Synthesis of Bi/Fe–N–C catalysts for efficient electrochemical CO₂-to-CO reduction

Abstract

Single atom Fe sites, doped in CN materials, exhibit outstanding electrochemical activity for CO₂-to-CO conversion. The pyrolysis of ZIF8 is a controllable method for fabricating isolated single atom metal sites. In this study, we propose a new strategy to increase the ratio of Fe in ZIF8 precursors by synergistically replacing Zn²⁺ with Bi³⁺ and Fe³⁺. After precursor pyrolysis, the obtained Bi/Fe–N–C catalysts, consisting of Bi sites and pyrrole-type Fe–N_x sites, serve as efficient electrocatalysts for the CO₂RR. The results show that the optimized catalyst loaded with 94.8 mg per kg_Cat Fe exhibits a high FE_CO of >90.1% over a wide potential range of −0.4 to −0.7 V_RHE (98.2% at −0.5 V_RHE). Insights into the electrochemical reaction mechanism show that this successful design of Bi/Fe–N–C catalysts can provide a stable catalytic site to form *COOH, thus achieving energy-efficient electrochemical CO₂ reduction to CO.

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1. Introduction

The pursuit of carbon neutrality stimulates efforts to absorb and recycle CO₂, which is a greenhouse gas that has a significant impact on climate change.^1–4 Electrochemical CO₂ reduction reaction (CO₂RR) is an attractive pathway to obtain value-added products (such as CO, CH₃OH, and CH₄), especially using renewable electricity.^5–8 Carbon monoxide (CO), as an economically viable product from the CO₂RR, has a high ratio of molecular weight per electron transferred.^9–11 To date, efforts toward CO₂RR catalysts have focused on the development of highly active, selective, and stable catalysts for the CO product. Different types of metals, such as metal oxides, metal hybrids, phthalocyanine molecules and doped carbon, have been reported as catalysts for the conversion of CO₂ to CO.^12–14 However, it remains a formidable challenge to achieve efficient and robust CO₂ electroreduction based on the currently known catalysts.

Carbon-based materials have received extensive attention for the CO₂RR due to their porous structure and controllable doping modification.^15,16 These materials have distinct intrinsic properties, such as abundant natural resources, environmental friendliness, customized structures, high surface area, tailored structures, high-temperature stability, superior electric conductivity, and chemical inertness to acids and bases.¹⁷ Despite the low CO₂RR activity of pure carbon-based catalysts, heteroatom doping can improve the current efficiency and conductivity of carbon-based materials owing to the strong electron connection between the heteroatom and carbon.¹⁸ The N-doped carbon-based materials outperform other doping atoms (S, B, F, and P) in terms of overall performance.^19,20 Based on the reported results,²¹ the graphitic and pyridinic N of N-doped carbon-based materials play a key role in increasing the selectivity and decreasing the overpotential towards CO₂-to-CO conversion. Meanwhile, calculations by density functional theory (DFT) confirmed that pyridinic N atoms with a lone pair of electrons can bind to CO₂.^22,23 Despite multiple promising results, the fatal flaw of metal-free catalysts is their poor catalytic activity, which limits further research on the CO₂RR process.

Porous carbon materials doped with N-coordinated metal sites (M–N–C), usually derived from pyrolyzing the precursors composed of metal-(M-), N- and C-containing complexes, are derivative electrocatalysts of N-doped carbon for the CO₂ reduction reaction (CO₂RR).^24–26 With remarkable catalytic activity and excellent selectivity, M/N-doped carbon has gained a lot of attention, especially for Fe–N–C catalysts.²⁷ Many studies have confirmed that the effective method for improving catalytic performance towards the CO₂RR is to achieve high exposure of Fe–N_x sites on Fe–N–C catalysts.^21,28–30 For this purpose, Fe-doped ZIF8 is widely selected as a precursor to design Fe–N–C catalysts.³¹ Much effort has been put into precisely designing ZIF8 precursor with high surface areas and rich micropores, such as surface functionalization with ammonium ferric citrate,³⁰ post-synthetic strategy,^32,33 and carrier modification.³⁴ However, the framework structure of ZIF8 determines that Fe³⁺ can only partially replace the Zn²⁺ center site, limiting the molar ratio of Fe³⁺/Zn²⁺.³⁵ Few studies have been reported to dramatically increase the Fe ratio in the ZIF8 precursor towards Fe–N–C catalysts. In pursuit of better CO₂RR performance, it is important to develop a method to increase the upper limit of the iron content in ZIF8 precursors.

