3.2.1 Lithium batteries

LIBs have been commercial for portable electronics and electric vehicles, and developing redox-active polymers are in consideration of the abundant sources, synthetic possibilities, lower-cost and sustainability of organic compounds. The carbonyl groups in anthraquinone-based POPs provide abundant active sites for electrochemical lithiation and delithiation. When anthraquinone functions as the only electrochemically active unit, a very important class of POP are CMPs. Different from COFs which are crystalline, CMPs are generally a special subclass of amorphous polymers which combine expanded π-conjugation. There is evidence that it can be used as an electrode material for batteries because of its permanent microporosity and substantial active groups (Yu et al., 2022). As discussed previously in the synthesis section, Ouyang (Ouyang et al., 2021) adopted a miniemulsion-solvothermal method to prepare IEP-11-E. Actually, with the increasing time of solvothermal treatment, except for the increased specific surface area, the textural properties of CMPs were also improved with more mesoporosity. Moreover, the solution dispersibility was also enhanced, which was beneficial to prepare homogeneous electrodes bearing smooth surfaces. By virtue of these factors, the overall battery performances based on IEP-11-E12cathode material were greatly improved with excellent rate (50% capacity retention at 30C, detailed data are listed in Table 2 entry 1) and cycle performances (90% capacity retention after 5000 cycles at 2C). With Sonogashira cross-coupling reaction as an effective tool to build CMPs, pyrene and tetraphenylethene units were also incorporated into CMPs (Py-A-CMP and TPE-A-CMP in Table 2 entry 2 and entry 3, respectivly) (Mohamed et al., 2021), which have greater conjugation length and twisted structure. When acted as cathode active materials, they showed excellent initial discharge capacity. However, the cycle performance of Py-A-CMP electrode needs further improvement.

Theoretically, acetylene motifs do not play electroactive roles in the skeleton structures. Luo et al. (Luo et al., 2021) prepared PyAq CMP (Table 1 entry 11) combining pyrene and anthraquinone units via Suzuki − Miyaura coupling. Without acetylene functions, PyAq would have a higher theoretical battery capacity (175 mAh g⁻¹), nevertheless, a smaller pore-size (microporosity). As illustrated in Fig. 6A, high redox activity and reversibility of carbonyl groups render PyAq CMP simultaneously to show great potentials as active cathode materials for both LIB and PIB (PIB would be discussed in the following section). Encouragingly, the PyAq cathode for LIB presented a high specific capacity of 169 mAh g⁻¹ (at 20 mA g⁻¹) which is basically close to its theoretical capacity, indicating ultrahigh carbonyl utilization (96.6%). The rate capability also performed very well (142 mAh g⁻¹) when the current density was elevated to 1A g⁻¹. To further improve the intrinsic conductivity of CMP materials, by virtue of the superior stability of ladder-type polymers meanwhile, Ouyang (Ouyang et al., 2021) designed a fully π-conjugated ladder-type CMP (BQbTPL in Fig. 4). When employed as active material for LIB cathode, the abundant quinone motifs and fused structure endows BQbTPL with high charge transport efficiency, thereby achieving superior rate capability as well as long-term cycle durability (94.8% capacity retention after 1500 cycles). Only anthraquinone functional groups are electroactive in the above discussed examples. In order to enhance the discharge capacity more effectively, it is a useful strategy to make every building unit electroactive as much as possible. Yang et al. (Yang et al., 2022) designed and fabricated a mesoporous polyimide-linked COF (HATN-AQ-COF) bearing multiple redox-active sites. As revealed in Fig. 6B, anthraquinone, imide, and hexaazatrinaphthalene subunits are all electroactive, therefore the theoretical capacity reaches up to 406 mAh/g. Together with the structural merits of inherent mesoporosity of 3.8 nm as well as high crystallinity, LIB with HATN-AQ-COF cathode presented high reversible capacity of 319 mAh/g and long-term cycle durability (80% after 3000 cycles), which ranks one of the best overall performances among reported COF-based electrodes.

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Fig. 6

(A) Schematic representation of the Li+/K + ions storage of PyAq (Luo et al., 2021). (B) Schematic representation of the potential lithium binding sites in HATN-AQ-COF (Yang et al., 2022).

