Wolfgang Oelerich

Metal oxides as catalysts for improved hydrogen sorption in nanocrystalline Mg-based materials

Abstract Nanocrystalline MgH2/MexOy- and Mg2NiH4/MexOy-powders were produced by high energy ball milling (MexOy=Sc2O3, TiO2, V2O5, Cr2O3, Mn2O3, Fe3O4, CuO, Al2O3, SiO2). The hydrogen absorption and desorption kinetics of the nanocomposite materials were determined with respect to a technical application. Some of the selected oxides lead to an enormous catalytic acceleration of hydrogen sorption compared to pure nanocrystalline materials. In absorption, the catalytic effect of TiO2, V2O5, Cr2O3, Mn2O3, Fe3O4, and CuO is comparable. Concerning desorption, the composite material containing Fe3O4 shows the fastest kinetics followed by V2O5, Mn2O3, Cr2O3 and TiO2. Only 0.2 mole% of the catalysts is sufficient to provide a fast sorption kinetics.

Comparison of the catalytic effects of V, V2O5, VN, and VC on the hydrogen sorption of nanocrystalline Mg

Abstract Nanocrystalline MgH 2 with additions of V 2 O 5 , VN, VC, or high-purity V was produced by high-energy ball milling. The hydrogen absorption and desorption kinetics of these novel materials were determined in order to compare the catalytic effects of the additions. The results show a significant enhancement of the hydrogen reaction kinetics under the chosen experimental conditions only for V 2 O 5 , VN, and VC, while the influence of high-purity V is negligible. This result is discussed with respect to previous reports.

Critical assessment and thermodynamic modeling of the Mg–H system

Abstract A comprehensive critical assessment has been made of the experimental data of the Mg–H system. Based on the selected original experimental phase diagram data from the literature, a set of thermodynamic functions for the Mg–H system was chosen and the parameters were optimized using the least squares method. Four different analytical descriptions were used to model the four different types of stable phases in the Mg–H system: gas, liquid, solid solution phase α -(Mg), and the stoichiometric compound β -MgH 2 . Most of the experimental information is in agreement with the modeling, especially the dissociation pressures of MgH 2 at different temperatures, the invariant equilibria, and the hydrogen solubilities in magnesium at one atmosphere. Calculation of the system has been used to thermodynamically analyze some experimental information.

Thermodynamic analysis of the hydriding process of Mg-Ni alloys

Abstract A consistent set of thermodynamic functions for the Mg–Ni–H system has been developed from experimental data, and the Mg–Ni–H phase diagram has been calculated thermodynamically. The extended ternary solubility of hydrogen in Mg 2 Ni, the ternary compound phase Mg 2 NiH 4 , and the effect of Ni addition on the hydrogen solubility in molten magnesium have been modeled thermodynamically. The ternary solid solution phases were extrapolated from the thermodynamic descriptions of the binary edge systems. The thermodynamic functions of the Mg–Ni–H system have been applied to study the hydriding process of Mg–Ni alloys.

Thermodynamics of the Ni–H system

Abstract Thermodynamic properties of the Ni–H system have been analyzed by means of the CALPHAD method. Thermodynamic models have been defined to describe the Gibbs energy of the individual phases, and the model parameters have been optimized from the original experimental hydrogen solubility data. Magnetic ordering in solid nickel has also been considered. The nickel hydride that only forms at very high hydrogen pressure has not been included. No secondary values, such as the hydrogen solubility data at 1 bar derived from those values measured at lower pressures, were used in optimization. The heat of solution of hydrogen in nickel is calculated in dependence of temperature. The effect of the magnetic ordering in solid nickel on the heat of solution of hydrogen and, consequently, on the hydrogen solubility, is discussed. The CALPHAD method is demonstrated to be a powerful tool for determining the heat of solution of gaseous atoms in metals.

Hydrogen Sorption of Nanocrystalline Mg at Reduced Temperatures by Metal‐Oxide Catalysts

Metal hydrides offer a safer alternative for storage of hydrogen for use as a zero-emission vehicle fuel than storage in compressed or liquid form. Mg hydride also has a high storage capacity by weight and is therefore favored for mobile applications. However, light metal hydrides have not been considered competitive because of their rather sluggish sorption kinetics. Recently, a breakthrough was achieved by preparing nanocrystalline hydrides using high-energy ball milling. The sorption kinetics can further be improved by adding catalysts to magnesium. In this communication results for MgH 2 with different oxide catalysts are compared. Special emphasis is put on the sorption kinetics at reduced temperatures, from 300 °C down to room temperature.

Nanocrystalline Mg-Based Hydrides: Hydrogen Storage for the Zero-Emission Vehicle

Nanocrystalline light metal hydrides are interesting for hydrogen storage in zeroemission vehicles. In this paper, recent detailed results on the sorption behaviour of nanocrystalline Mg and Mg-based alloys are presented. Different transition metals have been added to Mg to achieve a thermodynamic destabilisation of the hydride, thus lowering the desorption temperatures to about 230°C. A breakthrough in sorption kinetics was achieved by using nanocrystalline materials produced by high-energy milling. Additionally, new catalysts based on metal oxides have been added to further enhance the sorption reaction. As a result, hydrogen is absorbed and released within minutes instead of hours as in conventional Mg hydrides. The new materials fulfill the current requirements for the desired application with respect to both, capacity and kinetics.

Tailoring nanocrystalline materials towards potential applications

Abstract The high interface area in nanocrystalline materials leads to advanced structural and functional properties that are interesting for a variety of applications and products. Three distinct examples for potential applications are given: Nanocrystalline and submicron-sized light-weight intermetallics based on γ-TiAl exhibiting favourable deformation behaviour at reduced temperatures, nanocrystalline cermet coatings produced by thermal spray process exhibiting improved hardness and wear resistance, and nanocrystalline Mg hydride-based composites for hydrogen storage in future mobile applications exhibiting extremely high, reversible storage capacity and fast kinetics.