Yaqiong Zeng

Jian Zhao

Shifu Wang

Xinyi Ren

Yuanlong Tan

Ying-Rui Lu

Shibo Xi

Junhu Wang

Frédéric Jaouen
email: frederic.jaouen@umontpellier.fr

Xuning Li

Yanqiang Huang

Tao Zhang

Bin Liu
email: liubin@ntu.edu.sg

Unravelling the Electronic Structure and Dynamics of the Atomically Dispersed Iron Sites in Electrochemical CO2 Reduction

Single-atom catalysts with a well-defined metal center open unique opportunities for exploring the catalytically active site and reaction mechanism of chemical reactions. However, understanding of the electronic and structural dynamics of single-atom catalytic centers under reaction condition is still limited due to the challenge of combining operando techniques that are sensitive to such sites and model single-atom systems. Herein, supported by state-of-the-art operando techniques, we provide an in-depth study of the dynamic structural and electronic evolution during electrochemical CO2 reduction reaction (CO2RR) of a model catalyst comprising iron only as a high-spin (HS) Fe(III)N4 center in its resting state. Operando 57 Fe Mössbauer and X-ray absorption spectroscopies clearly evidence the change from a HS Fe(III)N4 to a HS Fe(II)N4 center with decreasing potential, CO2-or Ar-saturation of the electrolyte leading to different adsorbates and stability of the HS Fe(II)N4 center.

With operando Raman spectroscopy and cyclic voltammetry, we identify that the phthalocyanine (Pc) ligand coordinating the iron cation center undergoes a redox process from Fe(II)Pc to Fe(II)Pc -.

Altogether, the HS Fe(II)Pc -species is identified as the catalytic intermediate for CO2RR. Furthermore, theoretical calculations reveal that the electroreduction of the Pc ligand modifies the d-band center of the in situ generated HS Fe(II)Pc -species, resulting in an optimal binding strength to CO2 and thus boosting the catalytic performance of CO2RR. This work provides both experimental and theoretical evidence towards the electronic structural and dynamics of reactive sites in single-Fe-atom materials and shall guide the design of novel efficient catalysts for CO2RR.

Introduction

Revealing the nature of active sites in heterogeneous metal-based catalysts is essential for the fine control of the reaction kinetics and dynamics [START_REF] Pan | Boosting CO2 reduction on Fe-N-C with sulfur incorporation: Synergistic electronic and structural engineering[END_REF][START_REF] Ju | Unraveling mechanistic reaction pathways of the electrochemical CO2 reduction Fe-N-C single-site catalysts[END_REF][START_REF] Lin | Temperature-dependent CO2 electroreduction over Fe-N-C and Ni-N-C singleatom catalysts[END_REF] . However, the geometric and electronic structures of active sites are usually influenced by many parameters such as the nature of the metal atoms [START_REF] Han | Supported cobalt polyphthalocyanine for high-performance electrocatalytic CO2 reduction[END_REF][START_REF] Yi | Conductive two-dimensional phthalocyanine-based metal-organic framework nanosheets for efficient electroreduction of CO2[END_REF] , the nature of coordinating atoms (C, N, O, S, etc.) [START_REF] Liu | Tuning the spin state of the iron center by bridge-bonded Fe-O-Ti ligands for enhanced oxygen reduction[END_REF] , coordination numbers [START_REF] Zhu | Unravelling the structure of electrocatalytically active Fe-N complexes in carbon for the oxygen reduction reaction[END_REF] , secondary coordination spheres [START_REF] Qiu | Highly selective CO2 electroreduction to C2H4 using a metal-organic framework with dual active sites[END_REF][START_REF] Zhang | Conductive phthalocyanine-based covalent organic framework for highly efficient electroreduction of carbon dioxide[END_REF] , etc., which leads to great challenges in identifying structure-activity relationships and the underlying reaction mechanisms [START_REF] Xie | High carbon utilization in CO2 reduction to multi-carbon products in acidic media[END_REF][START_REF] Yang | Atomically dispersed Ni(I) as the active site for electrochemical CO2 reduction[END_REF] . Moreover, due to the dynamic behaviors of active sites during catalysis and the issues of current characterization techniques, the atomic-level understanding of the active sites during a chemical reaction remains challenging [START_REF] Liu | Structural regulation and support coupling effect of singleatom catalysts for heterogeneous catalysis[END_REF][START_REF] Li | Identification of the electronic and structural dynamics of catalytic centers in single-Fe-atom material[END_REF][START_REF] Li | Identification of durable and non-durable FeNx sites in Fe-N-C materials for proton exchange membrane fuel cells[END_REF][START_REF] Yang | Dynamic behavior of single-atom catalysts in electrocatalysis: identification of Cu-N3 as an active site for the oxygen reduction reaction[END_REF] .

