Thermodynamic properties of metal hydride nanostructures

Abstract

Hydrogen is considered a good energy carrier candidate for future automotive applications because of its high abundance and its potential role in a carbon-free cycle. The high gravimetric and volumetric storage capacities of metal hydrides make them ideal hydrogen carriers if the limitations associated with their slow hydrogen release kinetics, their high hydrogen release temperatures, and their poor thermal properties can be resolved. In this thesis, the thermodynamic and kinetic improvements on the hydrogen release properties of nanostructured metal hydrides are investigated both theoretically and experimentally. Four main results are presented in this work. The excess volume present in deformed regions is identified as the key factor in explaining the reduction in the enthalpy of formation observed experimentally in nanostructured materials. The impact of excess volume on the enthalpy of formation at OK is quantified using three equations of state, and it is found to be as important as the combined impact of surfaces, grain boundaries and the presence of metastable crystalline phases. Then, the findings on the properties of excess volume are generalized to high temperatures. It is demonstrated that the impact of temperature will be more favorable to a reduction of the enthalpy of formation if a large fraction of the metal hydride is in a state of small excess volume compared to a small fraction of the hydride in a state of high excess volume. The impact of a temperature increase on the enthalpy of formation of metal hydrides is found to offset the effect of the excess volume as calculated at OK.

(cont.) The stability of the regions containing excess volume over multiple hydriding/dehydriding cycles is also calculated. At high temperatures and large excess volumes, the free energy barrier created by the excess entropy reduces the recrystallization rate of the deformed regions by several orders of magnitude. A regime where the benefits of the excess volume on the enthalpy of formation can be maintained is thus identified. Finally, an experiment to study the cycling properties of metal hydrides was designed using Raman spectroscopy. It is demonstrated that the release temperature of hydrogen could be accurately measured using Raman spectroscopy.