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

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.

Dehydrogenation Properties and Catalytic Mechanism of the K2NiF6-Doped NaAlH4 System.

The K2NiF6 catalytic effect on the NaAlH4 dehydrogenation properties was studied in this work. The desorption temperature was studied using temperature-programmed desorption and exhibited a lower onset hydrogen release after doped with different wt % of K2NiF6 (5, 10, 15 and 20 wt %). It was found that the NaAlH4 doped with 5 wt % K2NiF6 showed the optimal value that can reduce the onset desorption temperature of about 160 °C compared to 190 °C for the milled NaAlH4. The NaAlH4 + 5 wt % K2NiF6 sample showed faster desorption kinetics where 1.5 wt % of hydrogen was released in 30 min at 150 °C. In contrast, the milled NaAlH4 only released about 0.2 wt % within the same time and temperature. From the Kissinger analysis, the apparent activation energy was 114.7 kJ/mol for the milled NaAlH4 and 89.9 kJ/mol for the NaAlH4-doped 5 wt % K2NiF6, indicating that the addition of K2NiF6 reduced the activation energy for hydrogen desorption of NaAlH4. It is deduced that the new phases of AlNi, NaF, and KH that were formed in situ during the dehydrogenation process are the key factors for the improvement of dehydrogenation properties of NaAlH4.