Enhancing the selectivity of C₂ hydrocarbons over Fe-based catalysts by controlling nitrogen doping in electrocatalytic CO₂ reduction

Abstract

Rational design of cost-effective materials as highly efficient catalysts for the electrochemical CO₂ reduction reaction (CO₂RR) to produce multi-carbon (C₂₊) hydrocarbons is highly desirable. Cu-based catalysts are widely recognized as effective for the electrosynthesis of multi-carbon products. Exploration of non-copper-based catalysts for C₂₊ products is very interesting and challenging. In this work, using ethylenediaminetetraacetic acid disodium salt and ethylenediamine as the ligands and N source, N-doped Fe₂O₃ catalysts containing FeO_1−xN_x (0.34 < x < 0.54) sites with and without a FeN₄ structure were fabricated. It was found that the catalyst without a FeN₄ structure converted CO₂ to C₂ hydrocarbons (ethane and ethylene). The faradaic efficiency (FE) of C₂ products and the current density of C₂ products reached 60.8% and 39.1 mA cm⁻², respectively, which is currently the best result for non-copper catalysts in an H-cell. However, the FE of the catalyst with a FeN₄ structure was much lower when producing C₂ products. Detailed study showed that the FeO_1−xN_x sites with suitable coordination of N with Fe was pivotal to the high FE of C₂ products. A combination of experimental and density functional theory studies indicated that the feasible coordination of N with Fe resulted in the deformation of the electron cloud around the Fe nucleus, which facilitated the charge transfer and promoted the production of C₂ products. This work provides a successful example of designing non-copper catalysts for producing C₂₊ products in the CO₂RR.

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Green foundation

1. We report N-doped Fe₂O₃ catalysts containing FeO_1−xN_x (0.34 < x < 0.54) sites with and without a FeN₄ structure. It was found that the performance for the catalyst without a FeN₄ structure was excellent for the electroanalytic CO₂ reduction reaction to produce C₂ hydrocarbons (ethane and ethene).

2. The faradaic efficiency (FE) and current density of C₂ products reached 60.8% and 39.1 mA cm⁻², respectively, which is currently the best result for non-copper catalysts in an H-cell. This work provides a successful example of designing non-copper catalysts for producing C₂₊ products from the CO₂RR.

3. In future studies, the utilization of flue gas as a direct raw material for carbon dioxide sources will be explored, considering the excellent ability of ionic liquid to capture CO₂. This type of approach could be more cost-effective in practical application. Moreover, realizing high efficiency for the formation of C₂₊ products in inexpensive, and more easily recyclable electrolytes over Fe-based catalysts is an interesting topic for future research. Furthermore, other efficient non-copper catalysts, such as multi-metal catalysts, should be designed for producing C₂₊ products, and the relationship between composition, structure, and efficiency should be disclosed.

Introduction

The conversion of carbon dioxide (CO₂) to high value-added chemicals or fuels under ambient temperature is considered as a promising and effective approach to solve the greenhouse effect problem.^1,2 Many efforts have been devoted to achieving carbonaceous products by constructing various electrocatalysts, such as Cu-, Fe-, Co-, Ni-, and Zn-based materials, for the electrochemical CO₂ reduction reaction (CO₂RR).^3–7 The use of earth-abundant catalysts for the electrocatalytic reduction of CO₂ to carbonaceous products is desirable. Iron is the most inexpensive and one of the most abundant transition metals, and iron-based catalysts have been extensively studied for CO₂RR, which mainly produces C₁ products such as CO with high efficiency and selectivity.^8–11 The further conversion of CO₂ into more energy-intensive, higher-value C₂ products is desirable, but still remains a challenge.

In recent years, the activity for generating C₂ products has been enhanced by introducing elemental dopants such as S, P, and N into Fe-based catalysts, which indeed promotes the production of C₂ products.¹² However, the efficiency of the CO₂RR in obtaining C₂₊ products is far from satisfactory. Additionally, the nature of the active sites responsible for C–C coupling in Fe-based catalysts continues to remain elusive to date.

