Hydrogenation behaviour of the new composite storage materials Mg- x wt.% CFMmNi 5

Abstract

Alloys with general formula Mg- x wt.% CFMmNi 2, (10, 10, 30, 40 and 50) have been successfully synthesized. The hydrogenation behaviour of these alloy has been extensively investigated. The as-synthesized composite materials have been activated at 550 ± 10 C under a hydrogen pressure of approximately 34 kg cm and their hydrogen storage capacities and desorption kinetics have been evaluated. The new composite hydrogen storage materials, in contrast to the native ingredient CFMmNi 5 (cerium-free misch-metal pentanickellide), have been found to posses much higher storage capacity and to exhibit favourable absorption-desorption kinetics. It has been found that the composite material Mg-30 wt.% CFMmNi 5 has an optimum storage capacity of approximately 5.6 wt.% at the optimum temperature of around 500°C. This is one of the highest reported storage capacities for any known hydrogen storage material. The composite materials Mg- x wt.% CFMmNi 5 also exhibit fast kinetics, represented by the rate of desorption of about 140 cm 1 min 1 which is about three to four times that of CFMmNi 5 alone. Another important hydrogenation behaviour, hysteresis, has been explored through the evaluation of(1/2 RT)In( P t/ P d). The free energy loss due to the hysteresis effect has been found to vary between 1566 J mol 1 at T = 673K and 3299 J mol 1 at T = 773K. The thermodynamic parametersδHandδS for the present optimum composite storage material have been evaluated and found to be 29.00 kJ mol 1H, and −112.14 J mol 1H 2K 1 respectively. The hydriding rate and the improved hydrogen storage capacity of these composite alloys have been found to be strongly correlated with the structural and microstructural characteristics as brought out through XRD, SEM and EDAX techniques. Based on the observed structural and microstructural characteristics, details of the hydrogenation behaviour have been outlined in terms of multiphasic nature and availability of free nickel on the surface. The present composite materials correspond to a new hydrogen storage material with one of the highest storage capacities (around 5.6 wt.%), suitable desorption kinetics and other amenable hydrogenation features, e.g. hysteresis and thermodynamic (δHandδS) parameters.