Changes in microstructure, phases, and hydrogen storage characteristics of metal hydro-borate and nickel-added magnesium hydride with hydrogen absorption and release reactions

摘要

In this work, Zn(BH4)(2) and/or Ni were added to MgH2 in order to improve the hydrogen absorption and release properties of MgH2. 99 wt% MgH2 + 1 wt% Zn(BH4)(2), 99 wt% MgH2 + 0.5 wt% Zn(BH4)(2) + 0.5 wt% Ni, and 95 wt% MgH2 + 2.5 wt% Zn(BH4)(2) + 2.5 wt% Ni samples [named MgH2-1Zn(BH4)(2), MgH2-0.5Zn(BH4)(2)-0.5Ni, and MgH2-2.5Zn(BH4)(2)-2.5Ni, respectively] were prepared by milling in a planetary ball mill in a hydrogen atmosphere. MgH2-0.5Zn(BH4)(2)-0.5Ni had the highest initial hydriding and dehydriding rates and the largest quantities of hydrogen absorbed and released for 20 min. MgH2-0.5Zn(BH4)(2)-0.5Ni dehydrided at the fourth cycle had small particles, large particles, and agglomerates. The sizes of the fine particles on the agglomerates were slightly smaller than those in the as-milled sample and quite flat surfaces of the agglomerates were not observed. MgH2-0.5Ni-0.5Zn(BH4)(2) dehydrided at 623 K under 1.0 bar H-2 at the 4th cycle contained Mg, MgO, and small amounts of beta-MgH2 and Mg2Ni. The initial hydriding rates at n = 2, 3, and 4 were higher than that at n = 1. The quantity of hydrogen absorbed for 60 min, H-a (60 min), decreased as the number of cycles, n, increased. The initial dehydriding rate increased and the quantity of hydrogen released for 60 min, H-r (60 min), decreased as n increased. Outside the particles and agglomerates, particles became finer due to expansion and contraction, while in their interiors cracks were believed to coalesce due to annealing effect. MgH(2)0.5Ni-0.5Zn(BH4)(2) had an effective hydrogen storage capacity (the quantity of hydrogen absorbed for 60 min) of about 5.5 wt% (5.52 +/- 0.10 wt% at 593 K under 12 bar H-2). The PCT curve of MgH2-0.5Ni-0.5Zn(BH4)(2) showed that the hydrogen storage capacity was 6.64 +/- 0.25 wt%. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.