| Confronted with the growing demand of safe and high-density storage of hydrogen in the utilization of hydrogen energy,developing novel solid state hydrogen storage materials with high capacities have been considered as an essential issue.Among the hydrogen storage materials,Mg?BH4?2 draws great attention from researchers all over the world due to its high hydrogen capacity of 14.9 wt.%and the abundance of Mg and B in nature.However,the dehydrogenation of Mg?BH4?2 could only take place at very high temperatures due to the stable B-H bonds,meanwhile the sluggish hydrogen desorption kinetics and poor reversibility also severely hinder the actual use of Mg?BH4?2.In this paper,several strategies such as catalyst doping,multi-hydride composition and nanoconfinement have been utilized in order to improve the hydrogen storage properties of Mg?BH4?2-based hydrogen storage materials,and in-depth investigations have been made to explain the modification effect.Two dual-cation transition metal fluorides K2TiF6 and K2NbF7 are introduced into Mg?BH4?2 by ball-milling to catalyze the dehydrogenation behavior of Mg?BH4?2.According to the results,the onset dehydrogenation temperature of Mg?BH4?2 doped with K2TiF6 and K2NbF7 are remarkably reduced to 105.4 and 118?,respectively.Meanwhile,the catalyzed systems could release more than 6 wt.%hydrogen under280?,showing an improvement in dehydrogenation amounts.Also,the reversible capacity of Mg?BH4?2-3%K2TiF6 system is 2.7 wt.%,which is enhanced comparing to that of pristine Mg?BH4?2.Further investigations reveal that K2TiF6 actually acts as a catalyst precursor to react with Mg?BH4?2,forming active hydrides KBH4 and TiH2,which further serve as "hydrogen pumps" ,facilitating the de/rehydrogenation of Mg?BH4?2.2D MXene Ti3C2 is synthesized through an acid etching method,and then used as a dopant to modify the dehydrogenation properties of Mg?BH4?2.The onset temperature of Mg?BH4?2 is significantly reduced to 124.6? due to Ti3C2 additive,and more than11 wt.%hydrogen could be released below 400? from Mg?BH4?2-Ti3C2 composites,indicating enhanced dehydrogenation capacities.Also,Ti3C2-catalyzed Mg?BH4?2systems could emit 10.71 wt.%hydrogen under 330? rapidly while pristine Mg?BH4?2 could only release 5.28 wt.%,showing much improved dehydrogenation kinetics.Deeper researches have found that on one hand Ti3C2 reacts with Mg?BH4?2to form metallic Ti active catalytic sites,on the other hand Ti3C2 keeps Mg?BH4?2particles from gathering and aggregation.This synergistic effect explains the mechanism of Ti3C2 catalyzing Mg?BH4?2.The strategy of multi-hydride composition is also conducted by introducing active metal hydride AlH3 into Mg?BH4?2 to form Mg?BH4?2-AlH3 binary composite and further introducing LiH to form Mg?BH4?2-AlH3-LiH ternary composite.Mg?BH4?2-AlH3 composite could start to desorb hydrogen from 130.8?,releasing11.9 wt.%hydrogen during the whole dehydrogenation process.Meanwhile the dehydrogenation peak temperatures of Mg?BH4?2 in Mg?BH4?2-AlH3 composite are reduced by more than 20? due to the active Al formed by AlH3 decomposition during dehydrogenation.Moreover,the addition of LiH into Mg?BH4?2-AlH3composite greatly improved the system's reversibility.Mg?BH4?2-AlH3-0.5LiH composite could release 3.95 and 3.80 wt.%hydrogen in the second and third dehydrogenation process.It is found that LiH could react with the dehydrogenation product MgAlB4 during rehydrogenation to generate LiBH4 and MgH2,improving the cycling capacities of the system.Hollow carbon nanospheres?HCNS?are synthesized by self-assembling method and then used for the nanoconfinement of Mg?BH4?2-LiBH4?LMBH?eutectic composite.It is found that after melting-infiltration into HCNS,the dehydrogenation peak temperatures of LMBH@HCNS composites are reduced to 318.6?,which is more than 40? lower than that of bulk LMBH.10.8 wt.%hydrogen could be released from LMBH@HCNS composite under 280?,showing enhanced dehydrogenation kinetics.Also the reversible hydrogen storage capacity of nanoconfined LMBH@HCNS composite is greatly improved to 4.5 wt.%.According to the structural observations of LMBH@HCNS composites with different LMBH loading amounts,it is discovered that the over-loaded LMBH could form a nano "coating layer" on the surface of HCNS,improving the system's actual hydrogen capacities while maintaining the modification effect of nanoconfinement.Mono-layered fluorographene?FG?has been synthesized by liquid phase exfoliation method,and then introduced into Mg?BH4?2-NaBH4 composite.The Mg?BH4?2-NaBH4-FG composite could start dehydrogenation from 114.9?,which is about 100? lower than that of Mg?BH4?2-NaBH4 composite.And more encouragingly,the whole dehydrogenation process could be finished within several seconds,with 6.9 wt.%hydrogen desorption capacity reached below 150?.Further investigations reveal that a 3D bowl-like structure is formed during ball-milling,which allow FG to effectively load Mg?BH4?2 and NaBH4 particles.Meanwhile,FG could also act as a reactant,forming NaMgF3 during dehydrogenation and lowering the dehydrogenation reaction enthalpy.This synergistic effect could explain the huge modification effect of FG to Mg?BH4?2-NaBH4 composite.Based on the research of the previous chapter,the less stable hydride LiAlH4 is choosed to replace NaBH4,and fluorographite?FGi?is used to replace FG.The new as-prepared Mg?BH4?2-LiAlH4-FGi ternary composite could start to release hydrogen from an ultra-low temperature of 68.2?,and 7.12 wt.%hydrogen could be rapidly emitted in seconds at this temperature.Such dehydrogenation behaviors have never been reported before,showing the optimal dehydrogenation properties in the field of Mg?BH4?2 research.Moreover,it is found that the modification effect of FGi to Mg?BH4?2-LiAlH4 composite could be explained in two aspects:Firstly the addition of FGi greatly lowered the dehydrogenation reaction enthalpies of Mg?BH4?2 and LiAlH4 to-135.7 and-173.4 kJ/mol H2,respectively.Also,the layered FGi structure could effectively load LiAlH4 and Mg?BH4?2 particles,and LiAlH4 could serve as a "micro-lighter" during dehydrogenation to trigger the whole system's dehydrogenation. |
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