Research progress on metal and covalent organic framework-based materials for high-performance supercapacitors

WANG Shuai, GUO Yu-zhe, WANG Fang-xiao, ZHOU Sheng-hu,ZENG Tian-yu, DONG Yu-bin

（College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, China）

Abstract： Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are both a series of crystalline porous materials.MOFs, COFs and their derivatives have attracted much attention in energy storage devices due to their highly ordered structures, large surface areas, tunable pore sizes and topologies, and well-defined redox-active porous skeletons.They must also have structural stability, an abundance of redox-active sites and high electronic conductivity for use in high-performance supercapacitor electrodes.We review the recent research progress on the design of MOFs and COFs, and their hybrids with conductive materials (e.g.conductive polymer, graphene and carbon nanotubes), and MOF- and COF-derived carbon materials.Their chemical and physical properties, capacitive performance and structure-property relationships are discussed.Finally, the challenges and prospects of MOF- and COF-based electrode materials are presented.

Key words: Metal-organic frameworks；Covalent organic frameworks；Hybrid；Derived carbon；Supercapacitors

1 Introduction

Supercapacitors (SCs) are attractive, clean and environmentally friendly storage devices owing to their fast charging-discharging feature and long cycle life[1].SCs have higher power densities but lower energy densities than batteries[2].The electrode materials are critical in producing high-performance SCs with high specific capacity, high voltage, low weight,excellent rate performance and excellent cycling performance.The physicochemical properties of electrode materials have a significant impact on SC performance.

SCs are usually divided into electrical doublelayer capacitors (EDLCs) and pseudocapacitors according to the energy storage mechanism.EDLCs store energy via fast electrosorption of electrolyte ions at the electrode surface, whereas pseudocapacitors store charge via fast and reversible surface and nearsurface redox reactions[3].Electrode materials with tailored porosity adapted to the size of electrolyte ions are required for EDLCs.Multitudinous porous carbon materials, which have high specific surface area and suitable surface polarity, are examples of such materials[4].The capacitance can be enhanced by introducing redox-active sites (e.g.conductive polymers,transition metal oxides or hydroxyls and heteroatoms)for the EDLCs by pseudocapacitive charge storage[5].The following aspects are required in designing electrode materials for SCs: (1) conductive frameworks that facilitate electron transport between the electrolyte and electrode; (2) open channels with suitable pore size to accelerate ion transport, which is necessary for the high-rate performance; (3) excellent stability to ensure that the structure will not collapse during multiple oxidation-reduction reactions and will not dissolve or be etched in organic and aqueous electrolytes; (4) large surface area and abundant exposed electrochemical active sites that provide high chargestorage capacity[6].In this regard, redox-active crystalline porous materials, such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs)and their derivatives, have attracted much attention in energy storage devices[7,8,9].

MOFs are an attractive class of crystalline porous coordination materials built by organic ligands and metal ions or clusters[10].MOFs have been extensively studied in electrochemical energy storage andconversion devices owing to their highly ordered structures, large surface areas and tunable pore sizes and topologies[11].Transition metal ions or clusters and redox-active conjugated ligand molecules can act as redox-active sites in electrochemical reactions.However, most pristine MOFs are insulators or semiconductors and unstable in the presence of heat or humidify[12].MOFs have both metal and carbon sources,in addition to standard morphologies, making them ideal templates/precursors for electrode functional materials[13].MOF-derived materials possess higher chemical stability and electronic conductivity, whose size, morphology, structure and other features can be well controlled, inherited and preserved by rational design[14].Well-defined MOF and MOF-derived materials with two-dimensional (2D)[15], three-dimensional 3D[16], hollow[17]and bimetallic[18]structures have been widely employed for energy storage.

COFs are a class of crystalline porous organic materials, which are constructed using organic building units via strong covalent bonds[19].COFs show ordered open channels and well-defined redox-active porous skeletons based on various building blocks,making them promising materials for energy storage material designs[20].Moreover, COFs demonstrate high gravimetric storage performance in energy storage devices due to the linkage of lightweight elements by robust covalent bonds.The strong covalent bonds in COFs make the frameworks stable in various electrolytes[21,22].Furthermore, many COFs with π-conjugated structures exhibit modest electronic conductivity[23].Recently, various COF composites and COF-based carbonaceous electrode materials with controllable porosities have been developed to improve their conductivity[24,25].

MOFs, COFs and their derivatives have attracted much attention in SCs due to their highly ordered structures, high surface areas, tunable pore sizes and topologies as well as well-defined redox-active porous skeletons.However, the biggest disadvantage of these materials as SC electrode materials is their low conductivity.Therefore, many efforts have been made to exploit the advantages and make up for the disadvantages[7].In this review, we introduce MOFs- and COFs-based materials as electrodes for SCs(Fig.1).The design strategy of MOFs and COFs based materials include pristine MOFs and COFs, the hybrid of MOFs or COFs with conductive materials(conductive polymer, graphene, carbon nanotubes),MOFs and COFs derived carbon materials.Their capacitive performance is emphasized.Finally, some suggestions on the development of MOFs- and COFsbased electrode materials are given.

2 MOFs-based materials for SCs

2.1 Pristine MOFs for SCs

2.1.1 Traditional MOFs

Traditional MOFs have great potential in SCs as electrode materials owing to their large surface area,adjustable pore size and rich redox active sites.Their high surface area ensures that the contact surface area between electrodes and the electrolyte is high, and that the metal ions can contribute to pseudocapacitance.

Ni-based MOF materials were studied earlier and widely used as electrode materials for alkaline SCs due to their unique structures with conjugate π bonds,superior electrolyte penetrability, and low steric hindrance[26-29].To improve the electrochemical performance of Ni-MOFs, MOFs with 2D and 3D macrostructures are designed to increase their active surface area.The 2D-layered Ni-MOF of [Ni3(OH)2(C8H4O4)2(H2O)4]·2H2O (C8H4O4=p-benzenedicarboxylic acid)was prepared in a N,N-dimethylacetamide (DMF)solution via the solvothermal method for high-performance SCs, which was achieved by the synergistic effect between Ni-MOF and Fe(CN)64?/Fe(CN)63?.Fe(CN)64?/Fe(CN)63?acts as an electron relay during the charge/discharge process by coupling Ni(II)/Ni(III) in the Ni-MOF electrodes.Moreover,the alkaline battery-supercapacitor hybrid device(NiMOF//CNTs-COOH) was assembled in 3 mol·L?1KOH and 0.1 mol·L?1K4Fe(CN)6.This mixed electrolyte shows an extended voltage window of 1.4 V,which resulted in a high energy density of 55.8 W h·kg?1at a power density of 7 000 W·kg?1.The unique 2D-layered crystal structure of Ni-MOF can provide ample space for mixed electrolyte ions[30].Yan et al.prepared a new-type of 3D accordion-like Ni-MOF superstructure of [Ni3(OH)2(C8H4O4)2(H2O)4]·2H2O as an electrode material for SCs.The Ni-MOF electrode showed a high specific capacitance of 988 F·g?1and an excellent capacitance retention rate of 96.5% after 5 000 cycles at 1.4 A·g?1in 3.0 mol·L?1KOH.More importantly, the accordion-like Ni-MOF and activated carbons were assembled into a high-performance flexible solid-state asymmetric supercapacitor (Ni-MOF //activated carbon).This device shows a specific capacitance of 230 mF·cm?2at a current density of 1.0 mA·cm?2, which can offer 92.8% capacity of the initial capacitance at 5.0 mA·cm?2after 5 000 cycles.The device can achieve a maximum energy density of 4.18 mW h·cm?3and a maximum power density of 231.2 mW·cm?3.Its excellent performances are due to a large number of nanochannels created by the nanosheet structures, which could provide a stable and porous framework for the electron/ion transport and ion insertion/detachment[31].

