The Absolute Electrode Potential

Although estimation of absolute electrode potentials falls outside of the realm of thermodynamics, it is of interest to theoreticians, especially those involved with the quantum-mechanical description of solids and liquids. These quantities allow one to define an absolute potential scale which is referenced to charge-free infinity. For example, when semiconductors are used as electrodes, it is helpful to compare the absolute potential for a redox couple in solution with that of the Fermi level in the electrode. 

Trasatti [2] has described the methods used to estimate the absolute electrode potential on the basis of suitable extrathermodynamic assumptions. The method presented here is the one which gives an estimate which can be related to the potential scale used by physicists. Moreover, the resulting estimates of the absolute values of the standard electrochemical potential are based on experimentally measured quantities. The analysis is illustrated here for cell (9.3.30), which contains a hydrogen electrode. An air gap is introduced into the cell, so that the solutions surrounding each electrode are separated. The resulting cell is 

If the Volta potential gap across the air gap is maintained at zero, then the potential drop across the cell is a compensation potential as described in section 8.7. Thus, one may write 

The two terms in square brackets may be identified with the absolute potentials of the silver silver chloride and hydrogen electrode, respectively. For the standard hydrogen electrode 

Both quantities needed to estimate may be obtained from experiment. 

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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...

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). 

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. 

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 ... 

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.

The determination of the real energies of solvation from measurements of the voltaic cells (Section VI) makes it possible to find the absolute electrode potentials in nonaqueous solvents owing to the relation... 

The emersed electrode, in principle, may be treated as the experimental realization of a single electrode. However, it is doubtful whether its liquid layer has the same bulk properties. This is probably the main reason for the different results of E°H(abs) found for emersed electrodes, e.g., -4.85 V.83 Samec et al. have found that emersion of electrodes in a nitrogen atmosphere decreases the Volta potential and therefore the absolute electrode potential by ca. 0.32 V relative to the value in solution. They have attributed this mainly to the reorientation of the water molecules at the free surface. 

The value of p defined by Eq. (29.6) is sometimes called the absolute electrode potential measured against vacuum. We must remember here that we are concerned with electrochemical potentials stated in electron volts rather than with electrostatic potentials stated in volts. Hence, this absolute potential, which can be determined... 

Thus the EMF has been separated into two terms, each containing a quantity related to a single electrode. If the surface potential of the electrolyte x(S) is added to each of the two expressions in brackets in Eq. (3.1.73), then the expression for the EMF contains the difference in the absolute electrode potentials for the absolute electrode potential of metal M we have... 

Unfortunately, as shown by Trasatti, because of water adsorption both these values cannot escape a certain ambiguity. The absolute electrode potential of the standard hydrogen electrode is most often reported as 4.44 0.02 V. Recent studies showed, however, that values between 4.2 and 4.8 V can also be considered. 

Neither electric fields nor absolute potentials can be directly measured in the interfacial region. Instead, potential differences are measured against a reference electrode. Although it cannot be directly measured, the absolute electrode potential may be defined as... 

Thus, the electrochemical potential difference between an electron in the solution and in the electrode is related to the absolute electrode potential. If the solution composition is assumed to be constant with potential, the chemical potential and dipole potential of the solution are constant. Thus, the ability of an electron to transfer across the interface for a given solution composition is controlled exclusively by the electrode potential. 

Since u,(M) and xsat are characteristic of specific combinations of electrodes and electrolyte solutions, they are constant. For an electrode S3 tem, thereby, the electrode potential is a function of the interfacial potential A u/s only. The electrode potential, E, defined in Eqn. 4—14 corresponds to what is called the absolute electrode potential. The reference zero level of the absolute electrode potential is set at the outer potential of the electrolyte solution in which the electrode is immersed. 

The relative electrode potential nhe referred to the normal (or standard) hydrogen electrode (NHE) is used in general as a conventional scale of the electrode potential in electrochemistry. Since the electrode potential of the normal hydrogen electrode is 4.5 or 4.44 V, we obtain the relationship between the relative electrode potentiEd, Ema, and the absolute electrode potential, E, as shown in Eqn. 4-36 ... 

Figure 4-25 compares the relative electrode potential, Etna, both with the (absolute) electrode potential, E, and with the real potential, of electrons in the... 

Fig. 4-25. Comparison between the real potential a.M

Is there any relevance of this new potential, work function, to electrochemistry The main idea is that because of its nature, the work function can be considered fingerprints of individual metals. If the electrode studied is a metal, then the work function is expected to be a relevant physical property in electrochemistry. It is involved in all electrochemical processes and accounts for effects observed on metals with different surface orientations. An example of these effects is given in Fig. 6.46. Obviously, different metals would have different chemical potentials, and that would account for the different values of d> in Fig. 6.46. But what about the differences observed, for example, for two of the crystalline faces of silver (Ag) For both crystals He is clearly the same thus the work function difference arises from different dipole layers at surfaces with different surface geometry. Another important involvement of in electrochemistry is in the determination of the absolute electrode potential, as will be explained in the next section. 

This equation defines the absolute electrode potential (Bockris and Argade, 1968). Table 6.1 shows a summary of the different definitions of potential that have been discussed and that are found in electrochemical systems. 

S. Trasatti, The Absolute Electrode Potential AnExplanatoryNote, Pure Appl. Chem. 58(7) 955 (1986). 

Then, an absolute electrode potential was defined, Z fabs). It was established that the absolute electrode potential for the reference hydrogen electrode has a value between -4.44 V and -4.78 V, and a scale of absolute potentials lor different reactions was obtained. This was an important step because knowledge of this scale allows one to predict the direction of electron flow when two electrodes are brought into electrical contact. 

The second and third terms on the right hands side of Eq. 9.8 remain constant for a given electrode-electrolyte system, and hence the electrode potential is a linear function of the interfacial potential A MIS of the electrode. This definition of the electrode potential holds valid for all electronic and ionic electrodes, whether the electrode reaction is in equilibrium or non-equilibrium. The potential defined by Eq. 9.8 is called the absolute electrode potential. 

In electrochemistry we have customarily employed, instead of the absolute electrode potential / abs scale, a relative scale of the electrode potential, E yila scale, referred to the standard or normal hydrogen electrode potential E m at which the hydrogen electrode reaction, 2H + 2e dox = H2(gas), is at equilibrium in the standard state unit activity of the hydrated proton, the standard pressure of 101.3 kPa for hydrogen gas, and room temperature of 298 K. Since Eniie is + 4.44 V (or + 4.5 V) in the absolute electrode potential scale, we obtain Eq. 9.9 for the relation between abs scile and [Refs. 4 and 5.] ... 

A a

chemical component is determined fey the difference of chemical potentials (pi is the -> chemical potential of the species i, and p, is its - electrochemical potential). Otherwise, when the points belong to two different phases, the experimental determination of the potential drop is impossible. In the literature, the term Galvani potential is also applied to the separate value of -> inner potential it was named initially as the -> absolute electrode potential. 

The Absolute Electrode Potential an Explanatory Note, (prepared for publication by S. Trasattl) PureAppL Chem. 58 (1986) 955. 

Trasatti S. (1986a), The absolute electrode potential—an explanatory note (recommendations 1986) , Pure Appl. Chem. 58, 955-966. 

Trasatti S. (1986b), Components of the absolute electrode potential—conceptions and misinterpretations . Mat. Chem. Phys. 15, 427-438. 

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