Nanoconfinement of LiBH4·NH3 towards enhanced hydrogen generation

Successful synthesis of LiBH₄·NH₃ confined in nanoporous silicon dioxide (LiBH₄·NH₃@SiO₂) was achieved via a new “ammonia-deliquescence” method, which avoids the involvement of any solvents during the process of synthesis. Compared to the pure LiBH₄·NH₃, the confined LiBH₄·NH₃@SiO₂ exhibited significantly improved dehydrogenation properties, which not only suppressed the emission of NH₃, but also decreased the onset dehydrogenation temperature to 60 °C, thus leading to an enhanced conversion of NH₃ to H₂. In the temperature range of 60–300 °C, the mole ratio of H₂ release for the confined LiBH₄·NH₃@SiO₂ is 85 mol % of the total gas evolved, compared to 2.66 mol % for the pristine LiBH₄·NH₃. Isothermal dehydrogenation results showed that the LiBH₄·NH₃@SiO₂ is able to release about 1.26, 2.09, and 2.35 equiv. of hydrogen, at 150 °C, 200 °C, and 250 °C, respectively. From analysis of the Fourier transform infrared, Raman, and nuclear magnetic resonance spectra of the confined LiBH₄·NH₃@SiO₂ sample heated to various temperatures, as well as its dehydrogenation product under NH₃ atmosphere, it is proposed that the improved dehydrogenation of LiBH₄·NH₃@SiO₂ is mainly attributable to two crucial factors resulting from the nanoconfinement: (1) stabilization of the NH₃ in the nanopores of SiO₂, and (2) enhanced combination of LiBH₄ and NH₃ groups, leading to fast dehydrogenation at low temperature.     

NH3BH3/LiBH4·NH3 modified by metal hydrides for advanced dehydrogenation

The significantly enhanced dehydrogenation performance of binary complex system, NH₃BH₃/LiBH₄·NH₃, were achieved through a chemical modification of LiH to form ternary composites of x (LiH–NH₃BH₃)/LiBH₄·NH₃. Among the studied composites, 3LiH–3NH₃BH₃/LiBH₄·NH₃ released ca. 10 wt. % high-pure hydrogen (>99.9 mol%) below 100 °C with fast kinetics, while less than 8 wt. % hydrogen, accompanied with a fair number of volatile byproducts, were released from 3NH₃BH₃/LiBH₄·NH₃ at the same conditions. Further investigations revealed that the hydrogen emission from x (LiH–NH₃BH₃)/LiBH₄·NH₃ composites is based on the combination mechanism of H^δ+ and H^δ− through the interaction between LiH–NH₃BH₃ and NH₃ group in LiBH₄·NH₃, in which the controllable protic hydrogen source from the stabilized NH₃ group played a crucial role in providing optimal stoichiometric ratio of H^δ+ and H^δ−, and thus leading to the significant improvement of dehydrogenation capacity and purity.     

Preparation and dehydrogenation properties of two cobalt-based ammine borohydrides: CoCl3·3NH3/3LiBH4 and CoCl2·3NH3/2LiBH4

Two new cobalt-based ammine borohydrides were prepared via ball milling of LiBH₄ and CoCl_n·3NH₃ (n = 3, 2) with molar ratios of 3:1 and 2:1, respectively. X-ray diffraction (XRD) results revealed the as-prepared composites having amorphous state. Thermogravimetric analysis-mass spectrometry (TG-MS) measurements showed that the two composites mainly release H₂, concurrent with the evolution of a small amount of NH₃. Further results showed that the excessive addition of LiBH₄ can suppress the liberation of NH₃, resulting in the release of H₂ with a high purity (>99 mol.%). By combination with the temperature-programmed-desorption (TPD) results, the CoCl₃·3NH₃/4LiBH₄ and CoCl₂·3NH₃/3LiBH₄composites can release 7.3 wt.% (4.2 wt.% including LiCl) and 4.2 wt.% (2.0 wt.% including LiCl) pure hydrogen, respectively, in the temperature range of 25–300 °C. Isothermal dehydrogenation results reveal that CoCl₃·3NH₃/3LiBH₄ shows favorable dehydrogenation rate at low temperatures, releasing about 5.2 wt.% (2.9 wt.% including LiCl) of hydrogen within 45 min at 80 °C.     

Effects of Mg and Al doping on the electronic structure and dehydrogenation of LiBH4·NH3

The electronic structures and bonding characters, the occupation energies of dopants, as well as the formation energies of Frenkel defects in pure LiBH₄·NH₃ and in Mg- and Al-substituted LiBH₄·NH₃ were investigated by using first-principles calculations. The occupation energies show that the substitutions with Mg and Al destabilize LiBH₄·NH₃ and that Mg substitution is easier than Al substitution. Substitution with Mg or Al partly reduced interactions between B–H and N–H atoms, thus improving the dehydrogenation property of LiBH₄·NH₃. At the same time, substitution with Mg or Al increases the interactions between metal and N atoms, which stabilize the NH₃ group and inhibit the release of NH₃ during dehydrogenation. The formation energy of Frenkel defects indicates that Mg or Al doping facilitates the formation of Frenkel defects. Our theoretical studies show that Mg and Al are good candidates but Al is better than Mg for improving the dehydrogenation property of LiBH₄·NH₃.     

LiBH4·NH3BH3: A new lithium borohydride ammonia borane compound with a novel structure and favorable hydrogen storage properties

Mechanically milling ammonia borane and lithium borohydride in equivalent molar ratio results in the formation of a new complex, LiBH₄·NH₃BH₃. Its structure was successfully determined using combined X-ray diffraction and first-principles calculations. LiBH₄·NH₃BH₃ was carefully studied in terms of its decomposition behavior and reversible dehydrogenation property, particularly in comparison with the component phases. In parallel to the property examination, X-ray diffraction and Fourier transformation infrared spectroscopy techniques were employed to monitor the phase evolution and bonding structure changes in the reaction process. Our study found that LiBH₄·NH₃BH₃ first disproportionates into (LiBH₄)₂·NH₃BH₃ and NH₃BH₃, and the resulting mixture exhibits a three-step decomposition behavior upon heating to 450 °C, totally yielding ∼15.7 wt% hydrogen. Interestingly, it was found that h-BN was formed at such a moderate temperature. And owing to the in situ formation of h-BN, LiBH₄·NH₃BH₃ exhibits significantly improved reversible dehydrogenation properties in comparison with the LiBH₄ phase.