Proximity-engineered Ru single-atom sites modulate Fe-N4 spatial distortion for enhanced acidic oxygen reduction reaction

doi:10.1016/S1872-2067(25)64813-3

Abstract

Abstract:

Fe-N-C catalysts are promising substitutes for precious-metal platinum in acidic oxygen reduction reactions (ORR), yet their moderate intrinsic activity and susceptibility to reactive oxygen species (ROS)-induced degradation hinder practical implementation. Herein, we fabricate a Ru-Fe dual-site catalyst (RuFe-N-C) through a two-step pyrolysis strategy. Structural characterization reveals atomic-scale proximity between Ru single atoms and Fe-N₄ moieties, exhibiting a projected distance of ~1.7 Å. This configuration induces Fe-N bond elongation accompanied by 2.5% lattice distortion. The optimized RuFe-N-C catalyst exhibits high ORR performance, with a half-wave potential (E_1/2) of 0.840 V and peak power density (P_max) of 938 mW cm^-2 under 150 kPa absolute H₂-O₂. These metrics signify substantial enhancements relative to conventional Fe-N-C benchmarks (+21 mV in E_1/2 and +42% in P_max). Moreover, the catalyst maintains outstanding stability, showing merely 17 mV E_1/2 decay after 10000 accelerated durability test (ADT) cycles. Experimental analyses reveal a bifunctional mechanism: (1) Adjacent Ru sites substantially enhance the intrinsic ORR activity of Fe-N₄ moieties, delivering a notable turnover frequency (TOF = 17.86 e site^-1 s^-1 at 0.85 V vs. RHE) that exceeds state-of-the-art Fe-N-C benchmarks by 1-2 orders of magnitude (< 1 e site^-1 s^-1); (2) Ru centers function as electron relays that facilitate ROS scavenging, thus suppressing degradation. This work establishes a paradigm for engineering bimetallic single-atom catalysts through synergistic electronic modulation to concurrently enhance activity and stability.

Key words: Oxygen reduction reaction, PGM-free catalyst, Ru-Fe dual site, Spatial distortion, Fuel cells

Shu-Hu Yin, Xiao-Yang Cheng, Yu Han, Ting Zhu, Zhong-Wei Yu, Rui Huang, Jun Xu, Yan-Xia Jiang, Shi-Gang Sun. Proximity-engineered Ru single-atom sites modulate Fe-N₄ spatial distortion for enhanced acidic oxygen reduction reaction[J]. Chinese Journal of Catalysis, 2025, 78: 343-353.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(25)64813-3

https://www.cjcatal.com/EN/Y2025/V78/I11/343

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Fig. 1. (a) Diagram for M-Fe dual sites (M = Mn, Co, Ni, Cu, Ru). The atomic tensile strain (ε) (b), Bader charge (c) and d-band center (d) of Fe atom for different M-Fe dual sites.

Fig. 2. (a) Synthetic route of the RuFe-N-C catalyst. (b) AC-STEM image of RuFe-N-C catalyst. (c) The intensity profiles obtained on dual Ru-Fe sites in yellow circled zone of (b). (d) Comfirmation of bimetallic Ru-Fe sites by EELS. (e) HAADF-STEM image and elemental mapping images.

Fig. 3. Local structure analysis of on bimetallic Ru-Fe sites. (a) Fe K-edge XANES spectra. (b) Ru K-edge XANES spectra. (c) Fe K-edge FT-EXAFS spectra. (d) Ru K-edge FT-EXAFS spectra. Fe K-edge EXAFS fitting analysis in R space for FePc reference (e) and RuFe-N-C (f). Fe K-edge (g) and Ru K-edge (h) WT-EXAFS contour plots of RuFe-N-C and standard samples.

Fig. 4. ORR activity evaluation of the catalysts. (a) LSV curves. (b) E1/2. jk at 0.82 V (c) and H2O2 yields (d) of different catalysts.

Fig. 5. Enhancement mechanism of ORR activity. (a) SD values. TOF at 0.80 and 0.85 V vs. RHE (b), jk at 0.82 V (c) and H2O2 yields (d) of different catalysts; Isoactivity plots at 0.80 V (c) and 0.85 V (d) comparing the ORR performance metric of the RuFe-N-C to recently reported five benchmark Fe-N-C [32].

Fig. 6. ORR stability evaluation of the catalysts. (a) LSV curves after stability tests with 10,000 cycles of Ru-N-C, Fe-N-C and RuFe-N-C. (b) Reaction between ROS and ABTS. (c) UV-vis absorption spectra of 0.1 mol L-1 HClO4 include Benchmark, Ru-N-C, Fe-N-C and RuFe-N-C. (d) The absorbance difference at 417 nm after subtraction of the benchmark value. (e) Possible stability enhancement mechanism of RuFe-N-C.

Fig. 7. Performance tests with RuFe-N-C, Fe-N-C and Ru-N-C as the cathodic catalysts in a single-cell PEMFCs. H2-O2 (a) and H2-air (b) PEMFCs polarization curves. Test conditions: cathode, Fe-N-C with 3.5 mgcat cm-2 loading; anode, commercial Pt/Canode with 0.4 mgPt cm-2 (Alfa Aesar, Johnson Matthey); Nafion 211 membrane; H2 0.3 L min-1 and O2 0.4 L min-1 feed; 100% relative humidity (RH); 150 kPa absolute partial pressure H2 and O2; cell temperature, 80 °C; electrode area, 4.41 cm2. The cell voltage and power density are not iR corrected.

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Proximity-engineered Ru single-atom sites modulate Fe-N₄ spatial distortion for enhanced acidic oxygen reduction reaction

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