impedance spectroscopy as a diagnosis tool for sofc stacks and

IWEslide: 1, 9/17/2009source: 2009-06-24 EIS Diagnosis for Stacks.ppt,

Universität Karlsruhe (TH)Institut für Werkstoffe der Elektrotechnik

International Symposium onDiagnostics Tools for Fuel Cell Technologies

Impedance Spectroscopy as a Diagnosis Tool for SOFC Stacks and Systems

André Weber, Dino Klotz, Volker Sonn, Ellen Ivers-Tiffée

Institut für Werkstoffe der Elektrotechnik (IWE)Universität Karlsruhe (TH)

Adenauerring 20b, 76131 Karlsruhe, Germany

IWE



ENBW, Delphi

Control an Diagnosis of SOFC-Stacks and SystemsStack Monitoring by Impedance Spectroscopy

Control of parameters critical for a failure free operation of stack and system

Monitoring of the internal resistance of the stack by electrochemical impedance spectroscopy

• stack performance and efficiency• stack temperature(s)• reformer temperature(s)• steam to carbon ratio / λPOx-value• fuel (reformate) composition• oxidant and fuel flow rates• oxidant and fuel temperatures at gas inlet• oxidant and fuel pressures at gas inlet• exhaust gas composition (remaining CO, HC´s)• ...

• stack performance and efficiency• stack temperature(s)• reformer temperature(s)• actual steam to carbon ratio / λPOx-value• fuel (reformate) composition• oxidant and fuel flow rates• oxidant and fuel temperatures at gas inlet• oxidant and fuel pressure inlet• exhaust gas composition (remaining CO, HC´s)• ...



Electrochemical Impedance Spectroscopy

H. Gerischer, Elektrodenpolarisation bei Überlagerung von Wechselstrom und Gleichstrom. Z. Elektrochem., 58, 9, 278, 1954

see above

Impedance Spectroscopy Materials, (Model-) Electrodes and Single Cells





Analysis of Solid Electrolytes by Impedance Spectroscopy

J. E. Bauerle, Study of solid electrolyte polarization by a complex admittance method, J. Phys. Chem. Solids 30, 2657, 1969

see above






Analysis of Solid Electrolytes by Impedance Spectroscopy

J. E. Bauerle, Study of solid electrolyte polarization by a complex admittance method, J. Phys. Chem. Solids 30, 2657, 1969

see above


Analysis of Electrode Microstructure and Degradation Behaviour

T. Kawada, N. Sakai, H. Yokokawa, M. DokiyaM. Dokiya, M. Mori, T. Iwata, Characteristics of Slurry-Coated Nickel Zirconia Cermet Anodes for Solid Oxide Fuel Cells, J. Electro-chem. Soc., 137, 3042, 1990



Impedance Spectrum of an Anode Supported Single Cell

IWE

0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.600.00

-0.05

-0.10

-0.15

-0.20

-0.25

cell type: ASCel. area: 1 cm²fuel: H2 (9.4% H2O), 250 sccmoxidant: air, 250 sccm

Z‘‘/

Ω⋅c

m²

Z‘ / Ω⋅cm²100 101 102 103 104 105

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

f / Hz

spectrum: -Z´´(f)

-Z‘‘

/ Ω⋅c

m²

• 2 or more electrochemical processes ???• high resolution impedance data analysis required !!!

anode supported single cell (Forschungszentrum Jülich)Ni-8YSZ substrate (50x50x1mm³)Ni-8YSZ anode functional layer8YSZ electrolyte (~ 10 µm)CGO interlayer (~ 7 µm)LSCF cathode (10x10mm²)



R1 R2

C2C1

γ

τ

⎟⎟⎠

⎞⎜⎜⎝

⎛

gesRR1 ⎟

⎟⎠

⎞⎜⎜⎝

⎛

gesRR2

τ1 τ2

ideal:

pol pol0

( )Z ( ) R d1 j

∞ γ τω = τ

+ ωτ∫ γ(τ): „Distribution function ofrelaxation times“

Process 1(RQ/CPE)

Process 2(RQ/CPE)

real:

γ

ττ1 τ2

Impedance Data AnalysisDistribution of Relaxation Times (DRT)

IWE

H. Schichlein et al., Deconvolution of Electrochemical Impedance Spectra for the Identification of Electrode Reaction Mechanisms in Solid Oxide Fuel Cells, J. Appl. Electrochemistry, 32, 8, 875, (2002)



