Anode-supported thin electrolyte cells are studied by electrochemical impedance spectroscopy ͑EIS͒. The aim is to describe how the losses of this type of cells are distributed at low current density ͑around open-circuit voltage͒ as a function of temperature. An equivalent circuit consisting of an inductance, a serial resistance ͑R s ͒, and five arcs to describe the polarization resistance is suggested. This equivalent circuit is based on previous studies of single electrodes in three-electrode and two-electrode symmetric cell setups. The equivalent circuit components have been assigned to the electrode processes, and the assignments were verified by extensive full cell studies in which the partial pressure of reactant gases on both the electrodes as well as temperature was systematically varied with the aim to identify frequency regions which are dominated by an electrode specific process. Furthermore, the model is applied on a good performing cell with area specific resistance ͑ASR͒ = 0.15 ⍀ cm 2 at 850°C and a poor performing cell with ASR = 0.29 ⍀ cm 2 at the same temperature. Both cells were fabricated using nominally the same procedure. The EIS analysis indicated that the difference in performance originates from microstructural differences on the cathode. This is further supported by the observation of large differences in the cathode microstructure by scanning electron microscope.

The degradation behavior of anode supported solid oxide fuel cells ͑SOFCs͒ was investigated as a function of operating temperature and current density. Degradation rates were defined and shown to be mainly dependent on the cell polarization. The combination of a detailed evaluation of electrochemical properties by impedance spectroscopy, in particular, and post-test microscopy revealed that cathode degradation was the dominant contribution to degradation at higher current densities and lower temperatures. The anode was found to contribute more to degradation at higher temperatures. Generally, the degradation rates obtained were lower at higher operating temperatures, even at higher current densities. A degradation rate as low as 2%/1000 h was observed at 1.7 A/cm 2 and 950°C over an operating period of 1500 h.

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The contributions of the individual components of the cell (anode, cathode, and electrolyte) to the cell resistance were determined experimentally, directly from impedance spectra obtained from a full cell. It was an anode supported thin electrolyte cell, consisting of a YSZ electrolyte, a Ni/YSZ cermet anode, and a LSM composite cathode. Additional, qualitative information was obtained using symmetric cells with LSM composite electrodes. The investigations were carried out in the temperature interval from 700 to 850 °C. The electrolyte and anode activation energies obtained were 0.9 and 1.1 eV, respectively, which is in relatively good agreement with literature values. The anode resistance was 0.24 Ω cm2 and the cathode resistance was 0.58 Ω cm2 at 700 °C, corresponding to 23% and 56% of the total resistance, respectively.

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