Improvement of hydrogen dehydrogenation performance of lithium amide pyrolysis by ball milling with magnesium

Abstract

Li–N–H system is one of the promising candidates for hydrogen storage. However, with the heat treatment, LiNH2 can release not only H2 but N2 and NH3. To improve the purity and amount of H2 in the products, LiNH2–Mg composite is prepared by ball milling. In the composite, Mg can absorb the gaseous byproducts, e.g. NH3 and N2, effectively when the LiNH2 pyrolysis. Finally, H is desorbed in the form of H2 and N is absorbed by Mg and forms Mg3N2. Besides, LiNH2 reacts with Mg directly to form LiMgN and release H2. After re-hydrogenation of the composite, H2 is stored into Li2MgN2H2 and LiH, thus the cyclic mechanism is different with the first dehydrogenation of the LiNH2–Mg composite. First-principles calculation shows that the NH3 molecule can be captured by Mg when it is adsorbed on top and bridge positions. After captured by Mg, the molecular structure of NH3 is changed and the barrier energy of NH3 dissociation is dramatically reduced. In addition, the barrier energy of NH3 and N2 dissociation on Mg (0001) plane is lower than that of H2, which means NH3 and N2 can be absorbed by Mg before Mg react with H2.