Cobalt-based MOFs are another type of SC electrode materials with great potential[32].Liu et al.synthesised a 2D-layered Co-based MOF (Co-LMOF)electrode for SCs with a specific capacitance of 2 474 F·g?1(1 A·g?1) and 1 978 F·g?1(2 A·g?1) in 1 mol·L?1KOH (Fig.2a, b)[33].In addition, after 2 000 cycles, the specific capacitance reached 94.3%(2 A·g?1) of the initial capacitance.The 2D-layered structure feature is favorable for the contact of electrolyte and active materials in the electrode, which could improve Faradaic reaction efficiency[34].The nanosize of Co-LMOF particles provides more exposed active sites for Faradaic reaction, which could also decreases the electrolyte ion diffusion distance due tothe increased surface area.The electric double-layer capacitance, which originates from the charge separation at the electrode-electrolyte interface, also contributes to the high specific capacitance.Introducing Na or Ni into Co-MOF framework is a remarkable strategy for high-performance SCs[35,36,37].More recently, due to the similarity of atomic radii and chemical valence states between Co and Ni, Ren et al.synthesised a series of compositionally adjustable bimetallic Ni/Co metal-organic skeleton materials using the one-solvent method by adjusting the initial ratio of Co2+to Ni2+(Fig.2c)[37].The Ni/Co-MOF retained the stability and electrical conductivity of Co-based MOF materials and demonstrated outstanding electrochemical performance with a high specific capacitance(1 230.3 F·g?1at 1 A·g?1), excellent rate capability(87.0%, from 1 to 10 A·g?1) and excellent cycle stability (80%, 4 000 cycles at 10 A·g?1).Besides, the assembled solid-state SC using the Ni/Co-MOF as the positive electrode and activated carbon as the negative electrode exhibited an ultrahigh energy density of 116 W h·kg?1at a power density of 0.795 kW·kg?1and superior stability with 92.1% initial capacity retention, even after 6 000 cycles at 10 A·g?1.These excellent results are attributed to synergistic contributions of the stability and electrical conductivity of Cobased MOF materials and the ultra-high theoretical capacitance of Ni-based MOF materials.

In addition to the frequently used metals Ni and Co, there are also Zn-, In-, Zr-based MOFs as feasible electrodes for SCs[38-40].Shinde et al.reported a series of [Mn(BDC)·nDMF]n(Mn-MOFs) with 2D-layered structure using the hydrothermal method at various temperatures for SCs[41].The obtained Mn-MOF electrode (140 °C) showed a high specific capacitance of 10.25 F·cm?2at 1 A·g?1in 2 mol·L?1KOH electrolyte, which shows a capacity retention of 92.3% over 5000 cycles.Moreover, the fabricated hybrid SC (Mn-MOF//rGO) demonstrated specific and volumetric capacitances of 211.37 F·g?1and 3.32 F·cm?3, respectively.The device showed a specific energy of 66 W h·kg?1at a specific power of 441 W·kg?1, with a capacity retention of 81.18% over 10 000 cycles.Their excellent performances were attributed to synergistic contributions from both the electrodes with enhanced electric conductivity and high specific surface area.The various inherent features of the Mn-MOF, including its layered structure,open space for efficient electrolyte access, and short ion diffusion pathways, might account for its excellent electrochemical performance.Moreover, the Mn species and H2BDC linker have been shown to be cost-effective and excellent energy storage materials for SCs.

2.1.2 Conductive MOFs

Pure MOFs have potential applications in SCs due to their high porous structure.However, most MOFs have low conductivity, limiting their applications.One of the most effective ways to improve the conductivity of MOFs is to design and develop conductive MOFs (c-MOFs).For example, Sheberla reported a novel c-MOF of Ni3(HITP)2with a high conductivity (5 000 S·m?1) as an electrode material for EDLCs, which shows a capacitance retention rate of over 90% after 10 000 cycles.However, it showed moderate gravimetric (ca.100 F·g?1) and areal (ca.18 mF·cm?2) capacitances due to only EDLC contribution[42,43].In another case, Li et al.created conductive MOF nanowire arrays (NWAs) and used them as a conductive additive and binder-free electrodes for solid-state SCs, achieving a capacitance of ～22 μF·cm?2.c-MOFs have 1D cylindrical channels that can facilitate ion transport and enable high ion conductivity.When composited with transition metal oxides, the c-MOFs can significantly improve the capacitance and rate performance[44].Duan et al.designed a series of MnO2@Ni-HHTP (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) nanoarrays with different lengths and explored the influence of lengths on the electrochemical energy storage performance.With regard to NHMO-5, not only ultrathin MnO2provided abundant redox active sites to ensure high pseudo-capacitance but also Ni-HHTP nanorod arrays that in-situ grew on the surface of MnO2along [001] can significantly improve the conductivity and provided enough ion transport pathways.When NHMO-5 was assembled with AC in an aqueous asymmetric SC(NHMO-5//AC), the device displayed a high energy density of 35.8 W h·kg?1at a power density of600 W·kg?1, and the capacitance retained 87.4% at 3 000 W·kg?1with the potential window of 1.2 V.Its high performance is ascribed to the high redox activity of MnO2and the excellent electron and ion conductivity in Ni-HHTP.After 3 000 cycles, NHMO-5//AC demonstrated a high capacitance of 233.1 F·g?1(149.7% of the initial capacitance) and a high columbic efficiency of 95.4%[45].More recently,Wang et al.reported phthalocyanine-based 2D c-MOF(Ni2[CuPc(NH)8]) nanosheets with rich active sites using the ball-milling mechanical exfoliation method.As a result, the micro SC device made of Ni2[CuPc(NH)8] nanosheets and exfoliated graphene exhibited excellent cycle stability and a superior surface capacitance of 18.9 mF·cm?2.

2D conductive MOFs (2D c-MOFs) had high active sites, large specific surface areas and fast ion transport, making them ideal for energy storage devices[46].Zhao et al.synthesised a 2D conductive ultrathin Cu3(HHTP)2(HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) film on the indium tin oxide/polyethylene terephthalate (ITO/PET) substrate for flexible transparent conductive electrodes (FTCEs) using the layer-by-layer assembly method.The large-areal ultrathin conductive frameworks contributed the low internal resistance for fast electron transport and ion diffusion, thus producing large capacity, superior rate capability, and bending flexibility.The photoelectric property of Cu3(HHTP)2-15/ITO/PET FTCEs was significant withT550 = 82.2% andRs = 49.1 Ω·sq?1and a high areal capacitance (CA) of 1 700 μF·cm?2.The corresponding symmetrical FTSCs exhibited an excellent capacitance of 939.2 μF·cm?2at a current density of 7 μA·cm?2, superior rate capability (63.9% at 150 μA·cm?2) and long-term cycling stability (85%after 3 000 cycles)[47].Conductive MOFs can overcome some drawbacks of traditional MOFs such as low conductivity.It is highly favorable to decrease the electron resistance by more conductive structures using modified mixed metal, conductive framework or combining appreciatively with other conductive and reliable electrochemical materials.

