| Magnesium-based hydrogen storage alloys possess many characteristics such as high hydrogen storage capacity,abundant resources and low cost,and are considered as one of the most promising metal materials to be widely used in automotive internal hydrogen storage systems.However,the disadvantages such as high hydrogen release temperature,slow hydrogen absorption kinetics and easy toxicity affect its mass use.Although the hydrogen storage properties of MgH2 have been effectively improved by using modifications such as alloying,surface modification and nanosizing,the long hydrogenation cycle of the process of preparing MgH2,the relationship between alloy microstructure and properties and the mechanism of catalytic modification and hydrogen absorption/exhaustion behavior are not clear enough.In this paper,based on the overall regulation idea,the intern al microstructure and surface properties of MgH2 were improved by using mechanochemical reaction and high-temperature and high-pressure hydrogenation regulation.By introducing TiF3 and Nb2O5 catalysts to improve the surface activity of MgH2;based on the idea of micro-alloying regulation,a small amount of alloying element Ni is introduced to improve the phase composition and reduce the thermodynamic stability of the material system while retaining the high-capacity advantage of Mg-based hydrogen storage alloy.The effects of mechanochemical reactions and hightemperature and high-pressure hydrogenation on the phase composition,microstructure,activation properties,hydrogen absorption/release kinetics and variable-temperature hydrogen release thermodynamics of magnesium-based hydrogen storage materials are investigated.The main conclusions are as follows:(1)Study the phase composition and microstructure of magnesium hydride composites synthesized by catalyst and alloying elements in the process of mechanochemical reaction.The results show that both the introduction of TiF3/Nb2O5 catalyst and Ni microalloying can significantly accelerate the hydrogenation process of mechanical chemical reaction and improve the synthesis degree of magnesium hydride,and the hydrogenation effect is better in the case of composite catalysis.The hydrogenation degree of pure magnesium powder in ball milling at 20 h is low.When TiF3 was added,the samples were basically hydrogenated at 12 h.When Nb2O5 was added,20 h samples were basically hydrogenated.In addition,the samples were nearly hydrogenated at 12 h.Ni microalloying can be completely hydrogenated in 8 h.(2)The phase composition and microstructure of magnesium hydride composites synthesized by catalyst and alloying elements under the condition of isothermal and isobaric hydrogenation were studied.The results show that the introduction of TiF3/Nb2O5 catalyst and Ni microalloying can accelerate the isothermal and isobaric hydrogenation process significantly.When the temperature is 350℃ and the hydrogen pressure is 6 MPa,it takes 72 h for the pure magnesium powder to achieve basic hydrogenation.When adding catalyst and alloying elements,the hydrogenation time can be shortened to 12 h,and the hydrogenation degree of Mg-Nb2O5 is higher.When Ni microalloying catalyzed the synthesis of magnesium hydride,complete hydrogenation can be achieved in 8 h.(3)Study the activation properties,hydrogen absorption and desorption kinetics and variable temperature hydrogen desorption properties of magnesium hydride composites.The results show that the mechanical chemical reaction has better activation performance,there is no incubation period,and the first hydrogen absorption capacity can reach 5 wt.%after adding the catalyst.At 300℃,the hydrogen absorption capacity of pure magnesium powder is 1.8 wt.%,and the hydrogen absorption capacity of Mg-TiF3,Mg-Nb2O5 and Mg-TiF3-Nb2O5 can reach 7.04 wt.%,7.08 wt.%and 6.52 wt.%,respectively.The hydrogen absorption performance of Nb2O5 is better when catalyzed by Nb2O5 alone.The activation energy of Mg-Ni-Nb2O5 decreases to 49.42 kJ/mol after the addition of alloying element Ni. |
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