DOI:
10.1039/D4SC02824D
(Edge Article)
Chem. Sci., 2024,
15, 12989-13000
Orbital electron delocalization of axial-coordinated modified FeN4 and structurally ordered PtFe intermetallic synergistically for efficient oxygen reduction reaction catalysis†
Received
27th April 2024
, Accepted 7th July 2024
First published on 8th July 2024
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 PtxFe/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 mgPt−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 OHabs intermediates and effectively accelerating the ORR catalytic kinetic process.
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 gPt kW−1, but there is still a considerable gap from the requirements of fuel cell vehicle industrialization (<0.1 gPt 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 (O2,abs, H2O2,abs, OHabs and OOHabs) 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 PtxFe/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 OHabs species (EOHabs = 2.51 eV) relative to FeN4C (EOHabs = 3.07 eV), FeN4CCl (EOHabs = 2.75 eV), and PtFe (EOHabs = 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 mgPt−1 and SA of 2.270 mA cm−2, respectively, surpassing the commercial Pt/C catalyst (0.165 A mgPt−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 PtxFe/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 PtxFe/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 PtxFe/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 PtxFe/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–Nx 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 PtxFe/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 (E1/2) 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 Hupd 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 mgPt−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 mgPt−1/0.899 mA cm−2), Pt3Fe/FeN4CCl (0.738 A mgPt−1/1.061 mA cm−2), ordered PtFe NPs (0.523 A mgPt−1/0.852 mA cm−2) and commercial Pt/C (0.165 A mgPt−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 (jk), 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 E1/2 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 PtxFe/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 E1/2) 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 E1/2 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 PtxFe/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 OHabs 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 O2,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 OHabs 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 OHabs (OHabs + H+ + e− → H2Oabs). Obviously, the PtFe/FeN4CCl–Pt site has a smaller Gibbs free energy change (ΔGpds) (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 OHabs intermediate species was investigated. The bond length of Fe–OH in optimized models of the FeN4C, FeN4CCl, and PtFe/FeN4CCl–Fe sites is LFe–OH = 1.82 Å, LFe–OH = 1.86 Å, and LFe–OH = 1.89 Å, individually. Identically, the bond length of Pt–OH of the PtFe and PtFe/FeN4CCl–Pt site is LPt–OH = 1.96 Å and LPt–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 (Eabs) of OHabs 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 Eabs = −3.07 eV, Eabs = −2.75 eV, Eabs = −3.12 eV, Eabs = −3.25 eV, and Eabs = −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 3dz2 orbitals and Cl 3pz orbitals contributes to strong dz2–pz hybridization. The electron delocalization between the 3dz2 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 dz2-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 OHabs 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 ΔE1/2 = 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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