Photoelectrochemical cells (PEC) combine a photovoltaic device for light harvesting and an electrolyzer for water splitting into a single system. These cells consist of a photoanode for water oxidation and oxygen evolution, a cathode where hydrogen is evolved, and an electrolyte. A major obstacle for the use of PEC cells is low photoanode efficiency. By some accounts, the rate of water oxidation has to be increased by more than an order of magnitude to keep pace with the production of electrons and holes. In addition, recombination losses can limit performance. For example, when particulate catalysts are used in the photoanode only about 5% of the generated carriers are available for the reaction. We explored two strategies to improve the photoanode performance: producing semiconducting oxides in the form of nanotubes and incorporating nanoscale gold onto their surfaces.
Highly ordered TiO2 nanotube (TiNT) arrays were fabricated using an anodization process. Gold nanoparticles were supported onto the TiNTs using a modified deposition precipitation method. The pH, aging time and Au precursor concentration were manipulated to increase the Au dispersion and loading on the nanotubes. Cyclic voltammetry was used to measure the total nanotube and Au electrochemical surface areas, and to study the location of traps and active states in the photocatalyst. The performance of photoanodes was evaluated in a 3-electrode cell with a 1.0 M KOH electrolyte using a solar simulator (1.5 AM) and potentiostat/galvanostat. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction were used to characterize key properties of the materials, and optical absorption measurements were used to determine their bandgaps.
Titania nanotubes with a range of morphologies have been produced and the resulting electrodes showed improved efficiencies perhaps as a consequence of decreased recombination losses by enhancing capture of the light and better separating and transporting charge carriers. The incorporation of Au nanoparticles resulted in a slight reduction of the bandgap, which has been attributed to the existence of impurity levels between the band edges of the oxide. Introduction of the gold nanoparticles resulted in a significant improvement in the electrocatalytic properties. The photocurrent normalized by the Au active area increased dramatically as the size of Au particles decreased below 5 nm. This increase may have been a consequence of more effective hole transfer from TiO2 to Au, thereby enhancing the H2O oxidation rates. Experiments are under way to determine the structural and compositional character of active sites involved in H2O oxidation.
Longer nanotubes provided higher photocurrents compared to TiO2 powders and short nanotubes. This was attributed to better separation of the charge carriers, as well as enhanced capture of the light. The incorporation of Au nanoparticles improved rates for water oxidation to produce hydrogen. Catalysts with smaller Au nanoparticles were significantly more active than those containing larger Au particles.&'
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