Single-atom iron doped BiOCl atomic layers to promote efficient CO₂ electroreduction towards formate

Abstract

The electrocatalytic CO₂ reduction reaction to valuable fuels is a promising strategy to simultaneously tackle the crises of fossil fuel shortage and carbon emission. Herein, a single-Fe-atom was introduced into the BiOCl catalyst by a facile hydrolysis method, which exhibited a greatly enhanced catalytic performance in the electrochemical CO₂RR to formate, exceeding 90% hydrogenation selectivity and maintaining above 90% activity for 10 hours.

---

The electrocatalytic CO₂ reduction reaction (CO₂RR) is one of the most promising CO₂ resource utilization approaches because of controllable reaction conditions and recyclable raw materials, along with easy modularization.^1–5 This technology harnesses renewable energy sources (e.g. solar energy, wind energy) to produce electrical energy, subsequently using electrocatalysis to convert CO₂ into high-energy-density fuel substances, like formic acid, for recycling.^6–8 The strategy can not only reduce CO₂ concentration, but also helps alleviate the strain on energy shortage to a certain extent. Most research on the CO₂RR into formic acid has focused on metal electrodes which often possesses a high hydrogen evolution potential (HER) and weak adsorption force for CO, such as Pb, Hg, In, Pa, etc.^9–11 Moreover, the high toxicity and cost make them less practical for large-scale industrial applications. To this end, the less toxic and cost-effective metal Sn has attracted attention.¹² Although the Sn metal exhibits high formic acid selectivity, it is easily deactivated by carbon deposition thus suffering from poor stability during the electrocatalytic reduction of CO₂.^13,14 Alternatively, Bi is an easily available, environmentally friendly metal with high selectivity toward formic acid production. Actually, in 1995, the Komatsu group first discovered that the metal Bi is active toward electroreduction of CO₂ in aqueous systems, and the efficiency of the product formate reached almost 100%.¹⁵ To date, there have been relatively few studies regarding the electrocatalytic reduction of CO₂ using either elemental Bi or Bi-based materials. Furthermore, the precedents have shown that Bi-based catalysts still exhibit a high overpotential. Preliminary investigations have indicated that traditional bulk metal electrodes typically possess a low electrochemical active area, leading to low current density, a high reaction overpotential, and susceptibility to electrode deactivation. In contrast, single-atom doping of catalytic materials can significantly increase the number of active sites,^16–18 which is beneficial to the CO₂RR, allowing the catalyst to exhibit better catalytic activity and target product selectivity.^19–23 Notably, the recent emergence of two-dimensional nanocatalytic materials has garnered considerable attention and is considered one of the most promising catalysts in the CO₂RR.²³

In this study, we have developed an Fe₁/BiOCl catalyst (denoted as FBOC) with atomically dispersed Fe on a two-dimensional BiOCl nanocatalyst. The presence of atomically dispersed Fe in BiOCl is verified by a collection of characterization techniques, including X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy and transmission electron microscopy (HAADF-STEM). Furthermore, the introduction of single-atom Fe enriches the electron density of Fe with respect to Bi, resulting in a modification of the electronic structure of Bi within the BiOCl catalyst. This alteration in the electronic structure of Bi influences the reaction pathway for the CO₂RR, leading to enhanced catalytic performance. Notably, FBOC not only demonstrates exceptional hydrogenation selectivity (exceeding 90% at −0.8 V vs. RHE) but also exhibits remarkable stability (maintaining above 90% activity at −0.8 V vs. RHE for 10 hours) during the CO₂RR.

Atomically dispersed Fe on two-dimensional BiOCl (FBOC) and BiOCl nanocatalysts (BOC) were synthesized based on a recently reported method. In brief, a transparent mannitol aqueous solution containing Fe(NO₃)₃·6H₂O, Bi(NO₃)₃·5H₂O and NaBr was transferred to a Teflon-lined stainless steel autoclave to perform a solvothermal process at 180 °C for 3 h. The solid product was collected by centrifugation and washed with deionized water and ethanol five times (Fig. 1a). The FBOC and BOC feature a two-dimensional BiOCl structure, indicating that the introduction of Fe does not change the geometric structure of BiOCl (Fig. 1b and S1†). XRD patterns show that the diffraction peaks of all the products were well indexed to the tetragonal phase BiOCl (JCPDF No. 06-0249) with negligible influence on the phase structures of the BiOCl sample upon the addition of Fe atoms (Fig. 1c). Also, no Fe nanoparticles or clusters were found on the surface of BiOCl, as demonstrated in the TEM and HR-TEM images of FBOC (Fig. 1d, e, S2a and c†). The existence of atomically dispersed Fe is further verified by the associated EDX mapping and line scan, where the Bi, O, Cl, and Fe elements are homogeneously distributed throughout the surface of the material (Fig. 1f, S2b, d, and S3†). The loading amount of Fe on FBOC was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) to be 0.51 wt%.

