Metal – Organic Frameworks and Their Derived Functional Materials for Supercapacitor Electrode Application

dual role as both a large of and the recent advances of MOFs and their as in supercapacitors, which are classi into pristine MOFs, MOF composites, and MOF-derived functional materials, on. Their synthetic routes and modi cation are summarized the relationship the diversity of the architectures and compositions of MOF-based materials their research extensive applications of MOFs their functional discharge process. However, the deep insights into the charge storage behavior of the MOF-based materials are still necessary to explore. 2) For the fabrication of MOF-derived functional materials for supercapacitors, precursors or templates involving several MOFs (e.g., ZIF-67, ZIF-8, MOF-74, MOF-5, MIL-101) are relatively limited. More outstanding precursors and templates need to be developed to achieve more novel functional materials with distinct architectures. Furthermore, the integration of these MOF-derived materials with other functional materials should be given considerable attention to realize synergistically enhanced electrochemical performance. 3) MOF-based materials as ﬂ exible electrodes often depend on other ﬂ exible substrates. It is of great importance to achieve a balance between electrochemical performance and mechanical properties. In addition, exploring a simple and versatile technology to achieve an ef ﬁ cient interaction between the two materials is also a promising direction, which is conducive to promoting the early realization of industrial production of these materials.

phosphates, metal selenides, metal carbides, etc., which are beneficial to efficient ion transportation. Meanwhile, MOF-derived materials can offer a rich metal and carbon source for large redox activity and high electrical conductivity. [5] By virtue of the distinct advantages, a large number of MOFs and their derivatives have been developed for energy conversion and storage applications, and some review articles have summarized their usage, especially the MOF-derived carbon and metal oxide materials with a specific architecture in supercapacitors. [6] In this review, we will comprehensively summarize the recent progress of pristine MOFs, MOF composites, and MOF-derived functional materials for supercapacitor electrode application. First, the synthetic strategies and modification methods will be discussed from the perspective of architectures (porous structure, dimensions, and morphologies) and compositions (electrode materials based on a single component/multiple components with various electrochemical behaviors). In the subsequent section, the application of varied types of MOF-based materials as supercapacitor electrodes is discussed. Therein, the correlation between the architecture and composition of the electrode materials and their electrochemical performance is specially emphasized. Finally, the challenges and perspectives of this field are featured.

Pristine MOFs
The electrochemical performance of pristine MOFs is significantly dependent on their architectures and compositions. The architecture, including the frameworks and porous structure, affects the specific surface area, transport channel, electrical conductivity, and structure stability of MOFs, which is strongly related to their transport capability of electrons and ions. And the composition (especially the metal center) determines the electrochemical activity and the number of active sites, demonstrating the pseudocapacitive behavior of MOFs. Herein, the synthetic strategies of pristine MOFs will be presented to manipulate their architectures and compositions.
MOFs are usually self-assembled by metal ions and organic ligands in a specific environment, including a suitable solvent (polar solvent or high-boiling-point solvent) and sufficient energy supply. Traditional heat sources such as an oven, as well as electric energy, microwave, ultrasonic irradiation, electromagnetic radiation, and mechanical grinding, can provide energy for the coordination process. In addition, the formation of the crystal structure is also related to the pH condition, molar ratios between the metal ion and organic ligand, the reaction time, and the solubility of the product. Interestingly, in the absence of any solvents, MOFs can also be produced via a vapor-phase growth route. [7] As shown in Figure 1a, a microporous zeolitic imidazolate framework (ZIF)-8 thin film has been fabricated through a chemical vapor deposition process, which includes two steps: a metal oxide vapor deposition and a consecutive vapor-solid reaction. [7a] The prepared ZIF-8 film presents a uniform and controlled thickness as well as a high-aspect-ratio feature. Typical architectures including 1D, 2D, and 3D micro/ nanosized structures of MOFs have been designed to promote their electrical conductivity and structural stability, which are beneficial for their improved electrochemical performance. [8] For the fabrication of 1D MOFs, several versatile ways, such as self-assembly, templating, the recrystallization process, microemulsion, and the lab-on-a-chip approach, have been developed. For instance, a metal ion-induced self-assembly template method has been used to prepare size-controlled 1D ZIF-8 nanorod, representing great potential for the design of other types of 1D MOFs (Figure 1b). [9] However, it is still a challenge to completely remove the template to achieve the ideal pristine 1D MOF materials. In recent years, 2D sheet-like MOF structures have stimulated significant attention because they possess larger specific surface areas, stronger quantum confinements, and richer exposed active sites compared with bulk MOFs. Especially, the ultrathin nanosheet structure can endow MOFs with increasing accessible active sites on their surface or in channels due to the ordered pores perpendicular to the side, contributing to their unique electron-and mass-transferring behaviors in many fields. Typically, the top-down method (physical/ chemical exfoliation) and bottom-up method (interfacial growth, three-layer synthesis, surfactant-assisted synthesis, etc.) are the common strategies to obtain 2D MOFs with a nanosized thickness and microscale transverse size. For example, He and co-workers have demonstrated a bottom-up approach for the fabrication of an ultrathin Zn-MOF nanosheet, which shows a thickness of only around 5 nm. [10] Compared with 1D and 2D MOFs, most MOFs are easily formed with 3D architectures due to the nonplanar characteristic of most of the ligands. The 3D MOFs possess the significant advantages of multiple porous structures, which provide not only a large specific surface area but also abundant channels for mass transport. [11] For instance, a type of 3D hierarchical porous Zr-MOF (HP-UiO-66) has been prepared via a solvothermal reaction followed by an etching process by hydrochloride acid, showing a much higher BET surface area of 874 m 2 g À1 than that of bare UiO-66 (490 m 2 g À1 ). [12] The obtained HP-UiO-66 can be directly utilized as a supercapacitor electrode with a capacitance of 849 F g À1 , which is 8.36 times larger than that of bare UiO-66 (101.5 F g À1 ), demonstrating its superior electrochemical performance.
In view of the significant influence of the metal center on the electrochemical performance of MOFs, so far, Co-, Zn-, Ni-, Fe-, Zr-, Cu-, and Cd-MOFs with different compositions have been explored as electrode materials for supercapacitors. Meanwhile, Ni-Co, Ni-Zn, Co-Mn, Co-Fe, and Zn-Zr bimetallic MOFs have also been developed for further increasing redox active sites and realizing the synergistic effect of multiple components. Most of them were prepared via a typical solvothermal reaction, the solvent system of which is either a single solvent or mixed solvents, such as N,N-dimethylformamide (DMF), methanol, water, DMF/ethanol, and water/ethanol/DMF. Interestingly, in addition to the solvothermal reaction, an anodic growth method was proposed to easily regulate the compositions of MOFs, and the related experimental setup is shown in Figure 1c. [13] In detail, Zn and Co foil were selected as electrodes and soaked into an electrolyte solution that had been heated and deoxidized with dissolved ligands under a voltage difference of 2.5 V between two electrodes. As a result, five different ZIFs (ZIF-4, ZIF-7, ZIF-8, ZIF-14, and ZIF-67) were obtained via such an anodic dissolution strategy.

