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Lewis acid-mediated In/In₂O₃ heterointerfaces with abundant oxygen vacancies boosting CO₂ electroreduction into formate

Dongxing Tan† *, Xianfang Yin†, Hengrui Kang, Jing Wang, Dan Zhang and Yuanyuan Feng*
Key Laboratory of Catalytic Conversion and Clean Energy in Universities of Shandong Province, School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, Shandong 273165, P. R. China. E-mail: tandx@qfnu.edu.cn; fengyy@qfnu.edu.cn

First published on 11th August 2025

Beyond conventional strategies (e.g., morphology control, composition optimization, and defect engineering), precise regulation of surface oxygen vacancies and construction of In/In₂O₃ heterostructures are pivotal for improving the selectivity of formate.^9–11 Oxygen vacancies could reduce the energy barrier and stabilize the critical OCHO* intermediate. Concurrently, the Schottky effect at In/In₂O₃ heterointerfaces modulates the electronic structure of the active sites, further promoting OCHO* formation and catalytic activity.¹² Therefore, integrating abundant, stable oxygen vacancies with well-defined In/In₂O₃ heterostructures effectively activates CO₂ and improves formate production activity.

Herein, we constructed a catalyst with abundant oxygen vacancies and stabilized In/In₂O₃ interfaces by a Lewis acidic metal-doping strategy. Lewis acid doping not only effectively suppresses the excessive reduction of In₂O₃ during the electrocatalytic process and maintains the stability of the In/In₂O₃ interfacial structure, but also increases the content of oxygen vacancies on the catalyst surface. Benefiting from the synergistic effect of In/In₂O₃ heterointerfaces and oxygen vacancies, the catalyst demonstrates superior formate selectivity. When paired with a commercial solar cell, the solar-driven synchronous CO₂ reduction and methanol oxidation electrochemical cell demonstrates a sustainable coelectrolysis process for formate production.

The target catalyst was synthesized via a three-step procedure (Fig. 1a). Initially, Al-doped MIL-68(In) metal–organic frameworks (Al–In MOFs) were synthesized solvothermally. Subsequently, Al-doped In₂O₃ was obtained by calcining the Al–In MOFs in air. Finally, Al-doped In₂O₃ underwent pre-reduction at −1.0 V for 1 h to stabilize its structure and composition. A reference catalyst was synthesized identically without Al(NO₃)₃·9H₃O addition. Scanning electron microscopy (SEM) revealed that the Al–In MOFs possess a well-defined hexagonal morphology (Fig. S1). Calcination decomposed the organic framework, yielding a porous Al-doped In₂O₃ structure assembled from nanoparticles (Fig. 1b), with Al doping exhibiting negligible morphological influence (Fig. S2). Transmission electron microscopy (TEM) confirmed nanoparticles with an average size of about 30 nm (Fig. 1c). The XRD patterns showed the absence of distinct Al-related peaks, suggesting the presence of amorphous Al species (Fig. S3).¹³ The slight shift of the diffraction lines toward higher angles confirms the substitution of smaller-radius Al³⁺ ions for In³⁺ within the In₂O₃ lattice. After pre-reduction at −1.0 V for 1 h, Al-doped In₂O₃ was partially reduced to metallic In, yielding coexisting In and In₂O₃ phases (denoted Al–In/In₂O₃) (Fig. S4). HRTEM revealed lattice spacings of 0.247 and 0.253 nm, corresponding to the In (002) and In₂O₃ (400) planes, respectively, and a clear grain boundary, indicating the formation of an In/In₂O₃ interface (Fig. 1d).¹⁴ This interface enhances CO₂ activation and facilitates rapid electron transfer, optimizing the electrocatalytic conversion.¹⁵ In contrast to Al-doped In₂O₃, pre-reduction almost completely converted In₂O₃ to metallic In (denoted R–In) (Fig. S5). Energy dispersive X-ray spectroscopy (EDS) elemental mapping of Al–In/In₂O₃ showed the homogeneous distribution of In, Al, O, and C (Fig. 1e).