Single-atom catalysts (SACs) with well-defined and atomically-dispersed metal sites are a promising platform for the precise identification of structure-activity relationships and, on the practical level, for the full utilization of the metal [START_REF] Qiao | Single-atom catalysis of CO oxidation using Pt1/FeOx[END_REF][START_REF] Jiang | Modulating the electronic metal-support interactions in single-atom Pt1-CuO catalyst for boosting acetone oxidation[END_REF][START_REF] Deng | Few-atom Pt ensembles enable efficient catalytic cyclohexane dehydrogenation for hydrogen production[END_REF][START_REF] Xiao | Atomically dispersed Pt and Fe sites and Pt-Fe nanoparticles for durable proton exchange membrane fuel cells[END_REF][START_REF] Liu | Atomically dispersed iron sites with a nitrogen-carbon coating as highly active and durable oxygen reduction catalysts for fuel cells[END_REF] . Recent studies highlighted that single-atom active sites can undergo dynamic evolution under reaction conditions, induced by applied temperature, electrochemical potential, adsorbates, and etc., challenging the identification of the true structure of the catalytic site, and of structure-catalysis relationships [START_REF] Jin | Emerging electrochemical techniques for probing site behavior in single-atom electrocatalysts[END_REF][START_REF] Ahmed | Nitrogenase-inspired atomically dispersed Fe-S-C linkages for improved electrochemical reduction of dinitrogen to ammonia[END_REF][START_REF] Huang | Adsorption energy as a promising single-parameter descriptor for single atom catalysis in the oxygen evolution reaction[END_REF][START_REF] Liu | Elucidating the electrocatalytic CO2 reduction reaction over a model single-atom nickel catalyst[END_REF] . Therefore, real-time monitoring the structural evolution of active sites in SACs under realistic operating conditions is highly desired.

The development of in situ/operando techniques and setups such as X-ray absorption spectroscopy (XAS), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, etc. for probing SACs offers new opportunities for identifying the active sites, capturing the reaction intermediates, and uncovering the catalytic mechanism [START_REF] Huang | Modifying redox properties and local bonding of Co3O4 by CeO2 enhances oxygen evolution catalysis in acid[END_REF][START_REF] Chen | Operando analysis of NiFe and Fe oxyhydroxide electrocatalysts for water oxidation: detection of Fe 4+ by mossbauer spectroscopy[END_REF] . For instance, relying on operando XAS, Hu's group showed with operando Fe K-edge XAS the high activity, selectivity and stability of Fe(III) sites in a Fe-N-C SAC for CO2RR to CO [START_REF] Gu | Atomically dispersed Fe 3+ sites catalyze efficient CO2 electroreduction to CO[END_REF] . The iron was shown to remain in +3 oxidation state down to -0.4 V vs. RHE, while at -0.5 V vs. RHE, its oxidation state decreased to +2 but the CO2RR activity concomitantly became unstable and lower. The maintained ferric oxidation state down to -0.4 V vs. RHE in this Fe-N-C material prepared by pyrolysis at 900 °C of Fe-doped ZIF-8 was assigned to a pyrrolic nitrogen environment, and an analogy made to the similarly low Fe(III)/Fe(II) redox potential of some iron porphyrins. In contrast, Li et al. concluded that H2O-Fe(II)N4 is the most active site for selective CO2RR to CO for a Fe-N-C SAC material derived from the flash pyrolysis at 1050 °C of Fe acetate and ZIF-8, on the basis of operando Fe K-edge XAS results and theoretical calculations [START_REF] Li | Volcano trend in electrocatalytic CO2 reduction activity over atomically dispersed metal sites on nitrogen-doped carbon[END_REF] . It is of note that the Zn content in ZIF-8 derived Fe-N-C materials is much higher after pyrolysis at 900 °C than at 1050 °C (Zn boiling point 907 °C), which could play a role on the carbon structure and, indirectly, on the detailed structure and reactivity of FeNxCy moieties sites. Recently, we studied another Fe-N-C catalyst derived from ZIF-8 via pyrolysis. Taking advantage of operando [START_REF] Liu | Theoretical investigation of the molecular, electronic structures and vibrational spectra of a series of first transition metal phthalocyanines[END_REF] Fe Mössbauer spectroscopy, we unveiled the in situ generation of the low-spin monovalent Fe(I) site at -0.9 V vs. RHE, and assigned it as the most reactive center in CO2RR [START_REF] Li | Unveiling the in situ generation of a monovalent Fe(I) site in the single-Fe-atom catalyst for electrochemical CO2 reduction[END_REF] . The contrasting and apparently inconsistent results from these works highlight the complexity of the real structure of catalytic sites under reaction conditions.

