Activation characteristics and microstructure of Mg_2Ni/Mm_(0.3)Ml_(0.7)Ni_(3.55)Co_(0.75)Mn_(0.4)Al_(0.3) composite hydrogen storage alloys prepared by two-step re-melting

摘要

In the present work, novel composites Mmo.3Mlo.7Ni_(3.55)Co_(0.75)Mn_(0.4)Al_(0.3)/x wt%Mg_2Ni(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_2Ni content on the activation characteristics was analyzed by electrochemical method. With the increasing content of Mg_2Ni, activation characteristics and maximum discharge capacities of composites increase first and then decrease. The composite with 5% Mg_2Ni 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 30wt% Mg_2Ni 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 /c = 60 mA/g and I_d = 60 mA/g. The XRD and SEM analysis show that, with the increasing Mg_2Ni 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_2Ni grows up.