Robust imidazole-linked Ni-phthalocyanine-based covalent-organic framework for CO₂ electroreduction in the full pH range

Abstract

The electroreduction of CO₂ to value-added chemicals is a promising approach to utilize CO₂ and mitigate greenhouse gas emission. Covalent organic frameworks (COFs) with abundant accessible active sites, tunable pore size, and large CO₂ adsorption capacity are considered promising electrocatalysts for CO₂ conversion. However, most COFs linked by reversible covalent bonds exhibit poor stability, which limits their application for CO₂ electroreduction reactions in acidic or alkaline electrolytes. Herein, a Ni-phthalocyanine-based COF linked by stable imidazole building blocks, named NiPc-Im-COF, was synthesized through the condensation reaction of 2,3,9,10,16,17,23,24-octa-aminophthalocyaninato Ni(II) and 4,4′-biphenyl dialdehyde. The obtained NiPc-Im-COF exhibits high chemical stability after soaking in concentrated HCl and KOH. When applied for the electroreduction of CO₂, the NiPc-Im-COF exhibits high CO selectivity (>90%) in electrolytes with different pH values. Specifically, the NiPc-Im-COF shows a high CO partial current density of 267 mA cm⁻² with a CO selectivity of 90% at −0.8 V vs. the reversible hydrogen electrode (RHE) in 5 M KOH solution, and meanwhile, over 90% of FE_CO and 6 hours of stability at −1.3 V in 0.5 M K₂SO₄ (pH = 1). The XRD patterns prove that the structure of the NiPc-Im-COF is not destroyed after CO₂ electroreduction at different pH values. This work presents a strategy to improve the stability of COFs via irreversible covalent linkage and offers an efficient CO₂RR in the full pH range, which paves a new pathway for the industrial application of COFs in CO₂ electroreduction.

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Introduction

Today, with the excessive use of fossil fuels, the rapid increase in the content of CO₂ in the atmosphere has led to a series of problems, including global warming and rising sea levels.^1–3 Various approaches, including photocatalysis, electrocatalysis, and thermal catalysis, have been explored to address this challenge.^4–10 Among these methods, the electrocatalytic reduction of CO₂ to fossil fuels driven by renewable energy could realize the recycling of carbon resources.^11,12 Nowadays, many kinds of electrocatalysts, including metal-based catalysts (Sn-, Ag-, and Cu-based catalysts); Fe-, Co-, Ni-based single-atom catalysts; and metal-free catalysts, were employed to promote the CO₂ electroreduction reaction (CO₂RR) and produce diverse chemicals and fuels, including CO, HCOOH, CH₄, and C₂H₄.^13–23 However, many problems remain to be solved in the CO₂RR, such as ambiguous active centers led by complex composition, low stability due to reconstruction of catalysts, low selectivity resulting from a multi-electron transfer process, and the competitive hydrogen evolution reaction (HER).^24–29 Hence, it is necessary to design highly efficient electrocatalysts with clear active centers. In addition, as is well known, to achieve highly efficient CO₂RR performance with high current density, alkaline and/or acidic electrolytes are usually required. Under alkaline conditions, CO₂ molecules readily react with OH⁻ and convert into carbonate/bicarbonate, which results in a low utilization of CO₂. Simultaneously, the generated carbonate/bicarbonate will block the pores of the electrode, reduce CO₂RR stability, and lead to the leakage of the electrolyte.³⁰ Although these problems can be avoided while using acidic electrolytes, the HER usually occurs predominantly under such conditions and far exceeds the CO₂RR. Therefore, it is important to develop and prepare stable electrocatalysts under such harsh conditions for the CO₂RR.

