Fe-N3 Defect As a Possible Active Site in Pyrolyzed ORR Electrocatalysts
We have performed density-functional-theory (DFT) based calculations for a variety of in-plane Fe-Nx (x=2-4). In contrast, to the in-plane Fe-N4 and Fe-N2 defect motifs, Fe-N3 defects (Fig. 1) show a significant out-of-plane motion of zFe=1.47 Å. The Fe-N bond distance is 1.87 Å, approximately 8% shorter than the sum of empirical covalent radii and shows that the Fe is bonded to nitrogen through covalent bonds. Previous work has shown that the formation energy of this defect is exothermic and that the underlying N3 defect motif has a high affinity for Fe.1
Thus, thermochemistry suggests that this defect could be present in the pyrolyzed ORR electrocatalyst. In order to compare the results directly to our XPS measurements we computed N1s core-level shift in the final state approximation relative to a pyridinic reference structure which is accessible in both experiment and theory, following previous work.2 All N1s core-level-shifts lead to higher binding energies, consistent with our XPS observations. For Fe-N2, Fe-N3, and Fe-N4 we obtain 1.0 eV, 1.5 eV, and 1.5 eV. This prediction suggests that XPS is unlikely to distinguish Fe-N3 and Fe-N4defects based on N1s core-level-shifts alone (Fig. 2). In contrast, our experimental XPS do show that the two defects can be distinguished through correlation with Fe 2p binding energies (Fig. 2). Furthermore, our PCA analysis indicates that electrocatalysts with this XPS signature show better ORR performance.
In conclusion, our results show that the synergy of experiment and theory provides new insights into performance of pyrolyzed ORR electrocatalysts. We present evidence for the presence of graphitic Fe-N3 defects directly through XPS spectroscopy. Furthermore, the DFT computed N1s core-level-shifts in combination with our experimental observations and PCA suggests that this type of defect can be related to increased ORR performance. These results suggest that ORR performance can be improved by increasing the concentrations of Fe-N3defects in pyrolyzed ORR electrocatalysts.
(1) Kattel, S., Atanassov, P., and Kiefer, B., Journal of Physical Chemistry C, 116, 8161-8166 (2012).