Journal of Power Sources 159 (2006) 132–134 Short communication Hydrogen uptake characteristics of mischmetal based alloy Ankur Jain, R.K. Jain, I.P. Jain ∗ Material Science Laboratory, Centre for Non-Conventional Energy Resources, 14 – Vigyan Bhawan, University of Rajasthan, Jaipur 302004, India Available online 23 May 2006 Abstract Hydrogen storage properties of Mm 39.2 Ni 42.1 Mn 4.9 Al 1.25 Co 10.2 Fe 2.35 alloy have been systematically studied in the present work. An attempt is made to relate the content of hydrogen with change in resistance. It is found that the resistance of material increases with the increase in value of H/M due to hydrogen absorption. Pressure composition (P-C-T) isotherm using water displacement method has been investigated in the temperature and pressure ranges of 308 ≤ T ≤ 338 K and 0.5 ≤ P ≤ 10 bar, respectively. The P-C isotherms show the presence of two single  and  regions one mixed  +  phase. The maximum H (wt%) was found to be around 1.53 at 308 K and around 6 bar. Since enthalpy is an index of thermochemical stability of metal hydride the thermo dynamical parameters viz., the relative partial molar enthalpy (H) and relative partial molar entropy (S) of dissolved hydrogen have been calculated by plotting the Van’t Hoff plot. The variation of H and S with the hydrogen concentration confirm the phase boundaries. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen Storage; P-C-T isotherm; Hydrogen absorption; Thermodynamics 1. Introduction Storing hydrogen in the form of reversible metal hydride is a viable alternative to low storage density forms such as high- pressure gas or cryogenic liquids. Hydrides offer low volume without disadvantages of the cryogenics required for liquids, or the hazards and the inefficiency associated with gas storage. Ide- ally, metal hydride for commercial applications should be able to release H 2 at relatively low temperature, having high volu- metric density, low mass density, rapid hydriding/dehydriding rate, high resistance to pulverization and low cost in both fab- rication and acquisition of raw material. The rare earth based hydrogen storage alloys have been widely used due to their high energy density, high rate of charge discharge ability, long charge discharge cyclic lifetime and environment capability [1–3]. The typical rare earth based hydrogen storage alloy is MmNi 5 , in which Mm denotes mischmetal (a commercial mix- ture consist of mainly rare earth elements La, Ce, Pr, and Nd). Now a days, two kinds of mischmetal, namely La rich and Ce rich mischmetal owing to different mineral resources and extractive metallurgical methods, have been utilized widely in commercial hydrogen storage alloys. Several authors [4–7] have reported that ∗ Corresponding author. Tel.: +91 141 2701602; fax: +91 141 2710880. E-mail addresses: ankurjainankur@sify.com (A. Jain), ipjain46@sify.com (I.P. Jain). the contents of La and Ce in mischmetal had a promising influ- ence on the hydrogen absorption desorption properties, thermo- dynamical parameters, plateau pressure, and cycle lifetime. The storage capacity of the MmNi 5 system is found to lie between 1.2 and 1.4 wt% [8]. One has to bear in mind the fact that one of the criteria for increasing the hydrogen storage capacity through replacement of Ni will be to find a more electron attrac- tive element taking the place of Ni [9]. Srivastav and co-workers [10] have studied Fe in low concentration in Ni (Ni 0.9 Fe 0.1 ) and found that the hydrogen storage capacity has been found to increase to a value of 1.66 wt%. However, when the Fe concen- tration is increased further the hydrogen storage capacity starts decreasing. Tang et al. [11] studied the microstructure and the electrochemical properties of Mm 0.7 Ni 2.8 Co 0.6 hydrogen stor- age alloy and found the highest concentration to be 1.58 wt%. In the present work the hydrogen absorption isotherms and reaction kinetics of Mm 39.2 Ni 42.1 Mn 4.9 Al 1.25 Co 10.2 Fe 2.35 alloy have been measured. The thermodynamical parameters, i.e. the enthalpy and entropy of hydride formation have also been cal- culated. 2. Experimental The alloy was supplied by Dr. Balachandra of Defence Metal- lurgical Research Laboratory (DMRL), Hyderabad, India and it was well characterized by him for its composition homogeneity. 0378-7753/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2006.04.002