KINETICS, MODELING, AND MECHANISTIC ANALYSIS FOR THE DEHYDROGENATION OF MAGNESIUM BOROHYDRIDE

Abstract

The continuing rise in global population, its growing dependence on technological advances, and the rising prices seen for both natural gas and crude oil, have led to an unrelenting strain on the energy resources of our planet. Increased utilization of fossil fuels, the primary energy source of humans for more than 200 years, has currently led to estimates for their depletion before the turn of the century. In addition to this, increased fossil fuel use results in higher concentrations of greenhouse gases (GHG) released into the atmosphere, contributing to climate change and global warming. Such issues therefore make it imperative to find alternative energies that are abundant, energy-dense, and also environmentally friendly. Hydrogen has long been considered such an ideal fuel because it is non-toxic, naturally abundant, and able to be produced from a wide variety of resources. It is also extremely energy-dense with the energy contained within the hydrogen atom to be the largest of any fuel. The energy derived from hydrogen is considered clean energy when produced from electrolysis, and when combined with O2 in a Polymer Electrolyte Membrane (PEM) fuel cell, produces electricity with the only waste products being heat and water. Its extensive use in everyday fuel applications, however, is constrained by its storage considerations in a safe, compact, and cost-effective manner. A considerable amount of research on hydrogen storage today focuses on using material-based methods of storage, for example, within chemical compounds such as magnesium borohydride, Mg(BH4)2 , an extremely promising candidate for hydrogen storage. This is due to its favorable thermodynamics during hydrogen release, its demonstrated reversibility, and its ability to store large quantities of hydrogen, both by volume and by weight. Unfortunately, as is true for most complex hydrides, slow kinetics appear to be the main obstacle to its utilization. To further understand these issues, this thesis investigates the kinetics of Mg(BH4)2 using line-fitting analysis to model isotherms produced at various temperatures during its dehydrogenation to Mg(B3H8)2 , a system known to be readily reversible under moderate conditions. The dehydrogenation kinetics were also investigated following the addition of tetrahydrofuran (THF) to the borohydride, resulting in rapid hydrogen release at lower temperatures compared to the regular borohydride. The dehydrogenation of Mg(BH4)2 with THF also led to the formation of a second product, the highly stable closoborane, MgB10 H10 . Prior work exploring the reversibility of this system indicated hydrogen cycling under even more mild conditions than that of the unsolvated borohydride. The implications of this claim led to its further investigation. Analyses conducted in this thesis utilized Pressure Composition Temperature (PCT) instrumentation, X-ray Diffraction (XRD), 11B/1H Nuclear Magnetic Resonance (NMR) spectroscopy, and Temperature Programmed Desorption (TPD) with Quadrupole Mass Spectrometry (QMS), the latter conducted at the National Renewable Energy Laboratory (NREL) in Golden, CO, USA.