Introduction to electrochemistry

The investigation of electroactive microorganisms requires an appropriate measurement system. To perform highly sensitive measurements we used a potentiostat. For the understanding of the mode of operation of a potentiostat it is necessary to define a few basic principles of electrochemistry. The following definitions come from (Harnisch, F. & Freguia, 2012):

• Anode:
  The electrode where an oxidation takes place.


• Cathode:
  The electrode where a reduction takes place.


• Current:
  The flow of electric charge.


• Capacitive Current:
  The current related to the change in the electrode surface charge, not related to an oxidation/ reduction reaction.


• Faradaic Current:
  The current generated from the oxidation (positive current) or reduction (negative current) of chemical species.


• Charge q [C]:
  Cumulative current flow (1C= 1A x 1s). Values can be determined by the integration of current-time curves.


• Formal Potential E^f [V]:
  Replaces the standard potential when the activities of the species involved and of the side-reactions are unknown or too complex. It is the favoured value for reactions that take place in a biological environment.


• Peak Current:
  The maximum current at the working electrode in a voltammetric measurement.


• Peak Potential:
  The potential of the working electrode at which the peak current in a voltammetric measurement is obtained.


• Potentiostat:
  An electronic amplifier that controls the potential drop between an electrode (the WE) and the electrolyte solution; it usually constitutes a reference electrode (RE) as a sensing component and a counter electrode (CE) for balancing the current flow.


• Reference electrode (RE):
  A non-polarizable (stable) electrode with a fixed potential that sets or measures the potential of the WE.


• Working electrode (WE):
  An electrode at which a given electrochemical reaction of interest is examined; its potential is controlled versus the RE in a three-electrode system.


• Scan rate [mV s^-1]:
  The speed of potential change per unit of time in a voltammetric experiment.

The Potentiostat

A potentiostat is an electronic control and measuring device for the study of electrochemical phenomenons. The instrument controls the voltage difference between a working electrode (WE) and a reference electrode (RE). Therefore the prevailing potential of the WE is measured in respect of the RE. Potential control and upkeep of constant voltage value is implemented by injecting current into the cell through a counter electrode (CE). (Gamry Instruments)
The fundamental principle is shown in Figure 1.

Figure 1: Principle of the circuit for potentiostatic measurements with a four electrode set up.

The potential difference between WE ad RE is measured at the entrance of the regulatory circuit (actual current) and compared to a target voltage. There are two modes of operation for the target voltage. It can be held constant by the 2 V DC-circuit or operated with a variable voltage, set by a function generator. If there occurs a difference between actual current and target voltage the regulatory circuit automatically sets the wanted current by sending an electric current into the electrobiochemical reactor system.
Thus the set up ensures that the measured potential at the WE is held constant at the wanted potential. The current gets isolated by the resistor R. The set up allows it to vary the target voltage and enables to apply linear increasing target voltages. This mode of operation allows the recording of a current-voltage chart by an oscilloscope that can be displayed and analyzed on a computer. (Hamann et al.,2007)

Cyclic voltammetry

Cyclic voltammetric measurements require a three electrode setup composed of a reference electrode, a counter electrode and a working electrode. Those three electrode setups are especially suitable for electrochemical measurements because only the potential changes at the working electrode are measured. Any variances at the counter electrode are not considered and are totally independent of the measurement. This allows to monitor specific reaction with maximal precision.(Gamry Instruments)
Cyclic voltammetry can be used to study oxidative and reductive reactions of chemical compounds. Therefore a linearly changing potential based on the reference electrode is applied to the working electrode up to a specific value. Afterwards the potential returns linearly to the initial value again. If the potential has reached the starting value one cycle is completed. During one measurement several cycles can be performed. The scan rate can be derived from the gradient of the curve.(Harnisch, F. & Freguia, 2012)
The course of potential during the measurement is shown in Figure 2.

Figure 2: Course of potential during a cyclic voltammetric measurement (screenshot of analysis program: Gamry Echem Analyst^TM).

Due to the applied potential the compound which should be analyzed runs through an oxidation-reduction cycle and a current-voltage chart is plotted (Figure 3).Therefore there is nearly no current flow in the initial phase of the linearly proceeding potential except the capacitive current. When reaching a certain potential the current flow increases up to the Faradaic current where the chemical compound gets oxidized. The resulting peak current indicates the peak potential which is needed to oxidize the compound. If the potential begins to return linearly back to the initial value the compound gets reduced again at a specific potential, the peak potential which is needed to reduce the compound. If both current peaks are measurable the chemical compound is reversible oxidable and reducible (Figure 3).(Harnisch, F. & Freguia, 2012)

Figure 3: Cyclo voltammetric measurement with peak current for oxidation (upper part of the curve) and peak current for reduction (lower part of the curve) (screenshot of analysis program: Gamry Echem Analyst^TM).

For our project we perform several cyclo-voltammetric measurements to characterize neutral red and bromphenol blue as mediators regarding their redox potentials. Therefore different parameters are tested like various electrode materials or scan rates.

Chronoamperometry

Chronoamperometry is a powerful analysis technique to determine many different electrochemical parameters like concentrations and diffusion coefficients. In this case electrochemical reactions and mechanisms influenced by diffusion can be monitored.(Gamry Instruments)
A chronoamperometric measurement is therefore performed by initially applying a potential to the electrode where the chemical compound undergoes no chemical reaction like oxidation or reduction. That means that no Faradaic current is measured. During the next step the potential is changed to a value where the chemical compound gets reduced or oxidized. At this potential a Faradaic current is measured. (Xiong, L. et al., 2012)
The course of potential is shown in Figure 4.

Figure 4: Course of potential during a chronoamperometric measurement (screenshot of analysis program: Gamry Echem Analyst^TM)

Over the whole process a current vs. time curve is plotted (Figure 5). Those curves show usually an initially large Faradaic current due to steep gradient of concentration at the electrode. Afterwards the current decreases because of the decline of redox active compounds near the electrode. (Xiong, L. et al., 2012)

Figure 4: Typically current vs. time curve for a chronoamperometric measurement (screenshot of analysis program: Gamry Echem Analyst^TM)

During our project chronoamperometry is performed to evaluate if the bacteria is able to accept and consume electrons. Therefore a one step chonoamperometric measurement is done which means that there is no potential change during the experiment. The whole process is therefore carried out at the reduction potential of neutral red under the given reactor conditions. This reduction potential has to be defined before through cyclic voltammetry.

References

• Harnisch, F. & Freguia, S., 2012. A Basic Tutorial on Cyclic Voltammetry for the investigation of Electroactive Microbial Biofilms. In: Chemistry – An Asian Journal, 7 (3), pp. 466–475.

• Gamry Instruments: Potentiostat fundamentals

• Hamann, Carl H., Hamnett, Andrew, Vielstich, Wolf (2007): Electrochemistry. 2^nd. edition. Weinheim: Wiley-VCH

• Gamry Instruments: Two-, Three-, and Four-Electrode Experiments
• Gamry Instruments: PHE200 Physical Electrochemistry Software
• Xiong, L., Aldous, L., Henstridge, M. C., Compton, G. (2012): Investigation of the optimal transient times for chronoamperometric analysisof diffusion coefficients and concentrations in non-aqueous solvents and ionicliquids Anal. Methods , 4, pp. 371