Study of the Hydrogen Storage Properties and Catalytic Mechanism of a MgH₂–Na₃AlH₆ System Incorporating FeCl₃

Abstract

In this work, the catalytic effects of FeCl₃ toward the hydrogen storage properties of the MgH₂-Na₃AlH₆ composite were investigated for the first time. The temperature-programed desorption results indicated that the onset temperature of the hydrogen release of a 10 wt % FeCl₃-doped MgH₂-Na₃AlH₆ composite was ∼30 °C lower than that of the undoped MgH₂-Na₃AlH₆ composite. The addition of FeCl₃ into the MgH₂-Na₃AlH₆ composite resulted in improved absorption and desorption kinetics performance. The absorption/desorption kinetics measurements at 320 °C (under 33 and 1 atm hydrogen pressure, respectively) indicated that within 10 min, the doped sample absorbed ∼4.0 wt % and desorbed ∼1.5 wt % hydrogen. By comparison, the undoped sample absorbed only ∼2.1 wt % and desorbed only ∼0.6 wt % hydrogen under the same conditions and time. Comparably, the apparent activation energy value of the doped composite is 128 kJ/mol, which is 12 kJ/mol lower than that of the undoped composite (140 kJ/mol). The formation of the new species of MgCl₂ and Fe in the doped composite was detected from X-ray diffraction analysis after heating and absorption processes. These two components were believed to play a vital role in reducing the decomposition temperature and kinetics enhancement of the MgH₂-Na₃AlH₆ composite.