Influence of sulfur in the precursor mixture on the structural composition of Fe-N-C catalysts

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 H₂SO₄. 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.