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Electrochemistry has multiple characterisation techniques, but perhaps the most widely used to date is cyclic voltammetry. Regardless of what your research is about, acquiring cyclic voltammograms as part of your characterisation is always a good place to start. For this reason, in this post we explain everything you need to know about the technique so that you can start applying as part of your experiments.
What is cyclic voltammetry?
In simple terms, cyclic voltammetry (also known as CV) is a technique that studies how the current at the working electrode changes as the potential applied is sweeped back and forth between two distinct voltages.
Cyclic voltammetry excitation waveform
When obtaining voltammetric data, the working electrode of the electrochemical cell needs to experience a series of applied potentials in a specific sequence known as the excitation waveform. The shape of this excitation waveform differs between the different techniques.
For cyclic voltammetry, this excitation waveform is triangular.
While this triangular excitation wave seems smooth, in practice, however, the slopes of the wave follow a staircase fashion. This makes it easier to program with today’s digital electronics and also allows further control over the final size of the data by controlling the height of the potential steps (Estep).
Cyclic voltammetry plots
The data originating from cyclic voltammetry experiments is typically plotted as current vs potential (i vs E) plots commonly referred to as cyclic voltammograms.
A typical example of a cyclic voltammogram is that of a reversible redox probe, like the one in the picture above, that displays an oxidation and a reduction peak a few millivolts apart in a “duck-like” shape.
Parameters of cyclic voltammetry
When programming a modern potentiostat to perform cyclic voltammetry there are a few parameters that need to be set up. Each of these parameters has a different effect on the resulting voltammogram, so it is important to understand what they are and what they do.
The parameters can be divided into 2 types of settings: pretreatment and cyclic voltammetry settings.
Stage 1: Pretreatment Settings
The pretreatment settings are a series of parameters that are applied to the electrochemical cell before the cyclic voltammetry measurement starts.
The purpose of the pretreatment setting is simple: minimize artifacts at the beginning of the measurement and to minimize variations between experiments.
Depending on the potentiostat model, these pretreatment settings can either be part of the cyclic voltammetry technique or need to be set up separately. But in general there are 2 types of pretreatments available:
- Conditioning: often also referred to as “quiet time”, this pretreatment equalizes the electrode potential at the beginning of the experiment by holding the working electrode at a specific potential for a set amount of time. This helps avoid variations between experiments and artifacts from differing resting potentials (OCP) due to uncontrollable experimental conditions such as impurities or surface charging. It is set up by providing the following parameters:
- Conditioning potential (V)
- Conditioning time (s)
- Deposition: this pretreatment is performed after the conditioning and is mostly used in experiments that require either stripping or preconcentration of the analyte. This pretreatment helps improve experimental reproducibility and may help increase signal. It is set up by providing the following 2 parameters:
- Deposition potential (V)
- Deposition time (s)
Stage 2: Cyclic Voltammetry Settings
- Equilibration time (s): this parameter determines the amount of time that the potentiostat will wait before starting the measurement. This is particularly useful when the measurement is intended to start from the open circuit potential (OCP) of the electrochemical cell, otherwise it will typically be 0.
- Starting potential (V): this parameter states the first potential to be applied to the electrochemical cell during the cyclic voltammetry measurement. It is important to select it carefully to avoid artifacts at the beginning of the measurement. Ideally, it should be equal to the last potential applied during the pretreatment (if any) or 0 when measuring against the OCP with a preset equilibration time. If multiple scans are used in a single measurement, the starting potential will be applied only once.
- Potential vertex 1 (V): this refers to one of the extremes of the cyclic voltammetry measurement range.
- Potential vertex 2 (V): this refers to the other of the extremes of the cyclic voltammetry measurement range.
- Potential step (V): this setting specifies the height of the potential step in the staircase wave of the excitation of the cyclic voltammetry measurement, as well as the final resolution of the voltammogram.
- Scan rate (V/s): this setting specifies how fast the triangular wave will be applied to the working electrode during the cyclic voltammetry measurement. Due to its time relationship, it will have an impact with the thickness of the diffusion layer that will interact with the working electrode during the measurement.
- Number of scans: this parameter states the amount of times that the triangular potential wave from vertex 1 to vertex 2 will be applied to the working electrode.
- Mesurement vs OCP or vs reference electrode: this setting states how the potential settings will be calculated. Normally, by default, potentials are calculated vs the reference electrode. But in some cases it may be interesting to read vs the OCP of the electrochemical cell (for example, for corrosion studies). In this cases, the OCP is first recorded according to two subparameters:
- Max OCP time (s): the maximum amount to wait for a stable OCP
- Stability criterion (mV/s): the criteria to determine that the OCP is stable
- IR drop compensation: this parameter allows the compensation for an internal resistance loss (IR drop). This parameter may not always be available as a setting since some potentiostats only allow IR compensation as a post-processing data transform. For those potentiostats that do allow IR drop compensation, when used they will require the compensated resitance in Ohms.
Who invented cyclic voltammetry?
The invetion of cyclic voltammetry is attributed to czech chemist and Nobel Prize winner Jaroslav Heyrovský. His work main field of work was devoted to the development of polarography, a primitive form of voltammetry that utilized a hanging mercury drop as a working electrode.
What is cyclic voltammetry used for?
Despite the simplicity of the concept behind cyclic voltammetry, it is one of the most powerful electrochemical techniques out there.
It is used to study the redox properties of electroactive molecular species and their interaction with the working electrode. By analyzing the resulting voltammograms the following key parameters may be determined:
- The electrochemical window of the electrolyte
- The working electrode capacity
- The oxidation and reduction potentials of electroactive molecular species
- The reversibility of redox molecules
- The concentration of the electroactive molecular species
- The diffusion coefficient of electroactive molecules
- The electron transfer rate of electrochemical systems (Nicholson’s Method)
As you can see, cyclic voltammetry is extremely versatile if utilized properly. That’s why it is often used as the first step in electrochemical characterisation.
