Efficient Photoelectrochemical Water Splitting and Glycerol Oxidation using Zn based Transition Metal Oxide Photoanode

Abstract

With the rapid expansion of energy demands for carbon reduction technology, molecular hydrogen is an attractive energy source, which can be used in various applications including transportation fuel and heating fuel. However, the most of hydrogen production process contributes carbon dioxide (CO2) emission from the reforming of fossil fuels at high temperature. Therefore, it is necessary to develop an eco-friendly hydrogen production using renewable energy source and feedstock. Photoelectrochemical (PEC) water splitting is considered one of the most attractive technologies for producing eco-friendly hydrogen as fuel by utilizing solar energy. Complementing PEC water oxidation, the photo-oxidation of organic and biomass from bio-diesel is a promising field for the hydrogen production and valuable chemicals. The production of bio-diesel has significantly increased, leading to generation of a large amount of glycerol as a byproduct. In this regard, the conversion of glycerol to functionalized feedstock can produce a variety of value-added materials as well as efficiently generate hydrogen gas at a low potential compared with water splitting. The focus of this thesis is an investigation of the synthesis and characterization of Zn based transition metal oxide photoanodes for PEC system. In the first part of this thesis, the gold nanoparticles decorated with BiVO4/ZnO nanowires (NWs) photoanode using sputter, hydrothermal, spin coater, and immersion methods are investigated. The ZnO NWs provide a direct pathway for efficient charge separation. In addition, the kinetics of oxygen evolution and photoactivity are improved due to the localized surface plasmon resonances and hot electron injection with Au nanoparticles oscillation. This optimized electrode shows that 1D heterostructures decorated with plasmonic metal particles can facilitate effective water oxidation for renewable energy. In the second part of this thesis, Fe-doped ZnS/ZnO NWs photoanodes via hydrothermal and ion exchange process for PEC glycerol oxidation. ZnS has been considered as a suitable material for the biomass decomposition because of its inexpensive cost and a proper band position for glycerol oxidation. By optimizing Fe doping concentration, the photoanodes showed the enhancement of light-harvesting at the visible region and high conversion efficiency of glycerol. As a result, the efficiency of photo-reforming of the optimized photoanode is greatly improved compared to the pristine photoanode. Also, we corroborated the selective oxidized products according to the various applied potential in PEC cells. In the third part of this thesis, CeO2 decorated with Ce-doped ZnO NWs photoanode realized by hydrothermal and pulsed laser deposition process for efficient PEC glycerol oxidation and hydrogen production is discussed. The Ce dopant in the ZnO array exhibits simultaneous improvements in charge separation and light-harvesting by providing oxygen vacancies. Moreover, the operation stability of our CeO2 decorated Ce-doped ZnO NWs photoanode was improved and its photocurrent density was retained even after 10 hours. Also, we corroborated the efficient hydrogen evolution using the Ce utilization. We believe that the dense, the Zn based transition metal oxide photoanodes fabricated by solution process will afford us opportunities to enhance the production of hydrogen gas and produce value-added chemicals. Our results also provide the inspiration for eco-friendly energy production using transition metal oxide photoanode.