Presentation on theme: "Potentiostat Basics. The Electrochemical Interface A potentiostat is an instrument which measures the current / voltage characteristics of an electrochemical."— Presentation transcript:
The Electrochemical Interface A potentiostat is an instrument which measures the current / voltage characteristics of an electrochemical (electrode/solution) interface. “Electrode” =
Why does a potentiostat have to be so complicated!? Why can’t a potentiostat be as simple and inexpensive as an ohmmeter?
Electrodes A potentiostat works with electrodes immersed in a conductive medium. l Working Electrode A sample of the corroding metal being measured. l Reference Electrode An voltage-sensing electrode with a constant electrochemical potential. l Counter Electrode A current-carrying electrode that completes the cell circuit.
Reference Electrode The reference electrode impedance must be kept low l High impedance electrodes pick up noise. l High impedance electrodes can cause oscillation To lower the impedance l Use large junction reference electrodes l Replace isolation frits regularly l Avoid narrow Luggin Capillaries
Potentiostat Definition Potentiostat An electronic device that controls and measures the voltage difference between a working electrode and a reference electrode. It measures the current flow between the working and counter electrodes.
Galvanostat Definition Galvanostat An electronic device that controls and measures the current flow between a working and a counter electrode. It measures the potential between the working electrode and a reference electrode.
ZRA Definition Zero Resistance Ammeter (ZRA) An electronic device that controls the voltage between two electrodes (usually at 0 V), and measures the current flow between them. A ZRA may also measure the potential of one of the electrodes versus a reference electrode. ZRA mode is used for electrochemical noise and galvanic corrosion measurements.
The Complete Potentiostat
Electrometer Characteristics The Ideal: l Perfectly Accurate l Infinite Input Impedance l Zero Input Capacitance l Infinitely fast l Infinite common mode rejection (crosstalk) The Real: (Series G 300) l DC Accuracy±0.3% of reading 1 mV l Input Current< 10 pA Input Resistance > 1 T l Input Capacitance< 5 pF l Bandwidth (-3 dB)> 4 MHz l CMMR> 80 to 3 Hz > kHz 3/28/2005 RSR
I/E Converter Characteristics Cell current is measured by passing the current through a known resistance, Rm. Cell currents vary widely, so an I/E Converter needs a wide range of resistor values (typically 1 to 10 M ). For the Series G 300: l Full-Scale Ranges 3 nA to 300 mA in decades l DC Accuracy 0.3% of range 50 pA Bandwidth (-3 dB)> 500 kHz (300 A to 300 mA) 3/28/2005 RSR
Control Amplifier The control amplifier is a high power device. Series G 300 Series G 750 Reference 600 Maximum voltage 20 volts12 V22 V Maximum current300 mA750 mA600 mA Modern potentiostats should be protected from damage due to misconnection of this output to any other cell connection. 3/28/2005 RSR
The Potentiostat - Control Amp, I/E Converter, Electrometer
Turning a Pstat into an Instrument
Putting It All Together
Grounding is Very Important!
Potentiostat Accuracy Contour Map Every point represents a frequency and an impedance. Plot iso-accuracy lines. Gain accuracy and phase accuracy can be plotted separately.
Trouble-Shooting a Gamry Potentiostat 1. Calibrate the instrument with an external 2000 ohm resistor (UDC3). The unit should calibrate with no errors. This test catches at least 75% of all possible problems. If the instrument fails to calibrate properly, examine the Calibration Results file. 2. Run a Polarization Resistance scan on the 2000 ohm calibration resistor. Compare the actual current to the calculated current using Ohm’s Law (E = iR). 3. A resistor has an Eoc of zero, which isn’t realistic. Attach a battery in series with the resistor for an “off-zero Eoc” dummy cell. Connect the Working to the resistor, the Counter between the resistor and battery, and Reference to the battery. Run a Polarization Resistance and verify correct Eoc. 4. Run an Electrochemical Impedance Spectroscopy (EIS) test on the Universal Dummy Cell. Fit the curve to the Randles model. Resistors should fit within 1.5% of nominal, cap within 6%. These tolerances are wide because of the dummy cell component tolerances (1% for the resistors and 5% for the cap). 5. Run an EIS curve on a capacitor (220 pF) from 100 kHz to 50 uHz. Look for phase breaks in the Bode plot. Phase should be 90 +/ 2 degrees until Z is above 10 9 ohms. Analyze the resulting curve by fitting to a parallel RC model. Cap should be within 5% and resistor should be greater than 100 GOhm (Series G) or 1 TOhm (Reference 600). This procedure tests all current ranges, noise levels, leakage currents, etc. 6. Unstable voltage oscillations are often caused by bad reference electrodes. Use 2 electrode configuration to check. 7. When in doubt, call Gamry!
Potentiostats and the Lunatic Fringe! The use of potentiostats is limited only by your imagination. Several real applications are: l Electrochemical experiments in molten salts at 450º C. l Corrosion experiments in an autoclave at 300° C. l Apply a constant current on one side of a metal electrode and a constant potential and measure the current on the other side of the electrode. l Potentiostatically control seven independent working electrodes in one solution with one reference electrode and one counter electrode.