Study On The Pyrolysis Kinetics Of LiAlH₄ And Its Catalytic Regulation Method

Hydrogen energy is one of the most powerful renewable energy sources.However,hydrogen storage technology is the main bottleneck that restricts the sustainable development of hydrogen energy.Currently,solid-state hydrogen storage has attracted much attention due to its safety and high volumetric energy density.Complex hydrides are the prospective candidates of the solid-state hydrogen storage materials for commercial utilization,especially LiAlH4.Its high hydrogen storage capacity and low initial hydrogen release temperature make it a favorable candidate to meet the conditions for the commercial application of hydrogen storage materials.But LiAlH4 still has the disadvantages of high thermodynamic stability and slow hydrogen absorption(release)kinetics.In this paper,with LiAlH4 as the principal research object,the modification effect of the mechanical ball milling method and doping method on its hydrogen release performance was investigated.And all kinetic parameters of the two-step thermal decomposition of LiAlH4 were obtained by combining the theoretical kinetic method and optimized kinetic method,which contributed to constructing perfect kinetic modeling of LiAlH4 decomposition.In addition,the effect of the addition of Fe,Fe2O3 and Fe-Fe2O3 compound on the hydrogen release properties of LiAlH4 was investigated and the intrinsic mechanism was illustrated by the experimental method and theoretical calculation method.Since the initial temperature of the third-step decomposition of LiAlH4 is greater than 350℃,which is not favorable for its practical application.Therefore,this paper mainly talks about the first two-step decomposition of LiAlH4 and the research contents are as follows:(1)LiAlH4 was prepared by the mechanical ball milling method for 30 minutes and 60 minutes.During the ball milling process,LiAlH4 could partially decompose into Li3AlH6 and Al.As the increase of ball milling time,the total amount of hydrogen released from the samples gradually decreases and the initial decomposition temperature of the samples also decreases to a certain extent.Moreover,mample power law(Pn)and nucleation and growth(An)are the most suitable models for the first-step and second-step decomposition of LiAlH4,respectively.The activation energies of the two-step decomposition of pure LiAlH4 are 57.79 kJ/mol and 127.94 kJ/mol,respectively.Meanwhile,the activation energy of the second-step decomposition of the samples ball-milled for 60 minutes is reduced by about 36%.In addition,the kinetic parameters of the system were derived by genetic algorithm and the experimental data was in good agreement with the fitting curves.(2)LiAlH4-Fe and LiAlH4-Fe2O3 composite systems were prepared by adding different contents of Fe and Fe2O3 into LiAlH4,respectively.Compared with the samples directly ballmilled,the total amount of hydrogen released from the LiAlH4-Fe composite system increases while the total amount of hydrogen released from the LiAlH4-Fe2O3 composite system significantly decreases.And the initial decomposition temperature of the LiAlH4-Fe2O3 composite system greatly decreases.In addition,the peak overlap phenomenon occurs in the LiAlH4-Fe2O3 composite system,which substantially reduces the exothermic heat of the thermal decomposition and improves the safety of the hydrogen release process.In terms of the modification mechanism,the addition of Fe could reduce the stability of the[AlH6]3-group and weaken the strength of the Al-H bond,which is favorable to the thermal decomposition of LiAlH4.And the addition of Fe2O3 might attach to the surface of the matrix and provide more channels for the diffusion of H atoms,which could improve the hydrogen release properties of LiAlH4.(3)The LiAlH4-Fe-Fe2O3 composite system was prepared by adding different contents of the Fe-Fe2O3 compound into LiAlH4.According to the XRD results,Fe-ions might dissolve in the lattice of Li3AlH6 and they have strong interaction with adjacent Al-ions,so that intermetallic compounds between iron and aluminum could be formed.On the other hand,Fe2O3 could react with Al and form Al2O3,which is often used as a catalyst carrier.Fe-Al2O3 could serve as active sites to provide more H-atom diffusion channels.