November 2008, 1 Potentiostats Principles of operation.

November 2008, 1

PotentiostatsPrinciples of operation

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

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

November 2008, 4

The potentiostat – a black box ?

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

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

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

November 2008, 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 crystalproblems occurring during dissolution of deposited metallic adlayer short-circuit in the cell

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!

November 2008, 10

Understanding the potentiostat

• Core element of a modern potentiostat The operational amplifier (op amp)

-

+

VS-

VS+

VoutV-

V+

Inverting input

Non inverting input

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

Vs = voltage of inverting input with respect to the non-inverting input

-

+

VS-

VS+

VoutV-

V+

VS out SV (V V ) G V G -

+

VoutV-

V+

VS

November 2008, 12


• 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 = )

November 2008, 13


• 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

November 2008, 14

Building block # 1 - Voltage follower

-

+Vout

Vin

VS

• Based on voltage feedback

out SV V G

S out inV V V

out out in out in

1V V V G V 1 V

G

o ouG t it nu

1lim V 1

GV V

November 2008, 15

Building block # 1 - Voltage follower

-

+Vout

Vin

• Based on voltage feedback

out inV V

Output of the voltage follower is always equal to the input voltage! Useless ? Input impedance =

Vin

Vin

Vin

November 2008, 16

Building block # 2 - Current Follower

• Based on current feedback

-

+Voutiin

Rf

if

S

@ S : i 0f ini i

out Sin

f

V Vi

R

And out SV V G

outout

in out in ff

VV 1G i V 1 i R

R G

out out in fG

1lim V 1

GV i R

November 2008, 17



-

+Voutiin

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!

November 2008, 18



-

+Voutiin

Rf

if

S

Vout -iin Rf

Output of the CF must match the input current (x Rf) at all times !

November 2008, 19



-

+Voutiin

Rf3

S

Rf2

Rf1

Automatic current ranging in the potentiostat

November 2008, 20

Automatic current ranging issues

• Relay settling time problem prevents ACR @ high sampling rate

1000 V/s linear scan100 uA current range

alkanethiol SAM on gold composed of a 10 bond ferrocenederived alkanethiol with 8-mercaptooctanol in a 1:20 ratio

November 2008, 21

Automatic current ranging issues

• Relay settling time problem prevents ACR @ high sampling rate

1000 V/s linear scan10 mA current range

alkanethiol SAM on gold composed of a 10 bond ferrocenederived alkanethiol with 8-mercaptooctanol

November 2008, 22

Building block # 3 - Scaler

-

+Vout

Rf

if

S

Rin

Vin

iin

Vout = -iin

Rf

fou

inin t in

inin

Vi

R

RV V

R


Scaling factor

November 2008, 23

Building block # 3 - Scaler

-

+Vout

Rf

if

S

Rin

Vin

iin


fout in

in

RV V

R

Output of the scaler is always equal to the inverted input multiplied by the scaling factor !

November 2008, 24

Building block # 4 - Adder

-

+Vout

Rf

if

S

R1

R2

R3

V1

V2

V3

i1

i2

i3Vout = -iin

Rf

f f fout 1 2 3

1 2

31 2in 1 2 3

1 32 3

VV Vi i i i

R R

R R RV V V V

R R RR

• Combination of scalers

November 2008, 25

Building block # 4 - Adder

-

+Vout

Rf

if

S

R1

R2

R3

V1

V2

V3

i1

i2

i3f f f

out 1 2 31 2 3

R R RV V V V

R R R

• Combination of scalers

Output of the adder is always equal to the inverted sum of the independently scaled input voltages!

November 2008, 26

Building block # 5 - The control amplifier

Vout

VA = -Vin

-

+

R1

R2

A

i0

i0

R

R

i

i

SVin

-Vin

Condition must be true at all times

21

out0 RR

Vi

2

in0 R

Vi

2

21inout R

RRVV

November 2008, 27


Vout

VA = -Vin

-

+

R1

R2

A

i0

i0

R

R

i

i

SVin

-Vin

2

21inout R

RRVV

Output of the control amplifier is set so that the potential of A is at – Vin w.r.t. ground at all times: potentiostat

