Abstract: Novel Mg-Co binary alloys with BCC (body-centered cubic) structure have been
successfully synthesized by means of mechanical alloying technique. The formation of BCC structure was confirmed by X-ray diffraction and transmission electron microscopy. Mg-Co alloys were found in the range of Co concentration between 37 and 80 atomic %. All the Mg-Co alloys synthesized absorbed hydrogen below 373K. The maximum hydrogen capacity of these alloys reaches 2.7 mass %. However, desorption of hydrogen at 373 K has not been observed yet. Mg- Co-X (X = B and Ni) ternary alloys with BCC structure have also been synthesized. The lattice
parameter of both alloys is lower than that of Mg-Co binary alloys, meanwhile the maximum hydrogen content of both alloys also decreased.
Abstract: In order to improve the cycling stability of a new rare-earth Mg-based hydrogen
storage alloys, La0.7Mg0.3Ni2.65-xMn0.1Co0.75Alx (x=0.0-0.4) alloys were prepared to investigate the structure and electrochemical properties of these alloys. XRD and Rietveld analyses reveal that the alloys consist of a (La,Mg)Ni3 phase with rhombohedral PuNi3-type structure and a LaNi5 phase with hexagonal CaCu5-type structure. Electrochemical studies on these alloys indicate that their maximum discharge capacities were decreased from 400.7 mAh/g (x=0.0) to 335.6 mAh/g (x=0.4). However, the cycling stability of the alloy electrodes was significantly
improved after Ni was partially replaced by Al. After 100 charge/discharge cycles, the discharge capacity retention was increased from 32.0% (x=0.0) to 73.8 % (x=0.3), which can be attributed to the formation of a dense oxide film on the alloy surface. Moreover, the high rate dischargeability measurements indicate that the electrochemical kinetic properties were deteriorated with increasing Al content owing to the presence of a dense oxide film of Al.
Abstract: Hydrogen desorption properties in multiwall carbon nanotubes(MWNTs) were
investigated from the view points of not only physical hydrogen molecules adsorption in nano-hole but also chemical hydrogen adsorption on graphite surface. MWNTs with intrinsic closed structure having the blocked hole and MWNTs with open structure were studied through hydrogen thermal desorption technique equipped with gas chromatograph. In order to grow MWNTs with open structure, novel method using oxygen added plasma(CH4/H2/O2) has been introduced. The structure of MWNTs was gradually changed with increasing oxygen addition amount, showing aligned MWNTs bundles and open tube cap at 10% oxygen addition. The precise analysis on thermal desorption spectra on MWNTs with closed structure showed that hydrogen gas was released at three major temperature ranges such as 100-230K, 290-350K, and 600-625K, where the evolved hydrogen amounts were about 1.65wt%, 0.64wt%, and 0.03wt%, respectively. However, in case of aligned and open MWNTs, the evolution peak around 290-350K was highly developed (about 1.9wt%) and the desorption peak at 600-625K was disappeared. The hydrogen desorption activation energy was also
calculated. The obtained hydrogen desorption activation energy of MWNTs with closed and open structure at ambient temperature was -18.5kJ/mol H2 and 16.5kJ/molH2, respectively. The hydrogen desorbed between 290 and 330K in MWNTs with open structure was the hydrogen physi-sorbed in nano-hole. And Ni-doping (dispersion) effect on hydrogen storage properties was investigated
comparatively. Ni nano-catalysts were expected to effectively dissociate hydrogen molecules in gas phase, providing atomic hydrogen possible to form chemical bonding on carbon surface. The metal nanoparticles were homogeneously dispersed using incipient wetness impregnation method. Hydrogen desorption spectra of MWNTs with Ni nanoparticles showed that about 2.8wt% of hydrogen was released in the range of 340-520K.
Abstract: Structural changes in C15 Laves phase DyCo2 on heating using a pressure differential scanning calorimeter (PDSC) in a hydrogen atmosphere between 0.1 and 5.0 MPa were investigated by a powder X-ray diffractometer (XRD), a differential scanning calorimeter under an argon flow atmosphere (Ar-DSC), a
transmission electron microscope (TEM) and a hydrogen analyzer. As the temperature of DyCo2 increases, the reactions such as hydrogen absorption in a crystalline state, HIA (hydrogen-induced amorphization), precipitation of DyH3 and decomposition of the remaining amorphous phase into β-Co + DyH3 occurred
exothermically for every hydrogen pressure. The mechanism of HIA in DyCo2 is discussed on the basis of the experimental results.
