sibee - presentation impedance 2013 seminar

8/9/2019 SIBEE - Presentation Impedance 2013 Seminar

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Metrohm AutolabInstruments for Electrochemical Impedance Spectroscopy



Outline

• The Autolab instrument• Cell connections and configurations• Basics of EIS

• Practical EIS• Data analysis• Applications



The Instrument



Instrumentation requirementsfor EIS

• PGSTAT• Impedance analyzer• Software for data acquisition and

analysis



The instrument

Analog PGSTAT with USBconnection to Windows software

FRA32M impedance analyzer module with1 MHz – 10 µHz range with PGSTAT



FRA32M Module

• Module for electrochemical impedancespectroscopy• 32 MHz to 10 µHz• 1 MHz to 10 μHz in combination

with the PGSTAT• DC range: up to 10 V

• Single sine and multi-sine



FRA32M

• New impedance analyzer module• Replaces the FRA2• Works with Autolab PGSTAT series

• Not available for µAutolabIII



FRA32M

• 32M stands for 32 MHz• !! MAXIMUM Frequency with

PGSTAT: 1 MHz !!• Future developments to raise the limit



FRA32M

• Direct DSG output

20 MHz 32 MHz



What happens above 1 MHz

• PGSTAT128N/FRA32M, 500 mV (top)Sharp decrease @ 700kHz

~80% attenuation @ 1 MHz

~99.9 % attenuation @ 10 MHz



FRA32M

• Uses on-board DSP• Crunches 1 M per channel down to

4096 points• And finally down to 1• Much faster than FRA2• No filtering



FRA2 vs FRA32M



FRA32M: wrapping up

• Very advanced module• Expected product lifetime 10+ years• All procedures are compatible!



The instrument

Analog PGSTAT with USBconnection to Windows software

FRA2 impedance analyzer modulewith 1 MHz – 10 µHz range



FRA2 module specifications

• Frequency range from 1 MHz to 10 µHz• Applied Frequency Resolution of 0.003%• AC amplitude: 0.2mV to 0.35 V (RMS)• Input range of 10 V

• Max. frequency limited by PGSTAT



Instrumental limitations

• Contour plot

Not possible to measureZ accurately at anyfrequency



Cell connections



Cell connections

• Autolab potentiostat / Galvanostat



Cell connections

• PGSTAT provides 4connections

• CE and WE (currentmeasurement)

• RE and S (voltagemeasurement)



Cell connections (2 electrodes)

2 electrode configuration precise control of theinterfacial potential is not critical

This configuration is often used in the characterizationof energy storage and energy conversion devices

Impedance measured between RE and S




3 electrode configuration most common for typicalelectrochemical measurements

The RE in close proximity of WE to control the interfacialpotential accurately





4 electrode configuration is used to study theliquid/liquid interface (two non-miscible electrolytes ortwo electrolytes separated by a membrane)




Electrochemical cellWE/S

RECE

Electrochemical

interface



Electrochemical interface• Interface

+ ++

+++

+

+

Electrode

e -

IHP OHP Diffusion layer1- 100 μm

Potential

IHP Inner Helmoltz plane: ion adsorbed onto metal surface

OHP Outer Helholtz plane: closest ions free to move within the electrolyte

Bulk Solution



Equivalent circuit

• Electrochemical interface model



Equivalent circuit

• Uncompensated resistance, Ru• Solution between RE and WE

• Concentration, cell setup, Temperature,conductivity, type of ions, mass transport,…



Equivalent circuit

• Typical values:• up to 100 Ω (aqueous solution)

KCl 0.1 mol/L 20°C = 85.69 Ω • up to 10 k Ω (organic solvent)• Between 1m Ω to 1 Ω Batteries



