Development of high hydrogen capacity complex and chemical hydrides for hydrogen storage

Abstract

The depletion of fossil fuel stimulates tremendous efforts in setting up a sustainable energy system. Because of its abundance, high energy output and zero emission hydrogen is recognized as the most prospective energy carrier for the future energy system. To utilize hydrogen as a fuel for transportation, a safe and efficient storage medium is needed.

Lithium alanate (LiAlH4) and ammonia borane (AB) possess high hydrogen content of 10.5 wt% and 19.6 wt%, respectively, and thus, are attractive for hydrogen storage. However, direct uses of these chemicals for hydrogen storage are not feasible due to their poor dehydrogenation kinetic and thermodynamic properties. Therefore, the aim of this study is to improve the dehydrogenation properties of LiAlH4 and AB by chemically altering (or modifying) their dehydrogenation thermodynamics.

Significant improvement in the dehydrogenation properties of LiAlH4 has been achieved by reacting it with NaNH2 or LiAl(NH2)4. As results of chemical alterations, LiAlH4 underwent different dehydrogenation pathways, releasing hydrogen rapidly under ambient temperature. In the dehydrogenation process, the large combination potential of H+ and H- to H2 together with the formation of thermodynamic favorable Al-N bond induce a direct interaction of the materials to form hydrogen, giving rise to the formation of Li-Al-N-H product.

In the chemical modification of AB by reacting it with calcium amide in THF solvent, high purity calcium amidoborane (CaAB) was yielded; whereas in a solid-state reaction of AB with Ca or Mg amides or imides, a new class of materials, namely metal amidoborane ammoniate, with high hydrogen content was discovered. These newly developed materials possess high hydrogen content of > 11 wt% and demonstrate attractive dehydrogenation behaviors. In the study, ammonia was found to play a vital role in stabilizing crystal structures of ammoniates, forming strong coordination with metal cation and establishing a complete dihydrogen bond network within the structures. As a consequence of dihydrogen bonding, N-H and B-H bonds are weakened, resulting in a significant reduction in dehydrogenation temperature. Furthermore, stoichiometric conversion of ammonia to hydrogen was achieved, allowing more hydrogen to be released in the dehydrogenation.

Due to the attractive properties of amidoborane ammoniates, mechanisms of their formation and dehydrogenation were studied by using isotopic labeling. It was found that the formation of ammoniate sample was attributed to the combination of one H+ from AB and [NH2]- unit in amide, i.e. Ca(NH2)2, to form NH3, and the remaining [NH2BH3]- unit of AB may bond to Ca2+ as a result of electrostatic force. In the investigation of the dehydrogenation mechanism, it was evidenced that the improved dehydrogenation properties in amidoborane ammoniate was resulted from the participation of NH3 molecule in the dehydrogenation of metal amidoborane.

Stimulate by the mechanistic study, the dehydrogenation thermodynamics of CaAB was chemically modified by reacting it with LiNH2 and the results showed that an even lower dehydrogenation temperature with a milder reaction enthalpy can be achieved in comparison to that of CaAB.

Overall, significant improved dehydrogenation properties have been achieved in the chemically modified materials as compared to the pristine materials (LiAlH4 and AB). A few new materials were synthesized which exhibit remarkable dehydrogenation properties.