Application of Carbon Materials in Magnesium-Based Hydrogen Storage Materials
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 consid-ered the key factor limiting the large-scale commercial application of hydrogen energy due to the low hydrogen storage density,high en-ergy 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％H2)and abundant resourc-es.However,high working temperatures(573～673 K)and sluggish hydrogen absorption and desorption rate limit its practical applica-tion.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 condi-tions,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 magne-sium-based hydrogen storage materials were reviewed in this work.The effects of carbon materials on hydrogen storage properties(hy-drogen storage capacity,hydrogenation/dehydrogenation kinetics,reaction activation energy,cycle stability,etc.)were comprehen-sively discussed with catalytic mechanism analyzed when loaded with different catalysts(such as active metals,intermetallic,transi-tion 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 MgH2).With regard to graphite,its addition reduced the oxidation of the materi-al 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 MgH2 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 alka-line metals(such as Li,K).Graphene with vacancy defects could also effectively prevent the agglomeration of metal atoms without re-ducing H2 adsorption capacity.In addition,attaching metals to the vacancy defects,the graphene would transfer a large amount of charge between Mg and MgH2,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 parti-cles 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 pri-mary 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 MgH2,thereby weakening the interaction between Mg and H at-oms 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/MgH2,reducing the stability of Mg-H bonds and the decomposi-tion energy barrier of H2.To sum up,carbon material mainly acted as a good catalyst and cocatalyst when introduced into the Mg/MgH2 system.When the carbon material was uniformly dispersed on the metal surface,it could promote the dissociation of hydrogen mole-cules and assist the diffusion of hydrogen atoms,provide active sites and accelerate the nucleation of MgH2 while reducing the desorp-tion 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 even-ly 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.
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