MOF Composites
Poor conductivity has always been the most remarkable obstacle to promoting the electrochemical performance of most MOFs, which can be optimized by hybridization with other functional materials with good electrical conductivity, such as carbon materials, conducting polymers, and metals. Among them, carbon materials, such as graphene (GE), carbon fibers (CFs), carbon cloth (CC), and carbon nanotubes (CNTs), are generally integrated with MOFs under a solvothermal or hydrothermal reaction. In a typical procedure to prepare MOF composites with carbon, metal ions, ligands, and solvents are mixed with the carbon materials, and then the MOF will in situ grow on the surface of the carbon substrate at a specific time and temperature. [14] The hybridization of a metal substrate such as nickel foam and MOFs can also be achieved via a similar process. [15] In addition, another electrodeposition route is also usually used to integrate MOFs with carbon materials and Ni form, which can easily realize the in situ growth of MOFs uniformly on the substrate surface. [16] On the other hand, the integration of MOFs with conducting polymers can be roughly divided into two categories. One is to grow MOFs on conducting polymer substrates, and the other is to grow conducting polymers on or into MOFs. The latter can be accomplished in a system containing a polymeric monomer, MOFs, an oxidant, and a solvent through a chemical or electrochemical synthesis. Especially, several novel routes have been developed for the preparation of metal oxides/MOF composites. For instance, a chemically in situ self-transformation route was demonstrated to fabricate a MnO x @MOF composite using MOF-Mn hexacyanoferrate hydrate nanocubes as the starting precursor. [17] In addition to binary composites, MOF-based ternary composites can also be prepared via a similar procedure. For example, a two-step hydrothermal approach was performed to grow Ni-based MOFs on CC with Co 3 O 4 . [18] A ternary graphene oxide (GO)/Cu-MOF/ poly(3,4-ethylenedioxythiophene) (PEDOT) composite has been fabricated via an electrochemical polymerization process. [19] Specially, an adhesive agent is usually beneficial for the integration of MOFs with other functional materials. In previous reports, dopamine is one of the most popular adhesive agents, which can be used to guide the synthesis of CFs@UiO-66/ polypyrrole (PPy). [20] In addition, small compounds can be directly introduced into the structure of MOFs during their growth process via solvothermal or hydrothermal reactions. [21]

MOF-Derived Functional Materials
MOFs can be used not only directly as active electrode materials, but also as templates or precursors to produce other functional electrode materials with a preserved porous structure. MOFderived carbon materials, metal oxides, metal hydroxides, metal Reproduced with permission. [7a] Copyright 2015, Springer Nature. b) Schematic drawing of a metal-induced self-assembly template method for the fabrication of 1D MOFs. Reproduced with permission. [9] Copyright 2020, American Chemical Society. c) The experimental setup used in an anodic growth method to prepare MOFs. Reproduced with permission. [13] Copyright 2016, Elsevier. d) An indirect route for the preparation of MOF-71-derived metal oxides. Reproduced with permission. [23] Copyright 2017, Elsevier. sulfides, metal phosphates, metal selenides, metal carbides, and their composites have been prepared via different routes. In the following, we will introduce the corresponding synthetic strategies for these MOF-derived functional materials. The production of carbon materials derived from MOFs usually undergoes a carbonization process at a high temperature under an inert atmosphere. Generally, during the carbonization process, metal ions are converted into corresponding metal species (e.g., metal, metal oxides, metal carbides), which can also be removed with different acidic solutions, such as hydrochloric acid and hydrofluoric acid. Zn and Cd may be directly evaporated from carbon materials under a high temperature because of their relatively low boiling point.
Although pyrolysis under a nitrogen or argon atmosphere can generate MOF-derived functional materials, this method is still time-consuming and it is a little difficult to control their architecture. Therefore, several mild strategies, such as microwaveassisted heating, CO 2 laser engraver, and alkaline hydrolysis, have been developed to prepare MOF-derived carbon, metals, and metal oxides (hydroxides). [22] For example, a microwaveassisted heating method has been developed to fabricate MOF-5-derived carbon materials. [22a] In addition, with a KOH activation and acidic washing process, 3D interconnected porous carbon can be achieved, which shows a huge BET surface area up to 1595 m 2 g À1 . Commercial CO 2 laser engraving is another efficient synthetic route to prepare MOF-derived materials, which has been performed to transform MOF-74(Ni) into Ni nanoparticles/porous carbon. [22b] The advantage of the laser scribing endows a rapid quenching rate, beneficial for the formation of Ni nanoparticles with a small size and high uniformity. On the other hand, an indirect alkaline hydrolysis reaction has been used to prepare MOF-derived metal oxides. As shown in Figure 1d, Co 3 O 4 was transformed from Co(OH) 2 that was derived from MOF-71 by an alkaline aqueous treatment, showing a higher Brunauer-Emmett-Teller (BET) surface area than those synthesized by a direct pyrolysis method. [23] Similarly, MOFderived metal hydroxides can be prepared via hydrolysis or chemical etching during a hydrothermal reaction or in the alkali aqueous solution. During the hydrothermal process, other mental ions can be added to produce mixed metal hydroxides and even layered double hydroxide (LDH) through ion coprecipitation or ion exchange. Recently, Sun and co-workers reported a unique route to prepare a NiCo 2 O 4 /β-Ni x Co 1Àx (OH) 2 /α-Ni x Co 1Àx (OH) 2 ternary composite with the assistance of alkaline hydrolysis followed by a selective oxidation process using H 2 O 2 as oxidant. [24] As for the production of MOF-derived metal sulfides, two main synthetic routes are proposed to offer a sulfur source. One is using thioacetamide (TAA), thiourea, and sodium sulfide as sulfur sources in a solution system under a hydrothermal (solvothermal) or wet chemical process at a specific temperature. During such reaction processes, both MOFs and their derivatives can be utilized as the precursors. The other strategy to prepare MOF-derived metal sulfides is using sulfur powder as sulfur source during a calcination process. The amount of sulfur powder and heating parameters are the key role to ensure uniform vulcanization of materials. Among them, the one-pot calcination-sulfurization process can avoid the sintered phenomenon-caused by a stepwise calcination and sulfurization process, which is more suitable for preparing metal sulfides.
In particular, an organic ligand, containing sulfur atoms, can also provide sulfur sources for MOFs. For instance, thiophene-2,5dicarboxylate has been used as both a ligand and a sulfur source for the synthesis of Co-MOF-derived Co 9 S 8 @S,N-doped carbon materials. [25] A similar calcination process to that for metal sulfides is necessary for the conversion from MOFs to metal phosphide, metal selenide, and metal nitride-based materials. [26]

Supercapacitor Applications
In this section, we highlight the remarkable progress on the promising applications of pristine MOFs, MOF composites, and MOF-derived functional materials in supercapacitor electrodes. Special emphasis is focused on the correlation between the architecture and composition of the electrode materials and their electrochemical performance. Generally, the charge storage mechanism of the supercapacitor electrodes can be classified into EDLC-, pseudocapacitive-, and battery-type behaviors. [27] Based on the physisorption of electrolyte ions on their internal surfaces and reversible redox reactions between the metal centers and electrolyte, the charge storage of the pristine MOFs can be attributed to both EDLC and pseudocapacitance mechanisms. For the MOF composites with complex components, a synergistic multiple charge storage mechanism is usually involved in the electrochemical reaction. On the other hand, a large variety of functional materials can be derived from MOF materials. MOF-derived nanoporous carbons are mainly related to the EDLC-type characteristic, while MOF-derived metal oxides, metal sulfides, metal phosphates, metal selenides, and metal hydroxides are mainly ascribed to the pseudocapacitive-and battery-type behaviors for supercapacitor electrode application.

