Covalent Organic Frameworks as Promising Platforms for Efficient Electrochemical Reduction of Carbon Dioxide: A Review

In current research, achieving carbon neutrality has become a primary focus through the utilization of various conversion technologies that transform carbon dioxide (CO2) into valuable chemicals or fuels. Covalent organic frameworks (COFs), as emerging crystalline organic polymers, offer distinct advantages in CO2 conversion compared to other materials. These advantages include controllable nanoscale pores, predefined functional units, editable framework structures, and rich conjugated systems. The unique characteristics of COFs make them highly promising electrocatalysts for CO2 conversion. This review provides a comprehensive overview of pioneering works and recent research on the utilization of COF‐based materials as electrocatalysts for electrochemical CO2 reduction reaction. This review offers a comprehensive analysis of the design principles for various reactive sites, skeleton structures, pore functionalities, 3D frameworks, morphologies, and composite materials of COFs, aiming to enhance electrocatalysis. Finally, this review presents some recommendations for material design, reaction mechanisms, and theoretical computations to enhance the understanding of reaction mechanisms further and facilitate the design of high‐performance COF‐based electrocatalysts.

skeletons and ordered nanopores. [33,34]Thus, COFs have the advantages of structural designability, functional adaptability, tailorable topologies, modifiable pore sizes and porosities, interchangeable building blocks, diverse covalent connections, and large surface areas. [35,36]][58][59][60] Especially since the pioneering reports on the utility of COFs for ECO 2 RR [61] by Yaghi and Chang et al. in 2015, the application of COFs-based materials in ECO 2 RR has become the domain of research activities.As heterogeneous catalysts, COFs not only provide the advantages of reusability and easy separation but also exhibit acceptable chemical stability. [61,62]In addition, the introduction of diverse organic units and the design of framework structures allow COFs to possess specific pore-reactive environments, [35,62] which enable the prominent surface area to be transformed into a large number of active sites.Also, the selectable organic aromatic building blocks, as well as 2/3D nets, would allow us to manifest the semiconducting behaviors and electronic configurations of COFs. [63,64]All these factors promote COFs as promising platforms to perform highly efficient ECO 2 RRs.
In most of the previous reviews, COFs are typically categorized as one kind of organic framework material, [65,66] 2D materials, [67] or heterogeneous molecular catalysts [21] , and are discussed together with other materials.Similarly, the discussion of COFs for ECO 2 RR is usually embedded within broader categories such as catalysis, [57,58] energy conversion, [68][69][70] or other applications. [71]This broad categorization has led to a lack of specificity in reviewing this field, making it challenging to delve into the detailed exploration of defined COF materials for ECO 2 RR.Currently, the reviews involving COFs for ECO 2 RR often classify them based on the type of active metal [65] or the types of linkages [72] without delving into a comprehensive discussion of the structure-activity relationship.Consequently, we intend to revisit these pioneering and representative efforts in this domain, classify different structural characteristics of COFs, and discuss the correlation between these characteristics and ECO 2 RR performance.
In this review, we will summarize the structure-performance relationships related to ECO 2 RR from various factors such as active sites, skeletons, pores, networks, morphologies, and composite materials within COFs (Figure 1).Subsequently, we will outline the strategies for achieving specific products based on COF materials.Finally, we will discuss current challenges and potential directions for enhancing the efficiency of CO 2 electrocatalytic conversion with COFs as electrocatalysts, aiming to provide valuable insights and inspiration for this exciting field.

COFs for Electrocatalytic CO 2 Reduction
Electrocatalysis is a catalytic process that promotes chemical reactions through the electron transfer process on the electrode surface. [73,74]For an electrocatalytic reaction, three basic steps are involved: 1) adsorption of reactant species at the active sites; 2) formation of reaction intermediates; and 3) desorption of reaction products.Due to the nonpolar linear structure of CO 2 as well as its small bond-forming atomic radius with robust σ and π bonds, CO 2 has a very stable chemical nature, requiring some specific conditions for activation. [75]Furthermore, since ECO 2 RR generally occurs in an aqueous solution, the HER, which exhibits a comparable electrochemical window, inevitably reduces the FE. [76,77]Moreover, ECO 2 RR is a multiprotoncoupled multielectron transfer process, and due to the thermodynamic and kinetic complexities of the reaction, it is very challenging to control the selectivity of specific products. [78,79]n addition to the intrinsic requirements of the reaction, ECO 2 RR is also influenced by the level of available protons in the electrolyte, [80] the formation of the *CO intermediate, [81,82] and the coverage of the catalyst surface. [76,83]These factors impose demands on the concentration of reactants in the microenvironment of the reaction.

