Switching the Oxygen Reduction Reaction Pathway via Tailoring the Electronic Structure of FeN4/C Catalysts

FeN4-type carbon-based materials are promising non-precious-metal catalysts for the oxygen reduction reaction (ORR). However, FeN4/C catalysts always exhibit different ORR activities and selectivities, and their structure- performance relationship remains elusive. Herein, we design a covalent triazine framework with abundant N-4 units (CTF-N-4) to anchor Fe ions to precisely prepare a FeN4-type precursor (CTF-FeN4) that undergoes the 2e(-) ORR pathway with high selectivity. Interestingly, such a 2e(-) ORR pathway can be switched to a 4e(-) route through the modulation of the electronic structure by a controlled-pyrolysis process. Both X-ray photoelectron and synchrotron X-ray absorption spectra verify that all of the samples maintain the atomically dispersed FeN4 type configuration before and after the heat treatment, but the nonplanarity of FeN4/C increases dramatically with the carbonization temperature. Density functional theory calculations reveal that the introduced Fe atoms and the enhanced nonplanarity enhance the binding energy of *OOH on C adjacent to the pyridinic N, which favors the 4e(-) ORR path. Our study provides a fundamental understanding of the ORR mechanism on FeN4/C with a tunable electronic structure, hence paving the way for the development of cost-effective electrocatalysts for specific applications.

Automated summary

In this study, a covalent triazine framework was designed to anchor Fe ions for the preparation of a FeN4-type precursor, which can switch from a 2e(-) to a 4e(-) ORR pathway through controlled pyrolysis. The XPS and XAS spectra confirmed the atomically dispersed FeN4 configuration in all samples before and after heat treatment, with a significant increase in nonplanarity at higher carbonization temperatures.