Sources of clean and sustainable energy are important vectors for economic growth and development. The current global energy supply depends heavily on fossil fuels, which in the future may have added costs of carbon dioxide emissions. This makes technology such as direct water splitting from harvesting solar energy in photo-electrochemical (PEC) systems potentially attractive. The principle of this technology utilises semiconductors to absorb photons of energy greater than their band gap energy, generating an electron-hole (absence of electron) pair. The hole could oxidise water to produce oxygen at the anode, while the electrons could reduce water to form hydrogen at the cathode. This project aims to design, model, characterise the performance and optimise a photo-electrochemical reactor that could efficiently harvest and store solar energy by splitting water to produce hydrogen and oxygen.
α-Fe2O3 ,which is a cheap and abundant material, has shown promise as a photo-anode material, so was chosen as the photo-anode in the development of the PEC reactor. α-Fe2O3 thin films were produced by spray pyrolysis of alcoholic FeIII solutions onto fluorine-doped tin oxide film on glass. Effects of deposition precursor, post deposition heat treatment and SnIV-doping were studied. Results showed that both SnIV-doping and heat treatment were required to produce the best results (photocurrent of ca. 1-2 Am-2 at applied potential of 0.5 V vs. HgO |Hg). A charge carrier transport model was developed to understand and predict the behaviour of the Fe2O3. The model suggested that the magnitude of photocurrent was dependent on the photo-electrochemical reaction rate at the electrolyte | electrode interface, and would be limited by the intensity of illuminated photon flux. Operating the reactor at higher temperatures favoured the electrolysis process in the absence of light, but experimental results showed it was unfavourable for the net photo-generation of charge.
A reactor system with a 0.1 m x 0.1 m photo-anode, Ti/Pt cathode and cation-permeable membrane was built to investigate the effects of operating parameters and operational issues of process scale up. COMSOL Multiphysics™ software was used to model the reactor and to study the reactor performance effects of fluid flow, light intensity and electrical potential drop in the thin conducting layer on glass. Results showed that at electrode area of 10-2 m2 scale, a significant electrical potential drop occurred across the photo-anode, due to its sheet resistance, resulting in non-uniform distribution of current density / rate of H2 (and O2) production (solar to hydrogen conversion efficiency of 0.16%), much of the photo-anode area being inactive. A reactor model was developed to provide a better understanding of larger-scale PEC reactor performance and was used to re-design and optimise the next reactor prototype
