Abstract
The structural and hydrogen storage properties were studied of nanostructured Mg thin films prepared by two different methods, namely plasma sputter deposition and pulsed laser deposition (PLD). Cross-sectional transmission electron microscopy (TEM) shows that in both cases the films grow in the shape of closely-stacked columns extending throughout the film thickness, while containing polycrystalline grains and grain boundary defects. Subsequent hydrogenation leads to a clear reduction in the presence of such defects. Selected area electron diffraction (SAED) on the films confirms the hcp-Mg to rutile tetragonal MgH 2 transformation upon hydrogenation, following the martensitic-like orientation relationship with Mg(0 0 0 2)//MgH 2(1 1 0)//Si(0 0 2). The hydrogen sorption temperatures reduce significantly from ∼670 to ∼475 K by capping the Mg films with a thin Pd layer, which plays a key role in enhancing the rate-limiting process of dissociating the hydrogen molecules at the sample surface. A maximum hydrogen uptake of 4–7.5 wt% is reached under optimum hydrogen loading conditions of hydrogen pressures between 0.25 and 1.0 MPa at a temperature of ∼470 K for both types of films. Nevertheless, cycling experiments showed that a clear reduction in hydrogen content occurs within only a few cycles due to partial delamination of the top Pd layer, which poses a clear limit in practical applications.
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