Decomposition mechanisms and the role of transition metal impurities in the kinetics of Ca(BH_4)_2-2NH_3

摘要

We present first-principles density-function theory studies of the formation and migration of native point defects in Ca(BH4)(2)center dot 2NH(3), which is a promising material for hydrogen storage. According to analysis of the energetics, structures, formation and migration of hydrogen-, calcium-, boron- and nitrogen-related defects, we find that the diffusion of hydrogen interstitial (HI) in the bulk is the rate-limiting step for decomposing Ca(BH4)(2)center dot 2NH(3). Our calculated results show that the activation energy for the decomposition process of Ca(BH4)(2)center dot 2NH(3) is 1.41 eV (in a closed vessel). Moreover, electrically active impurities such as Fe, Co and Ni can tailor the kinetics of dehydrogenation of Ca(BH4)(2)center dot 2NH(3) by shifting the Fermi level. These mechanisms provide an appropriate explaining for the experimentally observed improving of the kinetics for hydrogen desorption of Ca(BH4)(2)center dot 2NH(3) through doping transition metal chlorides in a closed vessel. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.