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

Abstract

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 H2 at the 4th cycle contained Mg, MgO, and small amounts of β-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, Ha (60 min), decreased as the number of cycles, n, increased. The initial dehydriding rate increased and the quantity of hydrogen released for 60 min, Hr (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. MgH2-0.5Ni-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 H2). The PCT curve of MgH2-0.5Ni-0.5Zn(BH4)2 showed that the hydrogen storage capacity was 6.64 ± 0.25 wt%.