Equivalent circuit

• Double layer capacitance, Cdl• Diffuse interfacial capacitance

• Concentration, ions, electrode surface,adsorbed species, …



Equivalent circuit

• Typical values:• Double layer bare metal 10-50 µF/cm 2

• Fe 2 O3 - Ni 2 O5 2 µF/cm2

• Al2O3 0.5 µF/cm 2 • Epoxy cast coating 6 pF/cm 2

• Asphalt coating 0.5 pF/cm 2Bernard Tribollet & Mark E. Orazem: Electrochemical Impedance Spectroscopy, Wiley-Interscience, 2008



Equivalent circuit

• Charge transfer resistance, Rct• Polarization resistance

• Electron transfer kinetics, Temperature,electrode composition, morphology, …



Equivalent circuit

• Typical values: whole measurable range( m Ω very fast kinetics up to G Ω slow

charge transfer)Silver 1.59 10 -8 Ωm, Copper 1.68 10 -8 ΩmSilicon 6.40 10 2 Ωm Glass10 10 to 10 14 Ωm

Teflon 1022

to 1024

Ωm



Equivalent circuit

• Diffusion impedance, W (Warburg)• Related to semi infinite diffusion

• T, concentration, diffusion coefficients,viscosity, electrode area

• Typical values: depends on the system



Electrochemical methods

• Large number of methods available• DC: cyclic voltammetry, chrono

amperometry, … • AC: electrochemical impedance

spectroscopy, …



Advantage of DC methods

• Ex: cyclic voltammetry• Most popular technique in

electrochemistry• Provides a complete overview of the

electrochemical interface• Immediately recognizable



Cyclic voltammetryPlatinum in HClO 4 0.1 M

Added EtOH

Anodic peaks attributedto the oxidation of EtOH



Disadvantage of DC methods

• System is driven away from equilibrium• Response is given by Σ individual

contributions• Information about the rate determining

step can be determined• No information on the internal dynamics

of the system



Disadvantage of DC methodsIron in sea waterNo direct information aboutelectrochemical interface



Dummy cell case study

• Assuming a simple dummy cell




• CV staircase methodUsing Ohm’s law and a linearregression:Slope = 0.000142411/Slope = 7021.98 Ω ≈ 102 Ω + 2230 Ω + 4700 Ω (7032 Ω )




• EISUsing direct reading of datashown in the Nyquist plot, R1,R2 and R3 are immediatelyknownR1 = 106 Ω R2 = 4844.1 – 106=4738.1 Ω R3 = 7028.7 – 4844.1=2184.6 Ω




• EISUsing indirect reading ofdata shown in the Nyquistplot, C1, and C2 are easy tocalculate

C1 = 9.95 nF using R2C2 = 10.77 µF using R3

max1

R C



Alternative: EIS

• Electrochemical impedancespectroscopy

‘Study of the variation of the impedanceof an electrochemical system with the

frequency of a small-amplitude ACperturbation’



EIS Basics



EIS Basics

• Ohm's law gives a simple relationbetweendc-potential (E) and dc-current (i):

I E R



EIS Basics

• EIS is a transfer function betweenpotential and current

te e sin t ti i sin t Z(ω )

0

e sin t sin tZ Z

i sin t sin t



EIS Basics

te e sin t

ti i sin t

The current responseleads or lags the voltageby a phase angle



EIS Basics

When ac-signals are involved therelation is:

Z ( ω )=

where Z ( ω ) is the complex impedance.

ΔE ( ω )

ΔI ( ω )



EIS Basics: complex number

• It is common practice to express Z asa complex function

j ( )0 0

Re Im

Z( ) Z e Z cos jsin

Z Z j Z



EIS Basics: complex number

• Real components are located on theabscissa

• Imaginary components are located onthe ordinates



EIS Basics: data presentation

• Nyquist plot• Plot of – Z” vs Z’, using iso -metric

axes

Z( ) Z'( ) j Z"( )

