Bulk and Surface Property Characterization of Novel Solid Oxide Fuel Cell Cathode Materials

The development of intermediate temperature solid oxide fuel cells (IT-SOFCs) with operating temperatures in the 500 – 700 °C range would provide cost savings, and thus commercial viability over traditional high temperature (800 – 1000°C) SOFCs. However, at intermediate temperatures, the electrochemical processes within the cell become significantly slower due to high activation energies for ion transport and reactions at the electrodes, leading to decreased power output. A fundamental understanding of cathode processes, such as oxygen transport, and detailed structural information for bulk cathode materials are necessary for the development of IT-SOFCs.

Electrical conductivity relaxation (ECR) was utilized to determine the oxygen surface exchange coefficient, kchem, and bulk diffusion coefficient, Dchem, for La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF 6428) materials at 1073 K (800 °C) in the 100 to 3.3% pO2 range. The value of kchem varies by almost one order of magnitude depending on the number of terms chosen in the data fit. However, a precise determination of kchem can be accomplished by performing ECR with dense thin film model cathodes, allowing Dchem to be eliminated from the analysis. LSCF 6428 samples were also individually surface-doped with La, Sr, Co, and Fe, as well as with LSCF 6428 nanoparticles (25 – 50 nm) to investigate trends in kchem and Dchem. Fe surface doping caused a decrease in kchem, while La, Sr, Co and nanoparticle doping did not affect the oxygen surface exchange or bulk diffusion coefficients.
PrBaCo2O5+δ (PBCO) layered perovskite polycrystalline thin films were deposited onto a polished polycrystalline SrTiO3 substrate using a spray pyrolysis technique. ECR was utilized to determine kchem in the 650 – 850 K (377 -577 °C) temperature range and 100% to 0.05% pO2 range. The resulting values are higher than the values reported for some traditional single perovskite materials at intermediate temperature.

Layered perovskites NdBaCo2O5+δ (NBCO) and PrBaCo2O5+δ (PBCO) were characterized using neutron powder diffraction under in-situ conditions: 550 – 825 °C and 10-1 - 10-4 atm oxygen. The tetragonal (P4/mmm) space group provided the best fit for the data. Transport of oxygen through both materials via the vacancy hopping mechanism likely involves the nearest-neighbor oxygen sites in the Co-layer, in addition to the more vacant oxygen sites in the Nd / Pr layer. Total oxygen stoichiometry values ranged from 5.51 – 5.11 and 5.57 – 5.17 for NBCO and PBCO, respectively. The tetragonal lattice parameters for both materials showed crystal expansion with temperature. However, the crystal shortened in the c-direction with decreasing pO2 as all sites shifted toward the increasingly vacant Nd/Pr-O layer.

Finally, formation of La0.6Sr0.4CoO3 (LSC64) nanorods was attempted via direct hydrothermal synthesis, calcination of hydrothermal synthesis products and topochemical conversion of La(OH)3 nanorod templates. LSC64 nanorods were not formed during hydrothermal synthesis as reported previously. LSC was produced by calcination and topochemical conversion, but the rod structure was lost due to coarsening at temperatures as low as 500 °C. These findings are discussed in terms of the kinetics and thermodynamics of the hydrothermal synthesis reaction, pointing to the thermodynamic instability of LaCoO3 at reaction conditions.