Showing papers in "Journal of Applied Electrochemistry in 1995"

Use of electrochemical impedance spectroscopy for the study of corrosion protection by polymer coatings

Abstract: A discussion of the requirements for hardware and software necessary for collection and analysis of electrochemical impedance spectroscopy data for polymer coated metals is presented. Most authors agree that a simple model can describe the frequency dependence of impedance spectra for polymer coated metals exposed to corrosive environments. The water uptake of the coating can be estimated from the time dependence of the coating capacitance C c, The pore resistance R po depends both on the resistivity ρ of the coating and the disbonded area A d. The polarization resistance R P of the corroding area under the coating and the corresponding capacitance C dl both depend on A d. The breakpoint frequency method is discussed in detail and the dependence of the breakpoint frequency f b on ρ and A d is derived. In addition to f b other parameters can be obtained which depend on the ratio A d/ρ or only on A d or ρ. Since these parameters can be obtained at frequencies exceeding 1 Hz without the need for an analysis of the impedance spectra in the entire frequency region, this approach is considered especially useful for corrosion monitoring. The concepts proposed for the analysis and interpretation of EIS data for polymer coated metals are illustrated using data for Al alloys, Mg and steel exposed to NaCl. For an alkyd coating on cold rolled steel the time dependence of A d and ρ during exposure to 0.5 m NaCl has been determined qualitatively using the modified breakpoint frequency method.

Anodic oxidation of phenol in the presence of NaCl for wastewater treatment

Abstract: The electrochemical oxidation of phenol in the presence of NaCl for wastewater treatment was studied at Ti/SnO2 and Ti/IrO2 anodes. The experimental results have shown that the presence of NaCl catalyses the anodic oxidation of phenol only at Ti/IrO2 anodes due to the participation of electro-generated ClO− in the oxidation. Analysis of the oxidation products has shown that initially organo-chlorinated compounds are formed in the electrolyte which are further oxidized to volatile organics (CHCl3).

Electrochemical codeposition of inert particles in a metallic matrix

Abstract: A survey on electrochemical codeposition of inert particles in a metallic matrix is given. Particles held in suspension in an electroplating bath are codeposited with the metal during electrodeposition. The particles used are inert to the bath and can be of different types, that is, pure metals, ceramics or organic materials. Combining this variety of types of particles with the different electrodeposited metals, electrochemical codeposition enables the production of a large range of composite materials with unique properties. Many experimental factors were found to influence the codeposition process, which led to some understanding of the mechanism. Models to predict the codeposition rate were developed, but were only partly successful.

Photoelectrochemistry and the environment

Abstract: This article builds upon a companion review on electrochemical techniques [1] and discusses photocatalytic methods for the treatment and analysis of pollutants in water and air. Organic, inorganic and microbiological pollutants are considered. Areas wherein further research and development are needed are identified. Finally, perspectives on commercialization of this technology are presented.

Applications of magnetoelectrolysis

Abstract: A broad overview of research on the effects of imposed magnetic fields on electrolytic processes is given. As well as modelling of mass transfer in magnetoelectrolytic cells, the effect of magnetic fields on reaction kinetics is discussed. Interactions of an imposed magnetic field with cathodic crystallization and anodic dissolution behaviour of metals are also treated. These topics are described from a practical point of view.

Physical and electrochemical characteristics of nickel hydroxide as a positive material for rechargeable alkaline batteries

Abstract: Nickel hydroxide is widely used as an active material in pasted-type nickel electrodes. Physical properties of several nickel hydroxide powders have been examined by laser diffraction, BET, X-ray diffraction, thermal analysis and SEM. Nickel hydroxide powder with a smaller crystalline size shows better charge-discharge cyclic characteristics. The chemical diffusion coefficients of the proton in the nickel hydroxide sample are measured by a current pulse relaxation method. Nickel hydroxide with a smaller crystalline size shows a higher proton diffusion coefficient, giving excellent cycling behaviour.

