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November 2008, 1 Potentiostats Principles of operation.

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Presentation on theme: "November 2008, 1 Potentiostats Principles of operation."— Presentation transcript:

1 November 2008, 1 Potentiostats Principles of operation

2 November 2008, 2 Overview The potentiostat – a black box ? Potentiostat (role) The operational amplifier –Voltage follower –Current follower –Scaler & Adder –Control amplifier Basic potentiostat construction How to make the most of your potentiostat

3 November 2008, 3 Overview A. Bard & L. Faulkner, Electrochemical Methods – Fundamentals and Applications, 2nd edition, John Wiley & Sons H. Girault, Analytical and Physical Electrochemistry, EPFL Press, Marcel Dekker C. Brett, A. M. Brett, Electrochemistry – Principles, Methods and Applications, Oxford University Press D. Pletcher, R. Greef, R. Peat, L. Peter, J. Robinson, Instrumental Methods in Electrochemistry, Horwood Publishing R. E. Simpson, Introductory Electronics – For Scientists and Engineers, Allyn and Bacon

4 November 2008, 4 The potentiostat – a black box ?

5 November 2008, 5 Difficulties of potential control It is not possible to measure the potential of the working electrode  potential difference w.r.t. reference electrode Reference electrode is always required Controlling potential is a lot more difficult than controlling current This increases the probability of an experiment going wrong

6 November 2008, 6 The role of the potentiostat – facts… The potentiostat controls the potential of the working electrode (relative to the reference electrode) The potentiostat controls the potential of the working electrode regardless of the characteristics of the cell The counter electrode is required for measuring the current only

7 November 2008, 7 …or fiction The potentiostat controls the potential of the working electrode (relative to the reference electrode) : false The potentiostat controls the potential of the working electrode regardless of the characteristics of the cell : false The counter electrode is required for measuring the current only : false

8 November 2008, 8 Is it important to know how it works ? Probably not but… Important for troubleshooting –Example #1 – V OVL warning at potentials well below the maximum value ? common problem with fast kinetics in resistive environments –Example #2 – Small counter electrode / QCM crystal problems occurring during dissolution of deposited metallic adlayer  short-circuit in the cell

9 November 2008, 9 The role of the potentiostat The default role of a potentiostat is to control/measure a potential difference (involves feedback mechanism) The instrument applies and maintains a given setpoint, regardless of the characteristics of the cell If the cell changes during time, the potentiostat changes its output in order to maintain the setpoint  At all times, the potential difference between the working electrode and the reference electrode must be controlled!

10 November 2008, 10 Understanding the potentiostat Core element of a modern potentiostat  The operational amplifier (op amp) - + V S- V S+ V out V-V- V+V+ Inverting input Non inverting input

11 November 2008, 11 The operation amplifier Role of the op amp  Amplify the voltage difference between the 2 inputs by a factor G G = Open loop gain V s = voltage of inverting input with respect to the non-inverting input - + V S- V S+ V out V-V- V+V+ VSVS - + V-V- V+V+ VSVS

12 November 2008, 12 The operation amplifier The ideal op amp: interesting properties –Infinite open gain loop (G =  ) Slightest input voltage difference V s drives the output to infinity –Infinite input impedance (input i = 0) –Zero output impedance (output i =  )

13 November 2008, 13 The operation amplifier The ideal op amp: interesting properties –Infinite open gain loop (G =  ) Slightest input voltage difference V s drives the output to infinity If op amp is used in any circuitry, then the 2 inputs must be (by design) at the same voltage ! The amplifier must be stabilized by feeding back part of its output to its input

14 November 2008, 14 Building block # 1 - Voltage follower - + V out V in VSVS Based on voltage feedback

15 November 2008, 15 Building block # 1 - Voltage follower - + V out V in Based on voltage feedback Output of the voltage follower is always equal to the input voltage! Useless ?  Input impedance =  V in

16 November 2008, 16 Building block # 2 - Current Follower Based on current feedback - + V out i in RfRf ifif S : And

17 November 2008, 17 Building block # 2 - Current Follower Based on current feedback - + V out i in RfRf ifif S V out  -i in  R f CF is a current-to-voltage converter Constitutes the basic element of a zero-resistance amperometer (ZRA) – no shunt resistance summing point S V S = - V out / G  0 V S is a virtual ground!