Fig. 1 (a) Schematic illustration of the synthesis process of the FBOC and BOC. (b) SEM image of the FBOC. (c) XRD patterns of FBOC and BOC. TEM image of the BOC (d) and FBOC (e). (f) The corresponding EDX mapping images for the Bi, O, Br, and Fe elements of FBOC.

XPS measurement was conducted to explore the states of the elements in FBOC and BOC. No zero valence components related to Fe particles were found in the Fe 2p XPS spectra (Fig. S4†), well in line with the atomic dispersion of Fe. In Bi 4f XPS spectra, two peaks appearing at the binding energies of 159 and 165 eV were observed, which corresponded to Bi 4f_7/2 and Bi 4f_5/2 of Bi³⁺ in BiOCl respectively.^24–26 Noticeably, the binding energies of Bi 4f have an obvious red shift after the introduction of single atom Fe. Similarly, the red shift of the Cl 2p peak located at 199 eV was also observed in XPS spectra for FBOC and BOC (Fig. 2b). To probe the local atomic structure and cation coordination resulting from the doping effect of single atom Fe, X-ray absorption fine structure (XAFS) was employed. The Bi L3-edge absorption edge position of FBOC shows a slight difference with that of BOC, suggesting a distinct local atomic structure due to Fe incorporation. The R-space spectra show that the peak attributed to Bi–O (around 1.5 Å) broadens and the peak position attributed to Bi–Bi increases slightly with the addition of Fe, indicating that Fe single atoms replace some of the Bi. Additionally, the Fe K-edge absorption edge position of FBOC is located close to Fe₂O₃ rather than the Fe foil, suggesting that the single Fe atom carries a positive charge with the valence state approaching +3 (Fig. 2e). Fourier transformed (FT) k³-weighted EXAFS spectra (Fig. 2f) show that the main peak at 1.51 Å for FBOC could be assigned to Fe–O coordination in the BiOCl lattice and FBOC does not show the peak of the Fe–Fe bond in reference to standard Fe foil and Fe₂O₃, confirming that the Fe species are isolated in the form of single atoms. Besides, the wavelet transform (WT) contour plots also demonstrated different Fe features in FBOC from those in the Fe foil and Fe₂O₃ (Fig. 2g–i) and the intensity maximum at 4.8 Å⁻¹ in FBOC could be ascribed to Fe–O. All above results qualitatively verify the distinct local atomic arrangement of FBOC relative to the BOC.

Fig. 2 (a) Bi 4f core-level XPS spectra and (b) Cl 2p core-level XPS spectra of the FBOC and BOC. Bi L₃-edge XANES spectra (c) and EXAFS spectra (d) for FBOC and BOC. Fe K-edge XANES spectra (e) and EXAFS spectra (f) of Fe foil, FBOC and Fe₂O₃. EXAFS wavelet transform of Fe foil (g), FBOC (h) and Fe₂O₃ (i), respectively.

The electrochemical CO₂ reduction activity of the as-prepared FBOC and BOC catalysts was assessed on a homemade three-electrode system. As displayed in Fig. 3a and S5,† a remarkable current difference was noticed for both FBOC and BOC in CO₂-saturated and Ar-saturated KHCO₃ solutions, suggesting that CO₂ could be electrochemically reduced on BOC and FBOC. Notably, FBOC possessed the highest catalytic activity regarding the largest CO₂ reduction current. The CO₂ reduction performance of FBOC and BOC catalysts was further investigated by constant potential electrolysis in CO₂-saturated 0.5 M KHCO₃ solution. As shown in Fig. 3b and c, FBOC exhibits the best CO₂RR activity with ca. 90% FE_formate at −0.8 V (vs. RHE). Moreover, FBOC also exhibits excellent catalytic stability with no obvious decay in activity (both current density and FE_formate) after 10 h continuous operation (Fig. 3d). All the above results indicate that the introduction of Fe single atoms into BiOCl nanosheets promotes the CO₂RR into formate.

Fig. 3 (a) Linear sweep voltammetry curves of FBOC and BOC catalysts acquired in CO₂-saturated and Ar-saturated 0.5 M KHCO₃ solutions. The catalyst was loaded on carbon paper with a loading amount of 0.5 mg cm⁻². Faradaic efficiency (FE) (b) and the corresponding potentiostatic current density (c) of FBOC and BOC catalysts. (d) Long-term electrolysis under −0.8 V (vs. RHE). The formate selectivity maintained above 90% over 10 hours of continuous operation. All measurements were carried out in 0.5 M CO₂-saturated KHCO₃ solution (pH = 7.3) at room temperature.

