Showing papers in "Journal of Applied Electrochemistry in 1989"

Heat-treated polyacrylonitrile-based catalysts for oxygen electroreduction

Abstract: Polyacrylonitrile (PAN), mixed with Co(II) or Fe(II) salts and high-area carbon and then heat treated, has been found to yield very promising catalysts for O2 reduction in concentrated alkaline and acid solutions. The catalytic activities are comparable to those for the heat-treated corresponding transition metal macrocycles and polypyrrole black-based catalysts. The addition of the transition metal to the nitrogen-containing polymer, either before or after the heat treatment with carbon, is an important factor for good activity. The nitrile nitrogen of the PAN is probably retained and converted to pyridyl nitrogen during the heat treatment, and this nitrogen is believed to provide binding sites for the transition metal ions, which then act as catalytic sites for oxygen reduction to peroxide and its decomposition.

New solid polymer electrolyte composites for water electrolysis

Abstract: A new procedure for the preparation of SPE-electrocatalyst composites has been developed. In this procedure, noble metal cationic species are chemically reduced within a solid polymer electrolyte. The metallic particles are not homogeneously distributed across the SPE thickness but predominate near its surfaces. The structure and the distribution of the precipitates, along the membrane thickness have been investigated by scanning electron microprobe analysis and transmission electron microscopy. Application to SPE water electrolysis has been performed using Nafion membranes and platinum-group electrocatalysts. The SPE-electrocatalyst composites prepared according to this new procedure present good electrochemical properties, low catalyst loadings, long-time stability and high energetic efficiencies.

Electrocoating of iron and other metals with polypyrrole

Abstract: Galvanostatic electrodeposition of polypyrrole from more than 20 nonaqueous and aqueous electrolytes containing anions such as BF 4 − , ClO 4 − , HSO 4 − , Tos−, HCO 3 − , H2PO 4 − , HPO 4 2− , H2BO 3 − and NO 3 − at the substrates Pt, Au, Cu, Ti, stainless steel/V2A and Fe was investigated. The passivating substrates Pt, Au, Ti and V2A could be electrocoated under a broad variety of conditions. Cu dissolved anodically. The same was true for Fe with only one important exception, namely aqueous nitrate. Smooth, well-adhering polypyrrole layers were obtained on iron at NO 3 − concentrations of 0.01–1 M and at current densities of 0.5–10 mA cm−2. The degree of insertion for nitrate ions was abouty=0.25, confirmed by elemental analysis. Current efficiencies with regard to thisy were about 90%. The layers were stable in air for a long time.

Mechanism of nickel-molybdenum alloy electrodeposition in citrate electrolytes

Abstract: The kinetics of the induced codischarge of Mo with Ni in citrate-ammonia electrolytes was investigated by means of polarization and a.c. impedance measurements. Three potential ranges were considered. At low polarization, hydrogen evolution resulting from citrate reduction is the main reaction. The impedance plots exhibit a large capacitive loop with a small high frequency inflection characteristic of the development of a porous layer and a low frequency inductive feature. At intermediate polarization, the partial currents for Ni and Mo discharge increase in the same proportion; the hydrogen evolution is first constant and then rapidly decreases. Then a large low-frequency capacitive feature is observed on the impedance plots, whose size decreases with increasing polarization. At still higher polarization, the Mo discharge becomes increasingly controlled by diffusion which generates an additional capacitive loop. A reaction scheme is proposed which accounts for the polarization data and the major impedance features.

On the conductivity of phosphoric acid electrolyte

Abstract: The specific electric conductivity and viscosity of 7.5–100 wt% aqueous phosphoric acid electrolyte have been measured in the temperature range 25–200°C. The product of conductivity and viscosity was found to decrease exponentially with increasing temperature. This suggests that the electric conduction in phosphoric acid follows the Grotthus proton switching mechanism. The measured conductivity data were correlated by the following empirical equation: $$\begin{array}{*{20}c} {k\mu = {\rm A} exp [BT]}\\ {A = 702.7X^{1.5} - 1734.2X^2 + 1446.5X^{2.5} - 350.7X^3 }\\ {B = - 0.010163 + 0.011634X - 0.08313X^2 }\\ \end{array}$$ wherek is conductivity in mho cm−1; μ is viscosity in centipoise;T is the absolute temperature in K; andX is mole fraction of phosphoric acid in the electrolyte. The standard deviation for the percentage difference between the calculated and experimental values of 188 data points was less than 7.5%.