Recently, porous crystalline covalent-organic frameworks (COFs) have garnered extensive interest owing to clear and accessible active centers and diverse adjustable chemical and pore structures, which have propelled them into the spotlight within the field of electrocatalysis.^31–35 However, to obtain crystalline structures, most COFs are constructed by dynamically reversible covalent bonds, such as C=N imine bonds. These unstable bonds usually lead to the decomposition of the crystalline framework during the CO₂RR process under harsh acid or alkaline conditions.^36–40 In contrast, the construction of COFs with irreversible covalent bonds could effectively enhance their stability under acid or alkaline conditions.^41–43 Ni phthalocyanines with active sites have been shown to promote electrocatalytic CO₂RR. However, most Ni phthalocyanine-based COFs are connected with dynamic bonds such as C=N and B–O. It is still challenging to construct crystalline robust metal phthalocyanine-based COFs with irreversible covalent bonds for efficient electrocatalytic CO₂RR in gas diffusion electrodes (GDE) with acid and alkaline electrolytes. Recently, nitrogen heterocyclic units were used to construct stable COFs due to the excellent chemical stability of nitrogen heterocycles.^44,45 Herein, a Ni-phthalocyanine-based COF linked by the stable imidazole building blocks (NiPc-Im-COF) is synthesized, and the as-prepared NiPc-Im-COF exhibited high selectivity of CO (>90%) from −0.5 V to −0.8 V vs. reversible hydrogen electrode (RHE) and the largest CO partial current density (J_CO) of 267 mA cm⁻² at −0.8 V when 5 M KOH was employed as the electrolyte in GDE. Besides, the faradaic efficiency of CO (FE_CO) reached 90%, and the J_CO arrived at 87 mA cm⁻² at −1.4 V under the conditions of potassium sulfate aqueous solution (pH = 1). The results clearly indicate the significant potential of the stable COF connected via irreversible covalent bonds for the electrocatalytic CO₂RR.

Results and discussion

As depicted in Fig. 1a, NiPc-Im-COF was synthesized by the reaction of 2,3,9,10,16,17,23,24-octa-aminophthalocyaninato nickel(II) (NiPc-(NH₂)₈) and 4,4′-biphenyl dialdehyde (BPDH) in the mixed solvent of N-methyl pyrrolidone (NMP) and mesitylene (MT). The powder X-ray diffraction (PXRD) pattern of NiPc-Im-COF, as shown in Fig. 1b, is in agreement with the simulated profile, suggesting the formation of crystalline NiPc-Im-COF most likely with an AA stacking model (Fig. 1a). Three main peaks located at 3.55°, 7.03°, and 10.6°, could be attributed to the (100), (200), and (300) crystallographic planes, respectively. The peak at 26.6° corresponding to the (001) facet is indicative of the π–π stacking between COF layers. Notably, the main diffraction peak of NiPc-(NH₂)₈ in the PXRD pattern was located at 5° (Fig. S1, ESI†), which was distinct from that of NiPc-Im-COF. As demonstrated by Material Studio 7.0, the NiPc-Im-COF possessed a unit cell with dimensions a = b = 25.2 Å and an interplane distance of c = 3.35 Å, in agreement with the experimental and the resulting XRD pattern (the R_p is 1.51% and the R_wp is 1.64%). To provide additional validation for the successful synthesis of the COF network structure, Fourier transform infrared spectroscopy (FT-IR), and solid-state ¹³C nuclear magnetic resonance (¹³C NMR) spectra were performed. The FT-IR spectra (Fig. S2, ESI†) revealed the band at 1688 cm⁻¹ attributed to the stretching vibration of the aldehyde-based C=O in BPDH, and the bands at 3220 cm⁻¹ and 3334 cm⁻¹, signifying the vibration of –NH₂ in NiPc(NH₂)₈, disappeared after the reaction. Simultaneously, new bands corresponding to the C–H vibration of benzene rings emerged at 2852 cm⁻¹ and 2921 cm⁻¹.^46,47 Besides, as illustrated in Fig. 1c, the solid-state ¹³C NMR spectra of NiPc-Im-COF exhibited a special chemical shift peak at 153 ppm, which was ascribed to the C of the NH–C=N in the imidazole ring in the framework.⁴⁸ Besides, the ultraviolet-visible (UV-vis) spectra substantiate a red shift in the Q band for NiPc-Im-COF, which suggested the extended conjugation within NiPc-Im-COF (Fig. S3, ESI†). These aforementioned results indicated the successful synthesis of NiPc-Im-COF.

Fig. 1 (a) Schematic illustration for the synthesis of the NiPc-Im-COF, with top and side views of AA stacking. (b) Simulated and experimental NiPc-Im-COF PXRD patterns. (c) Solid-state ¹³C NMR spectrum.