Processes for the production of mixtures of caustic soda and hydrogen peroxide via the reduction of oxygen

Abstract: The long history of the synthesis of hydrogen peroxide via the cathodic reduction of oxygen in caustic soda catholyte is reviewed. Recent progress is analysed on the electrochemical syntheses of mixtures of caustic soda and hydrogen peroxide in various by-weight ratios from 2.3: 1 NaOH to H2O2 to about 1 : 1. The analysis presented focuses primarily on published work concerning planar fuel cell type electrodes in membrane-divided cells and particulate bed electrodes in cells employing microporous separators with well-defined anolyte-to-catholyte flows. Potential ancillary technology for changing the ratios of products is also discussed. One configuration of the processes described encompasses the simultaneous near 50/50 use of two variations of generation technology. A highly desired product, for instance 1.2: 1 NaOH to H2O2, may be formed using the catholyte product of a membrane or diaphragm cell with a caustic anolyte as the catholyte feed stream for a membrane cell with an acidic anolyte. Although the particulate bed cathode approach has reached commercial trials, the planar cathode membrane cell approach may prove a difficult process to develop as the performance of electrodes optimized for realistic hydraulic depths may prove very different to that of electrodes used in small scale laboratory development.

Mass transfer and electrocrystallization analyses of nanocrystalline nickel production by pulse plating

Abstract: A comparison between the experimental process parameters employed for the pulse plating of nanocrystalline nickel and the solution-side mass transfer and electrokinetic characteristics has been carried out. It was found that the experimental process parameters (on-time, off time and cathodic pulse current density) for cathodic rectangular pulses are consistent and within the physical constraints (limiting pulse current density, transition time, capacitance effects and integrity of the waveform) predicted from theory with the adopted postulates. This theoretical analysis also provides a means of predicting the behaviour of the process subject to a change in the system, kinetic and process parameters. The product constraints (current distribution, nucleation rate and grain size), defined as the experimental conditions under which nanocrystalline grains are produced, were inferred from electrocrystallization theory. High negative overpotential, high adion population and low adion surface mobility are prerequisites for massive nucleation rates and reduced grain growth; conditions ideal for nanograin production. Pulse plating can satisfy the former two requirements but published calculations show that surface mobility is not rate-limiting under high negative overpotentials for nickel. Inhibitors are required to reduce surface mobility and this is consistent with experimental findings. Sensitivity analysis on the conditions which reduce the total overpotential (thereby providing more energy for the formation of new nucleation sites) are also carried out. The following lists the effect on the overpotential in decreasing order: cathodic duty cycle, charge transfer coefficient, Nernst diffusion thickness, diffusion coefficient, kinetic parameter (γ) and exchange current density.

Effect of thiourea, benzotriazole and 4,5-dithiaoctane-1.8-disulphonic acid on the kinetics of copper deposition from dilute acid sulphate solutions

Abstract: The effects of thiourea (TU), benzotriazole (BTA) and 4,5-dithiaoctane-1,8-disulphonic acid (DTODSA) on the deposition of copper from dilute acid sulphate solutions have been studied using potential sweep techniques. Tafel slopes and exchange current densities were determined in the presence and absence of these organic additives. TU and BTA were found to inhibit the copper deposition reaction; increases in the BTA concentration gave a systematic lowering of the exchange current density, whilst TU behaved in a less predictable manner. For BTA and TU concentrations of 10−5 mol dm−3,j0 values of 0.0027 ± 0.0001 and 0.0028 ± 0.0002 mA cm−2 were obtained compared to a value of 0.0083 ± 0.0003 mA cm−2 for the additive free acid sulphate solution. In contrast, in the presence of DTODSA, an increased exchange current of 0.043 ± 0.0003 mA cm−2 was observed. The presence of additives gave rise to measured Tafel slopes of −164, −180 and −190 mV for TU, BTA and DTODSA, respectively, compared to that of −120 mV for copper sulphate alone.