In this paper, we report a chemical assisting approach to synthesize Bi/Fe-doped ZIF8 precursors. The Bi element is explored to increase the Fe ratio in ZIF8. One-step thermal activation at 950 °C is applied to successfully prepare the Fe–N–C catalysts with isolated Fe–N_x sites anchored on N-doped carbon. The Fe–N–C catalyst with the highest CO₂RR activity was screened by regulating the Bi content in the mother liquor of the ZIF8 precursor. The Bi₄/Fe–N–C catalyst with the loaded 94.8 mg per kg_Cat Fe exhibits a high FE_CO above 90% over a wide potential range from −0.4 to −0.7 V_RHE (98% at −0.5 V_RHE). A series of experimental measurements and in situ characterization reveal that a high Fe ratio in Bi/Fe–N–C is conducive to promoting the dissociation of H₂O, accelerating the proton transfer process and forming *COOH intermediates.

2. Materials and methods

2.1 Reagents

Analytical grade zinc nitrate hexahydrate Zn(NO₃)₂·6H₂O, iron(III) nitrate nonahydrate Fe(NO₃)₃·9H₂O, 2-methylimidazole (mIm), bismuth nitrate pentahydrate Bi(NO₃)₃·5H₂O, and potassium bicarbonate (KHCO₃) were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd, China. Nafion (5%) was acquired from Sigma-Aldrich. All chemicals used in this experiment were of analytical grade and were used without further purification. Carbon Paper (Toray Carbon Paper, Baselayered, TGP-H-60, 12 × 12 cm²) was obtained from J&K Scientific Technology Co., Ltd, and ultrasonically cleaned in ethanol.

2.2 Synthesis of M-ZIF8 (M = Bi, Fe, Bi & Fe)^28,29

Typically, 1.232 g of 2-methylimidazole (mIm) was dissolved in 100 mL of methanol under ultrasound for 15 min to form solution A. Zn(NO₃)₂·6H₂O (1.116 g), Bi(NO₃)₃·5H₂O (x mg, x = 10, 20, 30, 40 and 50) and Fe(NO₃)₃·9H₂O (80 mg) were dissolved in 100 mL of methanol with different matches and then treated with ultrasound for 15 min to form solution B. Next, solution A was poured into solution B quickly in a 500 mL flask under intensive stirring; then, the flask was maintained at 60 °C in a water bath by refluxing for 6 hours. The obtained product was separated by centrifugation at 9000 rpm for 3 minutes, washed three times with methanol, and finally dried in a vacuum at 70 °C overnight. According to the dosage of Bi, the samples are sequentially denoted as Bi₁/Fe-ZIF8, Bi₂/Fe-ZIF8, Bi₃/Fe-ZIF8, Bi₄/Fe-ZIF8 and Bi₅/Fe-ZIF8.

2.3 Synthesis of Bi/Fe–N–C series

The synthesis of Bi/Fe-doped ZIF8 (Bi/Fe-ZIF8) is illustrated in Fig. 1.³² Typically, the powder of Bi/Fe-ZIF8 was placed in a tube furnace, kept at 950 °C for 3 hours at a heating rate of 5 °C min⁻¹ under flowing argon gas, and then naturally cooled to room temperature. The as-prepared samples of Bi/Fe–N–C were directly used without any post-treatment.

Fig. 1 Illustration of the Bi-doping chemistry strategy to fabricate Bi/Fe-doped ZIF8 precursors for catalyst preparation.

