Theory and EX Situ Studies of the Electrochemical Double Layer

The electrochemical interphase between a metal and electrolyte is examined both theoretically and experimentally on a molecular level. The electrochemical potential of conduction species is reviewed. The factors controlling double layer formation are neutrality within the interphase between the two conductors and in the bulk, and the equilibration or mobility of conduction species across the interphase. Bending of electron orbitals near the electrode surface is discussed from a unique thermodynamic viewpoint. The absolute potential of a half-cell, E(,abs), is derived and determined from the viewpoint of double layer formation, treating the half-cell by itself. E(,abs) is defined as the fermi level of the equilibrated half -cell electrode, which is equal to the potential drop across the double layer which forms during the equilibration of the ion between phases (plus the electrode work function which relates the fermi level to zero at (INFIN)). This potential is the driving force behind the double layer formation, which changes the fermi level of the electrode with respect to the solution during equilibrium. Its value is determined by measuring the difference in the vacuum levels of the half-cell electrode and solution, and adding the value of the electrode work function. This was done, and E(,abs) for the calomel and standard hydrogen electrodes were determined to be +4.698 and +4.456 volts respectively. The electrode emersion process is reviewed. Experimental observations of the electrochemical double layer and specific double layer species were made using ex situ surface-sensitive techniques via the electrode emersion process. Significant results of XPS and work function studies are shown, including a comparison of the electrochemical shift of adsorbing and non-adsorbing double layer species. For the first time, species at the inner Helmholtz plane can be distinguished experimentally from those of the outer Helmholtz plane. Also, the intrinsic surface dipole layer of 80% saturated KCl solution is measured as +.16 V. Also seen for the first time are infrared spectra of species within an ideally-polarizable double layer. Perchlorate and sulfate ions are readily observed, as are additional species due to CO(,2) contamination. The change in amount of species as a function of electrode potential is clear, while shifts in vibrational frequency are evident.