Electrode blocking

Artificial reasons due to (a) improper choice of the electrode morphology (reversible vs blocking electrode) and (b) inaccurate measurement of UWR. 

In conclusion When no catalytic reaction is taking place on the gas-exposed electrode surface, only poor experimentation (blocking electrodes, inaccurate measurement of Uwr> and of course O) can cause deviations from Eq. (5.18) in presence of ion backspillover. In presence of a catalytic reaction Eq. (5.18) still holds unless the reaction is severely mass transfer controlled or an insulating layer is built on the catalyst surface. 

In order to gain some additional physical insight on how spillover leads to the experimental equations (7.11) and (7.12) we will consider the solid electrolyte cell shown in Figure 7.10a and will examine the situation in absence of spillover (Equations (7.11) and (7.12) not valid) and in presence of spillover (Equations (7.11) and (7.12) valid). For simplicity we focus on and show only the working (W) and reference (R) electrodes which are deposited on a solid electrolyte (S), such as YSZ. The two porous, thus non-blocking, electrodes are made of the same metal or of two different metals, M and M. The partial pressures of 02 on the two sides of the cell are p02 and po2 Oxygen may chemisorb on the metal surfaces so that the work functions w(p02)and R(pb2). 

A constant phase element (CPE) rather than the ideal capacitance is normally observed in the impedance of electrodes. In the absence of Faradaic reactions, the impedance spectrum deviates from the purely capacitive behavior of the blocking electrode, whereas in the presence of Faradaic reactions, the shape of the impedance spectrum is a depressed arc. The CPE shows... 

Inhibition of Electron Transfer at Partially Blocked Electrodes... 

If we place an ionic conductor between parallel-plate blocking electrodes that produce an electric field E parallel to the x-axis, the electrostatic potential varies as — xE on passing from one electrode at x = 0 to the other. At equilibrium, the mobile-ion concentration Cj(x) is proportional to exp(qEx/kT), and the ionic drift-current density (7(E in the field is balanced by a diffusion current due to the concentration gradient (Fick s law) ... 

One of the major concerns which we have in relation to non-blocking electrodes is to understand the way in which the current crossing the interface varies with the overpotential tj defined as ... 

Fig. 10.13 Impedance plane diagrams for metal non-blocking electrodes with two mobile species in the electrolyte, (a) Interfacial impedance is only a Warburg impedance. (b) Interfacial impedance shows a charge transfer resistance semicircle.

The separator resistance is usually characterized by cutting small pieces of separators from the finished material and then placing them between two blocking electrodes. The separators are completely saturated with the electrolyte. The resistance (Q) of the separator is measured at a certain frequency by ac impedance techniques. The frequency is chosen so that the separator impedance is equal to the separator resistance. To reduce the measurement error, it is best to do multiple measurements by adding extra layers. The average resistance of single layer is determined from multiple measurements. The specific resistivity, ps cm), of the separator saturated... 

The principal features of the electrochemistry and construction of oxygen electrodes in alkaline solution, which were considered in Chapter 3 for primary cells, are common to secondary cells. If air is used rather than oxygen, it is then necessary to scrub the gas to remove carbon dioxide, since otherwise the electrolyte becomes progressively contaminated with carbonate which reduces the conductivity and may block electrode pores. 

Before presenting some applications of practical interest, the theoretical EHD impedance for partially blocked electrodes and for electrodes coated by a porous layer will be analyzed. 

In fact, the crucial point in the set-up, common to all variants, is the electromechanical part which must deliver at the output (i.e. at the RDE) a noise free modulation of the angular velocity indeed, regarding the theoretical requirements for some applications (e.g. partial blocked electrodes), well performing motors and servosystems are necessary. 

We will consider these cells, primarily the oxygen cells, under open circuit conditions and under load (or even short-circuit condition). In the transient and in the steady state it is not necessary to treat them all in detail, since (as outlined below) cells with one selectively blocking electrode and those with two of the same kind show far-reaching similarities (compare cell 3 with cell 4 and cell 5 with cell 6). The same is true if we compare cells with electrodes that are selectively blocking for electrons with cells that are specifically blocking for ions (compare cell 3 with cell 5 and cell 4 with cell 6) it is easy to show that the relations are symmetrical as regards the indices e" and O2" (see below and Appendix l).21011... 

Eq. (62) can take account of the dielectric response of a boundary (electrode, grain boundary) but would not be sufficient to describe bulk polarization phenomena (appearing at longer times or lower frequencies) induced by strongly selectively blocking electrodes or grain boundaries (see below). The latter effect will be touched upon in the next section. 

Electrochemical Polarization—The Effect of Selectively Blocking Electrodes... 

Figure 38. The voltage response on a galvanostatic polarization of a mixed conductor with ion-blocking electrodes ( eqc. (G3)3,15). Reprinted from J. Maier, Evaluation of Electrochemical Methods in Solid State Research and Their Generalization for Defects with Variable Charges , Z. Physik. Chemie N.F., 191-215, Copyright 1984 with permission from Oldenbourg Verlagsgruppe.

In the case of electron blocking the indices eon and ion have to be exchanged, but also the total equivalent circuit has to be complemented by circuit elements that take account of the bulk impedance of the blocking electrodes (e.g., YSZ). 

What has been ignored so far and will only be briefly mentioned is that a stoichiometric polarization is also caused by grain boundaries if the ratio of ionic and electronic conductivities differs from the bulk value, as it is usually the case.230 Figure 41 gives a clear example of this. In the general case of blocking electrodes and grain boundaries we expect even two stoichiometry polarization processes. 

Now we wish to consider the electrochemical polarization with the help of selectively blocking electrodes (connected with the neutral phase at x=L, while x=0 is the position of the reversible electrode contact) on a more fundamental level3 15 210 225 231 and refer, to be specific, to a galvanostatic experiment on cells 3 and 4. We start with the steady state. 

Herefrom the electronic conductivity is precisely arrived at by differentiating the current with respect to the oxygen potential at the contact of the blocking electrode (x=L). As... 

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