Covalent organic frameworks based on tetraphenyl-p-phenylenediamine and metalloporphyrin for electrochemical conversion of CO₂ to CO

Abstract

Electrocatalytic CO₂ reduction provides a possible method for carbon neutralization. Electrode materials with efficient electron transfer, high selectivity and large current density are highly desirable. Herein, we have developed a couple of tetraphenyl-p-phenylenediamine and metalloporphyrin-based 2D COFs for the electrocatalytic CO₂ reduction. TPPDA-MPor-COFs (M = Co and Ni) were obtained by the cross-condensation of tetraphenyl-p-phenylenediamine (TPPDA) and 5,10,15,20-tetrakis(4-formylphenyl)-metalloporphyrin (MPor). The as-prepared TPPDA-CoPor-COF shows high CO faradaic efficiencies of 87–90% from −0.6 to −0.9 V vs. RHE, and the largest CO partial current density (j_CO) of TPPDA-CoPor-COF (−22.2 mA cm⁻² at −1.0 V vs. RHE) exceeds those of most of the reported COF-based electrocatalysts. Notably, exfoliated TPPDA-CoPor-COF nanosheets (TPPDA-CoPor-COF-NSs) show much better electrocatalytic performance. The CO faradaic efficiencies of TPPDA-CoPor-COF-NSs are over 90% in a wider voltage range (−0.7 to −0.9 V), and the maximum j_CO reaches up to −29.2 mA cm⁻² at −1.0 V. Density functional theory calculations have been performed to rationalize the improved CO₂RR performance of TPPDA-CoPor-COF.

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Introduction

In order to achieve carbon neutralization, photocatalysis,^1–3 electrocatalysis^4–6 and thermal catalysis⁷ have been developed over the past decades. The electrocatalytic CO₂ reduction reaction (CO₂RR) is considered as a promising strategy among them, which is clean and mild and can be associated with renewable energy.^8,9 Varieties of homogeneous and heterogeneous catalysts have been developed for the efficient and selective electrocatalytic CO₂RR.^10–17 Although homogeneous catalysts have been proved to show low overpotential and high selectivity by means of smart design and functional group regulation,¹¹ inefficient electron transfer and poor stability in homogeneous organic catalytic systems restrain their practical application. Meanwhile, due to their aqueous compatibility and great catalytic activity, various heterogeneous catalysts have been investigated.^18,19

As promising porous crystalline materials, metal–organic frameworks (MOFs)^20–22 and covalent organic frameworks (COFs)^23,24 have been explored for the electrocatalytic CO₂RR. In particular, the building blocks of COFs can be manipulated precisely for specific activity and selectivity. Some active homogeneous molecular catalysts, such as porphyrins^25–31 and phthalocyanines,^32–34 could be integrated into COFs as building blocks for the electrocatalytic CO₂RR. Nevertheless, most of the reported COF materials exhibit limited current density and relatively low faradaic efficiency. Hence, novel COFs with high current density and excellent energy conversion efficiency are highly desirable.

Tetraphenyl-p-phenylenediamine, as a typical electron donor, has a high electron transfer capability and has been widely used in preparing electrochemically active materials.^35–39 On the other hand, metalloporphyrins (MPor) can function as excellent electron acceptors and charge transfer components due to their conjugated macrocyclic structures.^{27–29,40–42} Therefore, efficient intramolecular electron transfer paths might be constructed by integrating TPPDA and MPor into two-dimensional COFs, and enhanced electrocatalytic CO₂RR activities are expected for the obtained COFs.

Herein, MPor and TPPDA-based COFs (TPPDA-MPor-COFs, M = Co(II) and Ni(II)) have been synthesized for the first time (Fig. 1). TPPDA-CoPor-COF exhibits high CO faradaic efficiencies (FE_CO) and large CO partial current densities (j_CO), which can be further improved by physical ultrasonic exfoliation. The obtained TPPDA-CoPor-COF nanosheets (TPPDA-CoPor-COF-NSs) show a maximum FE_CO of 92% at −0.7 V and a j_CO of −29.2 mA cm⁻² at −1.0 V vs. RHE. The excellent electrocatalytic properties can be attributed to the large amount of accessible Co(II) sites and efficient electron transfer from TPPDA to CoPor blocks in these COF-based materials.

