1. Principles of operation
Potentiostats
Principles of operation

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. 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. The potentiostat – a black box ?

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. 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. …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. Is it important to know how it works ?
Probably not but…
Important for troubleshooting
Example #1 – VOVL 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. 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. Understanding the potentiostat
Core element of a modern potentiostat
 The operational amplifier (op amp) - +
VS-
VS+
Vout V- V+
Inverting input
Non inverting input

11. The operation amplifier
Role of the op amp
 Amplify the voltage difference between the 2 inputs by a factor G - +
VS-
VS+
Vout V- V+ VS - +
Vout V- V+ VS
G = Open loop gain
Vs = voltage of inverting input with respect to the non-inverting input

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

13. The operation amplifier
The ideal op amp: interesting properties
Infinite open gain loop (G = )
Slightest input voltage difference Vs 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. Building block # 1 - Voltage follower
Based on voltage feedback - VS
Vout
Vin +

15. Building block # 1 - Voltage follower
Based on voltage feedback
Vin -
Vin
Vout
Vin +
Vin
Output of the voltage follower is always equal to the input voltage! Useless ?  Input impedance = 

16. Building block # 2 - Current Follower
Based on current feedback - +
Vout
iin Rf if S
@ S :
And

17. Building block # 2 - Current Follower
Based on current feedback - +
Vout
iin Rf if S
Vout  -iin  Rf
CF is a current-to-voltage converter
Constitutes the basic element of a zero-resistance amperometer (ZRA) – no shunt resistance
summing point S
VS = - Vout / G  0 V
S is a virtual ground!

18. Building block # 2 - Current Follower
Based on current feedback - +
Vout
iin Rf if S
Vout  -iin  Rf
Output of the CF must match the input current (x Rf) at all times !

19. Building block # 2 - Current Follower
Based on current feedback
Rf1
Rf2
Rf3 -
Automatic current ranging in the potentiostat S
iin
Vout +

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. 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. Building block # 3 - Scaler
Based on current feedback Rf if
Rin
Vin - S
Vout
iin +
Vout = -iin  Rf
Scaling factor

23. Building block # 3 - Scaler
Based on current feedback Rf if
Rin
Vin - S
Vout
iin +
Output of the scaler is always equal to the inverted input multiplied by the scaling factor !

24. Building block # 4 - Adder
Combination of scalers R1 Rf V1 i1 if R2 V2 - i2 S
Vout R3 +
Vout = -iin  Rf V3 i3

25. Building block # 4 - Adder
Combination of scalers R1 Rf V1 i1 if R2 V2 - i2 S
Vout R3 + V3 i3
Output of the adder is always equal to the inverted sum of the independently scaled input voltages!

26. Building block # 5 - The control amplifierS
Vin -
Vout i + R i i0 R1
-Vin
VA = -Vin A i0 R2
Condition must be true at all times

27. Building block # 5 - The control amplifierS
Vin -
Vout i + R i i0 R1
-Vin
VA = -Vin A i0 R2
Output of the control amplifier is set so that the potential of A is at – Vin w.r.t. ground at all times: potentiostat

28. Building block # 5 - The control amplifierS
Vin -
Vout i + R i i0 R1
-Vin
VA = -Vin A
Vin (V) 1
VA (V) -1
R1 (Ohm)
100
10000
R2 (Ohm)
1000
Vout (V)
-1.1
-11 !! i0 R2
Max Vout = Compliance voltage

29. Compliance voltage problems

30. Compliance voltage problems

31. Basic potentiostat/e-cellR S
Vin -
Vout i + R i ce re
icell
-Vin
Vref = -Vin we ce Cd re RW we Rp ce re we

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. Basic potentiostat/e-cellR S
Vin -
Vout i + R i ce re
icell
-Vin
Vref = -Vin we
Problems of this potentiostat concept:
Current flowing through the reference electrode
No current measurement

34. Basic potentiostat/e-cell
Control amplifier S
Vin -
Vout + - ce
-Vin re
icell + we
Voltage follower

35. Basic potentiostat/e-cell
Control amplifier S
Vin -
Vout + - ce
-Vin re
icell + we
Voltage follower - S’
Vcurrent +
Current follower

36. Basic potentiostat/e-cellV1
Control amplifier S V2 -
Vout + V3
Adder - ce
-Vin re
icell + we
Voltage follower - S’
Vcurrent +
Current follower

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. Good enough for a homemade potentiostat?

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 – imean = 2 µC/1 µs = 2 A peak current can be higher !

40. Difficulties with potential control
Solution resistance – high current measurements
Compensated resistance (control amplifier)
Ru is the uncompensated resistance

41. Difficulties with potential control
Solution resistance – high current measurements
fwk
Ru is the uncompensated resistance
Rsol = RW + Ru
iRu
iRsol
fce
Ref
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. Difficulties with potential control
How to reduce Ru
Reduce total resistance (RW + Ru)
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. Electronic IRu compensation – positive feedback-
Vout + V3 ce -
-Vin re +
icell we
Vcurrent = -iRf - S’ +

44. Electronic IRu compensation – positive feedback
Automatic compensation of the iRu 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 (Rf)
feedback voltage is –ifRf
etrue (vs re) = e1 + e2 + e3 – ifRf + iRu V1 S V2 -
Vout + V3 ce -
-Vin re +
icell we
Vcurrent = -iRf - S’ +

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

47. Computer controlled potentiostat
DAC
Digital to analog converter
Resolution in bits: 16 bits – 216 = digital words - 10 V range/65536 = 150 mV resolution
Defines the smallest possible step
Multiple channels working as indipendent function generators

48. Computer controlled potentiostat
ADC
Analog to digital converter
Resolution in bits: 16 bits – 216 = digital words - 10 V range/65536 = 150 mV resolution
ADC is a digital filter
Multi-channel ADC to convert several analog signals into digital

49. Autolab potentiostat

50. Autolab potentiostat External (RDE, strirrer, T, …) 01001010… 0-10 V
DAC
P-stat …
0-10 V
MODULE …
0-10 V
ADC
External (QCM, spectro, pH, …)

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