Flow-through and flow-by porous electrodes of nickel foam. I. Material characterization

Abstract: This paper deals with the characterization of three nickel foams for use as materials for flow-through or flow-by porous electrodes. Optical and scanning electron microscope observations were used to examine the pore size distribution. The overall, apparent electrical resistivity of the reticulated skeleton was measured. The BET method and the liquid permeametry method were used to determine the specific surface area, the values of which are compared with those known for other materials.

Electrocatalytic oxidation of methanol on platinum dispersed in polyaniline conducting polymers

Abstract: Etude de l'oxydation electrochimique du methanol sur electrode de platine disperse dans du polyaniline. Les films de polyaniline sont obtenus en cyclant l'electrode dans une solution du monomere dans H 2 SO 4 0,5 M. Le courant d'oxydation du methanol obtenu reste remarquablement stable. D'apres les auteurs, les especes susceptibles de jouer le role de poison pour le catalyseur se chimisorbent difficilement du fait de la geometrie specifique des sites du catalyseur

Potential drops due to an attached bubble on a gas-evolving electrode

Abstract: It is shown how the various components of the overpotential due to an attached bubble on an electrode can be separated and estimated. By considering the resistance increments due to the presence on the electrode surface of a bubble, obtained from impedance measurements, it is possible to determine the predominant potential distribution which controls the gas evolution. A relationship between the measured overpotential and the diameter of the bubble is established. The time evolution of the overpotential due to a growing bubble is modelled in the case of the limitation of the bubble growth by dissolved gas diffusion in the solution. In agreement with previous experimental results a linear time variation is found.

Hydrogen evolution on iridium oxide cathodes

Abstract: IrOx electrodes prepared by thermal decomposition of IrCl3 dissolved in water have been prepared at temperatures betweeen 300 and 500°C. They have been characterized by voltammetric curves before and after each experiment to monitor changes in surface conditions. Hydrogen evolution from 1 mol dm−3 constant ionic strength perchlorate solutions at pH between 0 and 1 has been studied by quasi-stationary potentiostatic curves and reaction order determination. A Tafel slope close to 40 mV and a reaction order of 1.5 with respect to H+ are explained with a mechanism involving a step of reduction of the active sites and a pH dependence of the potential at the reaction plane which is typical of the acid-base properties of oxide surfaces. The results indicate that wetting-dewetting phenomena of the electrode surface, depending on the magnitude of the current, are operative during hydrogen evolution, and that the oxide surface is never reduced down to the metal, while no bulk reduction whatsoever takes place.

Ni and Mo coatings as hydrogen cathodes

Abstract: The catalytic activity of hydrogen cathodes based on Ni/Mo coatings prepared in different ways has been investigated under conditions of advanced alkaline water electrolysis in 10m KOH at 100°C, in the current density range 0.05–1.0 A cm−2. The activity of electrodeposited Ni/Mo and Ni/Mo/V coatings was quite low, apparently due to their low effective surface area. The activity of all thermally deposited Ni/Mo coatings, electrodeposited Ni/Mo/Cd coatings and of Raney nickel-Mo alloys has been found to be high. When accompanied by some current interruptions of various durations, however, it successively decreased during long-time electrolysis, especially when the residual potential of the electrode, after the current interruption, approached a certain threshold value. The rate of electrode deactivation depends on its mode of preparation as well as on electrolysis conditions, particularly on conditions during the current interruptions. The Mo content in the coating decreased quickly by a factor of 2–10 under operating values of both Tafel constants. The addition of Mo to the expressed as an increase in the absolute values of both Tafel constants. The addition of Mo to the Raney nickel improves its catalytic efficiency as long as Mo is not dissolved. The enhancing of catalytic activity by Mo in Raney nickel is partly caused by synergetic effects between Ni and Mo, as follows from their electronic structure, and partly by material stabilization as follows from comparison with the Raney nickel (Zn).

Removal of chromium from aqueous solutions by treatment with porous carbon electrodes: electrochemical principles

Abstract: The possibility of removing hexavalent chromium from waste water by electrochemical treatment using a graphite felt electrode and synthetic electrolytes is investigated. It is suggested that the process proceeds in two steps: electrochemical reduction of the hexavalent chromium to chromic ion followed by the formation of an insoluble chromic hydroxide in an electrochemically generated high pH environment. The chromic hydroxide adheres to the electrode surface as a charged colloidal particle. The electrochemical dissolution of the hydroxide layer by potential inversion is also discussed as a possible regeneration procedure.

