Discussion Examples for Cyclic Voltammetry | Redox Peaks and Reversibility

What Is Cyclic Voltammetry?

Cyclic voltammetry is a method in which the potential of the working electrode is changed at a constant rate over time, reversed at a certain potential, and then swept in the opposite direction while the resulting current is measured.
Because the potential is swept back and forth, both oxidation and reduction reactions can be observed in the same measurement.
The resulting current-potential curve is called a cyclic voltammogram.

In CV, the oxidation current increases at the potential where a substance is oxidized, producing an oxidation peak.
After the scan direction is reversed, a reduction peak appears if the oxidized substance is reduced.
The reversibility and reaction rate of the redox reaction can be discussed from the positions and magnitudes of these oxidation and reduction peaks.

Main Items to Include in the Results

In CV results, organize the sample concentration, supporting electrolyte, working electrode, reference electrode, counter electrode, potential range, scan rate, oxidation peak potential, reduction peak potential, oxidation peak current, reduction peak current, peak potential difference, peak current ratio, and other information.
Because the peak shape and position change depending on the measurement conditions, it is important to record the conditions specifically.

Main Items to Include in the Results

• Name of the sample measured
• Sample concentration
• Type of supporting electrolyte
• Supporting electrolyte concentration
• Solvent
• Type of working electrode
• Type of reference electrode
• Type of counter electrode
• Potential scan range
• Scan rate
• Oxidation peak potential Epa
• Reduction peak potential Epc
• Oxidation peak current ipa
• Reduction peak current ipc
• Peak potential difference ΔEp
• Peak current ratio ipa/ipc
• Peak shape
• Judgment of reversibility
• Causes of error and points for improvement

Reference Experimental Values and Calculation Examples for Cyclic Voltammetry

Here, the oxidation peak, reduction peak, peak potential difference, peak current ratio, and scan-rate dependence obtained in cyclic voltammetry measurements are organized using reference experimental values.

In CV, the potential is swept back and forth at a constant rate and the changes in current accompanying oxidation and reduction reactions are measured.
By comparing the oxidation peak potential, reduction peak potential, and magnitude of the peak currents, the reversibility of the redox reaction and the ease with which the electrode reaction proceeds can be discussed.

Reference Experimental Conditions

Example of Basic CV Measurement Results

First, the peak potentials and peak currents are organized for a measurement performed at a fixed scan rate of 100 mV/s.
An oxidation peak appears during the scan in the oxidation direction, and a reduction peak appears in the reduction direction after the scan is reversed.

Example Calculation of Peak Potential Difference

The difference between the oxidation peak potential and the reduction peak potential is called the peak potential difference.
For an ideal reversible one-electron transfer reaction, the peak potential difference is considered to approach approximately 59 mV.

Peak potential difference ΔEp = Epa − Epc

In this reference example, the oxidation peak potential is +0.284 V and the reduction peak potential is +0.214 V, so the peak potential difference is calculated as follows.

ΔEp = 0.284 − 0.214 = 0.070 V

0.070 V corresponds to 70 mV.
Although this is somewhat larger than the ideal value of 59 mV, the oxidation and reduction peaks are clearly observed and the magnitudes of the peak currents are also similar, so the redox reaction can be considered relatively highly reversible.

Example Calculation of Peak Current Ratio

When considering reversibility, the magnitudes of the oxidation and reduction peak currents are also compared.
Because the reduction peak current is often expressed as a negative value, the absolute value is used for comparison.

Peak current ratio = |ipc| ÷ ipa

In this reference example, ipa is 18.6 μA and ipc is -17.9 μA, so it can be calculated as follows.

|ipc| ÷ ipa = 17.9 ÷ 18.6 = 0.96

When the peak current ratio is close to 1, the oxidized substance is considered to be reduced again during the reverse scan.
Therefore, this provides evidence that the electrode reaction has high reversibility.

Measurement Results When the Scan Rate Is Changed

Next, the oxidation and reduction peak currents are compared when the scan rate is changed.
For a reversible diffusion-controlled reaction, the peak current tends to be proportional to the square root of the scan rate.