Abstract: Mg2Ni powder was heated up to 880°C which is higher than the melting point of Mg2Ni alloys (760°C) for 30 minutes and immediately cooled into water (14°C). And then, mechanical ball milling was conducted for 1,4,8,10,12,24 hours respectively. As milling time increased discharge capacity was increased to maximum of 700mAh/g at 12hours then decreased with increasing milling time. The best high rate dischargeability (HRD) characteristic also obtain at the 12 hours ball milling time. Therefore, the ball milling time could be shorten to 12 hours by heat treatment and rapid cooling. Praseodymium (0,0.2,0.5,0.7,1.0%Pr) was added to Mg2Ni alloys. These alloys were ground for 12 hours to investigate the improvement of cycle life. The cycle life was remarkably improved with increasing the amount of Pr. The best amount of Pr was 0.7 wt%.
Abstract: V- and Ti-Cr-based solid solutions with body-centered cubic (BCC) type
structure were investigated on hydrogen absorbing-desorbing cycle durability by using H2 without and with 10ppm CO (Hereafter they are expressed as H2 and CO/H2, respectively.). The solid solutions showed excellent cycle durability even after 1,000 cycles of hydrogen absorption and desorption under H2 atmosphere. On the other hand, the capacities decreased rapidly during hydrogen absorbing-desorbing cycles in the beginning of the tests under CO/H2 one. The solutions were not disproportionate though the stable monohydride phase increased, that is, stored hydrogen increased gradually. It was observed that not only microscopic pulverization but also nanoscopically fine-grained crystallization occurred in degraded particles. It is considered that the impurity CO influences bulk structure as an intrinsic factor as well as surface area as an extrinsic factor as regarded as ever. This causes the degradation of the cycle capacities.
Abstract: The pressure-composition (P-C) isotherms of protium and deuterium in Pd sponge particles have been measured over a temperature range from 278K to 323K. Based on these data and combined with literature data, the relationship between the Pd crystal type, particle size and thermodynamic properties in the Pd-H system was investigated. The saturation solubility of hydrogen in solid solution
region ( a-phase) and the absorption plateau pressure increase with the decreasing Pd particle size at ambient temperature, but the desorption plateau pressure does not change with Pd morphology. The effect of Pd morphology on above two parameters gets weaker at higher temperature and the difference of plateau pressure among several different Pd morphologies disappears at higher
temperature, such as 373K. The absolute value of phase transformation enthalpy and entropy from solid solution phase to hydride phase decrease with the decreasing particle size of Pd, but which are the smallest in single crystal Pd. The degree of hysteresis effect in Pd-H system depends on the background density in the sample, so it is the strongest in Pd sponge.
Abstract: Quantum mechanical calculations of the electronic structure of hydrogen storage
materials have undoubtly deepened our fundamental understanding of their basic properties. In addition, ab-initio calculations can provide a useful insight on the hydrogen storage properties that are crucial for applications. In this paper, we focuss on the latter aspects, we discuss the electronic and elastic properties of Haucke compounds of AB5 type (A=La or Y, B=Ni), in relation with their hydrogen absorption properties. The effects of substitutions of Ni by an s element of the 3d series, Cu, as well as by s-p elements of the IIIA (Al) and IVA series (Si, Ge, Sn) for several substitution rates are presented. and discussed in light of available experimental data on electronic, thermodynamic and elastic properties.
Abstract: Niobium metal is one of the promising material for hydrogen purification because of its
high hydrogen permeability. In order to design and develop a new palladium-free hydrogen permeable membrane, it is important to understand the effects of alloying elements on the hydrogen permeability through metals. However, to the niobium metal, the alloying effects still remain unclear. In the present study, using a DC-polarization technique under the diffusion limiting condition, the hydrogen permeability of Nb-5mol%M alloys were investigated in high precision at 573K. Here, M’s were 4d transition metals, Zr, Mo, Ru and Pd. The permeability of niobium is found to be varied with the addition of a small amount of alloying element. For example, the hydrogen permeability of niobium increases by the addition of Zr but decreases by the addition of Ru.