Manabu Ihara

The relationship between overpotential and the three phase boundary length

Porous La0.81Sr0.09MnO3 and LSM-YSZ composite electrodes of the solid oxide fuel cell (SOFC) were made, and the relationship between the overpotential and the three phase boundary (TPB) length was investigated by constructing a model. Three-dimensional distribution of TPB for the composite cathode was suggested. The effective thickness of it was observed to be less than 20 μm.

Cathodic reaction mechanism for dense Sr-doped lanthanum manganite electrodes

Abstract Dense La0.81Sr0.09MnO3 (LSM) electrode was prepared by a laser ablation method and electrochemical measurements were carried out. The electrode impedance was proportional to the film thickness and almost independent of the oxygen partial pressure. Steady-state polarization characteristics were measured and oxygen ionic conductivity of LSM as calculated using Hebb-Wagner polarization method. The slopes of log i − log a0 curves were calculated to be −1 2 . These results suggest that the cathodic reaction mechanism of the dense LSM electrode is quite different from that of porous electrode.

Competitive Adsorption Reaction Mechanism of Ni/Yttria-Stabilized Zirconia Cermet Anodes in H 2 ­ H 2 O Solid Oxide Fuel Cells

The reaction mechanism of the most commonly used anode material, Ni/yttria-stabilized zirconia (YSZ) cermets, in H 2 -H 2 O solid oxide fuel cells (SOFCs) was investigated. Because the reaction mechanism for the Ni/YSZ anode in H 2 has not been conclusively determined, we investigated the detailed dependence of dc polarization and interfacial conductivity of Ni/YSZ cermet anode on the partial pressure of hydrogen (p H2 ). Based on our experimental results, we developed a model that links the chemical reactions on the anode with the electrical characteristics of the anode such as the dc polarization and the interfacial conductivity. In our model, we assumed competitive adsorption equilibrium of H 2 , H 2 O, and O on Ni surfaces at the three-phase boundary, and assumed the rate-determining step to be Langmuir-type reactions of H with O. In SOFCs with Ni/YSZ anodes, reported dependencies of the current and interfacial conductivity on p H and p H2O differ among previous studies. Both our measured dependencies and previously published dependencies were successfully reproduced by our Langmuir reaction model. Furthermore, possible reasons for these different observed dependencies include the different adsorption equilibrium constants.

Quickly Rechargeable Direct Carbon Solid Oxide Fuel Cell with Propane for Recharging

A quickly rechargeable direct carbon solid oxide fuel cell (RDCFC) that uses a solid carbon fuel supplied by thermal decomposition of propane was developed. This RDCFC, after a fuel charging time of only 5 min, maintained a power density of 44.2-50.4 mW/cm 2 for 83 min, had a maximum power density of 52 mW/cm 2 , and exhibited these stable characteristics for at least six cycles of power generation and charging. This new type of solid oxide fuel cell can be used as a compact, portable power unit.

Oxidation mechanism and effective anode thickness of SOFC for dry methane fuel

Abstract Power generation experiments were carried out to investigate the effective anode thickness and to clarify the mechanism determining the threshold current density for CO 2 production. The production of CO 2 increased fuel efficiency and reduced anode overpotential. The effect of anode thickness, diameter of NiO powder for anode, concentration of fuel and operating temperature on anode potential and outlet gas composition were measured in this report. The anode overpotential decreased with increasing anode thickness up to 120 μm for pure hydrogen fuel and 70 μm for 4.6% of methane fuel. The result indicated that the effective three phase boundary could be distributed from the electrolyte surface into the anode to a depth of 120 μm for hydrogen fuel and 70 μm for methane fuel. The threshold current density was explained by the surface activity with fuel on the three phase boundary. The modelling of the anode reaction by a complete mixing reactor consistently interpreted all experimental results.