According to the preparation process of Bi/Fe-ZIF8 precursors, the mass ratios of Bi(NO₃)₃·5H₂O and Fe(NO₃)₃·9H₂O were configured to 1/8 (10 mg/80 mg), 2/8 (20 mg/80 mg), 3/8 (30 mg/80 mg), 4/8 (40 mg/80 mg) and 5/8 (50 mg/80 mg). After thermal activation, the obtained catalysts correspond to Bi₁/Fe–N–C, Bi₂/Fe–N–C, Bi₃/Fe–N–C, Bi₄/Fe–N–C and Bi₅/Fe–N–C, respectively.

2.4 Characterization

Powder X-ray diffraction patterns of the samples were recorded using Purkinjie XD-3 with Cu Kα radiation. High-resolution TEM and HAADF-STEM images of the corresponding electron energy-loss spectroscopy were recorded using an FEI Tecnai G2 F20 S-Twin high-resolution transmission electron microscope at 200 kV and a JEOL JEM-ARM200F TEM/STEM with a spherical aberration corrector working at 300 kV. Scanning electron microscopy (SEM) was carried out using CIQTEK SEM4000. Brunauer–Emmett–Teller (BET) analysis was conducted on Micromeritics ASAP2460 at 150 °C with N₂ as the adsorptive gas. X-ray photoelectron spectroscopy (XPS) was performed at the Catalysis and Surface Science End station at the BL11U beam line of the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. Elemental analysis of Fe, Bi, and Zn in the solid samples was performed using a ThermoFisher i CAPQ inductively coupled plasma mass spectrometer (ICP-MS) and ICS-600.

2.5 Electrochemical measurements

The experiments were performed in a gas-tight cell with two compartments separated by a cation exchange membrane (Nafion-R115, Dupont), and the cell was connected to an electrochemical station (CHI 660E). Each compartment contained 70 mL of electrolyte (0.5 M KHCO₃ made from ultrapure water) with approximately 30 mL of headspace. A Pt plate was used as a counter electrode. All potentials were measured against an Ag/AgCl (saturated KCl) reference electrode and converted to those against a reversible hydrogen electrode (RHE). In a typical preparation procedure for working electrodes,³⁶ the catalyst (4 mg) was dispersed in 40 µL of Nafion solution (5 wt%) and 1 mL of ethanol solution with sonication for 30 min to form a homogenous ink. Then, 140 µL of the homogeneous ink was loaded by dripping on the two sides of the carbon fiber paper and dried naturally to obtain the working electrode. The geometric area of the working electrode was confined within 1 × 0.5 cm². All chemicals were used without further purification.

During the CO₂ reduction experiments, the electrolyte in the cathodic compartment was stirred at 900 rpm. Linear sweep voltammetry (LSV) was performed with a scan rate of 10 mV s⁻¹ from −0.6 V to −1.8 V (vs. Ag/AgCl) in Ar-saturated 0.5 M KHCO₃ (pH = 8.8) and CO₂-saturated 0.5 M KHCO₃ (pH = 7.2) as the supporting electrolyte. The potentials in the study were reported versus RHE with the conversion E (vs. RHE) = E (vs. Ag/AgCl) + 0.1989 V + 0.0591 × pH.

The current density was obtained by normalizing it with the carbon fiber paper geometric surface area. CO₂ gas was delivered at an average rate of 20 mL min⁻¹ (at room temperature and ambient pressure) and routed directly into the gas sampling loop (1 mL) of a gas chromatograph. The gas phase composition was analyzed by GC every 30 min. The separated gas products were analyzed using a thermal conductivity detector (TCD) and a flame ionization detector (FID). The quantification of the products was performed using the conversion factor derived from the standard calibration gases. Liquid products were analyzed afterwards using quantitative NMR with dimethyl sulphoxide (DMSO) as an internal standard. The solvent pre-saturation technique was implemented to suppress the water peak.

