Development of novel technologies for the conversion and storage of energy has been actively investigated in recent years. The use of a combined approach consisting of direct electrochemical and photocatalytic oxidation reactions could allow the efficient utilization of energy resources. Direct electrochemical oxidation in a fuel cell could offer significant advantages over conventional combustion technologies, in light of their increased energy efficiency, reduction in emission of toxic pollutants, and overall process simplicity. The majority of fuel cell research has focused on the use of hydrogen, an environmentally friendly fuel characterized by high energy density and production of H2O byproduct. Despite these advantages, commercialization of hydrogen powered fuel cells is currently limited by difficulties in hydrogen production and storage.
The high operation temperature of the solid oxide fuel cell (700-1000 °C) facilitates the direct use of hydrocarbon and carbon fuels, avoiding the complex and expensive reforming processes for the generation of concentrated H2 fuel. Exposure of gaseous hydrocarbons to the fuel cell at these high temperatures provides a thermodynamically favorable pathway for formation of carbon deposits (i.e., coking) which can lead to rapid and irreversible anode electrode degradation. This dissertation presents a study of the use of a novel Cu/Ni-YSZ anode electrode that reduces the
formation of coke deposits and allows the energy efficient operation of the solid oxide fuel cell in hydrocarbon and carbon fuels. The microstructure of the Cu/Ni-YSZ anode electrode is extensively characterized by scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The performance of the Cu/Ni-YSZ anode and the energy efficiency during the operation on carbon (i.e., coke, a devolatized form of coal) is experimentally measured with the aid of in situ electrochemical analysis and, mass spectrometry (MS) and gas chromatograph (GC), demonstrating higher energy efficiencies compared with combustion technologies.
Photocatalytic reactions over semiconductor catalysts such as TiO2 have received significant attention due to their potential applications for conversion and storage of solar energy to chemicals and the degradation of harmful pollutants present in air streams and wastewaters. Excitation of photocatalysts by exposure to light of appropriate energy causes promotion of electrons from the valence band to the conduction band, resulting in the generation of electron/ hole pairs that can initiate redox reactions with species adsorbed on the surface of the photocatalyst. Hydrogen can be produced by the photocatalytic reduction of water (i.e. splitting reaction). Addition of alcohol molecules have been shown to improve the photocatalytic evolution of H2 from H2O due to hole scavenging oxidation reactions that limit electron/hole recombination. Detail knowledge of the mechanisms governing the photocatalytic oxidation of alcohols could facilitate the development of highly efficient photocatalysts for water splitting and degradation of volatile organic compounds (VOC).
The photocatalytic evolution of H2 from aqueous solutions containing methanol (CH3OH) hole scavenging reagents was studied by tracing the reaction of D2O over a Cu/S-TiO2 catalyst under UV illumination. Use of D2O/CH3OH produced higher formation rates of HD and D2 than that of H2. The low H2 formation rates indicate that the direct reaction of CH3OH with photogenerated holes does not proceed to an appreciable extent in the presence of high concentrations of D2O. The role of CH3OH in accelerating hydrogen formation can be attributed to its ability to produce an electron donor, injecting its electrons to the conduction band.
The photocatalytic oxidation of alcohols was further studied at 30 °C and 1 atm by in situ infrared methods, using ethanol as a model compound. Results from these studies have shown ethanol adsorbs on TiO2 in the presence of high contents of water as molecularly adsorbed ethanol (CH3CH2OHad), which exhibit a lower initial C-H scission and CO2 formation rate than ethoxy CH3CH2Oad produced from ethanol adsorbed low water content TiO2 catalysts. CH3CH2OHad photooxidation produced formic acid (HCOOHad) and formate (HCOO-ad) species, whereas CH3CH2Oad reactions proceed via formation of acetaldehyde (CH3CHOad) and acetate (CH3COO-ad). CH3CHOad was found to react on TiO2 via hydrogen abstraction of the -Carbon producing CH3COO-ad which can be further oxidized to HCOO-ad and CO2. The rate of ethanol photooxidation was found to decrease due to the accumulation of CH3COO-ad species on the TiO2 surface. In the presence of excess H2O, weakly adsorbed species (i.e., acetic acid CH3COOHad) can be redistributed in the surface and dissociated producing acetate.