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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 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 previousH2/3%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 Gas conversion TPB IT proces Gas diffusion
Test set-up for electrodes in a symmetric cell with typical area of 0.25 cm2
Two Risø three-electrode pelletsin one atmospere placed working to working electrode to aviod change in local gas composition tothe larges possible extent
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?
50 h Y Si Ti Na XPS of YSZ surface. After Badwell and Drennan, 1994.
The TPB CT process The old ”pure” ideas and actual practical reaction limitations
EIS of air/LSM-YSZ/YSZ electrodes. 1000°C, 0.4 cm2. 1 Hz 100 Hz 10 Hz 100 Hz 10 kHz 1 Hz
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
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