Morphology Control And Electrochemical Properties Of Ordered Mesoporous Carbons As Electrode Materials For Energy Storage

Ordered mesoporous carbon (OMC) materials with high surface area, ordered pore channel, uniform pore size, high stability and conductivity, have attracted considerable attention due to their potential applications in adsorption, catalysis, electrode materials, and hydrogen storage system. For a widely practical uses, the design of desired morphologies for mesoporous material is as important as the control of its internal structure and porosity. Not only different fields have different needs for the particle morphologies, but also the morphology control could open up new possibilities for their applications. Up to now, series of OMC materials with special morphologies, such as fibers, spheres and films, have been synthesized, however, the method is usually focused on the two-step hard template route. This traditional hard template way involves a time consuming and complicated multi-step synthesis procedure such as the preparation of hard template and repeated impregnating of carbon precursors, which may severely limit its applications. So it is necessary to search for simple method to control the morphology of the OMC materials. The one-step method combines the synthesis of hard template and filling the pores with carbon sources through a one-step organic-inorganic self-assembly method, which skips many time-consuming steps of the traditional hard template method and saves resources because the costly surfactant is not need to extracted, and may have great potential in the time-saving synthesis process.In this research, OMC materials with various morphologies, including spherical, rod-like, gyroid-shaped, plate-like and flower-type, were synthesized using different carbon sources and changing the synthesis parameters. The morphologies, structures and pore characteristics of carbon materials were investigated by scanning electron microscope, transmission electron microscope, X-ray diffraction, and nitrogen sorption techniques. In order to research the relationship among the capacitive behavior, pore structure and morphology, the electrochemical performances of the carbons as electrodes for energy storage were measured. Moreover, OMC/polyaniline (PANI) composites were synthesized using in situ polymerization method. The main results are as follows.Ordered mesoporous carbons with various morphologies, including rod-like, gyroid-shaped, plate-like and flower-type, were synthesized by changing hydrochloric acid concentrations in the formation of silica/triblock copolymer P123/glycerol composites and further carbonization. The mixture of triblock copolymer P123 and glycerol was used as both the structure-directing agent and carbon precursor. Tetraethoxysilicon (TEOS) was used as the silica source. The pore sizes of the mesoporous carbon materials are from 4.7 to 6.5 nm, and their surface area are from 970 to 1312 m2/g. The electric double layer capacitor (EDLC) properties of the carbon materials were measured. It was found that the sample possessing the largest specific surface area shows the highest capacitance of about 164 F/g at a current density of 100 mA/g, and decreases to 142 Fg-1 with the current density increased to 2000 mAg-1. The capacitance retention ratio is used to evaluate the ion transport behavior of the carbon materials, and the larger the retention ratio, the better the ion transport behavior. The capacitance retention ratio (relative to capacitance at a current density of 100 mAg-1) of the sample with short rod at a current density of 2000 mAg-1 (94.4%) is much higher than that with long rod (86.6%), which could be partly ascribed to its shorter pore channels. In addition, the morphology and ordered degree of the samples may also affect their electrochemical performances.Spherical OMC materials with Ia3d cubic mesostructure were directly synthesized from the carbonization of silica/triblock copolymer P123/butanol composites through one-step method in sulfuric acid system using P123 and butanol as both structure-directing agent and carbon precursor. The size of the sphere is from 2 to 10μm. The pore size and surface area is 3.0 nm and 1236 m2/g, respectively. In order to investigate the effect of butanol addition on the morphology and pore structure of the carbon materials, samples with different mass ratio of butanol to P123 were synthesized. It was found that when the mass ratio was 0:1, rodlike carbon materials with ordered structure were obtained; Fine spheres could be obtained only in a narrow range of mass ratio from 0.3:1 to 1.5:1, and with the increase of mass ratio, the ordered degree of the samples decreases; sample with 2:1 butanol to P123 mass ratio shows only big block morphology with disordered structure.Rod-type ordered mesoporous carbons were synthesized by direct carbonization of sulfuric-acid-treated silica/triblock copolymer composites, using P123 as the structure-directing agent and carbon precursor. By changing the synthesis parameters, the rod length of the carbon materials can be modulated from one to tens of micrometers. The pore sizes are centred at 3.6 nm, and the sample possessing two-pore system also has pores up to 14.2 nm. The EDLC measurements show that sample possessing two-pore system has the highest capacitance retention ratio of 92% at a current density of 2000 mA/g, which can be ascribed to the larger mesopores. The capacitance performances of samples with similar pore sizes show that short pore channels are facilitative for the ion transport behavior. The electrochemical properties of OMC electrodes for Lithium ion batteries were measured using a galvanostatic charge/discharge cycling method. It was found that carbon materials have the reversible capacities from 787 to 1158 mAh/g at a current density of 50 mA/g after 30 charge/discharge cycles, which is much higher than the theoretical capacity of graphite. The samples also exhibit excellent high-rate performance. At the current density of 1000 mA/g, the reversible capacities of the samples are ranged from 259 to 468 mAh/g, with about 32% to 44% capacity retained relative to that at a current density of 50 mAg-1. If the loss of first irreversible capacities could be further decreased, the ordered carbon materials may have greater potential in the field of Lithium ion batteries.Two kinds of OMC/PANI composite with different formation manner of PANI on the OMC were synthesized using in situ polymerization method by controlling the synthesis parameter, and by changing the mass ratio of OMC to aniline in each system, we also got series of composites. The capacitance properties of OMC/PANI composites were tested by galvanostatic charge/discharge method. It was found that both kinds of composites with 40:60 mass ratio of OMC to aniline have the highest capacitances, which are about 350-380 F/g at a current density of 100 mA/g.