2.6 Faradaic efficiency

The faradaic efficiency for CO production is calculated at a given potential as follows:
where Q_CO: partial current density for CO production; Q_total: total current density; N: the number of electrons transferred for product formation, which is 2 for CO; n_CO: the molar amount of CO during the measurement time (t); F: faradaic constant, 96 485C mol⁻¹; and FE_CO: faradaic efficiency for CO production.

2.7 Turnover frequency (TOF, h⁻¹)

The TOF for CO was calculated as follows:
where I_CO: partial current for a certain product, CO; N: the number of electrons transferred for product formation, which is 2 for CO; F: faradaic constant, 96 485C mol⁻¹; m_cat: catalyst mass in the electrode, g; w: Fe loading in the catalyst from ICP; and Fe: atomic mass of iron, 56 g mol⁻¹.

3. Results and discussions

We employed ZIF8 as precursors to provide N and C sources and used bismuth nitrate as an assistant to increase the iron content in ZIF8. After ultrasonic treatment, the raw material mixture was transferred to a round flask and kept at a reaction temperature of 60 °C for 6 hours in a water bath. Based on a reported paper,³³ some Zn²⁺ sites within the ZIF8 framework are exchanged by Fe³⁺ and Bi³⁺ ions to form uniform Bi/Fe–N₄ coordination. For comparison, Fe-doped ZIF8 and dopant-free ZIF8 were also synthesized under the same conditions. As shown in Fig. S1, the optical images of the precursor reflect an obvious yellow change in Bi₄/Fe-ZIF8. The corresponding XRD pattern of Bi₄/Fe-ZIF8 fits well with the Fe-ZIF8 structure (Fig. S2), indicating that these two Fe-doped ZIF8 samples have a similar crystal structure. A morphology comparison determined by SEM images from Bi₁/Fe-ZIF8 to Bi₅/Fe-ZIF8 is illustrated in Fig. 2a–e. From Bi₁/Fe-ZIF8 to Bi₄/Fe-ZIF8, the particle size shows a significant increase trend. The regular dodecahedral structure of ZIF8 is not found in Bi₅/Fe-ZIF8 samples. These findings suggest that Bi can promote the outward derivative growth of ZIF8. However, a high proportion of Bi also inhibits the formation of the ZIF8 framework. Based on the results of multiple analyses of EDS (Fig. 2f and S3–S7), the content of Zn in ZIF8 particles decreases, while the content of Fe and Bi elements increases. This indicates that Bi contributes to increasing the Fe content in ZIF8.

Fig. 2 (a–e) SEM images of Bi₁/Fe-ZIF8, Bi₂/Fe-ZIF8, Bi₃/Fe-ZIF8, Bi₄/Fe-ZIF8 and Bi₅/Fe-ZIF8. (f) Ratios of Zn, Bi, and Fe to N elements based on the EDS results.

After 2 hours of annealing at 950 °C under Ar protection, the N-coordinated Fe sites of Bi/Fe–N–C were successfully anchored on porous carbon, serving for the CO₂RR process as catalytically active sites. Meanwhile, part of the Zn was reduced by high-temperature carbonization and evaporated away (b.p. 907 °C).²⁶ For comparison, Bi–N–C and Fe–N–C were also treated with the same annealing process. As revealed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Fig. S8), the as-prepared Fe–N–C presents a uniform size (450–500 nm) and retains the polyhedral morphology of the reported ZIF-8.^31,35 As shown by EDS mapping images, all key elements (C, N, and Fe) are evenly distributed in the Fe–N–C samples. Importantly, no obvious metal nanoparticles are observed through the above characterization. Based on previous studies,^28–30 it could be concluded that the Fe element is anchored on porous carbon in the form of Fe–N_x.

