Solar Energy Conversion by Photoelectrochemical Processes

Abstract

This thesis reports on experimental work in the field of photo-electrochemistry and focusing on the photoelectrochemical water splitting process. This process is important as a part in the solar energy harvesting process, mimicking the natural photosynthesis process, to produce hydrogen which can be used as a carrier of clean energy in the fuel cells.

In chapters 1 and 2 the literature background for water splitting and semiconductor electrochemistry are summarised. Chapter 3 describes all key techniques for electrochemistry, surface analysis and spectrometry, and conditions employed for experimental methods.

The first main results chapter is chapter 4 with information about the formation and investigation of photoelectrochemically active WO3 films. Thin, nanostructured, WO3 films are grown onto conducting FTO substrates and shown to act as photocatalyst for water splitting under positive potential bias. The time dependence of photocurrents is studied by impedance, light pulse, and light modulation techniques, and the time constants for photodoping and electron transport are dissected. Clear evidence for trapping of diffusing electrons is obtained from intensity modulated photocurrent spectroscopy.

Chapters 5 to 8 focus on iron oxide films and their preparation. In chapter 5 the layer-by-layer assembly method is employed to form films from hematite nanoparticles and phytate binder molecules. Photoelectrochemical responses are observed as a function of film thickness and applied potential. A model based on differing electron and hole mobility in conjunction with recombination is applied to explain the phototransient responses.

In chapter 6 hematite films are formed in a chemical vapour deposition (CVD) growth process from a ferrocene precursor. Films are formed in variable thickness on FTO (fluorinedoped tin oxide) and ITO (tindoped indium oxide) substrates. Photoelectrochemical experiments are conducted and the effect of thickness and illumination direction discussed in the context of a water splitting process in a mesoporous Fe2O3 film. In contrast to the thin mesoporous films investigated in chapter 5, here thicker and denser films are studied. The photocatalytic efficiency is improved by a factor of 4 due to higher density and potential gradient effects.

In chapter 7 the spraypyrolysis formation of photoelectrochemically active films is introduced in order to introduce dopants and to affect morphology and structure of deposits. Here, addition of tetramethoxysilane (TMS) in varying amounts to iron oxide precursor systems for spray pyrolysis is investigated and the dramatic effect on the photoelectrochemical water splitting current in aqueous 1 M NaOH is discussed.

In chapter 8 a solid solution iron oxide similar to the perovskite family of materials SrTi1xFexO3y (STF) is studied. In this chapter a spray pyrolysis approach to prepare STF films on ITO substrates is employed. In preliminary data, the nanostructured films show good photoactivity with IPCEs of up to 11%.

Date of Award	1 May 2011
Original language	English
Awarding Institution	University of Bath
Supervisor	Laurie Peter (Supervisor) & Frank Marken (Supervisor)