PNNL

Abstract

The effects of high pressure and temperature on the bonding in ammonia borane (AB), NH3BH3 and decaborane (DB), B10H14 and their interactions with molecular hydrogen (H2) were investigated using Raman spectroscopy in a diamond anvil cell. At 0.7 GPa, AB becomes amorphous between 120 and 127 °C, indicating a positive Clapeyron slope. Heated to 140 °C, AB begins to undergo decomposition to polyaminoborane. The amorphous and decomposed AB does not recrystallize back to AB during slow cooling to room temperature or upon application of high pressure up to 3 GPa, underscoring the challenge of rehydrogenation of decomposed AB. The molecular Raman modes broaden in the reacted phase, and the NH3 modes show no pressure dependence. DB was studied at room temperature up to 11 GPa. The observed frequency dependence with pressure (dv /dP) and mode Grüneisen parameters varied for different spectral groups, and a new transition was identified at approximately 3 GPa. In both DB and heated AB, we found that they could store additional H2 with the application of pressure. We estimate that we can store approximately 3 wt % H2 in heated AB at 3 GPa and 1 wt % H2 in DB at 4.5 GPa. "T.A. acknowledges support from the U.S. Department of Energy, Office of Basic Energy, Division of Chemical Sciences, Biosciences and Geosciences. PNNL is operated by Battelle."