Non-blocking electrodes

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). 

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.

So far, the ionic conductivity of most ILs has been measured by the complex impedance method [116], In this method, charge transfer between carrier ions and electrode is not necessary. Therefore platinum and stainless steel are frequently used as blocking electrodes. However, it is often difficult to distinguish the resistance and dielectric properties from Nyquist plots obtained by the impedance measurement. In order to clarify this, additional measurements using non-blocking electrodes or DC polarization measurement are needed. 

Figure 10 shows a solid electrolyte with two non-blocking electrodes. For non-blocking electrodes, no accumulation of charge occurs at the electrode-electrolyte interface.The Nyquist plot is expected to show a semicircle. The equivalent circuit may take the form of a resistor connected in parallel with a capacitor. In the presence of R, the expected Nyquist plot together with its equivalent circuit is depicted in Figure 11. On the other hand, for blocking electrodes, charge accumulates at the electrolyte-electrode interfaces. This contributes to the double layer capacitances at the interfaces, A vertical spike is thus expected to arise in the Nyquist plot due to the double layer capacitance (Figure 12).

FIGURE 10 Expected Nyquist plot and equivalent circuit for a solid electrolyte with two non-blocking electrodes. 

Fig. 27.1. Wagner polarization experiments, (a) Current is sent from the non-blocking electrode to the blocking electrode. Both ionic and electronic currents can be sustained, (b) Current is sent from the blocking electrode to the non-blocking electrode. There is no electrode reaction to sustain the ionic current, so only an electronic current can be sustained.

Test cell with one ionically blocking and one non-blocking electrode ultimately permits only electronic charge carriers discrimination is possible between electrons and and holes. 

Similar to many complex physical quantities, the real and the imaginary part of the complex conductivity are interconnected via Kramers-Kronig-relations. This implies that (7 co) and contain the same information and can be transformed into each other, provided that the complete experimental spectrum is known. In the work described in this review, both the real and the imaginary part of the complex conductivity were experimentally determined, but the discussion will focus on the real part of the conductivity. The dc conductivity is defined as the conductivity of the ion conducting material, which one would measure in the limit w 0 under conditions where the interface to the electrodes does not block ion transport ( non-blocking electrodes). 

A feature of the behaviour of such concentrated systems is the preponderance of ion pairs and triple ions. The ionic transport then includes contributions from cations such as M2X+ and anions such as MX2. Most techniques for determining the individual conductivities of different species in fact only discriminate between two classes those that are reversible at a given non-blocking electrode and those that are not. If the test cell is configured so that the contribution of the cations to the total ionic conductivity can be determined, then the cationic transference number, x+, can be obtained. This may be defined as ... 

In the case of measurements with non-blocking electrodes, there is both a source and a sink for the mobile ionic species. As in the case of blocking electrodes, however, DC measurements do not easily provide accurate values for R, . This is because, although ions can cross the boundary, electrons and/or holes cannot. Since these are the conducting species within the electrodes, the charge-transporting process has to be transferred to and from electrons and ions at the two interfaces. 

Figures 14-16 show the time dependence of the polarization current at different potentials for the polymer electrolytes (1), (2), and (3), respectively. The decrease in the polarization current with time became pronounced with increasing potential. However, the decrease in the current of (2), which has the fixed anion sites, was considerably lower than those of the others [(1) and (3)]. The stability of the polarization current has been observed in other polyelectrolyte-type polymers [96]. Because the lithium electrode is a non-blocking electrode for Li ions, but is a blocking electrode for the anions, the decrease seemed to be mainly due to the polarization of the anions. The stability of the polarization current in (2) may be concerned with its structure. Quasi-steady-state current was observed within 1 h when the applied potentials were lower than 0.1 V for (1) and (3), and 0.5 V for (2). The steady-state current values were lower than the current values calculated from / h + Re found in the complex impedance spectra for (1) and (3), whereas these current values were consistent with the calculated values from R, + / , for (2).

Non-blocking metal electrodes - one mobile charge in the electrolyte... 

The interface structure for non-blocking interfaces is similar to that for related blocking interfaces. Thus the distribution of charge at the C/ Ag4Rbl5 interface will be similar to that at the Ag/Ag4Rbl5 interface. The major difference is that there is one particular interfacial potential difference at which the silver electrode is in equilibrium with Ag ions in the bulk electrolyte phase. At this value of A, there is a particular charge on the electrolyte balanced by an equal and opposite charge — on the metal. At any potential different from value of q different... 

Fig. 10.12 Expected complex plane impedance diagram for an electrolyte with one mobile species which is contacted by two non-blocking metal electrodes, e.g. Ag/Ag Rblj/Ag. is the bulk resistance of the electrolyte and R is the charge transfer resistance for the Ag/Ag Rbls interface.

Deposition of a non-electroactive film on the surface of an electrode blocks the electron transfer from solution-based ions to the electrode. The efficiency of such blocking depends on the permeability of the film and the nature and density of defects, and heterogeneous electron transfer is routinely used to address these problems366. Capacitance measurements of the blocked electrodes also give valuable information about the thickness and integrity of the monolayer. These applications are described in Section II.D. 

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