November 2008, 28


Vout

VA = -Vin

-

+

R1

R2

A

i0

i0

R

R

i

i

SVin

-Vin

2

21inout R

RRVV

Vin (V) 1 1 1

VA (V) -1 -1 -1

R1 (Ohm) 100 100 10000

R2 (Ohm) 1000 1000000 1000

Vout (V) -1.1 -1.0001 -11 !! Max Vout = Compliance voltage

November 2008, 29

0.1 0.2 0.3 0.4 0.5

-8.0x10-5

-6.0x10-5

-4.0x10-5

-2.0x10-5

0.0

2.0x10-5

4.0x10-5

6.0x10-5

8.0x10-5

250 mV/s 100 50 25 10 5

I /A

E /V

4.8mM ferrocene, 0.1M [NEt4][BF

4], MeCN

flow rate = 1ml/hr

Compliance voltage problems

November 2008, 30

Compliance voltage problems

-0.1 0.0 0.1 0.2 0.3 0.4

-1.0x10-4

-5.0x10-5

0.0

5.0x10-5

1.0x10-4

1.5x10-4

I /A

E /V

1000 mV/s

-0.1 0.0 0.1 0.2 0.3 0.4-0.1

0.0

0.1

0.2

0.3

0.4

2nd

sign

al E

/V

E /V

November 2008, 31

Basic potentiostat/e-cell

-

+

R

R

i

i

SVin

icell

Vout

ce

we

re-Vin

ce

we

reR

wece re

Rp

Cd

Vref = -Vin

pout in

p

R RV V

R

November 2008, 32


• 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

November 2008, 33


-

+

R

R

i

i

SVin

icell

Vout

ce

we

re-Vin Vref = -

Vin

pout in

p

R RV V

R

Problems of this potentiostat concept:

-Current flowing through the reference electrode

-No current measurement

November 2008, 34

-

+

S

icell

Vout

ce

we

re

Vin

-

+

-Vin

Basic potentiostat/e-cellp

out inp

R RV V

R

Voltage follower

Control amplifier

November 2008, 35

-

+

S

icell

Vout

ce

we

re

Vin

-

+

-

+

VcurrentS’

-Vin


out inp

R RV V

R

current cell fV i R

Current follower

Voltage follower

Control amplifier

November 2008, 36

-

+

S

icell

Vout

ce

we

re

V1

V2

V3

-

+

-

+

VcurrentS’

-Vin


out inp

R RV V

R

current cell fV i R

Current follower

Voltage follower

Control amplifier

Adder

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

November 2008, 38

Good enough for a homemade potentiostat?

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

November 2008, 40


• Solution resistance – high current measurements

Ru is the uncompensated resistance

Compensated resistance (control amplifier)

November 2008, 41


• Solution resistance – high current measurements

wk

ce

iRsol

Ref

iRu

Ru is the uncompensated resistance

Rsol = R + Ru

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)!

November 2008, 42


• How to reduce Ru

– Reduce total resistance (R + 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

November 2008, 43

-

+

S

icell

Vout

ce

we

re

V1

V2

V3

-

+

-

+

Vcurrent = -iRfS’

-Vin

Electronic IRu compensation – positive feedback

November 2008, 44

-

+

S

V1

V2

V3

-

+

Vcurrent = -iRfS’

-Vin

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 –ifRf

etrue (vs re) = e1 + e2 + e3 – ifRf + iRu

Electronic IRu compensation – positive feedback

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)

November 2008, 46

Computer controlled potentiostat

01001010…

10010100…

DAC

ADC

0-10 V

0-10 V

P-stat

External (RDE, strirrer, T, …)

External (QCM, spectro, pH, …)

November 2008, 47


DAC Digital to analog converter

- Resolution in bits: 16 bits – 216 = 65536 digital words- 10 V range/65536 = 150 V resolution

- Defines the smallest possible step

- Multiple channels working as indipendent function generators

November 2008, 48


ADC Analog to digital converter

- Resolution in bits: 16 bits – 216 = 65536 digital words- 10 V range/65536 = 150 V resolution

- ADC is a digital filter

Multi-channel ADC to convert several analog signals into digital

November 2008, 49

Autolab potentiostat

November 2008, 50

Autolab potentiostat

01001010…

10010100…

DAC

ADC

0-10 V

0-10 V

P-stat

External (RDE, strirrer, T, …)

External (QCM, spectro, pH, …)

MODULE

01001010…

0-10 V

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

November 2008, 1 Potentiostats Principles of operation.

Documents

Transcript of November 2008, 1 Potentiostats Principles of operation.