Polycrystalline electrodes

The interfacial chemistry of palladium electrodes (polycrystalline and single crystal) was surveyed by Soriaga etal. recently [103]. According to the authors, the remarkably rich interfacial chemistry of palladium may have its origin in the anomalously weak inter-metallic palladium-palladium bond, a circumstance that is expected not only to enhance lateral surface mobihties but also to facilitate the disruption of substrate-substrate bonds and/or the formation of substrate-adsorbate bonds. 

Fig. 28. Difference spectrum in a 0.01 M methanol solution in 0.5 M HCIO4 obtained using a continuous flow of electrolyte. Working electrode polycrystalline Pt. Sample and reference potentials 500 and OmV vs. SCE. (After [95]). Reprinted by permission of Pergamon Press.

For pc-Au/electrolyte interfaces, Clavilier and Nguyen Van Huong256 also concluded that the crystallographic inhomogeneity factor depends on ce. Later, the influence of the crystallographic inhomogeneity of pc and monocrystalline electrodes (with various surface defects) was discussed in many papers.75,152,154 156 247-259 It has been shown that the potential of the diffuse-layer capacitance minimum for a polycrystalline electrode does not correspond to Ea=Q of the whole surface, i.e., 2J 0,< 0 at E n. 

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)).

Electrochemical impedance spectroscopy (EIS) in a sufficiently broad frequency range is a method well suited for the determination of equilibrium and kinetic parameters (faradaic or nonfaradaic) at a given applied potential.268,269 EIS has been used to study polycrystalline Au, Cd, Ag, Bi, Sb, and other electrodes.152249 270-273... 

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. 

The value of E for pc-Cd10,637,643 as well as for pc-Zn lies between the Ea= i values of these two groups of planes. <7=0 for a cut Cd(0001)c electrode is in good agreement with that for electrochemically polished Cd(0001). For chemically treated Cd electrodes, a remarkable dependence of min on Cnbf has been observed.249 These results are confirmed by data for other electrodes with a polycrystalline surface structure,10,74 and can... 

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... 

Polycrystalline electrodes are still used in many electrochemical experiments. For this reason, polycrystalline metals are still included in compilations of pzc as in this chapter, although the physical significance of such a quantity is ambiguous (see Section I). 

As already shown on Figures 5.8 and 5.11, Eqs. (5.18) and (5.19) have been obtained experimentally with polycrystalline Pt, Ag and Au catalyst electrodes deposited on YSZ and (3"-Al203. 

Can one use STM to study spillover/backspillover phenomena and to confirm the origin of electrochemical promotion The answer is positive and the experimental setup used for the first demonstration of electrochemically controlled spillover/backspillover between a catalyst-electrode (Pt) and a solid electrolyte (p"-Al203) is shown in Figure 5.48.78,79 A polished Pt(lll) single crystal (lOmmxlOmmxlmm) was mounted on an appropriately carved polycrystalline p"-Al203 sample (20mmx20mmx3mm). 

Methanol oxidation on Ag polycrystalline films interfaced with YSZ at 500°C has been in investigated by Hong et al.52 The kinetic data in open and closed circuit conditions showed significant enhancement in the rate of C02 production under cathodic polarization of the silver catalyst-electrode. Similarly to CH3OH oxidation on Pt,50 the reaction exhibits electrophilic behavior for negative potentials. However, no enhancement of HCHO production rate was observed (Figure 8.48). The rate enhancement ratio of C02 production was up to 2.1, while the faradaic efficiencies for the reaction products defined from... 

A polycrystalline synthetic diamond electrode was used, values of A o range between 6-10 and 2.4-10 cm s . 

Poly crystalline silver layer electrodeposited on a polycrystalline platinum electrode. 

Gregory WB, Norton ML, Stickney JL (1990) Thin-layer electrochemical studies of the underpotential deposition of cadmium and tellurium on polycrystalline Au, Pt and Cu electrodes. J Electroanal Chem 293 85-101... 

Cathodic deposition of lead sulfide from acidic aqueous solutions of Pb(II) ions (nitrate salts mainly) and Na2S203 on various metallic substrates at room temperature has been reported. Stoichiometric PbS films composed of small crystallites (estimated XRD diameter 13 nm) of RS structure were obtained at constant potential on Ti [204]. Also, single-phase, polycrystalline thin films of RS PbS were electrode-posited potentiostatically on Ti, Al, and stainless steel (SS) [205]. It was found that the Al and Ti substrates promoted growth of PbS with prominent (200) and (111)... 

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