Synthesis and characterization of Tungsten-based and Bismuth-based photoelectrodes for solar energy conversion

James Curtis Hill, Purdue University

Abstract

The search for a sustainable and renewable source of energy is of utmost importance to offset increasing energy demands and the decreasing amount of finite resources. This has led to great interest in the utilization of solar energy, more specifically, converting solar energy to chemical energy, which allows for the storage and transport of energy. A particularly promising area in this field is solar water oxidation, or the conversion of sunlight and water to hydrogen and oxygen, which recombine in a fuel cell to release energy and water. To date, a suitable material for use as a photoelectrode in a photoelectrochemical cell for solar water splitting has not been identified. An ideal photoelectrode should have a band gap ca. 2.0 eV, and be inexpensive, stable, and benign. The electrochemical synthesis of thin film semiconductors is a facile and inexpensive synthesis method that allows for a high degree of tunability. There are many ternary and quaternary materials that have not been evaluated as photoelectrodes for solar water splitting. An understanding of band edge positions and the effect that doping, substitution, and solid solutions has on the position of the band edges is advantageous because it allows for a targeted search for an ideal photoelectrode for solar water splitting. This thesis focuses on two groups of materials for solar energy conversion, W-based and Bi-based photoelectrodes. Chapters 2, 3, and 5 focus on W-based photoanodes. Chapters 4 and 6 focus on Bi-based photoelectrodes. W-based photoanodes studied include WO3, CuWO4, Bi2WO6, and CuMoxW1-xO4. WO 3 is a compound that has great promise for use as a photoanode in solar water oxidation because it is inexpensive, non-toxic, and has a band gap of 2.7 eV, which allows it to absorb visible light. However, it is prone to degradation, evidenced by photocurrent decay due to incomplete water oxidation causing accumulation of peroxide on the surface. The stability and selectivity of the oxidation of water in various electrolytes was tested, and the products were qualitatively and quantitatively analyzed. Choosing a photo-electrolyte with the ideal anion and cation for solar water splitting enhances the efficiency and stability of solar water oxidation on WO3. The photoelectrochemical properties of WO3 could be improved by synthesizing ternary or quaternary W-based photoelectrodes that retain the advantages of WO3 and eliminate its disadvantages. CuWO4, Bi2WO 6, and CuMoxW1-xO4 are n-type photoelectrodes that retain some of the advantages of WO3 and were synthesized to address these shortcomings. All three had increased photo-stability and chemical stability. Bi2WO6 had a conduction band edge that was positioned more favorably for water splitting. CuWO4 and CuMoxW1-xO4 had smaller band gaps that allow them to absorb more sunlight and have higher theoretical efficiencies. Bi-based compounds are also very promising for solar water splitting because they typically have smaller band gaps and conduction band edge positions that are better positioned for water reduction. However, many Bi-based compounds have not been thoroughly studied. Therefore, CuBi2O4, BiVO4, and BiCu2VO6 were synthesized and their photoelectrochemical properties were examined because they have appropriately positioned band edges for solar water splitting that make them promising materials as photoelectrodes in a photoelectrochemical cell.

Degree

Ph.D.

Advisors

Choi, Purdue University.

Subject Area

Alternative Energy|Chemistry|Inorganic chemistry|Physical chemistry

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