Mitsunobu Kawano

Effect of SO2 Concentration on Degradation of Sm0.5Sr0.5CoO3 Cathode

A trace amount of SO2 in air is not a negligible factor on degradation of SOFC cathode. SO2 acts like Cr vapors which cause Cr-poisoning, and easily reacts with the strontium component in lanthanum-cobaltite based SOFC cathodes. Effects of SO2 concentration on degradation of an Sm0.5Sr0.5CoO3 (SSC) cathode were investigated in order to evaluate degradation of SSC cathode in an accelerated mode. Three different concentrations of SO2 (100 ppm, 1 ppm, and 5 ppb) were supplied to the cathode with air during cell tests at T = 1073 K Degradation of SSC cathode was accelerated with increasing SO2 concentration. In the thin SO2 conditions (1 ppm and 5 ppb SO2), SO2 in air was captured on the surface of the SSC cathode. SO2 reacted with the strontium component in the SSC cathode to form SrSO4. In the thick SO2 condition (100 ppm SO2), SO2 reacted with the strontium and samarium components. Too excess SO2 concentration in air was not suitable to examine cathode degradation in an accelerated mode because other degradation modes were appeared. One ppm - SO2 in air was applicable to accelerate the degradation of SSC cathode.

Internal Reforming of SOFCs Carbon Deposition on Fuel Electrode and Subsequent Deterioration of Cell

Several cermet anode materials were investigated for direct internal reforming operation of solid oxide fuel cells. The effects of steam and discharge conditions were examined for internal reforming operation with propane as a hydrocarbon fuel. Even at high steam-to-carbon (S/C) ratio, power generation characteristics with Ni-yttria-stabilized zirconia (YSZ) were deteriorated during internal reforming operation with propane at 1000°C, possibly due to carbon formation at low current densities, whereas stable generation behavior was observed for the cells with Ni-scandia-stabilized zirconia (ScSZ) or Ni-samaria-doped ceria (SDC) anode by feeding propane at low S/C = 0.8. The carbon deposition rate was measured for several cermet anode materials by feeding dry or humidified methane by the gravimetric technique. The carbon deposition rate over Ni-ScSZ was lower than Ni-YSZ, leading to better generation performance with propane at low S/C. Ni-SDC showed the highest carbon deposition rate among the cermets investigated, even for humidified methane. This implies that carbon formed over Ni-SDC could be effectively removed upon power generation.

Direct Internal Steam Reforming at SOFC Anodes Composed of NiO – SDC Composite Particles

The dependence of anode catalytic activities on the fuel species and the operating temperatures was investigated for use in thedirect internal steam-reformed power generation. Solid oxide fuel cells SOFCs employing Ni–samaria-doped ceria SDC cermetanode synthesized by spray pyrolysis were fabricated and operated in the temperature range 700–1000°C by feeding the humidi-fied hydrogen and methane as fuels. Cell performances with humidified hydrogen fuel were higher than those with humidifiedmethane fuel at every operation temperature. The performances at 700 and 800°C were much lower than that at 1000°C when thehumidified methane fuel was supplied. Considering the results of the electrochemical characterization and microstructural obser-vation of the anode, methane conversion and the deposited carbon appeared to significantly affect cell performances.© 2007 The Electrochemical Society. DOI: 10.1149/1.2712128 All rights reserved.Manuscript submitted October 13, 2006; revised manuscript received December 29, 2006.Available electronically March 22, 2007.

Development of Intermediate-Temperature SOFC Module Using Doped Lanthanum Gallate

An intermediate temperature solid oxide fuel cell (SOFC) module was developed using electrochemically active cells composed of (La, Sr)(Ga, Mg, Co)O 3 electrolyte, Ni-(Ce, Sm)O 2 anode, and (Sm, Sr)CoO 3 cathode. Seal-less planar type stack design was employed. The first generation module successfully provided the output power of I kW with thermal self-sustainability below 800°C. Maximum electrical efficiency obtained with this module was 43%[LHV] together with the corresponding fuel utilization of 78%. Dynamic performance tests demonstrated the capability of output power alteration from 0.6 to 1 kW while maintaining a high electrical conversion efficiency. Further testing and modification of the module for methane fuel utilization are in progress.

