Preparation And Hydrogen Sorption Properties Of Nanostructured Mg Based Bulk Materials And Mg-metal Oxides Composite Powders

Due to high hydrogen storage capacity(7.6wt%), low cost, environmental friendliness and abundancy on earth, Mg and Mg-based materials are widely regarded as promising hydrogen storage carriers. However, Mg hydrides have not been widely used practically due to their sluggish hydrogen sorption kinetics and high operating temperature(~300 °C). To overcome these drawbacks, arc plasma method was used to prepare Mg-MexOy(MexOy = TiO2, Fe2O3, NiO, NiO/CeO2) composite powders. In addition, high pressure torsion(HTP) combined with arc plasma method was also used to prepare nanostructured Mg based bulk materials(pure Mg, Mg-(2wt%)Fe mixture and Mg-(2wt%)Ni mixture). ICP, XRD, TEM and SEM techniques were used to characterize the composition, phase components and microstructure of the composite powders. The hydrogen sorption properties of the composite powders were investigated by DSC and PCT techniques.The nanostructured Mg based bulk material prepared by high pressure torsion possesses homogeneous nanostructure. Compared to pure Mg powder,nanostructured pure Mg bulk material shows lower desorption temperature(424℃)on DSC curve and lower activation energy of 55.1kJ/mol, while the corresponding values for pure Mg powders are 440℃ and 92.9 kJ/mol, respectively. The improvement of the desorption temperature and the activation energy of pure Mg powders is mainly attributed to the structural re?nement in Mg and the positive catalytic effect from dispersed ?ne MgO induced under the HPT treatment. The Fe added into Mg by high pressure torsion method does not show remarkable catalytic effect on Mg, while the Ni added into Mg by the same method improves the kinetics of hydrogen absorption at low temperature.In the process of preparing Mg-MexOy, some oxides were reduced by Mg and the others remained their original chemical states. During this procedure, Fe2O3, Ni O and CeO2 were reduced to Fe, Ni and Ce2O3 since the electronegativity of Fe, Ni and Ce are lower than Mg, while TiO2 was not reduced by Mg.The Mg-MexOy, which was not involved in reduction reactions in the process of synthesis, shows high maximum hydrogen storage capacity but the hydrogen absorption kinetics is quite moderate. At 400 ℃, the maximum hydrogen storage capacity of Mg-TiO2 composite powders can reach 6.92wt%. At 200 °C under an initial value of 3 MPa, Mg-TiO2 composite powders possess a hydrogen adsorption capacity of 3.8wt% in 2h, and the corresponding temperature of MgH2 dehydrogenation peak on the DSC curve is 418℃.During preparation, the Mg-MexOy reduced by Mg shows good kinetics of hydrogen absorption and desorption, but the maximum hydrogen storage capacity is lower than that of those which didn’t undergo the reduction process. At 400℃, the maximum hydrogen storage capacity of Mg-Ni composite powders is just 6.21wt%. However, Mg-TiO2 composite powders is able to absorb 5.4wt% hydrogen in 2h at 200°C under an initial hydrogen pressure of 3MPa, and the corresponding temperature of MgH2 dehydrogenation peak on the DSC curve is 380℃.The excellent kinetics of hydrogen absorption and desorption is due to the catalytic effect of Mg2 Ni. For Mg-Fe composite powders, at 400℃, the maximum hydrogen storage capacity is 6.67wt%. At 200 oC, Mg-Ni composite powder can absorb 3.7% hydrogen in 2h under an initial hydrogen pressure of 3MPa, and the corresponding temperature of MgH2 dehydrogenation peak on the DSC curve is 410℃. The kinetics of Mg-Fe composite powders is poorer than Mg-Ni composite powders because of the limited catalytic effect of Fe.The maximum hydrogen adsorption capacity can be increased by doping oxides of rare earth elements into the Mg-Ni system. For instance, Mg-Ni/Ce2O3 exhibits a high maximum hydrogen adsorption capacity of 6.67wt% at 400℃. It is able to adsorb 5.6wt% hydrogen within 2h at 200℃ under an initial value of 3MPa, and the corresponding temperature of MgH2 dehydrogenation peak is located at 379℃ on the DSC curve. The results indicate that Mg-Ni-RE oxides systems have the merits of high hydrogen storage capacity and fast hydrogen sorption rate.