Polycrystalline electrodes, double layer

Figure 10. Theoretical model for the electrical double layer at an electrode with a polycrystalline surface, (a) Model of independent diffuse layers [Eq. (S3)], and (b) model of common diffuse layer [Eq. (54)).

The electrical double layer at pc-Zn/fyO interfaces has been studied in many works,154 190 613-629 but the situation is somewhat ambiguous and complex. The polycrystalline Zn electrode was found to be ideally polarizable for sufficiently wide negative polarizations.622"627 With pc-Zn/H20, the value of Eg was found at -1.15 V (SCE)615 628 (Table 14). The values of nun are in reasonable agreement with the data of Caswell et al.623,624 Practically the same value of Eff was obtained by the scrape method in NaC104 + HjO solution (pH = 7.0).190 Later it was shown154,259,625,628 that the determination of Eo=0 by direct observation of Emin on C,E curves in dilute surface-inactive electrolyte solutions is not possible in the case of Zn because Zn belongs to the group of metals for which E -o is close to the reversible standard potential in aqueous solution. 

Clavilier eta/.196,794-796have studied CO adsorption on electrochemi-cally faceted Pt(lll) and Pt(110) electrodes and from the charge transients, with the provision that the CO dipole has a negligible contribution to the electrical double-layer potential these authors have provided a definite determination of ( =o- However, electrochemically faceted Pt(lll) electrodes have a polycrystalline surface structure, and thus the value of Eq-q for such electrodes lies between fiULO for terraces and forst s.197 786 787... 

Electrical double-layer (edl) properties of solid polycrystalline zinc (pc-Zn) electrodes and single-crystal zinc electrodes in aqueous solution were studied in many works, which are reviewed in Refs 1, 2. 

The electrical double-layer (edl) properties pose a fundamental problem for electrochemistry because the rate and mechanism of electrochemical reactions depend on the structure of the metal-electrolyte interface. The theoretical analysis of edl structures of the solid metal electrodes is more complicated in comparison with that of liquid metal and alloys. One of the reasons is the difference in the properties of the individual faces of the metal and the influence of various defects of the surface [1]. Electrical doublelayer properties of solid polycrystalline cadmium (pc-Cd) electrodes have been studied for several decades. The dependence of these properties on temperature and electrode roughness, and the adsorption of ions and organic molecules on Cd, which were studied in aqueous and organic solvents and described in many works, were reviewed by Trasatti and Lust [2]. 

The main properties of the double layer of solid lead electrodes have been already described in the Encyclopedia [1]. New achievements in this field have been the subject of reviews [for example [2-6]. Some of the new results relate to impedance of polycrystalline Pb electrodes in aqueous [7-9] and nonaqueous solvents (references in [3, 6[). Special attention has been paid to chemically and electrochemically polished polycrystalline electrodes, mainly in aqueous [10-12] and methanolic [13] fluoride solutions. 

Jusys and Bruckenstein [84] have used electrochemical quartz crystal microbalance to study adsorption/desorption of perchlorate and perrhenate ions on a bare polycrystalline Au electrode. The change in the equivalent mass was undoubtedly assigned to the adsorption of both anions in the double-layer region of Au electrode. 

Fig. 2 Differential double-layer capacitances of Ag(l 11), Ag(lOO), and Ag(l 10) in 10 mM NaF determined applying an ac technique / = (o/2jt = 20 Hz, scan rate = 5 mV s / The calculated curve corresponds to the model polycrystalline Ag electrode [24],...

Fig. 7.42. A potentiostatic transient. The current (A-B) ascends almost vertically after being switched on, because all of it goes to charge the double layer. In B-C, the current is increasingly used in the form of electrons crossing the double layer. After C the current should decline slowly as diffusion control sets in. In reality, at solid polycrystalline electrodes, in reactions involving adsorbed intermediates, there is often some further variation of /, owing to, e.g., surface crystalline rearrangements and the effect of impurities from the solution.

Any double layer model has to explain experimental results, for example in Fig. 3.4 for sodium fluoride at a mercury electrode. Until the 1960s measurements were made almost exclusively at mercury electrodes and models were developed for this electrode. The fact that mercury is an ideally polarizable liquid in the zone negative to the hydrogen electrode means that its behaviour is often different from solid electrodes (monocrystalline and polycrystalline). These models are, therefore, of a predominantly electrostatic nature. 

Already in this Modem Aspects of Electrochemistry series the theory and status of data for electrochemical double layers (dl) were detailed, but the sections devoted to results obtained with solid electrodes and their discussion were brief. Some aspects of the dl on polycrystalline metals, such as the potential of zero charge (pzc), were described." In this series a chapter was also devoted to the metal-gas interface certainly the comparison of this interface with that at an electrode is fruitful, although the local electric field in the latter case can be varied far more easily. 

As a further example for the meaning of ex situ investigations of emersed electrodes with smface analytical techniques, results obtained for the double layer on polycrystalline silver in alkaline solutions are presented in figme C2.10.3. This system is of seientifie interest, since thin silver oxide overlayers (thickness up to about 5 nm) are formed for sufficiently anodic potentials, whieh implies that the adsorption of anions, cations and water can be studied on the elean metal as well as on an oxide eovered surface [55. 56]. For the latter situation, a changed... 

E. Lust, Double layers at single crystal and polycrystalline electrodes. Thermodynamics and Electrified Interfaces, Encyclopedia of Electrochemistry (Eds. E. Gileadi, M. Urbakh), Wiley-VCH, Germany, Vol. 1, Chap. 2.4. forthcoming. 

Electrical Double Layers. Double Layers at Single-crystal and Polycrystalline Electrodes... 

Electrical Double Layers. Double Layers at Single-crystal and Polycrystalline Electrodes 209 Tab. 4 Electrical double-layer parameters of Bi and Sb [5, 15, 16, 22] electrodes in various solvents... 

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