Takehisa Fukui

Morphology Control of Ni-YSZ Cermet Anode for Lower Temperature Operation of SOFCs

Abstract A NiO–Y2O3 stabilized ZrO2 (YSZ) composite particles for solid oxide fuel cell (SOFC) anode was fabricated by advanced mechanical method in dry process. The processed powder achieved better homogeneity of NiO and YSZ particles, where submicron NiO particles were covered with finer YSZ particles. A Ni–YSZ cermet anode fabricated from the NiO–YSZ composite particles showed the porous structure in which Ni and YSZ grains of less than several hundred nano-meter as well as micron-size pores were uniformly dispersed. The cermet anode achieved high electrical performance at low temperature operation (

Solid Oxide Fuel Cells with Doped Lanthanum Gallate Electrolyte and LaSrCoO3 Cathode, and Ni‐Samaria‐Doped Ceria Cermet Anode

The electrode performance of a single solid-oxide fuel cell (SOFC) was evaluated using a 500 {micro}m thick La{sub 0.9}Sr{sub 0.1}Ga{sub 0.8}O{sub 0.3} (LSGM) as the electrolyte membrane. A doped lanthanum cobaltite, La{sub 0.6}Sr{sub 0.4}CoO{sub 3{minus}{delta}} was selected as the cathode material, and a samaria-doped ceria-NiO composite powder was used as the anode material. The spray-pyrolysis method was applied for synthesis of the starting powders of the cathode and anode. In this study, different microstructures of the cathode were obtained by varying the sintering temperature from 950 to 1200 C. High power density (the maximum power density of the cell was about 425 mW/cm{sup 2}, which is 95% of the theoretical value) of the solid oxide fuel cell at 800 C was achieved. The cell performance showed that, with a proper choice of electrode materials with optimized microstructure and LSGM as the electrolyte, a SOFC operating at temperatures T{sub op} {le} 800 is a realistic goal.

Ni-SDC cermet anode for medium-temperature solid oxide fuel cell with lanthanum gallate electrolyte

The polarization properties and microstructure of Ni-SDC (samaria-doped ceria) cermet anodes prepared from spray pyrolysis (SP) composite powder, and element interface diffusion between the anode and a La0.9Sr0.1Ga0.8Mg0.2O3−δ (LSGM) electrolyte are investigated as a function of anode sintering temperature. The anode sintered at 1250°C displays minimum anode polarization (with anode ohmic loss), while the anode prepared at 1300°C has the best electrochemical overpotential, viz., 27 mV at 300 mA cm−2 operating at 800°C. The anode ohmic loss gradually increases with increase in the sintering temperature at levels below 1300°C, and sharply increases at 1350°C. Electron micrographs show a clear grain growth at sintering temperatures higher than 1300°C. The anode microstructure appears to be optimized at 1300°C, in which nickel particles form a network with well-connected SDC particles finely distributed over the surfaces of the nickel particles. The anode sintered at 1350°C has severe grain growth and an apparent interface diffusion of nickel from the anode to the electrolyte. The nickel interface diffusion is assumed to be the main reason for the increment in ohmic loss, and the resulting loss in anode performance. The findings suggest that sintering Ni-SDC composite powder near 1250°C is the best method to prepare the anode on a LSGM electrolyte.

High performance electrodes for reduced temperature solid oxide fuel cells with doped lanthanum gallate electrolyte I. Ni-SDC cermet anode

The reduced temperature solid oxide fuel cell (SOFC) with 0.5 mm thick La0.9Sr0.1Ga0.8Mg0.2O3−α (LSGM) electrolyte, La0.6Sr0.4CoO3−δ (LSCo) cathode, and Ni-(CeO2)0.8(SmO1.5)0.2 (SDC) cermet anode showed an excellent initial performance, and high maximum power density, 0.47 W/cm2, at 800°C. The results were comparable to those for the conventional SOFC with yttria-stabilized zirconia (YSZ) electrolyte, La(Sr)MnO3-YSZ cathode and Ni–YSZ cermet anode at 1000°C. Using an LSCo powder prepared by spray pyrolysis, and selecting appropriate sintering temperatures, the lowest cathodic polarization of about 25 mV at 300 mA/cm2 was measured for a cathode prepared by sintering at 1000°C. Life time cell test results, however, showed that the polarization of the LSCo cathode increased with operating time. From EPMA results, this behavior was considered to be related to the interdiffusion of the elements at the cathode/electrolyte interface. Calcination of LSCo powder could be a possible way to suppress this interdiffusion at the interface.

Properties of NiO cathode coated with lithiated Co and Ni solid solution oxide for MCFCs

Abstract The short circuit of cells due to the dissolution and the deposition of NiO in a molten carbonate is a big problem that has to be resolved if we want to use Molten Carbonate Fuel Cells (MCFCs) for long-term operation. To resolve this problem, we have proposed a new cathode structure in which NiO grains have been coated with LiCoO 2 grains by usual tape casting method, because LiCoO 2 is more stable in a carbonate melt than NiO. CoO/NiO composites with 5 to 20 mass% Co could be formed by using the CoO/Ni composite particles. As confirmed by SEM, these composites consisted of NiO cores covered with the outer layer of Li(Co, Ni) oxide. The stability of the lithiated CoO/NiO composite with 5 mass% CoO was significantly improved in comparison with that of conventional NiO for MCFCs. The lithiated CoO/NiO composite was also successfully formed by in-situ oxidation and lithiation process. We thought that this composite cathode material consisting of NiO core covered with Li(Co, Ni) oxide will be adopted as the new cathode for MCFCs.

