In-Situ Diagnostic Methods for SOFC

G. Schiller, K.A. Friedrich, M. Lang, P. Metzger, N. Wagner

German Aerospace Center (DLR), Institute of Technical Thermodynamics, Pfaffenwaldring 38-48, D-70569 Stuttgart, Germany

International Symposium on Diagnostic Tools for Fuel Cell Technologies,Trondheim, Norway, June 23-24, 2009

Outline

IntroductionElectrochemical Impedance Spectroscopy on Stacks Spatially Resolved Measurements: Current density

VoltageImpedanceTemperatureGas Composition

Optical Spectroscopy X-Ray TomographyConclusion

Investigation of Degradation and Cell Failures

Insufficient understanding of cell degradation and cell failures in SOFC Extensive experimental experience is not generally available which would allow accurate analysis and improvementsLong term experiments are demanding and expensiveOnly few tools and diagnostic methods available for developers due to the restrictions of the elevated temperatures

Conventional Test Stand Diagnostics

Conventional test stand diagnostics: provide important and essential information about fuel cell performance and behaviour:

U(i) characteristics, OCV…EIS on single cellsCurrent interrupt methodsPerformance degradation with time U(t); i(t) …Cell voltage distribution Ustack= U1 + U2 + U3….Pressure loss / Gas tighness testGas utilization measurementTemperature distribution and control

„Sophisticated“ (non-traditional) in-situ Diagnostics

Electrochemical impedance spectroscopy on stacks

Spatially resolved measuring techniques for current, voltage, temperature and gas composition

Optical imaging

Optical spectroscopy

Acoustic emission detection

X-ray tomography

Challenges for EIS for Stack Investigations

Large areas (e.g. 100 cm2) lead to high current and low impedances of about 1 mOhm.Electrochemical processes appear at high frequencies (up to 100 kHz) due to the high reaction rates at high temperatures. Stacks generally contain metallic components leading to high frequency disturbances.Contacting of all cells and sensing in specific cells does not account for the voltage distribution in the stack.The sensor wires are at high temperatures: an optimization of the measurement system is not possible during operation.Strong overlapping of electrode processes; evaluation with equivalent circuits can be inaccurate.For system with current > 40 A no commercial equipment available.

Mitigation of EIS Problems

Reduction of the high frequency disturbances by optimization of the wiring of the electrical sensing of the SOFC stack.Variation of the operating conditions (gases, temperature) in order to determine the different impedances of the electrode processesModeling of the spectra by an equivalent circuit.Development of advanced EIS equipment for high currents / high frequencies in corporation with instrument manufacturer (ZahnerElektrik GmbH).

Experimental Set-up for EIS Measurements of Stacks at DLR

CSZ05-DGF09-CT, 750°C5H2+5N2+3%H2O / 20air (SLPM)

94h

0,0

0,2

0,4

0,6

0,8

1,0

1,2

0 200 400 600 800current density i [mA/cm²]

cell

volta

ge U

[V]

0

200

400

600

pow

er d

ensi

ty p

[mW

/cm

²]

cell 5 (top)cell 4cell 3cell 2cell 1 (bottom)

p

U

Performance of the 5-Cell Short Stack at 750°C(5 H2+5 N2+3%H2O / 20 air (SLPM), 94 h)

Cell 1-4 : 1.10 VCell 5 : 1.05 V

Cell 5: 404 mW/cm²Cell 4: 476 mW/cm²Cell 3: 447 mW/cm²Cell 2: 472 mW/cm²Cell 1: 415 mW/cm²

@ 3,5VPstack = 184 WFU = 37%

-0,5

-0,25

0

0,25

0,5

0,75

1

1,25

0 0,25 0,5 0,75 1 1,25 1,5 1,75 2 2,25

Re Z [Ohm*cm²]

Im Z

[Ohm

cm²]

