2D Metal–Organic Frameworks for Electrochemical Energy Storage

Metal–organic frameworks (MOFs) have been widely adopted in various fields (catalysis, sensor, energy storage, etc.) during the last decade owing to the trait of abundant surface chemistry, porous structure, easy‐to‐adjust pore size, and diverse functional groups. However, the limited active sites and the poor conductivity hinder the relative practical application. 2D MOFs can shorten the ion transport path with the merit of layered structure. The large surface area can increase the number of active sites as well as effectively utilize the sufficient active sites, exhibiting enormous potential in the field of energy storage systems (EESs). In this review, the characteristics of the 2D MOFs have been introduced, and the systematic synthesis methods (top‐down and bottom‐up) of 2D MOFs are presented, providing fundamental understanding for the construction of 2D MOFs. Moreover, the applications of 2D MOFs in energy storage fields such as supercapacitors and batteries are demonstrated in detail. Finally, the future development prospects have been proposed, offering guidelines for the rational utilization of 2D MOFs and promoting the understanding of 2D MOFs in EESs.


Introduction
With the continuous development of economy and society, the living standard has been greatly lifted during the last decade.[3] The exploitation of clean energy such as solar, wind, and tidal energy has been adopted to promote continuable economic development.Nevertheless, the unsustainability and instability hazards severely limit the corresponding large-scale application.Developing advanced electrochemical energy storage technologies (e.g., batteries and supercapacitors) is of particular importance to solve inherent drawbacks of clean energy systems.However, confined by limited power density for batteries and inferior energy density for supercapacitors, exploiting high-performance electrode materials holds the key to boost the manufactured processes of energy storage systems.
Metal-organic frameworks (MOFs) have attracted increasing attention endowed by the porous structure, thermal stability, adaptable surface chemistry, robust configuration, and high surface area, in which metal ions and organic ligands interconnect each other by the coordination bonds.The structures of MOFs are flexible and can be regulated by tuning the category of the metal cation and organic linkers.Moreover, synthesis process holds the enormous impact on morphology and structure of MOFs during the construction processes.Since 1995, layered cobalt-homophonic acid was synthesized and first named as metal-organic framework material, [4] more than 20 000 MOFs have been reported by the year of 2022, [5] and they have been widely utilized in catalysis, [6,7] sensing, [8,9] separation, [10,11] and energy storage systems [12] (Figure 1).However, most of the traditional 3D MOFs synthesized previously have the following inherent defects, seriously restraining the wide application in electrochemical energy storage.Moreover, the sensitive surface area and dramatically decreased capacity of 3D MOFs when exposed to moisture put forward higher requirements to environment.Additionally, the poor electrical conductivity induces inferior rate performance of 3D MOFs.Note that the narrow micropores of MOFs would limit the rapid diffusion of metal ions into the pores, thereby impeding their metal ion storage ability.Most importantly, the incomplete exposure of active sites in common existed morphologies of MOFs (3D frame), which limits the contact with diffusion ions, thereby impairing the output of electrochemical performance.On account of the above-mentioned shortcomings, 3D MOFs have rarely been exploited as energy storage materials directly.Fortunately, the porous skeleton structure and pore size structure of the materials are adjustable; thus, the electrochemical performance of MOFs as electrode materials for energy storage devices can be effectively improved by enhancing the conductivity of MOFs and improving the structure of MOFs.Since Novoselov's group used micromechanical stripping technology to peel 2D graphene materials with large specific surface area in 2004, [23] excellent optical transparency and good electrical conductivity have been delivered, and 2D materials have received increasing attention, various 2D MOFs have been developed, such as 2D transition metal disulfide compounds (TMDs), [24] 2D black phosphorus (BPs), [25] 2D precious metal nanomaterials. [26,27]A 2D material consists of single or multi-layered atoms/molecules held together by strong covalent or ionic bonds within the layers and stacked together by van der Waals forces between the layers.Due to the multiple chemical and physical properties such as low transmission resistance and high flux, 2D material has been widely used in electrochemistry.The combination of 2D structures and MOFs provides rapid charge transfer and mass transfer, and it can provide opportunities for enhanced electrochemical activities.
