Electrochemistry

Our Electrochemical Workstations ZENNIUM X / ZENNIUM Pro / ZENNIUM  base on an universal and modular data acquisition system. Together with the THALES software package we offer all standard electrochemical methods at a mouse click.

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Photo-Electrochemistry

CIMPS is an universal photo- and spectro-electrochemical workstation for a wide field of applications. You can easily extend the basic CIMPS system with various options for special fields of photo-electrochemical research.

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Energy Storage & Conversion

Our power potentiostats, electronic loads and multi-channels acquisition systems provide a smart system for research and investigation on batteries, fuel cells and super caps.

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Corrosion

Electrochemical noise has become popular as a new method of corrosion detection and prevention. ZAHNER has developed the CorrElNoise® technique, the first method to obtain both, potential noise and current noise from one electrochemical source.

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Artefacts in High-Ohmic Impedance Measurements

Like other physical techniques, Electrochemical Impedance Spectroscopy (EIS) requires a considerable amount of background knowledge - not only concerning the interpretation of the measured data, but also in the practical procedures, performing the experiments themselves.

If the experimenter takes into account potential stumbling blocks, this method is a highly reliable tool for determining physical and electrochemical parameters. Concerning practical EIS experiments, three kinds of artefacts must be considered depending on the magnitude of typical object currents.

Measuring high Ohmic objects, for instance coated metals, the so-called electric smog, i.e. the electromagnetic environmental noise, is the main source of complication performing an impedance measurement. This type of artefact is dominant in the low frequency part of the spectrum, assuming pure capacitive or (high Ohmic) resistive behavior which leads to high impedances at low frequencies and therefore to small signal amplitudes.

 

Figure 1: Typical equivalent circuit for barrier coatings.

 

Component

Meaning

Value

C1

Coating capacity

(150 pF)

R1

Pore resistance

(1 GΩ)

C2

Double layer capacity

(470 pF)

R2

Charge Transfer resistance

(10 GΩ)

 

The reason for this complication is the energy content of the electric smog reaching or exceeding the order of magnitude of the measured signal. As a rule of thumb and for typical electrochemical excitations, i.e. a few millivolts, this complication becomes dominant when the current drops below 1 µA. This value may differ, especially in different laboratories and even in the same laboratory, depending on the exact site of the measuring system. For example, assuming a sinusoidal excitation of ± 20 mV, a current of 10-6 A yields an impedance of 200 MΩ at the peak-values.

According to the equivalent circuit (EC) depicted in figure 1, two impedance measurements are plotted in figure 2. The EC of figure 1 is a kind of standard EC for the interpretation the electrochemical behavior of coated metals.

 

Figure 2: Impedance measurement using the EC of figure 1. left hand side: without-, right hand side: with an appropriate shielding (Faraday cage). (o): measured data, lines according to the result of the simulation.

 

Both spectra are measured under the same conditions, for instance an amplitude of 10 mV but with (right hand side) and without (left hand side) shielding. On the left hand side one recognizes immediately that the electric smog becomes dominant when the impedance reaches a value of about 100 MΩ. Since the impedance is plotted in a logarithmic scale, this effect becomes more obvious considering the course of the phase angle. It should be noted that an additional artefact is present in the diagram on the left hand side. This artefact is located at a frequency of about 50 Hz and derives from the power-line frequency (50 Hz in Germany). This artefact is also a very good indicator of improper shielding and increases with increasing impedance of the measured object.

On the right hand side however one can clearly see that even such difficult objects can be measured properly if an appropriate shielding technique is applied. Often, very simple shielding techniques may improve the results of the measurements significantly. For instance a ground plate underneath the measured object or a (grounded) metal mesh around the objected may be well suited. In really difficult systems like those depicted in figure 2, a hermetically closed Faraday cage is recommended. Only complete shielding results in a reliable measurement of really difficult objects like the measurement depicted on the right hand side. This includes shielding from air movement arising from people within the laboratory and resulting in a change in the local distribution of the charge of the air. At least, a proper grounding of the Faraday cage is a key step in shielding techniques. In this context it is sufficient to know that grounding of different, metallic tools required for the measurement must be accomplished at a single node. For that, a blank banana jack is mounted at the rear of the electrochemical workstation.

 

 

You can download a PDF-version of the application note.
The download link can be found on the top of this page.