Papers by Author: Simone Giusepponi

Abstract: Hydrogen desorption from hydride matrix is still an open field of research. Extensive abinitio molecular dynamics simulations are performed to characterize the desorption process at the interface MgH2-Mg. The numerical model succesfully repoduces the experimental desorption temperature for the hydride with and without Fe catalyst. Formation energies and work of adhesion are computed and linked to the desorption mechanism. Moreover a detailed analysis of the structural data reveals the role played by the catalysts in the lowering the desorption temperature.

Abstract: Magnesium is one of the most promising materials for hydrogen storage due to its high capacity and low cost. Unfortunately, practical applications are for the moment limited by the slow kinetics and the high operating temperature. Nanostructuring magnesium hydride MgH2, generally by ball milling, introduces plastic deformations and catalysts that highly enhances the H2 absorption and desorption. However a fundamental understanding of the role played by catalysts and interfaces in MgH2 is still lacking. Microscopic characterization of MgH2-Mg system with and without heavy metal catalysts, is achieved by combining accurate SEM observations of samples after partial desorption process and atomic level ab-initio molecular dynamics simulations of MgH2-Mg interfaces. The experimental method is based on low voltage SEM observations of cross sectional powder samples, prepared by a new specific metallographic process. Identification of nucleation sites of the sorption reaction and their correlation with the presence of catalyst particles is achieved by suitable experimental conditions. Moreover ab-initio molecular dynamics clarifies the interplay of interfaces and the deformations induced during desorption by the presence of catalysts that are able to lower binding energies and free hydrogen atoms toward interfaces. Both approaches confirm and characterize the nucleation step in the catalysts driven phase transformation.

Abstract: We studied the atomic-level structure of a model Mg-MgH2 interface by means of the Car-Parrinello molecular dynamics method (CPMD). The interface was characterized in terms of total energy calculations, and an estimate of the work of adhesion was given, in good agreement with experimental results on similar systems. Furthermore, the interface was studied in a range of temperatures of interest for the desorption of hydrogen. We determined the diffusivity of atomic hydrogen as a function of the temperature, and give an estimate of the desorption temperature.