Pristine MOFs for Supercapacitor Electrodes
Due to the good redox activity, tunable pore structure, and high specific surface areas, pristine MOFs can directly serve as electrode materials. Over the last few decades, a lot of research attempts have been devoted to improving their electrochemical performance.
Generally, the electrochemical performance of MOFs suffers from their poor electrical conductivity; therefore, many researchers focus on the investigation of conducting MOFs. Ni 3 (2,3,6,7,10,11-hexaiminotriphenylene) 2 (Ni 3 (HITP) 2 ), composed of stacked π-conjugated 2D layers, was fabricated as the electrode material in an electric EDLC. [28] Ni 3 (HITP) 2 is the first MOF used as an active material in supercapacitors without the addition of a conductive agent or other binders, delivering a superior areal capacitance to those of the majority of carbon-based materials. Figure 2 shows a phthalocyanine-based 2D conjugated MOF nanosheet with the conductivity of 0.01 S m À1 and thickness of %7 nm (%10 layers), as the flexible thin-film electrodes in supercapacitor. Figure 2a-c shows the corresponding synthetic strategy, scanning electron microscope (SEM) image, and atomic force microscope (AFM) images, respectively. Its outstanding chemical stability (acid/alkaline aqueous solution and other polar solvents), p-type semiconducting behavior, as well as ultrathin features endow this MOF material with excellent cycling stability and a high areal capacitance (18.9 mF cm À1 ) in the MOF//G device (Figure 2d-f ). [29] Bao and co-workers have studied the mechanism of high capacitance in a conductive MOF for supercapacitor application, demonstrating a pHdependent surface pseudocapacitive behavior based on ligandcentered redox activity. [30] In addition, charge storage and charging dynamics of the supercapacitors based on conductive MOFs (electrodes) and ionic liquid (electrolytes) at a molecular scale were also revealed by constant-potential molecular dynamics simulations, demonstrating that a better electrochemical performance can be achieved in an MOF/ionic liquid-based device than in most carbon-based cells. [31] To promote the electrochemical performance of MOF-based materials in supercapacitor electrodes, more strategies are focused on their compositional and structural tunability. From the composition point of view, the choice of metal center and ligand will lead to the difference of their properties. First, longer molecules as organic linkers may bring about larger exposed surface area and pore, offering the free path for charge transfer. Meanwhile the molecular dimensions of MOFs are related to their surface architecture. For example, Han and co-workers reported the preparation of three types of MOFs generated by the coordination of cobalt with benzendicarboxylic acid (BDC), 2,6-naphthalenedicarboxylic acid (NDC), and 4,4-biphenyldicarboxylic acid (BPDC), respectively. [32] The specific capacitance rose from 131.8 to 179.2 F g À1 at 10 mV s À1 with the increase of molecular length. Second, the functional groups on the ligands affect the redox activity and stability of MOFs by controlling the distribution of charges in the ligand, which is related to the electron-donating/withdrawing effect and the polarity of groups. Specifically, the metal center can be activated by an  [29] Copyright 2020, John Wiley and Sons.
www.advancedsciencenews.com www.advenergysustres.com electron-donating group due to its increased electron cloud density, which provides easier transfer of electrons and mitigates the polarization of the electrode. [33] In general, nonpolar groups are beneficial to enhance the structural stability, which has been proved by Liu and co-workers. [34] Compared with 1,4-naphthalenedicarboxylic acid (NDC), Ni-MOFs, with similar topology, based on the linker with nonpolar groups (9,10-anthracenedicarboxylic acid (ADC) and 2,3,5,6-tetramethyl-1,4-benzenedicarboxylic acid) tend to produce more stable structures in the presence of a second ligand (1,4-diazabicyclo[2.2.2]octane (DABCO)). The high structural stability of DABCO-MOF (DMOF)-ADC contributes to its excellent cycling stability (>98%, 16 000 cycles). It is worth pointing out that the structural stability mentioned is attributed to the inherited structure of the converted hydroxides from DMOF-ADC in strong alkaline electrolytes. In addition, structural stability is also related to the water content in MOFs and the central metal ions. [35] More importantly, the pseudocapacitance of MOFs is largely determined by the redox activities of the metal ions. As shown in Table 1, different metal ions (Ni, Co, Zr, Zn), along with BDC, are used to produce MOFs as electrode materials for supercapacitors, whose growth conditions and capacitive properties are shown in the table for comparison. [36] Especially, Eddaoudi and co-workers designed a Zr-based-MOF that contained a redox active core in the organic ligand, making a favorable contribution to the capacitance performance due to the increased redox activity, rigidity, and surface area. [37] Third, mixed-metallic MOFs generally display more advantages than single-metallic MOFs as supercapacitor electrode materials, which may be attributed to the increased pore volume, electrochemical active sites, electrical conductivity, crystallinity, and synergistic effect from multiple components.
In the previous literature, Co and Zn ions occupying the position of Ni ions in MOFs based on BDC or 1,3,5-benzenetricarboxylic acid (BTC) result in a significant improvement in capacitance, rate performance, and cycling life. [36g,38] Interestingly, 1,10-ferrocene dicarboxylic acid (FcDCA), including both Fe species and carboxyl groups, was designed into a Co-MOF based on 4,4 0 -bipyridyl (bpy) to achieve an inclined polycatenated {[Co 4 (FcDCA) 4 (bpy) 4 (H 2 O) 6 ]·11H 2 O} n , showing promising applications for supercapacitors and dye adsorption. [39] Fourth, the properties of an electron donor or acceptor can be improved by the inclusion of pyridyl rings, halogen and hydroxyl ions into MOFs, resulting in better specific capacity and cycle stability. [40] The improved electrochemical performance of MOFs can also be achieved from the perspective of structure and morphology. The growth time, reaction temperature, type of solvent, along with the synthetic method determine the morphology and crystal structure of MOFs. Typically, controlling the growth time enables MOFs to present various macroscopic sizes and microcosmic crystal structures. A nanorod-like MOF powder has been obtained by shortening the growth time of bulk MOFs, delivering a high specific capacitance of 2405 F g À1 (0.75 A g À1 ), which is seven times larger than that of a bulk single-crystal MOF electrode. [41] Wei et al. have reported two types of MOFs (Ni-MOF-12 and Ni-MOF-24) with 12 and 24 h growth time, respectively, exhibiting crystal structures with different largest exposed facets. [36b] A wider exposed (100) facet in Ni-MOF-24 compared to (020) facets in Ni-MOF-12 is more conducive to the improvement of the electrochemical performance. Furthermore, the size and morphology of MOFs can also be achieved by tuning reaction temperatures. For instance, four environmental temperatures (50, 70, 90, and 110 C) have been offered to support the growth of UiO-66. As a result, UiO-66 at 50 C with the morphology of smaller particles possessed a higher specific capacitance (1144 F g À1 , 5 mV s À1 ), which is attributed to its larger surface area (1047 m 2 g À1 ).
[36d] Meanwhile, the specific capacitance of these MOFs decreased with the increasing of the synthesis temperature. The type of solvent also produces an effect on the size and morphology of MOFs, which stems from the pH value of the solvent system. It is believed that the deprotonation rate of the organic linker plays a vital role in the rate of nucleation and growth of the MOF, further affecting its size and morphology. [42] A Co-MOF prepared in a DMF/ethanol solvent, with a higher pH value than DMF, DMF/H 2 O, ethanol, and DMF/H 2 O/ethanol, results in the morphology of nanosized needles and sharp-edge nanorods, accelerating the ion exchange and transportation for enhanced electrochemical behavior. [43] A solvent-tailoring strategy has been used to control the morphologies of a Ni-Co MOF by altering the water content in a mixed solvent system. Figure 3a shows the corresponding morphologies of samples in three typical solvent systems. Compared with the samples with morphologies of nanosheets and rhombus sheets, nanosheet-assembled hollow spheres that are synthesized in a mixing solution of DMF, ethanol, and a certain water display a superior specific surface area and porous characteristics, contributing to a higher specific capacitance of 1126.7 F g À1 at 0.5 A g À1 . [44] In addition to the solvent, some solid agents such as benzoic acid and sodium formate can also regulate the m orphologies of some MOFs by tailoring the nucleation rate. The lower concentration of solid agents corresponds to a higher nucleation rate, leading to smaller crystal sizes. [45] Recently, Xu and co-workers reported the fabrication of an ordered macro-microporous single-crystalline MOF (HKUST-1) via a monodentate-ligand-assisted template method. The pore architecture, SEM image, and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of HKUST-1 are separately shown in Figure 3b-d. [46] During the formation of HKUST-1 from the PS template, sodium formate not only induces the nucleation of the crystallization solution through the deprotonation of ligands, but also serves as a capping agent to limit the crystal sizes. Moreover, the addition of a surfactant and second metal ions can also change the morphology of MOFs. [38c,47] In general, MOFs are regarded as templates or precursors to prepare other functional materials. However, there are few reports on the fabrication of pristine MOFs using other target products as templates and precursors. Recently, Zhang and co-workers utilized Co(OH) 2 as both the template and precursor to fabricate a vertically oriented MOF electrode, which delivers a double specific capacitance and superior rate capability compared to MOFs in powder form. [48] In addition, some novel porous-structured MOFs can be prepared via an etching process. For instance, Zn/Zr MOFs were used as a precursor to prepare hierarchical porous Zr-MOFs. In detail, Zn-MOFs were removed in 1 M HCl aqueous solution to achieve the unique characteristics of rich surface defects, high pore volume, and large specific surface areas. Accordingly, the prepared Zr-MOFs with a unique characteristic represents a specific capacitance of 849 F g À1 , which is about eight times higher than that of traditional  In addition, some other MOFs with different structures or compositions have giant potential as electrode materials for supercapacitors, and the related modification studies need to be further explored. [49] 3.2. MOF Composites for Supercapacitor Electrodes MOF composites are designed to mitigate the shortcomings or expand the advantages of pristine MOFs; thus, the materials usually display ideal electrical conductivity, good chemical stability, large specific surface areas, and acceptable mechanical properties, which can be selected as the main research objects to construct an electrode material with a better electrochemical performance. [50] In general, the electrochemical performance of MOF composites mainly depends on three aspects: 1) the architecture and metallic ions of MOFs. The former affects the Figure 3. a) Schematic illustration of three synthetic routes and corresponding SEM images for the NiCo-MOFs. Reproduced with permission. [44] Copyright 2020, American Chemical Society. b) Schematic illustration of macropores and micropores, c) SEM image, and d) HAADF-STEM image along the [100] zone axis of HKUST-1. Reproduced with permission. [46] Copyright 2020, John Wiley and Sons.
www.advancedsciencenews.com www.advenergysustres.com diffusion and transport of electrolyte ions, and the valence variability of the latter is related to the pseudocapacitive activity.
2) The physical and chemical properties of the hybrid materials with carbon, conducting polymers, metal, metal oxides, etc. Especially, the conductivity and stability of these additional functional materials can compensate for the poor rate performance and short cycling life of pristine MOFs.
3) The synergistic effect between two different materials. The synergistic effect is reflected from an enhanced electrochemical performance from the electron transfer and interfacial effect between the two different components in the hybrids.