Electrodeposition of nickel/silicon carbide composite coatings on a rotating disc electrode

Abstract: Composite coatings suitable for protection against wear were prepared by electrodeposition from a nickel Watts solution containing silicon carbide particles maintained in suspension. To obtain a better understanding of hydrodynamic effects on the codeposition process a rotating disc electrode, immersed in a vertical rising flow, was used. The local concentration of embedded SiC along the radius of the disc electrode was studied as a function of suspension concentration, rotation rate and the particle mean diameter. The effect of a rheoactive polymer was also examined. Although it is generally admitted that the particle incorporation rate is governed by a two-step adsorption process, the experimental results show that it is also dependent on the spatial distribution of the wall fluid flow. The normal component of the fluid velocity promotes particle impingement, whereas the parallel component tends to eject the loosely fixed particles. The competition between the forces which tend to maintain particles attached to the surface and the shear force which tends to remove them, depends on several parameters, in particular the surface chemistry and the size of the particles, the flow rate and the current density.

Removal of formaldehyde from aqueous solutions via oxygen reduction using a reticulated vitreous carbon cathode cell

Abstract: The removal of formaldehyde from waste streams to <0.3 ppm has been demonstrated using a cell with a reticulated vitreous carbon cathode; the formaldehyde is oxidized by hydrogen peroxide, formed at the cathode by reduction of oxygen. In most electrolytes studied (e.g. NaOH, NaCl and Na2SO4), the formaldehyde is oxidised only to formic acid. On the other hand, the addition of a low concentration of an iron salt (i.e. 0.5 mm), catalyses the complete oxidation to carbon dioxide. The removal of formaldehyde can be achieved in media of low ionic strength (< 10 mm) although the use of iron salts necessitates the adjustment of pH to 3 to maintain the catalyst in solution.

Surface pH measurements during nickel electrodeposition

Abstract: To better understand the electrochemistry of nickel electrowinning from nickel chloride solutions at the cathode-electrolyte interface, the cathode surface pH was measured using a flat-bottom combination glass pH electrode and a 500 mesh nickel-plated gold gauze as cathode. The cell was a modification of that designed by Romankiw and coworkers. The pH electrode was positioned at the back of, and in direct contact with, the gauze cathode. As expected, the cathode surface pH was always higher than the pH in the bulk electrolyte, and if the current density was sufficiently large, it could cause the precipitation of insoluble Ni(OH)2(5) on the cathode surface. Lower bulk pH, higher nickel concentration, higher temperature, and the additions of H3BO3 and NH4Cl effectively suppressed the rise of the cathode surface pH. The results provide further evidence of the buffering action of H3BO3 and NH4Cl and of the enhancement of nickel deposition by H3BO3. At current densities less than 240 A m−2 additions of NaCl and Na2SO4 suppressed the rise of the cathode surface pH but to a much smaller degree.

Methanol electrooxidation on carbon-supported Pt-WO3−x electrodes in sulphuric acid electrolyte

Abstract: Electrooxidation of methanol has been studied in sulphuric acid electrolyte at 60°C on carbon-supported Pt-WO3−x electrodes employing varying amounts of WO3−x . It is found that the electrodes containing (3:1) Pt-WO3−x composite catalyst exhibit a higher catalytic activity towards methanol electrooxidation than platinized carbon electrodes without WO3−x . In the light of the XPS and XRD data on the carbon-supported (3:1) Pt-WO x sample, it is speculated that the WO x is present in the form of an oxyhydroxide, which can promote surface oxy-species on platinum by proton transfer.

Electrochemical mediators for total oxidation of chlorinated hydrocarbons: formation kinetics of Ag(II), Co(III), and Ce(IV)

Abstract: Mediated electrochemical oxidation (MEO) with Ag(n) and Co(iii) in 7m HNO3 and 3.5 M H2SO4 has been studied. High degradation rates were found for superchlorinated organic substances such as pentachlorophenol (PCP), Lindane and polychlorinated biphenyls (PCBs) using Ag(ii) in HNO3 as mediator. An investigation on the formation kinetics of Ag(ii), Co(iii), and Ce(iv) by means of a rotating disc electrode (RDE) led to a quantitative determination of limiting currents at various mediator concentrations. Reaction rate constants of water oxidation by Ag(ii) were determined at various temperatures and compared to reaction rates of Co(iii).