To investigate the detailed CO₂RR process over FBOC and BOC, the operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) measurements were conducted. Recorded in CO₂ saturated electrolyte, the absorption peak centered at 1390 cm⁻¹ can be assigned to O–C–O vibration in bidentate *OCHO, which is detected in the spectra of both FBOC (Fig. 4a) and BOC (Fig. 4b). Notably, the intensity of *C–H stretching between 2852 and 2975 cm⁻¹, a characteristic absorption peak in the CO₂RR to generate formic acid/formate, over FBOC is much stronger than that over BOC. On the other hand, the *CO absorption peak appearing at around 1851 cm⁻¹ on FBOC is significantly weaker with respect to that on BOC. Additionally, the *CO absorption peak starts to appear at −0.8 V vs. RHE for the BOC catalyst, while it only shows up when the applied potential is more negative than −0.8 V vs. RHE for the FBOC catalyst, indicating more thermodynamically favorable *CO formation on BOC. Based on the above results, the CO₂RR pathways over FBOC and BOC catalysts are proposed (Fig. 4c). Specifically, CO and formate are the main products over the BOC catalyst due to the favorable adsorption of both *CO and *OCHO. In contrast, the single Fe atoms effectively tuned the electronic structure of the BiOCl catalyst, suppressing the adsorption of *CO while favoring the adsorption of *OCHO, and therefore promote the selectivity of the CO₂RR toward formate production.

Fig. 4 Operando ATR-SEIRAS spectra recorded at different cathodic potentials in CO₂ saturated electrolyte: (a) FBOC and (b) BOC. (c) The proposed CO₂RR pathways over FBOC and BOC.

To sum up, we have developed a facile hydrolysis method to synthesize single Fe atom doped 2D BiOCl nanosheets at room temperature for the electrochemical CO₂RR. The incorporation of single Fe atoms can induce electron transfer from Fe to Bi, which effectively increases the valence state of Bi and as a result promotes the CO₂RR to formate, while suppressing the competing CO₂RR to CO and HER. This work provides a rational design guidance for single-atom doped 2D nanocatalysts in the application of the electrochemical CO₂RR.

Author contributions

Fengwu Tian: investigation, data curation, formal analysis and writing – original draft; Tian Tang: investigation and formal analysis; Xixi Di: investigation; Xiaosha Guo: formal analysis; Dong Liu, Yixuan Shi, Zheng Shen and Xiaohu Yu: writing – original draft and writing – review & editing; Xianzhao Shao: conceptualization, funding acquisition and project administration. All authors discussed the results and contributed to the final manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2021YFB4000700), the National Natural Science Foundation of China (22273053), and the Natural Science Basic Research Program of Shaanxi (2021JCW-20, 2023-JC-YB-107).

