Improved hydrogen storage properties of MgH2 doped with chlorides of transition metals Hf and Fe

The effects of HfCl4 and FeCl3 addition on the de/rehydrogenation properties of MgH2 were investigated. Both HfCl4 and FeCl3-doped MgH2 samples started to released hydrogen at about 270 °C, a decreased of about 70 °C and about 140 °C compared to as-milled and as-received MgH2, respectively. In terms of the desorption kinetics, the HfCl4-doped MgH2 sample showed significant improvement, with 6.0 wt.% hydrogen released within 10 min at 300 °C, while the FeCl3-doped MgH2 and undoped MgH2 samples released 3.5 and 0.2 wt.% hydrogen, respectively, under the same conditions. In terms of the absorption kinetics, 5.5 wt.% hydrogen was charged at 300 °C under 3.0 MPa hydrogen in 1 minute for the HfCl4-doped MgH2 sample, while 4.8 wt.% was absorbed by the FeCl3-doped MgH2 sample, compared to just 3.0 wt.% hydrogen for the undoped MgH2 sample under the same conditions. From the Arrhenius plot based on isothermal dehydrogenation kinetics at different temperatures, the apparent activation energy of as-milled MgH2 is calculated to be 166 kJ/mol, and this value is reduced by 64 and 36 kJ/mol after doping with HfCl4 and FeCl3, respectively. A cycling study of dehydrogenation at 300 oC shows that the hydrogen capacity of the HfCl4-doped MgH2 sample was maintained at about 6 wt.% after 10 cycles. Based on the X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy examinations, we believe that the significant improvement of MgH2 sorption properties in the doped samples is due to the catalytic effects of in-situ generated metal species and MgCl2 that were formed during the dehydrogenation process.