Multi‐scale structure engineering of covalent organic framework for electrochemical charge storage

Covalent organic frameworks (COFs), which are constructed by linking organic building blocks via dynamic covalent bonds, are newly emerged and burgeoning crystalline porous copolymers with features including programmable topological architecture, pre‐designable periodic skeleton, well‐defined micro‐/meso‐pore, large specific surface area, and customizable electroactive functionality. Those benefits make COFs as promising candidates for advanced electrochemical energy storage. Especially, for now, structure engineering of COFs from multi‐scale aspects has been conducted to enable optimal overall electrochemical performance in terms of structure durability, electrical conductivity, redox activity, and charge storage. In this review, we give a fundamental and insightful study on the correlations between multi‐scale structure engineering and eventual electrochemical properties of COFs, started with introducing their basic chemistries and charge storage principles. The careful discussion on the significant achievements in structure engineering of COFs from linkages, redox sites, polygon skeleton, crystal nanostructures, and composite microstructures, and further their effects on the electrochemical behavior of COFs are presented. Finally, the timely cutting‐edge perspectives and in‐depth insights into COF‐based electrode materials to rationally screen their electrochemical behaviors for addressing future challenges and implementing electrochemical energy storage applications are proposed.

expands the disparity between the increasing requirement in energy and the shortage of worldwide energy.2][3] In this context, exploring efficient devices for storing clean and discontinuous energy have gained considerable attentions. 46][7] Thus far, they are widely applied for both large-and small-scale energy storage applications.For EES devices, electrode materials play a crucial role to determine their charge storage capability. 8,9Conventionally, diverse carbonaceous materials such as graphene, carbon nanotubes (CNTs), and activated carbon 10,11 are employed as electrode materials, which show large rate capacity, high power density, and long lifespan. 12While they inevitably suffer limited charge storage and low energy density since the absence of electroactive sites for redox response.Alternatively, inorganic compounds such as metal oxides/nitrides/sulfides, which possess functionalities for reversible redox reaction, are currently the basis of active electrode materials to boost charge storage and density energy.Nonetheless, the further development of inorganic compounds is restricted by their partial dissolution in the electrolyte, weak ionic conductivity, sluggish charge transfer, meanwhile come with toxicity, hard to processing, and high cost. 13,146][17] However, they ineluctably show sloping voltage and dropping capacity upon repeated charging/discharging process, performing inferior cycling stability. 18In the near future, the everincreasing demand in energy storage requires advanced electrode materials which not only have optimal overall electrochemical performance, but also coupled with easy availability, sustainability, and safety.
Covalent organic frameworks (COFs) formed by linking organic building blocks into an extended structure at atomic scale, are newly emerged porous crystalline polymers with high porosity, diverse topologies, ordered open channels, and light weight. 19In particular, the predesign in the molecule of initial building blocks enables the functional tunability of COFs.Based on this, electroactive groups can be ingeniously incorporated into the framework, hence allowing a flexible modulation for redox response.That makes COFs exceptional and distinguish from traditional electrode materials.Specifically, compared with carbon materials, COFs possess diverse and dense redox-active moieties for charge storage contribution.Additionally, COFs composed by rigid and extended framework structures linked by stable covalent bonds, have better structural integrity and electrochemical stability superior to both inorganic compounds and conducting polymers.Additionally, the ordered open channels within the framework provide continuous pathways for efficient ions migration.Furthermore, COFs are considered as porous graphene due to their atomic-scale ordered structures with high in-plane π-conjugation, but having more active sites and higher porosity over graphene. 20,21Overall, COFs are appealing and promising electrode candidates, and the development over the past decades in chemistry, physics, and material science has witnessed their great potential in EES applications.Nonetheless, the intrinsic weak conductivity and the low utilization of active centers hamper the further step of COFs in EES research direction.To this regard, structure engineering of COFs from multiscale aspects has been carried out to address such shortfalls and further optimize the charge storage capability, which has made a great progress thus far.
In this review, we focus on scrutinizing the status of multi-scale structure engineering for COFs from intrinsic molecular structure (linkage and active site), topological structure (polygon skeleton), crystalline structure (aggregation state), and composite microstructure aspects, with respect to optimize their charge storage capability.First of all, the basic chemistry, synthesis principle, and structure characters of COFs are summarized in brief.Furthermore, in-depth understanding regarding the interplays between the different-scale structure modulations and the eventual electrochemical behavior of COFs are carefully discussed.Finally, we proposed the challenges and perspectives of this field to guide the function-oriented design of COFs in terms of their practical requirements for EES techniques.

