Cyclic Voltammetry: Introduction to Electrochemical Techniques
Cyclic voltammetry is an electrochemical technique for measuring the current response of a redox active solution to a linearly cycled potential sweep between two or more set values. It is a useful method for quickly determining information about the thermodynamics of redox processes, the energy levels of the analyte and the kinetics of electronic-transfer reactions.
Introduction to Potentiometry
Potentiometry is a way of measuring the electrical potential of an electrochemical cell under static conditions (i.e. no current flow).
For a general reduction or oxidation (redox) reaction the standard potential is related to the concentration of the reactants (A) and products (B) at the electrode/solution interface according to the Nernst equation:
where E is the electrode potential, E0′ is the formal potential, R is the gas constant (8.3145 J·K-1·mol-1), T is temperature, n is the number of moles of electrons involved and F is the Faraday constant (96,485 C·mol-1).
The term [B]b/[A]a represents the ratio of the concentrations of products to reactants, raised to their respective stoichiometric powers, which can be used in place of an activity term when the concentration is sufficiently low (< 0.1 mol·dm˗3).
Under standard conditions of temperature and pressure, the Nernst equation can be written as:
An electrochemical reaction is reversible in nature when the kinetics of electron transfer are sufficiently fast such that the concentration of oxidised species and the concentration of reduced species is in equilibrium.
What is Voltammetry?
Voltammetry is a technique where the current is measured as the potential between two electrodes is varied. The current generated is a result of electron transfer between the redox species and the electrodes, which is carried through the solution by the migration of ions.
Typically a three-electrode cell is used in voltammetry to separate the role of referencing the potential applied and balancing the current produced. This is because in practice it is very difficult to maintain a constant potential at the reference electrode while also passing current to counteract the redox events at the working electrode.
To measure and control the potential difference applied, the potential of the working electrode is varied while the potential of reference electrode remains fixed by a well-defined value electrochemical redox reaction. This is acheived by the reference electrode containing constant concentrations of each component of the reaction, such as a silver wire and a saturated solution of silver ions. No current passes between these electrodes.
The current observed at the working electrode is completely balanced by the current passing at the counter electrode, which has a much larger surface area.
The electron transfer between the redox species at the working electrode and counter electrode generates current that is carried through the solution by the migration of ions. This forms a capacitive electrical double layer at the surface of the electrode called the diffuse double layer (DDL). The DDL is composed of ions and orientated electric dipoles that serve to counteract the charge on the electrode.
The measured current response is dependent on the concentration of the redox species (the analyte) at the working electrode surface, and is described by a combination of Faraday’s law and Fick’s first law of diffusion:
where id is the diffusion-limited current, A is the electrode area, D0 is the diffusion coefficient of the analyte and (∂C0/∂x)0 is concentration gradient at the electrode surface. The product of the diffusion coefficient and concentration gradient can be thought of as the molar flux (mol·cm-2·s-1) of analyte to the electrode surface.
Inert ions are added to the electrochemical solution in molar excess to the analyte to provide enough ionic strength to the solution to obey the Nernst equation. The excess of electrolyte decreases the thickness of the diffuse double layer to ensure that the applied potential decreases to a negligible level within nanometers of the working electrode surface. The result is that the current response at the electrode surface is well defined.
Cyclic Voltammetry and Voltammograms
Cyclic voltammetry is a sophisticated voltammetric method. A potentiostat is used to linearly sweep the potential between the working and reference electrodes until it reaches a preset limit. It is then swept back in the opposite direction, switching potentials. This process is repeated multiple times during a scan and the changing current between the working and counter probes is measured by the device in real time.
The resulting ‘duck-shaped’ plot is called a cyclic voltammogram, an example of is displayed in Figure 1.
In Figure 1, the scan starts at -0.4V and sweeps forward to more positive, oxidative potentials. Initially the potential is not sufficient to oxidise the analyte (Figure 1, a).
As the onset (Eonset) of oxidation is reached the current exponentially increases (b) as the analyte is being oxidised at the working electrode surface. Here the process is under electrochemical control with the current linearly increasing with increasing voltage with a constant concentration gradient of the analyte near the electrode surface within the DDL.
The current response decreases from linearity as the analyte is depleted and the DDL grows in size. The current reaches peak maximum at point c (anodic peak current (ipa) for oxidation at the anodic peak potential (Epa). The process is now under mixed control: more positive potentials cause an increase in current that is offset by a decreasing flux of analyte from further and further distance from the electrode surface.
From this point the current is limited by the mass transport of analyte from the bulk to the DDL interface, which is slow on the electrochemical timescale and therefore does not satisfy the Nernst equation. This results in a decrease in current (d) as the potentials are scanned more positive until a steady-state is reached where further increases in potential no longer has an effect.
Scan reversal to negative potentials (reductive scan) continues to oxidise the analyte until the applied potential reaches the value where the oxidised analyte which has accumulated at the electrode surface can be re-reduced (e).
The process for reduction mirrors that for the oxidation, only with an opposite scan direction and a cathodic peak (ipc) at the cathodic peak potential (Epc) (f).The anodic and cathodic peak currents should be of equal magnitude but with opposite sign provided that the process is reversible.
The peak current, ip, of the reversible redox process is described by the Randles-Sevcik equation.
At 298 K, Randles-Sevcik equation is:
where n is the number of electrons, A the electrode area (cm2), C the concentration (mol·cm-3), D the diffusion coefﬁcient (cm2·s-1), and v the potential scan rate (V·s-1).
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How does Cyclic Voltammetry Work?
When performing cyclic voltammetry, the applied potential causes the chemical being tested to undergo either oxidation or reduction depending on the direction of the ramping potential. Oxidation and reduction are electron transfer processes. When a chemical undergoes oxidation, it loses an electron and is said to be oxidised. Likewise, when a chemical undergoes reduction, it gains an electron and is said to be reduced.
Cyclic Voltammetry of Ferrocene
To aid in the explanation of what occurs during the measurement, we shall use the example of ferrocene (Fc).
First, a positively ramping potential (the forward sweep) is applied between the working and reference electrodes. As the potential increases, Fc close to the working electrode is oxidised (i.e., loses an electron), converting it to Fc+. The movement of the electrons creates an electrical current.
As un-reacted Fc diffuses to the electrode and continues the oxidation process, the electrical current is increased and there is a build up of Fc+ at the electrode. This build up of reacted material is called the diffusion layer, and effects the rate at which un-reacted material can reach to the electrode. Once the diffusion layers reaches a certain size, the diffusion of Fc to the electrode slows down, resulting in a decrease in the oxidation rate and thus a decrease in electrical current.
When the potential ramp switches direction, the process reverses (the reverse sweep). Fc+ close to the working electrode reduces (i.e., gains an electron), converting it back to Fc. The electrical current flows in the opposite direction, creating a negative current. The Fc+ diffuses to the electrode, reducing to Fc and resulting in a increase in the negative current.
As with the forward sweep, a build up of material occurs near the electrode, eventually slowing down the diffusion of Fc+ and causing the negative current to decrease.
- Sevćik, A. Collection of Czechoslovak Chemical Communications 1958, 13, 349.