Abstract
Hydrogen technologies broadly classify any technology which uses hydrogen in its primary operation. Recent and intense international focus has emphasised the role of hydrogen as a promising energy vector to supplement or replace fossil fuels. Furthermore, the global appetite for hydrogen technology has grown within the past decade owing to reported near-room temperature hydrogen-based superconductivity within several hydrides. However, promising, practically accessing the technology-enabling properties is often hampered by the low density of hydrogen, even at cryogenic conditions and elevated pressures. This PhD study focuses on the development of microporous carbon-hydride heteroatom composites to achieve higher densities of hydrogen, which would allow the investigation of the magnetic properties in the search for superconductivity.Hydrogen sorption on sulfur encapsulated in single-walled carbon nanotubes (S@SWCNTs) produced by vapour infiltration was evaluated for the first time using gravimetric gas sorption and semi-empirical modelling. Excitingly, these composite materials demonstrated up to 35% improved gravimetric hydrogen uptake per unit of surface area and 74% greater volumetric hydrogen density compared to the undoped porous carbons. The enhanced properties are proposed to stem from the increased polarizability of the adsorbent surface, modified by the confined polysulfide species. However, it was determined that preservation of the porous network was necessary to achieve high hydrogen densification.
The magnetic moment of these S@SWCNTs was measured using superconducting quantum interference device magnetometry using a novel sample cell. It was found that the magnetic properties can be altered by exposure to hydrogen at cryogenic temperatures. Despite single-walled carbon nanotubes representing a simple carbon structure, deeply embedded ferromagnetic impurities dominate the magnetic response of the composite, thus complicating the search for superconductivity within these systems. Although superconductivity was not observed, our findings demonstrate the potential for hydrogen as a strategy for tuning the magnetic properties of S@SWCNTs. In the presence of H2 below 5 K, a strong enhancement to paramagnetic contributions was observed in bundled S@SWCNTs containing greater proportions of narrow, metallic SWCNTs and several mechanisms are suggested to account for this phenomenon. To avoid residual metal catalysts and lower hydrogen pressure requirements needed to observe superconductivity, palladium/porous carbon composites (Pd@C) were produced to explore the potential of superconductivity within confined nanoparticle systems. The 77K and 273K hydrogen uptakes up to 101:3 kPa were measured using volumetric gas sorption. Additionally, the magnetic properties of the Pd@C composites were studied using superconducting quantum interference device magnetometry.
Transition to a superconductive state was not observed above 2 K. However, interactions between nanoconfined Pd nanoparticles within a carbon scaffold led to the introduction of ferromagnetism and showed an apparent dependence on carbon microstructure and particle diameter, with hierarchical carbons and larger diameters demonstrating greater magnetisation. Following exposure to a H2 atmosphere during a multi-step absorption process, notable changes in the magnetisation occurred due to the formation of PdHx and further changes to magnetic ordering within the composite, resulting in an up to 95% reduction in the magnetic susceptibility.
In the search for superconductivity within heteroatom/porous carbon composites, the results from this thesis indicate plausible methods to improve H2 densities and the introduction and modification of hitherto unobserved magnetic properties within heteroatom/porous carbon composites. The results from this work may provide useful insight for hydrogen storage and magneto gas sensing and generally provide lessons which may eventually lead to hydrogen-based superconductivity within similar composite materials.
| Date of Award | 1 Oct 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Lui R Skytree (Supervisor), Sebastien Rochat (Supervisor) & Valeska Ting (Supervisor) |