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

Abstract

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°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°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°C, respectively, when the temperature was increased at a heating rate of 5°C/min up to 400°C under an argon flow. The enhanced hydrogen storage performance was also achieved during subsequent hydrding–dehydriding cycles.