Basis of Electrochemical Technique
- Part 1: Cyclic voltammetry - I
- Part 2: Cyclic voltammetry - II
- Part 3: Cyclic voltammetry - III
- Part 4: Cyclic voltammetry - IV
Part 1: Cyclic voltammetry - I
M.Sc. Takayuki Tezuka
1. Potential sweep technique (Potentiodynamic method)
The potential sweep technique refers to a measurement of the current change in function of the time (this is called “potential sweep” or “potential scan”) by changing the potential (applied potential) to the working electrode over time. If the sweep direction is one direction only, it is called linear sweep voltammetry. Once the potential is swept, the sweep direction is reversed to continue the potential sweep, and repeat the reversal, it is called cyclic voltammetry. The result is usually displayed as a current-potential curve (voltammogram) by combining two time-varying parameters (current, potential). At this time, the voltammetric response current represents the response to the applied potential of the electrode reaction, as well as the time course of the electrode reaction as in chronoamperometry.
Fig. 1.1 Cyclic voltammetry
2. Three electrodes electrochemical cell and current direction
Basically, in this voltammetric technique, three types of electrodes are connected to the potentiostat as shown in Fig. 1.2. The current flowing between the working electrode and the counter electrode is detected while controlling the electrode potential of the working electrode with respect to the potential of the reference electrode. This is done by detecting the current flowing between the two electrodes. When an oxidation reaction occurs at the working electrode interface, electrons flow from the working electrode to the counter electrode from the external circuit connected to the potentiostat, so current flows from the counter electrode to the working electrode.
A Orientation direction of response current flow during oxidation.
B Orientation direction of electrons flow during oxidation.
Fig. 1.2 Potentiostat and electrode connection
Part 2: Cyclic voltammetry - II
M.Sc. Takayuki Tezuka
3. Current - Potential curve
A typical cyclic voltammetry example is the current - potential curve of a solution containing ferricyanide ion (2 mmol/L potassium ferricyanide + 1 mol/L potassium nitrate). In this one-electron redox reaction, the ferricyanide ion Fe(CN)63− is an oxidant and the ferrocyanide ion Fe(CN)64− is a reductant.
Fig. 3.1 Shows the applied potential variation as a function of the time, and it sweeps at a scan rate of 100 mV/s in the negative direction (reduction direction), from 0.6 V to -0.1 V vs Ag/AgCl reference electrode. The result is shown in the cyclic voltammogram (Fig. 3-2). The current, which was about 0 A at the start of the measurement, starts to flow in the first segment of sweeping the potential in the negative direction, and the current for the reduction (the sign is minus in this figure) flows out, reaching the reduction peak at 0.229 V. The current observed at this time is called the reduction peak current. After that, the potential sweep direction is turned back at -0.1 V, and oxidation of ferrocyanide ion starts at around 0.1 V, and an oxidation peak is observed at 0.289 V. The difference between the oxidation peak potential and the reduction peak potential is called the peak potential difference, and the value is close to 0.059/n V (59/n mV) at 25°C. Note that n is the number electrons transferred. However, this equation can only be applied in case of fast electrons transfer.
where: ΔE is redox peak potential difference, Epa is peak anodic potential,
Epc is peak cathodic potential and n is number of electrons transferred.
The midpoint potential between the two peak potential (midpoint potential) Em (0.259 V) is approximate to the standard electrode potential Eo, because there is no difference between diffusion rate (diffusion coefficient) for oxidant and reductant. Thus cyclic voltammetry is often used to estimate the approximately standard electrode potential Eo.
where: Em is midpoint potential, Epa is peak anodic potential,
Epc is peak cathodic potential and E0 is standard reduction potential.
Fig. 3-1 Time change of sweep potential Fig. 3-2 Cyclic voltammetry
Part 3: Cyclic voltammetry - III
M.Sc. Takayuki Tezuka
4. Peak Current
The faradaic current at the peak value is referred to as the peak current, and it is known that the following equation can be adapted to the peak current of reversible reactions with fast electrons transfer. (Randles - Sevcik equation).
where: ip is peak current, F is Faraday constant (96485 C/mol),
A is working electrode surface area, D is diffusion coefficient,
c is concentration, v is scan rate, R gas constant (8.31 J.K-1.mol-1),
and T is temperature.
At 25°C, this equation summarizes the constants and is expressed as follows.
The peak current is proportional to the concentration c of the redox substance and the square root of the scan rate v.
Fig. 4-1 shows the result of cyclic voltammetry of 2 mmol/L potassium ferricyanide solution at scan rates of 0.01, 0.04, and 0.09 V/s. Fig. 4-2 plots the relationship between the square root of the scan rate and the reduction peak current, and it can be seen that there is a linear relationship. Also parameters such as the diffusion rate D can be get by drawing an approximate line for this plot and using its slope.
Fig. 4-1 Cyclic voltammogram of 2 mmol/L K3Fe(CN)6 varying the scan rate.
Fig. 4-2 Plots of reduction peak current versus square root of the scan rate.
Part 4: Cyclic voltammetry - IV
M.Sc. Takayuki Tezuka
5. Faraday current and charging current
In cyclic voltammetry, besides of the Faraday current caused by the redox reaction, the charging current, the current flows due to charge and discharge in the electric double layer formed at the electrode interface. This is generally corrected by subtracting the measured (background) cyclic voltammetry of the electrolyte containing no redox substance.
Fig. 5-1 shows the cyclic voltammogram (red line) and overwrited background (brown line), and the red line subtracted the background, the (blue line).
Fig. 5.1 Cyclic voltammogram before correction (red line), background (brown line) and after correction (blue line)
6. Quasi-reversible and non-reversible cyclic voltammetry
An electrochemical reaction with a slow electron transfer rate at the electrode interface is called as a non-reversible reaction. This is due to the reaction rate constant k0 (n is the number of electrons transferred).
Reversible: k0 > 0.3(nv)1/2
Quasi-reversible: 2 x 10-5 (nv)1/2 < k0 < 0.3(nv)1/2
Non-reversible: k0 < 2 x 10-5 (nv)1/2
Fig. 6-1 is a cyclic voltammogram of a quasi-reversible reaction, which the reaction rate k0 of cyclic voltammogram is changed using simulation software. In such cyclic voltammogram, although the peak potential difference spreads, the midpoint potential Em can be used to estimate the standard electrode potential E0 if the extent of spread is almost the same on the oxidation side and the reduction side. While, when a clear peak does not appear, it becomes difficult to estimate the standard electrode potential E0 from the midpoint potential Em.
Fig. 6-1 Cyclic voltammograms of varying reaction rate.
As described above, the criteria for classification of reversible and non-reversible reactions using reaction rate constants depend on the number of electrons tranferred (n) and the square root of the scan rate (v), respectively. Thus, for example, in the case of one-electron reaction of k0 = 0.05 cm/s, it is quasi-reversible at v = 0.1 V/s, but reversible at v = 0.01 V/s. Thus, even if the sample to be measured is the same, in some cases it become possible to analyze as a reversible reaction by reducing the scan rate of cyclic voltammetry.