Tuning Structural and Electronic Configuration of FeN4 via External S for Enhanced Oxygen Reduction Reaction

The Fe–N–C material represents an attractive oxygen reduction reaction electrocatalyst, and the FeN4 moiety has been identified as a very competitive catalytic active site. Fine tuning of the coordination structure of FeN4 has an essential impact on the catalytic performance. Herein, we construct a sulfur‐modified Fe–N–C catalyst with controllable local coordination environment, where the Fe is coordinated with four in‐plane N and an axial external S. The external S atom affects not only the electron distribution but also the spin state of Fe in the FeN4 active site. The appearance of higher valence states and spin states for Fe demonstrates the increase in unpaired electrons. With the above characteristics, the adsorption and desorption of the reactants at FeN4 active sites are optimized, thus promoting the oxygen reduction reaction activity. This work explores the key point in electronic configuration and coordination environment tuning of FeN4 through S doping and provides new insight into the construction of M–N–C‐based oxygen reduction reaction catalysts.

prepared through a polymerization process, and the S atoms were introduced through an ion adsorption process.Through a series of advanced characterizations, the coordination structure of S-modified Fe-N-C is clearly identified.The obtained S-modified Fe-N-C demonstrates superior ORR catalytic property with a high-onset potential of 0.99 V (vs reversible hydrogen electrode, RHE) in alkaline media, surpassing those of pristine Fe-N-C and Pt/C.Density functional theory (DFT) calculations elaborate that the highly electronegative S functionality with strong electron-attracting ability could affect the electronic structure of FeN 4 sites, hence optimizing the interaction between oxygenated reactive intermediates and active sites.This work provides new guidance for optimizing the coordination environment of FeN 4 moiety for further improving the ORR catalytic performance.

Results and Discussion
The construction of S-modified Fe-N-C catalyst involves three main steps (Figure 1): polymerization, ion adsorption, and carbonization.Firstly, m-phenylenediamine and Pluronic P123 are dissolved in water.The P123 with both hydrophilic and hydrophobic chains acts as the structure-directing agent and spontaneously forms uniform micelles. [33]eanwhile, the m-phenylenediamine molecules adsorb on the surface of the micelles through hydrogen bonding.With the addition of formaldehyde, the polymerization between m-phenylenediamine and formaldehyde initiates, resulting in the formation of P123 micelle@ m-phenylenediamine-formaldehyde resin (designated as P123@MPF) particles. [34]A multi-step heating process is employed to make the polymer product thermally stable and prevent structural collapse during calcining.For the ion adsorption step, FeCl 3 and KSCN are selected as the Fe and S sources, respectively.The Fe 3+ cations can be adsorbed on the MPF polymer through coordination interactions with the amino groups, while the SCN − anions are adsorbed as counter ions.The homogeneous distribution of amino groups on the MPF polymer at atomic scale as well as their strong coordination interactions with adsorbed Fe 3+ ensure the successful formation of single-atom Fe-N-C. [35]The as-obtained intermediate (P123@MPF/Fe 3+ /SCN − ) is then annealed at 800 °C in NH 3 , and the final product, S-modified Fe-N-C, is obtained.Without introducing KSCN during the ion adsorption process, the control sample, pristine Fe-N-C, is obtained.
To figure out the effect of P123 dosage, the amount of P123 is tuned from 0.0435 to 0.6525 g while the amount of mphenylenediamine and formaldehyde are fixed at 1.32 g and 0.915 mL (Supporting Information Figure S1), respectively.With the increase in surfactants, the polymer spheres gradually evolve from solid spheres to hollow spheres and the optimized hollow spherical morphology is obtained with a P123 dosage of 0.522 g (12 × P123).
The P123@MPF/Fe 3+ /SCN − intermediate is composed of monodisperse spheres with a diameter of ~100 nm (Supporting Information Figure S2).Tiny holes can be clearly seen on the surface in scanning electron microscopy (SEM, Supporting Information Figure S2a,b), demonstrating the porous feature of the P123@MPF/Fe 3+ /SCN − spheres.Transmission electron microscopy (TEM, Supporting Information Figure S2c,d) image of the intermediate sphere shows brighter contrast in the center and darker contrast at the edge, confirming the polymerization of MPF resin on the surface of P123 micelles.
