Ideally, we would like to measure or control the potential of the WE relative to the RE, where the reference electrode is just outside the diffuse double layer of the working electrode. Unfortunately, it is not possible to do so. There are two principal concerns: first, the degree to which the physical presence of the RE disturbs the current and potential distributions near the electrode, and second, the ohmic losses between the RE and WE. Given the dimensions of the double layer (∼nm) and the finite size of any RE, choosing where to place the electrode and how to correct for any iR differences takes some care.
Imagine that we have large planar WE and CEs separated by some distance. The current distribution is uniform, and the potential varies linearly. Figure 6.24 shows isopotential lines around a reference electrode placed far from the WE (a) and close to the WE (b); the WE is on the left. If the reference electrode is placed far from the WE, it does not interfere much with the potential distribution (and current) around the surface of the WE. However, there is clearly a difference between the measured potential and the actual potential just outside the double layer. The potential difference can be reduced by moving the RE closer to the WE. But what happens if we position the electrode adjacent to the WE? This placement is shown in Figure 6.24b. In contrast to the earlier case, the potential and current distributions are affected by the presence of the RE. Additionally, for systems where fluid flow is important, the reference electrode can change the velocity field. For instance, a RE placed near the disk of the RDE discussed in the previous section would completely alter the fluid flow and negate the key RDE feature of uniform accessibility.
To minimize the disturbance of the current and potential distributions by the RE, the reference electrode should be far away from the surface; however, we noted this may introduce a large error in the measured potential. Fortunately, with some careful attention, this dilemma can be largely addressed.
Figure 6.25 shows an equivalent circuit representation of this situation. The RC element on the right symbolizes the resistance to electron transfer (faradaic) and the double-layer capacitance (non-faradaic). The RE electrode is a finite distance away from the CE and, of course, we control the potential between the RE and WE. Compared to Figure 6.6, there exists an additional resistance between the RE and WE. This resistance is called the uncompensated resistance, RU. The true overpotential will be either higher or lower depending on the direction of current. Adjusting for this potential drop is called iR correction.
The approach to dealing with this resistance is first to minimize it as much as possible, and then to compensate for any iR that cannot be eliminated. There are several ways to minimize the uncompensated resistance:
- Use small electrodes. Microelectrodes are discussed in next section—essentially with small electrodes, the currents are small and therefore the ohmic losses are reduced.
- Supporting electrolyte can be added to increase conductivity of solution and therefore reduce the ohmic loss. The use of supporting electrolyte was discussed in Chapter 4, and this method is often used in electroanalytical chemistry. It does add additional ions that may be detrimental. Therefore, it is not always an option.
- Move the RE closer to WE by using a Luggin capillary, for example. A Luggin capillary (see Figure 6.26) is typically made of glass and consists of a long thin neck that is open at the end. The capillary is filled with the same electrolyte as in the system.
After we have reduced the uncompensated iR as much as possible, we can try to either correct for any remaining resistance or to compensate for the resistance. Both attempt to accomplish the same goal, but the key difference is that compensation is done automatically with electrochemical instrumentation, such as a potentiostat. There are several methods to make the necessary correction or compensation automatically during the experiment, and the method of choice will depend on the experiment that you are performing. Some of these methods require that the resistance of the electrolyte be known a priori; an AC measurement is typically used to find the required value in such cases. The easiest strategy to understand and implement is current interruption as discussed in Section 6.4. During operation, the current is interrupted very briefly and the change in potential due to ohmic loss is identified as discussed previously. The necessary correction is then made while running the experiment. While generally effective in iR compensation, automatic strategies can lead to instabilities and care should be taken when implementing such an approach.
Is there a way to know if iR correction or compensation is being done correctly? It’s not always straightforward, but let’s examine a typical cyclic voltammogram collected where there is significant iR drop between the working and reference electrodes. Figure 6.27 reproduces the CV shown in Figure 6.11, the true electrochemical response is shown with the solid line. Also plotted in this figure (dashed line) is the same CV, but where there is significant uncompensated iR. Note that the CV with uncompensated iR is skewed, and the peak potentials are no longer separated as predicted by Equation 6.20. One might be tempted to conclude from the uncompensated data that the reaction is quasi-reversible because of the large peak separation. Carefully compare the data for the CV with uncompensated iR with one for a quasi-reversible reaction (Figure 6.14) to convince yourself that the behaviors are different.
IR, CORRECTION, AND COMPENSATION
iR drop refers to the potential difference measured between two points in an electrolyte caused by the flow of current. iR is also called ohmic drop.
iR correction is a means of adjusting the potential of the WE relative to the RE to account for iR drop in the electrolyte. The correction may be positive or negative.
iR compensation is a technique that is used by typical potentiostats to automatically correct for iR drop in the electrolyte. Often, full compensation is not accomplished because of stability problems with the instruments.
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