In the last 5 years, researches of 2D MOFs have aroused intensive attention (Figure 2).Compared to traditional 3D MOFs, 2D MOFs have graphene-like structures, large p-p bonds, and enhanced electrical conductivity.The active sites of 2D MOFs are exposed to the surface rather than enclosed in the pore, which could promote deep interactions between the active sites and the substrate molecules.The layered structure and high aspect ratio of 2D MOFs can greatly improve anisotropy, provide larger specific surface area, and avoid aggregation during pyrolysis to a certain extent.The conductivity of the material can be increased by shortening the electron transport path as well.In addition, the unique layered structure of the 2D MOFs can deal with more local mechanical stress uniformly, resulting in optimum mitigation of volume changes and stable electrode structure during the energy storage/conversion processes. [28]Lately, numerous 2D MOFs have been prepared, exhibiting excellent electrochemical properties.For example, a conjugated copper (II) catecholate-based metal-organic framework can reach 479 F g À1 at 0.2 A g À1 .Additionally, copper-benzoquinoid (Cu-THQ) MOF delivers stable cycling property and remains a capacity of 340 mAh g À1 after 100 cycles as the lithium cathode material.Such remarkable results show that 2D MOFs possess broad application prospects in electrochemical energy storage field.However, until now, there are few systematic reviews on the design, preparation, and application of 2D MOFs in the energy storage systems.
Herein, various preparation strategies of 2D MOFs have been comprehensively summarized in this review, including top-down tactic and bottom-up method, which could provide the fundamental understanding for elaborately constructing 2D MOFs in advanced energy storage systems.Moreover, the benefits and disadvantages of each method have been illustrated, providing guidelines for the future research and preparation of new 2D MOFs.Moreover, the analysis of systematic research progress of 2D MOFs in energy storage fields during recent years has been conducted, especially their applications in supercapacitors and battery configurations.Finally, the shortcomings of current research as well as the future development directions of 2D MOFs in energy storage field are proposed and dedicated to promote the blossom of energy storage systems.

Synthesis Method of 2D-MOFs
2D MOFs possess the merits of high specific surface area, abundant active sites on surface, and strong coordination bonds between layers, which can be utilized to improve the electrochemical performance.Developing effective synthesis strategies and ameliorating the controllability of synthesis will greatly contribute to modifying the structure of 2D MOFs and the corresponding applications.In general, 2D MOFs can be prepared through top-down or bottom-up method.Figure 3 shows the timeline of the representative methods prepared for 2D MOFs, and the details of such methods will be mentioned in the next section.

Top-Down Method
Top-down method corresponds to the stripping of MOFs formed by weak interaction in 2D vertical layer direction, including physical stripping and chemical stripping.The most commonly used method is physical stripping, including mechanical stripping and ultrasonic stripping.By breaking the weak interaction between adjacent layers in situations such as ultrasound and ball milling and grinding, a single layer or several layers of 2D MOF structures can be prepared without ruining the covalent or coordination bonds in each layer of MOF structure.Chemical exfoliation has rarely been used and is generally adopted for 3D structures, including ion exfoliation and organic ligand exfoliation.
