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. 2022 May 26;13(1):2963.
doi: 10.1038/s41467-022-30702-z.

Iron atom-cluster interactions increase activity and improve durability in Fe-N-C fuel cells

Xin Wan  1 , Qingtao Liu  1 , Jieyuan Liu  1 , Shiyuan Liu  1 , Xiaofang Liu  1 , Lirong Zheng  2 , Jiaxiang Shang  1 , Ronghai Yu  1 , Jianglan Shui  3
Affiliations

Iron atom-cluster interactions increase activity and improve durability in Fe-N-C fuel cells

Xin Wan et al. Nat Commun. .

Abstract

Simultaneously increasing the activity and stability of the single-atom active sites of M-N-C catalysts is critical but remains a great challenge. Here, we report an Fe-N-C catalyst with nitrogen-coordinated iron clusters and closely surrounding Fe-N4 active sites for oxygen reduction reaction in acidic fuel cells. A strong electronic interaction is built between iron clusters and satellite Fe-N4 due to unblocked electron transfer pathways and very short interacting distances. The iron clusters optimize the adsorption strength of oxygen reduction intermediates on Fe-N4 and also shorten the bond amplitude of Fe-N4 with incoherent vibrations. As a result, both the activity and stability of Fe-N4 sites are increased by about 60% in terms of turnover frequency and demetalation resistance. This work shows the great potential of strong electronic interactions between multiphase metal species for improvements of single-atom catalysts.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Morphology characterization of FeSA/FeAC−2DNPC.
a, b SEM images; c TEM image; d, e HAADF-STEM image with zoom-in image showing an iron cluster (cyan circle) and its satellite iron atoms (red circles); f HAADF-STEM image and corresponding element mappings.
Fig. 2
Fig. 2. Active site structure analysis of FeSA/FeAC−2DNPC.
a, b High-resolution N 1 s (a) and Fe 2p (b) XPS spectra of FeSA−2DNPC, FeSA/FeAC−2DNPC, FeSA/FeNP−2DNPC. c Normalized Fe K-edge XANES spectra of FeSA/FeAC−2DNPC and references of Fe foil, FePc and Fe2O3. d, e k3-weighted Fourier transforms (d) and wavelet transforms (e) of the experimental EXAFS spectra of FeSA/FeAC−2DNPC and references of Fe foil, FePc and Fe2O3. FT-EXAFS fitting curve of FeSA/FeAC−2DNPC is present in (d). Source data are provided as a Source Data file.
Fig. 3
Fig. 3. Half-cell tests and quantitative analysis of active sites.
a ORR polarization curves and (b) H2O2 yields and electron transfer numbers of FeSA−2DNPC, FeSA/FeAC−2DNPC, FeSA/FeNP−2DNPC and Pt/C in O2-saturated 0.5 M H2SO4 (0.1 M HClO4 for Pt/C). c SD and TOF of Fe–N4 sites of the indicated Fe–N–C catalysts. Error bars correspond to the standard deviation of three-time measurements. d–f ORR polarization curves and H2O2 yields (inset) before and after 10,000 potential cycles (0.6–1.0 V vs. RHE) in O2-purged 0.5 M H2SO4. g Raman spectra of the catalysts. h Results of metal leaching experiments. ∆Fe%, relative amount of demetalation; v(∆Fe), demetalation rate. i The changes of kinetic current density (jk) and SD after the CV cycling. Source data are provided as a Source Data file.
Fig. 4
Fig. 4. PEMFC tests.
a–c Polarization and power density curves of the indicated catalysts (b, c) before and after the stability test at a constant voltage of 0.5 V under 1 bar H2–air (a). d Polarization curves of FeSA/FeAC−2DNPC (under 1 bar H2–O2) through 30,000 square-wave cycles between 0.6 and 0.95 V under H2–N2. e Polarization and power density curves of FeSA/FeAC−2DNPC under 1 and 2 bar H2–O2. Test conditions: cathode loading 1.5 mgFe–N–C cm−2 and, anode loading 0.2 mgPt cm−2, Nafion 211 membrane, 5 cm2 electrode, 80 °C, 100% relative humidity (RH), flow rates of 300/600 ml min−1 for H2–air polarization, 100/100 ml min−1 for H2–air stability test, and 300/400 ml min−1 for H2–O2 polarization. f Comparison of mass activity of the high-performing Pt-group-metal free catalysts in PEMFC under 1 bar H2–O2. The references to the data points are supplied in Supplementary Table 8. Source data are provided as a Source Data file.
Fig. 5
Fig. 5. Theoretical analysis of the activity and stability of the hybrid active site.
a Model structure of Fe–N4/Fe4–N6 used for theoretical calculation with a spontaneously formed OH ligand. b Schematic ORR process on the Fe–N4 site of Fe–N4-OH/Fe4–N6. c Free energy diagrams at 1.23 V for ORR over three types of active sites of Fe–N4, Fe–N4/Fe4–N6 and Fe–N4-OH/Fe4–N6. d Fe–N radical distribution function profiles of the Fe–N4 moiety in the models of bare Fe–N4 and Fe–N4/Fe4–N6 at 25 and 80 °C. Wavy arrows are used to indicate the amplitude of Fe–N bond-length fluctuation. e Snapshots of Fe–N4 and Fe–N4/Fe4–N6 obtained from MD simulations at 80 °C. The initial configuration (left) and an intermediate state (right) are provided to show the elongation of the Fe–N bond as marked by the yellow box. Source data are provided as a Source Data file.
See this image and copyright information in PMC

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