HYDROGENATION BEHAVIOR OF THE NEW COMPOSITE STORAGE MATERIALS MG-X WT.PERCENT CFMMNI(5)

Alloys with general formula Mg-x wt.% CFMmNi(5) (x = 20, 30, 40 and 50 ) have been successfully synthesized. The hydrogenation behaviour of t hese alloy has been extensively investigated. The as-synthesized compo site materials have been activated at 550 +/- 10 degrees C under a hyd rogen pressure of approximately 34 kg cm(-2) and their hydrogen storag e capacities and desorption kinetics have been evaluated. The new comp osite hydrogen storage materials, in contrast to the native ingredient CFMmNi(5) (cerium-free misch-metal pentanickellide), have been found to possess much higher storage capacity and to exhibit favourable abso rption-desorption kinetics. It has been found that the composite mater ial Mg-30 wt.% CFMmNi(5) has an optimum storage capacity of approximat ely 5.6 wt.% at the optimum temperature of around 500 degrees C. This is one of the highest reported storage capacities for any known hydrog en storage material. The composite materials Mg-x wt.% CFMmNi(5) also exhibit fast kinetics, represented by the rate of desorption of about 140 cm(3) min(-1) which is about three to four times that of CFMmNi(5) alone. Another important hydrogenation behaviour, hysteresis, has bee n explored through the evaluation of (1/2RT) In(P-f/P-d). The free ene rgy loss due to the hysteresis effect has been found to vary between 1 566 J mol(-1) at T = 673 K and 3299 J mol(-1) at T = 773 degrees K. Th e thermodynamic parameters Delta H and Delta S for the present optimum composite storage material have been evaluated and found to be -29.00 kJ mol(-1) Hz and -112.14 J mol(-1)H(2)K(-1) respectively. The hydrid ing 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 an d EDAX techniques. Based on the observed structural and microstructura l characteristics, details of the hydrogenation behaviour have been ou tlined in terms of multiphasic nature and availability of free nickel on the surface. The present composite materials correspond to a new hy drogen storage material with one of the highest storage capacities (ar ound 5.6 wt.%), suitable desorption kinetics and other amenable hydrog enation features, e.g. hysteresis and thermodynamic (Delta H and Delta S) parameters.