As the consequence of the simplicity and effectiveness, ultrasonic stripping in liquid phase has been widely employed for the synthesis of 2D MOFs.At the beginning of 2010, Zamora's group applied ultrasonic stripping to the preparation of 2D MOFs for the first time.This method breaks the weak interaction between p and p layers and obtains 2D ultra-thin MOF nanosheets [Cu 2 Br(IN) 2 ] IN (IN = iso-nicotinate) with a thickness of about 5 A. The successful synthesis of this material  [4,[13][14][15][16][17][18][19][20][21][22] Energy Environ.Mater.2023, 6, e12521 also inspired the subsequent use of ultrasonic stripping to synthesize new 2D MOF material. [29]Due to weak interactions of hydrogen bonding and van der Waals forces, suitable solvents or solvent mixtures can be used to overcome these interactions during ultrasonic stripping of bulk MOFs.Appropriate solvents can penetrate the interlaminar space of large MOF and stimulate the separation between layers, thus weakening the weak interlaminar interaction.However, improper solvents can reduce the stripping rate; hence, the solvent plays a major role in ultrasonic stripping.A mixture of solvents has been frequently wielded to achieve both high stripping rates and stability. [40,41]Nevertheless, solvent systems suited for one MOF system cannot be extended to other systems.This is because each MOF material has different properties due to different combinations of metal centers and ligand units.For example, Lotsch et al. noted that when the hydrophobic MOF was stripped, the stripping effect was not satisfactory if strong polar solvents such as H 2 O and DMF were used, since the solvent molecules could not effectively penetrate into the hydrophobic interlayer space.When THF, toluene, CHCl 3, and other solvents were used, the stripping effect decreased successively (Figure 4a-c). [38]In contrast, when MOF materials are stacked together by interlaminar hydrogen bonds (Figure 4d), they can be broken by solvents like ethanol and DMSO, leading to spontaneous stripping, which was founded by Moorthy's group.The stripping degree of different solvents is related to the ability of the solvent to accept hydrogen in the hydrogen bonds (Figure 4e-f). [42]Therefore, it is necessary to analyze the structural characteristics of different MOF systems when using ultrasonic stripping method to synthesize 2D MOF nanosheets, so as to select the most suitable solvent system and obtain MOF materials with better performance more efficiently.
In addition, mechanical stripping has been commonly deployed as top-down method for preparing 2D MOF nanosheets.In 2015, Abherve and co-workers successfully separated 2D nanosheets from bulk MOFs for the first time by using micromechanical stripping. [30]eng's research fabricated phthalocyanine-based 2D conjugated metal-organic framework (Ni 2 [CuPc(NH) 8 ]) through ball milling mechanical stripping approach in 2020 (Figure 4g).The average thickness of the nanosheet is about 7 nm, exhibiting high crystallinity and chemical stability.In addition, by virtue of its ultra-thin characteristics, the active site utilization rate of the nanosheet is high, and the solution is easy to process.Therefore, the microultracapacitor devices made by mixing the nanosheet and the exfoliated graphene show preeminent cycle stability and mechanical flexibility.An outstanding area capacitance of 18.9 mF cm À2 has been delivered, which is superior to most other micro-supercapacitors (MSCs) based on 2D materials. [37]Although there are few 2D MOFs obtained by mechanical stripping at present, this method offers great possibilities for the development of 2D MOFs with neat surfaces, satisfactory aspect ratio, and excellent crystallinity. [43]n addition to the above-mentioned two most common methods, the following stripping methods can also be utilized to prepare 2D MOF materials.In 2013, the group of Salome Delgado and Felix Zamora first reported the use of solvent-induced delamination to peel large MOFs materials.A 2D nanomaterial can be obtained by simply subsuming the main material in water (Figure 4h).It is found that the monolayer thickness of the crystal structure is about 8.5 A in the main structure, and the space between layers can be easily occupied by appropriate solvent molecules, forming weak hydrogen bonds.When the crystal is soaked in the solvent, it is easy to be completely peeled off. [31]The intercalation/chemical stripping method was developed by zhou et al.They first formed new intercalated MOFs by bonding 4,4 0 -dipyridine (DPDS) disulfide into stacked massive metallotetrakis (4-carboxyphenyl) porphyrin (M-TCPP) materials.Then, trimethylphosphine (TMP) was employed to react with disulfide bonds in DPDS to reduce disulfide bonds, so as to shear DPDS and destroy the interlayer interaction of MOFs.The combination of the two methods can improve the yield and finally obtain ultra-thin nanosheets of about 1 nm (Figure 4i). [32]In addition to organic compounds, lithium ions can be embedded as well.Xia and colleagues selected n-butyllithium as the embedded molecule to exfoliate La 2 (TDA) 3 (TDA means 2,2 0thiodiacetic acid).Lithium ion can enlarge the layer spacing and weaken the interlayer interaction, leading to the spalling of multilayer MOF nanosheets and the formation of single-layer MOF nanosheets with a thickness around 2.0 nm (Figure 4j1-k3). [44]However, most columnar layered MOF materials are structurally stable due to strong coordination bonds, so it is challenging to achieve selective fracture of interlaminar bonds.The electrochemical/chemical exfoliation strategy can be executed to exfoliate these types of MOFs.Zhang and colleagues prepared ultra-thin 2D MOF nanosheets by electrochemical oxidation.After electrolysis, McF-13 crystal prepared from Co(CH 3 COO) 2 and 2,3-dihydroxy-1,4-phthalic acid (H 4 dhbdc) was transformed into 2D MOF nanosheets. [45]he top-down method to prepare 2D MOF nanosheets has been endowed with high product yield as well as simple preparation process.However, due to the defect that the strength of external force can hardly be accurately adjusted, the thickness of the prepared MOF nanosheets can barely be controlled, and the nanostructure is difficult to be obtained uniformly, which is generally applicable to the layered MOF with weak interlayer interaction.

