Potentiostat BasicsPotentiostat Basics

The Electrochemical InterfaceThe Electrochemical Interface

A potentiostat is an instrument which measures the current / voltage characteristics of an electrochemical (electrode/solution) interface.

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Why does a potentiostat have to Why does a potentiostat have to be so #$@! complicated!?be so #$@! complicated!?

Why can’t a potentiostat be as simple and inexpensive as an ohmmeter?

ElectrodesElectrodesA potentiostat works with electrodes immersed in a conductive medium.

Working ElectrodeA sample of the corroding metal being

measured.

Reference ElectrodeAn voltage-sensing electrode with a constant

electrochemical potential.

Counter ElectrodeA current-carrying electrode that completes

the cell circuit.

Reference ElectrodeReference Electrode

The reference electrode impedance must be kept low High impedance electrodes pick up noise. High impedance electrodes can cause oscillation

To lower the impedance Use large junction reference electrodes Replace isolation frits regularly Avoid narrow Luggin Capillaries

Potentiostat DefinitionPotentiostat Definition

PotentiostatAn 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.

Potentiostat SchematicPotentiostat Schematic

Potentiostat

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T he Cell

Galvanostat DefinitionGalvanostat Definition

GalvanostatAn 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.

Galvanostat SchematicGalvanostat Schematic

G alvanostat

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ZRA DefinitionZRA 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.

ZRA SchematicZRA Schematic

ZRA

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The Complete PotentiostatThe Complete Potentiostat

Potentiostat

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I/E Converter

Contro l Am p

Electrom eter

Cell Sw itch

Electrometer CharacteristicsElectrometer CharacteristicsThe Ideal:

Perfectly Accurate Infinite Input Impedance Zero Input Capacitance Infinitely fast Infinite common mode rejection (crosstalk)

The Real: (Series G 300) DC Accuracy ±0.3% of reading 1 mV Input Current < 10 pA Input Resistance > 1 T Input Capacitance < 5 pF Bandwidth (-3 dB) > 4 MHz CMMR > 80 dB @DC to 3 Hz

> 60 dB @ 100 kHz

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I/E Converter CharacteristicsI/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:

Full-Scale Ranges 3 nA to 300 mA in decades

DC Accuracy 0.3% of range 50 pA

Bandwidth (-3 dB) > 500 kHz (300 A to 300 mA)

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Control AmplifierControl Amplifier

The control amplifier is a high power device.

Series G 300 Series G 750 Reference 600•Maximum voltage 20 volts 12 V 22 V•Maximum current 300 mA 750 mA 600 mA

Modern potentiostats should be protected from damage due to misconnection of this output to any other cell connection.

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The Potentiostat - Control Amp, The Potentiostat - Control Amp, I/E Converter, ElectrometerI/E Converter, Electrometer

Potentiostat

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I/E Converter

Contro l Am p

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Cell Sw itch

Turning a Pstat into an Turning a Pstat into an InstrumentInstrument

Signal G eneration

Signal Conditioning

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Data Presentation

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Signal GenerationSignal Generation

Signal G eneration

SBias

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DC O ffset

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High F idelitySinew ave

ReconstructionFilter

Signal ConditioningSignal Conditioning

Signal Conditioning

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Noise filterBackgroundSubtraction

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Putting It All TogetherPutting It All Together

Signal G eneration

Signal Conditioning

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Grounding is Very Important!Grounding is Very Important!

Signal Conditioning

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Signal G enerationBias

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Com puter

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Potentiostat Accuracy Contour Potentiostat Accuracy Contour MapMap

Every point represents a frequency and an impedance.

Plot iso-accuracy lines. Gain accuracy and phase accuracy can be plotted separately.

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100 mHz 100 kHz10 kHz1 kHz100 Hz10 Hz 1 Hz 10 Hz

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Trouble-Shooting a Gamry PotentiostatTrouble-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 <S/N> 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 109 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 Potentiostats and the Lunatic Fringe!Fringe!

The use of potentiostats is limited only by your imagination. Several real applications are:

Electrochemical experiments in molten salts at 450º C. Corrosion experiments in an autoclave at 300° C. 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.

Potentiostatically control seven independent working electrodes in one solution with one reference electrode and one counter electrode.