Metal Ions as the Active Site
[86] The inherent properties of a metal center, including its atomic size and electronegativity, basically determine its intrinsic selectivity and activity. [65]hus, selecting a metal active center with excellent potential ECO 2 RR activity is paramount.Based on the position and bonding of metal elements within COFs, metal centers can be introduced into COFs in three ways: 1) metal ions bonded to the coordinated sites of COFs, 2) metal ions coordinated to the linkers of the COFs, and 3) metal nanoparticles (NPs) embedded in COFs.
Metal Ions Bonded to the Coordinated Sites of COFs: Various metal porphyrins have been demonstrated to serve as superior catalysts for a diverse array of reactions. [87]COFs provide opportunities for the integration of these metal porphyrins, serving as centers of ECO 2 RR while providing both electronic and structural modifications.Therefore, it is imperative to emphasize the importance of the meticulous design of specific metal porphyrin cores as they dominate the quintessential determinants in ECO 2 RR.
In 2015, Yaghi et al., for the first time, reported their pioneering work on utilizing COFs to electrochemically reduce CO 2 into CO. [88]In this research, two frameworks (COF-366-Co and COF-367-Co) were prepared through the condensation of 5,10,15,20tetrakis(4-aminophenyl)porphinato]cobalt [Co(TAP)] with two different aldehydes (Figure 2a).Compared to Co porphyrin molecular catalyst, COFs exhibited superior electrochemical performance: at an overpotential of À0.55 V vs a revesible hydrogen electrode (RHE), COF-366-Co displayed an FE CO of 90% and turnover frequency (TOF) of 98 h À1 , representing a remarkable 26 times enhancement compared to the corresponding Co porphyrin molecular catalyst.Yaghi et al. also synthesized COF-367-Co(10%) and COF-367-Co(1%) with varying Co/Cu ratios (Co/ Cu = 1% and 10%).Performance tests indicated that for each electroactive Co site used for CO production, the TOF exhibited a marked trend of improvement with every tenfold dilution of Co loading (Figure 2b).However, this came at the expense of FE CO .X-Ray absorption spectroscopy (XAS) revealed that the Co K-edge XAS spectra of Co(TAP) and COF-367-Co showed similar curves with the Co(II) oxidation state observed in all the samples (Figure 2c).When the COFs were subjected to a reducing potential of À0.67 V under a CO 2 atmosphere, the XAS demonstrated that the changes of curves were consistent with partial reduction of Co(II) to Co(I).This was in contrast to the molecular Co(TAP) analog, which lacked these additional pre-edge features (Figure 2c, inset).This suggested that the COFs lattice provided a more delocalized electronic structure at the metal site, subsequently influencing the mechanistic pathway of CO 2 reduction.This study innovatively demonstrates the potential prospects of COFs in immobilizing molecular catalysts and enhancing their catalytic performance, further inspiring researchers' studies and applications of COFs in the ECO 2 RR domain.
In addition to Co, Ni and Fe atoms are also frequently employed as active species for various ECO 2 RR.Subsequently, in 2019, Kubiak et al. selected 5,10,15,20-tetra-(4-aminophenyl) porphyrin Fe(III) chloride and 2,5-dihydroxyterephthalaldehyde (Dha) as the organic linker units to construct COFs (FeDhaTph-COF, Figure 2d). [89]The as-obtained FeDhaTph-COF was coated onto a carbon cloth electrode via a solvent-free method.The asfabricated electrode demonstrated a TOF of 600 h À1 and an FE CO of 80% after 3 h at À2.2 V versus Ag/AgCl in acetonitrile (MeCN) containing 0.5 M trifluoroethanol (Figure 2e).Its performance in a 0.5 M KHCO 3 aqueous solution was unsatisfactory with FE H 2 higher than 80% and FE CO lower than 20%, which meant that competing HER dominated.Also, the electrochemical stability of the FeDhaTph-COF-coated electrode was relatively poor.Specifically, a decrease in Fe site activity of the FeDhaTph-COF electrode was observed after 3 h of electrochemical CO reduction reaction (ECORR) in MeCN and DMF, which might be ascribed to the degradation of the porphyrin moiety within the COF network.Zhuang et al. prepared a covalent triazine framework (CTF, namely, NiPor-CTF) with Ni porphyrin as the active unit for ECO 2 RR (Figure 2f ). [90]Compared to Ni/N-PC (obtained from the pyrolysis of NiPor-CTF under N 2 at 900 °C), NiPor-CTF with the atomically dispersed Ni-N 4 structure ensured superior CO 2 capture capability (at 298 K, 2.4 mmol g À1 for NiPor-CTF, compared to 1.6 for Ni/N-PC).This superiority was confirmed as NiPor-CTF could reach a maximum FE CO at À0.9 V versus RHE for 97% (Figure 2g) with a significantly enhanced current density of ≈52.9 mA cm À2 .
To further clarify the influence of transition metal elements on the metal porphyrin structure, Xie et al. studied the free energy changes of ECO 2 RR for MN 4 -Por-COF (M = 3 d metals from Sc to Zn) via density functional theory (DFT) (Figure 3). [91]alculations revealed that all MN 4 -Por-COF, except for M = Cu or Zn, prefer to perform ECO 2 RR over HER.Notably, Fe/Co/ NiN 4 -Por-COFs tend to produce CO, while the rest of MN 4 -Por-COF (M = Sc/Ti/V/Cr/Mn) favorably produced HCOOH The inset shows the pre-edge regime.Reproduced with permission. [88]Copyright 2015, AAAS.d) Schematic structural representation of FeDhaTph-COF.e) FE of CO and H 2 during controlled potential electrolysis of FeDhaTph-COF electrode in MeCN.Reproduced with permission. [89]Copyright 2019, American Chemical Society.f ) Schematic diagram of the synthesis of NiPor-CTF.g) CO FE for Por-CTF, NiPor-CTF, and Ni/N-PC at various specific potentials.Reproduced with permission. [90]Copyright 2019, Wiley-VCH.
(Figure 3g).The authors also designed an N-confused porphyrin metal structure (Figure 3a-c), and DFT analyses suggested that compared to its original structure, the N-confused porphyrin metal structure exhibited superior ECO 2 RR performance.This enhancement was attributed to the replacement of N with C atoms.This substitution increased the electron density of metal atoms (Figure 3d-f ) and elevated the d-band center, thereby stabilizing key intermediates and reducing the ΔG of each ratedetermining step (RDS).This study offers theoretical insights into the ECO 2 RR performance difference of metal-porphyrinbased COFs.It should be noted that there are always discrepancies between DFT results and actual experimental outcomes.This could be attributed to the difference between theoretical and actual structures as well as the neglect of the reaction environment, etc. Various measurements have been taken to consider and address these influences, including establishing solvent effect models and making corrections to DFT parameters. [92,93]At present, DFT has emerged as an effective tool in studying catalytic mechanisms.It is recommended that researchers attempt to use DFT judiciously and accurately to guide and interpret experiments, ultimately establishing creditable, systematic, and catalyst design strategies.
In addition to the porphyrin structure, phthalocyanine, another structure with a large π-conjugated system, can also coordinate with metal ions to become an active reaction center. [94,95]imilar to porphyrin, metal-phthalocyanine can also be integrated into the skeleton of COF to serve as an active site for ECO 2 RR.
In 2018, Jiang et al. developed a series of COFs with different pore sizes using Co phthalocyanine and boric acid as components through theoretical calculations (Figure 4a). [96]The results from molecular dynamics simulation showed that COFs with smaller pore sizes had greater CO 2 weight capacity under atmospheric pressure, and CO 2 tended to adsorb at the corner sites of the COFs.DFT calculations indicated that Co phthalocyanine exhibited a stronger binding energy stability with the ECO 2 RR reaction intermediates compared to Co porphyrin, consequently reducing the energy barrier for reduction.In 2020, Huang and Jiang et al. synthesized an all-π conjugated COF, named CoPc-PDQ-COF, by connecting the Co phthalocyanine through phenazine linkages (Figure 4c). [97]The phenazine linkage allowed for a topological connection of the Co phthalocyanine structures with a uniform distance of merely 2.2 nm, resulting in a stacked square lattice.This fully π-conjugated stacking structure enabled electrons to conduct along the skeleton in any axes, directing electrons to the catalytic sites across the shortest distance under driving force.Specifically, due to its extensive and continuous electronic conjugation, the prepared CoPc-PDQ-COF exhibited a bulk conductivity of up to 3.68 Â 10 À3 S m À1 .CoPc-PDQ-COF boasted an FE CO of 96% (Figure 4d), an astonishing TOF of 11 412 h À1 , and remarkable long-term stability (Figure 4e), demonstrating superior electrocatalytic performance.Figure 4f shows the diagram of the proposed reaction path calculated by DFT theory.Similarly, Cao et al. developed a conductive pyrazine-linked NiPc-COF through condensation reactions, utilizing tert-butylpyrene-tetraone as the linker (Figure 4b). [98]The steric hindrance from the tert-butyl groups promoted interlayer delamination of the layered 2D NiPc-COF.The COFs nanosheets had an average thickness of less than 0.74 nm, equivalent to a two-to three-layer structure, meaning more accessible active sites.NiPc-COF exhibited similar properties, including a higher conductivity (3.77Â 10 À6 S m À1 ) and remarkable electrocatalytic performance (with FE CO > 93% in a potential range from À0.6 to À1.1 V versus RHE).
an FE CO over 90% in a wide potential range from À0.6 to À1.2 V and reached nearly 100% between À0.8 and À0.9 V under illumination (Figure 5e).Building upon this, Lan et al. utilized 2,3,9,10,16,17,23,24-octaamino-phthalocyaninato nickel (II) and 2,6-diformylphenol to construct MPc-DFP-M COF (M = Co, Ni). [100]The MPc-DFP-M COF featured metal phthalocyanine sites and salphen pockets with metal-ion coordination (Figure 5f ).The NiPc-DFP COF, with metal-free salphen pockets, exhibited excellent catalytic performance, reaching an FE CO of 99.19% at À1.1 V versus RHE.In comparison, the NiPc-DFP-Co COF achieved an FE CO of 99.86% at À0.9 V and maintained an FE CO of above 99% over a potential range from À0.9 to À1.1 V, whereas the NiPc-DFP-Ni COF reached an FE CO of 97.81% at À1.1 V.The research team further assessed the PECR performance of these samples under illumination conditions.Results showed that NiPc-DFP-Co COF significantly enhanced current densities and TOF under illumination (Figure 5g,j) and achieved ≈100% FE CO from À0.7 V to À1.1 V (Figure 5h).The change of Tafel slope under radiation indicated that light illumination could enhance the electron transfer properties of the COFs (Figure 5i).In fact, phthalocyanine-based and porphyrin-based COFs have been extensively used in various photocatalytic studies. [101,102]This research highlights the potential applications of phthalocyanine-based COFs in photoelectrochemical catalysis.
Similarly, Huang et al. introduced hexadecafluorophthalocyanine (HDFP) as a novel C 4 symmetric structural unit [103] to synthesize a series of COFs with dual-metal Cu-Co phthalocyanines (Figure 5k).The CuPcF 8 -CoNPc-COF, with a larger pore, exhibited superior electrocatalytic performance with a peak FE of 97% and TOF of 2.87 s À1 at À0.62 V versus RHE (Figure 5L), along with notable stability over a duration of 50 h.(Figure 5m).DFT and experiment results suggested that ECO 2 RR was more likely to occur at the CoNPc site, while HER was more inclined to take place at the CuPcF 8 site.It is worth noting that, inspired by Lan's work, the authors tested the performance of all samples under illumination conditions.However, the results revealed no FE variations compared to dark conditions, indicating the weak photoelectrical performance of these COF materials.Thus, further clarity is required on the conditions of how COFs can be efficiently utilized in the PECR domain.6a). [104]As shown in Figure 6b, the NiPc-CoPor-imi-COF exhibited a higher FE CO than the singlemetal NiPc-imi-COF and CoPor-imi-covalent organic polymer  [96] Copyright 2018, Wiley-VCH.b) Schematic diagram of the synthesis of NiPc-COF.Reproduced with permission. [98]Copyright 2020, Wiley-VCH.c) Synthesis diagram of the synthesis of CoPc-PDQ-COF and top and side views of crystal structure (N: purple, Co: red, C: green, H: white).d) FE CO of CoPc-PDQ-COF (red), monomeric [NH 2 ] 8 CoPc (blue), commercial CoPc (green), and carbon fiber (black) at various potentials.e) Longterm stability test of CoPc-PDQ-COF in current density (blue) and FE (red) at À0.66 V versus RHE.f ) Proposed electrocatalytic reaction pathway for the reduction of CO 2 to CO by Co (II) phthalocyanine.Reproduced with permission. [97]Copyright 2020, Wiley-VCH.
(COP) and demonstrated an exceptional ECO 2 RR activity, with a maximum FE CO of 97.1% at À1.4 V versus Ag/AgCl.DFT calculations revealed that the NiPc-CoPor-imi-COF not only enhanced the CO 2 RR and suppressed the HER of the Ni sites but also improved the selectivity of the Co sites (Figure 6c).[107] Zang's work showcases the unique advantages of COFs in this regard, specifically in adjusting the distance and chemical environment of the dual-metal sites through the framework and coordinating ditions.e) FE CO of NiPc-TFPN COF in the dark and radiation conditions.Reproduced with permission. [99]Copyright 2021, Wiley-VCH.f ) Schematic diagram of the synthesis of NiPc-DFP COF.g) LSV curves, h) FE CO , i) Tafel plot, and j) partial current density for CO and TOF of NiPc-DFP-Co COF in the dark and radiation conditions.Reproduced with permission. [100]Copyright 2023, Chinese Chemical Society.k) Schematic diagram for the synthesis of CuPcF 8 -CoNPc-COF.l) FE CO for CuPcF 8 -CoPc-COF (blue) and CuPcF 8 -CoNPc-COF (red).m) FE CO (solid dot) and FE H 2 (hollow dot) tested for 50 h of CuPcF 8 -CoPc-COF (black) at À0.70 V and CuPcF 8 -CoNPc-COF (red) at À0.62 V. Reproduced with permission. [103]Copyright 2021, American Chemical Society.
monomers.However, current research suggests that not all active sites participate in the reaction, and a clear correlation between the density of metal atoms and catalytic performance has not been established.To clarify this relationship, Zeng et al. synthesized a metal-free H 2 Pc-COF using urea and pyromellitic dianhydride, and different Ni atom densities of NiPc-COF were prepared through the impregnation method (Figure 6d). [108]lectrochemical tests showed that when the ratio of NiPc unit to H 2 PC unit was 1, 0.5NiPc-COF had the best catalytic performance, exhibiting the maximum FE CO of 92.52% at À0.8 V and the highest partial current density for CO of 25.08 mA cm À2 (Figure 6e).This performance was better than other samples with different ratios.The TOF values of H 2 Pc-COF, 0.25NiPc-COF, 0.5NiPc-COF, 0.75NiPc-COF, and NiPc-COF calculated at À1.0 V were 1258.64,2790.41,4713.53,3749.88, and 3860.12h À1 , respectively, presenting a volcano-shaped curve (Figure 6f ).DFT  [104] Copyright 2023, Royal Society of Chemistry.d) Schematic diagram of the synthesis of XNiPc-COFs (H 2 Pc-COF, 0.25NiPc-COF, 0.5NiPc-COF, 0.75NiPc-COF, and NiPc-COF), respectively.e) FE CO and f ) TOF values for H 2 Pc-COF (black), 0.25NiPc-COF (pink), 0.5NiPc-COF (red), 0.75NiPc-COF (blue), and NiPc-COF (green) from À0.5 to À1.0 V in 0.5 M KHCO 3 under CO 2 atmosphere.Reproduced with permission. [108]calculations indicated that the H 2 Pc unit donated electrons to the NiPc unit in the framework, enhancing the electron density of the Ni sites.These Ni sites exhibited greater nucleophilicity and formed stronger interactions with the Lewis acidic CO 2 molecule, thereby promoting ECO 2 RR.This study provides a new perspective for modulating the density of catalytic sites in COFs.
Metal Ions Coordinated to the Linkers of the COFs: The abundant and adjustable adsorption sites, along with the pervasive nanoporous structure in COFs, offer a promising platform for loading various molecular catalysts.The framework not only provides a reliable carrier but also exerts various electronic controls and mechanical constraints on the adsorbed molecular catalysts.This significantly improves the reaction pathways, stability, selectivity, and overall performance of the molecular catalysts.
In another research, Lloret-Fillol et al. modified COFs with Mn(CO) 5 Br, resulting in a COF bpyMn with a high Mn load, where about 85% of the bipyridine coordinated with Mn (Figure 7c). [112]-ray near edge absorption fine structure (XANES) spectra indicated that the Mn center in the COF had an equivalent coordination environment as the homologous molecular complex (Figure 7d).Notably, both experimental results and theoretical calculations showed that the N atoms in the bipyridine were stacked in alternating directions.The hybrid COF bpyMn|NT with multiwalled carbon nanotubes (CNTs) has an excellent reaction overpotential (190 mV).When the current density surpassed 12 mA cm À2 , the integrated selectivity of the CO 2 reduction  [110] Copyright 2018, Royal Society of Chemistry.b) Schematic diagram of the synthesis of COF-Re, COF-Re_Co, and COF-Re_Fe.Reproduced with permission.[111] Copyright 2018, American Chemical Society.c) Schematic diagram of the synthesis of COF bpy and COF bpyMn .d) XANES spectra of Mn(bpy)(CO) 3 Br and COF bpyMn .e) Faradaic yield of COF bpyMn |NT under CO 2 at different overpotentials.f,i) CVs, g,j) ATR-IR absorbance potential-dependent spectra, and h,k) Nernst fittings for ATR-IR-SEC of COF bpyMn |NT and Mn(bpy)(CO) 3 Br|NT in solution (0.5 M NaHCO 3 ). For CF bpyMn |NT (g), the υ(CO) centered at 1937 cm À1 increases (in red), and bands at 2053 cm À1 disappear (in blue), while for the immobilized Mn(bpy)(CO) 3 Br|NT (j), vibrations at 1860 and 1836 cm À1 (in red) indicate the formation of Mn 0 -Mn 0 dimer.Reproduced with permission.[112] Copyright 2021, American Chemical Society.
product relative to H 2 was as high as 72%, with a TOF up to 1100 h À1 (Figure 7e).This performance was much better than that of Mn-based molecular catalysts, exceeding it by ten times.Importantly, in cyclic voltammetry (CV) scans and attenuated total refraction-infrared-spectroelectrochemical (ATR-IR-SEC) of COF bpyMn|NT (Figure 7f-k), there was no characteristic signal for the Mn 0 -Mn 0 dimer.Additionally, DFT calculations indicated that the Mn 0 -Mn 0 dimer in the COF structure was thermodynamically unstable.These results suggested that the alternating stack arrangement within the COF significantly influenced the redox behavior of the Mn molecular units.The Mn(CO) 5 Br dimer under alternating ordering needed to overcome an extremely high reaction barrier.Clearly, the characteristic of alternating ordering avoided the formation of the detrimental Mn 0 -Mn 0 dimer.This particular constraint effect reduced the overpotential and thereby improved the overall ECO 2 RR activity.This work innovatively demonstrates the potential of COFs to modify the performance of the original active species by understanding the structureactivity mechanism of coordinated metal species.
COFs could also effectively adsorb metal ions as active sites.In 2022, Qiao and colleagues synthesized a high-nitrogencontaining COF (N-COF) through the direct condensation of hexaaminotriphenylene hexahydrochloride (HATP) and hexaketocyclohexane octahydrate (HKH) (Figure 8a). [113]The abundant pyridine N sites in the skeleton provided the coordination environment for Co 2þ , where cobalt acetate hexahydrate was chosen as the Co source.After pyrolysis of the as-obtained COF at 300 °C, Co─N─COF was obtained.In electrochemical tests, Co─N─COF exhibited a notable ECO 2 RR performance, reaching an FE up to 97.4% for HCOOH at À0.75 V versus RHE and a remarkable current density up to 480 mA cm À2 at À1.05 V while still maintaining high selectivity for HCOOH.Additionally, Co─N─COF demonstrated excellent stability, maintaining an FE over 90% and a partial current density for HCOOH of ≈446 mA cm À2 after 100 h of testing.
In addition to adsorbing active species in the channels through specific adjacent N sites on the skeleton edge, active centers can also be placed on the plane of the 2D COF.Jiang and colleagues used 2,5-dimethylterephthalaldehyde (DMTP) and 3,5-dicyano-2,4,6-trimethylpyridine (DCTMP) as starting agents to synthesize the alkene-linked DCTMP-DMTP-COF (Figure 8b). [114]Among the structures, cobalt porphyrin can be anchored on the CN group of COF to form a new Co─N 5 active center (Figure 8c).Extended X-ray aborption fine struture (EXAFS) spectra and wavelet transform contour plot spectra showed that the N atoms of the CN groups in DCTMP-DMTP-COF were coordinated with CoPor (Figure 8d-f ), indicating that the COF@CoPor structure was successfully constructed.COF@CoPor demonstrated  [113] Copyright 2022, Elsevier.b) Schematic diagram of the synthesis of DCTMP-DMTP-COF.c) Structural diagram of COF@CoPor.d) Fourier-transformed radial distribution functions of k 3weighted χ(k)-function of Co EXAFS spectra for the Co foil (black curve), CoPor (blue curve), and COF@CoPor (red curve).e) The first shell fitting of Fourier-transformed radial distribution functions of k 3 -weighted χ(k)-function of Co EXAFS spectrum of COF@CoPor.f ) Wavelet transform contour plot of Co foil, CoPor, and COF@CoPor.Reproduced with permission. [114]Copyright 2022, Wiley-VCH.
excellent ECO 2 RR performance.Specifically, the FE CO at À0.6 V versus RHE was 94.3%, while the single CoPor species was only 75.9%.At À1.0 V, the maximum TOF of COF@Por reached 4578 h À1 , which was over 20 times greater than that of CoPor at 235 h À1 , highlighting the improvement effect of COF as a carrier.However, the adsorption of large molecules on the COF plane led to the potential problem of fewer active sites.Through inductively coupled plasma optical emission spectroscopy measurements, the Co content in COF@CoPor was 1.28 wt%.The interlayer interactions in 2D COFs cause layered stacking, inevitably resulting in kinetic and thermodynamic resistance for large molecules entering between the layers.The same problem will also limit the diffusion of subsequent reactants.This seems to be a poor choice.However, these limitations may change the reaction characteristics of some large molecules, leading to some unforeseen results.
Metal NPs Embedded in COFs: The inherently interconnected porosity of COFs offers ample opportunities for accommodating diverse metal clusters. [115]Moreover, the abundant adsorption sites within the framework are helpful for various metal ions to be adsorbed and reduced to metal clusters.Meanwhile, the nanoscale dimensions of the COF channels constrain the further growth of these metal clusters, thereby serving as a valuable tool for modulating their size and functional attributes.
In 2020, Cao et al. prepared an imidazolium-functionalized cationic CTF (CCTF) using 1,3-bis(4-cyanophenyl)imidazolium chloride as the monomer and deposited Cu NPs in CCTF through chemical reduction method, denoted as Cu/ICTF X . [116]ransmission electron microscope (TEM) showed that Cu/ICTF has highly dispersed Cu NPs with an average size of 5.2 nm.Specifically, Cu/ICTF 50 displayed superior catalytic performance in ECO 2 RR, achieving an FE up to 35% for C 2 H 4 at À1.30 V versus RHE.After reacting for 7 h, TEM characterization of Cu/ICTF 50 showed no aggregation of Cu NPs, and the average particle size was similar to that of fresh catalyst.This indicated that the N-heterocyclic carbene sites can stabilize Cu NPs, preventing their aggregation and deactivation.
In 2022, Shao et al. employed 2,6-diaminoanthraquinone and 2,4,6-triformylphloroglucinol as starting agents to prepare COFs and controlled the loading of Cu from single atoms (SAs) to nanoclusters (NCs) on COFs by adjusting the electrodeposition time. [117]Specifically, SA Cu-COF only had SA Cu loaded, while NC-SA Cu/COF had both SA Cu and NC Cu.Electrochemical tests showed that NC-SA Cu/COF and SA Cu-COF had similar CO 2 reduction current densities.However, NC-SA Cu/COF achieved a lower FE H 2 and a higher FE CH 4 at a potential range between À1.08 and À1.34 V versus RHE.In contrast, SA Cu-COF without Cu NCs showed both FE CO and FE H 2 of about 45%, indicating that Cu NCs might alter the ECO 2 RR pathway.The significant enhancement of CH 4 production was due to CO produced on Cu SA, quickly transferring to nearby Cu NCs and subsequently being reduced to CH 4 .

