碳质材料在镁基储氢材料中的应用

Application of Carbon Materials in Magnesium-Based Hydrogen Storage Materials

碳质材料因具有诸多优异的物理化学性质,在储氢研究领域受到广泛关注。综述了碳材料(石墨、石墨烯、碳纳米管)在镁基储氢材料中的应用现状与研究进展,讨论碳材料负载不同催化剂(活性金属、金属间化合物、过渡金属等)对镁基储氢材料性能(储氢容量、吸放氢动力学、反应活化能、循环稳定性等)的影响。在研究碳材料的催化机制时发现,碳材料在镁基储氢材料中不仅起到催化、助催化的作用,还能抑制晶粒团聚和生长。将碳材料应用于镁基金属氢化物时,两者的协同效应能使储氢体系表现出高活性;当碳材料负载催化剂时,还起到良好的分散剂的作用,有利于MgH2表面催化剂均匀分散,使储氢体系表现出高循环稳定性。可为储氢领域高性能材料的构建提供一种新的思路。

Hydrogen energy,as the clean energy with the most development potential in the 21st century,has been extensively used in transportation,power generation,fuel cells,and other fields because of its significant advantages such as zero-carbon emission,high efficiency,high calorific value,non-toxic and pollution-free.The process of hydrogen energy utilization mainly includes four links,namely hydrogen generation,storage,transportation,and application,among which,hydrogen storage technology is considered the key factor limiting the large-scale commercial application of hydrogen energy due to the low hydrogen storage density,high energy loss,and high material requirements for high-pressure containers,and thereby has aroused extensive research all over the world.Magnesium is regarded as a very promising material for hydrogen storage because of its high capacity(7.6%H_(2))and abundant resources.However,high working temperatures(573~673 K)and sluggish hydrogen absorption and desorption rate limit its practical application.To address this issue,carbon materials including graphite,graphene,and carbon nanotube,have been widely incorporated into Mg-based hydrogen storage materials due to the advantages,such as high reducibility,stable structure under high-temperature conditions,the porous and high specific surface areas(which provide significant superiorities for catalysis when being prepared in different physical forms to support nanoparticles),and the synergy effect when carbon material interacts with the supported functional groups or metals.Therefore,the application status and research progress of carbon materials(graphite,graphene,carbon nanotubes)in magnesium-based hydrogen storage materials were reviewed in this work.The effects of carbon materials on hydrogen storage properties(hydrogen storage capacity,hydrogenation/dehydrogenation kinetics,reaction activation energy,cycle stability,etc.)were comprehensively discussed with catalytic mechanism analyzed when loaded with different catalysts(such as active metals,intermetallic,transition metals,etc.).It was found that carbon materials play the role of catalyst,cocatalysts,and inhibition of aggregation and growth of magnesium-based hydrogen storage materials(such as MgH_(2)).With regard to graphite,its addition reduced the oxidation of the material surface and improved the hydrogen absorption capacity.When graphite embedded in the magnesium-based material matrix evenly and wrapped Mg particles through the ball-milling procedure,it would hinder the aggregation of MgH_(2) particles and provide additional edge location,and act as a catalyst for the hydrogenation/dehydrogenation reaction.With regard to graphene,it was easy to form a heat conduction network due to its unique two-dimensional structure,and thereby shortened the time for the reaction to reach the energy barrier and optimized the kinetic performance.When graphene was used as the carrier of catalyst,its surface chemistry,electronic structure,and structural characteristics had a significant impact on its activity and stability.The vacancy defects provided stronger binding sites for metal atoms and improved the binding energy of metals,especially transition metals(such as Ni,Ti,Nb)and alkaline metals(such as Li,K).Graphene with vacancy defects could also effectively prevent the agglomeration of metal atoms without reducing H_(2) adsorption capacity.In addition,attaching metals to the vacancy defects,the graphene would transfer a large amount of charge between Mg and MgH_(2),constructing the path that hydrogen atoms diffuse and release preferentially in the process of hydrogen absorption and desorption.Graphene could also be designed to construct a spherical shell structure to inhibit the agglomeration of particles and improve the cycling performance.Carbon nanotubes(CNTs),unlike graphene with two-dimensional layered structures,were sensitive to hydrogen molecules due to nanoscale hollows with larger specific surface area and excellent thermal conductivity.The primary catalytic effect of CNTs was to form hydrogen diffusion channels by inserting them into the matrix of hydrogen storage materials.The highly curved surface of CNTs changed the charge distribution of MgH_(2),thereby weakening the interaction between Mg and H atoms and improving the desorption kinetic performance.The electronic structure of the active component generated strong interaction with carbon nanotubes when carbon nanotubes were used as carriers of catalysts to support transition metals,intermetallic compounds,and multi-metal,which promoted the transfer of electrons between Mg/MgH_(2),reducing the stability of Mg-H bonds and the decomposition energy barrier of H_(2).To sum up,carbon material mainly acted as a good catalyst and cocatalyst when introduced into the Mg/MgH_(2) system.When the carbon material was uniformly dispersed on the metal surface,it could promote the dissociation of hydrogen molecules and assist the diffusion of hydrogen atoms,provide active sites and accelerate the nucleation of MgH_(2) while reducing the desorption temperature and optimizing the kinetic performance.At the same time,the synergistic effects between carbon material and active components made the hydrogen storage system show high activity.Through the intercalation or coating,carbon materials could be evenly dispersed in Mg-based hydrogen storage materials,presenting enhanced adsorption and excellent cyclic stability.This work could provide important references for the optimization and construction of high-performance Mg-based hydrogen storage materials to promote the development of solid-state hydrogen storage technology.