A Study Of Non-noble Metal Oxygen Reduction Catalysts Fe(Co)-N/C For Fuel Cells

Fuel cell is considered as one of the most promising new energy technologies in the twenty-first century thanks to its high energy density,large conversion efficiency and pollution-free feature.However,based on that the oxygen reduction reacton?ORR?at the cathode has the characteristics of slow reaction kinetics and multifarious pathways,the screening of suitable ORR electrocatalytic materials has become a hot topic in this field.The commercial application and mass production of the platinum catalyst?Pt/C?are limited because of its high cost and low stability.At present,non-noble metal catalysts with wide sources and high catalytic efficiency have attracted the attention of researchers all over the world.Transition metal-nitrogen-carbon?M-N-C?electrocatalysts have been thought as one of the most promising catalysts to replace the commercial Pt/C catalyst.In this paper,a series of non-noble metal Fe?Co?-N-C catalysts with high catalytic activity and stability were prepared via the space-confined role of sodium chlorides to control the morphology and structure,combined with graphdiyne,heme and 2,4,6-Tri?2-pyridyl?-1,3,5-triazine?TPTZ?as the raw materials.The effects of heat-treatment temperature,heat-treatment time and preparation methods on the ORR catalytic activity of M-N-C catalysts were investigated.Various characterization methods were used to analyze the size,structure,morphology,composition and the ORR electrocatalytic performance.The specific works of this thesis are as follows:?1?An iron and nitrogen co-doped porous carbon electrocatalyst?3D-Fe/N/C-700?with the three-dimensional nano-network structure was synthesized by using sodium chloride crystallines as a solid-state template and using hemin biomaterial as a single-source precursor.The morphology,structure and BET surface area of3D-Fe/N/C-700 were characterized by SEM,TEM,Raman spectroscopy,XRD,BET and XPS.The results show that the Fe/N/C-700 catalyst without using sodium chlorides as a template is composed of bulk particles with smooth surface,but the prepared 3D-Fe/N/C-700 catalyst displays a porous structure with three-dimensional nano-networks.A large number of hollow carbon spheres with thickness of 10 nm were formed inside 3D-Fe/N/C-700.In addition,the specific surface area and total pore volume of 3D-Fe/N/C-700 were significantly increased compared to the Fe/N/C-700.Electrochemical results show that the ORR onset potential and half-wave potential of 3D-Fe/N/C-700 were significantly higher than those of Fe/N/C-700,but its half-wave potential is only 30 mV lower than that of the Pt/C catalyst.It is found that the porous structure and high specific area are beneficial to the diffusion of oxygen molecules,which can promote the ORR electrocatalytic activity.The FeN₂and graphitic-N species formed after the heat-treatment may be the catalytic sites of the catalyst.Above results preliminarily demonstrate that the proposed new method of preparing the Fe-N-C catalyst in this chapter is reasonable and efficient.?2?Graphdiyne?GD?was used as a new carbon support to prepare ORR catalysts with a core-shell structure?Fe-PANI@GD-900?,in which the carbon-shell layers derived from polyaniline and iron species were formed by in-situ polymerization method and high-temperature treatment.The morphology and structure of Fe-PANI@GD-900 were characterized by TEM,SEM,XRD,Raman,etc.The oxygen reduction catalytic performance,stability and conductivity were characterized by electrochemical methods.The results show that the Fe-N-C shell layers formed on the graphydine surface effectively prevents the agglomeration of polyaniline during pyrolysis and reduces the charge transfer resistance of the ORR,which is beneficial to expose more catalytically active sites on the surface and largely improve the ORR catalytic activity.Electrochemical results indicate that the Fe-PANI@GD-900 catalyst exhibits unexpectedly high ORR activity with onset and half-wave potentials of 1.05V and 0.82 V?vs.RHE?,and follows a direct four-electron reduction pathway and its long-term stability is superior to the Pt/C.?3?A new cobalt and nitrogen co-doped three-dimensional porous network ORR catalyst?3D-Co-N-C?with high BET specific surface area was fabricated by using the precursor of Co-TPTZ complex with the molecular level coordination effect,and controlled by the confined space of self-assembled NaCl crystallines.The morphology,structure and catalytically active sites of 3D-Co-N-C were characterized by SEM,TEM,BET,Raman spectra,XRD,etc.The ORR performance,long-term stability and catalytic reaction kinetics of the 3D-Co-N-C catalyst in alkaline medium were studied by electrochemical methods.The results show that the Co-N structure is formed in the coordination process of cobalt and nitrogen atoms in the TPTZ,resulting in the enhancement of thermal stability in the Co-TPTZ complex.The space-confinement effect in self-assembled 3D-NaCl crystallines reduces the loss of nitrogen atoms during high-temperature pyrolysis and improves the nitrogen-doping efficiency.It is found that the Co-N active structure,high contents of pyridinic-and graphitic-nitrogen may be the catalytically active sites for the ORR in the prepared catalysts.Besides,high BET surface area,high mesoporous characteristics and excellent conductivity have a positive effect on the improvement of the ORR activity.Electrochemical tests show that the ORR electrocatalytic performance of 3D-Co-N-C is better than that of the Pt/C catalyst,but the former's stability is better,further confirming that the M-N-C catalyst has promising applications in the fuel cells.This work provides new ideas for the design of high performance ORR electrocatalysts and helps to understand the nitrogen-doped active sites and their catalytic mechanism.