Kohta Asano

Hydriding and dehydriding processes of LaNi5-xCox (x = 0-2) alloys under hydrogen pressure of 1-5 MPa

Abstract The hydriding and dehydriding behavior of LaNi5−xCox (0⩽x⩽2) was studied by the pressure differential scanning calorimetry (PDSC) at the hydrogen pressure range of 1–5 MPa in the temperature range from 323 to 573 K. In the heating run of the LaNi5–H2 system, two endothermic peaks were observed. One was the peak for the transformation from the γ phase (LaNi5H6 hydride) to the β phase (LaNi5H3 hydride). The other was the peak for the transformation from the β phase to the α phase (solid solution). In the cooling run, one exothermic peak for the transformation from the α phase to the γ phase was observed. In the range of x⩽0.5, PDSC curves similar to that of the LaNi5–H2 system were observed. However, in the range of x⩾1, one endothermic and one exothermic peaks were observed in the heating and cooling runs, respectively. Using Ozawas method, the activation energies for the dehydriding processes were estimated. The activation energy for the γ–β transformation was higher than that for the β–α transformation and the activation energy for the β–α transformation has a maximum at the composition of about x=1.

Hydrogenation and Dehydrogenation Behavior of LaNi5-xAlx (x=0-0.5) Alloys Studied by Pressure Differential Scanning Calorimetry

The hydrogenation and dehydrogenation behavior of LaNi 5 , LaNi 4.75 Co 0.25 and LaNi 3 Co 2 was studied by the pressure differential scanning calorimetry (PDSC) at the hydrogen pressure range of 1 to 5 MPa in the temperature range from 323 to 473 K with the heating and cooling rates of 2 to 30 Kmin -1 . In the heating run of the hydride of LaNi 5 , two endothermic peaks were observed. One was the peak for the transformation from the y phase (full hydride LaNi 5 H 6 ) to the β phase (hydride LaNi 5 H 3 ). The other was the peak for the transformation from the β phase to the a phase (solid solution). In the cooling run, one exothermic peak for the transformation from the α phase to the y phase was observed. These endothermic and exothermic peaks shifted to higher temperatures with the increase in hydrogen pressure. In the heating and cooling runs of the LaNι 4.75 Co 0.25 -H 2 system the PDSC curves similar to those of the LaNi 5 -H 2 system were observed. However, in the heating run of the hydride of LaNi 3 Co 2 only one endothermic peak was observed. Using Ozawas method, the activation energies for dehydrogenation of the hydrides were estimated. The activation energy for the γ-β transformation was higher than that for the β-α transformation. Substitution of cobalt for a part of nickel in LaNi 5 increased the activation energies for the phase transformations.

Preparation and Hydrogen Storage Properties of Mg-Ni-B BCC Alloys

Mg-Ni-B system alloys were prepared by the mechanical alloying (MA) method. Body centered cubic (BCC) structure alloys are obtained in some of the Mg-Ni-B compositions after the starting mixtures of raw elements were ball milled for 200 h. Mg50Ni45B5 and Mg48Ni48B4 alloys after ball milling are with single BCC structure, which is confirmed by electron diffraction patterns. From the results of X-ray diffraction and transmission electron microscope, the crystallite size of the alloys is calculated into nanometer scale. Mg50Ni45B5 and Mg48Ni48B4 BCC alloys can absorb hydrogen at 373 K with higher rate than Mg50Ni50 alloy prepared in the same conditions. And these two samples can reach a hydrogen absorption capacity of 1.94 and 1.93 mass%, respectively at 373 K without any activation process.

Structural Variation of Self-Organized Mg Hydride Nanoclusters in Immiscible Ti Matrix by Hydrogenation

Hydrogenation of nonequilibrium alloys may form nanometer-sized metal hydride clusters, depending on the alloy compositions and hydrogenation conditions. Here in the Ti-rich compositions of the immiscible Mg-Ti system MgH2 clusters are embedded in a Ti-H matrix. Our previous works have indicated that the interface energy between the two metal hydrides reduces the stability of MgH2. The aim of our study is to obtain the structural information on the nanometer-sized clusters. Indeed, MgD2 clusters embedded in a face-centered-cubic (FCC) Ti-D matrix is found in Mg0.25Ti0.75D1.65 by means of 2H magic angle spinning nuclear magnetic resonance (MAS NMR). The atomic pair distribution function (PDF) analysis of neutron total scattering data suggests that the MgD2 clusters have an orthorhombic structure, which is different from a rutile-type body-centered-tetragonal (BCT) structure of α-MgD2 observed in the Mg-rich compositions. Our results suggest that we can tune the thermodynamics of hydrogen absorption and desorption in Mg-H using the interface energy effect and accompanying stress-induced structural change, which contributes to the substantial development of lightweight and inexpensive hydrogen storage materials.

In-situ hydrogen gas loading setup for synchrotron X-ray total scattering

Hydrogen has been considered as a promising alternative fuel for transportation, provided we can find a way to store a large amount of hydrogen in a compact way. The realization of such a storage system can be achieved by developing materials that can easily absorb, safely store, and rapidly release hydrogen repeatedly. However, there is currently no material to meet all the requirements for on board storage. Great efforts have been made to understand hydrogenation properties of currently available materials to look for a way to improve properties or to prepare new materials. However, investigating the structure of some of these materials is challenging since their hydrides are only available under hydrogen gas pressure. Furthermore, many novel materials with improved properties often show heavily disordered or nanoscale structural features which are difficult to characterize using conventional crystallographic technique alone (crystallographically challenged hydrogen storage materials). In order to investigate the structural change in crystallographically challenged hydrogen storage materials during hydrogenation or dehydrogenation processes we have developed in-situ hydrogen gas loading setup for synchrotron X-ray total scattering experiments at the Japan Atomic Energy Agency (JAEA) beamline of BL22XU [1] at SPring-8. Coupled to an area detector [1,2], this setup allows us to obtain the atomic pair distribution function (PDF) [3] of metal hydrides either in equilibrium or in non-equilibrium state with hydrogen. In this poster, we will introduce our in-situ setup and present some preliminary results on AB5-type intermetallic compounds and Pd nanoparticles.