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The behavior of SOEC was investigated as compared with that of solid oxide fuel cell (SOFC) using small cells prepared on yttria stabilized zirconia (YSZ) planar discs. Ni-YSZ cermet negative electrode showed asymmetric behavior indicating the existence of diffusion limited process in the electrolysis direction, although the behavior was strongly dependent on the electrode preparation. The behavior of positive elec trodes made by using perovskite type oxides was also investigated.When polarized anodically, the positive electrode showed degradation behavior which ended up with electrode delamination from electrolyte. The degradation rate was remarkably decreased by improving the initial polarization performance of the elec trode or by using a mixed ceria intermediate layer between YSZ and electrode. Polarization measurements of SOEC were conducted at 1173K, 1223K and 1273K with various water content in hydrogen simulating the atmosphere of various water electrolysis rate. The cell was shown to work at a high current density and at high water electrolysis rate without suffering from diffusion limiting current.

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The effect of operating pressure on the performance of SOFC and SOEC was investigated electrochemically under pressurized and depressurized conditions (0.01-1.0 MPa) using practical size single cells. It was found that the diameter of low frequency semi-circle appears in impedance spectra was not so much affected by operating pressure with electrolyte supported cell as was expected theoretically for gas conversion impedance. On the other hand, the diameter significantly depends on the operating pressure with anode supported cell especially when the operating pressure becomes low, indicating the contribution of diffusion to the semicircle. Numerical simulation in which diffusion processes in electrodes were considered was proposed to explain the observed behavior.

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