The Overpotential

Once current passes tluough the interface, the Galvani potential difference will differ from that expected from the Nemst equation above the magnitude of the difference is temied the overpotential, which is defined heiiristically as... 

The overpotential is defined as the difference between the actual potential of an electrode at a given current density and the reversible electrode potential for the reaction. 

The overpotential required for the evolution of O2 from dilute solutions of HCIO4, platinum electrodes is approximately 0.5 V. 

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. 

Tertiay Current Distribution. The current distribution is again impacted when the overpotential influence is that of concentration. As the limiting current density takes effect, this impact occurs. The result is that the higher current density is distorted toward the entrance of the cell. Because of the nonuniform electrolyte resistance, secondary and tertiary current distribution are further compHcated when there is gas evolution along the cell track. Examples of iavestigations ia this area are available (50—52). 

In the overpotential deposition regime we observed that nanosized Al was deposited in the initial stages. Eurthermore, a transfer of Al from the scanning tip to the Al covered substrate was observed. We accidentally succeeded in an indirect tip-induced nanostructuring of Al on growing Al (Eigure 6.2-6). 

The displacement of the potential of an electrode from its reversible value is the overpotential tj, and... 

It should be noted that whereas E is always relative to a specified reference electrode this will not apply to the overpotential see equation 1.59). 

It is evident from these expressions that since in the Tafel region / (the current density actually determined) must be greater than /(, (the equilibrium exchange current density), the signs of the overpotentials will conform to equations 1.60 and 1.61, i.e. will be negative and will be positive. 

However, if the pipeline has multiple drain points separated by distance V and the aim is that the pipeline electrode potential should just reach the protection potential at the mid-point (x = a/2), then is equal to (the overpotential required just to achieve the protection potential). At X = a/2 the current in the line is zero and (d nx/dx), 2 also zero. Thus ... 

These considerations have been based entirely on thermodynamics and take no account of the overpotential, which is dependent on the rate of the process and the nature of the surface at which the reaction occurs. For this reason, the rate of reduction of HjO or HjO is usually low, and remains so to potentials from 0-5 to 1-OV below that given in equation 12.1. Even so, the instability of water is an insuperable obstacle to electrodepositing... 

In acid conditions oxide films are not usually present on the metal surface and the cathodic reaction is primarily that of hydrogen discharge rather than oxygen reduction. Thus, inhibitors are required that will adsorb or bond directly onto the bare metal surfaces and/or raise the overpotential for hydrogen ion discharge. Inhibitors are usually organic compounds... 

Thus -q the overpotential is defined as the departure of the potential of the electrode from its equilibrium value. 

If the overpotential is taken as —0-1 V it follows from equation 20.120 that = 10 atm. Although the arguments used in deriving these relationships do not give precise values of p they indicate that pressures of very high magnitude will be obtained at significant overpotentials. 

Bockris and Subramanyan during studies of the permeation of hydrogen through pure Fe and Fe-SNi alloy found that a normal permeation transient was obtained (Fig. 20.21), providing the overpotential was less than a critical value, and when the overpotential was less than it was possible to reproduce the normal permeation curve, i.e. apply the polarising current at a constant rj < t , allow J to attain a steady value, switch off, reapply... 

THE POTENTIAL DIFFERENCE AT A METAL/SOLUTION INTERFACE The overpotential ij is defined as... 

For an interfering redox reaction at an ion-selective membrane, the overpotential t B can be easily determined experimentally. It is the potential difference between the ion-selective membrane and an inert redox electrode in the same solution containing the measured ion and an interfering redox system. 

Overpotential. It has been found by experiment that the decomposition voltage of an electrolyte varies with the nature of the electrodes employed for the electrolysis and is, in many instances, higher than that calculated from the difference of the reversible electrode potentials. The excess voltage over the calculated back e.m.f. is termed the overpotential. Overpotential may occur at the anode as well as at the cathode. The decomposition voltage ED is therefore ... 

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