Effect of adding different percentages of HfCl4 on the hydrogen storage properties of MgH2

Abstract

Magnesium hydride (MgH2) is the best candidate material to store hydrogen in the solid-state form owing to its advantages such as good reversibility, high hydrogen storage capacity (7.6 wt%), low raw material cost and abundance in the earth. Nevertheless, slow desorption/absorption kinetics and high thermodynamic stability are two issues that have constrained the commercialization of MgH2 as a solid-state hydrogen storage material. So, to boost the desorption/absorption kinetics and to alter the thermodynamics of MgH2, hafnium tetrachloride (HfCl4) was used as a catalyst in this study. Different percentages of HfCl4 (5, 10, 15 and 20 wt%) were added to MgH2 and their catalytic influences on the hydrogen storage properties of MgH2 were investigated. Results showed that the 15 wt% HfCl4-doped MgH2 sample was the best composite to enhance the hydrogen storage performance of MgH2. The onset decomposition temperature of the 15 wt% HfCl4-doped MgH2 composite was decreased by ~75 °C compared to as-milled MgH2. Meanwhile, the desorption/absorption kinetic measurements showed an improvement compared to the undoped MgH2. From the Kissinger analysis, the apparent dehydrogenation activation energy was 167.0 kJ/mol for undoped MgH2 and 102.0 kJ/mol for 15 wt% HfCl4-doped MgH2. This shows that the HfCl4 addition reduced the activation energy of the hydrogen decomposition of MgH2. The desorption enthalpy change calculated by the van't Hoff equation showed that the addition of HfCl4 to MgH2 did not affect the thermodynamic properties. Scanning electron microscopy showed that the size of the MgH2 particles decreased and there was less agglomeration after the addition of HfCl4. It is believed that the decrease in the particle size and in-situ generated MgCl2 and Hf-containing species had synergistic catalytic effects on enhancing the hydrogen storage properties of the HfCl4-doped MgH2 composite.