Characterization and hydrogenation of CeNi5−xCrx (x = 0, 1, 2) alloys

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Abstract

The effect of partial substitution of Ni by Cr in CeNi5 intermetallic compound has been studied by pressure–composition isotherm measurements for different temperatures. The samples were prepared of high purity materials using the standard arc melting technique in argon atmosphere. The structure and the elemental composition of different alloys have been investigated by means of XRD, SEM and EDX techniques. The unit cell volume of the alloy was found to increase with increasing Cr content. In order to calculate the hydrogen storage capacity pressure–composition isotherm has been investigated for CeNi5−xCrx (x = 1, 2) alloys in the temperature and pressure ranges of 293  T  333 K and 0.5  P  35 bar, respectively. The P–C–T isotherm for different alloys clearly shows the presence of three regions α, α + β and β. The enthalpy and entropy for the systems has also been calculated using Van’t Hoff plot. The variation of enthalpy and entropy with hydrogen content has also been studied.

Introduction

Hydrogen is a promising alternative, non-polluting and rich source of energy. This makes hydrogen a subject of intensive research in recent years. Hydrogen establishes its inescapable worth in the field of renewable energy and has capability of replacing petroleum for use in automobile sector. Suitable means for hydrogen storage is one of the key requirements for hydrogen based energy technology, which is economically good enough and inexpensive. Metal hydride is one of the safest and reasonably priced way to store hydrogen but still a good quality material has to be investigated.

Intermetallic compounds (IMCs) can absorb large amounts of hydrogen and are practically infinite in numbers over wide ranges of composition. The most frequently followed route involves modifying the original alloy composition AnBm by partial replacement of A or B, or both, with a third, fourth or fifth element [1], [2], [3], [4]. The partial substitution can modify absorption–desorption equilibrium pressures without significant changes in hydrogen capacity. This substitution may affect the bulk properties of the IMCs like lattice constants, or interfacial behaviour such as corrosion resistance, rate and mechanism of the hydrogen evolution reaction [5]. In spite of its so many convenient features for hydrogen storage like high capacity, easy recovery, fast kinetics and convenient P–T relations, LaNi5 is too uneconomical for routine hydrogen storage because of its materials cost. Besides this the synthesis of this alloy involves highly specialized technology and environment control. To make it more useful and economical it is necessary and required to replace either one or both of La or Ni by less expensive metals. The Ce based, AB5 intermetallic compounds are typical hydrogen storage materials for high-pressure applications of metal hydride because of high capacity of hydrogen, cyclic durability and resistance against impurities [6]. The effect of partial substitution of B-component (Ni) with various metals (Cu, Fe, Co, Mn, Al, Si, and Sn) has been studied extensively [6], [7], [8], [9], [10]. It has been observed that Co-substituted composition have unique capability to reduce hysteresis without decreasing equilibrium pressure [10]. The substitution of nickel by copper causes a substantial decrease in the hydrogen dissociation pressure and also lowers the alloy cost [11]. The replacement of nickel by 10–35% Fe and 15% Mn in CeNi5 expands the lattice and produce a significant change in magnetic properties [12]. Cerium containing AB5 type high pressure hydrides have been characterized by an extremely large hysteresis effect [13] and the magnitude of the hysteresis in CeNi5 based systems strongly depends on any variation of intermetallic matrix composition [14].

The purpose of the present work is to study the effect of partial substitution of Ni by Cr in CeNi5. Pressure–composition isotherm studies have been undertaken for CeNi5−xCrx (x = 1, 2) in the temperature range 293 < T < 333 K and pressure range 0.5 < P < 35 bar, respectively. The enthalpy and entropy for the systems have also been calculated using Van’t Hoff plot. The variation of enthalpy and entropy of these two systems have also been studied by plotting them against hydrogen storage capacity.

Section snippets

Experimental

The CeNi5−xCrx (x = 0, 1.0, 2.0) alloys were prepared in an arc furnace under argon atmosphere of about 0.30 bar with stoichiometric proportion of constituent elements of purity 99.99% through the partial substitution of Ni by Cr, at UGC-DAE Consortium for Scientific Research, Indore, India. Each alloy ignot was turned over and remelted thrice for homogeneity. The weight loss percentage of these samples during their preparation was less than 0.5%. Then the ingot was mechanically crushed and ground

Results and discussion

X-ray diffraction (XRD) patterns of CeNi5−xCrx (x = 0, 1.0, 2.0) alloys have been shown in Fig. 1. It has been seen from the XRD data that all the alloys are crystallized as a single-phase compound in the hexagonal CaCu5 type structure. The lattice parameters and unit cell volume of alloys have been presented in Table 1. The values of lattice parameters and unit cell volume are found to increase with increasing Cr content in the alloy which is due to the large atomic radii of Cr (0.13 nm) as

Acknowledgment

Authors are highly grateful to University Grants Commission, New Delhi, India, for providing financial support for this work.

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