The scanning electron microscopy (SEM) image (Fig. 2a and Fig. S4a, ESI†) revealed that the bulk NiPc-Im-COF comprised stacked layers of nanosheets, aligning with the simulated structure. As shown in Fig. 2b and Fig. S4b, ESI,† the transmission electron microscopy (TEM) image revealed its porous structure. Additionally, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and the energy-dispersive X-ray spectroscopy (EDS) elemental mapping images (Fig. 2c) illustrated the uniform distribution of C, N, and Ni elements within the NiPc-Im-COF framework, with no discernible Ni nanoparticles (Ni NPs) in TEM images, aligning with PXRD findings. At the same time, atomic force microscopy images also confirmed a sheet-like structure of NiPc-Im-COF with a height of 1.4 nm (Fig. S5, ESI†). The inductively coupled plasma (ICP) optical emission spectrometry demonstrated the Ni content of 3.75 wt% in NiPc-Im-COF, which was consistent with the result of EDS (Tables S1 and S2, ESI†). The porous structure of NiPc-Im-COF was investigated by N₂ adsorption–desorption experiments at 77 K. As shown in Fig. 2d and Table S3, ESI,† NiPc-Im-COF exhibited a high Brunauer–Emmer Teller (BET) surface area of 360 m² g⁻¹ and a total pore volume of 0.32 cm³ g⁻¹. Furthermore, benefiting from the affinity of the imidazole ring for CO₂, NiPc-Im-COF displayed an impressive CO₂ uptake of 27 cm³ g⁻¹ at 298 K (Fig. 2e), highlighting its favorable CO₂ affinity, which is beneficial for enhancing the catalytic performance of CO₂RR. To evaluate the stability of NiPc-Im-COF, the NiPc-Im-COF powders were immersed in various solvents and solutions (such as N,N-dimethylformamide (DMF), ethanol (EtOH), tetrahydrofuran (THF), trichloromethane (TM) and acid/base such as concentrated HCl (1 M and 5 M), concentrated KOH (1 M and 5 M)) at room temperature for one week. As demonstrated in Fig. S6, ESI,† the PXRD patterns indicated that NiPc-Im-COF exhibited excellent acid/base and organic solvent stability. Besides, NiPc-Im-COF retained its crystallinity after soaking in THF, TM at 60 °C and DMF at 80 °C for 12 h (Fig. S7, ESI†). Additionally, the thermal stability of NiPc-Im-COF was substantiated by thermogravimetric analysis (TGA), revealing a decomposition temperature exceeding 320 °C (Fig. S8, ESI†).

Fig. 2 (a) SEM and (b) TEM images of the NiPc-Im-COF. (c) HAADF-STEM image of the NiPc-Im-COF and elemental mapping images showing the distribution of C, N, and Ni. (d) N₂ sorption curves for the samples at 77 K. (e) CO₂ adsorption curves for the samples at 298 K.

X-ray photoelectron spectroscopy (XPS) survey spectrum revealed the presence of C 1s, N 1s, and Ni 2p core levels in NiPc-Im-COF (Fig. S9a, ESI†). The resonance peak of O in the spectrum might come from guest molecules in the pore or the unreacted aldehyde groups at the edge of the framework, explaining the total atomic mass fraction being less than 100%. In the high-resolution C 1s spectrum (Fig. S9b, ESI†), two prominent peaks were located at 284.8 eV and 285.97 eV, corresponding to C=C/C–C and C=N, respectively.^49–51 As shown in Fig. S9c, ESI,† the high-resolution N 1s spectrum displayed peaks at 398.6 eV and 400.16 eV. The peak at 398.6 eV was assigned to the N=C–N bond. Notably, the binding energy is at 400.16 eV, higher than that of the Ni–N bond (399.8 eV) but lower than that of the N–H bond (400.5 eV) in the imidazole ring, suggesting an interaction of Ni–N and N–H.^52–54 Moreover, the Ni 2p XPS spectrum in Fig. 3a displays two sets of peaks with binding energies of 872.7 eV and 855.4 eV for Ni 2p_1/2 and Ni 2p_3/2, respectively, which indicates the presence of Ni(II) species and the absence of Ni nanoparticles.^55–57 X-ray absorption spectroscopy (XAS) was performed to determine the coordination environment and electronic structure of Ni sites. As illustrated in Fig. 3b, the X-ray absorption near edge structure (XANES) exhibited nearly identical curves for NiPc-Im-COF and NiPc, indicating minimal alterations in the coordination environment and electronic structure of Ni sites. In addition, the peaks at 8334 eV and 8340 eV were detected in NiPc and NiPc-Im-COF, which could be attributed to the dipole forbidden 1s to 3d transition and the shakedown satellite 1s to 4pz transition.^58,59 The XANES curves of NiPc-Im-COF and NiPc closely resembled that of NiO. These results indicated that the valence of Ni sites in NiPc-Im-COF is approximately +2, aligning with the results of XPS. The coordination environment of the Ni sites in the NiPc-Im-COF was analyzed by extended X-ray absorption fine structure (EXAFS). As depicted in Fig. 3c, compared with Ni foil, there was no evidence of a Ni–Ni bond at 2.2 Å in NiPc-Im-COF, indicating that Ni atoms in NiPc-Im-COF existed as single Ni sites, with no presence of Ni NPs. Besides, in comparison to NiO, NiPc and NiPc-Im-COF exhibited a prominent peak at 1.4 Å, indicative of the Ni–N bond, while NiO featured a primary peak at 1.6 Å, corresponding to the Ni–O bond. These results suggest the absence of nickel oxides within NiPc-Im-COF. The EXAFS fitting was carried out to obtain the quantitative structural parameters of Ni sites in NiPc-Im-COF. As shown in Fig. 3d and Table S4, ESI,† the fitting results of NiPc-Im-COF indicated that the average coordination number of Ni–N was 4.2, further demonstrating its Ni–N₄ coordination structure.