Bottom-Up Method
Abundant bottom-up methods have been deployed to prepare 2D MOFs.The bottom-up method is the straight synthesis of 2D MOFs through the coordination of metal source and organic ligand, which restricts the vertical growth of MOF without affecting the transverse growth.This method is endowed with low-cost merit and can directly control the generated products by adjusting the types and proportions of metal centers and organic ligands.However, the product size distribution is generally wide and the experimental process is complicated, which could be adopted for the synthesis of non-layered 2D MOFs.The commonly used methods include interface fabrication, surfactant-assisted strategy, three-layer synthesis, modulated synthesis, and sonication manufacture.
2D MOFs can be synthesized directly in solution, which can overcome the disadvantages of easy morphology/structure destruction and uneven structure of the stripping method, and can effectively adjust the size and thickness of the 2D MOFs as well.Surfactantassisted approach can be used to promote anisotropic crystal growth by attaching the surfactant to a crystal plane and reducing the surface energy. [46]For example, in 2015, series of suspended ultra-thin 2D M-TCPP (M = Zn, Cu, Cd or Co, TCPP = tetracarboxy phenyl) porphyrin nanosheets with a thickness below 10 nm by using surfactant-assisted method were prepared by Zhang's group (Figure 5a).PVP promotes anisotropic crystal growth by selectively attaching the crystal to the surface of M-TCPP and limiting the growth direction perpendicular to the plane. [33]In 2016, Zhang's group synthesized bimetallic MOF nanosheets less than 10 nm using TCPP (Fe) through surfactant-assisted method firstly and assembled them into multilayer films on the electrode by Langmuir-Schafer method (Figure 5b-e).This work provides a general method for preparing 2D ultrathin bimetallic MOF nanosheets with high yield. [47]In addition to surfactants, small molecule regulators are also adopted to prepare MOF nanosheets.Small molecules with functional groups similar to organic linkers can competitively bind with metal nodes to control dynamic growth. [48]For example, benzoic acid can promote central metal coordination of porphyrins, reduce h-stacking (face to face stacking) and j-aggregation (edge-to-edge accumulation) within layers, thus inducing anisotropic growth of MOF flakes.Pei et al. synthesized Cu-TCPP (BA) micron-level ultra-thin MOF flakes by adding benzoic acid into the reaction solution, which reveals the growth mechanism of ultra-thin MOF flakes and provides a feasible strategy for the synthesis of ultra-thin micron-level MOF flakes with similar thickness to graphene (Figure 5f-g).It greatly promotes the application of materials and provides ideas for the development of 2D materials. [49][31][32][33][34][35][36][37][38][39] The interfacial growth method is to prepare MOF nanosheets by limiting the coordination assembly of organic connectors and metal nodes to the interfaces of two different phases.Liquid-gas interfaces are formed by covering the liquid surface with a small amount of volatile organic solvent composed of a small number of organic ligands or metal ions, allowing the evaporation of organic solvents.Nucleation as well as growth kinetics of MOFs are well controlled when the reaction occurs on water surface.In 2013, Nishihara and co-workers successfully prepared nickel bis (dithiolene) nanosheets using gas-liquid interface synthesis. [38]They coated thin layers of ethyl acetate solution containing BHT flat on the surface of aqueous solution embodying Ni (OAc) 2 and NaBr.When the ethyl acetate layer evaporated, the 2D nano-1 nanosheet can be synthesized at the liquid-gas interface.Scanning tunneling microscopy imaging showed that 0.6 nm thickness of nano-1 nanosheet (Figure 5h-j).Later, the team found that by changing the concentration of ligand, the thickness of the nanosheet could be adjusted from 6 to 800 nm, [50] demonstrating the relative feasibility.In liquid-liquid