Theoretical calculation of hydrogen desorption energies of calcium hydride clusters

Recently calcium hydride has attracted attention as a possible component in ternary complex hydrides such as Ca(AlH4)2, Ca2SiHx and quaternary complex hydrides of the type Li–B–Ca–H. Although in bulk form CaH2 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® package for clusters CanH2n with n = 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 ∼1.4 eV per H2 for n up to 4, followed by a broad maximum of ∼1.8 eV per H2 around n = 12–14, and then tapers off to a nearly constant value of 1.6 eV per H2 approximating bulk behavior, which compares favorably with previously reported results. Comparison of these results with those of MgnH2n shows that CanH2n has a lesser potential as a hydrogen storage medium.