Characterisation of Solid Oxide Fuel Cells and Electrodes Using EIS

Mogens MogensenMaterials 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?or2) Physical-chemical models?

The answer is yes! Naturally, we should end up with 2), but 1) may be a necessary intermediate step.

SOFC EIS examplesIn 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 JørgensenMost 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

", cm

2

Z', cm2

TPB ITproces

Gasdiffusion

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

R RTF J x xg

i 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

/ c

m2

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-im

ag (

)

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