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Electrochemical characterization and performance evaluation Mogens Mogensen Fuel Cells and Solid State Chemistry Risø National Laboratory Technical University of Denmark P.O. 49, DK-4000 Roskilde, Denmark Tel.: +45 4677 5726; [email protected]

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Page 1: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

Electrochemical characterization and performance evaluation

Mogens MogensenFuel Cells and Solid State Chemistry Risø National LaboratoryTechnical University of DenmarkP.O. 49, DK-4000 Roskilde, DenmarkTel.: +45 4677 5726; [email protected]

Page 2: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Contents• Introduction• Characterization methods incl. Electrochemical Impedance

Spectroscopy, EIS• Examples of performance • SOFC cell degradation• Detailed analysis by EIS• Prevention of degradation• Recommended literature

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SOFC Summer School 2010

Introduction

ElectrolyteAnode

Cathode

ee--

ee--

OO22--

2O2O22--

+ 2H+ 2H22

⇔⇔ 2H2H22

O + 4eO + 4e--

OO22

+ 4e+ 4e--

⇔⇔ 2O2O22--

Air

Fuel

All contributing to the losses

Objective of electrochemical characterization:

• Gain further insight on the behaviour of each individual cell component

• Assist production

• Enable further development and performance optimisation

Main goal is:

•Increase knowledge

•Increase energy efficiency

•Knowledge to $$$$

• Electrolyte resistance

• Contact resistance on all interfaces

• Polarization resistance (electrodes)

• Gas diffusion limitations

• Gas conversion

• Leakage of all kinds

Page 4: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Electrochemical Impedance Spectroscopy, EIS• EIS is very strong tool in the process of break down the

total electrode impedance into the contributions from the various components of the cell.

• EIS does not replace i -V curves (current density vs. cell voltage)

• It is most often wise and often necessary to supplement (enhance) the electrical characterisation of the cell with microscopic or surface analysis examination methods

Page 5: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

IS of electrical RC parallel circuit

• The simplest equivalent circuit (model) of an electrode is a parallel connection between a capacitor and a resistor:

• The total current is the sum of two currents

• The Total impedance, Ztotal = 1/(1/ZR + 1/ZC )

• ZC is infinite for DC, i.e. no current goes through• ZC is 0 for infinite high frequencies

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SOFC Summer School 2010

Sinusoidal voltage applied onto this.

Angular frequency ω

= 2πf (rad/s)

φ is the phase shift of the voltage relative to the current. For a capacitor the voltage is always "behind" the current, and φ is negative

Page 7: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Very low frequency - phase angle is 0 - resistor

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SOFC Summer School 2010

Higher frequency - phase shift < O for capacitance containing circuit

Page 9: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Still higher frequency

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SOFC Summer School 2010

Very high frequency - phase angle is 0 again - capacitor

Page 11: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

An impedance is a complex number

Vector Z

Angle φ

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SOFC Summer School 2010

Equivalent circuits An equivalent circuit can consist of several, combined

elements, like resistors, capacitors, inductors and constant phase elements (CPEs)

An equivalent circuit can be developed to describe the system and separate the magnitude of the physical processes:

– Several impedance spectra are required, recorded at e.g. different temperatures and gas compositions

1Hz

Z' (kΩ)

0 100 200 300 400

Z'' (

kΩ)

0

100

2001Hz

R Q

( )11)(

−⎟⎠⎞⎜

⎝⎛ ⋅+−= niQRZ ωω

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SOFC Summer School 2010

Equivalent circuits and the cell

Unfortunately, the EIS of a solid oxide cell is much more complicated than the spectrum of the equivalent circuit above

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SOFC Summer School 2010

Electrical Circuits -Series and Parallel Connections

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SOFC Summer School 2010

Graphical representations of EIS spectra

Different, complementary information can be obtained by plotting the data in different forms, for example:

Nyquist plot

Orazem et al. 2006, J. Electrochem. Soc. 153 B129

0.0

0.5

1.0

1.0 1.5 2.0 2.5 3.0 3.5Zreal (Ohm cm2)

