Study of Dehydrogenation of Pressure-induced NaBH4 Structures Using Calculated NQCC Parameters

Abstract

Nuclear quadrupole resonance (NQR) spectroscopy is an accurate method for determination of electric charge distribution around quadrupolar nuclei. Using ab initio computational methods, it is possible to calculate the Nuclear Quadrupole Coupling Constants (NQCCs) with high accuracy and obtain the useful structural information by using these parameters. Sodium Borohydride, NaBH4, is a metal hydride complex which is a good candidate for being applied in fuel cells as hydrogen storage material with high capacity of 10.6 wt %. Despite the high capacity of hydrogen storage, hydrogen desorption occurs at high temperatures due to high stability and strong bonds of this compound. This problem limits the practical usage of NaBH4 in fuel cells. One way to overcome this problem is applying the high-pressure techniques and using the pressure-induced NaBH4 structures. Under ambient conditions, NaBH4 has a cubic structure (α-NaBH4) that can be converted to β- and γ-NaBH4 by increasing the pressure. In the present research, charge distribution of α-NaBH4 nanocrystal has been compared to that of high-pressure structures using calculated NQCCs, to study the effect of pressure on different NaBH4 structures and hydrogen desorption ability of them. Our results show the smaller value of 2H–NQCCs and higher value of 11B-NQCCs for β-NaBH4 respect to other structures. In other words, the B-H bond is weaker in β-NaBH4 and it is expected that dehydrogenation occur more feasible and at lower temperature in β-phase compared to other phases. NBO results are in agreement with Calculated NQCCs. Calculations performed using Gaussian 09 program in B3LYP/6-311G*.