Study On Hydrogen De/Absorption Kinetic And Thermodynamic Modification Of Magnesium-based Hydrogen Storage Materials By Carbon Nanotubes

Confronted with the significant requirement of high-density and safe hydrogen storage,studying on high-capacity hydrogen storage materials has been the core content of the current hydrogen storage technology,which attracts the attentions of researchers all over the world.Among the numerous hydrogen storage material systems,magnesium hydride(MgH2)is traditionally thought to be one of the most promising candidates due to its high hydrogen storage densities,low cost,abundant resource storage and environmental compatibility.However,its practical application is primarily limited by the drawbacks of poor kinetics and stable thermodynamics with a high operation temperature.To counter the problems above,reversible hydrogen storage performances of light Mg-based hydrogen storage materials are improved by catalytic doping and nanoconfinement by using carbon nanotubes(CNTs).Moreover,advanced structural analysis technologies are used to figure out the effect mechanisms of CNTs-based materials on improving the reversible hydrogen storage performances of MgH2.Co nanoparticles(NPs)loaded on carbon nanotubes(Co@CNTs)are synthesized by carbonizing zeolitic imidazolate framework-absorption 67(ZIF-67),and shows effective catalysis on improving the hydrogen and desorption kinetics of MgH2.MgH2-Co@CNTs displays complete release of hydrogen with capacity of 6.48 wt.%at 300 ? within 20 min,and the apparent activation energy(Ea)decreases to 130.36 kJ/mol.CNTs are ball milled into carbon pieces by the mechanical force and covered on the surfaces of the MgH2 particles,offering diffusion channels for the hydrogen atoms and preventing the growth and aggregation of MgH2 particles.Moreover,the reversible phase transformation between Mg2Co and Mg2CoH5 lowers the barriers of hydrogen absorption and desorption,which is an important reason for the enhanced hydrogen storage properties.Nano-sized bidirectional catalyst Co/Pd homogeneously loaded on the bamboo-shaped carbon nanotubes(Co/Pd@BCNTs)are synthesized by replacement reactions,which shows special "bidirectional catalyst" effect on the hydrogen storage properties of MgH2.MgH2-Co/Pd@BCNTs composite shows dramatically improved absorption and desorption kinetics,it can release hydrogen of 6.34 wt.%at 275 ?within 10 min,and the desorption activation energy reduced to 76.66 kJ/mol.For absorption kinetics,1.91 wt.%H2 is quickly taken up within 100 s at temperature as low as 50 ?."Bidirectional catalyst" is firstly proposed to explain the mechanism of Co/Pd@BCNTs on the improved absorption and desorption properties of MgH2 During hydrogenation,Mg/Pd interface plays dominant role in the preferential diffusion of hydrogen atom in the interface.Meanwhile,Mg2Co plays a certain role in accelerating the diffusion of hydrogen atoms.While in dehydrogenation,Mg/Pd interface together with phase transformation between Mg2Co and Mg2CoH5 becomes the crucial factors to facilitate the release of hydrogen atoms by decreasing the diffusion barrier.Novel 1D BCNTs with high specific surface area and high mechanical stability are firstly used as carriers for supporting Mg-based hydrogen storage materials.In spite of high loading up to 76.8%,the synthesized MgH,NPs shows small particle size(15-20 nm),uniform distribution and excellent reversible hydrogen storage properties.MgH2@BCNTs can release 5.70 wt.%H2 at 275 ? within 1 h.and quickly absorb 5.79 wt.%H2 at 250 ? under hydrogen pressure of 80 bar.The thermodynamic and kinetics are simultaneously improved,and the dehydrogenation Ea and ?H are reduced to 97.97 kJ/mol and 68.92 kJ/mol,respectively.With high pressure densification treatment under 750 MPa,MgH2@BCNTs achieves an exceptional volumetric capacity up to 65.90 g/L from 16.01 g/L without obvious performance degeneration,resulted from the special structure of the BCNTs with intervals inside inner space which play significant mechanical supporting role under huge pressureIn order to further improve the reversible hydrogen storage properties of MgH2 supported by BCNTs,the surfaces of carriers are doped by fluorine.In low degree of fluorination,C-F semi-ionic bonds,C-F covalent bonds and perfluorinated-CF3 bonds are detected on the surfaces of FBCNTs.As the degree of fluorination increases,only C-F covalent bonds and perfluorinated-CF3 bonds can be detected.MgH2@FBCNTs containing C-F semi-ionic bonds shows superior hydrogen absorption and desorption properties,due to the higher conductivity of C-F semi-ionic bonds than those of C-F covalent bonds and perfluorinated-CF3 bonds.MgH2@FBCNTs can release 5.36 wt.%H2 at 275 ? within 30 min,and quickly absorb 3.48 wt.%H2 at 100 ? under hydrogen pressure of 80 bar.After fluorination,cycling performance is significantly enhanced,which can be ascribed to the high electronegativity of F atoms,accelerating the dissociation or combination of hydrogen molecules,offering more active sites for the de/hydrogenation and preventing the growth and agglomeration during cycling.