Fluorine and Nitrogen Doped Fe-Based Non-Precious Metal Catalyst for Highly Efficient Catalytic Oxygen Reduction Reaction in Alkaline Electrolyte
Authors: Jung Won Suk , Yoon Ho Seok 
Published: December 2024

Anion exchange membrane fuel cells (AEMFCs) attract much attention since they play a crucial role in the hydrogen economy. The oxygen reduction reaction (ORR) at the cathode is a fundamental reaction in electrochemical energy conversion systems like AEMFCs. However, the kinetics of the ORR are sluggish and the precious metal catalysts such as Pt which are highly efficient for the ORR are scarce and costly. Owing to their disadvantages, it is required to replace the precious metal catalysts with abundant non-precious metal catalysts.

In the past decade, the transition metal nitrogen carbon catalysts (M-N-C) have been accentuated as possible alternatives to Pt catalysts due to their wide applicability to electrochemical catalysis and high ORR performance in alkaline media [1, 2]. Fe is generally known as the most efficient transition metal for the ORR [3, 4]. However, the ORR activity of Fe-N-C catalysts is not competitive with that of Ptcatalysts. To boost the activity and performance of theFe-N-C catalyst, doping additional heteroatoms (such as B, S, P, and F) to the Fe–N–C catalysts is widely investigated since it is a simple and practical strategy.

In this work, we synthesize Fe-N-F-C catalysts using chelating agents like ethylene diamine and ethylenediamine (EDA) and 1,1,1,5,5,5-hexafluoroacetylacetone (HFAc) as the N and F precursors, respectively. The annealing temperature and doped heteroatoms were systematically designed to arrange control samples to elucidate their synergistic effects on the ORR activity. We used high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), Raman spectroscopy, Inductively coupled plasma-optical emission spectrometry (ICP-OES), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) using the 1D-XRS KIST-PAL beamline of the PLS-II light source at the Pohang Accelerator Laboratory as well as a rotating ring disk electrode (RRDE) system.

Well-dispersed uniform iron carbide particles were detected in Fe@NFC700 by HR-TEM. Among the catalysts, the highest amount of heteroatom functional groups (i.e. semi-ionic C–F bonds and Fe–Nx) that expedite the ORR activity were observed in Fe@NFC700. In addition, Fe@NFC700 exhibited higher contents of the Fe–Nx, pyridinic-N, and graphitic-N than Fe@NC, which indicates that F doping influences the formation of the N functional groups. As a result, Fe@NFC700 showed a high onset potential (0.981 V) and a small Tafel slope (79.11 mV dec‒1) compared to the catalysts synthesized at different annealing temperatures. The F-doped Fe–N–C system is a highly potential candidate for electrochemical energy applications such as AEMFCs and metal-air batteries.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2020R1C1C1004206).

References

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