Ikuo Yonezu

Microstructures and hydrogen absorption/desorption properties of LaNi alloys in the composition range of La77.8 ∼ 83.2 at.%Ni

Abstract Microstructure and hydrogen absorption/desorption properties of La Ni alloys have been investigated as a function of alloy composition in the range of La 77.8 ∼ 83.2 at.%Ni, which corresponds to compositions between two intermetallic phases, La 2 Ni 7 and LaNi 5 . The intermetallic phase, La 5 Ni 19 of the Ce 5 Co 19 -type is found for the first time to exist as an equilibrium phase at a composition between La 2 Ni 7 and LaNi 5 . This phase is stable at high temperatures around 1000°C but decomposes into La 2 Ni 7 and LaNi 5 below 900°C. Hydrogen absorption/desorption properties described in terms of pressure-composition isotherms decline with decreasing Ni content (i.e. with increasing volume fraction of intermetallic phases other than LaNi 5 ). In particular, the plateau at the equilibrium pressure corresponding to the hydrogen absorption in the LaNi 5 phase is narrowed with decreasing Ni content and additional plateaus with higher equilibrium pressures come into existence. The degradation becomes more pronounced in the presence of La 2 Ni 7 than La 5 Ni 19 . This can be understood in terms of the ratio of the number of LaNi 2 (Laves) unit layers to that of LaNi 5 unit layers in the unit cell of the two intermetallic phases.

Homogenizing behaviour in a hydrogen-absorbing LaNi4.55Al0.45 alloy through annealing and rapid quenching

Abstract Homogenizing behaviour in a hydrogen-absorbing alloy with a composition of LaNi 4.55 Al 0.45 through annealing and rapid quenching was investigated to improve its equilibrium characteristics with hydrogen. Annealing has an effect on homogenizing the Al distribution in the dominant phase of CaCu 5 structure and decreasing the plateau slope in the P - C isotherms. The rapidly quenched alloy exhibited a flatter but narrower plateau region than the induction-melted and annealed alloy, which became wider after short-time annealing for 7.2 ks at 1323 K. The effect of the annealing in the rapidly quenched alloy was attributed to a decrease in lattice defects introduced by the rapid quenching process.

Relation between equilibrium hydrogen pressure and lattice parameters in pseudobinary ZrMn alloy systems

Abstract The influence of transition elements (M ≡ V, Fe, Co, Ni) in ZrMn2 − xMx alloy systems on the equilibrium hydrogen pressure and lattice parameters was studied in order to develop metal hydride materials for use in heat storage and heat transportation systems operating at temperatures between 100 and 250 °C, for which there is great demand in industrial heat processes. The equilibrium characteristics and lattice parameters of as-cast pseudobinary alloys ZrMn2 − xMx (M ≡ V, Fe, Co, Ni; 0 ⩽ x ⩽ 0.6) were evaluated by measuring pressure-composition isotherms at 200 °C and by analysing X-ray diffraction patterns using Rietvelds method. The crystal structures of all the pseudobinary alloys were the C14-type Laves phase. The decrease of the unit cell volume increased the equilibrium hydrogen pressure in the alloys modified by vanadium, iron or cobalt, but the alloys modified by nickel showed an opposite tendency. These results were interpreted in terms of Miedemas rule. Consequently, cobalt and vanadium were found to be the most effective elements for control of the equilibrium hydrogen pressure and therefore for extension of the available temperature range of pseudobinary ZrMn alloy systems of the C14-type Laves phase.

Development of a high-performance PEFC module operated by reformed gas

Abstract Improvements in the CO tolerance of the anode has been studied with the objective of developing PEFC modules operated with reformed gas. The CO tolerance of the Pt–Ru alloy anode was improved by reducing its thickness from 40 μm to 20 μm. Maintaining a saturated steam pressure of the fuel cell anode gas outlet is expected to decrease the influence of CO poisoning. When keeping sufficient water content in the feed fuel, it was established that the cell performance was stable over a wide range of air-humidification conditions. Successful operation was achieved with a 1 kW class system utilizing the improved CO-tolerant anode and fuel processor. An output power of 1.1 kW (average cell voltage: 0.62 V) was achieved at the current density of 0.3 A/cm 2 .