DESIGN PRINCIPLE, SYNTHESIS, AND STRUCTURAL DIVERSITY OF COFs
Integrating organic building blocks into extended periodic skeletons with ordered nanopores via step-growth polymerization forms a new class of crystalline porous polymers, that is, COFs. 22,23Fine controlling building blocks can guide the rigid topological evolution of framework.According to the geometry matching of building blocks (i.e., knot and linker) and the corresponding frameworks formed, a topology diagram for framework structural guidance of COFs is summarized (Figure 1A).Specifically, the self-condensation of C 2 -symmetric units with themselves (i.e., C 2 + C 2 + C 2 ), 24 and C 3 knots combined with themselves (C 3 + C 3 ) 25 or C 2 linkers (C 3 + C 2 ), 26,27 can form hexagonal COFs with mesopores but limited π-cloud delocalization region. 280][31] Especially, compared with those polygon skeletons mentioned above, C 6 + C 2 topology matching can generate triangular skeleton featuring the smallest pores meanwhile coupled with the highest π-electron density, characteristic of typical microporous materials. 32ovalent and noncovalent interactions are involved in such COF formation process, both are the driving forces to shape up the extended crystalline structures.For this, the chemical reactions applied to synthesize frameworks should have a high reversibility meanwhile coupled with self-healing ability for repairing the structural defects.Figure 1B shows the diverse reactions adopted to prepare COFs.Initially, the reaction of boronic acid and catechol, self-condensation of boronic acid, and selfcondensation of a difunctional monomer with borane and amine units, have been conducted to synthesize COFs linked by boronate ester five-membered ring, boroxine six-membered ring, and borazine six-membered ring, respectively. 29,33,34Due to the high reversibility of those reactions, the resulting COFs generally show a high crystallinity and have a good thermal stability, while they tend to be unstable in the presence of water, acids, and alcohols.In addition, COFs with boron-based linkages have a poor conjugated skeleton due to the non-π-conjugated boron-based linkages.To enhance the structural stability, stable but non-π-conjugated nitrogen-based linkages such as imide and hydrozone have been introduced into frameworks.To further extend the π-electron configuration, COFs with partially π-conjugated imine linkage are constructed via the condensation between aldehyde and amine.6][37][38] Furthermore, considering the poor electrical conductivity of COFs, fully π-conjugated linkages have been introduced into the skeletons of COFs, such as the phenazine linkage formed from the condensation of a quinone and ortho-substituted aromatic diamine. 39Especially, fully π-conjugated C═C bond linkage has been established for the synthesis of sp 2 -carbon COFs (sp 2 c-COFs) using Knoevenagal reaction between 1,3,6,8-tetrakis(4-formylphenyl)pyrene knot and 1,4-phenylenediacetonitrile linker. 40Impressively, the resulting frameworks show greatly improved structure stability and remarkably enhanced electrical conductivity, while they usually have a low crystallinity and porosity due to the low reversibility of a C═C bond formation reaction.Based on above, the chemical reactions employed play significant roles not only to ensure the organic units extended into lattice structures in an order manner but also affect the π-conjugated structure, which thereby determine the crystalline structure, porosity, structural stability as well as conductivity of COFs.
For COF formation, the dimensionality of building blocks, that is, two-dimension (2D) or three-dimension (3D), directs the elementary polygon skeletons evolution manner. 41That correspondingly generates 2D or 3D COFs.For 2D framework formation, 2D atomic layers are firstly formed from organic units linked by covalent bonds, which are further stacked into crystallize layered structures driven by the interlayer π-π interactions (Figure 1C, left). 42he presence of both intra-layer covalent bonds and interlayer noncovalent interactions stabilizes the resulting 2D frameworks, and further promoting them to be orientated and crystallized.However, for 3D COFs, they are difficult to crystallize and prefer amorphous phases with large void spaces as the absence of strong interlayer π−π interaction (Figure 1C, right). 43,44Nonetheless, 3D COFs still show some special merits such as more diverse pore structures beyond the only one-dimensional (1D) channels existed in 2D COFs, and high surface area as well as low density benefited from the void 3D frameworks. 45,46Based on those positive effects, currently, the post-functionalization and stability engineering of 3D COFs toward their applications in EES aspect have achieved a great progress.
In the case of COFs served as electrodes for rechargeable batteries, the capability of undergoing reversible electrochemical redox reactions is essential.As a matter of fact, COFs share a high similarity in the energy storage mechanism with that of electroactive organic electrodes. 51,524][55] Specifically, during the electrochemical process, n-type COFs are reduced by accepting electrons in the neutral state to form negatively charged anions, followed by subsequent oxidation reactions back to the neutral state.On the contrary, p-type COFs are oxidized to form positively charged cations followed by returning to the natural state.Especially, bipolar COFs can be either reduced to a negatively charged state or oxidized to a positively charged state, though typically the only one side of the reaction is utilized for charge storage applications.Taking n-type COFs served as LIB electrode as an example, along with the reduction of n-type COFs, Li + ions will be move toward the n-type COFs to balance the negative charged species.In the reverse oxidation, Li + ions are released into the electrolyte meanwhile the n-type COFs recover the neutral state.As such, a discharge/charge process of n-type COFs is completed.Beyond the above, another charge storage mechanisms such as ion intercalation to the aromatic rings can also be utilized to store charge when COFs used as battery electrodes.On the other hand, when COFs applied to supercapacitors, there are two major types of charge storage mechanisms: (1) electric charge accumulation on the electrode surface which corresponds to the electric double-layer capacitor (EDLCs), and (2) pseudocapacitance originates from the surface redox reactions occurred on COFs.The EDLC mainly depends on the unique crystalline porous structure of framework which not only offers a high surface area preferable for charge accumulation but also provides convenient pathways for ion diffusion.While the pseudocapacitance is dominated by the redox-active moieties which are involved in the surface redox reactions.
Either used for battery or supercapacitor, the charge storage capability of COFs is highly associated with the structure features of framework.An insightful understanding of COFs from structural perspectives, and correlate them with eventual electrochemical behavior is quite important to guide the pre-designed structures and tailormade functions for COFs toward EES applications.In principle, within the skeletons, the functional moieties involved in the redox reactions dominate the intrinsic electrochemical activity of COF.Ingeniously selecting appropriate linkers which bear certain oxygen-or nitrogen-containing functionalities allow them to form robust covalent linkages with other linkers, finally to form functional COFs with electrochemical activity.The incorporation of redox-active groups not only boosts the charge storage but also tunes the operating voltage of COFs to widen the working potential and thus enhancing the energy density.Except the active sites, polar linkages introduce heteroatoms with a powerful anchoring capability, which can trap electrolyte ions through adsorption interaction to offer extra charge storage.Nonetheless, the polar linkages are prone to collapse under acidic or alkaline conditions, as a result causing the poor cycling stability for COFs.On the contrary, frameworks with sp 2 -carbon linkage have high structural stability, meanwhile featured extended π-conjugation which allows a faster electron transfer throughout the skeleton.Consequently, sp 2 -carbon-linked COFs can achieve better stability and higher rate performance superior to those of COFs with polar linkages.However, they suffer inferior energy storage capability due to the absence of redox-active sites.
For COFs with different dimensionality, that is, 2D COFs versus 3D COFs, both have the advantages of modified skeletons, multiple open sites, and a highly porous structure with large pore volume, but showing different electrochemical behaviors when applied to EES applications.Specifically, 2D COFs formed by linking rigid and planar building blocks, have an extended large π-conjugated network with tight layer-by-layer stacking mode along the out-of-plane direction.Based on this, 2D COFs have vertical columnar arrays which provide regular and continuous channels for the efficient electron movement and ion migration.While for 3D COFs, the 1D conjugated segments interlace with each other that forms void 3D frameworks with irregular and long-range conjugated systems, which also offer transport pathways for electron transfer but within a limited region.Nonetheless, the diversity in pore structures, ultrahigh surface area and low density of void frameworks allow 3D COFs gaining considerable attentions for EES applications.
In general, bulk COFs have a low utilization of redoxactive sites and show poor rate performance due to their aggregated stacking.COF nanosheets (CONs) differing from bulk COF stacks are featured with shortened diffusion pathways and more accessible active sites, which benefit a fast ion migration and thus a good rate performance.Specifically, in the case of battery, the sheet-like structure has a capability of accommodating ions between layers to boost the capacity.While for supercapacitor, CONs have high porous nature and large specific surface area, which helps in the accessibility of electrode surface by electrolytes and absorbing ions, hence resulting in high specific capacitances.
Despite the outstanding advantages of COFs for EES applications, they still face shortcomings such as the poor conductivity and the low utilization of active sites buried in the frameworks.Both restrict their charge storage capability and further development in the EES research direction.Based on the structure-effect relationship of COFs for EES, structure engineering of COFs from intrinsic molecular structure (linkage and active site), topological structure (polygon skeleton), crystalline structure (aggregation state), and composite microstructure aspects, has been carried to address such shortfalls and improve the overall electrochemical properties.Next, we will focus on discussing the attempts which have been made on the different structure engineering of COF-based electrode materials in detail.In the review, all the COFs electrodes studied with different linkages and active sites, as well as their charge storage capability when applied to EES applications are summarized in Table 1.

Imine linkage
The planarity and conjugated structure of imine linkage affect the stability of electrochemical behavior for COFs. 56,57Imine-linked COFs, which are generally synthesized via Schiff base condensation reaction between amine and aldehyde functional groups, 58,59 possess robust framework with excellent resistance to chemical and thermal treatments.In 2015, Zha et al. 60 firstly reported a imine 2D COFs (DAAQ-benzene-1,3,5-tricarbaldehyde COF) with anthraquinone functionalities for charge storage.Afterwards, another redox-active and imine-linked COF was developed and processed into free-standing thin sheet electrode. 61As-made 2D COF (TpOMe-DAAQ) sheet electrodes showcase areal capacitance of 1600 mF cm −2 .Even after 100 000 cycles, there is no obvious decrease in capacitance or Coulombic efficiency.Additionally, two carbazole-based COFs with imine linkage, that is, Car-TPA COF hollow sphere and Car-TPP COF hollow microtube, were synthesized for charge storage. 62or both Car-COFs, they combine the electrical-doublelayer behavior and pseudocapacitive response, while still deliver inferior capacitance below 20 F g −1 at 0.2 A g −1 , arose from the low utilization of porous structures and electroactive sites.To boost the charge storage, a new porphyrin-based 2D COF linked by imine bond had been designed by combing 5,10,15,20-tetrakis(paraamino phenyl)porphyrin with 1,1,2,2-tetrakis(4-formyl-(1,1′-biphenyl))-ethane. 63The obtained COF, marked as PT-COF, features good crystallinity and high surface area up to 1998 m 2 g −1 .When applied to supercapacitor, impressively, PT-COF achieves the highest 1443 F g −1 capacitance at 1 A g −1 and remains 91% capacitance after 3000 cycles, outperforming most of imine-lined COF electrode materials reported.
Imine-linked COFs also have been served as anodes for rechargeable batteries.As has been demonstrated before, 64 an imine-linked 2D COF (DBA-COF3) with dehydrobenzoannulene (DBA) redox moiety was designed to be used as anodes for Li + ion storage, which could provide a stable capacity of 207 mAh g −1 at 50 mA g −1 .Dou and co-workers 65 further extended the Li + storage capability of imine-linked 2D COFs to a higher level via introducing high-density C═N functionalities into a hexaaminobenzene-based triangular framework (i.e., HAB-COF) (Figure 3A).Consequently, HAB-COF can provide a stable capacity of 1255 mAh g −1 at 1 A g −1 after 1100 cycles.Another two examples of imine-liked 2D COFs with alkynyl ligands, that is, TAEB-COF and DBA-COF3 with 1,3,5-tris(arylethynyl)benzene (TAEB) and DBA units, respectively, had been prepared and acted as anodes for KIBs. 66For both TAEB-COF and DBA-COF3, their alkynyl ligands can interact with K + ions that results in a 254 mAh g -1 capacity for TAEB-COF while a lower capacity of 76.3 mAh g -1 for DBA-COF3.The higher capacity of TAEB-COF is attributed to the conformational flexibility of TAEB linkers which facilitates the intercalation and further the storage of K-ions.