18 November 2008, 18 Building block # 2 - Current Follower Based on current feedback - + V out i in RfRf ifif S V out  -i in  R f Output of the CF must match the input current (x R f ) at all times !

19 November 2008, 19 Building block # 2 - Current Follower Based on current feedback - + V out i in R f3 S R f2 R f1 Automatic current ranging in the potentiostat

20 November 2008, 20 Automatic current ranging issues Relay settling time problem prevents high sampling rate 1000 V/s linear scan 100 uA current range alkanethiol SAM on gold composed of a 10 bond ferrocene derived alkanethiol with 8-mercaptooctanol in a 1:20 ratio

21 November 2008, 21 Automatic current ranging issues Relay settling time problem prevents high sampling rate 1000 V/s linear scan 10 mA current range alkanethiol SAM on gold composed of a 10 bond ferrocene derived alkanethiol with 8-mercaptooctanol

22 November 2008, 22 Building block # 3 - Scaler - + V out RfRf ifif S R in V in i in V out = -i in  R f Based on current feedback Scaling factor

23 November 2008, 23 Building block # 3 - Scaler - + V out RfRf ifif S R in V in i in Based on current feedback Output of the scaler is always equal to the inverted input multiplied by the scaling factor !

24 November 2008, 24 Building block # 4 - Adder - + V out RfRf ifif S R1R1 R2R2 R3R3 V1V1 V2V2 V3V3 i1i1 i2i2 i3i3 V out = -i in  R f Combination of scalers

25 November 2008, 25 Building block # 4 - Adder - + V out RfRf ifif S R1R1 R2R2 R3R3 V1V1 V2V2 V3V3 i1i1 i2i2 i3i3 Combination of scalers Output of the adder is always equal to the inverted sum of the independently scaled input voltages!

26 November 2008, 26 Building block # 5 - The control amplifier V out V A = -V in - + R1R1 R2R2 A i0i0 i0i0 R R i i S V in -V in Condition must be true at all times

27 November 2008, 27 Building block # 5 - The control amplifier V out V A = -V in - + R1R1 R2R2 A i0i0 i0i0 R R i i S V in -V in Output of the control amplifier is set so that the potential of A is at – V in w.r.t. ground at all times: potentiostat

28 November 2008, 28 Building block # 5 - The control amplifier V out V A = -V in - + R1R1 R2R2 A i0i0 i0i0 R R i i S V in -V in V in (V)111 V A (V) R 1 (Ohm) R 2 (Ohm) V out (V) !! Max V out = Compliance voltage

29 November 2008, 29 Compliance voltage problems

30 November 2008, 30 Compliance voltage problems

31 November 2008, 31 Basic potentiostat/e-cell - + R R i i S V in i cell V out ce we re -V in ce we re RR wece re RpRp CdCd V ref = -V in

32 November 2008, 32 Basic potentiostat/e-cell The potentiostat controls the potential of the working electrode (relative to the reference electrode) The potentiostat controls the potential of the working electrode regardless of the characteristics of the cell The counter electrode is required for measuring the current only

33 November 2008, 33 Basic potentiostat/e-cell - + R R i i S V in i cell V out ce we re -V in V ref = -V in Problems of this potentiostat concept: -Current flowing through the reference electrode -No current measurement

34 November 2008, S i cell V out ce we re V in - + -V in Basic potentiostat/e-cell Voltage follower Control amplifier

35 November 2008, S i cell V out ce we re V in V current S’ -V in Basic potentiostat/e-cell Current follower Voltage follower Control amplifier

