Investigation On Hydrogen Storage Properties Of Lithium Amidoborane And Single Aln Sheet

The stable structure of Lithium amidoborane LiNH₂BH₃（LAB） was investigatedby using the first-principles method based on the density functional theory （DFT） andthe Molecular Dynamics （MD） method. Our calculated results reveal the reason forthe improved hydrogen desorption property of LAB as compared with that of AB.Then hydrogen storage properties on a single AlN sheet under an applied electric fieldwas investigated by using the linear combination of atomic orbital （LCAO） methodbased on the DFT. It is found that the site on top of the N atom is the favorite site forH₂molecule adsorption without an applied electric field. When being applied anelectric field, H₂molecules can be adsorbed on both side of the AlN sheet. Bycontrasting the adsorption energy of H₂molecules, the bound length of H-Al and H-Nbonds and the corrsponding density of states, we reveal that the electric field can beused to enhance the stability and reversibility of hydrogen storage.The main results are shown in the following:（1） The decreased N-B bond length in LAB reveals a strong covalent interactionbetween N and B compared to that in ammonia–borane （AB）. The Li-N-B bond angleincreases obviously from single molecule to crystalline phase LAB. Our calculatedresults show that the increased interaction between N and B, the movement of thehydrogen energy level towards the Fermi energy （EF） level and the decreased energygap in the LAB are responsible for the decreased stability of hydrogen in LAB.（2） The configuration of H₂adsorption on the N site is the most stable one whilethe H₂molecule adsorption on the Al site is a metastable state without the electricfield. The H₂molecule can easily diffuse from the Al site to the N site at ambienttemperature with low diffusion barrier.（3） Double layers of H₂adsorption can be realized due to the electric-fieldinduced polarization. The most stable configuration for double layers of H₂adsorptionis that the H₂molecules are adsorbed on the N site above the AlN sheet and on the Alsite below the AlN sheet simultaneously under the downward electric field.（4） It is shown that the bond length of N-H, Al-H decreases and the bond lengthof H-H increases as the electric field increases within a certain range. The adsorption energy of the H₂molecule increases as well with the increasing electric field. Theelectronic structure shows the interaction between N and H increased when theelectric field is applied. These reveal that the stability of H₂molecules being absorbedon the AlN sheet is improved under the electric field.（5） The hydrogen sorption-desorption reversibility is obtained by adjusting thestrength of the applied electric field.