FAQ - Potentiostats

The cyclic voltammetry option in CorrWare enables the application of a potential sweep between fixed points (up to four set points), monitoring the current with respect to the swept voltage. The scan rate determines the potentiostat setting (found in Edit -Set-up Pstat/Gstat), the options available for the sweep type being either stepped or analogue. If the scan rate is increased beyond a certain level when using a stepped potentiostat, the response obtained will be spiked or stepped as the applied stepped potential is below the scan rate speed. It is advised that for all cyclic voltammetry the potentiostat is set to the analogue setting allowing smooth voltammograms to be produced.
The use of a potentiostat (1287A Electrochemical Interface) is sometimes necessary in certain applications as electrochemists/materials scientists need to apply a DC bias to a system prior to applying the AC perturbing signal. For example it is often the case in corrosion studies that the system will be held at different bias levels and the corrosion rate monitored via impedance measurement.
The procedure for checking potentiostats requires that measurements are made using the instrument independently. The standard test box (12861A) is connected to the front panel of the potentiostat. 

(a) With the test box connected there should be a 10 kohm resistance between RE1 and RE2. The polarization voltage level should be set at 1 volt. Measurements should be commenced and the voltage dV (1 volt) and the current (1 V/1 0Kohm = 100 mA) checked. The procedure can then be repeated, this time applying 1V, and the results checked to make sure that they are sensible. 

(b) Further checks can then be made by attaching an oscilloscope to the rear of the potentiostat and making sure that the correct voltage level appears on the output. 
The scope should then be attached to the current output, which should then give a voltage reading that is proportional to the current measured (i.e. the scaling factor is the reference resistor value proportional to the current input, selected on the current range). 

For example if 100 mA is being measured using the 1ohm current range, then the voltage output on the current terminal is 100 mV. 
If the electrochemical interface and frequency response analyzer are working independently, both providing meaningful results, then they can be coupled together (see instruction manual). The combined system can then be tested using the following techniques. 

(a) Set the FRA to apply a 1 volt AC signal, connected to the polarization input on the electrochemical interface. A 1 K resistor should then be attached between CE+RE1 and WE+RE2, and the V and I outputs on the interface monitored on an oscilloscope. The appearance of DC offsets should be monitored for as these will affect the measurement resolution of the FRA. 

(b) A further test which can be used involves using the FRA at 100 mHz and monitoring the display on the electrochemical interface. The display should show sine wave values over a period of ten seconds. 

(c) Finally the acquisition of data using ZPlot software should be attempted (dummy/test cells can be used, which should provide characteristic impedance plots). 
Three common possibilities: 

(a) Hardware problem. Check system using potentiodynamic sweep of a known resistor, if correct potential is not achieved, a problem requiring professional repair/calibration may be needed. If correct potential and current (based on resistor size) are achieved, problem is likely cell related. 

(b) The counter electrode may be too small in relation to the working electrode and it is starved for reactants. Use a larger counter electrode, or reduce the size of the working electrode (if possible). It is recommended that the counter electrode be 2X the size of the working electrode to insure adequate current handling capability. 

(c) The counter electrode may have a high resistance path between it and the working electrode causing the voltage to exceed the compliance limits (voltage between working and counter electrodes) of the instrument. To correct this, move the counter electrode closer to the working electrode and/or remove any high resistance pathways such as membranes between the electrodes if at all possible. It may also be caused by poor cable connections to your working or counter electrodes which again result in a high resistance path. Clean and/or reseat the connections. 
The minimum requirements to run SMaRT are as follows: 
PC with:
  • Microsoft Windows XP Professional (Service pack 2)
  • 1 GHz Pentium or equal processor
  • 512 Meg RAM
  • CD
  • (2) USB ports required if using 12XX FRA plus (2) IEEE-488.2 Interface (National Instruments GPIB Board) USB-GPIB controller cables OR
  • 10/100 BaseT Ethernet port (2 if connecting to a network) if using 1470E alone or coupled with 14XX FRA
  • For 12XX FRA/1296:- (2) USB Ports plus ( 2) IEEE-488.2 Interface (National Instruments GPIB Board) USB-GPIB controller cables
  • For 12XX FRA/1294:- Parallel port (LPT) (available on desktop PCs; not USB-to-parallel converter or parallel port on laptop docking station) and (1) USB port plus (1) IEEE-488.2 Interface (National Instruments GPIB Board) USB-GPIB controller cable
The minimum requirements to run CellTest are as follows: 
PC with: 
  • Microsoft Windows XP Professional (Service pack 2)
  • 1 GHz Pentium or equal processor
  • 512 Meg RAM
  • CD
  • (2) USB ports required if using 12XX FRA plus (2) IEEE-488.2 Interface (National Instruments GPIB Board) USB-GPIB controller cables OR
  • 10/100 BaseT Ethernet port (2 if connecting to a network) if using 1470E alone or coupled with 14XX FRA
Some users believe that cables are simple and therefore should be inexpensive. However, as critical components of the measurement system, this is an incorrect assumption. 

The cell connection cable is often overlooked when considering the quality of measurement, yet it can be just as important as the following signal processing stages in delivering high accuracy signal measurements, particularly at the extremes of an instrument’s capability. 

When the ModuLab-ECS system entered the initial design phase, it quickly became clear that ‘off-the-shelf’ cables from component suppliers would not be suitable for this research grade Potentiostat/FRA. We therefore worked in close partnership with a specialist cable company to develop a cable set unique to the ModuLab-ECS. Highlights of the design of these cables include: 
  • Stability resistors inserted inside the cable to greatly improve the performance of the control loop
  • LEMO connectors allowing multiple carrier signals in one cable
  • Triax shielded cables for ultra low current measurements
As an example of the effect of cables on a measurement, compare the two impedance results on a high impedance cell overleaf. The plots below clearly demonstrate that using a specially screened cable such as the ModuLab-ECS triax working electrode cable, significantly improves the performance of the measurement system. Note particularly the level of noise (especially in the phase values) of the measurement made using standard coaxial cable vs. that with the triax cable at frequencies below 0.1 Hz 

Key: 
Blue and Purple Lines = Triax (Magnitude and Phase) 
Red and Green Lines = Coax (Magnitude and Phase) 

Cell cables are an important part of a measurement system and consequently they are not inexpensive.