Synthesis And Hydrogen Storage Properties Of Mg-based Mg₂Ni-type Mg_2-xAl_xNi_1-yCo_y Hydrogen Storage Alloys

Mg-based Mg₂Ni-type hydrogen storage alloys were considered as one of the most promising lightweight hydrogen storage materials,due to numerous advantages such as high theoretical hydrogen storage capacity,abundant resources,low cost and relatively small density.However,Mg-based Mg₂Ni-type hydrogen storage alloys have not been fully used and commercialized because of its difficult synthesis,high hydrogenation temperature,poor reaction kinetics and thermodynamic properties to date.The crucial difficulty in synthesis of Mg₂Ni-type alloy by the conventional smelting method is due to large difference in melting point of magnesium and nickel and easy volatilization of magnesium in high temperature melting during conventional preparation process.Therefore,based on the analysis of present situation and existing problems of Mg₂Ni-type hydrogen storage materials,the synthesis and performance improvement of alloys were studied,and some important conclusions were obtained.Firstly,the Mg₂Ni hydrogen storage alloy was successfully prepared by low temperature solid phase sintering.Based on the influence of sintering temperature and sintering time on the structure and properties of the alloy,the optimum sintering conditions were as follows: sintering temperature of 893 K and sintering time of 14 h.In order to improve the performance of the alloy,the alloy was high-energy ball-milled and the effect of milling time on the structure and properties of the alloy was studied.The results showed that the maximum discharge capacity of the alloy increases and the cycling stability decreases with the increase of milling time.In addition,the effects of test temperature on alloy properties were investigated.Electrochemical studies showed that themaximum discharge capacity of the alloy increases significantly,but the capacity retention rate exhibits obvious reduction with the increase in test temperature.The thermodynamics analysis showed that the hydrogen evolution pressure of the alloy was gradually increased and the hydrogen evolution platform was flat and broadened.The calculated decomposition enthalpy of the hydride was-109.09kJ/mol H₂,indicating that metal hydride is not easy to be decomposed.Secondly,in order to improve the electrochemical performance of Mg₂Ni alloy,Mg element was replaced by Al element and the structure and performance of Mg_2-xAl_xNi alloy were studied.The results showed that the new phase of Mg₃AlNi₂ formed in replaced alloys.With the increase of x,the content of Mg₃AlNi₂ increases.When x=0.5,the alloy is consisted of a single Mg₃AlNi₂ phase,the maximum discharge capacity reaches the highest?117.58 mAh/g?,and the hydrogenation pressure and platforms are the highest and the widest,respectively.At the same time,the effects of milling time on the structure and properties of Mg_1.5Al_0.5Ni alloy were studied.The results showed that the diffraction peak of the alloy broadened obviously with the milling time,and some diffraction peaks disappear.Electrochemical analysis showed that,the maximum discharge capacity of the Mg_1.5Al_0.5Ni alloy is optimal?266.12 mAh/g?,when the ball milling time was 20 h,while the capacity retention rate exhibits obvious reduction.In addition,the effects of test temperature on Mg_1.5Al_0.5Ni alloy properties were studied.Electrochemical studies showed that the maximum discharge capacity of the alloy increases significantly,while the capacity retention rate exhibits obvious reduction with the increase of test temperature.The thermodynamics analysis showed that the hydrogen evolution pressure of the alloy was gradually increased and the hydrogen evolution platform was flat and broadened.The calculated decomposition enthalpy of the hydride was-36.67 kJ/mol H₂,which is less than that of the Mg₂Ni alloy.It implied that Al instead of Mg can effectively reduce the hydride stability of the alloy.Finally,in order to improve the cycling stability of Mg_1.5Al_0.5Ni alloy,Ni was replaced by Co element and the structure and performance of Mg_1.5Al_0.5Ni_1-yCo_y alloy were studied.The results showed that the new phase of MgCo₂ is formed in replaced alloys.Comparing with Mg_1.5Al_0.5Ni alloy,the maximum discharge capacity of Mg_1.5Al_0.5Ni_1-yCo_y alloy decreases,while the capacity retention rate of Mg_1.5Al_0.5Ni_1-yCo_y alloy increases.When y=0.2,the capacity retention of the alloy reaches to the best and the hydrogenation pressure and platforms are the highest and the widest,respectively.At the same time,the effect of milling time on the structure and properties of Mg_1.5Al_0.5Ni_0.8Co_0.2 alloy were studied.The electrochemical results showed that the maximum discharge capacity of the alloy at milling time of 20 h reaches the maximum?223.14 mAh/g?,while the capacity retention rate of the alloy exhibits obvious reduction.In addition,the effects of test temperature on Mg_1.5Al_0.5Ni_0.8Co_0.2 alloy properties were studied.Electrochemical analysis showed that the maximum discharge capacity of the Mg_1.5Al_0.5Ni_0.8Co_0.2 alloy increases significantly with the increase of test temperature.The thermodynamics analysis showed that the calculated decomposition enthalpy of the hydride of Mg_1.5Al_0.5Ni_0.8Co_0.2 was-39.45 kJ/mol H₂,which is more than that of the Mg_1.5Al_0.5Ni alloy.It implied that Co instead of Ni can increase the hydride stability of the alloy.