Principles of operation Potentiostats Principles of operation

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

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

The potentiostat – a black box ?

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

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

…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

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

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!

Understanding the potentiostat Core element of a modern potentiostat  The operational amplifier (op amp) - + VS- VS+ Vout V- V+ Inverting input Non inverting input

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

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 = )

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

Building block # 1 - Voltage follower Based on voltage feedback - VS Vout Vin +

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 = 

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

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 Voltage @ summing point S VS = - Vout / G  0 V S is a virtual ground!

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 !

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

Automatic current ranging issues Relay settling time problem prevents ACR @ 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

Automatic current ranging issues Relay settling time problem prevents ACR @ 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

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

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 !

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

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!

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

Building block # 5 - The control amplifier S 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

Building block # 5 - The control amplifier S Vin - Vout i + R i i0 R1 -Vin VA = -Vin A Vin (V) 1 VA (V) -1 R1 (Ohm) 100 10000 R2 (Ohm) 1000 1000000 Vout (V) -1.1 -1.0001 -11 !! i0 R2 Max Vout = Compliance voltage

Compliance voltage problems

Compliance voltage problems

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

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

Basic potentiostat/e-cell R 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

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

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

Basic potentiostat/e-cell V1 Control amplifier S V2 - Vout + V3 Adder - ce -Vin re icell + we Voltage follower - S’ Vcurrent + Current follower

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

Good enough for a homemade potentiostat?

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 !

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

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)!

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

Electronic IRu compensation – positive feedback - Vout + V3 ce - -Vin re + icell we Vcurrent = -iRf - S’ +

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’ +

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)

Computer controlled potentiostat External (RDE, strirrer, T, …) 01001010… 0-10 V DAC P-stat ADC 10010100… 0-10 V External (QCM, spectro, pH, …)

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

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

Autolab potentiostat

Autolab potentiostat External (RDE, strirrer, T, …) 01001010… 0-10 V DAC P-stat 01001010… 0-10 V MODULE 10010100… 0-10 V ADC External (QCM, spectro, pH, …)

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