Anode-supported solid oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolyte, Sr-doped LaMnO 3 (LSM)ϩ YSZ cathode, and Ni ϩ YSZ anode were fabricated and their performance was evaluated between 650 and 800ЊC with humidified hydrogen as the fuel and air as the oxidant. Maximum power densities measured were ϳ1.8 W/cm 2 at 800ЊC and ϳ0.82 W/cm 2 at 650ЊC. Voltage (V) vs. current density (i) traces were nonlinear; V vs. i exhibited a concave-up curvature [d 2 V/di 2 Ն 0] at low values of i and a convex-up curvature [d 2 V/di 2 Յ 0] at higher values of i, typical of many low temperature fuel cells. Analysis of concentration polarization based on transport of gaseous species through porous electrodes, in part, is used to explain nonlinear V vs. i traces. The effects of activation polarization in the Tafel limit are also included. It is shown that in anode-supported cells, the initial concave-up curvature can be due either to activation or concentration polarization, or both. By contrast, in cathode-supported cells, the initial concave-up curvature is entirely due to activation polarization. From the experimentally observed V vs. i traces for anodesupported cells, effective binary diffusivity of gaseous species on the anodic side was estimated to be between ϳ0.1 cm 2 /s at 650ЊC and ϳ0.2 cm 2 /s at 800ЊC. The area specific resistance of the cell (ohmic part), varied between ϳ0.18 ⍀ cm 2 at 650ЊC and ϳ0.07 ⍀ cm 2 at 800ЊC with an activation energy of ϳ65 kJ/mol.

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The effect of porous composite electrodes on the overall charge-transfer process in solid-state devices, such as solid oxide fuel cells, is theoretically examined by taking into account various parameters such as electrolyte thickness, intrinsic charge-transfer resistance, electrode thickness, and porosity. A model is presented that accounts for ionic transport within the electrolyte, electronic conduction through electrocatalyst, and charge-transfer at the electrolyte-electrocatalyst interface. Diffusion of gaseous species in porous electrodes is assumed to be rapid so as not to be rate limiting. The conduction of electrons in the electrocatalyst is assumed to introduce negligible resistance. The activation overpotential as a function of current density is assumed to be ohmic, and an effective charge-transfer resistance is defined. The transport equations are solved numerically in two dimensions using a finite difference technique and analytically in one dimension. The analysis predicts that the use of composite electrodes in devices employing solid electrolytes can significantly increase performance under conditions where the intrinsic charge-transfer resistance is high in comparison to the area-specific resistance of the electrolyte. The results indicate a low effective charge-transfer resistance is obtained for relatively thick electrodes with a fine microstructure as long as the porosity is sufficient to ensure negligible concentration polarization.Infroduction Electrochemical devices, such as fuel cells, batteries, and gas separators, based on solid electrolytes have the potential for commercial viability. The performance of an active electrochemical device is dictated by the sum of resistances

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Lake City, Utah 841 12Bi,O,-based cubic solid solutions containing 20% Er,O, undergo transformation to a rhombohedral phase when annealed at temperatures 1635°C. This transformation is generally very sluggish and is accompanied by a decrease in conductivity. The kinetics of this transformation were enhanced by the addition of CaO and suppressed by the addition of ZrO,. The time constants for transformation kinetics at 600°C for CaO-doped and undoped samples were -55 and -330 h, respectively, and the incubation periods ranged between -20 and 12000 h depending upon the dopant type, its concentration, and the temperature. This result is rationalized on the premise that cation interstitials are more mobile compared to cation vacancies. The same samples, originally cubic of CaF, type, when annealed below about 600°C for a short period of time (a few hours) undergo a degradation in conductivity without a change in XRD patterns. The kinetics of this conductivity decay were observed to be significantly faster than the kinetics of cubic -I rhombohedral phase transformation. A similar degradation of conductivity was also observed in samples containing other stabilizers such as Yz03, DyzOJ, and Yb,O,. The kinetics of conductivity decay without the formation of the rhombohedral phase were also found to depend upon the presence of aliovalent dopants. Specifically, ZrO, suppressed this decay. Electron diffraction showed the formation of a superstructure in samples annealed at temperatures <600"C. Analysis of the diffraction patterns suggests that the structure corresponds to a doubling of the unit cell with the ordering of cations responsible for the origin of the superstructure. The decay in conductivity in the ordered state is attributed to the expected differences in bonding between oxygen ions and the two different cations, Bi and RE.