Study On Kinetics Model And Mechanism Of Hydriding/Dehydriding Of Mg Based Alloys

Hydrogen storage material with good hydriding/dehydriding kinetics has been a hot topic for researchers.Mg based alloys have attracted much attention due to their abundant resources and high capacity.However,there are still a series of problems,such as poor kinetics performance,difficult activation and high hydrogen desorption temperature.Rare earth-magnesium based hydrogen storage alloys have the advantages of mild hydrogen storage,moderate platform pressure and abundant raw materials.Based on two control mechanism that the diffusion and interface speed control of the hydriding/dehydriding process to put forward a series of easy to use kinetics model formula,including relationship of reaction time and concentration,the optimum hydrogenation temperature and corresponding minimum characteristic time type.The rare earth-Mg based hydrogen storage alloys were prepared by powder sintering process and hydriding combustion synthesis,and hydriding/dehydriding kinetics of the alloys were studied systematically by using the derived model.In addition,the kinetic experimental data in some literatures are summarized,and hydriding/dehydriding kinetics of Mg alloys are studied by using the derived model.The correctness of the model are further verified.In the analysis of kinetic data of rich magnesium hydrogen storage alloy model,we found that the addition of Nd to improve the hydriding/dehydriding kinetics significantly for magnesium based hydrogen storage alloys.While improving the hydrogen storage capacity,the activation energy is reduced to 79.29 kJ/mol H₂,to complete hydrogen absorption that characteristics of the reaction time of the required minimum?83 s?under the optimum reaction temperature?696 K?and the condition of 4 MPaH₂.Magnesium rich hydrogen storage alloy consist of Mg₉₅Ni₅ with a small amount of Zr_0.7Ti_0.3Mn₂ formed.hydriding activation energy of 109.85 kJ/mol H₂,the characteristic time is only 49 s,than that of pure magnesium is significantly reduced.Mainly controlled by the interface,and the model fitting of R² up to 0.99045.Based on the fitting analysis of the experimental data of hydriding/dehydriding kinetics of three kinds of Mg₂Ni based hydrogen storage alloys,it can be concluded that the three alloys is controlled by diffusion.Among them,R² of Mg₂Ni alloy prepared by ball milling was the highest?0.98186?,and the activation energy was 110.326 kJ/mol H₂,the characteristic time was only 109 s under the optimum temperature of 707 K and 4 MPaH₂,and the overall effect was the best.The results of dehydriding kinetics show that: the three alloys prepared under different magnetic fields,the rate of dehydriding of 4 MPa H₂ alloy is the fastest,and the hydrogen can be released at 623 K and about 160 s.Much faster than the first two.R² of the three kinds of alloys is higher than 0.95,and the activation energy of the alloy is the lowest under 2 MPaH₂,which is 161.371 kJ/mol H₂.The kinetics fitting results of La-Mg composite hydrogen storage materials: the addition of Ni to La-Mg composite hydrogen storage materials to improve the kinetics of hydrogen absorption effect is the best,under 573 K and 2MPa H₂ that achieve to 3.87 wt.% maximum hydrogen storage capacity required for only 250 s,and can achieve the maximum hydrogen storage capacity of 90% in 80 s.After model fitting analysis found that the kinetic data under different pressure conditions can be fitting by the speed diffusion controlled model,R² is greater than 0.98,the hydrogen absorption of La₂Mg₁₇-Ni activation energy was 39.492 kJ/mol H₂,the characteristics of optimal temperature hydrogenation reaction time is 222 s,and the comprehensive performance optimal.The hydrogen storage alloy prepared in the experiment and literature of hydriding/dehydriding kinetics data by model fitting analysis,which verified the correctness and applicability of the model,and the information were obtained that the hydriding/dehydriding kinetic mechanism and apparent activation energy of different system alloy.For the hydrogen absorption process,it can also obtain the optimal reaction temperature and the characteristics of minimum reaction time of the system.The model provide new ideas and approaches to theoretical research on hydriding/dehydriding and similar gas-solid reaction for hydrogen storage alloy.