Magnesium and Doped Magnesium Nanostructured Materials for Hydrogen Storage - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 296

Direct decomposition of Grignard reagents in the presence of hydrogen was shown
to give pure magnesium hydride that can be used for dehydrogenation studies,
however purity varied among samples [ 20 ]. A recent example of a solvated metal
atom dispersion method followed by digestive ripening with organic surfactants
produced small (2-4 nm in diameter), monodisperse nanocrystals [ 21 ]. This syn-
thesis is arguably the most effective at making ultrasmall clusters of magnesium
that are expected to show the fastest kinetics at lower temperatures as well as
improved thermodynamics.
The solution-phase synthesis of nanoparticles has emerged as an elegant and
rational method which can yield a high level of control over size [ 22 ], shape [ 23 ,
24 ], surface properties [ 25 ], and connectivity [ 26 ]. Synthetic methods such as
electrochemical reduction from solution result in small particles exhibiting
improved kinetics, but are not amenable to the controllable addition of catalyst
particles [ 27 ]. Mg nanoparticles synthesized from solution have been tested for
hydrogen storage properties, but the early preparation methods were not designed
to control particle size, and the average size and surface composition of these
nanoparticles were not investigated, therefore, not providing a good description of
how much particle diameter can influence the material's properties [ 28 ]. Mg
nanowires have been synthesized in high yield, and show enhanced kinetics for
hydrogen sorption [ 29 ]. As mentioned previously, ball-milled Mg nanoparticles
show even faster kinetics, although there is a large size distribution in these
samples [ 30 ]. A solution-phase synthesis would provide a route toward narrow size
dispersion, high quality nanoparticles.
In a push toward lower energy methods for producing nanoscale magnesium,
there have been several recent reports of solution synthesis methods. The low
reduction potential of magnesium has allowed for electrochemical reduction to
produce magnesium nanocrystals out of THF [ 31 ]. A similar synthesis has pro-
duced both nanoparticles and nanowires, [ 32 ] and there is no indication that
impurities were a byproduct. Direct chemical reduction of magnesium salts to
magnesium metal in glyme at near-room temperature has been shown [ 33 ]. This
method has also demonstrated control over particle size with corresponding
enhanced kinetics. The powder X-ray diffraction characterization of this material
showed pure, crystalline magnesium, with significant peak broadening indicative
of the small size (Fig. 5 ). One final example of a solution method not only reduces
magnesium salts to magnesium metal at room temperature, but also encapsulates
the nanoparticles in a polymer matrix, which keeps the highly pyrophoric mag-
nesium air stable for weeks in air (Fig. 6 )[ 34 ].
2.2 Doped Magnesium Nanocrystal Synthesis
In addition to reducing particle size, dopants can be incorporated into the pure
magnesium nanocrystals to enhance the poor sorption kinetics of magnesium.
This chapter will not specifically address magnesium-based composites such as