MOF/Carbon Materials
Carbon materials possess outstanding electrical conductivity and chemical stability, and are considered promising candidates to be integrated with MOFs. Carbon materials not only provide the opportunity to optimize the electron transfer and ionic diffusion in MOF composites, but also can be utilized as supports to eliminate the aggregation of MOFs, and even act as a current collector.
Typically, the single-layer graphite structure of graphene renders its unique physical and chemical properties. Currently, graphene, graphene aerogel (GA), graphene oxide (GO), and reduced graphene oxide (rGO) all appear in the design of MOF composites. [51] The hybridization of these materials with MOFs depends on their electrostatic interaction, strong p-π interaction, and covalent bonding. As shown in Figure 4a, an amine-functionalized MOF (UiO-66-NH 2 ) was attached to carboxylate-functionalized graphene (CG) based on the formation of amide bonds, a high-resolution transmission electron microscope (HRTEM) image of which is shown in Figure 4b. [52] UiO-66-NH 2 nanocrystals between CG layers increase the Reproduced with permission. [52] Copyright 2020, John Wiley and Sons. d) Demonstration of the synthetic procedure of the MOF@COF hybrid structure. Reproduced with permission. [64] Copyright 2020, John Wiley and Sons.
www.advancedsciencenews.com www.advenergysustres.com specific surface area, and the π-conjugated structure dominated by amide linkage accelerates charge transportation, leading to a high capacitive performance of CG@UiO-66-NH 2 (651 F g À1 ) that exceeds that of most traditional graphene-based materials (Figure 4c). To explore the role of graphene in the electrochemical performance of the hybrids, the ratio of MOFs to graphene was explored. For example, Zhai and co-workers have reported three isostructural Ni-MOFs integrated with GO as efficient supercapacitor electrodes to boost their specific capacitance. [51c] And an optimal GO content of 3% in the Ni-MOF/GO hybrid is confirmed to achieve the highest specific capacitance (590 F g À1 ), which is determined by the balance between the high transport rate of electrons (from the high GO content) and high redox activity (from the high MOF content). For the preparation of an MOF/graphene composite, the introduction of dual metal ions into MOFs can bring about an enhanced electrochemical behavior because the presence of dual metal ions in MOFs may result in a synergistic effect between the metal species. [51d] In contrast, in an MOF/GA composite system, the metal-doped graphene can increase their interaction, which is able to influence the electric double-layer characteristics of the hybrid. [51b] Furthermore, calcination at an appropriate temperature can also improve the capability of the material to a certain extent. Similarly, CNTs and CFs are also distinct carbon materials that exhibit outstanding electrical conductivity, chemical stability, and mechanical properties. CNTs and CFs modified by carboxylic groups, as backbones, can be covered by MOFs evenly and also affect the morphology and array of MOFs. [53] A solid spherical Ni-MOF was transformed into a flower-like Ni-MOF in the presence of multiwalled CNTs (MWCNT@Ni-MOF), presenting a core-shell structure. This hierarchical architecture and porous nature endow the composites with a large surface area, contributing to more accessible active sites for electrolyte ions, and the addition of MWCNTs furnishes faster charge transport. As a result, the MWCNT@Ni-MOF delivers a specific capacity of 115 mA h g À1 (2 A g À1 ) and good rate properties, which are beyond those of the spherical Ni-MOF. In addition, conductive Cu-MOF nanowire arrays grown on CF papers as conductive additives and binder-free electrode have also been reported by Wang and co-workers. [54] Based on their electrochemical performance, it is reasonable to suggest that the morphologies of highly oriented nanowire arrays compared to powder are more favorable for electrochemical performance. The positive effect of CNTs on the electrochemical properties of MOF-based composites is also reflected in both CNT/Ni-MOF and CNT/Mn-MOF hybrids. [55]

MOF/Conducting Polymers
Conducting polymers, including PPy, polyaniline (PANI), and PEDOT, possess high theoretical capacitance, good electronic transmission capability, and a wide potential window, and are also considered promising electrode materials for supercapacitors. When conducting polymers are integrated with MOFs, they can serve as bridges to offer smooth charge pathways between the outer circuit and the internal surface of MOFs, and act as supports for evenly dispersed MOF nanostructures, leading to richer active sites and higher specific surface areas. [56] PPy, involving a conjugated molecular structure, shows higher density and better flexibility in comparison with most other conducting polymers. PPy can not only emerge as a substrate for the growth of MOFs, but also encapsulate the structure of MOFs. [57] Remarkably, PPy was introduced into a Zn/Ni-MOF for supercapacitor application. [58] It is notable that the polymerization of pyrrole benefits from the existence of the Zn/Ni-MOF (Lewis acid), which catalyzes the polymerization process using O 2 as the oxidant at room temperature. Based on the investigation of the different amounts of PPy in this composite, there is a trade-off relation between the ion diffusion resistance (low PPy content) and electrical conductivity (high PPy content). Differently, various kinds of MOFs as porous inorganic fillers could be added into a conducting poly(ortho-aminophenol) (POAP) to form a series of composites (Cu-bipy-BTC/POAP, ZIF-67/POAP, [Cu(1,2,4,5-benzenetetracarboxylate acid) 0.5 DMF]/ POAP), which exhibit improved electrochemical behavior. [59]

