Skeleton Engineering of Isostructural 2D Covalent Organic Frameworks: Orthoquinone Redox-Active Sites Enhanced Energy Storage

Open AccessCCS ChemistryRESEARCH ARTICLE1 Feb 2021Skeleton Engineering of Isostructural 2D Covalent Organic Frameworks: Orthoquinone Redox-Active Sites Enhanced Energy Storage Miao Li†, Jingjuan Liu†, Yusen Li, Guolong Xing, Xiang Yu, Chengxin Peng and Long Chen Miao Li† Department of Chemistry, Institute of Molecular Plus, and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 †M. Li and J. Liu contributed equally to this work.Google Scholar More articles by this author , Jingjuan Liu† Department of Chemistry, Institute of Molecular Plus, and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 †M. Li and J. Liu contributed equally to this work.Google Scholar More articles by this author , Yusen Li Department of Chemistry, Institute of Molecular Plus, and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 Google Scholar More articles by this author , Guolong Xing Department of Chemistry, Institute of Molecular Plus, and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 Google Scholar More articles by this author , Xiang Yu Analytical and Testing Center, Jinan University, Guangzhou 510632 Google Scholar More articles by this author , Chengxin Peng School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093 Google Scholar More articles by this author and Long Chen *Corresponding author: E-mail Address: [email protected] Department of Chemistry, Institute of Molecular Plus, and Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000257 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail The integration of redox-active sites into the skeleton of open-framework materials is an efficient strategy toward high-performance organic electrodes for energy storage devices. In this work, stepwise introduction of ketone (KT) groups to the skeletons of isostructural two-dimensional (2D) covalent organic frameworks (COFs) was realized by the condensation of 2,4,6-triformylphloroglucinol (Tp, as nodes) with a series of ditopic diamines, which contained none, one, two, and four KT moieties in each linker units, respectively. The precise control of the redox functionalities at the molecular level, combined with regular built-in channels in these KT-Tp COFs endowed them with superior capacitances and excellent rate capabilities for energy storage. In particular, 2KT-Tp COF and 4KT-Tp COF electrodes exhibited high capacitances of 256 and 583 F g−1 at a discharge rate of 0.2 A g−1, respectively, which outperformed most reported COF-based electrodes. More importantly, exceptional long-term cyclabilities (> 92% capacitance retention) were achieved even after 20,000 cycles at a high current density of 5 A g−1 for these KT-Tp COFs. Our results demonstrated that orthoquinone moieties rendered enhanced performance than the redox COFs with isolated carbonyl groups. Download figure Download PowerPoint Introduction Electrochemical energy storage (EES) devices, such as batteries, fuel cells, and supercapacitors (SCs), and others, have been recognized as promising sustainable energy source on the account of the efficient storage and/or conversion.1 Hitherto, SCs have attracted widespread attention as powerful electrochemical energy storage devices by virtue of their high power density, ultralong lifespan, and ultrafast charging/discharging process.2,3 As the core component of the SCs, electrode materials exert vital roles in determining the electrochemical performance of SCs.4 Although notable efforts have been devoted to carbonaceous materials5 assigned to electrochemical double-layer (EDL) capacitors, the lack of redox-active centers usually results in low capacitance. On the other hand, metal oxides,6 organic polymers,7 and others, have been demonstrated to afford high pseudocapacitance, while these SCs still suffer unsatisfied power density and inferior cyclability. We envisioned that exploiting new redox-active electrode materials by taking