β-Ketoenamine linkage
Firstly, Banerjee and co-workers 67 synthesized a βketoenamine-linked 2D COF with outstanding hydrolytic and chemical (acid/base) stability, meanwhile showing high redox response resulted from electron-rich N atoms.In 2013, succeeding β-ketoenamine-linked 2D COF (DAAQ-TFP) constructed from 2,6-diaminoanthraquinone (DAAQ) and 1,3,5-triformylphloroglucinol was proposed by Dichtel and co-workers and applied to supercapacitive energy storage, 68 while a small portion (∼3%) of electroactive sites is electrochemically accessible and being used.That limits DAAQ-TFP-based electrode materials presented as the form of either conductive substratessupported thin film (<250 nm) 69 or thicker films (1 μm) combined with conductive fillers. 70In this context, developing high-performance electrode materials from bulk COFs are important and meaningful.For this, thereafter, Dichtel and co-workers 39 reported a new phenazine-based and β-ketoenamine-linked 2D COF (DAPH-TFP COF), which has a higher electrical accessibility for redox sites over that of DAAQ-TFP analog.Accordingly, bulk DAPH-TFP used as supercapacitor electrode can deliver high power as well as energy density.To further boost the electrical storage capability of COFs, DAAQ-TFP COF had been extended as anodes for Li-ion hybrid supercapacitor. 71It turns out a 132 mAh g −1 capacity at 0.5 A g −1 and can retain a 108 mAh g −1 even with 20  times increase in current density, meanwhile showing a good stability in 1000 cycles.After assembling DAAQ-TFP COF anode into an aqueous Li-ion capacitor, the obtained device delivers a 224 F g −1 high specific capacitance at 0.1 A g −1 , a supercapacitor-level power density of ca.4000 W kg −1 , and long cycling stability.Moreover, in term of the increasing demand in cutting-edge wearable EES devices, β-ketoenamine-linked 2D COF (Dq1Da1Tp COF) thin sheets had been prepared from redox-active anthraquinone (Dq) and π-electron-rich anthracene (Da). 72The resulting Dq1Da1Tp COF thin sheets incorporate advantages of precisely integrated redox moieties, mechanically strength, and light weight, showing great potential to be used as self-supporting electrode materials for solid-state flexible supercapacitor devices.Except supercapacitor application, β-ketoenaminelinked COFs have also been applied to battery devices.For instance, a β-ketoenamine-linked 2D COF framework with abundant active sites (N═N and C═O), that is, Tp-Azo-COF, had been designed and served as the anodes for LIBs. 73The corresponding LIB device achieves a stable capacity of 305 mAh g -1 at 1 A g -1 even after 3000 cycles.Another truxenone-based 2D COF (COF-TRO) with β-ketoenamine linkage had been explored as cathodes for all-solid-state LIBs. 74It turns out the highest capacity of 268 mAh g −1 and maintains 99.9% capacity retention after 100 cycles at 0.1 C rate, showing an outstanding cycling stability.Such electrochemical performance combination surpasses most of COF-based cathodes reported for allsolid-state LIBs so far.Beyond LIBs, some preeminent researches imply that β-ketoenamine-linked COFs are great candidates as ideal electrode materials for other metal-ion batteries. 75,76For example, Lu and co-workers 77 suggested that DAAQ-COF could react with Na + ion to achieve a high capacity when acting as anodes in SIBs (Figure 3B).Additionally, for the first time, Banerjee and co-workers 78 explored β-ketoenamine-linked 2D COF (HqTp COF) as cathode material for rechargeable aqueous ZIBs.The assembled Zn/HqTP unit cell displays a discharge capacity up to 276 mAh g −1 at 125 mA g −1 , verifying that HqTp COF is a functional platform for Zn 2+ ions storage.

Triazine linkage
Covalent triazine frameworks (CTFs) known as triazinelinked COFs, are constructed through cyclization reaction of nitrile aromatic building blocks.Despite being classed as a branch of COF, CTFs feature low crystallinity or amorphous due to ultra-strong covalent bonds (aromatic C═N) from triazine linkage. 79CTFs are also regarded as an exciting new type of porous organic polymers with π-conjugated skeletons and permanent nanopores. 80ntriguingly, the presence of triazine units offers special advantages to CTFs, such as rich nitrogen content and high chemical stability.All those properties make CTFs potentially useful in energy storage application. 81For example, Zhi and co-workers 82 constructed a conjugated microporous CTF from trigonal-symmetrical nitrogencontaining monomer, that is, 1,3,5-tricyanobenzene.The obtained CTF shows significantly enhanced conductivity, rich nitrogen doping, and high microporosity.Assembling CTF into supercapacitor, the device could work at a high voltage of 3 V, accordingly achieving the highest energy density reaching 47.4 Wh kg −1 and power density of 7500 W kg −1 , meanwhile showing an outstanding cycling stability with 85% capacitance retention after 10 000 cycles at 10 A g −1 .Another, Deng and co-workers 83 prepared tetracyanoquinodimethane-derived conductive microporous CTFs (TCNQ-CTFs), which have high nitrogen content over 8% and specific surface area (3600 m 2 g −1 ).Those effects allow TCNQ-CTFs for fabricating high-performance supercapacitor.As expected, as-constructed device achieves a specific capacitance over 380 F g −1 and remarkable energy density of 42.8 Wh kg −1 , as well as demonstrates outstanding cycling performance without any degradation in capacitance even after 10 000 cycles.
To further improve the overall electrochemical properties, pyridine functionalities are incorporated into porous CTF, and the corresponding framework product is marked as p-CTF. 84Particularly, p-CTF shows well-controlled porous structure and nitrogen doping level by adjusting the reaction temperature.Specifically, high-temperature treatment at 800 • C enables the corresponding p-CTF-800 having a specific surface area of 2795 m 2 g −1 and a rich nitrogen doping level reaching 11.82%.In view of those features, p-CTF-800 exhibits high specific capacitances of 406 F g −1 in three-electrode system.Further combining p-CTF-800 with 1-ethyl-3-methylimidazolium tetrafluoroborate as electrolyte, the obtained supercapacitor device achieves the highest energy density up to 77 Wh kg −1 , and retains 94% energy density retention after 20 000 cycles at 4 A g −1 , presenting an impressive cyclic durability.Another, for the first time, Zhu and co-workers synthesized two CTFs, that is, CTF-0 and CTF-1, employed as anodes for KIBs.For them, the sheet-like structure and regular channels dominate the intercalation/deintercalation of K + into/from CTFs (Figure 3C). 85Based on this, CTF-0 with a small pore size proceeds an exothermic depotassiation process while the depotassiation process is endothermic in CTF-1 due to the relatively large pore size.That makes K-ions deintercalation from CTF-0 easier than that from CTF-1.As a result, CTF-0 delivers reversible capacity almost twice higher than that of CTF-1, specifically, 113 versus 60 mAh g −1 .In addition, Li and co-workers firstly proposed a biphenyl-based CTF (CTF-2) to be used for Li + storage material. 86They suggest that the reversible and fast electron in each aromatic ring allows CTF-2 with a super-lithiation performance with 4.4 Li + storage.As a result, CTF-2 could offer a capacity of 1527 mAh g −1 at 0.1 A g −1 , which retains 463 mAh g −1 at 10 A g −1 , performing a superior rate performance.Besides, even after 500 cycles at 1 A g −1 , a high capacity of 1321 mAh g −1 is well maintained, exhibiting an admirable cycling stability.

Phenazine linkage
Phenazine-linked COFs are generally synthesized by linking ortho-quinones and ortho-diamines.For phenazinelinked COFs, their fully extended π-conjugated system coupled with a long-range π-π orbital overlapping provide a continuous pathway for electron transfer, hence endowing relatively good conductivity.That places phenazinelinked COFs as special organic polymers. 87,88In particular, phenazine-linked COFs resemble porous graphitic frameworks featuring the porous, hetero graphene-like, and fully sp 2 -hybridized in-plane structure, which are promising candidates for advanced electrodes.For example, Eddaoudi and co-workers firstly designed a phenazinelinked 2D COF (Hex-Aza-COF3) with an aza-fused πconjugated framework (Figure 3D). 89When applied to supercapacitor under a three-electrode setup, Hex-Aza-COF3 could deliver a specific capacitance of 663 F g −1 , much higher than that of most COF-based electrodes reported.Further combining Hex-Aza-COFs with RuO 2 to construct an asymmetric supercapacitor, the device could work at a high potential window of 1.7 V, which thereby resulting in a high energy density of 23.3 Wh kg −1 at a power density of 661.2 W kg −1 .Furthermore, phenazinelinked COFs are also served as cathodes for LIBs.Zhang and co-workers 90 synthesized a phenazine-linked COF (PGF-1) as cathode for LIB.Due to the high structural regularity of framework facilitating Li-ion transportation and the continuous pore channels improving the accessibility of redox-active centers, PGF-1 displays stable electrochemical behavior with a reversible capacity of 842 mAh g −1 at 100 mA g −1 .

