Potential of the Emersed Electrode

In recent investigations, it appears that the interfadal potential between a metal electrode and an aqueous solution somehow survives after the electrode is taken out of the aqueous solution and into ultra high vacuum or an inactive gas phase [Wagner, 1993]. This circumstance is referred to as emersion . As shown in Fig. 4—26, the electrode potential E m of the emersed electrode is 

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Emersion has been shown to result in the retention of the double layer structure i.e, the structure including the outer Helmholtz layer. Thus, the electric double layer is characterised by the electrode potential, the surface charge on the metal and the chemical composition of the double layer itself. Surface resistivity measurements have shown that the surface charge is retained on emersion. In addition, the potential of the emersed electrode, , can be determined in the form of its work function, , since and represent the same quantity the electrochemical potential of the electrons in the metal. Figure 2.116 is from the work of Kotz et al. (1986) and shows the work function of a gold electrode emersed at various potentials from a perchloric acid solution the work function was determined from UVPES measurements. The linear plot, and the unit slope, are clear evidence that the potential drop across the double layer is retained before and after emersion. The chemical composition of the double layer can also be determined, using AES, and is consistent with the expected solvent and electrolyte. In practice, the double layer collapses unless (i) potentiostatic control is maintained up to the instant of emersion and (ii) no faradaic processes, such as 02 reduction, are allowed to occur after emersion. 

Double layer emersion continues to allow new ways of studying the electrochemical interphase. In some cases at least, the outer potential of the emersed electrode is nearly equal to the inner potential of the electrolyte. There is an intimate relation between the work function of emersed electrodes and absolute half-cell potentials. Emersion into UHV offers special insight into the emersion process and into double layer structure, partly because absolute work functions can be determined and are found to track the emersion potential with at most a constant shift. The data clearly call for answers to questions involving the most basic aspects of double layer theory, such as the role water plays in the structure and the change in of the electrode surface as the electrode goes frcm vacuum or air to solution. 

The emersed electrode, in principle, may be treated as the experimental realization of a single electrode. However, it is doubtful whether its liquid layer has the same bulk properties. This is probably the main reason for the different results of E°H(abs) found for emersed electrodes, e.g., -4.85 V.83 Samec et al. have found that emersion of electrodes in a nitrogen atmosphere decreases the Volta potential and therefore the absolute electrode potential by ca. 0.32 V relative to the value in solution. They have attributed this mainly to the reorientation of the water molecules at the free surface. 

These measurements have verified that the work function of an electrode, emersed with the double layer intact, depends only on the electrode potential and not on the electrode material or the state of the electrode (oxidized or covered with submonolayer amounts of a metal) [20]. Work function measurements on emersed electrodes do not serve the same purpose as in surface science investigations of the solid vacuum interface. At the electrochemical interface, any change of the work function by adsorption is compensated by a rearrangement of the electrochemical double layer in order to keep the applied potential i.e. overall work function, constant. Work function measurements, however, could well be used as a probe for the quality of the emersion process. Provided the accuracy of the measurement is good enough, a combination of electrochemical and UPS measurements may lead to a determination of the components of equation (4). 

In order to explain the changing optical properties of AIROFs several models were proposed. The UPS investigations of the valence band of the emersed film support band theory models by Gottesfeld [94] and by Mozota and Conway [79, 88]. The assumption of nonstoichiometry and electron hopping in the model proposed by Burke et al. [87] is not necessary. Recent electroreflectance measurements on anodic iridium oxide films performed by Gutierrez et al. [95] showed a shift of optical absorption bands to lower photon energies with increasing anodic electrode potentials, which is probably due to a shift of the Fermi level with respect to the t2g band [67]. 

Electrodes of 2 x 2 cm2 geometric area were used in the experiments. In the case of platinum, a pretreatment by fast triangular potential scans (200-300 V/s between 0.05 V and 1.5 V RHE) followed by heating up to 900 K in a 3 x 10 6 mbar 02 atmosphere was carried out. Electrodes, pre-treated in this way, can be emersed with a thin liquid film, which can easily be evaporated in the vacuum chamber. The heat treatment drastically reduces contamination of the platinum electrode by carbon. The roughness factor is usually in the order of three. 