After calcination in NH 3 , the pores on the surface can still be maintained (Figure 2a).Meanwhile, hollow cavities can be clearly discovered in the center of the S-modified Fe-N-C spheres (Figure 2b) owing to the decomposition of P123 at high temperature.Lowmagnification SEM and TEM characterizations (Supporting Information Figure S3) confirm the uniformity as well as hollow spherical structure of the S-modified Fe-N-C.High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterization at atomic level is carried out to explore the presence of single Fe atoms (Figure 2c).A number of bright dots indicated by red circles are clearly observed, demonstrating that atomically dispersed Fe atoms are anchored on the carbon matrix.As a further evidence, energy dispersive spectroscopy (EDS) elemental mappings (Figure 2d; Supporting Information Figure S4) prove that the Fe, N, C, and S are evenly distributed throughout the material.Besides EDS elemental mapping, more information on elemental composition can be obtained by inductivecoupled plasma atomic emission spectrometry (ICP-AES, Supporting Information Table S1) and CHNS (Supporting Information Table S2) analyses.X-ray diffraction (XRD) patterns of the S-modified Fe-N-C and pristine Fe-N-C catalysts are provided in Figure 2e and Supporting Information Figure S5.Both patterns exhibit a broad (002) diffraction peak at ~24°and an inconspicuous (100) diffraction at ~43°from the amorphous carbon matrix. [36]Except for the characteristic peaks from carbon, there is no peak from iron-containing compounds in XRD.Fourier-transform infrared (FTIR) spectra (Supporting Information Figure S6) confirm the successful conversion of polymer skeleton with rich functional groups to carbon with Fe-N bonds during the annealing in NH 3 . [37]o further probe the atomic structure of Fe sites, X-ray absorption fine structure (XAFS) analyses were performed.X-ray absorption near edge structure (XANES) spectra show that the Fe K-edge position of S-modified and pristine Fe-N-C are both located near that of FePc and away from that of Fe foil (Figure 2f).Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) spectra were used to obtain information on the coordination structure of Fe (Figure 2g).For S-modified Fe-N-C, pristine Fe-N-C, and FePc, the predominant Fe-N scattering path are located at ~1.52 Å.The Fe-Fe metallic path of Fe foil at higher R values is absent from the S-modified and pristine Fe-N-C, eliminating the possibility of metallic Fe. [38] Combined with the STEM observation, we can conclude that the Fe is atomically dispersed in both S-modified and pristine Fe-N-C.Interestingly, at a radial distance of 1.98 Å, another peak longer than the Fe-N is observed in S-modified Fe-N-C, which belongs to the Fe-S scattering path. [39]This phenomenon proves that S is coordinated to Fe in S-modified Fe-N-C.The existence of Fe-S bond in S-modified Fe-N-C can also be confirmed by comparing the XANES and FT-EXAFS results of S-modified Fe-N-C and FeS (Supporting Information Figure S7).Furthermore, the wavelet transform analysis of S-modified Fe-N-C, pristine Fe-N-C, and FePc demonstrate only one maximum intensity at approximately 5.0 Å−1 (Figure 2h), which suggest the configuration of mononuclear Fe center with FeN 4 coordination and the complete absence of metallic Fe. [29] Particularly, the contour intensity maximum for S-modified Fe-N-C is broader than those of pristine Fe-N-C and FePc, demonstrating the presence of additional bond (axial Fe-S bond) other than Fe-N bonds in S-modified Fe-N-C. [40]The EXAFS curve-fitting analyses (Figure 2i) are further conducted to reveal the exact coordination configuration of Fe center.For both S-modified and pristine Fe-N-C, the first coordination shell could be fitted by a Fe-N scattering path with a Fe coordination number of four.This implies that the Fe centers in both catalysts possess a FeN 4 configuration.Besides, the data for