36 November 2008, S i cell V out ce we re V1V1 V2V2 V3V V current S’ -V in Basic potentiostat/e-cell Current follower Voltage follower Control amplifier Adder

37 November 2008, 37 Summary The potentiostat does not control the potential of the working electrode! The potentiostat controls the potential of the counter electrode only (relative to the working electrode) The counter electrode is the most important electrode (followed by the reference electrode – the working electrode is never a problem) Compliance voltage limits are very important in the choice of the potentiostat / application With a few components you can build your own potentiostat

38 November 2008, 38 Good enough for a homemade potentiostat?

39 November 2008, 39 Difficulties with potential control Interfacial capacitance and solution resistance – High solution resistance has high impact on potential control, especially for large currents – Potentiostat must have enough power reserve to supply the necessary current – Ex: 1 V step in 1 µs on a 2 µF interfacial capacitance – i mean = 2 µC/1 µs = 2 A peak current can be higher !

40 November 2008, 40 Difficulties with potential control Solution resistance – high current measurements R u is the uncompensated resistance Compensated resistance (control amplifier)

41 November 2008, 41 Difficulties with potential control Solution resistance – high current measurements  wk  ce iR sol Ref iR u R u is the uncompensated resistance R sol = R  + R u For high currents, the voltage drop across the solution can reach ~ 100 V - The potentiostat must be able to supply enough power (  the compliance voltage must be high enough)!

42 November 2008, 42 Difficulties with potential control How to reduce R u – Reduce total resistance (R  + R u ) Increase the conductivity (supporting electrolyte, polar solvent) Reduce the viscosity Increase the temperature – Reduce the size of the we – Move the re as close as possible to the we Use a Luggin capillary

43 November 2008, S i cell V out ce we re V1V1 V2V2 V3V V current = -iRf S’ -V in Electronic IR u compensation – positive feedback

44 November 2008, S i cell V out ce we re V1V1 V2V2 V3V V current = -iR f S’ -V in Automatic compensation of the iR u drop can be attempted by feeding back a correction voltage proportional to the current flow to the input of the potentiostat The variable resistance can be trimmed to be set to a fraction f of the feedback resistance (R f ) feedback voltage is –ifR f e true (vs re) = e1 + e2 + e3 – ifR f + iR u Electronic IR u compensation – positive feedback

45 November 2008, 45 Computer controlled potentiostat Computer use digital signals (0 & 1) instead of analog signals (0-10 V) Interfacing a potentiostat with a computer requires translation back and forth Modern potentiostats have on-board DAC (digital to analog converters) and ADC (analog to digital converters)

46 November 2008, 46 Computer controlled potentiostat … … DAC ADC 0-10 V P-stat External (RDE, strirrer, T, …) External (QCM, spectro, pH, …)

47 November 2008, 47 Computer controlled potentiostat DAC Digital to analog converter - Resolution in bits: 16 bits – 2 16 = digital words - 10 V range/65536 = 150  V resolution - Defines the smallest possible step - Multiple channels working as indipendent function generators

48 November 2008, 48 Computer controlled potentiostat ADC Analog to digital converter - Resolution in bits: 16 bits – 2 16 = digital words - 10 V range/65536 = 150  V resolution - ADC is a digital filter Multi-channel ADC to convert several analog signals into digital

49 November 2008, 49 Autolab potentiostat

50 November 2008, 50 Autolab potentiostat … … DAC ADC 0-10 V P-stat External (RDE, strirrer, T, …) External (QCM, spectro, pH, …) MODULE … 0-10 V

51 November 2008, 51 Autolab potentiostat other D/A modules Scangen module: true linear scan generator –Generates an analog scan (no staircase) with scan rates up to 250,000 V/s FRA module: frequency response analyzer –Digital to analog sine wave generator Both modules are fed into the Adder circuit of the Autolab


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