Electrochemical characterization and performance evaluation · ’logaritmic’ Bode Plot. SOFC...
Transcript of Electrochemical characterization and performance evaluation · ’logaritmic’ Bode Plot. SOFC...
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]
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
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
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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
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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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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
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Very low frequency - phase angle is 0 - resistor
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Higher frequency - phase shift < O for capacitance containing circuit
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Still higher frequency
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Very high frequency - phase angle is 0 again - capacitor
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An impedance is a complex number
Vector Z
Angle φ
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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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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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Electrical Circuits -Series and Parallel Connections
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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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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
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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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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 (%
)
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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
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Questions here?
My question to you: Any proposal about what to do in order to get this pre-knowledge?
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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
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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
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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)
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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
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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
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“Point” electrodes of metal
Ni-wire
YSZ-single crystal
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Cone shaped "point" electrode of ceramics
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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
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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)
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Pattern electrodes
Thin electrode stripes
YSZ electrolyte
Also a counter and a reference electrode must be applied (not shown)!
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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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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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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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Three-electrode-set-up
One of them to be used as an auxiliary electrode
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Symmetrical cell
A symmetrical two-electrode cell arrangement for measurements at OCV
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-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
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-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
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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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Full cell vs. symmetric cellsTemperature
°CConditions
Air &Full cell B
Anode[Ω
cm2]
Sym. cell Anode
[Ω
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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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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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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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
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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 (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
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
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/ / 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
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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-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
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
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
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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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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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Post-test microscopy: Cell tested in air
• Smaller crater rings, blurred shapes • Foreign phases, nano-sized particles
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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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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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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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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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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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More EIS - DRT and ADIS to come
Any questions now?
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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Distribution of relaxation times (DRT)
Leonide et al. 2008, J. Electrochem. Soc. 155 B36
Nyquist representation
DRT representation
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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'' /
[Ω
·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'' /
[Ω
·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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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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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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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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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
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
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
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
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.
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