Nanoconfined light metal hydrides for reversible hydrogen storage
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Introduction Solid-state systems provide an attractive option for safe and compact hydrogen storage. High volumetric and gravimetric hydrogen densities are realized if hydrogen is allowed to react reversibly with light metals, such as magnesium, to form metal hydrides. However, none of the light metal hydrides meets all practical hydrogen storage requirements, especially with respect to having fast release, full reversibility, and an equilibrium pressure in the desired range. In the past decade, nanosizing metal hydrides using a porous scaffold has emerged as an effective strategy toward meeting these requirements.1–4 In this article, we give a brief overview of the impact of nanosizing, especially by employing a nanoscaffold, on the hydrogen storage properties of light metal hydrides, highlighting fundamental aspects, giving practical examples, and discussing possible future developments.
Particle size effects It is well known that the physicochemical properties of materials change if the particle size is reduced to the low nanometers range. A well-known example is gold, which
changes from a golden colored inert metal into a red colored, low melting temperature, reactive, and catalytically active material once the particle size is reduced below 10–20 nm.5 What can we expect when we decrease the size of metal hydride particles? In nanoparticles, a significant fraction of the atoms are located at the surface, while none of the interior atoms are very far from the surface. This means that we expect very fast kinetics for hydrogen release and uptake, as these are surface-mediated processes and typically involve diffusion through the solid state. Indeed this is generally observed, quite independent of the exact metal hydride system involved. Importantly, the equilibrium between metal and hydrogen on one side and the metal hydride on the other side changes. For particles 2–20 nm in size, this is mostly related to the significant contribution of the surface energy term to the overall stability of the nanoparticles. If the metal hydride has a much larger surface energy than the metal, the equilibrium will shift toward the metal with decreasing size; however, if the metal has a higher surface energy, the situation is reversed. This is illustrated in Figure 1, which depicts the theoretically predicted change in equilibrium hydrogen release temperatures
Petra E. de Jongh, Debye Institute for Nanomaterials Science–Utrecht University, The Netherlands; [email protected] Mark Allendorf, Sandia National Laboratories, CA, USA; [email protected] John J. Vajo, HRL Laboratories, CA, USA; [email protected] Claudia Zlotea, Institut de Chimie et des Materiaux−Paris, France; [email protected] DOI: 10.1557/mrs.2013.108
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MRS BULLETIN • VOLUME 38 • JUNE 2013 • www.mrs.org/bulletin
© 2013 Materials Research Society
NANOCONFINED LIGHT METAL HYDRIDES FOR REVERSIBLE HYDROGEN STORAGE
region is not constant, but increases with increasing hydrogen content. The amount of hydrogen stored in the interior of Pd nanoparticles is lower
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