On the mechanically pulverized MmNi4.6Fe0.4 as a viable hydrogen storage material

The present study deals with investigations on the AB5-type MmNi4.6Fe0.4 hydrogen storage alloy with improved storage capacity of ∼1.7 wt%. The as-synthesized MmNi4.6Fe0.4 intermetallic alloy has a storage capacity of ∼1.5 wt% with kinetics ≈25 cm3/min/g. In search of effects, which may lead to improvement of storage capacity and kinetics, we have carried out several possible material modifications including substitution of 3d-transition elements such as Fe, Co, Mn, etc. at Ni sites, surface treatment and ball-milling. It has been found that the last material modification, i.e. ball-milling technique gives optimum results. The ball-milling technique parameters like speed, time duration, ball to powder ratio and medium of milling has been optimized. The maximum storage capacity of ∼1.7 wt% with kinetics of ≈35 cm3/min/g is obtained when pulverization of the as-synthesized (RF melted) material employing an attritor mill. By varying the medium, duration and speed of ball milling, it was found that the optimum conditions and estimates correspond to medium: hexane, duration: 10 min, speed: 200 rpm. The PCT evaluation was carried out using Sievertʹs type apparatus. The structural and microstructural characterizations were explored using XRD and SEM. XRD explorations revealed that full-width at half-maximum (FWHM) before and after pulverizations are 0.20 and 0.25°, i.e. there is a broadening of ∼25% in the width of the peaks after pulverization. The microstructural investigations revealed that the average particle size of MmNi4.6Fe0.4 after pulverization was ∼2.5 times less than that of the as-synthesized alloy. Smaller particles together with fresh surfaces are the most likely cause of enhanced hydrogenation (storage capacity and kinetics) behaviour.