The Effective Double Layer

The obvious question then arises as to whether the effective double layer exists before current or potential application. Both XPS and STM have shown that this is indeed the case due to thermal diffusion during electrode deposition at elevated temperatures. It is important to remember that most solid electrolytes, including YSZ and (3 -Al2C)3, are non-stoichiometric compounds. The non-stoichiometry, 8, is usually small ( 10 4)85 and temperature dependent, but nevertheless sufficiently large to provide enough ions to form an effective double-layer on both electrodes without any significant change in the solid electrolyte non-stoichiometry. This open-circuit effective double layer must, however, be relatively sparse in most circumstances. The effective double layer on the catalyst-electrode becomes dense only upon anodic potential application in the case of anionic conductors and cathodic potential application in the case of cationic conductors. 

In a broad sense similar effective double layers can be formed via gaseous adsorption or evaporation (e.g. Na evaporated on Pt electrodes deposited on fT-A Ch has been shown to behave similarly to electrochemi-cally supplied Na). In other cases, such as the effective double layer formed upon anodic polarization of Pt deposited on YSZ, the electrochemically created effective double layer appears to be unique and cannot be formed via gaseous oxygen adsorption at least under realistic ( 300 bar) oxygen pressure conditions. 

An interesting observation about this effective double layer came from XPS, i.e. that the backspillover oxygen ions on the Pt surface are 

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. 

Vayenas, S. Bebelis, and S. Ladas, Dependence of Catalytic Rates on Catalyst Work Function, Nature 343, 625-627 (1990). 

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Figure 1.5. Schematic representation of a metal electrode deposited on a 02 -conducting (left) and on a Na -conducting (right) solid electrolyte, showing the location of the metal-electrolyte double layer and of the effective double layer created at the metal/gas interface due to potential-controlled ion migration (backspillover).

Thus the driving force for O2 backspillover from YSZ to the gas exposed, i.e. catalytically active, electrode surface exists and equals jJ02 (YSZ) - jl02- (M). It vanishes only when O2 backspillover has taken place and established the effective double layer shown in Fig. 3.6. 

Figure 5.18. Schematic representation of the density of states N(E) in the conduction band and of the definitions of work function d>, chemical potential of electrons p, electrochemical potential of electrons or Fermi level p, surface potential x> Galvani (or inner) potential

What is the physical implication of the experimental equations (5.26) to (5.28) They simply reflect the fact that the effective double layer formed (via ion spillover) at the metal/gas interface is, as every double layer, overall neutral. 

This does not imply that this double layer is at its point of zero charge (pzc). On the contrary, as with every other double layer in electrochemistry, there exists for every metal/solid electrolyte combination one and only one UWr value for which this metal/gas double layer is at its point of zero charge. These critical Uwr values can be determined by measuring the dependency onUWR of the double layer capacitance, Cd, of the effective double layer at the metal/gas interface via AC Impedance Spectroscopy as discussed in Chapter 5.7. 

Figure 5.58. Schematic of the effective double layer during C2H4 oxidation on Pt/YSZ (top) and Pt/p"-Al203.

In view of the assumed lack of individual lateral adsorbate-adsorbate interactions the only electrostatic energy to be accounted for in expressing the electrochemical potential, p j, of the adsorbate is the electrostatic energy of interaction of the adsorbate dipole with the effective double layer field. This is accounted for by ... 

As shown in Figure 5.26 and also Figs. 2.6 and 2.15 there is excellent agreement between Eq. (6.40) and experiment. Equation 6.40 is also in excellent qualitative agreement with rigorous quantum mechanical calculations (Fig. 5.56). This provides solid support for the effective double layer isotherm (Eq. 6.36). 

In general Figures 6.18 to 6.25, and in particular figures 6.18, 6.19, 6.20, 6.24 and 6.25 show, beyond any reasonable doubt, that the effective double layer model of promotion, expressed mathematically by Equations 6.65 and 6.66, grasps the essence of promotional kinetics. 

In Chapter 5 we have discussed in detail the nature of the effective double layer formed at the metal/gas interface of metals deposited on solid electrolyte. 

Figure 7.4. STM images (unfiltered) of a Pt(lll) surface interfaced with P"-A120j28 in ambient air showing the (a) sodium-cleaned and (b) sodium-dosed surface. Note (a) the Pt(l 1 l)-(2x2)-0 adlatice and the reversible appearance (b) of the Pt(l I l)-(12xl2)-Na adlayer (Ut = +100 mV, I, = 1.8 nA, total scan size 319 A).28 Reprinted with permission from Elsevier Science (c) STM images (unfiltered) of the effective double layer formed by the Nas+ (12x12) - Na adlayer on a Pt surface consisting mainly of Pt(l 11) planes and interfaced with p"-A1203.21,34 Each sphere is a Na atom. Reprinted with permission from The Electrochemical Society.

