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

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.

Nanomaterials in the advancement of hydrogen energy storage

“…Though the cost of synthesis and preparation time of colloid-Ti was too high, the method was no longer considered for the doping. Fan et al [33] investigated energy and hydrogen desorption thermodynamics of CeAl 4 -doped NaAlH 4 and found the separation level for second-step desorption to be 0.096 MPa at 120 • C. More recently synthesized Ti 2 and TiN catalysts for the synthesis of sodium-alanate through mechanical milling reported remarkably mild recharging conditions of 90 • C at 4 MPa H 2 pressure. The results were similar for high-pressure hydrogen (50 bar 2 ), suggesting good reversibility of TiN-doped NaAlH 4 [34].…”

Lightweight hydrides nanocomposites for hydrogen storage: Challenges, progress and prospects

“…Apart from Ti-containing compounds, other transitionmetal, MOFs, rare-earth-metal-based compounds have also been explored and investigated as dopants, which were proved to be able to enhance hydrogen storage properties of alanates [193,[235][236][237][238][239][240][241][242][243][244]. Bogdanović et al [235][236][237] revealed that CeCl 3 and ScCl 3 additives could remarkably promote the hydrogen desorption kinetics of NaAlH 4 and LiAlH 4 .…”

“…Hydrogen storage properties of additive-doped lightweight metal alanates Ti x specie. Fan et al[238,239] investigated the enhanced hydriding-dehydriding properties of Ce-Al species-doped NaAlH 4 . Assynthesized CeAl 2 -doped NaAlH 4 released around 70% of the hydrogen capacity in the first hydrogen evolution step at 160°C within 6 min, and could be hydrogenated under 12 MPa hydrogen pressure at 120°C in 20 min, with a hydrogen capacity of 4.9 wt%.…”

Catalytic Tuning of Sorption Kinetics of Lightweight Hydrides: A Review of the Materials and Mechanism

“…Similar to a TiCl 3 doped NaAlH 4 system, a CeCl 3 doped system exhibited the formation of Ce-Al species, which are considered as catalytically active for the decomposition of NaAlH 4 [205]. Thus, CeAl 2 and CeAl 4 alloys were directly mixed to NaAlH 4 and were found to be quite effective in reducing the activation energy of both steps, which were found in the range of 72-90 and 93-104 kJ/mol H 2 for first and second step decomposition [206,207]. Similar to NaAlH 4 system, several catalysts were developed for solving the kinetic problem of LiAlH 4 decomposition, which included Ti based halides [208][209][210][211], other metal halides [212][213][214][215][216][217][218], nano-sized oxides [219][220][221], carbon [222][223][224], etc.…”