Overview

There are a multitude of experiments that can be performed with the basic three-electrode setup described above. Through a series of examples, we will explore a few of the more common ones, highlighting the main information that is obtained from each and its advantages and disadvantages. The key features of the any experiment are (1) the geometry of the electrode and the system, (2) the flow of electrolyte, and (3) control of potential or current. The examples that we will treat in this chapter are shown in Table 6.1. Of course, the same principles illustrated in these examples can be applied to different situations. Hence, an understanding of these experimental techniques will help to equip you with the tools needed to analyze any system that may be of interest.

Table 6.1 Experimental Features Considered in This Chapter

Each of the features will be explored in more detail in the upcoming sections and are only briefly discussed here. Mass transfer has a central role in the analysis of these systems. Two key equations from Chapter 4 are the Nernst–Planck equation and mass conservation:

(4.3)

and

(4.10)

For the circumstances considered in this chapter, there is no homogeneous reaction (). We will also assume that there is a supporting electrolyte so that migration can be neglected. In addition, we assume that the diffusion coefficient is constant and that the fluid is incompressible. With these assumptions, the general material balance, Equation 4.10, reduces to

(4.25)

Alternatively, a binary electrolyte yields a similar equation by elimination of the potential gradient. The balance equation in this form can be applied to any geometry and will be starting point for our studies in this chapter. The time-dependent term on the left side is included only for transient experiments. The flow and therefore the velocity field, v, will either be zero or a well-defined known value. An initial condition is required for transient problems. In addition, we must specify boundary conditions. Since electrode reactions occur at the surfaces, electrode kinetics will enter as boundary conditions. Most often the concentration is known far from the electrode and can be used as one of the boundary conditions. For the second boundary condition, the flux is frequently specified at the electrode surface. Our approach is to identify the geometry, characterize the flow, and specify the boundary conditions. As we examine different electrochemical experiments, we will be interested in determining which reactions take place over a specified range of potentials. For these reactions, we want to know whether the reaction is controlled by kinetics or mass transfer or is under mixed control. If under mixed control, how much of the polarization is due to kinetics, ohmic resistance, and concentration overpotentials. Often, the objective is to measure kinetic and transport parameters for the system. Keep these types of things in mind as we describe several different types of experiments with their corresponding analyses and interpretations.