Hibiki Itoh

Thermal Expansion of Nickel‐Zirconia Anodes in Solid Oxide Fuel Cells during Fabrication and Operation

The thermal expansion of NiO-8 mol % Y{sub 2}O{sub 3}-stabilized ZrO{sub 2} (YSZ) composites and Ni-YSZ cermets in air and hydrogen have been investigated in the temperature range from 50 to 1,000 C. The average linear thermal expansion coefficient (TEC) of NiO-YSZ composites in air increased with NiO content over the entire composition range. While NiO in the composites was changed to Ni in the H{sub 2} stream, their expansions were governed by the reduction of NiO. For reduced, Ni-YSZ cermets, the TEC increases significantly with Ni content in the composition range > 60 vol% Ni. The TEC increased gradually during repeated thermal cycles between room temperature and 1,000 C. When cermets were measured in air, the Ni particles were fully oxidized to NiO above 900 C, and many cracks appeared in the samples.

Cubic-stabilized zirconia and alumina composites as electrolytes in planar type solid oxide fuel cells

The electrical, mechanical and thermal properties of the 8 mol% yttria-stabilized zirconia (8YSZ) and alumina composites have been examined as a function of the alumina content. The temperature dependence of bending strength of 8YSZ showed a minimum at 500°C. The strength at 1000°C was 250 MPa, almost comparable to that at room temperature (RT). The strength of the composites increased with increasing alumina content up to 20 wt%. The composite with 20 wt% Al2O3 (20A8YSZ) showed a 3-point bending strength of 320 MPa and an excellent thermal shock resistance. The thermal conductivity of the composite increased and the thermal expansion coefficient decreased with increasing alumina content. The electrical conductivity of the composites at 1000°C increased slightly with increasing Al2O3 content up to 1 wt% and decreased monotonically with alumina content. The conductivity of 20A8YSZ was 1.0×10−1S/cm at 1000°C.

Compatibility of alkaline earth metal (Mg, Ca, Sr)-doped lanthanum chromites as separators in planar-type high-temperature solid oxide fuel cells

The mechanical, electrical and thermal properties of alkaline earth metal (Mg, Ca and Sr)-doped LaCrO3 have been examined as separators in planar-type high-temperature solid oxide fuel cells. The maximum three-point bending strength at 1000°C in air was measured and found to be 186 MPa for LaCr0.9Mg0.1O3, 36 MPa for La0.9Ca0.1CrO3 and 77 MPa for La0.9Sr0.1CrO3. The La0.8Sr0.2CrO3 separator placed in both an oxidizing and a reducing environment at 1000°C showed almost the same electrical conductivities of the H2 atmosphere, and the conductivity was independent of sample thickness in the range 0.5–3.0 mm. For all the doped LaCrO3 perovskites, a difference between the thermal expansion behaviours of air and the H2 atmosphere was observed. In particular, the thermal expansion slope for the first heating cycle under the H2 atmosphere showed a marked change. The volume changes were due to the formation of oxygen defects in the perovskite structure.

Reaction mechanism between lanthanum manganite and yttria doped cubic zirconia

Abstract The reactivity between lanthanum deficient lanthanum manganite, La0.9MnO3, and yttria doped cubic zirconia (YSZ) has been examined at the temperature range between 1250°C and 1400°C. The reaction product of La2Zr2O7 was observed after some induced period, which depended on the reaction temperature and the yttrium content in YSZ. The induced period increased with increasing Y2O3 content in ZrO2. The reaction mechanism has been discussed with reference to the the difference of the solubility limit of Mn and La in YSZ. The solid solution of Mn saturated YSZ showed no reaction product even when reacted with La0.9MnO3 at high temperatures for long periods. All the reaction products formed by the reaction showed lower oxide ion conductivity than that of YSZ and had the different thermal expansion coefficient values from that of YSZ.

Evaluation of Ni and Ti-doped Y2O3 stabilized ZrO2 cermet as an anode in high-temperature solid oxide fuel cells

Abstract The compatibility of nickel and Ti-doped 8 mol% Y 2 O 3 stabilized ZrO 2 (Ni–Ti–YSZ) cermets has been discussed for use as an anode in high-temperature solid oxide fuel cells (SOFC). The solubility limit of TiO 2 in the YSZ fluorite was estimated to be at least 10 mol%. With increasing Ti content in the Ti–YSZ samples, the average linear thermal expansion coefficient was found to decrease when measured in an H 2 atmosphere. In an H 2 atmosphere, the oxygen vacancy concentration was observed to increase with increasing Ti content and temperature, and its value in the 8 mol% Ti-doped YSZ (8Ti–YSZ) sample was 0.0056 at 1000 °C. Although electronic conduction appeared in the Ti–YSZ samples at temperatures greater than ∼600 °C in the H 2 atmosphere, their electrical conductivities had a tendency to decrease with increasing Ti content. The mechanical strength of the Ni–Ti–YSZ cermets at room temperature were found to increase with increasing Ti content. Anodic polarization of the 40 vol.% Ni–8Ti–YSZ cermet at 1000 °C was 60 mV at 300 mA cm −2 in the H 2 atmosphere after firing at 1400 °C for 10 h.

Production cost estimation of solid oxide fuel cells

Abstract Production costs of two types of solid oxide fuel cells (SOFCs) were estimated to clarify their economical prospects. In this assessment 2000 kW of SOFC capacity is assumed to be manufactured per month. Single cells of a planar type with 0.21 W/cm2 are fabricated by co-firing of doctor blade layers and the separators are prepared by firing of press-moulding bodies. Stacks with 400 W consists of 20 cells and 21 separators. The materials cost of this type covers nearly 60% of the total (118 000 yen/kW). By comparison, the tubular SOFCs with 0.18 W/cm2 are assumed to be made using an electrochemical vapour deposition (EVD) method or a plasma spraying (PS) method on porous support tubes. The module with 20 tubes is built up by arranging the cells in a suitable order. The support tube is fired after continuous isostatic pressing. Including the cost of support tubes, the overall costs for tubular cell systems by the EVD method and by the PS method are about 617 000 yen/kW and 293 000 yen/kW, respectively.

Chemical stability between NiO/8YSZ cermet and alkaline-earth metal substituted lanthanum chromite

Abstract The reaction between the separator in solid oxide fuel cells, alkaline-earth substituted lanthanum chromite (LaCr 0.8 Mg 0.2 O 3 ,La 0.8 Ca 0.2 CrO 3 and La 0.8 Sr 0.2 CrO 3 ), and the fuel electrode, NiO8YSZ cernet, was investigated at high temperature. No chemical reaction was observed at the fuel electrode/separator interface of all samples annealed in a temperature range up to 1350°C. In the combination of fuel electrode and La 0.8 Sr 0.2 CrO 3 annealed at 1400°C, no diffusion of specific elements was observed at the interface even for an annealing period of 12 h. In the case of the fuel electrode and La 0.8 Ca 0.2 CrO 3 , which was annealed at 1400°C, a dissolution of Ca into 8YSZ of the NiO/8YSZ cermet was observed at the NiO/8YSZ side of the interface. For the couple of the fuel electrode and LaCr 0.9 Mg 0.1 O 3 treated at 1400°C, the Ni concentration increased and the Mg concentration decreased at the LaCr 0.9 Mg 0.1 O 3 side of the interface. It was expected that a LaCr 1− x Ni x O 3 solid solution was formed by the replacement reaction of Mg by Ni.