Electrochemical Phase Boundary Reactions

In electrolytic corrosion, an anodic partial reaction takes place according to Eq. (2-3) 

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The potential dependence of the velocity of an electrochemical phase boundary reaction is represented by a current-potential curve I(U). It is convenient to relate such curves to the geometric electrode surface area S, i.e., to present them as current-density-potential curves J(U). The determination of such curves is represented schematically in Fig. 2-3. A current is conducted to the counterelectrode Ej in the electrolyte by means of an external circuit (voltage source Uq, ammeter, resistances R and R") and via the electrode E, to be measured, back to the external circuit. In the diagram, the current indicated (0) is positive. The potential of E, is measured with a high-resistance voltmeter as the voltage difference of electrodes El and E2. To accomplish this, the reference electrode, E2, must be equipped with a Haber-Luggin capillary whose probe end must be brought as close as possible to... 

W. Lorenz and G. Sake, Mechanism of the Electrochemical Phase Boundary Reaction, Z Phys. Chem. (Leipzig) 218 259(1961). 

II Electrochemical syntheses are phase boundary reactions. The boundary problems, which are also familiar from heterogeneous catalysis, must be expected especially during continuous operation. 

In the case of dissolution of a number of semiconducting metal oxides in acid aqueous solutions, it can be calculated from the rate of solution, that a phase boundary reaction must be rate determining. Moreover, it can be shown, that this reaction is electrochemical in nature, for its rate depends on the potential of the dissolving oxide. As an example, Fig. 1 shows the influence of the electrode potential on the rate of dis -solution of FeQ g Oin sulfuric acid and hydrochloric acid. The rate of dissolution is expressed in terms of a dissolution current. A linear relation exists between the electrode potential... 

Wagner was well aware that these processes also may affect or even control the kinetics of oxidation, sulfidation, and so on, and he initiated research on the phase boundary reaction kinetics [10-18]. Some cases of surface reaction control are described in Sect. 6.2.3.2. The surface reactions generally have no electrochemical character, but as shown, electron transfer steps are involved [10-12]. Least is known about the reactions at the inner interface, but studies on sulfidation [13-18] have proven its role. 

Owing to the high disorder and diffusivi-ties in many sulfides, sulfidation is often controlled by phase boundary reactions. The surface reactions of oxidation in H2S and reduction in H2 have been studied on Ag2S [82] at 300 °C using the electrochemical cell Ag AgI Ag2S Pt. As described in Sect. 6.2.2.3.2, the chemical potential of Ag in Ag2S is given by... 

Another characteristic feature of microreactors derived from their much greater surface-to-volume ratios is that they make phase-boundary reactions such as gas-liquid, liquid-liquid, or solid-liquid reactions more efficient. This feature of flow microreactors is also advantageous for photochemical [66-75] and electrochemical [76-86] reactions, which have received significant attention from the viewpoint of environmentally benign syntheses. 

Let us now look at an example in which the rate of a phase boundary reaction is measured by an electrochemical method [33]. In order that only phase boundary effects are measured, it is necessary to ensure that all diffusion or convection processes are rapid in comparison to the phase boundary reaction. This condition may be achieved by proper choice of the dimensions of the sample, and, in the case of fluid phases, by forced or natural convection. The electrochemical method of measurement is particularly convenient and powerful, since the activities of the components as well as the reaction rates (as in the above example of the sulfidizing of Ni) can be directly measured electrically. 

In order to describe any electrochemical cell a convention is required for writing down the cells, such as the concentration cell described above. This convention should establish clearly where the boundaries between the different phases exist and, also, what the overall cell reaction is. It is now standard to use vertical lines to delineate phase boundaries, such as those between a solid and a liquid or between two innniscible liquids. The junction between two miscible liquids, which might be maintained by the use of a porous glass frit, is represented by a single vertical dashed line, j, and two dashed lines, jj, are used to indicate two liquid phases... 

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 ... 

The presupposition is that parallel electrochemical reactions (i.e., ion or electron transfer) occur across the phase boundary, if the measured ions and interfering ions are both present in the solution. A redox process in which electrons pass the phase boundary is also considered an interfering electrochemical reaction. 

During the determination of standard electrode potentials an electrochemical equilibrium must always exist at the phase boundaries, e.g. that of the elec-trode/electrolyte. From a macroscopic viewpoint no external current flows and no reaction takes place. From a microscopic viewpoint or a molecular scale, a continuous exchange of charges occurs at the phase boundaries. In this context Fig. 6 demonstrates this fact at the anode of the Daniell element. 

Nonpolarizable interfaces correspond to interfaces on which a reversible reaction takes place. An Ag wire in a solution containing Ag+ions is a classic example of a nonpolarizable interface. As the metal is immersed in solution, the following phenomena occur3 (1) solvent molecules at the metal surface are reoriented and polarized (2) the electron cloud of the metal surface is redistributed (retreats or spills over) (3) Ag+ ions cross the phase boundary (the net direction depends on the solution composition). At equilibrium, an electric potential drop occurs so that the following electrochemical equilibrium is established ... 

This electrochemical reaction contains the elementary step (4.1) and under conditions of backspillover can be considered to take place over the entire metal/gas interface including the tpb.1,15 18 This is usual referred to as extension of the electrochemical reaction zone over the entire metal/gas interface. But even under these conditions it must be noted that the elementary charge transfer step 4.1 is taking place at the three-phase-boundaries (tpb). 

Although NEMCA is a catalytic effect taking place over the entire catalyst gas-exposed surface, it is important for its description to also discuss the electrocatalytic reactions taking place at the catalyst-solid electrolyte-gas three phase boundaries (tpb). This means that the catalyst-electrode must also be characterized from an electrochemical viewpoint. When using YSZ as the solid electrolyte the electrochemical reaction taking place at the tpb is ... 

In solid electrochemistry electrochemical (change transfer) reactions take place primarily at the three-phase-boundaries (tpb) metal-electrolyte-gas, e.g. ... 

Porous electrodes are commonly used in fuel cells to achieve hi surface area which significantly increases the number of reaction sites. A critical part of most fuel cells is often referred to as the triple phase boundary (TPB). Thrae mostly microscopic regions, in which the actual electrochemical reactions take place, are found where reactant gas, electrolyte and electrode meet each other. For a site or area to be active, it must be exposed to the rractant, be in electrical contact with the electrode, be in ionic contact with the electrolyte, and contain sufficient electro-catalyst for the reaction to proceed at a desired rate. The density of these regions and the microstmcture of these interfaces play a critical role in the electrochemical performance of the fuel cells [1]. 

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