Is Cyclic Voltammetry a destructive technique?
In general, cyclic voltammetry is considered a non-destructive technique. This is because it only acts upon a small portion of the sample (i.e. the diffusion layer) and compared to the overall volume of the sample it may not show a significant difference in the overall concentration.
However, it must be noted that applying potentials to interact with electroactive materials that undergo irreversible oxidation/reduction reactions over long periods of time will eventually result in a decrease in the concentration. An example of this is the application of cyclic voltammetry for electropolymerization or electrodeposition processes where the electroactive species in solution undergoes an irreversible electrochemical reaction and ends up being deposited on the working electrode.
Analyzing cyclic voltammetry data
The i vs E plots arising from cyclic voltammetry measurements are information-dense datasets that need to be analyzed properly in order to extract valuable insights.
Types of currents in cyclic voltammetry data
The first thing that must be noted from cyclic voltammetry data is that it displays 2 types of currents: faradic currents and residual (capacitive) currents.
When we refer to Faradic currents we are talking about current readings due to electron transfer processes. That is, electrooxidation/reduction reactions, which can be reversible or irreversible.
These types of currents are normally observed as peaks and tend to be the basis of electrochemical energy storage in batteries, as well as redox-labeled biosensor assays.
Residual or capacitive currents
Besides currents due to oxidation and reduction processes, cyclic voltammograms also display a background or residual current. This background current is due to the charging of the double layer capacitance and has a rectangular-like shape.
Whilst for most planar electrodes the capacitive current is small and does not severely impair the detection of faradic currents, for highly porous materials it may end up interfering with the signal.
4 insights that can be obtained from cyclic voltammetry
Now that we know the types of currents that can be observed in voltammograms, let’s see what kind of information can we extract from cyclic voltammetry.
1. The Electrochemical Window
One of the first bits of information that can be obtained from cyclic voltammetry is the so-called electrochemical window.
The electrochemical window, in essence, the potential range where the electrolyte and the electrode of the cell remain stable and do not degrade.
In an ideal world, the cyclic voltammogram of just the electrolyte would show an recangular-like central region (the double layer capacitance of the electrode) and, at the extremes, an exponential increase in the absolute current. These currents correspond to the rapid degradation of the electrolyte which ends up decomposing into other smaller molecules due to the electrolysis of the electrolyte. In the case of water, these smaller molecules would be oxygen and hydrogen gas.
However, in the real world, electrode materials can also undergo chemical transformations and/or degrade, contributing to the shape of the voltammogram. For example, for gold electrodes at potentials above 1 V vs Ag/AgCl in neutral electrolytes generate a surface oxide, and in certain acidic electrolytes it can dissolve. In these two cases, the effective electrochemical window for the experimentation should avoid, where possible, the potentials where these electrode reactions ocurr.
2. Redox Potentials
Another important insight that can be obtained from cyclic voltammetry is the redox potentials of the electroactive species in the electrochemical cell.
Knowing the redox potential of a particular reaction is super important for the development and improvement of electrochemistry applications.
Redox potentials define the minimum potential required in the system to oxidize or reduce a molecule. This helps determine the best way to perform assay tests and identify potential interfering molecules.
Moreover, redox potentials and their patterns can be used to identify substances in a mixture. While not as powerful as other more sophisticated techniques such as LCMS, redox potentials have been used as point-of-care tests for analytes like heavy metals in water bodies.
3. Analyte concentration
While cyclic voltammetry is not normally the first choice to determine analyte concentration in point-of-care devices, it can certainly be used for this purpose.
The peak height of voltammograms can be linked to the concentration of a known redox marker and used to develop assays or troubleshoot experiments in the lab.
The concentration can be determined experimentally by performing a calibration curve or estimated utilizing Randles-Sevcik equation if all the diffusion parameters of the analyte are known.
4. Electrode capacitance
With cyclic voltammetry we can determine the capacitance of the electrochemical cell. This is a good starting point when characterizing capacitors and pseudocapacitors before moving to other charaterization methods more similar to their real-world operation.
5. Diffusion coefficient of redox species
Cyclic voltammetry can be used to characterize the diffusion coefficient of redox molecules. To do so, it is necessary to perform experiments at multiple scan-rates while keeping the concentration of the redox species and the area of the working electrode constant. With these conditions, the peak daa arising from the oxidation/reduction of the molecule can be fed to the Randles-Sevcik equation to calculate its diffusion coefficient.
Advantages and Disadvantages of Cyclic Voltammetry
The main advantages of cyclic voltammetry are:
- Simple technique
- High sensitivity
- Low cost
The disadvantages of cyclic voltammetry are:
- High capacitive background current
- Low discrimination of overlapping peaks
- Requires complex electronics
- Complex data analysis
While there are some disadvantages to this particular technique, there are other techniques that can compensate for them. Moreover, electronic limitations are being rapidly eliminated by the development of new miniaturized potentiostats that open new application avenues.
What equipment do you need to perform cyclic voltammetry?
Cyclic voltammetry can be performed in any electrochemistry lab. You will only need:
- An electrochemical cell consisting of:
- A counter electrode
- A working electrode
- A reference electrode
- An electrolyte
- A potentiostat to operate the electrochemical cell
- A computer to store and analyze the data.
In electrochemistry, cyclic voltammetry is a staple characterization technique used in labs around the world. In fact, it is how most electrochemical research and development starts. Therefore, understanding how it works and what it can do is extremely important. With this post, we have introduced the technique, its main parameters and applications so that you can grasp the tremendous versatility of cyclic voltammetry.