Tungsten oxide-based photoanodes: biomass valorization and the effects of oxygen vacancies

Tungsten oxide-based materials have unique and intriguing properties as photoanode materials for solar energy conversion. The first part of the thesis focuses on the use of WO3 photoanodes for the oxidation of 5-(hydroxymethyl)furfural (HMF). Chapter one lays down the general introduction to HMF oxidation and the unique properties of WO3 photoanodes. The direct oxidation (without any catalyst or a redox mediator) of HMF on WO3 photoanode is presented in chapter two. WO3 indeed shows activity toward the oxidation of HMF, but the product yield is very subtle. A kinetic model is developed and applied to the system to gain insight into the selectivity of HMF oxidation on the surface of WO3. Faster kinetics of the competing reactions have been found to be the main culprit for the low yield. In chapter three, the development of an analysis technique to predict the selectivity between the competing reactions in the photoelectrochemical cells as the function of applied potential is discussed. The analysis is based on derivative voltammetry and can predict the selectivity using only the information from the linear sweep voltammogram. For the oxidation of HMF on WO3, the analysis predicts the selectivity between the HMF oxidation and the competing water oxidation reaction, but the yield for the product yield remains unchanged regardless of the applied potential. To address this challenge, electrocatalysts are introduced to the surface of WO3. The effects of electrocatalysts are laid down in chapter four. Finally, in chapter five, the work is expanded to CuWO4. This ternary metal oxide retains many properties of WO3 but with a smaller band gap, allowing this material to harvest a larger proportion of the solar radiation. The investigation is be based on the effects of oxygen vacancy (OV), a common dopant for metal oxides. Many studies have shown that a small number of this dopant help improve the photocurrent while the excess poses detrimental effects. Nevertheless, the effort to understand the effect of OV on the photocurrent has been unequally given to the benefits side. The reason for the adverse effect posts by the excess OV is rarely discussed. In this chapter, the effect of the number of OVs on the parameters that affect the photocurrent in CuWO4 are discussed. To conclude this thesis presents the case for the use of tungsten oxide-based materials for solar energy conversion and gives a future outlook.