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Munshi, Sweta (2024) Investigating the dehydrogenation pathway for lithium borohydride / hydroxide system. PhD thesis, University of Nottingham.

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Abstract

Developing a cost effective and energy dense hydrogen storage system with a moder-ate operating condition is one the key challenges in hydrogen economy. Lithium boro-hydride (LiBH4) has gained much interest as a hydrogen storage material due to its high gravimetric and volumetric hydrogen density of 18.5 wt.% and 121 kg/m3. How-ever, key drawbacks include high stability, slow kinetics, and a high dehydrogenation temperature (400–600 ºC). In order to decrease the decomposition temperature and enhance the kinetics of LiBH4, a technique called thermodynamic tuning can be used. Thermodynamic tuning involves creating new, more favourable reaction pathways by adding a secondary material.

Complex hydride–metal hydroxide multicomponent hydrogen storage systems have high potential for hydrogen storage because their dehydrogenation thermodynamics can be tuned while keeping a high hydrogen storage capacity. Out of all the stoichio-metric ratios (1:1, 1:3 and 1:4) explored using lithium borohydride and lithium hy-droxide, a particularly promising system is LiBH4-3LiOH with a maximum hydrogen storage capacity of 7.47 wt.%. Thermal and diffraction studies along with in-situ neu-tron diffraction under a self-generated deuterium atmosphere were performed on the samples to explore the reaction pathway. The onset decomposition temperature for 1:3 system was reduced to 220 ºC from 400 ºC (for pure LiBH4) and releasing around 5.3 wt.% of H2 by 370 ºC. Lithium borate (Li3BO3) was found to be the main decomposi-tion product and no toxic side products were formed. Changing the milling parameters improved the reaction kinetics by releasing around 6 wt.% of H2 between 220-260 ºC, below melting temperature of LiBH4. Post that, no additional weight loss was detected for this system. The destabilization was achieved through the reaction in between Hδ- in [BH4]- and Hδ+ in [OH]-. In-situ neutron diffraction suggested the reaction occurring in several steps and the formation of two intermediates before the main decomposition reaction took place. The 1:4 system followed a separate reaction pathway influenced by altered reactant stoichiometry. Excess unreacted LiOH left after the main dehydro-genation step, decomposed after 450 ºC forming Li2O and H2O whereas for 1:1 stoi-chimetric system, excess LiBH4 decomposed above 350 ºC.

Finally, among all the catalysts investigated on 1:3 system, the addition of 5wt.% NiCl2 and CoCl2 catalysts led to further improvement in reaction kinetics. This result-ed in a decrease in the onset decomposition temperature to 75 ºC and the release of over 6 wt.% of H2 below 300 ºC for both catalysed systems. The outcomes have exhib-ited improvements in kinetics and operational temperature, which holds potential as a single use hydrogen storage material or portable storage material due to its light weight. However, inclusion of TiF3 and TiF4 had no such effect on the reaction kinet-ics of LiBH4:3LiOH system. However, it did prove to be beneficial in limiting the oc-currence of the side reaction at lower temperature.

This research opens promising avenues for the development of hydrogen storage sys-tems with improved kinetics and operational temperature ranges, potentially revolu-tionizing the hydrogen economy and making hydrogen a more accessible and practical energy carrier. The inclusion of hydroxide group, [OH]-, proved to be more efficient in destabilizing the system when compared to various LiBH4 - metal hydride configura-tions. Additionally, the insights into reaction pathways and the ability to control side reactions contribute significantly to the broader field of hydrogen storage research, of-fering a foundation for further advancements and innovations.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Walker, Gavin S. Grant, David M. Manickam, Kandavel
Keywords:	Hydrogen, Storage, Complex hydride, Metal hydroxide, Metal halides, Multi-component system, Thermodynamic tuning
Subjects:	T Technology > TP Chemical technology
Faculties/Schools:	UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID:	77164
Depositing User:	MUNSHI, SWETA
Date Deposited:	18 Jul 2024 04:40
Last Modified:	18 Jul 2024 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/77164

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