Influence of charge density and electrolyte concentration on the electrical double layer characteristics at rough cadmium electrodes

A new effective diffuse layer thickness dependent surface roughness model, based on the non-linear Poisson-Boltzmann theory, has been used for the int erpretation of impedance data for rough cadmium electrodes. The so-called e lectrochemical roughness function values at various electrode potentials an d surface charge densities have been calculated. It was found that the roug hness function values, the inverse value of Parsons-Zobel plot, as well as the fitting coefficient in the Valette-Hamelin approach, obtained from the electrochemical impedance data, art: effective (seeming) parameters and do not characterise the real geometrical structure of electrode surface. Howev er, they characterise the deviation of real experimental system from ideall y smooth (flat) surface. In the region of moderate rational electrode poten tials 0 less than or equal to \E\ less than or equal to 0.2 V and surface c harge densities (1 less than or equal to \a\ less than or equal to 4 muC cm (-2)) the roughness function rises with the decrease of the effective thick ness of diffuse layer. At higher electrode potentials and surface charge de nsities the surface roughness function levels of to the constant value, i.e . to the so-called geometrical surface roughness value. The new effective d iffuse layer thickness dependent surface roughness theory needs future corr ections taking into account the role of crystallographic inhomogeneity of t he geometrically rough electrode surface, i.e. the dependence of the local surface charge density on the crystallographic structure of energetically h omogeneous regions exposed on the macro-polycrystalline electrode surface. (C) 2000 Elsevier Science Ltd. All rights reserved.