Himeko Orui

Application of LaNi ( Fe ) O 3 as SOFC Cathode

LaNi(Fe)O 3 (LNF) is one of the promising cathode materials for intermediate-temperature solid oxide fuel cells (SOFCs). However, LNF is more reactive with ZrO 2 -based electrolyte than conventional La(Sr)MnO 3 in the sintering temperature region. In this paper, we examine the relationships among LNF sintering characteristics, reactivity, and cell performance to investigate the potential of LNF. We show that the current-voltage characteristics of cells using LNF as the cathode are drastically improved by preliminary loading at a very high current density. The cathodic overvoltage became one order of magnitude smaller after the preliminary loading, and the cells generated the maximum power density of 1.56 W/cm 2 at 800°C.

SOFC Cathodes Composed of LaNi0.6Fe0.4O3 and Pr-Doped CeO2

Solid oxide fuel cell (SOFC) cathodes consisting of LaNi 0.6 Fe 0.4 O 3 and Ce(Ln)O 2-δ (Ln = Pr, Sm, Gd) were investigated with the ac impedance method. The interface resistance of cathodes with Pr-doped ceria was approximately one-third of those with Sm- and Gd-doped ceria. Cathodes with Pr-doped ceria had frequency peaks in the imaginary part impedance spectra that were 100 times lower than those for cathodes without Pr-doped ceria. This low-frequency peak for the Pr-doped ceria was caused by its large interface capacitance. This suggests that triple-phase boundary is present over the entire surface of the Pr-doped ceria particles.

Development of Practical Size Anode-Supported Solid Oxide Fuel Cells with Multilayer Anode Structures

We have developed anode-supported solid oxide fuel cells (SOFCs) using LaNi(Fe)O 3 and scandia-alumina-stabilized zirconia (SASZ) for the cathode and electrolyte, respectively, and employing a mixture of NiO and SASZ for the anode. Anode-supported SOFCs were fabricated using two types of NiO powder with different sintering characteristics to control the anode structure. The cells with anodes made from fine NiO powder provide high power performance, while those with anodes made from coarse NiO powder show good gas diffusion characteristics. Combining the merits of these anodes, we successfully fabricated a cell that simultaneously exhibits high power generation characteristics and smooth gas diffusion through the anode substrate. Moreover, by controlling the cell construction, we minimized the cell warpage. The single cell stack performance when using a metallic alloy manifold showed that the power increased with reduced cell warpage. We fabricated 60, 100, and 120 mmφ cells and obtained high electric power from a single cell stack, which was nearly proportional to the cell size. These cells exhibited an electrical conversion efficiency of 53% lower heating value (LHV) when using hydrogen as a fuel. A long-term stability test was also successfully performed over 6000 h.

LaNi0.6Fe0.4O3–Ceria Composite Cathode for SOFCs Operating at Intermediate Temperatures

We fabricated single cells with a LaNi 0.6 Fe 0.4 O 3 (LNF) cathode and a scandia alumina-doped zirconia electrolyte. Cells with a cathode consisting of a current collecting LNF layer and an active layer were also fabricated by using a screen-printing technique. The active layer contained Ce 0.8 Sm 0.2 O 1.9 (SDC) or Ce 0.9 Gd 0.1 O 1.95 and LNF particles. The influence of the sintering temperature and particle size of LNF and SDC on the cathode properties was investigated. LNF cathodes with this active layer exhibited good levels of performance, including cathode potential, and interface resistance from the start of the operation at 800°C. This configuration also improved the adhesiveness between the cathode and the zirconia electrolyte. We found that the active layer is functional when the ceria particle size is equal to or smaller than that of the LNF. Scanning electron microscope observations of the active layer showed that a high sintering temperature tends to break weak bonds around the triple phase boundary and create strongly connected bonds of LNF and ceria particles in the active layer. AC impedance measurements revealed that this microstructural change reduced the interface resistance and ohmic resistance of the cathode. The reactivity of LNF and SDC in the sintering process was also investigated by X-ray diffraction analysis.