Conductive carbon-polypropylene composite electrodes for vanadium redox battery

Abstract: Carbon-polymer composite electrodes have been developed and investigated for application in the vanadium redox flow battery. Electrical, electrochemical, physical and mechanical properties as well as chemical resistance and solution permeability of a wide range of carbon-polymer composite materials were evaluated. Volume resistivities as low as 0.21 Ω cm were achieved with composites based on polypropylene, this being combined with excellent stability. Finally, the performance of a vanadium redox flow cell employing the best composite electrode was evaluated and voltage efficiencies as high as 91 % were achieved at a charge/discharge current density of 20 mA cm−2.

Electrodeposition of refractory metals (Ti, Zr, Nb, Ta) from molten salt electrolytes

Abstract: A selective review of the anodic and cathodic processes associated with electroextraction, electrorefining and plating of refractory metals (Ti, Zr, Nb and Ta) from molten salt-based electrolytes is presented with regard to the results obtained during the last two decades. Special attention is paid to the mechanism of cathodic reduction and its rate-limiting steps, as well as to the parallel chemical reactions of disproportionation and complexing. The anodic processes in corrosion and electrodissolution of Ti, Zr, Nb and Ta in molten electrolytes are also discussed. As a conclusion, a few contradictory subject-related questions are outlined and some new trends of investigation are suggested.

Hydrogen evolution reaction on Ni-Al-Mo and Ni-Al electrodes prepared by low pressure plasma spraying

Abstract: The hydrogen evolution reaction was studied on vacuum plasma sprayed Ni-Al and Ni-Al-Mo electrodes in 1m NaOH and 25% KOH at 70°C. It was found that Ni-Al-Mo electrode is more active in 25% KOH whereas Ni-Al electrode was more active in 1m NaOH solutions. A.c. impedance measurements showed two semicircles on the complex plane plots. Two different equivalent models were used to explain this behaviour. The results indicate that the formation of the second semicircle is connected with the hydrogen evolution reaction.

Sol-gel TiO2-SiO2 films as protective coatings against corrosion of 316L stainless steel in H2SO4 solutions

Abstract: Sol-gel TiO2-SiO2 films were deposited on 316L stainless steel by dip coating process from a sono-catalysed sol of composition 30TiO2-70SiO2 prepared from a mixture of Ti(OC2H5)4 and Si(OC2H5)4, absolute ethanol C2H5OH and glacial acetic acid CH3COOH as precursors and solvents. The films, densified at 800° C in air for 2 h, are composed of small orthorhombic titania (anatase) crystallites embedded in a SiO2 amorphous matrix as identified by X-ray diffraction. The temperature dependence of the film morphology was observed using scanning electron microscopy (SEM) and the content was determined by FTIR reflection spectroscopy. The corrosion behaviour of 316L stainless steel samples coated with densified 30TiO2-70SiO2 films was studied in 15% H2SO4 by potentiodynamic polarization curves at 25, 40 and 50°C. The measured corrosion rates show a considerable decrease for the protected steel samples in comparison to the bare substrate. The effect of time of heat treatment of the films on the corrosion parameters is also reported.

Electrochemical promotion of IrO2 catalyst for the gas phase combustion of ethylene

Abstract: The catalytic activity of IrO2 catalyst films for the gas phase combustion of ethylene can be increased by up to a factor of 10 via anodic polarization of the IrO2 catalyst relative to a Au electrode both deposited on Y2O3-stabilized ZrO2 solid electrolyte. The steady-state increase in the catalytic reaction rate is typically 200 higher than the expected rate increase of ethylene combustion calculated from Faraday's law. This is the first demonstration of the effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) using a metal oxide catalyst.

Sonovoltammetry at platinum electrodes: surface phenomena and mass transport processes

Abstract: Application of simultaneous ultrasound to representative solution-phase reversible voltammetric couples produce a step-shaped voltammogram at platinum electrodes of both ‘macro’ and ‘micro’ dimensions. The limiting current increases with ultrasonic power, but is not markedly affected by ultrasonic frequency in the 20–800kHz region. In contrast the complex voltammetry of a platinized platinum electrode surface within the hydrogen adsorption regime in aqueous acid medium is very little affected by sonication. Factors affecting the reproducibility of sonoelectrochemical experiments when employing ultrasonic sources are discussed.