- Zim

ag (O

hm c

m2 )

Rs

Rs + Rp

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SOFC Summer School 2010

Graphical representations of EIS spectra

• Different, complementary information can be obtained by plotting the data in different forms, for example:

Bode plots of impedance:

0.0

0.1

0.2

0.3

0.4

0.5

0 1 10 100 1000 10000 100000 1000000Frequency

- Zim

ag (O

hm c

m2 )

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 1 10 100 1000 10000 100000 1000000Frequency

Zrea

l (O

hm c

m2 )

Orazem et al. 2006, J. Electrochem. Soc. 153 B129

’logaritmic’ Bode Plot

Page 17: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

CNLS fitting • When an equivalent circuit has been developed, the

magnitudes of each of the elements can be calculated by CNLS fitting.

• By plotting the calculated values from the CNLS fitting, the ‘goodness’ of the equivalent circuit can be evaluated.

0.0

0.2

0.4

0.6

1.2 1.7 2.2 2.7 3.2Zreal (Ohm cm2)

- Zim

ag (O

hm c

m2 )

L Rs R1

C1

R2

CPE2

GE R4

C4

L Rs R2

CPE2

R3

CPE3

R4

C4

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SOFC Summer School 2010

0.0

0.2

0.4

0.6

1.2 1.7 2.2 2.7 3.2Zreal (Ohm cm2)

- Zim

ag (O

hm c

m2 )

CNLS fitting L Rs R2

CPE2

R3

CPE3

R4

C4

L Rs R1

C1

R2

CPE2

GE R4

C4

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

0 1 10 100 1000 10000 100000 1000000

Frequency (Hz)

Erro

r (%

)

Page 19: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Break down of the cell impedance by fitting to equivalent circuits

A series resistance + 4(RQ) in series +(RC) in series!As this can fit every elephant and octopus we must get a lot of pre-knowledge in order to do this in a credible manner

Ramos et al. 2008, ECS Transactions 13 235

Page 20: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Questions here?

My question to you: Any proposal about what to do in order to get this pre-knowledge?

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SOFC Summer School 2010

Electrode test strategies• Naturally, we would like to measure all relevant

properties of an electrode, e.g. electronic conductivity, ionic conductivity, electrocatalytic activity and electrochemical performance of a porous or even of a composite electrodes

• This cannot be done by testing of full cells. A rather tedious strategy is necessary

Page 22: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Specific SOC test problems• The detailed structure of the solid oxide electrode is

extremely important for the polarization resistance - this makes it difficult to assess the electro-catalytic effect of a potential electrode material using the technological type of composite electrodes

• Polarization resistance = overvoltage/current density (Ohm x cm2) is usually used instead of “overvoltage at a given cd” as SOC gives fairly linear responses

• For a given electrode - made as reproducible as possible - the polarization resistance may be very dependent on the thickness of the electrolyte and on the method of electrolyte fabrication

Page 23: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Specific SOC test problems

ASR measured on anode supported Ni/YSZ/LSM cells (open symbols, line) compared to ASR calculated from electrode and electrolyte data (closed symbols)

Page 24: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Test strategies• It is necessary to use a number of set-ups - more or less a

special set-up is required for each kind of property to be investigated

• Conductivity of materials may be measured in a classical 4- pooint set-up

• Electro-catalytic activity is tested using model electrodes

• Effect of structure may be tested in symmetrical 2-electrode cells

• Effect of overvoltage can only be studied accurately in a three-electrode set-up

• Measure EIS at many systematically varied conditions

Page 25: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Model electrodes• Two main types

– Pointed electrodes– Pattern electrodes

• The border line between them is not very sharp

• A “point electrode” may be defined as a circular (or elliptical) shaped contact, the radius of which is less than 0.1 times the thickness of the electrolyte

• The purpose of model electrodes is to know the exact contact area and three phase boundary length

Page 26: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

“Point” electrodes of metal

Ni-wire

YSZ-single crystal

Page 27: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Cone shaped "point" electrode of ceramics

