Hyrogn Storage Performance Of Li-Mg-B-H System Doped With Hydrothermal-synthesized Transition Metal Compounds

Lithium borohydride (LiBH4) is considered as a potential hydrogen storage material due to its high hydrogen capacity. However, the thermodynamic and kinetic limitations that are essentially imposed by its strong ionic and covalent bonds greatly restrain the practical application of LiBH4.Several methods have been proposed to improve the de/rehydrogenation properties of LiBH4. The reactive hydride composite of LiBH4 and MgH2 has been found to be a better hydrogen storage material due to the relatively high capacity, facile kinetics and much improved reversibility. For further enhancing the hydrogen storage performance, we synthesized ZrO2 and La(OH)3 nanorods as well as titanate nanotubes and nanosheets through a hydrothermal process and added these catalyst precursors into 2LiH+MgB2 for the reversible hydrogen storage reaction between 2LiH+MgB2 and 2LiBH4+MgH2.The present research shows that La(OH)3 nanorods can significantly enhance the dehydriding kinetics of the 2LiH+MgB2 composites. The X-Ray diffraction analyses clearly revealed the formation of LaB6 after the first hydrogenation. LaB6 may act as nuclei of MgB2 formation, thus enhanced the dehydriding kinetics.The 2LiH+MgB2 composites doped with the ZrO2 nanorods demonstrated superior and stable hydriding/dehydriding kinetics upon cycling. The XRD, SEM and HRTEM results revealed that the doped ZrO2 nanorods were transformed into ZrB2 nanoparticles with a size of a few nanometers during the first hydrogenation. The initial nanosize of the ZrO2 catalyst precursor accounted for the formation of extremely fine ZrB2 nanoparticles. It is believed that ZrB2 nanoparticles can act as nucleation agents for MgB2, thereby greatly enhancing the kinetics of dehydrogenation. The good cycling stability of the ZrO2-doped composite may be due to the chemical stability of ZrB2 at high temperatures.Furthermore, titanate nanotubes (TNTs) and sheetlike titanate (TNS) were also synthesized and employed as catalyst precursors for the 2LiH+MgB2 composite. The results show that both TNTs and TNS can significantly enhance the dehydriding kinetics of the 2LiH+MgB2 composite, resulting the formation of MgB2 without incubation during dehydrogenation. The catalytic performance of TNTs is superior to TNS. However, the TNTs-doped 2LiH+MgB2 composite revealed a slow degradation of capacity during hydriding-dehydriding cycles. The reason of the capacity degradation needs to be further investigated.The present study provides a new route for performance enhancement of complex hydride composites through manipulating the nanostructure of catalysts. The results also shed light on understanding the reaction mechanism and further improving the hydrogen storage properties of Li-Mg-B-H system.