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Apart from the structural factors of POPs, the pattern of stacking or dispersion of active materials, the interactions with conductive materials, and the loading amount of active materials on electrodes would have decisive impacts on the battery performances. Considering the long eclipsed stacking mode in COFs might cause sluggish diffusion rate and low availability of active materials, Wang et al. (Wang et al., 2017) exfoliated the long stacking COFs into few-layer nanosheets via ball milling (Fig. 7A). The limited few layers would shorten the migration pathways and facilitate ionic/electronic diffusion, leading to higher utilization efficiency (94.7%) of COFs as well as faster kinetics (DAAQ-ECOF in Table 2 entry 8). Except simply focusing on the polymers, the characteristics of porosity and high specific surface areas of anthraquinone-based POPs endow them with great potentials to integrate with other substances (e.g., nano-carbon materials), giving rise to more smart materials. In 2020, Molina et al. (Molina et al., 2020) fabricated a series IEP-11-Xy hybrid porous materials through the preparation of IEP-11 CMP in the presence of different carbon nanostructures, and then the corresponding composite materials were further assembled into self-supported buckypaper electrodes (Fig. 7B). This innovative electrode preparation method not only improved the interactions between POPs and conductive materials, but also greatly increased the mass loading (60 mg cm⁻²) of active materials on the electrode. By condition optimizations, the buckypaper cathode electrode based on IEP-11-S5R5@S10 showed the best performances with excellent electrochemical performances (detailed data see Table 2 entry 9), and the gravimetric capacity (83.7 mAh g_electrode⁻¹) and areal capacity (6.3 mAh cm⁻²) were the best reported values of organic electrode materials. Similarly, Amin et al. (Amin et al., 2020) adopted an approach of in situ polymerization with carbon nanotubes to improve the conductivity, and the electrochemical properties was also enhanced (AQ-COF@CNTs in Table 2 entry 10).

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Fig. 7

(A) Schematic diagram for the exfoliation of long-range stacking COFs into few-layer exfoliated COFs serving as cathode material for LIB (Wang et al., 2017). (B) Schematic diagram of the synthesis IEP-11-Xy hybrid porous materials as well as the preparation process to gain high-mass-loading electrodes by buckypaper approach (Molina et al., 2020).

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Actually, in addition to being positive active materials, anthraquinone-based POPs have been also investigated as anode active materials, which also obtains good battery performances. For instance, anthraquinone-based POPs (e.g., DAAQ-COF, rCTF, PAT in Table 2 entry 11–––13) and related hybrid materials (Co@2AQ-MnO₂ in Table 2 entry 14) all behave superior properties. Nonetheless, the contribution of anthraquinone structural unit to battery specific capacity is not the main factor since almost all the conjugate units would make contributions, such as anthraquinone, triazine, and benzene (Buyukcakir et al., 2020). These polymers do not exhibit the unique advantages of anthraquinone building blocks, therefore we do not discuss them in detail. By the way, anthraquinone-based COF can also play other roles. Guo et al. (Guo et al., 2022) added a small amount (1 wt%) of OH-AAn-COF as a binder microadditive in lithium-sulfur battery. The hollow tube of OH-AAn-COF not only strengthened the necessary properties of the binder, but also played key role in spontaneous immobilization and catalytic conversion of lithium polysulfides, leading to doubly enhanced reversible capacity. This study opens a new avenue for the applications of COFs in the field of energy storage.

3.2.2 Na- and K-ion batteries

SIB and PIBs, two kinds of alternative battery techniques to replace LIBs, have attracted considerable attention owing to the merits of low potentials (M⁺/M, M = Na, K), easy resource availability, and particularly the higher abundances of Na and K elements on the earth (Wang and Zhang 2018). Nevertheless, the larger ionic radiuses of Na⁺ and K⁺ would generally cause slow diffusion kinetics and serious structural deterioration of electrode materials upon repeated charging-discharging processes, thus leading to low capacity and poor cycle stability. The high surface area and inherent insolubility of redox-active POPs provide an effective approach to address this issue. As mentioned above, PyAq is endowed with high activity for LIB and PIB simultaneously (Luo et al., 2021). The PIB performances were pretty good containing high reversible capacity, good rate capability, and good cycling stability (Table 2 entry 5). Doubtlessly, the PIB performances were slightly reduced in comparison to those of PyAq-based LIB. Except for the ionic radius factor, much thicker solid electrolyte interphase (SEI) film and higher electrolyte concentration were also put forward to explain the decreased properties. With smaller building unit so as to produce more theoretical capacity, Luo et al. (Luo et al., 2022) investigated the effect of linking patterns of building blocks on the electrochemical performances. Three topological structures, including 1,4-, 1,3,5- and 1,2,4,5- linkage on benzene connecting with anthraquinone, were designed and prepared. It is found that increasing the number of linkages on benzene would not only enhance the specific surface area but also improve the conjugation length of resultant polymers. Therefore, PBAQ-3 with 1,2,4,5-linkage presented the best performances (Table 2 entry 16).