Page 28: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Point electrode

SRr

⋅⋅=

σ41

The area can be determined/estimated by

r is the radius, σ is the specific conductivity of the electrolyte material and RS is the series resistance

Thickness, t, of electrolyte: t > 10r

Page 29: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Model electrode• Determination of the electro-catalytic activity (for given

geometry and conditions) is possible in principle

• 1/Rp , where Rp is the polarization resistance, is a measure of the specific electro-catalytic activity for the electrode material in case of a well-defined electrode geometry

• The surface topography (and other surface properties) of both electrolyte and the electrode must be carefully controlled

• This means that it may be only possible in practice for a series of ceramic materials if the preparation of the cone electrodes is done by the very same person

• Dots made by e.g. pulsed laser deposition may be more reproducible (and have other problems)

Page 30: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Pattern electrodes

Thin electrode stripes

YSZ electrolyte

Also a counter and a reference electrode must be applied (not shown)!

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SOFC Summer School 2010

Tests of technological electrodes

Technologically relevant electrodes are usually composites e.g. Ni-YSZ and LSM-YSZ

• 3-electrode cells

• symmetric cells

• full cells

All have their advantages and disadvantages

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SOFC Summer School 2010

Reference electrode

Electrolyte pellet

Counter electrode

Alumina support

Weight load

Platinum wires

Working electrode

LSM pellet

Unsintered LSM tape

Three-electrode-set-up

The Risø

3E-pellet is a proper 3E-set-up, but there are other possibilities

It must be a thick electrolyte, a pellet like thing in case of good electrodes

Ref.: Winkler, Hendriksen, Bonanos, Mogensen, Geometric requirements of solid electrolyte cells with a reference electrode, J. Electrochem. Soc. 145

(1998)

1184-1192

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SOFC Summer School 2010

Three-electrode-set-up

Real reference electrode

– If e.g.

pure oxygen is reference gas, the reference electrode potential is constant

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SOFC Summer School 2010

Three-electrode-set-up

One of them to be used as an auxiliary electrode

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SOFC Summer School 2010

Symmetrical cell

A symmetrical two-electrode cell arrangement for measurements at OCV

Page 36: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

-0.2

-0.1

0.0

0.1

0.2

0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3

Zreal (Ohm cm2)

- Zim

ag (O

hm c

m2 )

0 h280 h

L Rs R1

C1

R2

CPE2

GE R4

C4

Equivalent circuit:

p Degradation/deactivation of symmetrical solid oxide cells

YSZ

LSM-YSZ

LSM-YSZ

Page 37: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

-0.01

0

0.01

0.02

0.03

0.04

0.1 1 10 100 1000 10000 100000 1000000

frequency (Hz)

Zim

ag 2

80 h

- Zi

mag

0 h

(Ohm

cm

2 )

L Rs R1

C1

R2

CPE2

GE R4

C4

p Degradation/deactivation of symmetrical solid oxide cells

YSZ

LSM-YSZ

LSM-YSZ

Page 38: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

268252,610Cathode 2

3371,5104,390Anode 2

8,25426,10026,100Cathode 1

7,36022,70034,500Anode 1 (?)

650°C750°C850°Cf summit [Hz]

Anode 1 (?) 34,500Anode 1 (?)

Symmetric cell data

For both symmetric cell with SOFC anodes and cathodes two ion transfer related arcs have been observed in the EIS. An example of data seen below.

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SOFC Summer School 2010

Full cell vs. symmetric cellsTemperature

°CConditions

Air &Full cell B

Anode[Ω

cm2]

Sym. cell Anode

cm2]

Full cell BCathode[Ω

cm2]

Sym. cell Cathode[Ω

cm2]

750 20% H2 O

3% H2 O

0.09

0.16 0.23

0.24

0.260.30

650 20% H2 O

3% H2 O

0.46

0.60 1.05

0.87

0.901.32

•• Symmetric cells exhibit consistently higher resistances Symmetric cells exhibit consistently higher resistances

•• The summit frequencies are generally higher in full cellsThe summit frequencies are generally higher in full cells

•• The differences are more marked for the anodeThe differences are more marked for the anode

••

What justifies these differences? Production? Different amountsWhat justifies these differences? Production? Different amounts

of of impurities? Overall different microstructure? Intrinsically diffimpurities? Overall different microstructure? Intrinsically different test erent test setup? Combination of previous?setup? Combination of previous?

