This paper deals with improvement in the hydrogenation behavior of LaNi₅ type alloys through substitution of Mm (Mischmetal) on La (Lanthanum) site and Fe on Ni site.
The hydrogen storage alloy (La Mm)(Ni_0.8Fe_0.2)₅ has been synthesized by RF induction melting (in 18 kW furnance) of stoichiometric mixtures of high purity individual elements. The amount of hydrogen desorbed was monitored by the volume displacement method using a Sievert’s type apparatus, developed in our laboratory.
The storage capacities for La_1-xMm_x(Ni_0.80Fe_0.20)₅ (x = 0.01, 0.02, 0.03, and 0.05) are 2.16, 2.08, 2.04 and 1.96 wt% respectively The rate of flow of hydrogen during desorption (desorption kinetics) of the optimum material are shown in Fig. 1. Desorption isotherms corresponding to the alloy La_0.99Mm_0.01(Ni_0.80Fe_0.20)₅ are shown in Fig. 2.
Generally increase of Fe concentration results in increase of capacity. However at the same time the surface catalytic activity decreases due to high Fe concentration. Therefore an optimum Fe concentration has been determined .This has been found to be (Ni_0.8Fe_0.2)₅.
We have synthesized AB₅-type materials with formula La_1-xMm_x(Ni_0.80Fe_0.20)₅ (x = 0.01, 0.02, 0.03, and 0.05). A maximum storage capacity of ~2.16 wt% at room temperature and faster kinetics 15 cc/g/min have been observed for the La_0.99Mm_0.01(Ni_0.80Fe_0.20)₅. It is about 44% higher than the capacity of the parent alloy LaNi₅ (~1.5wt%).