Arylamine linkage
Developing new linkages especially functional linkages is an efficient strategy to diversify COFs with additional tailored properties.Keep this in mind, in view of the high stability, extended π-conjugation system, and redox response, arylamine linkage stands out from traditional linkages for COFs.Furthermore, the diphenylamine moiety shares a high similarity with the repeating segments of polyaniline (PANI), confers the unprecedented arylamine-linked COFs capable of performing electrochemical performance such as PANI.Based on this, for the first time, Chen and coworkers synthesized a new 2D COF with arylamine linkage (Aam-TPB) via the condensation of dimethyl succinyl succinate with multitopic amines (Figure 3E). 91As-resulted arylamine-linked COF exhibits superior structural stability and has rich electroactive moieties of diphenylamine groups.As a result, Aam-TPB used as supercapacitor electrode can deliver a high capacitance of 271 F g −1 at a discharge rate of 1 A g −1 , surpassing most of COF-based electrode materials reported thus far.

sp 2 -carbon linkage
For conventional COFs linked by polarized covalent bonds, they usually exhibit relatively weak π-electron conjugation over the skeleton, as a result showing a poor electrical conductivity which severely impedes the practical utilization of COFs for energy storage.To this regard, Jiang and co-workers successfully designed a fully π-conjugated 2D COF by linking tetrakis(4-formylphenyl)pyrene and 1,4phenylenediacetonitrile with sp 2 -carbon linkage (C═C) via Knoevenagel condensation. 40The efficient π-electron delocalization capability of sp 2 -carbon linkage facilitates the resulting COFs with significantly enhanced electrical conductivity, presenting as a complement for the traditional 2D soft (semi-) conductors. 92Besides, the longordered porous structures combined with the regular open channels are beneficial for the fully exposure of active sites, fast mass transportation, as well as accelerated electron transfer.All of those together allow sp 2 -carbonliked π-conjugated COFs appealing and promising for EES applications.Thereafter, Zhang and co-workers synthesized a fully conjugated C═C-linked 2D COF (i.e., g-C 34 N 6 -COF) from building blocks of 1,3,5-triazine units and 3,5-dicyano-2,4,6-trimethylpyridine via Knoevenagel condensation. 93As-resultant g-C 34 N 6 -COF shows unique nanofibrous morphology, coupled with rich porous structure and excellent stability resistance to aggressive chemicals.After assembling g-C 34 N 6 -COF into a flexible microsupercapacitor, the device delivers an areal capacitance up to 15.2 mF cm −2 , a high energy density of 7.3 mWh cm −3 , and good rate capability.Beyond supercapacitor application, sp 2 -carbon-linked COF materials have been applied to LIBs.The first attempt using sp 2 c-COF as LIB cathode was proposed by Feng and co-workers in 2018. 94They constructed a novel and conjugated 2D framework with sp 2 -carbon linkage (i.e., sp 2 -carbon-linked conjugated polymer [CCP]-HATN, Figure 3F), which has a nitrogen-doped skeleton, electrochemical redox-active hexaazatrinaphthalene (HATN), dual pore structure, and high structure stability resistance to chemical and electrochemical treatments.Integrating those merits together, CCP-HATN cathode could offer a high capacity of 116 mAh g −1 and retain 91% retention of initial capacity after 1000 cycles, showing superb cycling stability.

Electroactive center chemistry
Electroactive center determines the overall electrochemical performances of COFs in terms of redox reaction type, charge storage capability, rate performance, or durability.For now, the types of electroactive centers for COFs mainly fall into nitrogen-enriched moieties, carbonyl-contained functionalities, and free radicals (Figure 4).In the following part, we will present the latest development of electroactive COFs with various redox sites used for EES techniques, with placing great emphasis on the interplays between diverse electroactive centers and eventual electrochemical behaviors.

Nitrogen-contained COFs
Nitrogen heteroatom can be easily incorporated into COF skeletons to function as redox-active site for charge storage.
Previously, Tang and co-workers prepared a 2D TaPa-Py COF with N-containing moiety, that is, pyridine, 95 which can proceed a 2H + /2e − transition reaction upon electrochemical process.That allows TaPa-Py COF delivering a high capacitance of 209 F g −1 at 0.5 A g −1 .Accordingly, the supercapacitor device assembled by TaPa-Py COF shows the highest energy density of 9.06 W h kg −1 at the power density of 100 W kg −1 and maintains 5.4 Wh kg −1 energy density at the highest power density of 1956 W kg −1 , meanwhile having a good cycling durability with 92% capacitance retention after 6000 cycles.Despite pyridine as electroactive center has been successfully introduced into COFs, the corresponding frameworks still suffer inferior charge storage capability due to the low-doping level of nitrogen atoms.Regarding to this, Yang and co-workers 96 combined nitrogen-enriched melamine (MA) and piperazinedicarboxaldehyde (PDC) to prepare a triazine-linked COF (PDC-MA-COF), in which the nitrogen content is up to 47.87%.As a result, when PDC-MA-COF served as supercapacitor electrode, it shows a maximum specific capacitance of 335 F g −1 at 1.0 A g −1 , and maintains 285 F g −1 capacitance after 20 000 charge/discharge cycles, showing a remarkable cycling lifespan.Additionally, N-enriched highly conjugated COFs have been designed and applied to LIB.For example, Yang et al. 97 developed a highly conductive 2D COFs (TThPP COF) by combining porphyrin and 4-thiophenephenyl moieties.The uniform and ordered alignment of 2D sheets allows a good in-plane conductivity of 2.38 × 10 −4 S m −1 for TThPP COF.That benefits the carrier transportation, and thus the high-rate performance.As a result, TThPP COF anode turns out a reversible capacity of 666 mAh g −1 at 200 mA g −1 and maintains a stabilized discharge capacity of 381 mAh g −1 after 200 cycles at 1 A g −1 .To further improve the Li-ion storage capability, Bai et al. 98 prepared two nitrogen-contained 2D COFs with fully conjugated framework, which are N2-COF and N3-COF.For them, the ordered stacking channel and small pore sizes features accelerate Li-ion transportation and electrolyte permeation.As a consequence, N2-COF and N3-COF served as anode materials in LIBs deliver discharge capacities of 735 and 732 mAh g −1 at 1 C, respectively.Furthermore, they could retain over 80% capacity after 500 cycles, show-ing a good stability.Additionally, Chen and co-workers 99 designed a nitrogen-enriched 2D COFs, that is, BQ1-COF, as cathode for LIBs (Figure 5A), which could give a reversible capacity of 502.4 mAh g −1 at 0.05 C.After repeating charging/discharging process for 1000 cycles, BQ1-COF cathode still maintains 81% retention of initial capacity, showcasing good rate capability as well as long lifespan simultaneously.The overall electrochemical performance for BQ1-COF outperforms most of polymerbased cathode materials reported so far, which is benefited from the following synergistic effects: (1) the abundant active sites of C═O and π-conjugated C═N groups contributing the charge storage, (2) the stable 2D conjugated network responsible for the excellent cycling life, and (3) the highly ordered channels and extended π-conjugated system expediting Li + diffusion and electron transfer, hence resulting in a remarkable rate performance.
Except Li-ion storage, Chen and co-workers reported a honeycomb-like 2D COF (triquinoxalinylene and benzoquinone units [TQBQ]-COF) with dual electroactive sites of pyrazine and carbonyl for Na + storage. 100Upon electrochemical process, TQBQ-COF involves 12 Na + redox reaction per segment (Figure 5B), as a result exhibiting a remarkably high specific capacity of 452.0 mAh g −1 .And it can retain ∼96% capacity after 1000 cycles, exhibiting good cycling stability.Further assembling TQBQ-COF into a pouch-type battery, the device could achieve gravimetric and volumetric energy density of 101.1 Wh kg −1 and 78.5 Wh L −1 , respectively.Nitrogen-enriched COFs are also available for Mg-ion storage.For the first time, Wang and co-workers 101 introduced triazine rings into frameworks which are capable of reacting with Mg 2+ ions (Figure 5C).When as-prepared COF used as cathode for MIB, it delivers a discharge capacity of 102 mAh g -1 at 0.5 C, a high power density up to 2.8 kW kg −1 , and a maximum energy density of 146 Wh kg −1 .Furthermore, upon 3000 electrochemical cycles, there is a very slow capacity decay rate (0.0196%) per cycle for the COF cathode (Figure 5D).Beyond above, especially, a 2D COF furnished with quinone carbonyl oxygen and pyrazine nitrogen functionalities, marked as QA-COF, had been developed for NH 4 + ion storage (Figure 5E,F). 102ring electrochemical process, NH 4 + ions interact with both carbonyl O and pyrazine N atoms on QA-COF driven by the H-bonds (N-H⋅⋅⋅O and N-H⋅⋅⋅N).That thereby induces the formation of six-coordination structure occupying the diagonal position, which further translates into 12-coordination structure per COF unit (Figure 5E).In this way, QA-COF achieves a high capacity of 220.4 mAh g -1 at 0.5 A g -1 , and retains 200 mAh g -1 capacity after 500 cycles at 0.5 A g -1 .Even undergoing 7000 cycles at a high current density of 6 A g -1 , there is a negligible attenuation in the capacity of QA-COF, verifying a good cyclic stability.