Fig. 3 (a) XPS spectra of Ni in Im-COFs; (b) the normalized Ni K-edge XANES spectra of Im-COFs, NiPc, Ni foil, and NiO; (c) Fourier transform EXAFS spectra of different samples; (d) first-shell fitting of the Fourier transformed EXAFS spectrum of Im-COFs.

To evaluate the CO₂RR performances of NiPc-Im-COF, the ink comprising NiPc-Im-COF and conductive carbon was drop-casted on 1 cm² carbon paper electrode. The electrochemical tests were firstly conducted in CO₂-saturated 0.5 M KHCO₃ aqueous solution within a standard H-type electrochemical cell containing a two-chamber and three-electrode separated by a proton exchange membrane. Fig. 4a illustrates the linear sweep voltammetry (LSV) curves of NiPc-Im-COF, demonstrating a notably higher current density than that of conducted in the corresponding N₂-saturated electrolyte. This discrepancy suggested the CO₂RR has occurred over NiPc-Im-COF. Besides, the LSV curve of NiPc-Im-COF also displayed a superior current density than that of NiPc-(NH₂)₈ in CO₂-saturated 0.5 M KHCO₃ electrolyte, which suggested that the porosity and conductivity of NiPc-Im-COF enhance the mass and electron transfer processes, thereby increasing the current density during the electrocatalytic CO₂RR. To further assess the selectivity of NiPc-Im-COF for CO₂RR, gas chromatography (GC) was employed to detect the gas products. As shown in Fig. 4b, Fig. S12 and S13, ESI,† NiPc-Im-COF showed high faradaic efficiencies of CO (FE_CO) (>90%) over a wide potential range from −0.7 V to −1.1 V vs. reversible hydrogen electrode (RHE) and exhibited nearly 100% at −0.7 and −0.8 V. It is important to note that all the potentials mentioned in this work are referenced to the reversible hydrogen electrode (RHE). Furthermore, the CO partial current density (J_CO) of NiPc-Im-COF gradually increased with the rising overpotential, reaching 19.3 mA cm⁻² at −1.1 V (Fig. 4c). Concurrently, the turnover frequency (TOF) increased with the potential and reached 1880 h⁻¹ at −1.1 V. To evaluate the stability of the catalysts in 0.5 M KHCO₃, the time-dependent total current density curves were obtained. The long-term stability of NiPc-Im-COF was analyzed at −0.8 V in CO₂-saturated 0.5 M KHCO₃ (Fig. S14, ESI†). Notably, the FE_CO remained consistently above 90% for a duration of 20 hours. Furthermore, the PXRD pattern of NiPc-Im-COF shown in Fig. S15, ESI† remained unchanged, indicating that good crystallinity was maintained even after undergoing the CO₂ reduction reaction. Besides, the valence of Ni sites in NiPc-Im-COF was assessed by XPS after the CO₂RR in 0.5 M KHCO₃. The XPS curves of NiPc-Im-COF demonstrated that bonding energies at 872.7 eV and 855.4 eV are assigned to 2p_3/2 and 2p_1/2 of Ni(II), respectively (Fig. S16, ESI†), suggesting the excellent stability of NiPc-Im-COF under neutral condition.

Fig. 4 The CO₂RR performances of the NiPc-Im-COF were studied in a H-cell. (a) LSV curves measured in N₂-saturated and CO₂-saturated 0.5 M KHCO₃ solution, scan rate = 10 mv s⁻¹; (b) FE for CO from −0.7 to −1.1 V vs. RHE; (c) CO partial current density; (d) TOF under different potentials.

