Category: 03. Electrochemical Kinetics

Several efficiencies are used to characterize electrochemical processes and systems. The faradaic efficiency was introduced in Chapter 1. Here, we present the current efficiency, which is slightly different: (3.39) Undesired reactions occur in both electrolytic and galvanic cells. These unwanted side reactions reduce the current efficiency. We can explore this concept with the charging of a lead–acid…

The purpose of this section is to help develop some initial experience and intuition with full electrochemical cells. By a full cell, we simply mean an electrochemical cell with the anode and cathode separated by some distance by an electrolyte. What’s more, we will consider the potential of the cell, namely, the potential of the…

In situations where current−voltage data are available only over a limited range that is not adequately addressed by one of the limiting cases above, a direct fit of the data to the full BV equation may be appropriate. This fitting can be done in a straightforward manner with use of a nonlinear solver or optimization…

Tafel Approximation The BV equation has two exponential terms, one that represents the anodic current (i > 0, ηs > 0) and the other that represents the cathodic current (i < 0, ηs < 0). What happens to the relative magnitude of the two terms as ηs becomes more positive? What about as ηs becomes more negative? When ηs is large and positive, the anodic term of the BV equation dominates…

We will find the Butler–Volmer formulation of kinetics extraordinarily useful for the study of electrochemical systems. Nonetheless, it is largely a phenomenological or empirical equation with three parameters, io and two transfer coefficients, αa and αc. We saw vast variations in exchange-current densities based on the reactions and the electrode surface. What causes one reaction to be facile and others…

The purpose of this section is to help you understand and learn how to use Equation 3.17, the Butler–Volmer equation. The BV equation is a relationship between the current density (i) and the charge transfer or surface overpotential (ηs). It contains three parameters: the exchange-current density (io), the anodic charge transfer coefficient (αa), and the cathodic…

Electrochemical reactions take place at the surface of the electrode as electrons are transferred to and from ions or neutral species in solution. The rate of reaction is influenced by the potential drop across the double layer. At open circuit (no external current), this potential drop reaches an equilibrium value where the forward and reverse…

A metal electrode in an electrolyte solution is typically charged, even at equilibrium. Excess charge is present on the outer surface of the electrode, adjacent to the electrode–electrolyte interface. The amount of charge in the metal electrode is a measure of the energy of the electrons in the metal and can be changed by using…