Study On Modification And Mechanism Of Magnesium-based Composite Hydrogen Storage Materials

With the increasing scarcity of fossil fuels and increasingly serious environmental problems,the development and utilization of clean energy is extremely important.Hydrogen energy is widely regarded as one of the most promising clean energy sources.How to store hydrogen safely and efficiently is currently practical for hydrogen energy.The main problem of globalization.MgH₂ has the advantages of high hydrogen storage capacity,abundant reserves,good reversibility and low cost.It is one of the most promising hydrogen storage materials at present,but MgH₂ has higher thermal stability and reaction activation energy barrier.As a result,the hydrogen absorption and desorption temperature is too high and the kinetic performance is poor,which limits its application in vehicle-mounted hydrogen storage systems.This paper systematically studied the effect of CaH₂ addition on the hydrogen storage performance of the12LiBH₄-La₂Mg₁₇Hy system,focused on the synergistic catalytic modification of MgH₂ by LaH₃ and LiBH₄ and the effect of La₂Ti₂O₇ catalyst addition on the hydrogen storage performance of MgH₂,and discussed the corresponding catalytic mechanism.The addition of CaH₂ to the LiBH₄/La₂Mg₁₇hy composite hydrogen storage material has no obvious effect on its thermodynamic properties,but it increases the apparent activation energy of the two-step hydrogen desorption reaction,thus increasing the operating temperature of the hydrogen absorption and desorption reaction,and reducing the isothermal hydrogen desorption reaction.It failed to realize the synergistic improvement of the material’s hydrogen absorption and desorption operation temperature and reaction rate.However,in the first step of the MgH₂hydrogen absorption and desorption reaction of the sample,due to the addition of CaH₂,the LaH₃ in the sample after the ball milling is enriched in the hydrogen desorption reaction,thereby catalyzing the re-hydrogenation of the product Mg in the first step of the hydrogen desorption reaction.The co-addition of LaH₃ and LiBH₄ has a good synergistic modification effect on the hydrogen absorption and desorption performance of MgH₂.The initial hydrogen release temperature of MgH₂-0.1LiBH₄-0.04LaH₃ is 40℃lower than that of pure MgH₂,which is about 30℃lower than the initial hydrogen release temperature of MgH₂-0.04LaH₃,and the apparent activation energy for hydrogen release is 100 k J/mol.The 134 k J/mol of MgH₂ is reduced by 30%.The sample after complete dehydrogenation of MgH₂-0.1LiBH₄-0.04LaH₃ began to absorb hydrogen from room temperature,and the hydrogen absorption reached 5.4 wt%H₂ at 120℃.The start and end temperatures were lowered by 90℃and 130℃respectively compared with pure MgH₂.JMA fitting reveals that the hydrogen absorption process of the sample is controlled by a one-dimensional phase boundary dynamic mechanism.The in-situ formed LaH_2.3,La B₆ and Li₃Mg₁₇ act as a"hydrogen pump",accelerating the transfer and association of hydrogen.On the other hand,The interface between the catalytically active material and the hydrogen storage matrix provides many ways for hydrogen diffusion,and these interfaces are the active nucleation sites of the hydride phase,resulting in a lower nucleation barrier.The addition of La₂Ti₂O₇ effectively improves the kinetic performance of hydrogen absorption and desorption of MgH₂.The initial hydrogen release temperature of MgH₂-7 wt%La₂Ti₂O₇ sample is 35℃lower than that of MgH₂.At 275℃,the average rate of isothermal hydrogen evolution of the sample within 30 minutes is 14times that of the original sample,and the apparent activation energy of the hydrogen desorption reaction is 78±4 k J/mol,which is about 40%lower than that of MgH₂.The MgH₂-7 wt%La₂Ti₂O₇ sample starts to absorb hydrogen at room temperature and can absorb 4.3 wt%when the temperature reaches 50℃;the isothermal hydrogenation rate at 150℃is 16 times that of MgH₂.When hydrogen is cyclically absorbed and released at 300℃,the amount of hydrogen released for the second time is 92%of the amount of hydrogen released for the first time,and the amount of hydrogen released basically remains unchanged after 2 to 10 cycles.La₂Ti₂O₇ generates Ti,Ti O₂ and La Ti O₃ after the hydrogen absorption and desorption reaction.The multivalent transition metal has a better catalytic effect than the single valence metal oxide,which promotes the electron transfer between magnesium and H₂and the dissociation of H₂.