Synthesis and enhanced hydrogen desorption kinetics of magnesium hydride using hydriding chemical vapor synthesis

This paper describes the hydriding chemical vapor synthesis (HCVS) of the hydrogen storage alloy MgH2 in a hydrogen atmosphere and the product's hydrogenation properties. Mg powder was used as a starting material to produce submicron MgH2 and uniformly heated to a temperature of 600 degrees C for Mg vaporization. The effects of deposited positions in HCVS reactor on the morphology and the composition of the obtained products were examined by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) analyses. It is clearly seen that after the HCVS process, the particle size of synthesized MgH2 was drastically reduced to the sub-micron or micrometer-scale and these showed different shapes (needle-like nanofibers and angulated plate) depending on the deposited position. The hydrogen desorption temperatures of HCVS-MgH2 were measured using a differential scanning calorimeter (DSC). It was found that after the HCVS process, the desorption temperature of HCVS-MgH2 decreased from 430 to 385 degrees C and, simultaneously, the smallest particle size and the highest specific surface area were obtained. These observations indicate that the minimum hydrogen desorption temperature of HCVS-MgH2 powder with needle-like form can be obtained, and that this temperature is dependent on the particle size and the specific surface area of the products. The thermogravimetric analysis (TGA) for HCVS-MgH2 with needle-like and angulated plate morphologies showed that hydrogen started to be released at 340 and 375 degrees C, respectively, when the temperature was increased at a heating rate of 5 degrees C/min up to 400 degrees C under an argon flow. The enhanced hydrogen storage performance was also achieved during subsequent hydrding-dehydriding cycles. (C) 2012 Elsevier B.V. All rights reserved.