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Dye-sensitized solar cells (DSCs) offer easy fabrication, flexibility of substrate material, and a comparable energy payback with respect to the conventional silicon solar cells, and thus are a promising low cost alternative. In DSCs, the iodide (I-)/tri-iodide (I3-) redox couple is used to shuttle charge between the photoanode and the counter electrode (CE). In order to reduce I3- to I- with a minimum energy loss on the CE, efficient electro-catalysts are required. During my Ph.D. study, I have worked on preparing and investigating the nanostructured materials for the development of CE in DSCs. This thesis includes several different topics.1: Mesoporous Nb-doped TiO2 for the control of Pt nanoparticlesTo control the Pt particle size and prevent its agglomeration, mesoporous Nb-doped TiO2 film was prepared by the sol-gel method on a transparent conducting fluorine doped tin oxide (FTO) glass. Pt nanoparticles were impregnated in the mesoporous TiO2 support substrate and tested for the CE in DSCs. The mesoporous Pt/Nb-doped TiO2 was characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The electrocatalytic activity of the Pt nanoparticles on different supports was studied by electrochemical impedance spectroscopy (EIS) for the I3-/I- redox reaction in acetonitrile, a common electrolyte solution for DSSC. By fabricating Pt/Nb-doped TiO2 electrode, the charge transfer resistance was reduced and the exchange current density was increased as the result of a larger active surface area of Pt in the mesoporous Nb-doped TiO2. This electrode could be used in other systems where there is a need to control the amount of Pt surface area and hamper Pt particle aggregation when operating at high temperatures. The impregnation of Pt in the mesoporous TiO2 can also improve the mechanical rigidity and stability of the electrocatalyst against abrasion or generally mechanical contact, a desired property for practical applications. 2: Electrocatalytic properties of graphene for triiodide reductionThere has been considerable interest in the use of graphene for a CE in DSCs. However; it is difficult to increase the electrocatalytic activity of graphene towards I3-/I- in the effective direction without understanding the fundamental electrocatalytic properties of graphene for the application to DSCs. This work reported the fundamental electrocatalytic properties of graphene films towards I-/I3- by cyclic voltammetry (CV) and EIS. The films were prepared from chemical or thermal reduction of graphene oxide. We demonstrated graphene films obtained under different reduction conditions exhibit different electrocatalytic properties. The mechanism was elucidated by the structural differences as confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. We also showed the electrocatalytic activity of graphene films could be tuned by surface modification with polyelectrolytes. However, graphene-based materials are still in the early stages of development and they require improvement to obtain an acceptable catalytic effect and mechanical stability. In this study, the composites of graphene and transition metal oxide nanoparticles were synthesized by employing graphene oxide as a metal-anchoring site, facilitating the nanoparticle nucleation and growth on graphene sheets. This system was tested for potential use as a highly efficient electrocatalyst for counter electrode in n- or p-DSCs. Compared to the bare graphene electrocatalyst, the transition metal oxide/graphene composites showed a high electrocatalytic activity for I3-/I– redox reaction, demonstrated by higher peak currents, lower values of peak to peak separation, and greater exchange current densities (J0), limiting diffusion current densities (Jlim) as well as slopes of the Tafel polarization curves. This highlighted the importance of the high specific surface area, good accessibility of reactant to the electrocatalyst, and appropriate conductivity of the graphene support, underlining a synergetic effect between the graphene support and transition metal oxide nanoparticles. The present transition metal oxide/graphene composites provide the new electrocatalysts with excellent electrocatalytic activity, which is of great significance in improving the electrocatalytic property of the transition metal oxides.3: Electrochemical milling for the synthesis of electrocatalystsIn the field of catalysis, the amorphous materials are sometimes catalytically more active and selective than the crystalline catalysts. Here we reported the amorphous CoS CEs deposited on flexible Ti substrate obtained by the sulfurization of amorphous CoO prepared by electrochemical milling (ECM) process. Using Co3O4 film as the starting electrode material in Co3O4/Li cells, Co3O4 can be electrochemically reduced and oxidized to form amorphous CoO in a controllable way. The influence of current density on the final product was also studied. It is found that the current density has pronounced effects on the morphology of the obtained CoO products. Amorphous CoO obtained by ECM process offers high catalytic activity toward sulfurization reaction to form amorphous CoS compared to crystalline CoO. The obtained amorphous CoS resulted in high electrocatalytic performance toward the reduction of I3- for DSCs. EIS and CV measurements revealed that the amorphous CoS counter electrodes deposited on flexible Ti substrate obtained by amorphous CoO with high applied current density exhibited better catalytic activity, fostering rate of I3- reduction, than that of the platinized and CoS electrodes obtained by conventional thermal decomposition and chemical bath deposition (CBD) supported on bare FTO substrates, respectively. The large surface area of the amorphous CoS, together with other microscopic features (e.g., high density of surface defects), potentially offers more active sites for I3- adsorption, thus significantly enhancing I3- reduction activity. The preparation of amorphous CoS electrode via ECM process with improved electrocatalytic properties are feasible to apply in flexible substrates, which is at most urgency for developing novel counter electrodes for lightweight flexible solar cells.Besides the above four projects, this work also includes the new approaches to the synthesis of carbide and hydride compounds. 1: Use of graphene oxide for low-temperature synthesis of carbidesSince graphene oxide (GO) contains only one atomic layer of carbon, thus it could be envisioned to facilitate the diffusion of carbon atom. Cobalt(II) carbide (Co2C) was synthesized by the ammonia evaporation induced method between cobalt(II) salt and GO followed by thermal reduction under H2-N2 mixture. The Co2C showed a BET surface area of 68 m2/g and its pore size was estimated to be ca. 4.0 nm. The low temperature (200 °C) of synthesis and amphiphilic behavior of GO was responsible for the high porosity and specific surface area and narrow pore size distribution of the Co2C. TEM analysis showed that the Co2C had a particle size of around 5 nm and revealed the mesoporous disordered structure with diameter of ca. 3 - 5 nm. The results suggest that the reaction occurs in two steps: reduction by H2 leading to graphene formation and carburization/reduction of the Co3O4 to Co2C. Compared to the traditional high-temperature methods of synthesis of metal carbides, the use of GO has provided the lowest reported temperature (200 °C) for a metal carbide synthesis.2: Ultrasonic synthesis of metal hydridesBy ultrasonic irradiation, the collapse of the cavitation bubbles in aqueous solutions accounts for sonochemical reductions; more specifically, sonochemically generated H• radicals within the bubbles are considered to act as reductants. This work reported the sonochemical synthesis of copper(I) hydride (CuH) by the ultrasonic irradiation of a copper(II) aqueous solution. A reaction mechanism based on the reduction of copper(II) by the ultrasound-generated hydrogen atoms is discussed. To our best knowledge, this is the first time that a metal hydride has been synthesized through sonochemistry. This study might potentially open up new opportunities for sonochemistry in materials synthesis, particularly considering the significant applications of metal hydrides in organic syntheses, Ni-metal hydride batteries and hydrogen storage. |
