Metal-electrolyte interface contact potentials difference

When a semiconducting electrode is brought into contact with an electrolyte solution, a potential difference is established at the interface. The conductivity even of doped semiconductors is usually well below that of an electrolyte solution so practically all of the potential drop occurs in the boundary layer of the electrode, and very little on the solution side of the interface (see Fig. 7.3). The situation is opposite to that on metal electrodes, but very similar to that at the interface between a semiconductor and a metal. 

When metal parts rub in an electrolyte, it is possible to form short-circuit galvanic microelements (Fig. 1.7). Potentials 1E3 and 2E3 appear at the metal-electrolyte interface and contact potential difference 1E2 in the contact sites of the parts. The electromotive force of these elements promotes electrode processes on the friction surfaces. The processes appear even though lEs = 2E3. because of the galvanic elements resulting from crevice corrosion in the friction zone. 

In contrast to the processes commonly considered in electrochemistry, electrochemical processes occur during friction under conditions of moving and deforming discrete contacts of individual microasperities. The participation of electrolytes as a liquid layer in the friction pair leads to potential leaps of 9 3 and 933 the metal-solution interface and to the contact potential difference 9 2 ill metal contacts (see Fig. 1.7) [22]. As a result, a short-circuited galvanic microelement appears with a probability of redox reactions on its electrodes. 

When two conducting phases come into contact with each other, a redistribution of charge occurs as a result of any electron energy level difference between the phases. If the two phases are metals, electrons flow from one metal to the other until the electron levels equiUbrate. When an electrode, ie, electronic conductor, is immersed in an electrolyte, ie, ionic conductor, an electrical double layer forms at the electrode—solution interface resulting from the unequal tendency for distribution of electrical charges in the two phases. Because overall electrical neutrality must be maintained, this separation of charge between the electrode and solution gives rise to a potential difference between the two phases, equal to that needed to ensure equiUbrium. 

What, therefore, is the potential difference to be used Is it MzfraP< ), the potential difference from the metal to the contact adsorption plane, or IHP (inner Helmholtz plane, see Fig. 6.88), or is it MzfOHP<[>, the potential difference from the metal to the OHP (outer Helmholtz plane, see Fig. 6.88), or MzfSpotential difference from the bulk of the metal to the bulk of the electrolytic solution In respect to P, does one consider it to multiply the whole potential difference across the interface or only a fraction of this potential difference Similarly, what concentrations of electron acceptors and donors must be fed into the basic equation Bulk values or the values at the OHP or the values at the contact-adsorbed species (Fig. 6.88) ... 

It is the electrode potential

electrochemical experiments it represents a potential difference between two identical metallic contacts of an electrochemical circuit. Such a circuit, whose one element is a semiconductor electrode, is shown schematically in Fig. 2. Besides the semiconductor electrode, it includes a reference electrode whose potential is taken, conventionally, as zero in reckoning the electrode potential (for details, see the book by Glasstone, 1946). The potential q> includes potential drops across the interfaces, i.e., the Galvani potentials at contacts—metal-semiconductor interface, semiconductor-electrolyte interface, etc., and also, if current flows in the circuit, ohmic potential drops in metal, semiconductor, electrolyte, and so on. (These ohmic drops are negligibly small under experimental conditions considered below.)... 

Let us consider the voltage first. When a metal electrode M (— the electrode whose interface with the solution we investigate henceforth referred to the woi k-ing electrode is dipped into an electrolyte solution and equilibrium is established, an electrostatic potential is established between the two phases. What is usually measured (see Fig. 17.1) is the potential difference between this electrode and a reference half cell, R—say a platinum electrode in contact with some fixed redox solution which in turn is connected by a capillary to the close neighborhood of... 

The simplest photoelectrochemical cells consist of a semiconductor working electrode and a metal counter electrode, both of which are in contact with a redox electrolyte. In the dark, the potential difference between the two electrodes is zero. The open circuit potential difference between the two electrodes that arises from illumination of the semiconductor electrode is referred to as the photovoltage. When the semiconductor and counter electrode are short circuited, a light induced photocurrent can be measured in the external circuit. These phenomena originate from the effective separation of photogenerated electron-hole pairs in the semiconductor. In conventional photoelectrochemical studies, the interface between the flat surface of a bulk single crystalline semiconductor and the electrolyte is two dimensional, and the electrode is illuminated from the electrolyte side. However, in the last decade, research into the properties of nanoporous semiconductor electrodes interpenetrated by an electrolyte solution has expanded substantially. If a nanocrystalline electrode is prepared as a film on a transparent conducting substrate, it can be illuminated from either side. The obvious differences between a flat (two dimensional) semiconductor/ electrolyte junction and the (three dimensional) interface in a nanoporous electrode justify a separate treatment of the two cases. 

