Electrode emersed

As a furtlier example for tire meaning of ex situ investigations of emersed electrodes witli surface analytical teclmiques, results obtained for tire double layer on poly crystalline silver in alkaline solutions are presented in figure C2.10.3. This system is of scientific interest, since tliin silver oxide overlayers (tliickness up to about 5 nm) are fonned for sufficiently anodic potentials, which implies tliat tire adsorjDtion of anions, cations and water can be studied on tire clean metal as well as on an oxide covered surface [55, 56]. For tire latter situation, a changed... 

Kolb D M, Rath D L, Wille R and Flansen W N 1983 An ESCA study on the electrochemical double layer of emersed electrodes Ber. Bunsenges. Phys. Chem. 87 1108-11 131... 

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. 

Figure 3. Sketch of an emersed electrode. M is the metal, S is the solvent (electrolyte solution), (a) < is the work to extract an electron from M through S. (b) The emersed electrode drags a liquid layer with it, through which the measurement of is apparently the same as in (a). The question mark is meant to cast doubts on that.

A value close to 4.8 V has been obtained in four different laboratories using quite different approaches (solid metal/solution Ay, 44 emersed electrodes,40,47 work function changes48), and is apparently supported by indirect estimates of electronic energy levels. The consistency of results around 4.8 V suggests that the value of 4.44 V is probably due to the value of 0 not reflecting the actual state of an Hg jet or pool. According to some authors,44 the actual value of 0 for Hg in the stream should be 4.8 V in that the metal surface would be oxidized. 

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. 

It is intriguing that upon emersion the value of A0 changes up to about 0.3 V compared with the immersed state.41 This has been attributed42,51 to the different structure of the liquid interfacial layer in the two conditions. In particular, the air/solvent interface is missing at an emersed electrode because of the thinness of the solvent layer, across which the molecular orientation is probably dominated by the interaction with the metal surface. 

The situation believed to exist at an emersed electrode is sketched in Fig. 4. It is seen that while A in the immersed state is given by Eq. (20) rewritten as... 

Figure 4. Sketch to illustrate the situation believed to exist at a metal surface upon adsorption of water from the gas phase (or at the surface of an emersed electrode). In particular, the layer thickness is so small that the orientation of solvent molecules at the external surface is strongly affected by the orientation at the internal surface.

Emersed electrode, 12 Energy scales and electrode potentials, 7 Energy transitions via polaronic and bipolaronic levels, 362 Engineering models, for fluorine generation cells, 539 Esin and Markov plots, 259-260 Experimental data comparison thereof, 149 on potential of zero charge, 56... 

The presence of this backspillover formed effective double layer is important not only for interpreting the effect of electrochemical promotion, but also for understanding the similarity of solid state electrochemistry depicted in Fig. 7.3 with the case of emersed electrodes in aqueous electrochemistry (Fig. 7.2) and with the gedanken experiment of Trasatti (Fig. 7.1) where one may consider that H2O spillovers on the metal surface. This conceptual similarity also becomes apparent from the experimental results. 

In summary, the creation via ion spillover of an effective electrochemical double layer on the gas exposed electrode surfaces in solid electrolyte cells, which is similar to the double layer of emersed electrodes in aqueous electrochemistry, and the concomitant experimentally confirmed equation... 

It is also worth noting that the one-to-one correspondence between change in (ohmic drop-free) catalyst potential and work function in solid-state electrochemistry,7,8 may also be applicable to the work function of liquid-free gas-exposed electrode surfaces in aqueous electrochemistry.8 Such surfaces, created when gases are consumed or produced on an electrode surface, may also play a role in the observed NEMCA behaviour. The one-to-one correspondence between eAUwR and AO is strongly reminiscent of the similar one-to-one relationship established with emersed electrodes previously polarized in aqueous solutions,9,10 as already discussed in Chapter 7. 

The contact angle between electrode and electrolyte solution can be determined using a solid and partially emersed electrode and observing the meniscus rise [68Mor, 69Mor, 71Mor]. (Data obtained with these methods are labelled CA). 

A relatively new arrangement for the study of the interfacial region is achieved by so-called emersed electrodes. This experimental technique developed by Hansen et al. consists of fully or partially removing the electrode from the solution at a constant electrical potential. This ex situ experiment (Fig. 9), usually called an emersion process, makes possible an analysis of an electrode in an ambient atmosphere or an ultrahigh vacuum (UHV). Research using modem surface analysis such as electron spectroscopy for chemical analysis (ESCA), electroreflectance, as well as surface resistance, electrical current, and in particular Volta potential measurements, have shown that the essential features (e.g., the charge on... 

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. 

Electrochemical reactions are driven by the potential difference at the solid liquid interface, which is established by the electrochemical double layer composed, in a simple case, of water and two types of counter ions. Thus, provided the electrochemical interface is preserved upon emersion and transfer, one always has to deal with a complex coadsorption experiment. In contrast to the solid/vacuum interface, where for instance metal adsorption can be studied by evaporating a metal onto the surface, electrochemical metal deposition is always a coadsorption of metal ions, counter ions, and probably water dipols, which together cause the potential difference at the surface. This complex situation has to be taken into account when interpreting XPS data of emersed electrode surfaces in terms of chemical shifts or binding energies. 

Fig. 3. Binding energy distribution of several oxygen species occurring on emersed electrodes. Binding energies are taken for different substrates from various authors. In part after [15, 18].

Very useful information concerning the surface of emersed electrodes, however, can be deduced from UPS spectra directly, like the electronic density of states at the Fermi level, the position of the valence band with respect to the Fermi level or possible band gap states. The valence band of UPD metals might help to explain the respective optical data (see Sections 3.2.1 and 3.2.5). 

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). 

While characterization of the electrode prior to use is a prerequisite for a reliable correlation between electrochemical behaviour and material properties, the understanding of electrochemical reaction mechanisms requires the analysis of the electrode surface during or after a controlled electrochemical experiment. Due to the ex situ character of photoelectron spectroscopy, this technique can only be applied to the emersed electrode, after the electrochemical experiment. The fact that ex situ measurements after emersion of the electrode are meaningful and still reflect the situation at the solid liquid interface has been discussed in Section 2.7. 

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. 

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. 

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. 

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... 

Fig. 4-26. (a) Electrode potential of an immersed electrode in aqueous solution and (b) electrode potential of an emersed electrode removed from aqueous solution M = metal electrode S = aqueous bulk solution Sm = aqueous solution adsorbed on emersed electrode V = vacuum or gas phase. 

Fig. 4-27. Work fimction 4> of emersed electrodes of gold, silver and platinum in ultra hig vacuum (UHV) as a function of electrode potential E at which the electrodes have been maintained in 0.1 M per chloric add solution before emersion Esm => normal hydrogen electrode potential = 4.5 or 4.44 V Cnhe = emersed normal h3rdiogen electrode potential in UHV = 4.85 V arrow = work f mction of free dean surfaces of gold, platinum and silver in vacuum. [From Kota-Neff-Moller, 1986.]...

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