A new bath for the electrodeposition of aluminium. I. conductivity measurements

The conductivity of mixed hydrides of aluminium chloride and lithium-aluminium hydride in a mixed solvent of tetrahydrofuran (THF) and toluene was measured with respect to the total concentration of aluminium and also to the molar ratio of LiAlH₄ to AlCl₃. The values obtained were compared with those of the THF-benzene mixed solvent and those of the NBS (National Bureau of Standards) bath –AlCl₃ and LiAlH₄ in dithyl ether. The results showed that a solution of AlCl₃ and LiAlH₄ with a molar ratio of 3:1, respectively, in THF-toluene (80 vol% toluene) with a total concentration of aluminium of about 1.0 moll^–1, has a suitable conductivity for the electrodeposition and dissolution of aluminium. In addition to its low price, the electrolytic bath obtained has low volatility and relatively good stability with respect to the other baths studied.     

The corrosion and electrochemical behaviour of pure aluminium in alkaline methanol solutions

The corrosion and electrochemical behaviour of pure aluminium in alkaline methanol solutions has been investigated. The results of hydrogen collection experiments showed that aluminium has a lower corrosion rate in alkaline methanol solutions compared to water based solutions and that the corrosion rate increases with increasing water content of the solution. Polarization and galvanostatic discharge experiments showed that there is a wide potential window of electrochemical activity and a better discharge performance in the alkaline methanol solutions with a certain amount of water. Scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDAX) showed that the passivation in the later stages of discharge in alkaline methanol solutions at relatively high current densities is due to the formation of a dense Al(OH)₃ layer on the surface of the anode.     

A new method for electrochemical screening basedon the rotating ring disc electrode and its applicationto oxygen reduction catalysts

A new method for electrochemical characterization of composite electrode materials is reported. A paste of the catalytic material in Nafion® is coated on a rotating ring disc electrode (RRDE) to partially simulate the working environment of a proton exchange membrane (PEM)/electrode composite as used in, for example, water electrolysis or PEM fuel cell operation. This allows direct comparison of a wide range of candidate electrocatalysts in a reproducible manner. Problems specific to these volumic electrodes are accommodated satisfactorily by rational modification of the standard expressions used in RRDE analysis. The value of the method is illustrated in studies of various cobalt complexes which show promise in dioxygen reduction; namely, cobalt tetramethoxyphenylporphyrin (CoTMPP), cobalt phthalocyanine (CoPC), and cobalt cyclam (CoCy), supported on a range of particulate carbons BP2000, Printex XE 2 and Vulcan XC-72. Typical electrochemical parameters have been measured or estimated, including half-wave potentials (E1/2), Tafel slopes (b), activation currents (Ia) and the average number of electrons transferred (n). The nature of the complex itself and the carbon support have a strong influence on electrode behaviour. Ligands with more aromatic character give better performance. Dramatic improvements in performance result from heat pretreatment, which is tentatively attributed to the formation of dimeric cobalt species via thermally-induced aggregation. In terms of the four electron reduction (to water), the best result was obtained for CoTMPP on Printex XE2 and rationalized on the basis of popular current views on the mechanism and catalyst functionality. CoPC on BP2000 is unusual in showing a strong change in n with reduction potential. Product selectivity ranges between mainly hydrogen peroxide (n=2) and water (n=4) with increasing overpotential.     

A new method for electrochemical screening basedon the rotating ring disc electrode and its applicationto oxygen reduction catalysts

