Preparation Of The Doped Carbon Catalysts And Their Electrocatalytic Activity For Oxygen Reduction Reaction

Low temperature fuel cells have been recognized as a kind of promising power sourcefor new energy vehicles, due to their advantages such as high power density, low operationtemperature and quick startup. However, high loading of Pt-based catalysts on the cathode isneeded to facilitate the sluggish oxygen reduction reaction （ORR） at the current technologystatus. The high cost and unsustainability of low temperature fuel cells resulting from theexpensive and scarce Pt are becoming one of the most important factors, which block thelarge-scale commercialization of this technology. Thus, developing the inexpensive non-Ptcathodic catalysts with both high performance and durability to substitute Pt-based catalystshas been regarded as a feasible way to reduce the cost of the fuel cells, and to realize theirlarge-scale commercialization. Among all the non-Pt cathodic catalysts, doped carbonmaterials have been widely studied for their high catalytic activity, less sensitivity formethanol cross-over and excellent stability.In this thesis, doped carbon nanomaterials with unique morphology have been preparedby the chemical vapor deposition method, hard template method, transfer doping method, andsoft template method, with some simple compounds as precursors. The performances andstructural characteristics of the catalysts, the structure of the catalytic active site, and theeffects of some preparation parameters on the catalysts have been investigated extensivelythrough the evaluation and the characterizations. The morphology, composition,microstructure of these catalysts have been revealed.Firstly, nitrogen-doped carbon nanotubes （N-CNTs） arrays have been prepared by achemical vapor deposition approach, using ferrocene as the catalyst and imidazole as thecarbon and nitrogen precursor. The N-CNTs have about100nm of outside diameter and32wall layers with a bamboo-like structure, the nitrogen content reaches high up to8.54at%.For the reduction of oxygen, the N-CNTs showed excellent electrocatalytic activity in bothacidic medium （0.5M H₂SO₄） and alkaline medium （0.1M KOH）, especially in alkalinemedium. The optimum temperature for the preparation of the catalysts, in terms of catalyticactivity, is750C. It is found that the pyridinic nitrogen plays most important role for thecatalyst, it may be the most important components to the catalytic active site.Secondly, Vesicular nitrogen doped carbon （VNC） material, with BET surface area ofhigh up to555m2·g1, has been prepared by pyrolyzing the polyaniline covering on the Fe₂O₃nanoparticles, followed by acid leaching with H₂SO₄and graphitization. Although thenitrogen content is low, only1.42at%, VNC exhibits excellent catalytic activity towards oxygen reduction reaction comparable to commercial20wt%Pt/C, with the onset potentialand half-wave potential reach to0.93V and0.82V （vs. RHE） in0.5M H2SO4. It isdemonstrated that the air/hydrogen single cell with VNC as cathode catalyst, has anopen-circuit cell potential of0.85V and191mW·cm^-2of maximum power density at celltemperature of70C. It is suggested that the high catalytic activity of VNC results from itshigh surface area and high active center density, which may be caused by the vesicularstructure. Furthermore, we believe that the iron may just participate in the construction ofactive sites, but not as a component of the active site.Thirdly, Well defined nitrogen-doped graphene （NG） has been prepared by a transferdoping approach, in which the graphene oxide （GO） is deoxidized and nitrogen doped by thevaporized polyaniline, and the GO is prepared by a thermal expansion method from graphiteoxide. The content of doped nitrogen in the doped graphene is high up to6.25at%, andoxygen content is lowered to5.17at%. The NG catalyst exhibits excellent activity towardsthe ORR, as well as excellent tolerance towards methanol. In0.1M KOH solution, its onsetpotential, half-wave potential and limiting current density for the ORR are-0.01V,-0.12V（vs. Ag/AgCl） and5.38mA·cm-2, respectively, which are comparable to those of commercial20wt%Pt/C catalyst. It is suggested that the high ORR catalytic activity of NG is attributedto the choice of polianiline as nitrogen source, as well as the large surface area, highgraphitization and special features of NG.Finally, the3D self-supported N, S-codoped carbon nanofiber network has been preparedby a procedure, including polymerization, addition of Fe, carbonization, acid leaching andgraphitization, with β-naphthalene sulfonic acids （β-NSA）, aniline and （NH）₄S₂O₈as startreactants. It was found that the concentration of β-NSA affected the structure of the materialssignificantly. With the concentration of β-NSA increasing from0.0125M to0.05M, thematerials structure is transformed from the coexistence of doped carbon nanotube andnanofiber into pure carbon nanofiber, along with the increase of diameter and length, and theemergence of3D structure. The catalyst derived from the precursor with molai ratio of4:6ofaniline to β-NSA contains highest contents of nitrogen, and shows the best ORR catalyticactivity. Adding3wt%of iron can further enhance the catalytic activity, the onset potential ishigh up to0.57V （vs. Ag/AgCl） in0.1M HClO₄for the material prepared at optimalconditions. It is suggested that the high catalytic activity is attributed to the synergistic effect of nitrogen, sulfur and carbon atoms in the material, as well as the more efficient diffusion ofoxygen in3D codoped carbon nanofiber network.