| Direct alcohol fuel cells (DAFC) have been one of ideal power source for a variety of small portable electronics with remarkable advantages of rich sources, high energy, safe transportation and storage. Up to date, methanol is more often used as a fuel in DAFC systems, however, the evidently poor kinetics reaction on the anode and the crossover of methanol from the anode to cathode remain great two challenges towards reliable application of this novel material in DAFC. Therefore, the search for alternative catalyst materials with excellent electrocatalytic performance is an urgent and crucial goal regarding the issue. Considering noble metal of platinum (Pt) and palladium (Pd) are frequently regarded as the first choice for dehydrogenation of organic molecules in industry, thus they are widely employed as an essential and effective catalyst in DAFC systems. In general, noble metals are commonly distributed on electronic conductive carbon supports integrated with large surface area, therefore the corresponding electrocatalytic behavior is closely related to the category and surface properties of the carbon supports. Furthermore, the dispersion, particle size and distribution of noble metal catalyst are also directly dependent upon the structure and composition of carbon supports. Recently, ordered mesoporous carbon (OMC) as a novel nano-structured carbon materials, has attracted tremendous attention from both experimental and theoretical communities because of it excellent physical and chemical properties. In comparison to other carbon materials, OMC is considered to be an ideal support material in DAFC due to continuous and ordered porous framework, adjustable pore size distribution, large surface area, good chemical inertness and electrical conductivity characteristics. In most cases, noble metals catalysts are directly loaded on the exterior surface of OMC, and the pore characteristics, electronic conductivity and superficial wetting ability of OMC still remain to be great limitations in some aspects, therefore, hindering its further applications as a carbon support in DAFC. As a consequence, the feasible fabrication of OMC with suitable porosity, high conductivity and excellent wetting ability, which will enable the tailoring of the utilization of the noble metals and the electrocatalytic performance, has great and important implication in the field of OMC.In this thesis, we have systemically reviewed the newest development in the research field of DMFC, noble metal catalysts, carbon support materials and OMC, respectively. Based on the OMC materials from the structural optimization, chemical doping and surface modification, etc. point of view, a series of functional OMC materials were fabricated via different hard template or soft template incorporated hard template approaches, respectively, which were employed as the carbon supports of Pt or Pd catalysts in DAFC. The resulting OMC-based nanocomposites were characterized by a variety of physical techniques including powder X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Transmission electron microscopy (TEM), N2 adsorption isotherm (BET) measurements. Most importantly, the effects of pore structure, electronic conductivity and wetting ability of the synthetic OMC-based catalysts on the electrochemical characteristics are systemically studied. The main contents are as follows:1. In order to explore the relationship between the pore characteristics and electrochemical performance, two types of ordered mesoporous carbons (OMC-FT and OMC-F) with different hexagonal pore structures were synthesized via two-phase and three-phase self-assembly template methods, respectively, two catalysts of OMC-FT/Pt and OMC-F/Pt were achieved via microwave-assisted reduction procedure. Additionally, the electrochemical performance of such two catalysts has been investigated regarding the oxidation of methanol in details. Structural characterizations revealed that OMC-FT possessed higher surface area (699 m2·g-1), broader mesopore range (6.5 nm), and more enhanced mesoporosity (0.89) compared with the counterpart of OMC-F. Moreover, in view of the similar Pt loading content, the more uniform distribution and homogeneous dispersion of Pt nanoparticles occurred in the case of OMC-FT. The electrochemical results also disclosed that the specific current density of OMC-FT/Pt catalysts was as high as 11.43 mA·cm-2 for electro-oxidation of methanol while it was only 6.01 mA·cm-2 for OMC-F/Pt catalyst. Even after 2000 s, the current density-time responses for methanol oxidation measured at a fixed potential of 0.65 V were observed that the current density of OMC-FT/Pt still maintained 33.3 %. The better cyclic activity and stability of the OMC-FT/Pt catalyst towards methanol electro-oxidation could be attributed to following reasons: the broad mesopore range, enhanced mesopority and high surface area of the OMC-FT carbon supports supplied to a good dispersion and efficient utilization of loaded Pt nanoparticles, a short ion-transport pathway through the walls and minimizing the inner-pore resistance. So the OMC-FT supports would give the reactants an efficient and facile access to the catalytic active sites, as a result, leading to the improvement of the mass transport in the cell systems.2. Highly ordered mesoporous carbon-silica architecture was prepared by organic-inorganic-surfactant tri-constituent co-assembly method. The structural characterizations including surface area, pore size and pore structure of the as-prepared carbon-silica composites were closely dependant upon the different dissolve times (6 h, 12 h, 24 h and 48 h) in HF solution. The synthetic carbon-silica composites/Pt catalysts were prepared via sodium borohydride reduction method. Systematical characterizations have been conducted on such novel Pt catalyst. The physical characterizations