Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Chemistry

Supervisor

Staroverov, Viktor N.

2nd Supervisor

Song, Yang

Abstract

High-pressure experiments on hydrogen-rich compounds provide crucial data for the rational design of hydrogen storage materials. Ethylenediamine bisborane (EDAB), BH3·NH2CH2CH2NH2·BH3, is one of the prime candidates for this role due to its high hydrogen content (10 wt%) and good kinetic stability under ambient conditions. Previous studies of EDAB using in situ Fourier-transform infrared (IR) spectroscopy, Raman spectroscopy, and synchrotron X-ray diffraction (XRD) techniques suggested that EDAB undergoes two possible phase transitions in the pressure range of 0 to 17 GPa. However, the crystal structures of the two new phases arising in these transitions remained unknown due to experimental challenges of in situ structural characterization under high pressures. In this study, we perform Kohn–Sham density functional theory (DFT) calculations and identify the structures of the two high-pressure phases of EDAB. Our results confirm that EDAB undergoes two structural transformations. The first one is at 1 GPa from the orthorhombic Pbca ambient-pressure structure (phase I) to the monoclinic P21/c structure (phase II). The second is at 8 GPa from phase II to another structure (phase III) of the monoclinic P21/c symmetry, which remains the dominant phase up to at least 17 GPa. The mechanism of these phase transitions is attributed to the formation of dihydrogen bonding frameworks, revealed by DFT calculations.

Summary for Lay Audience

Hydrogen-rich chemical compounds have long attracted attention as efficient carriers of elemental hydrogen―a renewable and environmentally friendly potential replacement for fossil fuels. Because hydrogen uptake and release by these compounds is controlled by varying temperature and pressure, it is essential to understand how they behave under compression. Several specific compounds that can store and release hydrogen have been proposed and investigated so far. One of them, called ethylenediamine bisborane (EDAB), was studied by Dr. Song's group at Western using optical spectroscopies and X-ray diffraction techniques. The experimental observations suggested that EDAB undergoes two possible structural changes when the pressure rises from atmospheric to 17 gigapascals (GPa). However, the crystal structures of those two new phases could not be determined due to the challenges of interpreting the experimental data. In this work, we performed quantum-mechanical calculations to identify the crystal structures of two new high-pressure phases of EDAB. We confirmed that EDAB undergoes two transformations near 1 and 8 GPa, respectively, and determined their crystal structures. Our calculations also shed light on how the molecules of EDAB re-orient themselves under applied pressure. These findings contribute to the fundamental understanding of the high-pressure chemistry of EDAB and will help the development of new hydrogen storage materials.

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