Remarkably, there are some materials (e.g., DAAQ-TFP) could be suitable for multiple kinds of metal-ion batteries. In addition to serving as cathode active materials for LIBs (Table 2 entry 8 and entry 10), DAAQ-TFP also exhibits high activities for SIBs and PIBs. Gu et al. (Gu et al., 2019) paid close attention to the radical intermediates during the charging and discharging of SIB. In order to balance the reactivity, kinetics and stability of radical intermediates, they stripped DAAQ-TFP into various samples bearing different stacking thickness (including 4–––12 nm, 50–––85 nm, 100–––180 nm and 100–––250 nm) via different exfoliation processes (e.g., ball milling and solvent stripping). With comprehensive techniques, it is found that the C-O· and α-C radical intermediates play subtle roles in the Na⁺ insertion/extraction processes (Fig. 8A), and the reduced thickness is beneficial for its contributive capacity as well as the stabilization of radical intermediates. The anode electrode with the thinnest stacking thickness (4–––12 nm) exhibited the highest specific capacity, optimal rate capability, and best perfect long-term cycle life (99% capacity retention after 10 000 cycles at high C-rate of 5 A/g). Duan et al. (Duan et al., 2022) adopted an in situ polymerization method to integrate a few-layered DAAQ-COF to carboxylated carbon nanotubes (DAAQ-COF@CNT in Table 2 entry 17), in which the few-layered DAAQ-COF acted as cathode active materials would provide more exposed active sites and shorten ion/electron diffusion length and the CNT nanofiber could improve electrical conductivity and mechanical properties to prevent large volume changes. Owing to the above-mentioned merits, when utilized as cathode active material for PIBs, DAAQ-COF@CNT presented enhanced performances than those of bare DAAQ-COF. The specific mechanism for K⁺ storage was also rationally speculated. As represented in Fig. 8B, unlike as anode materials in which all the carbonyl groups are electroactive, only the carbonyl groups in anthraquinone units play active roles when used as cathode material. For one repeating six-membered ring, each six K⁺ is accommodated step-by-step. The successes of these examples offer new design strategies for the fabrication of high-performance organic electrode materials for SIBs and PIBs.

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Fig. 8

(A) Redox sodiation mechanism of DAAQ-COF anode with the stacking thickness of 4–––12 nm (Gu et al., 2019). (B) K⁺ storage mechanism of DAAQ-COF@CNT for PIB cathode during cycling processes (Duan et al., 2022).

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3.2.3 Other batteries

The metal-ion batteries discussed above are primarily based on organic solvents as supporting electrolytes. In fact, aqueous rechargeable batteries could also provide attractive competitiveness in term of their intrinsic advantages of cost effectiveness, low volatility, low toxicity, and non-flammability of water-based electrolytes. However, the drawbacks of low energy density, high self-discharge and short service life limit their practical large-scale applications. Anthraquinone-based POPs could play positive roles in alleviating these restrictions. For example, as outlined in Table 2 entry 18–––20, AQ-CMP (Zhong et al., 2021), IEP-11 (Grieco et al., 2022), and Zn-AAn-COF (Guo et al., 2022) were assembled into the anodes of various aqueous rechargeable batteries such as rechargeable air batteries (RABs), alkaline rechargeable batteries (ARBs) and Zn-based aqueous batteries (ZABs), in which the POPs in the anodes served as active material, active material and support material, respectively. The performances of corresponding batteries were superior to their linear counterparts and other reported anode composite materials.

By comparing several types of batteries discussed above, from the POPs point of view, there are several common characteristics that contribute to the improvement of battery performances. Firstly, the intralayer π-conjugated skeletons or few-layer stacking structure greatly facilitate charge transfer. Secondly, the nanoscale open channels provide convenient and short diffusion avenues for mass transport. Thirdly, the porous structures supply sufficient space for electrolytes to contact with redox sites. Finally, the rigid polymer backbones together with highly crosslinked network structures ensure that POPs are impermeable to dissolution, which results in high cycle stability. The synergistic effect of these factors would play pivotal roles in improving material utilization efficiency as well as discharge capacity, enhancing rate performance, and prolonging the service life of batteries.

