The Hydrogen Storage Properties And Mechanisms Of Aluminum Nitride-based Nano-clusters:First-principles Study

Hydrogen is being considered as an ideal energy carrier due to its efficiency, abundance, and environmental friendliness. However, one of the most difficult challenges in realizing hydrogen economy is to find materials that can store hydrogen with high gravimetric and can reversibly adsorb/desorb hydrogen under ambient conditions. For more than a decade a large number of studies have been devoted to finding and designing ideal materials for hydrogen storage. The wide-gap semiconductors as the hydrogen storage materials have received continuing concern and the facing challenge is how to improve the ability of hydrogen storage at ambient condition. The purpose of this thesis is to investigate how to improve the capability of aluminum nitride-based nanostructures and discuss the micro-mechanism of the hydrogen storage. The main contents are as follows:The capability of Li-decorated (AlN)n (n=12,24,36) nanocages to store hydrogen has been studied by using density functional theory (DFT) with the generalized gradient approximation (GGA). It is found that each Al atom is capable of binding one H2molecule up to a gravimetric density of hydrogen storage of4.7wt%with an average binding energy of0.189.0.154, and0.144eV/H2in the pristine (AlN)n (n=12,24,36) nanocages, respectively. Further, we find that Li atoms can be preferentially decorated on the top of N atoms in (AlN)n (n=12,24,36) nanocages without clustering, and up to two H2molecules can bind to each Li atom with an average binding energy of0.145,0.154,0.102eV/H2in the Lin(AlN)n(n=12,24,36) nanocages, respectively. Both the polarization of the H2molecules induced by the charged Li atom and the hybridization of the Li-2p orbitals with the H-s orbitals contribute to the H2adsorption on the Li atoms. Thus, the Li-decorated (AlN)n (n=12,24,36) nanocages can store hydrogen up to7.7wt%. approaching the U.S. Department of Energy (DOE) target of9wt%by the year2015, and the average binding energies of H2molecules lying in the range of0.1-0.2eV/H2are favorable for the reversible hydrogen adsorption/desorption at ambient conditions. It is also pointed out that when allowed to interact with each other, the agglomeration of Li decorated (AlN)n nanocages would lower the hydrogen storage capacity.