S-modified Fe-N-C are fitted by a Fe-S coordination path, with a Fe coordination number of one (S).From the above characterizations, the FeN 4 configuration in pristine Fe-N-C and the square planar FeN 4 moieties with an axial S atom in S-modified Fe-N-C is confirmed.The proposed models for the S-FeN 4 and FeN 4 configurations are schematically shown in the inset of Figure 2i.Actually, the FeN 4 axially coordinated with a S ligand is responsible for the ORR in biological cells. [41,42]upporting Information Figure S8 displays the N 2 sorption results of as-obtained intermediates and final products.With the presence of surfactant P123 in the materials, the P123@MPF/Fe 3+ /SCN − and P123@MPF/Fe 3+ have relatively low-specific surface areas, 63 m 2 g −1 for the former, and 75 m 2 g −1 for the later.After P123 removal, the S-modified Fe-N-C and pristine Fe-N-C show significantly increased specific surface area, 374 m 2 g −1 for the former, and 420 m 2 g −1 for the later.Both catalysts manifest a typical Type-IV isotherm with a hysteresis loop at a relative pressure of 0.4 < P/P 0 < 0.9, indicating the presence of mesopores (Supporting Information Figure S8b).Meanwhile, the N 2 adsorption amount increases sharply at P/P 0 > 0.9, demonstrating the presence of rich macropores in the sample.Such large macropores and mesopores are originated from the hollow cavities and pores on the shells.When applied as the ORR electrocatalyst, such a unique porous structure will facilitate the mass diffusion significantly. [43,44]upporting Information Figure S9 shows the Raman spectra of S-modified and pristine Fe-N-C, and their I D /I G ratios are 1.03 and 0.99, respectively.The higher I D /I G ratio of S-modified Fe-N-C indicates the formation of more edge and topological defects. [45]Such defects would modulate the electronic and surface properties of S-modified Fe-N-C, and thus optimize the adsorption energies of electrocatalytic steps. [46]Thermogravimetric analysis (TGA, Supporting Information Figure S10) shows the mass loss of S-modified and pristine Fe-N-C reach 93.5 and 93.3 wt.%, respectively.Assuming the complete oxidation of Fe to Fe 2 O 3 during TGA, the net Fe content are determined to be 4.5-4.6 wt.%.
The ORR electrocatalytic property of the as-obtained catalysts and Pt/C were evaluated in O 2 -saturated 0.1 M KOH using a rotating disk electrode (RDE).Firstly, linear sweep voltammetry (LSV) curves were collected (Figure 3a).The S-modified Fe-N-C displays the most positive onset and half-wave potentials (E onset = 0.99 V, E 1/2 = 0.88 V vs RHE) among all catalysts (Figure 3b).The significantly improved E onset and E 1/2 of S-modified Fe-N-C compared to pristine Fe-N-C suggests the significant role of S doping on ORR activity.Impressively, the S-modified Fe-N-C outperforms most of the reported Fe-based ORR electrocatalysts in terms of E onset and E 1/2 (Supporting Information Figure S11, Table S3).Besides, cyclic voltammetry (CV) profile of the S-modified Fe-N-C shows the highest cathodic peak potential (Supporting Information Figure S12), indicating its highest intrinsic ORR activity.In addition to activity, durability is also an important indicator for ORR catalyst.After 40 000 s stability test (Figure 3c), the S-modified Fe-N-C demonstrates the highest retention ratio of initial current among the three samples, demonstrating its ideal long-term stability.To further explore the durability of the catalyst, CV and LSV cycling stability and resistance to methanol poisoning were provided.Interestingly, the ORR behavior of the S-modified Fe-N-C catalyst only differs slightly after 10 000 CV and LSV cycles (Supporting Information Figure S13a-c).Besides, compared with the sharp decrease of current density for commercial Pt/C, the S-modified Fe-N-C catalyst shows outstanding tolerance to methanol crossover, showing a stable current density after the methanol injection (Supporting Information Figure S13d).