3.3.1 Polymer materials engineering

The first anthraquinone-based POP for supercapacitor (DAAQ-TFP in Table 3 entry 1) was reported by DeBlase (DeBlase et al., 2013) in 2013. The high specific surface area and the reversible electrochemical processes render DAAQ-TFP capable of capacitive energy storage. Two charge storage mechanism, including electrical double-layer capacitance (EDLC) and surface-bound Faradaic capacitance (pseudocapacitance), were both found to contribute the final capacitance. Besides, except for conductivity and accessibility of materials, the EDLC and the pseudocapacitance are more directly related to the electroactive surface area and the nature of redox process, respectively. Owing to the polycrystalline structure as well as the random orientation of DAAQ-TFP COF particles, the utilization rate of active materials was pretty low (2.5%). He et al. (He et al., 2022) regulated the morphologies of polymers by modulating polymerized conditions, thus improving the material utilization rates. In their study, solvent effects and pre-polymerization methods were adopted to manipulate the different growth direction of COFs (Fig. 9A). Consequently, two well-crystalline COFs bearing different morphologies, that is flake-flower (COFs-F) and rod-flower (COFs-R), were prepared. Despite the slightly decreased specific surface areas, the unique morphologies make COFs-F and COFs-R have more available redox active sites and more regular pores, leading to higher specific capacitance of 407.7F/g and 486.3F/g for COFs-F and COFs-R, respectively. Moreover, kinetic analysis further revealed that the capacitance contribution rate of COFs-R reaches up to 95.3% (at 100 mV s⁻¹). On the contrary, Geng et al. (Geng et al., 2022) improved the performances of COF materials via post-modification method (DAAQ–TFP–COF in Table 3 entry 4). A pinpoint surgery was conducted on the β-ketoenamine linkages, in which the N─H bonds were deprotonated into N⁻ anions in high-pH aqueous electrolyte (pH = 11.5). Following this useful method, DAAQ–TFP–COF was assembled into an aqueous Li-ion capacitor and presented enhanced performances. These examples provide innovative strategies for improving the electrochemical performances of supercapacitors through front control and post-modification.

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Fig. 9

(A) Schematic illustration of the proposed self-assembly mechanism of flake flowers (COFs-F) and rod flowers (He et al., 2022). (B) Chemical structure of PAQTA. (C) Schematic diagram of the charge transfer in tetrathiafulvalene and anthraquinone-based donor–acceptor COF (Li et al., 2020).

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Polyaniline nanostructures have proved to be good polymer-based electrode materials for supercapacitors. Nonetheless, the swelling and shrinkage during charge–discharge cycles stemming from linear macromolecular structures leads to decreased cycle stability. With Buchwald–Hartwig coupling reactions as effective approach to yield C─N bonds, Liao (Liao et al., 2018) prepared a series of anthraquinonylamine-based POPs. Benefiting from the excellent redox properties of anthraquinone and aromatic amines, the POPs exhibited superior electrochemical energy storage features. Among various building blocks (e.g., triphenylbenzene, carbazole, tetraphenylmethane, spirobifluorene, and triphenylamine), the strongest electron donor triphenylamine standed out and the corresponding CMP (PAQTA in Fig. 9B and Table 3 entry 5) exhibited an exceptional specific capacity of 576F/g. Upon assembling into asymmetric two-electrode supercapacitors, the device based on PAQTA showed a wide working potential window (0–1.6 V), high energy density (60 Wh kg⁻¹), and high power density (1300 W kg⁻¹). By virtue of the donor–acceptor strategy to facilitate charge transfer, Li et al. (Li et al., 2020) fabricated another donor–acceptor COF (TTF-COF1 in Fig. 9C and Table 3 entry 6) in which tetrathiafulvalene served as strong donor unit, and the specific capacitance was further elevated to 752F/g. In order to enhance the electrical conductivity, Zhou (Zhou et al., 2018), Yan (Yan et al., 2019), and Song (Song et al., 2021) prepared diverse heteroatom-doped porous carbon materials (Table 3 entry 7–9) using pyrolysis method with anthraquinone-based POPs as precursors. The specific capacitance and the cycle performance are greatly enhanced, however, the basic structure of anthraquinone is destroyed as mentioned above, thus they would not be discussed in detail.

