Enhanced hydrogen uptake and release kinetics and capacity for magnesium nanocatalyst composites

The objective of this study was to investigate the hydrogen absorption kinetics and capacity of Mg-nano Ni composites. A dry particle coating technique (Theta Composer) was utilized for coating magnesium particles with nano-Ni. Subsequently, hydrogen absorption and desorption kinetics were evaluated by varying system parameters (i.e. coating time, catalyst loading, coating speed and heating rate). Hydrogen absorption curves plotted as a function of time showed that composites processed for longer periods of time exhibited significantly higher absorption rates. With increased coating time, the catalyst was more evenly distributed over the Mg surface, resulting in a product with increased hydrogen capacity and kinetics. A change in the rate limiting mechanism from the interfacial growth of MgH2 to diffusion into the Mg particle was observed.;SEM/TEM characterization verified that magnesium hydride forms from the surface rather than from random nucleation and growth in Mg. An analytical model based upon the shrinking core concept was developed to evaluate the dependence of hydrogen absorption on two important rate limiting mechanisms: diffusion and the interfacial growth of MgH2. Hydrogen absorption capacity was expressed as a function of temperature, hydrogen pressure and Mg particle size at the given hydrogenation time. The analytical solution agreed very well with the experimental data for magnesium hydride formation.;A high speed orbiting ball media (HSOBM) processor was developed and utilized to fabricate flake-shaped materials (thin metal flakes with large diameters). Several aspects of flake characteristics produced by the process were studied, including: flake diameter, thickness, morphology (as a function of processing time), ball media count and weight loading. Improvement in hydrogen storage kinetics and capacity of Mg by this novel approach utilizing high aspect ratio powders coated with Ni nanocatalysts was evaluated. The Mg flakes were fabricated by the HSOBM process and were coated with Ni nanocatalysts using the Theta Composer. The flakes thus produced possess more favorable hydrogen absorption/desorption characteristics and improved hydrogen storage capacity than spherical particles. Hydrogen absorption kinetics was identified to be more sensitive to variations in geometric shape as opposed to changes in grain size.