Actually, interfacial potential differences can develop without an excess charge on either phase. Consider an aqueous electrolyte in contact with an electrode. Since the electrolyte interacts with the metal surface (e.g., wetting it), the water dipoles in contact with the metal generally have some preferential orientation. From a coulombic standpoint, this situation is equivalent to charge separation across the interface, because the dipoles are... 

Identical metab in contact with solutions of different concentrations The metal dissolves from the electrode immersed in a dilute solution, and is deposited on the electrode that is immersed in a more concentrated solution. The corrosion stops when the electrolyte concentration is homogeneous at the interfaces of both of electrodes. The other type of electrochemical concentration cell is known as a differential aeration cell. The electrode potential difference in this case results from different oxygen aeration of the electrodes. This type of corrosion initiates crevice corrosion in aluminum or stainless steel when exposed to a chloride environment. 

The band-bending phenomenon, shown in Fig. 5(b) and (c), is by no means unique to the semiconductor-electrolyte interface. Analogous electrostatic adjustments occur whenever two dissimilar phases are in contact (e.g. semiconductor-gas, semiconductor-metal). An important point of distinction from the corresponding metal case now becomes apparent. For a metal, the charge, and thus the associated potential drop, is concentrated at the surface penehating at most a few A into the interior. Stated differently, the high elechonic conductivity of a metal cannot support an electric field... 

Whereas a direct measurement of the inner electric potential of a single phase is impossible, the difference, i.e., the Galvani potential difference of two phases A

common interface, is accessible when a proper reference electrode is used, i.e., a metal/electfolyte system, which should guarantee that the chemical potential of the species i is the same in both electrolytes, i.e., the two electrolytes contacting the metal phases I and 11. In addition, the absence of a junction potential between the two electrolytes is required. Under such circumstances it is possible to measure a potential difference, AE, that is related to A(p however, it always includes the A4> of the reference electrode. The latter is set to zero for the Standard Hydrogen Electrode (see below). In fact, the standard chemical potential of the formation of solvated protons is zero by convention. 

This figure shows the levels of the electric potential in the electrochemical cell represented in Figure 2.5. The voltage measured at the contacts is given hy U = f M" - f M > where f M is the electric potential of the metal M and 0 " that of the metal M". This measurement in no case offers information concerning the differences of potential at the electrode-electrolyte interface, f t, - 0 and 0 o> because 0, the electric potential of the electrolyte, is not available. 

The cell voltage Ucell is defined as the potential difference between the cathode and the anode. It is usually measured during fuel-ceU operation. The potential difference between the electrode and the electrolyte, which is caUed the anode or cathode potential in the following, is responsible for the electrochemical reaction occurring within the catalyst layers but cannot be measured directly. In the further text, we use electrolyte and membrane as equivalent expressions. While the electrode potential can be sensed from the bipolar plates, it is not feasible to sense the membrane potential directly, since each measurement equipment forms an interface between the membrane and the metal contact Two methods for the installation of a reference electrode within the ceU have been discussed in the Hterature, namely the reverse hydrogen electrode (RHE) [18] and the dynamic hydrogen electrode (DHE). In addition to ceU internal methods, a conventional... 

When a metal is placed in contact with an electrolyte, a potential difference is observed at the liquid-metal interface, as noted in Chapter 2. This is similar to the work-function potential difference which occurs when two dissimilar metals are brought into contact, or the potential difference associated with a semiconductor p n junction. The value of potential difference associated with a metal electrode-electrolyte interface is a function of the metal and contacting electrolyte. Theoretical treatment of this situation is complex and one should refer to a text on electrochemistry such as those by Macinnes (1961) or Newman (1973). Certain types of electrodes are extremely sensitive to various trace impurities in the contacting electrolyte and may react quite differently in seemingly similar circumstances. 

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