Catalytic Mechanism Of Carbon Nanotubes Supported Nickel On Hydrogen Storage Properties Of Magnesium Borohydride

High-capacity hydrogen storage material is a research hotspot in the field of hydrogen storage materials at present.Light metal complex hydrides,such as LiBH₄,NaBH₄,LiAlH₄ and Mg?BH₄?₂ et al,have attracted much attention due to their high theoretical hydrogen storage capacity.However,these materials usually have problems of unduly stable thermodynamic properties,poor dehydriding kinetics properties and low reversible capacity,which restrict their practical application.The functionalized carbon materials with microstructure based on carbon nanotubes?CNTs?,graphene?Gr?and graphite?C?can distinctly improve the hydrogen storage properties of materials.In order to improve the comprehensive properties of Mg?BH₄?₂hydrogen storage materials and investigate the variation of microstructure of hydrogen storage materials and the mechanism of the catalytic modification during the process of hydrogen absorption and desorption,CNTs,Gr and C were selected as additives combined with nano confinement effect and catalytic doping in this paper,which is based on domestic and overseas research progress of Mg?BH₄?₂.First of all,the modification of Mg?BH₄?₂ hydrogen storage materials through CNTs,Gr and C with different microstructures is studied.The results show that the hydrogen desorption property of the Mg?BH₄?₂-CNTs system is distinctly improved.The Mg?BH₄?₂-5wt%CNTs sample starts to release hydrogen at 120?and the desorption rate also increases compared with the pure Mg?BH₄?₂.It can be obtained from the analysis that the ball milling time influences the comprehensive property of the sample as well,and the best comprehensive property is obtained at the ball milling time of 10 h,which lays the foundation for further investigations of Mg?BH₄?₂ hydrogen storage materials.Secondly,the modification effect of CNTs with different doping amount on Mg?BH₄?₂hydrogen storage materials is investigated in depth on basis of the previous investigations.The results show that the Mg?BH₄?₂-xCNTs?x=5,10,25,50 wt%?hydrogen storage materials with different CNTs doping amount could be prepared through simple mechanical ball milling for10 h.The grinding aid effect of CNTs refine the Mg?BH₄?₂ particles to sizes of nanoscale,increasing the specific surface area and surface defects of the matrix,resulting the decrease of the thermodynamic stability of Mg?BH₄?₂.All the samples start to release hydrogen at temperature lower as 150°C after the doping of CNTs.CNTs also provide more hydrogen release channels,improving the dehydriding kinetics properties of Mg?BH₄?₂.Furthermore,the doping of CNTs changes the reversibility of Mg?BH₄?₂ to some extent.The Mg?BH₄?₂-5wt%CNTs sample start to release hydrogen at about 104°C,which is 170°C lower than the original sample?275°C?,and releases 6.04 wt%hydrogen in 2000 seconds at the constant temperature of 300°C.Mass spectrometry?MS?analysis demonstrates that the released gas is pure hydrogen without any other impurity gases.Further investigations of reversibility show that,the sample is able to absorb about 1.1 wt%hydrogen again within 1000 seconds at 300°C and 10 MPa hydrogen pressure after dehydration.The hydrogen absorption process lasts about6000 seconds,and the total hydrogen absorption is 2.3 wt%.Finally,the synergistic catalysis of Ni and CNTs is investigated.CNTs-xNi?x=20,40,60,80wt%?catalysts with different Ni loadings were prepared through in-situ reduction,which were doped into the Mg?BH₄?₂ through ball milling with a mass fraction of 5 wt%.The microstrutural characterization results show that the particles of matrix Mg?BH₄?₂ are uniform and refined after the doping of catalyst,which increases the grain boundary ratio and specific surface area;the Ni-based catalyst embedds in the matrix surface,that increased the efficiency of nucleation due to the increasing of surface defect amount and the shortening of the distance between matrix and catalyst,which provides more area for nucleation;the embedded carbon nanotubes also provides fast channels for hydrogen release,resulting the improvement of the hydrogen storage property.Among them,the 95Mg?BH₄?₂@5?CNTs-60wt%Ni?sample starts to release hydrogen at 93?and releases 5.2 wt%hydrogen at 300?with relative high desorption plateau pressure.According to thermodynamic calculations,the hydride decomposition enthalpy corresponding to Mg?BH₄?₂ platform is reduced from-49.1 kJ/mol H₂to-10.6 kJ/mol H₂.Its apparent activation energy is reduced to 119.6 kJ/mol,which is about332.0 kJ/mol lower than the original sample?451.6 kJ/mol?.The doping of CNTs-60wt%Ni decreases the thermodynamic stability of Mg?BH₄?₂ greatly and the dehydriding kinetics properties are improved.