Kinetics of Single Half-Cell Reactions

Regardless of the cause of the electron flow at the interface, deviations of the half-cell potentials along the interface from their equilibrium values are functions of the current density. These deviations reflect the polarization behavior of the reaction, a phenomenon of 

Polarization behavior relates to the kinetics of electrochemical processes. Study of the phenomenon requires techniques for simultaneously measuring electrode potentials and current densities and developing empirical and theoretical relationships between the two. Before examining some of the simple theories, experimental techniques, and interpretations of the observed relationships, it is useful to characterize the polarization behavior of several of the important electrochemical reactions involved in corrosion processes. 

Historically, Faraday observed that single-electrode half-cell potentials shifted from their equilibrium values when current passed through electrochemical cells. This deviation is referred to as overpotential or overvoltage. It is generally designated as q and is defined by the relationship  

It is shown subsequently that simple electrode-kinetics theory leads to the following equations for the oxidation and reduction half-cell reactions, respectively  

Substituting Eq 3.3(a) and the Nemst half-cell equation into Eq 3.4 gives an expression for the potential as a function of current density for the oxidation of a metal, M (M — Mm+ + me)  

---

The previous concepts may be summarized by briefly reviewing the experimental procedures for determining the kinetic parameters, i0, Pox, and Pred. If a single half-cell reaction is involved, the equilibrium half-cell potential will be measured against some reference electrode. If the electrode is now connected to a potentiostat and the potential increased in the positive or oxidation direction, the upper solid curve of Fig. 3.11 will be plotted. If the potential is decreased, the lower solid curve will be plotted. The higher current-density linear sections of each curve are then extrapolated through the value of the equilibrium poten-... 

The oxygen/water half-cell reaction has been one of the most challenging electrode systems for decades. Despite enormous research, the detailed reaction mechanism of this complex multi-step process has remained elusive. Also elusive has been an electrode material and surface that significantly reduces the rate-determining kinetic activation barriers, and hence shows improvements in the catalytic activity compared to that of the single-noble-metal electrodes such as Pt or Au. 

Bard and co-workers have reported on the attainment of equilibrium between the nanosized particles and an electrode in the presence of a redox mediator [25a]. The study refers to the production of a mediator (methyl viologen radical cation) that reduces water in the presence of colloidal gold and platinum metal catalyst. An electrochemical model based on the assumption that the kinetic properties are controlled by the half-cell reactions is proposed to understand the catalytic properties of the colloidal metals. The same authors have used 15 nm electrodes to detect single molecules using scanning electrochemical microscopy (SECM) [25b]. A Pt-Ir tip of nm size diameter is used along with a ferrocene derivative in a positive feedback mode of SECM. The response has been found to be stochastic and Ear-adaic currents of the order of pA are observed. 

---