To address the CO₂ mass transfer limitation in H-cells, a GDE-based alkaline flow cell was employed. The conductive ink was drop-casted on single carbon paper with an area of 0.8 cm² and a set of KOH solutions with gradient concentrations (1 M, 3 M, and 5 M) were used as electrolytes. As depicted in Fig. 5a, the LSV curves indicated that the total current density increased with the concentration of KOH, which resulted from the increased ion concentration and CO₂ mass transfer. The current density obtained under alkaline GDE conditions far exceeded that of the neutral H-cell. Furthermore, we conducted tests to evaluate the product selectivity under various conditions. As shown in Fig. 5b, the FE_CO of the catalyst maintained over 90% over a range from −0.5 V to −0.9 V under 1 M KOH. Similarly, the catalyst also showed high FE_CO in the presence of 3 M and 5 M KOH aqueous solutions. Furthermore, the CO partial current density of the catalyst improved significantly, with J_CO reaching 142 mA cm⁻² at −0.8 V when 1 M KOH was employed as the electrolyte, which was 7-fold higher than the highest J_CO in 0.5 M KHCO₃ (Fig. 5c). Similarly, the J_CO also increased with the concentration of KOH (3 M and 5 M KOH) and reached 231 mA cm⁻² and 267 mA cm⁻² at −0.8 V (Fig. 5c), respectively. Compared to previously reported materials of the same type, such a high current density is at the forefront (Fig. S17, ESI†). Likewise, the XPS spectrum shown in Fig. S18, ESI† proves that the valence of Ni sites in NiPc-Im-COF is still maintained at +2 after the CO₂RR in all kinds of electrolytes. FE_CO kept over 90% for a duration of 6 hours when the CO₂RR was conducted in 5 M KOH at −0.6 V (Fig. S19, ESI†). At the same time, the PXRD patterns performed after the CO₂RR suggested that the structure of NiPc-Im-COF remained stable (Fig. S20, ESI†).

Fig. 5 CO₂RR performances for the NiPc-Im-COF were studied in a flow cell. (a) LSV curves measured in KOH solutions with a scan rate of 10 mv s⁻¹; (b) FE for CO from −0.7 to −1.1 V vs. the RHE and (c) CO partial current density in 1 M, 3 M, and 5 M KOH; (d) FE for CO from −1.1 V to −1.4 V vs. the RHE and (e) CO partial current density in 0.5 M K₂SO₄ (pH = 1 and 2); (f) FE for H₂ in 0.5 M K₂SO₄ (pH = 1 and 2).

To improve the utilization of CO₂, CO₂RR for the robust NiPc-Im-COF was also performed in GDE with acidic electrolyte (0.5 M K₂SO₄, the pH value was adjusted to 1 and 2 by H₂SO₄). As shown in Fig. S21, ESI,† the total current density in K₂SO₄ (pH = 1) was higher than that in K₂SO₄ (pH = 2) and exhibited ion concentration dependence, with only a slight decrease in the selectivity for CO₂RR to CO. As illustrated in Fig. 5d, NiPc-Im-COF also exhibited high selectivity and the FE_CO exceeded 90% in K₂SO₄ (pH = 2) from −1.1 V to −1.4 V. When the pH was reduced to 1, FE_CO remained above 80% within the voltage range of −1.2 V to −1.4 V, and the maximum J_CO reached 87 mA cm⁻² (Fig. 5e). The electrocatalytic CO₂RR was also carried out in H₂SO₄ (pH = 1 and 2) without added K₂SO₄, and the current density significantly decreased (Fig. S22, ESI†). Besides, the FE_H2 was almost 100% in H₂SO₄ (Fig. 5f), demonstrating that the presence of K⁺ in the electrolyte effectively suppressed the HER. The pH value was tested after CO₂RR. As shown in Fig. S23, ESI,† the pH values of electrolytes with K₂SO₄ (pH = 1 and 2) were unchanged, whereas the pH values of electrolytes without K₂SO₄ (pH = 1 and 2) changed to 1.7 and 2.4. The pH changes further confirmed the dominance of CO₂RR in the presence of K⁺. The valence of Ni sites in NiPc-Im-COF and its crystallinity remained stable after CO₂RR, as confirmed by XPS and XRD (Fig. S24 and S25, ESI†). FE_CO remained stable (about 90%) for 6 hours at −1.3 V in K₂SO₄ (pH = 1) (Fig. S26, ESI†).

