Synthesis And Hydrogen Storage Properties Of Micro/Nano Mg Based Core-shell Structure

Pure Mg is one of the most studied and most promising solid state hydrogen storagematerials due to many advantages, such as high capacity (7.6wt.%), wonderful reversibility,abundance in earth crust, etc. However, its thermodynamics and kinetics are too stable to bepratical useful. Theoretical calculation indicates that elastic strain induced during Mgre/dehydrogenation could affect its thermodynamics. But there is little adequate experimentaldata to support the theory. In this case, we tried several methods to synthesize Mg basecore-shell structure to introduce elastic strain by covering the Mg core with a crust. UtilizingX-Ray Diffraction and SEM to characterize its structure and Sivert method to analyze PCIdesorption curve.Mg@Ni core-shell structure is unable to be synthesized by transmetalation method. Mgparticles produced by gas condensation has better performance in the transmetalation reactionthan its counterpart which is produced by high energy ball milling. However, distribution ofNi particles at Mg surface are neither uniform, where large plank area gets the least coveragerate. The growth of Ni layer is not in the form of continuous shell but accumulation ofparticles which has porous structure. Ni particle synthesized by Mg+NiCl2+THF system havediameter about500nm while the counterpart from Mg+Ni(acac)2+(OAm) system is no morethan30nm. Surface oxidization acts as a major obstacle hindering the transmetalation reaction,but there is no effective method to remove the MgO layer.40%hydrofluoric acid and NEt3.3HF are effective aqueous and non-aqueous fluorinationagent to synthesize Mg@MgF2core-shell structure. MgF2layer fluorated by hydrofluoric acidis too compact even for hydrogen to penetrate, hinders the de/absorption process. MgF2layerfluorated by NEt3.3HF becomes obviously thicker when reaction time reached144h. SEMpicture of Mg@MgF2particles crushed by10MPa pressure reveals that thickness of MgF2isstill no more than10nm. Desorption PCI curves show ascent in plateau pressure, especially inelevated temperature (380oC). Hydrogen desorption enthalpy decreases from73.4kJ/mol(pure Mg) to71.8kJ/mol, while desorption entropy remains128J/mol·k. Nano-Mg particle with diameter ranging from20nm to50nm is successfully synthesizedby K+MgCl2+THF system, in a rigid anhydrous environment. Remove of side product KClthrough multiple washing with DMF and THF will result in formation of DMF-Mg colloid.Washed Mg nano particles can be successfully fluorated by NEt3.3HF to form Mg@MgF2core-shell. Direct fluoration of DMF-Mg colloid only results in KMgF3, no Mg@MgF2isfound in the precipitation.