	Fig. 1 (a) Schematic illustration for preparation of the Al–In/In2O3 catalyst. (b) SEM image of the Al–In2O3 catalyst. (c) TEM image, (d) HR-TEM image, (e) EDS elemental mapping of the Al–In/In2O3 catalyst.

XPS was employed to investigate the surface electronic configurations and oxidation states of the catalysts. As shown in Fig. S6a, the high-resolution In 3d spectra of Al–In₂O₃ exhibit a 0.3 eV negative shift relative to In₂O₃. The high-resolution O 1s XPS spectra indicated that the Al–In₂O₃ surface possesses a higher content of oxygen vacancies compared to In₂O₃ (Fig. S6b). Furthermore, the high-resolution Al 2p spectrum indicated the presence of Al³⁺ species (Fig. S6c). These XPS results demonstrate that the introduction of Lewis acidic Al species effectively modulates the surface electronic structure of the catalyst.¹⁶ Following pre-reduction, significant changes occur in the surface chemical valence states. Compared to R–In, Al–In/In₂O₃ can maintain a higher ratio of In³⁺ species and oxygen vacancy content (Fig. S7).^17,18 EPR spectroscopy further confirmed that Lewis acidic metal doping significantly enhanced the concentration of surface oxygen vacancies on Al–In/In₂O₃ (Fig. S8). Semi in situ XRD patterns were used to track the derivation of the crystal structure of the catalyst during the pre-reduction process. Fig. S9 reveals the persistent coexistence of metallic In⁰ and In₂O₃ phases within Al–In/In₂O₃ after 1 h of reduction. These results indicate that the Lewis acid metal doping strategy effectively maintains the stability of the In/In₂O₃ heterointerfaces and preserves a high oxygen vacancy content during the electrocatalytic CO₂ reduction process. The In/In₂O₃ heterointerfaces and abundance of oxygen vacancies in the Al–In/In₂O₃ catalyst can enhance the activation of CO₂ and improve the selectivity of formate.¹⁹

The CO₂RR performances of the Al–In/In₂O₃ and R–In catalysts were systematically evaluated in a H-cell with a 0.1 M K₂SO₄ electrolyte. The linear scan voltammetry (LSV) curves revealed enhanced current densities in a CO₂-saturated electrolyte compared to an Ar-saturated electrolyte for both catalysts. Notably, the Al–In/In₂O₃ catalyst demonstrated a higher current density in the CO₂-saturated electrolyte than that of R–In, indicating enhanced CO₂RR activity (Fig. 2a). Gas chromatography (GC) and nuclear magnetic resonance (NMR) were employed to analyze the gas-phase and liquid-phase products of the CO₂RR, respectively. The reduction products of the electrocatalytic CO₂RR for Al–In/In₂O₃ and R–In catalysts were formate, CO, and H₂ from the competitive HER (Fig. S10). As expected, the Al–In/In₂O₃ catalyst exhibited higher selectivity for formate. The maximum FE for the Al–In/In₂O₃ catalyst is up to 94% at −1.0 V, and remains above 90% over a potential range of −0.8 to −1.3 V (Fig. 2b). The FE of H₂ for Al–In/In₂O₃ is only 3% at −1.0 V, which is significantly lower than that of R–In (14%) (Fig. S11). This observation suggests that the incorporation of Lewis acidic Al species could enhance the formate selectivity. Based on the high current density and FE, the Al–In/In₂O₃ catalyst exhibits higher partial current density (Fig. 2c).

	Fig. 2 (a) LSV curves acquired in Ar- or CO2-saturated 0.1 M K2SO4 electrolyte at a scan rate of 20 mVs−1, (b) FE of formate at various potentials, (c) partial current densities of formate, (d) the double-layer capacitances derived from the CV curves at various scan rates, (e) simulated EIS spectra, (f) LSV curves in Ar-saturated 0.1 M KOH for Al–In/In2O3 and R–In catalysts. (g) Performance comparison of Al–In/In2O3 with the reported electrocatalysts for electrocatalytic CO2RR into formate. (h) Long-term stability test at −1.0 V for the Al–In/In2O3 catalyst.

