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Potentiostat Basics I’m Max Yaffe

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1 Potentiostat Basics I’m Max Yaffe
V.P. & one of the 3 founders of Gamry Instruments. I’m an electrochemist, engineer, computer hacker, whatever it takes to create these instruments. So lets start with the part of this business you’re most interested in--

2 The Electrochemical Interface
= “Electrode” Corrosion is an electrochemical phenomenon. Charge is transferred between two chemical phases, in this case from a metal to a liquid. Charge is carried in the metal by electrons, in the liquid by ions. In the process, ions and molecules react. I’m going to use the ball and line symbol for an electrode as a circuit component. I think I learned this from Royce Murray but not many people use it. Anyway-- This is the heart of the matter. This is the thing we’re really trying to investigate. Corrosion engineers may be trying to inhibit the reaction, battery engineers may be trying to accelerate it, others may just be trying to study it. They all use potentiostats. A potentiostat is an instrument which measures the current / voltage characteristics of an electrochemical (electrode/solution) interface.

3 Why does a potentiostat have to be so #$@! complicated!?
Why can’t a potentiostat be as simple and inexpensive as an ohmmeter? This slide and the next three slides compare a potentiostat to an ohmeter. Some presenters find this helpful to explain why a pstat is relatively complicated. Others may choose to skip these slides and go directly to Slide 7.

4 Electrodes A potentiostat works with electrodes immersed in a conductive medium. Working Electrode A sample of the corroding metal being measured. Reference Electrode An voltage-sensing electrode with a constant electrochemical potential. Counter Electrode A current-carrying electrode that completes the cell circuit. A potentiostat generally requires three electrodes. That’s true for both field probes and lab cells. There are some places where 2 electrode potentiostats are useful. The working electrode represents the system being studied. It can be bare metal or coated. The reference electrode potential has a constant electrochemical potential as long as no current flows through it. The most common lab references are the SCE and the Ag/AgCl electrodes. In field probes, a pseudo-reference (a piece of the working electrode material) is often used. The counter electrode in lab cells is generally an inert conductor like platinum or graphite. In field probes it's generally another piece of the working electrode material. The current that flows into the solution via the working electrode leaves the solution via the counter electrode.

5 Reference 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 One of the most sensitive components is the reference electrode. Our website has several technical articles about reference electrodes that you can check out after you’re more comfortable with the equipment. Noise is a problem. A high impedance reference electrode acts like an antenna. You’d like as low an impedance as possible. If the reference impedance can also cause potentiostats to oscillate causing all sorts of bizarre behavior. I’ll talk about this in a bit. Reference electrode contents can pollute the chemistry. One common reference is Ag/AgCl in KCl. Chloride from the reference can activate corrosion of normally passive materials. You can check out our website for tech tips about reference electrodes and the nasty things they do to potentiostats

6 Potentiostat Definition
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. So in words, what is a potentiostat? Job1: Control the potential of the Working Electrode Job2: Measure the current Job 3: Measure the potential (self-check on job 1) A potentiostat, by classical definition, doesn’t have to measure anything! It just holds (stats) a potential. In reality, you are generally interested in knowing the current and it is a good idea to measure the potential although it isn’t required. In our previous software and all of our competitors software, it is assumed that the applied potential is the same as the measured so why bother measuring. In V3.0, we always measure the potential independently.

7 Potentiostat Schematic
So lets take our 3 electrodes, put them in a cell and wire up the potentiostat. Where I went to school, we called this the the “Deford Potentiostat” because he was a professor there. I’ve never heard the term used elsewhere. S is the incoming signal. The heavy lines form a feedback loop. The Power Amp in this loop tries to make its two inputs equal. One is the signal S, the other is Vmeasured = Vwe-Vref. Remember what I said about high impedance reference electrodes. Because of real electronics, and real chemistry, this feedback loop can be relatively slow and can oscillate. In addition to being used in the control loop, the potential is reported out. Follow the current path. It starts at the PA, goes through the CE, the cell, the WE, the ammeter, and finally is sunk to ground. The ammeter reports current. Pstats have 1 inny & 2 outies.

8 Galvanostat Definition
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. A lab potentiostat can also function as a glavanostat and a ZRA. A galvanostat is the mirror image of a potentiostat. The current between the working and counter electrodes is controlled and voltage is measured. Let’s look at the picture

9 Galvanostat Schematic
The galvanostat feeds back the current signal. This can cause some confusion because now the input signal is not the same as the cell voltage -- it is a voltage proportional to the required cell current -- Remember we are stating galvo’s. The proportionality constant is equal to Range Resistor which we’ll see in a second. Voltage is read and reported.

10 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. A ZRA is another name for the 2 electrode potentiostat. All potentiostats can function as a ZRA -- some better than others. It’s common to want an independent voltage measurement as well as the ZRA current measurement. It’s wired up like the next slide

11 ZRA Schematic The feedback loop (heavy line) is like the potentiostat but now we’re feeding back Vwe-Vcounter. Often you’ll see this as Vwe1 and Vwe2. The extra electrometer comes at no charge in the PC4. There is also an additional wire in the cell cable that is called “counter-sense”. I always wanted to add a wire going nowhere and call it “non-sense”.