interfaces, two insoluble liquids are usually employed to dissolve organic linkers and metal ions. [51]Recently, 2D Ni MOFs were prepared through a mild liquid-liquid interface method by controlling the molar ratio of metal precursors and linkers.A certain amount of terephthalic acid, an organic linker, was dissolved in a mixture of DMAC and CHCl 3 , and nickel acetate was spread around the deionized water as a metal precursor and dropped into the organic solution.When the metal precursor aqueous solution contacted the organic phase, plenty of light green products were formed on the interface of water-organic phase.TEM characterization showed that the product exhibited 2D parallelogram features with transverse dimensions ranging from 200-450 nm.AFM showed that the average thickness of 2D Ni MOFs was 5 nm (Figure 5k-l). [52]The main restriction of interface synthesis is the relatively low yield of 2D MOFs as the consequence of limited interfacial area.By controlling the growth process of the interface, arbitrary thickness of the MOF nanosheets can be generated, and the output of 2D MOF nanosheets is largely decided by the growth of MOFs on the interface area.
In addition, there are many other methods for synthesizing 2D MOF materials.In 2008, Professor R. Gref's research group applied ultrasonic synthesis method to the preparation of MOFs for the first  [38] d) Hydrogen bond mediated assembly of the layered MOF and ultrasonication induced liquid-phase exfoliation of Cd-TPA.e, f) Fluorescence emission spectra of Cd-TPA colloidal solutions and relative fluorescence intensities in various solvents. [42]g) The exfoliation of bulk 2D c-MOF Ni 2 [CuPc(NH) 8 ] into nanosheets. [37]) Solvent-assisted interaction fully exfoliated a coordination polymer. [31]i) Intercalation and chemical exfoliation approach to produce 2D MOF nanosheets. [32]j1, k3) The SEM and TEM image of MOF-La in bulk state, nanosheets made by ultrasonic for 30 minutes and nanosheets prepared by the method of Li-intercalation respectively. [44]ergy Environ.Mater.2023, 6, e12521 time.Small monodisperse nanoparticles were finally obtained, but in very low yields.Only microwave-assisted hydrothermal synthesis could successfully and quickly synthesize monodisperse nanoparticles with high yield. [53]Hiroshi Kitagawa's group synthesized Cu-TCPP, a MOF thin film with nanoscale structure, by adopting the strategy of "modular assembly" in 2012 (Figure 5m).This provided an original strategy for constructing highly oriented thin MOF crystals which can hardly be prepared by traditional techniques and enabled the simple integration of mixed MOF thin films with various functions, types, and orientations in order to meet the requirements of future MOF practical application as well. [36]In 2018, Bao et al. synthesized the first oxygen analogue M 3 (C 6 X 6 ) 2 (X = NH, S) series, namely Cu-HHB [Cu 3 (C 6 O 6 ) 2 ], using the competitive coordination reagent of ethylenediamine by kinetic control through Cu(II) and hexahydroxybenzene (HHB).With understanding the formation mechanism of Cu 3 (C 6 O 6 ) 2 , it was proved that 2D conductive MOF can be fabricated with abundant raw materials.THQ is a 2e À oxidation product of HHB.This scheme synthesized Cu 3 (C 6 O 6 ) 2 through HHB deprotonation and continuous oxidation.Therefore, Bao et al. substituted THQ for HHB and modified the reaction conditions slightly to synthesize a dark blue Cu-THQ product with the same plate-like morphology (Figure 5n), establishing the feasibility of simplifying the synthesis process of MOF and raising the possibility to eliminate the usage of unnecessary oxidizing agents, which can be expected to facilitate the synthesis of crystalline 2D conductive MOFs with air-stable and lowcost raw materials. [39]oth top-down and bottom-up methods possess the special advantages and limitations.In the future, combining the advantages and avoiding the disadvantages to prepare 2D MOFs with stable structure, excellent performance, and high yield are extremely desirable.