MOF/Other Materials
In addition to typical carbon and conducting polymer electrode materials with good electrical conductivity, MOF/metal, MOF/ metal oxides, MOF/metal hydroxides, MOF/COFs, and MOF/ small molecules are also attempted to achieve an ideal electrochemical performance, giving full play to the advantages of the two materials. [1a,b,60] In general, Ni foam (NF), as a collector, is broadly applied in the field of supercapacitors due to its low internal resistance. Therefore, NF covered with MOFs can be utilized as a selfsupported electrode, which avoids the addition of binders and conductive additives, resulting in large active areas and low contact resistance. Mn 0.1 -Ni-MOF/NF (6.48 C cm À2 ), Co-Mn-based bimetal MOF/NF (2.375 F cm À2 ), Co-MOF/NF (13.6 F cm À2 ), and Ni-MOF/NF (13.64 mF cm À2 ) have been reported in pioneering works, demonstrating that NF as a backbone for MOFs shows unique advantages. [15,16,61] In addition to improving conductivity, the integration of metal oxides with MOFs can achieve an increasing pseudocapacitive activity, specific surface area, and durability. [17,62] For example, MnO 2 , a typical pseudocapacitive material, has been introduced into MOF (manganese hexacyanoferrate hydrate nanocubes) to boost the pseudocapacitance of electrode materials, which can be assembled into a flexible solid-state hybrid supercapacitor to show an areal capacitance of 175 mF cm À2 at 0.5 mA cm À2 , superior to many previously reported flexible supercapacitor devices. [17] In another typical example, a wide voltage window of 2.0 V was achieved for a supercapacitor composed of Cu-MOF@d-MnO 2 and Na 2 SO 4 electrolyte. [62c] Then a high specific capacitance of 340 F g À1 at 1.0 A g À1 and an excellent cycling stability were achieved. Remarkably, our group has utilized electrospun preoxidized polyacrylonitrile nanofibers (PPNFs) with rich functional groups as the substrate to support the Ni-MOF nanosheets to realize a significantly increased specific surface area. [62e] This MOF-based composite presents a large capacitance of 702.8 F g À1 at 0.5 A g À1 and outstanding cycling stabilities over 10 000 cycles. In addition, Co, Zn, Cu, and Fe ions were introduced to Ni-MOF@PPNF for better electrochemical properties.
[62f] Among them, the introduction of Co ions brings about the greatest improvement to the capacitance and rate performance of this composite, which is due to the well-preserved morphology, high redox activity, and cooperative effects of Co and Ni. In addition to metal ions, molybdenum (VI) nanoparticles have also been reported to be confined in the nanopores of a Zr-MOF, offering an enhanced electrochemical behavior caused by more redox-active sites. [63] The integration of MOFs with covalent organic frameworks (COFs) is a newly developed direction to achieve high performance in supercapacitors. The architecture involved in the all-covalent nature renders COFs with higher thermal and chemical stability. An Aza-Diels-Alder reaction as postsynthetic modification was performed to fabricate a robust, functional porous Aza-MOF@COF hybrid material (Figure 4d). [64] The hybrid material shows a superior capacitor performance with a specific capacitance of 20.35 mF cm 2 and volumetric energy density of 1.16 F cm 3 . This performance is beyond most of the reported MOF@COF hybrids.

Ternary MOF-Based Composites
Compared with binary MOF-based composites, ternary MOF composites have stimulated more and more attention benefiting from the synergistic effect of three materials. [65] More comprehensive characteristics are gathered by various components. For example, ZIF-67 grown on CC was selected for electrochemical deposition of PANI. PANI plays the role of a bridge for charge transportation between the external circuit and the internal surface of MOFs. This flexible porous electrode material displayed an areal capacitance of 2146 mF cm À2 (10 mV s À1 ), which is attributed to EDLC capacitance produced from the internal surface areas (MOFs), pseudocapacitance (PANI), good flexibility, and high conductivity (CC), as well as the synergistic effect among all the components. [65a]

MOF-Derived Functional Materials for Supercapacitor Electrodes
MOFs have great potential to convert into other types of active electrode materials, which can be due to the following points. On one hand, the metal center and organic ligand can provide metal, carbon, metal oxides, metal hydroxides, metal sulfides, metal phosphates, metal selenides, metal carbides, etc. as active components for supercapacitors. Therein, nitrogen, sulfur, and phosphorus sources have also been used as dopants in carbon materials. On the other hand, a part of the framework and pore structure of MOFs can be preserved after a series of treatments, resulting in abundant exposed electrochemical active sites and favorable paths for ions transportation.

