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Townsend, William (2023) Redox-Active Species in Carbon Nanotubes. PhD thesis, University of Nottingham.

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Abstract

The work in this thesis investigates single-walled carbon nanotubes (SWNTs) as host for redox active materials towards energy applications. SWNTs provide a template for the formation of unique, nano-dimension morphologies of encapsulated materials, with extremely high surface area to volume ratios.

Firstly, SWNTs are tested for their electrocatalytic activity towards the oxygen evolution reaction (OER). Through various pre-treatment methods, it is revealed that the presence of metallic impurities inherent in many SWNT materials have a significant impact on the electrocatalytic activity of the SWNT. Even metallic impurities that are not initially accessible due to graphitic coatings can be revealed in-situ by the oxidation of carbon. A pretreatment methodology is developed to remove these metallic impurities, and these purified SWNTs are then used as hosts for metal oxides. Upon encapsulation, the confined metal oxides lead to an enhancement of the OER activity of the metal oxide/SWNT composite material. To investigate active site location, the ends of the nanotubes are blocked by in- situ filling of fullerene molecules, supressing mass transport of ions to the encapsulated metal oxide. All metal oxide/SWNT composites retain activity, showing that the active site is the carbon surface itself. The carbon is activated towards OER by the encapsulated metal oxides via charge transfer from the nanotube to the interior metal oxide, leading to a lower energy barrier for hydroxide adsorption, the first step of the OER. This interaction between the encapsulated metal oxide and SWNT is monitored using in-situ spectroelectrochemical Raman spectroscopy.

SWNTs are also used as host materials for the encapsulation of sulfur. Filling sulfur into two different sized SWNTs revealed large differences in the morphology of the confined sulfur, which had large impacts on their electrochemical properties. The larger SWNT host (1.5 nm) allowed for diffusion of lithium into the nanotube interior, with suppression of redox processes observed when limiting access to the nanotube interior via fullerene filling in the nanotube ends. The smaller SNWT host (1.0 nm) led to the formation of one-dimensional sulfur chains. These sulfur chains could be monitored with Raman spectroscopy due to a large resonance enhancement of nanoconfined sulfur chains. This allowed for in-situ investigation of these confined sulfur species during electrochemical lithiation, with interactions between the nanotube and sulfur supporting a through-wall charge compensation rather than lithium diffusion into the nanotube interior, as is the case for the larger SWNT host.

Finally, SWNTs are used as a conductive support for the immobilization of redox active viologen species. This led to the unexpected formation of a new redox process for the trifluoromethylphenyl viologen / SWNT composite material. This redox process was reversible, surface confined and pH dependent, the last of which was particularly surprising for a viologen species. AC-TEM revealed the formation of a polymeric viologen species on the surface of the nanotube. With this in-situ transformation that results in the formation of a new stable redox process, the composite material could find use as a chemical sensor or redox mediator.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Khlobystov, Andrei Newton, Graham Johnson, Lee
Keywords:	carbon nanotubes, electrocatalysis, energy storage
Subjects:	Q Science > QD Chemistry > QD450 Physical and theoretical chemistry
Faculties/Schools:	UK Campuses > Faculty of Science > School of Chemistry
Item ID:	73655
Depositing User:	Townsend, William
Date Deposited:	02 Jun 2025 08:15
Last Modified:	02 Jun 2025 08:16
URI:	https://eprints.nottingham.ac.uk/id/eprint/73655

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