First-Principles Study on Hydrogen Atom Hopping in NaAlH 4
- PDF / 118,783 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 41 Downloads / 208 Views
1216-W08-30
First-Principles study on Hydrogen atom hopping in NaAlH4 H. Wang, A. Tezuka, H. Ogawa and T. Ikeshoji Research Institute for Computational Sciences (RICS), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568, Japan
ABSTRACT Hydrogen vacancy effect on the activation energy for self-diffusion is investigated by NEB method. The path was calculated by moving a hydrogen atom from a nearby AlH4- complex into the vacancy in the AlH4- complex. Compared with the substitution enthalpy of hydrogen vacancy, the activation energy for self-diffusion is easier to achieve during the dehydrogenation process. INTRODUCTION Hydrogen is a very attractive energy source or energy mediator because it has the highest ratio of valence electrons to protons among all elements and it has very high energy gain per electron [1]. Owing to the high mass density of these light-element hydrides, many experimental and theoretical researches are being carried out to use them in a hydrogen storage system. However, hydrogen in these materials is held by strong covalent or ionic bonds. Consequently, the hydrogen desorption temperatures are high and the kinetics is slow. In recent times, sodium alanate, NaAlH4, has attracted attention as one of potential hydrogen-storage material with a 5.6wt% of hydrogen storage capacity. NaAlH4 can store and release hydrogen reversibly. In addition, it has a moderate hydrogen gravimetric capacity and relatively rapid hydrogenation kinetics. Higher temperature than the room temperature is, however, required for desorption of hydrogen. In order to make the absorption/desorption process reversible under practical conditions, NaAlH4 must be doped with a catalyst. Bogdanovic and Schwickardi experimentally showed that there were a reduction in the decomposition temperature of NaAlH4 and a reversible release of hydrogen doped with TiO2 [2]. The addition of small amount of TiCl3 to sodium alanate was also found not only to make the reaction reversible, but also to markedly lower the hydrogen desorption temperature [2]. On the basis of an experimental study, Palumbo et al. suggested that a model that aims at interpreting the decomposition reactions and kinetics of doped NaAlH4 should take into account the hydrogen mobility and trapping and the stoichiometric defects [3]. Íniguez et al. theoretically showed that Ti-doped NaAlH4 has an increased number of bound hydrogen atoms and an increased number of vacancies; further, decomposition of sodium alanate occurred at a lower temperature [4]. From these studies, we can expect the hydrogen vacancies (H vacancies) to play an important role in desorption of hydrogen from sodium alanate. All researches above have investigated the location of Ti and its effect on the dissociation of H2 and bond length with H atoms. However kinetic research on the diffusion, which connects the transfer of tetragonal NaAlH4 to hexahidride Na3AlH6, has not yet accomplished. Based on the description above, in this study, the main objective of this study is t
Data Loading...