To reveal the ORR pathway of S-modified Fe-N-C, LSV profiles were collected at different rotation rates (Figure 3d), and the electron transfer number (n) is determined to be ~4 from the Koutecky-Levich (K-L) curves (Supporting Information Figure S14).Rotating ring-disk electrode (RRDE) measurements also provide information on the catalytic pathway.From the ring and disk currents, the calculated n ranges from 3.86 to 3.98 and the yield of peroxide species (H 2 O 2 ) is below 7% within the 0.2-0.9V potential range (Figure 3e,f), suggesting an ideal Pt-like four-electron reduction pathway.Electrochemical impedance spectroscopy (EIS) results of the S-modified Fe-N-C, pristine Fe-N-C, and commercial Pt/C reveal that the S-modified Fe-N-C possess the lowest charge transfer resistance as reflected from its smallest semi-circle (Supporting Information Figure S15).In addition, the S-modified Fe-N-C also demonstrates better ORR performance in acidic electrolyte than Pt/C and pristine Fe-N-C (Supporting Information Figure S16).
To clarify the role of S doping, a variety of spectroscopy tests were carried out.First, the valence states and chemical environments of the constituting elements are investigated by X-ray photoelectron spectroscopy (XPS, Supporting Information Figure S17).After deconvolution, the Fe-N bond component is clearly identified from the rich N species (Figure 4a), suggesting the presence of Fe-N moiety in the S-modified and pristine Fe-N-C electrocatalysts. [47]Fe 2p XPS spectra of S-modified and pristine Fe-N-C (Figure 4b) show a noticeable difference.The Fe 2p XPS spectrum of pristine Fe-N-C displays Fe 2p 3/2 and 2p 1/2 peaks at 723.1 and 709.9 eV, respectively, while those of S-modified Fe-N-C are located at 723.5 and 710.3 eV, respectively. [48]he positive shifts corroborate that the introduction of S dwindles the local electron density of Fe.Notably, the Fe 2p XPS spectrum of S-modified Fe-N-C also presents a component for Fe-S bond, which is located at 711.9 eV.The S 2p XPS spectrum (Figure 4c) displays a component for the sulfide ions at the surface (161.9 eV) and another component for the metal-sulfur bonds (163.1 eV), further verifying that the Fe atoms are coordinated with S. [49,50] Electron energy loss spectroscopy (EELS) results of S-modified and pristine Fe-N-C are provided in Figure 4d,e and Supporting Information Figure S18, where the element-specific absorption edge of Fe for S-modified Fe-N-C is located at a higher position.The EELS results further confirm the more positively charged state of Fe in S-modified Fe-N-C.Both XPS and EELS results reveal the S attracts partial electrons from Fe center in S-modified Fe-N-C.A higher Fe valence state can optimize the adsorption of oxygen towards Fe sites, facilitating the reaction kinetics. [51]ltraviolet photoelectron spectroscopy (UPS) provides information on the work functions (Φ) of S-modified and pristine Fe-N-C, which are determined to be 4.3 and 4.7 eV, respectively (Figure 4f).The lower work function of S-modified Fe-N-C is beneficial for electrical conductivity, thereby the surface electron transport is facilitated. [52]lectron spin resonance (ESR) spectroscopy was further employed to investigate the change in electronic configuration (Figure 4g).With the introduction of S, the sample shows slightly shifted peak position and higher g factor (Supporting Information Figure S19).It is well known that the number and occupation of unpaired electron determines the shape of ESR spectra, and the change in g factor value directly reflects the change in angular momentum of unpaired electron. [53]More detailed information on electronic configuration can be obtained through the 57  ) (Figure 4i), respectively. [54]This finding is consistent with the ESR results and both ESR and 57 Fe Mössbauer spectra indicate the increase in unpaired electrons through S doping.Additionally, an increased quadrupole splitting (Qs) for S-doped Fe-N-C compared with that of pristine Fe-N-C (Supporting Information Table S4) certifies an increase in the coordination number, which is resulted from the Fe coordination with four in-plane N and an external axial S. [55,56] Actually, after S doping, the change from intermediate spin to high spin state makes the Fe(II) atom more electrophilic, which induces the endon O 2 adsorption. [47]The optimized unpaired electron number and electrophilicity of the S-modified Fe-N-C contributes to an optimized bond strength between oxygen species and the electrocatalyst, leading to better ORR