Activation characteristics and microstructure of composite hydrogen storage alloys prepared by two-step re-melting

In the present work, novel composites Mm 0.3 Ml 0.7 Ni 3.55 Co 0.75 Mn 0.4 Al 0.3 / x wt % Mg 2 Ni ( 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 Mg 2 Ni content on the activation characteristics was analyzed by electrochemical method. With the increasing content of Mg 2 Ni , activation characteristics and maximum discharge capacities of composites increase first and then decrease. The composite with 5% Mg 2 Ni has the least cycle number for activation and the highest discharge capacity. It is activated after only 6 cycles ( C n = 6 ) at room temperature and its maximum discharge capacity ( C max ) reaches 274.4 mAh/g. However, the composite contained 30 wt% Mg 2 Ni is difficult to be activated at room temperature. It is also found that it is easier to be activated for the composites at I c = 60 mA / g and I d = 60 mA / g than that at I c = 100 mA / g and I d = 60 mA / g , but their discharge capacity decay slightly at the condition of I c = 60 mA / g and I d = 60 mA / g . The XRD and SEM analysis show that, with the increasing Mg 2 Ni 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 Mg 2 Ni grows up.