Fe-N3 Defect As a Possible Active Site in Pyrolyzed ORR Electrocatalysts

Fuel Cells have become promising candidates for energy conversion technologies in particular for non-stationary applications. Materials which continue to attract significant attention are non-PGM electrocatalysts that are based on carbon embedded TM-N_x defect motifs. XPS observations support the presence of Fe-N_x (x=2 and 4) defect moieties. These defects promote the complete oxygen reduction at the cathode. XPS peaks are broad. Thus, due to the lack of appropriate reference materials structure/property relationships remain tentative. First-principles computations can provide important constraints for this effort and by providing core-level shifts for specific defect motifs. Thus, the synergy of this capability with experimental XPS observations and principal component analysis (PCA) can provide guidance as to what Fe-N_x defect motifs are responsible for increased ORR activity in this class of non-PGM ORR electrocatalyst and improved electrocatalyst design. In this presentation we will focus on Fe-N₃defects and use this strategy to address the presence of this defect type in pyrolyzed non-PGM ORR electrocatalysts and its relation to ORR performance.

We have performed density-functional-theory (DFT) based calculations for a variety of in-plane Fe-N_x (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 z_Fe=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.

References :

(1)     Kattel, S., Atanassov, P., and Kiefer, B., Journal of Physical Chemistry C, 116, 8161-8166 (2012).

(2)     Artyushkova, K., Kiefer, B., Halevi, B., Knop-Gericke, A., Schlögl, R., and Atanassov, P., Chemical Communications, 49, 2539-2544 (2013).