Dimensional engineering of covalent organic frameworks derived carbons for electrocatalytic carbon dioxide reduction

Covalent organic frameworks (COFs) have been developed as the precursors to construct porous carbons for electrocatalytic systems. However, the influences of carbon dimensions on the catalytic performance are still underexplored. In this work, we have first constructed COF‐derived carbons by template‐synthesis strategy in different dimensions to catalyze the carbon dioxide reduction (CO2RR). By using different templates, the one‐dimensional (1D), two‐dimensional (2D), and three‐dimensional (3D) COF‐derived carbons have been employed to anchor Co‐porphyrin to form the Co‐N5 sites to catalyze CO2RR. The 1D catalyst templated by carbon nano tubes presents high binding ability of CO2, more defective sites, and higher electronic conductivity, resulting in a higher catalytic activity for CO2 and selectivity of CO than 2D and 3D carbon‐based catalysts. The 1D catalyst delivers the turnover frequency values of 1150 h−1 and the FECO of 94.5% at 0.7 V versus RHE, which is significantly better than those of 2D and 3D carbon‐based catalysts.

1][42][43][44] More importantly, the stacking layers of COFs in the edge-on direction would convert into standing carbon sheets, which have high electrocatalytic activity. 35,36,45,46These catalytic carbons have been used in ORR, OER, and HER.Recently, the CoN 2 O 2 and CoN 5 sites were constructed in the COF-derived carbons for the electrochemical conversion of CO 2 . 21,36,46,47However, the direct pyrolysis of COFs yields only three-dimensional (3D) carbon with losing their two-dimensional (2D) structural and porous features, although COFs are 2D layered materials. 19,31,32Therefore, the structure control of COFderived carbon is challenged but important to obtain CO 2 RR catalysts with large porosity, high conductivity, and abundance of edge sites for doped heteroatoms.
Herein, the structure of COF-derived carbons was controlled in one-dimensional (1D) to 3D by using carbon nanotubes (CNTs) and graphene (Gr) as templates.The effects of dimensional engineering carbon on the CO 2 RR performance were comparatively investigated.

RESULTS AND DISCUSSION
As illustrated in Figure 1A,B, the 1D and 2D COF-derived carbons were synthesized by the template-pyrolysis methods with depositing COFs layer on CNTs and Gr, respectively.As control, the 3D carbon was synthesized by the direct carbonization of COF precursor (Figure 1C).TP-BPY-COF, which was synthesized from the trialdehyde phloroglucinol (TP) and [2,2'-bipyridine]-5,5'-diamine (BPY) by solvothermal methods based on previous reports, 35,36,48 was used as the COF precursors.
The COF layer was in situ grown on the surface of the templates to obtain COF@CNT and the COF@Gr, respectively.
F I G U R E 1 Synthesis of (A) Co-COF@CNT 800 and (B) Co-COF@Gr 800 by the template-pyrolysis method.(C) Synthesis of Co-COF 800 by direct carbonization of TP-BPY-COF.CoPor: Cobalt tetramethoxyphenylporphyrin.
The formation of COF structure was first investigated by Fourier transform infrared (FT IR) spectroscopy (Figure 2A).The band at 1450 cm −1 was assigned to the C=N linkages of TP-BPY-COF, and the bands at 1606, 1580, and 1268 cm −1 were ascribed to the vibrations of C=O, C=C, and C-N bonds, respectively.This suggests that TP-BPY-COF was successfully synthesized. 48In comparison, the peaks of C=C linkages slightly changed to 1571 and 1568 cm −1 in COF@CNT and COF@Gr, respectively, while no changes were observed from the peaks of C=O, C=N, and C-N.It confirms that the skeletons of TP-BPY-COF were well retained in COF@CNT and COF@Gr samples.
The crystal structures of the prepared TP-BPY-COF, COF@CNT, and COF@Gr were examined using powder x-ray diffraction (PXRD) measurements.The PXRD patterns of TP-BPY-COF (Figure 2B) exhibited a strong peak at 3.6 • with other two weak peaks at 6.1 • and 26.8 • , which were contributed to (100), (200), and (001) facets, respectively.This indicates that TP-BPY-COF has high crystallinity. 35,36Apart from the signals of template (Figure S1), the main peaks of TP-BPY-COF were significantly weakened in COF@CNT and COF@Gr.It is reasonable that the thickness of COF layer is too thin to reflect the diffraction.