The electrochemical active surface areas (ECSAs) of Al–In/In₂O₃ and R–In were further evaluated (Fig. S12). As shown in Fig. 2d, the double-layer capacitance of Al–In/In₂O₃ is 1.2 times that of R–In. This suggests that abundant oxygen vacancies and In/In₂O₃ interfaces can provide more active sites to enhance the CO₂ catalytic activity.²⁰ The smaller charge transfer impedance of Al–In/In₂O₃ also further indicates the rapid charge transfer kinetics (Fig. 2e).²¹ Hydroxyl (OH⁻) adsorption was utilized as an alternative test to investigate the binding affinity of the catalysts to the *CO₂^•− intermediate. Al–In/In₂O₃ exhibited a lower OH⁻ adsorption potential, indicating strong adsorption capacity for *CO₂^•− (Fig. 2f). This indicates that Al–In/In₂O₃ could stabilize the intermediate and further converts into formate.²² Benefiting from robust In/In₂O₃ heterointerfaces and abundant oxygen vacancies, Al–In/In₂O₃ achieves formate selectivity on par with the state-of-the-art catalysts reported (Fig. 2g and Table S1).^{10,11,15,23–36} Moreover, stability tests revealed no significant decay in current density and FE of formate during 30 h of continuous electrolysis, highlighting the good stability of the active sites (Fig. 2h). A series of Al-doped In₂O₃ catalysts with varying Al molar ratios demonstrate that Lewis acid metal doping effectively enhances the selectivity towards formate, with the selectivity exhibiting an initial increase followed by a decrease as the doping concentration increases (Fig. S13–S15).

Although Al–In/In₂O₃ showed excellent FE of formate in the H-cell, the low CO₂ solubility in the neutral electrolyte greatly limited the current density of the catalytic reaction. To overcome the mass transport bottleneck, the electrocatalytic CO₂RR activity of the catalysts was further investigated in a flow cell equipped with gas diffusion electrodes (GDE) (Fig. S16). This advanced reactor allows for the direct supply of gaseous CO₂ to the catalyst layer, effectively bypassing the limitations of CO₂ solubility. As anticipated, the current density of Al–In/In₂O₃ was dramatically enhanced in a flow cell, reaching 264 mA cm⁻² at −1.0 V, which is 6 times that of the H-cell (Fig. 3a). Moreover, the alkaline electrolyte further increases the catalytic activity (Fig. 3b, c and Fig. S17). Additionally, long-term stability tests revealed no significant decay in current density and FE of formate, confirming the exceptional stability of the catalyst's active sites (Fig. 3d).

	Fig. 3 (a) LSV curves, (b) FE of formate, (c) partial current density of formate for Al–In/In2O3 and R–In in a flow cell. (d) Stability test of the Al–In/In2O3 catalyst at −0.8 V in a flow cell.

Mn, Cd, and Ce-doped In₂O₃ catalysts were synthesized to elucidate the Lewis acid species doping in modulating catalytic performance. XRD patterns and SEM images confirm that the Mn, Cd and Ce-doped In₂O₃ catalysts exhibit morphology and crystal structure similar to Al–In₂O₃ (Fig. S18 and S19). The high-resolution O 1s spectra revealed significantly enhanced surface oxygen vacancy concentrations compared to In₂O₃ (Fig. S20). Furthermore, the positive correlation between oxygen vacancy content and catalytic activity demonstrates that Lewis acid doping enhances CO₂RR performance by facilitating oxygen vacancy formation (Fig. S21).

Although Al–In/In₂O₃ can effectively reduce CO₂ into formate, the slow kinetics of the anodic oxygen evolution reaction (OER) increases the energy input of the entire catalytic reaction. To address this limitation, we propose a coupled electrolysis strategy integrating the CO₂RR with the methanol oxidation reaction (MOR) to reduce energy consumption. The MOR is characterized by a low thermodynamic equilibrium potential (0.103 V) for efficient conversion of methanol into formate. According to the literature reported method, the Ni_0.2Mo_0.8N/F,N–C@NF heterogeneous catalyst was prepared (Fig. S22 and S23).³⁷ Ni_0.2Mo_0.8N/F,N–C@NF exhibited excellent MOR activity. The potentials required for the Ni_0.2Mo_0.8N/F,N–C@NF catalyst to achieve the same current density in the MOR are significantly lower compared to the OER (Fig. S24 and Table S2).