It must be emphasized that the effective double layer is overall neutral, as the backspillover species (O6, Na6+) are accompanied by their compensating (screening) charge in the metal.32,3,35,36 It must also be clarified that this backspillover formed effective double layer is not in general at its pzc (point of zero charge). This happens only at a specific value of the electrode potential, as in aqueous electrochemistry.37... 

The constancy of 0R with changing potential is also remarkable, as expected for a reference electrode. The deviation from Eq. (7.11) for negative potentials is due to the removal of O2 and concomitant destruction of the effective double layer. 

It is important to note that equation (7.11), and thus (7.12) is valid as long as the effective double layer is present at the metal/gas interfaces. Therefore equation (7.11) is valid not only under open-circuit conditions (which is the case for the Nernst equation) but also under closed-circuit conditions, provided, of course, that the working electrode effective double layer is not destroyed. Consequently the importance of equation (7.11) is by no means trivial. 

As shown in Chapter 4 (section 4.5.9.2), Equation (8.14) can also be derived via a rigorous electrostatic model which takes into account the presence of the effective double layer on the catalyst surface and gives in general ... 

That the synergistic action of an electron donor (Na8+) and electron acceptor (O5") promoter can cause dramatic enhancement in rate and selectivity. This is very likely due to the increase in the field strength, E, of the effective double layer discussed in Chapters 5 and 6 and to the concomitant enhanced interaction with the adsorbate dipoles, leading to more pronounced promotional behaviour (Chapter 6). 

As already discussed in Chapter 6 (Figure 6.25) the observed complex rate dependence of CO oxidation on pco, P02 and UWR (O) (Figs. 4.16, 4.31, 9.6 and 9.7) can be described in a semiquantitalive fashion by the effective double layer model presented in Chapter 6. The system provides an excellent paradigm of the promotional rules Gl, G2 and G3 which are summarized by the general inequalities (6.11) and (6.12) written specifically here for the CO oxidation system ... 

The reason is that the backspillover ions desorb to the gas phase directly from the three-phase-boundaries or react directly at the three-phase-boundaries (electrocatalysis, A=l) before they can migrate on the gas-exposed electrode surface and promote the catalytic reaction. The limits of NEMCA are set by the limits of stability of the effective double layer at the metal/gas interface. 

As indicated above, the Ap technique has been applied to several other phenomena involving Pt-based electrocatalysts. The first report of Ap applied to operating Pt electrocatalysts was based on Hads at anodic potentials. The nature of Ha on Pt, and its contribution to the effective double layer, had long been a matter of debate. " Ap analysis of Pt Lmn XANES showed the H to be highly delocalized, and hopping between one-fold and three-fold (fee) sites on the Pt surface. While prior research had pointed to such activity, the realistic extent in respect to potential was murky due to the nature of the analytical techniques (e.g., IR spectroscopy, UHV studies, etc.) employed. The study by Teliska et al., ... 

The effective double-layer concept is an important one, as it shows that electrochemical promotion, but also promotion and, as we shall see, metal-support-... 

Equations (29), (30), and (33) are valid both at open-circuit and under closed-circuit, i.e., NEMCA, conditions, in the presence or absence of catalytic reactions as long as the effective double layer is present at the metal-gas interface [140]. 

Deviations from Eqs. (29), (30), and (33) occur when the effective double layer at the metal-gas interfaces is destroyed [140]. This is the case for (1) very low temperatures (<250 °C for YSZ, <100 °C for )8"-Al203) where ion spillover-backspillover is kinetically frozen, or (2) very high temperatures (>500 °C for YSZ, >400 °C for P"-A 203) where the effective double layer desorbs (3) fast diffusion-controlled catalytic reactions, which again destroy the double layer (4) formation of insulating carbonaceous or oxidic deposits at the metal-gas interface which allow for the storage of electric charge [140]. 

The technique of AC impendance spectroscopy, one of the most common techniques in aqueous and solid-state electrochemistry, was used recently to confirm the formation of the effective double layer on metal surfaces interfaced with YSZ [43,95]. An example for the case of Pd/YSZ is shown in Figure 20. The semicircle labeled Ci is associated with the charge-transfer reaction... 

The hrst success of the effective double-layer isotherm [Eq. (48)] is that it predicts the experimentally observed linear variation of isosteric heat of adsorption or chemisorptive binding energy Ej with work function (Figures 11, 12, 24, 25) ... 

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