advantage of pseudocapacitance, together with appropriate EDL capacitance, might be an effective solution to addressing the aforementioned limitations.8–10 Covalent organic frameworks (COFs) are emerging porous and crystalline polymers constructed by integration of organic building blocks via strong covalent bonds.11–13 Due to their merits of lightweight, high-porosity, well-defined structure, and tailored functions, COFs have been investigated comprehensively toward gas storage and separation,14,15 catalysis,16–18 optoelectronics,19–21 sensing22,23 and drug delivery,24,25 and so forth. Particularly, COFs represent a burgeoning and promising platform for new electroactive energy storage materials, thanks to their following unique features26–29: First, the regular open nanochannels in COFs are beneficial for electron/proton transfer, thus, enhancing the rate capability and power density. Second, the rigid and extended skeletons of COFs ensure good stabilities in electrolytes, thereby, improving the cyclic durability. Last but not least, the predesignable building blocks with tunable redox activities facilitate the modulation of capacitive energy storage performance. Typical examples include the incorporation of paraquinone,30 imide,31 triazine,32 and phenazine33 into the skeletons of COFs, which resulted in the exhibition of decent electrochemical performance. However, most COFs-based electrodes employ the redox-active features without reckoning the utilization of regular built-in channels and encountering the challenges of relatively low capacitance, poor rate ability, or inferior durability.34 In this context, developing new electrochemical active COF electrodes with abundant and accessible redox-active sites, large surface areas, and regular nanochannels are highly desirable. Orthoquinone derivatives have been exploited widely as organic electrodes in terms of the highly reversible redox activity.35 Nonetheless, the severe dissolution of orthoquinone molecules into the electrolytes largely decreases the working lifetime and impedes their practical applications.36 The integration of orthoquinone units into COFs could effectively solve the problem of dissolution, meanwhile providing regular channels for mass transfer. Herein, we designed and synthesized two novel orthoquinone decorated linkers, 2KT-BD37 and 4KT-BD (Figure 1), for constructing 2KT-Tp COF and 4KT-Tp COF using 2,4,6-triformylphloroglucinol (Tp)38 as the nodes. Two comparative COFs owning similar skeletons but with only one ketone (1KT-Tp COF) or without carbonyl groups at the linker units (BD-Tp COF) were also synthesized for comparison. Both 2KT-Tp COF and 4KT-Tp COF exhibited not only enhanced capacitances but also improved cyclability, compared to BD-Tp COF39 and 1KT-Tp COF without the orthoquinone sites. Our results indicated that the integration of orthoquinone moieties afforded much better performance for energy storage than the incorporation of isolated carbonyl groups reported in previous studies.30,35 Figure 1 | Schematic illustration of the design and synthesis of Tp COFs via skeleton engineering strategy. Inset images: photographs of the corresponding four Tp COFs. Download figure Download PowerPoint Experimental Methods 1KT-Tp COF, 2KT-Tp COF, and 4KT-Tp COF were newly synthesized by solvothermal Schiff-base condensation of Tp (as the knot) with different redox linkers including 2,7-diamino-9H-fluoren-9-one (1KT-BD),40 2,7-diaminophenanthrene-9,10-dione (2KT-BD),37 and 2,7-diaminopyrene-4,5,9,10-tetraone (4KT-BD). The earlier reported control sample, BD-Tp COF, prototype was synthesized via polycondensation of Tp and benzidine (BD) according to the literature.39 We investigated further the possible electrochemical contribution of the carbonyl groups of the β-ketoenamines in these Tp-COF skeletons, BD-Tf COF,41 as another comparative, which bore the same framework structures but without any carbonyl groups in the skeleton ( Supporting Information Scheme S8). BD-Tf COF was