Discovery of Fluidic LiBH_4 on Scaffold Surfaces and Its Application for Fast Co-confinement of LiBH_4−Ca(BH_4)_2 into Mesopores

Abstract

Generation of fluidic LiBH_4 molecules, ƒ-LiBH_4, was demonstrated by NMR spectroscopy of LiBH_4 bulk powder mixed with silica scaffold surface materials under minor heat treatment. In the presence of the fumed silica or mesoporous MCM-41 and SBA-15, LiBH_4 shows increased translational mobility at relatively low temperature (ca. 95 °C) and becomes liquid-like by evidence from ^1H–^(11)B J-coupling in ^1H and ^(11)B MAS NMR or substantial line narrowing of ^7Li static NMR. This high diffusional mobility of LiBH_4 at the molecular level has never been seen for bulk LiBH_4, and the property is attributed to the interfacial interaction with the mesoporous scaffold surfaces. While ƒ-LiBH_4 facilitates the confinement of LiBH_4 itself into various scaffold materials, LiBH_4 migrates along the SBA-15 surface to reach other metal borohydride particles, Ca(BH_4)_2 in this case, and promotes the formation of similarly fluidic LiBH_4–Ca(BH_4)_2 composite (LC solid solution) for coconfinement into mesopores. In situ variable temperature (VT) NMR spectroscopy detects the co-infiltration process of eutectic LiBH_4–Ca(BH_4)_2 composite (LC) into mesopores of SBA-15. The infiltration rates measured for LiBH_4 bulk powder or LC composite showed dependence on pore sizes (MCM-41 vs SBA-15) and heat treatment conditions (static vs MAS).

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