Flow-through and flow-by porous electrodes of nickel foam. II. Diffusion-convective mass transfer between the electrolyte and the foam

Abstract: The work described here concerns the diffusion-convective mass transfer to flow-through and flow-by porous electrodes of nickel foam. Empirical correlations giving the product\(\bar k_d a_e\) of the mass transfer coefficient\(\bar k_d\) and the specific surface areaae of the material as a function of the pressure drop per unit electrode height and as a function of the grade characterizing the foam are proposed. The performance of various materials are compared in terms of\(\bar k_d a_e\) vs the mean linear electrolyte flow velocity.

Preparation of polycrystalline thin films of CuInSe2 by electrodeposition

Abstract: CuInSe2 thin films with the chalcopyrite structure were electrodeposited on titanium substrates from an acidic sulphate solution containing citrate ions as a complexing agent. These layers were characterized by X-ray diffraction, SEM, X-ray energy dispersive analysis and spectrophotometry of reflected light. The best deposits were obtained atE=−0.97V vs SSE, in a solution adjusted to pH 3.3. The deposition of the binary alloys was also investigated. It was shown that the formation of smooth layers of crystalline definite compounds is correlated with a slow surface process which induces a plateau on the polarization curves.

The electrochemical regeneration of Fenton's reagent in the hydroxylation of aromatic substrates: batch and continuous processes

Abstract: Fenton's reagent, regenerated by the electrochemical reduction Fe3++e-→Fe2+ has been used for the hydroxylation of benzene into phenol in both continuous and batch processes. In a batch process, the current yield for this reaction is relatively low because of the oxidation, in the aqueous phase, of phenol and its oxidation products by HO0 radicals. When the phenol is continuously extracted in the form of phenate ions, the current yield is found to increase from 20 to 70% for a nine-fold rise in the rate of benzene circulation. Varying the sulphuric acid concentration in the range 0.05 to 0.8m has little effect on current yield, while varying the Fe3+ concentration gives a maximum yield of about 70% for a concentration of 0.1m. At higher Fe3+ concentrations the rate of production of HO0 radicals becomes too high in comparison with the rate of benzene hydroxylation. The presence of a co-solvent like CCl4, although unreactive with HO0, reduces the yield. A numerical model taking into account the 13 reactions for which rate constants could be found or estimated, was developed for the continuous process. This model can only predict the general trends in yield and is not quantitatively predictive. Attempts at hydroxylating fluorobenzene using Fenton's reagent with electrochemical regeneration did not give appreciable yields of fluorophenols.

The importance of superficial (adatom) surface oxidation in the electrocatalytic behaviour of noble metals in aqueous media

Abstract: The conventional, activated chemisorption, model of electrocatalysis is unable to explain fully the surprisingly high level of activity of gold anodes in base for certain organic electrooxidation reactions or even the ability of platinum to catalyse certain oxygen insertion reactions, e.g. methanol or formaldehyde oxidation in acid, at low potentials (in the hydrogen or double layer region) where the surface is considered usually to be oxygen-free. It was demonstrated that such behaviour can be rationalized by postulating the involvement of a quite low coverage of hydrous oxide species, generated at low potential at adatom sites, which act as mediators or catalysts in many electrooxidation reactions. The converse situation was also shown to arise, i.e. the reduction of certain species, e.g. HNO3 or NO 3 − , which requires the involvment of adatoms, was shown to be inhibited as long as the latter existed in the oxidized, hydrous oxide, form. An appreciation of hydrous oxide electrochemistry was shown to be very useful in interpreting electrocatalytic effects at noble metal electrodes in aqueous media.

Scale-up of electrocarboxylation reactions with a consumable anode

Abstract: Efficient carboxylation of various organic halides can be achieved by electroreduction in aprotic solvents under an atmosphere of carbon dioxide when the selected anodic reaction is the electrodissolution of a magnesium electrode. Successful scale-up from 30 ml atmospheric pressure laboratory cells to a 400 l pressurized pilot unit clearly demonstrates the technical feasibility of this new process for large-scale production of valuable carboxylic acids.

The influence of mass transport on the deposit morphology and the current efficiency in pulse plating of copper

Abstract: The morphology of copper deposits formed by pulse plating from an acid sulphate electrolyte is investigated. The steady and non-steady state conditions of mass transport are controlled by use of a rotating hemispherical electrode. Below the limiting pulse current density (ipl), granular deposits are observed. Aboveipl, regardless of the individual values of the pulse parameters, dendritic deposits are formed. Measured current efficiencies are compared with a theoretical model, which predicts a rapid decrease of the efficiency with the increasing ofip/ipl forip/ipl greater than one, whereip is the applied pulse current density. For a given set of pulse parameters, the measured current efficiency increases with the deposit thickness due to the increase of the effective surface area. This effect is particularly important for dendritic deposits.