prepared by condensation of BD and 1,3,5-triformylbenzene (Tf). BD was commercially available, while 1KT-BD40 and 2KT-BD37 were synthesized according to the reported procedures. The new ditopic diamine linker 4KT-BD was prepared by nitration of pyrene-4,5,9,10-tetraone (PYT) using fuming nitric acid in concentrated sulfuric acid (conc. H2SO4), followed by sequential reduction using sodium hyposulfite (Na2S2O4) in overall 44% yield (two steps, Supporting Information Scheme S1). The synthetic details of these five isotructural COF analogues are summarized in the Supporting Information Schemes S4–S8. Standard three-electrode system was used to evaluate the electrochemical performance of these five isostructural COFs. The energy density and power density of 2KT-Tp COF and 4KT-Tp COF were evaluated further by assembling asymmetric capacitors. For detailed electrode preparation, electrochemical testing, and characterization methods, please refer to the supporting information Section 3. Results and Discussion Structure characterization Fourier transform infrared (FT-IR) spectra of these Tp-COFs and their corresponding monomers were compared to confirm the chemical composition and polycondensation efficiencies ( Supporting Information Figures S7–S9). The disappearance of the N–H stretching bands of these BD derivatives at approximately 3311–3473 cm−1 and the C=O strech of Tp (1638 cm−1) indicated high condensation efficiencies between Tp and the diamine linkers. In addition, the emergence of new strong C–N stretching bands at approximately 1253–1262 cm−1 confirmed the resonant structure of β-ketoenamines.38 The analysis of our 13C solid-state cross polarization-magic angle spinning nuclear magnetic resonance (CP-MAS NMR) spectra showcased the characteristic chemical shifts of enamine carbons (=C–NH) at 147.4 ppm for 1KT-Tp COF, 147.6 ppm for 2KT-Tp COF, and 148.2 ppm for 4KT-Tp COF ( Supporting Information Figure S11) and the α-enamine carbons at ∼102.3 ppm.38 On the other hand, the signals at 177.2–186.4 ppm could be assigned to the ketone carbons in the redox linkers and carbonyl groups of β-ketoenamines. Elemental analysis revealed that the experimental contents of C, H, N were consistent with the theoretical values for the infinite two-dimensional (2D) networks ( Supporting Information Table S1), which also confirmed highly efficient condensation between the formyl and amine groups. Scanning electron microscopy (SEM) revealed that both 1KT-Tp COF and 2KT-Tp COF adopted fibrillar morphology with large aspect ratio, while 4KT-Tp COF exhibited short rod-like structure (Figures 3c and 3d; Supporting Information Figure S12). Transmission electron microscopy (TEM) images revealed porous characteristics of all these KT-Tp COFs (Figures 3e and 3f; Supporting Information Figure S12). Figure 3 | Argon adsorption (solid) and desorption (open) isotherms at 87 K for (a) 2KT-Tp COF and (b) 4KT-Tp COF, insets are the pore size distribution curves. SEM images for (c) 2KT-Tp COF and (d) 4KT-Tp COF. TEM images of (e) 2KT-Tp COF and (f) 4KT-Tp COF. Download figure Download PowerPoint As reported before, BD-Tp COF was obtained as orange crystalline powders. In contrast, 1KT-Tp COF was isolated as claret-red solid, while both 2KT-Tp COF and 4KT-Tp COF were obtained as black powders (inset, Figure 1). Reasonably, BD-Tf COF without any carbonyl groups in the skeleton only showed light yellow color ( Supporting Information Figure S14). To our suprise, 2KT-Tp COF and 4KT-Tp COF exhibit distinctive absorption ranging from UV–visible light to near-infrared (NIR) region (250–1400 nm; Supporting Information Figure S14). Crystalline structure The crystallinity of the newly synthesized KT-Tp COFs was assessed by powder X-ray diffraction (PXRD) measurements and compared with structural simulations using Materials Studio 7.0 software package. As shown in the Figures 2a and 2b and Supporting Information Figure S5, all the KT-Tp COFs exhibited intensive peaks at ∼3.4°, assigned to the (100) planes. These results indicated that the three KT-Tp COFs featured the same pore size of ∼2.5 nm. In addition, these KT-Tp COFs displayed broad (001) facets peaks approximately 26.7°–27.1°, which implied that the three KT-Tp COFs possessed similar interlayer spacing at ∼0.33 nm. 2KT-Tp COF (Figure 2a) and 4KT-Tp COF (Figure 2b) exhibited better crystallinity with more diffraction peaks in the PXRD patterns than 1KT-Tp COF ( Supporting Information Figure S5). The diffractions at 5.9°, 6.9°, and 9.2° for 2KT-Tp COF corresponded to the 110, 200, and 210 reflections, while the peak at 6.7° for 4KT-Tp COF was assignable to the 200 reflection. The experimental PXRD patterns of both frameworks agreed well with the simulated AA stacking models, calculated by Forcite method (Bolvia Materials Studio Software, Accelrys, Tokyo, Japan). Pawley refinement results revealed negligible differences between the experimental and simulated data with reasonable Rwp and Rp values (2.50% and 1.84% for 1KT-Tp COF; 5.39% and 4.45% for 2KT-Tp COF; 4.41% and 2.93% for 4KT-Tp COF). According to the Pawley refinement, 1KT-Tp COF afforded a P6 hexagonal space group with unit cell parameters of a = b = 29.9131 Å, c = 3.3519 Å, α = β = 90°, and γ = 120°. Similarly, 2KT-Tp COF was also fitted with a P6 hexagonal space group with lattice parameters of a = b = 30.1229 Å, c = 3.3542 Å, α = β = 90° and γ = 120°, while 4KT-Tp COF was assigned to a P6/m hexagonal space group with unit cell parameters of a = b = 30.1326 Å, c = 3.3488 Å, α = β = 90°, and γ = 120°. Figure 2 | Experimental PXRD patterns (blue), Pawley refined patterns (red), difference plots (violet), simulated AA-Stacking patterns (orange), and simulated AB-Stacking patterns (cyan) for (a) 2KT-Tp COF and (b) 4KT-Tp COF. The graphic views for illustration of stored charges in (c) 2KT-Tp COF and (d) 4KT-Tp COF (blue, N; red, O; gray, C; white, H). Download figure Download PowerPoint Porosity analysis The permanent porosity of 2KT-Tp COF and 4KT-Tp COF was evaluated by argon adsorption analysis at 87 K. All KT-Tp COFs exhibited typical Type-IV adsorption isotherm curves (Figures 3a and 3b; Supporting Information Figure S6) indicating mesoporous characteristics.42 The calculated Brunauer–Emmett–Teller (BET) surface areas were 996 m2 g−1 for 1KT-Tp COF, 1402 m2 g−1 for 2KT-Tp COF, and 672 m2 g−1 for 4KT-Tp COF. Pore size distributions calculated using the nonlocal density functional theory (NLDFT) method were mainly centered on 2.49 nm for 1KT-Tp COF, 2.24 nm for 2KT-Tp COF, and 2.17 nm for 4KT-Tp COF and were consistent to their theoretical values (2.5 nm for 1KT-Tp COF, 2.5 nm for 2KT-Tp COF, and 2.3 nm for 4KT-Tp COF). On the other hand, the two comparative BD-Tp COF and BD-Tf COF also exhibited large porosity with surface areas of 1980 and 732 m2 g−1 and poresizes of 2.49 and 2.32 nm ( Supporting Information Figures S21 and S22), respectively, which were comparable with values from previous reports.39,41 Stability test Thermogravimetric analysis (TGA) verified that there was almost no weight loss below the temperatures of 250 °C for KT-Tp COFs ( Supporting Information Figure S10). The chemical stabilities of these KT-Tp COFs were examined upon treatments in acidic, alkaline, and common organic solvents for 24 h ( Supporting Information Figures S15–S17). Interestingly, KT-Tp COFs maintained both their crystallinity and chemical composition after immersion in 1.0 M H2SO4 and boiling water, but the diffraction peaks exhibited significant attenuation after treatments in concentrated sodium hydroxide (conc. NaOH) solution (pH = 14). Supercapacitor performance Encouraged by the intrinsically porous skeletons and excellent chemical stabilities in strong acidic conditions, the electrochemical behaviors of KT-Tp COFs were evaluated in 1.0 M H2SO4 using a three-electrode setup and comparing with that of BD-Tp COF and BD-Tf COF. First, cyclic voltammetry (CV) measurements were carried out to gain a preliminary understanding of the charge