Nanocarbon Composites Preparation And Hydrogen Storage Mechanism

The nanocarbon composites for hydrogen storage were prepared using anthracite, petroleum coke,active carbon,deposited carbon made from acetylene and magnesium as main raw materials,and Fe,Co,Ni,Mo,Al and TiO₂ as catalysts.The structure and properties of the materials were characterized by TEM,XRD,FTIR,DSC,and p-c-T test,and the hydrogen capacity was determined by the self-designed apparatus according to water volume exchange principle.Also,the thermodynamics and kinetics were analyzed.At last,the mechanism of hydrogenation/dehydrogenation was discussed.After pretreatment of deashing and carbonization,the iodine absorption value of the anthracite increased,which showed that it had larger specific surface area.According XRD patterns,its structure became more regular,and the aromatic carbon layers grew larger and formed crystallitic carbon similar to graphite.According to the Bragg equation,its layer distance of(002) crystal planes was about 0.35nm and every crystallitic carbon was packed by six layers each of which was composed of about thirty-five carbon rings.Too high Mg content in the nanocarbon composites can lower the dispersivity of the powder.The dispersivity is fine as the Mg content is less than 60wt.%,while the dispersivity become lower with the increase of Mg content more than 60wt.%.The hydrogen capacity is the highest as the Mg content was about 60wt.%～70wt.%,and too high or too low of Mg content decrease the hydrogen capacity.In addition,too high Mg content reduces the dehydrogenation rate of the materials.The ball milling time and hydrogen pressure are the two important factors influencing the structure and properties of the materials.The suitable ball milling time is 2～4h,and the appropriate pressure is 1～2MPa.The material 60CC40Mg,which was milled for 2h and the particle size was 10～20nm,dispersed evenly and had nanocrystal structure.With the increase of ball milling time,the particles began agglomerating.Only small part of particles milled for 4h dispersed and the material milled for 5h agglomerated seriously and most of the material became amorphous.Active metals added in nanocarbon composites for hydrogen storage not only increased their hydrogen capacities,but also improved their dehydrogenation properties.The hydrogen capacity of the material 36CC60Mg4Fe which was prepared by adding Fe during the carbonization is 7.37wt.%.The dehydrogenation rate of the material increased and the dehydrogenation time reduced because of adding Ni and Al during ball milling.The initial dehydrogenation temperature of the material 40CC50Mg10Co is the lowest,201.7℃.All these metals could be well-distributed on the surface of crystallitic carbon and nanocystalline magnesium,which made hydrogen molecules atomization or protonation.On the one hand,the hydrogenation rate and the hydrogen capacity of nanocrystalline magnesium increased,on the other hand,hydrogen atoms were stored which diffused to the plane nets of carbon or defects to form metastable structure.The thermodynamics and kinetics of nanocarbon composites for hydrogen storage were analyzed.Within the limits of 300～380℃,enthalpy change and entropy change of the material 29CC67Mg4Ni during the process of hydrogenation are -52.38kJ·mol^-1 and -98.92J·mol^-1·K^-1, respectively,which the absolute values are both less than those of pure magnesium,and are reduced by 42.04%and 36.58%,respectively.The rate constant of dehydrogenation of the materials became larger with the increase of temperature,and the higher the temperature of dehydrogenation,the larger the rate constant.So the reaction belongs to the first order reaction. It was found that the activation energy of MgH₂ reduced,when suitable magnesium was added in crystallitic carbon during reactive ball milling.Thereby,the rate of dehydrogenation of the materials increased,and kinetics properties of the materials were improved.The results are that the higher the content of crystallitic carbon in nanocarbon composites for hydrogen storage,the lower the activation energy of the materials,and the faster the dehydrogenation rate.The activation energy of nanocarbon composites for hydrogen storage prepared by hydrogen reactive ball milling is between 109.17kJ·mol^-1 and 129.36kJ·mol^-1.At last,the mechanism of hydrogenation/dehydrogenation of nanocarbon composites for hydrogen storage was discussed.Abundant lattice defects,such as defects of lattice plane nets of carbon,inside cavity of carbon hexagonal grid,marginal forcipated defects,clearance between lattice plane nets of carbon,were formed during the process of high energy reactive ball milling of crystallitic carbon.Also,the strong C-H chemical bond was formed.All of the above were in favor of hydrogen chemiadsorption and making crystallitic carbon becomging hydrogen storage body by the action of catalysts.Nanocrystalline magnesium is of good hydrogenation properties for some reasons:â‘ During the process of reactive ball milling,the size of magnesium particles became smaller and smaller,and then the surface energy and activity of magnesium particles were improved.Besides,the defects which could supply more active center were formed.All these were advantageous to the formation of MgH₂;â‘¡The lattice deformation of crystalline magnesium reduced the diffusion course of hydrogen atom, which was favorable for the diffusion of hydrogen atom;â‘¢The interface and crystal boundary among nanocrystalline magnesium were good channels for the diffusion of hydrogen atom;â‘£Metal catalysts were embedded on the surface of nanocrystalline magnesium,and then caused electron transfers and surface-catalyzed reactions.