A two-electrode electrolytic cell was constructed with Al–In/In₂O₃ as the cathode and Ni_0.2Mo_0.8N/F,N–C@NF as the anode to realize the CO₂RR-coupled MOR or OER. As shown in Fig. 4a, when the current density reaches 200 mA cm⁻², the required potential for the CO₂RR coupled MOR system decreases by 0.8 V compared to the CO₂RR coupled OER system, indicating that integrating MOR with CO₂RR can significantly reduce the input voltage. More importantly, the CO₂RR coupled MOR simultaneously produces high value-added formate at the cathode and anode. FE of formate at both electrodes is maintained above 70% at current densities ranging from 50 to 250 mA cm⁻² (Fig. 4b). This demonstrates that it is feasible to construct a two-electrode electrolytic cell for the simultaneous electrosynthesis of formate. Furthermore, utilizing the photovoltaic cell to drive the two-electrode electrolytic cell can construct a green carbon cycle system and realize the conversion of solar energy to chemical energy (Fig. 4c). The open-circuit voltage and short-circuit currents of the photovoltaic cell under simulated sunlight are 3.33 V and 163.8 mA, respectively. The intersection point of the LSV curves for the electrolytic cell and photovoltaic cell curves are 2.83 V and 159.1 mA, near the maximum power point (MPP, 2.85 V, 158.4 mA) of the photovoltaic cell (Fig. 4d). The solar driven two-electrode electrolytic cell exhibits rapid and uniform current responses when cycling light on and light off. Under prolonged illumination, the electrochemical cell demonstrates good stability with negligible current density decay and consistently maintained FE of formate, thereby robustly confirming the superior operational durability and product selectivity of the CO₂RR coupled MOR system (Fig. 4e and Fig. S25).

	Fig. 4 (a) LSV curves in 1.0 M KOH with and without the addition of 1.0 M methanol. (b) FEformate of the CO2RR and MOR at varied current densities. (c) Solar-driven overall CO2 splitting reaction configuration. (d) I–V curves of the photovoltaic cell and the two-electrode electrochemical cell. (e) Current–time curve of the solar-driven electrocatalytic CO2RR integrated with the MOR.

In summary, we engineered the surface structure of In₂O₃ through a Lewis acid doping strategy, achieving stabilized In/In₂O₃ heterointerfaces and improved oxygen vacancy concentration. The experimental results clearly demonstrated that the Lewis acid doped catalyst has shown significant potential for electroreduction of CO₂ into formate. Al–In/In₂O₃ achieves a maximum FE of 97% and maintains above 90% within a wide range of potentials. By further integrating the MOR, a two-electrode electrochemical cell for synchronous electrocatalytic generation of formate was constructed, which exhibits good catalytic efficiency and operational stability. This study demonstrated a rational approach to modulate the interfacial properties of catalysts by a Lewis acid doping strategy to enhance the generation of formate for the CO₂RR.

We acknowledge the financial support from the Natural Science Foundation of Shandong Province (ZR2024QB034 and ZR2024MB078).

