Preparationof The Transition Metal/Heteroatom Doped Carbon Materials And Their Catalysis Towards Oxygen Reduction Reaction

In recent years,fuel cells have attracted tremendous interest asrenewable and highly efficient energy conversion devices to deal withtheincreasingly prominent energy crisis.The researches on oxygen reductionreaction ?ORR? electrocatalysts arevital to improve theperformance of these fuel cells which is limited by the sluggishORR in the cathode.Although considered to be the most promisingORR catalysts, the Pt-based catalysts are still confinedto the naturalweakness of high cost and poor stability,which unfortunately impede their widespread application.In this case, the search for durable, efficient and commerciallyavailable non-precious ORR catalysts becomes imperative torealize the practical application of fuel cells. At present, there are mainly two approaches to enhance the activity of the ORR electrocatalysts:on the one hand, doping the heteroatoms into the carbon catalysts has been reported to obtain metal-free electrocatalysts with outstanding ORR activity; on the other hand, as non-precious-metal catalysts, the introductionof transitionmetal to heteroatom-doped carbon catalysts can leadto an impressive improvement in the ORR performanceIn the introduction, we have demonstrated the history of the fuel cells, their operational principle and challenges. The development and innovation of ORR electrocatalysts was emphasized especially. We have fabricated some transition metal/heteroatom doped carbon materials through a simple pyrolysisprocedure with some facile precursor and then researched their electrocatalytic activity towards ORR. This dissertation mainly consists of the following several aspects:?1?Nitrogen and sulfur co-doped onion-like mesoporous carbon vesicle ?NS-MCV? with multilayer lamellarstructure was synthesized as a metal-free catalyst through a convenient and economical procedure. The NS-MCV electrocatalysts were fabricated through one-step pyrolyzing the mixture of resol precursor, benzyldisulfide and cyanamide. The combination of sulfur and nitrogen doping, multilayer mesoporous structure has made a contribution to the superior performance for ORR. When employed as a metal-free electrocatalyst for the ORR, the NS-MCV exhibits high electrocatalytic activity with mainly four electron transfer pathway. Moreover, in contrast to the commercially available Pt/C catalyst, NS-MCV shows much better long-term stability and tolerance toward methanol crossover in an alkaline medium.?2? Nitrogen doped graphitic layers encased cobalt ?N-C@Co? nanoparticles as novel non-precious-metal catalysts for the ORR were fabricated by a facile method using cyanamide and cobalt nitrate as precursors. The N-doped graphitic shell preventsthe Co nanoparticles from dissolution and agglomeration, further improving the conductivity of the electrocatalysts.Meanwhile, the Co nanoparticle core plays a positiverole in activating the N-doped graphitic shell towards the ORR.The N-C@Co-2 catalyst manifests excellent ORR activity with a minor HO₂-yield in a four-electron transferpathway, which making it promising as a low-cost and effective non-preciousmetal-based ORR catalyst.?3? Dicobalt phosphide nanoparticles encased in boron and nitrogen co-doped graphitic layers ?BNC/Co₂P? were fabricated by a one-step pyrolysis method as novel non-precious metal catalysts for theORR.We found that the Co₂P nanoparticle core plays apositive role in activating the B, N co-doped graphitic shelltowards the ORR. Moreover, the doping of B into graphiticlayers promotes the formation of the B-N bonding species,which more efficiently facilitate the ORR activity of catalysts. The BNC/Co₂P-2exhibits similar excellent catalytic performance but superiordurability and electrocatalytic selectivity towards ORR comparedto Pt/C catalysts. The RRDE tests further confirm that the ORR procedure at the BNC/Co₂P-2 occursmainly via a predominant four-electron pathway with minor HO₂-yield.