the PGM-PPy films exhibited higher specific capacitance, better high-rate discharge ability and lower resistance, and were more promising for applications in supercapacitor than GM-PPy films. The specific capacitance (SC) of PGM-PPy films was 403 F g⁻¹ in 1 M H₂SO₄ electrolyte and 281 F g⁻¹ in 1 M NaNO₃ electrolyte.     

Preparation of Pt/multiwalled carbon nanotubes modified Au electrodes via Pt–Cu co-electrodeposition/Cu stripping protocol for high-performance electrocatalytic oxidation of methanol

Pt nanoparticles well dispersed on multiwalled carbon nanotubes (MWCNTs) were prepared for high-performance electrocatalytic oxidation of methanol in both acidic and alkaline media via the co-electrodeposition/stripping (CS) protocol, namely, co-electrodeposition of Pt and Cu followed by electrochemical stripping of Cu, as examined by cyclic voltammetry (CV), electrochemical quartz crystal microbalance (EQCM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The Pt catalyst prepared by the CS protocol on MWCNTs (Pt_cs/MWCNTs/Au) exhibited a specific electrocatalytic activity of 519 and 2210 A g⁻¹ toward cyclic voltammetric electrooxidation (50 mV s⁻¹) of methanol in 0.5 M CH₃OH + 0.5 M H₂SO₄ and 0.5 M CH₃OH + 1.0 M NaOH media, respectively, which are larger than those prepared by conventional electrodeposition from chloroplatinic acid on Au and MWCNTs/Au, as well as that by a CS protocol on Au. The Pt_cs/MWCNTs/Au electrode also possessed the highest stability, which maintained 91% and 90% of its initial catalytic activity after 120-cycle CV in 0.5 M CH₃OH + 0.5 M H₂SO₄ and 0.5 M CH₃OH + 1.0 M NaOH, respectively. The electrode kinetics of methanol oxidation is also briefly discussed. The nanosubstrate-based CS protocol is simple, convenient and efficient, which is expected to find wide applications in film electrochemistry and electrocatalysis.     

Two-step growth of HFCVD diamond films over large areas

This paper reports the results of a two-step hot filament chemical vapor deposition method to improve the quality of diamond films. Diamond films were deposited on a Si(100) substrate having an area of 45 cm² and a thickness of 60 μm, employing a HFCVD system. The first step is the growth of CVD diamond in the HFCVD reactor. In the second step, the samples were treated in a saturated solution of H₂SO₄:CrO₃ and rinsed in a (1:1) solution of H₂O₂:NH₄OH. After this procedure, a second diamond layer was deposited. The diamond films were analyzed by Raman scattering spectroscopy (RSS), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The films showed a high degree of purity with a thickness of 60 μm, presenting uniform characteristics over a large area.     

Potentiodynamical deposition of nanosized manganese oxides as high capacitance electrochemical capacitors

Nanosized manganese oxides powders were potentiodynamically deposition onto the Pb substrates by anodic oxidation in 0.5 mol L⁻¹ MnSO₄ and 0.5 mol L⁻¹ H₂SO₄ mixed solution at 40 °C. The chemical composition of the sample was determined by complex titration with ethylene diamine tetraacetic acid and redox titration. X-ray diffraction, scanning electron microscopy, cyclic voltammetry, and chronopotentiometry were employed to characterize the materials. The highest specific capacitance of the MnO_x composite electrode was up to 420 F g⁻¹ in 1.0 mol L⁻¹ Na₂SO₄ electrolyte at the scan rate 5 mV s⁻¹. The synthesized nanosized manganese oxide exhibited ideal capacitive behavior indicating a promising electrode material for electrochemical supercapacitors.     

Excitation of the uranyl ion in U(IV) oxidation with xenon difluoride in frozen aqueous sulfuric acid solutions: I. Role of phase transitions in 5 M H<Subscript>2</Subscript>SO<Subscript>4</Subscript>

Low-temperature oxidation of U(IV) with xenon difluoride, accompanied by formation of the UO ₂ ²⁺ ion in an electronically excited state, was examined in relation to the aggregation state and phase composition of 5 M aqueous H₂SO₄ solution. The reaction was found to proceed at a measurable rate in a supercooled liquid solution at T > 190 K and in a polycrystalline sample at T > 150 K. The chemiluminescence emitted by *(UO ₂ ²⁺ ) grows in intensity by several (up to 6) orders of magnitude during the exothermic phase transitions. The increase in the chemiluminescence intensity during phase transitions (crystallization of liquid, in particular, supercooled 5 M H₂SO₄ solutions) correlates with formation of the crystalline phase. Presumably, one of the factors accelerating the low-temperature reaction of U(IV) with XeF₂ during crystallization of the solution (along with concentration of the reactants in the intercrystallite spaces) is the catalytic activity exhibited by the freshly formed surface of the H₂SO₄ crystal hydrates.     

Corrosion behaviour of amorphous Ti48Cu52, Ti50Cu50 and Ti60Ni40 alloys investigated by potentiodynamic polarization method

Potentiodynamic polarization studies were carried out on virgin specimens of amorphous alloys Ti₄₈Cu₅₂, Ti₅₀Cu₅₀ and Ti₆₀Ni₄₀ in 0.5 M HNO₃, 0.5 M H₂SO₄ and 0.5 M NaOH aqueous media at room temperature. The value of the corrosion current density (I_corr) was maximum for Ti₄₈Cu₅₂ alloy in all the three aqueous media as compared to the remaining two alloys. The value of I_corr for the alloy Ti₄₈Cu₅₂ was maximum (I_corr = 2.6 × 10^{- 5} A/cm²) in 0.5 M H₂SO₄ and minimum (I_corr = 3.5 × 10^{- 6} A/cm²) in 0.5 M NaOH aqueous solutions. In contrast, the alloy Ti₆₀Ni₄₀ exhibited the least corrosion current density in 0.5 M HNO₃ (I_corr = 40 × 10^{- 7}A/cm²) and in 0.5 M NaOH (I_corr = 5.5 × 10^{- 7} A/cm²) aqueous media as compared to those for Ti-Cu alloys, while its value in 0.5 M H₂SO₄ was comparable to that for Ti₄₀Cu₅₀. It is suggested that the alloy Ti₆₀Ni₄₀ is more corrosion resistant than the alloys Ti₄₈Cu₅₂ and Ti₅₀Cu₅₀ in all the three aqueous media.