Nanoparticles and 3D Supported Nanomaterials

Many light metal hydride systems are discussed in this book. However, none of them is currently able to meet all the demands for practical on-board hydrogen storage: high volumetric and gravimetric density, reasonably high hydrogen equilibrium pressure at room- or fuel cell operating-temperature, fast kinetics for both loading and unloading and ample reversibility. A variety of strategies is being pursued to meet these goals: ball-milling to improve the kinetics and add catalysts, searching for new yet unknown material compositions, and mixing several different compounds (“reactive hydride composites” or “destabilized hydrides”) [1–6]. Some approaches are remarkably successful, such as ball-milling in general to improve the kinetics [2, 3], and the addition of a small amount of a Ti-based catalyst to improve the kinetics of both hydrogenation and dehydrogenation of NaAlH4 [1]. However although steady progress is reported, we are still far from meeting simultaneously all criteria for on-board storage. In this chapter we discuss an alternative approach: altering the properties of a given material by nanosizing and/or supporting the material. Although this approach is control over morphology and particle size than ball-milling. However, most important is that entering this size regime, one can expect important changes in hydrogen sorption properties, such as improved kinetics and reversibility, and, possibly, a change in thermodynamics. The study of unsupported clusters, nanoparticles and nanostructures is mostly aimed at advancing the fundamental knowledge and understanding of how these effects may be used to the benefit, involving studies on relatively simple binary ionic or interstitial hydrides. However, supporting or confining the materials might be especially relevant for the recently developed more complex systems. These bring new challenges such as slow kinetics and lack of reversibility due to phase segregation for multiple component systems, and the release of unwanted gasses such asNH3 and B2H6 for novel compositions that have a high hydrogen content, but also contain nitrogen or boron. relatively new for hydrogen storage applications, it has been known for a long time in other fields such as heterogeneous catalysis, where a high surface/volume ratio is essential. Interesting material classes are unsupported clusters, nanoparticles and nanostructures, and 3D supported (or scaffolded) nanomaterials. In general the crystallite size of the materials discussed is below 10 nm. This is a clear distinction from materials prepared by ball-milling, presently the most common processing technique, by which crystallite sizes of 10–30nm or above (depending on the material) are achieved. Furthermore, in general (though not always) the alternative preparation techniques used to obtain these nanosized materials (gas-phase deposition, melt infiltration, or solution-based synthesis techniques) allow a much better We will start this chapter by discussing the potential impact of size on the hydrogen sorption thermodynamics and kinetics for metal (hydride) nanoparticles. We will then turn to the experimental results, and, first, treat the literature that deals with the production of unsupported metal (hydride) clusters, nanoparticles and nanostructures and their hydrogen sorption properties. Small particles of light metals are especially difficult to prepare and stabilize, as the sensitivity to oxidation is enhanced by the large volume to surface ratio.We will briefly report on size effects for clusters of transition metals. Then we discuss in more detail Pd(H) nanoparticles. Although strictly speaking not a light metal hydride, extended research has been performed on Pd(H), and it is, hence, an interesting model system to illustrate size effects in metallic (or interstitial) hydrides. As the last type of materials we will treat the ionic hydrides, formed from alkali and alkaline earth metals. In detail we will show results on preparation strategies and first hydrogen sorption results for magnesium-based compounds, being the most investigated example of an ionic binary hydride. Until now, as far as we are aware, it has not been possible to prepare clusters or unsupported nanoparticles