Additional studies have verified that metal centers are crucial for the CO₂RR. It was reported that dispersed M–N_x (M = Mn, Fe, Ni) sites in electrocatalysts exhibited high faradaic efficiency (FE) for CO.¹³ It can be concluded that metal sites rich in elemental N are feasible for producing C₁ products, which was attributed to the desorption of *CO intermediates over M–N_x sites.^14–18 In a previous work,¹⁹ we also found that the variation of N coordination with Fe in iron-based catalysts resulted in different selectivity toward C₁ or C₂ products in the CO₂RR.

Based on experimental results and previous reports, it is speculated that the desorption of *CO prevents further transformations for C–C coupling to generate C₂₊ products.^20–23 Therefore, it is expected that developing catalysts to tune the adsorption ability of *CO will realize the reaction routes toward C–C coupling during the CO₂RR. In other words, the Fe–N_x sites that are in favor of desorbing *CO intermediates should be avoided in the catalysts. However, iron-based catalysts with N dopant can generally be obtained in one pot by annealing iron hydroxides at elevated temperatures in the presence of a N source. It is inevitable that certain single- or double-active centers will be generated. Moreover, Fe–N_x sites will usually form due to the feasible migration of N during the preparation. Precise regulation of N–Fe–O sites in the doping process to further tune the selectivity towards C₂₊ products is a promising method.

Herein, we designed an approach to control the appropriate Fe–N/O coordination by utilizing the competitive coordination of Fe²⁺ and Cu²⁺ precursors with ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, YH₂) and ethylenediamine (EDA) at different pH values. To distinguish the different structures of EDTA at different pH values, Y was used to represent EDTA with four COO⁻ units and YH₂ to represent EDTA with two COO⁻ units. The obtained precipitates were pyrolyzed under an N₂ atmosphere, and catalysts containing different N occupancies in N–Fe–O sites were acquired. It was demonstrated that the catalyst possessing N–Fe–O sites without a FeN₄ structure was very efficient for the CO₂RR to produce C₂ hydrocarbons, including ethane and ethylene. The faradaic efficiency (FE) of C₂ products and the current density of C₂ products reached 60.8% and 39.1 mA cm⁻², respectively, which is the best result for non-copper catalysts in an H-cell to date. However, the catalyst containing a FeN₄ structure showed much lower FE for C₂ products. The correlation of catalytic performances and various N–Fe–O sites with different N occupancies was explored by a combination of experimental results and density functional theory (DFT) study.

Results and discussion

The coordination between metal ions and chelators (YH₂ and EDA) is a dynamic process. YH₂ exhibits various conformations with different association constants under varied pH conditions, resulting in different abilities to chelate with metal ions.^24–26 Based on this principle, we propose a strategy to use competitive coordination to fabricate N-doped iron oxide.

YH₂ was dissolved in deionized water, and after the preferential addition of an excess amount of FeSO₄ into the YH₂ solution to ensure the saturated coordination of YH₂ with Fe²⁺, CuSO₄ and EDA were successively added to the solution. During the process, the reaction flask was sealed. The exact molar ratios are shown in Table S1.† As illustrated in Fig. 1a: stage I: the equilibrated coordination of Fe²⁺ with YH₂ was permitted to preferentially form a complex of Fe-YH₂. At this point, Fe²⁺ is in excess, and existing Fe²⁺ ions are not coordinated with EDTA-2Na. Stage II: CuSO₄ was added to the solution with pH = 2.83, leading to a coordination of Cu²⁺ with the COOH groups in the Fe-YH₂ complex to obtain Fe-YH₂-Cu, which was soluble in the solution. Considering that the ratio of –COOH groups to Cu²⁺ is approximately 2 : 1 (Table S1†), it is likely that one Cu atom may coordinate with two –COOH groups. Stage III: the addition of EDA resulted in a variation of pH from 2.83 to 10.83 in the solution.

