Modification Investigation On Hydrogen Absorption And Desorption Kinetics For Hydrogen Storage System Of Nanocrystalline Mg

As one of the most promising clean energy in the 21st century,hydrogen energy has many advantages,such as cleanliness,high calorific value and wide source.However,the large-scale applications of hydrogen energy are restricted by the development bottleneck of hydrogen storage technology.Mg H₂/Mg hydrogen storage system is considered as a solid hydrogen storage material with great application prospect due to its high hydrogen storage capacity(110 g/L,7.6 wt%),good reversibility and low acquisition cost.However,the practical application of Mg H₂/Mg is obstructed by the high reaction temperature and sluggish rate of hydrogen absorption/desorption.Hence,based on the investigation status of Mg-based hydrogen storage materials,the hydrogen storage properties of Mg H₂/Mg were systematically modified by methods of nanocrystallization and catalytic doping in this thesis.In addition,the corresponding modified mechanism was analyzed in depth.First,nanocrystalline Mg covered by carbon layers with average grain size of 22 nm was successfully prepared via facile wet chemical ball milling,vacuumize and subsequent hydrogenation heat treatment.The sample started to take up hydrogen at about 40?under3 MPa hydrogen pressure with the final hydrogen absorption capacity of 6.8 wt%.In addition,at 100?,the hydrogen absorption capacity of the sample exceeded 2 wt%and 3 wt%in the first 10 and 20 min,respectively.The apparent activation energy(E_a)of hydrogen absorption for nanocrystalline Mg was calculated to be 26.7±0.1k J/mol,decreased 64.3 k J/mol(70.7%)compared with 91 k J/mol of bulk Mg.Besides,hydrogen absorption and desorption cycling tests indicated that nanocrystalline Mg showed superior cycling properties.At 300?,the content and rate of hydrogen absorption and desorption of the sample after 10cycles occurred no obvious decline.Simultaneously,the mechanism investigation indicated that the existing of carbon hindered the reaction of air and moisture with nanocrystalline Mg,leading to a certain air stability of nanocrystalline Mg.In the meanwhile,refined nanocrystalline Mg can form more channels to enhance the adsorption and disintegration of hydrogen molecules,thus promoting the properties of Mg H₂.Mg-5 wt%transition metal material composite system(transition metal material=Mn,Co Cl₂,Ti H_1.971)was successfully synthesized by wet chemical ball milling in order to further improve hydrogen storage performance based on nanocrystalline Mg.Hydrogen storage performance tests indicated that the three composites all improved the hydrogen storage properties of nanocrystalline Mg and the Mg-5 wt%Ti H_1.971 composite showed the best hydrogen storage performance among the samples.Mg-5 wt%Ti H_1.971 could start to absorb hydrogen at about 20?under 3MPa hydrogen pressure,and the dehydrogenation onset temperature of the hydrogenated composite is about 218?,with the final capacity over 6.0 wt%.In addition,the hydrogen absorption kinetics property of the Mg-5 wt%Ti H_1.971composite was remarkably enhanced,compared with that of nanocrystalline Mg.The hydrogenation apparent activation energy E_a of Mg-5 wt%Ti H_1.971 was reduced to 20.9±1.8 k J/mol from 26.7±0.1k J/mol of nanocrystalline Mg,with a dramatic decrease of 21%.Moreover,Mg-5 wt%Ti H_1.971 composite also showed excellent cycling performance.At 300?,the hydrogenation/dehydrogenation rate of the sample had unconspicuous change during 10 cycles and the hydrogen storage capacity stably remained at about 6.0 wt%.Finally,three kinds of NiTiO₃ with the average particle size of 50-100 nm were successfully synthesized through hydrothermal method and catalyzed the nanocrystalline Mg.The investigation results revealed that NiTiO₃-C exhibited the most obvious improvement on the kinetics property of nanocrystalline Mg due to its most uniform particle dispersion.The Mg-NiTiO₃-C composite absorbed over 4 wt%H₂ in 60 min at 100?u nder 3 MPa hydrogen pressure.The apparent activation energy E_aof Mg-NiTiO₃-C was k J/mol,decreased by 52%and 72%,compared with nanocrystalline Mg and bulk Mg,respectively.Moreover,hydrogenated Mg-NiTiO₃-C began to release H₂ at about 193.2?and completed desorption process in 5 min at 325?with the final capacity of 6.6 wt%.Furthermore,the capacity recession of Mg-NiTiO₃-C was less than 0.3 wt%and kinetics properties of Mg-NiTiO₃-C appeared unconspicuous change after 20 cycles.In addition,the hydrogen desorption enthalpy of hydrogenated Mg-NiTiO₃-C is 78.6±0.8 k J/mol through Van't Hoff curve fitting calculation,5.3 k J/mol lower than 83.9±0.7 k J/mol of nanocrystalline Mg.The mechanism research found that the promotion on thermodynamic and kinetic properties of Mg-NiTiO₃-C was attribured to the synergistic effect of nano-NiTiO₃ particles catalysis and nanocrystalline Mg refinement.