Hydrogen Storage Properties And Mechanism Of LiBH₄-Mg Based Composite System

This doctoral dissertation has intensely reviewed the major difficulties andcurrent research status of hydrogen storage materials, mainly focus on Mg-basedalloys and LiBH₄, which both have high hydrogen gravimetric density. Byconstructing a MgH₂+LiBH₄composite system, MgB₂is formed duringdehydrogenation which effectively lower the enthalpy of MgH₂and LiBH₄, however,the poor kinetic of the system results in a relatively high dehydrogenation temperature.Our groups have discovered that nano-modified H-Mg₃Lacan significantly improvethe dehydrogenation property of LiBH₄（H-Mg₃Larefers to hydrogenated status,Mg₃Laalloy will change into MgH₂and LaH₃, so would other Mg₃RE alloys）, butMgB₂is not detected during low temperature dehydrogenation, the synergism effectbetween different hydrides is not fully understood either. On the basis of thoseprevious researches, this paper further illustrate the mechanism of hydrogen exchangeeffect and interfacial interaction between MgH₂and LiBH₄, along with the catalysisof Mg particles on LiBH₄. Accordingly we use different methods to tune the Mg-basedmaterial in order to enhance the dehydrogenation property of LiBH₄.Different measurements are performed in order to study the effect ofhydrogenated Mg₃RE （RE=La, Ce, Pr, Nd） alloys have on LiBH₄duringdehydrogenation, including isothermal dehydrogenation kinetic measurement,temperature-programed decomposition, X-rays diffraction （XRD） and scanningelectron microscope （SEM）. Several types of H-Mg₃RE alloys have significant effecton enhancing the dehydrogenation kinetic of LiBH₄, H-Mg₃Laranks the top. Theisothermal dehydrogenation kinetic curves of LiBH₄+H-Mg₃RE compositeconsistently show two stages. In the first stage, hydrogen mainly comes from thedecomposition of MgH₂, which is enhanced by the formation of RE hydrides. Butthose hydrides have little effect on LiBH₄which mainly decomposes in the secondstage. LiBH₄within the composite release its6wt%hydrogen340℃,which shows agreat improvement.Deuterium （D） is used to mark Mg to get an insight of the catalysis MgH₂haveon LiBH₄. Through Fourier Transform Infrared Spectrometer （FTIR）, XRD andTPD-MS, the transfer of H and D atoms along with the dehydrogenation temperature of LiBH₄and MgD₂in the composite is characterized. B-D bond is detected by FTIRisotope tracing, which indicates there is a H-D exchange effect between LiBH₄andMgD₂after ball milling. This effect can facilitate the diffusion of H and D atoms fromthe surface of LiBH₄and MgD₂which in terms reduce the induction period ofdehydrogenation, and it boosts with increasing temperature. Due to the concentrationdifference of H atoms between two phases, a net flow of H atoms from LiBH₄surfaceto MgD₂surface occurs, it increases the degree of chaos of H atoms because of theaccumulation of H atoms on the surface of MgD₂. When MgD₂dehydrogenate, Hatoms exchange from LiBH₄are released to form hydrogen gas which directlyfacilitate the dehydrogenation of LiBH₄, in the meantime, D atoms from MgD₂surfacecan bond with H atoms from LiBH₄surface to form HD gas, results in thesynchronization of H2and HD in TPD-MS measurement. Therefor the combination ofH and D atoms is beneficial to the dehydrogenation kinetic of LiBH₄. After the fastdehydrogenation which significantly reduce the concentration of H and D atoms at thesurface region, instead of the surface hydrogen exchange effect, the sluggish diffusionrate of H and D atoms becomes the main limiting factor of dehydrogenation.Consequently, the dehydrogenation become slow. It is also verified thatdehydrogenated Mg particle have certain catalysis on LiBH₄.Previous study have shown that the dehydrogenation kinetic of LiBH₄dependson the dehydrogenation of MgH₂. Although in-situ nano-size LaH3can improve theperformance of MgH₂, it sacrifices certain hydrogen capacity of the composite. Thuswe search for more effective ways to enhance the kinetic of Mg-base materials tofurther improve the dehydrogenation property of the LiBH₄+Mg-based composites.Mg sample with surface decorated Ti catalyst shows a great improvement indehydrogenation kinetic of MgH₂, and the treated MgH₂exhibits better catalyticeffect on the dehydrogenation of LiBH₄.Composite is able to release7wt%hydrogenat340℃in6hours which realizes a55%augment in dehydrogenation capacitycompared to its counterpart containing LiBH₄+H-Mg3La. The kinetic of LiBH₄is alsoimproved.