Fig. 1 Preparation of catalysts. (a) Details and mechanism for the preparation of catalyst p-N-Fe₂O₃. (b) Schematic diagram of the competitive coordination process in the preparation of p-N-Fe₂O₃ before pyrolysis. Notes: coordination constants in different stages: stage I, Fe(II)-YH₂: 1.568 ≈ Cu(II)-YH₂: 1.553, Cu(II)-COOH: 2.3; Fe(II)-COOH: non; stage II: Fe(II)–Y: 14.27 > Fe(II)-OH₂: 9.77 > Fe(II)–EDA: 7.65, Fe(II)-OH₂: 9.77 > Fe(II)-YH₂: 2.7, Cu(II)–EDA: 20 > Cu(II)–Y: 18.7 > Cu(II)-OH₂: 13.68.

Part I in stage III: in the process, the formation of COO⁻ groups derived from the COOH-Cu structure in the Fe-YH₂-Cu complex was retarded because the disassociation of Cu²⁺ from COOH-Cu should initially occur before the deprotonation process of COOH. As a result, part of Fe-YH₂ preferentially coordinated with the OH⁻ group instead of with the COO⁻ group (Fig. 1b), generating insoluble complexes including Y-Fe(OH)₂, which quickly precipitated. Part II in stage III: on account of the large molar ratio of EDA and EDTA (60 : 1) and the large coordination constants of EDA with Cu²⁺ (log K_Y–Cu < log K_EDA–Cu), the soluble complex of 2EDA–Cu formed and was dissociated from Fe-YH₂-Cu. The 2EDA–Cu complex was stable under conditions with a pH of 10,²⁷ and the residual amount of Fe²⁺ formed insoluble Fe(OH)₂ and EDA-Fe(OH)₂. In addition, soluble complexes containing Y–Fe, 2EDA–Fe, and 2EDA–Cu were simultaneously generated and then removed by centrifugation.

The obtained precipitates were washed and analyzed by infrared (IR) spectroscopy, which exhibited the characteristic peaks of Y and EDA, indicating the successful coordination of Fe²⁺ with Y and EDA (Fig. S1†). In addition, the detailed coordination behavior was explored by in situ XAS experiments. The peak centered around 1.7 Å was assigned to Fe–O(N) coordination due to the difficult distinction between Fe–O and Fe–N coordination in XAFS. During the preparation process before adding EDA, the XANES curves and Fourier transform curves showed that Fe and Cu were in the state of Fe²⁺ and Cu²⁺, respectively (Fig. S2 and S3†).^28,29 Excess EDA preferentially coordinates with Cu²⁺ and Fe²⁺, maintaining their oxidation states. Quantitative extended X-ray absorption fine structure (EXAFS) fitting analysis (Table S2†) revealed that there was a subtle variation in the coordination numbers of Fe–O/N after the addition of CuSO₄, demonstrating the stability of the Fe-YH₂ structure. The as-prepared precipitates were pyrolyzed under N₂ to obtain a catalyst, which was denoted as p-N-Fe₂O₃.

To study the effect of the amount of Fe on the ratio of different components in the precipitates, we chose 75 mg, 100 mg, 150 mg, and 300 mg of FeSO₄ to investigate the composition under the same conditions. No precipitates were obtained under the condition of adding 75 mg FeSO₄ with an equimolar amount of YH₂. The precipitation occurred when 100 mg, 150 mg, and 300 mg of FeSO₄ were used, and the precipitation precursors obtained were abbreviated as Fe-100, Fe-150, and Fe-300, respectively, and characterized using organic elemental analysis (OEA). The molar fraction of different components in different catalysts was calculated according to the OEA results, as shown in Fig. S4.†

The results demonstrated that the molar fraction of Y-Fe(OH)₂ increased with the amount of Fe, while the molar fractions of EDA-Fe(OH)₂ and Fe(OH)₂ slightly decreased. The excess amount of FeSO₄ resulted in an increasing molar fraction of Y-Fe(OH)₂ by inhibiting the formation of Cu-YH₂ and increasing the stability of Fe-YH₂-Cu. In addition, EDA was used to provide an alkaline environment for this method, and an excess amount of EDA was utilized to inhibit the formation of Cu(OH)₂ due to the larger coordination constant of EDA and Cu(II).

