Orbital electron delocalization of axial-coordinated modified FeN₄ and structurally ordered PtFe intermetallic synergistically for efficient oxygen reduction reaction catalysis

Abstract

Regulating the chemical environment of materials to optimize their electronic structure, leading to the optimal adsorption energies of intermediates, is of paramount importance to improving the performance of electrocatalysts, yet remains an immense challenge. Herein, we design a harmonious axial-coordination Pt_xFe/FeN₄CCl catalyst that integrates a structurally ordered PtFe intermetallic with an orbital electron-delocalization FeN₄CCl support for synergistically efficient oxygen reduction catalysis. The obtained Pt₂Fe/FeN₄CCl with a favorable atomic arrangement and surface composition exhibits enhanced oxygen reduction reaction (ORR) intrinsic activity and durability, achieving a mass activity (MA) and specific activity (SA) of 1.637 A mg_Pt⁻¹ and 2.270 mA cm⁻², respectively. Detailed X-ray absorption fine spectroscopy (XAFS) further confirms the axial-coupling effect of the FeN₄CCl substrate by configuring the Fe–N bond to ∼1.92 Å and the Fe–Cl bond to ∼2.06 Å. Additionally, Fourier transforms of the extended X-ray absorption fine structure (FT-EXAFS) demonstrate relatively prominent peaks at ∼1.5 Å, ascribed to the contribution of the Fe–N/Fe–Cl, further indicating the construction of the FeN₄CCl moiety structure. More importantly, the electron localization function (ELF) and density functional theory (DFT) further determine an orbital electron delocalization effect due to the strong axial traction between the Cl atoms and FeN₄, resulting in electron redistribution and modification of the coordination surroundings, thus optimizing the adsorption free energy of OH_abs intermediates and effectively accelerating the ORR catalytic kinetic process.

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1. Introduction

Proton exchange membrane fuel cells (PEMFCs) as a potential clean energy-efficient conversion technique have gained extensive attention owing to their characteristics of being environmentally friendly and high-efficiency as power sources for a variety of transportation systems, especially mid-size and heavy-duty vehicles and light railway transits.^1,2 For large-scale applications, a key developmental target for these PEMFCs is to reduce the extensive use of precious platinum in the Pt-based nanocatalysts.³ At present, the consumption of Pt in fuel cell stacks has decreased to 0.3–0.4 g_Pt kW⁻¹, but there is still a considerable gap from the requirements of fuel cell vehicle industrialization (<0.1 g_Pt kW⁻¹).^4,5 To fulfil this purpose, researchers have focused on the following: (1) developing a catalyst with optimized coordination circumstances to modify the electron structure of Pt and the adsorption energy of intermediates to accelerate the oxygen reduction reaction (ORR) kinetics; (2) designing a catalyst with a structurally ordered phase for eliminating the dissolution of less-noble metals at the atomic scale observed in the disordered counterpart; (3) conceiving a highly stable support to enhance the bond length to resist the corrosion of the carrier and ensure accessibility of sites to maximize Pt utilization.^6–8

For simultaneously realizing the enhancement of the mass activities and stability in fuel cells, great efforts towards the development of such advanced Pt-based catalysts have been dedicated.^9,10 Among them, the carbon-supported Pt-based alloy materials have gained tremendous attention because of the rapid ORR kinetics in acidic media, but they are not yet durable enough for practical commercial applications owing to the carbon corrosion during potential cycling.^11,12 Supported structurally ordered Pt-based electrocatalysts with a determined atomic arrangement and a surface composition that consists of a transition metal atomically dispersed in a nitrogen-coordinated carbon (M–N–C, M = Fe, Co, Ni, Mn, etc.) support and structurally ordered phase have emerged as promising candidates with improved durability and mass activity towards the ORR.^13,14 Unfortunately, the N atom in M–N–C is attacked by protons generated during the reduction reaction (M–N + H₂O → M(OH)₂ + (NH)₂), leading to the breaking of the M–N bond and the continuous dissolution of the metal from the single atom active center of M, seriously affecting the activity of the catalyst.^15,16 Hence, considerable effort has been devoted to addressing the above problems, with efforts concentrated, in part, on retouching the coordination environment and the chemical composition of M–N–C.^17,18 Previous DFT calculations have proven that the configuration coordination environment of M–N–C can determine the electronic structure of Pt and M, which remarkedly affects the adsorption free energy of the oxygen species (O_2,abs, H₂O_2,abs, OH_abs and OOH_abs) on the Pt center and thus gives rise to variations in the ORR intrinsic activity.^19,20 Thus, in this direction, a catalyst with an optimized coordination state for active Pt sites and a heterogeneous local environment for the M–N–C must be rationally designed and engineered.

