Studies On Cobalt Doped Polypyrrole Modified Carbon Catalyst For Oxygen Reduction Reaction

Oxygen Reduction Reaction (ORR) is the rate controlling reaction in fuel cells and metal-air batteries, dominationg their performance. Carbon supported platinum (Pt/C) is widely used as electrocatalyst for ORR owing to its excellent ORR catalytic activity. However, the scarcity and high expense of platinum are the major obstacles for fuel cell commercialization. R & D of non-platinum electrocatalyst thus have been paid much attention.Among the reported non-platinum catalysts, macrocyclic/non-macrocyclic M-N/C catalysts (M=transition metals, N=nitrogen containing ligand) draws a lot of intrest due to their convenient preparation, low cost and high catalytic activity for ORR. Co-N/C and Fe-N/C are considered the most promising M-N/C catalysts. However, the current preparation procedure of M-N/C catalysts generally requires heat treatment at high temperatures (calcination) to form M-Nx sites, difficult to control the catalyst composition and structure due to the loss of nitrogen and formation of non-active metal/metal carbide aggregates, leading to the decrease in the utilization of transition metals.In this work,β-Co(OH)2/C and β-Ni(OH)2/C catalysts are prepared using chemical deposition method, and studied comparatively for their catalytic activity towards ORR. p-Co(OH)2/C and β-Ni(OH)2/C both catalyze ORR through the medium of P-MOOH/β-M(OH)2 (M=Co, Ni) redox couple. P-CoOOH/β-Co(OH)2 has lower redox potential which is favorable for ORR catalysis. At room temperatures, the redox potential of β-NiOOH/β-Ni(OH)2 is too high, which hinders the catalysis of ORR. Elevation of working temperature enhances the redox process of β-NiOOH/β-Ni(OH)2, at 60℃ or higher temperatures, β-Ni(OH)2/C exhibits comparable performance with β-Co(OH)2/C.Co-PPy/C catalysts are prepared using hydrothermal method. The effects of hydrothermal temperature, doping transition metal and anion on the activity of the catalyst are investigated by meterials characterization and electrochemical analyses such as morphology, compositon and electrochemical activity evaluation, via scanning electron microscopy, Fourier transformed infrared spectroscopy, X-ray photoelectron spectroscopy, galvanostatic discharging, cyclic voltammetry, rotating disk electrode, etc.. According to the experiment results, it is found that Co exhibits better doping effect on ORR catalytic activity improvement than other transition elements such as Ni, Fe, Mn and Cu. The anion in the metal precursor also influences the activity of Co-PPy/C. SO42- hinders the Co doping and leads to the deterioration of ORR catalytic activity. Hydrothermal treatment at higher temperatures enhances the formation of Co-Nx and graphitic-N, activating the Co-Nx sites away from the carbon support to improve the activity of the catalyst.In addition, higher hydrothermal temperature is also benificial for the formation of oxygen containing groups like quinones on the carbon support, which facilitates the ORR catalysis. However, excessive formation of graphitic-N competes with Co-Nx formation, and lowers the activity of Co-PPy/C.Macroporous carbon (MPC) is synthesized in this work to study the influence of carbon support structure on the ORR catalytic activity of Co-PPy/C. Compared to BP2000, the macroporous carbon has continuous crystal structure and remarkbly lowers the charge transfer impedance. However, the inner pores in the macroporous carbon restrain the mass transfer of oxygen and water, leading to the decrease in performance. Pore size in the macroporous carbon is significantly enlarged by using sucrose as an alternative carbon source and 2-step pyrolysis for carbonization after spray drying of the precursor. The pores are interconnected to improve the mass transfer of oxygen and water. The fuel cell using MPC as the catalyst support for Co-PPy/C exhibits higher performance than that using BP2000, close to the commercial E-Tek 10wt% Pt/XC-72.