Zigui Lu

Electrochemical Performance and Stability of the Cathode for Solid Oxide Fuel Cells: III. Role of Volatile Boron Species on LSM/YSZ and LSCF

Boron oxide is a key component to tailor the softening temperature and viscosity of the sealing glass for solid oxide fuel cells (SOFCs). The primary concern regarding the use of boron-containing sealing glasses is the volatility of boron species, which possibly results in cathode degradation. In this paper, we report the role of volatile boron species on the electrochemical performance of LSM/yttria-stabilized zirconia (YSZ) and LSCF cathodes at various SOFC operation temperatures. The transport rate of boron, ~3.24 × 10 -12 g/cm 2 sec was measured at 750°C with air saturated with ~3% moisture. A reduction in power density was observed in the cells with the LSM/YSZ cathodes after the introduction of boron source to the cathode air stream. A partial recovery of the power density was observed after the boron source was removed. Results from post-test secondary-ion mass spectroscopy (SIMS) analysis showed that the partial recovery in the power density correlated with the partial removal of the deposited boron by the clean air stream. The presence of boron was also observed in the LSCF cathodes by SIMS analysis; however, the effect of boron on the electrochemical performance of the LSCF cathode was negligible. The coverage of triple phase boundaries in LSM/YSZ was postulated as the cause for the observed reduction in the electrochemical performance.

Electrochemical Performance and Stability of the Cathode for Solid Oxide Fuel Cells IV. On the Ohmic Loss in Anode-Supported Button Cells with LSM or LSCF Cathodes

Anode-supported solid oxide fuel cells with a variety of yttria-stabilized zirconia (YSZ) electrolyte thicknesses were fabricated by tape casting and lamination. Preparation of the YSZ electrolyte tapes with various thicknesses was accomplished by using doctor blades with different gaps between the precision machined, polished blade and the casting surface. The green tape was cut into disks, sintered at 1385°C for 2 h, and subsequently creep-flattened at 1350°C for 2 h. Either (La,Sr)(Co,Fe)0 3 (LSCF) with an Sm 0.2 Ce 0.8 O 1.9 (SDC) interlayer or an (La 0.8 Sr 0.2 ) 0.98 MnO 3 (LSM) + YSZ composite was used as the cathode material for the fuel cells. The ohmic resistances of these anode-supported fuel cells were characterized by electrochemical impedance spectroscopy at temperatures from 500 to 750°C. A linear relationship was found between the ohmic resistance of the fuel cell and the YSZ electrolyte thickness at all the measuring temperatures for both LSCF and LSM + YSZ cathode fuel cells. The ionic conductivities of the YSZ electrolyte, derived for the fuel cells with LSM + YSZ or LSCF cathodes, were independent of the cathode material and cell configuration. The ionic conductivities of the YSZ electrolyte were slightly lower than that of the bulk material possibly due to Ni doping into the electrolyte. The fuel cell with an SDC interlayer and an LSCF cathode showed a larger intercept resistance than the fuel cell with an LSM + YSZ cathode, which was possibly due to the imperfect contact between the SDC interlayer and the YSZ electrolyte and the migration of Zr into the SDC interlayer to form an insulating solid solution during cell fabrication. Calculations of the contribution of the YSZ electrolyte to the total ohmic resistance showed that YSZ was still a satisfactory electrolyte at temperatures above 650°C. Explorations should be directed to reduce the intercept resistance to achieve significant improvement in cell performance.

Enhanced Densification of SDC Barrier Layers

This technical report explores the Enhanced Densification of SCD Barrier Layers A samaria-doped ceria (SDC) barrier layer separates the lanthanum strontium cobalt ferrite (LSCF) cathode from the yttria-stabilized zirconia (YSZ) electrolyte in a solid oxide fuel cell (SOFC) to prevent the formation of electrically resistive interfacial SrZrO{sub 3} layers that arise from the reaction of Sr from the LSCF with Zr from the YSZ. However, the sintering temperature of this SDC layer must be limited to {approx}1200 C to avoid extensive interdiffusion between SDC and YSZ to form a resistive CeO{sub 2}-ZrO{sub 2} solid solution. Therefore, the conventional SDC layer is often porous and therefore not as impervious to Sr-diffusion as would be desired. In the pursuit of improved SOFC performance, efforts have been directed toward increasing the density of the SDC barrier layer without increasing the sintering temperature. The density of the SDC barrier layer can be greatly increased through small amounts of Cu-doping of the SDC powder together with increased solids loading and use of an appropriate binder system in the screen print ink. However, the resulting performance of cells with these barrier layers did not exhibit the expected increase in accordance with that achieved with the prototypical PLD SDC layer. It was determined by XRD that increased sinterability of the SDC also results in increased interdiffusivity between the SDC and YSZ, resulting in formation of a highly resistive solid solution.

Effect of A-site Non-stoichiometry on LSCF Cathodes

LSCF Cathodes were explored when effected with A-site non-stoichiometry. At 700-800 C, the operating temperatures of intermediate temperature (IT-) SOFCs have enabled the use of stainless steels in the SOFC framework and current collectors, allowing significant reductions in cost. However, the lower operating temperatures of IT-SOFCs also result in significant decreases in power densities of cells with LSM cathodes due to their high activation energies. LSCF is a mixed ionic electronic conducting perovskite that exhibits higher performance than LSM/YSZ composites and shows potential as a replacement cathode. This study investigates the effect of A-site stoichiometry on the performance of LSCF cathodes. Cell tests showed that A-site and Sr-deficient LSCF cathodes consistently outperformed stoichiometric LSCF cathodes, exhibiting up to 10% higher cell power densities. It was also observed that all stoichiometric, A-site, and Sr-deficient LSCF cathodes degraded over time at similar rates. Contributions of ohmic and electrode polarization losses to cell degradation rates were similar regardless of cathode composition.