Carbonyl-contained COFs
To be well known, carbonyl-containing compounds can store charges via the benzoquinone/hydroquinone transformation with excellent electrochemical reversibility, 103 while they suffer the severe dissolution in electrolytes that hampers the further application in EES applications.In this context, an efficient strategy is to integrate carbonyl groups into robust frameworks, in such way to pre-vent the dissolution meanwhile well preserve the highly reversible redox ability.The first attempt was DAAQ-TFP COF which was constructed from the self-condensation of 1,3,5-triformylpholoroglucinol. 68When used as electrode materials, DAAQ-TFP COF could provide capacitive energy storage generated from the reversible two-electron redox reaction occurred on anthraquinone subunits.While it still shows a poor electrochemical cycling stability.Afterwards, interlayer hydrogen bonding interaction is introduced into a carbonyl-containing framework to generate a new 2D COF, that is, TpOMe-DAAQ. 61The presence of interlayer hydrogen bonds generates the steric hindrance effect and hydrophobic protection, which thereby significantly improves the electrochemical cycling stability.When applied to supercapacitor, as-made TpOMe-DAAQ electrode demonstrates a hugely improved cycling stability with ca.65% capacitance and near 100% coulombic efficiency retained after 50 000 charge/discharge cycles.To further investigate the influence of ortho-quinone structure on the charge storage capability of COFs, Chen and co-workers synthesized three redox-active 2D COF analogs, that is, 1KT-Tp COF, 2KT-Tp COF, and 4KT-Tp COF, which possess one, two, and four carbonyl groups per their skeleton unit, respectively (Figure 6A). 104Comparative studies demonstrate that the denser carbonyl groups, the higher capacitance.Specifically, 4KT-Tp COF has the highest gravimetric capacitances of 583 F g −1 , higher than 256 F g −1 for 2KT-Tp COF and 61 F g −1 for 1KT-Tp COF.Besides, all the resulting COFs could retain over 90% capacitance after 20 000 cycles at 5 A g −1 , verifying their excellent electrochemical durability.Beyond capacitive energy storage, the potential of carbonyl-containing COFs in rechargeable battery has also been demonstrated.The metal-ion storage mechanism for those COFs is proposed.Upon discharging process, carbonyl O react with electrons to form unstable enol oxygen anions which are further interact with adjacent alkali metal ions.In reverse, during charging process, alkali metal ions detach.Previously, Jiang and co-workers firstly designed a redox-active mesoporous 2D COF with boronate linkage (D TP -A NDI -COF) from naphthalene diimide and used it as cathodes for LIBs in 2015 (Figure 6B). 105During lithiation/delithiation process, naphthalene diimide segments within D TP -A NDI -COF undergo a reversible two-electron redox reaction to store Li + ions.While D TP -A NDI -COF still delivers a lower specific capacity and experiences a weaker chemical stability due to the boronate linkages, compared with the existing polymer cathodes. 106To this regard, the second-generation carbonyl-containing COF cathode with robust imine linkage, corresponding to Tp-DANT-COF, was developed in 2016 (Figure 6C). 107The presence of strong covalent linkages prevents redox-active moieties from dissolving, thus enabling as-resultant COFs with enhanced cycling stability superior to that of COFs with boronate or broxine linkages.To further boost the Liion storage capability, another, a COF-TRO with dense carbonyl groups and highly-ordered crystalline structure had been designed and served as cathodes for all-solidstate LIBs. 74As expected, the resulting COF-TRO cathode could deliver a high specific capacity of 268 mAh g −1 , corresponding to ca. 97.5% of the theoretical capacity.After charging/discharging 100 cycles at 0.1 C, COF-TRO still shows 99.9% capacity retention, presenting an excellent cycling stability.Thereafter, more stable carbonylcontaining COF superior to imine-linked COFs, that is, the third-generation COFs with imide linkage have been exploited.Luo et al. synthesized a microporous imide 2D COF poly(imide-benzoquinone) (PIBN) enriched with 10 redox-active carbonyl groups in each repeat segment, and then used it as cathode for LIBs (Figure 6D). 108onsequently, PIBN cathode delivers a specific capacity of 244.8 mAh g −1 , ca. 87% of its theoretical capacity, suggesting a high-level utilization of carbonyl moiety.Thereafter, an ultrastable polyacrylimide 2D COF (marked as 2D-PAI) was synthesized for LIB cathode, 109 which has extremely long durability with no capacity attenuation even after 8000 cycles.The emerging of 2D-PAI represents a milestone in the development of imide COF cathodes.
Except LIBs, carbonyl-containing COFs also behave as anodes for ZIBs. 110,111For example, Feng and coworkers 112 reported a polyarylimide 2D COF (PI-COF) used as ZIB anode.The highly ordered pore channels benefit the high accessibility of redox-active carbonyl groups as well as the efficient ion diffusion (Figure 6E), which thereby allow a capacity of 92 mAh g -1 at 0.7 A g −1 and a long cycle life for as-constructed PI-COF.Furthermore, Alshareef and co-workers 111 suggested the potential of COFs for Zn-ion storage from mechanistic and practical aspects.They incorporate quinone groups into 1,4,5,8,9,12-hexaazatriphenylene-based 2D COFs (HAQ-COF) that significantly improves Zn-ion storage capability and elevating the average (dis-)charge potential.Assembling HAQ-COF into coin cells with 2.0 M ZnSO 4 as electrolyte, the obtained device achieves a high specific capacity of 344 mAh g −1 at 0.1 A g −1 , and shows remarkable cycling durability with 85% retention of initial capacity after 10 000 cycles at 5 A g −1 .

Free radical-enriched COFs
Organic free radicals possessing unpaired electrons responsible for rapid and reversible redox reaction, have been immobilized onto the pore walls of COF skeletons to boost their energy storage capacity and enhance the rate performance.Thus far, a few of free-radical COFs have been designed and used for active electrode materials, while their development is still in infancy. 113For the first time, Jiang and co-workers 114 modified the channel walls of imine-linked porphyrin 2D COFs (NiP-COF) with different amount of 4-azido-2,2,6,6-tetramethyl-1-piperidinyloxy, that is, TEMPO, to construct series of active radical COFs (Figure 7A).TEMPO radical capable of storing charges relays on the reversible reaction between neutral radical and its oxidation state of the ammonium cation (Figure 7B-D).Based on this, [TEMPO]100%-NiP-COF with 100% TEMPO decoration delivers the highest 167 F g −1 capacitance at 0.1 A g −1 .In addition, Si and co-workers 115 designed a 2D COFs with carbon-oxygen radicals (i.e., COR-Tf-DHzDM-COFs) to serve as dendrite-free alkali metal anodes (Figure 7E,F).Carbon-oxygen radicals with high redox reactivity endow COR-Tf-DHzDM-COFs with enhanced charge storage capability, but sacrificing the stability of skeleton.For now, balancing redox activity and stability is crucial for free radical COFs to achieve optimal overall electrochemical performance in terms of high energy density, excellent rate capability, and long-term lifespan, but it is still in grand of challenge.

Block building chemistry (donor-acceptor COFs)
A good carrier mobility for COFs is important when they are applied to energy storage, while it is still unsatisfied due to the limited π-conjugation.Given that the structures of organic frameworks can be predesigned via the ingenious selection of building blocks, it is highly promising to incorporate donor (D) and acceptor (A) segments into the skeletons alternately, as such to construct conjugated COFs with effective in-plane charge transfer systems and easily tunable both the highest occupied molecular orbital and the lowest occupied molecular orbital levels. 116,117reviously, Gu and co-workers constructed a D-A 2D COF (TTF-COF1) from electron D-type tetraformyltetrathiafulvalene (TTF) and electron A-type DAAQ via imine condensation (Figure 8A). 118The resulting D-A framework shows enhanced intramolecular charge transfer capability and improved redox response (Figure 8B).When used as electrode materials, TTF-COF1 demonstrates a greatly optimized electrochemical properties with a 752 F g −1 high capacitance at 1 A g −1 and a maximum energy density up to 57 Wh kg −1 .Another a polyaryletherbased COF were rationally synthesized from octahydroxyphthalocyanine (D) and hexadecafluorophthalocyanine (A). 119The resulting COF features layered crystalline structures, 1.4-nm small pore sizes, 0.65 eV narrow band gaps, and low resistance.Benefitted from those positive effects, the COF-based in-plane micro-supercapacitor exhibits a volumetric capacitance as high as 28.1 F cm −3 , and an outstanding stability with well-maintained capacitance after 10 000 charge-discharge cycles.The above two works report D-A COFs applied to supercapacitors with aqueous electrolytes.Very recently, for the first time, Van Der Voort and co-workers 120 combined the D-A type TTF-based 2D COFs (TTF-porphyrin COF) with ionic liquid (IL) electrolyte to fabricate COF-based electrical double-layer capacitors with improved electrochemical performance.As expected, the device achieves a high double-layer capacitance of 130 F g −1 , which is attributed to the dense packing of electron acceptor segments in TTF-porphyrin COF skeleton and the micropore-induced pore confinement of IL within framework.
In addition, Wang et al. 121 introduced a D-A 2D COF (TN-COF) coating layer onto CNT to fabricate a porous shell/core hybrid (TN-COF@CNT) used as photoelectrochemical cathodes for ZIB (Figure 8C).The presence of both n-type and p-type redox-active sites characters TN-COF as bipolar-type materials.As a result, TN-COF performs a four-step electrochemical reaction to store both the cation and anion (Figure 8D), thus achieving a high capacity coupled with fast kinetic and wide operating potential.Furthermore, the CNT core ensures the fast elec-tron transfer throughout the hybrid system.That facilitates the electron-enriching -C-O − and -C═N + groups formed for photo-electrochemical response, hence enabling asdesigned TN-COF@CNT hybrid capable of being used for solar-to-EES.