In order to gain a deeper insight into the mechanism of the electrocatalytic activity of NiPc-Im-COF, ¹³C-labeled CO₂ isotope experiments through gas chromatography–mass spectrometry (GC-MS) at −0.8 V in KH¹²CO₃ and KCl electrolytes were conducted. As shown in Fig. 6a and Fig. S27, ESI,† when the CO₂RR was conducted under ¹³C-labeling CO₂ in 0.5 M KH¹²CO₃, both the signals of ¹²CO and ¹³CO were observed. While only ¹³CO was observed when KH¹²CO₃ electrolyte was replaced by the KCl electrolyte. These results revealed that the source of carbon in the CO₂RR was the CO₂ dissolved in the KHCO₃ solution. Then, to confirm the intermediates during the CO₂RR, in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) at −0.8 V in 0.5 M KHCO₃ was performed. As shown in Fig. 6b, the real-time FTIR spectra of CO₂RR on NiPc-Im-COF electrodes exhibited two prominent bands at 1400 and 2330–2360 cm⁻¹, which were assigned to the *COOH and adsorption of CO₂, respectively.^60,61 These results suggested that *COOH plays a crucial role as the key intermediate in the formation of CO.

Fig. 6 (a) Mass spectra of CO in the ¹³CO₂-saturated 0.5 M KCl for NiPc-Im-COF; (b) operando ATR-FTIR of NiPc-Im-COF during CO₂RR at −0.8 V in 0.5 M KHCO₃; (c) free energy diagrams of NiPc-Im-COF for CO₂RR and (d) HER pathways with or without K⁺ in acidic electrolyte.

Based on the results of isotope experiments and ATR-FTIR, density functional theory (DFT) calculations were conducted to calculate the free energies of reaction intermediates for CO₂RR and HER. In general, the pathway for the formation of CO can be summarized as the adsorption of CO₂ to form *CO₂, then the generation of *COOH and *CO through the process of electron-coupled proton transfer, and finally, the CO desorption from active sites. From the free energy diagram in Fig. 6c and d, the energy barrier of the *COOH formation decreases from 2.49 eV to 1.02 eV, while the energy barrier of the *H formation increases from −0.16 eV to 2.38 eV when an acidic solution containing K⁺ is employed as electrolyte. The higher energy barrier of *H formation is consistent with the experimental results that CO is the major product for CO₂RR in acidic electrolytes with K⁺.

Conclusions

In summary, the robust NiPc-Im-COF linked by irreversible covalent bonds was successfully prepared and employed as a highly efficient electrocatalyst for CO₂RR. It exhibited a remarkable CO selectivity exceeding 90% within a broad potential range of −0.7 to −1.1 V. Moreover, due to the brilliant stability of the NiPc-Im-COF conferred by the imidazole ring in alkaline and acid solutions, KOH and acidic K₂SO₄ solutions were employed as electrolytes in GDE to break through the limitation of CO₂ mass transfer. NiPc-Im-COF exhibited FE_CO exceeding 80% in KOH solutions of varying concentrations and the J_CO reached 267 mA cm⁻² at −0.8 V in 5 M KOH. Furthermore, NiPc-Im-COF exhibited high efficiency in CO₂RR, with FE_CO exceeding 85% and a J_CO of 87 mA cm⁻² at −1.4 V in acidic K₂SO₄ (pH = 1), effectively suppressing the hydrogen evolution reaction (HER). Crucially, the structure of NiPc-Im-COF and the valence of Ni sites can remain stable after CO₂RR, which is beneficial for the accessibility of active sites. Our work provides an effective strategy to enhance the stability of COFs through the linkage of the imidazole ring for CO₂RR. This approach can shed light on the rational design of porous framework materials for the electrocatalytic CO₂RR in various environments.

Author contributions

X. Y., H. L., Y. H., and R. C. conceived the project and wrote the manuscript. X. Y. performed the experiments and collected the data. H. L., D. S., and Y. H. polished the manuscript. All authors discussed the results and commented on the manuscript.

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

The work was supported by the National Key Research and Development Program of China (2021YFA1501500, 2022YFA1505700), NSFC (U22A20436, 22071245, 22033008, 22220102005, 22201286), and Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2021ZZ103), Open Science Promotion Plan 2023 of CSTCloud. The authors thank the beamline BL14W1 station for XAS measurements at the Shanghai Synchrotron Radiation Facility, China.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qm01190a

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