Herein, we successfully synthesize a concordant Pt_xFe/FeN₄CCl catalyst that combines the axial-coordinated modified FeN₄ of an orbital electron delocalization effect with structurally ordered PtFe intermetallic for synergistically highly efficient ORR. The unique structure of the catalyst was fabricated by a pyrolysis reduction strategy at high-temperature conditions (Scheme S1†), in which Fe in the hemin precursor alloys with Pt atoms and forms axial-coordinated modified Fe single-atoms. X-ray adsorption fine spectroscopy (XAFS) analyses reveal that a Fe atom coordinates with one axial Cl atom and four N atoms, effectively authenticating the structure of the three-dimensional FeN₄CCl architectures. The orbital electron delocalization of chlorine-coordinated modified FeN₄ in the FeN₄CCl support was confirmed by configuring the Fe–N bond for ∼1.92 Å and Fe–Cl bond for ∼2.06 Å. Additionally, DFT calculations proclaimed that the PtFe/FeN₄CCl shows the lowest adsorption free energy of OH_abs species (E_OHabs = 2.51 eV) relative to FeN₄C (E_OHabs = 3.07 eV), FeN₄CCl (E_OHabs = 2.75 eV), and PtFe (E_OHabs = 3.25 eV). This is attributed to the electron redistribution and modification of the coordination surroundings, resulting from a strong axial traction effect between the Cl atoms and FeN₄. Benefiting from the chlorine-coordinated modified FeN₄CCl support and the synergistic catalysis with the structurally ordered PtFe intermetallic, the obtained Pt₂Fe/FeN₄CCl catalyst with optimum surface composition exhibits a higher MA of 1.637 A mg_Pt⁻¹ and SA of 2.270 mA cm⁻², respectively, surpassing the commercial Pt/C catalyst (0.165 A mg_Pt⁻¹ and 0.264 mA cm⁻²). Therefore, our work gives significant insight into designing high-performance electrocatalysts with synergistic catalysis by combining tailored axial-coordinated modified non-precious metal active sites with a structurally ordered Pt-based intermetallic.

2. Results and discussion

2.1 Morphology and structure characterization

The structures and crystallinity properties of the Pt_xFe/FeN₄CCl samples and commercial Pt/C were characterized by powder X-ray diffraction (PXRD), where X represents the weight ratio of Pt to contained Fe in the hemin precursor (X = 1, 2, and 3, respectively; more detailed information is given in the ESI†). All the characteristic diffraction peaks of the Pt_xFe/FeN₄CCl sample exhibit a slight positive shift to higher angles compared with the commercial Pt/C catalyst (PDF#04-0802), suggesting that the alloy phase has been attained by the introduction of smaller Fe atoms (Fig. 1a).²¹ Importantly, the ordered characteristic peaks at 2θ of 23.93° for the (001) plane, 33.90° for the (110) plane, and 41.07° for the (111) plane were further observed in the Pt_xFe/FeN₄CCl sample, indicating the formation of structurally ordered PtFe intermetallic (PDF#43-1359).^22,23 In addition, the ordered degree of all as-prepared catalysts was determined by calculating the peak intensity ratio of the characteristic planes (110) and (111), which demonstrated that Pt₂Fe/FeN₄CCl (0.275) shows a higher ordered degree than Pt₁Fe/FeN₄CCl (0.233) and Pt₃Fe/FeN₄CCl (0.266) (Table S1†).^24,25 The Pt₂Fe/FeN₄CCl catalyst with a higher ordered degree and favorable surface composition was synthesized by precisely manipulating the feeding weight ratio of Pt to the contained Fe²⁺ precursor. The shape and structure of the as-prepared Pt₂Fe/FeN₄CCl materials were further investigated by transmission electron microscopy (TEM), aberration-corrected high-angle annular dark field scanning transmission electron microscopy (AC-HAADF-STEM), and X-ray energy dispersive spectroscopy (EDS). The overall TEM images of the Pt₂Fe/FN₄CCl catalysts and their corresponding particle diameter histograms are presented in Fig. 1b and c, exhibiting nanoparticles uniformly distributed throughout the whole support, with an average edge length of 3.38 ± 0.2 nm. AC-HAADF-STEM analyses were performed to further characterize the catalyst structure and surface composition. As displayed in Fig. 1d, we can observe that abundant bright isolated dots are regularly scattered in the modified graphitic carbon matrix, which are attributed to Fe single atoms, indicating that Fe single sites co-coordinated with chlorine-nitrogen and embedded in carbon (Fe–N–C–Cl) were formed. Besides, as presented in Fig. 1e–h, the high-resolution TEM images of the Pt₂Fe/FeN₄CCl sample reveal the as-prepared catalyst nanoparticle composition of an ordered PtFe alloy phase and an Fe single atom substrate, further verifying that a multiple-structure catalyst has been triumphantly synthesized. The corresponding fast Fourier transform (FFT) further measures the lattice fringes with d-spacings of 0.183 nm for the characteristic planes of the (110) facet and 0.227 nm for the (111) facet, which is consistent with the XRD results. Meanwhile, the corresponding line scan profile analysis of the area of line 1 and line 2 displayed the formation of ordered PtFe intermetallic via the incorporation of smaller Fe atoms into the Pt crystal lattice.²⁶ Moreover, as shown in Fig. 1i, EDS element mapping images of Pt₂Fe/FeN₄CCl indicate that Pt atoms are primarily centralized on the whole ordered PtFe intermetallic. However, some Fe atoms manufacture the ordered intermetallic and others are allocated to generate single-atoms, demonstrating that the synergistic catalyst consists of an ordered PtFe intermetallic and Fe single-atom support. Furthermore, the atomic ratio of Pt/Fe is estimated as 32/68, confirmed by electron energy loss spectroscopy (EELS) (Fig. 1j), the EDS spectrum (Fig. 1k), and inductively coupled plasma optical emission spectrometry (ICP-OES) analysis (Table S2†). Simultaneously, the element of Cl was also detected in the Pt₂Fe/FeN₄CCl catalyst, indicating that the Cl atoms of the hemin precursor have been successfully transformed into the axial-ligand of the coordinated modified FeN₄CCl substrate.