POLYGON SKELETON (TOPOLOGICAL STRUCTURE) OF COFs (2D COF vs. 3D COF)
Thus far, most of COFs used as electrode materials for EES applications are in 2D topology.The development of 3D COFs used for EES is restricted by their limited building blocks, rigid synthesis conditions, and poor stability due to the empty frameworks and the absence of π-π stacking. 122espite those shortcomings, 3D COFs feature diverse pore structures, interconnected channels, high surface area, low density, and well-exposed functional moieties.They are amazing platforms for designing high-performance electrode materials.Previously, Huang and co-workers 123 prepared highly conjugated 3D COFs from tetrahedral tetraamine and terephthalaldehyde with spirobifluorene core structure as building blocks (Figure 9A).As-resultant 3D COF (3D-sp-COF) has narrow size distribution ranged at 500-700 nm and large surface area of 1016 m 2 g −1 .When applied to supercapacitor, 3D-sp-COF delivers a specific capacitance of 251 F g −1 at 0.5 A g −1 , and exhibits an enhanced capacitance of 316 F g −1 after 8000 cycles at current density of 3 A g −1 (Figure 9B).Of note, upon cycling process, the increase in specific capacitance is attributed to the wetting activation of hollow spheres, which further promotes the effective transportation of electrolyte ions within the ordered channels of frameworks.
In addition, Jiang and co-workers 124 combined eightconnected pentiptycene-based knot (DMOPTP) with fourconnected square-planar linkers to prepare two 3D COFs, that is, 3D-scu-COF1 and 3D-scu-COF2.For both frameworks, DMOPTP with 3D rigid quadrangular prism structure dominates a twofold interpenetrated scu 3D topology formed, which thereby ensures a high-connected per-manent porosity as well as large specific surface area.Moreover, the extended π-electron delocalization over the whole framework allows high conductivities ranged at 3.2-3.5 × 10 −5 S cm −1 for the resulting 3D COFs (Figure 9C).As a result, when served as cathodes for lithium-sulfur batteries (LSBs), both 3D COFs show high capacities ranging 1035-1155 mAh g −1 at 0.2 C, meanwhile come with good rate capability and impressive cycling stability with 71%-83% capacity retention after 500 cycles at 2.0 C. Such electrochemical performance outperforms most of organic LSB cathodes reported before (Figure 9D).

CRYSTALLINE STRUCTURES (AGGREGATION STRUCTURE) OF COFs
For electroactive COFs, most of their redox sites are stacked and buried deep in the long-range ordered framework.That inevitably causes the insufficient utilization, sluggish ions diffusion, and low actual specific capacity for COF-based electrodes.As a matter of fact, the accessibility and utilization of redox-active sites in bulk COFs are highly two-(2D), and three-dimensional (3D) nanomorphologies for covalent organic frameworks (COFs).Copyright, 2022 American Chemical Society. 125lated to their crystal nanostructures.Currently, COFs with diverse-dimensional crystalline structures have been designed, including zero-(0D), 1D, 2D, and 3D nanomorphologies (Figure 10). 125In this part, we will move to discuss the electrochemical behavior of different COF electrodes from crystalline nanomorphology perspective.

0D COF particle or hollow sphere
7][128][129] In this case, the defects and boundaries between particles restrict the electron transfer and ion transportation, that makes COF particles suffer a sluggish electrochemical response. 96For example, the DAAQ-TFP COF crystalline particles reported by DeBlase et al. showed a very poor rate performance when used as supercapacitor electrode. 68In a sharp contrast, well-defined COF hollow spheres with interconnected interior space, which is usually derived from highly conjugated building blocks via self-template strategy, have large surface area and high porosity, which are beneficial for effectively exposing more accessible active sites and shortening pathways for ion mobility.
In the field of rechargeable battery aspect, hollow spherical COFs demonstrate their great promising.Before, Zhang and co-workers synthesized porphyrin-based COF hollow spheres (marked as POF-HSs), which had been used as sulfur cathode for LSBs. 130Due to the polar chemical structure-induced adsorption and the hollow spherical morphology-dominated physical confinement, POF-HSs can sufficiently mitigate polysulfides shuttle, which thereby allows the corresponding LSB device hav-ing good rate capability and long cycling life.In addition, Paek and co-workers 131 reported a COF hollow nanosphere used as anode for SIBs, which could show a reversible capacity of 426.2 mAh g -1 at 0.1 A g -1 and maintain stable electrochemical performance after 2600 charge/discharge cycles.Furthermore, another SiO 2 -templated COF hollow nanospheres (TP-COF HSs) were synthesized and used for KIB anode (Figure 11A). 132Integrating the merits of abundant channels for charge carrier transportation, robust structure, highly exposed active sites, and wettability features, the templated TP-COF HSs perform excellent K-ion storage behavior, showing a stable capacity of 203 mAh g -1 and well-maintained coulomb efficiency near 100% after 500 cycles at 300 mA g -1 .

1D COF nanofiber
1D COF nanofibers with large specific surface area, highly porosity, and good formability, are promising electrode materials for EES techniques.Before, Wu and co-workers developed a 2D COF nanofiber (TPA-COF) used as supercapacitor electrode, which could achieve a high specific capacitance of 263.1 F g −1 at 0.1 A g −1 and retain 111% capacitance retention after 5000 charge-discharge cycles. 133hereafter, our research group further compared the electrochemical behaviors of COF nanofibers with that of COF particles and hollow spheres. 134Compared with COF particles and hollow spheres, COF nanofibers have larger BET specific surface area, higher total pore volumes, and more regular built-in vertical aligned channels.All the positive effects enable as-prepared COF nanofibers delivering the highest specific capacitance up to 235 F g −1 at 0.5 A g −1 and remarkable cycling durability with over 80% retention of initial capacitance after 10 000 cycles.Furthermore, another two-novel olefin-linked 2D COFs with well-defined nanofibrillar morphology, with diameter of ca.80 nm and length up to several micrometers, had been developed and further processed into interdigital microelectrodes for planar micro-supercapacitors (Figure 11B). 135As a result, the obtained device could deliver a areal capacitance of 44.3 mF cm −2 at a wide operating potential of 2.5 V, as a result achieving a high volumetric energy density reaching 38.5 mWh cm −3 .

2D COF nanosheets
Resembling the exfoliation of graphite, 136 bulk 2D COFs with stacked layer structure can also be exfoliated into few-layer CONs.As comparison to the original 2D COF bulk, the exfoliated CONs are featured with more exposed redox-active functionalities, shortened ion diffusion distance, and enhanced transfer kinetics, giving rise to deliver better electrochemical properties.For now, the strategies employed for 2D COF exfoliation mainly include mechanical exfoliation, liquid-assisted exfoliation, and chemical exfoliation (Figure 11C).All the approaches have been demonstrated their high efficiency in exfoliating bulk COFs into CONs. 137A typical mechanical strategy, namely, ball milling, has been widely applied to exfoliate bulk COFs.Previously, Wang et al. successfully exfoliated CONs from redox-active DAAQ-COF via mechanical ball-milling treatment. 138As-resultant CONs have an average thickness of ca. 5 nm, which thereby benefits short Li + diffusion pathways, fast kinetics, and high utilization efficiency of redox sites for the CONs.Specifically, the obtained CONs have a Li + diffusion coefficient of 6.94 × 10 −11 cm 2 s −1 much higher than 2.48 × 10 −11 cm 2 s −1 for pristine COFs, an enhanced specific capacity of 210 mAh g −1 , and a good cycling stability with retaining 98% capacity retention after 1800 cycles.Additionally, an atomic-layer COF (i.e., E-TP-COF) exfoliated by ball milling strategy also was developed (Figure 11D), 139 which features short Li-ion diffusion distance with low energy barrier and structure stability.When being used as cathode for LIBs, E-TP-COF shows a high capacity of 110 mAh g −1 and can maintain 87.3% capacity retention after 500 cycles.
Liquid sonication is another efficient and energy-saving strategy for the exfoliation of 2D COFs to fabricate CONs.Before, polyimide COFs (PI-COFs) with lamellar structure was successfully exfoliated into 2D polyimide nanosheets (PI-CONs) with a thickness of ∼1.2 nm and a large size of ∼6 μm via liquid sonication treatment (Figure 11E). 140uch thin and large PI nanosheets demonstrate their great potential as new sulfur host materials for LSB, which shows a high capacity of 1205 mAh g -1 at 0.1 C, remarkable rate capability with retaining 503 mAh g -1 at 4 C, and good cycling stability with 96% capacity retention at 0.2 C.
There are another series of exfoliated CONs with thickness of ca.20 nm prepared via a facile, scalable, and mild chemical exfoliation method (Figure 11F). 141The chemical treatment has negligible influence on the framework structure except facilitating the exfoliation effect.When assembling as-resultant CONs into supercapacitor cell, the device achieves a areal capacitance of 5.46 mF cm −2 at 1 V s −1 , high power density of 55 kW kg −1 , and excellent cycling stability with almost 100% capacitance retention even after 10 000 cycles.In addition, Chen et al. applied a chemical stripping strategy, which is, introducing MnO 2 nanoparticles in situ intercalated into frameworks to prevent the re-stacking and aggregation of nanosheets, to fabricate few-layered CONs (marked as E-1, 2, 4, 5-Tetrakis-(4-formylphenyl) benzene [TFPB]-COF). 137enefitting from the few-layer structure of E-TFPB-COF and the synergetic effect of MnO 2 nanoparticles intercalator, their composite used as anode could deliver a high capacity reaching 2423 mAh g −1 .After 300 cycles at 0.1 A g −1 , a reversible capacity of 1359 mAh g −1 can be well maintained.Except in situ intercalation, post-insertion treatment is also employed for the production of fewlayer CONs.For example, Haldar et al. 142 synthesized two anthracene-based COFs, both are further subjected to a thermal Diels-Alder reaction with maleic anhydride.In this way, the presence of maleic anhydride can generate steric obstruction between sublayers, which thereby weakens the layer interactions and expands layer distance, giving rise to assist the chemical exfoliation process.As a result, anthracene-based COFs are successfully exfoliated into ultrathin nanosheets with different thickness values ranging at 2-5 nm.When used as anode electrodes, the thinner nanosheets obtained can deliver a steady capacity of 790 mAh g −1 after 280 cycles at 100 mA g −1 , higher than 580 mAh g −1 capacity for the relative thick nanosheets, verifying the superiority of thin CONs for charge storage.

