Cyclic voltammetry

Cyclic voltammogram of a potassium cyanide solution.

The cyclic voltammetry , also cyclic voltammetry or triangular voltage method called analytical methods with which one different electrode processes can examine.

With cyclic voltammetry, first a linearly rising or falling potential and then a falling potential is applied to the working electrode in an electrolyte . The term triangular voltage method comes from this triangular or sawtooth-shaped voltage curve .

If there is an electrochemically active substance on the working electrode, it is oxidized or reduced at a characteristic potential . The voltage value between the oxidation and the reduction peak indicates the normal potential of an electrolytic reaction and thus inferences about the species contained. In addition to substance identification in analytical chemistry, cyclic voltammetry is an important method for characterizing electrochemical catalysts.

functionality

Experimental setup

Cyclic voltammetry is usually performed with three electrodes immersed in an ion-conducting electrolyte . The electrochemical processes to be investigated take place at the working electrode at a given potential. The potential of the working electrode is precisely determined by means of a so-called reference electrode . The current to build up the voltage, which corresponds to the electrochemical processes, is provided with the help of the counter electrode. A potentiostat connects the three electrodes and provides voltage control and current measurement. Electrons flow between the potentiostat and the electrically conductive electrodes. In the electrolyte, however, there is pure ionic conductivity.

Measurement process

Starting from the initial potential, for which often the open-circuit voltage ( engl. Open circuit potential, OCP) is selected, a triangular voltage traversed. This voltage curve is defined by two reversal potentials and the frequency (also referred to as the feed rate of the potential). Depending on the polarity of the voltage on the working electrode, it becomes the anode in the positive case and the cathode in the case of negative potentials. The reversal potentials can accordingly be referred to as cathodic or anodic reversal potential. The flowing current is recorded as a function of the voltage. In order to examine the time dependency of the running processes, several measurements are often carried out one after the other.

interpretation

Basically, the current response obtained can be divided into two contributions: The capacitive current is created by a change in the electrode potential and the associated reorganization of the electrochemical double layer . It is independent of the electrode potential and, as a first approximation, corresponds to the charge or discharge of a classic plate capacitor . The Faraday current is created by the electrochemical oxidation or reduction of a chemical species and is therefore dependent on the electrode potential. With higher measurement frequencies of the triangular voltage, the capacitive current becomes more prominent.

At a certain positive potential, a Faraday current flow begins. Particles are now oxidized directly on the surface of the electrode (e.g. Fe²⁺ to Fe³⁺). The ratio of oxidized to reduced particles immediately in front of the electrode changes according to the Nernst equation . If the concentration of the oxidized and reduced components in front of the electrode is the same, the potential ideally corresponds to the standard potential of a redox process of the Nernst equation. In reality, the peaks are shifted compared to the standard redox potential, since a certain overvoltage is required for the partial processes of electron transfer . Therefore, the oxidation and reduction peaks are not directly above one another. The mean of the two peaks corresponds to the standard redox potential. Both oxidation and reduction peaks can only be observed with a reversible reaction.

Depending on the frequency of the triangular voltage, the mass transport to the electrode is limiting. After a short time, a depletion zone forms on the electrode surface, which means a lower metabolism and thus a lower current. This drop in current leads to the formation of peaks in the voltammogram (dependence: square root of time). The subsequent transport of the redox species is limited by the diffusion. This also results in the diffusion overvoltage.

Theoretical description

The cyclic voltammogram of a peak then occurs (engl. Peak value, peak) on which one (as in the superimposed on the stepped cam polarography occurs) and a current characteristic curve of a diffusion-controlled electrode reaction ( Cottrell curve can interpret). The exact shape of this curve also depends on the rate of advance of the potential (frequency of the triangular voltage). But even without electroactive species, the electrochemical reaction of the electrode (often consisting of a metal) with the solvent, for example oxidation, can lead to a peak, and adsorption processes on the measuring electrode can also lead to peaks (“top layer diagram”).

In the case of a reversible electrode process, the potential at the electrode surface is determined by the Nernst equation . Because of diffusion control, Fick's 1st law applies . The current density can be calculated from this. The corresponding Randles-Sevčik equation for the anodic half cycle is: ${\displaystyle i_{P}}$

${\displaystyle i_{P}=2,69\cdot 10^{5}\cdot c_{\mathrm {red} }\,{\sqrt {n^{3}\cdot \nu \cdot D_{\mathrm {red} }}}}$

${\displaystyle n}$: Number of electrons

${\displaystyle \nu }$: Feed rate of the potential (mostly mV / s)

${\displaystyle D_{\mathrm {red} }}$: Diffusion coefficient of the reduced form of the investigated substance

${\displaystyle c_{\mathrm {red} }}$: Concentration of this substance

A corresponding formula applies to the cathodic half cycle.

A precise quantitative recording of the measurement is also possible, with different mathematical expressions resulting depending on the type of electrode processes (reversible or irreversible). However, both can also occur in combination.

With the help of cyclic voltammetry the kinetics of a chemical reaction can be elucidated. Normally one uses certain diagnostic criteria that are characteristic of different reaction mechanisms.

In addition to redox reactions, if the reaction is carried out appropriately, other reversible and irreversible processes can also be observed in a cyclic voltammetric experiment, such as the charging of the double-layer capacitance C, for which the current density j is calculated as j = - ν C.

Derived Methods

• In order to be able to examine the effects of the mass transport, the working electrode can be set in rotation. See Rotating Disc Electrode .
• Processes taking place on the working electrode can be observed with the aid of the electrochemical scanning tunneling microscope during the cyclic voltammetry.

literature

• J. Heinze: Cyclovoltammetry - the "spectroscopy" of the electrochemist . In: Angewandte Chemie . tape 96 , 1984, pp. 823-840 , doi : 10.1002 / anie.19840961104 .
• Bernd Speiser: Cyclic Voltammetry, Chemistry in Our Time, Volume 15, April 1981, Pages 62-67, doi: 10.1002 / ciuz.19810150206
• Allen J. Bard & Larry R. Faulkner: Electrochemical Methods: Fundamentals and Applications, 2nd Edition . Wiley, New York 2001, ISBN 978-0-471-04372-0 .
• Piero Zanello: Inorganic Electrochemistry: Theory, Practice and Application, 2nd Edition. RSC Publishing, 2011, ISBN 978-1-84973-071-6 .
• N. Elgrishi, KJ Rountree, BD McCarthy, ES Rountree, TT Eisenhart, JL Dempsey: A Practical Beginner's Guide to Cyclic Voltammetry . In Journal of Chemical Education . Volume 95, 2017, pp. 197-206, doi: 10.1021 / acs.jchemed.7b00361

Web links

• Cyclic voltammetry from the TU Clausthal (PDF; 153 kB)