Conflicts of interest

Data availability

1. Y. Hu, M. Asif, J. Gong, H. Zeb, H. Lan, M. Khan, H. Xia and M. Du, Chem. Commun., 2024, 60, 10618–10628 RSC .
2. J. Li, Y. Sun and Z. Zhang, Revealing active Cu nanograins for electrocatalytic CO₂ reduction through operando studies, SmartMat, 2024, 5, e1209 CrossRef CAS .
3. W. Guo, X. Tan, J. Bi, L. Xu, D. Yang, C. Chen, Q. Zhu, J. Ma, A. Tayal, J. Ma, Y. Huang, X. Sun, S. Liu and B. Han, J. Am. Chem. Soc., 2021, 143, 6877–6885 CrossRef CAS PubMed .
4. Y. Yao, W. Zhuang, R. Li, K. Dong, Y. Luo, X. He, S. Sun, S. Alfaifi, X. Sun and W. Hu, Chem. Commun., 2023, 59, 9017–9028 RSC .
5. J. Bi, P. Li, J. Liu, Y. Wang, X. Song, X. Kang, X. Sun, Q. Zhu and B. Han, Angew. Chem., Int. Ed., 2023, 62, e202307612 CrossRef CAS PubMed .
6. H. Liu, Y. Bai, M. Wu, Y. Yang, Y. Wang, L. H. Li, J. Hao, W. Yan and W. Shi, Angew. Chem., Int. Ed., 2024, 63, e202411575 CrossRef CAS PubMed .
7. Y. Liu, T. Wu, H. Cheng, J. Wu, X. Guo and H. Fan, Active plane modulation of Bi₂O₃ nanosheets via Zn substitution for efficient electrocatalytic CO₂ reduction to formic acid, Nano Res., 2023, 16, 10803–10809 CrossRef CAS .
8. Z. Fan, R. Luo, Y. Zhang, B. Zhang, P. Zhai, Y. Zhang, C. Wang, J. Gao, W. Zhou, L. Sun and J. Hou, Angew. Chem., Int. Ed., 2023, 62, e202216326 CrossRef CAS PubMed .
9. R. Paterson, L. Fahy, E. Arca, C. Dixon, C. Wills, H. Yan, A. Griffiths, S. Collins, K. Wu, R. Bourne, T. Chamberlain, J. Knighta and S. Doherty, Chem. Commun., 2023, 59, 13470–13473 RSC .
10. Q. Cheng, M. Huang, L. Xiao, S. Mou, X. Zhao, Y. Xie, G. Jiang, X. Jiang and F. Dong, ACS Catal., 2023, 13, 4021–4029 CrossRef CAS .
11. F. Ye, S. Zhang, Q. Cheng, Y. Long, D. Liu, R. Paul, Y. Fang, Y. Su, L. Qu, L. Dai and C. Hu, Nat. Commun., 2023, 14, 2040 CrossRef CAS PubMed .
12. Y. Liang, W. Zhou, Y. Shi, C. Liu and B. Zhang, Sci. Bull., 2020, 65, 1547–1554 CrossRef CAS PubMed .
13. W. Huang, Y. Wang, J. Liu, Y. Wang, D. Liu, J. Dong, N. Jia, L. Yang, C. Liu, Z. Liu, B. Liu and Q. Yan, Small, 2022, 18, 2107885 CrossRef CAS PubMed .
14. W. Wang, X. Wang, Z. Ma, Y. Wang, Z. Yang, J. Zhu, L. Lv, H. Ning, N. Tsubaki and M. Wu, ACS Catal., 2022, 13, 796–802 CrossRef .
15. B. Wulan, X. Cao, D. Tan, J. Ma and J. Zhang, Adv. Funct. Mater., 2022, 33, 2209114 CrossRef .
16. C. Wu, F. Lin, X. Pan, Y. Zeng, G. Chen, L. Xu, Y. Fu, Y. He, Q. N. Chen, D. Sun and Z. Hai, Adv. Funct. Mater., 2023, 33, 2215135 CrossRef CAS .
17. W. Yang, Y. Zhao, S. Chen, W. Ren, X. Chen, C. Jia, Z. Su, Y. Wang and C. Zhao, Inorg. Chem., 2020, 59, 12437–12444 CrossRef CAS PubMed .