While the pyrolytic preparation of Fe-N-C catalysts comprising only single-metal atom sites is now relatively well-established, such materials typically feature FeNxCy moieties with different structures and/or electronic states (Supplementary Fig. 1), as evidenced with ex situ and/or operando spectroscopic techniques [START_REF] Li | Identification of durable and non-durable FeNx sites in Fe-N-C materials for proton exchange membrane fuel cells[END_REF][START_REF] Gu | Atomically dispersed Fe 3+ sites catalyze efficient CO2 electroreduction to CO[END_REF][START_REF] Li | Volcano trend in electrocatalytic CO2 reduction activity over atomically dispersed metal sites on nitrogen-doped carbon[END_REF][START_REF] Li | Unveiling the in situ generation of a monovalent Fe(I) site in the single-Fe-atom catalyst for electrochemical CO2 reduction[END_REF][START_REF] Ni | In situ 57 Fe mössbauer study of a porphyrin based FeNC catalyst for ORR[END_REF] . In addition, the electrochemical utilization of FeN4 sites is usually low in pyrolyzed materials, according to either CO cryo chemisorption or nitrite stripping techniques applied for counting the active sites [START_REF] Primbs | Establishing reactivity descriptors for platinum group metal (PGM)-free Fe-N-C catalysts for PEM fuel cells[END_REF] . Only in the case of the recent CVD preparation of a Fe-N-C, was a full utilization of FeN4 sites achieved, featuring a single HS Fe(III)N4 site structure, as revealed by ex situ [START_REF] Liu | Theoretical investigation of the molecular, electronic structures and vibrational spectra of a series of first transition metal phthalocyanines[END_REF] Fe Mössbauer spectroscopy [START_REF] Jiao | Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites[END_REF] . The low utilization of FeN4 sites implies that operando spectroscopic changes are only partly related to surface sites, complexifying the analysis of the operando structure of active surface sites. More research efforts are therefore required towards the development of model Fe-based catalytic systems featuring a mono-structure and high electrochemical accessibility of the single-metal-atom sites, and the application on such model catalysts of operando characterization techniques with atomic scale sensitivity [START_REF] Lu | Identification of the active complex for CO oxidation over single-atom Ir-on-MgAl2O4 catalysts[END_REF][START_REF] Ni | Identification of the catalytically dominant iron environment in iron-and nitrogendoped carbon catalysts for the oxygen reduction reaction[END_REF] .

In this work, leaning on the long-demonstrated CO2RR activity of metal-phthalocyanines in general [START_REF] Kapusta | Carbon dioxide reduction at a metal phthalocyanine catalyzed carbon electrode[END_REF][START_REF] Wang | A hybrid co quaterpyridine complex/carbon nanotube catalytic material for CO2 reduction in water[END_REF][START_REF] Choi | Steric modification of a cobalt phthalocyanine/graphene catalyst to give enhanced and stable electrochemical CO2 reduction to CO[END_REF] , and Fe-phthalocyanines in particular [START_REF] Li | Electrocatalytic reduction of CO2 to CO over iron phthalocyanine-modified graphene nanocomposites[END_REF][START_REF] Manbeck | A review of iron and cobalt porphyrins, phthalocyanines and related complexes for electrochemical and photochemical reduction of carbon dioxide[END_REF] , we rationally designed a model single-Fe-atom catalyst by firstly synthesizing a well-defined FePc molecule (C32H16FeN8, with or without [START_REF] Liu | Theoretical investigation of the molecular, electronic structures and vibrational spectra of a series of first transition metal phthalocyanines[END_REF] Fe enriched) comprising of identical HS Fe(III) species, followed by immobilizing it on conductive carbon nanotubes. Operando 57 Fe Mössbauer, XAS, and Raman studies were conducted to investigate the dynamic evolution of the single-Fe-atom centers during electrochemical CO2RR, revealing that the initial HS O2-Fe(III)Pc species is irreversibly reduced to metallic iron in Ar-saturated environment, while in-situ generating the HS Fe(II)Pc -active site in CO2-saturated condition. The one-electron reduction of the Pc ligand stabilizes the Fe(II) species which, combined with the presence of the carbonate anion adsorption intermediate (Fe(II)Pc-CO2 -), prevents the over-reduction to Fe(I) and metallic iron. Density functional theory (DFT) calculations further revealed that the in-situ generated HS Fe(II)Pc -species with modified d-band center could result in an optimal binding to CO2 molecules, promoting the CO2RR.

