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• As a novel and effective way for yielding fuels, electrochemical carbon dioxide reduction reaction (CO₂RR) attracts much attention because it is a promising method to kill two birds with one stone to solve environmental problems and energy crisis [1-9]. Beneficial from the designable frameworks and abundant active sites, metal-organic frameworks (MOFs) are regarded as a series of good candidate for electrochemical CO₂RR [2]. However, the complexity of control conditions for the lattice planes and the insufficient exposure of metal sites of MOFs restrict their industrial and commercial application in electrochemical CO₂RR. Since the first report of single-layer graphene, two-dimensional (2D) materials have been proved to have tunable exposed lattice planes and unique electronic states [10, 11]. Therefore, compared to those of the three-dimensional MOFs, the specific lattice planes and metal sites of 2D-MOFs are easier to be exposed through reasonable design, which endows 2D-MOFs with more potential for electrochemical CO₂RR. Generally, two types of 2D-MOFs usually serve as electrocatalysts for CO₂RR, namely hexahydroxy-aromatic MOFs and phthalocyanine-based MOFs [2, 6, 12, 13]. For example, 2D Cu-THQ [14] (THQ = tetrahydroxyquinone) and Cu₂O@Cu-HHTP [15] (HHTP = 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene) revealed a Faradaic efficiency (FE) of 91% toward CO and a FE of 73% toward CH₄, respectively. Recently, we reported a Cu phthalocyanine-based MOF (PcCu-Cu-O) as an electrocatalyst producing C₂H₄ with a FE of 50% [2]. However, there are only a few reports on 2D-MOFs as electrocatalysts for CO₂RR. More 2D-MOFs remain to be developed as electrocatalyst for CO₂RR.

  Currently, several types of products have been obtained through electrochemical CO₂RR, including carbon monoxide (CO) [6, 14, 16, 17], hydrocarbons [2, 15, 18], carboxylic acid [19, 20] and alcohols [21]. Taking market prices of these products as well as the cost of electricity into account, CO and formic acid are the most promising products for industrial application [22]. According to previous reports, 2D MOFs with CuO₄ secondary building units tend to exhibit high selectivity toward CO [14, 15].

  Combining the advantages of 2D structures and CuO₄ unit, herein, for the first time, we designed and synthesized a triphene-based MOF nanosheet, Cu₃(HHTT)₂ (Cu-HHTT, HHTT = 9, 10-dihydro-9, 10-[1,2]benzenoanthracene-2, 3, 6, 7, 14, 15-hexaol) via the solvothermal treatment (Fig. 1). Powder X-ray diffraction (PXRD) pattern of the obtained Cu-HHTT commendably match the simulated one (Fig. S1 in Supporting information), indicative of the successful preparation of Cu-HHTT. Cu-HHTT exhibits an AAA inclined packing (Fig. 1b). The shortest distance between the oxygen atoms of CuO₄ units and the aromatic hydrogen atoms is 3.14 Å, indicative of the existence of interlaminar hydrogen bonds (Fig. S2 in Supporting information), which assist to improve the framework stability. The disappearance of the hydroxyl peak in Fourier Transform Infrared (FTIR) spectrum of Cu-HHTT in contrast to that of HHTT suggests the successful synthesis of Cu-HHTT (Fig. S3 in Supporting information). Thermogravimetric curve of Cu-HHTT indicates a thermal stability up to 160 ℃ (Fig. S4 in Supporting information). X-ray photoelectron spectroscopy (XPS) spectra reveals a mixed-valence state of the as-synthesized Cu-HHTT (Fig. 2a). As is demonstrated by transmission electronic microscope (TEM) and atomic force microscope (AFM), Cu-HHTT exhibits morphology of the ultrathin nanosheet with the thickness of 2.90 ± 0.22 Å (Figs. 2b and c, Fig. S5 in Supporting information). As been demonstrated in our previous work that, in contrast to bulk MOFs, MOF nanosheets can exhibit enhanced electrochemical CO₂RR performances [2]. Compared with the general hexahydroxy-aromatic MOFs and phthalocyanine-based MOFs, the Cu ions in Cu-HHTT are located on the channel wall, causing more exposure of the potential active sites to the environmental media, which might exhibit high performance of electrochemical CO₂RR to CO.

  Figure 1

  

  Figure 1.  (a) 2D layers of Cu-HHTT viewing along the c-axis and (b) the b-axis. Color code: Cu, purple; C, gray; O, red. Hydrogen atoms are omitted for clarity.

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  Figure 2

  

  Figure 2.  (a) Cu 2p spectra, (b) TEM image and (c) AFM image of Cu-HHTT.