Nonmetal Ions as the Active Site
In addition to transition metals as active sites, nonmetallic elements, including carbon, [118] nitrogen, [119,120] sulfur, [121] phosphorus, [122] and others, [123][124][125][126] have also been reported to exhibit ECO 2 RR activity.Unlike metals, intercharge effects among nonmetal elements can effectively facilitate electron communication, and different structures can achieve significant changes in catalytic performance.Within the framework, various nonmetallic sites can be introduced, serving as active centers and achieving tandem reduction of multiple active sites.These offer many chances for selective reduction of specific products.
In 2018, Wen et al. used tetrafluoroterephthalonitrile as the organic building unit to construct metal-free all-fluorinated FN-CTF-400 at 400 °C under the ionic thermal assistance of molten ZnCl 2 , with N acting as the active center. [127]Electrochemical tests showed that FN-CTF-400 with the highest F content could achieve an impressive FE CH 4 of 99.3% within a potential range from À0.7 to À0.9 V versus RHE.Meanwhile, FN-CTF-700 and FN-CTF-900, which contained lesser amounts of F, primarily produced H 2 and CO rather than CH 4 .These results strongly indicated that F played a pivotal role in the selective CO 2 reduction to CH 4 over N active sites.
In 2022, Lan and colleagues designed a Cu-Tph-COF-Dct material with a triazine side chain, where 2,4-diamino-6chloro-1,3,5-triazine (Dct) acted both as a functionalized stripping agent and as a CO 2 reduction promoter (Figure 9a). [128]FT results showed that the triazine groups in the framework served as the key active sites, preferentially adsorbing CO 2 and selectively reducing it to CO (Figure 9b,c).This enhanced the CO concentration around the porphyrin Cu active center.A high concentration of CO resulted in the selective reduction of CO to CH 4 by Cu porphyrin.This work rationally combines the nonmetallic active sites in the pores with the transition metal active sites in the framework to achieve high selectivity for the overall reduction of CO 2 to CH 4 .
N sites not only serve as active centers for CO 2 but also function as capture sites, activating CO 2 and promoting reduction reaction.In 2018, Deng and colleagues reduced C═N─C bonds in 3D COF-300 to C─NH─C bonds, resulting in COF-300-AR with amine-linked bonds (Figure 9d). [129]The authors further combined Ag with COF-300-AR, creating an electrode material with a COF─Ag interface (Figure 9f ).Electrochemical tests showed that compared to the Ag electrode, the COF-Ag electrode increased the FE CO from 13% to 53% at À0.70 V and from 43% to 80% at À0.85 V versus RHE, respectively.The results showed that only the interface between COF-300-AR and Ag can improve the ECO 2 RR performance (Figure 9h), while part of the CO 2 was adsorbed on COF-300-AR in the form of carbamate (Figure 9e).The authors speculated that CO 2 first reacted with the amine bond in the framework to form carbamate, which was then easily reduced to CO under the catalysis of Ag, thus enhancing ECO 2 RR (Figure 9g).This fully showcases the abundant functionality of COF and is expected to inspire researchers to use functional COFs to achieve specific product conversion of CO 2 .

