| Under the global implementation of the consistent goal of carbon peak and carbon neutrality,ship energy will be transformed from traditional fossil fuels to low-carbon hydrogen-rich energy.As a clean secondary energy,hydrogen energy has the characteristics of flammability,explosion and extremely low density,so safe and efficient hydrogen storage is particularly important.Magnesium hydride（Mg H2）has the advantages of safety,low price,good cycle reversibility,and high hydrogen storage capacity（7.6 wt%）,which make it a very promising hydrogen storage material.However,the stable thermodynamic properties and slow kinetic properties of Mg H2 still puzzled many scholars.Doping catalyst is the simplest and most effective strategy to improve Mg H2 kinetic performance.In this work,Ni-based catalysts with different morphologies were synthesized by controlling the reaction conditions and the ratio of raw materials.Flake Ni nano-catalyst was synthesized firstly.Then,considering the good role of carbon materials in the hydrogen absorption and desorption cycle of Mg H2,Ni/BC,Ni O/C and Ni Mo O4/r GO catalysts were synthesized respectively.Finally,the bimetallic nano-catalyst Ni3Fe/BC was synthesized by comprehensively considering the performance,cost and synergistic catalytic effect of different carbon materials.The above catalysts were introduced into the Mg H2 system by mechanical ball milling to systematically study their effects on the kinetic properties of Mg H2 hydrogen desorption and absorption,and to reveal the corresponding catalytic mechanism.In this work,flake Ni nano-catalyst was prepared for the first time by wet chemical ball-milling.The performance test shows that the Mg H2+5 wt%Ni composite starts to release hydrogen at 180℃,and can complete 6.7 wt%hydrogen release at 300℃within 3 min,while pure Mg H2 starts to release hydrogen at 350℃.After complete dehydrogenation,hydrogen absorption starts at 40°C and 3 MPa hydrogen pressure.In addition,4.6 wt%H2 can be absorbed within 20 min at 125°C and 3 MPa hydrogen pressure,while the initial hydrogen absorption temperature of pure Mg H2 is about 130°C.The apparent activation energy for the hydrogen absorption and desorption reactions of Mg H2+5 wt%Ni are 42.73 and 83.90 k J/mol,respectively.The characterization after hydrogen absorption and desorption showed that the Ni nanocatalysts generated Mg2Ni/Mg2Ni H4 in situ during the reaction,which acted like a"hydrogen pump"and significantly improved the hydrogen ab/desorption kinetics of the composites.The flake structure of Ni nanocatalysts covers the surface of Mg H2 particles,which enhances the contact between the catalyst and the Mg H2 matrix.When the nano-sheets are further pulverized during the hydrogen desorption/absorption cycle,more active centers can be formed.Theoretical analysis shows that after the addition of Ni catalyst,positive and negative charges accumulate,which weakens the energy of Mg-H bonds,leading to the extension of the length of Mg-H bonds,which is more conducive to hydrogen desorption.Unfortunately,the cycle performance of the Mg H2+Ni composite was poor,and the hydrogen storage capacity decreased from 6.7 wt%to 5.6 wt%after 20 cycles.In order to improve the hydrogen ab/desorption performance of Mg H2 and further enhance the cycle stability,Ni/BC nano-catalyst was prepared by self-reduction using biomass charcoal（BC）as carbon source.Mg H2+10wt%Ni/BC-3 began to release hydrogen at187.8℃,and the apparent activation energy of hydrogen release decreased from 154.90k J/mol to 72.41 k J/mol.Although the initial hydrogen desorption temperature of Mg H2+Ni/BC is slightly higher than that of Mg H2+Ni,it has lower hydrogen absorption temperature and better cycle stability.After the complete hydrogen evolution of Mg H2+10wt%Ni/BC-3,the hydrogen absorption can begin at 30℃.The apparent activation energy of hydrogen absorption of Mg H2+10 wt%Ni/BC-3 is 26.95 k J/mol lower than that of pure Mg H2.After 20 cycles,the composites doped with Ni/BC catalyst can still maintain 99%hydrogen storage capacity.Various characterizations show that Ni/BC is uniformly