0 mA/cm260 mA/cm2120 mA/cm2180 mA/cm2240 mA/cm2300 mA/cm2360 mA/cm2420 mA/cm2

80 Hz

5-Zellen Short Stack [CSZ-05-83-CT], cell 5T=750°C, Zellfl. 84cm²

Gas Concentration

CathodeAnode

50mHz

1 Hz

6 Hz

100 kHz

Nyquist Plot of one Cell of a 5-Cell Short Stack at Different Current Densities(750°C, 2.5 H2+2.5 N2 / 20 air (SLPM), 142 h)

0.14 - 0.17 Ωcm2 0.55 - 2.2 Ωcm2

Equivalent Circuit for the Fitting of the Impedance Spectra

Gas Concentration

InductivityCathodeOhmic

RΩ

Rp(C)

ZL

Anode

Cdl(A)

RP(A)

Cdl(C)

RN(A)

CN(A)

CSZ-05-83-CT, 750°C2,5H2+2,5N2 / 21 Air (SLPM)

cell 5, 142 h

0,0

0,2

0,4

0,6

0,8

1,0

1,2

0 50 100 150 200 250 300 350 400 450current density i [mA/cm²]

cell

volta

ge U

[V]

0

20

40

60

80

100

volta

ge lo

ss [m

V]

U cellΔU (Anode)ΔU (Cathode)ΔU (Gas Concentr.)ΔU (Ohm)

Cell5 @ 700mV : 380 mW/cm2

U cell

ΔU

Voltage Losses at one Cell of a 5-cell Short Stack at Different Current Densities(750°C, 2.5 H2+2.5 N2 / 20 air (SLPM), 142 h)

Contact resistances

Polarisation resistances

Motivation

Strong local variation of gas composition, temperature, current density Distribution of electrical and chemical potential dependent on local concentrations of reactants and products

Reduced efficiencyTemperature gradientsThermo mechanical stressDegradation of electrodes

H2 H O2

O2 O2

H2 H O2

O2

Measurement Setup for Segmented Cells

16 galvanically isolated segmentsLocal and global i-V characteristicsLocal and global impedance measurements

Local temperature measurementsLocal fuel concentrationsFlexible design: substrate-, anode-, and electrolyte-supported cellsCo- and counter-flow

Cell design and Testing Station

From a „simple“ cell design with manually controlled features

GC measurement

Flexible housing, impedance spectra with reduced interferences

Assembly and contacts

All cell conceptsImproved contactingReliable assemblyImpedance measurementTemperature measurement

Schematic Lay-out of the Electrical Circuit of the Segmented Cell Configuration

curr

ent b

usba

r eq

uipo

tent

ial l

ine

curr

ent b

usba

r eq

uipo

tent

ial l

ine

Internal cell resistances: Ri,j,

Resistances of the wires contacting the anode: RLA,j

Resistances of the wires contacting the cathode: RLK,j

Only segments 1, 2, 3, 16 are illustrated

OCV Voltage Measurement for Determination of Humidity

• Voltage distribution at standard flow rates:• 48.5% H2, 48.5% N2 + 3% H2O, 0.08 SlpM/cm² air

13 14 15 16

9 10 11 12

5 6 7 8

1 2 3 4

fuel gas air

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

22

20 lnHO

OHrevrev pp

pzFRTUU

Nernst equation:

Produced water:S4: 0.61%, S8: 0.72%, S12: 0.78%, S16: 3.30%

Variation of Load - Reformate

Anode supported cell, LSCF cathode, 73,96 cm², gas concentrations (current density equivalent): 54.9% N2, 16.7% H2, 16.5% CO, 6,6% CH4, 2.2% CO2, 3.2% H2O (0.552 A/cm²), 0.02 SlpM/cm² air

0,0

50,0

100,0

150,0

200,0

250,0

300,0

Segment 9 Segment 10 Segment 11 Segment 12

pow

er d

ensi

ty p

[mW

/cm

²]

0,0

15,0

30,0

45,0

60,0

75,0

90,0

fuel

util

isat

ion

fu [%

]

p(i) 100 mA/cm² p(i) 200 mA/cm² p(i) 400 mA/cm² p(i) 435 mA/cm²

fu 100 mA/cm² fu 200 mA/cm² fu 400 mA/cm² fu 435 mA/cm²

fu

100

200

400435

100

200

400435

Pow

er d

ensi

tym

W/c

m2

Fuel

util

isat

ion

(%)