2.2 MOFs composites for SCs

To overcome the poor conductivity of MOFs,MOFs are usually composited with some conductive materials, such as graphene, carbon nanotubes (CNTs)and conductive polymers.Their synergistic effect provides overall multifunctionality and also presents new chemical and physical properties[48].

2.2.1 MOF/graphene composites

Graphene nanosheets can be used as ideal conductive substrates to grow MOFs because of their 2D-layered porous structures and good electronic conductivity.Choi et al.prepared a series of nanocrystal MOFs (nMOFs) combined with graphene to assemble symmetric SCs.Among these MOFs, a zirconiumbased MOF, MOF-867, showed high gravimetric (726 F·g?1), stack (0.64 F·cm?3) and areal (5.09 mF·cm?2)capacitances, whose performance could be maintained for at least 10 000 cycles[49].However, extreme restacking of graphene sheets decreases the accessible surface area of the electrolyte, resulting in a decrease in the capacitance.To solve this issue, guest materials are incorporated into the composites.These materials can prevent restacking of the graphene sheets and positively affect the mechanical and electrical properties.Azadfalah et al.fabricated a twocomponent Co-based MOF/graphene nanocomposite material as the electrode material of SCs through a simple one-step in-situ process.Among them, the CoMG5electrode demonstrated the highest capacity of 549.96 F·g?1.The CoMG5//CA asymmetric SC with a voltage window of 1.7 V had a specific capacity of 50.2 F·g?1at 20 mV·s?1, an energy density of 8.1 W h·kg?1at a power density of 850 W·kg?1at 1 A·g?1.After 1 000 charge/discharge cycles, it also showed a suitable cycle stability of 78.85%.Electrochemical testing demonstrated that the synergistic effect of graphene and Co-MOF in nanocomposites improved SC performance[50].The nanoscale homogeneous distribution of Co-MOF particles on the graphene sheets prevented them from agglomeration.The synergic effect of graphene and Co-MOF in the nanocomposite improved the supercapacitive performance and facilitated the ions and electron transportation due to the improved electrical conductivity.The partially-expanded graphite paper (EGP) that consists of abundant graphene nanosheets in situ formed on the graphite paper, has the merits of easy accessibility of electrolyte ions, large specific surface area, good conductiv-ity, acid and alkali resistance and mechanic flexibility.It is thus widely used as an attractive support for growing active materials in high-performance supercapacitors.Liu et al.prepared EGP composed of graphene nanosheets with a 3D layered porous structure and good electronic conductivity by controlling cathodic exfoliation (Fig.3a).The accordion-like Ni/Co-based bimetallic organic framework (NCMOF)was grown on the surface of EGP to prepared NCMOF/EGP.NCMOF/EGP showed a high area specific capacitance of 2.41 F·cm?2at 0.5 mA·cm?2with a good rate performance of 72.6% at current densities from 0.5 to 20 mA·cm?2.In addition, the constructed NCMOF/EGP//activated carbon SC provided an energy density of 0.11 mW h·cm?2at a power density of 0.75 mW·cm?2and a long cycle life[51].The improved electrochemical performance of NCMOF/EGP could be attributed to the 3D structure of accordion-like NCMOF vertically grown on EGP, resulting in fast ion and electron transfer rate, uniform distribution and large surface area.

2.2.2 MOF/CNT composites

CNTs have many advantages, such as good conductivity and high surface area, which make it becoming one of the most candidate materials to design high conductive composites.It is novel and attractive to design hybrid structures of MOFs with CNTs for SCs.The Ni-MOF/CNT composite obtained by growing Ni-based MOF on the surface of CNTs (Ni-MOFs/CNTs) achieved a high specific capacitance of 1 765 F·g?1at 0.5 A·g?1.The asymmetric SC device manufactured with Ni-MOFs/CNTs as the cathode and RGO/C3N4as the anode exhibited a high energy density of 36.6 W h·kg?1at 480 W·kg?1with only 5%loss of capacitance of the initial value after 5 000 cycles[52].CNTs are applied as a robust backbone in the composites, whose surface can be easily modified to have carboxyl groups to facilitate uniform growthof MOFs on their surface, and whose excellent conductivity and nanometer tube diameter make the composite have low electron and ionic resistance.Thus Yang et al.reported a new hybrid structure of Ni-MOF@CNT on the graphene/Ni foam (GN) substrate and obtained a novel self-supporting material of Ni-MOF@CNT/GN (Fig.3b).Ni-MOF@CNT and a graphene/Ni foam (GN) substrate served as the main active component and the self-supporting carrier, respectively.CNTs can act as a current collector to shorten the pathway of electron transport.In addition,the synergistic effects of CNTs and the Ni-MOF can buffer the large volume change of active species during long-term cycling.As a new electrode material for all-solid-state SCs, the device achieved high capacitance values of 898 and 230 mF·cm?2at 1 and 20 mA·cm?2, respectively, with an ultrahigh energy density of 0.339 6 mW h·cm?2.After 4 000 cycles, it still showed a high retention value of 93%.Such a significant electrochemical performance may be ascribed to the high conductivity of CNTs and the graphene layer, porous characteristic of the Ni-MOF@CNT/GN material and anchoring effect of oxygen-containing groups for Ni-MOF in the hybrid structure[53].

In addition, carbon nanotube film (CNTF)provides a significantly thermal and chemical stability, high conductivity, strong mechanical properties and high surface area.And CNTF as both the electrodes and charge collectors can eliminate the metalcarbon charge collector-electrode interface, leading to a simplified and lightweight architecture.UiO-66 was synthesized by Fu et al.and coated onto CNTFs (U-C)interconnected by electrochemically co-deposited poly(3,4-ethylenedioxythiophene)-graphene oxide(PEDOT-GO) to prepare a flexible porous electrode(PEDOT-GO/U-C).As a result of the synergistic effects among UiO-66, PEDOT and GO, an exceptionally high areal capacitance of 102 mF·cm?2at 10 mV·s?1was obtained[54].

2.2.3 MOF/polymer composites

Conducting polymers can act as electron transport circuits for MOF crystals and provide pseudocapacitance owing to their high conductivity and great electrochemical properties.Chen et al.reported a core-shell structure composite (PPy@NiCo-CAT) assembled by polypyrrole and 2D conductive MOFs.The use of polypyrrole as a backbone effectively prevented the aggregation of MOFs.Its own hollow structure provided better conductivity while shortening the ion diffusion pathway.Benefiting from the rational structural design, PPy@NiCo-CAT had a high performance of 572.2 F·g?1at 1 A·g?1in an aqueous KOH electrolyte[55].The charging and discharging of the PPy@NiCo-CAT electrode were a joint action of diffusion-controlled and surface-controlled capacitive processes.The dominant diffusion control in the charging and discharging process of PPy@NiCo-CAT electrodes gradually transformed to the capacitive control one with increasing scan speed (0.2-1.0 V).Wang et al.synthesized ZIF-67 on a carbon cloth(CC) and further electrodeposited with polyaniline(PANI) to get a flexible conductive porous electrode PANI-ZIF-67-CC.ZIF-67 was interwoven with PANI chains to reduce the bulk electrical resistance.The PANI-ZIF-67-CC showed an high area capacitance of 2 146 mF·cm?2at 10 mV·s?1in a three-electrode system.Moreover, a flexible symmetrical solid SC (SSC)assembled by PANI-ZIF-67-CC achieved a high area capacitance of 35 mF cm?2and retained more than 80% of initial capacitance after 2 000 cycles[56].Yue et al.loaded a conductive Cu-CAT MOF onto a porous polypyrrole scaffold (p-PPy) to obtain a hybrid architecture.The p-PPy could provide sufficient loading areas and efficient conductive skeletons (Fig.3c).The symmetric flexible SC assembled by the combining the p-PPy/Cu-CAT hybrid and the CC achieved a specific capacitance of 233 mF·cm?2with a stable cycling life, excellent mechanical flexibility and wide operating temperature tolerance[57].The high density of conductive Cu-CAT crystals uniformly dispersed in the 3D conductive porous p-PPy, which promoted the capacitance performance of p-PPy/Cu-CAT.The EDLC of the porous structure and the Faradaic pseudocapacitance of PPy both contributed to the high performances of p-PPy/Cu-CAT hybrid.These results demonstrated that the synergistic effect of the hierarchically porous structure and the highly intrinsic conductivity of both p-PPy and Cu-CAT.

