Half-cell reactions kinetics

Fig. 3 Electrochemical framework for intergranular corrosion described by an Evans diagram depicting the anodic half-cell reaction kinetics for the grain boundary zone and the grain matrix. In this case, enhanced active dissolution occurs in both the grain boundary region and in the matrix. At a fixed potential, given by Egpp, the anodic dissolution rate is accelerated along the grain boundary compared to the matrix.

In this paper, we will discuss the thermodynamic principles involved in fuel cells as well as the kinetic aspects of their half cell reactions. In the kinetic considerations, we will also touch, briefly, on the fundamental problem of electrocatalysis. We will then proceed to describe different types of fuel cells and finally present the status of this new electrical generation device. 

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. 

Factors Involved in Galvanic Corrosion. Emf series and practical nobility of metals and metalloids. The emf. series is a list of half-cell potentials proportional to the free energy changes of the corresponding reversible half-cell reactions for standard state of unit activity with respect to the standard hydrogen electrode (SHE). This is also known as Nernst scale of solution potentials since it allows to classification of the metals in order of nobility according to the value of the equilibrium potential of their reaction of dissolution in the standard state (1 g ion/1). This thermodynamic nobility can differ from practical nobility due to the formation of a passive layer and electrochemical kinetics. 

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. 

Knowing that a given combination of anode and cathode half-cell reactions will proceed sponta-neonsly does not ensure that the electrode reaction rates will be sufficiently high for practical applications. Reaction kinetics at the anode and cathode and mass transfer of reactants/products to/from the electrodes may play important roles in an electrochemical cell and may influence the choice of cell design and operating conditions. These important points will be addressed later in this chapter. 

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

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-... 

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