On the synthesis and characterization of some new AB 5 type MmNi 4.3Al 0.3Mn 0.4, LaNi 5- xSi x ( x=0.1, 0.3, 0.5) and Mg− x wt% CFMmNi 5−y wt% Si hydrogen storage materials

Abstract

The viability and feasibility of Hydrogen Energy becoming the clean alternative to Fossil Fuel Energy through replacement of ‘Fossil Fuel’ with ‘Hydrogen’ (the Green Fuel) is inextricably interlinked with development of ‘Hydrogen Storage Systems’. Out of the high pressure gaseous hydrogen, liquid hydrogen, storage in glass microspheres, activated carbon, zeolites, hydrogen rich liquids and solid state hydrides, the last option is of implicit importance. Out of the AB (e.g. FeTi, storage capacity — 1.75 wt%), AB 2 or A 2B (Mg 2Ni — 3.8 wt%), AB 5 (LaNi 5, MmNi 5 — 1.5 wt%) and K 2 PtCl 6 type (Mg 2FeH 6 — 5.2 wt%); the AB 5 type holds potential promise due to easy activation, tolerance to impurities of charging H 2 gas and avid amenability towards material tailoring for improved and better hydrogenation characteristics. We have carried out synthesis, characterization of several of the AB 5 type storage materials. The present paper is aimed at describing and discussing some of our more recent efforts in regard to this. In the present study the hydrogen storage material (MH) has been synthesized through normal casting (Radio Frequency (RF) induction melting) and melt-spinning techniques. The improvements in basic alloys LaNi 5/MmNi 5 have been studied through structural, microstructural and hydrogenation characteristics. The main features revealed by XRD characterizations are the existence of the free Ni and Si together with AB 5 material in melt-spun alloy of LaNi 5- x Si x . These free Ni and Si were found to disappear, giving rise to a singular material after hydrogenation. Also in melt-spun alloy growth has taken place in a direction perpendicular to the c-axis. Melt-spun version was found to be superior over bulk version in regard to kinetics and activation process. For MmNi 4.3Al 0.3Mn 0.4 alloy, melt-spun version has higher storage capacity (1.2 wt%) than bulk version (0.96 wt%). Also the melt-spun version has faster kinetics and improved activation. For example, the kinetics corresponding to the melt-spun version of LaNi 4.7Si 0.3 is 60% faster than the corresponding bulk. Si substitution results in faster kinetics and improved activation. For example, the kinetics of the melt-spun version of LaNi 4.7Si 0.3 is 70% faster than the melt-spun form of MmNi 4.3Al 0.3Mn 0.4. These AB 5-type materials are particularly attractive in relation to MH cathode for high energy density Ni–MH batteries. In addition to the above type AB 5 storage materials, we have also investigated Mg based composites. Of particular interest is the composite material Mg− x wt% CFMmNi 5−y wt% Si ( x=48; y=2). The as-synthesized composite materials have been activated at 500°C under a hydrogen pressure of ∼40 bar and their hydrogen storage capacities and kinetics have been evaluated. It has been found that the composite material corresponding to Mg−48 wt% CFMmNi 5−2 wt% Si has a maximum hydrogen storage capacity of ∼5.0 wt% at ∼400°C. The small metalloid (Si) substitution in the present material in contrast to Mg− x wt% CFMmNi 5 improves the absorption and desorption kinetics. Extensive structural and microstructural studies before and after hydrogenation have been carried out to unravel the details of hydrogenation behaviour. These results will be described and discussed.