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SOFC Summer School 2010

Cell house, Alumina

H2 flow

Glass seal

ACC

Cell

CCC

200 μm Au foil(current collector)

Air flow

Anode current collector, Pt foil

ACC

CCC

Cell

Ni

Au

Full cell test

5x5 cm foot print4x4 cm active area

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SOFC Summer School 2010

A cell test strategy

Anode supported cell

Cathode gas distributor

Anode gas distributorFuel flow

Glass seal

Air flow

Old Risø set-up: Active cell area: 16 cm2.

Many other set-ups are possible

1. Full cell test1. Full cell test2. Fingerprinting with gas (anode and cathode) and current variations

• EIS (e.g. OCV, 0.25 & 0.5 Acm-2)

• i-V curves

• Fuel gas: pH2 O/pH2 from 0.04 to 1.00 at constant total flow

• Cathode gas: dilution series (pO2 from 0.02 to 1.00) at constant total flow

3. Symmetric cell testing

To get the single electrode EIS response

4. Data analysis

• ADIS

• DRT

• CNLS approximation to a model function (equivalent circuit)

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SOFC Summer School 2010

Purpose of "fingerprint"

• If used on all cells then it is possible to compare the start performance of all cells

• If the fingerprint is used again at the end of say a durability testing then the changes can be described in much more detail than a change in potential at a given current density

Page 43: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Cell performance

0

0.2

0.4

0.6

0.8

1

1.2

0.0 0.5 1.0 1.5 2.0 2.5Current Density / [A cm-2]

Cel

l Vol

tage

/ [V

]

0

0.5

1

1.5

2

Pow

er D

ensi

ty /

[W c

m-2

]

i - V 750 °Ci - V 700 °Ci - V 650 °Ci - P 700 °Ci - P 750 °Ci - P 650 °C

ASR (750 °C, 0.65V, FU corr) = 0.13 Ω·cm²ASR (700 °C, 0.60V, FU corr) = 0.19 Ω·cm²ASR (650 °C, 0.60V, FU corr) = 0.37 Ω·cm²

i - V and i - P curves for a Risø

SOFC anode supported Ni-YSZ/YSZ/CGO/LSC-CGO cell

Part of fingerprint

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SOFC Summer School 2010

SOFC (Ni-YSZ-LSM) degradation

300

500

700

900

0 400 800 1200 1600Time under current in h

Cel

l vol

tage

in m

V 0.75 A/cm2 oxygen

0.75 A/cm2 air

750 oC, synthesis gas, 75-80% FU

Page 45: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Ni-YSZ/YSZ/LSM-YSZ: Degradation rates vs. current density

After 300 h operating time -

mainly

reflecting anode degradationAfter 1500 h operating time -

mainly

reflecting cathode degradation

750 oC850 oC950 oC

750 oC850 oC950 oC

This and following are from A. Hagen et al., J. Electrochem. Soc., 156 (2006) A1165 –

A1171, and

SOFC-X, 2007, Nara, Japan

0

100

200

300

0 1 2

Current density in A/cm2

ΔU30

0/ Δt

in m

V/1

000

h

0

50

100

150

0 1 2

Current density in A/cm2ΔU

1500

/ Δt

in m

V/1

000

h

Page 46: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

/ / Degradation vs. cell polarization

750 oC850 oC950 oC

750 oC850 oC950 oC

Anode (300 h): Degradation rates nearly same at all temperatures (except at high polarization)•

Cathode (1500 h): Degradation rates at 750 oC

much larger than at the higher

temperatures

0

100

200

300

100 200 300 400Cell polarization in mV

ΔU30

0/ Δt

in m

V/1

000

h

0

50

100

150

100 200 300 400Cell polarization in mV

ΔU15

00/ Δ

t in

mV

/100

0 h

After 300 h operating time -

mainly

reflecting anode degradation

After 1500 h operating time -

mainly

reflecting cathode degradation

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SOFC Summer School 2010-0.10

