Study On The Kinetics And Thermodynamics Modification Of Li-(Mg)-B-H Hydrogen Storage Systems And Their Mechanisms

LiBH4 has received extensive attention for its high hydrogen storage density(18.5 wt%).However,its high thermodynamic stability,sluggish de/rehydrogenation kinetics and incredibly harsh rehydrogenation condition severely handicap its application and popularization as on-board hydrogen storage material.Combining LiBH4 and MgH2(with molar ratio 2:1)by the reactive destabilization will produce new Li-Mg-B-H reactive hydride composites(RHCs).The intrinsic reaction path and the dehydrogenation enthalpy of LIBH4 can be changed.The Li-Mg-B-H composite system will generate MgB2 after the dehydrogenation,leading to an extremely excellent reversibility.But the kinetics for the second step desorption reaction of Li-Mg-B-H composite system should still be improved.Based on the detailed investigation of the main research progress at home and abroad,several catalytic additions and loading materials have been designed and fabricated to modify the de/rehydrogenation kinetic and thennodynamic properties of Li-Mg-B-H hydrogen storage system,and their modified mechanisms have been further studied.Three kinds of uniform dispersed transition metal nanoparticles wrapped by few layers of carbon(TM/C,TM=Fe,Co,Ni)were designed and fabricated by pyrolysis of corresponding metal-organic frameworks(MOFs-74-TM),and the TM/C were successively introduced into MgH,and Li-Mg-B-H RHCs.Research shows that the Ni/C exerts a significant effect on the hydrogen storage of MgH2.In-situ generated Mg2Ni and Mg2NiH4 nanoparticles in the matrix play an important role in the enhancement of the kinetic property and cycling stability.Further study indicates that the TM/C nanoparticles can also promote the kinetics of Li-Mg-B-H system at various degree(Efficiency order:Ni/C>Co/C>Fe/C).Detailed analysis makes it clear that the Fe/C,Co/C and Ni/C will eventually transformed to FeB,CoB and MgNi3B,nanoparticles respectively.MgNi3B2 possesses the hexagonal crystal structure,the same as MgB2,which can efficiently promote the nucleation of MgB2,leading to a rapid kinetics and cycling stability.Porous rod-like TMTiO3(TM = Co,Ni)composite oxide catalytic materials,which contain two kind of transition metal elements were synthesized by Sol-Gel method and calcinations procedure,and the TMTiO3 were introduced into MgH2 and Li-Mg-B-H RHCs respectively.Firstly,the effect of porous rod-like TMTiO3 on the hydrogen storage of MgH2 has been investigated.It can be found that NiTiO3 doped MgH2 shows a better improved effect compared with CoTiO3.The initial hydrogen desorption temperature for NiTiO3 doped MgH2 reduces to 235 ?,and 10 times cycling tests do not show clear recession.Mechanism analysis indicates that Ni will eventually transform to Mg2Ni and Mg2NiH4,and Ti exists in the matrix with the multi-valence forms(Ti2+,Ti3+ and Ti4+)for the NiTiO3 doped MgH2,The synergistic actions can promote the electron transformation from H-to Mg2+ and accelerate the escaping procedure of hydrogen atomic from the MgH2 matrix,which have significant effects on the enhancement of kinetics and cycling stability for MgH2 system.Then,the Li-Mg-B-H RHCs doped with TMTiO3 are further systematically studied,NiTiO3 shows more powerful catalytic effciency compared with CoTiO3 for the hydrogen desorption kinetic performance.The NiTiO3 doped Li-Mg-B-H RHCs can finish the hydrogen desorption process completely within 50 min.Mechanism analysis shows that the NiTiO3 will eventually transform to MgNi3B2 and TiB2,which both possess the same hexagonal crystal structure as MgB2.The MgNi3B2 and TiB2 can efficiently promote the nucleation of MgB2,leading to an improved kinetics and cycling stability.N-doped porous carbon framework(NPC)was synthesized by pyrolysis of metal-organic frameworks(ZIF-8),and LiBH4 was injected into the porous channel by melting process.At the same time,porous carbon framework(PC)without the N element was used as a reference sample.Hydrogen storage property tests indicate that the LiBH4 confined into the NPC shows a lower desorption temperature than the LiBH4 confined into the PC.The initial and peak hydrogen desorption temperatures for the LiBH4@NPC sample are 212.9 ? and 344.1 ? respectively,reducing 194 ?and 105 ? compared with bulk LiBH4,and also 31 ? and 8.3 ? lower than the LiBH4@PC sample.Detailed analysis indicates that there are ternary impacts of NPC on the desorption of LiBH4:1)C element in the NPC can reduce the thermodynamic stability of LiBH4,making it starts to decompose before the melting point;2)N element in the NPC can further reduce the thermodynamic stability of LiBH4;3)nano porous structure in the NPC framework has an interspace confinement effect on LiBH4,resulting in a nano-size influence on the desorption of LiBH4.Combining high catalytic activity Ni element with non-metal two dimension materials(contained C and N elements such as GR and g-C3N4),composite additions such as Ni@GR and Ni@g-C3N4 were designed and firstly introduced to the Li-Mg-B-H RHCs.Hydrogen storage property tests demonstrate that the Ni@g-C3N4,contains N element,stimulates the desorption kinetics of Li-Mg-B-H RHCs,which is quicker than the Ni@GR doped Li-Mg-B-H system.The induction period for the second step desorption reaction of the Ni@g-C3N4 doped Li-Mg-B-H RHCs disappears completely.Mechanism analysis shows that Ni particles in the Ni@g-C3N4 composite addition eventually transform to MgNi3B2,facilitating the nucleation process of MgB2 and accelerating the desorption kinetics of Li-Mg-B-H RHCs.The non-metal carrier,g-C3N4,has dual influences on the Li-Mg-B-H system:1)it contains some H+((NH2)-1 group)in the g-C3N4 carrier,and the H+ will interact with the H-in LiBH4 to trigger the H2 release at 105 ?;2)g-C3N4 will destabilize LiBH4 and MgH2 to form Li3N and Mg3N2 respectively for the high electronegativity of N.During the rehydrogenation process,the Li3N and Mg3N2 will be hydrogenated to Mg(NH2)2 and LiH reversibly.The desorption pathway for partial hydrogen storage component is changed in the Li-Mg-B-H system,and its thermodynamic property can be modified.