Fig. 1 The synthesis of TPPDA-MPor-COFs (M = Co and Ni).

Results and discussion

Synthesis and characterization

TPPDA-MPor-COFs (M = Co and Ni) were synthesized via Schiff-base condensation of TPPDA and MPor (M = Co and Ni). The experimental powder X-ray diffraction (PXRD) patterns show that TPPDA-MPor-COFs have good crystallinity (Fig. 2a and S1†). Le Bail refinements of TPPDA-MPor-COFs were carried out on the basis of experimental PXRD data (Fig. S2†).⁴³ Eclipsed (AA) and staggered (AB) stacking models of TPPDA-CoPor-COF were constructed, where the former one matches well with the experimental data. Furthermore, the refined results of TPPDA-CoPor-COF show low residual values (R_p = 5.04% and R_wp = 7.50%) indicating the validity of the AA stacking model. The unit cell information of TPPDA-CoPor-COF has been obtained as follows: triclinic P1 space group, a = 24.56 Å, b = 25.08 Å and c = 4.63 Å, α = 66.76°, β = 104.04°, γ = 90.83°. In the experimental PXRD pattern of TPPDA-CoPor-COF (Fig. 2a), the strong diffraction signals at 3.77°, 5.09° and 5.54° are assigned to the (100), (110) and (1−10) planes, respectively. A couple of weak signals at 10.20° and 11.12° are attributed to the (220) and (130) planes, respectively. TPPDA-NiPor-COF (Fig. S1†) shows similar PXRD results.

Fig. 2 (a) Experimental PXRD patterns, (b) FT-IR spectra, (c) N₂ sorption isotherm at 77 K and (d) CO₂ sorption isotherm at 298 K for TPPDA-CoPor-COF.

In the FT-IR spectra of TPPDA-MPor-COFs (Fig. 2b and S3†), the peaks at 1622 cm⁻¹ reveal that the imine bond (C=N) formed successfully, along with a decrease of C=O (1699 cm⁻¹) and N–H (3458–3348 cm⁻¹ and 1620 cm⁻¹) vibrations for MPor and TPPDA monomers, respectively.⁴⁴ In the solid-state electronic absorption spectrum, the characteristic absorptions of the MPor moiety (i.e. Q-band, 541 nm; sorbet band 432 nm) can be observed, suggesting the successful integration of the cobalt porphyrin unit into TPPDA-CoPor-COF (Fig. S4†).

The XPS spectra reveal the presence of C, N and Co/Ni elements in the two COFs (Fig. S5a and S6a†), and the two COFs show three characteristic peaks corresponding to the C–N (pyrrolic nitrogen) bond (398.6 and 398.7 eV) of MPor, and the C=N bond (399.0 and 399.1 eV) and the C–N bond (399.6 eV) of TPPDA, respectively (Fig. S5b and S6b†), further indicating the formation of the imine bond.⁴⁵ In addition, the XPS analyses of Co 2p and Ni 2p indicate that all the metal sites in TPPDA-MPor-COFs are divalent (Fig. S5c and S6c†).^46,47 The N₂ sorption curves of the two COFs display type I-isotherms (Fig. 2c and S8†). The adsorption curves show a steep increase when P/P₀ < 0.01, corresponding to the presence of permanent micropores. The Brunauer–Emmett–Teller (BET) surface area and the total pore volume of TPPDA-CoPor-COF were 1209 m² g⁻¹ and 0.99 cm³ g⁻¹, respectively (Fig. 2c). TPPDA-CoPor-COF displays two pore sizes (1.2 nm and 1.5 nm) (Fig. S7†), being consistent with the simulated structure (Fig. 1). TPPDA-NiPor-COF shows similar N₂ sorption and porous properties (Fig. S8† and S9†). Besides, TPPDA-CoPor-COF and TPPDA-NiPor-COF show moderate CO₂ sorption capacities of 26.8 cm³ g⁻¹ and 28.1 cm³ g⁻¹ at 25 °C and 1.0 bar, respectively (Fig. 2d and S10†), indicating the CO₂ affinity of TPPDA-MPor-COFs.