3D COF flower
Compared with 0D COF particles or sphere, 1D COF nanofiber, and 2D COFs nanosheets, 3D COF flowers with hierarchical superstructures are newly emerged intelligent electrode materials, due to their ultrahigh specific surface area, low density, and high loading capacity. 143,144reviously, a 3D hierarchical porous COF microflower (COF-MF, Figure 11G) stacked by porphyrin-rich conjugated ultrathin nanosheets had been synthesized and served as a sulfur host for LSBs with high stability and sulfur loading. 145As-obtained COF-MF combines unique macro-, meso-, and micro-pores, presenting a superstructure with minimum nanosheet stacking and large accessible surface area.That maximizes the chemical adsorption of polysulfides and promotes the sulfur sufficient utilization for COF-MF.Furthermore, the corresponding LSB assembled from COF-MF exhibits an appealing areal capacity with 4.78 mAh cm −2 at a sulfur loading of 4.1 mg cm −2 , and superb cycling durability with only 0.047% decay rate after 1000 cycles at 1 C. Another 3D COFs with rod-flower morphologies (COFs-R) had been developed via using solvent effects combined with pre-polymerization steps to control the kinetic parameters. 143The resulting COFs-R used as supercapacitor electrode can offer a specific capacitance of 486.3 F g −1 , meanwhile showing outstanding cycling stability with 93.2% capacitance retention after 10 000 cycles at 0.5 A g −1 .Additionally, an all-in-one dioxinbased COF microflower (COF-316) composed by interconnected hollow petal had been prepared via a self-template strategy. 144And further applied to construct a flexible and transparent supercapacitor.The unique microflower structure of COF-316 benefits the corresponding device achieving an areal specific capacitance of 783.6 μF cm -2 at 3 μA cm -2 and long-term cycling stability.

COMPOSITE MICROSTRUCTURES OF COFs
For pure COF electrode materials, they usually suffer urgent matters of poor conductivity and low utilization of active sites, as a result showing inferior rate performance and poor power/energy density when they are applied to EES applications.To this regard, currently, COFs have been combined with highly conductive carbon additives, including 1D CNTs, 2D graphene, or 3D carbons.The carbon additives introduced serve as conducting skeleton to support the uniform growth of COF.That generates carbon-supported COF composites with enhanced electrical conductivity, larger specific surface area, and more redox-active exposed.All the positive effects allow COF/carbon composites capable of achieving higher rate performance and better charge storage capability superior to the pure COF electrodes.

1D CNT-supported COF composites
For now, CNTs have been widely used as conducting skeleton to support the uniform coating of diverse COF materials to prepare advanced composites for EES. 146or example, pyrene-4,5,9,10-tetraone COF (PT-COF) had been introduced onto CNT surface to yield a core-shell tube microstructure which showed enhanced conductivity and increased redox-active groups exposed. 147With the CNT loading content up to 50 wt%, PT-COF can still maintain the ordered structure with hexagonal-shaped pores on CNT surfaces.When as-synthesized CNT-supported PT-COF microstructure applied as a LIB electrode, each pyrene-4,5,9,10-tetraone unit on PI-COF involves an overall 4e -/4Li + redox process, that allows PT-COF/CNT composite electrode achieving 98% utilization of the redox functionalities (vs.71% for pure PT-COF).Accordingly, the composite electrode shows an ultrahigh capacity of 280 mAh g -1 at 200 mA g -1 and excellent rate performance with retaining 82% capacity at high current of 5 A g -1 , 45% and 2.1 times higher than those of pure PT-COF, respectively.In addition, Wang and co-workers 148 designed a CNT-supported few-layer COF (COF-10@CNT) for K + storage (Figure 12A,B).Benefited from the thin coating providing more active sites exposed and the π-cation interaction between π-conjugated benzene rings and K + ions, COF-10@CNT composite exhibits high K-ion storage with a stable capacity of 288 mAh g -1 after 500 cycles at 0.1 A g -1 , much larger than that (57 mAh g -1 ) for COF-10.Another, Wu and co-workers 149 conducted the in situ condensation of anhydride and amine in the presence of single-wall CNT (SWCNT), as a result coating a PI-COF layer onto SWCNT.
The synergistic effects between PI-COF providing numerous active sites and SWCNT ensuring fast electron transfer, allow a specific capacity of 438 mA h g −1 and excellent cyclic stability for the resulting polyimide COF@SWCNT composites when they were used as advanced anode for PIBs.

2D graphene-supported COF composites
Graphene has been combined with COFs not only to enhance the electrical conductivity, but also to modulate the microstructures of their composites.Before, Talyzin and co-workers 150 realized a flexible modulation in the microstructures of 2D-2D COF-graphene hybrid materials, from the thick COF platelets parallel to the surface of graphene oxide (GO) to the forest of ultrathin CONs perpendicular to the GO surface (Figure 12C).In addition, Chen and co-workers 108 in situ polymerized a 2D microporous polyimide COF (PIBN) in the presence of graphene to develop a composite electrode material, that is, PIBN-G.In this case, graphene nanosheets induce PIBN thin crystalline nanosheets grown along their surfaces, which is driven by π-π interaction.Such face-to-face thin crystalline nanosheet stacks favor the charge transfer between graphene and PIBN, which thereby facilitates the full access of both electrons and Li + to the abundant redoxactive sites.When served as a cathode material for LIB, the resulting PIBN-G can perform a two-step redox action which involves 10 Li + ions.That generates a large specific capacity of 271.0 mAh g −1 (corresponds to a utilization of 96.8%) at 0.1 C, meanwhile come with fast kinetics and long cycle life with coulombic efficiency of ∼100% after 300 cycles.That outperforms pure PIBN which has the specific capacity of 244.8 mAh g −1 (with a lower utilization of 86.1%).Based on above, the 2D microporous framework with abundant accessible carbonyls, combined with the strong interaction between PIBN and graphene, allow the high specific capacity, capacity retention, and good rate capability for PIBN-G.

3D carbon-supported COF composites
Developing free-standing 3D macroscopic object from COFs is of great significance but still in grand of challenge toward their development in wearable EES techniques.For this, previously, Thomas and co-workers 151 constructed an ultralight and self-supported TpDq-COF/graphene composite aerogel via a hydrothermal approach followed by frozen drying.The obtained composite aerogel can be directly used as a free-standing supercapacitor electrode, which delivers a high capacitance of 269 F g −1 at 0.5 A g −1 (vs.22.4 F g −1 for TpDq-COF) and well retain the capacitance after 5000 cycles.The high specific capacity and good cycling stability of the COF/rGO electrode is attributed to a synergistic effect of rGO providing conductivity and the COF providing a high surface area and redox-active sites, thus increasing the double layer and pseudocapacity, respectively.Thereafter, our group designed a carbon/rGOsupported COF composite foam (COF product obtained from DHA and TAB units [DAB]/graphene-wrapped carbon foam [GCF]) by in situ growing COF crystallites along the carbon skeleton surfaces (Figure 12D,E). 152The resulting composite foam used as self-supported electrodes for supercapacitor, can deliver a high capacitance of 129.2 F g −1 at 0.5 A g −1 , 47 times higher than that of DAB-COF (2.7 F g −1 ).Furthermore, there is no obvious decline in capacitance after 20 000 cycles at 10 A g −1 , indicating an excellent long cycling stability.Impressively, the composite foam shows good compressibility and fatigue tolerance due to the presence of strong carbon skeleton.That further allows as-made composite foams capable of being assembled into flexible supercapacitors.Consequently, the corresponding devices show stable charge storage performance even subjected to serious compressing or bending, confirming a great potential in wearable devices.Based on the results, we suggested that the carbon skeletonsupported the uniform coating of redox COFs, allows a combination of hierarchically porous structure, good electrical conducting balanced with mechanical robustness and elasticity for the DAB/GCF composite foam, which thereby displays a high capacitance, good energy-powder density integration, as well as outstanding cyclic stability.
Beyond carbon aerogel, carbon cloth is also employed to supporting COF coating.Hu et al introduced redoxactive COFs onto carbon cloth with strong covalent bond linkage. 153For this, DAAQ pillars were embedded on the carbon fiber surface firstly, which is aimed at controlling the evolution of frameworks.In this way, tentacle-like COF (DAAQ-Tp COF) arrays are grown along the carbon fiber surfaces vertically (Figure 12F,G).Due to the 3D hierarchical structure beneficial for effective electron transfer, stabilizing COFs structure, as well as promoting the active sites to be exposed and accessible, as a result, the carbon cloth-supported COF arrays achieve a specific capacitance as high as 1034 mF cm -2 (almost 50 times that of DAAQ-Tp COFs), meanwhile showing a good cycling durability with 98% capacitance retention after 20 000 cycles.Such electrochemical performance is ascribing to that the inherent porosity of COFs and abundant defects on the carbon cloth surface which provide the shortest conductive path for ions/electrons.In addition, the interconnected hierarchical holes and tentacle-like arrays increase surface wettability which further facilitates the ion transportation at electrode/electrolyte interface.