A new method for electrochemical characterization of composite electrode materials is reported. A paste of the catalytic material in Nafion® is coated on a rotating ring disc electrode (RRDE) to partially simulate the working environment of a proton exchange membrane (PEM)/electrode composite as used in, for example, water electrolysis or PEM fuel cell operation. This allows direct comparison of a wide range of candidate electrocatalysts in a reproducible manner. Problems specific to these volumic electrodes are accommodated satisfactorily by rational modification of the standard expressions used in RRDE analysis. The value of the method is illustrated in studies of various cobalt complexes which show promise in dioxygen reduction; namely, cobalt tetramethoxyphenylporphyrin (CoTMPP), cobalt phthalocyanine (CoPC), and cobalt cyclam (CoCy), supported on a range of particulate carbons BP2000, Printex XE 2 and Vulcan XC-72. Typical electrochemical parameters have been measured or estimated, including half-wave potentials (E1/2), Tafel slopes (b), ‘activation currents’ (Ia) and the average number of electrons transferred (n). The nature of the complex itself and the carbon support have a strong influence on electrode behaviour. Ligands with more aromatic character give better performance. Dramatic improvements in performance result from heat pretreatment, which is tentatively attributed to the formation of dimeric cobalt species via thermally-induced aggregation. In terms of the four electron reduction (to water), the best result was obtained for CoTMPP on Printex XE2 and rationalized on the basis of popular current views on the mechanism and catalyst functionality. CoPC on BP2000 is unusual in showing a strong change in n with reduction potential. Product selectivity ranges between mainly hydrogen peroxide (n=2) and water (n=4) with increasing overpotential.     

A new bath for the electrodeposition of aluminium. II. Kinetics and mechanism of the deposition and dissolution processes

The electrodeposition of aluminium was carried out successfully in AlCl₃-LiAlH₄/tetrahydrofuran-toluene solutions. The kinetic parameters controlling the cathodic deposition and anodic dissolution of aluminium were derived. The effect of the total concentration of aluminium and the molar ratio of AlCl₃ to LiAlH₄ on the current density-potential behaviour of the rotating disc aluminium electrode was also studied. A detailed mechanism for the electrode reactions was suggested.     

Some aspects of the electrochemical behaviour of the iron electrode in alkaline solutions

The anodic oxidation of iron in strong KLH solutions has been examined by Faradaic impedence and potential step techniques. The conclusions indicate that the rate determining film growth process is modified by the presence LI⁺ and S^2? in the solution.     

Electrochemical behaviour of manganese in the aluminium electrorefining bath

The study of the mechanism of the electrode reaction Mn(II)/Mn in the aluminium electrorefining bath free of aluminium fluoride (1·48 NaF-1·05 BaCl₂) has been carried out by chronopotentiometry with an inert graphite electrode, a manganese electrode and a platinum electrode; this study was supplemented by utilizing anodic chronopotentiometry with current reversal from a stationary state. The tests carried out at 800° C show that the exchange mechanism of Mn(II)/Mn is reversible, involves the exchange of two electrons and that the diffusion coefficient D_Mn(II)=(1·9±0·5)×10^–5 cm² s^–1.     

The estimation of the coulombic efficiency of zinc electrodeposition by measurement of current efficiencies at a rotating ring disc electrode

The rotating ring disc electrode (RRDE) offers a simple method for the determination of the current efficiency (CE) of zinc electrodeposition in acidic zinc sulphate electrolytes. The hydrogen evolved during zinc deposition can be detected at the ring. This allows estimation of the current due to hydrogen formation at the disc and hence CE for electrodeposition of zinc can be calculated. Investigations of the conditions under which reliable measurements of CE can be obtained are described. A correlation between CE, determined using the RRDE, and the coulombic efficiency (QE), determined by weighing the zinc deposit obtained after 2 h electrodeposition at constant current, is established for a range of electrolyte compositions. It is suggested that measurement of CE at a Pt–Zn RRDE provides an efficient means of estimating QE for zinc electrolytes.     

Investigation of the aluminium electrodeposition process in cryolite-based melts using a rotating ring-disc electrode: evidence for the existence of a subvalent intermediate species

The rotating ring-disc electrode technique has been used to investigate the reaction mechanism of the aluminium electrodeposition process in cryolite-based electrolytes. Laboratory studies using high temperature gold-molybdenum and platinum-molybdenum rotating ring-disc electrodes have provided evidence for the existence of a subvalent intermediate species (Al (I)). In a cryolite-alumina electrolyte (bath ratio: 1.5), two well separated convective-diffusion controlled oxidation processes were observed at both a gold and a platinum ring during aluminium electrodeposition at the disc. On the basis of the data presented, a reaction scheme involving reduction of A1(III) to A1(0) via A1(I), followed by chemical dissolution of A1(0) into the bulk electrolyte was proposed. The loss of current efficiency in aluminium smelting was primarily attributed to the chemical dissolution of A1(0), rather than to the formation of a subvalent intermediate species.     