-0.05

0.00

0.05

0.10

0.0 0.1 0.2 0.3 0.4 0.5

Z real in Ω cm2

-Zim

ag in

Ω c

m2

-0.10

-0.05

0.00

0.05

0.10

0.0 0.1 0.2 0.3 0.4 0.5

Z real in Ω cm2

-Zim

ag in

Ω c

m2

-0.10

-0.05

0.00

0.05

0.10

0.0 0.1 0.2 0.3 0.4 0.5

Z real in Ω cm2

-Zim

ag in

Ω c

m2

Impedance spectra under polarization: Test in air

300

500

700

900

0 400 800 1200 1600Time under current in h

Cel

l vol

tage

in m

V 0.75 A/cm2 oxygen

0.75 A/cm2 air

750 oC, synthesis gas, 75-80% FU

Air:• Continuous increase of both, serial and even more polarization resistance over 1500 h

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SOFC Summer School 2010

-0.10

-0.05

0.00

0.05

0.10

0.0 0.1 0.2 0.3 0.4 0.5

Z real in Ω cm2

-Zim

ag in

Ω c

m2

-0.10

-0.05

0.00

0.05

0.10

0.0 0.1 0.2 0.3 0.4 0.5

Z real in Ω cm2

-Zim

ag in

Ω c

m2

-0.10

-0.05

0.00

0.05

0.10

0.0 0.1 0.2 0.3 0.4 0.5

Z real in Ω cm2

-Zim

ag in

Ω c

m2

-0.10

-0.05

0.00

0.05

0.10

0.0 0.1 0.2 0.3 0.4 0.5

Z real in Ω cm2

-Zim

ag in

Ω c

m2

Impedance spectra under polarization: Test in oxygen

300

500

700

900

0 400 800 1200 1600Time under current in h

Cel

l vol

tage

in m

V 0.75 A/cm2 oxygen

0.75 A/cm2 air

750 oC, synthesis gas, 75-80% FU

Oxygen:• Almost constant serial resistance• Increase of polarization resistance only within the first ~100 hours, afterwards no changes until 1500 h

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SOFC Summer School 2010

SOFC anode and cathode degradation

300

500

700

900

0 400 800 1200 1600Time under current in h

Cel

l vol

tage

in m

V 0.75 A/cm2 oxygen

0.75 A/cm2 air

750 oC, synthesis gas, 75-80% FU

Anode degradation

Cathode degradation

Impedance spectroscopy tells us which electrode that degraded how much after a given test time

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SOFC Summer School 2010

Degradation of cell voltage - effect of pO2 and cell voltage

Apart from the fast initial degradation over first hundred hours (anode) no degradation until at least 1500 h is observed, i.e. no

cathode degradation in pure oxygen, at these conditions

300

500

700

900

0 400 800 1200 1600Time under current in h

Cel

l vol

tage

in m

V 0.75 A/cm2 oxygen

1.19 A/cm2 oxygen

0.75 A/cm2 air

750 oC, synthesis gas, 75-80% FU

Page 51: Electrochemical characterization and performance evaluationorbit.dtu.dk/files/2369271/Electrochemical characterisation and... · Electrochemical characterization and performance evaluation

SOFC Summer School 2010

Post-test microscopy: Removal of cathode

View on electrolyte surface after etching cathode away

Crater shaped imprints left by LSM particles

YSZ contact points

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SOFC Summer School 2010

Post-test microscopy: Imprints from LSM on electrolyte

Reference cell

After

test in oxygen

After

test in air

Sharp craters on reference and after test in oxygen

Small, blurred craters, wrinkled surface after

test in air

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SOFC Summer School 2010

Post-test microscopy: Cell tested in air

• Smaller crater rings, blurred shapes • Foreign phases, nano-sized particles

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SOFC Summer School 2010

Degradation mechanism on the SOFC cathode at 750 °C

Under reducing conditions at the LSM:•Redistribution of elements in LSM/electrolyte interface region under

conditions of high cathode polarization and low oxygen activity• Formation of nano-sized particles of isolating foreign phases

(LZO, silicates?)• Weakening of contact between LSM and electrolyteThis is in good accordance with M. Chen et. al., O 268

Reference cell

After test in air electrolyte

LSM? LZO? silicate

LSM

electrolyte

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SOFC Summer School 2010

Effects of impurities on the TPB• Many impurities (incl. H2 O) may degrade the electrode

performance, e.g.

