Improved hydrogen sorption performance of NbF5-catalysed NaAlH4
The effect of NbF5 on the hydrogen sorption performance of NaAlH4 has been investigated. It was found that the dehydrogenation/hydrogenation properties of NaAlH4 were significantly enhanced by mechanically milling with 3 mol% NbF5. Differential scanning calorimetry results indicate that the ball-milled NaAlH4–0.03NbF5 sample lowered the completion temperature for the first two steps dehydrogenation by 71 °C compared to the pristine NaAlH4 sample. Isothermal hydrogen sorption measurements also revealed a significant enhancement in terms of the sorption rate and capacity, in particular, at reduced operation temperatures. The apparent activation energy for the first-step and the second-step dehydrogenation of the NaAlH4–0.03NbF5 sample is estimated to be 88.2 kJ/mol and 102.9 kJ/mol, respectively, by using Kissinger’s approach, which is much lower than for pristine NaAlH4, indicating the reduced kinetic barrier. The rehydrogenation kinetics of NaAlH4 was also improved with 3 mol% NbF5 doping, absorbing ∼1.7 wt% hydrogen at 150 °C for 2 h under ∼5.5 MPa hydrogen pressure. In contrast, no hydrogen was absorbed by the pristine NaAlH4 sample under the same conditions. The formation of Na3AlH6 was detected by X-ray diffraction on the rehydrogenated NaAlH4–0.03NbF5 sample. Furthermore, the structural changes in the NbF5-doped NaAlH4 sample after ball milling and the hydrogen sorption were carefully examined, and the active species and mechanism of catalysis in NbF5-doped NaAlH4 are discussed.