Interface reactions in the NiO–SDC–LSGM system

Abstract The reactivity of NiO–SDC (samaria-doped ceria) anode material with a Sr- and Mg-doped lanthanum gallate (LSGM) electrolyte was studied by X-ray diffraction (XRD) and electrical measurements. It was found that a LaNiO3-based compound in hexagonal structure formed in binary powder mixtures of NiO and LSGM after firing at 1150°C. Reaction between SDC and LSGM was also observed. Several SDC peaks merged with the adjacent LSGM peaks during firing, and a SrLaGa3O7 compound was identified as a reaction product. Reaction between LSGM and SDC could cause more than 50% loss in the ionic conductivity of LSGM–SDC electrolytes sintered at 1350°C. The measured conductivity of an LSGM electrolyte with a NiO–LSGM anode prepared at 1350°C was extremely low, indicating that the LaNiO3-based new phase is highly insulating. The reaction between NiO and SDC was not so obvious in comparison with NiO–LSGM and SDC–LSGM binary mixtures.

Interactions of a La0.9Sr0.1Ga0.8Mg0.2O3−δ electrolyte with Fe2O3, Co2O3 and NiO anode materials

Abstract In this study, the interactions of a Sr- and Mg-doped lanthanum gallate (LSGM with composition La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3− δ ) electrolyte with Fe 2 O 3 , Co 2 O 3 and NiO as the anode starting materials were investigated. It was found that the order of reactivity of the LSGM with the three oxides was Co 2 O 3 >NiO>Fe 2 O 3 , and La-containing oxides were detected in these binary powder mixtures after firing. The anode performance was greatly influenced by the interaction. The Fe 2 O 3 –LSGM anode, mixed with 40 vol.% LSGM powder and sintered at 1150°C, exhibited the highest initial performance in comparison with NiO–LSGM and Co 2 O 3 –LSGM anodes. It seems that Fe 2 O 3 is a possible anode starting material for a LSGM-based solid oxide fuel cell.

Performance and stability of SOFC anode fabricated from NiO/YSZ composite particles

Abstract NiO/YSZ composite particles with 20–35 mol% Y2O3 stabilized ZrO2 (YSZ) were synthesized by the spray pyrolysis (SP) method. The NiO/YSZ composite particles showed morphology of NiO grains partially or fully covered with fine YSZ grains. The composite particles also had good interface connection between nickel oxide and yttria stabilized zirconia (YSZ). The morphology of the Ni/YSZ cermet anode fabricated from composite particles is noticeably influenced by the YSZ content of the composite particles. Ni/YSZ cermet anodes with 25 mol% YSZ show the highest electrochemical activity and lowest internal resistance (IR). The electrochemical activity and IR of the anode in solid oxide fuel cells (SOFCs) highly depend on the three-phase boundary structure based on the metal (Ni) and ceramic (YSZ) interface as well as the network structure of each Ni grains. Moreover, the cell voltage of a single cell with the Ni/YSZ cermet anode kept almost constant value during 7200 h of operation. Consequently, it is concluded that the performance and stability of the Ni/YSZ cermet anode is improved by controlling the morphology of the NiO/YSZ composite particles.

Synthesis of NiO–YSZ composite particles for an electrode of solid oxide fuel cells by spray pyrolysis

Abstract NiO–YSZ (YSZ: Y2O3-stabilized ZrO2) composite particles for a Ni–YSZ cermet anode in solid oxide fuel cells (SOFCs) were synthesized via spray pyrolysis (SP). The formation mechanism of the composite particles by this process was analyzed. The internal microstructure of the particles synthesized during SP processing was observed at each heating temperature of 200, 300, 400 and 1000 °C, and then the formation mechanism of the composite structure was discussed. As a result, it was found that NiO–YSZ composite particles were formed through the following steps. Firstly, during the evaporation stage up to 200 °C, a filled particle with Ni(CH3COO)2 and YSZ fine grains were formed from the atomized droplet containing Ni ion and dispersed YSZ sol by volume precipitation. Secondly, during the continuous thermolysis stage up to 400 °C, YSZ grains were formed and moved to the surface of the composite particle by the outgas and the oxidation of Ni(CH3COO)2. Finally, the NiO–YSZ composite particle that has NiO grains uniformly covered with fine YSZ grains was formed after the final sintering stage up to 1000 °C.

Direct EPD of YSZ Electrolyte Film onto Porous NiO-YSZ Composite Substrate for Reduced-Temperature Operating Anode-Supported SOFC

Electrophoretic deposition (EPD) was used for the fabrication of anode-supported yttria-stabilized zirconia (YSZ) electrolyte films. For the EPD, thin layers of graphite were pre-coated on the surface of a nonconducting porous NiO-YSZ composite anode substrate. Uniform YSZ green films were formed on the reverse sides, which did not have the graphite layers. The specimens were transformed into dense bodies ∼5 to 10 μm thick after being co-fired with the substrates. The cell performance of the ∼5 μm thick dense YSZ films supported on the anode substrates was tested using a La(Sr)Co(Fe)O 3 cathode. Maximum output power densities of ∼0.19, ∼0.61, and ∼1.02 W/cm 2 were attained at 600, 700, and 800°C, respectively.