Prepartion Of Iron-based Bi-metal/Nitrogen Co-doping Carbon Catalysts And Their Electrocatalytic Activity For Oxygen Reduction Reaction

Fuel cells are attracting increasing attention as efficient and clean energy-conversion. The oxygen reduction reaction（ORR） at the cathode is an important factor influencing the performance of a fuel cell. Currently, platinum based materials are the best-known catalyst in fuel cell technology for ORR; however, the Pt-based catalysts suffer from the disadvantages of high cost and low methanol resistance, are the key limitations to the commercialization of fuel cells. Transition-metal-nitrogen-carbon（TM-N/C） catalysts have been considered as the best non-precious cathode catalysts most likely to replace Pt-based catalysts for oxygen reduction reaction in fuel cells due to their high catalytic activity, excellent methanol tolerance and cheapness.In this work, the ultrasonic method was to applied to synthesize Fe M-N/C-t（M=Co, Cu, Ni） catalysts. Dicyanodiamine was used as nitrogen source; cobalt（Ⅱ） acetate, copper（Ⅱ） acetate, nickel（Ⅱ） acetate and ferric chloride were used as metal precursor; BP2000, pre-treated by HNO₃, was used as carbon support. And then the catalysts annealed treatment under an inert atmosphere in the range of 700-1000℃, which showed a good electrocatalysis performance of oxygen reduction reaction. X-ray diffraction（XRD）, fourier transform infrared spectrum（FT-IR） and X-ray photoelectron spectroscopy were used to characterize the catalyst in terms of the structure and composition in order to know the catalytic active sites of the catalysts. The influencing factors of catalysis performance, active site and dynamics mechanism toward ORR on the catalysts as-prepared in this paper were studied and discussed. The results are listed as follows:（1） Catalysts Fe M-N/C-t displayed excellent electrolysis activity for ORR, which were synthesized by heat treatment under N₂ atmosphere in the range of 700-1000 ℃, using dicyanodiamine as nitrogen source, Fe Cl₃·6H₂O, Co（OAc）₂·4H₂O, Cu（OAc）₂·H₂O and Ni（OAc）₂·4H₂O as metal source, BP2000 as carbon support.（2） The metal intrinsic is the important influence factor for ORR performance in this paper. Electrocatalysis ability of bimetal catalyst is higher than single one, indicating that the binary Fe M-N/C-t catalysts had enhanced activity towards the ORR. The heat treatment temperature and the metal contents had an important effect on catalytic activity for ORR.（3） The electrocatalytic activity of Fe M-N/C-t（M=Co, Cu, Ni） is close to conventional JM20%Pt/C in terms of onset potential. And the catalyst Fe M-N/C-t（M=Co, Cu, Ni） had better methanol resistance and durability for V ORR than that of commercial JM20%Pt/C catalyst.（4） XRD and FT-IR techniques were used to characterize the structure of Fe M-N/C-t（M=Co, Cu, Ni）, it was concluded that Me-N and C-N may be the active sites of catalysts Fe M-N/C-t（M=Co, Cu, Ni）. XPS and linear sweep votammetry were used to characterize the structures of the surface active sites and the electrochemical activity of the catalysts 10%Fe₃Ni₁-N_0.4/C-800, 10%Fe-N_0.4/C-800, 10%Ni-N_0.4/C-800, _0.4/C-800. XPS analysis revealed that there was not apparent correlation between the catalytic activity for ORR and the nitrogen surface content, and the structures of Me-Nx（Fe-Nx, Ni-Nx） and C-N were present on the catalyst particle surface. N_0.4/C-800 catalyst prepared with no metal shown high retained nitrogen content but the ORR activity lower than 10%Fe₃Ni₁-N_0.4/C-800 catalyst, indicating that metal is an active site component. Through XPS analysis and fitting corresponding N 1s peak in the catalysts 10%Fe₃Ni₁-N_0.4/C-800, 10%Fe-N_0.4/C-800, 10%Ni-N_0.4/C-800 and N_0.4/C-800, the type of nitrogen in Me-Nx active site was the cation of metal coordinated with pyridinic-nitrogen on the carbon support and the structure of Me-N-C is the main part of active sites.（5） Linear sweep voltammetry was used to obtain the overall ORR electron transfer number. According to Koutechky-Levich equation, we calculated the electron transfer number of ORR on the catalysts Fe M-N/C-800（M=Co, Cu, Ni） and the overall electron transfer number for ORR catalyzed by the Fe M-N/C-800 was determined to be 3.3 to 4, suggesting that the ORR was a mixture of 2-electron and 4-electron transfer pathways but dominated by a 4-electron transfer pathway that produces H₂ O. That is to say, the mainly catalytic ORR process of the catalysts prepared in this paper directly transformed oxygen to the water.