• H2 O in case of some LSM type of cathodes• CrO3 vapour and other Cr (VI) containing vapours• High pH2 O in the Ni-YSZ anode• Sulphur containing electrodes• +++

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SOFC Summer School 2010

Durability as f(test details)

Cell voltage vs time at 750 °C and 0.75 A/cm2 for test A: “reference” test; test B: H2 gas cleaning applied; test D: after 440 h at OCV and (H2 O)/p(H2 )=0.4/0.6, without H2 gas cleaning; and test E after 332 h of OCV testing at p(H2O)/p(H2 )=0.4/0.6 H2 and H2 gas cleaning.

From: Hauch & Mogensen,

SSI 181 (2010) 745–753

Pure O2 at the cathode - thus anode investigation

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SOFC Summer School 2010

Ni-YSZ electrode degradation at high pH2 O

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

90 140 190 240

Time [hours]

R p,N

i [Ω

cm2 ]

850 °C

Increase in Rp,Ni

as a function of time at OCV

as measued by EIS in 98% H2

O and 2% H2

. The fit of the type (1-exp(-t/τ)) shown gives a time constant, τ, of 38 hours

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SOFC Summer School 2010

A

C

B

Reference

C

AB

Tested

SEM images of the YSZ-Ni/YSZ interface. Reference cell (left) and tested cell (right). A: Ni particle, B: YSZ in electrode, and C:

YSZ

electrolyte.

Ni-YSZ electrode degradation - high pH2 O 98% H2

O and 2% H2

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SOFC Summer School 2010

More EIS - DRT and ADIS to come

Any questions now?

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SOFC Summer School 2010

Distribution of relaxation times (DRT)

• Distribution of relaxation times is gained by a Fourier transform of the impedance data, giving a clearer picture of the number of physical processes and their nature

Schichlein et al. 2002, J. Appl. Electrochem. 32 875

0.0

0.5

1.0

1.5

2.0

1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06

Frequency (Hz)

DR

T

0.0

0.5

1.0

1.0 1.5 2.0 2.5 3.0 3.5Zreal (Ohm cm2)

- Zim

ag (O

hm c

m2 )

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SOFC Summer School 2010

Distribution of relaxation times (DRT)

Leonide et al. 2008, J. Electrochem. Soc. 155 B36

Nyquist representation

DRT representation

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SOFC Summer School 2010

Analysis of differences in impedance spectra (ADIS)

• An impedance spectrum often changes when the temperature or gas composition is changed. When analysing the differences between spectra, the number and nature of the changes can be analysed

0.0

0.2

0.4

0.4 0.6 0.8 1.0 1.2 1.4 1.6Z' / [Ω·cm²]

-Z'' /

·cm

²]

4% H2O8% H2O17% H2O25% H2O33% H2O42% H2O50% H2O

650 °C

0.0

0.2

0.4

0.4 0.6 0.8 1.0 1.2 1.4 1.6Z' / [Ω·cm²]

-Z'' /

·cm

²]

4% H2O8% H2O17% H2O25% H2O33% H2O42% H2O50% H2O

650 °C

Jensen et al. 2007, J. Electrochem. Soc. 154 B1325Hjelm et al. 2008, ECS Transactions 13 285

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SOFC Summer School 2010

Analysis of differences in impedance spectra, ADIS

Top: EIS - O2 diluted with 0, 20, 50, or 75 vol % N2 to LSM/YSZ, 50 %H2 -50 vol % H2 O to Ni/YSZ. Bottom: EIS - H2 with 5, 20, or 50 vol % H2 O to Ni/SZ electrode and pure O2 to LSM/YSZ.