A new correlation for characteristic velocity in rotating disc contactors

On the basis of present experimental results along with those of available literature (1539 experimental results for 12 systems from 13 different sources) with a wide range of variables, a general correlation for the prediction of characteristic velocity, involving phase flow rates, rotor speed (in terms of Froude number), column geometry and also the physical properties, is presented. The present proposed correlations are more accurate than those previously reported.     

Electrochemical behaviour of pure iron in the aluminium refining electrolyte

The reduction of iron dissolved in the aluminum refining industrial melt was studied using cyclic voltammetry and chronopotentiometry. The required solutions were obtained by adding ferrous or ferric fluoride to the Gadeau electrolyte. The experimental arrangement permitted molten fluoride electrolysis as well as the introduction of additional salts under an inert argon atmosphere. Platinum, iron and graphite electrodes were used. Chemical analysis proved that the stable ionic valency of iron in the melt at 750° wasII. The reduction reaction of the Fe(II) ions was reversible and involved two electrons. The diffusion coefficient of the Fe(II) ions was calculated.     

A pretreatment with galvanostatic etching for copper electrodeposition on pure magnesium and magnesium alloys in an alkaline copper-sulfate bath

A pretreatment with galvanostatic etching is recommended to obtain an adherent and uniformly covered copper deposit on pure magnesium and magnesium alloy specimens (AZ31 and AZ61) in an alkaline copper-sulfate bath. The effect of galvanostatic etching on the surfaces of Mg and Mg alloy specimens can be realized by their potential variation during galvanostatic etching, in which four distinct stages could be distinguished. Galvanostatic etching to stage III, an activated surface of Mg or Mg alloy, was obtained for electroplating a uniformly covered Cu deposit in the alkaline Cu-sulfate bath. The Cu-deposited Mg or Mg alloy was used as the substrate for further Cu and then Ni electrodeposition in acid plating baths to obtain a protective Ni/Cu coating. The proposed electroplating baths are environmentally friendly, and the electrodeposition process is easy to conduct to achieve a protective coating for Mg and Mg alloys.     

The electrochemical behaviour of silver sulphide in sulphuric acid solutions

The electrochemical behaviour of synthetic silver sulphide (acanthite) electrodes in sulphuric acid solutions has been investigated using several techniques including cyclic voltammetry, anodic polarization and constant potential experiments. Under anodic polarization the dissolution has been attributed to the reaction Ag₂S=2Ag⁺+S+2e which occurs in two sequential, single electron transfer steps. A kinetic model for this stepwise anodic dissolution process at lower overpotential, where the current is a function of potential, is provided. At high dissolution rates (i.e. high currents) the slightly soluble silver sulphate salt is formed on the surface due to the saturation of the electrolyte near the Ag₂S interface. This observation is supported by the influence of electrolyte composition on the cyclic voltammetry and the polarization curve. A paralinear film growth model has been found to describe the formation and growth of the silver sulphate product layer indicating an initial region of parabolic kinetics which gradually changes to linear kinetics as the rate of film dissolution approaches that of film formation.     

The electrochemical behaviour of copper in sodium salicylate aqueous solutions

Cyclic voltammetry has been used to investigate the behaviour of copper surfaces in sodium salicylate aqueous solutions. The observed copper anodic passivation dependence on the presence of salicylate ion in solution is explained. The analysis of the experimental data supports the formation of a complex passivating film formed by a Cu₂O inner layer and a mixed cupric oxide/cupric salicylate outer layer; this film provides a partial passivation of the copper surface and can be completely removed upon excursion to negative potentials values; the composition of the passivating layer depends on the electrolyte nature, i.e. sodium salicylate ion and solution pH, and on the potential programme the copper electrode is subjected to.     

