Comparison of the Hydrogen Release Properties of Zn(BH4)2-Added MgH2 Alloy and Zn(BH4)2 and Ni-Added MgH2 Alloy

Abstract

Samples with compositions of 99 w/o MgH2+1 w/o Zn(BH4)2 (designated MgH2-1Zn(BH4)2) and 96 w/o MgH2+2 w/o Zn(BH4)2+2 w/o Ni (designated MgH2-2Zn(BH4)2-2Ni) were prepared by milling in a planetary ball mill in a hydrogen atmosphere (reaction-involving milling). The hydrogen release properties of MgH2- 1Zn(BH4)2 and MgH2-2Zn(BH4)2-2Ni were compared. A larger quantity of additives and the addition of Ni decreased the temperature at which MgH2 decomposes in the as-milled samples. Activation processes were not required for these two samples. A larger quantity of additives and the addition of Ni by milling in a hydrogen atmosphere increased the quantity of hydrogen absorbed in 60 min (U (60 min)), the initial hydrogen release rate, and the quantity of hydrogen released in 60 min (R (60 min)). MgH2-2Zn(BH4)2-2Ni had an effective hydrogen storage capacity of about 5.5 w/o at the cycle number, CN, of one (CN=1). A ZnH2 phase, which has not been reported to be formed, was found in the MgH2-1Zn(BH4)2 sample and the Zn(BH4)2 and Ni-added MgH2 sample after hydrogen uptake-release cycling. Mg2Ni was formed in the Zn(BH4)2 and Niadded MgH2 sample after hydrogen uptake-release cycling. The rapid hydrogen release rate of the Mg2NiH4 formed in MgH2-2Zn(BH4)2-2Ni is believed to increase the initial hydrogen release rate of MgH2-2Zn(BH4)2-2Ni. The Mg2Ni decomposed from Mg2NiH4 might facilitate the nucleation of a Mg-H solid solution in the MgH2-2Zn(BH4)2-2Ni sample. There is also a slight possibility that the contraction due to the relatively rapid hydrogen release of the Mg2NiH4 provides passages for the hydrogen released from neighboring MgH2. (Received December 20, 2017; Accepted January 9, 2018)