Electrochemical Cells, Instrumentation, and Some Practical Issues

The basic electrochemical cell used for analysis of electrochemical systems consists of three electrodes: the working electrode (WE), the counter electrode (CE), and a reference electrode (RE). The three-electrode setup is shown in Figure 6.1. The working electrode is where the electrochemical reaction being studied occurs. Typically, the potential or current of the working electrode is controlled. Of course, an electrochemical cell must contain at least two electrodes. The second electrode is called the counter electrode. Sometimes this electrode is referred to as an auxiliary electrode. The counter electrode carries current and completes the circuit. The detailed reactions that take place on the counter electrode may or may not be of interest, depending on the experiment. Finally, whenever possible, we want to include a reference electrode in an electrochemical cell in order to allow us to separate the processes that occur on the working electrode from those at the counter electrode. The reference electrode does not participate in the reactions (no current is passed through the reference electrode), but provides a reference point for measuring and/or controlling the potential. Because there is no current flow to the reference electrode, its potential stays as close as possible to the reversible potential of the electrode.

There are two basic modes of operation for the three-electrode setup. The first mode is called potentiostatic control. Here the current flowing through the WE is adjusted until the desired potential is measured between the working and reference electrodes. Generally, we prescribe the potential as a function of time and measure the current that is required to achieve this potential. The second mode of operation is galvanostatic control. In this case, the value of the current is set and the potential of the WE is measured relative to the RE. We can specify the current as constant or as a function of time. There is need for both modes of operation and, fortunately, modern instrumentation makes this control straightforward.

The high-precision instrument used for such measurements is called a potentiostat/galvanostat. The typical potentiostat may be limited to no more than a few amperes of current. Quite often, with laboratory fuel cells, for instance, the engineer may require currents on the order of 50 A or more. A battery for an automobile may draw hundreds of amperes when a short burst of power is needed. In these situations, an electronic load is used rather than a potentiostat. It may be possible to operate in a near-potentiostatic mode, but often there is no RE included and the precision is reduced. Also consider the case when we are cycling batteries or electrochemical capacitors. It is common to have dozens if not hundreds of cells operating at the same time. Providing the level of control that a high-quality potentiostat offers is not economically feasible. In these instances, automatic cyclers are used.

There are several factors that need to be considered when analyzing an electrochemical system. Suppose that we are examining the oxidation and reduction of a particular species in the electrolyte. In this situation, the WE provides a surface on which the electron-transfer reaction can take place, but does not react itself. Noble metals like Pt or Au that are stable (nonreactive) over a wide range of conditions are often used for this purpose. Conversely, if we investigate the active material of a battery, the WE is the active material, which would participate directly in the electrochemical reaction. Similarly for corrosion studies (Chapter 16), the working electrode participates in the reaction.

As introduced in Chapter 2, a reference electrode should consist of well-defined materials and be capable of carrying out a specific, known electrochemical reaction. Ideally, the reference electrode reaction should involve an ion present in the electrolyte so that liquid junctions are avoided and additional corrections to the potential are not needed. Selection of reference electrodes is explored in Problem 6.1. Another consideration is the placement of the reference electrode. With current flow between the working and counter electrodes, there will be a potential drop through the electrolyte. As already noted, essentially no current flows through the RE; consequently, there is no polarization of the RE (this is just another way of saying that the surface overpotential of the reference electrode is zero). Nevertheless, the measured potential is affected by the potential drop in solution, and therefore depends on the placement of the reference electrode. Generally, we cannot eliminate the impact of the potential drop on the measured potential, but we can correct for it. This correction is examined in Section 6.9. The ideal situation is to have an infinitesimally small reference electrode located just outside of the double layer so that no correction for ohmic drop in solution is necessary.

Similarly, the counter electrode should be selected judiciously. This choice will vary according to what we are trying to accomplish. Frequently, it may be a relatively inert material such as Au or Pt. At times, the reaction at the counter electrode may not be important, and we may electrolyze the solvent or corrode the CE. For example, the evolution of oxygen (anodic) or hydrogen (cathodic) is commonly done at the CE. In all cases, you should know what reaction is occurring at the CE, and you should be aware of issues that may result from these reactions. A minute dissolution of metal from the CE may be transported to and interfere with the reaction at the WE. For example, Pt like Au is generally inert, but when studying oxygen reduction on a nonprecious metal catalyst, Pt would be a poor choice for the CE. Typically, the area of the CE should be much larger than the area of the WE; when this is true, the current density at the CE is significantly lower than that at the WE so that the cell potential and overall behavior of the cell are not limited by processes at the counter electrode.

In addition to the electrodes, there are factors related to the electrolyte that should be considered. In Chapter 4, we introduced the concept of a supporting electrolyte. As we saw before, the mathematical analysis is greatly simplified with a supporting electrolyte because we can neglect migration. In addition, the resistance of the solution is lowered. Supporting electrolyte is used with a number of electroanalytical techniques where it is beneficial and where the introduction of the additional electrolyte does not compromise other aspects of the experiment. It is also common to remove oxygen from the electrolyte. For instance, if oxygen reacts under the conditions (potential and composition) present, it may interfere with the reactions of interest. Oxygen removal is accomplished by bubbling an inert gas such as nitrogen or argon through the electrolyte to strip out any dissolved oxygen in solution (Figure 6.1). In contrast, if we are studying the reduction of oxygen, we would bubble oxygen through the solution to saturate the solution.

What exactly are we doing when we use a potentiostat to apply a potential? The physical situation is shown in Figure 6.2, which shows the CE, RE, and WE connected to a potentiostat. The current through the WE (measured with the I/E converter) and potential difference between the WE and RE (measured with the electrometer) are sensed. To change this potential, the instrument passes current between the working and the counter electrodes. When you select a value of E_app on the potentiostat, the control amplifier drives whatever current is needed through the WE so that the measured potential matches E_app. If that potential is greater than the equilibrium potential, then the resulting current will be anodic at the working electrode. Conversely, application of a potential below the equilibrium potential would drive the cathodic reaction at the working electrode.

What precisely does E_app represent? E_app can be expressed as follows:

(6.1)

 is the potential difference between the solution at the location of the reference electrode and a location just outside the double layer of the working electrode. This potential drop is well defined if the current distribution is uniform, or nearly so, and must be accounted for because it is not practical to locate the reference electrode just outside the double layer (see Section 6.9). The influence of the concentration overpotential is often neglected and can be reduced through the use of convection. Finally, a note about the sign of the current. According to the convention that we have used and Equation 6.1, you might expect a positive current when E_app is greater than the equilibrium potential. This is not necessarily the case and will depend on the instrument that you are using. A voltage greater than the equilibrium voltage will always yield an anodic current, but that current may be displayed as a negative current by some potentiostats.