Design of Skeleton
Many properties of COFs are influenced by their structures and composition, including crystallinity, intrinsic conductivity, and flexibility.Therefore, the design of the framework is crucial for creating COF materials with high ECO 2 RR performance.
Taking inspiration from the improved catalytic performance induced by appropriate defect structures in other materials, He and colleagues synthesized a Co-phthalocyanine-linked COF with abundant framework defects, termed D-P-CoPc (Figure 10a). [130]Different from the defect-free P-CoPc synthesized from pyromellitic dianhydride, D-P-CoPc was prepared by phthalic anhydride, where the missing anhydride on one side led to the termination of the framework, resulting in framework Reproduced with permission. [128]Copyright 2022, Wiley-VCH.d) Synthesis scheme for reduction of COF-300 to obtain COF-300-AR.C: gray, N: blue.Only the H atoms on the imine and amine linkage are shown in pink.e) 13 C CP-MAS NMR of COF-300-AR during ECO 2 RR process.f ) Schematic diagram of the active interface between COF-300-AR and Ag foil.g) Schematic diagram of the mechanism of ECO 2 RR through carbamate formation at the interface.h) The FE for CO on the electrode of COF-300-AR and Ag at various potentials, using bare Ag foil and Nafion solution as control benchmarks.Reproduced with permission. [129]Copyright 2018, Elsevier.
defects.In electrochemical tests, D-P-CoPc and P-CoPc demonstrated similar performance.At À0.43 V versus RHE, D-P-CoPc and P-CoPc exhibited FE CO of 55% and 40%, respectively, indicating a slight improvement in D-P-CoPc performance.D-P-CoPc showed the highest FE CO of 97% at À0.61 V versus RHE.However, in powder X-ray diffraction (PXRD), both D-P-CoPc and P-CoPc displayed amorphous characteristics, suggesting some potential issues in these samples.
Designing efficient electrocatalysts with high conductivity is essential for electrochemical reactions. [132]Although some COFs exhibit high selectivity for ECO 2 RR, their inferior conductivity and inefficient charge transfer typically result in low current densities, limiting the potential for further activity enhancement.For 2D π-conjugation, they exhibit a stacked planar π-conjugated configuration.However, the polarized covalent bonds within the framework lead to poor π-electron delocalization, resulting in a wide bandgap and unsatisfactory conductivity. [64]To address this problem, researchers have designed and synthesized numerous 2D COFs with high charge-transport capabilities. [133,134]As mentioned earlier, Jiang and colleagues used Co phthalocyanine as a structural unit, integrating phenazine linkage into the COF and endowing the COF with an exceptional conductivity of 3.68 Â 10 À3 S m À1 . [97]Moreover, Cao et al. employed 2,3,6,7tetra(4-formylphenyl)-tetrathiafulvalene (TTF-Ph-CHO) as the foundational monomer, successfully incorporating the tetrathiafulvalene structure into a 2D Co porphyrin-based COF (Figure 10c). [135]In terms of conductivity, TTF-Por(Co)-COF showcased an excellent conductivity of 1.32 Â 10 À7 S m À1 , compared to COF-366-Co of 6.5 Â 10 À9 S m À1 .TTF-Por(Co)-COF demonstrated optimal CO 2 conversion activity at À0.7 V versus RHE, with FE CO reaching up to 95%, notably higher than COF-366-Co under the same condition (70.8%).Subsequently, Cao et al. selected thieno[3,2-b]thiophene-2,5-dicarbaldehyde as building unit to synthesize TT-Por(Co)-COF for the characterization of its charge transport properties (Figure 10d). [136]TT-Por(Co)-COF displayed an enhanced conductivity of 1.38 Â 10 À8 S m À1 .At À0.6 V versus RHE, TT-Por(Co)-COF exhibited a high FE CO of 91.4%, surpassing COF-366-Co at the same potential (67.3%).Similarly, Jiang et at.chose tetraphenyl-p-phenylenediamine (TPPDA) as the electron donor in the skeleton to build 2D porphyrin-based COFs (TPPDA-MPor-COF, M = Co, Ni) (Figure 10e). [137]TPPDA-CoPor-COF exhibited a high FE CO of 87-90% in the range from À0.6 to À0.9 V versus RHE and reached the current density of 22.2 mA cm À2 at À1.0 V, while TPPDA-NiPor-COF exhibited a comparatively lower FE CO , ranging from 60% to 76% In addition to the linker, the linkage chemistry used to assemble COFs also plays a crucial role in their performance.Zeng et al. constructed CoPc-DNDS-COF with thiomorpholine linkage and CoPc-DSDS-COF with dithiine linkage using Co-hexadecafluorophthalocyanine as the node and diaminobenzene-1,4-dithiol or 1,2,4,5-benzenetetrathiol as the linker (Figure 11a). [138]oPc-DNDS-COF reached the highest FE CO value of 96.3% at À0.8 V versus RHE, while CoPc-DSDS-COF reached the highest FE CO value of 96.5% at À0.9 V (Figure 11b).Despite having similar FE CO , CoPc-DSDS-COF reached the highest CO partial current density of 30.42 mA cm À2 at À1.0 V vs RHE, while CoPc-DNDS-COF was only 19.6 mA cm À2 (Figure 11c).The superior performance may be attributed to the better conductivity of the dithiine bond than the thiomorpholine bond (CoPc-DSDS-COF: 7.8 Â 10 À3 S m, CoPc-DNDS-COF: 6.2 Â 10 À3 S m).Moreover, Zeng et al. synthesized TFPc-BFT/PBBA-COFs for ECO 2 RR with controllable boronic ester linkage content by controlling the raw material ratio (Figure 11d).[139] Fluorine atoms were introduced into the framework to enhance the hydrophobicity, thereby improving the water resistance and chemical stability of the COFs.As the content of boronic ester linkage increased, the conductivity of the COFs gradually increased.The conductivity of TFPc-BFT-COF was 3.3 Â 10 À5 S m À1 , while that of TFPc-BFT/PBBA-COF and TFPc-PBBA-COF increased to 3.7 Â 10 À5 and 4.6 Â 10 À5 S m À1 , respectively.Electrochemical tests showed that the FE CO of COF samples gradually increased with the rise of boronic ester bond content (Figure 11e).Compared with TFPc-BFT/PBBA-COF and TFPc-PBBA-COF, the TOF value of FPc-PBBA-COF increased by 26.1% and 59.8%, respectively, to 1695.3 h À1 (Figure 11f ).DFT calculations indicated that the B atoms in the COFs enhanced the binding ability of Co atoms to the intermediate *COOH, effectively reducing the free energy change barrier and thus improving the catalytic activity.
However, in 2D COFs, effective interlayer interactions such as π-π conjugation and hydrogen bonding will inevitably lead to highly stacked interlayer structures, making it difficult for reactive active sites to be exposed in the reaction environment.Thus, an ultrathin structure seems to be a good solution to address this issue.Ye et al. used squaric acid (SA) to synthesize a COP-SA with an ultrathin structure (Figure 12a). [140]In TEM and atomic force microscopy (AFM) tests, COP-SA showed a nanosheet structure with an average thickness of just 1.7 nm.In COP-SA, the less reversible covalent interactions induced its amorphous character, and such amorphousness weakened the interlayer affinities due to lattice inconsistencies.Moreover, the zwitterionic ion structure in the SA unit enhanced the interaction of this COF with polar solvents, promoting solvent exfoliation.In electrochemical tests, COP-SA reached a peak FE CO of 96.5% and a partial current density of 8.16 mA cm À2 at À0.65 V versus RHE without degradation in performance after 12 h stability tests.
In addition to employing a "top-down" design approach, postsynthetic modification provides an alternative path for customizing the frameworks. [141]Based on their previous research, Zeng and colleagues reduced the C═N bond in the highly crystalline CoTAPP-PATA-COF (abbreviated as COF) to the C─NH─C bond.Subsequently, they ionized the tertiary amine group to obtain N þ -NH-COF (Figure 12b). [142]N þ -NH-COF exhibited superior catalytic performance compared to COF, with a maximum TOF value of 9923 h À1 at À1.0 V and the highest FE CO of 97.32% at À0.8 V vs RHE, compared to 4166 h À1 and 81.75% of the original COF.
Here, it is crucial to emphasize the impact and challenge of pH values on the stability and catalytic performance of frameworks.The stability of the framework is a prerequisite for the long-term stability of COFs under electrochemical conditions.The linkers and linkages of COFs must maintain structural integrity in both acidic and alkaline conditions; otherwise, the framework will inevitably break.COFs connected through irreversible covalent bonds often have excellent stability.Recently, COFs containing linkages considered irreversible have also been successfully synthesized, including olefins, [143] sp 2 -carbon, [144,145] truxenes, [146] triazines, [147,148] phenazines, [97,149] oxazoles, [150,151] dioxins, [99,152] and C─C linkages. [153]Although the COFs containing irreversible covalent linkages are beyond the scope of this discussion, this review provides a brief introduction and aims to inform readers about the frontier direction of this research.
In the electrocatalytic industry, it is common to employ acidic or alkaline electrolytes to achieve higher current density.However, current research efforts mainly focus on performance under neutral conditions.Therefore, it is imperative to explore and develop COF catalysts that exhibit efficient ECO 2 RR under both acidic and alkaline conditions.Yaghi's work demonstrated that COF catalysts exhibited no rate dependence on pH values, suggesting that the rate-limiting step involved the participation of a CO 2 molecule and no proton. [88]In addition, Lan's work reported that the overpotential values and FEs of COFs were significantly affected in acidic and alkaline environments. [154]As the pH value increased, the overpotential at 1 mA cm À2 of COFs decreased from ≈410 mV (pH = 4.8) to ≈340 mV (pH = 5.8) and finally increased slightly to ≈350 mV (pH = 6.8).At À0.7 V and CoPc-DNDS-COF (blue) at various potentials.Reproduced with permission. [138]Copyright 2023, American Chemical Society.d) Chemical structure of monomers, TFPc-BFT-COF, TFPc-BFT/PBBA-COF, and TFPc-PBBA-COF.e) the FE CO values and f ) TOF values of TFPc-BFT-COF (black), TFPc-BFT/ PBBA-COF (blue), and TFPc-PBBA-COF (red) from À0.5 to À1.0 V. Reproduced with permission. [139]Copyright 2021, Elsevier.b) Schematic representation of reduction reactions, Menshutkin reactions, and multiple postsynthetic modifications.c) Schematic diagram of structure and synthesis of N þ -COF, NH-COF, and N þ -NH-COF from CoTAPP-PATA-COF.Reproduced with permission. [142]Copyright 2023, Springer nature.
versus RHE, the FE CO of COFs dropped from 91.3% (pH = 6.8) to ≈40% (pH = 5.8) and finally to ≈20% (pH = 4.8).The pH value significantly affects the concentration of proton source and CO 2 , which will have an impact on ECO 2 RR.Cao et al. synthesized Por(Co)-Vg-COF by connecting the dicationic 4,4 0bipyridinium core (Vg 2þ ) with cobalt porphyrin via C─N bonds (Figure 13a). [155]CV tests revealed that the valence state of the viologen moieties linker changed under negative potential due to the two-electron redox reaction.This change in valence state led to an increase in electronic conductivity.As shown in Figure 13b, the semicircle value of Nyquist plots for Por(Co)-Vg-COF decreased with the increase of negative potential, which may be attributed to the enhanced π-conjugation through the C═C bond of the reduced Vg structure.Within the potential range from À0.6 to À1.1 V versus RHE and under neutral conditions, Por(Co)-Vg-COF exhibited exceptional selectivity with an FE CO over 98% (Figure 13c).Furthermore, Por(Co)-Vg-COF demonstrated excellent catalytic performance under both acidic and alkaline conditions.Under acidic conditions (0.06 M H 2 SO 4 and 0.5 M K 2 SO 4 ) and alkaline conditions (1 M KOH), Por(Co)-Vg-COF exhibited a high FE CO and a significantly enhanced CO partial current density (Figure 13d-g).Liao et al. synthesized an imidazole-linked COF via 2,3,9,10,16,17,23,24-octaaminophthalocyaninatonickel (II) and terephthalaldehyde, denoted as PcNi-im (Figure 13h). [156]For comparison, the structurally similar pyrazine-linked PcNi-pz and dioxin-linked PcNi-tfpn were also synthesized (Figure 13h).Under acidic conditions (0.01 M H 2 SO 4 and 3 M KCl), PcNi-im exhibited a similar FE CO but a highest CO partial current density across the potential range than PcNi-pz and PcNi-tfpn with a maximum of 320 mA cm À2 at À1.4 V versus RHE, indicating the superior ECO 2 RR (Figure 13i,j).However, compared to neutral conditions, the performance of PcNi-pz and PcNi-tfpn remained virtually unchanged, while the CO partial current density of PcNi-im decreased from 320 to 258 mA cm À2 (Figure 13k,l).The performance comparison suggested that the ECO 2 RR performance of the PcNi active sites should be nearly identical under acidic and neutral conditions, while the framework of PcNi-im promoted ECO 2 RR under acidic conditions.However, compared to PcNi-pz and PcNi-tfpn, PcNi-im did not have advantages in terms of electrical conductivity, interfacial charge-transfer resistance, and the number of active sites, indicating that the framework enhanced catalytic performance by promoting the ECO 2 RR rather than other factors.DFT calculations suggested that upon protonation of PcNi-im under acidic conditions, the energy barrier for the formation of the key intermediate *COOH (* þ CO 2 !*COOH) significantly decreases from 1.82 to 1.53 eV, while the energy barrier for the *CO desorption step (*CO !* þ CO) significantly increases from 0.82 to 0.93 eV (Figure 13m,n).These lower-energy barriers promoted ECO 2 RR, allowing PcNi-im to exhibit optimal performance.The works of Cao and Liao demonstrate the transformation of COF structures under acidic, alkaline, and reduction potential conditions, which greatly enhances catalytic performance.Their results provide new insights for the development of COF catalysts with superior performance under acidic and alkaline conditions.