3.3.2 Electrode engineering

As mentioned above, whether it is EDLC or pseudocapacitance, they are closely related to the electrical conductivity of materials as well as the electrochemical accessibility to active sites. The low conductivity of POPs and the poor accessibility of active sites are generally the main reasons for low electrochemical performances. Therefore, improving the conductivity and the accessibility is very valuable topic. Electrode engineering would be beneficial to ameliorate these problems. For example, a certain amount of conductive materials (e.g., acetylene black and graphite) as well as binder (e.g., PVDF) are usually added to the active materials to improve the conductivity of active materials and the contact with electrodes. However, the poor electrical contact to insoluble COF samples is still a major problem. This section will briefly introduce several methodologies to improve the electrical conductivity and electrochemical utilization of materials.

DeBlase et al. (DeBlase et al., 2015) used gold, which has good electrical conductivity, as the working electrode. In their study, an in-situ polymerization of DAAQ-TFP COF was conducted in the presence of Au substrates, and TFP monomer was slowly introduced to DAAQ solution during the preparation process so as to form oriented thin films (Fig. 10A). The resultant COF thin film-modified Au was directly utilized as working electrode for capacitance test. This particular electrode preparation method would make more facilitated electrical conductivity not only between active material and electrode but also between different layers of COF crystallites. As a result, 80–––99% of the anthraquinone motifs are electrochemically accessed depending on the thickness of thin films (<200 nm), and the specific capacitance (3.0 mF cm⁻²) is almost an order of magnitude enhancement over the conventional COF microcrystalline powder (0.40 mF cm⁻²). Although the electrochemical performance is enhanced by using gold electrode, the adsorption mode as well as modest conductivity determine that the active material can only behave well at thin film thickness and slow charge–discharge rate. The same group addressed this limitation by further introducing conductive polymer with high electron mobility (Mulzer et al., 2016). 3,4-Ethylenedioxythiophene (EDOT) in CH₃CN solution was electropolymerized within the pores of DAAQ-TFP COF (Fig. 10B, left), and the electrical conductivity was drastically enhanced. As a consequence, the PEDOT-modified COF film working electrode performed nearly quantitative utilization rate of their redox-active groups and maintained high performances at thick film (1 μm) and fast charging rates (up to 1600C). The corresponding volumetric energy density and volumetric power density increased 30-fold and 12-fold, respectively, in comparison to the as-synthesized COF film.

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Fig. 10

(A) Schematic illustration of the solvothermal growth of oriented DAAQ-TFP COF thin film on Au electrode (DeBlase et al., 2015). (B) Schematic representation of the preparation of PEDOT-modified DAAQ-TFP film (left, Mulzer et al., 2016) and the electron highways of PEDOT@AQ-COF (right, Wu et al., 2019)(Mulzer et al., 2016). (C) Schematic diagram of fabrication for DAAQ-COFs/GA composite (An et al., 2021).

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To overcome the tedious processes of electrode preparation including COF-modified Au precursor and electropolymerization, Wu (Wu et al., 2019) developed an in situ solid-state polymerization approach. AQ-COF (the same skeleton structure with DAAQ-TFP COF) was firstly prepared by conventional solvothermal method, and the resulting polymer was subsequently carried out in situ solid-state polymerization with 2,5-dibromo-3,4-ethylenedioxythiophene. The solvent- and catalyst-free polymerization mode could tightly confine the conductive PEDOT chain within the nanochannels of AQ-COF, leading to an electron highway upon external electrochemical stimulus (Fig. 10B, right). Therefore, the PEDOT@AQ-COF nanocomposite showed billion-fold enhanced electrical conductivity (1.1 S cm⁻¹) and remarkably improved capacitance performances (Table 3 entry 12).

