Synthesis Of Polymer-derived Hollow Doped Carbon Spheres And Its Electrocatalytic Performance For Oxygen Reduction

With the rapid development of the global economy,traditional fossil energy can no longer meet the requirements of sustainable development.Therefore,the development of clean,efficient and renewable energy has become a hot spot of social concern and research.Among new energy sources,fuel cell devices are considered to be one of the important strategic ways to solve energy and environmental pollution problems in the future due to their high conversion efficiency,cleanliness and sustainability.Fuel cells have not been widely promoted because of many technical problems that have not been resolved.The cathode oxygen reduction reaction(ORR)of fuel cell has slow reaction kinetics,which is the reaction control step of the entire electrochemical process,and requires highly active catalyst to catalyze the reaction.Currently,platinum-based catalysts are still the main catalysts for fuel cell electrode reactions.However,the high price,scarcity,and poor durability of the platinum-based catalysts have severely hindered the wide application of fuel cell systems in practice.Therefore,the development of inexpensive and highly active non-precious metal ORR catalysts is one of the most effective methods to solve the slow ORR kinetics,and it is also one of the most promising strategies to achieve commercial applications of fuel cells.In this thesis,poly(o-phenylenediamine)(PoPD)hollow carbon spheres with openings on the surface were used as the carbon and nitrogen source and heteroelements,transition metal doping and transition metal oxide were employed to further improve the catalytic activity and cycle stability of the PoPD-based catalyst.The main contents and results of the thesis are shown as follows:(1)In the first part,the PoPD hollow spheres with higher nitrogen content were selected as precursors,and nitrogen-doped hollow carbon spheres(N-Carbon)with opening on the surface were obtained via direct carbonization.The controlled synthesis of N-Carbon was achieved by carbonization at different temperatures.The morphology of the as prepared nanomaterial was characterized by scanning electron microscope(SEM)and transmission electron microscope(TEM),Fourier infrared spectrometer,powder X-ray diffraction(XRD),Raman spectroscopy,X-ray photoelectron spectroscopy(XPS)and nitrogen absorption and desorption curve comfirmed the successful synthesis of N-Carbon materials and the properties of N-Carbon(such as the degree of graphitization,bonding state,specific surface area and pore size)obtained at different temperatures.Moreover,the cyclic voltammetry curve and linear scan curve were measured to study the electrocatalytic performance of the N-Carbon material.The results show that N-Carbon exhibites intact morphology and good dispersion,and the carbonization temperature of 900? was demonstrated to be the best condition for oxygen reduction reaction(ORR)catalytic activity.(2)On the basis of the first part,the phosphorus element were introduced,and the hollow nanospheres of PoPD were carbonized in the presence of the phosphorus-source to synthesize phosphorus and nitrogen co-doped hollow carbon spheres(P,N-Carbon)with openings on the surface.To study the differences in the morphology and electrocatalytic performance of P,N-Carbon synthesized by different phosphorus sources(NaH2PO2,H3PO4 and phytic acid(PA))and the related mechanism,the morphology of the nanomaterials were characterized by SEM and TEM.XRD and Raman spectroscopy demonstrated the successful synthesis of P,N-Carbon material.The electrocatalytic test comfired that P,N-Carbon-PA has superior activity in alkaline medium,and both its half-wave potential(0.85 V)and cycle stability are better than commercial Pt/C.XPS and density functional theory(DFT)were than carried out to elucidate the underlying reasons for the enhanced catalytic activity of ORR caused by heteroatom doping.(3)On the basis of the first part,the iron element was introduced.This chapter focuses on the synthesis of highly reactive iron and nitrogen co-doped hollow carbon spheres(Fe,N-Carbon)with openings on the surface by controlling the addition sequence of the iron source.For the first way,Fe(NO3)3�9H2O was added during the synthesizing process of PoPD,and the amount of Fe(NO3)3�9H2O was regulated to ensure that the material was structured with opening hollow nanosphere,and then the obtained material was carbonized under nitrogen at 900? to obtain Fe,N-Carbon-1.For the second method,PoPD hollow nanospheres with openings on the surface were first synthesized,followed by the mixing with Fe(NO3)3�9H2O,and finally the above material was charred under nitrogen protection at 900? to obtain Fe,N-Carbon-2.The morphology of the materials were analyzed by SEM and TEM,XPS and synchrotron radiation were used to characterize the bonding state of the material and the valence state of Fe,and the nitrogen adsorption and desorption curve was used to study the specific surface area and pore size of the material.It is found that Fe,N-Carbon-2 has higher ORR catalytic activity as demonstrated by electrochemical tests.Moreover,Fe,N-Carbon-2 shows better stability than Pt/C.(4)On the basis of the electrocatalytic performance of P,N-Carbon and Fe,N-Carbon,PoPD hollow nanospheres,Fe(NO3)3 and PA were employed as raw materials to construct iron phosphide and nitrogen co-doped carbon(FeP,N-Carbon)hollow Nanospheres and iron,phosphorus,nitrogen co-doped carbon(Fe,P,N-Carbon)hollow nanospheres.Firstly,PoPD hollow microspheres are synthesized,and the PoPD hollow nanospheres were calcined in the presence of Fe(NO3)3 and PA to obtain FeP,N-Carbon.To prepare Fe,P,N-Carbon,the PoPD hollow nanospheres were first calcined in the presence of Fe(NO3)3 to obtain Fe,N-Carbon,and then another cycle of calcination is performed in the presence of PA to synthesize Fe,P,N-Carbon.The SEM,TEM,high resolution transmission microscope(HRTEM)and energy dispersive X-ray(EDS)spectra were used to verify the morphology and structure of the as prepared materials.The XRD,Raman,XPS and nitrogen adsorption and desorption curves proved the successful synthesis and properties of the material.According to the electrocatalytic test,Fe,P,N-Carbon has higher ORR catalytic activity,and both of its half-wave potential(0.90 V)and limiting current density(5.82 mA cm-2)are better than FeP,N-Carbon.In addition,the long-term stability of the as synthesized catalyst is better than that of Pt/C,The results show that Fe,P,N-Carbon has high potential as an efficient non-noble metal ORR electrocatalyst.(5)The PoPD and all the doped carbon materials obtained from the previous four chapters were loaded with active manganese oxides,and the doped carbon and manganese oxide(MnO)composites were formed after carbonization.With PoPD as the precursor,the MnO2/PoPD/MnO2 composite material can be obtained taking advantage of the redox reaction between PoPD and KMnO4.The MnO2/PoPD/MnO2 can then be directly carbonized to achieve MnO/N-Carbon/MnO composite material.The effect of different carbonization temperature on the morphology and catalytic performance of the materials were systematically investigated.Through TEM,particle size analysis and electrocatalytic performance test,it can be known that when the carbonization temperature was 800?,the composite material showed the best ORR electrocatalytic activity with the most moderate MnO particle size and the largest specific surface area.The doped carbon materials as precursors,KMnO4 and doped carbon materials were employed to prepare MnO2 and doped carbon composite materials were carried out to confirm,which were then directly carbonized at 800? to form MnO and doped carbon composite materials.A series of characterizations were carried out to confirm the successful synthesis of the material,and electrochemical tests indicated that MnO/Fe,P,N-Carbon/MnO showed good ORR electrocatalytic activity.