Preparation And Catalytic Mechanism Of Novel Carbon Based Metal-free Electro-catalysts For Oxygen Reduction Reaction

Fuel cells have been attracting considerable interest as a future power source for the high energy conversion efficiency and environmental benefit compared to traditional energy resource.In the development of proton exchange membrane fuel cells and alkaline fuel cells,a big challenge is the exploration of the advanced electrocatalysts with low cost,high performance and good stability for oxygen reduction reaction(ORR).Based on the state of the art progress in the field of carbon-based nanomaterials as well as the research basis in our own group,this dissertation concentrates on the design,synthesis,characterization and catalytic mechanism of the novel carbon based metal-free ORR electro-catalysts including boron-doped carbon nanotubes(BCNTs),boron&nitrogen codoped carbon nanotubes(BNCNTs)and nanocages(BNCNCs).The main contents of this dissertation are summarized as follows:1.Injection chemical vapor deposition has been developed to prepare the electron-deficient boron-doped carbon nanotubes(BCNTs)with benzene and triphenylborane as precursors.BCNTs with tunable boron contents 0-2.24 at%have been obtained,which has been applied as ORR electrocatalyst in alkaline medium.Experimental results indicate that the activities of BCNTs get enhanced obviously with increasing boron contents,and the BCNTs with boron content 2.24 at%demonstrates the best ORR performance.These catalysts present the negligible methanol crossover effect and strong resistance to CO poisoning,which is essential for fuel cells.This progress extends the exploration on metal-free carbon-based electrocatalysts from electron-rich dopants(e.g.N)to electron-deficient dopants(e.g.B).To understand the origin of the ORR activity,DFT calculations were performed for BCNT before and after O2 adsorption.The results show that B dopant plays the crucial role in collecting electrons from ?conjugation and localizing some ?*electrons in conjugated carbon system.B dopant also creates a tunnel between the conjugated electrons and the chemisorbed O2 and transfers partial charge to O2,which effectively weakens the O=O bond and facilitates ORR.According to these experimental results and theoretical calculations,we summarized a key strategy to transform sp2 carbon into metal-free ORR electrocatalysts by doping:(1)breaking the electroneutrality of sp2 carbon to create charged sites favorable for O2 adsorption;(2)activating carbon n electrons for effective utilization by O2.This can be achieved by doping sp2 carbon materials despite of the electron-rich or electron-deficient features for the dopants.This progress provides some new train of thought in exploring the metal-free electrocatalysts in the heteroatom-doped sp2 carbon by mono-or multi-doping with the dopants such as B,N and P,which is a hopeful for developing the advanced ORR electrocatalysts for fuel cells.2.Two kinds of B&N codoped carbon nanotubes dominated by bonded or separated B and N are intentionally prepared,which present distinct oxygen reduction reaction(ORR)performances.The experimental results indicate that the separated case can,while the bonded case cannot,turn the inert CNTs into ORR electrocatalysts.Theoretical results indicate that,for the bonded case,the lone-pair electrons from N dopant are largely neutralized by the vacant orbital of B dopant,thus the activation of carbon ? electrons could hardly occur to catalyze ORR.This progress demonstrates the crucial role of the doping microstructure on ORR performance,which is of significance in exploring the advanced C-based metal-free electrocatalysts.Meanwhile,the strategies to turn sp2 carbon into metal-free ORR electrocatalysts proposed in Chaper 2 are still applicable for the B and N co-doped situations,which suggests a certain universality of the strategy.3.A novel kind of carbon nanomaterial,i.e.the boron and nitrogen co-doped carbon nanocages(BNCNCs),with close N content(7.1-8.5 at%)and tunable B content(0-2.44 at%)have been successfully prepared by MgO template method.BNCNCs demonstrate superior ORR performance in both alkaline and acid media,as well as excellent stability.The catalytic sites in BNCNCs have been theoretically studied,and the results show that BN2 micro-structures facilitate the occurrence of ORR in acid electrolyte.Such understanding is beneficial to the further exploration of advanced carbon-based metal-free ORR catalyst in acid medium.4.By making use of the anchoring effect of N species,nitrogen doped carbon nanocages(NCNCs)have been used as the support to immobilize the Fe3O4 nanoparticles by impregnation method.The nanocomposite catalyst of Fe3O4/NCNCs has been conveniently constructed.In alkaline medium,Fe3O4/NCNCs shows good ORR performance,and its onset potential,limited current and stability are comparable with those for the commercial Pt-C catalyst.Such nanocomposite catalysts can realize the high dispersion of the active species for the large surface area and N anchoring effect of NCNCs.Moreover,the ORR performance of NCNCs itself could also be utilized.This is an effective way to build nanocomposite catalysts for potential applications.