Synthesis And Hydrogen Sorption Mechanism Of Several Core-shell Structured Mg Based Composites

Magnesium has been considered as one of the most promising hydrogen storage materials due to its series merits like high gravimetric hydrogen storage capacity?7.6 wt%H₂?,environmental-friendly properties and rich sources.However,some drawbacks such as stable thermodynamics,sluggish kinetics and oxygen sensitivity hindered its widely application in practice.Target improving the hydrogen storage properties of Mg based materials,nanocrystallization,addition of catalysts and controlling the morphology of the particles were used in this study.The core-shell structure can enlarge the contact area between catalysts and Mg substrate and support more nucleation sites of Mg/MgH₂ during the hydrogen sorption processes,thus enhancing the catalytic effect of catalysts.By liquid electroless plating,generic solid and solvent evaporation deposition methods,the catalysts including transition metals?TM=Ti,Co,V,Pt,Pd?and NaBH₄ can be in-situ coated on the surface of Mg ultrafine powder which prepared by a DC arc plasma method.Thus,a series of core-shell structured Mg based materials were prepared and their hydrogen storage properties were investigated carefully.The ternary core-shell structured Mg@Ti@Ni and Mg@Co@V composites were prepared via the combination methods of a DC arc plasma method and electroless plating method.After hydrogenated,the corresponding TiH₂&Mg₂NiH₄ and Mg₂CoH₅&Mg₃CoH₅&V₂H layers covered on the surface of MgH₂ substrate,respectively.The hydrogenation enthalpies were calculated to be-67.1?-70.0 kJ/mol H₂,respectively,higher than that of binary core-shell structured Mg@Ti?Mg@Ni?Mg@Co?Mg@V composites and pure Mg ultrafine powder.The hydrogenation activation energies?63.7?73.3 kJ/mol H₂?and the dehydrogenation temperatures?350?323 ^oC?were the lowest among the ternary,binary composites and pure Mg/MgH₂ powder?118.2 kJ/mol H₂,383 ^oC?.The improvements can be attributed to the co-effect of the“spillover”effect from the stable hydrids TiH₂,V₂H and the“hydrogen pump”effect from Mg₂NiH₄,Mg₂CoH₅&Mg₃CoH₅.The core-shell like structured Mg@Pt,Mg@Pd composites with the Pt and Pd layers covered on the surface of pure Mg ultrafine powder were obtained using a generic solid method.The two de/hydrogenated composites showed faster hydrogen ab/desorption rate compared to pure Mg/MgH₂.In addition,the measured de/hydrogenation activation energy values?152.8/82.4,138.8/92.2 kJ/mol H₂?and tested dehydrogenation temperatures?287&374,246&363 ^oC?of the two composites from TPD and DSC were all decreased compared to those of pure Mg/MgH₂?333&408 ^oC?.Based on the results observed by TEM,after hydrogenated,Mg₃Pt and PdH_0.706/1.33 covered on the surface of MgH₂ substrate,and then transformed to Pt and Mg₆Pd after dehydrogenated for the Mg@Pt and Mg@Pd composites,respectively.According to the in-situ observation based HRTEM images obtained under electron irradiation of 3 nA,Mg₃Pt might play a“hydrogen pump”role to transfer H atoms during the dehydrogenation process of hydrogenated Mg@Pt composite,thus improving the hydrogen desorption properties.In addition,it can be deduced that the hydrogen sorption properties were improved due to the“spillover”effect of Pt during the initial stage of hydrogenation process.For the Mg@Pd composite,PdH_0.706/1.33 and Mg₆Pd acted as“hydrogen pump”to transfer H atoms in the hydrogen absorption and desorption processes,thus enhancing the hydrogen sorption properties.Mg@NaBH₄ with core-shell like structure was prepared using solvent evaporation deposition method.After activated under 350 ^oC,the shell layer transformed from NaBH₄ to MgB₂,thus the tested composite was actually Mg@MgB₂.The hydrogen absorption rate was faster than that of pure Mg at the same temperatures.In addition,the hydrogenation activation energy?60.1 kJ/mol H₂?of the composite and the dehydrogenation temperature?245 ^oC?of the hydrogenated composite were lower than those of pure Mg/MgH₂.The hydrogen sorption kinetics enhancements can be ascribed to the fact that the coverd MgB₂ may support more nucleation sites of Mg/MgH₂ and the solid of Na in Mg lattice may increase the hydrogen diffusion paths.However,the hydrogen sorption thermodynamics of the composite was not improved.