Influence of ammonia concentration on anodic deposition of nickel oxide

Nickel oxide was prepared by anodic deposition in a basic solution comprising nickel chloride, ammonium chloride and ammonia. The influence of ammonia was investigated using galvanostatic reduction techniques and cyclic voltammetry (CV). It was found that the oxidization peak potentials shift positively with decreasing ammonia concentration, while the oxidization peak currents firstly increase with decreasing ammonia concentration and then decrease when the ammonia concentration is lower than 1.67 mol L⁻¹. This is mainly attributed to the fact that the equilibrium concentration of Ni(OH)_2(aq) varies with the ammonia concentration. A potential plateau at about 0.75 V was observed in galvanostatic reduction curves for nickel oxide, and the length of the potential plateau in solution with 1.67 mol L⁻¹ ammonia concentration was longer than that in the other solutions, which agreed well with the CV results.     

Photo-electrochemical investigation of anodic oxide films on cast Ti-Mo alloys. I. Anodic behaviour and effect of alloy composition

The anodic behaviour of cast Ti-Mo alloys, having different Mo contents (6-20 wt.%), was investigated in acidic and neutral aerated aqueous solutions. All sample showed a valve-metal behaviour, owing to formation and thickening of barrier-type anodic oxides displaying interference colours. Growth kinetics of passive films is influenced by both anodizing electrolyte and composition of the starting alloy. This last parameter was found to change also the solid-state properties of the films, explored by photoelectrochemical and impedance spectroscopy experiments. Thicker films (U_f = 8 V/MSE) grown on alloys richer in Mo showed more resistive character and a photocurrent sign inversion under negative bias, that revealed an insulating character, whereas corresponding films grown on alloys with lower Mo content, as well as thinner films, behaved as n-type semiconductors. Results are discussed in terms of formation of a mixed Ti-Mo oxide phase.     

铋膜玻碳电极阳极溶出伏安法测定异烟肼的研究

研究了异烟肼在铋膜玻碳电极上的电化学行为检测方法。在0.1mol/L pH=4.5的HAc—NaAc缓冲溶液的底液中,通过富集,用铋膜玻碳电极进行阳极溶出伏安法测定异烟肼。异烟肼的阳极峰电位为-1.02V(vs.SCE),峰电流与异烟肼的浓度在1.6×10~(-7)～8.0×10~(-5)mol/L范围内呈良好的线性关系。该方法的检出下限为8.0×10~(-8)mol/L。对异烟肼含量用本法进行了测定,获得了满意的结果。本方法的优点是成本低,操作方便,重现性好以及检测下限低。     

Impedance study of the charge transport at poly-o-phenylenediamine film electrodes

The measured low-frequency capacitance of the polymer film was proportional to the film thickness and showed a maximum at the formal potential of the polymer. We analyzed the relation between the capacitance and potential by taking into account the contribution of an interaction between electroactive sites. The Warburg coefficient and the width of a linear 45 ° region in an impedance diagram showed each their minima near the formal potential. These impedance data were interpreted in terms of the diffusion-migration transport of both electron and anion through the film. The results of the impedance analysis suggested that electron transport was ensured by interchain electron hopping. From the combinations of the measured kinetic parameters, we inferred that one of the two charge carriers moved much faster than the other. The obtained diffusion coefficient showed a maximum in the vicinity of a voltammetric current peak; this corresponded with the potential dependence of a coupled diffusion coefficient expected in the extreme case of electron-transport control.     

Study of anodic oxide films formed on solid-state sintered SiC-ceramic at high anodic potentials

The present work studies the formation, chemical composition, and structure of an oxide layer formed on the technical solid-state sintered ceramic (EKasic®D) in a strong alkaline solution (1 M NaOH at pH 14) at high anodic potentials (30 V vs. 3 M Ag/AgCl). The observed formation of oxide films on SiC in alkaline solution is in contradiction to the thermodynamic laws (Pourbaix-diagram). The film thickness was determined by SEM/EDX measurements using the specific thin film analysis tool “AZtec” (Oxford Instruments) as well as the transmission electron microscopy. The thickness of the oxide film formed at 30 V amounts to 30 nm that corresponds with a field strength of E = 10 MV cm^?1, which corresponds with the formation according to the high-field mechanism. The chemical composition was studied by EDX-analysis in a transmission electron microscope as well as by X-ray photoelectron spectroscopy (XPS). The oxide layer is completely amorphous and consists of non-stoichiometric SiO_x and SiO_xC_y. The layer is assumed as graded with a higher amount of SiO_x in the outermost regions and an increased amount of SiO_xC_y in the inner region of the passive layer. Additionally, the passive layer is doped by a small amount of aluminum originating from a sinter additive used in the manufacture of the SiC ceramic and completely incorporated into the SiC grains.     