activity. [57]ensity functional theory calculations were employed to unravel the role of optimizing coordination environment of active centers on ORR activity.Based on the XAFS, XPS, and Mössbauer spectra, two models for the Fe-N system with and without external S (S-FeN 4 and FeN 4 ) have been constructed.The Fe atom in the S-FeN 4 moiety shows reduced electron density (blue isosurfaces) when compared to the Fe in FeN 4 (Figure 5a), demonstrating the electron-attracting ability of external S. The accumulation of electrons is clearly seen on the surface of S, and it reveals the redistribution of charge density between S atom and FeN 4 .The Bader charge analysis (Supporting Information Figure S21) was also provided to study the electronic properties.60] The ORR free energy diagrams of S-FeN 4 and FeN 4 were calculated (Figure 5b).At U = 0 V (vs RHE), the 4e − transfer processes are all thermodynamically downhill for the two catalysts, which means the steps are exothermic and favorable. [61]For FeN 4 , the final e − transfer Energy Environ.Mater.2024, 7, e12560 process, namely from *OH to OH − , is the rate determining step (RDS).In contrast, for S-FeN 4 , the RDS turns to be the formation of *OOH (Figure 5c). [62]In addition, the limiting potentials of FeN 4 and S-FeN 4 are 0.45 and 0.60 V, respectively, demonstrating the enhanced ORR performance of more positively charged Fe sites.To figure out the origin of catalytic activity from the perspective of electronic structure, the density of states (DOS) of catalysts are depicted.Clearly, increased DOS near the Fermi level can be identified for S-FeN 4 , owing to the significant enhancement of projected density of states (PDOS) on Fe atom (Figure 5d).The results suggest that the S-FeN 4 has a more active electron transport ability, [63,64] which matches well with EIS results.Virtually, the interaction of O p z with Fe d z 2 orbitals can form σ bonds which regulates the adsorption kinetics of oxygenated intermediates. [65]Compared with that of FeN 4 , the introduction of S p z orbital could make the Fe d z 2 orbital of S-FeN 4 away from the Fermi level (Supporting Information Figure S22).Therefore, the bonding interaction between O p z with the Fe d z 2 orbital is weakened and thus the desorption of oxygenated intermediates can be accelerated.The results rationalize that higher Fe valence states and electron delocalization around the Fe centers modulate the adsorption energy of ORR intermediates and thus facilitate reaction kinetics (Supporting Information Figure S23). [39]n-air batteries (ZABs) are assembled to assess the ORR performance of S-modified Fe-N-C in practical devices (Figure 6a).An air cathode composed of the as-prepared catalyst coated on carbon paper and a zinc foil anode operates in aqueous electrolyte (0.2 M ZnCl 2 and 6.0 M KOH).For comparison, ZABs with Pt/C (20 wt%) electrocatalyst was also fabricated.When operated in atmospheric air, the ZABs show good open circuit voltage (OCV) stability after 10 000 s (Figure 6b).The OCV of the constructed devices based on S-modified Fe-N-C reaches 1.48 V, which is slightly higher than that of commercial Pt/C-based device (1.45 V).Two ZABs connected in parallel are able to light up 60 light emitting diodes (LEDs) (Figure 6c).Tested at 10 mA cm −2 (Figure 6d), the S-modified Fe-N-C-based ZAB shows a discharge capacity of 766.5 mAh g −1 , higher than that of the Pt/C-based ZAB (702.2 mAh g −1 ). Figure 6e presents discharge polarization curves and corresponding power density of the ZABs.At low current density, the discharge voltage of S-modified Fe-N-C-based battery is slightly higher than the Pt/C-based cell.Additionally, the peak power density of Smodified Fe-N-C-based ZAB reaches 117.2 mW cm −2 , outperforming the Pt/C-based ZAB (66.5 mW cm −2 ).For the rate performance tested at different current densities (Figure 6f), the ZABs show very stable discharge voltage platforms.When the current density increases from 2 to 10 mA cm −2 , the discharge voltage decreases slowly and the voltage platform can return to the initial level after 8 h testing, indicating the satisfactory rate capacity of the S-modified Fe-N-C-based battery.Finally, the long-term charge-discharge cycling at 10 mA cm −2 shows that the Pt/C + RuO 2 battery deteriorates significantly after ~20 h, while the S-modified Fe-N-C + RuO 2 battery demonstrates good stability during 100 h cycling (Figure 6g).The outstanding ZAB performance further verify the ideal ORR electrocatalytic activity of Smodified Fe-N-C.

