Theoretical calculation of hydrogen desorption energies of calcium hydride clusters Deepesh Kumar Dixit*, Krishna Gandhi, Brajesh Kumar Dixit Department of Physics, Bipin Behari P.G. College, Jhansi 284001, India article info Article history: Received 28 February 2011 Received in revised form 31 May 2011 Accepted 2 June 2011 Available online 8 July 2011 Keywords: DFT calculations Calcium hydride clusters Hydrogen storage Desorption energies abstract Recently calcium hydride has attracted attention as a possible component in ternary complex hydrides such as Ca(AlH 4 ) 2 , Ca 2 SiH x and quaternary complex hydrides of the type LieBeCaeH. Although in bulk form CaH 2 decomposes reversibly above 600  centigrade we were motivated to see whether calcium hydride in cluster form has properties suitable for hydrogen storage. We report here the results of DFT calculations using VASP Ò package for clusters Ca n H 2n with n ¼ 1, 2, 3, 4, 6, 8, 10, 12, 14, 16, 20 to get the ground state geometries, energies, bond lengths, and desorption energies, after molecular dynamics optimization. The desorption energy vs. cluster size n curve showed that the desorption energy goes up sharply to w1.4 eV per H 2 for n up to 4, followed by a broad maximum of w1.8 eV per H 2 around n ¼ 12e14, and then tapers off to a nearly constant value of 1.6 eV per H 2 approximating bulk behavior, which compares favorably with previously reported results. Comparison of these results with those of Mg n H 2n shows that Ca n H 2n has a lesser potential as a hydrogen storage medium. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction Metal Binary Hydrides have been investigated both theoreti- cally and experimentally for hydrogen storage purposes. Recently calcium hydride has attracted attention as a possible component in ternary complex hydrides such as Ca(AlH 4 ) 2 , Ca 2 SiH x and quaternary complex hydrides of the type LieBeCaeH [1e4]. Calcium hydride decomposes partially and reversibly at temperatures above 600  C. Calcium hydride is insoluble in most organic solvents. Upon direct contact with water calcium hydroxide [Ca(OH) 2 ] and pure hydrogen gas are produced in a violent reaction and self-ignition is possible. One kg of CaH 2 liberates approx. 1 m 3 of hydrogen. Calcium hydride is used primarily as a source of hydrogen, as a drying agent for liquids and gases, and as a reducing agent for metal oxides. For reversible hydrogen storage, however, CaH 2 in bulk form is thermodynamically too stable to be used. Recently CaH 2 has attracted some attention as a possible component in ternary complex hydrides such as Ca(BH 4 ) 2 , Ca(AlH 4 ) 2 and quaternary complexes of the type CaeAleCaeH, CaeAleLieH, CaeAleNaeH, CaeAleKeH. Our interest in binary metal hydride systems derives from the application of the concept of clusters to the theoretical calculation of hydrogen storage properties. A full band structure calculation of bulk properties is time-consuming and computationally intensive, whereas the main proper- ties of interest can easily be obtained from the calculation of properties of clusters. We, therefore, were motivated to see whether in cluster form CaH 2 has thermodynamic properties useful for hydrogen storage. We have attempted to study how the scaling of the size of the clusters affects the prop- erties of hydrides, primarily the hydrogen desorption enthalpies and the stability of the clusters. We have attempted to explain these in terms of the structure (atomic * Corresponding author. Tel.: þ91 9450080163. E-mail address: dpsd2004@yahoo.co.in (D.K. Dixit). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 37 (2012) 3767 e3771 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.06.004