Overpotential types

Overvoltages for various types of chlor—alkali cells are given in Table 8. A typical example of the overvoltage effect is in the operation of a mercury cell where Hg is used as the cathode material. The overpotential of the H2 evolution reaction on Hg is high hence it is possible to form sodium amalgam without H2 generation, thereby eliminating the need for a separator in the cell. 

Charge Transport. Side reactions can occur if the current distribution (electrode potential) along an electrode is not uniform. The side reactions can take the form of unwanted by-product formation or localized corrosion of the electrode. The problem of current distribution is addressed by the analysis of charge transport ia cell design. The path of current flow ia a cell is dependent on cell geometry, activation overpotential, concentration overpotential, and conductivity of the electrolyte and electrodes. Three types of current distribution can be described (48) when these factors are analyzed, a nontrivial exercise even for simple geometries (11). 

The various types of overpotentials are dealt with in more detail in Section 20.1 but it is appropriate here to outline the significant factors in relation to their importance in controlling the rate of corrosion reaction. 

It is clear that for a half-cell the sum of the overpotentials should be as low as possible. Depending on their origin, a distinction has to be made between few different types ... 

Both the frequency of the well and its depth cancel, so that the free energy of activation is determined by the height of the maximum in the potential of mean force. The height of this maximum varies with the applied overpotential (see Fig. 13). To a first approximation this dependence is linear, and a Butler-Volmer type relation should hold over a limited range of potentials. Explicit model calculation gives transfer coefficients between zero and unity there is no reason why they should be close to 1/2. For large overpotentials the barrier disappears, and the rate will then be determined by ion transport. 

In general, the physical state of the electrodes used in electrochemical processes is the solid state (monolithic or particulate). The material of which the electrode is composed may actually participate in the electrochemical reactions, being consumed by or deposited from the solution, or it may be inert and merely provide an interface at which the reactions may occur. There are three properties which all types of electrodes must possess if the power requirements of the process are to be minimized (i) the electrodes should be able to conduct electricity well, i.e., they should be made of good conductors (ii) the overpotentials at the electrodes should be low and (iii) the electrodes should not become passivated, by which it is meant that they should not react to form on their surfaces any compound that inhibits the desired electrochemical reaction. Some additional desirable requirements for a satisfactory performance of the cell are that the electrodes should be amenable to being manufactured or prepared easily that they should be resistant to corrosion by the elements within the cell that they should be mechanically strong and that they should be of low cost. Electrodes are usually mounted vertically, and in some cases horizontally only in some rare special cases are they mounted in an inclined manner. 

The anode potential is so positive, due principally to the activation overpotential, that the majority of the impurity metals (Fe, Cu, Co, etc.) in the anode dissolve with the nickel sulfide. In addition, some oxygen is evolved (2 H20 = 02 + 4 H+ + 4 e ). The anodic current efficiency reduced to about 95% on account of this reaction. Small amounts of selenium and the precious metals remain undissolved in the anode slime along with sulfur. The anolyte contains impurities (Cu, Fe, Co) and, due to hydrogen ion (H+) liberation, it has a low pH of 1.9. The electrolyte of this type is highly unfit for nickel electrowinning. It is... 

In some cell types, especially those in which electrolysis generates gas at an electrode, the phenomenon of overvoltage may occur, which means that the voltage to be imposed must be higher than the emf plus an overvoltage the term overpotential must be strictly used for the single electrode. 

The third type of activation energy is the activation energy at constant overpotential (rj RT/F), defined by the equation... 

The first two terms on the right-hand side of this equation express the proper overpotential of the electrode reaction rjr (also called the activation overpotential) while the last term, r)c, is the EMF of the concentration cell without transport, if the components of the redox system in one cell compartment have concentrations (cOx)x=0 and (cRed)x=0 and, in the other compartment, Cqx and cRcd. The overpotential given by this expression includes the excess work carried out as a result of concentration changes at the electrode. This type of overpotential was called the concentration overpotential by Nernst. The expression for a concentration cell without transport can be used here under the assumption that a sufficiently high concentration of the indifferent electrolyte suppresses migration. 

It is clear from the calculated limiting-current curves in Fig. 3a that the plateau of the copper deposition reaction at a moderate limiting-current level like 50 mA cm 2 is narrowed drastically by the surface overpotential. On the other hand, the surface overpotential is small for reduction of ferri-cyanide ion at a nickel or platinum electrode (Fig. 3b). At noble-metal electrodes in well-supported solutions, the exchange current density appears to be well above 0.5 A/cm2 (Tla, S20b, D6b, A3e). At various types of carbon, the exchange current density is appreciably smaller (Tla, S17a, S17b). 

At illuminated p-type semiconductors, light energy brings about a shift of the applied cathode potential at which C02 reduction takes place toward a less negative potential by the photovoltaic effect.94 Thus, light energy can be used to reduce the apparent overpotential of C02 reduction. 

The photoelectrochemical reduction of C02 at illuminated p-type semiconductor electrodes is also effective for C02 reduction to highly reduced products. The combination of photocathodes with catalysts for C02 reduction leads to a marked decrease in the apparent overpotential. At present, however, light to chemical energy conversion efficiencies are still very low, and negative in some cases. 

A correlation between the spacing of striae and convection downstream of protrusions does not fully describe the process. The initial protrusions arise far from transport control and cannot be attributed to a diffusive instability of the type described in the previous section. Jorne and Lee proposed that striations formed on rotating electrodes by deposition of zinc, copper and silver are generated by an instability that arises only in systems in which the current density at constant overpotential decreases with increasing concentration of metal ion at the interface [59]. 

Figure 7.6 Gerischer diagram for a redox reaction at an n-type semiconductor (a) at equilibrium the Fermi levels of the semiconductor and of the redox couple are equal (b) after application of an anodic overpotential.

FIGURE 2.14 (a) Influence of NiO particle size on the anode overpotential r (at a constant current density of 250 m A/cm2), anode ohmic resistance Rn, and anode interfacial resistance, RE, for cermets made from YSZ and three different types of NiO NiO-1, NiO-2, and NiO-3, of which the particle size is 1, 5, and 10 pm, respectively, (b) Influence of TZ3Y particle size on the anode overpotential rj (at a constant current density of 250 mA/cm2) for cermets made from 600°C-calcinated NiO-1 and six different TZ3Y powders with different particle sizes. (From Jiang, S.P. et al., Solid State Ionics, 132 1-14, 2000. Copyright by Elsevier, reproduced with permission.)... 

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