Synthesis Of Phosphorus-doped Carbon Materials As Efficient Electrocatalyst For Oxygen Reduction Reaction

Fuel cell and metal-air battery,as energy conversion devices,are promising in portable,residential and especially transportation power source application due to their high energy density,low operation temperature and environmental compatibility.It is well known that the oxygen reduction reaction(ORR)at the cathode is a slow kinetics process,which severely limits the efficiency of fuel cell and metal-air battery.Currently,platinum-base electrocatalysts are considered as the best ORR catalysts and widely used for ORR.However,Pt catalyst suffers from some drawbacks,such as limited reserve,high cost,poor stability and low tolerance for methanol crossover,which seriously impede the large-scale commercial application of fuel cell.In this thesis,we focused on the carbon materials with abundant resources and low cost.P-doped,P and other heteroatoms co-doped carbon materials were synthesized,and the ORR acticities of which were studied.We investigated the effects of morphology,microstructure,co-doping with other atoms and the binding environment of heteroatoms on the ORR catalytic activity of P-doped carbon materials.The new approaches for the preparation of hetaroatoms doped-carbon materials were developed.The catalytic active sites and catalytic mechanism towards ORR of P-doped carbon were investigated.In order to investigate the effect of P-doping on the structure and the activity of carbon,P doped porous carbon materials were synthesized via hydrothermal and sol-gel polymerization methods followed by a pyrolysis and P doping process,in which sucrose,resorcinol,formaldehyde,phosphorous acid and cobalt nitrate were used as the precursors.We investigated the effects of P-doping,assisting doping of Co atom and specific surface area on the ORR catalytic activity of carbon materials.It is found that the amount of P incorporated into carbon instead of specific surface area plays an important role in improving the ORR performance and modifying the ORR pathway.Considering the important role of triple-phase inferface of catalysts,the P-doped carbon hollow sphere,P-doped carbon fiber tube and P-doped hierarchical porous carbon were synthesized by using a template method,in which tetraphenylphosphonium bromide(C24H20Br P)was used as the P source.The significant influence of the special structure on the mass transport at the triple-phase(gas,liguid and solid)interface as well as ORR catalytic performance was explored.The as-synthesized catalysts exhibit high electrocatalytic activities towards ORR.The ORR activity enhancement not only arises from the highly active catalytic sites induced by P doping into the carbon lattice but also ascribes to the efficient mass transport of reactants and products in the triple-phase region promoted by the special structure.Based on the mono P doping,we have successfully synthesized for the first time P and N binary co-doped graphene through one-step in-situ method by pyrolysing the complex of urea and tetraphenylphosphonium bromide.The as-prepared catalysts exhibit the remarkable electrocatalytic activity in alkaline solution for ORR.The half-wave potential of the cataltst is only about 46 m V lower than that of Pt/C.It provides a new method for preparing hetaroatoms(N,S and B)-doped graphene.P and B dual-doped graphene was synthesized via an electrostatic self-assemebly method followed by a pyrolysis process,in which ionic liquid containing P atom(C24H20Br P,C19H19 Br P and C16H36 Br P)and ionic liquid containing B atom(C24H20BNa)were used as precursors.We investigated the effect of the different binding environment of P on the electrical conductivity of graphene,specific surface area,electron spin density as well as the resulting ORR catalytic performance.On the basis of dual-doping,P,N and S ternary co-doped porous carbon was synthesized by carbonizing of tetraphenylphosphonium bromide and ammonium persulfate.Compared with mono P doping,P,N and S ternary doping significantly promoted the ORR electrocatalytic activity of carbon.The enhancement in ORR activity is mainly attributed to the synergistic effect of P,N and S doping in carbon.Based on the above studies,the ORR catalytic activity of P and transition metals(Co,Fe,Ni,Mn)co-doped porous carbon was investigated.It shows that the introduction of transition metals benefits the ORR activity improvement.The ORR activity increases as follows: P-Co-C>P-Ni-C>P-Fe-C>P-Mn-C>P-C.The ORR activity of P-Co-C is close to that of commercial Pt/C.We have proposed the catalytic active sites(TM-Px)of P and transition metals co-doped carbon materials for ORR.