Results and Discussion

Structural Characterization

The single-Fe-atom catalyst with well-defined N-coordination structure for CO2RR was built by immobilizing the FePc molecule on conductive carbon nanotubes (CNT) via π-π interaction (Supplementary Fig. 2) [START_REF] Liu | Elucidating the electrocatalytic CO2 reduction reaction over a model single-atom nickel catalyst[END_REF][START_REF] Ren | Electron-withdrawing functional ligand promotes CO2 reduction catalysis in single atom catalyst[END_REF] . Fourier-transform infrared (FT-IR) spectroscopy was used to confirm the skeleton information of the as-synthesized FePc molecule. As shown in Supplementary Fig. 3, the main peaks located at 727, 1080 and 1117 cm -1 can be assigned to out-of-and in-plane C-H bond vibrations from the Pc ring respectively, while the bands at 1162 and 1331 cm -1 can be assigned to C-N and C-C stretching vibration from the isoindole structure [START_REF] Wang | Iron phthalocyanine-graphene donor-acceptor hybrids for visible-light-assisted degradation of phenol in the presence of H2O2[END_REF][START_REF] Sun | Improved synthesis of soluble metal-free/metal phthalocyanine tetracarboxylic acids and their application in the catalytic epoxidation of cyclohexene[END_REF][43] . The XRD pattern of the as-prepared FePc-CNT catalyst shows the characteristic peaks of both CNT and FePc, which verifies the successful loading of FePc on CNT (Supplementary Fig. 4) [START_REF] Baranton | How does α-FePc catalysts dispersed onto high specific surface carbon support work towards oxygen reduction reaction (orr)?[END_REF] . Furthermore, FePc uniformally covers the CNT surface, as revealed by scanning electron microscopy (SEM), transmission electron microscopy (HR-TEM) and EDX elemental mappings (Fig. 1b,c and Supplementary Fig. 5a). Meanwhile, the scanning transmission electron microscopy (STEM) and the corresponding elemental mapping images reveal the atomically dispersed Fe elements that are uniformly distributed on the entire CNT (Fig. 1c). As shown in Fig. 1d,e, the high density of bright spots in the aberration-corrected high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) images confirm the atomically dispersed Fe species in FePc-CNT. FePc-CNT exhibits extremely similar electronic structure and environmental characteristics to pure FePc (Supplementary Fig. 678). The experimental X-ray absorption near edge structures (XANES) spectrum shows that the edge energy of FePc-CNT is close to that of commercial O2-Fe(III)Pc (O2-Fe(III)Pc-c) (Fig. 1f), supporting the ferric state and, the preedge peak at 7114.3 eV can be assigned to a dipole forbidden 1s→3d transition, suggesting a deviation from the FeN4 structure due to the axial coordination of Fe cation by O2. This can be further confirmed by the simulation of XANES spectrum using the finite difference method near-edge structure (FDMNES) calculation based on the model structure of O2-FeN4 (Supplementary Fig. 9). Fig. 1g shows the Fourier transformed extended X-ray absorption fine structure (FT-EXAFS) spectra of FePc-CNT and the spectra of reference compounds. The main peak at 1.62 Å (distance not corrected for phase shift) can be assigned to the Fe-N in the first coordination shell, and is in line with the Fe-N bond distance in iron phthalocyanine, further indicating atomic dispersion of Fe species on the CNT surface. As shown in Fig. 1h, the FT-EXAFS spectrum of FePc-CNT can be well fitted using the structural model of O2-FeN4 (optimized by DFT), with O2 adsorbed on Fe in end-on mode. The fitted structural parameters for FePc-CNT are listed in Supplementary Table 1. The electronic state of Fe species in the as-prepared FePc-CNT was further determined by room-temperature [START_REF] Liu | Theoretical investigation of the molecular, electronic structures and vibrational spectra of a series of first transition metal phthalocyanines[END_REF] Fe Mössbauer spectroscopy. As shown in Fig. 1i, the Mössbauer spectrum of FePc-CNT can be well fitted using one quadrupole doublet with an isomer shift (IS) of ca 0.35 mm s -1 and a quadrupole splitting (QS) of ca 0.71 mm s -1 , which can generally (based in particular on the low QS value), but not unambiguously, be assigned to a high spin (HS) Fe(III) species. The fitted parameters of the Mössbauer spectrum of FePc-CNT are shown in Supplementary Table 2. In the class of iron phthalocyanine materials, a HS ferric phthalocyanine was synthesized and studied by Mössbauer spectroscopy by Burtsev et al., with reported IS and QS values of 0.3 and 0.7 mm s -1 respectively, at room temperature [START_REF] Burtsev | Synthesis and spectral properties of iron(III) tetra-tert -butylphthalocyanine complexes[END_REF] . These values match well with those observed in the present work, and, combined with the other results, support the successful synthesis of FePc-CNT with atomically dispersed HS O2-Fe(III)N4 species in ex situ conditions. c) and Fe foil standards for comparison. h, Experimental and fitted EXAFS spectrum in R-space for FePc-CNT catalyst (R range, 1-2.2 Å). i, Room-temperature [START_REF] Liu | Theoretical investigation of the molecular, electronic structures and vibrational spectra of a series of first transition metal phthalocyanines[END_REF] Fe Mössbauer spectrum of FePc-CNT.