Carbon Materials and Their Composites
Carbon materials derived from MOFs show superiority to conventional carbon materials because of their distinct compositions and unique porous structures. Typically, organic ligands in MOFs as precursors not only offer a rich carbon source, but also grant carbon materials with various heteroatom doping, contributing to a high specific capacitance, which is related to the pseudocapacitive effect caused by N, P, and O dopants. [66] In addition, the wettability of carbon materials can be ameliorated. Especially, nitrogen dopants can even enhance the electron transfer ability of materials, leading to better rate performance. [67] The type and content of heteroatom doping have an impact on the electrochemical performance of the carbon material. As a typical example, self-doping of N (2.2 at%), O (30.9 at%), and P (7.5 at%) in carbon materials generated from a hexakis(4carboxylphenoxy)-cyclotriphosphazene-based MOF are beneficial for high-performance supercapacitor electrodes. The synergistic effect between different heteroatoms is conductive to good electrochemical behavior, including a large charge storage capacity (1258.7 F g À1 at 8 A g À1 ), excellent rate performance (1087.1 F g À1 at 20 A g À1 ), and superior cycling stability (110% after 50 000 cycles). [68] In another example, a Zn-MOF constructed with 2-ethylimidazole and 5-methyltetrazole as a double ligand was chosen as the precursor for the preparation of a ultrahigh-content (14.23 wt%) nitrogen-decorated nanoporous carbon, demonstrating that pyridinic N and pyrrolic N play an important role in pseudocapacitance. [69] Generally, the nitrogen content of carbon materials is related to calcination temperatures. The higher the temperature, the lower is the nitrogen content of carbon materials. Taking carbon materials derived from IRMOF-3 as an example, when the temperature increases from 600 to 950 C, the nitrogen content reduces from 7% to 3.3%. [70] Therefore, the electrochemical performance of the MOF-derived carbon materials is usually related to the calcination temperature, one reason for which is attributed to the content of heteroatom dopants. In addition to the doping to tailor their composition, MOF-derived carbon materials are also able to possess oxygen-containing functional groups, which are beneficial for improved wettability, contributing to increased pseudocapacitance. However, carboxyl groups with large molecular structure may become an obstacle to improving their capacitance due to the limited migration of ions in the carbon materials. [71] In addition to nonmetal elements, metal-doped carbon materials can be obtained from MOFs via a one-pot calcination process. The excellent electrochemical performance of this kind of electrode material benefits from the large electrochemical activity and high electrical conductivity of metal nanoparticles, as well as the stability of carbon materials, resulting in their potential as both positive and negative materials in supercapacitors. For example, Koner and co-workers designed a cobaltembedded N-doped carbon nanostructure as an efficient electrode material for supercapacitors because of the synergistic effect of the Co nanoparticle-embedded graphitized carbon matrix and nitrogen dopant. [72] The size and structural ordering of pores, determined by the framework of MOFs and calcination temperature, are other important factors affecting their electrochemical performance. [31,73] The size effect on electrochemical properties of an electrode material is reflected in MOF-derived carbon materials, which can be realized by tailoring the size of both the MOFs and the derivatives. Small-sized carbon materials may correspond to higher specific surface areas, but they face the issue of easy aggregation because of their strong interactions. Thus, there is a balance on the characteristic caused by the size of MOFs. Poly(ethylene glycol), polyvinylpyrrolidone, and benzoic acid are usually used to control the size of different types of www.advancedsciencenews.com www.advenergysustres.com MOFs, which can be used as templates for the preparation of porous carbon materials with controllable sizes. [71a,74] Micropores and mesopores make a significant contribution to both high energy density and high power density. [75] The ordered porous structure facilitates the ion transport behavior. In a typical work, four types of carbon materials with different porous structures and morphologies were prepared using BDC, trimesic acid, 4,4 0 -biphenyldicarboxylate, and 1,3,5-triphenylbenzene as ligands, respectively. [76] As a result, MOFs with shorter struts can maintain their initial morphologies after pyrolysis at 950 C, and a long-range ordered porous structure was achieved for carbon materials derived from MOFs, which led to excellent cycling stability (98%, 5000). In another typical example, MOF-5 was carbonized at 530, 650, 800, 900, and 1000 C to produce five porous carbon materials with all pore size distributions centered at %3.9 nm. [77] The sample calcined at 650 C delivered the highest specific capacitance, even exceeding those of samples with higher specific surface areas and better electrical conductivities. Similarly, ultramicroporous carbon nanoparticles (UiO-67), [73f] mesoporous carbon nanospheres (MOF-5), [78] and interconnected mesoporous carbon sheets (MOF-5) [73g] have also been studied as electrode materials for supercapacitors, which display a favorable electrochemical performance. Acid etching is another versatile route to prepare porous MOF-derived functional materials. For instance, most MOF-derived metal species in a carbon material can be removed to increase the pores and specific surface area as well as induce oxygen vacancy and defect of carbon. [66c] As an example, HCl was utilized to etch Ni nanoparticles in a Ni-MOF-derived Ni/carbon hybrid material to produce porous carbon. The as-prepared mesoporous carbon exhibited lower series resistance and higher specific capacity than those of the untreated sample, which resulted from the porous structure and increased surface area and oxygen vacancy. [79] In addition to acid etching, alkaline activation is also a feasible strategy to enhance the electrochemical behavior of carbon materials. [66b,80] Furthermore, to prevent the excessive usage of KOH to lead to large leakage and corrosion of carbon, a new strategy of K þ in situ activation was systematically investigated recently to prepare hierarchical porous carbons derived from Zn 8 (adeninate) 4 (biphenyldicarboxylate) 6 O·2Me 2 NH 2 , showing a large specific capacitance of 230 F g À1 and superior cycling stability with 97% retention over 10 000 times. [81] Recently, many researchers have paid attention to the novel architecture and specific morphology of MOF-derived carbon materials, including hollow, tubular, layered, core-shell structure, and flower-like, waffle-like, sponge-like, ribbon-like, polyhedron, cuboid, bubble, and microsphere morphologies that are remarkably beneficial for improved electrochemical performance. [10,82] As shown in Figure 5a, Yamauchi and co-workers reported the preparation of 3D interconnected N-doped carbon tubes (NCTs) derived from ZIF-8, using electrospun polyacrylonitrile nanofibers/Zn(Ac) 2 as templates. [82a] Figure 5b displays the energy-dispersive spectrum (EDS) elemental mapping of the NCTs. As a binder-free electrode, the tubular architecture not only shortens the diffusion paths for both charges and ions, but also offers accessible surface areas to accommodate ions, leading to the ultrahigh capacitive deionization performance exhibited in Figure 5c. A sandwiched MOF/ LDH/MOF structure was used to fabricate waffle-like carbons with high nitrogen contents and enriched mesopores. [82b] During calcination, the structure can mitigate the severe aggregations and nitrogen loss, contributing to a specific capacitance of 300.7 F g À1 (1 A g À1 ). In addition, ZnO nanosheets were utilized as self-sacrificial templates for the synthesis of 2D  ZIF-8-derived carbon nanosheets, [82c] and ZIF-8-derived hollow carbonaceous materials have also been prepared through the collapse of the MOF internal structure caused by the acid provided by the hydrolysis of glucose, with both products exhibiting excellent electrochemical performance as supercapacitor electrodes. [82d] Especially, graphene nanoribbons were generated from carbon nanorods (rod-shaped MOF-74) through a sonochemical treatment followed by thermal activation. As a result, the graphene nanoribbons displayed a higher specific capacitance than those of carbon nanorods and microporous carbon, which stems from the improved dimensions. [83] As mentioned earlier, MOF-derived carbon materials show favorable electrochemical performance, but their specific capacitance is still much lower than that of the pseudocapacitive electrode materials. To enhance the specific capacitance of the MOFbased carbon materials, an efficient route is preparing their composites integrated with other types of carbon, conducting polymers, metals, metal oxides, metal sulfides, etc. Other types of carbon materials that are introduced to be hybridized with MOF-derived carbon materials can offer smooth pathways for the transportation of electrons and ions. In this way, 3D hierarchical porous carbon derived from both a Zn-MOF and glucose, [84] 3D graphene-based nanoporous carbon derived from a graphene/Al-MOF composite, [85] and "brick-and-mortar" sandwiched porous carbon derived from a MOF-5/GO composite [86] have been prepared for supercapacitor application. In addition, N-doped carbon polyhedrons (NCPPs) derived from ZIF-8 on graphitic carbon nitride sheets (g-CN) have also been prepared through a calcination and carbonization process of a mixture of urea, glucose, and NCPPs. This composite reached a specific capacitance of 349.7 F g À1 at 0.5 A g À1 , which exceeds that of both the NCPP and g-CN samples. [87] Furthermore, MOFderived carbon can be supported or linked by other functional carbon materials, granting them great potential as binder-free (self-supporting) electrode materials with good stability. For example, copper-based hierarchical carbon material with a helical tube derived from a Cu-MOF on calcinated cotton and hierarchical porous carbon film derived from an MOF/CNT have been prepared for the construction of binder-free supercapacitor devices, which showed excellent specific capacitance and stability. [88] From another point, it seems that a stable structure and favorable electron transfer capacity can also be supplied by MOF-derived carbon materials with conducting polymers, metals, metal oxides, metal sulfides, etc., which can bring an enhanced capacitance to MOF-derived carbon materials. [89] In a previous work, ZIF-8-derived nanoporous PANI with a coreshell structure showed a superior specific capacitance and rate property over MOF-derived carbon and bare PANI material. [89b] In addition to the pseudocapacitance of PANI, the 3D network structure from ZIF-8-derived carbon and PANI with a remarkable specific surface area contribute to the excellent electrochemical performance. To be integrated with metals, a Ni-Co-MOF has been grown on the surface of a coconut leaf sheath derived nitrogen-doped carbon framework, followed by a pyrolysis process. [90] The prepared Ni and Co nanoparticles encapsulated in N-doped coral-shaped carbon delivered a high specific capacity of 308 mA h g À1 at a current density of 1 A g À1 in an alkaline solution. Focusing on the nature of long cycle life and fast charge of the Nb 2 O 5 material, Nb 2 O 5 quantum dots have been embedded in ZIF-8-derived N-doped porous carbon, leading to a great advance in the rate property and long-term cycling stability. [91] Recently, MOF-derived carbon materials have also been reported to be integrated with metal sulfides to achieve a high-performance supercapacitor application. For instance, ZIF-8-derived microporous carbon grown on MoS 2 nanosheets offered a stable surface for the storage of electrolyte ions, leading to outstanding cycling stability (98%, 3000 charging-discharging cycles) as a supercapacitor electrode. [89e]

