Abstract
In response to the needs of modern society and emerging ecological concerns, one of the biggest challenges in 21st century is to develop powerful electrochemical energy conversion and storage devices, such as supercapacitor, lithium-ion battery, and dye-sensitized solar cell. The introduction of ordered mesoporous carbon in the 1999 opens a new chapter in material sciences, and the significant progress made during the past decade in the design and application of ordered mesoporous carbon has provided fresh incentives for further innovations. A noteworthy is the fact that the required disruptive improvement in energy and environmental science has motivated the design of porous architecture and functionality of porous carbons through the aid of suitable templates and by introducing procedures for carbon framework functionality. The major goal of this study is to establish the synthesis protocols, the soft- and hard-templating strategies, in the fabrication of hierarchically-ordered porous carbons with graphitic nanostructures and ordered mesoporous titanium based materials-carbon composites, and to evaluate their physicochemical properties by many characterization methods. The soft- and hard-templating strategies are employed to fabricate porous carbon materials with hierarchically ordered porous structure and ordered mesoporous TiO2-carbon composites. Herein, the amphiphilic triblock copolymer Pluronic F127 and micro-sized polystyrene sphere were used as mesopore and macropore template, respectively. The phenol-formaldehyde resins were used as carbon precursor. The formation of ordered mesoporous structure was relied on organic-organic (PF resin-F127) or organic-organic-inorganic (PF resin-F127-TiO2 precursor) evaporation-induced self-assembly process (EISA). Three-dimensional, magnetically-separable, and hierarchically ordered porous carbon (HOPC) with designed porous textures has been successfully fabricated by dual-templating method with EISA. In addition, in order to obtain graphitic structure in hierarchically ordered porous carbon, the Ni species was used as catalyst for graphitization. The roles of Ni catalyst and pyrolysis temperature (600-1200ºC) and atmosphere (N2 or H2/N2) in the microstructures of hierarchically ordered porous carbon were elucidated. The synthesized HOPC-Ni-1000-g material (g=graphitic, 1000pyrolysis temperature of 1000°C) exhibits well-crystallized graphitic domains, excellent magnetic properties (3.8 emu/g), and designed porous textures. The designed porous textures of the hierarchically ordered porous carbons are composed of highly ordered, macroporous (220 nm), interconnected porous structures, including macroporous windows, hexagonally ordered mesopores (4.7 nm), and useful micropores. The HOPC with graphitic nanostructure has designed porous texture, serving as an ion-buffering reservoir, an ion-transport channel, and a charge-storage material, and is expected to be advanced an electrode material for high-rate supercapacitor and dye-sensitized solar cells (DSSCs). In supercapacitor, HOPC-Ni-1000-g has a low specific surface area (296 m2/g) and a low gravimetric specific capacitance (73.4 F/g at 3 mV/s), but improved electrical conductivity, better rate performance (47.1 F/g at 200 mV/s), higher electrolyte accessibility (24.8 μF/cm2 at 3 mV/s), and excellent cycling performance (>5400 cycles). In DSSCs, HOPC-Ni-1000-g counter electrode exhibits higher electrocatalytic activity towards I3- reduction. The photovoltaic conversion efficiency of the cell using HOPC-Ni-1000-g counter electrode reaches 5.2 % at one sun (AM 1.5G, 100 mW/cm2) which is close to that (6.7 %) of cell using conventional Pt counter electrode. The ordered mesoporous titanium based materials (anatase, rutile, Magneli phases, TiN and TiC)-carbon composites (Ti-C) have directly fabricated by supramolecular self-assembly with in-situ crystallization process. The titanium based materials are embedded into the frameworks of carbonaceous matrix. Importantly, the carbon content (25, 35, and 50 wt%), pyrolysis temperature (450-1200ºC), and pyrolysis atmosphere (N2 and Ar) have significant effects on the thermal stability, crystalline phase and crystallinity of Ti-based materials. The crystalline phase changes from anatase, rutile, Magneli phases, and then to TiN (pyrolysis in N2) or defect carbide TiCx (x< 1, pyrolysis in Ar) as the carbon content in nanocomposites is lower than 35 wt%; the crystalline phase of Ti-C composites at 50 wt % carbon content changes directly from anatase to TiN (pyrolysis in N2) or TiC (pyrolysis in Ar). Magneli products, TiN, or TiC materials were formed as the carbothermal reduction of TiO2 at high pyrolysis temperature, unfortunately, the composites lose the ordered mesostructures. The results allow us to elucidate the microstructural changes of titanium based materials inside the ordered mesoporous carbon matrices and open an avenue to the design and synthesis of cooperatively functional ordered mesoporous nanomaterials-carbon composites. In addition, it is advantageous to use ordered mesoporous TiO2-carbon composites as electrode materials for rechargeable Li-ion battery. A series of Ti-C composites with various weight percentages of carbon (25, 35, and 50 wt%) pyrolyzed at 600°C were utilized to evaluate the Li-ion storage performance. The 65Ti-35C material, containing 65 wt% TiO2 and 35 wt% carbon, shows a high capacity of 500 mAhg-1 at 0.1 C after 80 cycles. Moreover, it exhibits a good cyclability and rate capability. The reversible capacity remains at 98 mAh/g at a high rate of 5.0 C, and then recoveries to 520 mAh/g at 0.1 C after 105 cycles. In conclusion, the materials fabricated in this study are attractive materials and ideal candidates for manifold applications. The versatility and feasibility of these materials have been demonstrated, especially their application to energy storage and conversion. Much effort has to be devoted to systematic studies on the relationship between physicochemical properties of these materials and their performances in energy conversion and storage so that information on the fabrication strategies, properties, and potential applications of these materials can be obtained to stimulate further developments in this fascinating area.