storage behavior of these COF-based electrodes. As shown in Figure 4a, 2KT-Tp COF electrode showcased a typical symmetrical curve with a pair of highly reversible redox peaks at 0.27/0.22 V (vs Ag/AgCl) at a scan rate of 1.0 mV s−1. In addition, CVs of 4KT-Tp COF electrode exhibited two pairs of redox peaks at 0.28/0.24 and 0.37/0.33 V, respectively. Upon increasing the scan rates from 1 to 1000 mV s−1, the CV profiles of 2KT-Tp COF and 4KT-Tp COF electrodes remained symmetrical, with only slight shifted redox peaks ( Supporting Information Figures S23 and S24), demonstrating excellent electrochemical reversibility and promising capacitive behaviors even at high scan rates. In contrast, the 1KT-Tp COF electrode with isolated carbonyl groups exhibited very weak current response with small redox peak at a scan rate of 1.0 mV s−1. BD-Tp COF and BD-Tf COF electrodes only exhibited negligible current under identical conditions due to the lack of redox functionality (Figure 4a). These results indicated that the stable redox-active orthoquinone moieties in 2KT-Tp COF and 4KT-Tp COF could not only afford dense redox-active sites but also enabled simultaneous participation of the adjacent carbonyl groups in the redox process and enlarged the conjugation upon reduction, which exerted more important roles for improving the capacitive performance. Figure 4 | Electrochemical performance of 1KT-Tp COF, 2KT-Tp COF, and 4KT-Tp COF electrodes in 1 M H2SO4 electrolyte. (a) CVs of KT-Tp COFs and contrastive BD-Tp COF and BD-Tf COF electrodes at 1.0 mV s−1. (b) GCD profiles of KT-Tp COFs electrodes at 0.2 A g−1. (c) Gravimetric capacitance dependence on scan rates. (d) The Nyquist plots of EIS profiles. Inset: High-frequency region and the equivalent circuit of KT-Tp COFs electrodes. (e) Long-term cyclic performances at 5 A g−1. Inset: GCD profiles of 4KT-Tp COF in the first and last five cycles. Download figure Download PowerPoint The enhanced capacitance of the orthoquinone-type COFs was evaluated further by galvanostatic charge–discharge (GCD) measurements. The BD-Tp COF and BD-Tf COF contrastive electrodes exhibited classic “shark-fin” GCD profiles indicative of EDL capacitances but with low capacitances of 20 and 25 F g−1, respectively, at a discharge rate of 0.2 A g−1 ( Supporting Information Figures S29 and S30). In sharp contrast, as shown in Figure 4b, 2KT-Tp COF and 4KT-Tp COF displayed symmetrical GCD curves with a plateau at ∼0.3 V, corresponding to the redox reaction of electroactive orthoquinone moieties. Additionally, 2KT-Tp COF and 4KT-Tp COF-based electrodes delivered exceptional high gravimetric capacitances of 256 and 583 F g−1, respectively, at a discharge rate of 0.2 A g−1, much higher than that of 1KT-Tp COF (61 F g−1). Further, the discrete building blocks of phenanthrenequinone (PQ) and pyrene-4,5,9,10-tetraone (PYT) with same orthoquinone motifs as that of 2KT-Tp COF and 4KT-Tp COF were employed at the same conditions to evaluate the roles of orthoquinone moieties on charge storage. The GCD curves of PQ and PYT showcased apparent plateau ranging from ∼0.2 to 0.4 V, which were consistent with the curves of KT-Tp COFs. Besides, the discharging capacitances of PQ and PYT were 222 and 507 F g−1, respectively ( Supporting Information Figure S31), which were lower than the values obtained for 2KT-Tp COF and 4KT-Tp COF, indicating that the extended rigid structures with intrinsic built-in channels were prerequisites for high capacity. Except for high capacitances, these KT-Tp COFs electrodes also exhibited superior rate capabilities, attributable to their intrinsically ordered mesoporous channels (Figure 2c and 2d). When the load current reached up to 1 A g−1, the capacitances of 2KT-Tp COF and 4KT-Tp COF were 119 and 274 F g−1, respectively (Figure 4c), which were much higher than the building blocks (76 F g−1 for PQ and 88 F g−1 for PYT, Supporting Information Figure S32). Even at a high current of 10 A g−1, the 4KT-Tp COF electrode still retained a