Reiichi Chiba

An investigation of LaNi1-xFexO3 as a cathode material for solid oxide fuel cells

Abstract We prepared La(Ni–M)O 3 (M=Al, Cr, Mn, Fe, Co, Ga) samples and investigated their characteristics as solid oxide fuel cell (SOFC) cathode materials. We found that the LaNi 1− x Fe x O 3 satisfies the important requirements for a cathode, if x in this system is around 0.4. This system has its highest electronic conductivity of 580 S/cm at 800°C, when x in LaNi 1− x Fe x O 3 is 0.4. This value is more than three times higher than that of La 0.8 Sr 0.2 MnO 3 (180 S/cm). The average thermal expansion coefficient of LaNi 0.6 Fe 0.4 O 3 from 30 to 1000°C is relatively low (11.4×10 −6 K −1 ) and closer to the 0.92 ZrO 2 –0.08 Y 2 O 3 (YSZ) value (10.0×10 −6 K −1 ) than La 0.8 Sr 0.2 MnO 3 (12.0×10 −6 K −1 ).

Cr Poisoning Suppression in Solid Oxide Fuel Cells Using LaNi ( Fe ) O3 Electrodes

We have investigated the effect of Cr poisoning on various cells which consist of either LaNi 0 . 6 Fe 0 . 4 O 3 (LNF) or La 0 . 8 Sr 0 . 2 MnO 3 (LSM) as a cathode and either yttria-stabilized zirconia (YSZ) or alumina-dopedscandia stabilized zirconia (SASZ) as the electrolyte. It was found that the cathodic overvoltage was almost the same for cells with LNF cathodes, either in the presence or absence of Cr-containing alloy (Inconel 600). LSM cathodes in the presence of Inconel 600 exhibited a very steep increase in voltage curves after applying the current. These results indicate that the LNF cathode is electrochemically stable for Cr poisoning compared with the LSM cathode.

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.

Properties of La1−ySryNi1−xFexO3 as a cathode material for a low-temperature operating SOFC

Abstract We synthesized Sr-doped La(Ni–Fe)O 3 , or La 1− y Sr y Ni 1− x Fe x O 3 (LSNF), and investigated the composition and temperature dependence of the electronic properties and crystal structure of LSNF. The perovskite structure of LSNF is stable even at 1400 °C and in a single phase when its Fe concentration is higher than 0.5. The crystal structure and electronic conductivity of LSNF strongly depend on the composition. When the total concentration of Ni and Sr in this system is less than 0.4, it is in the orthorhombic phase. But the crystal structure symmetry improves (to the rhombohedral and then cubic phase) as the total concentration of Ni and Sr increases. The electronic conductivity values at 600 and 800 °C have sharp peaks when the composition is used as a parameter. The peak values are obtained when the total concentration of Ni and Sr is about 0.6 in this temperature range. The LSNF conductivity when the Ni and Sr concentration is high exhibits metallic temperature dependence. The conductivities drop sharply when the temperature exceeds about 600 °C. This implies that the oxygen vacancy concentration in the samples increases sharply at 600 °C or higher.

Temperature dependence of ionic conductivity in (1 − x)ZrO2−(x − y)Sc2O3−yYb2O3 electrolyte material

The temperature dependence of the ionic conductivity of (1 − x)ZrO2−(x − y)Sc2O3−yYb2O3 (x = 0.07–0.15, y = 0.00–0.03) was examined to clarify the origins of the ionic conductivity decrease at low temperature. For y = 0.02–0.03 in this system, the cubic phase was stabilized at room temperature and discontinuity in the ionic conductivity disappears. The Arrhenius plots of the ionic conductivity for the stabilized samples were curved. The grain boundary resistance for (1 − x)ZrO2−(x − 0.03)Sc2O3−0.03Yb2O3 (x = 0.07–0.15) was separated from the total resistance with the AC impedance method. This allowed us to conclude that grain boundary resistance is not the cause of the curvature in the Arrhenius plots for this system.

Long-term chemical stability of LaNi(Fe)O3 as a cathode material in solid oxide fuel cells

Chemical reactivity of LaNi 0.6 Fe 0.4 O 3 (LNF) with Cr 2 O 3 has been investigated in order to examine the long-term stability of the LNF phase at 1073 K, which shows a high performance as the cathode of intermediate-temperature solid oxide fuel cells (IT-SOFCs). The chemical stability of LNF was compared with that of La 0.8 Sr 0.2 MnO 3 (LSM) under the existence of Cr 2 O 3 . The LNF powder, a powder mixture of LNF and Cr 2 O 3 , and a powder mixture of LSM and Cr 2 O 3 were, respectively, heated at 1073 K, and they are analyzed by X-ray powder diffraction with the Rietveld refinement. We found that the LNF phase maintains the hexagonal crystal system heated in air up to 1000 h. In the LNF-Cr 2 O 3 mixture, the LNF phase kept its pristine crystal structure while a new phase, NiCr 2 O 4 , was detected, which was produced by the reaction between Cr 2 O 3 and the residual NiO contained in the starting LNF powder. In the LSM-Cr 2 O 3 mixture, LSM reacted with Cr 2 O 3 and produced a significant amount of MnCr 2 O 4 . LNF showed a much better chemical stability against Cr 2 O 3 than LSM. LNF can serve as a long-life cathode in the IT-SOFC system.

Ionic conductivity and morphology in Sc2O3 and Al2O3 doped ZrO2 films prepared by the sol–gel method

Abstract We used the sol–gel method to deposit zirconia films doped with Sc2O3 and Al2O3 on alumina substrates. We investigated the annealing temperature dependence of the ionic conductivity and morphology in the films whose composition was 0.85ZrO2–0.11Sc2O3–0.04Al2O3. The film thickness was controlled from about 0.1 to 1.0 microns by controlling the coating time. The films prepared by this method are stabilized in the cubic phase. Annealing at 1200°C produces isotropic and well-sintered films. The ionic conductivity of the annealed film was 7.6×10−2 S/cm at 800°C, which is comparable to that of bulk samples prepared by solid reaction at 1620°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.