Study On Catalytic Modification And Mechanism Of Iron Group Nanocomposite Catalysts For Magnesium Hydride Hydrogen Storage Materials

Magnesium hydride?MgH₂?is widely regarded as one of the most promising solidhydrogen storage materials due to its high hydrogen storage capacity(⁷.6 wt%/¹10kg H₂ m^-3),good reversibility and low price.However,the high hydrogen absorption/desorption temperature,the slow hydrogen absorption/desorption kinetics and the limited cycling stability severely restrict its practical application.The researches have found that the addition of transition metal compounds can effectively improve the hydrogen storage performance of MgH₂.In this paper,based on the review of magnesium hydride from around the world,the nanocomposites?CoB/CNTs,FeB/CNTs,CoFeB/CNTs,FeNi₃/CNTs,and MoSe₂@FeNi₃?of transition metal boride?FeB and CoFeB?and transition metal intermetallic compound?FeNi₃?with CNTs or MoSe₂ were prepared by redox and hydrothermal,introduced into MgH₂ using traditional dry ball milling and wet milling using cyclohexane?CYH?as the solvent,effectively improving the hydrogen absorption and desorption performance of MgH₂.In addition,the related mechanism have been systematically studied.First,the effects of dry ball milling and CoB/CNTs nanocatalyst on the hydrogen storage performance of MgH₂ were studied.The results show that the initial dehydrogenation temperature of commercial MgH₂ is as high as 380?.After adding10 wt%CoB/CNTs catalyst and ball milling,the initial dehydrogenation temperature of the sample is significantly reduced to 214?.The results of dehydrogenation kinetics studies show that the maximum dehydrogenation rate of commercial MgH₂ at300?is only 0.04 wt%H₂/h.After high-energy ball milling and CoB/CNTs catalysis,the maximum dehydrogenation rate o f MgH₂ was respectively increased to6.4 wt%H₂/h and 22.9 wt%H₂/h.In addition,the apparent hydrogen activation energy?Ea?of commercial MgH₂ is 208.5±3.4 kJ mol^-1,while the apparent hydrogen activation energy of ball milled MgH₂ and MgH₂?10 wt%CoB/CNTs samples is reduced to 115.6±2.7 kJ mol^-1 and 89.1±4.4 kJ mol^-1 H₂.The results of hydrogen absorption performance studies show that the MgH₂?10 wt%CoB/CNTs sample can absorb 5.7 wt%H₂ in only 26 s at 300?and 5 MPa H₂.However,MgH₂ without CoB/CNTs can only absorb 1.8 wt%H₂,and the hydrogen absorption of commercial MgH₂ is lower?0.5 wt%?.Moreover,the MgH₂?10 wt%CoB/CNTs sample also has excellent hydrogen absorption and desorption cycling stability.Ball milling can significantly reduce the grain and particle size of the sample,which is beneficial to shorten the hydrogen diffusion distance and increase the reactive sites to improve hydrogen absorption and desorption performance.Due to the introduction of CoB/CNTs,Co₃MgC generated in situ can provide active and nucleation sites for hydrogen absorption and desorption reactions,and can effectively inhibit sample particle agglomeration.Based on this research,the effect of wet ball milling using cyclohexane as the ball milling solvent on the hydrogen absorption and desorption performance of MgH₂ was further explored.The results show that the initial dehydrogenation temperature of the MgH₂?without CYH?sample is as high as 310?,and the temperature of the MgH₂is reduced by 32?after wet ball milling.The maximum dehydrogenation rate of the MgH₂?without CYH?sample is only 1.8 wt%H₂/h at 300?.After wet ball milling,the rate of the sample has increased by more than 2 times?3.7 wt%H₂/h?.MgH₂?without CYH?samples begin to absorb hydrogen at 140?,but the temperature of the MgH₂?CYH?sample is only 95?.This is mainly because wet ball milling can make the particle size of MgH₂ smaller and uniform.In addition,boron compounds?FeB and FeB/CNTs?of Fe element,which is the eighth group and has similar properties as Co,were synthesized and introduced into MgH₂ by wet ball milling to further improve its hydrogen storage performance.Hydrogen absorption and desorption performance studies have shown that the introduction of CNTs,FeB and FeB/CNTs can significantly improve the hydrogen storage performance of MgH₂.Among them,the catalytic effect of FeB/CNTs is the most significant.Specifically,FeB/CNTs can further reduce the initial dehydrogenation temperature of MgH₂ from310?to 196?and increase the maximum dehydrogenation rate by 12.8 times to23.1 wt%H₂/h;MgH₂?10 wt%FeB/CNTs?CYH?The initial hydrogen absorption temperature of the sample is only 39?.More importantly,the dynamic performance of MgH₂?10 wt%FeB/CNTs?CYH?sample after 10 cycles of hydrogen absorption and desorption is basically stable.This is because FeB in FeB/CNTs can react with MgH₂ to change its decomposition path.At the same time,in-situ Fe and B can provide more active and nucleation sites for the reaction.In addition,the good curvature and thermal conductivity of CNTs can effectively improve the heat transfer and hydrogen diffusion