Influence of sulfur in the precursor mixture on the structural composition of Fe-N-C catalysts
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Fe-N-C catalysts were prepared by a new synthesis protocol at 800 ∘C with subsequent acid leaching. The effect of sulfur was investigated by a systematic study in which the molar S/Fe ratio in the precursor was varied from 0.0 to 2.45. The obtained catalysts were evaluated for their ORR activity in 0.1 M H2 SO 4. In addition, the specific BET surface area was determined from N2 sorption measurements and structural characterization was made by Mößbauer spectroscopy. Catalysts contain FeN4 moieties and inorganic iron species. Structure activity correlation indicate a dominance of the ferrous low-spin FeN4 site for the ORR activity. This is in agreement with previous findings. In addition, the optimum in terms of ORR activity is in the same S/Me range as found for porphyrin-based catalysts. However, in contrast to previous conclusions of an avoidance of iron carbide formation by sulfur addition, a very high S/Fe ratio is required to obtain a catalyst free of iron carbide. Further work is required to identify the parameter that indeed enables inhibition of iron carbide formation.
KeywordsFe-N-C catalysts Electrocatalysis Mößbauer spectroscopy Oxygen reduction reaction (ORR)
Financial support by the German Research Foundation (DFG) for the Graduate School of Excellence Energy Science and Engineering (GSC1070), the Federal Ministry of Education and Research (BMBF) via the projects NUKFER (05K16RD1) and StRedO (03XP0092) is gratefully acknowledged.
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