Nitrogen-doped Carbon Nanofibers for the Oxygen Reduction Reaction
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Summary of thesis: One of the major obstacles for the commercialization of fuel cells is the necessity for high cost noble metals to facilitate the electrode reactions. Especially the oxygen reduction reaction (ORR) is slow and requires high Pt loadings on the fuel cell cathodes. In this context, it is vital to develop new catalysts for the ORR based on inexpensive and abundant materials with similar catalytic activity and stability as the traditional Pt/C catalysts. Several types of non-noble metal catalysts have been explored over the years, with carbon nanomaterials containing nitrogen and trace amounts of transition metals being the most promising alternative. However, in acidic electrolyte the ORR activities reported so far have not been as high as for Pt/C. Furthermore, the role of the transition metals on the ORR activities of these nitrogen-doped carbon catalysts has not been clarified. In this study oxygen reduction catalysts based on nitrogen-doped carbon nanofibers (N-CNFs) have been developed and the origin of the catalytic activity for the ORR investigated. A chemical vapor deposition method using simple gaseous precursors was employed to grow N-CNFs from Fe and Ni particles on the surface of expanded graphite. The N-CNFs prepared from Fe exhibited a notable activity for the oxygen reduction in both acidic and alkaline electrolyte, in addition to demonstrating a high durability after 1600 cycles. It was discovered that the iron growth catalyst consisted of different iron carbides depending on the carbon activity of the synthesis feed and influenced the structure and level of nitrogen doping of the N-CNFs. The best catalytic activity and selectivity was achieved when the N-CNFs were grown from Hägg carbide, χ-Fe5C2, suggesting that this carbide phase favors the incorporation of active sites into the N-CNFs. Post treatment of the N-CNFs with KOH was used to remove iron and nitrogen from the N-CNF surface. The results showed that the presence of iron and nitrogen was important for the oxygen reduction activity of the N-CNFs, although only a fraction of the Fe and N directly contribute to ORR activity. In addition, the KOH treatment increased the porosity of the catalyst which seemed to improve the ORR by increasing the accessibility of O2 to the active sites or by introducing carbon edges active for oxygen adsorption. Treatment of the N-CNFs with HNO3 removed iron carbides, metallic iron and 50% of the nitrogen initially present in the N-CNFs without affecting the oxygen reduction activity. Analysis of the acid treated N-CNFs revealed that porphyrin-like Fe-N4 sites were incorporated into the N-CNF structure during growth and were resistant to acid leaching. The Fe-N4 moieties were identified as active sites for the ORR in the N-CNF catalysts.