Phase reversal electrodes

Figure 4.14 Three different electrooptic waveguide devices [1] (a) Mach-Zehnder modulator with push-pull arrangement of the electrodes, (b) electrically controlled directional coupler, (c) coupler with phase-reversal electrodes.

In practice, for a ternary system, the decomposition voltage of the solid electrolyte may be readily measured with the help of a galvanic cell which makes use of the solid electrolyte under investigation and the adjacent equilibrium phase in the phase diagram as an electrode. A convenient technique is the formation of these phases electrochemically by decomposition of the electrolyte. The sample is polarized between a reversible electrode and an inert electrode such as Pt or Mo in the case of a lithium ion conductor, in the same direction as in polarization experiments. The... 

The detection of the AC component allows one to separate the contributions of the faradaic and charging currents. The former is phase shifted 45° relative to the applied sinusoidal potential, while the background component is 90° out of phase. The charging current is thus rejected using a phase-sensitive lock-in amplifier (able to separate the in-phase and out-of-phase current components). As a result, reversible electrode reactions yield a detection limit around 5 x 10 7m. 

Thus, the Volta potential may be operationally defined as the compensating voltage of the cell of Scheme 16. However, it should be stressed that the compensating voltage of a voltaic cell is not always the direct measure of the Volta potential. The appropriate mutual arrangement of phases, as well as application of reversible electrodes or salt bridges in the systems, allows measurement of not only the Volta potential but also the surface and the Galvani potentials. These possibilities are schematically illustrated by [15]... 

The case of the prescribed material flux at the phase boundary, described in Section 2.5.1, corresponds to the constant current density at the electrode. The concentration of the oxidized form is given directly by Eq. (2.5.11), where K = —j/nF. The concentration of the reduced form at the electrode surface can be calculated from Eq. (5.4.6). The expressions for the concentration are then substituted into Eq. (5.2.24) or (5.4.5), yielding the equation for the dependence of the electrode potential on time (a chronopotentiometric curve). For a reversible electrode process, it follows from the definition of the transition time r (Eq. 2.5.13) for identical diffusion coefficients of the oxidized and reduced forms that... 

To arrive at an understanding of the distribution of charge and potential near an interface, it is helpful to consider an electrode. A reversible electrode is one in which each of the phases contains a common ion that is free to cross the interface. The system Ag-Agl-aqueous solution is an example of a reversible electrode. A polarizable electrode, on the other hand, is impermeable to charge carriers, although charge may be brought to the surface by the application of an external potential. The system metallic Hg-aqueous solution is an example of a polarizable electrode we discussed the relationship among the applied potential, the interfacial tension, and the adsorption of ions in Chapter 7, Section 7.11. 

Electrodes can be reversible for chemical components and/or reversible for electrons. The case of electron reversible electrodes has already been treated in Section 4.4.2 (polarization cell). If the decomposition voltage of the phases located between inert electrodes is surpassed, their inertness is lost and they behave as if both electrons and components are available. This will be discussed further below. Let us first refer to Figure 8-12. If the electrodes are detached from the reacting system, the con-... 

The basic elements of a new electrochemical approach to saline water demineralization under study at the University of Oklahoma for the past three years are two porous electrodes, one of which is responsive to cations and the other to anions. When an appropriate voltage is applied to such an electrode pair immersed in saline water, cations are removed by the former and anions by the latter. In the regeneration phase, reversal of voltage gives up these ions to a reject solution. 

For reversible electrode reactions and no solution phase reactions other than disproportionation, the following reaction scheme (EE mechanism) applies (see also Sect. 3.3) ... 

Example. The surfaces of dispersed Agl particles can be considered similarly to an Ag-Agl-aqueous solution reversible electrode (i.e., each phase contains a common ion that can cross the interface). Here both Ag+ and I- will be potential determining ions because either may adsorb at the interface and change the surface potential. In this case, NaN03 is an example of an indifferent electrolyte as far as the electrode potential goes. 

The capillary plasma reactor consists of a Pyrex glass body and mounted electrodes which are not in direct contact with the gas flow in order to eliminate the influence of the cathode and anode region on CO2 decomposition. Analysis of downscaling effects on the plasma chemistry and discharge characteristics showed that the carbon dioxide conversion rate is mainly determined by electron impact dissociation and gas-phase reverse reactions in the capillary microreactor. The extremely high CO2 conversion rate was attributed to an increased current density rather than to surface reactions or an increased electric field. 

Figure 17 shows different mechanistic pathways for the oxygen reduction at the LSM cathode on YSZ electrolyte. The adsorbed, partially fully ionized oxygen may move along the surface to the three phase boundary where it is transformed into the electrolyte. (In principle it may also reach this place directly via the gas phase.) The oxygen may also reach the electrolyte by diffusion through the LSM bulk via a counter motion of O2 and 2e . Note that LSM sandwiched between Pt (serving as a reversible electrode) and YSZ... 

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. 

This technique can be even applied if the conditions (ii), (iii), and (iv) are not observed. In the latter case, however, regression analysis of the I-U dependencies requires to define explicit relationships between chemical potentials of all components, their concentrations, and mobilities. In practice, experimental problems are often observed due to leakages, non-negligible -> polarization of reversible electrodes, indefinite contact area between solid electrolyte and electronic filter, formation of depletion layers and/or phase decomposition of the electrolyte. 

The experimentally measured reversible electrode potential, E q, includes not only the above emf but also the potential difference at the metal-platinum contact. The electrons are the electromotively active particles at this junction, and it may be assumed that at equilibrium an electrical potential difference exists between the two metals which equalizes the electrochemical potential of the electrons in the two phases. As is well known, it is equivalent to the Volta potential difference and is given by the following ... 

For the ideally polarizable interphase, they are all independent. For the ideally nonpolarizable interphase, only two can be controlled independently. We recall that an ideally nonpolarizable electrode is a reversible electrode. By setting the concentrations (more accurately, the activities) of ions in the two phases, we determine the potential. Alternatively, by selling the potential, we determine the ratio of concentrations of this ion in the two phases. We conclude that the electrocapillary equation for the nonpolarizable interphase must have one less degree of freedom. 

In exceptional cases it might be possible that the transition of ions from the surface to the solution or in the inverse direction needs an activation energy. That such a barrier at the interface of two phases may metimes l>e present is suggested by certain phenomena (overvoltage, etc.) observed in electrolytic processes. In that case adjustment of the charge would occur slowly, and the assumption that the double layer charge is a constant, independent of the particle distance, would then be a more suitable approximation h In a case like that of Agl, behaving as a perfectly reversible electrode, and in many other systems, the assumption -Pq — constant will be more correct. 

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