No abstract

A significant improvement in hydrogenation/dehydrogenation properties of 2LiH/MgB 2 can be achieved by adding NbF 5 . The results show that the NbF 5 additive is effective for enhancing the de/hydrogenation kinetics of the Li−Mg−B−H system and reducing the desorption temperatures of MgH 2 and LiBH 4 . For the 2LiH− MgB 2 −0.03NbF 5 sample, About 9.0 wt % hydrogen capacity is obtained rapidly under cyclic conditions of rehydrogenation within 20 min at 350 °C and dehydrogenation within 20 min at 400 °C; thus, catalytic improvement persists well in the subsequent reversible dehydrogenation cycles. Moreover, the sample could reversibly reabsorb and release more than 9.0 wt % hydrogen even at 250 and 375 °C, respectively. Microstructure analyses reveal that the NbF 5 additive in improving the de/hydrogenation properties of Li−Mg−B−H system could be ascribed to the synergistic effect of in situ formed nano NbH particles acting as "active gateways" facilitating the diffusion of hydrogen, and the "favorable thermodynamic destabilization" from the reversible transition of LiH 1−x F x caused by functionality of F-anion substitution. This fundamental understanding provides us with insights into the design and optimization of the catalytic method and species for the catalyzed Li−Mg−B−H system.

No abstract

No abstract

The CeAl4-doped NaAlH4 has been synthesized by mechanical milling NaH/Al mixture with 4 mol % CeAl4 as catalyst under hydrogen pressure of 3 MPa. The hydrogen desorption thermodynamics and kinetics of as-synthesized NaAlH4 were systematically investigated. The enthalpies for the first and second dehydrogenation steps of CeAl4-doped NaAlH4 system are estimated to be 40.56 ± 1.62 and 51.48 ± 1.92 kJ/mol H2, respectively. By regulating the desorption temperatures, the two dehydrogenation steps were studied separately under a constant hydrogen backpressure of 0.1 MPa. The apparent activation energy, E a, for the first and second step is estimated to be 87.9 and 103.6 kJ/mol, respectively, by using Arrhenius equation. Isothermal dehydrogenation measurements show that no induction period is observed in the first step or the second step under the measuring conditions. Both of the decomposition steps conform to the Johnson–Mehl–Avrami (JMA) formalism with Avrami exponent n ≈ 1, indicating that the nucleation of decomposition process is of the site saturation type. Detailed modeling study presents that the first-step dehydrogenation kinetics is most likely controlled by the reaction at a moving boundary, whereas the second-step decomposition follows the first-order reaction mechanism. Change in the dehydrogenation temperature does not alter the nature of decomposition mechanism.