| Hydrogen is a promising alternative for the fossil fuels,and the development of safe,efficient,and affordable hydrogen storage approaches is particularly important for future mobile applications.Compared to pressurised gaseous or cryogenically liquefied hydrogen,storing hydrogen in solid-state materials has attracted much more attention due to their unique safety characteristics and high energy densities.Among all the developed candidates,LiBH4 offers a theoretical hydrogen capacity of as high as 18.5 wt%and a volumetric hydrogen density of 121 kg H2/m3.However,the strong covalent and ionic bonds in the LiBH4 structure cause a high decomposition temperature and poor reversibility,consequently limiting its practical applications.Appreciable hydrogen release of LiBH4 has occurred above 400?,and re-loading of hydrogen has required 600? and 350 bar of hydrogen pressure.Based on the previous development research of LiBH4 as the hydrogen energy medium,we mainly focus on the modification in the kinetics and thermodynamics to reduce the onset dehydrogenation temperature and improve the hydrogen storage performances of LiBH4 through the compositing and nanoconfinement.Firstly,flake graphite was introduced to LiBH4 by high energy ball milling method.The catalytic effects of graphite on the hydrogen storage behaviors of LiBH4were systematically investigated.It was found that adding 20 wt%graphite could significantly improve the de/hydrogenation performances of LiBH4 system.The onset and main hydrogen desorption temperatures of LiBH4-20 wt%graphite reduced to260? and 383?,which were decreased by 15? and 50? relative to the ball-milled LiBH4,respectively.After dehydrogenation,the product was subjected to hydrogenation under a hydrogen pressure of 100 bar in non-isothermal mode.It was observed that the dehydrogenated graphite-containing sample took up 6.3 wt%of hydrogen at 500? for 10 h,exhibiting a good reversibility.LiBH4-20 wt%graphite sample could release approximately 8.7 wt%hydrogen within 150 min at 370?.The apparent activation energy?Ea?was calculated to be 127.2 kJ/mol for the dehydrogenation.This value was 22%lower than that of ball-milled LiBH4.The Raman spectra revealed that the graphite in the composite possessed more disordered structure and defects.The disordered structure and defects of the graphite acted as active centers and maintained under the majority of LiBH4 dehydrogenation process,consequently improved the hydrogen desorption behaviors of LiBH4.Secondly,the effects of the addition of graphene on hydrogen storage properties of the LiBH4 were further investigated.The results revealed that the LiBH4-20 wt%graphene could release hydrogen from 240?,and the main hydrogen desorption peak was decreased to 383?,which were 35? and 73? lower than those of the additive-free LiBH4,respectively.About 9.7 wt%of hydrogen was released from the sample at the temperature range of 240? to 530?.At 340?,approximately 9.1wt%of hydrogen was released within 600 min.The dehydrogenated sample could absorb 6.9 wt%of hydrogen while being heated to 500? under 100 bar of hydrogen pressure.The structural and morphology characterization indicated that the graphene obtained more defects during the ball milling process.On the other hand,the graphene with large specific surface areas could increase the contact areas with LiBH4.It should be noted that the defects could be stable under the majority of hydrogen released from LiBH4,which facilitated the hydrogen dissociation and enhanced the desorption performance of LiBH4.DFT calculations indicated that strong electronic interaction occurred between LiBH4 and graphene with defects,which induced height location of the LiBH4 molecular to become broadening by DOS analysis,and this reasonably explained the much higher catalytic activity of ball-milled graphene with LiBH4.Subsequently,the LiBH4 confined in unique double-layered carbon nanobowls prepared by a facile melt infiltration process was studied.Taking into consideration of both desorption temperature and hydrogen capacity,the sample DLCB-2?LiBH4loading weight 80 wt%?was the optimal combination in the present study.DLCB-2started dehydriding from 225? and peaked at 353?,which were 50? and 112? lower than bulk LiBH4,respectively.Further volumetric measurements confirmed the improvement and determined 10.9 wt%of total H2 desorption capacity when heated to500?.Approximately 9.0 wt%H2 was released by DLCB-2 at 300?.Ea value was calculated to be 121.4 kJ/mol for DLCB-2,and approximately 30%reduction was resulted for the kinetic barriers of hydrogen release by nanoconfining LiBH4.Moreover,DLCB-2 exhibited a reversible charge and absorbed of 8.5 wt%H2 at300? under 100 bar of hydrogen pressure.The samples at different stages were collected for XRD,FTIR and NMR analyses.The results shown that nanoconfinement did not change the dehydriding/hydriding pathway of LiBH4.Reduction of particle size down to nanoscale resulted in high specific surface areas as well as short diffusion distances,which led to kinetic enhancement of DLCB-2 sample.A volumetric hydrogen density of 82.4 g H2/L was calculated for the sample DLCB-2,and 1.6-fold that set by US DOE for practical on-board applications?50.0 g H2/L?.Afterward,uniform and small size CeVO4 nanoparticle with 15 nm diameter was successfully synthesized through a solvothermal method and firstly introduced to improve the hydrogen storage properties of LiBH4 system.The results revealed that the LiBH4-30 wt%CeVO4 sample exhibited optimal hydrogen storage properties,and the onset and peak temperatures of hydrogen dehydrogenation were significantly reduced to 175? and 348?,respectively,which were reduced by 100? and 85? relative to those of the additive-free LiBH4.About 4.0 wt%hydrogen was released before the melting temperature of LiBH4,and the hydrogen release amounted to 9.2wt%at the temperature range of 175? to 500?.At 340?,the hydrogen desorption completed within approximately 1000 min.The rehydrogenation was conducted at 300? under 100 bar of hydrogen pressure,and 7.3 wt%of hydrogen could be absorbed.The compositional and structural investigation revealed that CeB6and VB2 were formed after the reaction between CeVO4 and LiBH4 in the heating process,which was the reason for the favorable dehydrogenation thermodynamics and kinetics of LiBH4.Finally,the hydrogen desorption behaviors of LiBH4?H2O with the strong affinity of protic H(H?+)and hydridic H(H?-)were systematically studied.Unique 2D leaf-like nanosheet morphology with 20-30 nm thickness was observed for the prepared LiBH4·H2O with the freeze-drying preparation process.The prepared LiBH4·H2O nanosheets started releasing hydrogen even below 50?,and quantitative measurement by a volumetric method indicated that hydrogen desorption amounted to10 wt%when increasing the sample temperature to 110?.When dwelling at 70?,the hydrogen desorption of the prepared LiBH4·H2O nanosheets completed within approximately 30 min.To shed light on why hydrogen could be released from the prepared LiBH4·H2O nanosheets at such low temperatures,density function theory?DFT?calculations were conducted.Further analyses of the electron density difference revealed an obvious charge transfer from the negatively charged H?-in the BH4 group to the positively charged H?+in the H2O molecule due to their coulomb attraction interaction,indicating electron depletion of the BH4 group.Such electron depletion subsequently weakened the chemical bonding of B and H.The bonding character between H?-of BH4 and H?+of H2O was also observed.Thus,combining LiBH4 with H2O to form LiBH4·H2O not only weakened the B-H bonding but also created local dihydrogen bonding.These were believed to be the most important reasons for the significantly reduced dehydrogenation temperature of LiBH4·H2O. |
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