Fig. 1 (a) XRD patterns of the commercial Pt/C, Pt₁Fe/FeN₄CCl, Pt₂Fe/FeN₄CCl, and Pt₃Fe/FeN₄CCl samples. (b and c) TEM and the corresponding particle size distribution images of Pt₂Fe/FeN₄CCl. (d and e) AC-HAADF-TEM images of the Pt₂Fe/FeN₄CCl sample and a single particle. (f and g) HR-TEM and the corresponding FFT images in the Pt₂Fe/FeN₄CCl sample and (h) the corresponding line scan profiles marked in g, corresponding with the area of line 1 (top) showing only Pt atoms as well as line 2 between Pt and Fe atoms (bottom). (i) HAADF-STEM images of the Pt₂Fe/FeN₄CCl sample and corresponding elemental mapping image. (j) EELS profile. (k) EDS spectrum of the corresponding element mapping.

X-ray adsorption fine spectroscopy (XAFS) analysis was carried out to further analyze the local coordination environment and electronic structure at the atomic level. The Pt L₃-edge X-ray absorption near-edge structure (XANES) spectra of Pt₂Fe/FeN₄CCl with Pt foil and PtO₂ as references are shown in Fig. 2a.²⁷ The XANES of the white line intensity (∼11 569 eV), edge energy, and shapes in the Pt₂Fe/FeN₄CCl catalyst are similar to those of Pt foil, suggesting that the chemical states are approximately zero-valence. Previous studies have proven that a higher content of Pt (0) in the alloy catalysts is beneficial to improving the ORR catalytic kinetics.²⁸ Additionally, the Fourier transforms of the extended X-ray absorption fine structure (FT-EXAFS) spectra (Fig. 2b) demonstrate that the relatively prominent peak at ∼1.5 Å should be assigned to Pt–N bonds, declaring the strong metal-support interaction.^29,30 Moreover, due to heteroatomic interactions in Pt–Fe alloying, a shorter radial distance in comparison to that of Pt foil is ascribed to the contribution of the Pt–Pt/Pt–Fe bond at ∼2.63 Å, further indicating the construction of a PtFe intermetallic interface.³¹ Compared with the R, k and q space diagrams of Pt foil and PtO₂ (Fig. S1†), the fitting information presented in Fig. 2c for Pt₂Fe/FeN₄CCl further supports that the Pt–Pt bond length is shortened to induce a compressive effect due to the PtFe alloy formation. The scattering peak at ∼1.5 Å is from the contribution of Pt–N bonds, suggesting the strong metal-support interaction between the PtFe intermetallic and the FeN₄CCl support. In addition, the fitting results of the EXAFS curve also reveal that the coordination number is approximately 6.25 for the Pt–Pt shell, approximately 4.14 for the Pt–Fe shell and approximately 1.46 for the Pt–N shell. The optimized coordination environment and compressive strains described above enhance the catalytic activity of the ORR (Fig. S2 and Table S3†). Wavelet transform EXAFS spectroscopy could afford resolution in both radial distance and k-space. Incidentally, the scattering peak at ∼2.05 Å is more likely assigned to the Ramsauer–Townsend effect, which is caused by the quantum effect rather than the superposition of the sine function because of the existence of heavy-metal elements. The Pt L₃-edge WT contour plots in the Pt₂Fe/FeN₄CCl catalyst show that the intensity maximum color with Pt–Pt/Pt–Fe bonds at ∼9.5 Å⁻¹ is shifted upwards compared to Pt foil and PtO₂ (Fig. 2d and S3†).^32,33 Meanwhile, the XANES characterizations of the Pt₂Fe/FeN₄CCl catalyst and Fe foil, FeO, and phthalocyanine (FePc) are shown in Fig. 2e. The white line intensity of the Fe K-edge in Pt₂Fe/FeN₄CCl (∼7122 eV) is located among those of Fe foil and FePc, indicating that the chemical state of Fe species in the Pt₂Fe/FeN₄CCl samples is probably concentrated on the metal Fe and oxidated Fe. Moreover, FT-EXAFS at the Fe K-edge presents primary peaks at ∼1.5 Å, ∼2.09 Å and ∼2.72 Å, which are reasonably in line with the scattering paths of Fe–N/Fe–Cl,^34,35 Fe–Fe,³⁶ and Fe–Pt,³⁷ respectively (Fig. 2f). It is worth noting that the scattering paths Fe–N at ∼1.5 Å probably offer evidence for the existence of the square-planar Fe–N₄ configuration with a porphyrin-like structure.^38,39 As a verification, the coordination configuration of the Fe atoms was analyzed by quantitatively fitting the EXAFS spectra. As shown in Fig. 2g, the fitted structural parameters suggest that a Fe atom is coordinated with one axial Cl atom at ∼2.06 Å and four N atoms at ∼1.92 Å, individually (Fig. S4 and Table S4†), substantiating that the three-dimensional FeN₄CCl architecture was attained (Fig. S5†). The fitting scattering path information benchmarked against Fe foil, FeO and FePc is plotted in Fig. S6.† The Fe K-edge WT contour plots of the Pt₂Fe/FeN₄CCl catalyst signify that the intensity maximum is ∼4 Å⁻¹ for Fe–N/Fe–Cl and ∼6.1 Å⁻¹ for FeFe/Fe–Pt and similar to the wavelet-transform contour plots of Fe foil, FeO and FePc, respectively (Fig. 2h and S7†).^40,41 These combined results further confirm the successful construction of FeN₄CCl moieties, the optimized axial-coordinated effect and the suitable electronic interface for intermediate adsorption and desorption.

