Lithium Hydrazinidoborane: A Polymorphic Material with Potential for Chemical Hydrogen Storage

Abstract

Metal amide and hydrogen (MNH2-H2) system is recognized as a promising reversible hydrogen storage system due to its high hydrogen capacity and lower operating temperature. However, slow reaction rate for the Li system with the highest hydrogen capacity is an important issue to be solved for practical use. In this thesis, modification of the reaction properties for the LiNH2-H2 system is carried out from thermodynamic and kinetic points of view. Particularly, the novel ammonia synthesis technique is proposed by applying the LiNH2-H2 system and Amide-imide system. Lithium hydride-Potassium hydride (LiH-KH) complex synthesized by ball-milling has been focused in order to modify the kinetic properties of the reaction between LiH and ammonia. The LiH-NH3 system is recognized as one of the most promising hydrogen storage system because it generates hydrogen at room temperature by ammonolysis reaction. Moreover, the starting system can be regenerated below 300 °C and possesses more than 8.0 wt.% hydrogen capacity. From the experimental results, it is confirmed that the hydrogen generation from the reaction between ammonia and the LiH-KH complex shows much higher reaction rate than that of the simple summation of each component as a synergetic effect. Then, a double-cation amide MNH2 (LiK(NH2)2) phase, which could not be assigned to any reported amides so far, is formed as the reaction product. Moreover, in the hydrogenation of LiK(NH2)2, two processes were confirmed at the different temperatures. After the low temperature hydrogenation, KH-lithium amide (LiNH2) composite is generated as the hydrogenated product. It is noteworthy that the hydrogenation temperature of the composite is dramatically lower than that of LiNH2 itself, which should be due to the interaction between LiNH2 and KH such as a eutectic melting phenomenon. II An ammonia synthesis technique from lithium nitride (Li3N) based on the reactions of "Amide-imide system" and "Amide-hydrogen system" is proposed. Namely, Li3N is hydrogenated below 300 °C under 0.5 MPa hydrogen atmosphere, and then LiNH2 and LiH are formed as products. Furthermore, the reaction between LiNH2 and hydrogen proceeds below 250 °C under 0.5 MPa of hydrogen flow condition, which results in the formation of ammonia and LiH. In this study, a new method of ammonia synthesis is proposed at laboratory scale using above two reactions. This method is capable of being operated under more moderate conditions than those of Haber-Bosch process. The proposed method is investigated for various reaction system such as open system, closed gas circuit system, and closed gas exchange system using couple of hydrogen storage alloys. As a result, it is experimentally clarified that the ammonia can be synthesized below 300 °C and 0.5 MPa with realistic reactions rate by non-equilibrium reaction field under certain hydrogen flow rate even in the closed system.

In situ Synchrotron X-ray Thermodiffraction of Boranes

“…The patterns obtained for the temperature range 81-301 K were indexed in the monoclinic phase with the space group P2 1 /c ( Figure S14). The unit cell parameters (Table 1) are in good agreement with those reported elsewhere [13,15]. The unit cell parameter c presents a negative coefficient of thermal expansion (Figure 8).…”

“…For example, the polymorphic derivative lithium hydrazinidoborane (LiHB) is obtained by ball-milling HB and LiH in equimolar amounts, and depending on the milling conditions, two different phases can be obtained. The β phase, β-LiHB, is orthorhombic with a space group Pbca [13]. It is in fact the low-temperature phase.…”

Hydrazine Borane and Hydrazinidoboranes as Chemical Hydrogen Storage Materials

“…This result is surprising, as even for amidoboranes, such an unexpected substitution has never been observed [21]. Moury et al [66] recently argued that they were also working on lithium hydrazinidoborane when Wu et al [32] published their experimental report. However, the former researchers were not able to compare favorably the structure of lithium hydrazinidoborane they mechano-synthesized to the compound proposed by later authors.…”

“…However, the former researchers were not able to compare favorably the structure of lithium hydrazinidoborane they mechano-synthesized to the compound proposed by later authors. Moury et al [66] demonstrated that lithium hydrazinidoborane is a polymorphic material with a stable low-temperature phase with orthorhombic Pbca (61) space group, atoms standing in the 8c sites, and a metastable high-temperature phase as described by Wu et al [32]. The former was called the β phase because discovered after the metastable phase, then called α.…”

“…The solid state of lithium hydrazinidoborane is explained by the presence of an intermolecular head-to-tail H δ+ ···H δ− interaction that form chains (i.e., parallel plans on which the H δ+ ···H δ− network extends). In the β phase, the network was identified according to the definition of Klooster et al [70], with a B-H···H angle slightly bent (106.8°), a N-H···H angle almost linear (171.2°) and a H···H distance of 2.25 Å [66]. For both phases, the lithium is in tetrahedral environment (Figure 7).…”