Impedance Spectrum of an Anode Supported Single CellDistribution of Relaxation Times

IWE

0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.600.00

-0.05

-0.10

-0.15

-0.20

-0.25

cell type: ASCel. area: 1 cm²fuel: H2 (9.4% H2O), 250 sccmoxidant: air, 250 sccm

Z‘‘/

Ω⋅c

m²

Z‘ / Ω⋅cm²100 101 102 103 104 105

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

f / Hz

spectrum: -Z´´(f)DRT: g(f)

-Z‘‘

/ Ω⋅c

m²,

g(f)

/ Ω

⋅sec

• high resolution data analysis by the DRT• 5 processes resovable

→ perform impedance measurements at varying operating conditions !A. Leonide et al., J. Electrochem. Soc, 155 (1), pp. B36-B41, (2008).



Analysis of Electrochemical Processes in an ASCVariation of Operating Temperature

IWE

1 10 1000.00

0.01

0.02

0.03

0.04

0.05

g(f)

/ Ohm

*sec

f / Hz

730740750770800860

T / °C

T = 600 ... 850 °Cfuel: H2, 250 sccmp(H2O) = 0.635 barox.: air, 250 sccm

• up to 5 different electrochemical processes resolvable• impedance values in between 10 and 1000 mΩ⋅cm²

P3A

P2AP2C

P1A

103 104 105 0.9 1.0 1.1

0.01

0.1

1

850 800 750 700 650 600

ASR

/ Ω

⋅cm

²1000 K/T

temperature/°C

R2A+R3A: anode electrochemistry

R1A: gas diffusion R2C: cathode electrochemistry

Ea,2A+3A=1.30 eV

Ea,2C=1.43 eV

A. Leonide et al., J. Electrochem. Soc, 155 (1), pp. B36-B41, (2008).



Analysis of Electrochemical Processes in an ASCImpedance Model of a Single Cell

IWE

cell type: ASCel. area: 1 cm²fuel: H2 (9.4% H2O), 250 sccmoxidant: air, 250 sccmT = 717 °COCV

impedance spectrum

anodiccathodic

oxygen surface exchange kinetics and O2--diffusivity

p(O2), T10...500 Hz, 8...50 mΩcm2P2C

p(H2), p(H2O), T12...25 kHz,10...130 mΩcm2P3A

gas diffusion coupled with charge transfer reaction and ionic transport (AFL: anode functional layer)

p(H2), p(H2O),T2...8 kHz, 10...50 mΩcm2P2A

gas diffusion (anode substrate)p(H2), p(H2O)4...20 Hz, 30...150 mΩcm2P1A

gas diffusion (<< 10 mΩ⋅cm² in air)p(O2)0.3...10 Hz, 2...100 mΩcm2P1C

electrode process / physical origindependencefr , ASRAbbreviation

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

-0.1

-0.2

Z‘‘/

Ω⋅c

m2

Z‘ / Ω⋅cm2

CPE2 CPE3 CPE1

R3A R2A R2C R1A R1C

Q3A Q2A Q1C

R0

CNLS-Fit0( )G

ZZk j

ω =+ ω

( )tanh ( )

( )W W

j TZ R

j T

α

α

ωω

ω

⎡ ⎤⎣ ⎦= ⋅

0

1( )( )nQ

j Yω

ω=

P3AP2A P2C P1A

A. Leonide et al., J. Electrochem. Soc, 155 (1), pp. B36-B41, (2008).



IWE

Electrochemical Impedance Spectroscopy for StacksImpact of Cell and Stack Size

anode supported single cell1 cm² active electrode area

ASR: 0.08 ... 2 Ω⋅cm²→ 80 mΩ ... 2 Ω

single cell 100 cm² → 0.8 ... 20 mΩ

• impedance of a few mΩ per cell• gradients in temperature, gas

composition etc.• gas conversion impedance• additional noise and error sources



Electrochemical Impedance Spectroscopy for StacksTesting Equipment

websites and datasheets of manufacturers

3001020± 5 / 10 / 2050100max. bias voltage[V]

10005020± 40 / 20 / 10 25100max. bias current [A]

1500.5n.s.0.250.1253max. power diss. [kW]

1 % / 1°2 % / 2°n.s.0.25 % (f, |Z| not specified)

30 % / 30°(@ 10 mΩ)

1 %accuracy (error at 1 mΩ / 100 kHz)