Total impedance Real componentImaginary component




• Bode presentation• Plot of |Z| vs log F and Φ vs log F

j ( )Z( ) Z e

Total impedance Modulus

Phase angle




• Bode presentation




• 3D projection



Circuit Element Review



Ohm’s law holds at all frequencies for apure resistor

Z( ) R

0

Pure resistor (R)



Ohm’s law holds at all frequencies for apure resistor

Typical examples: Ohmic resistance,

charge transfer resistance

There is no phase shift

Z( ) R

0

Pure resistor (R)



Resistor



Impedance (reactance) is frequencydependent

1Z( )

C

2

Pure capacitor (C)



Capacitor

• Capacitor

• Typical examples: double layercapacitance, coating capacitance

C1 1Z j

j C C

90

phase shift



Capacitor



• Inductance

• Typical examples: instrumentalartifacts, adsorption phenomena

LZ j L

90

Inductance



• Impedance of element in series =Σ impedance of elements

R C1

Z Z Z R j

C

RC, series

R Ci i i

R CE E E 1Z( ) R R

0

2



RC, series



1 1Z( )1CR

02

Impedance of element in parallel =1/R (1/impedance of elements)

R C

1 1 1 1 j CZ Z Z R

RZ

1 Rj C

RC, parallel

R Ci i i R CE E E



RC, parallel



Impedance vs admittance



Φ0

Φ 0

Randles Cell – Electrochemical cell model

R Ω + Z f

RΩ

∞ > ZC dl > 0Capacitivecontribution

Pure resistive behavior,

no phase change.

Pure resistive behavior,

no phase change.



Fmax

Randles Cell – Electrochemical cell model

R Ω + Z f

RΩ



EIS back to Basics



Linearity condition

• The applied AC amplitude must be

small so that the response of the cellcan be assumed to be linear (in 1 st approx)

• Important choice of the appliedamplitude



Linearity condition

At fixed DC potential (or current), a small amplitude sinusoidal potential(or current) perturbation is applied. The response of the cell is a small

amplitude sinusoidal current (or potential)



Linearity condition

All systems are nonlinear and the response

depends on the system



Linearity condition



Linearity conditionPotentiostatic or galvanostatic control?Z is independent of the m o d e

At what DC potential or current? Always at OCP?

Depends on the systemRequires knowledge of the system

What should the AC amplitude be?



Linearity condition

• FAQ # 1

• How to choose the amplitude?• Small enough to stay linear• Big enough to measure a response



Stability condition

• Corrosion of the electrode, adsorption

or formation of oxides, discharge ofthe device, changes in theelectrochemical interface, …

• Electrochemical system are unstable



Stability condition

• Min. time for Z at 10 µHz: 10.000 s

• Scan from 1 kHz to 1 mHz, 61frequencies, log. distribution

• 2 h, 47 minutes (estimated)

• Scan from 1 MHz to 1 Hz, 61frequencies, log. Distribution• 7 minutes (estimated)



Stability condition

• Duration

The experimentduration is dominatedby the low frequencies



Avoiding traps

• Powerful method but not stand alone

• Knowledge of the system is required• 3 conditions must be respected• Instrument is not a black box



What’s wrong with this picture?

Bit of scattering in the low frequency range?



Measurement sequence

• Step 1: AC signal is applied on the cell

• Step 2: AC response is recorded• Step 3: signals are sent to the

impedance analyzer




Raw signals (50 Hz contamination) Lissajous plot




• FFT is computed for both signals

• Impedance is calculated fromfrequency domain data




Fourier transformed signals (50 Hz)



Nyquist and Bode plots

• Updated in the software



Hidden information

• Nyquist and Bode plots do not show

all the information• The raw signals are insteresting to

consider

• Lissajous plots test the linearity



Lissajous plotsPure R Pure C Real cell



Lissajous plots

• Measurement of impedance

O

B

ADO

B

AD

OAVZ 1M

i OB

OD

sin 33OA



Raw signals in resolutionThe raw data can beused to verify the dataquality in terms oflimit of detection



Kramers-Kronig test

• Basic test for data validation

• Equations linking real and imaginarycomponents of complex quantities forsystems fulfilling the conditions of

linearity , causality and stability



Kramers-Kronig test

• Diagnostic tool for impedance data

• Test the data for consistency• Recalculates the real/imaginary part

based on the measured imaginary/real

part



Practical EIS: frequency scan

• Frequency range?