The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that TPPDA-MPor-COFs have layered morphologies with sizes of 100–500 nm (Fig. 3a, b and S11†). The element energy dispersive spectroscopy (EDS) mapping images (Fig. 3c and S12†) show the uniform distribution of metal ions (Co or Ni), C and N elements in TPPDA-CoPor-COF and TPPDA-NiPor-COF, respectively. The total Co content (3.35%) of TPPDA-CoPor-COF and Ni content (3.20%) of TPPDA-CoPor-COF were measured by ICP tests (Table S1†), and are reasonably lower than the calculated metal contents. Thermal gravimetric analyses (TGAs) were carried out under a nitrogen atmosphere (Fig. S13†). TPPDA-MPor-COFs only displayed a slight weight loss till 500 °C owing to the loss of residual solvents, suggesting the high thermal stability of the COFs.

Fig. 3 (a) SEM, (b) TEM and (c) EDS mapping images of TPPDA-CoPor-COF.

The Co K-edge XANES spectra of TPPDA-CoPor-COF and CoPc exhibit similar curves, suggesting that the coordination environment of Co atoms in TPPDA-CoPor-COF are the same as that of CoPc (Fig. 4a). In the Fourier-transform (FT) EXAFS curves, TPPDA-CoPor-COF displays a strong signal at 1.53 Å corresponding to the Co–N scattering path (Fig. 4b),⁴⁸ and no signal of Co–Co bonds was detected. EXAFS fitting for TPPDA-CoPor-COF was conducted using Co–N₄ coordination models. The result also suggests that the Co site in TPPDA-CoPor-COF is coordinated with four nitrogen atoms (Fig. 4c and Table S2†). TPPDA-NiPor-COF shows similar results (Fig. S14†).

Fig. 4 (a) Co K-edge XANES spectra of TPPDA-CoPor-COF, CoPc, CoO and Co foil. (b) FT EXAFS spectra of TPPDA-CoPor-COF, CoPc, CoO and Co foil. (c) The EXAFS fitting curve of TPPDA-CoPor-COF.

Electrocatalytic performance

To investigate the catalytic performances of the COFs, a H-cell with a standard three-electrode system and 0.5 M KHCO₃ electrolyte solution was used. In order to avoid the accidental factors, three parallel experiments were conducted. All potentials are presented relative to the reversible hydrogen electrode (RHE) in this work.

Linear sweep voltammetry (LSV) curves were obtained in CO₂ and Ar-saturated 0.5 M KHCO₃, respectively (Fig. 5a). The onset potential of TPPDA-CoPor-COF (−0.46 V) is more positive than that of TPPDA-NiPor-COF (−0.72 V). TPPDA-CoPor-COF also exhibits larger current densities in CO₂-saturated solution than in Ar-saturated solution from −0.5 to −1.0 V, suggesting greater electrocatalytic CO₂RR activity than the hydrogen evolution reaction (HER) activity. Short-term electrolysis tests and the corresponding gas chromatography analyses show that the products of the electrocatalytic CO₂RR were carbon monoxide and hydrogen (Fig. S15 and S16†). The nuclear magnetic resonance experiment shows that no liquid product is generated during the reduction process (Fig. S17†). Only a negligible amount of CO was detected when the control electrolysis was conducted in an Ar-saturated 0.5 M KHCO₃ electrolyte (Fig. S18†). Furthermore, carbon cloth decorated with Vulcan XC-72R carbon black and Nafion shows almost no electrocatalytic CO₂RR activity in the CO₂-saturated electrolyte (Fig. S19†). According to the above results, the source of CO and the electrocatalytic CO₂RR activity of TPPDA-CoPor-COF can be confirmed.

Fig. 5 (a) LSV curves, (b) FEco, (c) j_CO, (d) TOF and (e) Tafel plots of TPPDA-MPor-COFs. (f) Lasting stability test for TPPDA-CoPor-COF at −0.8 V.

