Activation characteristics and microstructure of Mg2Ni/Mm0.3Ml0.7Ni3.55Co0.75Mn0.4Al0.3 composite hydrogen storage alloys prepared by two-step re-melting

In the present work, novel composites Mm0.3Ml0.7Ni3.55Co0.75Mn0.4Al0.3/xwt%Mg2Ni (x=0,5,10,30) for hydrogen storage were prepared by two-step re-melting and their activation characteristic and microstructure were investigated. The influence of Mg2Ni content on the activation characteristics was analyzed by electrochemical method. With the increasing content of Mg2Ni, activation characteristics and maximum discharge capacities of composites increase first and then decrease. The composite with 5% Mg2Ni has the least cycle number for activation and the highest discharge capacity. It is activated after only 6 cycles (Cn=6) at room temperature and its maximum discharge capacity (Cmax) reaches 274.4 mAh/g. However, the composite contained 30 wt% Mg2Ni is difficult to be activated at room temperature. It is also found that it is easier to be activated for the composites at Ic=60mA/g and Id=60mA/g than that at Ic=100mA/g and Id=60mA/g, but their discharge capacity decay slightly at the condition of Ic=60mA/g and Id=60mA/g. The XRD and SEM analysis show that, with the increasing Mg2Ni content, the microstructure of the composites varies gradually from lamellar (x=5), acicular (x=10) to massive (x=30), and the activity of the composite declines as a result of the grain size of phase Mg2Ni grows up.