Metal Oxides and Their Composites
Compared with the metal oxides prepared by other methods, MOF-derived metal oxides exhibit some unique merits. First, the diversity of composition and morphology of metal oxides derived from MOF precursors enables metal oxides to achieve more possibilities as high-performance electrode materials in supercapacitors. Second, the high surface areas and novel architecture of MOF-derived metal oxides provide an efficient diffusion path for electron and ion transfer. As a pseudocapacitive or battery-like material, metal oxides possess abundant redox active sites, whose amount and activity make an important impact on their specific capacitances. Furthermore, the design of MOF-derived metal oxides with novel architectures and compositions is also suggested to improve the electrochemical performance.
A large number of transition metal oxides (Co 3 O 4 , FeO x , MnO x , NiO, CeO 2 , CuO x, and ZnO) can be derived from different kinds of MOFs as electrode materials for supercapacitors. [23,36f,92] Their redox activities are related to metal species and valence states. Among these suitable metal oxides, manganese oxides, involving various oxidation states, are one the most promising electrode materials for supercapacitors due to their fast redox charge transfer, which can be realized from MOFs under different calcination temperatures. [93] The different calcination process can also achieve diversity of chemical composition and morphology of the MOF-derived MnO x . For example, Mn-MIL-100derived manganese oxides/carbon nanocomposites calcined at 400, 300, and 200 C are assigned to Mn 2 O 3 , Mn 3 O 4 , and Mn 3 O 4 /C, respectively. The sample calcined at 200 C for 2 h showed a superior capacitance to other derivatives. [94] Perovskite-type metal oxides have also been proven to be a type of promising electrode materials for supercapacitors. To address the issue of the poor chemical homogeneity of precursors on the basis of single-metal MOFs, an MOF gel was used as a precursor for the preparation of LaFeO 3 with a mesoporous structure. [95] A specific capacitance of 241.3 F g À1 (1 A g À1 ) and capacitance retention of 68% (20 A g À1 ) were achieved for this perovskite-type metal oxide. Compared with single-metal oxides, mixed-metal oxides, involving different metal species, display richer redox reactions and higher electrical conductivity because of the relatively low activation energy for electron transfer between cations, usually leading to an increased capacitance. In a typical report, a bimetallic Zn-Co MOF was selected as the precursor for the fabrication of nanoporous carbon (negative electrode) and ZnCo 2 O 4 (positive electrode). The asymmetric device based on the two electrodes reached an energy density of 28.6 W h kg À1 with a power density of 100 W kg À1 . [96] Among a variety of mixed-metal oxides, metal oxides with spinel structures possess higher electrical conductivity and ideal electrochemical behavior. MOF-derived spinel mixed oxides with various structures and components have been designed as electrode materials for supercapacitors, and the crucial roles of the metal species and spinel oxides in the electrochemical performance have been discussed in detail. [97] Recently, some MOF-based composites have also been chosen as precursors to fabricate metal oxide-based composites for supercapacitor applications. For instance, Co 3 O 4 /CoMoO 4 nanocomposites have been obtained from polyoxometalate (POM)-based MOFs, delivering a larger lithium storage capacity (%900 mA h g À1 ) than those of individual Co 3 O 4 and CoMoO 4 . [98] Similarly, a spinel Co 3 O 4 -MnCo 2 O 4 nanocomposite has also been derived from MnO 2 /ZIF-67, which showed a specific capacity of 614 mA h g À1 even at 5000 mA g À1 . [99] The introduction of oxygen vacancies, by surface engineering, can make a great contribution toward the electrochemical performance of metal oxides due to the regulation of band structures, along with increased active sites. [100] In general, the higher oxygen vacancy content corresponds to better performance of electrodes. Two strategies are usually conducted to produce oxygen vacancies: annealing in a reducing gas atmosphere and the utilization of a reducing agent at a normal temperature condition. For example, oxygen-deficient dodecahedral Co 3 O 4 derived from ZIF-67 can be fabricated with a NaBH 4 treatment. [101] Compared with untreated samples, NaBH 4 -reduced Co 3 O 4 shows higher specific capacitance and rate properties due to the existence of oxygen vacancies. Interestingly, the amount of oxygen vacancies in metal oxides is also significantly related to their morphology. Recently, An and co-workers reported the preparation of three types of Co 3 O 4 derived from ZIF-67 in water, DMF, and DMF/water mixing solvents, which are dominated by morphologies with thick microplates, irregular nanoparticles, and nanomeshes. [102] Interestingly, the Co 3 O 4 nanomeshes showed a much higher atomic ratio of Co 2þ /Co 3þ , which is associated with the larger content of oxygen vacancies. The introduced vacancies promote ion transport, resulting in a superior electrochemical performance to that of Co 3 O 4 microplates and nanoparticles.
Over the past few years, many strategies have been devoted to the design of MOF-derived metal oxides with different architectures and morphologies, offering more exposed redox active sites. Hollow structures, due to the superior mass diffusion and ability to relieve volume expansion, have attracted significant attention. [103] Templates and surfactant-assisted approaches are usually applied for the fabrication of these structures, and some can be achieved only through a calcination process. The heating temperature plays an important role in the formation of hollow structures. For instance, MOF-derived graphene-wrapped multishelled NiGa 2 O 4 hollow spheres and yolk-shell NiFe 2 O 4 hollow spheres (MOF) have been prepared via a solvothermal reaction followed by a pyrolysis process, which can be selected as positive and negative electrode materials in a supercapacitor to show a high energy density of 118.97 W h kg À1 and good durability. [104] Figure 6a, b shows the schematic illustration of the NiGa 2 O 4 and NiFe 2 O 4 structure formation process. It was found that unique multishelled NiGa 2 O 4 hollow spheres can be achieved at a calcination temperature of 500 C, while the NiFe 2 O 4 samples that were calcined at 150, 300, and 400 C resulted in morphologies of rigid spheres, an evident shell and a rigid core, and rougher core-shell nanoarchitecture, respectively (Figure 6c-f ). As a result, an asymmetric device consisting of NiGa 2 O 4 as a positive electrode and NiFe 2 O 4 as a negative electrode shows an outstanding electrochemical performance with an exceptional cyclability and superior energy density compared to many reported devices (Figure 6g,h). Similarly, a double-shelled NiO/ZnO hollow sphere has also been prepared by Han and co-workers, exhibiting a high specific capacitance (497 F g À1 at 1.3 A g À1 ) and remarkable cycling stability, which is attributed to the free space in the hollow structure and rich active sites in the metal oxides. [105] In addition, MOF-derived metal oxides with some other nanoarchitectures, such as nanowires, nanomeshes, nanobars, and nanotubes, were also extensively studied as electrode materials for supercapacitors. [106] MOF-derived metal oxides can be further integrated with other functional materials, including CC, GE, GO, CNFs, CNTs, NF, and metal oxides, to provide a favorable architecture and achieve a synergistic effect between different components. [28,31,107] Interestingly, CoFe 2 O 4 nanorods (derived from a Co-Fe MOF)/MXene nanosheets have been prepared as supercapacitor electrodes. The MXene layer not only acts as a binder and conductive additive, but also contributes to the excellent flexibility, promoting charge transfer and ion transport in this composite. A large volumetric capacitance of 2467.6 F cm À3 was achieved for the electrode, and the flexible symmetrical supercapacitor displayed a specific areal capacitance of 356.4 mF cm À2 . [108] Indium-doped tin oxide (ITO) glass has also been chosen as substrate for the in situ growth of a hierarchical porous NiO film derived from MOF-74, which was used as an efficient electrode material for a supercapacitor. [109] Because of the strong binding force between the two materials and the large surface areas, the electrode showed excellent cycling stability for 15 000 cycles.

Metal Hydroxides and Their Composites
Transition metal hydroxides show similar electrochemical features to metal oxides, such as large theoretical capacity, low electrical conductivity, and modest reaction kinetics. The phase transformation process from MOFs to metal hydroxides plays a key role in their electrochemical performance; thus, the related parameters, such as the etching time, temperature, and concentration of the alkaline solution, have been extensively studied. [110] The crystallinity and porosity of Ni(OH) 2 were optimized by regulating both the concentration of KOH aqueous solutions (6 M) and reaction time (6 h) for a high specific capacity of 830.6 C g À1 . [111] The transformation process is usually accompanied by the formation of a hollow structure, which stimulates electron/ion transfer. [112] A porous hollow NiCoMn-OH polyhedra was reported using ZIF-67 as the template and cobalt source. During the etching process, the ligand of ZIF-67 was replaced by the protons obtained from the hydrolysis of the metal cation, and meanwhile a part of Co 2þ in ZIF-67 was oxidized to Co 3þ due to the existence of NO À 3 and O 2 . Finally, metal cations were coprecipitated to generate hollow NiCoMn-OH polyhedra with an outstanding cycling stability (100% retention, 10 000 cycles)[112a]. [112] Similarly, Co(VO 3 ) 2 -Co(OH) 2 and Co@Co hydroxide nanocages/graphene composites also presented a hollow structure, showing excellent electrochemical performance in a supercapacitor application[112b,c]. [112] Typically, LDH, which can be represented by [MII 1Àx MIII x (OH) 2 ] zþ (A nÀ ) z/n ·yH 2 O, consists of divalent (MII, Ni 2þ , Zn 2þ , Co 2þ , Mg 2þ , or Fe 2þ ) and trivalent (MIII, Fe 3þ , Ga 3þ , Cr 3þ , or Al 3þ ) metal ions in brucite-like layers, offering 2D ordered structures as well as tunable composition. A nÀ , an anion, which compensates the positive charge, exists in the interlayer. [113] For example, NiCo-LDH microspheres have been prepared from Ni-MOFs. The morphologies of NiCo-LDH derived from Ni-MOFs are much correlated with the reaction time. With the reaction time increasing, Ni-MOFs gradually changed from solid spheres to flower-like microspheres with sheet-like structures on their surfaces. As a result, the NiCo-LDH electrode material with a reaction time of 10 h showed the maximum specific capacity (1272 C g À1 at 2 A g À1 ), which is due to the fast electrolyte ion transportation provided by the large surface area and large dimension of LDH sheets. [114] Furthermore, the synergistic effect between various metal species contributes to redox reactions because of the multiple valance transitions. Typically, Co-Ni LDH nanosheets derived from ZIF-67 have been grown on Zn-Ni-Co nanoneedle arrays to produce a core-shell structure, overcoming the problem of severe aggregation of LDH and showing high specific surface areas. [115] The rapid electrolyte ion transportation resulted in the high specific capacitance of 2866 F g À1 at 1 A g À1 , which is superior to those of other MOF-derived LDH with core-shell structure, such as CoS x / Ni-Co LDH nanocages (1562 F g À1 , 1 A g À1 ), [116] hierarchical MnO 2 nanotubes@NiCo-LDH/CoS 2 nanocage (1547 F g À1 , 1 A g À1 ), [117] and MnO 2 @Co-Ni LDH (1436 F g À1 , 1 A g À1 ). [118]