• Depends on the system’s time constants • 10 kHz to 100 mHz is a good start

(always high to low)• Above 10 kHz effects from cables,

reference electrode and connections




Wave type

Amplitude

Frequency scanproperties Frequency

distribution




• Direction: high to low frequency

Frequency scan is performed from high tolow frequency (stability condition)




X




• # frequencies and distribution:

10/decade and logarithmic• ! Duration: dominated by the low

frequency

• Alternative: multisine



Multisine vs single sine

• Multisine: linear combination of

single sines5 sines signal,contain a basefrequency and 4

higher harmonics




• Multisine: linear combination of

single sines15 sines signal,contain a basefrequency and 14

higher harmonics




• 1000 Hz – 0.001 Hz, 61 Freq, single:

2h, 47 min• 1000 Hz – 0.001 Hz, 13 Freq, 5 sine:

50 min

• 37 min with 15 sines• Faster but less accurate!



Data analysis



Data analysis• Analysis: crucial part of the EIS

• Different tools are available• Difficult and time consuming• Pitfall: extract too much information



Analysis tools• Find circle (quick fit): used to quickly

fit a semi-circle in the Nyquist• Provides a quick estimate using a

R(RQ) circuit



Analysis tools• Find circle (quick fit)



Analysis tools• Circuit description based on

B.A.Boukamp:• Solid state Ionics 20 (1986) 31-44 &

Solid State Ionics 18&19 (1986)

136-140



Analysis tools• Elements



Analysis tools• Fitting tool comes with circuit editor

Elements can be connectedusing split and join elements



Analysis tools

Initial conditions,boundaries, fixedparameters can beedited in the dedicated

panel



Fitting progressIterative processSeveral iterationsrequired in order toreach convergence



Report



Problems with data fitting• Knowledge about the system is

required• EIS never to be considered as stand

alone!



Practical examples



Case 1 – coating on steelRE

CE

WE

Coating

Steel

NaCl

3 electrode setup

Z measuredbetween RE and WE



Case 1 – coating on steel

Nyquist BodePure R

Pure C



Case 1 – coating on steel• Interpretation?

RECE

WE

Coating

Steel

NaClRu

Cdl




Poor fit



Case 1 – problem?• Element Q

• Used to describe an imperfectbehaviour of a reactance

Q n

0

1Z

Y j

C j

ZC

n = 1 capacitorn = 0 resistorn = 0.5 uniform diffusion0.5 < n < 1 rough electrode0 < n < 0.5 porous electrode



Case 1 – Element Q

Smooth electrode: C dl = C Rough electrode: C dl ≠ C Q



Case 1 – Element Q

n ↑




Good fit



Case 2 – Partially blockedsurface

Electrode

Surface film Cdl

Re

Csf Rct

Electrolyte

ll bl k d



Case 2 – Partially blockedsurface



Case 3 – Coating revisitedRE

CE

WE

Coating

Steel

NaCl

3 electrode setup

Z measuredbetween RE and WE



Case 3 – Coating revisited

Nyquist BodePure R

Pure C

Two semi-circles



Case 3 – Coating revisited• Fitting?



Case 3 – Coating revisited• Knowledge of the system is required

• Not the same coating as case 1• What would happen if the coating

fails?