Alternatively, An et al. (An et al., 2021) improved material conductivity and active site utilization by the introduction of 3D graphene aerogels. Specifically, a binder-free DAAQ-COFs/GA composite electrode (the chemical structure of DAAQ-COF is the same with DAAQ-TFP COF) was fabricated by an electrostatic self-assembly of cationic surfactant modified-DAAQ-COF and negatively charged graphene oxide nanosheets under hydrothermal conditions (Fig. 10C). The hierarchical porous structure of COF together with aerogel and the rapid redox reactions of anthraquinone are conducive to enhance the utilization rate of active sites and to facilitate the ion/electron transfers. Accordingly, DAAQ-COFs/GA electrode presented not only high specific capacitance (378F/g at 1 A g⁻¹) but also fast kinetics with nearly 93.4% (at 3 mV s⁻¹) capacitive contribution. Through similar strategy, Zha et al. (Zha et al., 2015) prepared capacitive electrode via on-surface synthesis of COF_DAAQ‑BTA thin film on 3D graphene. Nevertheless, the cycle stability was not ideal owing to the electrostatic repulsion accumulation and the loss of active materials during repeating cycles. These meaningful explorations provide rich instructions for improving material utilization efficiency and optimizing electrode preparation process.

3.3.3 Flexible supercapacitors

Flexible supercapacitors are in great demand with the rapid development of wearable and flexible electronic equipment, which requires light-weight electrodes bearing high surface area, precisely integrated redox functional groups, and flexible free-standing capability with high mechanical strength. This section will discuss how to achieve these goals through appropriate material design and electrode preparation optimizations.

From the viewpoint of active materials, instead of β-ketoenamine-based DAAQ-TFP, Halder et al. (Halder et al., 2018) prepared an imine-based TpOMe-DAQ COF through convenient mechano-chemical grinding followed by drop casting (Fig. 11A). Due to the rich interlayer C─H···N H-bonding, TpOMe-DAQ not only showed superior chemical stability under harsh conditions but also could generate free-standing thin sheets (∼200 μm) in centimeter scale uniformly and continuously. The COF thin film was directly utilized as capacitive electrode with H₂SO₄ (2 M or 3 M) as an electrolyte, and the device exhibited extraordinarily high areal capacitance of 1600 mF cm⁻² and excellent cycle stability (Table 3 entry 15), which are both important indicators for flexible supercapacitor. Focusing on the intermolecular interactions, the same group further designed and synthesized free-standing flexible thin films by virtue of solid-state molecular baking strategy (Khayum et al., 2018). π-electron-rich 2,6-diaminoanthracene was incorporated to copolymerize with redox-active and electron-withdrawing anthraquinone to enhance the mechanical strength of resulting thin sheet via the optimization of noncovalent interactions between crystallites (Fig. 11B). Among different compositions, Dq₁Da₁Tp COF was found to show balance properties between good mechanical strength and specific capacitance (111F/g). Both TpOMe-DAQ and Dq₁Da₁Tp were assembled into symmetric solid-state supercapacitors. However, the performances of corresponding devices were not very good with low energy density or low power density. Predictably, the low conductivity of COFs might be the main reason.

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Fig. 11

(A) Schematic diagram of the synthesis procedure of the TpOMe-DAQ thin film and its self-standing feature (Halder et al., 2018). (B) Schematic illustrations of the modulation of redox activity and mechanical strength of convergent COFs bearing different ratios of monomers (Khayum et al., 2018).

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In order to improve the electrical conductivity of COF thin films, a variety of carbon nanomaterials, which bear excellent electrical conductivity, have been attempted to hybrid with COF. Carbon nanofibers (CNF), which have high aspect ratio and are easy to interpenetrate across each other so as to form flexible films, were included into COF precursor matrixes through in situ solid-state mechanomixing technique (Fig. 12A) by Mohammed (Mohammed et al., 2019). In spite of the weak intermolecular π···π interaction between CNF and COFs, the resulting COF-CNF hybrids yet behaved good electrical conductivity (DqTp-CNF: 2.5 × 10⁻⁴ S cm⁻¹ and DqDaTp-CNF: 5.2 × 10⁻⁵ S cm⁻¹) and high performances for electrochemical energy storage (DqTp-CNF: 464 mF cm⁻² and DqDaTp-CNF: 364 mF cm⁻² at 0.25 mA cm⁻²). In particular, the electrical conductivity of hybrid showed more than 10⁹-fold increment in comparison to the pristine COFs. With better mechanical robustness, DqDaTp-CNF hybrid material (with 20% CNF) was assembled into a flexible supercapacitor device that presented a high areal capacitance of 167 mF cm⁻² (at 0.5 mA cm⁻²), and the energy density as well as the power density were also improved (Table 3 entry 17). By the way, carboxylated carbon nanotubes (c-CNTs), bearing high aspect ratio but hollow structure, could also be used to fabricate flexible supercapacitors (c-CNT@COF-3 in Table 3 entry 18) (Kong et al., 2021).

