Low-temperature active site formation for oxygen reduction reaction in (Fe, N)-doped carbon black via rapid thermal annealing

Abstract

Nitrogen- and iron- doped carbon-based materials are promising oxygen reduction reaction (ORR) catalysts that can replace precious platinum-based catalysts. Among carbon-based materials, carbon black (CB) is attractive owing to its high surface areas, high wettability and high intrinsic electrical conductivity, and it has been reported that high annealing temperatures such as 900 °C is required to introduce FeNx active sites to CBs. In this study, we used defective acid-treated CBs that are suitable for FeNx accommodation, and employed direct nitrogen doping of iron-added defective CBs by rapid thermal annealing in ammonia. As a result, it was shown that the FeN_x active sites can be formed at a low temperature of 500 °C. We then comparatively explored the relationship between the annealing temperatures (500 °C, 700 °C and 900 °C) and the characteristics of the products including the ORR activity, and found that the highest ORR activity is obtained with a sample annealed at 700 °C. The Raman spectroscopy, transmission electron microscopy and X-ray absorption spectroscopy indicate that the FeN_x species transform into FeN₄ configurations in the edge with the optimized defect levels at 700 °C, resulting in high ORR activity. Further increasing the temperature to 900 °C leads to the evolution of FeNx species into FeN₂ configurations along with increased Fe nanoparticle aggregation, which diminishes catalytic performance.