Interface, electrode potential

In addition to metal-electrolyte interfaces, electrode potentials can be produced by ion transport through an ion-selective semipermeable membrane. In this case, the membrane is interposed between two liquid phases. Reversible transfer of a selected ion occurs through the membrane. For an ideal membrane, the developed electrode potential E is given by the Nernst equation... 

However, as during corrosion no net current will pass the interface, the theory of electrochemical reaction kinetics will have to be applied in order to calculate the current density under free corrosion conditions. This current density is called the corrosion current density. For a corroding surface under simple electrochemical conditions (no mass transfer effect), the relation between the current density and its driving force, the potential drop across the interface (electrode potential), is given by the Butler-Volmer equation... 

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

This handbook deals only with systems involving metallic materials and electrolytes. Both partners to the reaction are conductors. In corrosion reactions a partial electrochemical step occurs that is influenced by electrical variables. These include the electric current I flowing through the metal/electrolyte phase boundary, and the potential difference A( = 0, - arising at the interface. and represent the electric potentials of the partners to the reaction immediately at the interface. The potential difference A0 is not directly measurable. Therefore, instead the voltage U of the cell Me /metal/electrolyte/reference electrode/Me is measured as the conventional electrode potential of the metal. The connection to the voltmeter is made of the same conductor metal Me. The potential difference - 0 is negligibly small then since A0g = 0b - 0ei ... 

Thus the tendency for an electrochemical reaction at a metal/solution interface to proceed in a given direction may be defined in terms of the relative values of the actual electrode potential E (experimentally determined and expressed with reference to the S.H.E.) and the reversible or equilibrium potential E, (calculated from E and the activities of the species involved in the equilibrium). 

The thermodynamic and electrode-kinetic principles of cathodic protection have been discussed at some length in Section 10.1. It has been shown that, if electrons are supplied to the metal/electrolyte solution interface, the rate of the cathodic reaction is increased whilst the rate of the anodic reaction is decreased. Thus, corrosion is reduced. Concomitantly, the electrode potential of the metal becomes more negative. Corrosion may be prevented entirely if the rate of electron supply is such that the potential of the metal is lowered to the value where it is found that anodic dissolution does not occur. This may not necessarily be the potential at which dissolution is thermodynamically impossible. 

It is apparent that since the electrode potential of a metal/solution interface can only be evaluated from the e.m.f. of a cell, the reference electrode used for that purpose must be specified precisely, e.g. the criterion for the cathodic protection of steel is —0-85 V (vs. Cu/CuSOg, sat.), but this can be expressed as a potential with respect to the standard hydrogen electrode (S.H.E.), i.e. -0-55 V (vs. S.H.E.) or with respect to any other reference electrode. Potentials of reference electrodes are given in Table 21.7. 

Controlled-potential (potentiostatic) techniques deal with the study of charge-transfer processes at the electrode-solution interface, and are based on dynamic (no zero current) situations. Here, the electrode potential is being used to derive an electron-transfer reaction and the resultant current is measured. The role of the potential is analogous to that of the wavelength in optical measurements. Such a controllable parameter can be viewed as electron pressure, which forces the chemical species to gain or lose an electron (reduction or oxidation, respectively). 

If the concentration of the metal ion is not negligible at the potential of zero charge, the electrode potential varies linearly with log c according to Eq. (2) and there is no distinctive sign of the situation where the charge at the interface vanishes. The Nemst approach is obviously unsuitable for defining the nature and the amount of the charge at an interface. If the concentration of the metal ion at the pzc is small or very small, the behavior of the interface becomes that of a polarizable electrode. 

Equation (17) shows the relationship between electrode potentials and electronic energy. The electrode potential is measured by the electron work function of the metal, modified by the contact with the solution (solvent). This establishes a straightforward link, not only conceptually but also experimentally, between electrochemical and UHV situations.6,32 In many cases, electrochemical interfaces are synthesized in UHV conditions55-58 by adding the various components separately, with the aim possibly of disentangling the different contributions. While the situation can be qualitatively reproduced, it has been shown above that there may be quantitative differences that are due to the actual stmctural details. 

