E-Thesis 554 views 155 downloads
The Generation of Solar Fuels in Deep Eutectic Solvents / MICHAEL ALLAN
Swansea University Author: MICHAEL ALLAN
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PDF | E-Thesis – open access
Copyright: The author, Michael Allan, 2023. Distributed under the terms of a Creative Commons Attribution 4.0 License (CC-BY).
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DOI (Published version): 10.23889/SUthesis.63185
Abstract
New reaction systems that can efficiently convert solar energy into fuels is of major scientific importance to aid in alleviating the global use of fossil fuels. Hydrogen is viewed as the ideal solar fuel as it can be used in a clean carbon free energy cycle. Research into reaction systems which can...
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Swansea, Wales, UK
2023
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| Online Access: |
https://cronfa.swan.ac.uk/Record/cronfa63185 |
| Institution: | Swansea University |
| Degree level: | Doctoral |
| Degree name: | Ph.D |
| Supervisor: | Kuehnel, Moritz F. |
| URI: | https://cronfa.swan.ac.uk/Record/cronfa63185 |
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| Abstract: |
New reaction systems that can efficiently convert solar energy into fuels is of major scientific importance to aid in alleviating the global use of fossil fuels. Hydrogen is viewed as the ideal solar fuel as it can be used in a clean carbon free energy cycle. Research into reaction systems which can evolve hydrogen with the help of photocatalysts and sunlight mostly focuses on material development, with not a lot of thought given to the reaction solvent, even though the solvent is in constant contact with the photocatalytic components. This thesis focuses on deep eutectic solvents (DESs) as alternative reaction media for photocatalytic hydrogen evolution. Type III DESs based on choline chloride are favourable solvents due to their cost, ease of preparation, stability, and low toxicity. While DESs have found some uses in the literature, they have never been investigated as reaction media for photocatalytic hydrogen evolution. The work presented herein shows that tuning of the solvent media is key to controlling solar hydrogen production at heterogenous and homogenous photocatalysts. By varying water content of the solvents, the results show that DESs can compete with standard aqueous solutions in terms of photocatalytic solar fuel generation, and under some conditions, catalysts show greater activity in DESs compared to conventional aqueous solutions. Through the use of co-catalytic Pt, sacrificial electron donors and a redox mediator, the H2 evolution performance in DESs can be better understood to maximise hydrogen production activity. This thesis also shows that DESs are effective as reaction media for aerobic hydrogen production. The motivation behind aerobic hydrogen evolution lies in the fact that solar hydrogen production systems need to exhibit photocatalytic activity in the presence of oxygen, but to date tests on many photocatalytic systems in aerobic conditions is unknown, and those catalysts which show tolerance to oxygen are typically much lower than their inert counterpart. The work in this thesis shows that DESs allow photocatalysts which are inactive in the presence of oxygen to be photocatalytically active in atmospheric levels of oxygen. In fact, in some cases over 90% of original activity can be retained simply through solvent choice, versus 8% in water. This increase in activity under air results from the fact that DESs possess lower rates of oxygen diffusion and concentration relative to water, and it is electrochemically determined that through lowering of the diffusion coefficient and concentration of oxygen in the solution, the hydrogen evolution activity can be increased through a supposed decrease in competing oxygen reduction. This is also demonstrated through solvent engineering of aqueous solvents, and electrochemical measurements show that increase in the salinity of water can aid in increasing aerobic hydrogen evolution through a lowering of diffusion and concentration of oxygen. Finally, from knowing that solvent tuning can aid in increasing the photocatalytic activity, natural hydrogenase enzymes are tested for their photocatalytic hydrogen production in both inert and aerobic conditions. The work presented shows that choice and tuning of the solvent aids in increasing the activity of the hydrogenase in conjunction with a TiO2 light absorber. The increase in activity of performance in aerobic conditions in DESs relative to water again results from the lowering of the diffusion and concentration of dissolved oxygen. The presence of DESs again aids in increasing hydrogen production and it is thought this is the first demonstration of photocatalytic hydrogenase activity in organic media. In some reaction systems with DES, the performance of the enzyme exceeds water. This again can be explained electrochemically, whereby hydrogenases at a working electrode show good proton reduction currents in DESs. The electroanalytical technique of microwire chronoamperometry probes the proton diffusion and concentration in the DESs, with the difference in performance of the photocatalytic performance between the solvents rationalised. It is proposed that the hydrogen bonding network in the DESs aids in increasing the mobility solutions, with the activity of the hydrogenase affected by the solvent environment.This work shows that DESs are viable reaction media for the photocatalytic hydrogen production reaction and shows a new alternative of enabling highly efficient oxygen tolerance of catalysts, and that reaction conditions can be systematically and rationally tuned to aid in the increase of solar fuel production. |
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| Keywords: |
Photocatalysis, hydrogen evolution, deep eutectic solvents, solar fuels, hydrogenase. |
| College: |
Faculty of Science and Engineering |