efficiency of the sample.Moreover,the stable presence of Fe and B generated in situ during the cycle makes MgH₂?10 wt%FeB/CNTs?CYH?samples have good cycle stability.In order to combine the advantages of CoB/CNTs and FeB/CNTs,new catalysts CoFeB and CoFeB/CNTs were synthesized by redox method.After introducing them into MgH₂ through wet ball milling,it was found that both CoFeB and CoFeB/CNTs can significantly improve the hydrogen absorption and desorption performance of MgH₂.Among them,CoFeB/CNTs have excellent catalytic effect.The specific performance is as follows:CoFeB/CNTs can further reduce the onset dehydrogenation temperature of MgH₂ to 177?;at 300?,the maximum dehydrogenation rate of MgH₂ can be increased to 29.7 wt%H₂/h;the Ea of MgH₂ in the MgH₂?10 wt%CoFeB/CNTs?CYH?sample was further reduced to 83.2±0.5 kJ mol^-1;and the onset hydrogen absorption temperature of MgH₂ was also reduced to 30?.In addition,MgH₂?10 wt%CoFeB/CNTs?CYH?sample has more excellent cyclic hydrogen absorption and desorption kinetic stability,and its capacity retention rate has been further improved to 98.5%.The reason why CoFeB/CNTs has excellent catalytic effect is that the in-situ generated Co₃MgC,B,Fe and CoFe can provide more active sites for the MgH₂ absorption reaction and nucleation.CoFeB/CNTs have a smaller particle size,and CoFeB from CoFeB/CNTs can be distributed tightly around CNTs.The reason why MgH₂?10 wt%CoFeB/CNTs?CYH?samples have outstanding cycle stability is that on the one hand,in-situ generated Co₃MgC,Fe and CoFe can continuously provide active and nucleation sites during the cycle,on the other hand,the elements of Mg,Co,Fe,B and C can be kept evenly distributed during the cycling.The intermetallic compounds FeNi₃ and FeNi₃/CNTs were successfully prepared by the redox method and introduced into MgH₂ by wet ball milling.Hydrogen absorption and desorption performance tests show that both FeNi₃ and FeNi₃/CNTs can significantly improve the hydrogen storage performance of MgH₂.Among them,the effect of FeNi₃/CNTs is more excellent.The hydrogen release test shows that the introduction of 10 wt%FeNi₃/CNTs can reduce the started dehydrogenation temperature of MgH₂ to 168?,and can also increase the maximum dehydrogenation rate of MgH₂ to 34.6 wt%H₂/h.In addition,the dehydrogenation Ea of MgH₂ in MgH₂?10 wt%FeNi₃/CNTs?CYH?samples is only 80.5±2.8 kJ mol^-1.Regarding the hydrogen absorption performance,the temperature of the MgH₂?10 wt%FeNi₃/CNTs?CYH?sample starts to absorb hydrogen is only 16?below room temperature.This is currently the lowest hydrogen absorption temperature of the MgH₂ hydrogen storage material we know.Moreover,the sample can absorb up to 6.0 wt%H₂ in only26 s at 150?and 5 MPa H₂.More importantly,it also has excellent hydrogen absorption and desorption kinetic stability and the capacity retention rate of 98.5%after 20 cycles.The mechanism analyses show that the MgH₂?10 wt%FeNi₃/CNTs?CYH?sample has good hydrogen release performance because FeNi₃ can not only react with MgH₂ to change its hydrogen release path,but also in-situ Fe and Mg₂Ni can provide active site;during hydrogen absorption,on the one hand,the stable presence of Fe can continue to provide active and nucleation sites,on the other hand,Mg₂Ni can absorb hydrogen and convert it to Mg₂NiH₄,which can effectively stimulate hydrogen absorption reaction of Mg.Moreover,the stability of the phase and particle size of the MgH₂?10 wt%FeNi₃/CNTs?CYH?sample during cycling and the uniform distribution of the contained elements are important reasons for its excel ent cycling stability.MoSe₂ hollow nanospheres were successfully prepared by hydrothermal method and FeNi₃ nanoparticles were attached to the surface of MoSe₂ hollow nanospheres by oxidation-reduction method to obtain MoSe₂@FeNi₃ composite catalyst.In order to prevent the wet milling solvent cyclohexane from entering the MoSe₂ hollow nanospheres and causing drying difficulties,dry ball milling was used to introduce FeNi₃,MoSe₂ and MoSe₂@FeNi₃ into MgH₂.The hydrogen absorption and desorption tests show that the MoSe₂@FeNi₃ composite catalyst has the most excellent catalytic effect.It can reduce the onset dehydrogenation temperature of MgH₂ to 194?,increase the maximum hydrogen desorption rate to 35.7 wt%H₂/h and reduce the onset hydrogen absorption temperature to 73?.In addition,after 10cycles,the kinetic performance and hydrogen storage capacity of MgH₂?10 wt%MoSe₂@FeNi₃ sample did not decrease significantly.The mechanism analyses indicate that on the one hand,MoSe₂@FeNi₃ can react with MgH₂ to decompose it into a more prone path;on the other hand,Mo,MgSe,Fe and Mg₂Ni generated in situ can provide the required activity and nucleation sites.During hydrogen absorption,Mg₂Ni can absorb hydrogen and be converted into Mg₂NiH_0.3 or Mg₂NiH₄.These reactions can promote the occurrence of hydrogen absorption the sample.