Hydrogen release in composite complex hydride systems

Zhong, Yanyong (2025) Hydrogen release in composite complex hydride systems. PhD thesis, University of Glasgow.

Abstract

Pollution and global warming can result from the rapid usage of fossil fuel resources. In the future, fossil fuels will be run out. As a result, renewable and clean energy sources such as hydrogen technologies may be a good choice to be developed. Many renewable energy sources, have drawbacks, such as high storage and transition costs. One method is to store energy chemically as a clean fuel. Hydrogen have one of the greatest possibilities due to its availability, high energy density, and ability to be produced sustainably.

Currently, the most mature hydrogen storage technologies are compressed gas tanks and liquid hydrogen storage. However, both approaches face certain limitations. For compressed gas tanks, hydrogen’s low density requires a large storage volume; applying high pressure can reduce the volume, but this necessitates tank materials with exceptional compressive strength. Liquid hydrogen storage, on the other hand, requires cryogenic conditions at approximately -253 °C, which poses significant technical and economic challenges. In this context, the development of solid-state hydrogen storage emerges as a promising alternative, as it offers greater stability compared with gaseous or liquid hydrogen and enables higher volumetric storage capacity relative to weight. Nevertheless, the practical application of solid state storage still faces critical challenges, including limited reversibility, high desorption temperatures, and insufficient hydrogen release rates. Therefore, this study investigates five representative solid-state hydrogen storage systems to elucidate their properties and characteristics, thereby providing insights for the future development of solid-state hydrogen storage technologies.

This thesis investigated the dehydrogenation kinetics and reaction mechanisms of five hydride systems: NaH-NaOH, NaAlH4-NaOH, NaBH4-NaOH, Ca4Mg3H14-NaH, and NaAlH4/MgH2-Guanidine(CH5N3). The selection of these systems was based on their potential for practical hydrogen storage. Sodium-based hydrides and hydroxides are inexpensive, lightweight, and exhibit relatively high theoretical hydrogen capacities, making them promising candidates for large-scale applications. The NaH-NaOH, NaAlH4-NaOH, and NaBH4-NaOH systems were studied to evaluate the influence of hydroxide incorporation on the hydrogen release behaviour of simple and complex sodium hydrides. The Ca4Mg3H14 NaH system was chosen as a mixed alkaline composite, where synergistic effects may arise from multicomponent interactions. In addition, the NaAlH4/MgH2-Guanidine(CH5N3) system was introduced as an organic-inorganic hybrid, providing a new strategy to improve dehydrogenation pathways through organic molecular. Through thermal treatment, ball milling, catalyst addition, and variations in molar ratios, the dehydrogenation behaviour of these systems was systematically optimized. Thermal analysis, X-ray diffraction, and Raman spectroscopy were employed to monitor phase change and deduce the corresponding reaction mechanism.

In the NaH-NaOH system, ball milling significantly improved dehydrogenation kinetics, enabling a two-step mechanism. The first step involves the formation of a solid solution, NaH1-x(OH)x, between 170 °C and 210 °C, followed by the decomposition of this intermediate to release hydrogen. Under optimal ball milling conditions (400 rpm, 2 hours) and at a NaH:NaOH molar ratio of 1.15:1, the system exhibited the lowest dehydrogenation peak temperature (346 °C) and the highest hydrogen release (3.08 wt.%). Prior to catalyst optimization, the activation energy of the ball-milled NaH–NaOH system was determined to be 75.85 kJ mol-1. Subsequent addition of catalysts significantly reduced the activation energy, with 5 wt.% Ni and SiC showing the most pronounced effects, lowering the activation energy to 41.24 kJ mol-1 and 46.79 kJ mol-1, respectively. SiC primarily reduces the activation energy via physical mechanisms. As a milling aid, SiC decreases particle size, increases the contact area of reactants, and shortens hydrogen diffusion pathways, thereby facilitating the reaction. In terms of the Arrhenius relationship, this process enhances the pre-exponential factor (A) and increases the probability of effective molecular interactions, which is reflected in an apparent reduction of the activation energy (Ea). Ni nanoparticles may disperse uniformly across the NaOH-NaH interface, providing active sites that facilitate hydrogen atom desorption.

For the NaAlH4-NaOH system, the mechanistic insights gained from the NaH-NaOH system were applied to enhance the kinetics of NaAlH4 decomposition. This led to changes in the third step of NaAlH4 decomposition, which is crucial for improving the overall kinetics. In the hand-mixed NaAlH4-NaOH system, the following reactions were observed:

NaAlH4 + 4NaOH ⇋ 1/3Na3AlH6 + 2/3Al + H2 + 4NaOH
1/3Na3AlH6 + 4NaOH ⇋ NaH + 1/3Al + 1/2H2 + 4NaOH
NaH + Al + 4NaOH ⇋ Na5AlO4 + 5/2H

Notably, the first step of the dehydrogenation reaction transitions from an exothermic to an endothermic process with the addition of NaOH. When even more NaOH is added (e.g., in a 1:4 NaAlH4:NaOH ratio), the second step of the dehydrogenation reaction also shifts from exothermic to endothermic. Under ball-milling conditions, NaOH reacts with NaAlH4 to form Na3AlH6-x(OH)x, As the NaOH content increases, the Na3AlH6-x(OH)x further decomposes to NaH.

The reaction between NaBH4 and NaOH produces Na-B-O-H intermediates. When ball-milled for 10 hours, the 1:4 NaBH4:4NaOH mixture can be completely converted Na-B-O-H intermediates. Prior to heating to 300°C, these intermediates undergo a phase change at 240 °C-250 °C, and the nature of this phase change varies depending on the molar ratio of NaOH added. For instance, ball-milled (2h) 1:3 NaBH4:NaOH yields Na3BO3, Na, NaOH, and H₂ when heated to 400°C, while ball-milled (2h) 1:4 NaBH4:NaOH produces Na3BO3, Na2O, NaOH, and H2 under the same conditions.

For the Ca4Mg3H14-NaH system, ball milling a 1:1 Ca4Mg3H14:NaH mixture at 400 rpm for 2 hours enables NaH to alter the reaction pathway, leading to the formation of Ca4Mg2H14 and NaMgH3. Upon heating to 348 °C, the interaction between NaMgH3 and Ca4Mg3H14 results in a significant reduction of the overall dehydrogenation peak temperature by 102 °C (from 450 °C to 348 °C). This interaction not only facilitates the earlier decomposition of Ca4Mg3H14 but also promotes the premature decomposition of NaMgH3 (from 400 °C to 348 °C).

For the NaAlH4/MgH2-Guanidine (CH5N3) system, the thermal decomposition of CH5N3 alone follows the reaction:

3CH5N3→ C3H6N6 + 3NH3 T = 179 ℃

Subsequently, C3H6N6 primarily evaporates at approximately 300 °C, with a minor fraction undergoing decomposition. In the NaAlH4-CH5N3 system, a reaction occurs at 150 °C, resulting in the release of hydrogen gas and the formation of Al and an amorphous Na-C-N compound. In contrast, the reaction between MgH₂ and CH5N3 is significantly more complex, and at present, only the possible reaction pathways can be proposed based on available data.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QD Chemistry
• Colleges/Schools: College of Science and Engineering > School of Chemistry
• Supervisor's Name: Gregory, Professor Duncan H.
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85517
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Oct 2025 15:32
• Last Modified: 14 Oct 2025 15:32
• Thesis DOI: 10.5525/gla.thesis.85517
• URI: https://theses.gla.ac.uk/id/eprint/85517