In addition, a control sample named N-Fe₂O₃ was also prepared under conditions free from CuSO₄ addition to investigate the competitive effects during coordination (Fig. S5†). Under the circumstances, the saturated coordination of Fe²⁺ with YH₂ was permitted to form the complex of Fe-YH₂ in the presence of YH₂ and FeSO₄. However, the addition of EDA contributed to the change in solution to alkaline, which resulted in feasible coordination of the COO⁻ group with Fe²⁺ instead of coordination of the OH⁻ group with Fe²⁺ (Fe(II)–Y: 14.27 > Fe(II)-OH₂: 9.77). As a result, soluble complexes of Y–Fe and 2EDA–Fe were generated, and precipitates consisting of EDA-Fe(OH₂) and Fe(OH)₂ were obtained without Y-Fe(OH)₂. After separation and pyrolysis, the N-Fe₂O₃ catalyst was prepared. The obtained sediments showed no signal of symmetrical stretching vibration of C–O (1390 cm⁻¹) in the IR spectra, indicating the absence of a Y structure in N-Fe₂O₃, which was attributed to the nonexistence of the Y-Fe(OH)₂ complex in the precipitates before pyrolysis (Fig. S1†).

The as-prepared precipitates were pyrolyzed under an N₂ atmosphere to generate catalysts that were characterized by X-ray diffraction (XRD), as shown in Fig. 2a. The XRD patterns of all the catalysts were consistent with the characteristic peaks of pristine γ-Fe₂O₃ (PDF #39-1346), indicating that the main component of the obtained catalysts was γ-Fe₂O₃, and slight differences were observed between those of p-N-Fe₂O₃ and N-Fe₂O₃. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to study the morphologies of the catalysts. The morphologies of p-N-Fe₂O₃ and N-Fe₂O₃ exhibited a bulk structure composed of nanoparticles with a size distribution of 50–100 nm, as shown in Fig. 2b, and Fig. S6, S7,† respectively.

Fig. 2 Characterizations of the catalysts. (a) XRD patterns of p-N-Fe₂O₃ and N-Fe₂O₃. (b) TEM image of p-N-Fe₂O₃. (c) STEM image of p-N-Fe₂O₃ and the corresponding STEM-EDS elemental mapping images of Fe, O, and N of the region. (d) HRTEM images of p-N-Fe₂O₃. (e) Fe K-edge XANES spectra of p-N-Fe₂O₃, N-Fe₂O₃, Fe₂O₃, FeO, and Fe foil. (f) The first derivatives for p-N-Fe₂O₃, N-Fe₂O₃, Fe₂O₃, FeO, and Fe foil. (g) Fourier-transformed EXAFS spectra in R-space of p-N-Fe₂O₃, N-Fe₂O₃, Fe₂O₃, FeO, and Fe foil. (h) XPS spectra of p-N-Fe₂O₃ and N-Fe₂O₃ in Fe 2p.

Scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) demonstrated the existence of elemental Fe, N, and O in p-N-Fe₂O₃ and N-Fe₂O₃ (Fig. 2c and Fig. S8†). The element Fe was the main component in the catalyst, and the results were similar to those detected by inductively coupled plasma optical emission spectrometry (ICP-OES) (Table S3†), and the element Cu was not detected by ICP-OES. In the high-resolution TEM (HRTEM) of p-N-Fe₂O₃ (Fig. 2d), the emerged fringe spacings of 0.208 nm and 0.295 nm were well matched with the (400) and (220) lattice planes of γ-Fe₂O₃, and the results were in accordance with those of XRD.

The detailed coordination environment of Fe atoms in the catalysts was explored using X-ray absorption spectroscopy (XAS).^30,31 The K-edge X-ray absorption near edge structure (XANES) curves and the derivatives of the catalysts (Fig. 2e and f) showed that the element Fe was in the state between Fe²⁺ and Fe³⁺. The edge of the XANES spectra of the N-doped Fe₂O₃ samples (p-N-Fe₂O₃ and N-Fe₂O₃) shifted towards a lower energy as compared to that of pristine Fe₂O₃, suggesting a negative charge state of Fe atoms in the as-prepared catalysts.