Design of Pores
Pore-functionalized COFs have been widely used in various fields, including gas storage and separation, [157][158][159][160] water treatment and purification, [161][162][163] biological applications, [164,165] etc.In ECO 2 RR, CO 2 , electrolyte, and solvent molecules transport through the pores to reach the active center for electrochemical reactions.Additionally, the products also need to be transported out through the pores.Therefore, the pore size, pore distribution, affinity of pores for various compounds, and the synergistic effect of various functional groups within the pores play a crucial role in ECO 2 RR.
Generally, the pore size is directly correlated with the size of the framework unit.When discussing the influence of pore size on catalytic performance, Yaghi et al. designed COF-367-Co similar to COF-366-Co using biphenyl-4,4 0 -dicarboxaldehyde (BPDA) as a linker instead of 1,4-benzenedicarboxaldehyde. [88]OF-366-Co had a pore width of 21 Å and an interlayer spacing of 4.4 Å, while those of COF-367-Co were 24 and 4.8 Å, respectively.Similarly, both showed comparable Brunauer-Emmett-Teller (BET) surface areas (COF-366-Co: 1360 m 2 g À1 ; COF-367-Co: 1470 m 2 g À1 ).CV tests indicated that COF-367-Co exhibited enhanced concentrations of surface-active Co porphyrins (2 Â 10 À9 mol cm À2 ), corresponding to the exposure of 8% of the Co sites in the bulk material.In comparison, COF-366-Co exhibited a lower concentration, recorded at 1 Â 10 À8 mol cm À2 , corresponding to the exposure of just 4% of the Co sites in the material.Similarly, at À0.67 V versus RHE, COF-367-Co exhibited 2.2 times the current density and a higher FE CO (91%) compared to COF-366-Co (90%).The authors believed that the enhanced ECO 2 RR activity was attributed to the lattice expansion, enabling more effective diffusion of reactants to electroactive sites.
The porous structure in the framework affects the diffusion of reactants, while the framework itself is involved in charge transfer. [166,167]Therefore, it's crucial to consider the combined influence of both these factors on catalytic activity.Jiang and colleagues introduced a 2D Co phthalocyanine-based COF connected through polyimide linkage. [168]In their work, CoPc-PI-COF-1 and CoPc-PI-COF-2 were synthesized by reacting Co phthalocyanine with 1,4-phenylenediamine (PD) or 4,4 0biphenyldiamine (BD), respectively (Figure 14a).Electrochemical tests suggested that within the potential range from À0.60 to À0.90 V versus RHE, CoPc-PI-COF-1 displayed a notably enhanced current density, reflecting superior ECO 2 RR activity compared to CoPc-PI-COF-2 (Figure 14b-e).CoPc-PI-COF-1 and CoPc-PI-COF-2 had similar structures and active centers.The unique structural difference was that the linker length of CoPc-PI-COF-2 was longer than that of CoPc-PI-COF-1, which reduced the electrical conductivity and thus damaged the electrocatalytic performance.Based on the conclusions of the two above studies, designing a framework with high conductivity and appropriately expanding the porous structure appears to be a more favorable design strategy.
Functionalization of the COF pore structure can significantly affect the metal active centers and the reaction microenvironment.Yaghi et al. introduced various electron-withdrawing groups on the 1,4-benzenedicarboxaldehyde linker unit in  [155] Copyright 2023, Royal Society of Chemistry.h) Structures of PcNi-im, PcNi-pz, and PcNi-tfpn.i) FE CO and FE H 2 and j) partial current density of CO and TOFs of PcNi-im, PcNi-pz, and PcNi-tfpn at various potentials in 0.01 M H 2 SO 4 þ 3 M KCl electrolyte.k) FE CO and FE H 2 and l) partial current density of CO of PcNi-im, PcNi-pz, and PcNi-tfpn at various potentials in 0.5 M KHCO 3 electrolyte.Gibbs free energy change from COOH* to CO* (m) and CO* to COþ* (n) for different catalysts.Reproduced with permission. [156]Copyright 2023, Wiley-VCH.
COF-366-Co (Figure 14f ). [169]First, the authors prepared the oriented thin films of COF-366-Co, which could reduce CO 2 to CO with high selectivity (FE CO of 87%) and a partial current density of CO (45 mA mg À1 ).When XAS results with hypothetical square-planar ligand field theoretically predicted the incorporating inductive effects, the sequence of increasing electronwithdrawing effects on the linker was confirmed to be: COF-366-Co, COF-366-(OMe) 2 -Co, COF-366-(F) 4 -Co, and COF-366-F-Co (Figure 14i-m).However, this sequence didn't correspond to the differences in electron-withdrawing character on the cobalt center of COFs (Figure 14g,h).Notably, compared to the original species Co(TAP), COF-366-Co exhibited a stronger electron-withdrawing effect on the Co center, leading to the enhanced performance of the COF (Figure 14j,l).In the electrochemical tests, with electrodes measured at À0.67 V versus RHE, a partial current density of CO increased from 45 mA mg À1 for COF-366-Co to 46 mA mg À1 for COF-366-(OMe) 2 -Co and further to 65 mA mg À1 for COF-366-F-Co.The trend wasn't observed for COF-366-(F) 4 -Co, which could be attributed to the elevated hydrophobicity of the framework and potentially hindered electrolyte access to active sites.This study underscores that groups within the pore can participate in electronic communication with the framework, thereby affecting the active centers.
The work from Yaghi's team involved the influence of the hydrophilic and hydrophobic side chains in the pore channels on the reaction microenvironment.Jiang and colleagues prepared crystalline COFs with side chains of different solvent affinities. [170]As expected, the contact angles of CoP-BDT-COF, CoP-BD TEO -COF, and CoP-BDT HexO -COF with water were 98.8°, 105.5°, and 126.4°respectively, indicating hydrophobicity, while CoP-BDT PEO -COF was immediately permeated by water, demonstrating super hydrophilicity.Copyright Year, American Chemical Society.
However, the excessive hydrophobic structure might inhibit the diffusion of proton sources, thereby affecting the activity, as reported by Yaghi et al.Thus, further research is needed to design appropriate hydrophilic and hydrophobic functional groups to influence the reaction microenvironment and promote ECO 2 RR.
As described in Section 2.1.2,nonmetallic N sites can also serve as active sites for ECO 2 RR.Thus, active nonmetallic functional groups can be introduced into the side chains to enhance the catalytic performance.Based on their previous work, Lan et al. introduced the 2,4-diamino-6-chloro-1,3,5-triazine (denoted as Dct) into the pore channels of Cu-Tph-COF-OH, synthesizing Cu-Tph-COF-Dct. [128]In terms of electrocatalytic performance, it maintained an FE CH 4 of over 68% in a wide potential range from À0.8 to À1.0 V versus RHE and reached a maximum of about 80% at À0.9 V, while the maximum FE CH 4 for Cu-Tph-COF-OH was only 48.8 AE 2.6% at À1.0 V. To determine the role of the Dct group, a Cu-free Tph-COF-Dct was synthesized, which still exhibited an FE CO of 22% at À0.7 V, whereas Tph-COF-OH showed no ECO 2 RR activity, indicating that Dct structure acted as an active site during reaction.

