Yusuke Shiratori

Alternative electrocatalyst support materials for polymer electrolyte fuel cells

Durability of electrocatalysts under severe operational conditions of polymer electrolyte fuel cells (PEFCs) is one of the most important technological issues to be improved. Alternative electrocatalyst support materials are desired, the use of which may solve problems such as oxidation-induced carbon support corrosion. One interesting solution is to use conductive oxide-based catalyst support. We focus our attention to SnO2, known as an oxide semiconductor with a high electronic conductivity. Nano-carbon support materials are also studied as an another option of electrocatalyst support design. Technical merits and remaining issues in alternative electrocatalyst support materials selection are discussed.

Chlorine Poisoning of SOFC Ni-Cermet Anodes

Poisoning effects by chlorine compounds, including Cl 2 and HCl, to typical electrolyte-supported solid oxide fuel cells (SOFCs) with Ni-scandia-stabilized zirconia cermet anodes have been evaluated. The degradation rate of cell voltage poisoned by 5 ppm Cl 2 was estimated to be ca. 3%/1000 h. Chlorine degradation rate increased with increasing Cl 2 concentration. Microstructural change associated with the formation of nickel nanoparticles on zirconia grains, probably via NiCl 2 (g) sublimation, was observed after 150 h poisoning tests, whereas a partial recovery of cell voltage by removing chlorine from the fuels indicates that the chlorine poisoning is partially reversible. Thermochemical calculations, microstructural analysis, and electrochemical characterizations have revealed that the poisoning phenomena for Ni-based cermet anodes caused by chlorine compounds can be explained by the mixed adsorption-type and sublimation-type poisoning mechanism.

Phosphorus Poisoning of Ni-Cermet Anodes in Solid Oxide Fuel Cells

Chemical degradation of anodes in solid oxide fuel cells (SOFCs) induced by phosphorous impurities has been investigated by thermochemical calculations, electrochemical characterization, and microstructural analysis. Thermochemical calculations indicate that Ni phosphides (Ni m P n ) such as Ni 5 P 2 are readily formed under SOFC operational conditions even in the case where the concentration of phosphorous impurities is as low as ppb level. Phosphorous impurities in fuels resulted in the formation of the secondary phase Ni phosphides during operation especially at an operating temperature ~ 1000°C. The formation of Ni phosphides and change in anode microstructure were confirmed by using a field-emission-scanning electron microscope coupled with an energy dispersive X-ray analyzer, scanning transmission electron microscope, and X-ray diffraction, leading to the fatal cell performance degradation in the manner that anodic overpotential increased, current incorporation was obstructed, and internal fuel reforming reaction was deactivated.

Degradation of Solid Oxide Fuel Cell Cathodes Accelerated at a High Water Vapor Concentration

The influence of water vapor in air on power generation characteristic of solid oxide fuel cells was analyzed by measuring cell voltage at a constant current density as a function of water vapor concentration at 800°C and 1000°C. Cell voltage change was negligible at 1000°C, while considerable voltage drop was observed at 800°C accelerated at high water vapor concentrations of 20 wt % and 40 wt %. It is considered that La 2 O 3 formed on the (La 0.8 Sr 0 . 2 ) 0 . 98 MnO 3 surface, which is assumed to be the reason for a large voltage drop.

Discharge characteristics of planar stack fuel cells

A fuel cell system with planar stack configuration is studied with a view of portable applications. A mathematical model to estimate the discharge characteristics of the system is presented in this research to clarify the effect of the number of segmented cells on the output power and energy losses generated in the system. In the isothermal case of this analysis, we revealed that output power of a planar stack SOFC increases with increasing the number of cell segments because the current distribution in the electrolyte becomes more uniform by increasing the number and this results in a reduction of energy losses in the system.

Exchange Current Density of Solid Oxide Fuel Cell Electrodes

It is desired to develop computational procedures to simulate internal current density, anode/cathode gas concentrations, and temperature distribution in solid oxide fuel cell (SOFC) systems. In this study, the influences of various operational conditions on the exchange current density, the essential parameter to simulate SOFC performance, are revealed and discussed. The anodic exchange current density depended strongly on the humidity of H2-based fuel gas, and it exhibited the highest value at around 40% H2O. The cathodic exchange current density was strongly affected by the operational temperature. Parameters necessary to describe dependencies of exchange current density on various operational parameters were determined by fitting measured exchange current density values with empirical equations.