Relationship Between Scan Rate and Peak Current

As the scan rate increases, the absolute values of both the oxidation and reduction peak currents increase.
This is because faster potential changes produce larger concentration changes near the electrode surface, and mass transfer by diffusion is reflected in the peak current.

For example, at a scan rate of 25 mV/s, the oxidation peak current was 9.1 μA, whereas at 400 mV/s it was 37.5 μA.
Because the peak current increases as the scan rate increases, the current response can be said to depend on the scan rate.

Relationship Between Peak Current and the Square Root of Scan Rate

In a diffusion-controlled reaction, the peak current tends to be proportional not to the scan rate itself but to the square root of the scan rate.
Therefore, examining the relationship between ip and v^1/2 makes it easier to discuss whether the reaction is diffusion controlled.

Because the values of ipa / v^1/2 are nearly constant, the oxidation peak current in this reference example can be considered approximately proportional to the square root of the scan rate.
Therefore, the electrode reaction can be explained as proceeding mainly under diffusion control.

Change in Peak Potential Difference with Scan Rate

As the scan rate becomes faster, the peak potential difference ΔEp becomes larger.
This is because the electrode reaction and mass transfer can no longer completely follow the speed of the potential scan.

At lower scan rates, the peak potential difference is small and the behavior is close to that of a reversible reaction.
On the other hand, at higher scan rates, the peak potential difference increases, suggesting that delays in electron transfer or diffusion may have an effect.

Main Points for Judging Reversibility

When discussing reversibility from CV results, it is important not only to check whether peaks are present but also to make a judgment by combining multiple indicators.

Example of How to Write the Results

When CV measurement was performed at a scan rate of 100 mV/s, the oxidation peak potential Epa was +0.284 V and the reduction peak potential Epc was +0.214 V.
The peak potential difference ΔEp was 70 mV.
In addition, the oxidation peak current ipa was 18.6 μA and the reduction peak current ipc was -17.9 μA, giving |ipc|/ipa = 0.96.

When the scan rate was varied from 25 to 400 mV/s, the absolute values of both the oxidation and reduction peak currents increased as the scan rate increased.
In addition, the oxidation peak current was approximately proportional to the square root of the scan rate.
This suggests that, under these measurement conditions, the electrode reaction proceeded mainly under diffusion control.

Points for Connecting the Results to the Discussion

In a CV discussion, it is important not only to list peak potentials and peak currents but also to explain the reversibility of the redox reaction, whether the reaction is diffusion controlled, and how the results change with scan rate.

• Were both the oxidation and reduction peaks clearly observed?
• Was the peak potential difference ΔEp small and close to the value expected for a reversible reaction?
• Was |ipc|/ipa close to 1, with the oxidation and reduction currents corresponding to each other?
• Did the peak current increase as the scan rate increased?
• Was the peak current proportional to the square root of the scan rate, indicating diffusion control?
• Can the reason why the peak potential difference widened at faster scan rates be explained?
• Could contamination of the electrode surface, solution resistance, concentration, supporting electrolyte, or insufficient deaeration have affected the results?

Example Discussion

In this experiment, both the oxidation and reduction peaks were clearly observed by cyclic voltammetry.
At a scan rate of 100 mV/s, the peak potential difference was 70 mV, which was somewhat larger than the approximately 59 mV expected for an ideal reversible one-electron reaction.
However, the peak current ratio |ipc|/ipa was 0.96, close to 1, so the oxidized species was considered to have been reduced again during the reverse scan.
Therefore, this redox system can be judged to have relatively high reversibility.

As the scan rate increased, the absolute values of both the oxidation and reduction peak currents increased.
In addition, the oxidation peak current was approximately proportional to the square root of the scan rate.
This was considered to result from the supply of reactant to the electrode surface being controlled mainly by diffusion.
Therefore, the peak current in this experiment can be said to reflect a diffusion-controlled electrode reaction.