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Fig. 12

(A) Schematic diagram of the preparation of DqTp-CNF and DqDaTp-CNF COFs. The COFs stack on the graphene layers of the CNF via π···π interactions (Mohammed et al., 2019). (B) Schematic illustration of the preparation of functionalized AC-COFs (He et al., 2022). (C) Schematic representation of the mechanism of COF@rGO for improving ion transportation (Yao et al., 2022). (D) Schematic depiction of the H-bonding interactions between PDT chain and Ti₃C₂T_x sheets and the facilitated electron transfer in the hybrid materials (Wu et al., 2019).

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Although improving in conductivity, another problem arises because of the weak interactions between COF and CNF would lead to performance degradation (76% after 4500 cycles for DqDaTp-CNF). To address this issue, He et al. (He et al., 2022) integrated redox-active COFs onto the surface of carbon fibers (AC-COFs in Fig. 12B) via strong covalent bridging method. As shown in the figure, partial DAAQ monomer was grafted onto carboxylated carbon fibers to serve as nucleation sites to manipulate the morphology and the growth orientation of DAAQ-TFP COF, and then the polymerization was conducted in situ on the surface of AC-DAAQ. The resulting AC-COFs exhibited a structure of vertical tentacle-like arrays, which noy only could be benefit to capture more electrolyte ions but also could prevent COF from aggregating/collapsing. Accordingly, the capacitance, the cycle stability, the energy density, and the power density were all greatly enhanced (Table 3 entry 19). Xu et al. (Xu et al., 2020) found another approach to enhance the interactions by introducing hydroxyl-ended hyperbranched polymer (OHP) in which the abundant hydroxyl groups intensively interacted with active material and conductive material. Nevertheless, the charge–discharge stability decreased to 80% upon long-term cycles (COF@OHP in Table 3 entry 20), which was probably due to the existence of a large number of weak fatty ester bonds in the OHP.

Graphene bearing sheet structure represents another attractive conductive carbon material on account of its synthetic diversity, good conductivity, and high specific surface area. Moreover, π···π interactions between graphene sheets and other guests allow them to form flexible and highly conductive films. Considering the diverse advantages of graphene, Yao et al. (Yao et al., 2022) introduced reduced graphene oxide (rGO) sheets into DAAQ-TFP COF via in situ synthesis method. On one hand, the incorporation of rGO is helpful to improve the conductivity of materials. On the other hand, by virtue of the intermolecular π − π interaction between rGO nanosheets and COF layers, the few-layer COF plays a role of nanospacer to prevent the rGO nanosheets from accumulating, leading to effective electrolyte ion transport (Fig. 12C). Through the synergistic effect as mentioned above, the prepared flexible COF@rGO microsupercapacitor exhibited superior electrochemical energy storage (Table 3 entry 21). To prevent the re-stacking of graphene sheets, Jang et al. (Jang et al., 2022) modified the graphene via acid reflux activation. The porosity, surface area, and flake size of graphene could be well modulated by varying the acid treatment times. Therefore, the performances of corresponding devices were further enhanced based on anthraquinonylamine-based CMP and activated graphene (CMAP@A4G in Table 3 entry 22).

In addition to carbon materials, Wu et al. (Wu et al., 2019) firstly introduced nonmetallic inorganic material to gain highly flexible supercapacitor electrode. In their study, anthraquinone-based PDT CMP was in situ composited with layered MXene (Ti₃C₂T_x) that has high metallic conductivity. The PDT polymer and the MXene could form strong hydrogen bonds between the N─H or C═O bonds of PDT and the oxygen, fluorine or hydroxyl terminal groups on the Ti₃C₂T_x sheets surfaces (Fig. 12D). The H-bonding interactions not only supply precise pathways for charge transportation but also effectively prevent the device from swelling and shrinking upon long-term charge–discharge cycles, giving rise to high capacitance and excellent cycling stability. Remarkably, the film supercapacitor showed outstanding flexibility and cycle stability (97% capacitance retention) through 0 − 90° static bending test for 10,000 cycles. Considering the diversity of anthraquinone-based POPs and nonmetallic sheet materials, this work provides a novel pathway to fabricate flexible supercapacitors with excellent electrochemical performances and superior mechanical toughness.