Electrochemical double -layers

Figure C2.10.3. Ex situ investigation of the electrochemical double layer on Ag after hydrophobic emersion from 1 M NaClO + 0.1 M NaOH. (a) Peak deconvolution of the XPS 01s signals after emersion at +0.2 V A surface...

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... 

Flecht D and Strehblow FI-FI 1997 XPS investigations of the electrochemical double layer on silver in alkaline chloride solutions J. Electroanal. Chem. 440 211-17... 

Z. Borkowska, J. Stafiej, J. P. Badiah. Simple description of ionic solution at electrified interfaces. Proceedings of the Symposium on the Electrochemical Double Layer, Montreal, 1997, pp. 120-130. 

More than with continuous improvement in the preparation of clean surfaces in electrochemistry, the measurement of 0 is rather casual in surface science at present. In particular, work functions are mostly measured for d-metals rather than for sp-metals, which are more common in electrochemical double-layer studies. As a consequence, compilations of work function values report data for sp-metals that are 20 to 30 years old.63,856,857 This does not imply that the data are unreliable, but imparts to the situation a sense of frustration related to the immobility in one of the variables. 

Thus, as will be shown in this book, the effect of electrochemical promotion (EP), or NEMCA, or in situ controlled promotion (ICP), is due to an electrochemically induced and controlled migration (backspillover) of ions from the solid electrolyte onto the gas-exposed, that is, catalytically active, surface of metal electrodes. It is these ions which, accompanied by their compensating (screening) charge in the metal, form an effective electrochemical double layer on the gas-exposed catalyst surface (Fig. 1.5), change its work function and affect the catalytic phenomena taking place there in a very pronounced, reversible, and controlled manner. 

In summary AC impedance spectroscopy provides concrete evidence for the formation of an effective electrochemical double layer over the entire gas-exposed electrode surface. The capacitance of this metal/gas double layer is of the order of 100-300 pF/cm2, comparable to that corresponding to the metal/solid electrolyte double layer. Furthermore it permits estimation of the three-phase-boundary length via Eq. 5.62 once the gas exposed electrode surface area NG is known. 

The unique characteristic of the effective double layer is that it is directly accessible to gaseous reactants. Thus electrochemical promotion is catalysis in the presence of a controllable (via current and potential) electrochemical double layer. The theoretical implications and practical opportunities are obvious and numerous. 

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... 

If an electrode is brought into contact with an electrolyte solution or a molten electrolyte, the establishment of the electrochemical double layer will be accompanied by a transfer of electrical charge. In a suitable arrangement this charge can be measured as an external current. If the contact is made in a way which adjusts the electrode potential upon immersion exactly to the value of Epzc, the current will be nil. Various methods briefly described below have been devised to detect exactly this situation. 

Molecular dynamics simulations have also been used to study the effect of the presence of surface defects and the distribution of ions at the electrochemical double layer. The classical approach described previously has been challenged in recent times through the use of models that involve the calculation of both atomic and the electronic structures of the interface, as made by J. W. Halley et al. (1998). 

Electrochemical Double Layer In the absence of specific adsorption, the reciprocal of the capacitance C of the electrochemical double layer can be written as... 

FIGURE 35.5 Solvent contribution to the capacitance of the electrochemical double layer as obtained from a Monte Carlo simulation. (From Aloisi et ah, 1989, with permission from the American Institute of Physics.)... 

The main idea of a lattice model is to assume that atomic or molecular entities constituting the system occupy well-defined lattice sites in space. This method is sometimes employed in simulations with the grand canonical ensemble for the simulation of surface electrochemical proceses. The Hamiltonians H of the lattice gas for one and two adsorbed species from which the ttansition probabilities 11 can be calculated have been discussed by Brown et al. (1999). We discuss in some detail MC lattice model simulations applied to the electrochemical double layer and electrochemical formation and growth two-dimensional phases not addressed in the latter review. MC lattice models have also been applied recently to the study the electrox-idation of CO on metals and alloys (Koper et al., 1999), but for reasons of space we do not discuss this topic here. 

Skulason E, Karlberg GS, Rossmeisl J, Bligaard T, Greeley J, Jonsson H, Nprskov JK. 2007. Density functional theory calculations for the hydrogen evolution reaction in an electrochemical double layer on the Pt(lll) electrode. Phys Chem Chem Phys 9 3241-3250. 