0

0,05

0,1

0,15

0,2

0,25

0,3

Segment 9 Segment 10 Segment 11 Segment 12

Gas

konz

entra

tion

/ %

H2 CO CH4 CO2 H2O

KS4X050609-7 in Metallischem Gehäuse; Substrat: Anodensubstrat, aktive Zellfläche:73,78 cm²,A: 542 µm NiO/YSZ, E: 14 µm YSZ + YDC,

K: 28 µm LSCF, Kontaktierung: 30 µm LSP16+Pt3600,Integral, Gasflüsse: 0,552 A/cm² Stromdichteäquivalent (54,9% N2, 16,7% H2,

16,5% CO, 6,6%CH4, 2,2%CO2, 3,2% H20) // 0,08 SlpM/cm² Luft, 800 °C, 0 mA/cm²

Reformate: Changes of the Gas Compositionat 0 mA/cm²

Metallic housing, anode substrate, active area 73.78 cm²Anode: 542 µm NiO/YSZ, Electrolyte: 14 µm YSZ + YDC,Cathode: 28 µm LSCFOperation conditions: 0.10 A/cm² - Anode λ = 5.52(54.9% N2, 16.7% H2, 16.5% CO, 6.6% CH4, 2.2% CO2, 3.2% H2O0.08 Nlpm/cm² Air, 800°C)

Con

cent

ratio

n/ %

9 10 11 12

H2

CO

H2OCO2

CH4

Alteration of the gas composition at 435 mA/cm²

0

0,05

0,1

0,15

0,2

0,25

0,3

Segment 9 Segment 10 Segment 11 Segment 12

Gas

konz

entra

tion

/ %

H2 CO CH4 CO2 H2O

Con

cent

ratio

n/ %

H2

10 11 12

CO

CH4

H2O

CO2

9

Combined Experimental and Modeling Approach

Objectives of the study:Better understanding of the local variations

Identification of critical conditionsOptimisation of cell components

Experiments on single Experiments on single segmented SOFCsegmented SOFC

Electrochemical model of Electrochemical model of local distributionslocal distributions

H2H2/CO

CH4

H2OCO2

anode

electrolyte

cathode

O2/N2 N2 xy

xy

elyt eldegas z

interconnector

interconnector

Potential for Optical Spectroscopies

Raman spectroscopyLaser Doppler Anemometry (LDA)Particle Image Velocimetry (PIV)Fast-Fourier Infrared (FTIR)Coherent Anti-Stokes Raman Spectroscopy (CARS)Electronic Speckle Pattern Interferometry (ESPI)

Digital CCD camera

Distance microscope(resolution1 µm)

Quarz window

Transparentflow field

Imagingspectrograph

Lenses/filter

Pulsed Nd:YAG laser(532 nm, 10 ns)

Open tube(5 mm)

a) In situ microscopy b) In situ Raman laser diagnostics

15 cm

Heat & radiation shield

SOFC

neutrontomography

in-situ synchrotronradiography

in-situ neutronradiography

Tomography Diagnosis of PEM Fuel Cells

Investigation of water management under operating conditions

X-Ray Tomography (CT) Facility at DLR

3 dimensional non intrusiveimaging of SOFC cassette

X-Ray CT Facility v|tome|x L450 at DLR Stuttgart

Summary

The operating conditions (elevated temperature) reduce significantly the possibilities for in-situ SOFC diagnostic methods.EIS will remain the main diagnostic probe of the state of SOFC.Non-traditional in-situ diagnostics methods can provide additional important information:

Spatially resolved measurements to obtain local distributionof cell properties (current, voltage, impedance, gas compo-sition, temperature)Combined analytical and modeling approach

Large future potential for optical spectroscopies (e.g. Ramanspectroscopy) and x-ray tomography.