Electrochemical performance

The electrochemical CO2RR performance was first evaluated by linear scan voltammetry (LSV) in 0.5 M KHCO3 electrolyte. As shown in Fig. 2a, a significantly higher current density is observed with FePc-CNT at ca -0.3 V vs. RHE in CO2-saturated condition than in Ar-saturated condition, assigned to CO2RR. The current density is also much larger and at much higher potential for FePc-CNT vs H2Pc-CNT, indicating the critical role of Fe in catalysis (Fig. 2a). Cyclic voltammetry (CV) curves of FePc-CNT and H2Pc-CNT in Ar-saturated electrolyte were recorded to explore the redox process of the catalysts. As shown in Fig. 2b, FePc-CNT features two redox peaks, and the reduction peak positions are at around +0.34 and -0.36 V vs. RHE, whereas no redox peak over H2Pc-CNT can be observed within the -0.5 to +0.9 V vs. RHE potential window. The highest redox was assigned to Fe(III)Pc/Fe(II)Pc transition, while the low redox might be attributed to the Fe(II)Pc/Fe(II)Pc - transition [START_REF] Alsudairi | Resolving the iron phthalocyanine redox transitions for ORR catalysis in aqueous media[END_REF] . Electrochemical CO2RR activities and selectivities of FePc-CNT, H2Pc-CNT were further evaluated in an H-cell. H2 and CO were the only detected gas-phase products, and no liquid products were detected (Supplementary Fig. 10). As shown in Fig. 2c, the Faradaic efficiency of CO (FECO) over FePc-CNT reaches a maximum value of ~92% at -0.45 V vs. RHE, which is much higher than that over H2Pc-CNT (~31% at -0.45 V vs. RHE). Within the whole range of potential, the bare CNT displays a H2 favorable reaction pathway (Supplementary Fig. 11). The CO partial current density (jCO) further proves the higher activity of FePc-CNT in CO2RR compared to H2Pc-CNT (Fig. 2d, red and black curves). By depositing FePc-CNT on a gas diffusion electrode (GDE), the CO2RR performance of FePc-CNT was further evaluated in a flow-cell. At -0.45 V vs. RHE, jCO reaches 23 mA cm -2 (Fig. 2d, blue curve) with FECO of ~97%. The Tafel slope of FePc-CNT of 79 mV dec -1 supports that the rate determining step of CO2 to CO electroreduction is the first proton transfer (Supplementary Fig. 12). Besides initial activity and selectivity, the FECO after 10 h of continuous CO2RR at -0.45 V vs. RHE remained higher than 88%, while the current density decreased by only 8.5%, indicating good catalytic stability of FePc-CNT towards CO2RR (Supplementary Fig. 13).

Operando X-ray absorption spectroscopy

Operando XAS measurements were performed to reveal the evolution of the electronic structure and coordination environment of the single-Fe-atom species in FePc-CNT during the CO2RR process.