2.2.4 MOF/carbon nanofibers

Carbon nanofibers (CNFs) are attractive candid-ates to combine with MOFs, due to their good electrical conductivity, high specific surface area and large aspect ratio.Srimuk et al.coated rGO-HKUST-1 on a flexible CNF paper (CFP) to assemble a solidtype SC for practical application[58].The 10%rGO/HKUST-1 had a high specific surface area(1 241 m2·g?1) and specific pore volume (0.78 cm3·g?1).It had an proper average pore size of 8.2 nm to allow uptaking and releasing of electrolytes.The manufactured flexible symmetric SC displayed a high capacitance of ～193 F·g?1during a continuous test of 60 000 s at a potential window of ～1.6 V with a capacitance retention of 98% after 2 000 cycles.Co nanoparticles modified CNFs were used as supports to grow MOFs[59].Co nanoparticles embedded into CNFs not only enhanced the graphitization degree of CNFs, but also provided Co doping in MOF, leading to improved conductivity and pseudocapacitance.The prepared composite (C-Co@MOF) showed a hierarchical core-shell structure.More importantly, an asymmetric solid-state SC was assembled using CCo@MOF and nitrogen-doped CNTs derived from polyaniline as positive and negative electrode materials, respectively.KOH-PVA was selected as solidstate electrolyte.This device represented a high energy density of 37.0 W h·kg?1with a wide voltage window of 1.5 V.Different MOFs-based composites have been increasingly employed in SCs.MOFsbased composites provide high active sites and surface area.The electrochemical performance of MOFs is greatly improved by compositing MOFs with conductive materials (graphene, CNTs, CNFs, conductive polymer, etc.) by improving the conductivity of the pristine MOFs.

2.3 MOFs-derived carbon materials for SCs

The applications of MOFs-derived oxides/sulfides in the field of SCs have recently been reviewed in detail[60].In this study, only the latest MOFs-derived carbon materials are introduced.

Due to the unique structure of MOFs, MOFs are one of the most promising candidates to prepare porous carbons with various morphologies through carbonization and activation.Liu et al.synthesized a new hexagonal columnar Zn-MOF as a precursor for preparing a porous carbon (Fig.4a).The derived carbon showed a specific surface area of 1 464 m2·g?1and a pore size of 3.9 nm.Due to its richly porous structure,the derived porous carbon electrode exhibited excellent SC properties with high specific capacitance and excellent cycle stability.In addition, the assembled two symmetrical EDLCs with different electrolytes provided high energy densities of 22.4 W h·kg?1(1 mol·L?1Na2SO4) and 13.7 W h·kg?1(6 mol·L?1KOH).After 10 000 cycles at 20 A·g?1, their energy retention rates were 80.0% and 89.4%, respectively[61].Chen et al.presented an asymmetric SC device using the MOFs-derived urchin-like amorphous carbon anchored on nickel foam (UAC@NF) as positive electrode and the activated graphite carbon felt (GF500)as a negative electrode (Fig.4b).The derived unique Co/C heterostructure showed a uniformly anchored of Co (4.48%) on the special porous carbon, which can provide pseudocapacitance and improve the electronic conductivity of UAC@NF.This ASC device achieved a high energy density of 0.036 mW h·cm?3at a power density of 0.984 mW·cm?3.And it demonstrated excellent cycling performance with a 91.4%capacitance retention after 10 000 cycles[62].MOFs are also ideal precursor or templates to prepare CNTs and graphene.He et al.designed a nitrogen-doped carbon skeleton (NCS) assembled by CNTs and graphene layers utilizing a layer-shaped humate-based zeolitic imidazolate framework (ZIF) (HA-CoFe-ZIF) as a template or precursor (Fig.4c)[63].The synthesized NCS was mainly composed of graphitized carbon with a few hydroxyl groups on its surface, simultaneously doped by 9.5% nitrogen in the forms of pyridinic and pyrrolic N.The rich mesoporous structure entitled it to a high specific surface area of 427 m2·g-1and suitable average pore diameter of 3.14 nm.The NCS had a high capacitance of 324 F·g-1at 1 A·g-1, superior rate capability (a capacitance retention of 71% from 5 to 100 A·g-1,) and excellent cycling stability (capacitance retention of 96%and 87% after 5 000 and 10 000 cycles, respectively)at 6 mol·L?1KOH.The fabricated NCS//AC asymmetric SC also exhibited a high capacitance of 93 F·g-1at 1 A·g-1, a large energy density of 10.3W h·kg-1at 331 W·kg-1, and good cycling performance(a capacitance retention of 88% after 5 000 cycles).

The design and development of high performance EDLC-type carbon materials with the effective synergistic effect of high conductivity, tailored porous structure, and high surface area still remain challenging.Pokharel et al.firstly used functional groups such as hydroxyl and carbonyl groups on the polymer polyvinyl alcohol/polyvinylpyrrolidone (PVA/PVP) to replace the groups on the Cu-MOF particles, and improved the interaction between the Cu-MOF and the polymer through hydrogen or π-π staking[64].Then, the obtained PVA/PVP/MOF hydrogel after the hydrothermal process was carbonized to obtain carbon called CPM.CPM showed micro and mesoporous structures with a combination of highly conductive electronic pathways and rich ionic storage units in three-dimensional network morphology, leading to high specific capacitance of EDLC.It exhibited an extremely high specific capacitance of ～ 385 F·g?1at 0.1 A·g?1, and can still maintain a capacitance of 303 F·g?1at 10 A·g?1.In addition, the assembled EDLC provided a high energy density of 10.51 W h·kg?1and a power density of 5.454 kW·kg?1at a current density of 10 A·g?1in an aqueous electrolyte.The introducing of non-metallic heteroatoms (N, P, S and B)into the frameworks of carbon materials can further improve the SC performance of carbon materials.Gang et al.reported a nitrogen-rich spherical porous carbon by directly carbonizing a Zn-MOF based on Zn-2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine.Zn atoms could sublimate completely at＞850 °C to create pores.During the high-temperature carbonization process, Zn atoms, carboxyl groups and other functional groups were removed, forming hierarchical pores with a surface area of 1 826 m2·g?1and a nitrogen content of 11.37%.This N-rich spherical porous carbon exhibited a high specific capacitance of 386.3 F·g?1at 1 A·g?1and an excellent cycle stability of 97.8% after 100 000 cycles at 150 A·g?1, which is attributed to the high content of graphite N and spherical structure.In addition, the specific capacitance values of nitrogen-rich spherical porous carbon-based symmetric SCs at 1 and 150 A·g?1were 186.9 and 98.6 F·g?1, respectively.The device provided a considerable energy density of 50.9 W h·kg?1at a power density of 1.6 kW·kg?1under the optimal voltage window of 1.4 V in 6 mol L?1KOH[65].Porous carbonaceous materials are widely used as electrodes in SCs,owing to their good stability, great conductivity and relatively high surface area.The presence of organic linkers inside MOF structures provides a good carbon source to synthesize the MOF-derived carbons.More interestingly, the resulting carbons can show different textural properties based on precursors MOFs.The MOF-derived carbon materials not only inherit the merits from the parent MOFs but also strengthen the chemical and mechanical stabilities.The notable SC performances of MOFs-based materials discussed in this review are summarized in Table 1.

