Regulating single-Fe-atom spin-state via implanting second-shell symmetrical sulfur coordination

doi:10.1016/S1872-2067(26)65184-4

Abstract

Abstract:

Noble-metal-free iron-nitrogen-carbon (Fe-N-C) catalysts with well-defined FeN₄ active sites are promising alternatives to replace platinum (Pt) catalysts for oxygen reduction reaction (ORR). However, the precise design of the peripheral heteroatom-doped FeN₄ centers and the clarification of the underlying electrocatalytic ORR mechanism remain a significant challenge. Herein, we employ a coordination engineering strategy to implant two sulfur (S) atoms into the second coordination shell of the FeN₄ center (FeN₄S₂), which effectively modulates the Fe spin state via d_z²-p, d_xz-p, and d_yz-p interactions. The symmetrical S-doping in FeN₄S₂ can weaken the spin polarization of FeN₄ that induces a transition of single-Fe-atom from a high-spin to a medium-spin state, thereby optimizing the oxygen intermediates adsorption and transforming the ORR potential-determining step from *O → *OH over FeN₄ to *O₂ → *OOH over FeN₄S₂ with a lower free energy barrier, which greatly boosts the ORR performance in both alkaline and acidic media. The zinc-air battery with a FeN₄S₂ air electrode delivers a peak power density as high as 248.2 mW cm^-2 with exceptional charge-discharge stability, significantly outperforming the state-of-the-art Pt/C air electrode.

Key words: FeN₄ active sites, Sulfur-doping, Second coordination shell, Spin state, Oxygen reduction reaction

Xuebi Rao, Huiling Fang, Jialin Sun, Liqun Liu, Yongkang Zhu, Junxiang Chen, Shiming Zhang, Bin Liu. Regulating single-Fe-atom spin-state via implanting second-shell symmetrical sulfur coordination[J]. Chinese Journal of Catalysis, 2026, 89: 453-463.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)65184-4

https://www.cjcatal.com/EN/Y2026/V89/I10/453

Figures/Tables 5

Fig. 1. (a) Schematic illustration of the synthesis procedure. (b) TEM image of the Fe-N-S/CF catalyst and elemental mapping images showing distribution of C (blue), N (yellow), S (green), and Fe (red) elements. (c) Magnified AC-HAADF-STEM image of Fe-N-S/CF. (d) XRD patterns of N/CF, Fe-N/CF, and Fe-N-S/CF. (e) Raman spectra of Fe-N/CF and Fe-N-S/CF. (f) Pore size distribution of N/CF, Fe-N/CF, and Fe-N-S/CF.

Fig. 2. (a) N 1s XPS spectra of N/CF, Fe-N/CF, and Fe-N-S/CF. (b) Fe K-edge XANES spectra. (c) k3-weighted Fourier-transformed EXAFS spectra of Fe-N-S/CF and reference materials. (d) EXAFS fitting curves of Fe-N-S/CF in R-space and the corresponding fitting paths. (e) Wavelet transform of k3-weighted EXAFS spectrum for Fe-N-S/CF. (f) 57Fe M?ssbauer spectrum of Fe-N-S/CF. (g) χm-T plots and the μeff of Fe-N-S/CF and Fe-N/CF. (h) Schematic diagram showing spin-electron filled states of Fe2+ 3d orbital.

Fig. 3. Electrochemical performance. (a) LSV curves. (b) Comparison of E1/2 and jk at 0.85 V vs. RHE. (c) Comparison of E1/2 and jk at 0.85 V vs. RHE. (d) Tafel plots. (e) LSV curves of Fe-N-S/CF recorded at different rotation speeds (inset: K-L plots at the potential of 0.7, 0.8, and 0.85 V vs. RHE). (f) Electron transfer number (n) and H2O2 yields. (g) A radar chart comparing Eonset, E1/2, jk, Tafel slope, and n. (h) Amperometric i-t curves recorded at 0.7 V (vs. RHE) at a rotation speed of 1600 rpm. (i) LSV curves of Fe-N-S/CF before and after 15000 ADT cycles. Catalyst loading: 400 μg cm-2 for Fe-N-S/CF and 20 μg-Pt cm-2 for Pt/C, electrode rotation speed: 1600 rpm, electrolyte: 0.1 mol L-1 KOH solution.

Fig. 4. ZAB performance. (a) Schematic diagram of a typical ZAB device. (b) OCV curves (inset: the optical image of a Fe-N-S/CF ZAB). (c) Discharge polarization curves with peak power density indicated. (d) Discharge specific capacities at 10 mA cm-2. (e) Comparison of galvanostatic discharge curves at different current densities. (f) Galvanostatic discharge/charge cycling of ZABs evaluated at a current density of 5 mA cm-2.

Fig. 5. Reaction mechanism. (a) Free energy diagrams of ORR over FeN4, FeN4S2-2 at 1.23 V and FeN4, FeN4S2-1, FeN4S2-2, FeN4S2-3, FeN4S2-4 at 0 V. (b) The adsorption models of O2 and oxygen intermediates during the ORR process. PDOS of Fe 3d orbitals for FeN4 (c) and FeN4S2-2 (d). PCOHP analysis of the Fe-O bond formed after adsorption of *OH over FeN4 (e) and FeN4S2-2 (f). (g) In-situ Raman spectra recorded over Fe-N-S/CF at different potentials. Peak intensity at 1153 cm-1 (h) and 1532 cm-1 (i) versus applied potentials (vs. RHE).

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