Takaharu Gamo

Proton Conductive Properties of Gadolinium-Doped Barium Cerates at High Temperatures

Abstract Proton conductive properties of Gd-doped BaCeO 3 ceramics were investigated by electrochemical methods (hydrogen permeation test and hydrogen-oxygen fuel cell test) in the temperature range of 600°C to 1000°C. As a result of electrochemical hydrogen- permeation tests, it was verified that the proton was the only conductive species in BaCe 1− x Gdi x O 3−α in the absence of oxygen at 600°C–1000°C. Next the relationship between conductivity and Gd-substitution value of x BaCe 1− x Gd x O 3−α (0.5 x 0.8 Gd 0.2 O 3-α ceramics with a thickness of 0.5 mm as a solid electrolyte was constructed. The short-circuit currents of this cell were 1.2AA cm -2 at 1000°C, and 0.8 A cm −2 at 800°C, which were the best performances in the fuel cell using perovskite-type electrolyte instead of YSZ. The transference numbers of mobile ions (protons and oxide ions) were determined from the evolution rate of water vapor at each electrode when the fuel cells were discharged. Regarding the temperature dependence on the transport numbers of proton and oxide ions, it was found that protonic conduction was fully dominant at 600°C. On the other hand, oxide ions were the main charge carriers at 1000°C and mixed conduction was observed in the temperature range of 600–1000°C.

Life properties of TiMn alloy hydrides and their hydrogen purification effect

The effects of pressure-induced absorption-desorption cycling on the degradation properties and the hydrogen purification ability of TiMn alloy hydrides were studied. The lines in the characteristic X-ray diffraction pattern were much broader and weaker in the cycled alloys but the C14 hexagonal structure was maintained and no second phase was observed. The hydrogen capacity was reduced by 30% for TiMn binary alloy and by 20% for TiMn multicomponent alloys after about 10 000 cycles in contrast with the marked reduction observed for LaNi5. In some cases slight recovery could be produced by heating at 500 °C for 1 h in a vacuum after cycling. Gas chromatography measurements on hydrogen released from TiMn1.5 hydride showed that the hydrogen purity was better than 99.9999% (except for an H2O impurity) after a purge release of only a few per cent of hydrogen when the purity of the commercial grade hydrogen absorbed by the hydride was 99.99%. It can be concluded from the results of these measurements that hydrogen purification using TiMn alloy is related to two factors. The removal of N2, CO and CO2 proceeds by surface poisoning (i.e. oxidation, physisorption or chemisorption), whereas the removal of CH4, O2, H2O and Ar is controlled by surface poisoning and concentration. The hydrogen purification effect of TiMn alloy hydrides showed little degradation even after a large number of absorption-desorption cycles.

Control of hydrogen equilibrium pressure for C14-type laves phase alloys

Abstract It is very important to control continuously the hydrogen equilibrium pressure P -temperature T characteristics of hydrogen storage alloys to have them applicable in a wide range of temperatures. The relation between the alloy phases and the P-C-T characteristics was determined for C14 (MgZn 2 ) type Laves phase alloys based on Ti(Zr)Mn 2 by changing the alloy composition. The P-C-T characteristics, hysteresis and plateau pressure were changedsubstantially by suchtituting zirconium for titanium in the A site and other elements such as chromium, copper or nickel in the B site. In this case, the crystal lattice constants and hydrogen equilibrium pressure were changed more effectively by substituting zirconium for titanium in the A site. The range of equilibrium pressures covered was from 10 to 10 8 Pa at 20°C (this corresponds to the range from −50°C to +250°C at 1.013×10 5 Pa). The equilibrium pressure changes particularly markedly for lattice constants a =(4.84–5.05)×10 −10 m and c =(7.94–8.28)×10 −10 m. These results show that the C14-type Laves phase alloys with excellent flatnessof the plateau pressure and small hysteresis, are applicable in these ranges of temperatures and pressures for hydrogen storage. We have confirmed these results by fabricating a prototype heat pump with ahydrogen storage alloy that utilizes high temperature waste heat.

Electrode characteristics of C15-type laves phasealloys

Abstract Extensive studies on hydrogen storage alloys such as LaNi5, MmNi5, Ti(Zr)−Ni etc. have been actively carried out for the negative electrodes of Ni−H2 batteries. The present authors, on the contrary, have focused on C14-type and C15-typeLaves phase alloys of the AB2 type based on TiCr2, TiMn2 and ZrMn2, and have succeeded in optimizing them for electrode materials which can supply a high energy density by inspecting the relation between the composition of the alloys, the P−C−T characteristics and the electrode performance. The Mn:Cr ratio, A:B ratio and addition of vanadium to the C15-type Lavesphase ZrMn0.6Cr0.2Ni1.2 were examined to investigate the alloy phases, hydride characteristics (P−C−T) and electrode performance. As a result, electrodes composed of such alloys as ZrCr0.8Ni1.2 andZrMn0.5Cr0.2V0.1Ni1.2 were found to provide a large discharge capacity in a cell test with an abundant electrolytic solution.