In principle, a measurement of upon water adsorption gives the value of the electrode potential in the UHV scale. In practice, the interfacial structure in the UHV configuration may differ from that at an electrode interface. Thus, instead of deriving the components of the electrode potential from UHV experiments to discuss the electrochemical situation, it is possible to proceed the other way round, i.e., to examine the actual UHV situation starting from electrochemical data. The problem is that only relative quantities are measured in electrochemistry, so that a comparison with UHV data requires that independent data for at least one metal be available. Hg is usually chosen as the reference (model) metal for the reasons described earlier. 

Figure 16. Activation barrier A for the formation of a breakthrough pore in a thin surface oxide film on metal as a function of electrode potential at two different surface tensions, om, of the metal/electrolyte interface.7The solid lines indicate the values of A b against Aand the dotted lines correspond to die critical potentials for the pore formation. ACd= 1 F m-2, a = 0.01 J m-2, h = 2 x 10-9 m, a, am = 0.41 J m 2 b, am 0.21 J m 2 (From N. Sato, J. Electmchem. Soc. 129, 255, 1982, Fig. 3. Reproduced by permission of The Electrochemical Society, Inc.)...

By tradition, electrochemistry has been considered a branch of physical chemistry devoted to macroscopic models and theories. We measure macroscopic currents, electrodic potentials, consumed charges, conductivities, admittance, etc. All of these take place on a macroscopic scale and are the result of multiple molecular, atomic, or ionic events taking place at the electrode/electrolyte interface. Great efforts are being made by electrochemists to show that in a century where the most brilliant star of physical chemistry has been quantum chemistry, electrodes can be studied at an atomic level and elemental electron transfers measured.1 The problem is that elemental electrochemical steps and their kinetics and structural consequences cannot be extrapolated to macroscopic and industrial events without including the structure of the surface electrode. 

Figure 27. Minority charge carrier profiles near the semiconductor/electrolyte junction. calculated for a silicon interface at two different electrode potentials. Uf- -0.25 V and Uf= 5.0 V10 ((//= forward bias = t/ - Ufl>).

Equation (40) relates the lifetime of potential-dependent PMC transients to stationary PMC signals and thus interfacial rate constants [compare (18)]. In order to verify such a correlation and see whether the interfacial recombination rates can be controlled in the accumulation region via the applied electrode potentials, experiments with silicon/polymer junctions were performed.38 The selected polymer, poly(epichlorhydrine-co-ethylenoxide-co-allyl-glycylether, or technically (Hydrine-T), to which lithium perchlorate or potassium iodide were added as salt, should not chemically interact with silicon, but can provide a solid electrolyte contact able to polarize the silicon/electrode interface. 

The reason for the exponential increase in the electron transfer rate with increasing electrode potential at the ZnO/electrolyte interface must be further explored. A possible explanation is provided in a recent study on water photoelectrolysis which describes the mechanism of water oxidation to molecular oxygen as one of strong molecular interaction with nonisoenergetic electron transfer subject to irreversible thermodynamics.48 Under such conditions, the rate of electron transfer will depend on the thermodynamic force in the semiconductor/electrolyte interface to... 

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. 

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. 

The interface between a liquid metal and an electrolyte solution can be vibrated by applying an oscillating external pressure variation. An electric signal picked up at the oscillating interface as a function of the applied electrode potential can be registered. It shows a particular dependency, at the signal vanishes [83Mly]. (Data obtained with this method are labelled VE). 

In addition to the exchange current density the transfer coefficient a is needed to describe the relationship between the electrode potential and the current flowing across the electrode/solution interface. From a formal point of view a can be obtained by calculating the partial current densities with respect to the electrode potential for the anodic reaction ... 

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