As shown in Fig. 3a, the oxidation state of Fe is higher in CO2-saturated condition than that in Arsaturated condition at open circuit potential (OCP), which is possibly due to a spontaneous charge transfer from the central Fe to the adsorbed CO2 molecule. When a potential of -0.4 V vs. RHE was applied, the Fe K-edge of FePc-CNT shifted to lower energies, in both Ar-and CO2-saturated 0.5 M KHCO3 solution (Fig. 3a). As the applied potential decreased, the central iron of FePc-CNT was gradually reduced from Fe(III) into Fe(II). When the electrochemical potential was removed, the XANES spectrum of FePc-CNT was slightly different compared to the initial OCP condition (Fig. 3b), for measurements in CO2 saturated electrolyte. This indicates that the Fe(III) to Fe(II) reduction is not completely reversible, possibly as a result of a change in the axial ligand present atop Fe before and after CO2RR (Fig. 3b). The evolution of the pre-edge region in the operando Fe K-edge XANES spectra further reveals the structural dynamics of single-Fe-atom species. As shown in Fig. 3c, under OCP condition, the pre-edge exhibits identical feature of O2-Fe(III)N4 (pre-edge peak B) [START_REF] Lu | Operando elucidation on the working state of immobilized fluorinated iron porphyrin for selective aqueous electroreduction of CO2 CO[END_REF] . At -0.4 V vs. RHE, the intensity of the pre-edge peak B decreased, while a new pre-edge peak C appeared at 7118.3 eV. The latter is ascribable to 1s→4pz transition and can be indexed to square planar D4h symmetric structure of Fe(II)Pc, indicating the structural switching from non-planar to planar structure when the axial ligand is removed. With further decreasing the potential to -0.8 V vs. RHE, a nonnegligible pre-edge peak (A) at the lower energy of 7112.5 eV was observed, which can be assigned to a distorted square planar structure with axial ligand [START_REF] Gao | Atomically dispersed iron with densely exposed active sites as bifunctional oxygen catalysts for zinc-air flow batteries[END_REF][START_REF] Guo | The synthesis and synergistic catalysis of iron phthalocyanine and its graphenebased axial complex for enhanced oxygen reduction[END_REF] . The peak A is more intense at -0.8 than at -0.4 V vs. RHE, and disappears after removing the potential (labelled as AFT in the figure), suggesting that the peak A is most likely related to a catalytic intermediate. To determine the exact structure of the catalytic intermediate, we used the FDMNES calculation method to simulate the XANES spectra obtained at OCP and -0.8 V vs. RHE. The geometric configuration and atom coordinates for the calculated models are shown in Supplementary Fig. 14 and Supplementary Table Operando 57 Fe Mössbauer spectroscopy Operando 57 Fe Mössbauer spectroscopy was performed to quantitatively monitor the operando formed catalytic intermediates in [START_REF] Liu | Theoretical investigation of the molecular, electronic structures and vibrational spectra of a series of first transition metal phthalocyanines[END_REF] Fe enriched FePc-CNT during CO2RR. Fig. 4f displays the currenttime curves during the operando Mössbauer measurements. The 57 Fe Mössbauer spectra acquired at different applied potentials in both CO2-and Ar-saturated 0.5 M KHCO3 solution are presented in Fig. 4a-e. The quadrupole doublet (labeled D1) with an IS value of 0.33 mm s -1 and a QS value of 0.69 mm s -1 detected at OCP is similar to the doublet observed in the ex situ spectrum, and can be assigned to the HS Fe(III) in O2-Fe(III)Pc (Fig. 4a). At -0.4 V vs. RHE, two new doublets (IS = 1.19 mm s -1 , QS = 1.80 mm s -1 (D2a), and IS = 1.18 mm s -1 , QS = 2.50 mm s -1 (D2b)) are observed (Fig. 4b), both of which can be unambiguously assigned to HS Fe(II) species on the sole basis of their high IS values [START_REF] Filoti | Mössbauer study of the hyperfine interactions and spin dynamics inα-iron(II) phthalocyanine[END_REF][START_REF] Kuzmann | Mössbauer studies of the interaction of oxygen with solid β-FeIIphthalocyanine[END_REF] .

These QS values are in line with those recently experimentally reported in situ for HS Fe(II)N4 sites (with IS values  0.7 mm s -1 in all cases) of pyrolyzed Fe-N-C catalysts developed for oxygen electroreduction reaction 14 , (QS of ca 2.0 mm s -1 , measured at 0.2-0.4 V vs. RHE in O2-free acidic electrolyte, QS of ca 2.41-2.46 mm s -1 measured at 0.5-0.7 V vs. RHE in O2-saturated alkaline electrolyte [START_REF] Li | Identification of the electronic and structural dynamics of catalytic centers in single-Fe-atom material[END_REF] , QS of ca 2.1 mm s -1 at 0.2 V vs. RHE in O2-free acidic electrolyte [START_REF] Ni | Identification of the catalytically dominant iron environment in iron-and nitrogendoped carbon catalysts for the oxygen reduction reaction[END_REF] ). The fitted parameters of the Mössbauer spectra of FePc-CNT acquired at different potentials (OCP, -0.4, -0.8 V vs. RHE, in CO2 saturated electrolyte) in the present work are listed in Supplementary Table 6, and the corresponding spectra are shown in Fig. 4a-c. The relative signal intensity for D1 decreases with decreasing potential in CO2 -saturated KHCO3, from 100% at OCP to 20% at -0.8 V vs. RHE. The relative signal intensity for D2a is 55% at -0.4 V vs. RHE in CO2-saturated KHCO3, and almost unmodified at -0.8 V vs. RHE (53%). In contrast, the relative signal intensity for D2b is only 12% at -0.4 V vs. RHE CO2-saturated KHCO3 but increases to 26% at -0.8 V vs. RHE. These trends are visualized in Fig. 4g, and mainly indicate the reduction of HS Fe(III) to HS Fe(II) as a consequence of the applied potential, in the condition of CO2RR. The larger QS value for D2b vs. D2a in the present work implies increased asymmetry around the central Fe sites, which may result from the in situ formation of an axial intermediate of FePc-CO2 -, well consistent with the operando XAS results. After removing the applied potential, the relative signal intensity of D1 species increased back to 51%, the relative signal intensity for D2a remained quite unchanged (slight decrease from 53 to 49%), while the signal for D2b species disappeared (Fig. 4d and Supplementary Table 6). When comparing the Mössbauer spectra at -0.4 V vs. RHE in CO2-saturated vs. Ar-saturated 0.5 M KHCO3 solution, a sextet with hyperfine field of 324 kOe was detected (Fig. 4e), indicating the partial transformation of FeN4 sites into metallic iron in CO2-free electrolyte. This indicates the stabilization of Fe(II)Pc -by a CO2 adsorbate or by another CO2RR intermediate. Fig. 4h summarizes the dynamic evolution of the single-Fe-atom species in FePc-CNT during catalysis in CO2-and Ar-saturated electrolytes.

