A Scale of Absolute Surface Potentials of Metals. Part I

Abstract

The problem of the absolute electrode potentials of metals is considered as a problem that encompasses both the thermodynamics of electrode reactions on metals and the surface state of a metal. Within the framework of the physical chemistry of the surface, an adsorption model of the surface layer (SL) of a metal is developed, differing by using the value of the excessive Gibbs surface energy $$\Delta {{G}_{{\text{S}}}}.$$ For the low-index facet (hkl) of the metal, the magnitude of the absolute surface potential is introduced as Es = $${{\Delta {{G}_{{\text{S}}}}} \mathord{\left/ {\vphantom {{\Delta {{G}_{{\text{S}}}}} {zF}}} \right. \kern-0em} {zF}}$$ (z is the valence of metal), which, like the magnitude, depends on the electrode potential. On the other hand, a statistical model is used that relates the autoadsorption of point defects, the surface atomic vacancies V(S) in SL, and adatoms with the surface energy as $$\Delta {{G}_{{\text{S}}}}$$ = –RTlnNV(S)= –RTlnNad. For an uncharged metal, the uncharged metal value $$\Delta G_{{_{{\text{S}}}}}^{0}$$ is maximum, which gives a minimum of the mole fraction of Nad = NV(S) and leads to a zero charge potential formula as $$E_{N}^{0}$$ = – $${{\Delta G_{{\text{S}}}^{0}} \mathord{\left/ {\vphantom {{\Delta G_{{\text{S}}}^{0}} {zF}}} \right. \kern-0em} {zF}}$$ . The ideal polarization of the electrode relative to EN values of $$\Delta G_{{\text{S}}}^{{}}$$ and Gadsto zero that corresponds to the maximum of NV(S) or Nad. These extreme points of the surface activity of atoms determine the scale of absolute values of Es calculated using known table values $$\Delta {{G}_{{\text{S}}}}$$ (hkl) at T = 0 K obtained by the DFT method. When assessing the effect of temperature or potential, the change in the ΔGS and NV(S) (or Nad) values in thermal and electrochemical processes is considered. In Part 2, an application of this scale to the processes of evolution of hydrogen and passivation of metals is considered.