On the other hand, as the scan rate increased, the peak potential difference widened from 57 mV to 108 mV.
This was considered to result from electron transfer and mass transfer being unable to completely follow the potential change under faster scan conditions, thereby increasing the potential difference between the oxidation and reduction peaks.
Therefore, under high-speed scan conditions, the behavior may have shifted from a reversible reaction toward quasi-reversible behavior.

In addition, the measurement results may be affected by contamination of the working-electrode surface, solution resistance, supporting-electrolyte concentration, sample concentration, and dissolved oxygen.
In particular, when the electrode surface is contaminated, the peak current may decrease or the peak shape may become distorted.
Therefore, to obtain reproducible CV measurements, it is important to thoroughly polish and clean the electrode and keep the measurement conditions constant.

Summary

In cyclic voltammetry, the reversibility of a redox reaction and the characteristics of mass transfer can be evaluated from the positions and magnitudes of the oxidation and reduction peaks.

In this reference example, the peak current ratio was close to 1 and the peak potential difference was small at low scan rates, so the redox reaction was considered to have relatively high reversibility.
In addition, because the peak current was approximately proportional to the square root of the scan rate, the reaction can be explained as proceeding mainly under diffusion control.

What Is an Oxidation Peak?

An oxidation peak is a current peak that appears when a reduced species in the sample loses electrons and becomes an oxidized species as the potential is swept in the positive direction.
For example, in a reaction where Red is oxidized to Ox, Red transfers electrons to the electrode, causing an oxidation current to flow.
When the potential reaches a value sufficient for oxidation, the current increases and eventually shows a peak because of the effects of diffusion and other factors.

The oxidation peak potential Epa serves as an indicator of the potential at which the oxidation reaction readily occurs.
When the oxidation peak is sharp and clear, the oxidation reaction is considered to proceed relatively distinctly at the electrode surface.
On the other hand, if the peak is broad, shifted, or small, the effects of electron-transfer rate, diffusion, electrode-surface condition, concentration, resistance, and other factors must be considered.

Red → Ox + ne-

What Is a Reduction Peak?

A reduction peak is a current peak that appears when the oxidized species accepts electrons and returns to the reduced species as the potential is swept in the reverse direction.
If the oxidized species produced during the forward scan is reduced again during the reverse scan, a reduction peak is observed.
The reduction peak provides important information for judging whether the redox reaction readily proceeds in the reverse direction.

The difference between the reduction peak potential Epc and the oxidation peak potential Epa provides a clue for considering the reversibility of the reaction.
If a clear reduction peak is observed, the oxidized species is considered to have remained near the electrode without decomposing within the measurement time and to have been reduced.
If the reduction peak is small or disappears, decomposition through a chemical reaction, diffusion, adsorption, or an irreversible reaction should be considered.

Ox + ne- → Red

Peak Potential Difference and Reversibility

The difference between the oxidation peak potential Epa and the reduction peak potential Epc is called the peak potential difference ΔEp.
ΔEp is an important indicator for evaluating the reversibility of a redox reaction.
For an ideal reversible one-electron reaction, ΔEp is considered to be close to approximately 59 mV near 25°C.

In actual experiments, ΔEp is often larger than the ideal value because of the effects of solution resistance, electrode-surface condition, electron-transfer rate, scan rate, and other factors.
A larger ΔEp may indicate slower electron transfer or nonideal measurement conditions.
When judging reversibility, it is important to consider not only the peak potential difference but also the peak current ratio and scan-rate dependence.

ΔEp = Epa – Epc

Discussion of Reversible Reactions

A reversible reaction is a reaction in which electron transfer between the oxidized and reduced species is fast and the reaction quickly follows changes in electrode potential toward a state close to equilibrium.
In CV, when both the oxidation and reduction peaks are clearly observed, the peak potential difference is small, and the peak current ratio is approximately close to 1, the reaction is considered highly reversible.