Operating Properties of Solid Oxide Fuel Cells Using BaCe0.8Gd0.2 O 3 − α Electrolyte

The performance and the long-term operating properties of solid oxide fuel cells (SOFCs) using BaCeo 0.8 GdO 0.2 O 3-α (BCG) ceramics as the electrolyte have been investigated. A hydrogen-air fuel cell using BCG electrolyte with a thickness of 0.5 mm exhibited good performance at 800°C. Its short-circuit current was 0.9 A cm -2 , and there was hardly any degradation of BCG electrolyte in 1600 h of operation. When a quasi-fuel gas containing 8% CO 2 , which was expected by the reformation of the city gas (CH 4 ) at 800°C, was supplied to this cell, its short-circuit current was 0.63 A cm -2 and the cell worked stably under the discharge current density of 100 mA cm -2 for 2500 h. Some sintered metals (Fe, Co, Ni) were examined as a anode material for this electrolyte. Ni is a promising anode material for BCG electrolyte from the point of view of performance and durability.

Sensing Properties of an Oxygen Sensor Using BaCe0.8Gd0.2 O 3 − α Ceramics as Electrolytes

Limiting-current-type oxygen sensors using BaCe{sub 0.8}Gd{sub 0.2}O{sub 3{minus}{alpha}} (BCG) ceramics as electrolytes were constructed on a trial basis and their sensing properties investigated in order to develop a new oxygen sensor to replace the zirconia type. BCG ceramics exhibited high conductivity in the temperature range 200--1,000 C, and it was verified that the oxide ion could be a conductive carrier in BCG in oxygen at low temperatures (300 C). The oxygen sensors using BCGs worked at 300 C, and their output currents linearly increased with an increase in oxygen concentration in the range 1--22%. They could respond within 30 s between 1 and 21%, and humidity only slightly affected sensing performance. BCG seems to be a promising electrolyte material for an oxygen sensor operating at low temperatures (300 C).

Operating properties of solid oxide fuel cells using BaCe{sub 0.8}Gd{sub 0.2}O{sub 3{minus}{alpha}} electrolyte

The performance and the long-term operating properties of solid oxide fuel cells (SOFCs) using BaCeo 0.8 GdO 0.2 O 3-α (BCG) ceramics as the electrolyte have been investigated. A hydrogen-air fuel cell using BCG electrolyte with a thickness of 0.5 mm exhibited good performance at 800°C. Its short-circuit current was 0.9 A cm -2 , and there was hardly any degradation of BCG electrolyte in 1600 h of operation. When a quasi-fuel gas containing 8% CO 2 , which was expected by the reformation of the city gas (CH 4 ) at 800°C, was supplied to this cell, its short-circuit current was 0.63 A cm -2 and the cell worked stably under the discharge current density of 100 mA cm -2 for 2500 h. Some sintered metals (Fe, Co, Ni) were examined as a anode material for this electrolyte. Ni is a promising anode material for BCG electrolyte from the point of view of performance and durability.

The Exchange Current Density of Oxide Cathodes in Molten Carbonates

The exchange current density (i 0 ) for oxygen reduction has been measured by using a potential step technique on nonporous oxide electrodes in an Li/K(62/38 mole percent) carbonate eutectic melt at 923 K. Nonporous oxides were fabricated by a hot isostatic pressing method to be as dense as possible to study the kinetic parameters of the oxygen reduction on smooth oxide electrodes. However, the nonporous oxides had slightly rough surfaces. Accordingly, to allow for electrode roughness, the area of the nonporous oxide electrodes were corrected from the double-layer capacity values to make a comparison between our nonporous oxide electrodes and those used by other researchers

Effects of nonmetal addition on hydriding properties for Ti–Mn Laves phase alloys

Abstract The effects of nonmetal addition such as B, C, O, S and Se, on hydriding properties were investigated for Ti 0.9 Zr 0.1 Mn 1.4 Cr 0.4 V 0.2 Laves phase alloys. It was found that for both as-cast and annealed samples, the addition of nonmetal elements such as S, Se and C greatly increases the plateau pressure, improves the plateau slope factor, and expands the hysteresis. The effect of increasing the plateau pressure was strongest for the S addition. These phenomena are possibly related to the appearance of manganese-rich minute particles as observed by scanning electron microscopy. The above effects pave the way for increasing the hydrogen storage capacity of the Ti–Mn Laves phase alloys.

Fabrication and properties of combined electrode/electrolyte tape for molten carbonate fuel cells

A process which would simplify the fabrication procedure of a Molten Carbonate Fuel Cell (MCFC) is described. Green tapes of the combined electrode/electrolyte type were fabricated by a double doctor blade method. Then the combined tapes were burnt-out in-cell by elevating the temperature to 650°C. During the burn-out process, under stack compression (0.15 MPa), the electrolyte composites became dense (2.24 g cm−3) and gas-tight. On the other hand, the anode tape and the cathode tape were sintered orin situ oxidized to form porous plaques. Single cells made by this process exhibited performance characteristics of 0.80 V at 150 mA cm−2 (H2/CO2/H2O 66/16/18Uf=65%, air/CO2 70/30U0=61%). Though there remain some problems, such as thickness decrease of the tape and cross-leak during the burn-out process, this new process may be attractive for MCFCs.