Jimmi Nielsen

Comparison of the Degradation of the Polarisation Resistance of Symmetrical LSM-YSZ cells, with Anode Supported Ni-YSZ/YSZ/LSM-YSZ SOFCs

Symmetrical cells are a useful tool for screening electrode performance [1]. The goal of this study was to investigate how the results collected from symmetrical cells relate to results obtained on a corresponding full cell. Therefore, electrochemical impedance spectra (EIS) from a symmetrical cell and full cell were collected periodically over a longer period of time, at open circuit voltage and 650°C in air (cathode) and humidified (4%) hydrogen (anode). The symmetrical cells were screen-printed lanthanum strontium manganite yttria stabilized zirconia composite cathodes [LSM25.5-YSZ composites, where LSM25.5 = (La0.75Sr0.25)0.95MnO3±δ and YSZ = ZrO2 with 8 mol% Y2O3] on stabilized zirconia, YSZ, electrolyte. The full cells were planar anode supported Ni-YSZ/YSZ/LSM25.5-YSZ cells. These materials have been chosen as they are of continuing interest in the field [1, 2, 3]. The impedance was affected by degradation over time in the same frequency range for both cells (~10 Hz), possibly indicating that the same physical process was affected in both types of cell. To break down the losses of the cells, equivalent circuits were applied to the EIS data from both types of cells [4, 5]. However, deconvolution of the impedance data was not straightforward. Applying slightly different n- values for the constant phase elements; n-values proposed in literature or n-values determined in this study, in an otherwise identical equivalent circuit, led to very different results for the breakdown of losses of the full cell, showing the high sensitivity of the fitting procedure. Moreover, both sets of n- values delivered unexpected and non physical breakdown of losses. Hence, further data on the symmetrical cells at different temperatures and partial oxygen pressures have been obtained. Refinement of the equivalent circuits will be done applying analysis techniques like ADIS [6] (analysis of difference in impedance spectra) and DRT [7] (distribution of relaxation times). References [1] S.C Singhal and K. Kendall, editors. High Temperature Solid Oxide Fuel Cells Funda- mentals, Design and Applications. Elsevier, 2003. [2] S.P. Jiang. J. Mater. Sci. 43:6799, 2008. [3] S.B. Adler. Chem. Rev. 104:4791, 2004. [4] M.J. Jorgensen, S. Primdahl, C. Bagger, and M. Mogensen. Solid State Ionics, 139:1, 2001. [5] R. Barfod, A. Hagen, S. Rasmousse, P.V. Hendriksen, and M. Mogensen. Fuel Cells, 6:141, 2006. [6] S.H. Jensen, A. Hauch, P.V. Hendriksen, M. Mogensen, N. Bonanos, and T. Jacobsen. Journal of the Electrochemical Society, 152:B1325, 2007. [7] H. Schichlein, A. Muller, M. Voigts, A. Krugel, and E. Ivers-Tiffee. Journal of Applied Electrochemistry, 32:875, 2002.

Oxygen Sorption and Desorption Properties of Selected Lanthanum Manganites and Lanthanum Ferrite Manganites

Temperature-programmed desorption (TPD) with a carrier gas was used to study the oxygen sorption and desorption properties of oxidation catalysts and solid-oxide fuel cell (SOFC) cathode materials (La(0.85) Sr(0.15)0.95 MnO(3+δ) (LSM) and La(0.60) Sr(0.40) Fe(0.80) Mn(0.20) O(3-δ) (LSFM). The powders were characterized by X-ray diffractometry, atomic force microscopy (AFM), and BET surface adsorption. Sorbed oxygen could be distinguished from oxygen originating from stoichiometry changes. The results indicated that there is one main site for oxygen sorption/desorption. The amount of sorbed oxygen was monitored over time at different temperatures. Furthermore, through data analysis it was shown that the desorption peak associated with oxygen sorption is described well by second-order desorption kinetics. This indicates that oxygen molecules dissociate upon adsorption and that the rate-determining step for the desorption reaction is a recombination of monatomic oxygen. Typical problems with re-adsorption in this kind of TPD setup were revealed to be insignificant by using simulations. Finally, different key parameters of sorption and desorption were determined, such as desorption activation energies, density of sorption sites, and adsorption and desorption reaction order.