In a reversible reaction, the peak potential difference does not tend to change greatly even when the scan rate is changed, and the peak current tends to be proportional to the square root of the scan rate.
However, even for a reversible reaction, the peak potential difference may become larger than the theoretical value if the experimental conditions are not ideal.
Reversibility is judged comprehensively from multiple indicators.

Discussion of Quasi-Reversible Reactions

A quasi-reversible reaction is a reaction in which electron transfer is not completely fast and the reaction proceeds with some delay relative to changes in electrode potential.
In CV, both the oxidation and reduction peaks are observed, but the peak potential difference becomes larger than that of a reversible reaction, and ΔEp may widen as the scan rate increases.

In a quasi-reversible reaction, the electron-transfer rate is comparable to the measurement time scale, so the peak positions and peak shapes are more strongly affected by the scan rate.
If both oxidation and reduction peaks are present but the peak potential difference is large and the peak current ratio deviates from 1, quasi-reversibility can be considered.

Discussion of Irreversible Reactions

An irreversible reaction is a reaction in which, even if oxidation or reduction occurs, the reverse reaction hardly occurs within the measurement time.
In CV, only an oxidation peak may be observed without a reduction peak, or only a reduction peak may be observed.
This may occur because the oxidized or reduced species produced is unstable and decomposes or moves away from the electrode.

In an irreversible reaction, the peak potential may shift greatly depending on the scan rate.
A slow electron-transfer reaction or a chemical reaction following electron transfer may also produce an irreversible waveform.
The absence of a reduction peak does not necessarily mean the measurement failed; the possibility that the reaction itself is irreversible should be considered.

Meaning of Peak Current

Peak current is a value representing the magnitude of the redox reaction occurring at the electrode surface.
The higher the sample concentration, the greater the amount of redox species reaching the electrode surface and the larger the peak current becomes.
The peak current also becomes larger when the electrode area is larger or diffusion is faster.

For a reversible diffusion-controlled reaction, the peak current tends to be proportional to the concentration and to the square root of the scan rate.
Therefore, peak current can be used to quantify sample concentration or determine whether a reaction is diffusion controlled.
However, if adsorption or electrode-surface reactions are dominant, a different dependence is observed.

Effect of Scan Rate

Scan rate is the speed at which the electrode potential is changed.
Increasing the scan rate causes the potential to change more rapidly per unit time, so the peak current generally becomes larger.
In a reversible diffusion-controlled reaction, the peak current is considered proportional to the square root of the scan rate.

On the other hand, if the scan rate is too fast, electron transfer or mass transfer may not be able to follow the potential change and the peak potential difference may widen.
In addition, when a chemical reaction follows the electrode reaction, changing the scan rate may change whether the reduction peak appears and how large it is.
Scan-rate dependence provides important information for considering reversibility and reaction mechanisms.

Discussion of Diffusion Control

A common behavior observed in CV is diffusion control, in which redox species in the solution diffuse to the electrode surface and react.
As the potential is swept, the reactant is consumed at the electrode surface, creating a concentration difference between the region near the surface and the bulk solution.
This concentration difference causes the reactant to diffuse toward the electrode, and the rate of this supply affects the current.

Under diffusion control, the peak current tends to be proportional to concentration and electrode area and proportional to the square root of the scan rate.
Therefore, if the relationship between ip and v^1/2 is linear, the reaction is highly likely to be diffusion controlled.
Distinguishing between diffusion control and adsorption control is an important part of CV discussion.

Discussion of Adsorption Control

When redox species adsorb onto the electrode surface and react, the CV behavior differs from diffusion-controlled behavior.
Under adsorption control, the amount of material present on the electrode surface determines the current, so the peak current may tend to be proportional to the scan rate.
The peak may also become sharp.

When using substances that strongly adsorb onto the electrode surface or surface-modified electrodes, adsorption control must be considered.
However, in practice, both diffusion and adsorption may be involved.
Examining scan-rate dependence and changes in peaks during repeated measurements makes it easier to consider whether adsorption is occurring.

