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

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 nuclear quadrupole coupling constants (NQCCs) with high accuracy, and obtain the useful structural information using these parameters. Sodium borohydride, NaBH4, as a metal hydride complex, is an appropriate candidate for the use 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 their hydrogen desorption ability. 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 occurs more feasible at lower temperatures in β-phase compared to other phases. NBO results are in agreement with calculated NQCCs. Calculations were performed using Gaussian 09 program in B3LYP/6-311G*.