Operando Raman spectroscopy

To gain more insights into the role of ligands coordinated to single-Fe-atom species under reaction condition, operando Raman spectroscopy measurements were carried out. The Raman peak at 1532 cm -1 can be clearly observed over FePc-CNT at OCP under Ar-/CO2-saturated conditions (Fig. 5a,b,

Density functional theory calculations

DFT calculations were conducted to provide theoretical insights into the underlying reaction mechanism of electrochemical CO2 reduction over single-Fe-atom catalyst. As shown in Supplementary Fig. 19, the single point energy of Fe(II)Pc -is much lower than that of Fe(I)Pc (ΔE = EFe(I)Pc -EFe(II)Pc -= 0.231 eV), confirming the preferred reduction of Pc ring with the formation of Fe(II)Pc -. Furthermore, the delocalized electrons on the Pc ring lead to increased orbital energy levels, which can shift the d-band center of the central Fe from -4.925 eV to -4.575 eV (Fig. 6a) and decrease the CO2 adsorption energy from -0.816 eV to -2.047 eV, resulting in the transformation of the adsorption configuration from physical adsorption to chemisorption (Fig. 6b). Obviously, the dz 2 orbital of Fe(II) was changed into a more delocalized state as the electron added to the Pc ring and thus forming of a strong covalent bond with CO2 easily (Fig. 6c). The electronic structures of Fe(II)Pc and Fe(II)Pc -were further investigated by charge density difference and electron spin density distribution.

As shown in Supplementary Fig. 20, compared to Fe(II)Pc, a larger electronic density is delocalized at the vertical direction of Fe atom in Fe(II)Pc -. In Fig. 6d, the electron spin density distribution of Fe(II)Pc is mainly distributed around the central Fe and the N atoms in the first coordination shell, while that of Fe(II)Pc -is mainly distributed in the second coordination shell. Simultaneously, the electron spin density distribution of the Fe atom increases in the Z direction, which shall benefit CO2 activation at the HS Fe(II)Pc -site. Subsequently, the electron spin density distribution in the second coordination shell of FePc remained quite stable when bond to different intermediate species. Fig. 6e shows the energy-level correlation diagrams of Fe(II)Pc, Fe(II)Pc -, and Fe(II)Pc-CO2 -. It is shown that the transfer of electrons to the Pc ring with the formation of Fe(II)Pc -pushes up the total d orbitals, especially the dz 2 orbital of Fe, which leads to a stronger charge-transfer interaction from the Fe dz 2 to CO2 π* orbital, accelerating the formation of Fe(II)Pc-CO2 -intermediate and boosting the CO2 reduction reaction. All these theoretical insights are well consistent with the results obtained from operando measurements, clearly illustrating the electronic structure and dynamics of the atomically dispersed iron sites in CO2RR.

Conclusions

In summary, with the integration of state-of-the-art operando techniques on a model single-Fe-atom catalyst comprising of uniform HS Fe(III) center, we have provided a thorough study of the dynamic structural evolution of single-Fe-atom species under CO2RR condition. In CO2-saturated electrolyte, the transformation of O2-Fe(III)Pc to Fe(II)Pc was observed and the in-situ formation of an axial catalytic intermediate of Fe(II)Pc-CO2 -was evidenced. In contrast, in Ar-saturated electrolyte, the initial HS Fe(III)Pc species started to be irreversibly transformed to metallic iron when the electrochemical potential was decreased to -0.4 V vs. RHE. Meanwhile, the Pc ligand coordinated to single-Fe-atom center experiences a one-electron reduction, in combining with the adsorption and activation of CO2, that prevent the over-reduction of Fe(II) to Fe(I) species and metallic iron. DFT calculations further reveal that the in-situ generated HS Fe(II)Pc -species with modified d-band center results in a stronger binding with CO2 molecules, which can effectively boost the CO2RR. This work provides an in-depth understanding of the electronic structure and dynamic evolution of single-Featom center in CO2RR and shall shed new light for the future design of high-performance single atom catalysts.

Fig. 1 |

 1 Fig. 1 | Structural characterization of the as-prepared FePc-CNT. a, Synthesis strategy of the

Fig. 2 |

 2 Fig. 2 | Electrochemical CO2RR. a, Linear sweep voltammogram (LSV) curves recorded over FePc-

3

 3

- 5 .

