Development of advanced materials for photoelectrochemical systems: Production, characterization and integration

In conventional batteries, the storage takes place in the static solid electrode. The storage capacity is directly proportional to the volume of the electrodes and hence, the energy storage capacity is limited. Redox flow batteries, in which energy is stored in liquid electrolytes, have large-scale energy storage capacity. The storage capacity is directly related with the electrolyte volume, concentration and redox couple, whereas the power density is determined by the electrode area and current density. Properties such as flexible design, high efficiency and long lifetime make the redox flow battery a substantial storage method. Various redox couples have been studied to date, and vanadium redox flow battery has found one of the most prominent among them. Unlike other redox couples, the use of the same type of redox couples (V2+-V3+ and V4+-V5+) in the anolyte and catholyte brings many advantages. Vanadium redox flow batteries are generally used to store solar energy. The conversion and storage of solar energy in different systems creates a low-efficiency integration problem by increasing the cost and complicating the system. Due to this problem, the conversion and storage of solar energy in a single system has become a popular research topic. In this system, by integrating the Vanadium redox flow cells with the photoelectrochemical cell, the sunlight absorbed by the photoelectrode is converted into electrochemical energy. Simultaneously, it can be stored in the cell in the form of chemical energy. In the first chapter of the thesis, moderate band gap and non-toxic indium sulfide (In2S3) and hybrid titanium dioxide-indium sulfide (TiO2-In2S3) semiconductors are integrated as photoelectrode materials into the solar vanadium redox flow batteries. The dense and mesoporous layer of TiO2 semiconductor have been deposited via RF magnetron sputtering and spin coating method, respectively. On the other hand, the In2S3-based photoelectrodes have been fabricated via the ultrasonic spray pyrolysis (USP) method, which is a cost-effective, non-vacuum solution-based method. Basic characterizations of the thin films have been performed to correlate between material properties and photoelectrochemical performance in solar vanadium flow batteries. Maximum performance in photoelectrochemical tests has been obtained with 75 pass In2S3 photoanode. Moreover, resistance to the acidic nature of the vanadium solution increased after depositing the TiO2 layer. In the last chapter of the thesis, X-ray absorption and interatomic bond lengths of ZnO thin films have been analyzed. ZnO semiconductors have been prepared in different morphologies via solution-based chemical bath deposition. Zinc oxide (ZnO), one of the most common metal oxide materials, is a semiconductor with many properties such as high conductivity, and non-toxicity. Electronic and crystal structure properties of metal oxide materials are determined by X-ray absorption spectroscopy (XAS). In this method, it is absorbed by the atom by excitation by X-ray. X-ray absorption varies according to the structure of the atom and the bond length. The absorption spectrum gives information about many properties such as coordination number of atoms in the material, distance between neighboring atoms, chemical bond structure. In this context, the effects of the change in nanostructure on X-ray absorption have been determined by processing X-ray absorption data and the bond lengths between atoms have been compared. KEYWORDS: Solar vanadium redox flow battery, Indium sulfide, Titanium dioxide, Spray pyrolysis, XAS, EXAFS, XANES