Study Of The Effect Of Bimetallic MOFs-derived Catalysts On The Properties Of Mg-based Hydrogen Storage Materials

Mg H₂has the disadvantages of high working temperature and poor hydrogen absorption kinetics,which limit its application.In the past decades,adding catalyst is considered as one of the most effective ways to optimize the Mg-based hydrogen storage materials.Among many catalysts,carbides showed excellent catalytic activity.However,most of the current researchers focus on obtaining the stoichiometric catalysts containing specific elements from the regulation of MXene phase components.In this paper,Ni₃Zn C_0.7/Ni/Zn O multiple catalysts were obtained through the Ni/Zn-MOFs precursor.Subsequently,Ni₃Zn C_0.7,Ni and Zn O unit catalysts were obtained by regulating the MOFs precursor,and the best catalytic activity species among the three catalysts were determined.The main findings of this paper are as follows:1.Different shaped Ni/Zn-MOFs precursors were obtained by controlling different hydrothermal times,and in order to test the stability of Ni₃Zn C_0.7and Zn O,they were calcined under hydrogen for different times.Studies have found that the carbides and the Zn O were stable.After testing,the Mg H₂-5 wt%Ni₃Zn C_0.7/Ni/Zn O composite initially releases hydrogen at about 503 K and can release 5 wt%H₂within1 h at 573 K.Interestingly,at 353 K,it can still absorb 0.78 wt%H₂within 60 min.The activation energies（E_a）of hydrogen absorption and desorption of the composite was reduced to 44.85 and 93.06 KJ/mol,respectively.The experimental results show that Ni₃Zn C_0.7/Ni/Zn O composites react with Mg H₂during the absorption and desorption cycle,and in situ generated Mg₂Ni/Mg₂Ni H₄acts as a“hydrogen pump”to promote H₂dissociation and reduce the reaction barrier,provide more diffusion channels.Moreover,trace amounts of Zn/Mg Zn₂can promote Mg/Mg H₂formation,providing more routes for hydrogen diffusion.To determine the specific catalytic phase in the multiple catalysts,a unit catalyst study was conducted below.2.3D porous Ni₃Zn C_0.7was prepared by simple filtration and calcination.Then,different amounts of Ni₃Zn C_0.7（2.5,5.0 and 7.5 wt%）were added to the Mg H₂by ball milling to form the Mg H₂-Ni₃Zn C_0.7composite material.It was found that Mg H₂-5wt%Ni₃Zn C_0.7composite has the best hydrogen storage properties.After testing,the composite initially released hydrogen at about 383 K,and the hydrogen absorption capacity reached 2.34 wt%H₂at 353 K within 1 h.In addition,the composite can release about 5.36 wt%H₂at 573 K.And the E_aof the composite were reduced to37.28 and 84.22 KJ/mol H₂,respectively.In situ generated Mg₂Ni H₄/Mg₂Ni can act as a“hydrogen pump”to provide more activation sites and hydrogen diffusion channels during hydrogen absorption,facilitating H₂dissociation.Furthermore,the evenly distributed Zn/Mg Zn₂in Mg/Mg H₂promotes nucleation and provides more hydrogen diffusion channels.This attempt clearly demonstrates the efficient catalytic activity of non-stoichiometric bimetallic carbons and provides promising catalyst design for Mg-based materials.3.Ni was prepared by a method similar to that of the prepared Ni₃Zn C_0.7,in which large particles（Ni particles）were uniformly coated with worm tubular particles（carbon nanotubes）.Then,different amounts of Ni（2.5,5.0 and 7.5 wt%）were added to the Mg H₂by ball milling.After performance testing,the Mg H₂-5 wt%Ni composite has the best performance,with 423 K absorbing 5.2 wt%H₂in 200 s and about 3.75 wt%H₂at 573 K in 1000 s.The E_aof the composite were reduced to 87.63and 44.39 KJ/mol H₂,respectively.During the cycle,Mg₂Ni/Mg₂Ni H₄is generated in situ,which promotes H₂dissociation and provides more diffusion channels,and has a good catalytic effect Moreover,the presence of carbon nanotubes,restricts the agglomeration and growth of particles in the cycling process to some extent,thus improving the performance of Mg H₂.This simple Ni synthesis method provides a direction for the better design of high-performance transistors.4.The Zn O was prepared by similar method.The performance tests showed that the Mg H₂-2.5 wt%Zn O composite was the best among the three samples with different content.The composite can absorb 1.91wt%H₂even at 473 K for 1 h.Meanwhile,the composite can release 5.05 wt%H₂at 30 min and 573 K.Furthermore,the E_aof the Mg H₂-2.5 wt%Zn O composite were 72.00 and 107.58 KJ/mol H₂,respectively.After Zn O is added to Mg H₂,the phase changes in the cycle and Mg Zn₂is generated to catalyze Mg.By comparison the results,it is concluded that the main catalyst phases of Ni₃Zn C_0.7/Ni/Zn O multi-catalysts are Ni₃Zn C_0.7and Ni.The synergistic catalyst mechanism of Ni₃ZNC_7.0/Ni/Zn O multi-catalysts and the catalytic mechanism of adding unit catalysts were also revealed.