Calculation of the impedance of noncylindrical pores Part I: Introduction of a matrix calculation method

Abstract: In this paper a new method is proposed for the calculation of the impedance of arbitrary electrodes containing noncylindrical pores and/or having place-dependent impedances. The method is based on splitting up the pore and the surrounding material into N discs. For the equivalent circuit of each disc a transmission line with constant impedances is adopted. By matrix calculations the impedance of the porous electrode can be obtained. A comparison is made between this, very general, matrix method and a recursion method developed by Keiser et al. for purely capacitive interface behaviour of pores in an electrode material with negligible impedance. It is shown that the matrix method requires much smaller N-values owing to the use of transmission lines for each disc. This makes it more appropriate to be used in curve fitting procedures. Moreover, it is shown that the typical behaviour of the pore impedance at low penetration depths is much better simulated with the matrix method. Furthermore, an attempt is made to provide more general knowledge about the impedance behaviour of noncylindrical pores as a function of the penetration depth of the a.c. signal. Finally, the theory is enlarged using constant phase elements instead of capacities to describe the behaviour of the electrode/electrolyte interfaces.

Influence of zeolite on electrochemical and physicochemical properties of polyethylene oxide solid electrolyte

Abstract: A composite polymer electrolyte of polyethyleneoxide-LiBF4 containing fine particles of zeolite was studied using electrochemical impedance spectroscopy, cyclic voltammetry, differential scanning calorimetry, infrared spectroscopy and scanning electron microscopy. When compared with the polymer electrolyte without zeolite, the specific conductivity of the composite electrolyte film is higher by about two orders of magnitude at room temperature. The increase in specific conductivity is explained as due to increased amorphocity which is reflected in the thermal studies. The nature of the cyclic voltammograms and infrared spectra is discussed.

Electrocatalytic properties of mixed transition metal tellurides (Chevrel-phases) for oxygen reduction

Abstract: Pseudobinary molybdenum cluster tellurides (Chevrel phases) with mixed transition metal clusters of the type M x Mo6−xTe8 (M = Pt, Os, Ru, Rh) and the binary phase Mo6Te8 have been synthesized, structurally characterized and electrochemically investigated with respect to their electrocatalytic properties for oxygen reduction. A correlation of the number of electrons per cluster unit (NEC) with the open circuit potential (o.c.p.) in saturated oxygen electrolyte was found. The limitation of the o.c.p. values are due to the mixed potential developed with each electrode material due to molybdenum oxidation. The lowest corrosion tendency and highest catalytic activity are found in the telluride phases when NEC (in the valence band) approaches 24.

In-situ ftir characterization of the electrooxidation of glassy-carbon electrodes

Abstract: The electrooxidation of glassy carbon electrodes in acid and neutral solution has been investigated using in situ FTIR spectroelectrochemical techniques. The formation and transformation of intermediate oxide species in different potential regions has been observed. The results show that in the lower anodic potential region (i.e., + 1.65 V vs SCE), the electrooxidation of carbon is a combination of electrochemical and chemical oxidation. A more detailed electrooxidation mechanism is proposed based on the experimental results.

Improved corrosion resistance of pulse plated nickel through crystallisation control

Abstract: When electrodeposition of nickel is used for corrosion protection of steel two aspects are important: the porosity of the coating and the resistance against corrosion provided by the coating itself. Using simple pulsed current (PC) plating, the size of the deposited crystals can be significantly smaller, thereby reducing porosity correspondingly. This usually also leads to improved hardness of the coating. Introducing pulse reversal (PR) plating, the most active crystals are continuously dissolved during the anodic pulse, providing a coating with improved subsequent corrosion resistance in almost any corrosive environment. This correlation between film texture and corrosion resistance will be discussed.

Performance characteristics of a new type of lead dioxide-coated titanium anode

Abstract: A new type of PbO2-coated metal anode, useful for electrochemical processing requiring the use of high-overpotential anodes, was prepared by applying to a titanium substrate an undercoating of Ti-Ta oxides, then covering the undercoating with an intermediate coating of stress-free α-PbO2 deposit, and finally covering the intermediate coating with a Ta2O5 particle-admixed β-PbO2 deposit. The anode thus produced was found to be superior to the anodized Pb or platinized Ti anode in respect of oxygen overvoltage, and corrosion resistance, even in solutions containing organic solvents. The possibility for use in chromium plating is also indicated.