High-throughput study of the anodic oxidation of Hf-Ti thin films

A comprehensive study of the anodic oxide formation on Hf-Ti alloys over the entire range of composition was conducted. Combinatorial thin film libraries were prepared by co-sputtering. HRSEM and XRD were used to characterize the thin films and to confirm that a continuous change of the composition was obtained for this fully miscible system. Electrochemical investigations were performed by means of an automatic scanning droplet cell with a droplet radius of 100 μm. Subsequent potentiodynamic scans with intermittent impedance measurements allowed a quantitative determination of film formation factor and dielectric constant for each composition with a resolution of 0.5 at.%. Mott-Schottky analysis of the oxides was used to evaluate the relationship between parent metal composition and oxide properties, namely flat band potential and donor density. The structure-property and composition-property relationships are discussed in detail.     

Simplified point defect model for growth of anodic passive films on iron

We present a simplified point defect model to describe the growth of the primary passive oxide film on the surface of iron. The model postulates a reduced set of elementary interfacial reactions to describe the formation and dissolution of the oxide film. By casting the model in dimensionless form, we obtain a relatively small set of parameters that must be assigned values. Parameter values are set by matching the film thickness predicted by the model with one experimental data point. The model is then used to predict variations in film thickness with time, temperature, and applied potential, yielding reasonable agreement with experimental data. The model also gives the correct qualitative trend in the dependence of film thickness on electrolyte pH. Although the model parameters used in our comparisons are probably not unique, they suggest that physical picture embodied in the model provides a suitable starting point for modeling the growth of passive films on Fe.     

Influence of silicon on the growth of barrier-type anodic films on titanium

Amorphous anodic titania, stabilised by incorporation of silicon species, is shown to grow to high voltages on sputter-deposited, single-phase Ti-Si alloys during anodizing at a constant current density in ammonium pentaborate electrolyte. The films comprise two main layers, with silicon species confined to the inner layers. An amorphous-to-crystalline transition occurs at ∼60 V on the Ti-6 at.% Si alloy, while the transition is suppressed to voltages above 140 V on alloys with 12 and 26 at.% silicon. The crystalline oxide, nucleated at a depth of ∼40% of the film thickness, is associated with the presence of a precursor of crystalline oxide in the pre-existing air-formed oxide. The modified structure of the air-formed oxide due to increased incorporation of silicon species suppresses the amorphous-to-crystalline transition until the onset of dielectric breakdown. The transport numbers of cations and anions during growth of the anodic oxides are independent of the concentration of silicon species in the inner layer, despite the marked change in the field strength.     

Anodic stripping voltammetric measurement of trace heavy metals at antimony film carbon paste electrode

The antimony film carbon paste electrode (SbF-CPE) was prepared in situ on the carbon paste substrate electrode as a “mercury-free” electrochemical sensor. Its aptitude for measuring some selected trace heavy metals has been demonstrated in combination with square-wave anodic stripping voltammetry in non-deaerated model solutions of 0.01 M hydrochloric acid with pH 2. Some important operational parameters, such as deposition potential, deposition time, and concentration of antimony ions were optimized, and the electroanalytical performance of the SbF-CPE was critically compared with both bismuth film carbon paste electrode (BiF-CPE) and mercury film carbon paste electrode (MF-CPE) using Cd(II) and Pb(II) as test metal ions. In comparison with BiF-CPE and MF-CPE, the SbF-CPE exhibited superior electroanalytical performance in more acidic medium (pH 2) associated with favorably low hydrogen evolution, improved stripping response for Cd(II), and moreover, stripping signals corresponding to Cd(II) and Pb(II) at the SbF-CPE were slightly narrower than those observed at bismuth and mercury counterparts. In addition, the comparison with antimony film electrode prepared at the glassy carbon substrate electrode displayed higher stripping current response recorded at the SbF-CPE. The newly developed sensor revealed highly linear behavior in the examined concentration range from 5 to 50 μg L⁻¹, with limits of detection (3σ) of 0.8 μg L⁻¹ for Cd(II), and 0.2 μg L⁻¹ for Pb(II) in connection with 120 s deposition step, offering good reproducibility of ±3.8% for Cd(II), and ±1.2% for Pb(II) (30 μg L⁻¹, n = 10). Preliminary experiments disclosed that SbF-CPE and MF-CPE exhibit comparable performance for measuring trace concentration levels of Zn(II) in acidic medium with pH 2, whereas its detection with BiF-CPE was practically impossible. Finally, the practical applicability of SbF-CPE was demonstrated via measuring Cd(II) and Pb(II) in a real water sample.     