Fig. 2 (a–c) XAFS diagram of Pt L₃-edge XANES for Pt foil, PtO₂ and Pt₂Fe/FeN₄CCl (a), the Fourier transforms of EXAFS spectra for Pt foil, PtO₂ and Pt₂Fe/FeN₄CCl (b), the EXAFS fitting curve Pt L₃-edge of Pt₂Fe/FeN₄CCl catalyst at R-space (c) and wavelet transform of the Pt L₃-edge of corresponding EXAFS spectra for Pt₂Fe/FeN₄CCl catalyst (d). (e–h) XAFS diagram of Fe K-edge XANES for Fe foil, FeO, FePc and Pt₂Fe/FeN₄CCl (e), the Fourier transforms of EXAFS spectra for Fe foil, FeO, FePc and Pt₂Fe/FeN₄CCl (f), the EXAFS fitting curve Fe K-edge of Pt₂Fe/FeN₄CCl catalyst at R-space (g) and wavelet transform of the Fe K-edge of corresponding EXAFS spectra for Pt₂Fe/FeN₄CCl catalyst (h). (i–l) XPS spectra of N 1s (i), Pt 4f (j), Fe 2p (k) and Cl 2p (l).

X-ray photoelectron spectroscopy (XPS) further examined the surface chemical composition and electronic valence state of the as-prepared catalyst. The presence of Pt, Fe, Cl, N, C, and O elements was confirmed by the XPS survey of the Pt_xFe/FeN₄CCl sample (Fig. S8 and Table S5†), which is consistent with the EDS profiles. The fitted peak of C 1s contains C–N peaks in contrast with the commercial Pt/C catalyst, indicating presumable nitrogen-doping in the as-prepared catalyst support (Fig. S9 and Table S6†). Notably, the fitted peaks of the characteristic N 1s spectra contain Fe–N_x peaks, further identifying the occurrence of FeN₄ sites, which matched with the XAFS analysis results (Fig. 2i and Table S7†).⁴² Based on the peak fitting of Pt 4f (Fig. 2j, S10a and Table S8†), the Pt (0) mainly concentrates on the chemical state of the metal in commercial Pt/C, Pt₁Fe/FeN₄CCl, Pt₂Fe/FeN₄CCl, Pt₂Fe/FeN₄CCl and ordered PtFe NP catalysts, benefiting the acceleration of the entire ORR cycle.⁴³ Importantly, compared with the standard Pt 4f of Pt/C, the electron density of Pt (positive shift +0.12 eV for Pt₁Fe/FeN₄CCl, +0.19 eV for Pt₂Fe/FeN₄CCl and +0.08 eV for Pt₃Fe/FeN₄CCl, and negative shift −0.11 eV for ordered PtFe NPs, respectively) indicates the strong interactions between PtFe and FeN₄CCl.⁴⁴ However, the Pt₂Fe/FeN₄CCl catalyst was modified with the FeN₄CCl support, which drew plenty of electrons due to the strong electronegativity of the Cl ligands (3.16), thus resulting in a positive binding energy shift. Furthermore, a negative binding energy shift was exhibited for Fe 2p on Pt₂Fe/FeN₄CCl (−0.26 eV) as compared to the FeN₄C SACs at 707.03 eV, demonstrating that the axial-coordinated traction effect of the Cl ligands regulates the electron structure (Fig. 2k, S10b and Table S9†). Meanwhile, the Fe 2p of ordered PtFe NPs (0.14 eV) suggests a positive binding energy shift compared with Pt₂Fe/FeN₄CCl at 706.77 eV, in line with the Pt 4f results. Combined with the Cl 2p characteristic spectrum, the Cl species mainly exist in the form of Fe–Cl coordination (Fig. 2l and Table S10†), suggesting that the Cl atoms are axial-coordinated with the FeN₄ site, benefiting the adsorption/desorption conversion process.