The Fourier transform (FT) curves at the Fe K-edge of the extended X-ray adsorption fine structure (EXAFS) spectra were further fitted to study the coordination information for Fe atoms, as shown in Fig. 2g. The peak at 2–3 Å was well matched with the Fe–Fe bond in Fe₂O₃.^32,33 The doping of N in Fe₂O₃ resulted in the wide peaks of Fe–O/N and Fe–Fe in p-N-Fe₂O₃, which shifted towards a higher energy as compared to that of Fe₂O₃, indicating the variation in the electronic states of the element Fe in the catalysts.^34,35 It is worth noting that signals of elemental Cu and corresponding peaks were not observed. Furthermore, the XAS data were fitted to explore the detailed coordination information of the element Fe in different catalysts (Table S4†). It was discovered that the coordination number of Fe–O/N in p-N-Fe₂O₃ and N-Fe₂O₃ was lower compared to pristine Fe₂O₃, suggesting the presence of oxygen vacancies and defects in both of the obtained catalysts.

The electronic states of Fe in the catalysts were also analysed by XPS. The XPS spectra of p-N-Fe₂O₃ showed the existence of the elements Fe, O, N, and C in the catalyst (Fig. S9a†), and there was no characteristic peak of Cu in the spectra (Fig. S9b†), which was in accordance with the results characterized by XAS and ICP methods. As illustrated in Fig. 2h, the peaks at 710.8 eV and 724.8 eV in the Fe 2p spectra indicated that Fe atoms in the catalyst existed in the state of Fe³⁺.^36,37 The high-resolution O 1s spectra of p-N-Fe₂O₃ and N-Fe₂O₃ were assigned to three components corresponding to lattice oxygen (approximately 530 eV), oxygen vacancy (531.5 eV), and surface-adsorbed oxygen species (533.5 eV) (Fig. S9c†).^38,39 The N dopant resulted in facile generation of surface defects, such as oxygen vacancy, which was observed in the high-resolution O 1s spectra of these two catalysts. As displayed in Fig. S9d,† the elemental N in the catalysts mainly existed in the form of pyridinic N (398.1 eV) and Fe–N coordination with pyridine (399.6 eV).⁴⁰

⁵⁷Fe Mössbauer spectroscopy is sensitive to the electronic states of Fe with a similar coordination environment, which is an efficient method for the study of the valence state and the magnetic structure of Fe in catalysts.⁴¹ The p-N-Fe₂O₃ and N-Fe₂O₃ catalysts were further characterized by ⁵⁷Fe Mössbauer spectroscopy, and the results are illustrated in Fig. 3. The spectra of p-N-Fe₂O₃ was fitted into two types of sextets that were attributed to the γ-Fe₂O₃ phase and N-doped Fe₂O₃. According to the values of isomer shift (IS) and quadrupole splitting (QS) (Table S5†), N-doped Fe₂O₃ was denoted as FeO_1.5−xN_x (0.34 < x < 0.54) sites in p-N-Fe₂O₃.

Fig. 3 ⁵⁷Fe Mössbauer spectroscopy of the catalysts. (a) p-N-Fe₂O₃. (b) N-Fe₂O₃.

The hyperfine field (H) in the components of the FeO_1.5−xN_x (H = 46.08) phase was slightly different from that of γ-Fe₂O₃ (H = 49.63), which was attributed to the incorporation of a small amount of N in the γ-Fe₂O₃. Similar sextets attributed to FeO_1.5−yN_y (0.34 < y < 0.54) sites in N-Fe₂O₃ were also detected. However, a doublet attributed to the FeN₄ phase was observed in the Mössbauer transmission spectra of N-Fe₂O₃.⁴² The absence of the doublet component in the Mössbauer spectrum of p-N-Fe₂O₃ demonstrated the absence of the FeN₄ phase. In addition, it is noteworthy that obvious differences in quadrupole splitting (QS) were found for N–Fe–O sites in the different catalysts, as shown in Table S5.†