CONCLUSIONS AND PERSPECTIVES
In view of high surface area for rich reaction centers, large porous volume for accessibility, tunable pore size for mass transfer/diffusion and covalently-linked stable framework for long lifespan, COFs are promising candidates as active electrode materials for energy storage devices such as rechargeable batteries and supercapacitors thus far.In particular, the pre-designable feature of COFs allows the construction of frameworks with custom-made charge storage behavior via the ingenious choose of building blocks combined with suitable linkage.In this review, we summarize recent advances on COFs regarding their working principle, synthesis method, and effective design strategies in energy storage applications including supercapacitor and various metal-ion battery.Furthermore, the current research frontiers on the structure engineering of COFs for boosting the EES are presented, mainly focusing on the modulation in linkage, redox moiety, and building block, all of which are in high relation to the electrochemical behavior of COFs.Another factors such as polygon skeleton with different dimensionality (2D vs. 3D) and crystalline nanostructures (referring to the aggregative state), and their corresponding effects on EES capability of COFs, are also studied.Additionally, COFs combined with various conductive additives to fabricate advanced composites with different microstructures and improved electrochemical performances are summarized.Overall, the in-deep understanding of structure-property correlations provides useful feedback on the design of frameworks to explore high-performance redox-active COFs.Despite the outstanding advantages and great potentials of newly emerged COFs for energy storage, the future for COFs with custom-made structure and desirable functions toward practical energy storage application still faces several important challenges.First, most of redox-active COFs reported so far are mainly constructed by combining redox-active and inactive building blocks.That renders a low theoretical capacity for COFs which is usually less than that of the redox-active building block in monomer form.Therefore, efficient molecular design that contains a high density of redox-active sites to maximize the redox activity of COFs while maintaining physical and chemical stability is necessary.Second, the low electronic conductivity is always a critical flaw to overcome for COF-based electrodes.COFs designed with an extended π-conjugated skeleton and strong interlayer orbital stacking, meanwhile coupled with dense redox-active sites, are desirable and expected to exhibit superior electronic properties and rate performance to compete with the state-of-the-art electrode materials.Third, most of assynthesized COFs have partial disordered stacking and defects in their structures, which impede the long-range ordered pathways formed and thus restricting the efficient charge and mass transportation.Therefore, constructing COF single crystals with high quality and crystallinity is of special importance, while remains a major challenge as the lack of a general method or protocol.Fourth, although COFs possess a uniform porous structure, the nanochannels formed by the aligned COF stacks exhibit poor ions mobility.That limits the utilization of redoxactive sites.Exfoliation of COF into nanosheets has been demonstrated efficient to improve the accessibility of the redox-active sites within the frameworks.While the deep understanding of the interactions between electrons with mono-, few-, multi-layer or bulk COFs is of significance.Fifth, considering the pores and nanochannels confined within the void space of COFs.Modifications to this space by incorporating other substances may be more efficient and favorable to enhance the electrochemical properties, which needs further investigation.Based on above, we envision that, the detailed investigation of recent advances, proposed promising opportunities, and key challenges, all together would help in accelerating the development of COFs for advanced energy storage applications.

A C K N O W L E D G M E N T S
This work was financially supported by the Hubei Provincial Natural Science Foundation of China (2022CFB555) and the Open Project of State Key Laboratory of New Textile Materials and Advanced Processing Technologies (FZ2021003).

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

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I G U R E 1 (A) Topology diagram presenting the polygon skeletons of covalent organic frameworks (COFs) guided by building blocks.(B) Diverse reactions adopted and the corresponding different linkages for COFs.(C) Stacking manner of 2D COFs (left, Copyright, 2022 American Chemical Society 42 ) and 3D COFs (right, Copyright, 2022 American Chemical Society 43 ).

F I G U R E 4
Various redox-active groups for covalent organic frameworks (COFs).

F I G U R E 5
(A) BQ1-COF as a cathode material for Li-ion batteries (LIBs).Copyright, 2020 Elsevier.99(B) Two-step sodiation/desodiation electrochemical behavior for triquinoxalinylene and benzoquinone units (TQBQ)-COF analyzed by molecular electrostatic potential (MESP) method.Copyright, 2020 Springer Nature.100(C) Mg-ion storage mechanism and (D) the corresponding redox equations of organic covalent organic framework (COF) cathodes in chloride-containing and chloride-free electrolytes (above), and the long-term cycling performance of as-assembled device (below).Copyright, 2020 American Chemical Society.101(E) The NH 4 + ion storage mechanism in QA-COF COF.(F) Cyclic stability of QA-COF COF.Copyright, 2021 American Chemical Society. 102

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I G U R E 8 (A) Synthetic scheme of TTF-COF1.(B) The charge transfer in TTF-COF1.Copyright, 2020 The Royal Society of Chemistry. 118(C) Schematic diagram of TN-COF@CNT applied to solar-to-electrochemical energy storage.(D) Scheme representation of dual-ions storage electrochemistry of the TN-COF@CNT cathode in a Zn-organic battery.A continuous four-step electrochemical reaction is involved.Copyright, 2022 Wiley. 121CNT, carbon nanotubes; COF, covalent organic framework; TTF, tetraformyl-tetrathiafulvalene.

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I G U R E 9 (A) Schematic illustration for the synthesis of 3D-Sp-COF.(B) Long-term cycling durability of 3D-Sp-COF at 3 A g −1 in 6 M KOH electrolyte.Copyright, 2020 Springer Nature. 123(C) Schematic illustration of 3D-scu-COF with good conductivity and high connectivity for Li-S battery.(D) Comparison of the performance of 3D-scu-COF1 and 3D-scu-COF2 to other COF-based cathodes in lithium-sulfur batteries (LSBs).Copyright, 2022 American Chemical Society. 124COF, covalent organic framework.

F I G U R E 1 1
(A) Schematic illustration of TP-COF hollow nanospheres and the charge storage mechanism.Copyright, 2023 Elsevier.132(B) Covalent organic framework (COF) ultralong nanofibers for micro-supercapacitor with high power supply.Copyright, 2020 Elsevier.135(C) Schematic diagram of the synthesis routes for COF nanosheets (CONs).Copyright, 2020 The Royal Society of Chemistry.20 (D) E-TP-COF thin nanosheets as cathodes for Li-ion battery (LIB).Copyright, 2021 Wiley.139(E) Construction of ultrathin polyimide (PI)-CONs materials and their corresponding electrochemical processes.Copyright, 2021 American Chemical Society.140(F) The different charge storage behaviors of COFs and exfoliated COFs (e-COF).Copyright, 2020 Wiley.141(G) Schematic illustration of the preparation of COF-MF, and its featured structure for energy storage.Copyright, 2019 Elsevier.145

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I G U R E 1 2 (A) COF-10@CNT for potassium storage.(B) Transmission electron microscopy (TEM) image of COF-10@CNT showing a core-shell microstructure.Copyright, 2019 American Chemical Society. 148(C) Schematic diagram for COF1 nanosheets vertically grafted on graphene oxide (GO) and platelets parallel to GO. Copyright, 2017 Wiley. 150(D) Synthesis route for COF product obtained from DHA and TAB units (DAB)/graphene-wrapped carbon foam (GCF) and (E) scanning electron microscopy (SEM) images showing uniform coating of DAB-COF on 3D carbon skeleton.Copyright, 2022 Elsevier. 152(F) Schematic diagram of the functionalized AC-COFs.(G) Schematic illustration of electron transport in AC-COFs.(H and I) SEM images of AC-COFs.Copyright, 2022 American Chemical Society. 153CNT, carbon nanotube; COF, covalent organic framework.

COFs materials Linkage Active site Dimensionality Capacitance/capacity Electrode type Ref.
TA B L E 1 Diverse covalent organic frameworks (COFs) served as electrode materials for supercapacitors and metal-ion batteries.