decent capacitance of 152 F g−1, which outperformed most reported COF-based electrode materials ( Supporting Information Table S6). Electrochemical impedance spectrometry (EIS; Figure 4d) was conducted to assess the diffusion resistance of these KT-Tp COFs electrodes. Obviously, the nearly vertical line at lower frequencies indicated good ion diffusion and almost ideal capacitive behaviors, revealing that the built-in vertical aligned channels within KT-Tp COFs were favorable for ions transfer, and thus, enabled fast charging/discharging performance. EIS fitting data showed that the charge transfer resistance at the electrode/electrolyte interface (Rct) of 4KT-Tp COF (0.31 Ω) was significantly lower than those of 2KT-Tp COF (1.06 Ω) and 1KT-Tp COF (1.31 Ω), indicating a fast interface charge transfer. More interestingly, long-term cycling tests revealed that 4KT-Tp COF, 2KT-Tp COF, and 1KT-Tp COF exhibited excellent electrochemical durability with 92%, 94% and 96% capacitance retention after 20,000 cycles at 5 A g−1 (Figure 4e), respectively. Such exceptional long-term cyclability at high rates of active materials represented one of the prerequisites for high-performance electrodes. The large capacitances of 2KT-Tp COF and 4KT-Tp COF mainly originated from both the pseudocapacitance derived from the redox-active orthoquinones in the skeletons and the EDL capacitance attributable to the high surface areas and ordered built-in channels of these frameworks. We were compelled to shed new insights into the charge storage mechanism for these KT-Tp COFs, the capacitance contributions of pseudocapacitance, and EDL capacitance via an evaluation by Trasatti analysis43 and confirmation by Dunn method44 (Figure 5, for detials, see Supporting Information Figure Section 1). Owing to the substantial contribution to fabrications involving the redox KT moieties, the pseudocapacitance percentages for 4KT-Tp COF and 2KT-Tp COF were extraordinary, achieving as high as 82% and 63% (Figure 5c and Supporting Information Figure S34c), respectively, at the scan rate of 10 mV s−1, verifying that the faradaic process induced by redox-active orthoquinones dominated the capacities of 2KT-Tp COF and 4KT-Tp COF, regarding energy storage. Figure 5 | Calculation of capacitance contributions for 4KT-Tp COF. (a) Plots of the reciprocal of gravimetric capacitance (C−1) against square root of scan rate (v0.5). (b) Plots of the gravimetric capacitance (C) against the reciprocal of the square root of scan rate (v−0.5). (c) Pseudocapacitance and electrical double-layer (EDL) capacitance contributions of 4KT-Tp COF electrode at a scan rate of 10 mV s−1. The pink region outlined the pseudocapacitance contribution. (d) Percentage of the capacitance contribution of 4KT-Tp COF electrode evaluated at different scan rates. Download figure Download PowerPoint The predominant pseudocapacitance charge storage mechanism inspired us to further explore the electron transfer process of KT-Tp COFs during charge–discharge cycles. The control experiments of contrastive COFs (BD-Tp COF and BD-Tf COF) verified the electrochemical inactivity for the imine linkages.30,45 Additionally, the CVs of the representative discrete monomers (PQ and PYT) were carried out to evaluate the redox nature of KT moieties (Figure 6). PQ showcased a pair of highly reversible redox peaks at 0.21/0.19 V (vs Ag/AgCl), in line with the values of 2KT-Tp COF (0.27/0.22 V), which were assigned to the reversible proton-coupled electron transfer process (2H+/2e−) of hydroquinone/orthoquinone (I/II) moieties (Figure 6b). Furthermore, the symmetric PYT bearing four carbonyl sites exhibited complete and reversible conversion of four protons/electrons. CV profile of PYT exhibited two distinctly reversible redox switching at 0.22/0.20 and 0.34/0.31 V (vs Ag/AgCl), which were attributable to the transformation from hydroquinone (I) to intermediate semiquinone (II) and subsequent convertion to orthoquinone (III) (Figure 6a). These redox peak