The QS describes the axially symmetric deviation from the spherical shape of the positive charge distribution in the nucleus, which could exert an ulterior influence on the charge around the nucleus. A positive QS value (QS > 0) indicates that the charge distribution is elongated at the poles (prolate shape), exhibiting a cigar shape. In other words, if the charge distribution is compressed at the poles (oblate shape) showing a pancake shape, the value of the QS is negative (QS < 0).⁴³ The distortion of the Fe–O coordination derived from the doping of N into Fe₂O₃ may have an obvious influence on the QS.^44–47 Herein, the Fe ion spin states did not change according to the small QS value (QS < 1 mm s⁻¹). In p-N-Fe₂O₃, the QS value of FeO_1.5−xN_x (QS = −0.02) was negative, which was different from that of FeO_1.5−yN_y (QS = 0.01). It can be concluded that the differences were caused by various N-doping sites in Fe₂O₃. The different N-doping strategies resulted in various iron-coordination components and Fe–O–N sites. An H-type cell was used to evaluate the performance of the catalysts for the CO₂RR. Ionic liquid (IL)-based electrolyte (1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF₆/acetonitrile (MeCN)/H₂O (w/w/w = 30/65/5))) was used. It was previously reported that IL exhibited the ability to reduce the reaction barrier for CO₂ activation and suppress the hydrogen evolution reaction (HER).⁴⁸ The linear sweep voltammetry (LSV) curves of different catalysts in CO₂- or N₂-saturated electrolytes are illustrated in Fig. S10.† In the CO₂-saturated electrolyte, higher current densities were obtained at the potential ranging from −0.6 V to −2.8 V vs. Ag/Ag⁺, indicating that the catalysts were highly active for CO₂RR. Therefore, the potential ranging from −1.9 V to −2.6 V vs. Ag/Ag⁺ was selected as the reaction potential.

The gaseous and liquid products were collected and analyzed by gas chromatography (GC) and ¹H NMR, respectively. Only gas products were detected in the experiment. As shown in Fig. 4a, H₂, CH₄, C₂H₄, and C₂H₆ were produced over p-N-Fe₂O₃. At −2.0 V vs. Ag/Ag⁺, the FE of C₂ products reached 60.8%, and the current density of C₂ products was 39.1 mA cm⁻², which is the highest result for non-copper catalysts in an H-cell (Fig. S11†).^8,12,49–52 The HER was the predominant side reaction in this process as the potential became more negative. Additionally, the FE of C2 products gradually decreased with enhanced H₂ production.

Fig. 4 Electrochemical properties study and the charge density difference of catalysts. (a) Catalytic performances of p-N-Fe₂O₃. (b) Long-term stability of CO₂ electroreduction over p-N-Fe₂O₃ and N-Fe₂O₃. (c) In situ X-ray emission spectroscopy for p-N-Fe₂O₃ at the potential ranging from −1.9 V to −2.6 V vs. Ag/Ag⁺. (d) In situ X-ray emission spectroscopy for N-Fe₂O₃ at the potential ranging from −1.9 V to −2.6 V vs. Ag/Ag⁺. (e) The corresponding differential charge densities and Bader charges for p-N-Fe₂O₃ and N-Fe₂O₃. The yellow and cyan surfaces correspond to the charge gain and lost regions, respectively.

During the preparation of p-N-Fe₂O₃, a reduction in the iron content resulted in a significant enhancement of the HER and an obvious decrease in carbonaceous products (Fig. S12†). For comparison, we evaluated the performance of N-Fe₂O₃ under the same conditions. For the N-Fe₂O₃ catalyst, the FE of C₂ products was 20%, and the current density decreased to 10 mA cm⁻², with CO as the main product (Fig. S13†). A control experiment was conducted under the same conditions using N₂-saturated electrolyte, but no carbonaceous products were detected. The CO₂RR origin of the products was confirmed by measuring isotope-labelled ¹³CO₂ (Fig. S14†).

We also evaluated the catalytic performance of p-N-Fe₂O₃ in 0.1 M KHCO₃ (Fig. S15†) and observed the formation of multi-carbon products. However, because of the pronounced HER, the FE of C₂ products was relatively low. This further demonstrated that ionic liquid played a critical role in suppressing the HER. Thus, the p-N-Fe₂O₃ catalyst containing FeO_1.5−xN_x sites boosted the production of C₂ products. Significantly, the p-N-Fe₂O₃ catalyst without FeN₄ sites exhibited no production of CO. However, the N-Fe₂O₃ catalyst containing partial FeN₄ sites exhibited a tendency to generate CO. Furthermore, the C₂ selectivity was obviously low.