Construction of 3D Frameworks
Due to the face-to-face stacking modes of 2D COFs, the active sites between layers cannot be fully exposed to the reaction environment, leading to inefficient mass transfer.In contrast, 3D COFs have spatially separated structural units, allowing active sites within the framework to be exposed to the microenvironment, resulting in lower mass transfer resistance.

Morphology Control
In addition to employing 3D frameworks, low-dimensional materials can also be synthesized through morphology control to increase the number of active sites exposed to the reaction environment. [173]gure 15.a) Synthesis route and structural diagram of synthesis of MPc-PI-COF-3.Reproduced with permission. [171]Copyright 2022, Wiley-VCH.b) Schematic representation of the synthesis and simulation framework of 3D-Por(Co/H)-COF.Reproduced with permission. [172]Copyright 2022, Royal Society of Chemistry.
Lan et al. reported the synthesis of AAn-COF and OH-AAn-COF featuring adjustable 1D structures utilizing 1,3,5-triformylphloroglucinol with 1,5-diaminoanthraquinone (AAn) and 1,5-diamino-4,8-dihydroxyanthraquinone (OH-AAn), respectively (Figure 16a). [174]The COFs exhibited similar ordered nanofiber morphologies.AAn-COF-Cu had a diameter of 40 nm and a length of ≈0.5 μm, while OH-AAn-COF-Cu had a diameter of about 200 nm, a length of 1 μm, and a wall thickness of 18 nm.For AAn-COF, scanning electron microscope (SEM) images revealed that after monomer condensation, microcrystals formed, followed by rapid aggregation into sheet-like materials.For OH-AAn-COF, the monomers initially formed nanosheets, which were then self-assembled into larger, curled nanosheets.Although OH-AAn-COF and AAn-COF shared similar fibrous structures, different formation mechanisms led to different sizes.This may be due to the different affinities of hydroxyl groups in OH-AAn to the reaction solvent.In electrochemical tests, AAn-COF-Cu achieved an FE CH 4 of 77% at À0.9 V versus RHE, while OH-AAn-COF-Cu exhibited a maximum FE CH 4 of only 61% at À1.0 V.
that controlling the morphology can enhance the electrocatalytic ECO 2 RR process.

COFs-Based Composite Materials
COFs can be an excellent electrocatalytic material with high ECO 2 RR selectivity.It is undeniable that they inherently have low current density and poor stability compared to other materials.To address these limitations, introducing additional functional components to enhance the unsatisfactory performance is a viable and remarkably effective strategy.
Based on the work of Yaghi's team, Zhu et al. combined CNTs with 2D Co porphyrin COF at a ratio of 1:0.1 to synthesize the COF-366-Co@CNT series catalysts. [176]Different COFs in the series have different side chains in their pore and are respectively named COF-366-(OH) 2 -Co@CNT, COF-366-(F) 4 -Co@CNT, and COF-366-(OMe) 2 -Co@CNT (Figure 17a).High-resolution TEM images of COF-366-(OMe) 2 -Co@CNT revealed that COF nanolayers are uniformly wrapped around CNTs, with an average thickness of ≈0.9 nm.Contrary to Yaghi's report, COF-366-(OMe) 2 -Co@CNT exhibited the best ECO 2 RR activity with the highest FE CO of 93.6% at À0.68 V versus RHE.The difference in the best samples was due to the electronic communication between CNTs and COF.XPS tests showed that after combining with CNT, the Co 2p 3/2 peak of COF-366-(OMe) 2 -Co demonstrated the most significant shift in binding energy, which was 0.52 eV (compared to COF-366-Co: 0.22 eV, COF-366-(OH) 2 -Co: 0.32 eV, and COF-366-(F) 4 -Co: 0.17 eV), supporting the viewpoint that CNT altered the electronic structure of the Co metal center.
Similar to COFs, metal-organic frameworks (MOFs) are a type of porous network formed by the interconnection of metal centers with organic ligands. [177,178]Due to similar skeletons and different types of metal atoms, MOFs and COFs can form a composite framework with complementary functions and stable structure.Lan's team utilized HMUiO-66-NH 2 and COF-366-OH-Cu to synthesize the MOF@COF composite series (MCH-X, X = 1-4) (Figure 17b). [179]The MCH composite exhibited a honeycomb-like morphology, and as the amount of HMUiO-66-NH 2 increased, the thickness of the COF shell showed a volcano-like trend.Among the MCH-X, MCH-3 demonstrated the best ECO 2 RR performance, reaching an FE CH 4 of 76.7% and an impressive current density of 398.1 mA cm À2 at À1.0 V vs RHE, while HMUiO-66-NH 2 and COF-366-OH-Cu only exhibited 15.9% and 187.8 mA cm À2 , 37.5% and 374.4 mA cm À2 at À1.0 V, respectively.However, this work lacks a discussion on the synergistic effects between MOF and COF.The interfacial interactions and the mechanism of whether HMUiO-66-NH2 could enhance the performance of COF-366-OH-Cu or not remain unclear.
Reviewing the development process of COF-based functional electrocatalytic materials in ECO 2 RR, Co/Ni-based porphyrins and phthalocyanines are considered to be superior ECO 2 RR active centers, usually producing C 1 products.To further enhance catalytic performance, functional side chains are introduced to modify the active sites and construct tandem reaction sites.In addition to optimizing the intrinsic performance of reaction sites, researchers are gradually focusing on the charge transfer performance of materials and the diffusion resistance of reactants.To achieve high conductivity in COFs, electron enrichment and delocalization in the skeleton are essential.Therefore, linkers with a rich π-conjugated system are commonly chosen for constructing COF skeletons.Appropriate pore sizes and the shortest possible diffusion distances are necessary to minimize the diffusion resistance of reactants.In addition, coupling with other high-performance materials is a wise choice to improve ECO 2 RR performance.The modification between materials can significantly enhance the catalytic performance of COF-based composite materials.
However, in current research, due to the mature research of porphyrins and phthalocyanines, there are few reports on other metalized COF-based functional materials (such as introducing metal NPs or metal oxides into COFs) in ECO 2 RR.Additionally, in existing reports, the efficient use rate of active sites is usually low, generally below 10%.The understanding of the reaction microenvironment within COFs nanoscale pores is unclear, and the mechanism of how to effectively expose active sites to enhance catalytic activity remains ambiguous.In addition, existing research often overlooks product selectivity and typically uses DFT to infer reaction mechanisms.However, poor crystallinity of COF, inappropriate models and parameters, and the neglected electrocatalytic solution environment often led to distortion in computational results, which usually lack experimental data support.Therefore, it is crucial and indispensable to use some necessary operando characterization equipment to analyze the reaction mechanism.

Design Strategies for Specific Carbon-Rich (C 2þ ) Products
ECO 2 RR is fundamentally identified as a complex multielectron transfer process.Consequently, it will produce a variety of reduction products, including C 1 compounds like CO, CH 4 , COOH, and CH 3 OH, along with C 2 products such as C 2 H 4 and other carbon-rich compounds. [180]][183][184] Thus, they hold vast market and application potential.186][187][188][189] However, this C─C coupling places strict requirements on the affinity of the catalyst for *CO.192] Although COFs have been extensively studied for ECO 2 RR catalysis with significant achievements, most of the products are the easily obtained two-electron reduced product, CO.In contrast, C 2þ products impose more stringent requirements on reaction kinetics.In order to achieve C 2þ products, a more elaborate design of the catalyst is necessary, including micro-nano structure, active sites, and reaction interfaces. [193,194]In this section, the design strategies for selectively reducing different C 2 products in existing COFs studies will be systematically summarized and analyzed.

