Presentation on theme: "Principles of operation"— Presentation transcript:
1 Principles of operation PotentiostatsPrinciples of operation
2 Overview The potentiostat – a black box ? Potentiostat (role) The operational amplifierVoltage followerCurrent followerScaler & AdderControl amplifierBasic potentiostat constructionHow to make the most of your potentiostat
3 OverviewA. Bard & L. Faulkner, Electrochemical Methods – Fundamentals and Applications, 2nd edition, John Wiley & SonsH. Girault, Analytical and Physical Electrochemistry, EPFL Press, Marcel DekkerC. Brett, A. M. Brett, Electrochemistry – Principles, Methods and Applications, Oxford University PressD. Pletcher, R. Greef, R. Peat, L. Peter, J. Robinson, Instrumental Methods in Electrochemistry, Horwood PublishingR. E. Simpson, Introductory Electronics – For Scientists and Engineers, Allyn and Bacon
5 Difficulties of potential control It is not possible to measure the potential of the working electrode potential difference w.r.t. reference electrodeReference electrode is always requiredControlling potential is a lot more difficult than controlling currentThis 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 cellThe counter electrode is required for measuring the current only
7 …or fictionThe potentiostat controls the potential of the working electrode (relative to the reference electrode) : falseThe potentiostat controls the potential of the working electrode regardless of the characteristics of the cell : falseThe counter electrode is required for measuring the current only : false
8 Is it important to know how it works ? Probably not but…Important for troubleshootingExample #1 – VOVL warning at potentials well below the maximum value ?common problem with fast kinetics in resistive environmentsExample #2 – Small counter electrode / QCM crystalproblems 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 cellIf 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+VoutV-V+Inverting inputNon 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+VoutV-V+VS-+VoutV-V+VSG = Open loop gainVs = voltage of inverting input with respect to the non-inverting input
12 The operation amplifier The ideal op amp: interesting propertiesInfinite open gain loop (G = )Slightest input voltage difference Vs drives the output to infinityInfinite input impedance (input i = 0)Zero output impedance (output i = )
13 The operation amplifier The ideal op amp: interesting propertiesInfinite open gain loop (G = )Slightest input voltage difference Vs drives the output to infinityIf 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-VSVoutVin+
15 Building block # 1 - Voltage follower Based on voltage feedbackVin-VinVoutVin+VinOutput of the voltage follower is always equal to the input voltage! Useless ? Input impedance =
16 Building block # 2 - Current Follower Based on current feedback-+VoutiinRfifS@ S :And
17 Building block # 2 - Current Follower Based on current feedback-+VoutiinRfifSVout -iin RfCF is a current-to-voltage converterConstitutes the basic element of a zero-resistance amperometer (ZRA) – no shunt resistancesumming point SVS = - Vout / G 0 VS is a virtual ground!
18 Building block # 2 - Current Follower Based on current feedback-+VoutiinRfifSVout -iin RfOutput of the CF must match the input current (x Rf) at all times !
19 Building block # 2 - Current Follower Based on current feedbackRf1Rf2Rf3-Automatic current ranging in the potentiostatSiinVout+
20 Automatic current ranging issues Relay settling time problem prevents high sampling rate1000 V/s linear scan100 uA current rangealkanethiol SAM on gold composed of a 10 bond ferrocenederived alkanethiol with 8-mercaptooctanol in a 1:20 ratio
21 Automatic current ranging issues Relay settling time problem prevents high sampling rate1000 V/s linear scan10 mA current rangealkanethiol SAM on gold composed of a 10 bond ferrocenederived alkanethiol with 8-mercaptooctanol
22 Building block # 3 - Scaler Based on current feedbackRfifRinVin-SVoutiin+Vout = -iin RfScaling factor
23 Building block # 3 - Scaler Based on current feedbackRfifRinVin-SVoutiin+Output of the scaler is always equal to the inverted input multiplied by the scaling factor !
24 Building block # 4 - Adder Combination of scalersR1RfV1i1ifR2V2-i2SVoutR3+Vout = -iin RfV3i3
25 Building block # 4 - Adder Combination of scalersR1RfV1i1ifR2V2-i2SVoutR3+V3i3Output of the adder is always equal to the inverted sum of the independently scaled input voltages!
26 Building block # 5 - The control amplifier SVin-Vouti+Rii0R1-VinVA = -VinAi0R2Condition must be true at all times
27 Building block # 5 - The control amplifier SVin-Vouti+Rii0R1-VinVA = -VinAi0R2Output 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 amplifier SVin-Vouti+Rii0R1-VinVA = -VinAVin (V)1VA (V)-1R1 (Ohm)10010000R2 (Ohm)1000Vout (V)-1.1-11 !!i0R2Max Vout = Compliance voltage
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 cellThe counter electrode is required for measuring the current only
33 Basic potentiostat/e-cell RSVin-Vouti+Ricereicell-VinVref = -VinweProblems of this potentiostat concept:Current flowing through the reference electrodeNo current measurement
34 Basic potentiostat/e-cell Control amplifierSVin-Vout+-ce-Vinreicell+weVoltage follower
35 Basic potentiostat/e-cell Control amplifierSVin-Vout+-ce-Vinreicell+weVoltage follower-S’Vcurrent+Current follower
37 SummaryThe 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 / applicationWith a few components you can build your own potentiostat
39 Difficulties with potential control Interfacial capacitance and solution resistanceHigh solution resistance has high impact on potential control, especially for large currentsPotentiostat must have enough power reserve to supply the necessary currentEx: 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 measurementsCompensated resistance (control amplifier)Ru is the uncompensated resistance
41 Difficulties with potential control Solution resistance – high current measurementsfwkRu is the uncompensated resistanceRsol = RW + RuiRuiRsolfceRefFor high currents, the voltage drop across the solution can reach ~ 100 VThe potentiostat must be able to supply enough power ( the compliance voltage must be high enough)!
42 Difficulties with potential control How to reduce RuReduce total resistance (RW + Ru)Increase the conductivity (supporting electrolyte, polar solvent)Reduce the viscosityIncrease the temperatureReduce the size of the weMove the re as close as possible to the weUse a Luggin capillary
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 potentiostatThe variable resistance can be trimmed to be set to a fraction f of the feedback resistance (Rf)feedback voltage is –ifRfetrue (vs re) = e1 + e2 + e3 – ifRf + iRuV1SV2-Vout+V3ce--Vinre+icellweVcurrent = -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 forthModern potentiostats have on-board DAC (digital to analog converters) and ADC (analog to digital converters)
47 Computer controlled potentiostat DACDigital to analog converterResolution in bits: 16 bits – 216 = digital words - 10 V range/65536 = 150 mV resolutionDefines the smallest possible stepMultiple channels working as indipendent function generators
48 Computer controlled potentiostat ADCAnalog to digital converterResolution in bits: 16 bits – 216 = digital words - 10 V range/65536 = 150 mV resolutionADC is a digital filterMulti-channel ADC to convert several analog signals into digital
51 Autolab potentiostat other D/A modules Scangen module: true linear scan generatorGenerates an analog scan (no staircase) with scan rates up to 250,000 V/sFRA module: frequency response analyzerDigital to analog sine wave generatorBoth modules are fed into the Adder circuit of the Autolab