Solid electrolytes cells

In a typical NEMCA experiment the reactants (e.g. C2H4+02) are co-fed over a conductive catalyst which also serves, at the same time, as the working electrode in a solid electrolyte cell ... 

Wagner was first to propose the use of solid electrolytes to measure in situ the thermodynamic activity of oxygen on metal catalysts.17 This led to the technique of solid electrolyte potentiometry.18 Huggins, Mason and Giir were the first to use solid electrolyte cells to carry out electrocatalytic reactions such as NO decomposition.19,20 The use of solid electrolyte cells for chemical cogeneration , that is, for the simultaneous production of electrical power and industrial chemicals, was first demonstrated in 1980.21 The first non-Faradaic enhancement in heterogeneous catalysis was reported in 1981 for the case of ethylene epoxidation on Ag electrodes,2 3 but it was only... 

Electrocatalysis Again by definition, an electrocatalyst is a solid, in fact an electrode, which can accelerate a process involving a net charge transfer, such as e.g. the anodic oxidation of H2 or the cathodic reduction of 02 in solid electrolyte cells utilizing YSZ ... 

Electrochemical promotion or NEMCA is the main concept discussed in this book whereby application of a small current (1-104 pA/cm2) or potential ( 2 V) to a catalyst, also serving as an electrode (electrocatalyst) in a solid electrolyte cell, enhances its catalytic performance. The phenomenology, origin and potential practical applications of electrochemical promotion, as well as its similarities and differences with classical promotion and metal-support interactions, is the main subject of this book. 

M. Stoukides, and C.G. Vayenas, The effect of electrochemical oxygen pumping on the Rate and Selectivity of Propylene Oxidation on Silver in a Solid Electrolyte Cell, J. Electrochem. Soc. 131(4), 839-845 (1984). 

CATALYST WORK FUNCTION VARIATION WITH POTENTIAL IN SOLID ELECTROLYTE CELLS... 

The implications of Equation (4.30) for solid state electrochemistry and electrochemical promotion in particular can hardly be overemphasized It shows that solid electrolyte cells are both work function probes and work function controllers for their gas-exposed electrode surfaces. 

I. Riess, and C.G. Vayenas, Fermi level and potential distribution in solid electrolyte cells with and without ion spillover, Solid State Ionics, in press (2001). 

J. Yi, A. Kaloyannis, and C.G. Vayenas, High Temperature cyclic voltammetry of Pt electrodes in solid electrolyte cells, Electrochim. Acta 38(17), 2533-2539 (1993). 

Work function, a quantity of great importance in surface science and catalysis, plays a key role in solid state electrochemistry and in electrochemical promotion. As will be shown in Chapter 7 the work function of the gas-exposed surface of an electrode in a solid electrolyte cell can be used to define an absolute potential scale in solid state electrochemistry. 

Solid electrolyte cells can be used to alter significantly the work function catalytically active, catalyst electrode surface by polarizing the catalyst-solid electrolyte interface. 

Furthermore, to the extent that the entire solid electrolyte cell under consideration is overall neutral, i.e. carries no net charge, one can show using Gauss s law of electrostatics, that the constant C in Eq. (5.27) is zero, i.e. that ... 

Figure 5.20. Left Schematic of an O2 conducting solid electrolyte cell with fixed P02 and PO2 values at the porous working (W) and reference (R ) electrodes without (top) and with (bottom) ion backspillover on the gas exposed electrodes surfaces, showing also the range of spatial constancy of the electrochemical potential, PQ2-, of O2. Right Corresponding spatial variation in the electrochemical potential of electrons, ]Ie(= Ef) UWR is fixed in both cases to the value (RT/4F)ln( P02 /pc>2 ) also shown in the relative position of the valence band, Ev, and of the bottom of the conduction band, Ec, in the solid electrolyte (SE) numerical values correspond to 8 mol% Y203-stabilized-Zr02, pc>2=10 6 bar, po2=l bar and T=673 K.32 Reproduced by permission of The Electrochemical Society.

Consider the solid electrolyte cell shown in Figure 5.20. For simplicity we consider only a working (W) and reference (R) electrode deposited on a solid electrolyte, such as YSZ or p"-Al203. The two 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 workfunctions w and R(p 02). 

Equations (5.18) and (5.19), particularly the latter, have only recently been reported and are quite important for solid state electrochemistry. Some of then-consequences are not so obvious. For example consider a solid electrolyte cell Pt/YSZ/Ag with both electrodes exposed to the same P02, so that Uwr = 0. Equation (5.19) implies that, although the work functions of a clean Pt and a clean Ag surface are quite different (roughly 5.3 eV vs 4.7 eV respectively) ion backspillover from the solid electrolyte onto the gas exposed electrode surfaces will take place in such a way as to equalize the work functions on the two surfaces. This was already shown in Figs. 5.14 and 5.15. 

The experiments were carried out in ambient air.78 79 STM images were obtained at 300 K following current, I, or potential, Uwr, application in ambient air at 550 K. Figure 5.49 shows an unfiltered atomic resolution image of the Pt (111) surface after assembling the solid electrolyte cell before any current or potential application. 

Figure 7.5. (a) Solid electrolyte cell consisting of an YSZ disk with working (Pt), reference (Au, Ag) and counter electrodes (Au). (b) Schematic diagram of the electrochemical reactor.21 Reprinted with permission from The Electrochemical Society. 

In summary, the creation via ion spillover of an effective electrochemical double layer on the gas exposed electrode surfaces in solid electrolyte cells, which is similar to the double layer of emersed electrodes in aqueous electrochemistry, and the concomitant experimentally confirmed equation... 

Equations (7.11) and (7.12) provide a firm basis for understanding the effect of Electrochemical Promotion but also provide an additional, surface chemistry, meaning to the emf of solid electrolyte cells in addition to its usual Nerstian one. 

One can then examine YSZ solid electrolyte cell with metal electrodes,... 

N.G. Torkelsen, and S. Raaen, Work function variations and oxygen conduction in a Pt/ZrC>2(Y203)/Pt solid electrolyte cell, Appl. Surf. Sci. 93, 199-203 (1996). 

Fermi level of electrons and absolute potential, 346 distribution in a solid electrolyte cell, 219, 357... 

AgsSBr, /3-AgsSI, and a-AgsSI are cationic conductors due to the structural disorder of the cation sublattices. AgsSI (see Fig. 5) has been discussed for use in solid-electrolyte cells (209,371, 374,414-416) because of its high silver ionic conductivity at rather low temperatures (see Section II,D,1). The practical use seems to be limited, however, by an electronic part of the conductivity that is not negligible (370), and by the instability of the material with respect to loss of iodine (415). 

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