Discussion of Peak Current Ratio

The ratio between the oxidation peak current ipa and the reduction peak current ipc serves as an indicator for considering reaction reversibility and product stability.
In a reversible reaction, the oxidized substance is reduced again in almost the same amount during the reverse scan, so the peak current ratio is often close to 1.

If the peak current ratio deviates greatly from 1, possible causes include decomposition of the oxidized or reduced species during measurement, adsorption onto the electrode surface, progression to another chemical reaction, or changes in diffusion conditions.
The peak current ratio is used together with the peak potential difference to evaluate reversibility.

Peak current ratio = ipa / ipc

Discussion When Peaks Broaden

When CV peaks broaden, possible causes include slow electron-transfer rates, contamination of the electrode surface, high solution resistance, overlapping multiple redox reactions, adsorption, or accompanying chemical reactions.
An ideal reversible reaction shows relatively distinct peaks, but poor experimental conditions make the peaks more gradual.

Broad peaks make it difficult to read peak potentials and peak currents and also affect judgment of reversibility.
Improvement may be possible by polishing the electrode, adding sufficient supporting electrolyte, reducing solution resistance, and optimizing the measurement conditions.

Discussion When Peaks Shift

Causes of peak potentials shifting from expected values include reference-electrode drift, liquid-junction potential, solution resistance, electrode-surface condition, insufficient supporting electrolyte, pH changes, complex formation, and the effect of scan rate.
Even for the same substance, the redox potential may change when the solvent, supporting electrolyte, or pH changes.

Particularly in redox reactions involving protons, the peak potential shifts with pH.
In metal complexes, ligands and solvents may also change the redox potential.
A shift in peak potential may indicate not only measurement error but also a change in the reaction environment.

Discussion When No Reduction Peak Appears

If an oxidation peak is observed but no reduction peak appears, the chemical species produced by oxidation may have been unstable and decomposed before the reverse scan.
Other possible causes include diffusion of the oxidized species far from the electrode surface, progression to another chemical reaction, strong adsorption onto the electrode surface, or an insufficient potential range.

If no reduction peak appears, the reaction may be irreversible.
However, the peak may simply not be visible because of measurement conditions, so the scan rate, potential range, sample concentration, and electrode-surface condition should be checked.
Increasing the scan rate may allow the product to be reduced before it decomposes, making the reduction peak visible.

Role of the Supporting Electrolyte

In CV, it is important to add the supporting electrolyte at a sufficiently high concentration.
The supporting electrolyte increases the conductivity of the solution and reduces solution resistance.
It also makes the movement of the analyte occur mainly by diffusion and reduces the effect of migration in the electric field.

If the supporting electrolyte is insufficient, the solution resistance becomes large and the peak potentials may shift or the peaks may broaden.
The current response may also become unstable and reproducibility may decrease.
As a general rule, the supporting electrolyte should be selected so that it does not directly participate in the reaction.

Effect of the Working-Electrode Surface

Because CV measures reactions occurring at the electrode surface, the surface condition of the working electrode greatly affects the results.
If dirt, oxide films, adsorbed substances, or scratches are present on the electrode surface, electron transfer may be hindered, causing peaks to become smaller, broader, or shifted.
Polishing and cleaning before measurement are particularly important for glassy carbon and platinum electrodes.

If reaction products adsorb onto the electrode surface during repeated measurements, the peak current may decrease.
When the electrode surface changes, it becomes difficult to reproduce the same waveform even with the same sample.
Therefore, the electrode treatment before and after measurement must be standardized.

Effect of the Reference Electrode

Peak potentials obtained in CV are measured relative to the reference electrode.
Therefore, if the type or condition of the reference electrode changes, the measured peak potentials also change.
It is necessary to clearly state which reference electrode is used, such as an Ag/AgCl electrode, saturated calomel electrode, or standard hydrogen electrode.

If the reference electrode has deteriorated, the internal solution is contaminated, or the liquid junction is clogged, the potential may become unstable and cause peak-potential shifts or noise.
When comparing with literature values, differences in the reference electrode must be considered.

