Mechanical activation and thermodynamic destabilization of the lithium amide and lithium hydride system

Date of Completion

January 2009

Keywords

Engineering, Materials Science

Degree

Ph.D.

Abstract

The practical application of hydrogen storage in fuel cells for transportation purposes in hydrogen powered vehicles has been one of the major challenges. Complex metal hydrides have been considered as potential materials for practical on-board hydrogen storage applications because of their reversible storage and release of hydrogen, and moderately high gravimetric and volumetric hydrogen capacities. Within the last decade, the Li-N-H or (LiNH2+LiH) system has been investigated intensively as a candidate for practical on-board hydrogen storage. Despite of its reversible storage reaction with a theoretical capacity of 6.5 wt. % H2, there are two primary issues that prevent the utilization of this system for the on-board hydrogen storage: (1) the NH3 emission and (2) a relatively high operating temperature (∼280°C) for reversible hydrogen absorption and desorption at 1 bar of H2. ^ The NH3 emission results in hydrogen fuel contamination which can damage the PEM fuel cells and consequently degrade the hydrogen storage capacity. The utilization of the mechanical activation via the high-energy ball milling has been shown to be an effective way to address the NH 3 emission issue. This is due to the decrease in particle and crystallite size, increase in surface area, and better mixing of LiNH2 and LiH. The dehydriding reaction of the (LiNH2+LiH) mixture was also substantially enhanced by high-energy ball milling. The peak temperature for releasing large amounts of H2 from the mixture was reduced by ∼ 100°C via ball milling at room temperature for 180 min. ^ To lower the hydrogen absorption and desorption temperatures further and increase the H2 equilibrium pressure of the (LiNH2+LiH) system, the thermodynamic destabilization of the lithium amide through partial substitution of Li by Mg in lithium hydride has been pursued and confirmed that the (2LiNH 2+MgH2) mixture has a higher thermodynamic driving force for dehydrogenation than the (LiNH2+LiH) mixture, as reported by many other studies. This higher thermodynamic driving force results in a lower onset temperature for the dehydrogenation of the (2LiNH2+MgH 2) mixture than that of the (LiNH2+LiH) mixture. Furthermore, the isothermal hydriding and dehydriding cycling performance of the Li-Mg-N-H (1:2) system, starting with Li2MgN2H2 at 200°C has been examined in this study. This system exhibits a slow hydriding rate controlled by diffusion and a fast dehydriding rate that exhibits two distinct stages consisting of a very fast release at the beginning followed by a slow release. The hydriding and dehydriding reaction pathways during the cycling have been proposed. ^

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