The results verified the conclusion that the coexistence of FeN₄ in γ-Fe₂O₃ nanoparticles resulted in facile production of CO and was not favorable to C–C coupling. As reported, the CO selectivity over FeN₄ sites was attributed to the adsorption of CO₂ on the FeN₄ sites together with the acceleration of *COOH intermediate formation.^53,54 The long-term stability over p-N-Fe₂O₃ and N-Fe₂O₃ were estimated at −2.0 V vs. Ag/Ag⁺. The FE of C₂ products and the current density over p-N-Fe₂O₃ showed no obvious change after 16 h, indicating the excellent stability during the CO₂RR. However, when using N-Fe₂O₃ as the electrode, the FE clearly decreased after a period of time, as shown in Fig. 4b.

To explore the nature of the performance of various catalysts during the CO₂RR, in situ X-ray emission spectroscopy (XES) was also performed. As displayed in Fig. 4c, the signals of p-N-Fe₂O₃ inconspicuously changed at different potentials, indicating that elemental Fe maintained the Fe³⁺ state during the CO₂RR. However, the signals of Fe in N-Fe₂O₃ gradually shifted to high energy at more negative potentials, signifying that partial Fe³⁺ was continuously reduced (Fig. 4d). The various N occupancies resulted in different distortions of Fe–O coordination and the oxygen vacancy in the two catalysts, which influenced the stability of the catalysts.

To further investigate the stability of p-N-Fe₂O₃, we characterized the catalyst using XRD, TEM, SEM, and EDS mapping before and after the reaction (Fig. S16†). The results indicated that the difference in the structure and morphology of the catalyst before and after the reaction was not notable, which signified the excellent stability of p-N-Fe₂O₃ during the reaction process. Additionally, the coordination number of Fe–O in FeO and Fe₂O₃ is 6, indicating that the coordination number is due to oxygen vacancies and defects rather than a mixture of iron oxides not being Fe₂O₃.⁵⁵

The electrochemically active surface area (ECSA) of the catalysts was estimated by fitting the double electric layer capacitance of the different catalysts (Fig. S17†). The higher double-layer capacitance value of p-N-Fe₂O₃ (53.57 mF cm⁻²) compared with N-Fe₂O₃ (42.9 mF cm⁻²) indicates that there is a higher electrochemical surface area for p-N-Fe₂O₃.⁵⁶ The Nyquist plots under open circuit potential in CO₂-saturated electrolyte were used to determine the properties of the electrode/electrolyte interface (Fig. S18†). The values for the interfacial charge transfer resistance (R_ct) were obtained by fitting the experimental impedance data using Randles equivalent circuit R (C(R(Q(RW)))), as illustrated in Fig. S19.† ⁵⁷ The interfacial charge transfer resistance was lower for the p-N-Fe₂O₃ catalyst as compared with N-Fe₂O₃, indicating the facile charge transfer over p-N-Fe₂O₃, which was attributed to the appropriate N occupancies in Fe₂O₃.

The results were also in accordance with the electronic states of Fe characterized by ⁵⁷Fe Mössbauer spectroscopy. Various N–Fe–O components were formed due to the different N neighbours of Fe atoms in the lattice, which resulted in a different iron electron structure, leading to the different activities and selectivities of the CO₂RR. FeO_1−xN_x sites in p-N-Fe₂O₃ displayed a negative QS value, resulting in a compressed charge distribution around the poles of the Fe nucleus, which facilitated the charge transfer. However, the N-Fe₂O₃ catalyst containing N–Fe–O sites with other N occupancies in Fe₂O₃ displayed an elongated charge distribution around the poles of the Fe nucleus. Therefore, the electronic states of Fe exhibited a low ability for charge transfer. In addition, the coexistence of FeN₄ sites facilitated the desorption of *CO,⁵⁸ resulting in C₂ production with low efficiency.