positions of PQ and PYT monomers were in accordance with the aforementioned CV results for 2KT-Tp COF and 4KT-Tp COF (Figure 4a), indicating a similar electron transfer process during the charge–discharge cycles. According to the previous studies on the orthoquinone derivatives and corresponding polymers46–49 combined with the electrochemical behaviors of PQ and PYT monomers, the possible redox mechanisms of 2KT-Tp COF and 4KT-Tp COF are summarized in Supporting Information Figure S33. Figure 6 | CVs of (a) PYT and (b) PQ monomers at 50 mV s−1 and the proposed redox behaviors through reversible orthoquinone to hydroquinone transformation. Download figure Download PowerPoint Finally, we exploited the potential application of these orthoquinone-type COF electrode materials in asymmetric supercapacitor devices (2KT-Tp COF//AC and 4KT-Tp COF//AC), fabricated using active carbon (AC) as anode and 2KT-Tp COF or 4KT-Tp COF as cathode, followed by investigating the electrochemical performance of the 4KT-Tp COF//AC and 2KT-Tp COF//AC devices using CV, GCD, EIS, and long-term cycling tests, performed under a electrode potential range from 0 to 0.6 V ( Supporting Information Figures S35–S36). The calculated capacitances for the 4KT-Tp COF//AC and 2KT-Tp COF//AC devices were 251 F g−1 and 86 F g−1, respectively, at a current density of 0.8 A g−1. The capacitance, power density, and energy density of 4KT-Tp COF//AC and 2KT-Tp COF//AC devices are illustrated in Supporting Information Figure S38 and Table S7. Notably, the Ragone plot indicated that 4KT-Tp COF achieved a maximum energy density of 12.5 W h kg−1 with the power density of 240 W kg−1 under the current density of 0.8 A g−1 ( Supporting Information Figure S38). Moreover, 4KT-Tp COF//AC and 2KT-Tp COF//AC devices showcased almost no deterioration (99%) of the initial capacitances after 10,000 cycles at a current density of 5 A g−1 ( Supporting Information Figures S35f and S36d). Conclusion We synthesized a series of isostructural mesoporous COFs with or without orthoquinone moieties via a skeleton engineering strategy, followed by systematic comparison of their electrochemical properties. Among them, 2KT-Tp COF and 4KT-Tp COF exhibited good chemical stabilities, decent crystallinity, and superior electrochemical performance by virtue of the dense redox-active orthoquinone moieties and permanent built-in vertical aligned channels. 2KT-Tp COF and 4KT-Tp COF exhibited excellent capacitance up to 256 and 583 F g−1, respectively, accompanied by remarkable cycling stability (> 92% capacitance retention after 20,000 cycles). These metrics are much superior to those of contrastive COFs with isolated carbonyl active sites (1KT-Tp COF) or redox-inactive groups (BD-Tp COF and BD-Tf COF), and surpassed most reported COF-based electrode materials. Our work demonstrated an effective skeleton engineering strategy, realized via incorporation of highly reversible redox-active orthoquinone sites, which enhanced the capacitive performance greatly, and thus, opens a new avenue for rational design of high-performance organic electrode materials. Supporting Information Supporting Information is available. Conflict of Interest The authors declare no competing financial interests. Acknowledgments This work was supported financially by the National Key Research and Development Program of China (2017YFA0207500), National Natural Science Foundation of China (51973153), and Natural Science Foundation of Tianjin City (17JCJQJC44600). C. Peng is grateful for the funding from the National Natural Science Foundation of China (21875141), Shanghai Pujiang Program (18PJ1409000), and the Opening Project of State Key Laboratory of Advanced Chemical Power Sources (SKL-ACPS-C-23). References 1. Winter M.; Brodd R. J.What are Batteries, Fuel Cells, and Supercapacitors?Chem. Rev.2004, 104, 4245–4269. Google Scholar 2. Chu S.; Majumdar A.Opportunities and Challenges for a Sustainable Energy Future.Nature2012, 488, 294–303. Google Scholar 3. Pomerantsev