| Fuel cells have drawn much attention because of their high energy conversion efficiency and environment friendliness. As a complex reaction, the slow reaction kinetics of oxygen reduction reaction (ORR) is the critical issue restricting the performance of fuel cells. The traditional catalyst for ORR of PEMFC is Pt./C owing to its excellent catalytic activity. However, the scarcity and high price of Pt. hinder its application. Therefore, development of non-noble metal catalyst for ORR is of great significance. In this work, we prepared a series of cheap and high-performance catalysts for ORR. Their catalytic structure and activity were investigated and discussed. Finally we applied the best catalyst into the direct sodium borohydride fuel cell, mainly worked on its performance and stability.In this work, we respectively use BP2000 and macro porous carbon (MPC) which was made by nano-CaCO3 as template and glucose as carbon source as our carbon support. Co-IAA modified BP2000/MPC catalysts are prepared through hydrothermal method. The effects of carbon support, doping transition metal and anion, doped-nitrogen content, and heat treatment temperature are investigated by material characterization and electrochemical methods such as morphology, surface composition and content, crystal texture via scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), etc.; the electrochemical activity evaluation via cyclic voltammetry (CV) and rotating disk electrode (RDE).These studies show that the carbon support exhibits significant effect on the catalytic activity of Co-IAA/C. MPC has large specific surface area and conductivity. Its pore structure can overcome shortcomings of the traditional carbon support, such as particles migration and reunion. It is found that the use of MPC significantly increases the limiting current density of oxygen reduction reaction of MPC supported catalyst. Through doping different content of IAA, we found that the catalytic activity of Co-IAA/C catalysts has an optimal IAA doped content. After studies of nitrates with different metal cation (Co(Ⅱ), Fe(Ⅲ), Ni(Ⅱ)), and cobalt salts with different anions (NO3-, Cl-, SO4-), It is found that the effect of cation on catalytic activity of the synthesized catalysts ascends Ni(Ⅱ)<Fe(Ⅲ)<Co(Ⅱ) order, and that of anion descends follows NO3>Cl->SO4-. Co(NO3)2 shows the most significant effect on the enhancement of catalytic activity. Heat treatment promotes formation of graphitic nitrogen and Co-N, which favors to catalyze ORR via 4 electron transfer pathway, thereby significantly improving the stability and oxygen reduction activity of the catalyst.By changing the sinter temperature of glucose and CaCO3 to 650℃, we got a new carbon support, MPC(650) and the synthesized catalyst Co-IAA/MPC(650)-900 through hydrothermal method and heat treatment. Compared with Co-IAA/MPC(9000)-900, the test results revealed the high performance of synthesized catalyst. Precipitating different ratio of nano-CaCO3 to MPC(650) resulted in the deterioration of catalytic activity, which verified the effect of temple (CaCO3) on the catalytic activity. The experimental results show that the mass transfer performance of the catalyst becomes very poor after the precipitation of CaCO3. Besides, the catalytic performance of the catalyst decreases with the increasing precipitation ratio of CaCO3. Then we found that the performance of Co-IAA/MPC(650)-900 toward ORR can also further improved after optimizing the heat treatment temperature and doping nitrogen into carbon support.The direct sodium borohydride fuel cell (DBFC) with the synthesized non-noble catalyst as the cathode catalyst demonstrated a high maximum output power density 178 mW cm"2 at room temperature, a little lower than the power density 192 mW cm-2 of commercialized 28.6wt.%Pt/C as cathode catalyst on the same condition. Besides, the cathode life test of 53h showed the good stability of this synthesized catalyst under alkaline environment. |
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