Microwave-assisted Synthesis of Metal-free Highly N-doped Carbon Porous Materials and Their Applications as Cathodic Electrocatalysts in Direct Methanol Fuel Cells

• Main Authors: Yu-Siang Kao, 高郁翔
• Other Authors: Tuan-Chi Liu
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/62gk7h

碩士 === 國立臺灣科技大學 === 化學工程系 === 100 === The objectives of this study are the synthesis development and application of novel, metal-free electrocatalyst materials for direct methanol fuel cell (DMFC). In continuation of our previous investigation using the conventional hydrothermal synthesis route to fabricate heteroatom (such as nitrogen) doped carbon porous materials (CPMs), the highly N-doped CPMs report herein was synthesized by first mixing the N-rich (? 66.7%) melamine (C3H6N6) with formaldehyde (CH2O) to form the melamine-formaldehyde resin oligomer (MFRO). By utilizing the MFRO as primary carbon precursor, tri-block copolymer surfactant (P123) as soft template, and sodium silicate (NaSi3O7) as the hard template, various N-CPMs were fabricated by co-condensation method in conjunction with microwave heating. By varying the synthesis parameters, such as the temperature (60 ? 100 oC) and duration (1 ? 24 h) of the microwave pre-treatment, the extent of polymerization between the MFRO and the soft and hard templates may be manipulated, thereby affecting the structural properties of the carbon-silicon (C-Si) composite during carbonization at elevated temperatures (800 ? 1,000 oC). Subsequently, the C-Si composites were subjected to acid leaching with acid (HF) solution to remove the silica template, followed by thorough washing and drying of the resultant substrates. Finally, various N-CPMs with different N content, physicochemical, and electronic properties may be obtained. The N-CPMs so fabricated were characterized by a variety of different analytical and spectroscopic techniques, such as powdered x-ray diffraction (PXRD), nitrogen adsorption/desorption isotherm measurements, elemental analysis (EA), thermogravimetric analysis (TGA), and x-ray photoelectron spectroscopy (XPS). Finally, these metal-free N-CPMs were utilized as electrocatalysts for DMFC at cathode. Their electrocatalytic performances during oxygen reduction reaction (ORR) were further evaluated by linear sweep voltammetry (LSV) and compared with those prepared by the conventional hydrothermal synthesis method. It is found that, variations in parameters during the microwave-assisted synthesis procedures lead to changes in the distribution of different N species in the N-CPM catalysts. Among them, pyridinic-N (denoted as N6) and pyrrolic-N (denoted as N5) species have relatively higher (ca. 40% each) contents than the quarternary-N (denoted as NQ; ca. 15%) and oxidized pyridinic-N+-O?{ (denoted as NX; ca. 5%) species. By keeping the final carbonization temperature fix at 900 oC, the effects of microwave pre-treatment temperature and duration time on the distribution of various N species in N-CPMs were investigated by means of XPS. It was found that the NX content is insensitive to the synthesis parameters, as anticipated. Pretreatment at low temperature (60 oC) led to an increase in the amount of N6, which was accompanied by a marginal decrease in N5. Upon lifting the temperature to 100 oC, a notable increase in NQ amount was observed at the expanses of decreasing N6 and N5. Further extending the treatment duration from 1 to 24 h resulted in a decrease in N5 (from 43% to 30%) and a notable increase in NQ (from 12% to 20%) while the amount of N6 remained practically unchanged at ca. 45%. Apparently, the NQ species appears to have a relatively higher thermodynamic stability than N6 and N5. It was found that, the ORR activity decrease rapidly as the NQ content in N-CPM was lower than 10% . Since N6 and NQ species are known to play the key roles during ORR, it is hypothesized that the NQ sites were responsible as the primary active centers while the N5、N6 sites play a synergetic counterpart to proton electron donation. In terms of the textural properties, microwave-assisted synthesis also facilitates a control in the pore structure of N-CPMs, typically, a pore size distribution of ca. 20 ? 40 nm was observed for samples pre-treated at 60 oC and below 20 nm for those treated at 100 oC. In comparison with the conventional hydrothermal method, microwave-assisted synthesis can not only largely reduce the preparation time of N-CPMs, but also enhance their structural stability, thus, in turn promote a higher ORR activity. For example, when pre-treated at 100 oC for 6 h followed by carbonization at 1000 oC, the resultant N-CPM prepared by the microwave-assisted method is capable of retaining a satisfactory surface area (206 m2/g) as well as high total N content as high as 7 wt%. By means of LSV measurements, a maximum current density of ?{0.49 mA was observed for this sample during ORR, which is clearly more superior over the sample prepared by hydrothermal method (?{0.11 mA; under a pre-treatment time of 24 h). Although, the ORR activities observed for these N-CPMs are indeed lower than that of typical Pt-based commercial catalysts, these novel metal-free highly N-doped CPMs not only have a total tolerance over methanol crossover effect, but also can be fabricated more easily and cost-effectively, and thus should have prospective applications as electrocatalyst materials for DMFC and various sensors.