2.2 ORR performance of the Pt_xFe/FeN₄CCl system

The primary ORR activity of the FeN₄C SACs, Pt₁Fe/FeN₄CCl, Pt₂Fe/FeN₄CCl, Pt₃Fe/FeN₄CCl, ordered PtFe NPs and commercial Pt/C catalysts was investigated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) in N₂⁻ and O₂⁻ saturated HClO₄ solutions, respectively. As shown in Fig. S11a and b†, the CV curves show that the hydrogen adsorption/desorption peaks appear in the region of 0–0.4 V and the Pt oxidation/reduction peaks appear at 0.6–1.2 V, respectively. Moreover, as shown in Fig. 3a and S11c,† the Pt₂Fe/FeN₄CCl shows a higher ORR activity with a half-wave potential (E_1/2) of 0.936 V vs. reversible hydrogen electrode (RHE) relative to the FeN₄C SACs (0.851 V), Pt₁Fe/FeN₄CCl (0.909 V), Pt₃Fe/FeN₄CCl (0.920 V), ordered PtFe NPs (0.903 V) and commercial Pt/C (0.876 V) catalysts. The Tafel slope as a significant kinetic parameter is displayed in Fig. 3b and S11d.† The value of the Tafel slope for the Pt₂Fe/FeN₄CCl (57.75 mV dec⁻¹) sample is lower than that of the FeN₄C SACs (117.24 mV dec⁻¹), Pt₁Fe/FeN₄CCl (65.11 mV dec⁻¹), Pt₃Fe/FeN₄CCl (70.53 mV dec⁻¹), ordered PtFe NPs (72.35.35 mV dec⁻¹) and commercial Pt/C (77.59 mV dec⁻¹) counterparts, indicating the higher ORR kinetics of the Pt₂Fe/FeN₄CCl one. The electrochemical surface area (ECSA) was measured to further evaluate the ORR intrinsic activity by CO-stripping experiments, instead of hydrogen underpotential deposition (HUPD), considering the suppression of H_upd adsorption on Pt–M alloy catalysts.⁴⁵ The mass activity (MA) and specific activity (SA) at 0.9 V vs. RHE of all the catalysts were calculated by normalizing the Pt loading and ECSA (Table S11†). As shown in Fig. 3c, d and S11e,† the MA/SA of Pt₂Fe/FeN₄CCl was 1.637 A mg_Pt⁻¹/2.270 mA cm⁻², which is about 2.71/2.53, 2.22/2.14, 3.13/2.66 and 9.92/8.60 times higher than that of Pt₁Fe/FeN₄CCl (0.605 A mg_Pt⁻¹/0.899 mA cm⁻²), Pt₃Fe/FeN₄CCl (0.738 A mg_Pt⁻¹/1.061 mA cm⁻²), ordered PtFe NPs (0.523 A mg_Pt⁻¹/0.852 mA cm⁻²) and commercial Pt/C (0.165 A mg_Pt⁻¹/0.264 mA cm⁻²), respectively. For the FeN₄C SACs, the ORR activity was further evaluated by the half-wave potential and the kinetic current density (j_k), indicating that all of the as-prepared Pt-based catalysts exhibited superior ORR performance (Fig. S11f†). In addition, Fig. 3e and S12† show that the average electron transfer number (n) was calculated as 3.89 for Pt₂Fe/FeN₄CCl, 3.85 for Pt₁Fe/FeN₄CCl, 3.83 for Pt₃Fe/FeN₄CCl and 3.71 for commercial Pt/C by the Koutecky–Levich (K–L) equation, demonstrating the unabridged four-electron (4e⁻) ORR pathway with the reduction of O₂ to H₂O directly.⁴⁶ The stability of the as-prepared catalysts and commercial Pt/C for the ORR was investigated via an accelerated durability test (ADT) in O₂⁻saturated 0.1 M HClO₄ solution. Fig. S13† displays a larger negative shift (49 mV) of E_1/2 for commercial Pt/C before and after 30 000 potential cycles. Further, the calculated MA and SA of commercial Pt/C seriously decreased by 38.25% and 57.94% relative to the original values, respectively. In contrast, the as-prepared Pt_xFe/FeN₄CCl catalyst exhibited enhanced ORR performance (Table S12†). The Pt₂Fe/FeN₄CCl sample acquired more favourable retention of catalytic activity (only 16 mV negative shift of E_1/2) after 40 000 sweeping cycles (Fig. 3f) and afforded a slight loss of 7.80% in MA and 8.82% in SA (Fig. 3g and h). Meanwhile, the LSV curves of the as-prepared Pt₁Fe/FeN₄CCl and the Pt₃Fe/FeN₄CCl catalyst exhibit 21 mV and 24 mV negative shift of E_1/2 before and after 40 000 cycles (Fig. S14†), along with exiguous drops of 12.50% and 17.61% for Pt₁Fe/FeN₄CCl as well as 15.41% and 19.30% for Pt₃Fe/FeN₄CCl in MA and SA, respectively. Moreover, the ORR performance of the as-prepared Pt₂Fe/FeN₄CCl catalyst with optimum surface composition is superior to the reported literature (Table S13†).