The porous structures of TP-BPY-COF, COF@CNT, and COF@Gr were examined using N 2 sorption at 77 K.The TP-BPY-COFs showed type-IV isotherms with a Brunauer-Emmett-Teller surface area (S BET ) of 1318 m 2 g -1 and a pore volume of 1.44 cm 3 g -1 (Figure 2C).The S BET for COF@CNT and COF@Gr declined to 108.44 and 429.80 m 2 g -1 , with the total pore volumes of 0.32 and 1.55 cm 3 g -1 , respectively (Figure 2D).Additionally, CNT and graphene showed S BET of 24.76 and 129.25 m 2 g -1 , with the total pore volumes of 0.32 and 0.43 cm 3 g -1 , respectively (Figure S2).
The morphology of samples was measured by the scanning electron microscopy (SEM, Figure S3).TP-BPY-COF exhibited uniform fiber-like morphology with the width of ca.40 nm.In the case of COF@CNT and COF@Gr, the original tubular and lamellar morphologies of CNTs and graphene (Figure S4) were maintained, while the surface became rough due to the deposition of COF layer.
The TP-BPY-COF, COF@CNT, and COF@Gr were then pyrolyzed at 800 • C under N 2 for 1 h to obtain COF 800 , COF@CNT 800 , and COF@Gr 800 , respectively.The PXRD patterns of COF@CNT 800 posted two peaks at 26.0 • and 43.0 • , corresponding to (002) and (101) facets of graphitic carbon, respectively, while only one broad peak of (002) facet of graphitic carbon was observed for COF@Gr 800 and COF 800 (Figure 2E).The defects of the carbons were characterized by the Raman spectroscopy (Figure 2F and Figure S7).The peaks at 1346 and 1585 cm -1 were ascribed to the D and G bands of sp 2 carbon, 36 respectively.The intensity ratios (I D /I G ) of COF 800 , COF@CNT 800 , and COF@Gr 800 were 1.24, 1.41, and 1.03, while the I D /I G values of CNT and Gr were 1.49 and 0.92 (Figure 2F and Figure S7).The decreased I D /I G of COF@CNT 800 related to CNT indicates that the F I G U R E 3 Scanning electron microscopy (SEM) images of (A) Co-COF 800 , (B) Co-COF@CNT 800 , and (C) Co-COF@Gr 800 .Transmission electron microscopy (TEM) images of (D) Co-COF 800 , (E) Co-COF@CNT 800 , and (F) Co-COF@Gr 800 .(G) N 2 sorption isotherm profiles at 77 K (inset: pore size distribution curves) and CO 2 sorption curves (H) of Co-COF 800 , Co-COF@CNT 800 , and Co-COF@Gr 800 .
1D topology is beneficial to form more defects on pyrolytic carbon.
The chemical states of these carbons were studied by x-ray photoelectron spectroscopy (XPS).The nitrogen content of COF 800 was determined as 9.34 wt.%, which declined to 1.98 and 1.43 wt.% for COF@CNT 800 and COF@Gr 800 , respectively (Figure S8).The high-resolution N 1s spectra of these carbon were decomposed into four distinct peaks, namely, the pyridinic N peak (N1, 397.45 eV), the pyrrolic N peak (N2, 398.80 eV), the graphite N peak (N3, 400.09 eV), and the quaternary N peak (N4, 402.40 eV) (Figure S9). 47Among them, the contents of N1 and N2 were increased from 34.84% and 2.44% for COF 800 to 35.82% and 24.28% for COF@CNT 800 and 35.96% and 16.50% for COF@Gr 800 .The higher contents of N1 and N2 indicate more binding sites for metal anchoring.
To obtain CO 2 RR catalysts, the CoPor was introduced into these carbons via the impregnation method, which was named as Co-COF 800 , Co-COF@CNT 800 , and Co-COF@Gr 800 .The PXRD patterns exhibited that the new peaks were assigned to CoPor (Figure S10).
