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