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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A range of solid electrolyte cell geometries has been evaluated in terms of their suitability for measurements of the working electrode polarization resistance in a three-electrode configuration. The potential and current density distributions in the cells are calculated numerically. If the current density at the working electrode is not uniform, the standard procedure of evaluating the electrode polarization resistance from the experimental data is inadequate and may lead to erroneous results. Simulations are presented for cells with thin electrolytes and for pellet-based geometries with large electrolyte thickness. Working electrode C Solid electrolyte Counter electrode 1184

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Many processes contribute to the overall impedance of an electrochemical cell, and these may be difficult to separate in the impedance spectrum. Here, we present an investigation of a solid oxide fuel cell based on differences in impedance spectra due to a change of operating parameters and present the result as the derivative of the impedance with respect to ln(f) . The method is used to separate the anode and cathode contributions and to identify various types of processes.

This paper deals with degradation mechanisms of Ni-YSZ electrodes for solid oxide cells, mainly solid oxide electrolysis cells (SOECs), but also to some extent solid oxide fuel cells (SOFCs). Analysis of literature data reveals that several apparently different and even in one case apparently contradicting degradation phenomena are a consequence of interplay between loss of contact between the Ni-YSZ (and Ni-Ni particles) in the active fine-structured composite fuel electrode layer and migration of Ni via weakly oxidized Ni hydroxide species. A hypothesis that unravels the apparent contradiction and explains qualitatively the phenomena is presented, and as a side effect, light has been shed on a degradation phenomenon in solid oxide fuel cells (SOFCs) that has been observed during a decade.