Conversion of CO 2 to Ethylene
[201][202] As a heterogeneous carrier, COFs can precisely construct distinct SA coordination environments, facilitating an in-depth study of the dynamic evolution of Cu active sites during the ECO 2 RR process.
Based on this idea, Yang et al. synthesized Cu-CTF-4.8%loaded with SA Cu using the wet impregnation method.SA Cu had an initial CuN 2 Cl 2 structure with a concentration of 4.8 wt%  [176] Copyright 2020, American Chemical Society.
(Figure 18a). [203]At a potential of À1.3 V versus a standard hydrogen electrode (SHE), the main gas products were CO and H 2 .However, as the overpotential increased, there was a continuous decline in the FE for CO and H 2 .In contrast, the FE for hydrocarbons, including CH 4 and C 2 H 4 , significantly increased (Figure 18b).Time-dependent FEs suggest that during the reaction of Cu-CTF-4.8%,the FE of ECO 2 RR gas products, including CH 4 , C 2 H 4 , and CO, progressively increased, whereas the FE of H 2 steadily decreased (Figure 18c).This trend suggested a potential structure transition of the SA Cu.EXAFS results indicated a decreasing trend in the peak intensity of SA Cu, while the intensity peak of Cu─Cu bonds steadily increased and finally reached a relatively stable level as the reaction progressed (Figure 18d-g).This strongly indicated the formation of Cu atomic clusters (Cu-ACs) within the CTF-Cu during the electrochemical process.It revealed that the size of these Cu-ACs was ≈1.09 nm, closely matching the pore width of the CTF, which meant that the pore size may play a role in restricting the growth of Cu-ACs.Notably, when an oxidation potential of 0.2 V versus SHE was applied to the sample with Cu-ACs, it was observed that the Cu 2þ dissociated from the Cu-ACs were able to reform robust Cu-pyridine coordination bonds.This observation suggested  [203] Copyright 2020, American Chemical Society.h) Synthesis schematic of PTF(Ni) and PTF(Ni)/Cu.Operando ATR-FTIR spectra of I,j) PTF(Ni)/Cu and k,l) PTF/Cu during ECO 2 RR at À1.1 V versus RHE in the solution of 0.1 M KHCO 3 and 0.1 M KCl mixed.Reproduced with permission. [204]Copyright 2021, Wiley-VCH.
that an appropriately chosen oxidation potential can promote the structural transition from Cu-AC to SA Cu, demonstrating the potential for controlling material structure through electrochemical means.
It is well established that the RDS for the CO 2 -to-C 2 H 4 pathway is the C─C coupling step of intermediate CO* on the catalyst surface.Therefore, in order to enhance the selectivity of the CO 2to-C 2 H 4 pathway, it is promising to design tandem catalysts with two distinct active sites.One of these sites should facilitate the CO 2 -to-CO pathway, while the other should mediate the CO-to-C 2 H 4 pathway.Cao et al. synthesized a Ni-porphyrin-based CTF with Cu NPs loaded, named PTF(Ni)/Cu (Figure 18h). [204]TF(Ni)/Cu was loaded with Cu NPs with an average size of 4.0 nm.At À1.1 V versus RHE, the optimal FE for C 2 H 4 was 57.3%, with a partial current density for C 2 H 4 of 3.1 mA cm À2 .In contrast, the primary product of PTF/Cu was CH 4 .Moreover, PTF(Ni) exhibited an FE for CO exceeding 90% within a potential range from À0.9 to À1. Cao's work showcases the potential applications of tandem ECO 2 RR.Indeed, tandem ECO 2 RR offers competitive energy efficiency (EE) and single-pass carbon efficiency (SPCE).Current systems converting CO 2 into CO have already proven industrially viable (EE > 85%, SPCE up to 45%). [205]Therefore, the development of an efficient CORR system is crucial.To address this, Sinton and colleagues designed and realized a catalyst/COF bulk heterojunction (CCBH). [206]According to their experiments and calculations, cations tend to accumulate on the cathode surface in alkaline electrolytes, occupying active sites, thus hindering CO adsorption and leading to HER (Figure 19a).OH À ions simultaneously migrate from the cathode to the anode, leading to a reduction in the alkalinity of the cathode and thereby diminishing the drive for C─C coupling (Figure 19b).Within this design, the hydrophobic COF with π-conjugation not only restricted diffusion of K þ through cation-π interactions but enhanced the adsorption of OH À and gas reactants at the catalyst surface, achieving asymmetric ion migration-adsorption (AIM-A) (Figure 19c).TEM images showed uniform dispersion of COFs on copper NPs (Figure 19d-g).During ECORR tests, the bare Cu catalyst only achieved a peak SPCE C 2þ of 59% at EE C 2þ þ of 25% and a C 2þ current density of 137 mA cm À2 .In contrast, the CCBH catalyst attained an impressive SPCE C 2þ of 95% at EE C 2þ þ of 41% and a C 2þ current density of 210 mA cm À2 .The CCBH catalyst also exhibited outstanding reaction stability; the current sustained at 240 mA cm À2 over 200 h, maintaining an average SPCE C 2þ of 95% and EE C 2þ products of 40%.Such catalysts with AIM-A functionality can effectively adjust the microenvironment of the catalyst by promoting interfacial CO enrichment and C─C bond coupling, thereby significantly improving the generation of C 2 products.

Conversion of CO 2 to Ethanoic Acid
In the reaction pathways from CO 2 to C 2 H 4 or C 2 H 5 OH, C─C coupling between *CO species or between *CO and *CHO is required, typically necessitating two adjacent active sites.However, in the pathway from CO 2 to CH 3 COOH, the reaction involves CO 2 with *CH 3 , where isolated single active sites are more favorable for selectivity.Liao et al. synthesized a 2D COF, termed PcCu-TPFN, with a singular Cu phthalocyanine center (Figure 20a). [207]PcCu-TPFN demonstrated excellent performance in ECO 2 RR for CH 3 COOH.At À0.8 V versus RHE, PcCu-TPFN exhibited outstanding selectivity for acetate, achieving an FE of 90.3% and a current density of 12.5 mA cm À2 .To clearly understand the influence of the singular Cu active sites, the authors synthesized an MOF named PcCu-Cu-O, which has a structure similar to PcCu-TPFN (Figure 20b).In addition to the shared Cu phthalocyanine structure in both, PcCu-Cu-O also has CuÀO 4 sites within its framework.ECO 2 RR tests revealed that the primary product of PcCu-Cu-O was C 2 H 4 , while in CORR, PcCu-TPFN mainly produced C 2 H 4 and C 2 H 5 OH.These outcomes suggested that, with a second active site that generates CO or in the presence of CO molecules, the Cu phthalocyanine active site will prefer to produce C 2 H 4 and C 2 H 5 OH during ECO 2 RR in CO 2 atmosphere.
In order to further study the mechanism of PcCu-TFPN, the authors chose two recently reported catalysts that also have similar isolated CuÀN 4 sites, SA Cu catalyst (CuSAC), and Cu-porphyrin-based COF (Cu-porphyrin) for DFT analysis.Among them, the ECO 2 RR reduction product of CuSAC is CO, and the product of Cu-porphyrin is CH 4 .DFT results showed that Cu atoms of three structures had different electron densities: 2.71 e Å À3 for PcCu-TFPN, 2.51 e Å À3 for CuSAC, and 2.56 e Å À3 for Cu-porphyrin, respectively (Figure 20c,d).In CuSAC, the *CO intermediate exhibits unstable adsorption, leading to *CO being desorbed rather than further hydrogenation (Figure 20e).The barrier to couple *CH 3 with CO 2 to form *OOCCH 3 in Cu-porphyrin (0.29 eV) was much higher than in PcCu-TFPN (À0.55 eV) (Figure 20f ), indicating that Cu-porphyrin was more favorable for the production of CH 4 rather than CH 3 COOH.
Liao and his colleagues elucidated the pivotal role of isolated active sites in CH 3 COOH selectivity and interpreted the interactions of Cu atoms with intermediates from an electronic structure perspective.Their research provides new perspectives on the development of catalysts with enhanced selectivity for CH 3 COOH production.

Challenges and Future Strategies for Efficient ECO 2 RR
After nearly a decade of research, significant progress has been made in the development of high-performance COFs for ECO 2 RR.Despite these advances, the performance of current materials remains below commercial standards.This suggests that specific challenges remain and require attention in future research.We outline the following perspectives and potential solutions to address the current challenges facing the application of COFs in the electrochemical conversion of CO 2 into chemical products.

Novel Catalyst Discovery
Most importantly, there is a demand for developing COF-based electrocatalysts with high current density, exceptional selectivity, and robust stability.From an economic perspective, the current density should be no less than 300 mA cm À2 , with FE exceeding 80-90%, a cell voltage below 1.8 V, and demonstrable long-term stability. [208]his implies that the ideal COFs should have superior electrical conductivity, high catalytic performance, and superb reaction stability.As mentioned in the prior discussion, the conductivity of a framework is typically associated with the delocalization and range of π-electrons.Therefore, assembling COFs using π-rich conjugated components is a promising strategy.For COFs, an undesirable electrocatalytic efficiency is predictable because the organic components in crystalline COFs are either electrochemically inert or have low catalytic efficiency.Consequently, incorporating species with inherent catalytic activities becomes an inevitable choice.To realize superior ECO 2 RR performance in COFs, it's crucial to innovate and design novel active site structures.Enriching our understanding of the structure-activity relationship for ECO 2 RR and overcoming experimental technical limitations are paramount to achieving more catalytic possibilities.COFs provide abundant coordination environments and rich anchoring sites for a diverse set of active species.Performance adjustment can also be achieved through electronic communication and mechanical constraints from the framework.COFs can act as promising platforms for SA metal catalysts and can serve as carriers for various conventional molecular catalysts.The porous structure provides sufficient space for the embedding of NPs.
to analyze CO reduction preference to OCCOH* and CHO* under varying K þ concentrations and OH* coverages.In this correlation, ΔG A low value of ΔC C 2 ÀC 1 signifies conditions that are conducive to the production of C 2 products.c) A schematic representation of the CCBH catalyst, it displays Cu NPs uniformly coated with 2D, hydrophobic, and π-conjugated Hex-Aza-COF nanosheets.The COF layer coated to the Cu surface provides a hydrophobic, continuous pathway for electron transfer and cation diffusion, facilitated by cation-π interactions.d) SEM and e) TEM images of bare Cu NPs.f ) SEM and g) TEM images of the CCBH catalyst with 15 wt% COF loading.The Hex-Aza-COF nanosheets cover the surface of Cu NPs, resulting in a porous morphology.Reproduced with permission. [206]Copyright 2023, Springer nature.
Finally, one of the most pressing challenges is the chemical and electrochemical instability of COFs.However, irreversible covalent bonds with chemically inert are usually difficult to integrate into COFs due to their poor reversibility.Although many COFs containing irreversible covalent linkages have been developed, the understanding of the synthesis mechanism is insufficient, and it is not easy to have a universal synthesis strategy, which still requires the continued efforts of scientific researchers.

Profound Cognition to Reaction Microenvironment
While the material is crucial for ECO 2 RR, electrochemical performance can be influenced by other factors.Like traditional molecular sieves, reactants and products need to diffuse within COF materials.Unlike thermal catalysis, ECO 2 RR mostly occurs in a liquid phase with salt solutions as electrolytes.In the electrolyte, H 2 O or H 3 O þ acts as a proton source, and cations/anions form an electric double layer near the catalytic surface due to electrostatic interactions.The rapid diffusion of reactants is essential for efficient catalysis.However, current research indicates that only about <10% of active sites in COFs used are accessible for ECO 2 RR due to CO 2 mass transfer limitations.For CO 2 , effective strategies to enhance mass transfer include reducing diffusion distances, increasing mass transfer pressure, and improving mass transfer kinetics.The type of electrolyte can significantly affect catalytic performance, while the size and concentration of cations can affect product selectivity.Currently, ionic liquids have shown a potential to activate and stabilize CO 2 and dissolve free radicals and reactive species in solution.Under the influence of the electric double layer, the diffusion and Reproduced with permission. [207]Copyright 2022, Wiley-VCH.distribution of multiple species within the nanochannels collectively determine the electrocatalytic microenvironment of COFs.Hence, it's imperative to understand how factors like the pore structures of COFs, as well as the type and concentration of electrolytes, affect this reactive microenvironment.In order to improve this reaction microenvironment, the ECO 2 RR device must optimize the electrode structure, flow dynamics in the electrolytic cell, and related film-forming technologies to enhance ECO 2 RR activity and selectivity.

