Magnesium for Hydrogen Storage : from Micrometer to Nanometer

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

Hydrogen Storage in Magnesium Clusters: Quantum Chemical Study

Magnesium hydride is cheap and contains 7.7 wt % hydrogen, making it one of the most attractive hydrogen storage materials. However, thermodynamics dictate that hydrogen desorption from bulk magnesium hydride only takes place at or above 300 degrees C, which is a major impediment for practical application. A few results in the literature, related to disordered materials and very thin layers, indicate that lower desorption temperatures are possible. We systematically investigated the effect of crystal grain size on the thermodynamic stability of magnesium and magnesium hydride, using ab initio Hartree-Fock and density functional theory calculations. Also, the stepwise desorption of hydrogen was followed in detail. As expected, both magnesium and magnesium hydride become less stable with decreasing cluster size, notably for clusters smaller than 20 magnesium atoms. However, magnesium hydride destabilizes more strongly than magnesium. As a result, the hydrogen desorption energy decreases significantly when the crystal grain size becomes smaller than ~ 1.3 nm. For instance, an MgH2 crystallite size of 0.9 nm corresponds to a desorption temperature of only 200 degrees C. This predicted decrease of the hydrgen desorption temperature is an important step toward the application of Mg as a hydrogen storage material

Sulfur Speciation of Crude Oils by Partial Least Squares Regression Modeling of Their Infrared Spectra

Research has been carried out to determine the feasibility of partial least-squares regression (PLS) modeling of infrared (IR) spectra of crude oils as a tool for fast sulfur speciation. The study is a continuation of a previously developed method to predict long and short residue properties of crude oils from IR and near-infrared (NIR) spectra. Retention data of two-dimensional gas chromatography (GC GC) of 47 crude oil samples have been used as input for modeling the corresponding IR spectra. A total of 10 different PLS prediction models have been built: 1 for the total sulfur content and 9 for the sulfur compound classes (1) sulfides, thiols, disulfides, and thiophenes, (2) aryl-sulfides, (3) benzothiophenes, (4) naphthenic-benzothiophenes, (5) dibenzothiophenes, (6) naphthenic-dibenzothiophenes, (7) benzonaphthothiophenes, (8) naphthenic-benzo-naphthothiophenes, and (9) dinaphthothiophenes. Research was carried out on a set of 47 IR spectra of which 28 were selected for calibration by means of a principal component analysis. The remaining 19 spectra were used as a test set to validate the PLS regression models. The results confirm the conclusion from previous studies that PLS modeling of IR spectra to predict the total sulfur concentration of a crude oil is a valuable alternative for the commonly applied physicochemical ASTM method D2622. Besides, the concentration of dibenzothiophenes and three different benzothiophene classes can be predicted with reasonable accuracy. The corresponding models offer a valuable tool for quick on-site screening on these compounds, which are potentially harmful for production plants. The models for the remaining sulfur compound classes are insufficiently accurate to be used as a method for detailed sulfur speciation of crude oils

The Preparation of Carbon-Supported Magnesium Nanoparticles using Melt Infiltration

Magnesium dihydride contains 7.7 wt % hydrogen. However, its application for hydrogen storage is impeded by its high stability and slow kinetics. Bringing the size of Mg(H2) into the nanometer range will not only enhance the reaction rates but has also been theoretically predicted to change the thermodynamic stability and destabilize the MgH2 with respect to Mg. However, the preparation of such small particles is a major challenge. We identified a method to prepare large amounts of nanometer-sized nonoxidized magnesium crystallites. The method is based on infiltration of nanoporous carbon with molten magnesium. The size of the Mg crystallites is directly influenced by the pore size of the carbon and can be varied from 2–5 to less than 2 nm. The majority of the nanocrystallites is not oxidized after preparation. No bulk magnesium was detected in the samples with nanoparticle loadings up to 15 wt % on carbon. These 3D supported nanomaterials present interesting systems to study how nanosizing and support interaction can steer the hydrogen sorption properties of metal hydrides