Structure and hydrogen storage properties of the first rare-earth metal borohydride ammoniate: Y(BH4)3•4NH3

The ammine complex of yttrium borohydride Y(BH4)(3)center dot 4NH(3), which contains a theoretical hydrogen capacity of 11.9 wt.%, has been successfully synthesized via a simple ball milling of YCl3 center dot 4NH(3) and LiBH4. The structure of Y(BH4)(3)center dot 4NH(3), determined by high resolution powder X-ray diffraction, crystallizes in the orthorhombic space group Pc2(1)n with lattice parameters a = 7.1151(1) angstrom, b = 11.4192 (2) angstrom, c = 12.2710(2) angstrom and V = 997.02(2) angstrom(3), in which the dihydrogen bonds with distances in the range of 2.043 to 2.349 angstrom occurred between the NH3 and BH4- units contribute to the hydrogen liberation via the combination reaction of N-H center dot center dot center dot H-B. Thermal gravimetric analysis combined with mass spectrometer results revealed that the decomposition of Y(BH4)(3)center dot 4NH(3) consists of three steps with peaks at 86 degrees C, 179 degrees C and 279 degrees C, respectively, in which the first and second steps mainly release hydrogen accompanied by a fair amount of ammonia emission, while the third one accounts for a pure hydrogen release. Isothermal dehydrogenation results revealed that over 8.7 wt.% hydrogen was released for Y(BH4)(3)center dot 4NH(3) at 200 degrees C, which are improved significantly in terms of both capacity and kinetics comparing to Y(BH4)(3), in which the hydrogen capacity is only 3.2 wt.% at the same temperature. The favorable dehydrogenation properties presented by the Y(BH4)(3)center dot 4NH(3), i.e., lower dehydrogenation temperature and higher nominal hydrogen contents than that of Y(BH4)(3), enable it to be a promising candidate for hydrogen storage. In addition, in situ high resolution X-ray diffraction, differential scanning calorimetry, solid-state B-11 nuclear magnetic resonance and Fourier transform infrared spectroscopy measurements were employed to understand the dehydrogenation pathway of Y(BH4)(3)center dot 4NH(3).