Case 3 – Failing coatingRE

CE

WE

Coating

Steel

NaClRu

Cdl

Pores



Case 3 – Failing coating

Coating + pores

Steel

Ru

Cdl



Case 3 – Failing coating• Fit result



Case 3 – Failing coating• Knowledge of the system is required

when fitting the data

C 4 C ti ith



Case 4 – Coating withblisters

Electrode

Surface film

Blister with electrolyte

Cdl

Re

Rct

Rep

Csf Z

wa

Electrolyte

C 4 C ti g ith



Case 4 – Coating withblisters



Case 5 – Two layer coating

Electrode

Surface film 2 Cdl

Re

Rct

Rsf2

Csf2

Rsf1

Csf1

Surfacefilm 1

Electrolyte



Case 5 – Two layer coating



Case 6 – Mass transfer• Semi infinite diffusion

Platinum working electrode[Fe(CN) 6 ] 4- /[Fe(CN) 6 ] 3-



Case 6 – Mass transfer• Warburg diffusion element, W

• |ZW’| = |ZW”| 45 °• - Φ = π /4

• W depends on the concentration,

diffusion coefficient and temperature

WZ j 2 2

O O R R

RT 1 1n F A 2 D C D C



Case 6 – Mass transfer• Semi infinite diffusion


Semi-infinite diffusioncontribution



Case 7 – Forced convection• Finite diffusion


Hydrodynamic conditions

Angle of 45°Indication of semi-infinite diffusion

Falls off towards afinite impedance

Case 7 Mass transfer



Case 7 – Mass transfer

through a thin layer, a coating• Finite diffusion layer

• O element

O0

1Z tanh B j

Y j

O W0

1Z Z

Y j

Typically used for:- Diffusion through thin layer- Ion selective electrodes- Diffusion through a coating

O element is identical to Wwhen tanh argument > π



Red O 2Blue Air

Case 7 – Fuel Cell



Air O 2

Case 7 – Fuel Cell



Case 7 – Forced convection

• Finite diffusion layer

• O element

BD

1/3 1/6

RDE

1.61 D For a rotating disc

electrode




• Finite diffusion


Finite diffusion contribution




• Variation of rotation rate


At higher rotation rates diffusion layer thickness,δ decreases ZO decreases




• Variation of rotation rate


600RPM 1200RPM 2400RPMB B B

O O O

600RPM 1200RPM 2400RPMY Y Y

O

0

1Z tanh B j

Y j



Case 8 – Bounded diffusion

• Tangent hyperbolic diffusion

• Impedance of finite-length diffusionwith reflective boundary• Typical of film with electro-active

species, porous electrodes, Li Batteries ω jBthω jYωY 0

→

( ) [ ]ωω

ω→

j B j Y

Z coth1

=0




• Tangent hyperbolic diffusion• B = time (second) take for a reactant to

diffuse from one side of the layer to theother.

• Thickness of the thin layer δ

• Diffusion coefficient DD

B δ

=





>45 °

“T” element



Case 9 – Gerischer impedance

• Chemical reaction in bulk solution

• Model for porous electrodes• Lithium batteries

j k Y Z

0

1

jωkYωY 0




• Model for porous electrodes




• Li-Thionyl battery

Summary of mass transport



W, semi-infinitelinear diffusion

O, finite-length diffusion

T, bounded Warburg,linear finite diffusion

G, bulk chemical reaction

Summary of mass transport

elements



WT= Zinc – Air battery

W 45 0

T>45 0

d



Case 10 Inductance

Z = j ω L

• High frequencies inductance

• Low frequencies inductance



L f i I d



Low frequencies Inductance

• Adsorption process at electrode surface,surface reaction or chemical modification

L f i I d



Low frequencies Inductance

• Adsorption process at electrode surface,surface reaction or chemical modification

