The exploitation of (complex) metal hydride nanoparticles induces several advantages. Nanoparticles may have significantly different properties as compared to bulk materials due to (i) increased surface area of the reactants, (ii) nanoscale diffusion distances, and (iii) increased number of atoms in the grain boundaries and (iv) intimate contact between different reacting solids or a melt. These nanoscale properties facilitate release and uptake of hydrogen and enhance reaction kinetics. The thermodynamics may also be improved. The most widely used technique for preparation of hydride nanoparticles is ball milling. In this top-down approach the size of the bulk material is reduced mechanically, in some cases to less than 100 nm. However, the samples are often contaminated with traces of metals from vial and balls. Furthermore, nanosized particles may grow into larger particles upon hydrogen release and uptake cycles [1].

Confinement of metal hydrides in a nanoporous material is a bottom-up approach, which creates nanoparticles. This limits the particle size of the hydride to the pore size of the scaffold material, which allows direct production of smaller particles than mechanically obtainable. Furthermore, particle growth and agglomeration may be hindered by the compartmentalization of nanoparticles within the scaffold material and also limit the mobility of the decomposition products and keep them in intimate close contact. Thereby, nanoconfinement may improve both hydrogen release and uptake properties of metal hydrides [2-4]. Some results are outlined in Figure 1.

Fig. 1. (Left) Hydrogen storage properties can be improved and even tailored on the nanoscale through the design of the porous scaffold material in which it is embedded [2]. (mid) Several hydrides can react inside the nano pores of the scaffold with faster kinetics and improved stability [3]. (right) The combination of nanoconfinement and the addition of a catalyst synergetic effects and extremely favorable properties of the nanoconfined hydride [4].

[1] T. K. Nielsen, T. R. Jensen and F. Besenbacher, Nanoscale, 2011, 3, 2086-2098.

[2] T. K. Nielsen, K. Manickam, M. Hirscher, F. Besenbacher and T. R. Jensen, Acs Nano, 2009, 3, 3521-3528.

[3] T. K. Nielsen, U. Bösenberg, R. Gosalawit, M. Dornheim, Y. Cerenius, F. Besenbacher and T. R. Jensen,    

      Acs Nano, 2010, 4, 3903-3908.

[4] T. K. Nielsen, M. Polanski, D. Zasada, P. Javadian, F. Besenbacher, J. Bystrzycki, J. Skibsted and T. R. Jensen, Acs

      Nano, 2011, 5, 4056-4064.