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Cyclic Voltammetry: how to obtain good results…

Latest updated: February 12, 2020

Cyclic voltammetry (CV) is the most widely used technique for acquiring qualitative information concerning electrochemical reactions. CV provides information on redox processes, heterogeneous electron-transfer reactions and adsorption processes. It enables the user to rapidly identify redox potential of the electroactive species and kinetic information. To put simply, the cell is stressed by a voltage scan and the resulting current is measured.

Although, it is used on a daily basis, some parameters that have an effect on measurement quality that need to be clarified.

The aim of this article is to present these key parameters and describe how to correctly tune them in order to get the best out of your instrument.

 

At what voltage should you start?

We recommend you start the measurement at the Open Circuit Voltage (OCV). This will keep the cell in its initial state. The cell will not be disturbed before the voltage scan.

 

How the voltage scan is performed

Nowadays, potentiostat/galvanostats are based on digital technology, so the voltage scan is carried out in successive voltage steps (dE). In order to mimic a linear scan, the voltage step, dE, has to be as small as possible. The limiting factor is the voltage control resolution of the potentiostat.

 

 

The controlled voltage resolution can be improved by using a narrow voltage range. For example, the voltage resolution is about 300 µV with a voltage range of +/-10 V and a 16 bit DAC. It can be improved by a factor of 3 by setting a voltage range of +/-2.5 V to obtain a resolution of around 100 µV.

Moreover, in order to smooth out the current signal, we recommend you average the current response over several steps. Because of the accuracy of the reference electrode which is around +/-1 mV, one data point every 1 mV is a good target.

For electroanalytical applications in solution, we recommend you set a voltage range of +/-2.5 V and average 10 times.

NOTE: some instruments offer the possibility to apply a “true” analog ramp (see premium’s range option).

 

How the current is sampled

The current is sampled along the whole voltage step. The potentiostat samples an overall current that is the sum of the capacitive, IC, and faradaic current, IF. As the decay of the capacitive current is faster than the faradaic current after the pulse (see equations (1) and (2)), depending on where the current is sampled, the contribution of the faradaic current will be different on the measured overall current.

 

The faradaic current is given by the following equation:

$I_\text F = nFAC \sqrt { \dfrac{D}{\pi t}}$

 

Where n is the number of electrons involved in the redox process, F the Faraday constant, A the surface of the electrode, C the concentration of the electroactive species, D the diffusion coefficient of the electroactive species, t the time after the application of the pulse at which the current is sampled.

The capacitive current is given by the following equation:

 

$I_\text C = \dfrac {E}{R}\exp\left(\dfrac{-t}{RC_\text{dl}}\right)$

 

 

With E the pulse potential, R the ohmic resistance between the working electrode and the reference electrode, Cdl the double-layer capacitance.

 

 

Typically, 50% of the end of the step is a good setting for electroanalytical purposes. If the user is also interested in obtaining a larger capacitive current, a wider step can be set.

 

TIP: it is possible to remove the capacitive current by subtracting the CV of the blank (performed in the same condition i.e. same electrode, same voltage sweep.) from the CV of interest.

 

Current ranging…

During the shift from one current range to another, you will see a transient current. Therefore, it does not make sense to record these data. It will induce a peak that will make the post process more difficult to manage.

For scan rates faster than hundreds of mV/s, we recommend you set a fixed current range.

 

Ohmic drop compensation

The CV can be associated with a technique that compensates for the Ohmic Drop. So the CV can be preceded by an Ohmic drop compensation technique. See application notes 27-29.

 

Bandwidth

More information can be found in « Understanding bandwidth & its effect on measurements » but basically, the faster you scan, the faster the bandwidth should be.

 

Application note 27

Application note 28

Application note 29

 

CV Cyclic Voltammetry Voltage scan