Magnesium for Hydrogen Storage : from Micrometer to Nanometer

Abstract

Energy systems of the foreseeable future will have to be more reliable, flexible and cost-efficient and have a higher availability to meet the increasing energy demand. Especially considering greenhouse gas emissions, combustion of fossil fuels will be replaced by cleaner energy production. The “Hydrogen Society” is a scenario in which hydrogen (H2) is used as an energy carrier for mobile applications and for energy-load balancing. For widespread use of hydrogen, progress is required in several fields, of which H2 storage is one of the most tenacious. Metal-hydrides are promising candidates for safe, compact and efficient hydrogen storage for mobile applications. However, none of the presently known materials meets all requirements in terms of hydrogen sorption conditions, hydrogen storage capacity and reversibility. Magnesium hydride (MgH2) can store hydrogen up to a weight fraction of 7.7%. However, the major impediment for MgH2 is its H2 desorption temperature of 300 C. The research described in this thesis explores, both theoretically and experimentally, the possibilities to decrease the desorption temperature of MgH2 by decreasing the particle size. First, hydrogen sorption rates and durability of magnesium hydride were enhanced by a bulk-applicable process, comprising fluoridation of the surface and application of a Pd catalyst. Analogous to alloys and thin films, nm-sized metal hydrides are expected to behave differently from the bulk materials. Such particle size-effects on H2-sorption behavior were experimentally observed for a palladium-carbon model system. A theoretical study showed a particle size dependency of the hydrogen sorption thermodynamics of magnesium hydride. Since MgH2 destabilizes stronger than Mg with decreasing particle size, the hydrogen desorption energy decreases when the crystal grain size becomes smaller than ~1.3 nm. These results imply that sub-nm MgH2 crystallites should have a significantly decreased desorption temperature; for instance an MgH2 crystallite of 0.9 nm would already desorb hydrogen at 200 C. This predicted decrease of the H2- desorption temperature is an important step towards the application of Mg as a hydrogen storage material. Since such small particles would coalesce or sinter upon repeated hydrogen charging and discharging, a support material is needed for stabilization. Inert and low weight carbon matrices were tested as a support material for nanoscale magnesium. Mg/C-nanocomposites with 25% weight in Mg were prepared by infiltrating molten Mg into carbon matrices under argon and hydrogen atmospheres. With different TEM techniques Mg(O) nanoparticles of 3nm diameter and smaller were detected in the nanocomposites. Hydrogen-sorption measurements with a high-pressure magnetic suspension balance show 76% of the initial Mg still accessible for reversible hydrogen storage. Up to one third of the magnesium in the composites had increased hydrogen sorption pressures with a corresponding absorption enthalpy of 45 kJ•mol[H2]-1. For this part H2-desorption temperatures were determined around 175 C, 125 C lower than for bulk-MgH2. These results of making the thermodynamics of H2-sorption more favorable by lowering the desorption energy with downsizing could have a major impact on the efficiency of magnesium-based hydrogen storage materials and can most likely be extended to other hydrogen storage materials and other areas of science.