Controllable Synthesis Of Porphyrin Nanomaterials With Nitrogen And Phosphorus Co-Doping For Oxygen Reduction Reaction

Fuel cells have attracted extensive attention due to their advantages such as high conversion efficiency,high energy density,high safety,long-term continuous power supply and environmental friendliness recently.Nevertheless,the rate of the cathodic oxygen reduction reaction of the fuel cell is six to seven orders of extent slower than the hydroxide oxidation reaction rate,which becomes the rate-limiting reaction that refines the whole system.Pt-based catalysts is widely used in the ORR at present,but its scarcity and high cost,susceptible to CO poisoning and poor methanol tolerance hinder the widely commercialization of Pt-based electrocatalysts for fuel cells.Therefore,it is crucial to promote the development of full cells by developing cheaper,durable and high-activity non-noble metal catalysts.Among the many non-noble metal electrocatalysts reported,heterogeneous atom-doped carbon materials,especially transition metals?M: Fe,Co,Mn,etc.?and nitrogen?N?co-doped carbon?M-N_x/C?are a kind of oxygen reduction catalytic materials with high activity and good catalytic stability.Porphyrin is a planar macrocyclic molecules with highly delocalized ? electrons,and can effectively coordinate with most of the metal ions to form a natural high catalytic activity M-N₄ structure,which makes it become an ideal candidate for constructing a highly active non-noble metal oxygen reduction catalyst.However,the hightemperature pyrolysis is often needed to increase its electrical conductivity due to the poor intrinsic electrical conductivity of porphyrin,while the limited thermal stability and low char yield always lead to the metal aggregation.What's more,the pyrolysis will destroy the original structures of porphyrin and affect catalytic activity to some extent.Therefore,how to effectively improve the thermal stability,inhibit the volatilization and decomposition of materials during high-temperature pyrolysis,maintain and enhance the overall activity of catalysts have become the focus of people's research.According to scientific questions above,the thesis aims to synthesize nitrogen/phosphorus co-doping nanospheres with monodisperse and three-dimensional spatial network structure through covalent coupling of porphyrin with phosphazene.The introduction of phosphazene and the resultant three-dimensional network structure can effectively improve the stability of porphyrin molecules during the high-temperature pyrolysis,which ensures that the size and morphology keep intact with monodisperse,and high activity.Besides,the non-precious metal catalytic materials can improve the ORR performance.The research has been carried out form the following two aspects:?1?In order to improve thermal stability,phosphazene was introduced as the comonomer.The ultrasonic polymerization to controllable fabrication of THPP-HCCP nanospheres was carried out and the morphology variation mechanism through adjusting the concentration of reactants,different ways of reaction and solvent.were explored.The ORR performance was also examined by changing the heating temperature,heating rate,and N or P doping using the nanospheres with hexagonal closest packed.And a series of characterizations have been employed to verify the N,P doped monodisperse microspheres with good thermal stability after pyrolysis.The THPP-HCCP-800°C showed great ORR activity with a positive half-wave potential?E1/2?of 0.78 V?vs.RHE?and a large diffusion limited current density?Jd?of 5.28 m A·cm^-2,the electron transfer number?n?derived from the Koutecky-Levich?K-L?equation for the THPP-HCCP-800°C also reconfirms a 3.68 transfer pathway in the ORR process,and it has a better methanol resistance and stability than 20 wt % Pt/C.?2?In order to build high concentration of catalytically active sites.Cobalt porphyrin?Co THPP?with a clear Co-N4 active site was synthesized to polymerize with phosphazene to form nanospheres?Co THPPHCCP?.With the suitable spacing of center by phosphazene and stabilized the active center,the retention of active sites during the heat treatment process was achieved,and more active centers with Co-N_x structure were obtained.In addition,a series of characterizations confirmed that the microsphere morphology remained intact,the char yield was as high as 80%,and the thermal stability was further improved.The Co THPPHCCP-800°C demonstrated the excellent ORR activity with a positive half-wave potential?E1/2?of 0.854 V?vs.RHE?and better than 20% Pt/C 4 m V,and a larger diffusion limited current density?Jd?of 5.64 m A·cm^-2,the n derived from the Koutecky-Levich?K-L?equation for the Co THPP-HCCP-800°C also reconfirms a four-electron transfer pathway in the ORR process,durability as well as the tolerance to methanol relative to 20 wt % Pt/C.