Table 1 Selected properties of MOFs-based materials for SCs.

3 COFs-based materials for SCs

3.1 Pristine COFs for SCs

Covalent organic frameworks (COFs) are an emerging class of electrode materials due to their adjustable redox-active frameworks and controllable pore sizes.Basically, the structures of COFs include the linkers (building units) and the linkages (bonds between building units).The deployment of redoxactive linkers as building blocks for the construction of porous COFs could be a promising route for the attainment of SC electrode materials.The active sites in COFs-based SC electrode materials are usually quinones, imine, and conjugated heterocycles and so on.

The imine groups are important kinds of active sites in COFs, which are usually obtained by Schiffbase reaction[66].Schiff base reaction is the most common strategy for synthesizing COFs.Halder et al.synthesized an imine-based, redox-active and hydrogenbonded COF (TpOMe-DAQ), using 4,6-trimethoxy-1,3,5-benzenetricarbaldehyde (TpOMe) and 2,6-diaminoanthraquinone (DAQ) as building units(Fig.5a-d)[67].These linker units could provide electrochemical stability and reversible redox response due to the quinone/hydroquinone transformation(DAQ amine) for SCs.Due to the ‘locking’ effect of H-bonding, TpOMe-DAQ possesses a further improved stability, which elongates the life of the electrode materials.TpOMe-DAQ could be fabricated as thin sheets, which shows a high areal capacitance of 1 600 mF·cm?2in H2SO4.Khayum M et al.synthesized convergent COF with free-standing flexible sheets structure using a solid molecule baking strategy[68].Redox-active anthraquinone (Dq) and πelectron-rich anthracene (Da) were used as two different linkers to construct a β-ketoenamine-linked 2D COF.The COF thin sheets exhibited improved the mechanical property and redox activity by precisely integrated anthraquinone moieties (Fig.5e).Additionally, the prepared Dq1Da1Tp COF was selected as the electrode to prepare a highly flexible symmetric SC.The area capacitance of a single electrode was 8.5 mF·cm?2, and the energy and power densities were 0.30 μW h·cm?2and 960 μW·cm?2, respectively.Therefore, convergent COFs with high crystallinity and porosity can be applied as electrode materials for flexible SC devices.

COFs with N-containing heterocycle as redoxactive units are also commonly used in supercapacitors as electrode materials[69-72].Li et al.prepared a novel 2D COF (PDC-MA-COF) with redox-active triazine units via the Schiff-base condensation.Melamine (MA) and 1,4-piperazinedicarboxaldehyde(PDC) were used as building units[69].PDC-MA-COF exhibited a maximum specific capacitance of 335 F·g?1at 1 A·g?1in the three-electrode system, which still showed a relatively high capacitance of 248 F·g?1at 10 A·g?1, maintaining a specific capacitance of 74%.Xiong et al.synthesized TPA-COFs with a triphenylamine structure by the Schiff base poly-condensation method using a solvothermal method[70].Notably,the pseudocapacitance characteristics of the SCs that the TPA-COFs exhibited are due to the presence of redox-active triphenylamine units.EL-Mahdy et al.synthesized COFs through Schiff-based condensations of 1,3,5-triformylphloroglucinol (TFP-3OHCHO) with three tris-(aminophenyl)-presenting derivatives under solvothermal conditions.The obtained COFs exhibited excellent electrochemical performance, owing to their conjugated enamine structures with redox-active triphenyl amino, carbazole and pyridine moieties.Note that the hydrogen bonds can also be found in the structure between the carbonyl and amino groups[71].

Recently, COFs containing N/S heterocycle redox-active units show attracting electrochemical performances in SCs[73-75].A 2D electron donor-acceptor COF (TTF-COF1) was constructed by the imine condensation of electron donor tetraformyl-tetrathiafulvalene (TTF-fo) and electron acceptor 2,6-diaminoanthraquinone (DAQ) (Fig.6a)[73].TTF-COF1 had intramolecular charge transfer, offering capacitance of 752 F·g?1at 1 A·g?1in 3 mol·L?1KOH.The asymmetric SC (AC//TTF-COF1) showed an energy density of 57 Wh·kg?1at a power density of 858 W·kg?1.The highly porous structure and numerous donor-acceptor active groups (Tetrathiafulvaleneand p-benzoquinone) of TTF-COF1 contributed to the high pseudocapacitance.Li et al.constructed a 2D benzobisthiazole-linked COF (PG-BBT) with intramolecular hydrogen bonds for high-performance SCs (Fig.6b-e)[74].The empty d-orbitals of the sulfur atom in benzobisthiazole (BBT) polymer chains could decrease energy of the π-π*transition and improve delocalization within the π-orbitals, making them efficient electron transfer materials.The 2D full planar structure with formed intramolecular hydrogen bonds,redox-active benzobisthiazole unit and ordered porous structure made PG-BBT a promising electrode material for SCs, which exhibits a high capacitance of 724 F·g?1and an excellent energy density of 69 W h·kg?1.

COFs with highly conjugated structures can speed up the electron conductivity during electrochemical storage processes.The Knoevenagel reaction is a facile method for synthesizing sp2carbonlinked COFs with full π-conjugated structures.The first example of olefin-linked 2D conjugated COFs was synthesized using the Knoevenagel reaction(Fig.7a)[76].Through a variant of the Knoevenagel reaction, a fully conjugated COF (g-C34N6-COF) linked by unsubstituted C＝C bonds was reported by Xu et al.(Fig.7b).3,5- dicyano-2,4,6-trimethylpyridine and 1,3,5-triazine units were integrated into the molecular frameworks, leading to the enhanced π-electron communication and electrochemical activity.g-C34N6-COF possessed unique nanofibrous morphology with a specific surface area of 1 003 m2·g?1.The g-C34N6-COF nano-fiber assembled with CNTs can be made into a flexible thin-film electrode for a micro-supercapacitor (MSC).The MSC had an area capacitance of～15.2 mF·cm?2, a high energy density of ～7.3 mW h·cm?3and a significant rate performance[77].Zhang et al.prepared two novel olefin-linked 2D COFs through the Knoevenagel condensation of 2,4,6-trimethyl-1,3,5-triazine with tritopic triazine-cored aldehydes(Fig.7c-f).Moreover, this COF nano-fibers can be easily combined with CNTs to form g-C30N6-COF and g-C48N6-COF films.The density and mechanical strength of the film can be improved by hot-pressing and then assembled with an ionized gel electrolyte to form a planar MSC.The capacitor with g-C30N6-COF as an electrode material exhibited an area capacitance of 44.3 mF·cm?2, a large voltage window of 2.5 V, a high-volume energy density of 38.5 mW h·cm?3and good cyclic stability[78].All three instances construct 2D COFs with the linking of olefin.A breakthrough in the production of planar MSCs can be expected.

