Studies on nanostructured transition and post-transition metal compound electrodes for photoelectrochemical cells

Abstract

Photoelectrochemical cells are electrochemical cells that generally comprise semiconductor photoelectrode, electrocatalytic counter electrode, and electrolyte. There are two types of photoelectrochemical cells according to the function of the electrolyte

(i) photovoltaic cell with regenerative redox electrolyte and (ii) photosynthetic cell with sacrificial electrolye. Mesoscopic sensitized solar cells and photoelectrochemical water splitting cells are the typical examples of each kind, respectively. In this thesis, nanostructured electrodes composed of transition and post-transition metal compound for those cells were investigated. The main issues in the development of photoelectrodes, which are mainly metal oxide films, are improvements of light harvesting and charge collection. In contrast, replacement of high-cost and rare noble metal, typically platinum, with an economic and earth-abundant material has been the main objective of the research works on counter electrodes. Therefore, in this study, nanostructured metal oxides with intrinsically fast charge transport property and state-of-the-art nanomaterials with enlarged surface area were utilized as photoelectrodes. Also, nanostructured low-cost carbide and nitride materials with facile preparation methods were employed as electrocatalytic counter electrodes. Besides the discussions on quatitative improvements, in-depth physicochemical analyses for characterization of materials and photoelectrochemical cells were performed. After brief explanations of history, backgrounds, and previous research works on photoelectrochemical cells in chapter 1, in chapter 2 and 3, nanostructured metal oxide photoelectrodes with high carrier mobility are discussed. Mesoporous ZnO nanowire arrays with large surface area were synthesized by electrochemical anodization in a mild condition, and highly uniform SnO2 nanochannels were prepared by ultrasonic-assisted anodic oxidation. These nanostructured electrodes were employed as photoanode in quasi-solid dye-sensitized solar cells (DSCs), and fair energy conversion efficiencies were obtained, with further improvements by atomic layer deposition of TiO2 shells. In the next two chapters, strategies for efficient light harvesting by increased surface area were discussed. In chpater 4, TiO2 coated wrinkled silica nanoparticles were utilized as scattering centers in photoanode of DSCs. Superior performances to the DSCs with conventional sphere-shaped TiO2 scatterers were achieved due to the enlarged surface area, and futher observations on the relationship between spectral scattering properties and interwrinkle distances were made. In chapter 5, electrochemically anodized titanium and iron foams prepared by freeze-casting were used as photoanodes of DSCs and photoelectrochemical water splitting cells, respectively. By the anodic oxidation processes, one-dimensional TiO2 nanoatube arrays were formed on the titanium foam surface, and vertically aligned two-dimensional iron oxide nanoflakse were generated on the surface of the iron foam. These multidimensional structures had following three advantages

(i) large surface area for light harvesting, (ii) low-dimensionally confined semiconductor structure for enhanced charge transport, and (iii) three-dimensionally extended current collector with very low resistnace. Based on these properties, large photocurrent densities were obtained in both DSCs and iron oxide based photoelectrochemical water splitting cells. In chapter 6, nanostructured tungsten carbide synthesized by electrochemical anodization followed by heat treatment in carbon monoxide atmosphere was used as a counter electrode for DSCs employing cobalt bipyridyl redox electrolyte. The transformation of oxide into carbide was successfully and completely done due to the amorphous nature of anodic oxide materials. Moreover, superior electrocatalytic activity and photovoltaic performances were oberseved, which were attributed to the well known platinum-like electronic structure and nanoporous morphology. In chapter 7 and 8, nanostructured nickel nitride and cobalt nitride were fabricated by reactive sputtering of nickel and cobalt in nitrogen atmosphere, respectively. By physicochemical charaterizations based on electron microscopy and X-ray analyses, the films were addressed to Ni2N and CoN with cauliflower-like morphologies. When these electrodes were used as counter electrodes of quantum dot-sensitized solar cells (QDSCs), both electrodes displayed superior performances to Pt. In addition, the energy conversion efficiency of DSC employing cobalt nitride counter electrode was comparable to that with platinum. Furthermore, in QDSCs, CoN exhibited higher stability than the state-of-the-art Cu2S counter. Photocurrent densities of QDSC with CoN counter electrode exceeded that with Cu2S within 20 min, though the initial performance was higher in QDSC employing Cu2S.