However, the collapsed edge and dispersive nanoparticles of Bi₄/Fe–N–C are clearly presented in the SEM and TEM (Fig. 3a and b). The morphology of the nanoparticles displays a core–shell nanostructure covered with graphite carbon (Fig. S23). The other major part, without agglomeration, remains the structure of porous carbon. This is also consistent with the results of XRD, as depicted in Fig. S9. The Bi₄/Fe–N–C pattern has no characteristic diffraction peaks, which are similar to Fe–N–C and Bi–N–C. Two broad diffraction peaks are located at 25.3° and 43.7°, which correspond to graphitic carbon.^37,38 There is no other obvious iron-containing phase in Bi₄/Fe–N–C catalysts. The analysis of EDS shows the distributions of N, C, Bi and Fe, which directly proves that the aggregation of Fe results in the formation of the Fe nanoparticles (NPs) (Fig. S23(b–f)). Other bulk parts of the Bi₄/Fe–N–C catalyst without Fe NPs also show a uniform distribution of Fe. The atomically dispersed Fe sites were evidenced by the aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM), showing numerous bright dots on the low-atomic number hosts (Fig. 3c and d). C and N, as the two dominating elements, are uniformly distributed on the carbon matrix (Fig. 3f and h), indicating the successful synthesis of porous N-doped carbon. Compared with the EDS mapping results of Fe–N–C (Fig. S8c–f), the Fe region of Bi₄/Fe–N–C is similar to that of C and N (Fig. 3e), indicating the different content of Fe in different catalysts.

Fig. 3 (a) SEM, (b) TEM and (c and d) HAADF-STEM image of Bi₄/Fe–N–C and the corresponding EDS mappings of Fe (e), C (f), Bi (g), N (h) and Zn (i).

The elemental information of Bi–N–C, Fe–N–C and Bi₄/Fe–N–C surfaces is obtained from XPS (Fig. 4a). It is evident that the main difference is the appearance of the Fe signal and the disappearance of the Zn signal for the Bi₄/Fe–N–C pattern. The details of the Fe 2p pattern (Fe 2p_3/2, 711.9 eV; Fe 2p_3/2, 725.2 eV) are displayed clearly in Fig. 4b. The Bi₄/Fe–N–C catalyst shows a peak of Fe 2p, as opposed to Bi–N–C and Fe–N–C. This demonstrates that the Fe content of the Bi₄/Fe–N–C surface is significantly higher than those of Bi–N–C and Fe–N–C. Other support from the ICP results also confirms the increase in Fe content in the Bi₄/Fe–N–C catalyst (Fig. 4f). Thus, it also proves that the Bi-doping method can effectively increase the content of Fe in the ZIF8 precursor. In contrast, the high-resolution XPS Zn 2p spectra show an opposite result to the above (Fig. 4c). At 1021.9 eV, the Zn 2p signal of Bi₄/Fe–N–C is smooth without any peak. Combined with the ICP results of the Zn element, the Zn content decreases remarkably in the Bi₄/Fe–N–C catalyst, which may result from the formation of Fe NPs.³⁹ The agglomeration of Fe destroys the N–C structure, resulting in more Zn being reduced and then volatilized. As Fe metal evaporates at a higher temperature than Zn, Fe–N_x sites are more easily anchored on carbon than Zn–N_x sites. Thus, the residual amount of Zn (Zn–N_x) is affected by the Fe-doping content.

Fig. 4 (a) XPS spectra of Bi₄/Fe–N–C, Fe–N–C, and Bi–N–C. High resolution of (b) Fe 2p, (c) Zn 2p, (d) Bi 4f, and (e) N 1s XPS spectra of Bi₄/Fe–N–C, Fe–N–C, and Bi–N–C. (f) ICP results of Bi₄/Fe–N–C, Fe–N–C, and Bi–N–C.

