Thermodynamics, Kinetics, and Modeling Investigation on the Dehydrogenation of CeAl4-Doped NaAlH4 Hydrogen Storage Material

The CeAl 4 -doped NaAlH 4 has been synthesized by mechanical milling NaH/Al mixture with 4 mol % CeAl 4 as catalyst under hydrogen pressure of 3 MPa. The hydrogen desorption thermodynamics and kinetics of as-synthesized NaAlH 4 were systematically investigated. The enthalpies for the first and second dehydrogenation steps of CeAl 4 -doped NaAlH 4 system are estimated to be 40.56 ± 1.62 and 51.48 ± 1.92 kJ/mol H 2, 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.