Inhibition of corrosion of α-brass (Cu-Zn, 67/33) in acid chloride solutions by some amino pyrazole derivatives

Abstract: The inhibition of corrosion of commercial 67/33 brass in 0.1m HCl acid solutions by some amino pyrazole derivatives was tested using polarization, capacitance and weight-loss measurements. The amino pyrazole derivatives were 3(5)-amino, 5(3)-phenyl pyrazole; 3(5)-amino, 5(3)[4--chlorophenyl] pyrazole; 3(5)-amino, 5(3)[4--methoxy phenyl]pyrazole; 3(5)-amino, 5(3)[4--methyl phenyl]pyrazole; 3(5)-amino, 5(3)-cyano pyrazole; and 3(5)-amino, 4-phenyl-5(3)-methyl pyrazole. The results showed that these compounds predominate as cathodic inhibitors and inhibition efficiencies up to 91% can be obtained. The inhibition was assumed to occur via chemisorption of the inhibitor molecules fitting a Temkin isotherm. The influence of the substituent group on the inhibition efficiency of the inhibitor was explained in terms of the density of the electron cloud on the pyrazole ring and the mode of adsorption. Although increase in temperature was found to decrease the corrosion in the absence of the inhibition, it increased in the presence of inhibitor, probably via desorption of the inhibitor molecules.

Investigation of water electrolysis by spectral analysis. I. Influence of the current density

Abstract: The potential (or current) fluctuations observed under current (or potential) control during gas evolution were analysed by spectral analysis. The power spectral densities (psd) of these fluctuations were measured for hydrogen and oxygen evolution in acid and alkaline solutions at a platinum disk electrode of small diameter. Using a theoretical model, some parameters of the gas evolution were derived from the measured psd of the potential fluctuations, such as the average number of detached bubbles per time unit, the average radius of the detached bubbles and the gas evolution efficiency. The influence of the electrolysis current on these parameters was also investigated. The results of this first attempt at parameter derivation are discussed.

Mechanisms of mass transfer of dissolved gas from a gas-evolving electrode and their effect on mass transfer coefficient and concentration overpotential*

Abstract: Dissolved gases in the boundary layer of gas-evolving electrodes, in constrast to all other substances generated or consumed at electrodes, undergo two superimposed mass transfer mechanisms. Their interaction results in an increase of the effective mass transfer coefficient of disolved gas. A correction factor to the coefficient calculated from available mass transfer equations is proposed. The effect on the concentration overpotential is discussed.

Characterisation of different grades of commercially pure aluminium as prospective galvanic anodes in saline and alkaline battery electrolyte

Abstract: The effect of calcium chloride and sodium chloride on the electrochemical properties of various grades of aluminium namely 2s, 3s, 26s and 57s in alkaline citrate solution has been examined by studying the self corrosion, open circuit potential, anodic polarization and anode efficiency. It has been found that among the different grades of aluminium the 57s grade is found to be the most promising galvanic anode material in the 4N NaOH containing 20% wt/vol. of sodium citrate and 2.5% wt/vol. of CaCl2·2H2O. Further, it has been found that the above alkaline citrate electrolyte required for aluminium air batteries can be prepared from tap water, saline water and even sea water. The presence of chloride ions in the electrolyte, even up to a concentration of 2.5% wt/vol. as CaCl2·2H2O, is not found to alter the electrochemical properties of the different grades of aluminium as anode materials, though 57s is found to be the best.

High-temperature C1-gas fuel cells using proton-conducting solid electrolytes

Abstract: With proton-conducting solid electrolytes based on SrCeO3 or BaCeO3, high-temperature fuel cells were constructed and cell performances were examined. Mixtures of water vapor and some C1 gases such as methanol vapor or methane were used as fuel by internal reforming to liberate hydrogen in the anode compartment. These fuel cells worked stably above 900°C. Addition of water vapor to the fuel was necessary to prevent carbon deposition at the anode. The performances of the cells were limited mainly by ohmic resistance of the solid electrolyte.

Biomass electrochemistry: anodic oxidation of an organo-solv lignin in the presence of nitroaromatics

Abstract: The temperature required for the anodic conversion of an organo-solv spruce lignin, in aqueous alkaline solution, may be lowered by the addition of nitrobenzene or 1,3-dinitrobenzene. The additives cause significant and reproducible changes in the Tafel plot for the reaction. Preparativescale experiments are less reproducible although the range of phenolic products is generally similar to that produced by conventional nitrobenzene oxidation or by anodic oxidation at higher temperatures.

