Thermodynamics of electrochemical phase formation and underpotential metal deposition

Abstract

A new thermodynamic theory of the electrochemical phase formation and the underpotential deposition (UPD) of metals was developed. The half-crystal position of the two-dimensional crystal phase is introduced in order to characterise the electrochemical potential of the solid phase. It is shown that this half-crystal position depends on the specific adsorption of foreign species. New results are obtained for the equilibrium electrode potentials of underpotentially deposited metal layers onto crystalline substrates with different surface structures. Relations are derived for the adatom activity upon different substrates and its dependence on the supporting electrolyte and the specific adsorption. The obtained results explain quite well the position of the peaks in the voltammograms as well as their shape. The problem of the Gibbs free energy of nucleation and its dependence on the overvoltage of nucleation are comprehensively treated. A clear thermodynamic difference is shown between this overvoltage and the total value of the overvoltage. The influence of the supporting electrolyte and the specific adsorption on the UPD is explained. The condition for the transition from underpotential to overpotential deposition is shown. The obtained results are in very good agreement with all known experimental facts concerning the UPD of metals and lead to original conclusions which could be proved experimentally.