As was discussed above, it is essential to determine the effect, if any, that the emersion process has on the double layer. To do this, Wilhelm and colleagues have performed the definitive type of blank experiment. CO was adsorbed onto the Pt working electrode from sulphuric acid electrolyte. After adsorption, the CO-saturated solution was replaced with pure electrolyte. The potential of the electrode was then ramped in order to oxidise off the adsorbate, as C02, and the voltammogram so obtained is shown in Figure 2.118(a). The experiment was then repeated CO was adsorbed as before, but the electrode was emersed and transferred into the UHV chamber, before being re-immersed and the potential ramp applied. The voltammogram so... 

The intimate relationship between double layer emersion and parameters fundamental to electrochemical interfaces is shown. The surface dipole layer (xs) of 80SS sat. KC1 electrolyte is measured as the difference in outer potentials of an emersed oxide-coated Au electrode and the electrolyte. The value of +0.050 V compares favorably with previous determinations of g. Emersion of Au is discussed in terms of UHV work function measurements and the relationship between emersed electrodes and absolute half-cell potentials. Results show that either the accepted work function value of Hg in N2 is off by 0.4 eV, or the dipole contribution to the double layer (perhaps the "jellium" surface dipole layer of noble metal electrodes) changes by 0.4 V between solution and UHV. 

These results are remarkable Coupled with other results for silver and platinum (19J they show that the emersed electrode work function cam be independent of electrode material (even oxide coated) and electrolyte. The tracks < g one-to-one over a large potential region, even after placement in UHV. The apparatus used allowed for emersion and placement in UHV without exposure to air at any time. 

This equation applies also to the electrodes emersed from aqueous solution into inactive gas (Fig. 4-28 b) as shown in Eqn. 4-40, which gives the relationship between the electrode potential Em of an emersed electrode and the potential Ejcp of the Kelvin probe ... 

Finally, IRRAS of the electrode-electrolyte interface can be measured using the emersed electrode technique. This technique is based on the fact [445] that the compact DL remains intact upon removal of the electrode from elecfiolyte. To overcome the loss of the potential confiol upon emersion it was suggested [446] that an electrode that is in partial contact with electrolyte be rotated. In a more advanced double-cell technique [447], a cell similar to that shown in Fig. 4.45 is employed. The difference is that the thin internal tube holding the working electrode (Pt disk) serves also as a container for the second ceU. Both the main outer and internal auxiliary cells are filled with the same solution and have reference and counter electrodes inside. Since the Pt electrode is a good conductor its surfaces are equipotential. Therefore, any potential applied on the electrode back side is established at the outer surface that is pressed against the window for IR measurements. 

FIGURE 3.14. Experimental device for measuring the Volta-potential difference between an electrode surface emersed from an electrolyte (cell) and a reference surface (vibrator). The inert atmosphere in the stainless steel chamber is water-saturated. The position of the Kelvin vibrator in front of the withdrawn electrode surface is adjusted with a micrometer drive. 

Composition (nmol cm ) of the surface of an hydrophobically emersed Ag electrode from IM NaC104 + 10- M HCIO4 solution as a function of the applied electrode potential E, (a) n(ClOi), (b) n(Na+), (c) w(C104) -n(Na ) charge balance with indication of pzc. (From Hecht, D., Thesis Heinrich-Heine- Universitat, Diisseldorf, Shaker Verlag, Aachen, Germany, 1997.)... 

A third experimental configuration was proposed by Kolb and Hansen40 emersed electrodes. If an electrode is emersed from a solution while the control of the potential is maintained, the solvent layer dragged off with the metal (Fig. 3) would reproduce UHV conditions, but with potential control and at room temperature, as in the actual electrode situation. This appears to be the most convenient configuration for measuring 0. However, there are doubts that the solvent layer retains the properties of a bulk phase. It has in fact been demonstrated41 that a contact potential difference exists between an electrode in the emersed state and the same electrode regularly immersed in solution. 

On the other hand, potential measurements at the free surface of purified water have shown50 that the value for a flowing surface differs by about 0.3 V from that for a quiescent surface, as a result of adsorption of surface-active residual impurities in the solution (probably also coming from the gas phase). Since emersed electrodes drag off the surface layer of the solution as they come out of the liquid phase, the liquid layer attached to emersed solid surfaces might also be contaminated. 

Z. Samec, B.W. Johnson, and K. Doblhofer, The absolute electrode potential of metal electrodes emersed from liquid electrolytes, Surf. Sci. 264, 440-448 (1992). 

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