Synthesis And Modification Of Mg-ni Based Hydrogen Storage Alloys By Mechanical Grinding

The energy crisis and the using of fossil fuels has environment negative impacts, together with infinite demands of human beings, a new environment-friendly energy was urgently needed, especially the hydrogen resource. The hydrogen storage is one of the difficulties when to develope hydrogen energy technology.In all kinds of hydrogen storage, Mg-based hydrogen storage alloys have been considered as the most promising materials of hydriding, because of its high hydrogen storage capacity (MgH2 was about 7.6wt%, Mg₂NiH4 was about 3.6wt %), low density (only 1.74g/cm2), rich resources (Mg was the sixth metal element of the world), relatively low cost and environment-friendly. However, it usually takes a long time and a high temperature to absorb/desorb hydrogen. Besides, it has slow desorption kinetics, all these factors hinder its application.In this paper, all kinds of samples were synthesized by mechanical grinding, including those modified the surface performances by multi-phase grinding. The final process parameters for mechanical grinding were affirmed as: using stainless steel pot to grinding, the atmosphere was argon, rotational speed was 600rpm, ball-to-powder ratio was 20:1, and the agglomeration of the sample was crushed up per 2h in the glove box filled with argon, and then continue to grind.It was carried out for element substitution and multi-phase surface modification by mechanical grinding. The prepared samples were: Mg₂Ni,Mg₂-xTi_xNi/Mg₂ZrxNi1-x (x=0.2,0.4),2MgNi-xM(M=TiO₂,V₂O₅; x=10%,15%) and 2MgNi-xM(M=C, B;x=10%, 15%). The phases in the alloys and the hydriding/dehydriding properties of alloys were investigated by the measurement of XRD and Pressure-concentration-isotherms (p-C-T), respectively. Besides, the Differential scanning calorimetry (DSC) and simultaneous thermal analysis (TG) were employed to study on thermal behavior of the prepared alloys.It was indicated that the main absorbed phases were Mg₂Ni and Mg of the ternary alloys in which Mg was partially substituted by Ti and Ni by Zr. The speed of hydriding was improved after elements substitution, and the desorbing temperature was reduced while Ni was substituted by Zr.According to the results of the measurement of p-C-T at different temperature and Van′t Hoff equation, it is easy to establish the relationship between LnPH2 and 1000/T. It can be concluded that the values of enthalpy change (ΔH) would be decreased while increasing the content of substitutive elements, which could make the instability of hydrides and was helpful to desorbing hydrogen at a relatively low temperature.After multi-phase surface modification performances in 2MgNi-xM(M=TiO₂,V₂O₅; x=10%, 15%) alloys, it was indicated that metal hydrides of Mg₂NiH_x (x=1,4) existed after absorbing hydrogen. At 673K, the alloy which concluding 10%V₂O₅ has the highest hydroding capacity, with the number of 3.33wt%. Besides, TiO₂ has a better catalytic effect for the absorption and desorption of hydrogen.After adding the non-metal elements of 2MgNi-xM(M=C,B;x=10%, 15%) alloys, it was concluded that C and B could improve the hydrinding capacity, reduce the dehydrinding temperature to some extent, which could modify the hydrinding/ dehydriding properties of the alloys.