| In view of high hydrogen storage capacity,good cycle stability,and poor dynamic and thermodynamic performance of Mg/Mg H2system hydrogen storage materials.In this paper,to improve the comprehensive hydrogen storage properties of Mg/Mg H2system,Mg nanoparticles were prepared via chemical reduction method by using Li,naphthalene and Mg Cl2as raw materials,and the effects of milling time,catalyst doping and transition metals Ni,V,Ti loaded on carbon materials on the hydrogen storage performance of nano-Mg were studied.Firstly,the hydrogen storage properties of nano-Mg under different milling time?0-90 min?were studied.It was found that nano-Mg had the best hydrogen storage properties when it was milled for 60 min.The addition of graphene?G?could effectively inhibit the agglomeration of nano-Mg particles,improved the milling efficiency,and further promoted the hydrogen storage performance of the material.The Mg+G composite hydride could release 6.0 wt%H2when it was heated to 295?at a heating rate of 1?/min,which was 32?lower than that of undoped sample.And the dehydrogenation apparent activation energy of Mg+G composite hydride was 153 k J/mol,which was 22 k J/mol lower than that of Mg H2?175 k J/mol?.Secondly,Ni and Ni@G nanocomposites were introduced to improve the hydrogen storage properties of nano-Mg by mechanical alloying.When absorbed 4 wt%H2,the temperature for the as-milled nano-Mg sample was238?at a heating rate of 1?/min,while Mg+Ni composite and Mg+Ni@G composite only need to be heated to 109?and 80?,respectively.The initial dehydrogen temperature of Mg+G,Mg+Ni and Mg+Ni@G composites was253?,234?and 222?,respectively,which was 71?,90?and 102?lower than that of undoped sample.The hydrogen storage of nano-Mg can be improved effectively by doping with catalysts.Especially,the hydrogen storage performance of nano-Mg was significantly improved under the conbined action of Ni and graphene.For example,at 325?,the dehydrogenation rate of Mg H2was 1.04×10-3wt%s-1,it increased to 1.342×10-2wt%s-1under the catalytic action of Ni nanoparticles.While the Ni@G was used as a catalyst,the dehydrogenation rate of Mg+Ni@G composite increased to 2.564×10-2wt%s-1,which was 24.6 times and 1.91 times quicker than that of Mg H2and Mg+Ni composite.Meanwhile,the dehydrogenation activation energy of sample decreased from 206.84 k J/mol to 83.68 k J/mol afer it doping with Ni@G nanocomposite.In order to further improve the thermodynamic and kinetic properties of nano-Mg,TM@G?TM=V,Ti?nanocomposites were introduced to improve the hydrogen storage properties of nano-Mg by machenical alloying.The results showed that the hydrogen storage performance of Mg/Mg H2was remarkly improved under the catalytic action of TM?TM=V,Ti?composites.For example,when absorbed 4 wt%H2,Mg+Ti@G composite need to be heated from room to165?with a rate of 1?/min,and the temperature for the Mg+V@G composite was 157?as if it uptake a similar amount of hydrogen under the same condition.In the process of dehydrogenation,the dehydrogenation tempareture of Mg H2also can be decreased when TM?TM=V,Ti?composites were used as the catalysts.The rehydrogenated Mg sample desorbed 60%H2need to heat to 359?,while for the Mg+V@G and Mg+Ti@G composites just only need to raise the temperature to 251?and 275?,respectively.The activation energy for Mg+V@G and Mg+Ti@G composites was 150.41 KJ/mol and 151.16 k J/mol,respectively,which was far lower than that of Mg H2?206.84k J/mol?,suggesting that TM@G?TM=V,Ti?nanocomposites have a good catalytic effect on the hydrogenation of Mg H2.Finally,to further study the effect of the carbon material on the hydrogen storage performance of as-prepared nano-Mg,acetylene black supported with TM?TM=Ni,V,Ti??marked as TM@ACET?was prepared by Rieke method,and the catalytic action of TM@ACET?TM=Ni,V,Ti?on the hydrogen storage property of Mg H2were studied.Experimental results show that the dehydrogenation temperature of Mg H2was significantly reduced under the catalytic action of ACET,Ni@ACET,V@ACET and Ti@ACET nanocomposites,especially the V@ACET nanocomposites showed an excellent catalytic effect on the dehydrogenation of Mg H2.Compared with Mg H2,Mg+ACET,Mg+Ni@ACET and Mg+Ti@ACET composites,the dehydrogenation temperature of Mg+V@ACET composite decreased by 100?,84?,26?and 26?,respectively.In addition,the addition of ACET,Ni@ACET,V@ACET and Ti@ACET nanocomposites can significantly improve the kinetic performance of Mg H2dehydrogenation,and its catalytic effect sequence on the kinetic performance of Mg H2was ACET<Ti@ACET<Ni@ACET<V@ACET.The catalytic effect of V@ACET composite was superior to any other composites.The dehydrogenation activation energy of Mg+V@ACET composite hydride was 134.62 k J/mol,which was 62.2 k J/mol and 58.5 k J/mol lower than that of Mg+Ti@ACET and Mg+Ni@ACET composite hydride,respectively. |
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