18. S. Wang, Q. Gao, C. Xu, S. Jiang, M. Zhang, X. Yin, H. Peng, B. Liu and Y. Song, Nano Res., 2023, 17, 1242–1250 CrossRef .
19. W. Wu, Y. Tong and P. Chen, Small, 2023, 20, 2305562 CrossRef PubMed .
20. D. Yang, Q. Zhu, X. Sun, C. Chen, W. Guo, G. Yang and B. Han, Angew. Chem., Int. Ed., 2019, 59, 2354–2359 CrossRef PubMed .
21. P. Wu, W. Ma, Y. Ge, J. Xu, M. Lu, X. Li, L. Ma, S. Bai and T. Wang, ACS Appl. Nano Mater., 2025, 8, 1718–1726 CrossRef CAS .
22. S. Li, S. Hu, H. Liu, J. Liu, X. Kang, S. Ge, Z. Zhang, Q. Yu and B. Liu, ACS Nano, 2023, 17, 9338–9346 CrossRef CAS PubMed .
23. Z. Niu, X. Gao, S. Lou, N. Wen, J. Zhao, Z. Zhang, Z. Ding, R. Yuan, W. Dai and J. Long, ACS Catal., 2023, 13, 2998–3006 CrossRef CAS .
24. L. Xiao, R. Zhou, T. Zhang, X. Wang, R. Zhou, P. Cullen and K. Ostrikov, Energy Environ. Mater., 2023, 7, e12656 CrossRef .
25. H. Shang, T. Wang, J. Pei, Z. Jiang, D. Zhou, Y. Wang, H. Li, J. Dong, Z. Zhuang, W. Chen, D. Wang, J. Zhang and Y. Li, Angew. Chem., Int. Ed., 2020, 59, 22465–22469 CrossRef CAS PubMed .
26. X. Du, Y. Qin, B. Gao, K. Wang, D. Li, Y. Li, S. Ding, Z. Song, Y. Su and C. Xiao, Appl. Surf. Sci., 2021, 563, 150405 CrossRef CAS .
27. L. Pardo Pérez, D. Teschner, E. Willinger, A. Guiet, M. Driess, P. Strasser and A. Fischer, Adv. Funct. Mater., 2021, 31, 2103601 CrossRef .
28. J. Zhang, R. Yin, Q. Shao, T. Zhu and X. Huang, Angew. Chem., Int. Ed., 2019, 58, 5609–5613 CrossRef CAS PubMed .
29. Z. Zhu, B. Zhao, S. Hou, X. Jiang, Z. Liang, B. Zhang and B. Zhao, Angew. Chem., Int. Ed., 2021, 60, 23394–23402 CrossRef CAS PubMed .
30. P. Lu, X. Tan, H. Zhao, Q. Xiang, K. Liu, X. Zhao, X. Yin, X. Li, X. Hai, S. Xi, A. Wee, S. Pennycook, X. Yu, M. Yuan, J. Wu, G. Zhang, S. Smith and Z. Yin, ACS Nano, 2021, 15, 5671–5678 CrossRef CAS PubMed .
31. H. Liu, H. G. Wang, Q. Song, K. Küster, U. Starke, P. Van Aken and E. Klemm, Angew. Chem., Int. Ed., 2022, 61, e202117058 CrossRef CAS PubMed .
32. W. Luo, W. Xie, M. Li, J. Zhang and A. Züttel, J. Mater. Chem. A, 2019, 7, 4505–4515 RSC .
33. C. Fang, L. Huang, W. Gao, X. Jiang, H. Liu, R. Hu, X. Li, J. Yu and W. Zhou, Adv. Energy Mater., 2024, 14, 2400813 CrossRef CAS .
34. Z. Wang, Y. Zhou, C. Xia, W. Guo, B. You and B. Xia, Angew. Chem., Int. Ed., 2021, 60, 19107–19112 CrossRef CAS PubMed .
35. Z. Wang, Y. Zhou, D. Liu, R. Qi, C. Xia, M. Li, B. You and B. Xia, Angew. Chem., Int. Ed., 2022, 61, e202200552 CrossRef CAS PubMed .
36. Y. Zhang, J. Lan, F. Xie, M. Peng, J. Liu, T. Chan and Y. Tan, ACS Appl. Mater. Interfaces, 2022, 14, 25257–25266 CrossRef CAS PubMed .
37. D. Tan, X. Yin, J. Wang, Z. Zhang, X. Zhu, H. Kang and Y. Feng, J. Mater. Chem. A, 2025, 13, 267–274 RSC .