Introduction

Voltammetry is an electroanalytical method. The information will be gotten from a measurement where current is measured as a function of potential. Voltammetry uses electrochemical cell for the measurements. ^[1]

Cyclic voltammetry (CV) is usually used to examine organic and metalorganic systems. Cyclic voltammetry studies the mechanisms of the system and rates of reduction and oxidation processes^[1] (redox reactions^[2]). Redox reactions are electron-transfer reactions, which happen everywhere in our environment, and therefore they are an important part of chemistry and need to be studied carefully^[2]. Mechanisms of chemical reactions are also important to study, for example, in order to design more efficient reactions and to produce more pure products^[3].

Since CV has been more and more understood and developed over the past years it can be used to quantitatively study also some complex chemical systems^[4]. CV can also used to study electron transfer reactions that include a catalysis^[5].

Cyclic voltammograms

Cyclic voltammograms are the measurement results of cyclic voltammetry. With cyclic voltammograms peak potential and peak current data can be measured as function of potential. With cyclic voltammograms scientists can interpret data from heterogenous electron transfers or homogenous chemical processess. ^[4]Cyclic voltammograms have generally a certain shape, sometimes referred as a "duck"-shape^[5]. The typical shape for reversible cyclic voltammograms come from the mass transport and diffusion processes in the solution^[4]. When the mass transport is an important factor in the solution the diffusion layer will increase causing a peak in the cyclic voltammogram^[6]. Better spectroscopic or the use other methods of detection might be required with CV considering the fact that CV might have lower resolution of structural information. Therefore, complex schemes with CV should be studied with caution. ^[4]

CV utilises triangular excitation signals. The potential is first increased linearly and afterwards the potential will be decreased linearly with the same slope to the original starting point. The excitation signal is directed to an excitation cell, which has a microelectrode. CV process can be repeated many times. This complete cycle can take only one second or even minutes. ^[1]

Scan rate is an important parameter in cyclic voltammetry. Scan rate tells how fast is the scan of the potential applied. If cyclic voltammetry is examined with a scan rate of 90 mV/s it should inform to you that the potential in the experiment was linearly varied at the speed of 90 mV per second. In other words, with a scan rate and a potential range the time the measurement will take can be predicted.  ^[5]

The potential range used in cyclic voltammograms is called switching potentials. The important parameters in cyclic voltammograms are cathodic peak potential E_pc, anodic peak potential E_pa, cathodic peak current i_pc and anodic peak current i_pa.^[1]These parameters can be seen in Figure 1 which is a schematic cyclic voltammogram where current is measured as a function of potential. The measurement is usually started when the potential is 0. The third star at right side of Figure 1 describes the point where the potential is switched to opposite direction. The arrow shows the direction of the measurement. Elgrishi et al. got the same kind of cyclic voltammogram as Figure 1 when they had a scan rate of 100 mV/s^-1 and they did a reversible reduction of a 1 mM Fc⁺ (ferrocenium) solution to Fc (ferrocene). Potential is varied on Figure 1 from 0.21...-0.25 V.  So altogether, cyclic voltammograms describe how the analyte will react to changing potential at different scan rates (how the measured current changes).^[5] The peaks of a reversible cyclic voltammograms should be almost equal, but opposite signs. 0.0592/n is the difference between the peaks of the cyclic voltammogram, where n is how many electrons were used in the reaction^[1].

Figure 1. Schematic figure of a cyclic voltammogram. (Original figure: Elgrishi et al.^[5], ACS AuthorChoice license, modified by Pinja Räisänen).

How the data can be reported

The data for cyclic voltammograms can be reported as US or IUPAC convention. The difference between these conventions is that the data is 180 degrees rotated. The arrow shows the beginning of the first segment. For example, Figure 1 uses the US convention. The difference between these conventions can be seen in Figure 2. ^[5]

Figure 2. US and IUPAC Conventions for cyclic voltammograms.  (Figure: Elgrishi et al.^[5], ACS AuthorChoice license).

Equations and laws behind cyclic voltammograms

Reversible voltammograms are based on two laws ^[4]: 

1.  Fick's law for a planar electrode which describes the mass transport to electrode.

   
   

   \[ \begin{equation} \label{(1)} \frac{d[A]}{dt}=D\frac{d^2[A]}{dx^2} \end{equation} \]

   where

   t     time

   D   diffusion coefficient

   x    distance from the electrode surface. ^[4]

   
   

2. Nernst's law decsribes the surface concentrations of redox agents for a reduction process.

   \[ \begin{equation} \label{(2)} \frac{[A]_{x=0}}{[B]_{x=0}}=e^{\frac{nF}{RT}(E(t)-E _c^{\Theta'})} \end{equation} \]

          where

          [A]_x=0  describes the surface concentration of oxidated redox reagents for a reduction process A + n e- ⇄  B'

          [B]_x=0  describes the reduced form redox reagents for the reduction process A + n e- ⇄  B as a function of potentials E(t) and E_c^Θ′ 

          t          time

          n         the number of electrons transferred when A reacts with the electrode surface

          F         Faraday constant

          R         ideal gas constant 

          T         absolute temperature. ^[4]

Nernst equation can be applied to electrochemically reversible systems and the peaks are related to Nernst equation. Nernst equation helps to predict how a system will react if the concentration of the species in solution or electrode potential changes. The reversibility in chemical reaction refers to that if the analyte is stable during reduction and whether it can later be reoxidized. Reversibility in electrochemistry tells us the kinetics of the electrode and the analyte. ^[5]

Randles-Sevcik equation helps to understand the diffusion of the analyte in the solution. This equation is used to understand the linearity of peak current i_p (A) and square root of the scan rate v^1/2 (Vs^-1). ^[5]

Electrochemical cell

As discussed above, CV utilises an electrochemical cell. In CV, electrode has to be an electrical conductor^[5]. Usually platinum is used as a microelectrode^[1]. The electrochemical cell used in CV can be seen in Figure 3. 