Nickel-based MOFs have conjugate p bonds, which makes the steric hindrance scarcity in electrochemical reactions, benefiting the diffusion of the electrolyte.However, on account of low constitutive conductivity, poor mechanical, and chemical stability during charge and discharge, the electrochemical applications are less frequent.Lu et al. deployed an in situ growth tactic to interpenetrate carbon nanotubes with carboxyl into ultra-thin 2D nickel MOF nanosheets (Figure 6a-c).Carbon nanotubes have anisotropic properties and can shorten the ion diffusion path.By controlling the participation of C-CNTs, the thickness and specific surface area of 2D nanosheets can be availably regulated.The fabricated Ni-MOF/C-CNTs nanosheets delivered a specific capacity of 680 C g À1 and excellent capacity retention at 1 A g À1 .A maximum energy density up to 44.4 Wh kg À1 of the hybrid device can be obtained at the power density of 440 W kg À1 , demonstrating the potential of the material in the application of supercapacitors. [62]i 3 (HITP) 2 is the first 2D conductive MOFs for electrochemical double-layer capacitors as the sole electrode material.The surface area of this MOF far exceeds that of activated carbon material, and it settles the drawback of poor electrical conductivity of MOFs (Figure 6d,e).The bulk conductivity of the material is greater than 5000 S m À1 , much higher than that of activated carbon materials, and the electrical conductivity of the material is similar to that of graphite.It can make additional contributions to charge storage through reversible redox reactions and thus have greater application advantages than carbon materials.The capacity retention rate of the material exceeds 90% after 10 000 cycles, indicating the great application potential of 2D conductivity MOFs in capacitor field. [16] cost-effective layered Ni-p-Phenylenediamine (Ni-pPD) was fabricated by Thomas's group (Figure 7a-g).Tetraethylammonium tetrafluoroborate/acetonitrile organic electrolyte is the mostly adopted electrolyte in symmetric supercapacitors, which provides a voltage window of 2.5-2.7 V. [63] Nevertheless, in symmetric supercapacitors, the energy storage process is mainly realized through an adsorption-desorption electric double layer mechanism, bringing that the capacitance of single-electrode is usually less than 160 F g À1 .The capacitance of an electrochemical double-layer capacitor is highly dependent on the surface area of the active material; thus, the material is used in asymmetric supercapacitors, where anions and cations in an organic electrolyte with different sizes as well as electrochemical properties, resulting in low electrode densities and low areal performance.When the material was used as anode and the activated carbon as cathode to form an asymmetric supercapacitor, the Ni-pPD anode with a thickness of 230 microns exhibited great quality capacitance (259 F g À1 ) and excellent area capacitance (2.9 F cm À2 ) at discharge current density of 2 A g À1 .A broad potential window of 2.85 V has been presented in the tetrafluoroborate tetraethylammonium/acetonitrile organic electrolyte, showing excellent cycle stability over 12 000 cycles, and the actual usable energy of the whole capacitor is up to 0.9 mWh cm À2 .The material possesses high kinetics especially at high potential due to the enlarged chip spacing under large polarization, which opens up original method for the design of large cationic energy storage devices. [20] [62]d, e) Capacitance loss in a symmetric Ni 3 (HITP) 2 supercapacitor cell before cycling and after cycling.CV traces measured within 0-1.5 V, and 0-1.0 V. [16] Energy Environ.Mater.2023, 6, e12521 Bao's group developed 2D MOFs with high conductivity based on hexaaminobenzene (HAB) ligands; black crystalline-shaped HAB MOFs were obtained through reaction of the Ni(II)/Cu(II) salts and HAB linker in water, remarking as Cu-HAB and Ni-HAB.The CV spectra of the two materials were significantly different, indicating that the electrochemical behavior of HAB-based MOFs relied on the metal center (Figure 7h-m).It was proved that HAB-based MOFs could exhibit good chemical stability as well as great compatibility with water and electrolyte. [64]Submillimeter-thick HAB MOFs exhibited a volume capacitance of 760 F cm À3 , superior to most materials, while achieving stable  [20] h, i) Cyclic voltammetry profiles for Cu-HAB and Ni-HAB electrodes gathered at different scan rates.j) Weight and volume-specific properties of Ni-HAB particle electrodes with different area densities (9, 35, and 65 mg cm À2 ).k) Surface properties of Ni-HAB electrode under different weight loads.l) Capacitance retention data gathered by galvanostatic charge-discharge at 10 A g À1 .m) Comparison of volume and area capacitance of Additive-free Ni-HAB electrodes with other materials. [64]edox behavior and high weight capacitance as well.Ni-HAB, for example, achieved more than twice the weight capacitance of the best porous carbon materials reported so far, approaching redox-active conductive polymers. [65,66]The HAB-MOFs electrodes presented good cycling stability with a capacitance retention of 90% after 12 000 cycles at the current density of 10 A g À1 .This work demonstrated the importance and superiority of redox-active ligands in charge storage properties and highlighted the potential as electrodes of redox-active EES.
