Solid electrolytes work function

Relationships are shown between the electrolyte redox couple (H /H2), the Helmholtz layer potential drop (V ), the semiconductor band gap (Eg), electron affinity Of), work function (sc), band bending (V0), and flat-band potential (Uf ). The electrochemical and solid state energy scales are shown for comparison. is the electrolyte work function. 

By 19884 it became obvious that the NEMCA effect, this large apparent violation of Faraday s law, is a general phenomenon not limited to a few oxidation reactions on Ag. Of key importance in understanding NEMCA came the observation that NEMCA is accompanied by potential-controlled variation in the catalyst work function.6 Its importance was soon recognized by leading electrochemists, surface scientists and catalysis researchers. Today the NEMCA effect has been studied already for more than 60 catalytic systems and does not seem to be limited to any specific type of catalytic reaction, metal catalyst or solid electrolyte, particularly in view of... 

Thus, as will be shown in this book, the effect of electrochemical promotion (EP), or NEMCA, or in situ controlled promotion (ICP), is due to an electrochemically induced and controlled migration (backspillover) of ions from the solid electrolyte onto the gas-exposed, that is, catalytically active, surface of metal electrodes. It is these ions which, accompanied by their compensating (screening) charge in the metal, form an effective electrochemical double layer on the gas-exposed catalyst surface (Fig. 1.5), change its work function and affect the catalytic phenomena taking place there in a very pronounced, reversible, and controlled manner. 

D.A. Emery, P.H. Middleton, and I.S. Metcalfe, The effect of electrochemical current pumping on the work function of solid electrolyte supported catalysts, Surf. Sci. 405, 308-315(1998). 

C.G. Vayenas, and D. Tsiplakides, On the work function of the gas-exposed electrode surfaces in solid state electrolyte cells, Surf. Sci. 467, 23-34 (2000). 

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. 

The Work Function of Catalyst Films Deposited on Solid Electrolytes... 

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. 

Over a wide range of conditions (again i.e., as long as ion backspillover from the solid electrolyte forms a double layer at the metal/gas interface)30"33 the potential difference eUWR is equal to the difference in work functions between the two electrodes... 

Figure 5.10. Transient response of catalyst work function O and potential Uwr upon imposition of constant currents I between the Pt catalyst (labeled26 C2) and the Pt counter electrode p"-A1203 solid electrolyte T = 240°C, p02 = 21 kPa Na ions are pumped to (I<0) or from (I>0) the catalyst surface at a rate I/F.26 Reprinted with permission from Elsevier Science.

Figure 5.19. The physical origin of NEMCA When a metal counter electrode (C) is used in conjunction with a galvanostat (G) to supply or remove ions [O2 for the doped Zr02 (a), Na+ for P"-A1203 (b)] to or from the polarizable solid electrolyte/catalyst (or working electrode, W) interface, backspillover ions [O6 in (a), Na5+ in (b)] together with their compensating charge in the metal are produced or consumed at the tpb between the three phases solid electrolyte/catalyst/gas. This causes an increase (right) or decrease (left) in the work function of the gas-exposed catalyst surface. In all cases AO = eAUWR where AUWr is the overpotential measured between the catalyst and the reference electrode (R).

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 significant point is that PEEM, as clearly presented in Figures 5.45 to 5.47, has shown conclusively that follows reversibly the applied potential and has provided the basis for space-and time-resolved ion spillover studies of electrochemical promotion. It has also shown that the Fermi level and work function of the solid electrolyte in the vicinity of the metal electrode follows the Fermi level and work function of the metal electrode, which is an important point as analyzed in Chapter 7. 

THE WORK FUNCTION OF CATALYST FILMS DEPOSITED ON SOLID ELECTROLYTES RATIONALIZATION OF THE POTENTIAL -WORK FUNCTION EQUIVALENCE... 

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). 

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). 

When ions migrate through a solid electrolyte, they diffuse from this onto the gas-exposed surface of the metal electrode. These ions form a double layer (and hence a potential difference) at the metal/gas interface. I Iowcver, this potential difference (which varies with the electrode potential) in turn changes the work function at the gas/metal interface. The ease of availability of electrons in the bonding of radicals adsorbed from the gas phase onto the electrode increases as the electronic work function of the solid decreases. The chemical reaction rate of the catalyzed reaction depends on the bonding strength of these radicals to the electrode catalyst, which involves electrons from the metal and is therefore dependent on the work function of the metal this itself is a function of the electrode potential. In this way, a dependence of the rate of the chemical reaction upon the potential of the working electrode can be rationalized. 

In this section the use of amperometric techniques for the in-situ study of catalysts using solid state electrochemical cells is discussed. This requires that the potential of the cell is disturbed from its equilibrium value and a current passed. However, there is evidence that for a number of solid electrolyte cell systems the change in electrode potential results in a change in the electrode-catalyst work function.5 This effect is known as the non-faradaic electrochemical modification of catalytic activity (NEMCA). In a similar way it appears that the electrode potential can be used as a monitor of the catalyst work function. Much of the work on the closed-circuit behaviour of solid electrolyte electrochemical cells has been concerned with modifying the behaviour of the catalyst (reference 5 is an excellent review of this area). However, it is not the intention of this review to cover catalyst modification, rather the intention is to address information derived from closed-circuit work relevant to an unmodified catalyst surface. 

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