催化剂掺杂对MgH2储氢性能的改进研究

Hydrogen Storage Properties of MgH;with Doping Catalyst

催化剂掺杂被认为是一种非常有效改善MgH_(2)储氢性能的途径。研究结果表明,过渡金属对氢原子具有很强的亲和力,在氢分子的解离或者氢原子的重组过程中,过渡金属的d电子和氢原子/氢分子轨道上的电子发生转移填充,由此产生的相互作用力促进氢分子的解离和氢原子的重组;掺杂过渡金属氧化物同样可以有效地催化MgH_(2)的吸放氢反应,而且在研磨过程中还可以作为润滑剂和分散剂,防止MgH_(2)颗粒的团聚,细化MgH_(2)颗粒尺寸,加速MgH_(2)的氢解吸动力学,催化MgH_(2)的吸放氢反应;过渡金属卤化物在吸放氢过程与MgH_(2)反应生成的过渡金属氢化物能够促进氢分子的解离和氢原子的扩散、在氢化过程中促进形核、减小吸放氢反应的活化能;金属硫化物或金属氢化物与MgH_(2)在球磨过程中的反应产物具有较高的催化活性,能在一定程度上解决脱氢/氢化动力学缓慢的问题,并且MgS能够提供丰富的成核活性位点。碳基材料的添加,能促进Mg/MgH_(2)相形核,细化颗粒尺寸,改善氢吸咐动力学,然而效果弱于金属基催化剂。但掺杂金属基和碳基复合催化剂,MgH_(2)能同时获得掺杂金属基和碳基催化剂的效果,即脱氢温度和脱氢活化能显著降低,又表现出优异的循环稳定性。

As a potential hydrogen storage material for solid hydrogen storage,the binary hydride MgH_(2) had the advantages of high hydrogen storage density and reversible hydrogen absorption and desorption.However,the applications of MgH_(2) were restricted due to the slow rate of hydrogen absorption and desorption,and the high working temperature.To date,in order to meet the requirements of solid hydrogen storage materials for vehicular hydrogen storage,many methods to improve the hydrogen storage performance of MgH_(2) were developed by scientific researchers such as alloying,nanocrystallization,and catalyst doping.These methods reduced the hydrogen desorption temperature of the system,increased the hydrogen absorption and desorption cycle stability.Among them,catalyst doping was the most extensively studied one,and it was considered to be a very effective way to improve the performance of magnesiumbased hydrogen storage.A mechanical ball milling process was used to add the catalyst.During the ball milling process,the catalyst might react with the magnesium hydride in a solid state.Ball milling was a very important method of structural modification.It could not only conveniently control the composition of the alloy,but also could directly obtain materials with metastable structures such as nanocrystalline,amorphous and supersaturated solid solutions.In this method,metals(such as Fe,Co,Ni,Cu,etc.),metal compounds(such as Nb_(2)O_(5),CeO_(2),TMTiO_(3)(TM=Ni,Co),etc.),transition metal halides(such as CeF_(4),CeF_(3),LaCl_(3),etc.),carbonbased materials(such as graphite,graphene,AC,CFs,etc.),metal-based and carbon-based composites were used as catalysts,and added single or in combination to magnesium-based alloys by mechanical ball milling.The research results showed that transition metals had a strong affinity for hydrogen atoms.During the dissociation of hydrogen molecules or the recombination of hydrogen atoms,the d electrons of transition metals and the electrons on the hydrogen atom/hydrogen molecular orbital were transferred and filled,which resulted in the interaction forces to promote the dissociation of hydrogen molecules and the recombination of hydrogen atoms.At the same time,after the metal catalyst was doped,many defects were generated on the surface of MgH_(2),which were conducive to charge and heat transfer.Furthermore,the metal oxides were cheap and easy to prepare.Doping transition metal oxides could effectively catalyze the hydrogen absorption and desorption reaction of MgH_(2).It could also be used as a lubricant and dispersant during the grinding process to prevent the agglomeration of MgH_(2)particles and refine the size of MgH_(2)particles,accelerate the hydrogen desorption kinetics of MgH_(2),catalyze the hydrogen absorption and desorption reaction of MgH_(2).Transition metal halides could react with MgH_(2)in the process of hydrogen absorption and desorption to generate transition metal hydrides.These transition metal hydrides could promote the dissociation of hydrogen molecules and the diffusion of hydrogen atoms,promote nucleation and reduction in the hydrogenation process.The reaction product of metal sulfide or metal hydride and MgH_(2)in the ball milling process had high catalytic activity,which could solve the problem of slow dehydrogenation/hydrogenation kinetics to a certain extent.In addition,MgS could provide abundant nucleation active sites,and Fe element could stabilize MgH_(2).LaF_(3)could react with Mg to form MgF_(2)and LaH;phases during hydrogenation ball milling,which significantly increased the amount and rate of hydrogen desorption of magnesium.The catalytic mechanism was that LaH;acted as a hydrogen pump.The addition of carbon-based materials could promote the nucleation of the Mg/MgH_(2)phase,refine the particle size,and improve the hydrogen absorption kinetics.Unfortunately,the effect was weaker than that of metal-based catalysts.Doped with metal-based and carbon-based composite catalysts,MgH_(2)could simultaneously obtain the effects of doped metalbased and carbon-based catalysts,such as the low dehydrogenation temperature,low dehydrogenation activation energy and excellent cycle stability.It could be seen that doped metal-based catalysts could generally promote the formation and decomposition of MgH_(2)bonds by generating a large number of defects or new phases,and then enhancing the dehydrogenation/hydrogenation kinetics of the hydrogen storage material system.Carbon-based materials doped with non-metals could effectively prevent MgH_(2)particles from agglomeration and grain growth during the ball milling process.In addition,its low density could maintain the high hydrogen storage capacity of MgH_(2)materials.In order to meet the practical standard,it was necessary to develop new technology and find new experimental methods.One of the important strategies to improve the hydrogen storage performance of MgH_(2)was synergistic utilization of metal-based materials and carbon-based materials as catalysts.To carry out composite catalysis research and to study the synergistic effect of different catalysts could help to improve the hydrogen storage performance of MgH_(2).With the help of simulation calculation or first principles,the mechanism of chemical reaction of MgH_(2)in the process of hydrogen absorption and desorption after catalyst doping could be further clarified.Therefore,the uniqueness and potential advantages of metal hydride hydrogen storage technology could be fully exploited.