Normalization of Structure-Activity Relationships
The reaction pathways of ECO 2 RR are complex and involve intermediates that can be converted into each other.Additionally, the catalysts may experience structural changes during the reaction.Consequently, a comprehensive understanding of reaction mechanisms remains a formidable challenge.Operando characterization techniques can reliably identify the genuine catalytic active sites and intermediates generated in the ECO 2 RR pathway through interactions among reactants, intermediates, and catalytic active sites.These distinct dynamic changes of the catalyst and intermediates during the catalytic process provide insights into structure-activity relationships, thereby revealing the reaction mechanisms.
However, the credibility of these results is compromised due to the low coverage of intermediates, transient changes in structures, and challenges in confirming the signal of intermediates.Therefore, a significant challenge is how to closely couple multiple characterizations on the same time and spatial scales to obtain more convincing conclusions.
Theoretical calculations have emerged as an effective research tool in ECO 2 RR.Such calculations, which are now extensively used in research, have played an unparalleled role in predicting and elucidating material structures, speculating on physicochemical properties, clarifying electronic and band structures, and investigating reaction pathways and transition states.These calculations can enable insights at an atomic and electronic level.At present, the rapid advancement of artificial intelligence (AI) offers new possibilities for the progression of the scientific community.AI based on machine learning could potentially streamline and integrate research related to ECO 2 RR electrocatalysts more efficiently.This is evident in the efforts of the Yaghi team, who employed ChatGPT to extract synthesis reaction information for MOFs from literature automatically. [209]sing AI for high-throughput calculations related to catalyst properties and reaction pathways, results can be obtained more effectively.However, there are still challenges in establishing computational models, refining AI's judgment of calculation outcomes, and integrating AI with experimental conclusions.

Economical Cost of COF Synthesis
The high synthesis cost of COFs severely restricts their industrial applications, and therefore, it is crucial to achieve low production costs.The high costs of COFs materials have been attributed to several factors such as expensive raw materials for COFs, relatively poor product yields, and high energy and time consumption during the reaction process.From the perspective of raw materials, most of the monomers used for COF synthesis are expensive.The cumbersome purification steps and unsatisfactory yields of these expensive monomers hinder the Gram-scale synthesis of specific COFs.From a perspective of synthesis, unoptimized synthesis conditions always result in amorphous products and low yields of COFs.In particular, COFs based on irreversible coupling reactions have become a challenge to developing new COFs materials.Due to the poor reversibility and extremely poor versatility of irreversible coupling reactions, it is usually necessary to explore a wide range to obtain the optimal conditions for the synthesis of such COFs.Furthermore, the product needs to be activated to remove impurities trapped in the pores to obtain COFs with permanent porosity.Therefore, how to activate COFs quickly and cost-effectively needs to be taken into consideration.From a scale-up synthesis perspective, the uniformity and repeatability of porous COFs are central to commercialization, which requires a sustainable approach that allows for scalable production and batch-to-batch reproducibility.In conclusion, developing a green, efficient, and generic strategy to obtain well-defined, uniform, and repeatable COFs remains a highly challenging task.

Summary and Outlook
ECO 2 RR is one of the most promising technologies for achieving a low-carbon economic target using renewable energy.As an emerging class of porous materials, COFs have been used in the ECO 2 RR domain and demonstrated exceptional performance.
This review presents the latest developments of COFs as ECO 2 RR catalysts and provides systematic summaries based on the characteristics of COFs materials.The systematic summaries are mainly categorized into six major groups, including active sites, skeletons, pores, 3D frameworks, morphologies, and composite materials within COFs.
The active sites strictly determine the intrinsic potential activity of the material, and hence, superior active sites are essential.Designs within the skeleton and pores can significantly influence the performance of active sites, the conductivity of COFs, and the mass transfer efficiency of the reactants.3D frameworks and morphology control improve microscopic and macroscopic mass transfer resistance, respectively.Also, the overall catalytic performance of the electrode can be synergistically enhanced by compounding COFs with other high-performance materials.In short, the rational design of active sites, skeleton, and pores, control of 3D frameworks and morphology, and the construction of composite materials are all crucial to the catalytic efficiency of COF electrodes.Specific designs contribute to the generation of carbon-rich products.Notably, achieving C─C coupling and directing intermediates along a particular reaction pathway is crucial to enhancing the selectivity for carbon-rich products.Despite many efforts, the synthesis of COF materials with high selectivity, large current density, low overpotential, and longterm stability remains a significant challenge.
With the advancement of research, a number of COFs with novel metal coordination structures, organic monomers, and topologies are being developed.The rapidly evolving characterization techniques and theoretical calculations offer fresh tools for exploring the relationship between structure and performance, swiftly broadening researchers' understanding of highperformance materials.Furthermore, emerging AI presents boundless possibilities for the development of novel materials.Considering the immense potential of COFs across various domains, we sincerely hope that this review could enlighten future researchers, inspiring them to delve deeper into their studies and realize the potential applications of COFs in ECO 2 RR.

Figure 1 .
Figure 1.Schematic illustration of various reduced products of ECO 2 RR and classification of design strategies for COFs in ECO 2 RR.

Figure 9 .
Figure 9. a) Structural diagram of CuÀTphÀCOF À Dct.b) Reaction pathways and intermediate structures for the pathway of CO2 -to-CO on the Dct structure of CuÀTphÀCOF À Dct.c) Gibbs free energy of CO 2 -to-CO and CO 2 -to-CH 4 reaction pathways on Dct structure and porphyrin-Cu structure, respectively.Reproduced with permission.[128]Copyright 2022, Wiley-VCH.d) Synthesis scheme for reduction of COF-300 to obtain COF-300-AR.C: gray, N: blue.Only the H atoms on the imine and amine linkage are shown in pink.e)13 C CP-MAS NMR of COF-300-AR during ECO 2 RR process.f ) Schematic diagram of the active interface between COF-300-AR and Ag foil.g) Schematic diagram of the mechanism of ECO 2 RR through carbamate formation at the interface.h) The FE for CO on the electrode of COF-300-AR and Ag at various potentials, using bare Ag foil and Nafion solution as control benchmarks.Reproduced with permission.[129]Copyright 2018, Elsevier.

Figure 13 .
Figure 13.a) Schematic diagram of the synthesis of Por(Co)-Vg-COF.b) Nyquist plots from À1.1 to À1.4 V of Por(Co)-Vg-COF.c) FE CO value of Por(Co)-Vg-COF and COF-366-Co from À0.6 to À0.9 V. d,f ) Partial current density of CO and (e, g) FE CO of Por(Co)-Vg-COF and COF-366-Co in acidic condition (0.06 M H 2 SO 4 with a 0.5 M K 2 SO 4 , red line) and alkaline condition (1 M KOH, blue line) under CO 2 flow at various potentials.Reproduced with permission.[155]Copyright 2023, Royal Society of Chemistry.h) Structures of PcNi-im, PcNi-pz, and PcNi-tfpn.i) FE CO and FE H 2 and j) partial current density of CO and TOFs of PcNi-im, PcNi-pz, and PcNi-tfpn at various potentials in 0.01 M H 2 SO 4 þ 3 M KCl electrolyte.k) FE CO and FE H 2 and l) partial current density of CO of PcNi-im, PcNi-pz, and PcNi-tfpn at various potentials in 0.5 M KHCO 3 electrolyte.Gibbs free energy change from COOH* to CO* (m) and CO* to COþ* (n) for different catalysts.Reproduced with permission.[156]Copyright 2023, Wiley-VCH.

Figure 18 .
Figure 18.a) Schematic diagram of CTF-B preparation (C: gray; N: blue; O: red; H: green).b) FEs of different gas products during the ECO 2 RR at various potentials.c) Time-dependent FEs of different gas products on CTF-Cu-4.8%at À1.45 V versus SHE.Real-time operando XAS measurements of CTF-Cu-4.8%at À1.45 V versus SHE: d) Cu K-edge XANES spectra, e) first-order derivatives of the XANES spectra, f ) Fourier-transformed EXAFS spectra, and g) first-shell Cu-Cu and Cu-X (N/O/Cl) coordination numbers.Reproduced with permission.[203]Copyright 2020, American Chemical Society.h) Synthesis schematic of PTF(Ni) and PTF(Ni)/Cu.Operando ATR-FTIR spectra of I,j) PTF(Ni)/Cu and k,l) PTF/Cu during ECO 2 RR at À1.1 V versus RHE in the solution of 0.1 M KHCO 3 and 0.1 M KCl mixed.Reproduced with permission.[204]Copyright 2021, Wiley-VCH.
4 V.It is worth noting that, in electrochemical CO reduction reaction (ECORR) tests, both PTF(Ni)/Cu and PTF/Cu displayed comparable FE for C 2 H 4 at À1.1 V (29.3% and 26.7% respectively), indicating that Cu NPs effectively converted CO to C 2 H 4 .ATR-FTIR tests revealed the presence of *CO and C─H bonds on PTF(Ni)/Cu, which were absent on PTF/Cu, suggesting that the high concentration of CO generated by PTF(Ni) increased the *CO coverage on Cu NP and consequently shifted the production from CH 4 to C 2 H 4 (Figure 18i-l).The results suggested that PTF(Ni)/Cu achieved a tandem design of two active sites, synergistically promoting C 2 H 4 generation.

Figure 19 .
Figure 19.a) The influence of K þ and OH À concentrations on the performances of bare Cu catalysts for EE C 2þ and SPCE C 2þ at different CO feedstock flow rates.b) A free energy difference map, represented asΔC C 2 ÀC 1 ¼ ΔG C 2 OCCOH Ã Àð2CO Ã þH þ þe À Þ À ΔG C 1 CHO Ã ÀðCO Ã þH þ þe À Þto analyze CO reduction preference to OCCOH* and CHO* under varying K þ concentrations and OH* coverages.In this correlation,ΔG C 2 OCCOH Ã Àð2CO Ã þH þ þe À Þ represents the Gibbs free energy of 2CO* þ H þ þ e À !OCCOH* pathway, while ΔG C 1 CHO Ã ÀðCO Ã þH þ þe À Þ .represents the Gibbs free energy of CO* þ H þ þ e À !CHO* pathway.A low value of ΔC C 2 ÀC 1 signifies conditions that are conducive to the production of C 2 products.c) A schematic representation of the CCBH catalyst, it displays Cu NPs uniformly coated with 2D, hydrophobic, and π-conjugated Hex-Aza-COF nanosheets.The COF layer coated to the Cu surface provides a hydrophobic, continuous pathway for electron transfer and cation diffusion, facilitated by cation-π interactions.d) SEM and e) TEM images of bare Cu NPs.f ) SEM and g) TEM images of the CCBH catalyst with 15 wt% COF loading.The Hex-Aza-COF nanosheets cover the surface of Cu NPs, resulting in a porous morphology.Reproduced with permission.[206]Copyright 2023, Springer nature.