 5 As shown in Fig.3dand Supplementary Fig.15-17, the pre-edge peaks A, B, and C match well with the structure model of Fe(II)Pc-CO2 -, O2-Fe(III)Pc, and Fe(II)Pc, respectively. These assignments are also in line with the fact that the redox wave for the Pc ligand reduction to Pc -in Fe(II)Pc-CNT is situated at ca -0.4 V vs. RHE (Fig.2b), and could explain the enhanced adsorption of CO2 on Fe(II)Pc - at -0.8 V vs. RHE. All of the above results clearly show the transformation of O2-Fe(III)Pc to Fe(II)Pc and the in situ formation of Fe(II)Pc-CO2 -species as a key catalytic intermediate during the CO2RR.

Fig. 3 |

 3 Fig. 3 | Operando XAS characterization. a, Normalized Fe K-edge XANES spectra of FePc-CNT,

Fig. 4 |

 4 Fig. 4 | Operando 57 Fe Mössbauer characterization. Operando 57 Fe Mössbauer spectra of FePc-CNT

Fig. 5 |

 5 Fig. 5 | Operando Raman characterization. a, Operando Raman spectra of FePc-CNT recorded at

Fig. 6 |

 6 Fig. 6 | Theoretical calculations. a, Density of states (DOS) for Fe-3d orbitals in O2-Fe(III)Pc,

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (No. 2021YFA1500502), the National Natural Science Foundation of China (22102176, U19A2015 and 21925803), CAS Project for Young Scientists in Basic Research (YSBR-051), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB36030200), Ministry of Education of Singapore (Tier 1: RG4/20 and Tier 2: MOET2EP10120-0002), Agency for Science, Technology and Research (AME IRG: A20E5c0080), and GJTD-2020-08 (K. C. Wong Education Foundation). The XAS data of commercial O2-Fe(III)Pc and Fe(II)Pc were collected by Jingkun Li at the beamline 8-ID (ISS) of the National Synchrotron Light Source II, a DOE Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under contract no. DE-SC0012704. The authors gratefully acknowledge the support of Photon Science Center for Carbon Neutrality.

Supplementary Fig. 18), which can be assigned to the C=N vibration peak [START_REF] Lokesh | Synthesis and characterization of tetra-substituted palladium phthalocyanine complexes[END_REF][START_REF] Martin | Iron phthalocyanine electrodeposited films: characterization and influence on dopamine oxidation[END_REF][START_REF] Alessio | Iron phthalocyanine in non-aqueous medium forming layer-by-layer films: growth mechanism, molecular architecture and applications[END_REF] . In Supplementary Fig. 18a,b, under an Ar atmosphere, within the potential range from 0.5-0.3 V vs. RHE (covering the redox transition from Fe(III) to Fe(II) in FePc-CNT), no obvious change in the general features of the spectra was observed, indicating the lack of effect of the iron cation oxidation state on the vibrational spectra.

Notably, at -0.4 V vs. RHE, which is within the second reduction region of FePc-CNT, the intensity of the C=N vibration in FePc-CNT was significantly increased (Fig. 5a), suggesting that the change of the vibration peak was derived from the reduction of the Pc ring, instead of the redox transition from Fe(II) to Fe(I) [START_REF] Gunasekara | Solid phase FePc catalysts for increased stability of oxygen reduction reaction intermediates at the cathode/electrolyte interface in lithium air batteries[END_REF] . Negligible changes could be observed at -0.8 V vs. RHE and after removing the applied voltage (as compared to the case at -0.4 V vs. RHE), indicating that the Pc ring could be kept in the reduced state (Fig. 5a,b). Under CO2-saturated condition, similar trends were obtained as compared to those in Ar-saturated condition, where the intensity of the C=N vibration peak increased when the cathodic polarization decreased to -0.4 V and -0.8 V vs. RHE (Fig. 5c,d and Supplementary Fig. 18e). Interestingly, the intensity of the peak at 750 cm -1 (belonging to the C-N-C bending coupled to the Fe-N stretching vibration [START_REF] Chen | Operando characterization of iron phthalocyanine deactivation during oxygen reduction reaction using electrochemical tip-enhanced raman spectroscopy[END_REF][START_REF] Liu | Theoretical investigation of the molecular, electronic structures and vibrational spectra of a series of first transition metal phthalocyanines[END_REF] ) dramatically decreased compared to its neighbor at 682 cm -1 as the electrochemical potential decreased (Fig. 5c,d), which can be ascribed to the adsorption of CO2 molecule on the single Fe atom site with in situ formation of Fe(II)Pc-CO2 -intermediate. After removing the applied potential, the intensity of the peak at 750 cm -1 increased back to its initial value (AFT curve in Fig. 5c). This can be explained by the loss of the intermediate species. Considering that mainly Fe(II) species and a minority Fe(III) species could be observed from operando Mössbauer measurements under CO2 condition, the absence of stable Fe(I) species can be explained on the basis of the preferred one-electron reduction of the Pc ring as compared to the Fe(II) to Fe(I) reduction.

Notes

The authors declare no competing financial interests.

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