Climent V, Gomez R, Orts JM, Aldaz A, Fehu JM. 1997. The potential of zero total charge of single-crystal electrodes of platinum group metals. In Korzeniewski C, Conway BE, eds. The Electrochemical Society Proceedings (Electrochemical Double Layer). Pennington, NJ The Electrochemical Society, pp. 222-237. 

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. 

Although not the subject of this article, double layer studies are briefly discussed in this paragraph in order to demonstrate that ex situ XPS studies indeed provide information about the state of the electrode exposed to an electrochemical environment at a defined potential. A crucial step in any ex situ experiment is the emersion of the electrode. Here the question arises whether the electrochemical double layer or part of it is preserved at the interface after emersion and transfer. Winograd et al. [10,11] first demonstrated that the electrode under UHV conditions still remembers the electrode potential applied at the time of emersion. These authors investigated oxide formation on Pt and the underpotential deposition of Cu and Ag on Pt by means of XPS and proved that the electrochemically formed oxide layer and... 

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). 

The above effects are more familiar than direct contributions of the metal s components to the properties of the interface. In this chapter, we are primarily interested in the latter these contribute to M(S). The two quantities M(S) and S(M) (or 8% and S m) are easily distinguished theoretically, as the contributions to the potential difference of polarizable components of the metal and solution phases, but apparently cannot be measured individually without adducing the results of calculations or theoretical arguments. A model for the interface which ignores one of these contributions to A V may, suitably parameterized, account for experimental data, but this does not prove that the neglected contribution is not important in reality. Of course, the tradition has been to neglect the metal s contribution to properties of the interface. Recently, however, it has been possible to use modern theories of the structure of metals and metal surfaces to calculate, or, at least, estimate reliably, xM(S) and 5 (as well as discuss 8 m, which enters some theories of the interface). It is this work, and its implications for our understanding of the electrochemical double layer, that we discuss in this chapter. 

Anions may exhibit a tendency toward specific adsorption at the O/S interface. This may be related in some way to the complexing affinity. This effect, occurring at the inner Helmholtz plane of the electrochemical double layer, may significantly change the charge transfer situation [cf. Section III(5(iii))]. 

The problem of ion transfer across the interface has been treated in detail by Sato,26,27 Scully,28 and also Valand and Heus-ler,29 following the general theory of Vetter.30 Valand and Heusler assumed the same type of activation-controlled charge transfer kinetics, except that the dominant charge here is that on the O2-ions (or OH- ions) obtained by splitting water at the interface. The electrochemical double layer here is of the usual type for aqueous systems and the equilibrium p.d. is determined by the main charge transfer reaction... 

In the above considerations, the O/S interface was taken to be a clear-cut boundary between the oxide and the electrolyte. In reality, however, the outer part of the oxide is likely to be hydrated and penetrated by the electrolyte. Hence, the true O/S interface is likely to be withdrawn from the surface to a sufficient depth such that some oxide is left without any electric field imposed across it. This is especially true of thick porous oxide layers, but it can occur with compact layers as well. For example, Hurlen and Haug35 found a duplex film in acetate solution (pH 7-10), composed of a dry barrier-type part and a thicker hydrated part consisting of A1203 H20. Although the hydrated part becomes thinner with decreasing pH and seems to practically vanish at low pH, even a thickness of less than a nanometer is sufficient for the surface oxide to stay outside the electrochemical double layer. 

It is much less clear how the adsorption leads to such a dramatic change as a potential decay of several hundred volts, occurring within milliseconds. This short time is difficult to associate with film thinning, as assumed in the adsorption mechanism of pit initiation. It is not only that the mechanism of dissolution changes so much that the current efficiency falls from virtually 100% to virtually zero, but also that the resistance of the oxide decreases by orders of magnitude. The control of the process is, to a great extent, taken over by the events at the O/S interface, judging from the capacitance values measured,115 which approach those typical of the electrochemical double layer (cf. Fig. 22). 

Mayer S.T., Pekala R.W., Kaschmitter J.L. The aeorocapacitor an electrochemical double-layer energy-storage device. J Electrochem Soc 1993 140 446-51. 

---