0.1 mΩ ... 15 Ω

n.s.n.s.1 µΩ ... 1 kΩ10 µΩ ... 1 kΩ0.1 ... 100 mΩ

impedance range

200 µHz ... 100 kHz

10 µHz ... 10 kHz

(10 µHz ... 1 MHz)

10 µHz ... 200 kHz

10 µHz ... 100 kHz

1 mHz ... 1 MHz

frequency range

TrueData-EISCLB 500VersaSTAT4 + Power Booster

IM6 + PP 2xx1260/1287 + Power Booster

target values

3 kWel SOFC stack60 cells a 100 cm²Ustack = 42 VIstack = 71.4 A

→ limitations due to the testing equipment



Analysis of Electrochemical Processes in an ASCGas Conversion Impedance (at decreased fuel flow rate)

IWE, A. Leonide, presented at European SOFC Forum, Lucerne 2008

0.01

0.02

0.03

0.04250 ml/min

150 ml/min100 ml/min

50 ml/min

g(f)/

Ωs

00.01 0.1 1 10 100 1000 10k 100k 1M

f/Hz

anodecathode(+ 2nd peak ofanode gas diffusion)

gas diffusion(anode substrate)

gasconversion(gas channel)

→ the gas conversion impedance will be included in the stack impedance



IWE

Electrochemical Impedance Spectroscopy for Stacks2-dimensional Impedance Model

...

...

...

...

...

...

R2A(x)

R1A(x)

R1C(x)

U

R2C(x)

R0(x)

P3A(x)

H2+ 20% H2O H2+ 80% H2O

oxidant (air)

fuel

electrolytecathode

anode

1 A. Leonide et al., J. Electrochem. Soc, 155 (1), pp. B36-B41, (2008).

• based on the impedance model of a single cell1

• single cell impedance units along a gas channel

• impact of the gas utilization on the local impedance is considered

• dynamics of gas conversion have to be taken into account

D. Klotz et al., accepted for publication in ECS Transactions (2009)



2-dimensional Impedance ModelLocal Impedance Spectra and Stack Impedance

Ucell = 750 mV

simulated fuel utilization along the gas channel

simulated local impedance spectra and impedance spectrum of the stack

D. Klotz et al., accepted for publication in ECS Transactions (2009)

IWE



2-dimensional Impedance Model for StacksSpace and Time Dependence of the Fuel Utilization βf

operating parameters

Ucell 0,725 V

Uampl. 70 mV

fAC 50 Hz

L 10 cm

vgas 3 m/s

βf,in 20%

βf,out 80%

t [s]time / ms

IWE



IWE, presented at Solid State Ionics 15 (2005)

ExperimentalSulzer Hexis Stack Test Bench

Sulzer Hexis stack test bench• 5 cell stack• 100 cm² electrode area• operating on pipeline gas• desulfurization (disengageable)• catalytic partial oxidation• controlled gas flows• controlled stack temperature

(not thermal self-sustaining)• variable interconnect /

flow field geometry• testing of different MEAs and

MICs possible

Impedance spectroscopy ?




Sulzer Hexis Stack Test BenchModifications for Impedance Spectroscopy

EIS-Equipment• Zahner IM6 & PP200 potentiostat• impedance range: 1 µΩ ... 1 kΩ• frequency range: 10 µHz ... 100 kHz• dc current: 20 A

Significant interferences at frequencies above 10 kHz

initial state

frequenzy / kHzim

peda

nce

/ mΩ

phase / °




Sulzer Hexis Stack Test BenchModifications for Impedance Spectroscopy

Modifications• adaptation of voltage probes & current lines to

decrease mutual inductances• impedance converters for dc voltage metering• multiplexer for single cell / stack impedance

measurement

Low interferences over the whole frequency range

modified

frequenzy / kHzim

peda

nce

/ mΩ

phase / °




Stack DiagnosisImpedance Spectra of Stacks and Cell Units

0.06

0.04

0.02

00 0.02 0.04 0.06 0.08 0.1 0.12

Z´/ Ohm

-Z´´

/ Ohm

Stack A: Sulzer Hexis MEAs & MICs 2002

5 cell stackelectrode area: 100 cm²oxidant: air, 1000 g/htemperature: 930 °Cfuel: natural gas, 20 g/hfuel processing:CPO, λPOx: 0.28