Metal Sulfides and Their Composites
According to the synthetic routes, metal sulfides derived from MOFs can be classified into two categories. One is obtained through a solvothermal reaction. The inherent characteristics of the original MOFs, including the morphology, high surface area, and porous structure, can be preserved after their conversion to metal sulfides. The other is generated through a calcination process, corresponding to a metal sulfide/carbon composite with better electrical conductivity. Over the last few years, MOF-derived metal sulfides and their composites with controllable compositions www.advancedsciencenews.com www.advenergysustres.com and specific architecture have been proven to show excellent electrochemical performance for supercapacitor application. A series of metal sulfides with hollow structure derived from MOFs has been designed through an anion exchange strategy by serving TAA as the sulfur source. MOFs are utilized as both the precursors and the self-sacrificing templates. During the solvothermal process, the decomposition of TAA at high temperature provided S 2À for the reaction with outward diffused metal ions on the surface of MOFs to form a hollow structure. In recent years, NiCo 2 S 4 hollow cages, [119] hollow cobalt sulfide nanocage array/graphene-like MnO 2 nanosheets, [120] hollow NiS 2 /ZnS nanospheres, [121] and hollow Ni-Zn-Co-S nanosword arrays, [122] derived from the corresponding MOFs, have been applied in supercapacitors. Especially, in our recent research, MOF-derived Ni nanoparticles/carbon was selected as a precursor for hierarchical Ni/Ni 3 S 2 -decorated carbon nanofibers, as shown in Figure 7a. [123] During this process, the carbonization temperature has a vital effect on the crystallinity of Ni and the graphitization degree of the carbon material, further affecting the formation of the Ni 3 S 2 as well as the redox activities and electrical conductivity of the electrode material (Figure 7b-e). Furthermore, the favorable contribution of the MOF as the template to the large capacitance was revealed in this work.
Similarly, a 2D CoS 1.097 /nitrogen-doped carbon has been prepared using sulfur powder as the sulfur source from simultaneous sulfidation and carbonization during a calcination process, representing a specific capacitance of 360.1 F g À1 at 1.5 A g À1 and good rate performance. [124] In the previous reports, there are many impressive discussions on MOF-derived metal sulfides, such as the advantages of crystal planes, the optimal metal species, the solvent effect, and the synergistic effect between different components. [125] Especially, metal species generated from MOFs play a crucial role in the fabrication of metal sulfide. For example, Zn and Cd are usually used to generate porous structure; Ni is beneficial for the catalytic graphitization of carbon materials. [125a]

Other MOF-Derived Materials
Along with the rapid development of MOF-derived materials for supercapacitors, a large variety of other MOF-derived materials, such as metal phosphides, metal selenides, metal nitrides, and metal carbides, have stimulated significant consideration of researchers.
In general, the lower electronegativity of P than O in their respective compounds is conductive to electron transport and Figure 7. a) Schematic diagram for the fabrication route and b) SEM images of Ni/Ni 3 S 2 /CNFs. c) XRD patterns, d) cyclic voltammetry (CV) curves, and e) galvanostatic charge-discharge (GCD) curves of Ni/Ni 3 S 2 /CNFs calcined at 500, 700, and 900 C. Reproduced with permission. [123] Copyright 2019, Royal Society of Chemistry.
www.advancedsciencenews.com www.advenergysustres.com redox reactions, resulting in the ideal electrical conductivity and high theoretical capacitance of the transition metal phosphide. ZIF-67, a typical template and precursor, has been chosen for the preparation of Zn 0.33 Co 0.67 P/NF, [126] CoP N-doped carbon polyhedrons/rGO, [26a] and Ni-doped CoP@C/CNT, [26b] delivering specific capacitances of 2115.5 1 , 466.6, and 708.1 F g À1 at 1 A g À1 , respectively. Similarly, Co-MOF-derived metal selenides were successfully prepared via an etching reaction process as efficient electrode materials in consideration of high electrochemical activity, good electrical conductivity, and favorable stability. [26c] The etching in H 2 O/ethanol solution endowed the prepared metal selenides with a porous structure, contributing to an improved capacity and rate behavior. In addition, Ni-doped Co-Co 2 N and sheet-like Cr 3 C 2 derived from the corresponding MOFs have also been investigated as electrode materials, showing good electrochemical behaviors. [26d,e]

Conclusion
In this review, the detailed synthetic strategies of MOFs, MOF composites, and MOF-derived functional materials as electrode materials for supercapacitors have been intensively introduced.
We have further revealed the relationship between the growth parameters/treatment conditions/precursors/additional materials and the features of materials, along with the effects of the architecture and composition on the electrochemical performance of these electrode materials. For bare MOFs as supercapacitor electrodes, the choice of ligands, metal ions, solvents, growth time, reaction temperature, and synthetic method plays a vital role in their electrical conductivity, redox activity, specific surface area, porous structure, and wettability, further affecting the electrochemical performance of MOFs. Among a variety of MOFs, conducting MOFs shows unique advantages and promising potential as electrode materials for supercapacitors.
To enhance the electrochemical performance of MOF-based supercapacitor electrodes, MOF composites are usually fabricated by an in situ growth process to achieve a synergistic effect. Many conducting materials, including carbon, conducting polymers, and metals, have been integrated with MOFs to not only compensate for the poor electrical conductivity of MOFs, but also act as supports to achieve a higher specific surface area and even flexibility, and thus improved pseudocapacitive activity and durability can also be achieved.
MOF-derived functional materials, including carbon, metal oxides/hydroxides/sulfides/phosphates/selenides/nitrides/carbides and their composites, are another promising type of supercapacitor electrode material with superior electrochemical behavior. A significant advantage of the usage of MOF-derived materials for supercapacitors is that they can preserve the unique characteristic features of MOFs, including the high specific surface areas and abundant porous structures. Especially, the superiority of the hollow structure with a facile etching process during the MOF conversion is greatly beneficial for promoting mass transfer and improving electrochemical performance.
Undoubtedly, in recent years, a large variety of MOFs and their derived functional materials have been developed to realize the improvement of electrochemical behavior. However, there are still many challenges and opportunities for scientists to develop this promising topic in the future. 1) Bare MOFs have displayed exceptional electrochemical performance as supercapacitor electrodes. However, the physical and chemical stability of bare MOFs in the electrolyte and during the charge-discharge process should be further improved. Furthermore, the change of the microstructure and compositions of MOFs during electrochemical reactions requires advanced in situ characterization measurements. Over the last few years, a large variety of in situ characterizations, such as in situ X-ray powder diffraction, in situ X-ray absorption spectroscopy, in situ nuclear magnetic resonance (NMR) spectroscopy, and in situ infrared spectroelectrochemical technique, have been used to evaluate the storage mechanism of supercapacitor electrodes during the charge/ discharge process. [127] However, the deep insights into the charge storage behavior of the MOF-based materials are still necessary to explore. 2) For the fabrication of MOF-derived functional materials for supercapacitors, precursors or templates involving several MOFs (e.g., ZIF-67, ZIF-8, MOF-74, MOF-5, MIL-101) are relatively limited. More outstanding precursors and templates need to be developed to achieve more novel functional materials with distinct architectures. Furthermore, the integration of these MOF-derived materials with other functional materials should be given considerable attention to realize synergistically enhanced electrochemical performance. 3) MOFbased materials as flexible electrodes often depend on other flexible substrates. It is of great importance to achieve a balance between electrochemical performance and mechanical properties. In addition, exploring a simple and versatile technology to achieve an efficient interaction between the two materials is also a promising direction, which is conducive to promoting the early realization of industrial production of these materials.