Studies On Anodic Catalysts For Direct Methanol Fuel Cell Supported On Carbon Fibrous Mats

To satisfy the ever-increasing energy demands and tackle the environmental issues, one has to consider alternative energy sources to replace the currently dominant fuels and develop more effective energy conversion devices. Among various systems of energy conversion and storage, fuel cells have become a primary research focus since they convert the chemical energy directly into electrical energy with high efficiency and low emissions of pollutant. Direct methanol fuel cells (DMFCs), which supply the electrical energy by converting methanol to energy, have attracted enormous interest for applications in low emission vehicles, portable electronic devices and distributed power generators because of the high power density, low operating temperature, simple structure of the fuel cell system, low cost of methanol and easy storage and transport of the liquid fuel. At present, platinum and its alloy are usually used as the anode and cathode catalysts for the methanol oxidation and oxygen reduction in DMFCs, respectively. However, there are two key problems restricting the widespread commercialization of DMFCs:the high cost of catalysts and the low methanol electrocatalytic oxidation kinetics caused by the catalyst poisoning. Thus, it is necessary to improve the catalytic activity of platinum for methanol electrooxidation, minimize its use simultaneously and reduce the cost of DMFCs subsequently.In this dissertation, carbon fibrous mats (CFMs) formed by carbon nanofibers have been fabricated by thermally treating the electrospun polyacrylonitrile fibers. Then platinum nanoparticles are loaded on the CFMs by using various methods. The electrooxidation of methanol on Pt/CFMs electrodes is also investigated.(1) The CFMs with high conductivity (55 S cm-1), which are formed by carbon nanofibers with an average diameter of about 150 nm, have been fabricated by combining the electrospinning and thermally treating. A large number of gaps in the mats are beneficial to disperse catalysts and diffuse reactants. The value of D/G (the intensity ratio of D-band to G-band) obtained from the Raman spectrum is 1.16 for the CFMs in present work, which is smaller than that of the literature reported previously (the value of D/G is 3.41 and 1.51 at 1000℃and 1500℃, respectively), indicating that the CFMs contain more graphite.(2) The specific surface area and pore parameters of the CFMs and commercial carbon paper (CP) are determined on a mercury injection apparatus. The results show that the specific surface area, total pore volume and porosity of the CFMs are 22.84 m2 g-1,2.94 mL g-1 and 65%, respectively, which are larger than the specific surface area (4.64 m2 g-1), total pore volume (0.24 mL g-1) and porosity of (29%) of the commercial CP. The results also show that the commercial CP contains mainly the large pores whose size are larger than 2μm, while the CFMs contain not only the large pores whose sizes are larger than 2μm(55%) but also the small pores whose sizes are smaller than 2μm (45%). And the tensile, compressive and rupture strength of CFMs are about 12.09,16.02 and 38.67 MPa, respectively, which are slightly larger than the tensile, compressive and rupture strength of CP: 11.49,15.56 and 35.85 MPa. The data of mechanical strength indicate that the CFMs exhibit enough mechanical strength to endure the pressure to make the MEA electrodes.(3) The electrocatalytic activity of commercial Pt/C supported on the CFMs and the commercial CP for methanol oxidation in a sulfuric acid solution has been investigated by cyclic voltammetry, chronoamperometry, quasi-steady state polarization, and electrochemical impedance spectroscopy methods. The results show that the commercial Pt/C supported on the CFMs exhibits higher electrocatalytic activity, more stability, larger exchange current and smaller charge transfer resistance than that on the commercial CP, which reveals that the CFMs could be developed as highly efficient supporting materials for Pt/C catalyst in the future for high-performance DMFCs due to their special porous and continuous fibrous structures.(4) The platinum nanoparticles loaded on the CFMs are prepared by using multi-cycle cyclic voltammetry method, an aqueous reductive method with formaldehyde as reducer, and the method of combining the process of physical absorption and thermal treatment with formaldehyde vapor, or formic acid vapor or ethylene glycol. The obtained Pt/CFMs catalytic electrodes exhibit high performance in the aspects of electrocatalytic activity and long-term stability towards the oxidation of methanol. Additionally, the kinetic studies of methanol oxidation on the electrodes show that the Pt/CFMs electrodes exhibit higher exchange current density and smaller charge transfer resistance than Pt/C-CFMs electrodes.(5) As a result, the platinum nanoparticles supported on the CFMs by combining the process of physical absorption and thermal treatment with formaldehyde vapor offer many advantages such as improved electrocatalytic abilities, long-term stability, easy fabrication, environmental benignity and low cost. Therefore, the process in which formaldehyde vapor is used as the reducer can be developed as a new means for preparing noble metallic catalysts.