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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

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Characterisation of Solid Oxide Fuel Cells and Electrodes Using EIS


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    1. 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

    2. 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

    3. 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.

    4. 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

    5. Examples of H2/3%H2O/Ni-YSZ at 1000°C. a) 50/50 vol % Ni/YSZ fine powders, b) Risø ”standard”. 1Hz 1 Hz

    6. Q = Y0(j)n, Y0 and n are constants,  = 2f For a given electrode n1, n2, n3 …. should be constant or develop smoothly with varied parameter!!!

    7. Error plots for the two previousH2/3%H2O/Ni-YSZ EIS using this equivalent circuit, (n1, n2, n3) = (0.8, 0.75, 1).

    8. The Risø three-electrode set-up with a separate reference gas for the reference and counter electrodes

    9. Impedance spectrum of Ni -YSZ Gas conversion TPB IT proces Gas diffusion

    10. Test set-up for electrodes in a symmetric cell with typical area of 0.25 cm2

    11. Two Risø three-electrode pelletsin one atmospere placed working to working electrode to aviod change in local gas composition tothe larges possible extent

    12. Gas conversion impedance

    13. 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?

    14. 50 h Y Si Ti Na XPS of YSZ surface. After Badwell and Drennan, 1994.

    15. The TPB CT process The old ”pure” ideas and actual practical reaction limitations

    16. EIS of air/LSM-YSZ/YSZ electrodes. 1000°C, 0.4 cm2. 1 Hz 100 Hz 10 Hz 100 Hz 10 kHz 1 Hz

    17. May consist of at least five arcs*: Arc A and B is charge transfer Arc C is oxygen reduction / oxidation Arc D is oxygen diffusion Arc E is some kind of activation i.e. depassivation EIS of O2/O2-/ LSM+YSZ/YSZ

    18. EIS on a 16 cm2 AS-SOFC at 850 °C

    19. 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