International Journal of Hydrogen Energy 32 (2007) 2971 – 2976 www.elsevier.com/locate/ijhydene Thermodynamical, structural, hydrogen storage properties and simulation studies of P–C isotherms of (La , Mm) Ni 5-y Fe y Rajesh Kumar Singh a , M.V. Lototsky b, c , O.N. Srivastava a , ∗ a Department of Physics, Banaras Hindu University, Varanasi-221005, India b Institute for Energy Technology, P.O. Box 40, N-2027 Kjeller, Norway c Institute of Mechanical Engineering Problems NAS, Kharkiv 61046, Ukraine Received 27 September 2006; received in revised form 30 January 2007; accepted 30 January 2007 Available online 23 March 2007 Abstract The present work is focussed on the synthesis, structural/microstructural characterization, hydrogenation behaviour and computer simulation studies of P–C isotherms of a new series of alloys represented by La x Mm 1-x Ni 5-y Fe y (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 and y = 0.1), where Mm denotes Ce-rich mischmetal. The alloys have been analysed using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. The alloy corresponding to LaNi 4.9 Fe 0.1 has been found to have high storage capacity of ∼1.68 wt% which is one of the highest reported capacity for AB 5 type storage systems. A feasible mathematical model has been developed to simulate the P–C isotherms. The model describes the asymmetry of the ‘pressure–composition’ isotherms, temperature-dependent plateau slopes and smooth transitions between ,  +  and -regions. P–C isotherms, enthalpy and entropy changes (H and S ) have been evaluated experimentally for the alloys La x Mm 1-x Ni 4.9 Fe 0.1 in order to have input data for the simulation. Special software was developed to simulate the P–C isotherms using the said model. The simulated and experimental curves have been found to match closely.  2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen storage materials; Computer simulation; Storage capacity; P–C–T 1. Introduction Hydrogen, the most abundant element on earth, scores over all renewable fuels regarding reversibility, energy density and compatibility with the environment. The interest in hydrogen as a fuel has grown dramatically. In order for hydrogen to be potentially used, hydrogen storage technologies must be sig- nificantly improved if a hydrogen storage based energy system for transport sector is to be established. Many metals and inter- metallic compounds are capable of reversibly absorbing large amounts of hydrogen. In the families of metal hydrides, (AB, A 2 B, AB 2 and AB 5 ) alloys derived from LaNi 5 (AB 5 ) show some very promising properties including fast and reversible sorption kinetics with small hysteresis, plateau pressure of few bars at room temperature and good cycling life (>1000 cycle). LaNi 5 having a storage capacity of ∼1.5 wt% is the state of art ∗ Corresponding author. Tel./fax: +91 542 2368468. E-mail address: hepons@yahoo.com (O.N. Srivastava). 0360-3199/$ - see front matter  2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.01.017 alloy of AB 5 type. However, La is more expensive than mis- chmetal (mixture of rare earths Ce, La, Nd, Pr). In the present study, efforts were made to substitute Mm in place of La so as to obtain a viable storage alloy. However, it is known that sub- stitution of Mm in place of La decreases the storage capacity of LaNi 5 [1]. To take care of this, efforts have been made to carry out another substitution on Ni site so that this can compensate the said decrease in storage capacity. Most of the studies aimed at improving hydrogen storage characteristics of AB 5 type al- loys are confined to material tailoring through substitution on A or B site [2–6]. Keeping in view the fact that one of the criterion for increasing the hydrogen storage capacity through substitu- tion for Ni is to bring a more electron attractive element at the Ni site [7]. It has been found that Fe substitution, because of the higher electron attractive power of the Fe when substituted in small concentration, leads to enhancement of the hydrogen storage capacity [8]. In the present study, we have synthesized La x Mm 1-x Ni 4.9 Fe 0.1 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) al- loys and studied their structural/microstructural features and