Reaction Mechanism of Solid Carbon Fuel in Rechargeable Direct Carbon SOFCs with Methane for Charging

Rechargeable direct carbon fuel cells (RDCFC) can be expected as portable power sources. The performance of power generation from RDCFCs was investigated here for various Ar flow rates into the anode. The fuel of solid carbon was charged by thermal decomposition of pure dry methane on the anode. The power generation time of RDCFCs depended on the flow rate of Ar fed to the anode to control the partial pressure of oxygen in the anode. The power generation time increased with increasing Ar flow rate in the range of 4.6-9.2 STP mL/min, peaked at an Ar flow rate of about 4-11 STP mL/min, and then decreased at 11.0-92.0 STP mL/min. When CO 2 was supplied into the anode after a charging of carbon fuel, CO was generated. The reaction gas of CO 2 generated by electrochemical oxidation of carbon reacted with the deposited carbon (Boudouard corrosion) and produced CO. This CO can be used as fuel. The generation of CO permitted the efficient utilization of carbon during the power generation. By increasing the charging time up to 6 h, the power density increased to a maximum of 55 mW/cm 2 .

Diamond deposition on silicon surfaces heated to temperature as low as 135°C

Diamond films were deposited on the silicon wafer at as low a temperature as 135 °C by filament‐assisted chemical vapor deposition. Silicon wafer substrate scratched with diamond powder was cooled by a stream of water flowing around a substrate holder. The films were identified as diamond by Raman spectroscopy. Clearly faceted crystals were shown in scanning electron micrographs.

Correlation between nucleation site density and residual diamond dust density in diamond film deposition

Diamond was deposited on substrates pre‐etched with diamond powder using either a microwave plasma chemical vapor deposition method or a hot‐filament‐assisted chemical vapor deposition method. Density of residual diamond dust (i.e., number of diamond particles per unit area on the surface of a substrate) on the pre‐etched substrates was determined using field emission scanning electron microscopy, and ranged from 3.3×107 to 6.6×1010 ♯/cm2. The diamond nucleation‐site density (i.e., number of nucleation sites per unit area on the surface of a substrate) ranged from 1.5×106 sites/cm2, typical of the deposition on a substrate etched with diamond paste, to 1.1×1010 sites/cm2, sufficient to create nanostructured diamond films. We found that the nucleation site density was about 10% of the residual dust density. Our results also show that the residual diamond dust is the main source of nucleation sites for diamond growth on diamond‐etched substrates.

Effect of the Steam‐Methane Ratio on Reactions Occurring on Ni/Yttria‐Stabilized Zirconia Cermet Anodes Used in Solid‐Oxide Fuel Cells

Power‐generation experiments of solid‐oxide fuel cells with Ni/yttria‐stabilized zirconia cermet anodes were carried out by changing the water‐to‐methane ratio in the fuel. To separate the electrochemical reactions occuring on the three‐phase boundary of the anode from the steam‐reforming reactions, premixed gases corresponding to the composition at thermal equilibrium were used as the fuel. The reactions on the anode were quantitatively clarified at different steam‐to‐methane ratios by making a mass balance of the elements and electrons and by determining the anode exit gas composition. For dry (i.e., no water) methane fuel, the carbon deposited on the three‐phase boundary from the methane mainly reacted with oxide ions . With increasing steam‐carbon (S/C) ratio from S/C = 0.5 to 1, hydrogen became the dominant reactant. The anode potential vs. current density for S/C = 1 agreed with that for hydrogen diluted with argon. At S/C ratios higher than 1.5, CO in the inlet gas reacted with oxide ions.

Hydrogen dissociation on hot tantalum and tungsten filaments under diamond deposition conditions

The electric power consumed by hot tantalum and tungsten filaments used to dissociate hydrogen molecules into hydrogen radicals was measured at filament temperatures of 2000, 2300, and 2500 °C and hydrogen pressures from 0.5–100 Torr. The measured power consumption at pressures above 30 Torr was well represented by a model that assumed thermodynamic equilibrium between H2 and H near the filament. With decreasing pressure, however, the dissociation of H2 shifted from an equilibrium‐controlled regime to a surface‐reaction‐rate controlled regime. The relationship between the power consumption and the pressure in the surface‐reaction‐rate controlled regime was correlated with the surface dissociation probability, which was determined to range from 0.18 to 0.94.