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A 2.25 at.% V decorated Mg nanoblade array has been fabricated by a dynamic shadowing growth technique. It can absorb and desorb hydrogen rapidly at temperatures T>500 K after activation by one hydrogenation cycling, with low hydrogen absorption activation energy of 35.0 +/- 1.2 kJ/mol H2 and desorption activation energy of 65.0 +/- 0.3 kJ/mol H2. The saturated hydrogen content in Mg nanoblades decreases gradually with the number of cyclings, which is mainly caused by the formation and growth of Mg2Si hillock defects at the nanoblade/substrate interface even though a diffusion barrier layer consisting of a 500 nm Ti nanorod array and 500 nm Ti film was deposited between the Mg nanoblades and the Si substrate. The improved hydrogen sorption kinetics is attributed to both the catalytic effect of the V coating and the unique nanoblade morphology with large surface area and small hydrogen diffusion length.

Abstract: We carry out a computational investigation to show how the exponential concentration dependence of hydrogen diffusion, which was recently verified in a combined experimental and analytical study, could affect the characteristics of hydrogenation and dehydrogenation of a vanadium-coated magnesium nanoblade. A reaction model is built that separates hydrogen surface sorption and interior diffusion during the hydrogenation/dehydrogenation process. For the hydrogenation process, the hydrogen surface adsorption is much faster than the hydrogen diffusion, resulting in high hydrogen concentration buildup at the surface at a relatively low temperature. With increasing temperature, the hydrogen diffusion time decreases more rapidly than the hydrogen surface adsorption time. This leads to a relatively low-gradient diffusion field in the nanoblade during most time of the hydrogenation process, and no shell–core structure with a finite hydride layer is observed. However, for the dehydrogenation process, when hydrogen molecules are released at the surface, a hydride core is formed inside the nanoblade and the interface recedes gradually. The receding rate of the hydride core is determined by the hydrogen molecule release rate. In a two-dimensional simulation with decorated vanadium catalyst islands on the surface, isolated interior hydride islands are sometimes observed before the hydride core entirely fades away. The hydride core boundary is sharper at lower temperature when the surface reaction rate is high relative to the interior diffusion rate. [Copyright &y& Elsevier]

Abstract: Using a glancing angle (co)deposition technique, ∼4.6at.% V has been coated on the surface of individual Mg nanoblades and doped into Mg nanostructures fabricated at different deposition angles. The hydrogen storage properties of the formed V-decorated and V-doped Mg nanostructures depend strongly on how the nanocatalyst V is surrounded by the host Mg. The V-doped Mg sample has lower activation energies for hydrogen absorption (E a a =35.3±0.9kJ/mol H2) and hydrogen desorption (E a d =38.9±0.3kJ/mol H2) than the V-decorated Mg sample when deposited at the same deposition angle of θ =70°. The activation energies of the doped samples increase gradually with the decrease of the θ angle. We also find that the porosity of the Mg nanostructures plays a secondary role. A phenomenological model based on a heterogeneous reaction is proposed to explain the different hydrogen desorption activation energies obtained for different V–Mg nanostructured samples. [Copyright &y& Elsevier]

The hydrogenation of Mg film and Mg nanoblade array fabricated on Ti coated Si substrates has been studied and compared. The nanoblades start to absorb hydrogen at a temperature between 250 and 300 °C, which is much lower than 350 °C for Mg film. However, the saturated total hydrogen uptake in nanoblades is less than half of that in the film, resulting from MgO formation by air exposure. The nanoblade morphology with large surface area and small hydrogen diffusion length, and the catalytic effect of Ti layer, are two main reasons for the nanoblade hydrogenation behavior. [ABSTRACT FROM AUTHOR]

Using an oblique angle codeposition technique, 0–8 at. % Ti has been doped into Mg and aligned nanostructure arrays, composed of hexagonal Mg single crystals with preferred orientations, are formed. Ti doping produces nanorod structures instead of the nanoblade structures that are formed from pure Mg deposition. This is due to the pinning effect of Ti for Mg adatom diffusion. There is a linear shift of the Mg diffraction peaks toward the higher 2θ angles with the Ti dopant concentration. This is possibly caused by the partial substitution of large Mg atoms by small Ti atoms. [ABSTRACT FROM AUTHOR]