The solubility and electrochemistry of alkali metal oxides in the molten eutectic mixture of lithium carbonate-sodium carbonate-potassium carbonate

Abstract: Metal oxides as solutes in molten electrolytes are important in metal winning as well as intermediates or reaction products in a number of new energy related technologies. The systematic behaviour of oxide ions in carbonate melts has received little attention. The results of electroanalytical measurements have shown that the solubilities of alkali metal oxides in the ternary eutectic mixture Li2CO3-Na2CO3-K2CO3 (43.5∶31.5∶25 mol %) in the temperature range 400–700°C are less than 0.5 mol %. The solubilitiesX (mole fraction) are given by: $$X_{oxide} = 0.0538\exp \left[ { - \frac{{5775}}{{RT}}} \right]$$

Chemical and electrochemical inhibition studies of corrosion and hydrogen surface embrittlement. I: Fe0.78B0.13Si0.09 amorphous alloy in molar HCl

Abstract: Diorthoaminophenoldisulfane (DOAPD) was synthesized for use as a corrosion inhibitor and in order to test its effect on the hydrogen surface embrittlement of Fe0.78B0.13Si0.09 alloy. Results of electrochemical and gravimetric measurements were consistent and confirmed the beneficial role of this inhibitor. The amorphous state of the alloy was confirmed by X-ray measurements and differential thermal analysis. The maximum inhibition of corrosion was attained in the presence of 10−4 M DOAPD. This inhibitor concentration also gave rise to a diminution of metal hydrogenation. A mechanism explaining the mixed role of the disulfane was confirmed.

Stability of zinc/bromine battery electrolytes

Abstract: L'experimentation realisee montre l'utilite de l'evaluation de la stabilite d'une gamme de composes de bromure d'ammonium quaternaire (QBr) dans un electrolyte de batterie zinc-brome simule pendant une periode etendue

A Nafion¬-bound platinized carbon electrode for oxygen reduction in solid polymer electrolyte cells

Abstract: A full-factorial statistical method has been used to optimize the amount of binder, the compaction load and the compaction time in order to fabricate Nafion®-bound carbon electrodes containing platinum and pyrolysed cobaltmeso-tetramethoxyphenylporphyrin (CoTMPP). These electrodes are designed for oxygen reduction in solid polymer electrolyte fuel cells, and it has been found that incorporation of Nafion® gel with the carbon substrate facilitates its binding with the Nafion® membrane of such a fuel cell. The polarization behaviour of a Nafion®-bound carbon electrode containing platinum is compared to that containing pyrolysed CoTMPP.

Electrodeposition of chromium from Cr (III) electrolytes in the presence of formic acid

Abstract: Chromium coatings deposited from sulphate, chloride and perchlorate electrolytes based on the [Cr(H2O)4(HCOO)]2+ complex ion were investigated. The current efficiency reached 30% in the case of chloride electrolyte for various current densities in the range 4–10 A dm-2 depending on pH. Such a large current efficiency is due to the catalytic effect of the chloride ions on the electroreduction of the chromium complex to metallic chromium. Deposition of chromium from sulphate electrolyte took place with a current efficiency of 16% which rose for higher pH and lower current densities. Semi-bright and bright coatings with thickness of approximately 10μm of good adhesion to a copper electrode were deposited from chloride and sulphate electrolytes.

The interaction of pyrite with solutions containing silver ions

Abstract: Linear potential sweep voltammetry and X-ray photoelectron spectroscopy have been used to determine the products of the interaction of pyrite with sulphuric acid solutions containing silver ions. Silver sulphide was found to be the principal product for all reaction times. Initially, some sulphur excess (metal deficiency) in the sulphide lattice was associated with the formation of silver sulphide. The presence of elemental silver in addition to silver sulphide after extended reaction times was evident from the characteristics of voltammograms. This elemental silver was not detected by electron spectroscopy because it was formed as crystallites occupying only a very small area of the surface of the silver sulphide-covered pyrite. Silver sulphide was the only surface silver species present on a pyrite surface during acid iron(III) leaching; elemental sulphur was also identified on such surfaces.

Gas induced bath circulation in aluminium reduction cells

Abstract: Gas induced bath circulation in the interpolar gap of aluminium cells was studied in a room temperature physical model and by computer simulation. The circulation velocity increased with increasing gas formation rate, increasing angle of inclination and decreasing bath viscosity, while it was less affected by anode immersion depth, interpolar distance (in the normal range), and convection in the metal. A typical bath velocity near the cathode was 0.05 m s−1. The flow velocity decreased with decreasing bubble size. The results were fitted to a simple semi-empirical expression, and the velocities measured in the model experiments were in good agreement with the findings of the computer simulation.