Figure 3. Electrochemical cell for cyclic voltammetry. (Figure: Elgrishi et al. ^[5], ACS AuthorChoice license).

Electrolyte solution

Electrolyte solution needs a good solvent and a supporting electrolyte ^[5]. 

Salt is dissolved in the solvent as a supporting electrolyte to prevent excess resistance in the solution. The mixture of a supporting electrolyte and solvent is usually called electrolyte solution. Supporting electrolyte can be, for example, ammonium salts which are usually used for inorganic experiments performed in organic solvents. The solute can be, for example, CH₃CN (acetonitrile), H₂O or [Bu₄N][ClO₄]. ^[5]

Electrodes

Different electrodes are necessary for CV. These necessary electrodes are working electrode, counter electrode and reference electrode. The actual current goes between working and counter electrodes. The reference electrode refers the reaction to a stable reference reaction. ^[5]

Working electrode (WE) has to be composed of a redox inert material which tolerates the experiment potential range. Working electrode can be for example platinum, mercury or coal. If glassy carbon electrodes are used as a working electrode, they have to be polished before use and usually also between measurements. The material studied in CV is usually placed on working electrode. ^[5]

Reference electrode (RE) should have a stable equilibrium potential since it is used as a reference point for an electrochemical cell. Usually, applied potential is reported V vs. reference electrode, as V vs. SCE in Figure 4, where SCE (saturated calomel electrode) is the reference electrode. Some other reference electrodes are standard hydrogen electrode (SHE) and silver based AgCl/Ag electrode. ^[5]

Counter electrode (CE) is used in an electrochemical cell to complete the circuit. The electrons flow between working and counter electrode. If something is studied at working electrode, oxidation takes place at counter electrode and therefore counter electrode should be as inert as possible. ^[5]

What else should be taken into account before measurements?

When the electrochemical cell has been assembled, the CV measurement can start when oxygen has been sparged and the ohmic drop has been minimized. The presence of oxygen in the electrolyte can cause unwanted reactions with the analyte. Consequently, dissolved oxygen has to be removed. The oxygen can be sparged from the solution with an inert gas, such as nitrogen gas.^[5]

Electrolytic solutions are not ideal. Ohmic drop (or IR drop ^[4]) describes the intrinsic resistance R_sol the solution has in the electrochemical cell. Ohmic drop comes from the fact that the potential experienced by the analyte in the solution might not be the same that the instrument records since there is resistance difference R_u between working and reference electrode. ^[5]Ohmic drop can be decreased, for example, with these methods: ohmic drop can be decreased by bringing the reference electrode closer to working electrode ^[4]and one could decrease current i by reducing the size of the working electrode or using slower scan rates^[5].

A practical example of a cyclic voltammogram

Benmoussa et al. studied sol-gel vanadin pentoxide V₂O₅ thin film's electrochromism with linear-sweep CV. They set their potential range for their CV experiments from -0.4 V to 1.2 V (vs. SCE). The cyclic voltammetry was reversible. The group noticed two reduction (B, A) and two oxidation (B' & A') peaks. The scan rates they used were: (a) 2 mV/s, (b) 10 mV/s, (c) 20 mV/s, (d) 30 mV/s and (e) 50 mV/s. The results obtained by Benmoussa et al. are shown in Figure 4.^[7]Note that Figure 4 uses the IUPAC convention and the data is rotated by 180 degrees compared to the US convention.

Figure 4. Linear-sweep cyclic voltammogram for V₂O₅ thin films in Benmoussa et al study. (Figure: Benmoussa et al.^[7], License CC BY 3.0).

Fast-scan cyclic voltammetry

Some molecules can be examined by cyclic voltammetry since they are electrochemically active^[8]. Common CV has too slow a time scale for some fast biological changes. Fast-scale cyclic voltammetry (FSCV) can be used to measure these changes since it has a thousand times faster scan rate than CV usually has (FSCV scan rate is around 400 V/s).^[9]FSCV can be used for molecules which undergo redox reactions. These molecules are, for example, serotonin, dopamine and norepinephrine. The potential range used in the measurement depends on the structure of molecule examined. ^[8]

Figure 5 shows how adenosine has been studied with FSCV. A) shows the applied potential waveform. The potential range is from -0.4 V to 1.45 V. The scan rate is 400 V/s and the time between scans is 100 ms. The timescale of one measurement is 9.5 ms. B) shows how the backround charging current in the buffer and addition of adenosine which makes the cyclic voltammogram large. C) has been substracted of background and shows the oxidation of adenosine. One can see that the main oxidation has been occurred at 1.4 V and the second oxidation has been occurred at 1.0 V. The detection of primary peak current is important since it is linked of how much adenosine is detected at the electrode. D) shows false color plot of multiple cyclic voltammograms which have been substacted of backround. The color in the plot describes the current measured. ^[10]

Figure 5. Detection of adenosine with fast-can cyclic voltammetry. (Figure: Nguyen and Venton ^[10], data adapted from Nguyen, Lee, Ross et al. previous article^[11]) Both articles are licensed under CC BY 4.0 license.

References