The morphologies of Co-COF 800 , Co-COF@CNT 800 and Co-COF@Gr 800 were characterized by SEM and transmission electron microscopy (TEM).According to the SEM images, the main morphologies of COF, COF@CNT and COF@Gr were well maintained after the treatments of pyrolysis and CoPor introduction (Figure 3A-C Co-COF 800 (Figure 3D).In comparison, the TEM images of Co-COF@CNT 800 showed the presence of a large number of standing carbon layers with an interlayer distance of 0.36 nm in the carbon matrix (Figure 3E).The standing carbon layers, derived from the carbonization process of TP-BPY-COF, indicates the reactivity for CO 2 RR, as the standing carbon was shown to be more active in electrocatalytic reaction than the plane carbon. 35,36In the case of Co-COF@Gr800, the ordered carbon was also observed, which is parallel to graphene surface (Figure 3F).
The XPS Co 2p3/2 spectrum of Co-COF 800 contains the main peaks of Co-N (779.80 eV) and Co-O (781.86 eV), which delivers relative contents of 51.96% and 15.14% for Co-N and Co-O (Figure 4B).Compared to Co-COF 800 , Co-COF@CNT 800 has the relative contents of Co-N and Co-O of 39.32% and 13.95%, and the relative contents for Co-N and Co-O of Co-COF@Gr 800 are 28.17% and 13.65%.The electronic environment and atomic states of Co were further confirmed using x-ray absorption finestructure (XAFS) analysis.Figure 4C shows the XANES curves at Co K-edge of Co-COF 800 , Co-COF@Gr 800 , Co-COF@CNT 800 , CoPor, and Co foil.The Co K-edge exhibited a similar near-edge structure to that of CoPor but the curve had a minor left shift.It indicates that the valence of Co atom in Co-COF 800 , Co-COF@Gr 800 , and Co-COF@CNT 800 are slightly lower than that of CoPor. 21he EXAFS spectra showed the peak position of Co at 1.5 Å, corresponding to the Co-N first coordination layer (Figure 4D).In addition, the best EXAFS fitting results of Co-COF 800 , Co-COF@Gr 800 , and Co-COF@CNT 800 showed that the distance of Co-N for Co-COF 800 , Co-COF@Gr 800 , and Co-COF@CNT 800 are1.97,1.96, and 1.98 Å, respectively (Figure s S16-S18 and Table S1).The Co-N coordination numbers are evaluated to be 4.6, 4.6, and 4.5, respectively.The wavelet transform (WT) plot of Co-COF 800 , Co-COF@Gr 800 , and Co-COF@CNT 800 showed the WT maximum at 3.9 Å -1 , which further confirmed Co-N bonding by comparing with CoPor (Figure 4E).No intensity wavelet corresponding to Co-Co was observed, and the Co atoms were coordinated by five N atoms (Co-N 5 ). 49Therefore, the newly catalytic sites (CoN 5 ) were successfully constructed by immobilizing CoPor on the COF-derived carbons.
Given that the CoN 5 sites showed higher catalytic activity than that of CoN 4 toward CO 2 RR, 21,36,49 the catalytic performance of these catalysts was investigated.First, their catalytic performance for CO 2 RR was checked by linear scanning voltammetry (LSV) measurements in 0.5 M KHCO 3 saturated aqueous solution of Ar or CO 2 at a scan rate of 10 mV s -1 from 0 to −1.0 V versus RHE (Figure 5A).The LSV curves for all these catalysts exhibited that the current densities under the Ar atmosphere (dash line) were much lower than those under the CO 2 atmosphere (solid curve) at the same potentials, which suggested that their activities were due to CO 2 RR.The Co-COF@CNT 800 and Co-COF@Gr 800 displayed higher current densities than those of Co-COF 800 from −0.4 to −1.0 V versus RHE, which indicated their higher catalytic activity.Additionally, the onset potential (E o ), the current density of 1 mA cm -2 , for Co-COF@CNT 800 and Co-COF@Gr 800 was 0.40 V, which was more positive than that of Co-COF 800 (0.48 V) in CO 2 .It suggests that Co-COF@CNT 800 and Co-COF@Gr 800 are more active than Co-COF 800 .
The corresponding Tafel slope was 224 mV dec -1 for Co-COF@CNT 800 and Co-COF@Gr 800 , which was smaller than that of Co-COF 800 -Co (267 mV dec -1 ) (Figure 5B).This indicates the favorable kinetics of the formation of CO over Co-COF@CNT 800 and Co-COF@Gr 800 .
