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

Abstract

The ammine complex of yttrium borohydride Y(BH₄)₃·4NH₃, which contains a theoretical hydrogen capacity of 11.9 wt.%, has been successfully synthesized via a simple ball milling of YCl₃·4NH₃ and LiBH₄. The structure of Y(BH₄)₃·4NH₃, determined by high resolution powder X-ray diffraction, crystallizes in the orthorhombic space group Pc2₁n with lattice parameters a = 7.1151(1) Å, b = 11.4192(2) Å, c = 12.2710(2) Å and V = 997.02(2) ų, in which the dihydrogen bonds with distances in the range of 2.043 to 2.349 Å occurred between the NH₃ and BH₄⁻ units contribute to the hydrogen liberation via the combination reaction of N–H⋯H–B. Thermal gravimetric analysis combined with mass spectrometer results revealed that the decomposition of Y(BH₄)₃·4NH₃ consists of three steps with peaks at 86 °C, 179 °C and 279 °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)₃·4NH₃ at 200 °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)₃·4NH₃, 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 ¹¹B nuclear magnetic resonance and Fourier transform infrared spectroscopy measurements were employed to understand the dehydrogenation pathway of Y(BH₄)₃·4NH₃.