Hydrides for storage
Hydrogen Storage is the bottleneck for on-board vehicle applications. Magnesium hydride is one of the most promising candidates for solid-state hydrogen storage due to its light weight, low cost and highly reversible hydrogen storage capacity of 7.6 mass% in MgH2. The high thermodynamic stability and sluggish reaction kinetics limit its practical applications. But making magnesium into nanostructures along with appropriate transition metal catalyst addition could solve the problems.
Nanoblade
Researchers at the University of Georgia, US, have designed and fabricated a vanadium-decorated magnesium nanoblade array structure by coating a thin layer of vanadium onto the two sides of individual magnesium nanoblades. The structures were made using a dynamic shadowing growth (DSG) technique, which is based on a physical vapor deposition method and combines oblique angle deposition (OAD) with substrate manipulation and source control.
Performance
The nanoblades are extremely thin magnesium-based structures, but their width gives them an incredible amount of surface area for their size. The role of vanadium coating as catalytic in the formation and decomposition of MgH2 and the unique nanoblade morphology with large surface area and small hydrogen diffusion length contribute to an overall improvement in hydrogen sorption performance. Specifically, the hydrogen sorption activation energy is reduced from 120–150 kJ/mol H2 for magnesium films or powders to ~35 kJ/mol H2, the hydrogen uptake and release temperatures are reduced even to room temperature, and the hydrogen loading and unloading times are reduced from 50 hours to several minutes.
Unlike three-dimensional springs and rods, nanoblades are extremely thin, with very large surface areas. They also are surprisingly spread out for a uniform nanomaterial, with one to two micron meters in between each blade. For hydrogen storage a large surface area is needed to provide room for the material to expand as more hydrogen atoms are stored. Arranging the nanoblades with microscopic spaces between one another allows them to expand and contract as they absorb hydrogen. The vast surface area of each nanoblade, coupled with the large spaces between each blade, could make them ideal for this application.
This finding could have applications for on-board vehicle applications and in energy storage and fuel cell technology. The nanoblades are also fully recyclable, an important criteria that was established by the Department of Energy for any hydrogen storing material.
Fabrication
To create the nanoblades, the researchers used oblique angle vapor deposition which is a widely used technique for building nanostructures by vaporizing a material, magnesium in this case and allowing the vaporized atoms to deposit on a surface at an angle. As the deposition angle changes, the structure of the material deposited on the surface also changes.
When deposited at zero degrees, the blades obtained are flat, flakey structures overlapping one another and when the deposition angle was increased the blade-like nature of these new nanomaterials becomes apparent. On further increase, the structures first tilted away from the magnesium vapor source instead of the expected inclination toward the source. The blades then quickly curved upward to form nearly vertical structures resembling nanoscale razorblades.