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This study is an attempt to improve the performance of Pt/ZrO2 high-temperature air cathodes by imitating porous electrodes of liquid electrolyte fuel cells. Ion conductive oxides were introduced into platinum electrodes in expectation of playing the same electrochemical role as the liquid electrolyte penetrating the porous electrodes. The oxide-mixed platinum electrodes prepared were modeled on a pore whose inner wall was covered with a gas permeable electrolyte film. An analysis based on the pore model predicted that the performance increases with increasing electrode thickness. Electrodes containing (Bi203)0.7 (Er203)0.3 exhibited the expected trend, resulting in a much higher performance than simple Pt/ZrO2 electrodes. However, additions of (ZrO2)0.92(Y203)0.08 or (CeO2)0.8 (SmO1.~)0.2 led to no marked performance improvements. These different effects of oxide additives are explained in terms of the degree of contact between the platinum particles.

The contact area between the platinum paste electrode and the zirconia electrolyte was estimated on the basis of a linear relationship between the peak area of linear sweep voltammetry and the actual contact area. The latter area was measured by microscopic observation of the contact area from one side of a transparent zirconia single-crystal disk on the other side of which the platinum paste had been sintered. Applying the peak area vs. contact area relationship obtained to the same platinum electrode sintered on polycrysta]line zirconia disks, the contact area was estimated from the voltammogram peak. The electrode performance increased monotonically with increasing contact area, but there was a small deviation from the proportional relationship. This deviation was corrected by using the length of the three-phase boundary, instead of the contact area, as a morphological parameter.The electrode used for solid oxide fuel cells (SOFC) is a porous electrode which is usually prepared by sintering powder material onto a solid electrolyte. The electrode performance therefore is strongly affected by the morphology of the electrode as well as the properties of the material used. To compare different electrode materials on an equal basis, the effect of morphology must be eliminated. Singlepoint contact electrodes have been used to approach this problem. 1 For porous electrodes, however, no direct measurements of the morphological parameters have been made.To evaluate the effect of morphology, the capacitance of the electrode has been measured from the ac impedance or potential response to the pulsed currentY This method is based on the doubled-layer model having parallel plates of area S and spacing d. The double-layer capacity Cdt is expressed by Cdt = eS/d, where 9 is the dielectric constant of the space between parallel plates. Inserting 9 = 8.85 ~F 9 A/cm 2 (permittivity of free space), S = 1 cm 2 and d = 1 A (an approximate closest approach of O 2-ion), we obtain Cdl = 8.85 ~F/cm 2. However, the experimental values obtained for SOFC electrodes are one order of magnitude or more greater than expected above. 7 This discrepancy has been explained as being due to the pseudocapacitance arising from the faradaic process of adsorbed oxygen, s'8 or due to the adsorption of large polarizable ions in the electrolyte to the electrode-electrolyte interface. 8 The latter interpretation was supported experimentally for AgI and AgBr electrolytes, 9 but this is less likely for the more compact 02-ion having an adsorption behavior far from that of such large halide ions.If the capacitance is associated with any faradaic process, the meaning of the plate area is indefinite, depending on the location of the rate-determining reaction, e.g., the dissociation of O2 gas on the three-phase boundary (TPB), the charge transfer of O~ over the whole interface area between electrode and electrolyte, and various intermediates between these two extremes. At the present time, the interpretation of capacitance remains ambiguous and controversial, a...

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