RATIONAL SELECTION OF ADVANCED SOLID ELECTROLYTES FOR INTERMEDIATE TEMPERATURE FUEL-CELLS

Discussed here is a strategy for selecting perovskite-based solid electrolytes with the potential for achieving high ionic conductivities at intermediate temperatures (almost-equal-to 600-degrees-C). Activation energies for anionic transport (either O2- or proton) have been shown to be influenced by: (1) the average metal-oxygen bond energy in the perovskite, (2) the lattice free volumes, obtained by subtracting the ionic volumes of cations and O2- in the unit cell from the overall unit cell volume, (3) the parameter r(critical) (r(c)) which corresponds to the radius of the opening between the two A-site cations and one B-site cation through which the mobile anion must pass. and (4) the lattice polarizability. Low activation energies for anion migration appear to favor: (1) that the overall lattice possess a moderate metal-oxygen binding energy, (2) perovskite solid electrolytes posses free volumes of 30-35 angstrom3, (3) that the lattice minimally polarizes the mobile anion, and (4) preferred (r(critical)/r0(2-))2 ratios for A-A-B saddle points congruent-to 0.5. High ionic conductivities have also been achieved for the perovskite-related brownmillerites A2B2O5 which possess a high intrinsic population of anion vacancies in their lattice. Solid electrolytes evolving from this complimentary rationale, which has included BaTh0.9Gd0.1,O3, Sr2Gd2O5 and Sr2Dy2O5, have been incorporated into fuel cells operating at intermediate temperatures. La0.9Sr0.1CoO3, BaCo0.8Fe0.2O3, Ag and Ru have been found candidate cathodes for the intermediate-temperature fuel cell application.