Carbon nanotube-modified glassy carbon electrode for anodic stripping voltammetric detection of Uranyle

A multi-wall carbon nanotube (MWNT) modified glassy carbon electrode (GCE) is described for the measurement of trace levels of uranium by anodic stripping voltammetry. In a pH 4.4 NaAc-Hac buffer containing 0.010 mol L⁻¹ Mg(NO₃)₂, UO₂ ²⁺ was adsorbed onto the surface of a MWNT film coated glassy carbon electrode and then reduced at −0.40 V vs. Ag/AgCl. During the positive potential sweep the reduced uranium was oxidized and a well-defined stripping peak appeared at +0.20 V vs. Ag/AgCl. Low concentrations of Mg²⁺ significantly enhanced the stripping peak currents since they induced UO₂ ²⁺ to adsorb at the electrode surface. The response was linear up to 1.2 × 10⁻⁷ mol L⁻¹ and the relative standard deviation at 2.0 × 10⁻⁸ mol L⁻¹ uranium was 5.2%. Potential interferences were examined. The attractive behavior of the new “mercury-free” uranium sensor holds promise for on-site environmental and industrial monitoring of uranium.     

In situ photoelectrochemistry and Raman spectroscopic characterization on the surface oxide film of nickel electrode in 30 wt.% KOH solution

The oxide films of nickel electrode formed in 30 wt.% KOH solution under potentiodynamic conditions were characterized by means of electrochemical, in situ PhotoElectrochemistry Measurement (PEM) and Confocal Microprobe Raman spectroscopic techniques. The results showed that a composite oxide film was produced on nickel electrode, in which aroused cathodic or anodic photocurrent depending upon polarization potentials. The cathodic photocurrent at −0.8 V was raised from the amorphous film containing nickel hydroxide and nickel monoxide, and mainly attributed to the formation of NiO through the separation of the cavity and electron when laser light irradiates nickel electrode. With the potential increasing to more positive values, Ni₃O₄ and high-valence nickel oxides with the structure of NiO₂ were formed successively. The composite film formed in positive potential aroused anodic photocurrent from 0.33 V. The anodic photocurrent was attributed the formation of oxygen through the cavity reaction with hydroxyl on solution interface. In addition, it is demonstrated that the reduction resultants of high-valence nickel oxides were amorphous, and the oxide film could not be reduced completely. A stable oxide film could be gradually formed on the surface of nickel electrode with the cycling and aging in 30 wt.% KOH solution.     

Thermal expansion coefficient tailoring of LAS glass-ceramic for anodic bondable low temperature co-fired ceramic application

The Li–Al–Si glass-ceramics were prepared by conventional glass-ceramic fabrication method. The influences of Na₂O content on the sintering property, microstructure, and coefficient of thermal expansion were investigated. The results show that the coefficient of thermal expansion of LAS glass-ceramics can be tailored to match that of silicon by the addition of Na₂O content. Na₂O has a remarkable influence on the crystallinity of Li–Al–Si glass-ceramic. The coefficient of thermal expansion of Li–Al–Si glass-ceramic is thus tunable between that of glass phase and crystal phase. The Si–O bond length change in stretch vibration modes introduced by Na₂O also contributes to the variation of coefficient of thermal expansion of the Li–Al–Si glass-ceramics. The coefficient of thermal expansion of the Li–Al–Si glass-ceramic with 1.5 wt% Na₂O addition is about +3.34 ppm/°C at 350 °C and shows a good compatibility to that of silicon in a wide temperature range, which makes it a promising candidate for anodic bondable low temperature co-fired ceramic substrate applications.     