Effect of Dissolved Oxygen

If oxygen is dissolved in the solution, a reduction current for oxygen may appear in the CV.
Particularly on the negative-potential side, a current derived from O2 reduction may be observed and may overlap with the reduction peak of the target substance.
Therefore, nitrogen or argon may be used to deaerate the solution in order to avoid the effect of oxygen.

When dissolved oxygen has an effect, the background current may increase and peak analysis may become difficult.
If an unexpected peak or increase in current is observed on the reduction side, reduction of dissolved oxygen should be considered.
Whether or not deaeration is performed greatly affects the reproducibility of CV measurements.

O2 + 4H+ + 4e- → 2H2O

O2 + 2H2O + 4e- → 4OH-

Discussion of Background Current

In CV, in addition to the redox current of the target substance, capacitive current resulting from charging and discharging of the electrical double layer and background current caused by reactions of the solvent or supporting electrolyte also flow.
Particularly when the peak current is small, the effect of the background current may not be negligible.

Measuring a blank solution allows the current response in the absence of the target substance to be confirmed.
By subtracting the blank contribution from the CV of the sample solution, the redox peak of the target substance can be evaluated more accurately.
If the background current is large, the potential window of the solvent and the electrode-surface condition should be checked.

When Peaks Change During Repeated Measurements

When CV is measured over multiple cycles, the peak current or peak position may change.
This may occur because reaction products adsorbed onto the electrode surface, the electrode surface was activated or contaminated, the sample was consumed, or the redox products decomposed.
Changes during repeated measurements provide clues for considering reaction stability.

If the peak current decreases with each cycle, electrode-surface contamination, sample consumption, or adsorption may be considered.
Conversely, if the peak current increases, the electrode surface may have become activated during measurement.
Recording changes over multiple cycles makes it possible to evaluate the stability of the electrode reaction.

Causes of Error in Cyclic Voltammetry

Causes of error in CV include contamination of the working-electrode surface, instability of the reference electrode, insufficient supporting electrolyte, solution resistance, dissolved oxygen, errors in sample concentration, incorrect scan-rate settings, inappropriate potential range, background current, noise, and temperature changes.
Because CV measures reactions at the electrode surface, even slight differences in surface condition can greatly affect the waveform.

Possible causes of small peak currents include contamination of the electrode surface, insufficient sample concentration, decomposition of the redox species, and an insufficient potential range.
Possible causes of peak-potential shifts include reference-electrode drift, solution resistance, differences in pH or solvent conditions, and the effect of scan rate.
Causes of error are easier to organize when separated into peak current, peak potential, and peak shape.

When the Results Can Be Considered Good

CV results can be considered good when the oxidation and reduction peaks are clearly observed, the peak potentials and peak currents are reproducible, and the reversibility of the reaction can be explained from the peak potential difference and peak current ratio.
In addition, if changing the scan rate produces the expected dependence of peak current, the reaction mechanism becomes easier to discuss.

In a reversible diffusion-controlled reaction, both oxidation and reduction peaks are observed, the peak current ratio is approximately close to 1, and the peak current tends to be proportional to the square root of the scan rate.
Even if the measured values deviate from ideal values, the discussion is sufficient if the causes can be explained in terms of solution resistance, electron-transfer rate, surface condition, and other factors.

Example Discussions When the Experiment Did Not Go Well

When CV does not go well, possible causes should be considered from results such as no peaks appearing, small peaks, broad peaks, shifted peak potentials, no reduction peak, large noise, or waveforms changing from cycle to cycle.
Organizing the causes according to the electrode, solution, reference electrode, supporting electrolyte, deaeration, and scan conditions makes the discussion easier.

How to Write Points for Improvement

In a CV discussion, writing not only the causes of error but also points for improvement makes the report easier to organize.
Points for improvement can be organized according to electrode treatment, solution preparation, measurement conditions, and data analysis.

