Characterisation of Solid Oxide Fuel Cells and Electrodes Using EIS
Mogens Mogensen
Materials Research Department, Risø National Laboratory
DK-4000 Roskilde, Denmark
Presentation at the International Workshop ”Impedance Spectroscopy for Characterisation of Materials and Structures
Warsaw, 24 – 28 September, 2003
Introduction
• Electrochemical impedance spectroscopy, EIS, is in many respects a strong tool for SOFC characterisation, but it certainly has its limitations
• Difficult to interpret one or few spectra unless other information is available • Very little physical and chemical information available in one spectrum
How to model or fit EI spectra?
1) Equivalent circuits?
or
2) Physical-chemical models?
The answer is yes! Naturally, we should end up with 2), but 1) may be a necessary intermediate step.
SOFC EIS examples
In the following important features of the efforts of revealing the SOFC electrode mechanisms here at Risø is given; mainly the Ni-YSZ-H2/H2O is used as example. Details are found in the ph.d. theses of
• Søren Primdahl
• Karin Vels Jensen (now Hansen)
• Mette Juhl Jørgensen
Most of it has been and some is being published in the open literature
Examples of H2/3%H2O/Ni-YSZ at 1000°C. a) 50/50 vol % Ni/YSZ fine powders, b) Risø ”standard”.
1Hz
1 Hz
Q = Y0(j)n, Y0 and n are constants, = 2f
For a given electrode n1, n2, n3 …. should be constant or develop smoothly with varied parameter!!!
Error plots for the two previous H2/3%H2O/Ni-YSZ EIS using this equivalent circuit, (n1, n2, n3) = (0.8, 0.75, 1).
The Risø three-electrode set-up with a separate reference gas for the reference and counter electrodes
Impedance spectrum of Ni -YSZ
0.0 0.1 0.2 0.3 0.4
0.0
0.1
0.2
I II III
1 Hz
100 Hz10 kHz-Z
", c
m2
Z', cm2
TPB IT
procesGas
diffusion
Gas conversion
Test set-up for electrodes in a symmetric cell with typical area of 0.25 cm2
Two Risø three-electrode pellets in one atmospere placed working to working electrode to aviod change in local gas composition to the larges possible extent
Gas conversion impedance
RRT
F J x xgi i H O i H
4
1 12
2 2, ,
C
F PV
RT Ax x
g
i H O i H
4 11 1
2
2
2 2, ,
The TPB IT (ion transfer) process?- Literature on H2/Ni/YSZ -
• Extreme disagreement
• Activation energies from 0.8 - 1.7 eV!
• Dependencies on partial pressures of water and hydrogen vary a lot. For hydrogen both negative and positive dependencies have been found
• Do people study different systems even though they believe that they are identical?
XPS of YSZ surface. After Badwell and Drennan, 1994.
Y
SiTi
Na
50 h
The TPB CT processThe old ”pure” ideas and actual practical reaction limitations
EIS of air/LSM-YSZ/YSZ electrodes. 1000°C, 0.4 cm2.
1 Hz
1 Hz
100 Hz
100 Hz10 kHz
10 Hz
May consist of atleast five arcs*:
•Arc A and B is charge transfer
•Arc C is oxygenreduction / oxidation
•Arc D is oxygen diffusion
•Arc E is some kind of activation i.e. depassivation
1E4 1E
3 1E2
1E1
1.00 1.05 1.10
0.00
0.05
ECA B
b
a
1E4
1E3
1E1
1E0
0.92 0.96 1.00
0.00
0.04
DC-Z
imag
/
cm2
Zreal / cm2
EIS of O2/O2-/ LSM+YSZ/YSZ
-0.04
-0.03
-0.02
-0.01
0
0.010 0.02 0.04 0.06 0.08
Z-real ( )
Z-i
ma
g (
)
6% water24% water
9% H2/N2
9% H2/He
1Hz10Hz100H z
Rs
EIS on a 16 cm2 AS-SOFC at 850 °C
Conclusion
You do not know in details which processes you are studying as the starting point in real research
Therefore, forget about first-principle-modelling until you have done the many experiments which show the nature of the operating processes
