In Situ High-Pressure and Low-Temperature Study of Ammonia Borane by Raman Spectroscopy

As a potential hydrogen storage material, ammonia borane (NH3BH3) has received extensive investigation among many solid-state chemical hydrides over the past few decades. 4 Ammonia borane is a lightweight molecular complex with a high hydrogen content (19.6 wt %) that exceeds the 2015 U.S. Department of Energy target (9 wt %) for on-board hydrogen storage systems. Consequently, substantial research efforts were made toward the understanding of hydrogen chemistry involving ammonia borane and related B N compounds. 9 The hydrogen release mechanism has been extensively studied for ammonia borane, and it was found that the three thermolysis steps required different high temperatures and exhibited very slow kinetics. Therefore, the hydrogen storage application using ammonia borane and its derivatives has been examined with an extended list of materials and conditions. For instance, a class of composition modified ammonia borane derivatives, such as alkali-metal amidoboranes, was found to release hydrogen at relatively lower temperatures. More recently, Li et al. showed that hydrogen can be released from ammonia borane confined in the metal organic framework with enhanced kinetics and at lower temperatures (e.g., ∼30 C). The structures of ammonia borane have been studied using X-ray diffraction, neutron diffraction, and theoretical calculations under ambient conditions or at low temperatures. 19 Under ambient conditions, ammonia borane adopts the disordered body-centered tetragonal structure (space group I4mm with two molecules per unit cell, Figure 1a) with cell parameters a = 5.255 and c = 5.048 Å. At low temperatures, an ordered orthorhombic structure (space group Pmn21 with two molecules per unit cell, Figure 1b) with cell parameters a = 5.517, b = 4.742, and c = 5.020 Å was identified by Bowden et al. using X-ray diffraction at 90 K and by Klooster et al. using single crystal neutron diffraction at 200 K. In their studies, the unconventional dihydrogen bonding was reinforced due to the strong dipole dipole intermolecular interactions and the short distances between the hydrogen atoms with N H being the proton donor and H B being the proton acceptor. The application of high pressure to materials may induce significant changes in molecular structures and associated properties, such as enhanced hydrogen storage capacities, and therefore, a wide range of hydride complexes (e.g., ammonia borane, diborane, calcium borohydride, and sodium amide) as potential hydrogen storage materials have been investigated under high pressures. 24 Using vibrational spectroscopy, X-ray diffraction, and neutron diffraction as well as ab initio calculations, in particular, ammonia borane has been extensively studied under high-pressure conditions as well. 31 Early spectroscopy studies demonstrated that NH3BH3 undergoes two phase transitions upon compression to 40 kbar. Later, Lin et al. and Xie et al. performed independent highpressure studies on ammonia borane up to 20 GPa using Raman spectroscopic and combined Raman/IR spectroscopy,