Improvements to Electrode Treatment

• Polish the working electrode before measurement
• Thoroughly clean the electrode surface
• Ensure that no contamination remains after polishing
• Accurately determine the electrode area
• Check the surface condition before repeated measurements
• Regenerate the electrode if adsorbed substances are present

Improvements to Solution Preparation

• Prepare the sample concentration accurately
• Add the supporting electrolyte at a sufficient concentration
• Keep the solvent and pH conditions constant
• Deaerate with nitrogen or argon when necessary
• Perform a blank measurement
• Avoid introducing bubbles or impurities into the solution

Improvements to Measurement and Analysis

• Check the condition of the reference electrode
• Set the potential range appropriately
• Measure at multiple scan rates
• Measure under low-noise conditions
• Read peak potentials and peak currents using the same criteria
• Check the relationship between ip and v^1/2
• Perform multiple measurements to confirm reproducibility

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of cyclic voltammetry, simply writing that “a peak appeared” or “the reaction was reversible” results in a superficial discussion.
A good discussion relates the redox reaction, peak potential difference, peak current ratio, scan-rate dependence, diffusion control, and electrode-surface condition.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of cyclic voltammetry.
Adjust the necessary parts according to your own experimental results.

• In CV, oxidation and reduction reactions can be observed in the same measurement by sweeping the potential back and forth.
• The oxidation peak corresponds to the reaction in which the reduced species loses electrons and becomes the oxidized species.
• The reduction peak corresponds to the reaction in which the oxidized species accepts electrons and returns to the reduced species.
• The peak potential difference ΔEp is an indicator for evaluating the reversibility of a redox reaction.
• When the peak current ratio is close to 1, the interconversion between the oxidized and reduced species is considered relatively reversible.
• When the peak potential difference is large, a slow electron-transfer rate or the effect of solution resistance may be considered.
• If the peak current is proportional to the square root of the scan rate, the reaction is highly likely to be diffusion controlled.
• If the peak current is proportional to the scan rate, adsorption control may be involved.
• If no reduction peak is observed, decomposition of the oxidation product or an irreversible reaction may be considered.
• The CV waveform is affected by the electrode surface, supporting electrolyte, dissolved oxygen, reference electrode, and scan rate.

Points to Check When Discussing Cyclic Voltammetry

Checking the following points before writing the report makes the discussion easier to write.

• Is the principle of CV explained?
• Are the meanings of the oxidation and reduction peaks described?
• Are the oxidation peak potential Epa and reduction peak potential Epc organized?
• Has the peak potential difference ΔEp been calculated?
• Is reversibility considered from the peak current ratio?
• Are the differences among reversible, quasi-reversible, and irreversible reactions explained?
• Is the effect of scan rate discussed?
• Is it considered whether the reaction is diffusion controlled or adsorption controlled?
• Is the effect of the working-electrode surface considered?
• Are the effects of the supporting electrolyte and dissolved oxygen considered?
• Is the potential reference provided by the reference electrode clearly stated?
• Do the points for improvement correspond to the causes of error?

Summary

Cyclic voltammetry is a method for investigating redox reactions by measuring current while sweeping the potential back and forth.
When an oxidation peak is observed during the positive-direction scan and a reduction peak during the reverse scan, the redox species in the sample are considered to be exchanging electrons at the electrode surface.
The oxidation peak potential, reduction peak potential, peak current, and peak potential difference provide important information for considering the properties of the reaction.

In a reversible reaction, the oxidation and reduction peaks are clear, the peak potential difference is small, and the peak current ratio tends to be close to 1.
In a quasi-reversible reaction, the peak potential difference becomes larger and the reaction is more strongly affected by the scan rate.
In an irreversible reaction, only one of the peaks may appear.
In addition, the scan-rate dependence of the peak current can be used to discuss diffusion control and adsorption control.

In a report, rather than simply writing that “a peak appeared,” organize and discuss the meaning of the redox peaks, reversibility, peak potential difference, peak current ratio, scan-rate dependence, diffusion control, adsorption, electrode-surface condition, supporting electrolyte, dissolved oxygen, causes of error, and points for improvement.
CV is an important electrochemical measurement that allows the properties of redox reactions to be understood visually.