Metathesis of LiNH2BH3 and FeCl3 in THF solution was investigated in detail. Instead of formation of expected Fe amidoborane i.e., 3LiNH2BH3 + FeCl3 → 3LiCl + Fe(NH2BH3)3, 1.5 equiv. H2/LiNH2BH3 together with LiCl and a black precipitate was produced as a result of salt metathesis and reduction of Fe3+ by BH3. The hydrogen was desorbed in two steps involving a homogeneous interaction of the two starting chemicals to form [Fe(H2NBH2)3] precipitate and subsequent solid-state dissociation of [Fe(H2NBH2)3] to yield a polymeric product, [Fe(HNBH)3]n, respectively. FTIR evidenced the persistence of B–H and N–H stretches in the above two solid products and following the dissociation of [Fe(H2NBH2)3] to release 1 equiv. H2/LiNH2BH3 the B–N bond strengthened. Mössbauer and XAFS both indicated that Fe atoms in these solids are in very similar chemical environments, linking to the neighbouring N and B atoms and bearing slightly positive charge. Most likely, H2NBH2 in [Fe(H2NBH2)3] binds to Fe as a π-bound ligand. The mechanism of salt metathesis and reduction of Fe3+ was confirmed based on simulation work on the homogeneous reaction process.

Promotion of dehydrogenation based on the interaction of [BH4]− and [NH2]− sources has been demonstrated to be one of the most effective approaches in developing an advanced borohydride/amide hydrogen storage combined system. The Ca(BH4)2–2Mg(NH2)2 and Ca(BH4)2–2Ca(NH2)2 composites are thereby synthesized in the present work. It is found that the binary combined systems exhibit an onset dehydrogenation temperature of ∼220 °C, which is ∼100 °C lower than that of pristine Ca(BH4)2. The hydrogen release measurements for Ca(BH4)2–2Mg(NH2)2 and Ca(BH4)2–2Ca(NH2)2 samples below 480 °C show desorption amounts of 8.3 and 6.8 wt % hydrogen, respectively. The dehydrogenation of both samples is accompanied by an ammonia emission of <1.4 mol %. The characterizations such as X-ray diffraction and nuclear magnetic resonance on the postdehydrogenated samples indicate that the dehydrogenation reactions are in the pathways of Ca(BH4)2 + 2 Mg(NH2)2 → 1/3 [Ca3Mg6(BN2)6] + 8 H2 and Ca(BH4)2 + 2 Ca(NH2)2 → 1/3 Ca9(BN2)6 + 8 H2, respectively. Compared with pristine Ca(BH4)2 sample, both possess lower activation energy for dehydrogenation. Further investigation reveals that the interaction of B–H and N–H may be one of main driving forces for dehydrogenation of borohydride/amide combined system.

Improving effects of LiH and Co-catalyst on the dehydrogenation of Li4BN3H10 were investigated in this work. It was found that 9.6 wt % or 5 equiv. moles of H2 can be evolved from the promoted sample at 250 °C, which is 110 °C lower than that of pristine Li4BN3H10. More importantly, NH3 emission is depressed dramatically, with its concentration in hydrogen decreasing from 12 mol % to 0.008 mol % (or 80 ppm). Melting of Li4BN3H10 is essential for one unit of LiNH2 fleeing from the structure and interacting with LiH to evolve 1 equiv. mole of H2, in the meantime, Co-catalyst takes effect on the dissociation of remaining Li3BN2H8, resulting in the evolution of another 4 equiv. moles of H2 at temperatures close to that of LiNH2−LiH.

In this paper, two LiAlH4–NaNH2 samples with LiAlH4 to NaNH2 molar ratio of 1/2 and 2/1 were investigated, respectively. It was observed that both samples evolved 2 equiv H2 in the ball milling process, however, the reaction pathways were different. For the LiAlH4–NaNH2 (1/2) sample, Li3Na(NH2)4 and NaAlH4 were formed through cation exchange between reactants. The NaAlH4 formed further reacts with Li3Na(NH2)4 and NaNH2 to give H2, NaH and LiAlN2H2. For the LiAlH4–NaNH2 (2/1) sample, Li3Na(NH2)4, LiNH2 and NaAlH4 were formed firstly through the same cation exchange process. The resulting LiNH2 reacts with the remaining LiAlH4 and produces H2 and Li2AlNH2.Mechanical ball milling LiAlH4–NaNH2 mixture with LiAlH4 to NaNH2 molar ratio of 1/2 and 2/1 both result in evolution of 2 equiv hydrogen. However, FTIR revealed different reaction pathways for hydrogen release.

Alkali metal hydroxide and hydride composite systems contain both protic (H bonded with O) and hydridic hydrogen. The interaction of these two types of hydrides produces hydrogen. The enthalpy of dehydrogenation increased with the increase of atomic number of alkali metals, i.e., −23 kJ/molH2 for LiOH-LiH, 55.34 kJ/molH2 for NaOH-NaH and 222 kJ/molH2 for KOH-KH. These thermodynamic calculation results were consistent with our experimental results. H2 was released from LiOH-LiH system during ball milling. The dehydrogenation temperature of NaOH-NaH system was about 150 °C; whereas KOH and KH did not interact with each other during the heating process. Instead, KH decomposed by itself. In these three systems, NaOH-NaH was the only reversible hydrogen storage system, the enthalpy of dehydrogenation was about 55.65 kJ/molH2, and the corresponding entropy was ca. 101.23 J/(molH2·K), so the temperature for releasing 1.0 bar H2 was as high as 518 °C, showing unfavorable thermodynamic properties. The activation energy for hydrogen desorption of NaOH-NaH was found to be 57.87 kJ/mol, showing good kinetic properties.The enthalpy of dehydrogenation increases with the increase of atomic number of alkali metals in these hydride and hydroxide composites. These thermodynamic calculated results were consistent with our experimental results.Download full-size image

Metathesis of LiNH2BH3 and FeCl3 in THF solution was investigated in detail. Instead of formation of expected Fe amidoborane i.e., 3LiNH2BH3 + FeCl3 → 3LiCl + Fe(NH2BH3)3, 1.5 equiv. H2/LiNH2BH3 together with LiCl and a black precipitate was produced as a result of salt metathesis and reduction of Fe3+ by BH3. The hydrogen was desorbed in two steps involving a homogeneous interaction of the two starting chemicals to form [Fe(H2NBH2)3] precipitate and subsequent solid-state dissociation of [Fe(H2NBH2)3] to yield a polymeric product, [Fe(HNBH)3]n, respectively. FTIR evidenced the persistence of B–H and N–H stretches in the above two solid products and following the dissociation of [Fe(H2NBH2)3] to release 1 equiv. H2/LiNH2BH3 the B–N bond strengthened. Mössbauer and XAFS both indicated that Fe atoms in these solids are in very similar chemical environments, linking to the neighbouring N and B atoms and bearing slightly positive charge. Most likely, H2NBH2 in [Fe(H2NBH2)3] binds to Fe as a π-bound ligand. The mechanism of salt metathesis and reduction of Fe3+ was confirmed based on simulation work on the homogeneous reaction process.