distributed around Mg H2,and the Mg2Ni/Mg2Ni H4 formed in situ by Ni nanoparticles promotes the diffusion of hydrogen along the Mg/Mg H2 interface during hydrogen ab/desorption.The good conductivity of the carbon layer accelerates the electron transfer,and the synergistic effect of the carbon layer and the carbon layer improves the kinetic performance of Mg H2 and the cycle stability of the hydrogen storage material.In this thesis,biochar was introduced into Mg H2system for the first time,which provided a reference for the development of low-cost carbon borne hydrogen storage material catalyst.In view of the simple preparation of metal oxides and their good stability in air.Ni O/C catalysts were prepared by a simple ultrasonic vibration and calcination method,and Ni O/C was compounded with Mg H2 by a ball milling method.Mg H2 doped with Ni O/C can start dehydrogenation at 196℃,which is 154℃lower than pure Mg H2.After complete dehydrogenation,Mg H2+9 wt%Ni O/C begins to absorb hydrogen at 50℃.The apparent activation energies of hydrogen desorption and hydrogen absorption of Mg H2+9 wt%Ni O/C are 84.64 and 48.20 k J/mol,respectively.Mg H2+9 wt%Ni O/C can maintain 98.8%hydrogen storage capacity after 20 cycles.Studies on the catalytic mechanism show that Ni O/C catalyst is uniformly distributed on the surface of Mg H2.In the process of hydrogen absorption and discharge cycle,Mg2Ni/Mg2Ni H4 is generated in situ accelerate the diffusion of hydrogen.Theoretical analysis further reveals the influence of Ni O and Mg2Ni on the mean orientation shift of H element.Both Ni O and Mg2Ni can increase the diffusivity of H by more than 50%and maintain good H releasing ability.Fiber spherical bimetallic oxide Ni Mo O4 was synthesized by hydrothermal and calcination methods,and Ni Mo O4/r GO catalyst was synthesized by combination with reduced graphene oxide（r GO）.Experimental results show that Ni Mo O4 has better catalytic effect than Ni O/C.After adding Ni Mo O4,the initial dehydrogenation temperature of Mg H2 decreases to191℃,which is lower than that of Mg H2+Ni O/C.After complete dehydrogenation,Mg H2+10 wt%Ni Mo O4 can start to absorb hydrogen above 40℃,and its apparent activation energy of hydrogen absorption is 30.10 k J/mol lower than that of pure Mg H2.Due to the addition of r GO,the agglomeration of Mg H2/Mg was effectively prevented,and the hydrogen storage materials maintained high cyclic stability.Mg H2+10 wt%Ni Mo O4/r GO can still retain 96%hydrogen storage capacity after 20 cycles.Characterization analysis showed that Mg2Ni and Mo were formed after the first hydrogen desorption reaction of Ni Mo O4,Mg2Ni/Mg2Ni H4 and Mo promoted the desorption and diffusion of H in the process of hydrogen desorption and desorption.Moreover,Mo can effectively promote Mg2Ni/Mg2Ni H4mutual conversion.The synergistic effect of Mo and Ni can accelerate the performance of hydrogen storage materials.Bimetallic nanocatalyst Ni3Fe/BC was prepared in one step by solid-phase reduction using cheap biochar as carbon source.The starting hydrogen release temperature of Mg H2+10wt%Ni3Fe/BC is reduced to 184.5℃,and complete hydrogen desorption can be achieved at300℃within 7 min.After complete hydrogen desorption,hydrogen absorption can be started again at 30℃.After 20 cycles,the hydrogen storage capacity of Mg H2+10 wt%Ni3Fe/BC basically did not decay.Characterization analysis showed that Ni3Fe/BC was uniformly distributed on the Mg H2 after ball milling.Subsequently,the Ni3Fe/BC catalyst generated Mg2Ni/Mg2Ni H4 and Fe in situ during the hydrogen absorption and desorption reaction,and the synergistic catalytic effect of Mg2Ni/Mg2Ni H4 and Fe jointly ensured the hydrogen uptake and release performance of Mg H2.Meanwhile,Fe can also accelerate the interconversion of Mg2Ni/Mg2Ni H4,achieving a two-way promotion effect.Ni3Fe/BC bimetallic nanocatalyst synthesized by solid-phase self-reduction method is simple in preparation and low in price.It can realize Mg H2 hydrogen absorption and desorption at low temperature and good cycling performance. |
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