Overvoltage effect

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. 

Figure 8. Overvoltage effect on the rates of ethylene epoxidation (i = 1, 0) and deep oxidation (i = 2, 9). At constant overvoltage the relative rate increases are proportional to Pqi/Pet at 400°C.

The rate and extent of the hydrolysis reaction [Eq. (9.25)] is suppressed under alkaline conditions, and the lifetime of 02 is increased such that the l/2 potential may be shifted to more negative potentials. This is the result of thermodynamic effects via the Nemst equation rather than kinetic (overvoltage) effects. The overall post-electron-transfer chemistry is given by the relation... 

Development of the high-temperature steam electrolysis process should be pursued. The issues of materials durability, reduction of overvoltages, effects of the operating pressure, and separation of gas products in an efficient and safe manner should be investigated. 

Therefore, any couple of E° larger than 1.23 volts can oxidize water, provided kinetic ( overvoltage ) effects are absent. Since, under these conditions, E° must be less than 1.23 volts, must be less than 5.7 eV. 

In appl5nng Jprgensen s approach, it should also be remembered that there are usually kinetic factors (so-called overvoltage effects ) which results in hydrogen and oxygen being evolved only at potentials beyond the range of the thermodynamic ones [81). There is also often the question of the effect of complex ion formation. 

At present, electrolysis cells basically consist of two electrodes separated by an asbestos diaphragm impermeable to gases. 20 to 30% potassium hydroxide is dissolved in the electrolyte to increase its conductivity. The electrolysis is carried out at 80 to 85°C. The theoretical decomposition potential is 1.24 V with 1.9 to 2.3 V being used in practice due to overvoltage effects etc. Oxygen is produced at the anode and hydrogen at the cathode ... 

The second type of cell is a mercury pool type. A mercury cathode is particularly useful for separating easily reduced elements as a preliminary step in an analysis. l or example, copper, nickel, cobalt, silver, and cadmium are readily separated from ions such as aluminum, titanium, the alkali metals, and phosphates. The precipitated elements dissolve in the mercury little hydrogen evolution occurs even at high applied potentials because of large overvoltage effects. A coulomet-ric cell such as that shown in Figure 24-5b is also useful for coulometric determination of metal ions and certain types of organic compounds as well. 

The overvoltage or overpotential over is inserted in Eq. (3.20) to adjust for other processes that compete in the system and make electrodeposition less than ideally efficient. These processes are irreversible and include the effects of the decomposition of water, other solutes, and imperfections in the electrode surface. Because of these processes, a greater potential difference than calculated from the reference potential and the ionic concentration must be applied in order to achieve deposition. For the same reason, spontaneous deposition, inferred from a positive value of E°, may not occur if the overvoltage exceeds it. Overvoltage effects occur at both the cathode and the anode. 

Hg(<15ppm) 0 0 0 Increase the cathodic overvoltage, effect gennally reversible. 

Here n is the nnmber of moles of electrons involved in the reaction the rate constants have been assumed to be thermally activated and kj and kl are potential-independent rate constant parameters. The potential-dependent concentrations CRed and Cox are evaluated at their points of closest approach to the electrode, taken here as the outer Helmholtz plane. Further, a is here a dimensionless symmetry factor often assumed to be 0.5 (the symmetrical barrier case), and is the charge transfer overvoltage effective in driving the reaction away from eqnilibrium (for Rmh = 0, J = 0). The Butler-Volmer equation is usually a good approximation for both biased and unbiased conditions. 

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