Research On The Synergistic Effects And Mechanisms Of Confinement-Catalytic On The Dehydrogenation Properties Of Magnesium-Based Hydrogen Storage Materials

Achieving the safe and efficient utilization of hydrogen energy is of great significance in solving the environmental pollution and energy depletion problems in the world.However,the inflammable,explosive and diffusible characteristics of hydrogen have brought many difficulties to its storage and extensive application.Magnesium hybrid(MgH2),as a solid hydrogen storage medium,has become one of the hottest hydrogen storage materials because of its high hydrogen storage capacity,low cost,abundant resource and safety in use.However,the high dehydrogenation temperature and slow dehydrogenation rate of MgH2 caused by the high thermodynamic stability and poor kinetics limit its practical application.In this thesis,the MgH2,carbon materials and transition metals or their compounds were mixed evenly by high-energy ball milling.Using the confinement effect of carbon materials and the catalysis effect of transition metals or their compounds,the dehydrogenation thermodynamic and kinetic properties of magnesium-based hydrogen storage materials were modified by synergetic confinement and catalysis effects.The modification mechanisms were also investigated by theoretical calculations.The detailed research results are as follows:(1)The doping of carbon materials(graphene,graphite)effectively inhibited the agglomeration of MgH2 particles during ball milling and made the particles size uniform.Carbon materials attached to the surface of MgH2 particles played the structural confinement role for MgH2,which decreased the dehydrogenation temperature and accelerated the dehydrogenation rate of MgH2.The doping of transition metals or their compounds(Ni,NiC12,NiF2,NiPc)exhibited the catalytic role in MgH2 dehydrogenation.It also decreased the initial dehydrogenation temperature and accelerated the dehydrogenation rate of MgH2.(2)The co-doping of carbon materials and transition metals or their compounds can combined the confinement effect of carbon materials and the catalyze effect of transition metals or their compounds to achieve the synergetic modification on the dehydrogenation properties of MgH2,which further improved the dehydrogenation properties of MgH2.In addition,the doping order of carbon materials and transition metals or their compounds had a certain influence on the dehydrogenation properties of MgH2.For example,in Ni and graphene sequential-doped MgH2 system,the Ni added firstly can be partially solid-soluted into MgH2 lattice,resulting in the deformed lattice and decreased structural stability of MgH2.The graphene added later and the remaining Ni have synergetic confinement and catalysis effects on MgH2.Thereby,the dehydrogenation properties of MgH2 are significantly improved.(3)First-principles calculations showed that the essential reasons for the improvement of the dehydrogenation properties of MgH2 could be ascribed to the interface charge transfer between MgH2 and dopants,which resulted in the structural deformation of MgH2 in different degrees and the weakened Mg-H bond strength.This eventually led to the decreased dehydrogenation enthalpy and activation energy of MgH2.