Improved dehydrogenation of LiBH 4 supported on nanoscale SiO 2 via liquid phase method
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A wet loading method was developed to produce nano-sized LiBH4 combined with nanoSiO2 templates. The multicomponent LiBH4/SiO2 material synthesized by the wet method has been found to dehydrogenate at much lower temperatures than the pure LiBH4, as well as LiBH4/SiO2 mixtures prepared by ball milling. For example, the onset of dehydrogenation was decreased to about 200 C for a wet-treated LiBH4/SiO2 mixture with a mass ratio of 1:1, and the majority of the hydrogen could be released below 350 C. The improved dehydrogenation of the wet-treated LiBH4/SiO2 mixtures can be attributed to the destabilization of SiO2, resulting in the formation of lithium metasilicate (Li2SiO3) upon heating, and the confinement of LiBH4 to form nanoscale particles.
I. INTRODUCTION
Material for hydrogen storage is one of the key problems in technology currently, which remains a bottleneck for on-board application of hydrogen energy throughout the world. The challenge of hydrogen storage is to find a lightweight storage system with higher hydrogen density. LiBH4 has attracted wide attention due to its high gravimetric and volumetric hydrogen densities (18.6 wt%). Unfortunately, this material is quite stable thermodynamically, and pure LiBH4 will not release hydrogen until heated up to 400 C.1–3 Furthermore, reversibility has been observed only under conditions of high pressure and temperature.4,5 Despite these drawbacks, substantial experiments have already confirmed the potential for improvement in its hydrogen storage performance through modification in various ways. It was reported that enhancement of the hydrogen storage properties of LiBH4 can be achieved through destabilization if it is mixed with various additives, such as metals (Pt6 and Al7), oxides (TiO28 and SiO21,2,9), metal hydrides [V-based bodycentered cubic solid solution,10 and MgH211,12], etc. These additives can promote the hydrogen storage properties of LiBH4 in one or more aspects and ameliorate the thermodynamic, kinetic, or reversible deficiencies of LiBH4. Furthermore, efforts based exclusively on minimization of the particle size, which brings about the size effect, can also improve the re/dehydrogenation properties of LiBH4. For example, by encapsulating LiBH4 into nanoporous carbon scaffolds, Gross et al.13 demonstrated an enhancement of dehydrogenation rate 50 times as fast a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0301 J. Mater. Res., Vol. 25, No. 12, Dec 2010
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as that in the bulk material, when measured at 300 C in a nanostructured hydride formed by filling a porous carbon aerogel host with LiBH4. The aim of this study is to combine the two aforementioned destabilizing techniques by induced crystallization of LiBH4 on nano-sized SiO2 templates. The confirmed enhancement of dehydrogenation due to the formation of lithium metasilicate upon addition of SiO2,9 together with the more favorable kinetics on the basis of the “nm effect” due to the
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