On the basis of the above analysis, DFT studies were carried out to identify and characterize the atomic sites in iron-based catalysts for promoting the selectivity of C₂ products. According to the properties of the N–Fe–O sites obtained by various experiments, two catalyst models were constructed based on the QS values derived from the ⁵⁷Fe Mössbauer spectroscopy, as shown in Fig. S20.† The desorption free energy of the *CO intermediate over p-N-Fe₂O₃ is higher than that over N-Fe₂O₃, as illustrated in Fig. S21.† The difficult desorption of *CO leads to low selectivity for CO, which is consistent with the experimental results. Additionally, the corresponding charge density difference of the various catalysts is shown in Fig. 4e, which indicates that different N occupancies in Fe₂O₃ result in an alteration of charge distribution around the Fe atom. The delocalized electron transfer from the Fe atoms to the N–Fe–O interface was also confirmed by Bader charge analysis (Fig. 4e). Images from other views are provided in Fig. S22.†

An electron-rich interface for p-N-Fe₂O₃ is formed because of the delocalized electron transfer from Fe atoms at N–Fe–O sites, which greatly facilitates the adsorption and activation of the *CO intermediate promoting C–C coupling. Thus, the electronic effects of the N–Fe–O sites resulted in increased selectivity toward C₂ products compared with the N-Fe₂O₃ catalyst. Additionally, the Fe-d states of the catalysts distinctly changed as a result (Fig. S23†). Thus, the FeO_1−xN_x sites in the p-N-Fe₂O₃ catalyst, which have compressed charge distribution around the poles of the Fe nucleus, facilitate the adsorption of *CO intermediate and C–C coupling in the CO₂RR.

To further exclude the effect of copper on the production of C₂ products, we prepared corresponding catalysts using Ni and Cd salts as substitutes for Cu salt, and conducted characterization and performance tests (Fig. S24 and S25†). The characterization indicated that there were no remaining Ni/Cd elements in the catalysts. The faradaic efficiency of C₂ products over the two catalysts prepared with elemental Ni and Cd was 55.31% and 59.7%, respectively, which were comparable to those of p-N-Fe₂O₃. This further confirmed that the production of C₂ products was not attributed to Cu.

Conclusions

The p-N-Fe₂O₃ catalyst containing FeO_1−xN_x sites and free of FeN₄ sites is very efficient for producing C₂ product in the CO₂RR. The FE of C₂ products can reach 60.8% with a current density of 39.1 mA cm⁻², which is currently the best result for non-copper catalysts in an H-cell. In contrast, the N-Fe₂O₃ catalyst possessing FeN₄ and FeO_1−yN_y sites with different N occupancies in Fe₂O₃ showed an FE_C2 of 20% with a current density of 10 mA cm⁻² under the same reaction conditions, which is much lower than that of the catalysts without FeN₄ sites. The high selectivity of C₂ products was attributed to the electron-rich interface of the p-N-Fe₂O₃ catalyst that is formed by delocalizing electron transfer from Fe atoms in N–Fe–O sites. Thus, the adsorption and activation of the *CO intermediate are greatly facilitated. As a result, the process of C–C coupling is promoted. The relationship of N occupancies and selectivity towards C₂ products over the iron-based catalyst is very helpful for exploration of new non-Cu electrocatalysts for the CO₂RR to produce C₂ products.

Author contributions

Peng Chen and Pei Zhang: catalyst synthesis and characterisations. Xinchen Kang, Jun Ma, Chunjun Chen, and Xing Tong: electro-reduction of CO₂. Xueqing Xing, Zhonghua Wu, and Lirong Zheng: collection of in situ XES and XAS data. Zhimin Liu and Shiqiang Liu: DFT calculations. Pei Zhang and Buxing Han: overall design and direction of the project. Peng Chen, Pei Zhang, and Buxing Han: preparation of the manuscript with assistance from all authors.

Data availability

All data generated or analyzed in this study are included in this manuscript and the ESI.† The datasets used and/or analyzed in this study are available upon request from the corresponding authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the National Basic Research Program of China (2022YFA1504904), and the National Natural Science Foundation of China (22073104, 22003070, 22072156, 22273108, 22233006, 22121002). The XAS (1W1B), (1W2B) and SAXS (1W2A) measurements were performed at the Beijing Synchrotron Radiation Facility, China.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5gc00678c

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