Carbon Deposition over Ni–ScSZ Anodes Subjected to Various Heat-Treatments for Internal Reforming of Solid Oxide Fuel Cells

The influence of the preparation condition of Ni-scandia-stabilized zirconia (Ni-ScSZ) cermet on the tolerance to carbon deposition was investigated in the wide temperature range of 800-1000°C. The mixture of NiO-ScSZ was subjected to preheat-treatments prior to use as a cermet electrode. Although the inhibitory effect of ScSZ in Ni-based cermet for carbon deposition is well known, the present study revealed that the preparation method and firing condition influenced the carbon deposition behavior. The Ni-ScSZ cermet obtained from the oxide mixture heated at 1400°C showed peculiar behavior to carbon deposition. The initial carbon deposition rate at 800-900°C for this sample was larger than that at 1000°C, whereas other samples exhibited a reduction in deposition rate with decreasing temperature. The cubic phase stabilization in ScSZ was confirmed for the sample treated at 1400°C, though ScSZ was in the rhombohedral phase before the heat-treatment with NiO. These results suggested that a strong interaction between NiO and ScSZ was induced by the high-temperature heat-treatment.

Influence of Preparation Methods on the Carbon Deposition and Reduction Behavior of Ni–ScSZ Cermet

Nickel oxide and scandia-stabilized zirconia (NiO-ScSZ) composite powders were prepared through two different processes: mechanical powder mixing (PM) and coprecipitation (CP). The effect of calcination temperatures on the crystalline structure of as-prepared powders and the carbon deposition behavior over the cermets were investigated. Due to the finer precipitates obtained from the CP method, the solid-state reaction between NiO and ScSZ in the as-prepared powder was promoted even at low calcination temperatures, resulting in the stabilization of the cubic phase of ScSZ. The temperature-programmed reduction indicated the strong interaction between NiO and ScSZ in the composite subjected to high temperature treatment regardless of the preparation methods, CP and PM. Phase identification was also conducted for the composites after reduction treatment at high temperatures of 1000 and 1400°C. After reducing at 1400°C, a part of ScSZ in the composite from the CP method transformed from the cubic to the rhombohedral phase, whereas the cubic phase was stable for the composite from the PM method. In addition, the carbon deposition over the sample from the CP method was promoted due to the large surface area of Ni. However, the electrochemical performance of single cells was independent of the cermet anodes from different methods with a supply of both H 2 and CH 4 fuels.

SOFC Module and System Development by Means of Sealless Metallic Separators with Lanthanum Gallate Electrolyte

The third-generation 1-kW e -class module was developed with an automatic control system. A conversion efficiency of 48% ac/lower heating value [ac/LHV] was achieved with an exhaust heat recovery unit. An endurance test using the third-generation 1-kW e module was done for over 1000 h and no degradation of the power generation performance was observed. In parallel, a single-cell unit, which includes one cell and two metallic separators, was tested for over 10000 h and the degradation rate of the terminal voltage was found to be 1-2%/1000 h. In the direction of scale-up, a triple-stack module of 3-kW e output was developed. A partial load as well as excess loads on the module were tested and the output power of 1-5 kW e was attained under thermally self-sustainable conditions. It was found that a high efficiency of 55% dc/lower heating value [dc/LHV] was obtained under stable operation. Ongoing research of the fourth-generation 1-kW e module has resulted in the conversion efficiency of 58% [dc/LHV].

Carbon Dioxide Reforming of Methane on Ni-ceria-based Oxide Cermet Anode for Solid Oxide Fuel Cells

Carbon dioxide reforming of methane on Ni-ceria-based oxide cermet anode of solid oxide fuel cells was carried out. Electrochemical characterizations revealed that direct internal carbon dioxide reforming operation was strongly influenced by the operation temperature. Direct internal carbon dioxide reforming operation resulted in sufficient level of conversion for power generation with methane at 750aC, while it gave insufficient level of conversion at 700 and 650aC. The results of gas analysis at 750°C also reveal that methane was effectively converted to hydrogen and carbon monoxide in the radial direction of the cell, and that hydrogen and carbon monoxide were effectively used as fuel species for electrochemical oxidation reactions.