The development of new functional linkages can endow COFs with additional tailored properties besides the building units.Yang et al.reported two new arylamine-linked COFs (AAm-TPB and AAm-Py)[79],which were prepared by condensing dimethyl suc-cinyl succinate (DMSS) with corresponding multitopic amines (TPB-NH2and Py-NH2).AAm-TPB had a specific surface area of 403 m2·g?1with a dominant pore size of 2.99 nm.The AAm-TPB electrode exhibited a high pseudocapacitance of 271 F·g?1in a threeelectrode setup at 1 A·g?1in 1.0 mol L?1H2SO4.This performance may be caused by the abundant redoxactive diphenylamine moieties in the COF skeletons.COFs present great potential for SCs, whose highly conjugated structures and redox-active sites could be beneficial to the high-performance SCs.Nevertheless,due to the low conductivity and the slow charge transfer rate of organic structures, the electrochemical performance of COFs is far from satisfactory.

3.2 COF composites for SCs

3.2.1 COF/polymer composites

Despite rich active sites, open channels and high surface area, the poor conductivity and low electrochemical accessibility of COFs result in low electrochemical efficiency and specific capacitance.Hence,compositing with conductive materials is an effective strategy for COFs towards achieving excellent electrochemical performance attributed to the synergistic effects of the individual components.

Wu et al.incorporated high conductivity and stable poly (3,4-ethylene dioxythiophene) (PEDOT)units into the nano-pores of oxidation-reduction AQCOF by solid-state polymerization technology to prepare PEDOT@AQ-COF[80].PEDOT@AQ-COF nanocomposites can form electronic fast channels in the network, and had high performance in electrochemical applications such as Faraday energy storage.PEDOT@AQ-COF showed a maximum specific capacitance of 1 663 F·g?1at 1 A·g?1in 1 mol·L?1H2SO4.During the charge discharge process, PEDOT chains were reorganized in the nanochannels, resulting in more effective H+around the redox active system,thus improving the porosity.The composite showed improved conductivity and strong chemical stability.Liu et al.successfully designed and prepared all organic nano-composite COF@PANI with a complex network structure by combining porous and redox-active TpPA COF with conductive polyaniline using solvothermal in-situ polymerization[81].The strong redox peak on the cyclic voltammetry curve of TpPACOF@PANI was different from the ideal rectangularshape of EDLCs, indicating the obtained capacitance consisted of pseudocapacitance and electric double layer capacitance.The capacity retention rate of the SC fabricated with this material as the electrode was as high as 83% after 30 000 cycles.This result proves that TpPA-COF@PANI composites have high potential applications in SCs.Furthermore, MOF@COFLZU1 was synthesized using aza-Diels-alder cycloaddition reaction and modified to produce aza-MOFs@COFs hybrid porous materials with extended delocalization[82].As a hybrid architecture, it combined the unique characteristics of a single component to generate materials with novel physical and chemical properties.The obtained composites maintained high crystallinity and porosity and had high robustness.As an application in SC devices, the obtained aza-MOFs@COFs displayed a specific capacitance of 20.35 mF·cm?2and a high volumetric energy density of 1.16 F·cm?3.

3.2.2 COF/graphene composites

Dichtel et al.novelty combined 2D COF with monolayer graphene under solvothermal conditions,which has become a research hotspot in this field[83].Due to the strong van der Waals interaction, graphene nano-sheets stacked together to decrease the charge storage efficiency, seriously affecting their performance as SC electrodes.Herein, a hybrid material was designed by the interaction of COF with reduced graphene oxide (rGO) film or fibers.Two-dimensional-COF nano-sheets contain abundant mesopores,which prevent the stacking of rGO nano-sheets and realize efficient electrolyte ion mass transfer[84].An anthraquinone-based COF/graphene composite aerogel (DAAQ-COF/GA) electrode was prepared by electrostatic self-assembly of negatively charged graphene oxide nano-sheets and positively charged DAAQ-COF nano-materials.The obtained suspended electrode with a 3D cross-linked conductive network effectively solved the limitations of slow electron transfer and low utilization of active sites in the organic framework (Fig.8a-d)[85].In addition, the unbonded DAAQ-COF/GA and pure graphene aerogel electrodes were assembled into an asymmetric SC,whose energy density was as high as 30.5 W h·kg?1ata power density of 700 W·kg?1.The hybrid of COFs and conductive materials can further improve their conductivity and then enhance the electron transport during charge and discharge processes.For example,Sun et al.reported a COF/NH2-rGO composite prepared by a one-step reaction of the aldehyde group in 1,3,5-trimethylbenzene, and NH2-graphene and amine groups in 1,4 diaminobenzene[86].The synthesized conjugated COF/graphene composite had better supercapacitance performance than COFs and graphene.It can be used as a substitute electrode for supercapacitors.The results show that the synergistic effect of redox-active COFs and conductive graphene carrier improves their electrochemical performance.Similarly, COFs were grown on the surface of 3D graphene by interfacial polymerization, in which the redox-active part (anthraquinone) was deliberately selected to improve the electrochemical activity[87].Fullerene has become an attractive candidate electrode material for energy storage devices because of its unique electron conjugation system and high electron affinity.However, due to the poor structural characteristics of fullerene nanostructures, their low electrochemical accessibility limits their performance in energy storage applications.Ordered porous fullerene C60with COF templates was manufactured by nano-template technology to improve its electrochemical accessibility,which opened up new possibilities for electrode design of high energy density SCs (Fig.8e)[88].An asymmetric SC device ([C60]0.05-COF//rGO) optimized the operating voltage window as high as 1.8 V and provided an excellent energy density of 21.4 W h·kg?1at a power density of 900 W·kg?1.

3.2.3 COF/CNT composites

Flexible SCs in modern electronic equipment need light-weight electrodes with high specific surface area, accurately integrated redox parts and strong mechanical flexibility and independence.While maintaining flexibility and slip capacity to increase elongation, developing the mechanical stable and tensile strength high SCs from COFs is still a significant challenge to meet industrial applications[89-92].CNT films (CNTF) have commendable flexibility, slip ability and modifiable surface area.Therefore, Xu et al.constructed flexible SCs with covalent organic skeleton composite membrane electrodes.First, they prepared COF complexes with hydroxyl hyperbranched polymer (OHP) as a template by simple solid-state mechanical mixing.Afterwards, COF@OHP was impregnated into the microporous CNTF membrane to construct a composite membrane for the electrode of a flexible SC.The composite film had good tensile strength and elongation.This method also provides a new direction and strategy for preparing wearable flexible SCs with high mechanical strength[93].

Additionally, the interfacial synthesis of ordered and stable COFs on amino functionalized CNTs enhances the electrochemical properties[94].Xu et al.assembled single-wall CNTs with a nano-fiber COF to obtain a flexible thin-film electrode for MSCs.The MSC based on the COF showed an extraordinary prospect in manufacturing portable and wearable highperformance electronic systems[77].Yang et al.described the synthesis of NKCOFs with intrinsic proton conduction by the Grotthuss mechanism.After insitu hybridization with CNTs, the proton-coupled electron transfer reaction of azo groups was significantly promoted.The composite exhibited a high threeelectrode specific capacitance of 440 F·g?1(at 0.5 A·g?1).The assembled asymmetric two-electrode SC showed both high power density (42 kW·kg?1) and high energy density (71 W h·kg?1)[95].Constructing composites by rationally combining the merits of COFs and conductive materials is an effective strategy for COFs to achieve high-performance SCs.The designed conductive network could improve electron transfer and increase utilization of active sites in organic skeleton.The synergistic effect of redox-active COFs and conductive materials can be attributed to the enhanced SC performance.