Fig. 3 (a) LSV curves of Pt₁Fe/FeN₄CCl, Pt₂Fe/FeN₄CCl, Pt₃Fe/FeN₄CCl, and commercial Pt/C catalysts in O₂-saturated 0.1 M HClO₄ solution, with sweep rate 10 mV s⁻¹, and rotation rate 1600 rpm. (b) Tafel plots of Pt₁Fe/FeN₄CCl, Pt₂Fe/FeN₄CCl, Pt₃Fe/FeN₄CCl, and commercial Pt/C catalysts. (c and d) MA and SA of Pt₁Fe/FeN₄CCl, Pt₂Fe/FeN₄CCl, Pt₃Fe/FeN₄CCl, and commercial Pt/C catalysts. (e) ORR polarization curves of Pt₂Fe/FeN₄CCl catalysts at various rotation rates and Koutecky–Levich plots at various electrode potentials. (f) ORR polarization curves of the Pt₂Fe/FeN₄CCl catalysts before and after 40 000 sweeping cycles (the inset shows the CV curves of Pt₂Fe/FeN₄CCl). (g and h) The changes of MA and SA of Pt₁Fe/FeN₄CCl, Pt₂Fe/FeN₄CCl, Pt₃Fe/FeN₄CCl, and commercial Pt/C catalysts before and after ADT in different cycles. (i) CO stripping voltammetry in 0.1 M HClO₄ electrolyte of the Pt₂Fe/FeN₄CCl catalyst compared with commercial Pt/C.

To further verify the morphology after ADT, CO stripping experiments of Pt₁Fe/FeN₄CCl, Pt₂Fe/FeN₄CCl, Pt₃Fe/FeN₄CCl and commercial Pt/C are shown in Fig. 3i and S15.† The onset potential of the CO oxidation peak of all as-prepared catalysts is much lower than that of commercial Pt/C and fractionally deviated before and after 40 000 CV cycles, indicating the maintenance of the electronic structure and surface composition.⁴⁷ Such results demonstrate that the enhanced ORR activity and stability of the as-prepared Pt_xFe/FeN₄CCl catalyst was attributed to the construction of an FeN₄CCl moiety with optimized orbital electron-delocalization, the formation of an ordered phase with a definite atomic distribution and surface composition, and strong interaction between the PtFe intermetallic and FeN₄CCl substrate.

2.3 Density functional theory computational analysis

Density functional theory (DFT) was carried out to clarify the influence of the PtFe/FeN₄CCl structure on the ORR catalytic process. The constructed structural models (after optimization) of FeN₄C, FeN₄CCl, PtFe and PtFe/FeN₄CCl, based on the.

AC-HAADF-TEM images, XAFS and the corresponding fitting results, and XPS signal analysis, are shown in Fig. S16.†Fig. 4a and S17† exhibit the two-dimensional electron localization function (2D ELF) of the FeN₄ part in FeN₄C, FeN₄CCl and PtFe/FeN₄CCl, as well as the PtFe part in PtFe/FeN₄CCl, in which the FeN₄ part possesses a symmetrical electron localization distribution. The bright region on the Fe site of FeN₄CCl and PtFe/FeN₄CCl becomes darker compared to FeN₄C, testifying that the orbital electron delocalization effect exists induced by the axial-coordinated traction of Cl atoms.^48,49 In comparison, the overall color of Pt atoms is brighter compared to the Fe atoms in the PtFe part, which is caused by the significant charge polarization by the electron delocalization degree of both Pt and Fe atoms with different electronegativity.⁵⁰ A similar conclusion is drawn from the differential spin density distribution (Fig. S18†). Fig. 4b displays the charge density difference (CDD) of FeN₄C, FeN₄CCl, FeN₄CCl–PtFe, and PtFe–FeN₄CCl, in which the metal site evidently confirms an inclination to bereft electrons. There appear extremely palpable charge separation/transfer effects around the PtFe intermetallic and the FeN₄CCl substrate, exemplifying its interaction with the supports and accordingly benefiting the first adsorption of O₂ in conjunction with the following four-electron step process. More precisely, compared with FeN₄C and Fe₄CCl, the FeN₄CCl substrate in the PtFe/FeN₄CCl catalyst can absorb a significant number of electrons from the PtFe intermetallic and form a remarkable positive charge area due to the occurrence of the axial-coordinated Cl atoms. Previous research studies have manifested that electron deficient (or positively charged) metal sites are instrumental in attenuating the adsorption free energy of OH_abs intermediates.⁵¹ Bader charge quantifiable analysis in Fig. 4c and S19† further confirms this viewpoint. When a Cl atom axially coordinated the modified FeN₄ site, the Fe center electron transfer of both PtFe/FeN₄CCl (1.18|e|) and FeN₄CCl (1.16|e|) is clearly more than in FeN₄C (1.11|e|). Consistently, while the ordered PtFe nanoparticles embedded into the atomically dispersed chlorine-nitrogen-doped Fe single-atom support, the Pt center in the PtFe/FeN₄CCl catalyst showed charge accumulation less than that of PtFe without a modified Cl atom. These numerical results attest that the axial-coordinated Cl atoms can regulate the electronic structure of the active site, in line with the observation of the above CDD data.^52,53 The effect of axial-coordinated Cl atoms on the electron distribution was further investigated by partial density of states (PDOS) calculations. As shown in Fig. 4d, e and S20,† the d-band center of Fe-3d in FeN₄CCl and PtFe/FeN₄CCl (−0.95 eV and −1.41 eV) shifts down in comparison to FeN₄C (−0.89 eV). The PtFe/FeN₄CCl (−2.47 eV) exhibits a downshifted d-band center compared with PtFe (−2.33 eV). It is evidently demonstrated that the axial-coordinated Cl atoms arouse Fe center electron transfer, inducing the occupancy of orbital electron delocalization,^54,55 which is consistent with the calculated magnetic moment (Table S14†) and spin density distribution.