Influence of annealing on hydrogenation characteristics and microstructure of LaNi4.55Al0.45 alloy

Abstract The influence of annealing time and temperature on the plateau slope and microstructure of a hydrogen-absorbing LaNi4.55Al0.45 alloy was investigated. The homogenizing behaviour during annealing was also analysed to clarify the effects of annealing quantitatively. Electron probe microanalysis and X-ray powder diffraction analysis indicated that the plateau slope is caused mainly by the fluctuation of Al concentration in the dominant phase. The dependence of the homogenizing behaviour on the annealing conditions is reasonably explained with a non-steady-state Al diffusion model where Al is distributed periodically.

Electrochemical characteristics of graphite, coke and graphite/coke hybrid carbon as negative electrode materials for lithium secondary batteries

Electrochemical characteristics of various carbon materials have been investigated for application as a negative electrode material in lithium secondary batteries with long cycle life. Natural graphite electrodes show large discharge capacity in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC). However, their charge/discharge performance is largely influenced by electrolytes. There is a possibility that a rapid rise in the discharge potential of the natural graphite electrode at the end of the discharge would cause a side reaction such as decomposition of the electrolyte because of an unequal reaction over an electrode. In order to improve the cycle performance of natural graphite electrodes, mixtures of graphite and coke electrodes are prepared by adding coke to natural graphite. It is found that the mixture of graphite and coke electrode shows a better cycle performance than that of a natural graphite or coke electrode. The deterioration ratio of the mixture of graphite and coke negative electrode measured by using AA-type test cells is 0.057%/cycle up to the 500th cycle. The mixture of graphite and coke is a promising material for a negative electrode in long-life lithium secondary batteries for energy storage systems because of its excellent cycle performance and large discharge capacity.

Development of thermal energy storage technology using metal hydrides

Abstract The rate of heat transfer is important in heat storage technology using metal hydrides because the rate of hydrogen transfer depends on the rate of heat transfer. A new metal hydride container with heat pipes which were able to transfer heat rapidly was constructed and was used to operate the heat storage system with a hydrogen flow of 1.51 min−1 between 3.5 kg beds of CaNi5 and LaNi5. The metal hydride beds were located inside cylindrical heat pipes 66 cm long with an inside diameter of 4 cm.

Stability of LaNi5−x Alx alloys (x=0∼0.5 during hydriding and dehydriding cycling in hydrogen containing O2 and H2O

Cycling performance of hydrogen storage LaNi5−x alloys under O2 and H2O was investigated to develop a hydrogen storage system connected with a solid polymer electrolyte water electrolysis device. The best Al content was found to be between 0.5 and 0.8 through analysis of the initial hydrogen storage capacity decaying curves measured under impure hydrogen gas containing 100 ppm of O2, introducing a criteria for maximizing the total amount of hydrogen storage for long-term utilization of up to 10 000 cycles.

Cycle performance of a hydrogen-absorbing La0.8 Y0.2 Ni4.8 Mn0.2 Alloy

Abstract A long-term cycle test up to 30,000 cycles and two series of cycle tests under different cycle conditions were performed on a La 0.8 Y 0.2 Ni 4.8 Mn 0.2 alloy developed for a heat pump. The alloy kept 61% of its initial hydrogen capacity after 30,000 cycles. A degradation model was checked with experimental results and showed good agreement. Influence of the temperature and pressure difference conditions on degradation of the alloy was also investigated to estimate the degradation rate alloy under actual operating conditions in the heat pump and the alloy was concluded to have enough durability for practical use in application systems.

A method for designing a hydrogen absorbing LaNi5−x−yMnxAly alloy for a chemical refrigeration system

Abstract A new method for designing a composition of LaNi 5− x − y Mn x Al y alloy for a chemical refrigeration system was examined to give maximum hydrogen transfer under given operating conditions. The composition was optimized by analyzing experimental parameters in P – C isotherms of La–Ni–M (M=Mn, Al) alloys and by simulating those of La–Ni–Mn–Al alloys with a numerical P – C isotherm model. The optimized parameters agreed well with the measured parameters, indicating the validity of the optimization method.