Current-overpotential relationship

The conceptual development of limiting-current measurement was advanced substantially by Agar and Bowden (A2), who investigated the current-overpotential relationship for oxygen evolution at nickel electrodes in fused sodium hydroxide. Here water transport is the limiting step ... 

Figure 6.9. Four regions in the general current-overpotential relationship 1, linear 2, exponential 3, mixed control 4, limiting current density region.

Anodic and cathodic limiting currents are exhibited because of the rate limiting mass transfer between the electrode and the gas phase. As the potential of the electrode is changed the rate of mass transfer will remain constant and, hence, this behaviour will be manifested as a plateau in the current-overpotential relationship. 

If the electrocrystallization is controlled by formation of two- or three-dimensional isolated nuclei, the current—overpotential relationship has a stronger dependence on 17 than predicted by the Butler—Volmer equation for charge transfer control [151]... 

The passage of a net current through an electrode implies that the electrode is no longer at equilibrium and that a certain amount of overpotential is present at the electrode-electrolyte interface. Since the overpotential represents a loss of energy and a source of heat production, a quantitative model of the relationship between current density and overpotential is required in design calculations. A fundamental model of the current-overpotential relationship would proceed from a detailed knowledge of the electrode reaction mechanism however, mechanistic studies are complicated even for the simplest reactions. In addition, kinetic measurements are strongly influenced by electrode surface preparation, microstructure, contamination, and other factors. As a consequence, a current-overpotential relation is usually determined experimentally, and the data are often fitted to standard models. 

We can usefully take this discussion a little further by developing a current-overpotential relationship for a quasireversible mechanism having the pattern of... 

Fig. 8.4 Cathodic current-overpotential relationship for hydrogen evolution reaction obtained on AISI 4340 steel membrane in an electrolyte containing 0.5 M HCIO4 and 0.25 M NaCI04 (pH 0.3) [3]. Reproduced by the permission of The Electrochemical Society.

For Pt oxidation, the oxidation current-overpotential relationship is given by... 

If 0 = 0Q, the Heyrovsky current-overpotential relationship can be expressed as Equation 3.40 ... 

This is the Butler-Volmer equation, which describes the current-overpotential relationship when mass-transfer effects are negligible. This may be a useful approximation of (18) when the eurrent is less than 10% of the limiting current. 

As T] increases away from zero, one of the two terms of the current-overpotential relationship (representing either reduction or oxidation) will dominate ... 

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