The Preparation Of Transition Metal （Fe,Co,Ni） Based Compounds And Effects On The Hydrogen Storage Properties Of MgH₂

Hydrogen has become an ideal energy carrier with high combustion calorific value,environmental friendliness and renewability.However,the commercial application of hydrogen energy requires the development of efficient and safe hydrogen storage technology.Mg has attracted much attention due to its high theoretical hydrogen storage capacity（7.6 wt%）,low cost,and abundant resources,which is considered to be a very potential hydrogen storage material.Nevertheless,the overly stable thermodynamic and slow kinetic properties of MgH₂ limit its practical application.In view of the above problems,in this paper,a series of transition metal（Fe,Co,Ni）-based catalysts were designed and prepared by catalytic modification method.The catalytic mechanism and microstructure changes of the transition metal catalysts on the hydrogen absorption and desorption properties of Mg/MgH₂ hydrogen storage system were systematically studied.MgNi₂ alloy was prepared by high temperature sintering method,and combined with Mg through mechanical ball milling and hydrogen combustion synthsis method,and the promotion mechanism of MgNi₂ on MgH₂ hydrogen absorption and desorption kinetics was studied.The results show that Mg reacts with MgNi₂ to form Mg₂Ni H₄ during the hydrogenation process,and Mg₂Ni is generated in situ during the dehydrogenation process,constructing a Mg/MgH₂-Mg₂Ni/Mg₂Ni H₄ composite hydrogen storage system.The induction effects of Mg₂Ni and Mg₂Ni H₄ can significantly improve the kinetic properties of the Mg/MgH₂ system and promote hydrogen absorption and desorption.At 473 K,the Mg-Mg₂Ni composite can absorb 2.5 wt%hydrogen within 1 h,and at 573 K,the amount of hydrogen desorption increases by 2.6 wt%,which is much higher than that of pure Mg.Facilitated by Mg₂Ni H₄,the initial hydrogen desorption temperature of the composite is 100K lower than that of pure MgH₂（623 K）.The in-situ formed Mg₂Ni and Mg₂Ni H₄ anchored on Mg/MgH₂ act as a"hydrogen pump",which accelerates the transfer of hydrogen,weakens the energy barrier of Mg-H,and reduces the temperature of hydrogen absorption and desorption.The NiFe alloy nanoparticle product NiFe@CNT supported by bamboo-like carbon nanotubes derived from MOF materials through a simple pyrolysis process.The experimental results show that the MgH₂-NiFe@CNT composite can absorb 6.67 wt% H2 within 60 s at 573 K,and when the temperature is lowered to 373 K and 348 K,the hydrogen absorption capacities are 4.06 wt% and 3.25 wt%,respectively,while the hydrogen absorption of ball-milled MgH₂ at 423 K is only 0.82 wt%.More importantly,the introduction of NiFe@CNT into the Mg/MgH₂ system in situ generates two different catalytic phases,Mg2 Ni and α-Fe.The synergistic effect of the "hydrogen pump" function of Mg2Ni/Mg2 Ni H4 and the "hydrogen channel" function of the catalytically active material α-Fe,as well as the good dispersion function of carbon nanotubes in the hydrogen storage system,contribute to the excellent hydrogen storage performance of MgH₂-NiFe@CNT.CNTs not only serve as supporting materials to disperse the catalytic active substances,but also effectively disperse the hydrogen storage system and hinder particle agglomeration when broken into carbon fragments by ball milling.The ternary transition metal sulfide FeNi₂S₄ with hollow spherical structure was synthesized by solvothermal method,and it was introduced into MgH₂ by the methods of mechanical ball milling and hydrogen combustion.Three catalytic phases,Mg2 Ni H4,Fe and Mg S,appeared in the process of combining FeNi₂S₄ with MgH₂,thus constructing the MgH₂/Mg2 Ni H4-Mg S/Fe system.The results show that the hydrogen absorption and desorption performance of MgH₂ is significantly improved due to the synergistic catalysis of the active materials Mg2Ni/Mg2 Ni H4,Mg S and Fe derived from the MgH₂-FeNi₂S₄ composite hydrogen storage material.The hydrogen uptake of the composite at 373 K reaches 4.02 wt% within 1 h,which is 6 times that of ball-milled MgH₂（0.67 wt%）.In terms of hydrogen desorption,the initial dehydrogenation temperature of the composite is 80 K lower than that of MgH₂,and the activation energy of dehydrogenation is 95.7 k J mol-1 lower than that of MgH₂（161.2 k J mol-1）.Mg2Ni/Mg2 Ni H4 acts as a "hydrogen pump" to drive Mg/MgH₂ to absorb and desorb hydrogen rapidly,and Mg S and Fe provide catalytic active centers to accelerate the transfer of H,which is beneficial to improve the dynamic performance and cycle stability of the composite.A carbon-loaded transition metal sulfide FeCoS@C was designed and prepared by solvothermal and high-temperature sintering methods.The synergistic catalysis of MgH₂ by in situ formed carbon-dispersed heterogeneous catalysts Mg Co2,α-Fe,Co3Fe7 and Mg S was investigated.It is found that MgH₂-FeCoS@C can rapidly absorb 6.78 wt% H2 within 60 s at 573 K and can absorb 4.56 wt% H2 within 900 s at 473 K.In addition,after adding FeCoS@C,the initial hydrogen desorption temperature of MgH₂ decreased from 620 K to 550 K.Studies have shown that Mg2 Co,α-Fe,and Co3Fe7 can act as "hydrogen channels" to accelerate the transfer of H due to the presence of transition metals,while Mg S provides a strong and stable catalysis for the composite hydrogen storage system.Furthermore,the carbon skeleton obtained by carbonization of ZIF-67 can not only serve as the dispersion medium of the heterogeneous catalytic system,but also provide more active sites for the catalyst.The carbon material disperses the catalytic phase and also effectively reduces the agglomeration of the composite hydrogen storage material particles,thereby improving the reversible hydrogen storage performance of the system.