Development of nanostructured metal oxides for solar fuels

Student thesis: Doctoral ThesisPhD

Abstract

Global warming is a major concern in the current world owing to its contribution to natural disasters. CO2 is one of the main greenhouse gases causing global warming and its anthropogenic release in the environment mainly comes from the use of fossil fuels as a source of energy. Due to the increase in both the world’s population and living standards, the amount of energy consumed throughout the last 50 years has almost doubled, resulting in an associated increase of CO2 emissions in the environment. This increase in CO2 emissions is against the recommendation recently reported in the 2018 intergovernmental panel on climate change report, in which it was suggested that CO2 emissions should fall by about 45 % of 2010 levels by 2030 and achieve ‘net zero’ emissions by 2050. A feasible approach to fulfil this requirement is to change the current energy portfolio to a more sustainable one. Although several renewable sources of energy such as wind and solar are currently becoming a part of the energy mix, they suffer from intermittent cycles in which the energy production and demand are not decoupled. In this regard, photoelectrochemical (PEC) water splitting, where solar energy is used on photoelectrodes to split water to form H2 and O2, could be a viable complement to current renewable sources of energy and most importantly, an alternative to fossil fuels based sources of energy. H2 is considered to be the energy vector that could contribute to a fully sustainable society.
In this PhD thesis, Chapter 1 describes the evolution of the energy consumed and sources of energy used over the last 50 years and sets the scene of the necessity to use solar fuels, in particular H2, as an alternative to fossil fuels. A list of current methods of solar H2 production is reviewed highlighting the benefits of the PEC technology. In Chapter 2, the fundamentals of PEC technology including requirements, processes and mechanisms involved in the PEC technology are reviewed along with the state-of-the-art of the main materials currently employed with their advantages and limitations. Chapter 3 illustrates an overview of the aerosol-assisted chemical vapor deposition (AACVD) method, a highly versatile method for the synthesis of nanostructured thin films, that is widely used in the experimental part of this thesis. Finally, Chapters 4, 5 and 6 cover the experimental work performed in this thesis. In particular, in Chapter 4 a facile approach for the synthesis of Mo-doped TiO2 photoanodes prepared by spray pyrolysis from a polyoxotitanium oxo/alkoxy cluster is presented. This chapter investigates the role of molybdenum in the TiO2 lattice structure and its contribution to the enhanced PEC performance. Chapter 5 illustrates the advantages of using AACVD and a different titanium oxo/alkoxy cluster for the synthesis of highly nanostructured TiO2 photoanodes with preferential facets exposed. Characterization techniques such as TEM, XRD, linear sweep voltammetry and time-resolved microwave conductivity measurements reveal the preferential exposure of the anatase {0 1 0} facet, the excellent photocurrent
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performance and the main recombination pathways of the photogenerated charges. Furthermore, it is also shown and discussed how the metastable anatase TiO2 phase is maintained up to 1000 °C after annealing in air offering an alternative application in the smart tile ceramic industry. In Chapter 6, Zn-doped Fe2TiO5 photoanodes prepared by AACVD are studied for the first time. Through the use of characterization techniques such as linear sweep voltammetry, impedance spectroscopy and ultraviolet photoelectron spectroscopy measurements it is revealed that the improved PEC performance over pristine Fe2TiO5 photoanodes originates primarily from improved charge separation and injection efficiencies along with an increase in carrier concentration and better charge transfer kinetics.
Finally, the last chapter of this thesis summarizes the main conclusions of this work and gives a general overview of future research pathways in the field of PEC water splitting.
Date of Award19 Feb 2020
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorSalvador Eslava Fernandez (Supervisor) & Andrew Johnson (Supervisor)

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