Formation of anodic films on sputtering-deposited Al-Hf alloys

The growth of barrier-type anodic films at high efficiency on a range of sputtering-deposited Al-Hf alloys, containing from 1 to 95 at.% Hf, has been investigated in ammonium pentaborate electrolyte. The alloys encompassed nanocrystalline and amorphous structures, the latter being produced for alloys containing from 26 to 61 at.% Hf. Except at the highest hafnium content, the films were amorphous and contained units of HfO₂ and Al₂O₃ distributed relatively uniformly through the film thickness. Boron species were confined to outer regions of the films. The boron distributions suggest that the cation transport number decreases progressively with increasing hafnium concentration in the films, from ∼0.4 in anodic alumina to ∼0.2 for a film on an Al-61 at.% Hf alloy. The distributions of Al³⁺ and Hf⁴⁺ ions in the films indicate their similar migration rates, which correlates with the similarity of the energies of Al³⁺-O²⁻ and Hf⁴⁺-O²⁻ bonds. For an alloy containing ∼95 at.% Hf, the film was largely nanocrystalline, with a thin layer of amorphous oxide, of non-uniform thickness, at the film surface. The formation ratios for the films on the alloys changed approximately in proportion to the hafnium content of the films between the values for anodic alumina and anodic hafnia, ∼1.2 and 1.8 nm V⁻¹ respectively.     

Combinatorial investigation of Hf-Ta thin films and their anodic oxides

A co-sputtering technique was used for the fabrication of a thin film combinatorial library (Hf-21 at.% Ta to 91 at.% Ta) based on alloying of Hf and Ta. The microstructure and crystallography of individual metallic alloy compositions were analyzed using SEM and XRD mapping, respectively. Three different zones of microstructure were identified within the range of alloys, going from hexagonal to tetragonal through an intermediate amorphous region. The local oxidation of Hf-Ta parent metal alloys at different compositions was investigated in steps of 1 at.% using an automated scanning droplet cell in the confined droplet mode. Potentiodynamic anodisation cycles combined with in situ impedance spectroscopy provide basic knowledge regarding the oxide formation and corresponding electrical properties. Dielectric constants were mapped for the entire composition range and XPS depth profiles allowed investigation of the oxide compositions.     

Morphological and structural studies of nanoporous nickel oxide films fabricated by anodic electrochemical deposition techniques

Porous nickel oxide films are directly deposited onto conducting indium tin oxide coated glass substrates by cyclic voltammetric (CV), galvanostatic, and potentiostatic strategies in a plating bath of sodium acetate, nickel sulfate, and sodium sulfate. By tuning the deposition parameters, it is possible to prepare nickel oxide films with various morphologies and structures. Film formation relies on the oxidation of dissolved Ni²⁺ to Ni³⁺, which further reacts with the available hydroxide ions from a slightly alkaline electrolyte to form insoluble nickel oxide/hydroxide deposits on the substrate. A compact film with particularly small pores is obtained by CV deposition in a potential range of 0.7-1.1 V. A galvanostatically deposited film is structurally denser near the surface of the substrate, and becomes less dense further away from the surface. Interestingly, a potentiostatically deposited film has pores distributed uniformly throughout the entire film. Therefore, for obtaining a uniform film with suitable pore size for electrolyte penetration, potentiostatic deposition technique is suggested. In addition, except for CV deposition, the deposited films resemble closely to cubic NiO when the annealing temperature exceeds 200 °C.     

