Hydrogen Uptake and Release Characteristics of Mg-xTaF5-xVCl3 (x=1.25, 2.5, and 5)

Abstract

TaF5 and VCl3 were chosen as additives to enhance the hydrogen uptake and release rates of Mg. The total content of the additives was not more than 10 wt% since too high content reduces the fraction of Mg and thus the hydrogen storage capacity of the alloys. Samples with compositions of Mg-x wt% TaF5-x wt% VCl3 (x=1.25, 2.5, and 5) were prepared by reactive mechanical grinding. The temperatures at which the asmilled Mg-xTaF5-xVCl3 (x=1.25, 2.5, and 5) began to release hydrogen quite rapidly were 538, 613, and 642 K, respectively. Activation of the samples was not needed. In the first cycle (n=1), Mg-2.5TaF5-2.5VCl3 had quite a high effective hydrogen storage capacity (the amount of hydrogen absorbed for 60 min) of 5.86 wt%. Among the three samples, Mg-1.25TaF5-1.25VCl3 had the best hydrogen release properties. In n=4, Mg-1.25TaF5- 1.25VCl3 had the largest quantity of hydrogen released for 60 min at 593 K in 1.0 bar H2, releasing 0.23 wt% H for 5 min, 0.34 wt% H for 10 min, and 3.31 wt% H for 60 min. After hydrogen uptake-release cycling, Mg- 1.25TaF5-1.25VCl3 had the smallest particle size. In n=5, Mg-1.25TaF5-1.25VCl3 released 2.01 wt% H for 5 min, 3.78 wt% H for 10 min, and 4.89 wt% H for 60 min at 623 K in 1.0 bar H2. (Received May 30, 2018; Accepted June 15, 2018)