12 The Complete Potentiostat
A few more pieces go into the control section of the potentiostat. Here’s the actual ammeter which we call an I/E converter. It’s made up of a Range Resistor and a Voltmeter. The range resistor is selectable. In a PC4-300 there are 9 decade ranges from 300 mA full scale down to 3 nA. Most techniques automatically set the appropriate range for the current you’re trying to measure. The cap controls the speed of the I/E converter. There is a noise/speed tradeoff. Too much noise? Slow it down. There is another cap across the control amp which is also tweaked depending on the experiment. Finally there is a cell switch. Actually there are two -- a Solid State one, and a relay. This is used for current interrupt and to keep from frying your electrode when the pstat is in some indeterminate state.

13 Electrometer Characteristics
The 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 TW Input Capacitance < 5 pF Bandwidth (-3 dB) > 4 MHz CMMR > 80 to 3 Hz > kHz The electrometer is not just a volt meter! One common problem is when a customer puts a DVM between the Reference & Working electrode and wonders why everything changes. The electrometer must have a very high input resistance. We don’t usually want any current flowing through the reference electrode. It must make its measurement on top of a varying current measurement. It needs to be fast and accurate. It is one of the most highly engineered parts of a potentiostat. I’d like to invite you to check out our website for a discussion of potentiostat non-idealities. 3/28/2005 RSR

14 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 W to 10 MW). 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 mA to 300 mA) This is a typical set of I/E characteristics The PC4 has 9 current ranges. As I said, the lower current ranges are slower. 3/28/2005 RSR

15 Control Amplifier The control amplifier is a high power device.
Series G 300 Series G 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. Control amp power specifications are one thing that distinguish between potentiostats. The specification on the slide are for a different Gamry Potentiostats. The compliance voltage is the maximum voltage that can be applied at the counter electrode. If more voltage is required, the potentiostat cannot comply. High compliance voltage is needed when passing high current through poorly conductive solutions. The compliance current is the maximum cell current. Large currents are required for large samples that corrode quickly. 3/28/2005 RSR

16 The Potentiostat - Control Amp, I/E Converter, Electrometer
A few more pieces go into the control section of the potentiostat. Here’s the actual ammeter which we call an I/E converter. It’s made up of a Range Resistor and a Voltmeter. The range resistor is selectable. In a Series G 300 there are 9 decade ranges from 300 mA full scale down to 3 nA. Most techniques automatically set the appropriate range for the current you’re trying to measure. The cap controls the speed of the I/E converter. There is a noise/speed tradeoff. Too much noise? Slow it down. There is another cap across the control amp which is also tweaked depending on the experiment. Finally there is a cell switch. Actually there are two -- a Solid State one, and a relay. This is used for current interrupt and to keep from frying your electrode when the pstat is in some indeterminate state.

17 Turning a Pstat into an Instrument
It takes more to make an instrument than what you’ve seen. There are some other sections that can be very complicated. We need to create the input signal. We need to analyze (read) the output signals. We need to present and store the results.

18 Signal Generation How about the input signal, the one the potentiostat is supposed to match? We have several signal sources built into the G Family. There are DC levels, and arbitrary waveform generators which we know as Bias & Scan. There is also a 300 kHz high fidelity sine wave generator that we use in impedance. You can also stick on an external signal generator.

19 Signal Conditioning So what happens to the signals that leave the control amp section? In our Potentiostats, the I & V signals go through matched filters. These can be selected for 5 Hz, 1KHz or 200 KHz. The 5 Hz is for slow DC tests, with out current interrupt iR corrections. The 1 KHz is for faster tests such as CV’s. The very fast setting is a “get outa my way”. It’s used for high frequency impedance. There is also Offset & Gain. This is useful for taking a closer look at a small signal riding on top of a DC offset. There can be other filters for reasons we’ll get into in a bit.

20 Putting It All Together
So lets put the pieces together, again. One last, very important piece is some way to present and store the data. You’ll notice that I haven’t said anything about a computer here. Actually, it isn’t too important for this discussion. Whether or not a pstat is digital, all of these pieces can be useful. Where the computer is really mandatory is making all of these components work together seamlessly.

21 Grounding is Very Important!
One more issue that turns out to be critical in many corrosion engineering applications and lab apps too. What if you want to measure an electrode connected to ground such as a process pipe or a scanning microscope stage? Or what if your counter electrode is grounded like it would be in an autoclave? The potentiostat itself must float. Our Potentiostats do. Even though they are inside the computer they are optically isolated from the computer ground.

22 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. Finally: here is a novel context for thinking about potentiostat performance. It’s called an accuracy contour map. I’m plotting accuracy vs. Impedance and frequency. Remember how I said there was a trade-off? The upper limit is controlled by the Electrometer input impedance. The upper sloping line is controlled by the I/E converter stray capacitance. The lower one by the electrometer common mode. The lower limit by the maximum current. The right hand limit is controlled by the signal generator. The left hand limit is controlled by the operator’s life expectancy.

23 Trouble-Shooting a Gamry Potentiostat
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. 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). 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. 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). 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. Unstable voltage oscillations are often caused by bad reference electrodes. Use 2 electrode configuration to check. When in doubt, call Gamry!

24 Potentiostats and the Lunatic 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.

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