| Up to now, high cost is still the most vertical issue that blocks the commercialization of proton exchange membrane fuel cells(PEMFCs), thus, developing suitable high-performance catalysts to substitute traditional Pt-based catalysts will be of great importance for the practical applications of PEMFCs. In addition, compared to the hydrogen oxidation reaction in the anode of a PEMFC, oxygen reduction reaction(ORR) occuring at the cathode has a much more sluggish kinetic process, resulting in that most of the Pt catalysts are used in the cathode to catalyze the ORR. Therefore, developing high performance ORR electrocatalysts will be the priority among priorities during the commercialization of PEMFCs. Among the ORR catalysts obtained to date, carbons exhibit the highest catalytic performance. Due to their high ORR performance and other special features such as high stability, low-cost, great potential to replace traditional Pt-based catalysts in PEMFCs, etc., carbon-based ORR catalysts have attracted great attention and become one of the hottest topics in new energy field recent years. In this thesis, we designed and prepared a series of high-performance carbon-based ORR catalysts by using doping, self-assemble and nanocasting-pyrolysis procedures. The main results obtained are summarized as follows:(1) We obtained uniform nitrogen and sulfur co-doped carbon nanospherical catalysts through a one-step pyrolysis procedure by using polyacrylonitrile and sulfur as precursors. The catalyst exhibited remarkable ORR performance, which can be approaching that of commercial Pt/C in an alkaline medium, as well as outstanding stability and methanol tolerance. By studying the catalysts derived from precursors with various sulfur contents,we discovered that sulfur played a vertical role during the formation of uniform nanospherical morphology and porous structures. By simply changing the sulfur contents in the precursors, we successfully tunned the sulfur proportion as well as the porous structures and the BET surface areas of the final products. For the optimal catalyst, its sulfur contents and BET surface areas can be up to 9.5% and 652 m2g-1, respectively.(2) We constructed a material with a high surface area and hierarchical porous structures through a template-free self-assemble approach by using polystyrene(PS) foam,melamine, ferric chloride as the precursors. The as-prepared material exhibited excellent ORR performance. Compared to the commercial Pt/C catalyst, its half-wave potential was 20 m V more positive. Besides its excellent ORR performance, it also showed remarkable stability, outstanding methanol tolerance, and high catalytic efficiency(nearly 100% selectivity for the four-electron ORR process). By studying the relationship between catalyst loading and catalyst’s ORR performance, we found that the ORR occurred on our catalyst follows a apparent four-electron route. By investigating the effects of N dopants and Fe residue on the catalyst’s ORR performance, we found that Fe residue plays an important role in the enhancement of ORR catalytic activity, which can be even comparable to that of active nitrogen species.(3) We developed a facile process for preparing a high-performance carbon-based ORR catalyst using polyacrylonitrile, ferric chloride, and melamine as precursors. The catalyst had a fog-like, fluffy, porous structures and exhibited an excellent ORR performance, with a half-wave potential 27 m V more positive than that of a commercial Pt/C catalyst (-0.120 and-0.147 V, vs. Ag/Ag Cl) and a higher diffusion-limiting current density than that of Pt/C(5.60 and 5.33 m A cm-2, vs. Ag/Ag Cl) in an alkaline medium. Besides excellent ORR performance, the catalyst also showed outstanding methanol tolerance,remarkable stability, and nearly 100% selectivity for the four-electron ORR process. By comparing catalysts derived from the precursors and containing different amounts of melamine, we found that adding melamine cannot only give the catalyst fluffy and porous structures but also modify the N content and the compositions of each N species in the final catalyst, which we believe should be the origins for the catalyst’s excellent ORR performance.(4) We developed a new type of doped carbon catalyst, with well-defined ordered mesoporous structures, high surface area(1217 m2g-1) through a nanocasting-pyrolysis procedure by using poly(4-vinylpyridin), iron chloride as the precursors and SBA-15 as the templates.Excitingly, the catalyst exhibited excellent ORR performance in both alkaline and acid media. In an alkaline medium, its half-wave potential(-0.083 V, vs. Ag/Ag Cl) was 64 m V more positive than that of commercial Pt/C catalyst(-0.147 V, vs. Ag/Ag Cl). When tested in 0.1 M HCl O4 solution, it exhibited an onset potential and a half-wave potential of up to 0.63 and 0.47 V(vs. Ag/Ag Cl), respectively, which were quite close to those of commercial Pt/C(0.67 and 0.52 V, vs. Ag/Ag Cl, respectively). Besides excellent ORR performance in both alkaline and acid media, our catalyst also illustrated remarkable performance as the cathode catalyst in a single H2-air single PEMFC, yielding a maximum output power of 300 m W cm-2. To our knowledge, it is among the best carbon-based ORR catalysts to date in terms of the ORR performances in an acid solution and in a single fuel cell. By changing the pyrolysis temperature, we successful tuned the amount and proportion of N in the resulting catalysts, and found that the various amounts and proportion of each N species did play a vertical role in enhancing catalysts’ ORR performance. Interestingly, by comparing the catalysts derived from precursors containing different amounts of SBA-15, we found that the added SBA-15 can not only provide the catalysts with well-defined ordered mesoporous structures but also seem helpful for maintaining the total N content without significantly changing the proportion of each N species. |
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