International Journal of Hydrogen Energy 29 (2004) 1151–1156 www.elsevier.com/locate/ijhydene Studies on improvement of hydrogen storage capacity of AB 5 type:MmNi 4:6 Fe 0:4 alloy Akanksha Singh, B.K. Singh, D.J. Davidson, O.N. Srivastava * Physics Department, Banaras Hindu University Varanasi, 221005, India Accepted 30 October 2003 Abstract This paper deals with enhancement of hydrogen storage capacity for AB5 type alloy MmNi4:6Fe0:4 from ∼ 1:5to ∼ 2:04 wt%. It has been shown that through suitable processing involving ball milling with specic parameters (200 rpm, 20 min), the alloy particles besides getting fractured to smaller particles also become strained. The maximum strain, increase in lattice constant and unit cell volume are achieved for ball milling parameters of ∼ 200 rpm and 20 min. It has been suggested that the increase in the unit cell volume results in increase of interstitial hole sizes. This induces higher number of interstitials in unit cell of the material MmNi4:6Fe0:4 to be occupied by hydrogen atoms resulting in increase of hydrogen storage capacity from its value of ∼ 1:5 wt% obtained in the as-synthesized material to ∼ 2:04 wt% for ball-milled materials which embody strained particles. ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen storage capacity; Ball milling; Unit cell volume; Interstitial hole size 1. Introduction Hydrogen, the third most abundant chemical element on earth maintains a premier position in the list of renewable fuels. Hydrogen has the highest energy density per unit weight and has a diversied number of uses from inter- nal combustion engines to fuel cells. The major problem of utilizing hydrogen as a fuel is to store it economically and safely. Although we can store hydrogen in liquid and gaseous form, they have their own limitations [1,2]. The most attractive way to store hydrogen safely and econom- ically is in metal hydrides. In metal hydrides, hydrogen can be stored indenitely and released whenever needed. Regarding the types of these storage materials, they are basically AB5 (typied by LaNi5, MmNi5, etc.), AB (ex- emplied by TiX, where X=Fe, Co, Ni, Cr, V, Mn), A2B and AB2 (elucidated by Mg 2 Ni and ZrM2 where M=V, Cr, Mn, Fe, Co and Mo) types [2]. Even though AB and ∗ Corresponding author. Tel.: +91-542-23684-68; fax: +91-542- 23684-68. E-mail address: hepons@yahoo.com (O.N. Srivastava). AB2 materials have a higher hydrogen capacity than AB5 materials, they require special activation processes [3,4]. The AB5 type material is the material of choice in many cases and generally has a storage capacity of about 1:5 wt% at room temperature [5]. There is, therefore, always an attempt to enhance the storage capacity to be greater than 1:5 wt%. Apparently there are two routes for increasing the hydrogen storage capacity. One of these corresponds to nding altogether new materials as e.g. graphitic nanobres and sodium alnates [6,7]. The other re- lates to modifying the state of the art storage materials e.g. LaNi5= MmNi5 to enhance the storage capacity. This pa- per reports the studies of enhancement of storage capacity through modication of MmNi5 storage material through Fe substitution on Ni sites coupled with pulverization of the as-synthesized material. The modication of MmNi5 has been done through Fe substitution on Ni sites and pulverization has been carried out through ball milling. It has been found that was compared to the storage capacity of ∼ 1:50 wt% [8] for the present material MmNi5, for the cationic substituted material MmNi4:6Fe0:4 subjected to pulverization (ball milling at 200 rpm and 20 min), the 0360-3199/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2003.10.014