Absolute electrode potentials

Fig. 4-12. Electron energy levels in electron transfer from a standard gaseous electron throu an electrol3rte solution into an electrode a,(M/sn)) = real potential of electron in electrode E = electrode potential (absolute electrode potential).

Fig. 4-16. Energy levels of metal ion and electron in an ionic electrode of metal ion transfer 4Cjn i = sublimation energy of solid metal /m" = ionization energy of gaseous metal atoms > >s = outer potential of electrolyte solution E s electrode potential (absolute electrode potential).

In the cell used for measuring the electrode potential, in which the two electrodes are immersed in a single phase of electrolyte solution, the outer potential, tps, ofthe test electrode-solution is equal to the outer potential, ips, of the reference electrode-solution as shown in Fig. 4—24. The difference in the Fermi level of electrons, CFtu)- between the test electrode M and the reference electrode M , then, is represented by the difference in the real potential of electrons, M/aw) - .(M0/ V). tuid hence by the difference in the electrode potential (absolute electrode potential), AE = E-E°, between the two electrodes. This difference also equals the difference in the work function, 4>no/3/v - 4>ji/s/v> between the two electrodes. Thus, the potential E of the test electrode relative to the reference electrode is the difference in the electrode potential (absolute electrode potential) between the two electrodes as indicated in Eqn. 4-35 . 

Fig. 4-24. Electron energy levels for electrode potential relative to a reference electrode E = electrode potential (absolute) E = relative electrode potential Ps = outer potential of electrolyte solution of test electrode = outer potential of electrolyte solution of reference...

Absolute single electrode potential is a characteristic property of a metal. All metals have characteristic electrode potentials. Absolute single electrode potentials cannot be measured directly. They can be measured with respect to a standard electrode such as standard hydrogen electrode (SHE). The standard hydrogen electrode is a widely used standard electrode. It is arbitrarily assumed to have zero potential at all temperatures by definition. Thus, it provides a zero reference point for electrode potentials. The construction of the standard hydrogen electrode is shown in Fig. 2.10. As shown in the figure. 

Fig. 10.9 Diagram illustrating the source of the IR error in potential measurements on a cathodically protected structure. BA is the absolute electrode potential of the structure CD is the absolute electrode potential of the anode and CB is the field gradient in the environment due to cathodic protection current flux. A reference electrode placed at E will produce an IR error of EFin the potential measurement of the structure potential. If placed at G the error will be reduced to GH. At B there would be no error, but the point is too close to the structure to permit insertion of a reference electrode. If the current is interrupted the field immediately becomes as shown by the dotted line, and no IR is included...

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. 

H J. Reiss, The Absolute Electrode Potential. Tying the Loose Ends, J. Electrochem. Soc. 135, 247C-258C (1988). 

Z. Samec, B.W. Johnson, and K. Doblhofer, The absolute electrode potential of metal electrodes emersed from liquid electrolytes, Surf. Sci. 264, 440-448 (1992). 

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. 

It will also be shown that the absolute electrode potential is not a property of the electrode but is a property of the electrolyte, aqueous or solid, and of the gaseous composition. It expresses the energy of solvation of an electron at the Fermi level of the electrolyte. As such it is a very important property of the electrolyte or mixed conductor. Since several solid electrolytes or mixed conductors based on ZrC>2, CeC>2 or TiC>2 are used as conventional catalyst supports in commercial dispersed catalysts, it follows that the concept of absolute potential is a very important one not only for further enhancing and quantifying our understanding of electrochemical promotion (NEMCA) but also for understanding the effect of metal-support interaction on commercial supported catalysts. 

Trasatti14 16 has done a very thorough and lucid work in clarifying the concept of absolute electrode potentials in aqueous electrochemistry. He has pointed out that at least four different absolute, or single , electron potentials can be defined, depending on the choice of the reference state of electrons. 

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. 

Thus in order to utilize Eq. (7.23) and to work with absolute electrode potentials, one only needs to choose a reference temperature and to know U°2(abs). 

It is thus clear from the previous discussion that the absolute electrode potential is not a property of the electrode material (as it does not depend on electrode material) but is a property of the solid electrolyte and of the gas composition. To the extent that equilibrium is established at the metal-solid electrolyte interface the Fermi levels in the two materials are equal (Fig. 7.10) and thus eU 2 (abs) also expresses the energy of transfering an electron from the Fermi level of the YSZ solid electrolyte, in equilibrium with po2=l atm, to a point outside the electrolyte surface. It thus also expresses the energy of solvation of an electron from vacuum to the Fermi level of the solid electrolyte. 

Equation (7.32) underlines the pinning of the Fermi levels of metal electrodes with the solid electrolyte and reminds the fact that the absolute electrode potential is a property of the solid electrolyte and of the gaseous composition but not of the electrode material.21... 

S. Trasatti, The absolute electrode potential An explanatory note, Pure and Applied Chemistry 58, 955-966 (1986). 

S. Trasatti, The "absolute" electrode potential - The end ofthe story, Electrochim. Acta 35, 269-271 (1990). 

Can we measure the absolute electrode potential in solid state electrochemistry ... 

Does the concept of absolute electrode potential, defined in chapter 7, allow one to measure the absolute electrical potential difference, A(p, at a metal/electrolyte interface, one of the famous unresolved problems in electrochemistry ... 

Measurement of the differential capacitance C = d /dE of the electrode/solution interface as a function of the electrode potential E results in a curve representing the influence of E on the value of C. The curves show an absolute minimum at E indicating a maximum in the effective thickness of the double layer as assumed in the simple model of a condenser [39Fru]. C is related to the electrocapillary curve and the surface tension according to C = d y/dE. Certain conditions have to be met in order to allow the measured capacity of the electrochemical double to be identified with the differential capacity (see [69Per]). In dilute electrolyte solutions this is generally the case. 

Knowledge of the Volta potential of a metal/solution interface is relevant to the interpretation of the absolute electrode potential. According to the modem view, the relative electrode potential (i.e., the emf of a galvanic cell) measures the value of the energy of the electrons at the Fermi level of the given metal electrode relative to the metal of the reference electrode. On the other hand, considered separately, the absolute value of the electrode potential measures the work done in transferring an electron from a metal surrounded by a macroscopic layer of solution to a point in a vacuum outside the solotion. ... 

Figure 8. The physical meaning of the absolute electrode potential.

Knowledge of the value of ij (abs) makes it possible to convert all relative values of electrode potential to an absolute scale. For instance, the standard electrode potentials of the oxygen electrode, the zero charge of mercury, and the hydrated electron, in the absolute scale are equal to -5.67,. 25, and 1.57 V, recpectively. ... 

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