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  In order to examine the affinity of Cu-HHTT to CO₂, we performed CO₂ adsorption measurements. The activated Cu-HHTT exhibits a saturation uptake of 136.39 cm³/g toward CO₂ at 195 K and 1 bar (Fig. S6 in Supporting information). Since the CO₂ uptake of most MOFs are 20–100 cm³/g, we anticipate that Cu-HHTT exhibits more affinity to CO₂, indicative of a potential activity for electrochemical CO₂RR. Electrochemical measurements of Cu-HHTT were subsequently performed. According to cyclic voltammetry (CV) curves, Cu-HHTT exhibits a higher current density in CO₂-saturated solution than that in Ar-saturated solution (Fig. 3a), suggesting the existence of electrochemical CO₂RR, which is also corroborated by the apparent positive shift of onset potential of the linear cyclic voltammetry (LSV) curve recorded in CO₂-saturated electrolyte compared to that recorded in Ar-saturated electrolyte (Fig. 3b). An impressive Faradaic efficiency (FE) of 96.6% and a turnover frequency (TOF) of 7.3 s⁻¹ toward CO with a current density of 18 mA/cm² of Cu-HHTT at the potential of −0.6 V vs. RHE (Figs. 3a-d) were observed. No liquid product was observed in ¹H nuclear magnetic resonance (NMR) spectra before and after the electrocatalysis (Fig. S7 in Supporting information). The electrochemical performance of Cu-HHTT surpasses most MOF-based electrocatalysts (Table S1 in Supporting information) [6, 14, 16, 23-27]. Actually, to our best knowledge, it is the highest selectivity toward CO among all the Cu-MOF electrocatalysts.

  Figure 3

  

  Figure 3.  (a) Cyclic voltammetry curves and (b) LSV curves of Cu-HHTT in Ar- and CO₂-saturated 0.1 mol/L KHCO₃ electrolyte. (c) Faradaic efficiencies of CO and H₂ for Cu-HHTT. (d) Turnover frequency for Cu-HHTT.

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  For the evaluation the electrochemical durability, Cu-HHTT was applied with continuous potential of −0.6 V vs. RHE for at least 4 h. Almost no current attenuation was observed during the long-time durability evaluation (Fig. S8 in Supporting information), and after that, the FE value for CO product slightly decreased to 91.3%. The slight decrease in performance may be ascribed to the minor sloughing of the catalyst from the electrode. According to PXRD patterns and TEM images, no Cu, Cu₂O, or CuO clusters were observed after electrochemical CO₂RR (Figs. S9 and S10 in Supporting information). The XPS spectra of Cu-HHTT indicate that electrochemical CO₂RR causes negligible changes of the electrocatalyst (Fig. 2a and Fig. S11 in Supporting information).

  As is illustrated in the literatures, the mechanism of electroreduction of CO₂ to CO generally involves the *COOH and *CO intermediates [22]. To capture the information of intermediates during CO₂RR, the attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) measurements were carried out (Fig. 4a). As a result, the peak at 1222 cm⁻¹ corresponds to the C−O stretching in *COOH [2]. The absorption peak at 1488 cm⁻¹ can be attributed to the symmetric vibration of *COOH intermediate. To further determine the reaction pathway, density functional theory (DFT) was employed to determine the reaction free energy. The changes of free energy for the desorption of CO and hydrogenation of *CO intermediate are −0.33 and 0.81 eV, respectively (Fig. 4b). Therefore, the generation of CO takes precedence of alcohols and/or alkanes, which coincides perfectly with the high selectivity toward CO obtained in the electrochemical measurements.

  Figure 4

  

  Figure 4.  (a) In-situ ATR-FTIR spectra of electrochemical CO₂RR on Cu-HHTT. (b) Free energy diagrams for the conversion of CO₂ into CO on Cu-HHTT and the optimized structure of each intermediate.

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  In summary, a 2D MOF Cu-HHTT with CuO₄ unit was successfully designed and synthesized for the first time. Its ultrathin nanosheet exhibits an impressive selectivity for electrochemical CO₂RR for yielding CO. The exposed CuO₄ sites and ultrathin nanosheet morphology play an important role in the excellent performance. This work proposes a new strategy to design 2D MOFs for the industrial conversion of CO₂ into CO.

  Declaration of competing interest

  The authors declare no conflict of interest.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (NSFC, Nos. 21890380 and 21821003), Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (No. 2017BT01C161), Science and Technology Key Project of Guangdong Province, China (No. 2020B010188002), Guangzhou Science and Technology Project (No. 202002030291), and Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2018B030306009).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.01.027.

  
  
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  Figure 1  (a) 2D layers of Cu-HHTT viewing along the c-axis and (b) the b-axis. Color code: Cu, purple; C, gray; O, red. Hydrogen atoms are omitted for clarity.

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  Figure 2  (a) Cu 2p spectra, (b) TEM image and (c) AFM image of Cu-HHTT.

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  Figure 3  (a) Cyclic voltammetry curves and (b) LSV curves of Cu-HHTT in Ar- and CO₂-saturated 0.1 mol/L KHCO₃ electrolyte. (c) Faradaic efficiencies of CO and H₂ for Cu-HHTT. (d) Turnover frequency for Cu-HHTT.

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  Figure 4  (a) In-situ ATR-FTIR spectra of electrochemical CO₂RR on Cu-HHTT. (b) Free energy diagrams for the conversion of CO₂ into CO on Cu-HHTT and the optimized structure of each intermediate.

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