3.3 CTFs and their carbonized derivatives for SCs

Covalent triazine frameworks (CTFs) are a subclass of COFs built by 1,3,5-triazine units, which possess N-dopped conjugated skeleton with high stability and specific surface areas[96,97,98].CTFs have been used in gas storage, catalysis and energy storage[99,100].Several strategies exist for preparing CTF materials,including ionothermal trimerization, phosphorous pentoxide and superacid catalysis and Friedel-Crafts reactions[99].

Thomas et al.introduced ionothermal trimerization to synthesize CTFs using molten ZnCl2as the solvent and catalyst for the cyclotrimerization of aromatic nitriles at relatively high temperatures[101].The ionothermal temperature of CTFs is usually 400 °C using ZnCl2.The higher reaction temperature(500-900 °C) inevitably causes partial carbonization of the skeleton to form CTF-based porous carbons.The high temperature could increase the surface area and improve the porous structure.The N-rich skeleton and high porosity make CTFs and their carbonized derivatives perform excellently as electrode materials for SCs[102,103].

The porous structure and nitrogen content of the CTF-based materials can be controlled by the ionothermal/carbonization temperature and the percentage of ZnCl2in this strategy.Zhi et al.constructed a conjugated microporous CTF using (1,3,5-tricyanobenzene) as the building block through trimerization reactions[104].Several porous triazine-based frameworks were prepared by the structural evolution of a 2DCTF.The nitrogen incorporation and microporosity of the CTF-based materials could be well defined by different temperatures, making them a model-like system to research the effects of hetero-atom and micropores in SCs.The results revealed that nitrogen doping could improve the electrochemistry performance by affecting the relative permittivity of the electrode materials.Also, the contribution of nitrogen doping is more obvious than that of micropores in ionic liquidbased SCs.

The doped nitrogen of CTFs comes from the triazine unit and designed building block.Wu et al.used 2,6-pyridinedicarbonitrile as the building block to synthesize a CTF with repeated pyridine units by ionothermal trimerization[105].The in-situ nitrogendoped pyridine units in the skeleton could provide more pseudo-capacitance as redox-active groups.The introduction of some other hetero-atoms atoms into the framework of nitrogen-enriched CTFs effectively enhances their performance in SCs.Wang et al.introduced halogen (F/Cl) into CTFs to promote their electrochemical performances[106].This work indicated that the doped halogen could block the stack of two adjacent CTF layers through their atomic repelling effects, which could facilitate electron transport.The fluorinated CTFs (FCTFs) were synthesized based on 2,3,5,6-tetrafluoroterephthalonitrile by ionothermal strategy at 400 °C.The hierarchical FCTF exhibited a high specific capacitance in the three-electrode(379 F·g?1at 1 A·g?1) and the two-electrode systems(148 F·g?1at 1 A·g?1) with excellent rate capability and cycling stability.The carbonized derivatives of FCTF with rich F (7.8%) and N (12.1%) also possessed a similar performance[107], indicating that the hetero-atom atoms could improve the SC performance of CTFs and their carbonized derivatives.

Unlike small-molecule nitrile, macromolecular aromatic nitrile precursors for ionothermal synthesis could promote structural disorders and create an optimum hierarchical structure.Ye et al.used macromolecular polyethynylbenzonitrile as precursors to produce a CTF with a hierarchical pore structure by ionothermal synthesis for SCs[108].The optimized CTF-based material (CTF-800) showed an optimum micropore-to-meso/macropore surface area ratio and high electronic conductivity, which reflected superior supercapacitive properties with high energy and highrate performance.CTF-800 exhibited a high specific capacitance of 628 F·g?1at 0.5 A·g?1in a three-electrode system with 1 mol·L?1H2SO4aqueous electrolyte.A symmetric SC device fabricated with CTF-800 exhibited an exceptional energy density of 15.5 W h·kg?1in an aqueous electrolyte.The device also exhibited a high energy density in [EMIM][BF4](70 W h·kg?1) and LiPF6(78 W h·kg?1) electrolytes.CTFs based on the trimerization of small-molecule nitrile could improve their porous structure by KOH activation for high-performance SC electrode materials[109].

In the ionothermal trimerization, molten ZnCl2was used as the solvent, porogen and catalyst for the cyclotrimerization of aromatic nitriles, which should be washed to obtain CTFs and their carbonized derivatives after synthesis.Kaskel et al.synthesized a pyridine-based CTF material through ionothermal trimerization.ZnCl2, which should be removed, was used as the electrolyte salt directly.An aqueous ZnCl2electrolyte appeared in-situ in the CTF pores after water addition.The manufactured symmetrical SCs showed a specific capacitance of 141 F·g?1.

Using building blocks with triazine units could also construct triazine-based COFs.Melamine is a common nitrogen dopant with a simple triazine unit.Yang et al.used melamine and 1,4-piperazinedicarboxaldehyde to prepare a triazine-based COF (PDCMA-COF) by Schiff-based condensation.The PDCMA-COF showed better conductivity than conventional COFs[110].Also, the interlayer H-bonding in the skeleton could improve the PDC-MA-COF stability,which showed good cycle stability as a SC electrode material.Zhang et al.reported a conjugated COF (g-C34N6-COF) with unsubstituted C＝C bond linkages using 1,3,5-tris-(4-formylphenyl)triazine[73].The 3,5-dicyano-2,4,6-trimethylpyridine and 1,3,5-triazine units enhanced the framework’s electron conductivity and electrochemical activity.Kuo et al.used 1,3,5-tris-(4-formylphenyl)triazine to prepare triazine- and benzobisoxazole-based COFs with a hollow tubularstructure for SCs[111].Bhaumik et al.designed COFs using 1,3,5-tris-(4-aminophenyl)triazine and 2,6-diformyl-4-methylphenol by the Schiff base condensation[112].The N-doped π-conjugation framework and inherent porosity made the CTFs and their carbonized derivatives potential electrode materials for energy storage devices.

3.4 COFs-derived carbon materials for SCs

3.4.1 Carbon materials obtained by direct carbonization of COFs

Compared with pristine COFs and COF composites, COFs-derived carbon materials possess higher chemical stability, electronic conductivity and electrochemical performance.Zhuang et al.synthesized a 2D poly (phenylacetylene)-based COF (2DPPV) connected with olefin by Knoevenagel poly-condensation reaction, and it had a full sp2-bonded skeleton of chainlike cyanide groups.The porous carbon nanosheet 2DPPV-800 obtained by carbonizing the COF had a specific surface area of 304 m2·g?1and a total pore volume of 0.16 cm3·g?1.It can be used as the electrode materials for SCs (with a specific capacitance of 334 F·g?1at 0.5 A·g?1)[72].This work provides a green economy non-template approach to prepare two-dimensional porous carbon materials for energy-related applications.Wang et al.synthesized an interpenetrating polymer network with resorcinol formaldehydezinc tartrate and poly (styrene-maleic anhydride).A 3D hierarchical porous carbon material was simultaneously obtained after carbonization with micro-,meso- and macro-pores.It showed a high specific surface area of 742 m2·g?1and an average pore diameter of 3.1 nm.The SC assembled by this material had a wide potential window (3.5 V) in EMIMBF4electrolyte and achieved a high energy output[113].Therefore,COFs-derived carbon materials can be regarded as potential electrode materials for SCs.