As shown in Fig. 4d, there are no characteristic peaks in the Bi 4f XPS spectrum. The details of the chemical composition are confirmed by ICP (Fig. 4f). Therefore, the Bi element can be doped into ZIF8 but only in extremely small amounts. In spite of this, it is key for Fe-doping into ZIF8. Moreover, the high-resolution XPS N 1s spectra of all the samples were deconvoluted into pyridinic (398.5 eV), pyrrolic (400.9 eV), graphitic (402.0 eV), and oxidized (404.0 eV) N species (Fig. 4e).^26,28,40 The porphyrin-like moieties at 399.7 eV correspond to metal-nitrogen (M–N) coordination. Visibly, the M–N peak area of Bi₄/Fe–N–C is higher than those of Fe–N–C and Bi–N–C. Since the major metal increase is the Fe element after Bi-doping in Fe-ZIF8, the main part of the M–N site in the Fe–N–C catalyst should be the Fe–N_x site. It is generally evidenced to be an active site for the CO₂RR. This reflects the superiority of the Bi-doping strategy for the synthesis of abundant Fe–N_x sites.

The CO₂ electroreduction activities of Bi₄/Fe–N–C, Fe–N–C and Bi–N–C were tested using a three-electrode H-cell with a 0.5 M KHCO₃ solution as the electrolyte. As revealed by linear sweep voltammetry (LSV) shown in Fig. 5a, Bi₄/Fe–N–C exhibits the best CO₂RR activity with a current density of 8.55 mA cm⁻² at −0.60 V_RHE in CO₂-saturated electrolyte, which is roughly 1.3 and 6.6 times as high as Fe–N–C and Bi–N–C. Besides, in an Ar-saturated KHCO₃ electrolyte, Bi₄/Fe–N–C still presents a very similar trend of CO₂RR activity with a relatively small current density, indicating that the remarkable activity of Bi₄/Fe–N–C was saturated with CO₂. The gas products of carbon dioxide electroreduction analyzed by on-line gas chromatography (GC) and off-line nuclear magnetic resonance (NMR) were identified as only CO and H₂. The CO faradaic efficiencies (FE_CO) of Bi₄/Fe–N–C were compared in potentials ranging from −0.4 to −0.7 V_RHE (Fig. 5c). The maximum FE_CO was achieved at approximately 98% at −0.5 and −0.55 V_RHE, while the FE_CO slowly decreased at a higher potential matched with the current–time response of Bi₄/Fe–N–C (Fig. 5b).

Fig. 5 (a) LSV curves of Bi–N–C, Fe–N–C and Bi₄/Fe–N–C in 0.5 M KHCO₃ solution. (b) Stability test of Bi₄/Fe–N–C. (c) v_co and FE_CO. (d) TOF of Bi₄/Fe–N–C.

As shown in Fig. 5d, the calculated TOFs for Bi₄/Fe–N–C and Fe–N–C present a clear distinction at different potentials. At −0.5 V_RHE, the TOF of Bi₄/Fe–N–C is calculated to be 468.6 h⁻¹, which is 8.1 times higher than that of Fe–N–C. Moreover, the disparity of TOF between Bi₄/Fe–N–C and Fe–N–C gradually increases with the increase in potential. It also proves that an increase in Fe–N_x density promotes the activity of CO₂ electroreduction.^12–14

To accurately understand the effect of Bi on the Bi/Fe–N–C catalyst, different ratios of Bi/Fe were designed to prepare precursors of Bi/Fe–N–C. After annealing activation, the pyrolyzed M–N–C materials with various Fe contents were characterized by XRD, as illustrated in Fig. S15, and showed no significant differences. The ICP results (Fig. S16) reflect the opposite two-sided effect of Bi on Fe content, suppressing at a low Bi ratio and improving under a high Bi ratio. The CO₂ electroreduction activities present similar results (Fig. 6a). CO yield (activity) and FE_CO (selectivity) improve with increasing Fe content. However, the destruction of ZIF8 morphology and the agglomeration of Fe are inevitable and increase during the pyrolysis of precursors with a higher Bi ratio, which can greatly reduce the formation of active sites (Fe–N_x) for CO₂-to-CO conversion. Thus, the activity of the electrochemical CO₂RR is significantly reduced at a high Bi ratio.

