Controlled Synthesis And Property Study Of Carbon Nanomaterial Based Anode Catalysts Of Direct Methanol Fuel Cells

Due to the continuously escalating energy crisis and environmental pollution, direct methanol fuel cells (DMFCs) as potential alternative power sources have attracted great attention in both academic and engineering circles because of their high-energy conversion efficiency and low pollutant emission. However, the large-scale commercialization of DMFC has still been precluded by the lack of low-cost and high-performance anode catalysts. In recent years, carbon nanomaterials (such as carbon nanotubes and graphene) have emerged as the promising supporting materials for constructing advanced composite catalysts owning to outstanding physicochemical properties and unique nanometer size effects. In this dissertation, a series of novel strategies were applied for the controlled synthesis of different carbon nanomaterial-based anode electrocatalysts of DMFCs. Meanwhile, the influence of the microstructures, morphology and composition of the resulting nanocomposites on the electrocatalytic properties for methanol oxidation was also carefully investigated. The main contributions of this dissertation can be described as follows:1. Controlled synthesis and property study of low-defect carbon nanotube (CNT) supported Pt and PtCo alloy catalysts.In order to introduce oxygen-containing groups onto the external walls of CNTs, we utilized nitric acid from hydrolysis of platinum nitrate to slightly oxidize CNTs. Since the concentration of nitric acid was much lower than that of traditional acid treatment process, very few structural defects would be formed on CNT surface, which leads to a much higher electric conductivity. When compared with the Pt-acid-treated CNT electrocatalysts, the Pt-low-defect CNT catalyst showed good electrocatalytic activity as well as excellent antipoisoning ability. Based on this, PtCo alloy nanoparticles were also deposited on the surface of low-defect CNTs by using the similar method. The existence of Co in the composites could not only place Pt in a more reduced state, but also provide plenty of oxygen sources for CO oxidation. Therefore, the resulting PtCo-low-defect CNT catalyst exhibited much more enhanced catalytic activity in comparison with Pt-low-defect CNT and commercial PtRu-Vulcan XC-72.2. Controlled synthesis and property study of low-defect graphene supported Pt and Pd catalysts.Essentially, graphene can be considered as an unrolled CNT, which means it has similar physical characteristics but larger surface areas compared with CNTs. In this context, the design concept of low-defect CNT supported composites was further extended to the fabrication of Pt-low-defect graphene catalysts. Owning to the much softer synthetic conditions, the dispersion of Pt nanoparticles on low-defect graphene was much better than that of Pt nanoparticles on chemically reduced graphene oxide (RGO) supports. More importantly, the almost intact graphene structure could remarkably lower the charge transfer resistance of composites and improve the electrocatalytic activity of Pt nanoparticles. Moreover, given the relatively low price of Pd, we replaced Pt with Pd to reduce the manufacturing cost of low-defect graphene-based anode catalysts. Impressively, the as-obtained Pd-low-defect graphene had unusual catalytic activity for both formic acid oxidation in acid media and methanol oxidation in alkaline media. Comparatively speaking, the latter possessed better stability.3. Controlled synthesis and property study of MnO2-modified graphene supported Pt and Pd catalysts.The direct redox reaction between the carbon atoms of graphene framework and MnO4-could result in the formation of MnO2-graphene composites. The use of MnO2-graphene as a supporting material could efficiently improve the deposition of Pt and Pd nanoparticles and prevent the metal nanoparticles from agglomeration. During the electrocatalytic reactions, the incorporation of MnO2was able to assist in the diffusion of electrolyte and simultaneously enable the fast transportation of the poisoning species and protons from the electrode surface. As a consequence, the catalytic properties of the resulting MnO2-modified graphene supported catalysts were tested to be much better than those of the unmodified graphene and Vulcan XC-72supported catalysts.4. Controlled synthesis and property study of three-dimensional (3D) porous graphene-carbon nitride (g-C3N4) hybrid aerogel supported Pt catalysts.Nitrogen-doping has been demonstrated to be an effective protocol to enhance the electrochemical performance of graphene-based catalysts. In this section, we succeeded in constructing a3D porous architecture composed of graphene and g-C3N4nanosheets. By adjusting the ratios between graphene to g-C3N4nanosheets, we could prepare graphene-g-C3N4hybrid aerogels with different nitrogen contents. After the subsequent loading of Pt nanoparticles, the as-obtained nanocomposites were measured to be anode electrocatalysts for methanol oxidation reactions. Electrochemical measurements revealed that the graphene-g-C3N4hybrid aerogel supported Pt catalysts displayed high electrocatalytic activity, exceptional poison tolerance and reliable stability, far surpassing the reference samples.