1. Defining the cell
The voltammetric methods we have described thus far involve the application of a potential from an external power source to a working electrode (WE) relative to that of a reference electrode (RE), and the measurement of current that flows as a result of that applied potential. In most instances, the electrochemical cell in which this process is carried out involves a solid working electrode immersed in a solution of charged electrolyte along with a reference half-cell of fixed composition.
As was discussed in the Electrochemical thermodynamics section of this module, current passing in the cell as a result of applied potential can be of two basic types: 1) nonfaradaic, in which a change in the potential at the WE causes a rearrangement of electrolyte charges at each electrode/solution interface to counter the charge at the electrode surface; 2) faradaic, in which electrons are transferred across the electrode/solution interface to a solution species which is either oxidized or reduced. The two-electrode cell shown in Figure 32 is suitable as long as very little current passes during the experiment. This is the case for very low concentration solutions, or for very small working electrode dimensions (ultramicroelectrodes). In most practical applications, however, the circuit is designed to include a third electrode called the auxiliary electrode (AE). The device applying the potential and measuring the current (the potentiostat) electronically isolates the reference electrode so that little or no current passes through it, while allowing current to pass through the working and auxiliary electrodes. In this way, the activities of the reference half-cell components and thus the reference potential are prevented from changing during the experiment (a desirable characteristic!). The potentiostat will be discussed in a later section.
2. Positioning of the electrodes
For the remainder of our discussion, we will consider only the three-electrode electrochemical cell. The primary consideration for the relative location of the three electrodes in a cell is the minimization of the solution resistance. This is most usually accomplished by keeping the tips of the three electrodes as close together as feasible, while not interfering with the current paths between one another. Close approach is especially critical for the reference and working electrodes. The solution resistance between these electrodes leads to an iR drop that manifests itself as an error in the measured potential difference between them. In addition, the potentiostat is normally unable to electronically compensate for this resistance, as it can for the resistance between the reference electrode and the auxiliary electrode. A Luggin capillary arrangement is often employed, in which the RE is placed inside a tube drawn at the end to a fine capillary allowing very close positioning relative to the WE.
A second important consideration involves the shape and size of the auxiliary electrode relative to the working electrode. The AE should be at least as large in area as the WE, and positioned symmetrically with respect to the WE so that the current density and potential experienced along its entire length is constant.
In most voltammetric experiments, the three electrodes can be placed together in the same solution. Under certain conditions, however, either the AE or the RE (or both) may need to be physically isolated from the solution containing the WE. For example, if the goal of the experiment is the total electrochemical conversion of the bulk electroactive material to product (bulk electrolysis), products produced at the AE may be detrimental to the reaction occurring at the WE. Other times, small amounts of leakage of RE components, like Ag+ or even water from a Ag/AgCl reference, may cause undesirable reactions in the electrolyis solution, especially in experiments that involve non aqueous solvents. These problems can generally be avoided by the use of a glass frit or Vycor membrane (which allow charge to pass but not mixing of electrolyte) to separate the compartments of the electrochemical cell.
3. Other considerations
Most experiments require that dissolved oxygen be removed from the cell. Not only is it electrochemically active across the cathodic potential range, it is very likely to react with products formed by electron transfer. Typically, the solution is sparged with an inert gas like nitrogen or argon for 5 – 10 minutes prior to the experiment, and a “blanket” of inert gas maintained above the solution during the experiment.
Cell volumes are quite variable and range from microliters to many mL, depending upon the goal of the experiment. Designs for cells can be quite simple – a glass vial with a screw cap drilled with holes for each electrode – to quite complex, requiring significant glass-blowing and/or machining skills. Cells are available from numerous electrochemical suppliers, some with very specific applications.
Two common cell configurations for quiescent solution voltammetry are shown in Figure 33. On the left, the AE has been fashioned to be symmetrical to the WE. At right, a cell is shown with RE isolated from the solution containing the WE using a Luggin capillary. The AE is also isolated in a fritted compartment. Both cells have a fritted sparge tube to allow deoxygentation of the solution with inert gas.