TPPDA-CoPor-COF exhibits high CO faradaic efficiencies (FE_CO) of 87–90% in the range of −0.6 to −0.9 V (Fig. 5b), while TPPDA-NiPor-COF shows lower FE_CO (i.e. 60–76% in −0.7 to −0.9 V). The j_CO values of TPPDA-CoPor-COF increase with elevated applied potential and reach up to −22.2 mA cm⁻² at −1.0 V, which is 5.7-fold that of TPPDA-NiPor-COF (−3.9 mA cm⁻²) (Fig. 5c) and surpasses most of the reported COFs (Fig. S20 and Table S3†). Moreover, in comparison with TPPDA-CoPor-COF, the maximum FE_CO of the CoPor monomer is only 76% at −0.8 V, and the maximum j_CO value is only −11.8 mA cm⁻² at −1.0 V (Fig. S21†), and the TPPDA monomer shows nearly 100% FE of H₂ in the range of −0.7 to −1.0 V (Fig. S22†). The above results indicate that the TPPDA unit plays an important role in promoting the electrocatalytic CO₂RR activity of TPPDA-CoPor-COF. Besides, the turnover frequency (TOF) of TPPDA-CoPor-COF was calculated to be 1.4 s⁻¹ at −0.9 V and 2.0 s⁻¹ at −1.0 V (Fig. 5d).

The Tafel slopes of TPPDA-CoPor-COF and TPPDA-NiPor-COF are tested to be 224 mV dec⁻¹ and 226 mV dec⁻¹, respectively (Fig. 5e). The results imply the slightly superior reactivity of Co over Ni sites. The Nyquist plots of the electrochemical impedance test illustrate that TPPDA-CoPor-COF has a smaller charge-transfer resistance than TPPDA-NiPor-COF during the electrocatalytic CO₂RR process (Fig. S23†), confirming its more efficient electron transfer from the catalyst surface to the CO₂ molecules. To compare the electrochemically active surface areas (ECSAs) of TPPDA-MPor-COFs, electrochemical double-layer capacitances (C_dl) were acquired (Fig. S24†). TPPDA-CoPor-COF presents a C_dl value of 2.61 mF cm⁻², which is larger than 1.31 mF cm⁻² for TPPDA-NiPor-COF, further indicating that TPPDA-CoPor-COF shows higher inherent catalytic activity.

Chronoamperometric tests were performed for TPPDA-CoPor-COF to evaluate the durability at −0.8 V in the H-cell. The corresponding FE_CO values remained higher than 80% in the 10 h electrolysis experiment (Fig. 5f), demonstrating that TPPDA-CoPor-COF has an acceptable electrochemical catalytic stability in spite of a decrease of the current density (∼20%). The stability of TPPDA-CoPor-COF was further demonstrated by the almost unchanged XPS (Fig. S25†) and PXRD (Fig. S26†) data after 2 h and 10 h of electrocatalysis at −0.8 V. The SEM and TEM (Fig. S27†) images of TPPDA-CoPor-COF after 2 h and 10 h of electrolysis suggest that the layered morphology of the catalysts is retained well. The above results well disclose its good electrochemical catalytic stability.

Based on previous reports,^27,28 the high stability and performance of two-dimensional TPPDA-CoPor-COF motivated us to ultrasonically exfoliate the materials to further improve their catalytic activity. TPPDA-CoPor-COF was converted into TPPDA-CoPor-COF-NSs with a thin-layered morphology (Fig. S28†) and a thickness of ∼7 nm as proved by atomic force microscopy (AFM) measurements (Fig. 6a and b). Almost identical PXRD curves indicated that the periodic structure of TPPDA-CoPor-COF was retained in the nanosheets after ultrasonic exfoliation (Fig. S29†). To evaluate the electrocatalytic CO₂RR performance of TPPDA-CoPor-COF-NSs, the same short- and long-term electrolysis tests were conducted (Fig. S30 and S31†), and the corresponding FE_CO and j_CO were calculated. The FE_CO values of TPPDA-CoPor-COF-NSs are above 90% in a wider range (−0.7 to −0.9 V) than those of TPPDA-CoPor-COF (Fig. 6c). The maximum FE_CO value of TPPDA-CoPor-COF-NSs is 92% at −0.7 V and the j_CO reaches up to −29.2 mA cm⁻² at −1.0 V (Fig. 6d), which are higher than those of the unexfoliated one. The promoted CO₂RR performance of TPPDA-CoPor-COF-NSs could be attributed to the more exposed Co active sites.^27,28 Besides, a 10 h chronoamperometric test of the nanosheets at −0.7 V was carried out. Same as the unexfoliated one, the corresponding FE_CO can be kept above 80% in the whole electrolysis process as well (Fig. S29†), indicating that TPPDA-CoPor-COF-NSs also have tolerable electrochemical catalytic stability, although accompanied by a certain degree of current density attenuation.

