Perovskite compositions in the system La1M1Co1_8Fe8O3_, (M = Sr, Ba, Ca) exhibited high electronic and ionic conductivity. Substantial reversible weight loss was observed at elevated temperatures as the materials became increasingly oxygen deficient. Thisloss of lattice oxygen at high temperatures, which tended to increase with increasing acceptor content, resulted in a decrease in the electronic conductivity. In an oxygen partial pressure gradient, oxygen flux through dense sintered membranes of these materials was highly dependent on composition and increased with increasing temperature. The increase in oxygen flux with increasing temperature was attributed to increases in the mobility and concentration of lattice oxygen vacancies. The calculated ionic conductivities of some compositions exceeded that of yttriastabilized zirconia.

The structural stability and oxygen permeation properties of Sr 3Ϫx La x Fe 2Ϫy Co y O 7Ϫ␦ with 0 р x р 0.3 and 0 р y р 1.0 have been studied at high temperature (800 р T р 900°C) and in the oxygen partial pressure range 10 Ϫ5 р pO 2 р 0.21 atm. These phases have a perovskite-related intergrowth structure with tetragonal symmetry, which does not change with temperature up to 1000°C in the range 10 Ϫ5 р pO 2 р 0.21 atm. The oxygen permeation flux of Sr 3Ϫx La x Fe 2Ϫy Co y O 7Ϫ␦ membranes increases with increasing Co content, decreases with increasing lanthanum content, and does not change with time. Measurements of oxygen permeation flux as a function of membrane thickness for Sr 2.7 La 0.3 Fe 1.4 Co 0.6 O 7Ϫ␦ at 900°C indicates the oxygen transport is bulk limited for this composition. Assuming that this observation is also valid for other compositions, the ionic conductivity, i , and the vacancy diffusion coefficient D v have been estimated. The structural stability and permeation properties of Sr 3Ϫx La x Fe 2Ϫy Co y O 7Ϫ␦ are compared with those of the perovskite phases SrFe 0.2 Co 0.8 O 3Ϫ␦ and Sr 0.6 La 0.4 Fe 0.2 Co 0.8 O 3Ϫ␦ .

Reduction and reoxidation kinetics of Ni-based solid oxide fuel cell ͑SOFC͒ anodes were investigated over a range of temperatures between 600 and 800°C. Dense ͑no open porosity͒ two-phase NiO + YSZ ͑yttria-stabilized zirconia͒ samples, with and without small amounts of oxide additives ͑CaO, MgO, TiO 2 ͒, were fabricated and then reduced in a hydrogen-containing environment. The time dependence of the reduced layer thickness at various temperatures was measured. Reoxidation studies were conducted on fully reduced anodes that were subsequently reoxidized in air over a temperature range between 650 and 800°C. A simple theoretical model was developed to describe the kinetics of reduction and reoxidation based on two series kinetic steps: diffusion and interface reaction. It was observed that the reduction kinetics was linear ͑interface-controlled͒, while the reoxidation kinetics was nearly parabolic ͑diffusion-controlled͒. Also, the kinetics of reduction was thermally activated with an activation energy of ϳ95 kJ/mol. By contrast, over the temperature range investigated, the kinetics of reoxidation was essentially independent of temperature. The interface control of the reduction process implies that gas-phase diffusion through porous Ni + YSZ, formed upon reduction of NiO to Ni, is considerably faster than the kinetics of the actual reduction reaction occurring at the interface separating the pristine and the reduced regions. By contrast, diffusion control of the reoxidation process was attributed to slow, gaseous diffusion on account of the very small amount of porosity that remains when Ni reoxidizes to NiO, developed presumably due to a slight shape change of Ni particles that may occur at high temperatures. Doping the anodes with stable oxides, such as CaO and MgO, significantly reduced both the reduction and reoxidation kinetics of Ni-based anodes.

The effective diffusivity of O 2 -N 2 in porous media was measured at high temperatures ͑650-800°C͒ using an electrochemical concentration cell. Porous membranes having total porosity between 29 and 48 vol% were fabricated from Sr-doped LaMnO 3 ͑LSM͒ with 20 to 30 wt% carbon added as a pore former. The O 2 -N 2 effective binary diffusivity, D O 2 -N 2 eff , at 800°C increased from ϳ0.016 to ϳ0.12 cm 2 /s with increasing open porosity between 15 and 44 vol%. The D O 2 -N 2 eff exhibited a nonlinear dependence on open porosity and increased dramatically for samples with greater than 35 vol% open porosity. The estimated effective Knudsen diffusivities of O 2 and N 2 , D O 2 K eff and D N 2 K eff , at 800°C were an order of magnitude higher than the effective binary diffusivity, D O 2 -N 2 eff . Thus, O 2 -N 2 transport through the porous membranes was governed by the effective binary diffusivity, D O 2 -N 2 eff . The effects of O 2 -N 2 effective binary diffusivity, D O 2 -N 2 eff, on concentration polarization of cathodes for solid oxide fuel cells were assessed. The nonlinear behavior of the O 2 -N 2 effective diffusivity as a function of open porosity indicates that a critical amount of porosity in the cathode is necessary to ensure that the overpotential due to concentration polarization is small. The temperature dependence of D O 2 -N 2 eff was investigated between 650 and 800°C, which was found to be in accord with the Chapman-Enskog model.The contribution of concentration polarization to the total polarization at both the anode and cathode of solid oxide fuel cells ͑SOFCs͒ can be substantial. 1,2 The effects of concentration polarization are most pronounced in electrodes of low porosity and at high current densities. In electrode-supported SOFCs, the contribution of ohmic polarization ͑IR drop͒ of a thin electrolyte to the total overpotential of the cell is small as compared to that of an electrolytesupported SOFC. In fact, anode-supported SOFC have been reported wherein the contribution of the electrolyte ohmic polarization ͑IR drop͒ is less than that of the combined polarization losses at the two electrodes. Thus, in the case of electrode-supported SOFCs, the maximum current density attainable is largely limited by polarization losses at the electrodes. In electrode-supported SOFCs, one of the two electrodes, either anode or cathode, is fabricated of a thickness large enough to function as a support and thereby lend mechanical integrity. However, this does not mean that concentration polarization of the support electrode is necessarily greater than that of the thin electrode. For example, anode-supported SOFCs are routinely fabricated with anodes consisting of NiO and YSZ composites that are subsequently reduced in situ resulting in highly porous Ni and YSZ cermets. Measurements conducted on anode-supported SOFCs show that overpotential at the thick anode can often be less than that at the thin cathode. Typically, cathodes are fabricated from an electrocatalyst such as Sr-doped LaMnO 3 or Sr-doped LaCoO 3 ....