The current densities of CO (j CO ) were further measured (Figure 5D).j CO of Co-COF@CNT 800 significantly increased with the increase of the applied potentials, delivering the maximum j CO of 25.8 mA cm -2 at −1.0 V, which is much higher than that of Co-COF@Gr 800 (19.5 mA cm -2 ) and Co-COF 800 (7.6 mA cm -2 ).In addition, the turnover frequencies (TOF) of the catalysts for the production of CO production were obtained at different potentials (Figure 5E).And the highest value of 1151 h −1 at −1.0 V for Co-COF@CNT 800 was obtained, which was evidently larger than the highest TOF of Co-COF@Gr 800 (762 h −1 ) and three times the maximum TOF value of Co-COF 800 (323 h -1 ).Furthermore, the stability of Co-COF@CNT 800 was tested by a 20-h test at −0.7 V (Figure S21), which kept stable with less than 10% degradations on both current density and FE CO value during testing.We have used PXRD patterns and FT IR spectra to further investigate the stability of the structure of samples.The PXRD patterns showed no new peaks after CO 2 RR test (Figure S22).And the FT IR spectra showed that all peaks from the catalysts were well retained (Figure S23), which further confirmed their long-term stability performance.Thus, the Co-COF@CNT 800 showed high activity, excellent selectivity, and outstanding stability in CO 2 RR reaction.
Electrochemical active surface area (ECSA) was essential in an electrochemical reaction.As shown in Figure 5F, Co-COF 800 showed a smaller ECSA of 4.40 mF cm -2 than Co-COF@CNT 800 (8.15 mF cm -2 ) and Co-COF@Gr 800 (8.93 mF cm -2 ), in line with the catalyst reactivity in CO 2 RR.According to the Nyquist plots (Figure S28), Co-COF@CNT 800 displayed much smaller charge transfer resistance (9.76 Ω) compared to Co-COF 800 (18.30Ω) and Co-COF@Gr 800 (16.04 Ω).This demonstrates faster charge transfer kinetics of Co-COF@CNT 800 , which is favorable for the electrocatalytic CO 2 RR performance.Thus, the better catalytic performance of Co-COF@CNT 800 was attributed to higher binding ability of CO 2 , more defective sites, and higher electronic conductivity.
To confirm the roles of COF-derived carbons in the catalysts, the control sample of Co-CNT 800 without TP-BPY-COF was investigated.Co-CNT 800 had an E o value of 0.44 V (Figure S29).Moreover, Co-CNT 800 showed the largest Tafel slope (336.6 mV dec -1 ) among all catalysts and low FE CO of 72.7%, 73.3%, 70.2%, 56.8%, and 42.4% at -0.6, -0.7, -0.8, -0.9, and -1.0 V, respectively (Figure s S30 and S31).Thus, the COF-derived carbon shell on the CNT enabled improvement of the CO 2 RR activity and CO selectivity.

CONCLUSION
In summary, we have demonstrated the dimensional engineering of COF-derived carbons to catalyze CO 2 RR by controlling the carbon structure from 1D to 3D.The COFderived carbon provided abundant N sites to form CoN 5 catalytic centers.The COF-based 1D catalyst presented high binding ability of CO 2 , more defective sites, and high electronic conductivity, leading to higher CO 2 RR activity and CO selectivity than the 2D and 3D catalysts.This work provides a new insight into the development of efficient COF-based catalysts.

R E F E R E N C E S
).The CNTs and graphene, acting as hard templates, are helpful to control the dimension of COF-derived carbons.The SEM and TEM energy-dispersive spectrometer (EDS) mapping revealed well-dispersed C, N, O, and Co elements in the catalysts (Figure s S11-S14).No ordered carbon structure was observed from the TEM image of F I G U R E 4 XPS (x-ray photoelectron spectroscopy) spectra of N 1s (A) and Co 2p (B) for Co-COF 800 , Co-COF@CNT 800 , and Co-COF@Gr 800 .XANES spectra (C), Co K-edge k3-weighted Fourier transform spectra from EXAFS (D) and WT-EXAFS (E) of Co foil, CoPor, Co-COF 800 , Co-COF@CNT 800 , and Co-COF@Gr 800 .

F
I G U R E 5 (A) Linear scanning voltammetry (LSV) curve in carbon dioxide (solid line) and argon atmosphere (dotted line), (B) Tafel slope, (C) FE CO over a potential range of −0.6 to −1.0 V versus RHE, (D) the current density of CO, (E) the TOF values, and (F) ECSA curves of Co-COF 800 , Co-COF@CNT 800 , and Co-COF@Gr 800 .