Effect of ZrCrCo alloy on hydrogen storage properties of Mg
Introduction
Magnesium and its alloys have number of benefits to be considered as potential candidate for hydrogen storage [1]. Pure Mg contains 7.6 wt% hydrogen in its hydrogenated form i.e. MgH2 [2]. Its low environmental impact and abundant availability makes it very attractive for hydrogen community. However, hard interaction with hydrogen as seen from its high working temperature and slow kinetics limit its potential technically [3], [4], [5]. During last several years, many efforts have been made to improve sorption kinetics and to lower the sorption temperature. Nano-structuring of MgH2 is one of the most adopted methods to improve the hydrogenation performance. Mechanical milling has been used as a major tool to modify the crystallite and particle size of MgH2 [5], [6], [7]. Zaluska et al. [6] correlate this kinetic improvement with the existence of defects and grain boundaries, which allows easy diffusion and penetration of hydrogen into the material. This method allows kinetic improvement of sorption properties without any significant alteration in thermodynamics. The thermodynamical alteration i.e. the destabilization of MgH2 is possible only when the particle size is less than 5 nm, which has not yet been achieved using mechanical milling [8], [9]. The addition of catalysts such as Co, Ti, Zr, Fe, Ni [3], [4], [10], Nb2O5 [11] and Cr2O3 [12] in the process of mechanical milling has shown further enhancement in the reaction kinetics by reducing the activation energy to dissociate H2. Bobet et al. [3] showed significant increase in the amount of MgH2 formed during sorption process by Co addition. In another work the merits of Zr addition to Mg is claimed by Zaluska et al. [4]. They have shown that nano-crystalline magnesium with 1.5 at% Zr is capable to desorb hydrogen at lower temperature (∼280 °C) within few minutes. Another approach of alloying Mg with the transition metal to form Mg2Ni [13], Mg2Co [14], Mg–La [15] succeeded to destabilize MgH2 and the enthalpy is reduced from −76 kJ mol−1 for MgH2 to −64 kJ mol−1 for Mg2NiH4 [13]. However, the addition of heavier transition metal lowered the hydrogen capacity too. Recently low temperature hydrides such as LaNi5 [16], MmNi5 [17], Ti–V based BCC alloy [18], [19] and Zr based AB2 alloys [20] have been explored as a composite material with Mg in order to improve the sorption behavior of Mg greatly, These low temperature hydrides possess dual characteristics i.e., acting both as catalyst and hydrogen sorption species at the same time, thereby improving the kinetics and maintaining the effective capacity [21], [22]. ZrCr2 can have 3.4 H/f.u. for hexagonal structure while 4.0 H/f.u. for cubic structure. Fernandez et al. showed the improvement in the hydrogenation/dehydrogenation kinetics of Mg by adding ZrCr2 [23]. However the storage capacity was too low in this work. In another work by Wang et al. [24], hydrogen storage capacity could be increased up to 4.25% combined with excellent kinetics, when ZrFe1.4Cr0.6 was used as a composite element. ZrCrNi is another important alloy capable to strongly modify the properties of Mg [25], [26]. Yang et al. [25] have shown the superiority of amorphous ZrNi1.6Cr0.4 and ZrNiCr systems over their crystalline states respectively. They found that Mg–35 wt% ZrNi1.4Cr0.6 composite can release 4.3 wt% H2 at 300 °C in 30 min. Recently, our group investigated the hydrogen storage properties of a number of Mg–ZrCrM composite systems [25], [26], [27], [28]. For Mg–ZrCrNi system [26], we concluded that 10 wt% ZrCrNi alloy content is sufficient to attain improved characteristics. However, such a low amount of ZrCrM is not found suitable for other elements i.e. Fe and Cu, where the system shows remarkable kinetic improvement when 25% or higher amount of ZrCrM alloy is used. The dual property of ZrCrFe alloy, catalytic at high temperature and hydriding at low temperature is found to be responsible for higher content of 4.5 wt% hydrogen in Mg–ZrCrFe composite [28]. In case of Mg–ZrCrCu composite, the formation of Mg2Cu alloy due to diffusion of Mg in ZrCrCu phase during sorption cycles helps in improved kinetics by providing diffusion paths and nucleation sites for hydrogen [27]. These results suggest that changing M in ZrCrM systems has varying effects on hydrogenation properties of Mg. In order to get a better understanding of these systems, we investigate effect on ZrCrCo alloy on structural, morphological and hydrogenation properties of Mg.
Section snippets
Experimental
To prepare ZrCrCo alloy, Zr (99.8%), Cr (99.9%), and Co (99.8%) are mixed in the stoichiometric ratio and melted on the copper hearth of an arc furnace under Ar flow. Titanium ball is used to absorb the residual oxygen/water content presented even in the carrier gas Ar. To get better homogeneity, the alloy is remelted 4 times after inverting it upside down after each melting. The prepared alloy button is crushed in powder and this powder is used for X-ray diffraction measurement and composite
Results and Discussion
Fig. 1(a) illustrates the XRD pattern of powdered ZrCrCo alloy. The pattern is fitted by hexagonal C14 type structure i.e. ZrCrCo follow the same structure of its parent alloy ZrCr2. The lattice parameters of the alloy are calculated as a = 5.013 Å and c = 8.209 Å, which is in close agreement with a report by Hirosawa et al. [21]. The XRD patterns of 5 h milled Mg–x wt% ZrCrCo (x = 25, 50) composites are shown in Fig. 1(b) and (c). The peak intensities corresponding to the ZrCrCo phase is comparatively
Conclusions
Arc Melting method is successfully employed to prepare ZrCrCo alloy, which crystallize in C14 hexagonal structure. High energy ball milling is used for the preparation of Mg–x wt% ZrCrCo composite. Mg does not react with any element of the ZrCrCo alloy during milling. Even after several hydrogenation cycles, both counterparts maintained their identity and converted in to their respective hydride state only. Micro-structural studies confirmed the composite nature of alloy particles showing Mg
Acknowledgement
Authors gratefully acknowledge financial support from Department of Science and Technology, New Delhi, India under FAST Track Scheme. We are thankful to Mr. Amedeo Masci, ENEA-Casaccia, Rome for making SEM images for this work.
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