Electrochemistry, solid-state

ULRICH SUMMING and HENGYONG TU (Part A) Technische Universitat, Munchen, Germany 

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H. Rickert, Solid State Electrochemistry An Introduction, Springer-Verlag, Berlin, 1982. 

Chapter 7 introduces the concept of absolute electrode potential in solid state electrochemistry. This concept has some important implications not only in solid state electrochemistry but also, potentially, in heterogeneous catalysis of supported catalysts. 

D. Tsiplakides, and C.G. Vayenas, Electrode work function and absolute potential scale in solid state electrochemistry, J. Electrochem. Soc. 148(5), E189-E202 (2001). 

C.G. Vayenas, On the work function of the gas exposed electrode surfaces in solid state electrochemistry, J. Electroanal. Chem. 486, 85-90 (2000). 

There is an important point to be made regarding UWr vs t transients such as the ones shown in Fig. 4.15 when using Na+ conductors as the promoter donor. As will be discussed in the next section (4.4) there is in solid state electrochemistry an one-to-one correspondence between potential of the working electrode (UWr) and work function (O) of the gas exposed (catalytically active) surface of the working electrode (eAUwR=AO, eq. 4.30). Consequently the UWr vs t transients are also AO vs t transients. 

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. 

P.J. Gellings, and HJ.M. Bouwmeester, eds., The CRCHandbook of Solid State Electrochemistry, CRC Press, Boca Raton (1997). 

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. 

Equation (5.18) plays a key role in understanding and interpreting the NEMCA effect and it is therefore important to discuss it in some detail. Equation (5.19) is discussed in detail in Chapter 7 in connection with the absolute potential scale of solid state electrochemistry. 

It must be emphasized that Equations (5.24) and (5.25) stem from the definitions of Fermi level, work function and Volta potential and are generally valid for any electrochemical cell, solid state or aqueous. We can now compare these equations with the corresponding experimental equations (5.18) and (5.19) found to hold, under rather broad temperature, gaseous composition and overpotential conditions (Figs. 5.8 to 5.16), in solid state electrochemistry ... 

What do we leam from this comparison of the general theoretical equations (5.24) and (5.25) with the specific experimental equations (5.19) and (5.18) of solid state electrochemistry The answer is mathematically obvious and physically important. In solid state electrochemistry one has ... 

In section 5.4.3 we have discussed the physical meaning and range of validity of the potential-work function equivalence equations of solid state electrochemistry ... 

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. 

Figure 5.22 reveals the ability of solid state electrochemistry to create new types of adsorption on metal catalyst electrodes. Here oxygen has been supplied not from the gas phase but electrochemically, as 02 via current application for a time, denoted tj, of 1=15 pA at 673 K, i.e. at the same temperature used for gaseous O2 adsorption (Fig. 5.21). Figure 5.23 shows the effect of mixed gaseous-electrochemical adsorption. The Pt surface has been initially exposed to po2 =4x1 O 6 Torr for 1800 s (7.2 kL) followed by electrochemical O2 supply (1=15 pA) for various time periods ti shown on the figure, in order to simulate NEMCA conditions. 

In view of the potential-work function equivalence of solid state electrochemistry (Eq. 4.30 or 5.18) and of the fact that for non-activated adsorption, AEd>Pt=0=A AHo,pt, where AHo.pt is the enthalpy of chemisorption of O on Pt, these equations can also be written as ... 

Several approaches have been proposed to measure the three phase boundary (tpb) length, Ntpb in solid state electrochemistry. The parameter Ntpb expresses the mol of metal electrode in contact both with the solid electrolyte and with the gas phase. More commonly one is interested in the tpb length normalized with respect to the surface area, A, of the electrolyte. This normalized tpb length, denoted by Ntpb,n equals Ntpt/A. 

When doing in situ XPS in solid state electrochemistry one must be aware of the following experimental realities 6,56 62... 

For reasons which will become apparent below, such experimental problems are minimized in solid state electrochemistry so that both the definition and the direct measurement of absolute electrode potentials is rather straightforward. 

This, at first perhaps surprising fact, is important to remember as the same situation arises in solid state electrochemistry. To understand its validity it suffices to remember that the definition of the reference (zero) energy level of electrons for the she scale is simply the state of an electron at the Fermi level of any metal in equilibrium with an aqueous solution of pH=0 and pH2=l atm at 25°C. 

ENERGY LEVEL OF ELECTRONS IN SOLID STATE ELECTROCHEMISTRY... 

Similarly to aqueous electrochemistry, potentials in solid state electrochemistry utilizing YSZ are expressed in terms of the potential of a reference metal electrode exposed to P02 = 1 atm at the temperature T of interest. Thus a standard oxygen electrode scale (soe) can be defined. Similarly to equation (7.2) one has ... 

Before discussing the experimental results, which by themselves suggest a unique choice of the reference (zero) state of electrons in solid state electrochemistry, which is the same with the choice of Trasatti for aqueous electrochemistry,14 16 it is useful to discuss some of the similarities and differences between aqueous and solid electrochemistry (Fig. 7.3). 

The presence of this backspillover formed effective double layer is important not only for interpreting the effect of electrochemical promotion, but also for understanding the similarity of solid state electrochemistry depicted in Fig. 7.3 with the case of emersed electrodes in aqueous electrochemistry (Fig. 7.2) and with the gedanken experiment of Trasatti (Fig. 7.1) where one may consider that H2O spillovers on the metal surface. This conceptual similarity also becomes apparent from the experimental results. 

The implications of Equations (7.11) and (7.12) are quite significant. They allow for the establishment and straightforward measurement of a unique absolute electrode potential scale in solid state electrochemistry. 

Ion spillover. The temperature is now increased to the point that ionic mobility on the electrode surfaces is high, so that now there is ion spillover. This is the usual case in solid state electrochemistry. 

Thus the key experimental observation Equation (7.11), is satisfied in presence of spillover. When an external overpotential AUWR is applied, with a concomitant current, I, and O2 flux I/2F, although UWR is not fixed anymore by the Nemst equation but by the extremally applied potential, still the work function Ow will be modified and Equations (7.11) and (7.12), will remain valid as long as ion spillover is fast relative to the electrochemical charge transfer rate I/2F.21 This is the usual case in solid state electrochemistry (Figs. 7.3b, 7.3d) as experimentally observed (Figs. 5.35, 5.23, 7.4, 7.6-7.9). 

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