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SOFC Summer School 2010

ADIS cont.

ΔZ' spectra for gas shift to the LSM/YSZ electrode from pure O2 to O2 diluted in 0, 20, 50, or 75 vol. % N2 . The bold line 0% is a background noise measurement. 50/50 % H2/ H2 O to the Ni/YSZ.

Højgaard et al., J. Electrochemical Society, 154 (2007) B1325

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SOFC Summer School 2010

Equivalent circuit model

Having data from symmetric cells for both the SOFC anode and cathode plus ADIS + DRT then an equivalent circuit may be established (see e.g. Barfod et al., FUEL CELLS, 06 (2006) No. 2, 141) that can model the cell behaviour relatively precise.

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SOFC Summer School 2010

Electrochemical model validation: 750°C, 20% H2 O, air

56,000 Hz10,000 Hz

790 Hz

110 Hz19 Hz

0.00

0.05

0.10

0.15

0.20

0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80

Z' [Ω cm2]

-Z'' [

Ω c

m2 ]

FitCat IAno ICat IIDiffusionConversionCell #A

43,000 Hz5,500 Hz 680 Hz

56 Hz18 Hz

0.00

0.05

0.10

0.15

0.20

0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80

Z' [Ω cm2]

-Z''

[ Ω c

m2 ]

FitCat IAno ICat IIDiffusionConversionCell #B

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SOFC Summer School 2010

Cell B, 750°C, pO2 variations, 20% H2 O anode

43,000 Hz5,500 Hz 680 Hz

56 Hz18 Hz

0.00

0.05

0.10

0.15

0.20

0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80

Z' [Ω cm2]

-Z''

[ Ω c

m2 ]

FitCat IAno ICat IIDiffusionConversionCell #B

31,000 Hz 2,900 Hz

220 Hz56 Hz 18 Hz

0.00

0.05

0.10

0.15

0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70

Z' [Ω cm2]

-Z''

[ Ω c

m2 ]

FitCat IAno ICat IIDiffusionConversionCell #B

AirAir

OO22

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SOFC Summer School 2010

Cell B, 750°C, pH2O variations, air cathode

43,000 Hz

5,500 Hz 680 Hz

56 Hz18 Hz

0.00

0.05

0.10

0.15

0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70

Z' [Ω cm2]

-Z''

[ Ω c

m2 ]

FitCat IAno ICat IIDiffusionConversionCell #B

37,000 Hz5,200 Hz 650 Hz

79 Hz16 Hz

0.00

0.05

0.10

0.15

0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70Z' [Ω cm2]

-Z''

[ Ω c

m2 ]

FitCat IAno ICat IIDiffusionConversionCell #B

20% H20% H22

OO

40% H40% H22

OO

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SOFC Summer School 2010

Break down of losses for Risø 2G Ni-YSZ/YSZ/LSM-YSZ cells

00.10.20.30.40.5

0.60.70.80.9

11.1

690 710 730 750 770 790 810 830 850Temperature in oC

Res

ista

nce

in Ω

*cm

2

ASR Rtot_imp Rcathode Ranode Rconc Relec

Cathode

Diffu./conver.Electrolyte

Anode

AnodeElectrolyte

Diffu./conver. Cathode

750750°°CC 850850°°CC

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SOFC Summer School 2010

Prevention of degradation

• Do not load the cell too hard - find the allowable current density for your cathode

• Do not go to fuel utilisation (high steam partial pressure) above ca. 90 %. Again test the limit for your cell.

• Take care of removing or scavenging (e.g. CrO2 (OH)2 - H2 S) potential poisons in the feed gases and in the raw materials.

• Make stable electrode structures of stable materials - this is however a long story, which, hopefully, my colleagues teachers have informed you about.

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SOFC Summer School 2010

Literature:

Mogensen, Hendriksen, "Testing of Electrodes, Cells and Short Stacks", Chapter 10 in High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications, Eds. Singhal and Kendall, pp. 261 -290, Elsevier 2003.

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Thank you for your attention