Stack-0.03

-0.02

-0.01

00 0.01 0.02 0.03 0.04 0.05

Z´/ Ohm

Z´´/

Ohm

cell 1cell 2cell 3cell 4cell 5

Stack A

0 0.01 0.02 0.03 0.04 0.05Z´/ Ohm

-0.03

-0.02

-0.01

0Z´

´/ O

hm

Stack B cell 1cell 2cell 3cell 4cell 5

Single Cell Units 1 to 5

Impedance spectra of the stack provide an averaged ohmic and polarisation resistanceImpedance spectra of the individual cell units provide more detailed information

Stack B: Sulzer Hexis MEAs & MICs 2004



System DiagnosisImpedance Data Analysis by parametric extended DRT


-0.01

00.015 0.02 0.025 0.03 0.035

Re Z / Ω

ImZ

/ Ω 12 mHz10 Hz

100 kHz

impedance spectra

τ

γ(τ)

Z(DRT)

τRQ , nRQ, RRQ

RQ - element

LL RL

Series resistanceinductance

0

0.002

0.004

0.006

-2 0 2 4log(1s/τ)

γ(τ)

/ Ω

⋅s

gas conversionimpedance

DRT




System DiagnosisImpact of CPOx Reformer Temperature on cell voltage and nRQ

0.86

0.87

0.88

0.89

Tpox / °C

U /

V

Cell 1

0.9

0.905

0.91

0.915

0.92

0.925

575 625 675 725

n CPE

Tpox / °C

no dependency in cell voltage

Variation in fuel composition is detected

theory: n 1 for pure H2

RQ exponent nRQ

voltage cell 1

575 625 675 725



IWE

Conclusions and OutlookFuture Research Activities - Diagnostic Tools for FC-Technologies

• Impedance spectroscopy is a powerful tool to analyze SOFC stacks but

• The available testing equipment hardly fulfills the requirements

• The stack testing facilities have to be designed for impedance spectroscopy

• Complex impedance models are required to understand the stack impedance

• To do´s:

• Development of impedance analyzers for (SOFC-) stacks

• Development of tools for stack impedance modeling and data analysis

• Standardized testing procedures for stack impedance measurement and data analysis



Impedance Spectroscopy and Impedance Data AnalysisIWE-References

IWE

• H. Schichlein, M. Feuerstein, A. C. Müller, A. Weber, A. Krügel and E. Ivers-Tiffée, "System identification: a new modelling approach for SOFC single cells", Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells (SOFC-VI), pp. 1069-1077 (1999).

• H. Schichlein, Experimentelle Modellbildung für die Hochtemperatur-Brennstoffzelle SOFC, Aachen: Verlag Mainz (2003).

• E. Ivers-Tiffée, A. Weber and H. Schichlein, "Electrochemical impedance spectroscopy", in W. Vielstich, H. A. Gasteiger, and A. Lamm (Eds.), Handbook of Fuel Cells - Fundamentals, Technology and Applications, Chichester: John Wiley & Sons Ltd, pp. 220-235 (2003).

• E. Ivers-Tiffée, A. Weber and H. Schichlein, "O2-reduction at high temperatures: SOFC", in W. Vielstich, H. A. Gasteiger, and A. Lamm (Eds.), Handbook of Fuel Cells - Fundamentals, Technology and Applications, Chichester: John Wiley & Sons Ltd, pp. 587-600 (2003).

• H. Schichlein, A. C. Müller, M. Voigts, A. Krügel and E. Ivers-Tiffée, "Deconvolution of electrochemical impedance spectra for the identification of electrode reaction mechanisms in solid oxide fuel cells", Journal of Applied Electrochemistry 32, pp. 875-882 (2002).

• V. Sonn, A. Leonide and E. Ivers-Tiffée, "Towards Understanding the Impedance Response of Ni/YSZ Anodes", ECS Transactions 7, pp. 1363-1372 (2007).

• A. Leonide, V. Sonn, A. Weber and E. Ivers-Tiffée, "Evaluation and Modelling of the Cell Resistance in Anode Supported Solid Oxide Fuel Cells", ECS Transactions 7, pp. 521-531 (2007).

• A. Leonide, V. Sonn, A. Weber and E. Ivers-Tiffée, "Evaluation and modeling of the cell resistance in anode-supported solid oxide fuel cells", J. Electrochem. Soc. 155, p. B36-B41 (2008).



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impedance spectroscopy as a diagnosis tool for sofc stacks and

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