Dynamic Fluctuation in Heat Treatment Time Dependence of Activity and Reaction Kinetics of Active Centers in Fe/N/C Oxygen Reduction Reaction Catalyst

We elucidate dynamic fluctuation of activity characterized by the existence of multiple local maxima or a significant inflection point in the growth rate of an Fe/N/C oxygen reduction reaction (ORR) catalyst, with this fluctuation depending on the heat treatment (HT) time used in the catalyst's preparation. Since its HT time increased from 45 seconds to 10 hours at 750 degrees C HT, its activity exhibited a dynamic fluctuation and multiple local maxima, which are separated into two different timescales: several minutes and several hours. At 600 degrees C HT, a significant inflection point appears around several tens of minutes instead of the multiple local maxima. These results contrast with the conventional idea of how activity depends on heat treatment time, i.e. an initial monotonic increase and then a plateau or monotonic decrease. It suggests that the best HT condition cannot be found by only investigating a narrow range of HT time or a course plot of HT time dependence of activity in general. Based on a simple reaction model, the origin of the dynamic fluctuation of activity depending on HT time is explained by the existence of multiple types of Fe-N active centers with individual formation and decomposition rate constants during HT. These results will contribute to a comprehensive understanding of how a Metal/N/C catalyst achieves its ORR activity through HT, which is essential for designing yet higher-performance catalysts from the viewpoint of the number of constitutive active centers.