Fabrication and Characterization of Ni-Doped Ceria Anode-Supported Cells Using Lanthanum Gallate-Based Electrolyte

Lanthanum gallate-based compounds (LSGM) are drawing attention as electrolyte materials of intermediate temperature SOFC that operates with 600 800oC (1). The ionic conductivity of LSGM at 800oC is comparable to that of yttria-stabilized zirconia (YSZ) at 1000oC. On the other hand, the attempt to apply YSZ to intermediate temperature operation SOFC is widely done. It is known that good performance can be obtained at even 800oC or lower because IR drop of the electrolyte is minimized by stacking and co-sintering of the NiO-YSZ anode sheet and YSZ electrolyte sheet (2,3). It is expected that the cell with higher performance can be obtained by thinning the electrolyte even when LSGM is used as an electrolyte. However, it has a problem that the reaction product having high electric resistance (LaNiO3based material) is formed by the reaction of Ni in the anode and La in the electrolyte during co-sintering of the Ni-containing anode sheet and the LSGM electrolyte sheet. Then, the investigation on controlling the diffusion of La was carried out by inserting the La-doped ceria (LDC) interlayer between the Ni-containing anode and the LSGM electrolyte (4). If the doped-ceria compound with higher ionic conductivity is used, the cell performance is expected to improve further, because the ionic conductivity of LDC is known to be relatively low among the rare-earth doped ceria compounds (5). In this study, therefore, the anode-supported cells with the doped-ceria compounds with higher ionic conductivity than LDC and with LSGM electrolyte were fabricated and the cell performances were measured. NiO-Ce0.8Sm0.2O1.9 (SDC) anode substrate sheet were prepared by doctor-blade technique. NiO and SDC powder mixture was ball-milled with organic binder and plasticizer to obtain the slurry. The slurry was cast and dried to be the seat. The typical thickness of the resulting sheets was 0.32 mm. The sheet was cut to be disks of 42 mm in diameter, and the slurry of the doped-ceria compound for the interlayer was screen-printed on one side of the disks (described as Sheet A). The slurry of LSGM (La0.9Sr0.1Ga0.8Mg0.2O2.85, prepared by solid-state reaction method of each oxide) was screen-printed on Sheet A, and co-sintered at 1400oC, which was described as Sample 1. On the other hand, Sheet A was sintered at 1500oC, and then LSGM slurry was screen-printed and sintered at 1400oC (Sample 2). The slurry of Sm0.5Sr0.5CoO3-δ (SSC) cathode was screenprinted on LSGM and sintered at 1100oC. The samples after cathode sintering of Sample 1 and 2 are described as Cell 1 and 2, respectively. Fig. 1 shows the SEM image of the cross-section of Cell 1. Good performance was expected as the electrolyte was dense in spite of the pores in the interlayer. However, the cell performance was lower than that of the standard LSGM cell. In order to confirm the chemical reaction of the electrolyte and the anode, the surface of the LSGM before cathode sintering was analyzed by Xray diffraction. The XRD pattern looked like that of LaNiO3 rather than that of LSGM, therefore, the highly resistive phase was formed by the interdiffusion of La and Ni. Then, instead of co-sintering, the sheet composed of the anode sheet and the interlayer was sintered at the higher temperature than that of Cell 1 before the screen-printing of the LSGM slurry. Fig. 2 shows the SEM image of the cross-section of Cell 2. The interlayer of Cell 2 was not so dense sintered even at 1500oC, and the performance of Cell 2 was lower than that of the standard cell. The doped-ceria powder of higher sinterability is demanded, and the improved cell is being fabricated. The result using the improved cell will be presented at the meeting.

Residual Stress Analysis in Lanthanum Gallate-Based Cells before and after Fuel Cell Operation

The distribution of residual stress in cells with the practical size of 120 mm diameter was measured by X-ray diffraction utilizing synchrotron radiation at Super Photon ring 8 GeV (SPring-8). The residual stress was determined by the constant penetration depth method. When NiO-samaria-doped ceria (SDC) was sintered on (La,Sr)(Ga,Mg,Co)O 3-δ (LSGMC), the tensile stresses resided in NiO and SDC, and the compressive stress resided in LSGMC. The reduction of the anode by hydrogen decreased tensile stresses in the anode. The residual stresses in the electrodes did not change at any X-ray penetration depth. The sintering of the cathode slightly affected the residual stresses of the cells.