Conclusion
In conclusion, we propose a potential strategy to tune the coordinative and electronic configuration of FeN 4 via external S introduction.The local coordination environment of the Fe center in Fe-N-C can be precisely controlled.Combined experimental and computational results disclose that the S doping can lead to higher Fe valence states and electron delocalization around the Fe center, which accelerate the ORR dynamics.The more positively charged Fe sites optimize the O 2 adsorption capacity and the increase of unpaired Fe 3d electrons also facilitate the desorption of oxygen species.The obtained S-modified Fe-N-C electrocatalyst delivers better ORR activity than the pristine Fe-N-C and Pt/C catalysts and better ZAB performance than Pt/C catalysts.This contribution proposes a unique strategy for adjusting the local coordination structure of M-N-C catalysts to achieve high ORR performance.

Experimental Section
Detailed information related to the synthesis of active electrodes, physicochemical characterization, and electrochemical evaluation of bifunctional electrodes towards UOR and supercapacitor application is provided in Supporting Information.

Figure 3 .
Figure 3. Oxygen reduction reaction (ORR) electrocatalytic performance.a) Linear sweep voltammetry (LSV) curves at 1600 rpm, b) comparison of half-wave and onset potentials, and c) the i-t curves at 0.85 V vs reversible hydrogen electrode (RHE) of S-modified Fe-N-C, pristine Fe-N-C, and Pt/C.d) LSV curves at different rotation rates, e) j r and j d curves, and f) electron transfer numbers (n) and H 2 O 2 percentage of S-modified Fe-N-C.

Figure 4 .
Figure 4. a) N 1s and b) Fe 2p XPS spectra of S-modified and pristine Fe-N-C, c) S 2p XPS spectrum of S-modified Fe-N-C.d) HAADF-STEM images, e) EELS spectra, f) UPS spectra, and g) X-band ESR spectra of S-modified and pristine Fe-N-C.h) 57 Fe Mössbauer spectrum of S-modified Fe-N-C.i) Schematic representation for the spin transition of Fe(II) in S-modified and pristine Fe-N-C.The scale bars in (d) are 50 nm.

Figure 5 .
Figure 5. a) Spatial charge density difference isosurfaces of S-FeN 4 and FeN 4 .Yellow and blue isosurfaces represent electron accumulation and electron depletion, respectively.b) Oxygen reduction reaction free energy diagrams, c) free energy change, and d) Density of states (DOS) diagrams of S-FeN 4 and FeN 4 .

xy 2 d yz 1 d xz 1 d z 2 1 d x 2
Fe Mössbauer spectra.The spectrum of pristine Fe-N-C (Supporting Information Figure S20) is assigned to a D1 doublet and the spectrum of S-modified Fe-N-C (Figure 4h) shows an extra D2 doublet.The D1 and D2 doublets belong to FeN 4 species with an intermediate spin state (d xy 2 d yz 2 d xz 1 d z 2 1 ) and high spin state (d Ày 2 1

Figure 6 .
Figure 6.Electrochemical performance of ZABs.a) Schematic of a ZAB.b) OCV stability of S-modified Fe-N-C and Pt/C.c) Digital photographs showing the open-circuit voltages of two ZABs and two ZABs light a light board.d) Discharge curves of S-modified Fe-N-C-and Pt/C-based ZABs at 10 mA cm −2 .e) Discharge polarization curves and corresponding power density of S-modified Fe-N-C-and Pt/C-based ZABs.f) Rate performance at different current densities and g) long-term cycling curves at 10 mA cm −2 of S-modified Fe-N-C-and Pt/C-based ZABs.