Catalytic effects of iron on magnesium hydride-hollow silica spheres composites for hydrogen storage /
Mohammed Faraj Saeid Othman
- xxii, 209 pages : illustration ; 30 cm.+ 1 CD-ROM
Faculty of Chemical and Process Engineering Technology
Thesis (Doctor of Philosophy) -- Universiti Malaysia Pahang Al Sultan Abdullah - 2026
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Hydrogen is a clean and sustainable energy carrier, but developing safe and efficient storage materials remains challenging. Current storage methods include compressed gas, cryogenic liquid, and solid-state systems, with solid-state storage, particularly in metal hydrides being the most promising due to their high capacities. Magnesium hydride (MgH2) possesses a high theoretical hydrogen storage capacity (7.6 wt.%) but suffers from slow kinetics and high thermal stability. This study aims to address these limitations by investigating the potential of incorporating iron (Fe) into hollow silica spheres (HSS) with different loading to evaluate their effect on the hydrogen adsorption performance of HSS. Subsequently, Fe-HSS composites were introduced as functional additives to MgH2 to enhance its hydrogen sorption kinetics and overall hydrogen storage performance. HSS were synthesized via a sol-gel/emulsion method, systematically varying ethanol-to-water (E/W) ratios and CTAB concentrations to optimize surface characteristics. The best-performing sample, prepared at an E/W ratio of 0.4 with 5.3 mM CTAB, exhibited a well-defined hollow morphology and a high BET surface area of 1045 m2/g. Hydrogen adsorption studies of HSS-based hydrogen storage material were conducted using a Sieverts-type fixed-bed reactor equipped with a gas chromatograph (GC) for precise quantification, while the hydrogen absorption and desorption performance of MgH2 composites was evaluated using a Pressure-Composition-Temperature (PCT). This optimized HSS demonstrated a hydrogen adsorption capacity of 1.59 wt.% at 50 °C and 1.0 bar. To further enhance performance, Fe nanoparticles were incorporated into HSS at varying loadings (3-10 wt.%), the 5 wt.% Fe-HSS composite exhibited the highest hydrogen uptake of 2.42 wt.% at 250 °C, along with excellent cycling stability, maintaining over 83% capacity after four cycles. The Fe-HSS composite was then introduced into MgH2 via ball milling. Remarkably, the addition of 10 wt.% of 7Fe-HSS significantly lowered the onset of dehydrogenation temperature to 320 °C, representing a 100 °C reduction compared to pristine MgH2, and facilitated rapid hydrogen absorption (6.1 wt.% within 2 minutes at 320 °C). Moreover, the activation energy and enthalpy for desorption were reduced to 115 kJ/mol and 58.4 kJ/mol, respectively. The MgH2-7wt.%Fe-HSS composite exhibited five cycles of hydrogenation/dehydrogenation, confirming its good cyclability. Additionally, XRD analysis revealed the in-situ formation of iron disilicide (FeSi2), which contributed to enhanced dispersion and cycling stability. Mechanistic analysis revealed that Fe nanoparticles catalyze hydrogen dissociation and recombination, while HSS nanoconfinement enhances diffusion and structural stability, reducing desorption temperature and improving reversibility of MgH2. These results demonstrate that incorporating Fe into HSS enhances the scaffold’s hydrogen adsorption, significantly improves MgH2 storage performance, and underscores the promise of silica-based materials for efficient, sustainable hydrogen storage.
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