Fig. 4 (a) ELF of the center Fe site (the top shows FeN₄C, FeN₄CCl, and PtFe/FeN₄CCl) and the center Pt site (the bottom shows PtFe and PtFe/FeN₄CCl). Scale bars, 0–1. (b) Top and side views of the 3D charge density differences for FeN₄C, FeN₄CCl, FeN₄CCl ↔ PtFe and PtF ↔ eFeN₄CCl, respectively. (c) The value of transferred Bader charges of FeN₄C, FeN₄CCl, PtFe, and PtFe/FeN₄CCl at the metal site (Fe and Pt) and the surrounding atoms in the corresponding structural models. (d) PDOS of Fe-3d in FeN₄C and FeN₄CCl. (e) PDOS of the Fe and Pt sites in PtFe/FeN₄CCl. (f and g) After the PtFe/FeN₄CCl system interacts with O₂, the DOS diagram of the adsorption metal site (Fe and Pt) with O1 and O2, respectively. (h) (top) Free energy diagrams of different ORR intermediates at 1.23 V and (bottom) ORR catalytic process involved in the H₂O₂ intermediate over the PtFe/FeN₄CCl–Pt site and PtFe–Pt site as well as the PtFe/FeN₄CCl–Fe site, FeN₄C, and FeN₄CCl. (i) Scheme of the pathway in the ORR cycle of PtFe/FeN₄CCl based on DFT.

To further verify the synergistic ORR catalysis mechanism, we calculated the density of states (DOS) of the active site and O₂ molecule to perceive the activation mechanism of O_2,abs. According to molecular orbital theory, the DOS of both O1 and O2 atoms in the O₂ molecule is entirely symmetric (Fig. S21†).⁵⁶ When an O₂ molecule adsorbed on the Fe sites in PtFe/FeN₄CCl (Fig. 4f, S22a and b†), FeN₄C (Fig. S23a–c†), and FeN₄CCl (Fig. S23d–f†), the DOS shows significantly discrete Fe 3d and O 2p orbital states. In contrast, when an O₂ molecule adsorbs on the Pt sites in both PtFe/FeN₄CCl (Fig. 4g, S22c and d†) and PtFe (Fig. S23g–i†), the DOS shows a symmetric arrangement in the spin channels (spin-up and spin-down). Meanwhile, the O 2p orbital splits into discrete levels and the intensity of the localized Pt 3d states is significantly reduced, suggesting that the O₂ molecules are better activated.⁵⁷ Utilizing the free energy step diagram, the potential-determining-step (PDS) from OH_abs to H₂O in the ORR transversion on PtFe/FeN₄CCl is more likely to occur than in FeN₄C, FeN₄CCl and PtFe (Fig. 4h and S24†). When the voltage is 1.23 V, the PDS of the PtFe/FeN₄CCl–Pt site is the desorption of OH_abs (OH_abs + H⁺ + e⁻ → H₂O_abs). Obviously, the PtFe/FeN₄CCl–Pt site has a smaller Gibbs free energy change (ΔG_pds) (0.21 eV) relative to the FeN₄C (0.75 eV), FeN₄CCl (0.37 eV), PtFe (0.88 eV), and PtFe/FeN₄CCl–Fe site (0.71 eV), indicating that the synergistic effect of the PtFe intermetallic and FeN₄CCl support enhances the catalytic efficiency and accelerates the ORR cycle. In addition, DFT calculations were carried out to clarify the potential determining step (PDS) of the free energy step diagram for the ORR catalytic process based on electrochemical tests.⁵⁸ The DFT calculation results showed that the HOOH intermediates at Fe sites can release and migrate to the contiguous Pt sites for successive reactions, thereby effectuating integral oxygen reduction (Fig. 4i and S25†).^59,60 Such a result proved that the synergistic effect between the PtFe intermetallic and FeN₄CCl sites in the ORR process actualizes the four-electron transfer pathway.

