The underlying principle used in this type of analysis is that a quantitative amount of material is either deposited or removed from a surface or from an amalgam electrode. Because the coulombs passed can be measured very accurately, information about the surface area of an electrode or about the concentration of a metal species in solution can be determined precisely. A couple of illustrative examples will be discussed here, but there are many other applications where a similar analysis is useful.

To illustrate, we consider a method that is used to measure the surface area of a platinum catalyst in a porous electrode. At low potentials and relatively low temperatures, carbon monoxide adsorbs strongly on the surface of the catalyst. If the electrode is held near the potential of the hydrogen electrode, even a small amount of CO (100 ppm, for instance) will completely cover the surface of the electrode. The chemisorption is so strong that, even after removing the exposure to CO, the surface will remain covered until either the temperature is elevated enough to drive the adsorbed CO off or the potential is increased to oxidize the CO:

(6.15)

We’ll use removal by oxidation to quantitatively determine the amount of adsorbed CO. Specifically, after the surface is covered with CO, a linear sweep of potential is used, up to approximately 1 V relative to hydrogen. Beginning about 0.6 V, the adsorbed CO oxidizes to CO₂ resulting a sharp peak as shown in Figure 6.15. This peak has some resemblance to the peaks seen with cyclic voltammetry, but those were the result of a mass-transfer limitation. Here, the peak occurs because there is a finite amount of CO on the surface. The current versus time can be measured. This current represents both the charge needed for the oxidation of CO and that needed for charging the double layer. In this case, the formation of oxides on the surface of Pt also consumes a small amount of charge, which we will ignore. The double-layer charging (Equation 6.11) must be subtracted and the coulombs associated with CO oxidation are

Next we need to relate the charge measured to the surface area of the platinum. Pt has an FCC structure with a lattice parameter of 0.392 nm. From this information, the number of Pt atoms per unit area is calculated. As an example, for the (100) surface, there are 1.3 × 10¹⁹ atoms per square meter. We can then relate the area to coulombs of charge passed using the stoichiometry of reaction (6.15) and assuming one CO molecule per surface atom of Pt:

We have assumed that the CO is linearly bonded to the Pt. If the Pt surface is initially completely covered with CO, and given that there is 4.17 C·m⁻² of Pt surface, the surface area of platinum can be determined as simply

(6.16)

This is described as the electrochemically active surface area (ECSA) because, in contrast to BET, for instance, the metal particles must be in electronic and ionic contact to be detected with this technique. Fortunately, this is the value that is most relevant to electrochemical systems.

ILLUSTRATION 6.3

Using the data from Figure 6.15, calculate the ECSA per gram of Pt (also known as the specific surface area) from the CO adsorption experiment. What is the double-layer capacity? The platinum loading of the electrode is 0.028 mg·cm⁻² (based on the superficial area), and the scan rate is 20 mV·s⁻¹.

SOLUTION:

The solid line on the plot corresponds to the current in the absence of adsorbed CO during the potential sweep. The charge associated with the oxidation of CO is the area under the CO peak (centered around 0.8 V) minus the charge associated with double-layer charging and oxidation of the platinum; in other words, it is the peak area between the solid and dashed lines. Each increment on the y-axis is 1 A·m⁻². The peak area corresponds to about 75 rectangles of dimensions 1 A·m⁻² by 0.02 V.

Since , we need to convert the voltage increment to a time. This can be done readily noting that the voltage is scanned at a constant rate of 20 mV s⁻¹. Therefore, 0.02 V is equivalent to 1 s. The charge associated with CO oxidation is therefore

This is the amount of charge used to oxidize CO per superficial area (same area upon which the current density is based). We can now use the relationship above to calculate the ECSA per superficial area as follows:

The specific surface area is then

The capacitance of the double layer is estimated from the current around 0.5 V, above the hydrogen region but before oxidation of platinum or carbon. The current density is about 14 A·m⁻²; therefore, from Equation 6.11,

The area here refers to the superficial electrode area.