Hydrogen absorption in catalysed nanostructured metal hydrides
Example: Nb catalysed and nanostructured MgH2
Magnesium hydride currently has the highest reversible capacity in wt.%. This makes the material highly interesting for hydrogen storage purposes. A drawback of the material is the large enthalpy of formation, which results in high operating temperatures ~300 oC. In addition the kinetics of the loading and unloading reaction is slow. When loading hydrogen, this is mainly a result of the formation of a closed shell of MgH2 around a Mg metal core. The mobility of H in bulk MgH2 is very low due to the lack of H vacancies. In Mg metal the mobility of a dissolved H atom is very high.
The reaction of hydrogen gas with magnesium metal is enhanced significantly by the addition of catalysts such as Nb and V and by using nanostructured powders. In situ neutron diffraction on MgNb0.05 and MgV0.05 powders gives a detailed insight on the magnesium and catalyst phases that exist during the various stages of hydrogen cycling. During the early stage of hydriding (and deuteriding), a MgH0.8<x<2 phase is observed, which does not occur in bulk MgH2 and, thus, appears characteristic for the small particles. The abundant H vacancies will cause this phase to have a much larger hydrogen diffusion coefficient, partly explaining the enhanced kinetics of nanostructured magnesium.
It appears that under relevant experimental conditions, the niobium catalyst is present as NbH1. Second, a hitherto unknown Mg-Nb perovskite phase could be identified that has to result from mechanical alloying of Nb and the MgO layer of the particles (a small percentage of MgO is always present in Mg starting materials). All added Nb is accounted for in the Rietveld fits, so there is no indication of amorphous niobium containing phases. This is different in the Vanadium containing material. Vanadium metal nor its hydride is visible in the diffraction patterns, but electron micrographs show that the V particle size becomes very small, 2-20 nm. In analogy to the Mg-Nb-O Perovskite phase case there are Mg-V-O spinel phases known; unfortunately these have strongly overlapping diffraction patterns with the MgO, so no definite conclusions could be drawn on their occurrence here.
Nanostructuring and catalyzing the Mg enhance the adsorption speed that much that now temperature variations inside macroscopic sample amounts effectively limit the absorption speed and not, as for bulk, the slow kinetics through bulk MgH2 layers: the slight rise of temperature upon hydrogen absorption is enough to raise the equilibrium pressure.
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- CCLRC Highlight 2004/2005, publication of the Council for the Central Laboratory of the Research Councils, Oxfordshire, UK [PDF]