Fig. 6 (a) v_co and FE_CO of Bi/Fe–N–C at −0.5 V_RHE. (b) LSV difference of the samples under different saturated atmospheres (LSV(CO₂)–LSV(N₂)).

To compare LSV in Ar- and CO₂-saturated electrolyte, the difference of value is calculated using the following formula: δ = LSV(CO₂) − LSV(N₂). As shown in Fig. 6b, similar results tend to coincide except for Bi–N–C. The results indicate that the current density in Ar-saturated electrolyte increases with iron content.⁴¹ Herein, the hydrogen evolution reaction (HER) is the main competitive reaction of the CO₂RR.^24,42 However, in a CO₂-saturated electrolyte, the Fe–N_x sites can provide an active site for the second CO₂ activation and decrease the barrier of CO₂(g) → CO(g).⁴³ According to the reported DFT calculations, CO₂ absorbed in the electrolyte can be adsorbed and reduced on the active site (Fe–N_x) for reaction.^26,44 This means that the enhancement of CO₂ activation results in high activity and FE_CO for electrochemical CO₂RR on Fe–N sites. Thus, a similar value of δ proves that a high iron content in Bi₄/Fe–N–C can markedly improve electrochemical CO₂RR and FE_CO in a CO₂-saturated electrolyte.

Furthermore, in situ ATR-FTIR spectroscopy was performed under CO₂RR conditions to identify and monitor the reaction intermediates and provide mechanistic insights into CO production. It was conducted using a Thermo-Fisher Nicolet iS20 system equipped with a liquid nitrogen-cooled HgCdTe (MCT) detector and a PIKE electrochemical three-electrode cell (Fig. S17). According to the activity of electrochemical CO₂RR, −0.5 V_RHE was selected as the operating voltage. During the reaction process, the ATR-FTIR spectra for Bi₄/Fe–N–C catalysts show several peaks (Fig. 7). The peaks at 1490 and 1060 cm⁻¹ correspond to the atop and bridge configurations of *COOH adsorption, respectively, which increase with time.⁴⁵ This consequence indicates that *COOH can be stably adsorbed on the catalyst surface during the electrochemical process of CO₂(g) → CO(g). The increased peaks of CO and CO₂ further support the above speculation. For the CO₂RR, the pathway of CO₂ protonation first adsorbs CO₂ on the surface of the Fe–N–C catalyst, and the adsorbed CO₂ converts to *COOH. Next, *COOH is broken to form CO.

Fig. 7 In situ ATR-FTIR of Bi₄/Fe–N–C catalysts under CO₂RR conditions at −0.5 V_RHE.

4. Conclusion

In summary, we successfully fabricated a Bi/Fe–N–C catalyst with Fe–N_x sites anchored on porous carbon. The Fe content within M–N–Cs can be increased within a certain range by controlling the Bi-doping ratio in the Fe-ZIF8 precursor, as proved by the ICP and XPS results. The Fe content is significantly improved at high ratios of Fe and Bi. The tests of the electrochemical CO₂RR clearly indicate that the Bi/Fe–N–C catalyst exhibits high activity and selectivity for CO towards the CO₂RR within limited Fe content. The best catalyst with the loaded 94.8 mg per kg_Cat Fe exhibits high selectivity with CO faradaic efficiency above 90% over a wide potential range from −0.4 to −0.7 V_RHE (98% at −0.5 V_RHE). In situ ATR-FTIR measurements show that the Bi/Fe–N–C catalyst boosts the proton transfer from CO₂ to *COOH and subsequently improves the selectivity of CO.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this study's findings are available from the corresponding author upon reasonable request.

Supplementary information is available. See DOI: https://doi.org/10.1039/d5se01074h.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 52570130), the Basic Science (Natural Science) Research in Higher Education in Jiangsu Province (No. 23KJA610003), and the High-level Scientific Research Foundation for the introduction of talent in Nanjing Institute of Technology (YKJ202335).

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Footnote

† These authors contributed equally to this work.

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