Direct Internal Reforming Characteristics of SOFC with a Thin SASZ Electrolyte and a LNF Cathode

Power generation characteristics and internal reforming characteristics of anode-supported solid oxide fuel cells (SOFCs) with a thin scandia-alumina-stabilized zirconia (SASZ) electrolyte and a lanthanum nickel oxide (LNF) cathode were investigated under the condition of low steam/methane ratios or steam/carbon ratios (S/C). Exhaust gas flow rate of H 2 and CO increased and the rate of CH 4 decreased with increasing current density at S/C = 0.5. The ratio of the gas flow rate increase of H 2 to CO was approximately constant (H 2 /CO = 2), and no apparent carbon formation on the surface of anode was observed. We calculated the exchange current densities from results of anodic overpotential measurements at S/C = 0.5 and 2.0. The exchange current density at S/C = 0.5 differed from the exchange current density at S/C = 2.0 which was similar to the exchange current density in case of using H 2 fuel. These results suggest that the electrochemical reaction in anode at S/C = 0.5 involves the partial oxidation of CH 4 .

Reactivity of LaNi0.6Fe0.4O3 With Samaria Doped Ceria

LaNi 0.6 Fe 0.4 O 3 (LNF) is one of the promising cathodes for solid oxide fuel cells, but reacts with a zirconia-based electrolyte. To prevent this undesirable reaction, a ceria phase has been introduced in between the LNF cathode and electrolyte. On the other hand, the ceria phase itself could react with lanthanum-based perovskite oxides. We examined the reactivity of LNF and Ce 0.8 Sm 0.2 O 2-δ (samaria doped ceria (SDC)) in this study. The mixtures of LNF and SDC were sintered at temperatures between 1123 K and 1623 K and the resultants were analyzed by X-ray diffraction together with the Rietveld analysis. We also measured the activity of electrochemical cells with a LNF-SDC composite layer in between the LNF cathode and zirconia-based electrolyte. The lattice parameters of each phase are clarified and a possible reaction scheme is proposed. The cell activity was high, but was influenced by the sintering temperature of the composite. Both chemical stability and physical property of the cathode can affect the cell activity.

An SOFC Cathode Composed of LaNi 0.6 Fe 0.4 O 3 and Ce(Ln)O 2 (Ln

We fabricated single cells with a cathode consisting of a LaNi 0.6 Fe 0.4 O₃-Ce 0.8 Sm 0.2 O 1.9 composite (LNF-S20DC composite) active layer and an LNF current collecting layer on a 0.89ZrO₂-0.10Sc₂O₃ -0.01Al₂O₃ electrolyte sheet. The cathode layers were prepared by the screen-printing method. The cathode properties of these cells were measured by the AC impedance method at 800℃. The cathodes with the ceria-LNF composite active layer exhibited high power performance prior to current loading. We investigated the influence of the mixture ratio of LNF and S20DC on the cathodes properties. The Sm in the ceria particles of the composite cathode was substituted with other rare-earth elements. Cathodes with Pr and Gd co-doped ceria in the active layer provided the better performance than those with Sm- or Gd-doped ceria.

Thermochemical Stability and Polarization Resistance of La(Ni0.6Fe0.4)O3 Cathode

Polarization resistance of the LaNi0.6Fe0.4O3 (LNF) cathode has been investigated in relation to the electrical conductivity and thermochemical stability. Electrochemical properties of a solid oxide fuel cell using LNF as the cathode were compared to those using La0.8Sr0.2MnO3 (LSM). Temperature dependence of the cathodic polarization resistance was discussed in terms of the thermochemical stability. The electrical conductivity of bulk LNF sample was measured with the four-probe DC method at different oxygen partial pressures. Phase identification for the LNF powder samples treated at 900oC in the atmosphere of oxygen and argon was studied with time dependency of heat treatment by X- ray diffraction analysis. Although the rhombohedral crystal structure of the starting phase was maintained, the electrical conductivity of LNF sensitively changed with the partial pressure of oxygen.