C 11 Bi 2



Case 11 Bisquert2

• Transmission line element

• Model for a porous or mixed-phase

electrode of thickness L

C 11 Bi 2



Case 11 Bisquert2

XX

X1 X X1 X

X 3

X

X 3

X

X2

X1 X X1 X

X 3

X

X 3

X

X2 X2

C 11 Bi 2



Case 11 Bisquert2

L

anh L L Z B cotsinh

2XX

21

2

2

2

1

21

212

21

3X Where λ is given by

C 11 Bi t2



Case 11 Bisquert2

• DSSC V@MaxPP

C 11 Bi t2



Case 11 Bisquert2

• DSSC V@ OCP

S f ll l t



Summary of all elements

• Resistance R R

• Capacitance C C

• Inductance L L

• Constant Phase element Q Y 0 and n

S f ll l t




• Warburg diffusion W Y 0

• Finite diffusion O Y 0 and B

• Bound diffusion T Y 0 and B

• Gerischer impedance G Y 0 and Ka

S f ll l t




• Transmission line B 2

R1 Q1 n1, R2 Q2 n2, R3 Q3 n3, L

If R or Q is “0” means that is not available



Summary



Summary

• Potentiostat setting

• FRA setting

Summary Potentiostat setting



y g

• OCP?

• Potentiostat or Galvanostat?

• Fix DC potential (current) or potentialscan?

Potential scan



Intercept gives Flat Band Potential

Slope gives number

of dopants.

• Potential Scan: Mott Schottky plot of asemiconductor

Summary FRA setting



• Frequency range?

• Amplitude value?

• Single sine or Multi sine?

Example of frequency range:



1MHz -100mHz

Example of frequency range:



10KHz -1mHz

Summary FRA setting



Amplitude value?

Summary FRA setting



Amplitude value?

Δ 50 mV = 42.1 µA

Δ 10 mV = 6.4 µA

Summary FRA setting



Δ 200mV = 0.39 µA

Δ 100mV = 0.18 µA

Amplitude value?

Summary FRA setting



Z=1 GOhm10 mV leads to I=10 pA!!

• Amplitude value?

Take home message



Take home message

• Modern electrochemicalinstrumentation and software allowseasy and fast data acquisition

• Powerful data handling and analysistools are available

• Data analysis remains a difficult task

High current density



g y

measurementControl of programmable electronic loadsor power supplies for high current or

high voltage measurements

PEM fuel cell stack at 200 ADC current

DSSC devices measurements



IMPS and IMVS measurement on

photovoltaic device through modulation

of light intensity

DSC IMVS measurement at 625 nm,OCP, E = 1.652 mW/cm2, DE = 10 %




DSSC charge transfer TiO 2 /dye/electrolyteDiffusion process of I - /I 3

-




DSSC charge transfer TiO 2 /dye/electrolyteDiffusion process of I - /I 3

-

Monitoring of impedance in



g pfunction of time

Fuel cell measurement, Z measured for1800 seconds

Battery (LiPo)



Battery (LiPo)

Take home message



Take home message

! System knowledge is always required

! Impedance cannot be considered as astand alone! Careful attention to the data

acquisition settings

EIS only one form of many



EIS only one form of many

• Electrochemical impedancespectroscopy one form of impedance

• Rotation rate, mass, temperature,light intensity, charge, etc…

References



References

Evgenij Barsoukov & J. Ross Macdonald: ImpedanceSpectroscopy: Theory, Experiment, and Applications,Wiley-Interscience, 2005

The reference handbook on impedancespectroscopy

References



References

Bernard Tribollet & Mark E. Orazem: ElectrochemicalImpedance Spectroscopy, Wiley-Interscience, 2008

Good reference book, specificallyfocussed on ElectrochemicalImpedance Spectroscopy

References



References

Allen J. Bard and Larry R. Faulkner,Electrochemical Methods: Fundamentals andApplications, Wiley, 2001

The reference handbook onelectrochemistry, also coversimpedance spectroscopy

www.metrohm-autolab.com



www.metrohm autolab.com

Service and Support



Service and Support

• All instruments covered by 3 yearsfactory warranty



Metrohm Autolab

sibee - presentation impedance 2013 seminar

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