Tetra-n-butylammonium Borohydride Semiclathrate: A Hybrid Material for Hydrogen Storage

Abstract

In this study, we demonstrate that tetra-n-butylammonium borohydride [(n-C₄H₉)₄NBH₄] can be used to form a hybrid hydrogen storage material. Powder X-ray diffraction measurements verify the formation of tetra-n-butylammonium borohydride semiclathrate, while Raman spectroscopic and direct gas release measurements confirm the storage of molecular hydrogen within the vacant cavities. Subsequent to clathrate decomposition and the release of physically bound H₂, additional hydrogen was produced from the hybrid system via a hydrolysis reaction between the water host molecules and the incorporated BH₄⁻ anions. The additional hydrogen produced from the hydrolysis reaction resulted in a 170% increase in the gravimetric hydrogen storage capacity, or 27% greater storage than fully occupied THF + H₂ hydrate. The decomposition temperature of tetra-n-butylammonium borohydride semiclathrate was measured at 5.7 °C, which is higher than that for pure THF hydrate (4.4 °C). The present results reveal that the BH₄⁻ anion is capable of stabilizing tetraalkylammonium hydrates.