Fig. 6 (a) The atomic force microscopy topographical image. (b) Corresponding height curves of (a). (c) FE_CO of TPPDA-CoPor-COF-NSs. (d) Lasting stability test of TPPDA-CoPor-COF-NSs at −0.7 V.

DFT calculations

The frontier molecular orbitals of the repeat unit in TPPDA-MPor-COFs were calculated by DFT calculations (Fig. S32 and S33†). The HOMO is located on the TPPDA moiety, while the LUMO is on the MPor moiety, implying the electronic donor–acceptor configuration of the two COFs, which usually means decreased band gaps and enhanced electron transfer properties for organic semiconductors.

To understand the superior CO₂RR performance of TPPDA-CoPor-COF, the CO₂RR and HER mechanisms were calculated together with those of TPPDA-NiPor-COF and the CoPor monomer (Fig. 7a–c). According to the calculated relative free energies, the rate determining steps (RDSs) for the CO₂RR and HER are the formation of *COOH and *H intermediates, respectively.⁴⁹ The ΔG_RDS values of the CO₂RR for TPPDA-CoPor-COF (0.98 eV), TPPDA-NiPor-COF (1.10 eV) and CoPor (1.13 eV) are all lower than the respective ΔG_RDS values of the HER (1.11 eV, 1.24 eV, and 1.15 eV), indicating their preferable CO₂RR activity. TPPDA-CoPor-COF exhibits dramatically reduced ΔG_RDS of the CO₂RR relative to the CoPor monomer, suggesting that the introduction of the TPPDA unit and the formation of COFs could promote the CO₂RR activity of the CoPor core. Mulliken population and frontier orbital analyses afforded consistent results (Fig. 7d and Fig. S34†).⁵⁰ The Mulliken atom charge of the Co sites in TPPDA-CoPor-COF (0.76) is lower than that in the CoPor monomer (0.77), indicating the more electron-rich environment of the Co atom in TPPDA-CoPor-COF. The LUMO level of TPPDA-CoPor is −2.29 eV, which is higher than that of CoPor (−2.42 eV), suggesting the higher reducibility of TPPDA-CoPor-COF in the electrocatalytic process. TPPDA-NiPor-COF displays a higher ΔG_RDS of the CO₂RR than that of TPPDA-CoPor-COF, suggesting the harder formation of *COOH intermediates on TPPDA-NiPor-COF. Furthermore, the desorption of CO from TPPDA-NiPor-COF (*CO → *+CO) is endothermic, while this step for TPPDA-CoPor-COF is exothermic (Fig. 7b), indicating that the Co site shows superior CO₂RR activity than the Ni site in the COFs, which is consistent with the experimental results.

Fig. 7 (a) The proposed mechanism of the electrocatalytic CO₂RR for TPPDA-MPor-COFs. (b) The calculated energy diagrams of CO₂ reduction to CO and (c) the calculated energy diagrams of the HER for TPPDA-MPor-COFs and CoPor. (d) The Mulliken charge of the Co site in TPPDA-CoPor-COF and CoPor.

Conclusions

In summary, metalloporphyrin- and TPPDA-based two-dimensional COFs were explored for the electrocatalytic CO₂RR in the H-cell. The obtained TPPDA-CoPor-COF exhibits a high FE_CO of 87–90% in the range of −0.6 to −0.9 V and a maximum j_CO of −22.2 mA cm⁻² at −1.0 V. The exfoliated TPPDA-CoPor-COF-NSs show further improved FE_CO and j_CO than the as-prepared TPPDA-CoPor-COF. DFT calculations reveal that the integration of the TPPDA block enhances the electron transfer ability of TPPDA-MPor-COFs and reduces the CO₂RR energy barrier of the CoPor core. This work would be conducive to the rationally designed novel COF-based electrocatalysts towards the CO₂RR.

Author contributions

Lei Gong, Baotong Chen, Ying Gao and Yinhai Wang performed experiments under the guidance of Yongzhong Bian and Jianzhuang Jiang. Dongdong Qi, Bin Han, Chenxiang Lin and Baoqiu Yu provided assistance for data acquisition and analysis. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The financial support from the National Natural Science Foundation of China (21631003), Beijing Natural Science Foundation (2202028) and Fundamental Research Funds for the Central Universities (FRF-BD-20-14A) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and additional figures and tables. See DOI: https://doi.org/10.1039/d2qi00336h

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