The influence of suspended particles on the mass transfer at a rotating disc electrode. Non-conducting particles

The influence of suspended SiC particles on the mass transfer at an RDE (rotating-disc electrode) was investigated for the reduction of K₃Fe(CN)₆ and ZnO in alkaline electrolyte. A model for the increased mass transfer is developed, based on the decrease of the diffusion layer thickness by rotation of the particles in this layer. In experiments with different volume fractions of suspensions a critical value in the rotation speed and in volume fraction is found. The influences of hydrodynamic conditions, gravity, particle size and adhesion between particles and electrode were studied to investigate the origin of both critical values.     

The simulation of the homogeneous catalytic reaction at a monolayer-film covered rotating disc electrode

A simple and fast numerical method for the simulation of linear scan voltammetric response of the homogeneous catalytic reaction at the electroactive-monolayer-film covered rotating disc electrodes has been developed. The linear relationship between the voltammetric peak current and the concentration of dissolved reactant ions has been predicted. The proportionality factor of this relationship is very similar to the one predicted for steady-state conditions. The voltammetric peak current is, also, linearly proportional to the square roots of scan rate and rotation rate of the disc electrode, but no cumulative relationship on these parameters is predicted.     

Transient solutions of potential steps at the rotating disc electrode with steady state initial concentration profiles for one electron transfer reactions

In this paper the transient solution of a potential step at a rotating disc electrode (RDE) for irreversible and quasireversible one electron transfer reactions is derived by Nernst diffusion layer approximation and separation of variables. This is then compared to finite element simulation results. For the initial conditions steady state concentrations are chosen, such that with this theory it is possible to fit and simulate quasi steady-state linear sweep RDE measurements or other quasi steady-state sequences of potential steps.It was found that it is possible to derive accurate closed form solutions for the initial parts of the transient response. However, the Nernst diffusion layer approximation leads to inaccuracies in the intermediate times with relative errors of up to 10%.By fitting the initial transient to the closed form solution it is possible to extract steady state background currents. Additionally, we use the potential step theory to derive an expression for kinetically controlled transition times and show that these can exceed the mass transport controlled transition time.     

Use of the rotating ring disc electrode for electrochemically-induced redox initiation of aqueous polymerization

The redox polymerizations of acrylic acid, acrylamide, methacrylic acid and acrylonitrile were conducted in aqueous solution on a micro scale with the rotating ring disc electrode. The redox couple was $\text{Cu(I)}\text{S}{}_2\text{O}{}_8{}^{\mathrm{2?}}$, and the Cu(I) was generated at the disc of the electrode system, whereas the ring current reflected the Cu(I) which reached the ring. In essence the Cu(I) and monomer compete for the SO₄^? radical ion, and the relative rates and rate constants of these steps are deduced from the electrochemical measurements. The relative reactivities of the monomers towards the SO₄^? were thus determined and are indicative of the initiation step in the respective polymerizations.     

A new model of mass transfer in a rotating disc contactor

A new model of oxygen transfer using a rotating disc contactor, half immersed in wastewater in a trough, is presented. The boundary-layer theory is used to estimate the liquid-film thickness on the disc while submerged. The oxygen transfer rate calculated from the model showed good agreement with observations, hence showing improvement compared to the previous model which has not been supported by observations.     

Kinetics of oxidation of benzidines in acetonitrile at the rotating platinum disc electrode

N,N′-Diphenylbenzidine, N,N′-di-p-anisylbenzidine, N,N′-dimethyl-N,N′-diphenylbenzidine, and N,N,N′,N′-tetraphenylbenzidine were oxidized at a rotating platinum disc electrode in acetonitrile containing 0.1 M LiClO₄. Equilibrium potentials, standard rate constants, transfer coefficients and diffusion coefficients were determined using a method extending the one of Jahn and Vielstich[1]. With each of the benzidines, the standard rate constants for the oxidation to the semiquinoneimine cations are equal to the rate constants for the further oxidation to the diphenoquinonediimine dications. The standard rate constants for the different benzidines grow in the order given above.At positive potentials, the N,N′-di-p-anisyldiphenoquinonediimine dication is oxidized, probably at the methoxy group. The resulting intermediate decomposes in a homogeneous reaction of the first order with the rate constant k₁ = 5.5 s^?1, and the second intermediate is further oxidized yielding the yet unknown final product.