Preparation And Properties Of Fe-NC Oxygen Reduction Catalyst

The high cost of noble metal-based catalyst for oxygen reduction reaction (ORR) has hindered the commercialization process of fuel cells. Therefore, the development of non-precious metal catalyst with high performance and low cost is extremely urgent, especially for ORR catalyst. Among the non-precious metal catalysts for ORR. metal-carbon-nitrogen composite (M-N-C, M=Fe. Co. Ni) is considered as one of the most promising catalysts, and polyaniline is the most used precursor. Howerver. M-N-C catalysts are facing problems including low catalytic activity, high cost or toxicity of nitrogen precursor, or complicatedly syntheitic process.In this thesis, to overcome the shortcoming of low activity, Fe-N-C catalyst with high activity and excellent stability was synthesized through heat-treatment using polyaniline as nitrogen and carbon source, coiled carbon nanotubes (CNC) with high specific area as carbon support. On this basis, firstly vitamin B2 (VB2) with low cost and nontoxicity is proposed to be used as nitrogen and carbon sources, Fe-N-C catalyst with high activity and excellent stability was prepared through one step heat-treatment of VB2 and transition metal compounds. The effects of heat-treatment temperature, transition metal precursors, and nominal transition metal precursor content on the performance of catalysts are mainly investigated. Thermogravimetric analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR) analysis, X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM), Raman, Linear Sweep Voltammetry (LSV) and Chronoamperometry were carried out to study the structure, morphology and electrochemical performance of the catalysts.From the TEM images, the CNC has high degree of graphitization and coiled mesoporous structrues which favor electron transfer and diffusion of oxygen to active sites. As compared with Fe-N-XC72 catalyst, the Fe-N-CNC catalyst exhibites higher catalytic activity and more excellent cycling stability, a high current density of 3.5 mA/cm-2 at 0.3 V (vs Ag/AgCl) and 40 mV negative shift in half-wave potential after 1000 cyclic voltammetry (CV) cycles. The excellent performance of Fe-N-C catalyst is benefited from the three-dimensional structure which can promote the transportation of reactant/product. increase the amount of active sites, and maintain the stability of electrode structure.Fe-N-C catalyst was prepared through one-step heat-treatment using riboflavin as nitrogen and carbon sources, anhydrous ferric chloride as transition metal precursor. The ORR activities of Fe-N-C catalyst in acid and alkaline medium were studied. From the experiment results, heat-treatment temperature and nominal transition metal content are considered as the most important factors to impact the performance of catalysts, the activity of catalyst exhibites similiar tendency in acidic and alkaline medium. The Fe-N-C-7 catalyst prepared with 7 wt% nominal Fe content at 800? for 150 min exhibites the most excellent performance. LSV curves show that current density reaches 3.54 mA/cm2 at a potential of 0.3 V (vs Ag/AgCl) in acidic medium and 4.16 mA/cm2 at -0.2 V (vs Hg/HgO) in alkaline medium, which is comparable to the commercial Pt/C catalyst. Furthermore, before and after 1000 CV cycles, the LSV curves indicate that the negative shift in half-wave potential is 10 mV in acid medium and 5 mV in alkaline medium. Chronoamperometry (i-t) results show that, after continuous testing for 18000 s, the current density retentions for the commercial Pt/C catalyst are 80% and 60% in acid and alkaline medium, but for the Fe-N-C-7 catalyst, they are respectively 95% and 97%. The cycling stability and chronoamperometry results indicate that Fe-N-C-7 catalyst demonstrates much better stability than Pt/C catalyst. The electron transfer numbers of Fe-N-C-7 catalyst are respectively 3.7 and 3.9 in acid and alkaline medium, calculated by Koutecky-Levich equation. Those indicate that the Fe-N-C-7 catalyst mainly catalyzes oxygen to H2O through a four-electron mechanism.In summary, the Fe-N-C catalyst can be prepared via one-step pyrolysis of VB2 in the presence of anhydrous ferric chloride. The ORR performance of Fe-N-C is comparable to the commercial Pt/C catalyst, but its stability is much more excellent than the commercial Pt/C catalyst. Therefore, advantages of nontoxicity and low cost of VB2, simple procedure of catalyst preparation, good ORR acitivity and excellent stability of the catalyst will have the potential prospect that the Fe-N-C catalyst substitutes the noble metal catalysts to be practically applied in low temperature fuel cells.