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.

Solid oxide fuel cells produced at Risø National Laboratory have been tested as electrolysis cells by applying an external voltage. Results on initial performance and durability of such reversible solid oxide cells at temperatures from 750 to 950°C and current densities from −0.25 A/cm 2 to − 0.50 A/cm 2 are reported. The full cells have an initial area specific resistance as low as 0.27 ⍀cm 2 for electrolysis operation at 850°C. During galvanostatic long-term electrolysis tests, the cells were observed to passivate mainly during the first ϳ100 h of electrolysis. Cells that have been passivated during electrolysis tests can be partly activated again by operation in fuel cell mode or even at constant electrolysis conditions after several hundred hours of testing.

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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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