Introduction to electrochemistry

The investigation of electroactive microorganisms affords 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) of reduction (negative current) of chemical spezies.


• 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 usally constitutes a reference electode (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:
  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 prevailling potential of the WE is sensed in respect of the RE. The potential control and set to a constant 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.

Cyclic voltammetry

Chronoamperometry

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