Preparation of solid polymer electrolyte composites: investigation of the precipitation process

Abstract: A model for the chemical reduction of platinum tetramine in perfluorinated Nafion® membranes using sodium borohydride as reducer is proposed. A Nernst-Planck equation is employed for the description of ion transport by diffusion and migration and a reaction term accounts for the in situ chemical reduction. Time-dependent concentrations of the diffusing species within the membrane are obtained numerically by an iterative technique until completion of the precipitation. The model assumes that mass transport is limited by diffusion and migration within the membrane and that the concentrations remain constant at the interfaces during the precipitation. The model shows the effect of (i) the reducer concentration in the solution, (ii) the number of precipitation cycles and (iii) the rate of chemical reduction. To check the validity of the model, metallic platinum concentration profiles across the membrane thickness are obtained by electron microprobe analysis. Values of the diffusion coefficients of the diffusing species within the membrane are obtained from conductivity and permeation measurements.

Effect of surface modification using various acids on electrodeposition of lithium

Abstract: Sulface modification of lithium was carried out using the chemical reaction of the native film with acids (HF, H3PO4, HI, HCl) dissolved in propylene carbonate (PC). The chemical composition change of the lithium surface was detected using X-ray photoelectron spectroscopy. The electrodeposition of lithium on the as-received lithium or the modified lithium was conducted in PC containing 1.0 mol dm−3 LiClO4 or LiPF6 under galvanostatic conditions. The morphology of electrodeposited lithium particles was observed with scanning electron microscopy. The lithium dendrites were observed when lithium was deposited on the as-received lithium in both electrolytes. Moreover the dendrites were also formed on the lithium surface modified with H3PO4, HI, or HCl. On the other hand, spherical lithium particles were produced, when lithium was electrodeposited in PC containing 1.0 mol dm−3 LiPF6 on the lithium surface modified with HE However spherical lithium particles were not obtained, when PC containing 1.0 mol dm−3 LiClO4 was used as the electrolyte. The lithium surface modified by H3PO4, HI, or HCl was covered with a thick film consisting of Li3PO4, Li2CO3, LiOH, or Li2O. The lithium surface modified with HF was covered with a thin bilayer structure film consisting of LiF and Li2O. These results clearly show that the surface film having the thin bilayer structure (LiF and Li2O) and the use of PC containing 1.0 mol dm−3 LiPF6 enhance the suppression of dendrite formation of lithium.

Electrochemical behaviour of Al-In alloys in chloride solutions

Abstract: The present work is concerned with the study of the activation mechanism of Al in alloys produced by indium. The electrochemical behaviour of aluminium in NaCl solutions containing In 3+ ions and the dissolution of Al-In and In-Al alloys were studied using potentiostatic, galvanostatic and potentiodynamic techniques, complemented by SEM. It was concluded that the aluminium activation is obtained only when indium comes into a true metallic contact with aluminium within an active pit and in the presence of chloride ions. Polarization curves for dissolution of Al-In and In-Al alloys were compared. The results suggest that the initial step in the dissolution mechanism of the Al-In alloy can be interpreted through chloride ion adsorption on a surface In-Al alloy. This adsorption occurs at more electronegative potentials than that of pure aluminium, thus avoiding repassivation.

Electrodeposition of Ni-Mo alloys with pulse reverse potentials

Abstract: With the aim of improving the protective properties of Ni-Mo alloy layers, pulse reverse electro-deposition has been investigated. The anodic pulses were applied in the potential range where hydrogen desorption and oxidation occur. The alloy composition was shown to depend on the pulse parameters, especially on the anodic pulse duration. For long anodic pulses a preferential dissolution of molybdenum in the electrodeposited alloy occurs, while bulk Ni-Mo alloys do not undergo any dissolution. For anodic pulses longer than a certain threshold the electrocrystallization process becomes blocked. The morphology and microstructure of the layers are mainly determined by the molybdenum content rather than by the pulse parameters.