demonstrated that an average pore size, specific surface areas and pore volume of the carbon-silica composites increased with prolonging the dissolve time from 6 h to 48 h, while the ordered degree within the architecture collapsed gradually. Additionally, the size of the loaded Pt nanoparticles became more and more large incorporated with the enhanced dispersion on the supports. On the other hand, the electrochemical results disclosed that the ordered mesoporous carbon-silica composites/Pt catalysts with the soaking time of 24 h exhibited the best electrocatalytic performance towards methanol electro-oxidation. For example, the catalytic activity reached as high as 18.7 mA·cm-2, and the current density-time responses for methanol oxidation measured at a fixed potential of 0.70 V were observed that it still remained 10.2 mA·cm-2 even after 1400 s. The reason is that the introduction of rigid silica composition makes the oxygen species to attack carbon atoms on the edge of mesoporous carbon difficultly in the long electrochemical testing process, which effectively alleviates the corrosion of carbon materials; in addition, evenly dispersed Pt nanoparticles increase the contact area with the electrolyte and other factors, which are the stability of the catalyst to be significantly improved.3. How to tune the electronic conductivity of OMC systems has been a great challenge toward broad application of using these novel materials in material concerned science. Regarding this topic, boron-doped OMC composites were prepared by the carbonization of phenol-formaldehyde resin and boric acid confined in mesoporous SBA-15 silica template. By changing the concentration of boric acid (0.002 g, 0.004 g, 0.008 g and 0.012 g), it could be feasible to tune the pore characteristics of the as-yield boron-doped OMC composites. Using formaldehyde as a reducing agent, the boron-doped OMC composites/Pt catalysts were successfully fabricated. The effect of boron concentration on chemical and electronic structure of the supports, as well as on the dispersion of Pt nanoparticles was systematically investigated. As the amount of boric acid within the whole reactants increased, the ordered architecture of the boron-doped OMC systems gradually became weaken, and the specific surface area, pore volume and average pore size decreased. In the case of the boron concentration was 0.012 g, the ordered framework within the boron-doped OMC composites would be lost. On the other hand, the electronic conductivity of the boron-doped composites increased from 8.743 S·m-1 to 89.276 S·m-1 with the increase of boron content. The primary electrochemical measurements demonstrated that the best case, with the boron concentration up to 0.008 g, had the current density of 29.9 mA·cm-2 for methanol oxidation and it still reached 22.3 mA·cm-2 even after 500 cycles, nearly 3 times of pure OMC catalysts. On the basis of the above descriptions, we speculated the possible mechanism toward the enhanced electrochemical properties: the intercalation of boron into the framework of mesoporous carbons would give rise to the formation of conductive covalent bond on Fermi level, in this way resulting in reducing the charge-transfer resistance of electrochemical reaction incorporated with accelerating the electron transfer capabilities. As a result, the significantly improved the electronic properties of boron-doped OMC composites enhanced the electrochemical activity and stability in fuel devices.4. In order to enhance the wetting ability of OMC, functional poly(sodium-p-styrenesulfonate) (PSS) moieties were grafted onto the surface of OMC, followed by loading Pd catalysts via a microwave-assisted chemical reduction route. We have also systematically investigated the influence of PSS content (30 mg, 50 mg, 70 mg) on the corresponding electrochemical properties regarding the electro-oxidation of formic acid. With increasing the amount of PSS, the loading of Pd nanoparticles on the carbon supports increased, whereas the Pd dispersion gradually increased and then decreased. Electrochemical measurements also revealed that with increasing the content of PSS, the catalytic activities and CO tolerance ability became increased and then decreased. Comparably, when the PSS content was up to 50 mg, the current density of Pd catalysts supported on modified OMC reached 11.72 mA·cm-2, which was almost 1.8 times of pure OMC catalysts for formic acid oxidation. Moreover, both the stability of Pd catalysts and the ability of CO tolerance were better than pure OMC catalysts, suggesting that catalysts had higher catalytic activity and better stability for formic acid oxidation than that of the counterpart. The reaction mechanism of enhanced electrochemical performance with the assist of PSS was further discussed. On the other hand, in order to further improve the wetting ability property of OMC, the functional 4-aninobenzene groups were firstly successfully modified onto the surface of OMC using solvent-free method. With the assist of sodium borohydride, Pd nanoparticles were dispersed on these functional carbon supports toward formic acid electro-oxidation. The corresponding results showed that the 4-aninobenzene groups were grafted onto the surface of OMC, and the surface of modified OMC was uniformly covered with a certain amount of Pd nanoparticles. In most cases, the Pd nanoparticles were uniformly dispersed on the carbon supports, and almost had a similar particle size of approximately 3.0～10.0 nm. The electrochemical results indicated that the current density of Pd catalysts supported on modified OMC was up to 48.6 mA·cm-2 for formic acid oxidation, while it was only 21.4 mA·cm-2 for OMC catalyst. Therefore, the stability of Pd catalysts supported on modified OMC was also significantly improved by introducing 4-aninobenzene groups to OMC systems.
|
PDF Full Text Request