Towards efficient hydrogen production using water splitting

Although there is no shortage of supply of fossil fuels at the moment, the necessity to reduce green house gas emission and growing difficulties in fossil fuel recovery raise great challenges for the scientific community to develop efficient, low cost alternative energy sources. Hydrogen is sought by many as a way to store and transport energy produced from renewable sources and as a fuel hydrogen produces only water on burning and is not toxic in any way. The main pathways to produce hydrogen can be classified as thermal, electrolytic, and photolytic processes. Most of the hydrogen is currently produced via thermal processes, which use the energy from fossil fuels stored in natural gas, coal or biomass to release hydrogen. Although photolytic processes are very attractive due to the zero greenhouse gas emissions, they can be used for commercial hydrogen production only if limitations related to low efficiency and poor stability can be resolved. State-of-the-art hydrogen producing photoelectrochemical cells have 12.4% efficiency under visible light irradiation and combine several semiconducting materials in a monolithic device. Although efficient, this cell is able to split water only for a few days, making the possibility of commercial application daunting. Thus, the general aim of the project is to develop a novel structure for a stable photo-electrochemical device for water splitting applications. Having high efficiencies for photo electrochemical energy conversion, metal sulfides are promising candidates for use in commercial water splitting systems if their long-term stability can be improved. Cadmium sulfide was chosen for our investigations as a representative of the metal sulfide family, due to its well known properties. In the photo-electrochemical cell developed in this work the light harvester is separated from the electrooxidation and reduction processes that occur in the water splitting cell. The quantum confinement effect observed for semiconducting nanoparticles significantly alters electrical properties of materials that allow for engineering of the desired electrical properties. A range of nanoparticles and nanostructures were prepared in this work in order to investigate the influence of dopant and quantum size effects innanoparticles on the energy structure of the material and their potential to be utilized in the water splitting and electroluminescent applications. In order to address the high costs of production of thin film semiconductors, in this work we have developed a novel method for low cost, efficient deposition of high quality metal sulfide semiconductors and their alloys utilizing electrodeposition from ionic liquids at high temperatures. The structure of the proposed photoelectrochemical cell was created using electrochemical deposition as well as photo-driven electrochemical deposition, which allows in situ deposition of catalyst for water oxidation. It was shown that a multilayered structure of the device based on metal sulfides provides high corrosion resistance of the cell during photo-electrochemical water splitting leading to significant extension of the cell lifetime.