3.4.2 COFs-derived carbon materials doped with heteroatoms

In general, the electrochemical performance of carbon materials can be improved by doping heteroatoms into the porous network.For example, highlevel doped nitrogen can enhance the surface polarity of the electrode, which could improve its wettability and further enhance its electrochemical capacitance of the electrode.COFs could be used as precursors to obtained N-doped carbons.The nitrogen can be from the heteroatom groups in building units or covalent bonds after carbonization.Zhao et al.prepared a porous aromatic skeleton (PAF) material containing nitrogen atoms via a Suzuki coupling reaction.After carbonization at different temperatures, the covalent bond maintained the nitrogen atoms in structures, and the specific surface area of the materials increased.The COF-derived carbon (LNU-18-800) showed a nitrogen content of 3.06% and a specific surface area of 380 m2·g?1.LNU-18-800 also showed good performance as supercapacitor electrodes (a specific capacitance of 269 F·g?1at 0.5 A·g?1) in a typical three-electrode system[114].

Kim et al.directly carbonized a 2D molecular network COF (ACOF1) connected by azide bonds to prepare a microporous N-doped carbon, in which the azide bonds could generate nitrogen group in the carbon framework during pyrolysis.The carbonized derivative of ACOF1 had a high specific surface area(1 596 cm2·g?1), uniform micropores (less than 1 nm)and a nitrogen-doped (1.6%) graphite carbon structure.Meanwhile, the high nitrogen content of the carbon material also increased the specific capacitance and rate performance significantly (Fig.9a-b)[115].Zhu et al.used a COF with a high nitrogen content based on Schiff-base reaction as a carbon precursor.The nitrogen-doped porous carbon had uniform geometry,high specific surface area (1 478 m2·g?1), high pore volume (0.76 cm3·g?1) and high nitrogen content(8.71%).It showed a high specific capacitance and good rate performance as an electrode material of SCs[116].Xue et al.used a layered triazine COF (COFTP) connected by -NH- bond using 2,4,6-trichloro-1,3,5-triazine andp-phenylenediamine as a raw material to prepared a N-doped porous carbon(COF?TP?C) (Fig.9c-e).COF?TP?C showed a specific surface area of 398.15 m2·g?1and a pore size of about 1 nm.The carbonized electrode material had a high capacitance (a specific capacitance of 283 F·g?1at 1 A·g?1) and good cycling performance[117].The addition of boron (B atom) to carbon skeleton can not only modify its physicochemical structure and electronic structure, but also enhance its electrochemicalperformance.For the first time, Huang et al.successfully prepared porous B-doped carbons derived from COF-5 via a molten-salt (MS) approach (Fig.9f-h).Inorganic molten salt ZnCl2was used as a high-temperature reaction medium and pore-forming agent.The micropore and mesoporous ratios of carbons can be adjusted by the percentage of ZnCl2.With increasing the percentage of ZnCl2, the specific surface areas of the carbons increased.The B-doped carbon BCMS-700-14 had the largest specific surface area(1 460 m2·g?1) and total pore volume (1.76 cm3·g?1),which also had good electrochemical performance (a specific capacitance of 160 F·g?1at 10 mV·s?1)[118].Umezawa et al.directly carbonized a boron-containing covalent organic skeleton to obtain a boron-doped porous carbon (WCCOF-5).WCCOF-5 was pulverized into nanoparticles (PCCOF-5), which were suitable to manufacture electrode films for SCs.PCCOF-5 showed a specific surface area of 510 m2·g?1with the total pore volume of 0.38 cm3·g?1.The pseudocapacitance effect introduced by the doped boron atoms made PCCOF-5 have a higher charge density(15.3 mF·cm?2) than that of activated carbon(～6.9 mF·cm?2)[119].

3.4.3 Metal functionalized COFs and their derived carbon materials

Zhou et al.synthesized low-cost COFs (Cu@BN-COFs) containing rich nitrogen and boron content through a simple one-pot method using copper as both a chemical balance control agent and a catalyst.The B-N co-doped hollow mesoporous carbon B-N-C-1000 could be derived from Cu@B-N-COFs.B-N-C-1000 had abundant nitrogen (6.29%) and a few boron atoms (0.29%), with a specific surface area of399.4 cm2·g?1.B-N-C-1000 showed excellent performance as SC materials.The specific capacitance was 230 F·g?1at 5 A·g?1[120].Li et al.successfully prepared a 2D conductive covalent organic skeleton (Ni-COF) with square Ni (II) coordination (Fig.10).Ni-COF had a high Ni content (17.6%), a high specific surface area (362 m2·g?1), and pore sizes concentrated in 1-1.6 nm.The π-conjugation Ni(II)-Salphen units were favorable for the formation of the 2D layered structure.The highly conjugated structure and numerous redox sites increased the electrochemical performance: a specific capacitance of 1 257 F·g?1at 1 A·g?1and a capacitance retention of 94% after 10 000 cycles.The assembled asymmetric SC showed a high energy density of 130 W h·kg?1at a power density of 839 W·kg?1[121].Jorge Romero et al.used polyiminebased COFs to adsorb several metal ions (Fe2+, Co2+and Ni2+) in their cavities to produce N-doped graphene sheets by carbonization.These highly corrugated and hierarchical N-doped porous graphene sheets exhibited excellent electrochemical properties as electrode materials for SCs[122].

The above work results show that COFs-derived carbon materials could be used as electrode materials for SCs, and their electrochemical properties can be further enhanced by bonding with other functional elements.Meanwhile, these works provide a basis for developing high-performance SCs.Some notable SC performances of COFs-based materials discussed in this review are summarized in Table 2.

Table 2 Selected properties of COFs-based materials for SCs.(Continued).

Table 2 Selected properties of COFs-based materials for SCs.

4 Conclusions

This review has highlighted the promising scope of MOFs and COFs-based porous materials for SCs,including pristine MOFs and COFs directly used as electrode materials, hybrids of MOFs and COFs with conductive materials (conductive polymer, graphene and carbon nano-tube) and MOFs- and COFs-derived carbon materials.The basic parameters for electrode materials to achieve high-performance SCs include high-electric conductive continuous skeleton with open channels, high stability in organic/aqueous electrolytes and high specific surface areas with exposed electrochemical active sites.Based on these parameters, continuous efforts have been devoted to enhancing the efficacy of SCs.However, several challenges still exist that need special attention in the design of electrode materials.

(1) The flexible molecular design makes the MOFs and COFs’ variable structures based on various building blocks.The introduction of electrochemical redox-active units on skeletons increases the electrochemical performance.However, the non-redoxactive linkers in the frameworks decrease the capacitances and energy densities.Therefore, the density of the structures of redox-active units in frameworksshould be improved.

(2) Most MOFs and COFs exhibit poor structural stability in organic electrolytes (such as ionic liquids), which decreases the voltage window and the energy density of SCs.Hence, it is necessary to prepare MOFs and COFs with good structural good stability in organic electrolytes.

(3) MOFs and COFs could be obtained on a conducting support or composites could be prepared to improve their electrical conductivity.The multi-component synergies and heterogeneous structures complicate the structure-performance relationship.Their energy storage mechanisms still need to be deeply explored.

(4) The industrial applications of MOFs and COFs-based materials in SCs are cost-limited.The cost of the organic ligands used to prepare the crystal structure is high.The production of porous crystalline materials is largely at the laboratory scale.Hence, it is highly desirable to develop new synthesis methodologies to realize large-scale and low-cost production.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China(21701103).

Declaration of interests

There are no conflicts to declare.