Performance improvement of pasted nickel electrodes with an addition of ball-milled nickel hydroxide powder

Spherical β-Ni(OH)₂ was modified by a low-cost method of normal ball milling (NBM), and the physical properties of both ball-milled and un-milled Ni(OH)₂ were characterized by transmission electron microscopy, specific surface area, particle size distribution and X-ray diffraction. It was found that NBM could obviously increase the surface area, decrease the particle and crystallite size, and reduce the crystallinity of β-Ni(OH)₂, which were advantageous to the improvement of the electrochemical activity of Ni(OH)₂. NBM also lowered the packing density and flowability of Ni(OH)₂, as revealed by the measurements of tapping density and angle of repose. Electrochemical performances of pasted nickel electrodes with an addition of ball-milled Ni(OH)₂ to spherical Ni(OH)₂ as the active material were investigated, and were compared with those of the pure spherical Ni(OH)₂ electrodes. Charge/discharge tests showed that ball-milled Ni(OH)₂ addition could enhance the charging efficiency, specific discharge capacity, discharge voltage and high-rate capability of the electrodes. This performance improvement could be attributed to a more compact electrode microstructure, better reaction reversibility and lower electrochemical impedance, as indicated by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. Thus, it was an effective method to modify the microstructure and improve the electrochemical properties of pasted nickel electrodes by adding an appropriate amount of ball-milled Ni(OH)₂ to spherical Ni(OH)₂ as the active material.     

Thermal plasma-sprayed nickel catalysts in the clean-up of biomass gasification gas

Plasma spraying is a potential catalyst preparation method for hot gas clean-up which requires very durable catalytic coatings. In this work, the multi-layer composite coating powders consisted of a gibbsite or boehmite core together with a hydrotalcite coating. We used the coating powders in the preparation of the powder form Ni catalysts, which were characterized by XRD, BET, XPS, and XRF. The coating powders were also used in the preparation of plasma sprayed metal substrate supported Ni catalysts. Selected plasma-sprayed nickel containing catalysts were characterized by XRD and SEM. In addition, for comparative purposes we prepared and used a ceramic monolithic Ni catalyst on modified ZrO₂. The performance of all catalysts were studied in the clean-up of synthetic gasification gas, i.e. the decomposition of tar, ammonia, and methane, at 700 and 900 °C, without and with the addition of H₂S. Catalysts with a gibbsite-core showed higher activity and better sulfur resistance than those with a boehmite-core. The high activity of the gibbsite-core Ni catalyst with a sulfur-containing gas together with the promising mechanical and thermal strength makes the plasma spraying method an interesting alternative for gas clean-up catalysts in biomass gasification processes.     

Selection of measurement frequency in Mott-Schottky analysis of passive layer on nickel

The measurements of charge space capacity versus potential have been carried out on passive nickel electrode. The Mott-Schottky relationship was obtained for different measurement frequencies. The shape of this characteristic depends on selection of the measurement frequency. Due to that fact, the accounted defect concentration depends on frequency also. The Mott-Schottky relationship obtained by means of dynamic electrochemical impedance spectroscopy does not possess this disadvantage.     

Structure and electrochemical properties of nickel hydroxide electrodes with cobalt additives

In this article,cobalt additives are introduced into nickel hydroxide electrodes by two incorporation methods—co-precipitated cobalt hydroxide during the nickel hydroxide synthesis or post-added CoO with nickel hydroxide. The results of X-ray diffraction, cyclic voltammetry, electrochemical impedance spectroscopy, and charge–discharge tests indicate that (i) the diffraction peaks show a decrease in intensity and increase in the half peak breadths for Ni(OH)₂ with co-precipitated cobalt hydroxide; (ii) the electrochemical activity of nickel hydroxide can be improved by both incorporated cobalt and the effects of post-added CoO are more notable; (iii) CoOOH derived from post-added CoO is not stable in the KOH electrolyte when the potential of the Ni(OH)₂ electrode is lowered and its reduction product may be inactive, thus results in an irreversible capacity loss of nickel-metal-hydride battery after over-discharge-state storage.     

Influence of cathodic overpotential on grain size in nanocrystalline nickel deposition on rotating cylinder electrodes

Nanocrystalline (nc) nickel was electrodeposited using rotating cylindrical electrodes (RCD) from a Watt’s bath containing saccharin. The effects of cathode rotation speed and saccharin concentration on the cathodic overpotential and grain size were studied. The grain sizes are presented as a function of the cathodic overpotential. All reported cathodic overpotentials were corrected for ohmic overpotentials. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to determine the deposit grain size. In addition, the influences of cathode rotation speed and current density on the morphology of nanocrystalline nickel were observed by scanning electron microscopy (SEM).