Investigations on hydrogen desorption from the mixture of Mg(NH2)2 and CaH2

Abstract

A new ternary Metal-N-H sample of Mg-Ca-N-H has been prepared by mechanical ball milling-Mg(NH2)2 and CaH2 (1:1) in a planetary ball mill machine. The structure and hydrogen desorption properties were investigated. The results show that pure H2 gas can be released during ball milling. After 72 h ball milling, ∼3.8 H atoms (∼3.88 wt%) were found to be detached from the starting material, which is higher than most of the traditional metal hydrides. Moreover, volumetric release and soak tests on the sample collected after 12 h of ball milling show that hydrogen desorption starts at a temperature around 50 °C. DSC measurement on the post-12 h milled sample reveals that the overall heat effect of hydrogen desorption is ∼28.2 kJ/mol H2, implying that Mg-Ca-N-H system could be a potential lower temperature hydrogen storage system. In addition, the high-pressure release testing shows that 3.0 wt% of hydrogen can be desorbed from the post-12 h milled sample at a temperature of 380 °C although 80 bars of hydrogen was applied, indicating that the Mg-Ca-N-H possesses high equilibrium desorption pressure. However, recharging the sample with hydrogen is rather difficult, which may be due to the high kinetic barrier. © 2006 Elsevier B.V. All rights reserved.