Notes and references

1. H. B. Yang, S.-F. Hung, S. Liu, K. D. Yuan, S. Miao, L. P. Zhang, X. Huang, H.-Y. Wang, W. Z. Cai, R. Chen, J. J. Gao, X. F. Yang, W. Chen, Y. Q. Huang, H. M. Chen, C. M. Li, T. Zhang and B. Liu, Nat. Energy, 2018, 3, 140.
2. J. C. Zhang, J. Ding, Y. H. Liu, C. L. Su, H. B. Yang, Y. Q. Huang and B. Liu, Joule, 2023, 8, 1700.
3. J. Ding, F. H. Li, J. C. Zhang, Q. Zhang, Y. H. Liu, W. J. Wang, W. Liu, B. B. Wang, J. Cai, X. Z. Su, H. B. Yang, X. Yang, Y. Q. Huang, Y. M. Zhai and B. Liu, J. Am. Chem. Soc., 2023, 145, 11829.
4. H. B. Yang, C.-Q. Xu, S. Baskaran, Y.-R. Lu, C. D. Gu, W. Liu, J. Ding, J. C. Zhang, Q. L. Wang, W. Chen, J. Li, Y. Q. Huang, T. Zhang and B. Liu, EES Catal., 2023, 1, 774.
5. T. Wang, J. D. Chen, X. Y. Ren, J. C. Zhang, J. Ding, Y. H. Liu, K. H. Lim, J. H. Wang, X. N. Li, H. B. Yang, Y. Q. Huang, S. Kawi and B. Liu, Angew. Chem., Int. Ed., 2023, 62, e202211174.
6. Y. Y. Wang, J. K. Zhao, C. Cao, J. Ding, R. Y. Wang, J. Zeng, J. Bao and B. Liu, ACS Catal., 2023, 13, 3532.
7. W. J. Zhang, S. S. Liu, Y. Yang, H. F. Qi, S. B. Xi, Y. P. Wei, J. Ding, Z.-J. Wang, Q. X. Li, B. Liu and Z. P. Chen, Angew. Chem., Int. Ed., 2023, 62, e202219241.
8. Q. Zhang, H. J. Tsai, F. H. Li, Z. M. Wei, Q. Y. He, J. Ding, Y. H. Liu, Z.-Y. Lin, X. J. Yang, Z. Y. Chen, F. X. Hu, X. Yang, Q. Tang, H. B. Yang, S.-F. Hung and Y. M. Zhai, Angew. Chem., Int. Ed., 2023, 62, e202311550.
9. Y. Hori, Mod. Aspects Electrochem., 2008, 42, 88.
10. Y. Hori, H. Wakebe, T. Tsukamoto and O. Koga, Surf. Sci., 1995, 335, 258.
11. Y. Hori, H. Konishi, T. Futamura, A. Murata, O. Koga, H. Sakurai and K. Oguma, Electrochim. Acta, 2005, 50, 5354.
12. A. D. Castillo, M. A. Guerra, J. S. Gullón, A. Sáez, V. Montiel and A. Irabien, J. CO2 Util., 2017, 18, 222.
13. W. Luc, C. Collins, S. W. Wang, H. L. Xin, K. He, Y. J. Kang and F. Jiao, J. Am. Chem. Soc., 2017, 139, 1885.
14. Z. M. Wei, J. Ding, X. X. Duan, G.-L. Chen, F.-Y. Wu, L. Zhang, X. J. Yang, Q. Zhang, Q. Y. He, Z. Y. Chen, J. Huang, S.-F. Hung, X. Yang and Y. M. Zhai, ACS Catal., 2023, 13, 4711.
15. S. J. Komatsu, T. Yanagihara, Y. K. Hiraga, M. C. Tanaka and A. Kunugi, Electrochemical Reduction of CO₂ at Sb and Bi Electrodes in KHCO₃ Solution, Japan Science and Technology Agency, 1995, vol. 63, p. 217.
16. L. Liu, P. Zhou, X. Su, Y. Liu, Y. Sun, H. Yang and S. Zheng, Sens. Actuators, B, 2022, 351, 130983.
17. L. Liu, T. Xiao, H. Fu, Z. Chen, X. Qu and S. Zheng, Appl. Catal., B, 2023, 323, 122181.
18. L. Liu, C. Mao, H. Fu, X. Qu and S. Zheng, ACS Appl. Mater. Interfaces, 2023, 15, 16654–16663.
19. J. Ding, Z. M. Wei, F. H. Li, J. C. Zhang, Q. Zhang, J. Zhou, W. J. Wang, Y. H. Liu, Z. Zhang, X. Z. Su, R. Z. Yang, W. Liu, C. L. Su, H. B. Yang, Y. Q. Huang, Y. M. Zhai and B. Liu, Nat. Commun., 2023, 14, 6550.
20. J. Ding, Z. Y. Teng, X. Z. Su, K. Kato, Y. H. Liu, T. Xiao, W. Liu, L. Y. Liu, Q. Zhang, X. Y. Ren, J. C. Zhang, Z. Y. Chen, O. Teruhisa, A. Yamakata, H. B. Yang, Y. Q. Huang, B. Liu and Y. Zhai, Chem, 2023, 9, 1017.
21. J. Ding, J. Huang, Q. Zhang, Z. M. Wei, Q. Y. He, Z. Y. Chen, Y. H. Liu, X. Z. Su and Y. M. Zhai, Catal. Sci. Technol., 2022, 12, 2416.
22. J. Ding, H. B. Yang, X. L. Ma, S. Liu, W. Liu, Q. Mao, Y. Huang, J. Li, T. Zhang and B. Liu, Nat. Energy, 2023 DOI:10.1038/s41560-023-01389-3.
23. J. D. Wu, X. Zhao, X. Q. Cui and W. T. Zheng, Chin. J. Catal., 2022, 43, 2802.
24. J. Ding, Z. Dai, F. Tian, B. Zhou, B. Zhao, H. P. Zhao, Z. Q. Chen, Y. L. Liu and R. Chen, J. Mater. Chem. A, 2017, 5, 23453.
25. J. Ding, Z. Dai, F. Qin, H. P. Zhao, S. Zhao and R. Chen, Appl. Catal., B, 2017, 205, 281.
26. Z. Dai, F. Qin, H. Zhao, J. Ding, Y. Liu and R. Chen, ACS Catal., 2016, 5, 3180.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cy01523h

---