Carbon-free Pt Electrocatalysts Supported on SnO2 for Polymer Electolyte Fuel Cells

Oxidation-induced carbon support corrosion especially in cathode electrocatalysts for polymer electrolyte fuel cells (PEFCs) is one of the important technological issues to be solved. We focus our attention to SnO2, known as a broad-band oxide semiconductor with a high electronic conductivity, as a possible electrocatalyst support material. In this study, nanostructure, electrochemical properties and durability of carbon-free electrocatalysts, Pt/SnO2, Pt/Nb-SnO2, Pt/Al-SnO2, are investigated.

Effect of Water Vapor and SOx in Air on the Cathodes of Solid Oxide Fuel Cells

Ambient air is used as an oxygen source in SOFCs to be commercialized. Various chemical species which can lead to poisoning of SOFC cathodes are included as minor constitutions in air, such as water vapor, SOx, NOx and NaCl etc. However, their effects on the cathode performance have not yet well known, even though they are expected to cause a degradation of the electrode performance and to reduce the long-term durability of SOFCs. Therefore, in this study, we focused on the poisoning caused by water vapor and SOx in the oxygen source to clarify their effects on SOFCs performances and to reveal the degradation mechanism of cathodes. SOFCs with typical electrolyte-supported structure were used in this work, which were composed with ScSZ (10 mol% Sc 2 O 3 , 1mol% CeO 2 , 89 mol% ZrO 2 ) plate with the thickness of 200 µm as electrolyte, NiO-ScSZ (mixture of 56 wt% NiO and 44 wt% ScSZ) porous layer as anode, and two cathode layers of LSM ((La 0.8 Sr 0.2 ) 0.98 MnO 3 ) and LSM-ScSZ (mixture of 50 wt% LSM and 50 wt% ScSZ). Power generation characteristics of the cells had been analyzed by measuring cell voltage at a constant current density (200 mA/cm 2 ) and by comparing changes in cell impedance, upon supplying the artificially-contaminated air with water vapor or SOx, to the SOFC cathodes at various operational temperatures. High-resolution FESEM (S-5200, Hitachi) was used to analyze microstructural changes caused by the impurities. Mg Kα radiation from a monochromatized X-ray source was used for XPS measurements (ESCA-3400, KRATOS). AC impedance was measured at various temperatures under the open circuit voltage condition by an impedance analyzer (Solatron 1255B/SI 1287, Solatron), in a frequency range from 0.1 to 105 Hz with an amplitude of 10 mV.

Fuel Cells (SOFC): Alternative Approaches (Electroytes, Electrodes, Fuels)

While we may realize commercialization of SOFC systems in the very near future, continuous challenges will be needed to develop advanced fuel cells with higher efficiency, higher durability, better flexibility, as well as lower production cost. For realizing next-generation SOFCs, we may apply a broad range of knowledge related to (1) electrolyte and (2) electrode materials for cell development as well as (3) explore alternative fuels. This entry gives an overview on possible alternative approaches for these three important aspects to realize advanced high-performance SOFCs in a future generation.

Biogas production from local biomass feedstock in the Mekong Delta and its utilization for a direct internal reforming solid oxide fuel cell

Fuel-flexible solid oxide fuel cell (SOFC) technologies are presently under study in a Vietnam-Japan international joint research project. The purpose of this project is to develop and demonstrate an SOFC-incorporated energy circulation system for the sustainable development of the Mekong Delta region. Lab-scale methane fermentation experiments in this study with a mixture of biomass feedstock collected in the Mekong Delta (shrimp pond sludge, bagasse and molasses from sugar production) recorded biogas production yield over 400 L kgVS-1 with H2S concentration below 50 ppm level. This real biogas was directly supplied to an SOFC without any fuel processing such as desulfurization, methane enrichment and pre-reforming, and stable power generation was achieved by applying paper-structured catalyst (PSC) technology.