Abstract
Aiming at the development of less energy-consumptive process for producing hydrogen storage materials, an experimental study was carried out on absorption-desorption characteristics of hydrogen for the RmNi5 compounds which were produced from various mischmetal oxides (Rm2O3) by the reduction-diffusion (R-D) method. In the R-D process, a mischmetal oxide was reduced by metallic calcium in the presence of nickel particles and the reduced mischmetal elements diffused into the nickel particles to directly form a RmNi5 compound.The hydrogen pressure-composition isotherms (PCT) were determined by a Sievert's apparatus for hydrides of (a) lanthanum-base RmNi5, (b) cerium-base RmNi5, (c) cerium-base RmNi4.5 Coos, RmNi4.5 Al0.3 and RmNi4.5 Mn0.5, and (d) cerium-base RmNi3.55 Co0.75 Mn0.5 Al0.3. The hydride of the lanthanum-base RmNi5 presented a plateau region extending up to nH (=H/M) of about 1, where the equilibrium PH2 in the desorption isotherm was 4.5 atm at 20°C. On the other hand, a small hydrogen absorbability was shown for the cerium-base RmNi5 and no plateau region was observed in the PCT diagram. The hydrogen absorbability increased significantly by replacing some nickel sites of lattice of the cerium-base RmNi5 by cobalt, aluminium or manganese and forming RmNi4.5 Co0.5, RmNi4.5 Al0.5 or RmNi4.5 Mn0.5. The cerium-base RmNi3.5 5 Co0.75 Mn0.4 Al0.3 cornpound also showed favourable characteristics amenable to the use for a hydrogen storage material. It absorbed hydrogen at ambient temperatures so fast that there seemed to be no need of an activation procedure for the hydrogen absorption.
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