A.J. Calandra

Potentiodynamic current/potential relations for film formation under OHMIC resistance control

Abstract Theoretical potentiodynamic current/potential curves for a film formation process under an ohmic resistance control are given. Results are compared with experimental data involving the formation of insoluble parathiocyanogen on platinum and the precipitation of a solid CuCl layer during copper dissolution in aqueous HCl solution.

The electrochemistry of gold in acid aqueous solutions containing chloride ions

Abstract Results obtained with differently prepared gold electrodes in acid solutions containing chloride ion at temperatures ranging from 22 to 65°C are reported. Potentiostatic techniques and rotating disc electrodes were employed. The anodic dissolution, the onset of passivity and the cathodic deposition of the metal are investigated under different experimental conditions. The anodic dissolution of Au is discussed in terms of possible reaction mechanisms involving the participation of different adsorbed species. The onset of passivation is related to the depletion of chloride ion at the reaction interface. The experimental voltammograms can be reporduced by means of an equation which considers the diffusion of chloride ion, the activation polarization related to the dissolution of Au and the establishment of passivity.

Transient changes of intermediate species formed during the electrooxidation and surface electroreduction of gold within a monolayer thickness range

Summary Evidence for species formed during the anodic oxidation and electroreduction of a gold electrode surface within the range of a monolayer thickness is given by means of triangular potential sweeps run under different experimental conditions. The results are correlated with those obtained with the modulated potential sweep technique. Species related to the ageing of the oxide film are assigned and the corresponding rate is measured. The individual stages of the whole E/I profile which imply different kinetic characteristics, are interpreted in terms of different oxidation states of the electrode surface.

The electrochemical behaviour of different graphite/cryolite alumina melt interfaces under potentiodynamic perturbations

Abstract The anodic reaction in molten cryolite alumina systems has been studied in a wide range of potentials by applying both repetitive and single pulse linear sweep voltammetry, varying the sweep rate between 0.04 V/s and 600 V/s, and the alumina concentration between 0.05 and 1.5 wt%. The experimental results obtained at slow sweep rates were interpreted taking into account that the hydrodynamic conditions which prevail at the anodic interface are defined by the gas flow caused by the anodic reaction. At fast sweep rates, under conditions of linear diffusion, a mechanism for the oxide ion discharge is postulated. The low value of the diffusion coefficient of the oxide ion species, calculated from fast sweep rate experiments indicates a prior chemical equilibrium step. At fast sweep rates, a new voltammetric peak is reported, probably associated with the COF 2 formation.

The influence of ageing in the kinetics of the electroreduction of gold oxide films of the order of a monolayer thickness

Abstract Gold oxide films of the order of a monolayer thickness are electroreduced under different potentiodynamic conditions. The electrochemical process is coupled to a chemical dissolution of the film and a solid phase transformation. Under controlled ageing the kinetics of the electroreduction reaction obey simple laws which are derived from reaction models already discussed in previous publications.

The kinetics of hydrogen evolution on black platinum electrodes during the electrolysis of molten potassium bisulphate. The formation of a reversible hydrogen electrode

Abstract The discharge of hydrogen ions on black platinum electrodes during the electrolysis of molten potassium bisulphate was examined by means of potential/current-density curves and observation of the decay of overpotential. The results indicate that the mechanism of reaction involves a recombination reaction with hydrogen atoms on the black platinum surface as the rate-determining step. Due to the reversibility of the electrode reaction, a reversible hydrogen electrode in the melt was obtained, the potential of which was determined against a silver/silver-ion reference electrode.

A contribution to the theory of the potential sweep method: charge transfer reactions with uncompensated cell resistance

Abstract The effect of uncompensated cell resistance on current-potential curves obtained with the potential sweep method is presented for simple electrochemical reactions in conditions of a constant ionic activity and with the product entirely adsorbed on the electrode surface under either quasi-equilibrium or Tafel conditions. First and second order processes are considered. Results obtained for the latter are referred to the electrochemical reduction of platinum oxide film in 1 N sulphuric acid at room temperature.

The kinetics and mechanism of the cathodic reduction of monomolecular oxide layers electrochemically formed on platinum in bisulphate melts under constant ageing conditions

Summary The kinetics of the electrochemical reduction of a monolayer oxide film anodically formed on platinum in bisulphate melts is studied by means of single potential sweep voltammetry. Under constant ageing conditions, the kinetics of the process is discussed in terms of an irreversible process comprising a simple consecutive reaction scheme involving a second order process as the rate determining step.

Experimental and theoretical analysis of the anode effect on graphite electrodes in molten sodium fluoride under potentiodynamic perturbations

Abstract The anode effect on graphite electrodes in pure molten sodium fluoride was investigated by applying various potential/time perturbation functions and the triangular potential sweep technique. The experimental potentiodynamic current/potential curves were compared with those obtained from equations derived from a reaction model involving the electrochemical formation of two surface passivating species coupled with a simultaneous thermal decomposition. A comparison is made between the kinetic behaviour of the anode effect in sodium fluoride and in cryolite melts.

Electrochemical kinetics of molten potassium thiocyanate on platinum. Mechanisms of anodic film formation and its cathodic dissolution

Abstract The kinetics and mechanism of the anodic film formation and its cathodic dissolution on platinum, through electrolysis of molten potassium thiocyanate, were studied. Experiments were made at 190°C by applying both repetitive and single pulse linear sweep voltammetry. The kinetics of the anodic film formation is a film-growth-rate controlled process, whose mechanism comprises various steps: discharge, nucleus formation and growth. The discharge step is a fast process and the second step is postulated as rate-determining at low potential sweep rates; the corresponding Tafel slope is RT/2F . At higher potentials the larger reaction rates achieved make reaction sites undistinguishable. This fact reflects a change of reaction mechanism involving a different Tafel slope for the anodic film formation, which is twice the former slope. The corresponding E/I curves are theoretically evaluated with parameters derived from the postulated mechanisms. The dissolution process is not definitely established although the likely pathway is envisaged. The catholic reaction is characterized by a Tafel slope of RT/F . The dissolution comprises a fast electron-transfer step followed by thiocyanate-ion formation; the latter appears as the slowest step.