Jaime González Velasco

A study at the molecular level of the mechanism of surface diffusion at electrode–electrolyte interfaces

Abstract Using a model for the adsorption of water on electrode surfaces in which only two orientations are considered, it was possible to deduce the exponential dependence between surface diffusion coefficients and potential, which has been experimentally found when measuring the change with time of the roughness factors of columnar structured platinum and gold electrodes in contact with acidic electrolytes. The linear dependence between the logarithm of surface diffusion coefficient and the potential is explained by introducing an empirical coefficient, δ , proportional to the rate of variation of the activation enthalpy for surface diffusion with the potential. An explanation for the change of sign of δ , as well as a semiquantitative estimation of it, can be given using the theoretical description developed here.

A kinetic study of the origin of electroluminescence in porous silicon layers

Using the principles of electrochemical kinetics it is possible to describe the rate of formation of precursors able to inject electrons in the conduction band of porous silicon layers submitted to galvanostatic creation of holes in the valence band. In this way it becomes possible to explain the shape of the electroluminescence peaks emitted in porous silicon layers, both in the presence and in the absence of H2O2. The kinetic arguments give account of the experimental dependences found between the intensity of the electroluminescence and the potential at which a maximum intensity is observed on the H2O2 concentration. The theoretical description is demonstrated to be valid, notwithstanding the model accepted for explaining the origin of luminescence in porous silicon.

A theoretical explanation of the surface diffusion mechanism in metal electrodes in contact with electrolytes

A theoretical explanation for the surface diffusion mechanism observed in columnar structured metal electrodes in contact with electrolytes is given. The potential energy of a surface metal atom on which ions forming part of the supporting electrolyte are adsorbed is described by means of an anharmonic oscillator curve whereas the energy of a surface metal atom liberated from any adsorption interaction is approximated by a harmonic oscillator energy fuction. Geometric arguments allow to define a symmetry factor δ for which experimental values were previously obtained. A qualitative interpretation of the value of δ has been made.

SURFACE DIFFUSION IN ELECTROCHEMICAL SYSTEMS. THE MEANING OF THE FACTOR δ

A revision has been made of the supposition according to which the surfaces of solid electrodes behave in many electrochemical reaction mechanisms as inimitable. In fact, the distribution of the surface energy in different kinds of surface sites like, terraces, steps, kinks or edges as well as other surface defects, together with the relaxation induced by the adsorption on individual surface sites of molecules of the solvent, ions belonging to the supporting electrolyte and reactants, intermediates and products of the electrochemical reactions can produce significant changes in the surface topography of the electrodes due to surface diffusion processes whose existence has been put in evidence through a number of experiments.

A comparative study of the electrochemical and the spectroscopic behaviour of trisbipyrazyl ruthenium [Ru(bpz)32+]

A comparative study of the ultraviolet-visible spectrum and of the electrochemical behavior of the [Ru(bpz)32+] ion has been made. An examination of the recorded UV spectrum allows us to determine the wavelengths at which different electronic transitions take place. The energies at which these transitions take place correspond to the energy differences between the molecular orbitals in the ruthenium complex which are involved in the electronic transitions observed. With these data, and the wavelength of the maximum of the emission spectrum also recorded, drawing of the Jablonski diagram for the [Ru(bpz)32+] ion is possible. By comparing the energy differences between the molecular levels identified in the Jablonski diagram with the differences in potential values at which the various redox processes recorded in the cyclic voltammogram take place, it is possible to identify the electronic levels in the ruthenium complex which are involved in the redox reactions responsible for the cyclic voltammetric waves registered in the electrochemical study. The close correlation found between the spectroscopic and the electrochemical results confirms the first definition given for the cyclic voltammetry as electrochemical spectroscopy and allows the undoubted identification of the molecular orbitals that are involved in the different charge transfer reactions with the electrode.

On the mechanism of electroluminescence emission by porous silicon layers

The analytical treatment of a model considering the electrooxidation of p-porous silicon layers under galvanostatic conditions is able to give account of experimental facts such as the shape and location of the electroluminescence peak as well as of the spectral shift of the electroluminescence peak produced by oxidation. The proposed model considers electroluminescence to be the result of electron injection into the conduction band by an adsorbed intermediate produced by electrooxidation of the surface coverage with hydrogen or siloxene of the silicon nanocrystallites. The access of holes to the surface is made possible by low accumulation layer conditions and is the rate determining step in the electroluminescence mechanism. In this way it is possible to give a satisfactory explanation to the shift towards the blue experimented by the electroluminiscence emission maximum as a consequence of electrooxidation.

ELECTROCATALYTIC SOLUTIONS TO SOME ENERGY AND ENVIRONMENTAL QUESTIONS

SUMMARY After and introduction in which the concept of electrocatalysis in discussed, the electrochemical systems are defined and divided into two main classes: substance producers and energy producers. Then, an exposition is made of the fundamental aspects of electrocatalysis and applications as well as of the relation between Electrochemistry and environment by giving some examples obtained from industrial applications of electrocatalytic systems. Environmental strategies in which electrocatalysis may play a role and some research fields in environmental problems related to electrochemistry are also discussed.