To trace back the reason for the higher activity of PtFe/FeN₄CCl, the interaction between the catalytic site and OH_abs intermediate species was investigated. The bond length of Fe–OH in optimized models of the FeN₄C, FeN₄CCl, and PtFe/FeN₄CCl–Fe sites is L_Fe–OH = 1.82 Å, L_Fe–OH = 1.86 Å, and L_Fe–OH = 1.89 Å, individually. Identically, the bond length of Pt–OH of the PtFe and PtFe/FeN₄CCl–Pt site is L_Pt–OH = 1.96 Å and L_Pt–OH = 2.27 Å, respectively, which reveals that activation of the O₂ molecule is easier to implement on the PtFe/FeN₄CCl–Pt active sites (Fig. 5a–c). Synchronously, the adsorption free energy (E_abs) of OH_abs species on the Fe sites of FeN₄C, Fe sites of FeN₄CCl, Fe sites of PtFe/FeN₄CCl–Fe, Pt sites of PtFe, and Pt sites of PtFe/FeN₄CCl–Pt are E_abs = −3.07 eV, E_abs = −2.75 eV, E_abs = −3.12 eV, E_abs = −3.25 eV, and E_abs = −2.51 eV, respectively. Due to the axial-coordinated Cl atoms pulling more electrons to deviate from the central metal atoms, the electronic circumstances were directly revised by orbital electron delocalization, which weakens the binding strength between the catalyst surface and the adsorbates. As shown in Fig. 5d, the interaction between the Fe 3d_z² orbitals and Cl 3p_z orbitals contributes to strong d_z²–p_z hybridization. The electron delocalization between the 3d_z² orbital in the FeN₄ moiety and the 2p orbitals of the oxygen intermediates could tune the energy levels, splitting into bonding orbitals and antibonding states. The d_z²-state energy level of the active site modified with axial-coordinated Cl atoms commonly down-shifts from the Fermi level.⁶¹ Additionally, according to d-band theory, the alteration of the d-band center could affect its absorption capability of oxygen-containing intermediates.^62,63 Pt is alloyed with a non-precious Fe element, which can exhibit a lattice contraction and a surface strain effect compared with pure Pt (Fig. 5c). When oxygen-containing intermediates hybridize with a broader d band, the adsorbate state splits into a localized bonding orbital and antibonding orbital.⁶⁴ The more electron occupancies of the antibonding orbitals could downgrade the energy level of the d-band center, thereby weakening the active site adsorption energy. Besides, the PtFe intermetallic emanates the structurally ordered atomic arrangement and uniquely local geometrical properties, thus tremendously heightening higher ORR stability and activity.

Fig. 5 (a–c) The bond lengths and corresponding adsorption free energy with an oxygen intermediate on active sites (Fe and Pt). (d) The axial-coordinated Cl atoms lead to an orbital electron delocalization effect of the Fe 3d orbitals. (e) The schematic diagram of alloying Pt with a non-precious Fe element, which can exhibit a lattice contraction compared with pure Pt. The alteration of the d-band center could affect its absorption capability of oxygen-containing intermediates.

3. Conclusions

In summary, a synergistic ORR catalyst with an orbital electron delocalization axial-coordinated effect was successfully synthesized by utilizing hemin as a precursor in high-temperature pyrolysis. The unique structure of the as-prepared catalyst, such as the ordered atomic arrangement and FeN₄CCl support with atomically dispersed Fe single-atoms, was confirmed by the AC-HAADF-STEM images. XAFS analysis further demonstrates the existence of the FeN₄Cl moiety with an Fe single-atom structure and the axial chlorine-coordinated coupling induction of the FeN₄CCl substrate by configuring the Fe–N bond for ∼1.92 Å and Fe–Cl bond for ∼2.06 Å. More importantly, ELF and DFT further determine an orbital electron delocalization effect between the Cl atoms and the FeN₄, resulting in electron redistribution and coordinated surroundings modification, thus optimizing the adsorption free energy of the OH_abs intermediates for accelerating the electrocatalytic kinetics. Specifically, the obtained Pt₂Fe/FeN₄CCl catalyst with optimum surface composition displayed a higher catalytic performance, which was 9.92 times in MA, and 8.60 times in SA relative to the commercial Pt/C catalyst, respectively. The LSV loss of ΔE_1/2 = 16 mV was observed after 40 000 cycles before and after ADT, along with affording a marginal loss of 7.80% in MA and 8.82% in SA at a potential of 0.9 V vs. RHE, respectively. The heightened catalytic performance can be attributed not only to the orbital electron delocalization of the axial-coordinated modified FeN₄CCl substrate, but also to the synergistic catalysis with the structurally ordered PtFe intermetallic. Our work manifests the crucial role of regulating the chemical environment of materials to optimize their electronic structure in catalysis and provides an alternative insight into designing synergistically efficient ORR catalysts.

4. Experimental section

Experimental procedures, material characterization, electrochemical measurements, and DFT computational details are provided in the Electronic ESI.†

Data availability

The authors declare that the data are available within the paper and its ESI† file.

Author contributions

Chenzhong Wu: conceptualization, investigation, writing – original draft. Meida Chen: data curation, methodology validation. Bin Wang: data curation, visualization. Leqing Luo: data curation. Qian Zhou: data curation. Guangtao Mao: resources, visualization. Yuan Xiong: resources, visualization. Qingmei Wang: supervision, writing – review & editing, funding acquisition.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This research work was financially supported by the National Natural Science Foundation of China (No. 22169005, 22068009 and 22262006), the Science and Technology Support Project of Guizhou Provincial Science and Technology Department (No. ZK[2023]050 and [2023]403), and the Open Project of Institute of Dual-carbon and New Energy Technology Innovation and Development of Guizhou Province (No. DCRE-2023-06).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4sc02824d

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