Yaghi and coworkers synthesized Cu-CAT for the first time by linking 2,3,6,7,10,11-Hexahydroxytriphenylene (H 12 C 18 O 6 , HHTP) with copper ions. [67,68]Subsequently, Xu et al. grew Cu-CAT onto carbon fiber to construct crystalline nanowire arrays (NWAs) under a controlled method and presented the preparation of 2D conductive MOF NWAs for the first time (Figure 8a-d).The application of NWAs as sole electrode for solid supercapacitors was also reported.The preparation of this electrode could be applied without the requirement of conductive additives or adhesives.Cu-CAT NWAs can manifest great area capacitance of approximately 22 lF cm À2 for supercapacitors by effectively utilizing the high porous structure and preeminent conductivity of nano-structured Cu-CAT NWAs in solid state supercapacitors, which was similar to most carbon materials (according to the quality of the electrode). [69]ecently, a high-performance 2D conductive MOF of conjugated copper (II) catechol-based metal-organic skeleton (Cu-DBC) has been fabricated by chen and colleagues.Due to the fact that the material is a p-d conjugated skeleton, it has typical semiconductor properties and delivers a conductivity of around 1.0 S m À1 in room temperature.Based on the excellent conductivity of Cu-DBC and the redox reversibility of the copper center, the mass capacitance of the material can reach 479 F g À1 at the discharge current density of 0.2 A g À1 (Figure 8e,f).In addition, Cu-DBC symmetric solid-state supercapacitors display high area capacity and volume capacity of 879 mF cm À2 and 22 F cm À3 , respectively.These performances are better than most of the reported MOF-based supercapacitors and show a promising application in energy storage devices. [22]n 2020, Wu's research group developed surfactant-assisted method to restrict the growth of MOF to a specific plane and successfully isolated 2D ultra-thin CoTCPP-PZ nanosheets with thickness less than 10 nm from 3D MOF CoTCPP-PZ bulk.As excellent microelectrode materials with high capacitance for precision all solid state MOF devices, 2D ultra-thin CoTCPP-PZ nanosheet MSCs were named MN-MSCs and 2D massive CoTCPP-PZ nanosheet MSCs were named MB-MSCs.When MN-MSCs work in EMIMBF 4 ionic liquid electrolyte, the area capacitance is as high as 28.3 mF cm À2 , and the volume capacitance is 15.7 F cm À3 in 0.2 mA cm À2 .The device has a high energy density of 8.7 mWh cm À3 and a good cycle life, with a capacity retention rate of 96.0% after 10 000 charge-discharge cycles, both higher than that of MB-MSCs.At the same time, compared with MB-MSCs, MB-MSCs present better mechanical flexibility, uniformity and excellent module integration.This strategy for preparing ultra-thin MOF nanosheets provides a new functional approach for the wide application of MOF.It also indicates the promising application advantage of 2D MOFs in supercapacitors. [70]n addition to single metal 2D MOFs, bimetal 2D MOFs are widely utilized in energy storage configurations as well.Liu et al. reported that the use of two or more metal elements in high-performance  [69] e) Rate capability curves of the areal and volumetric capacitances of Cu-DBC symmetric solid-state cell.f) Ragone plot of the Cu-DBC and other reported materials symmetric solid-state cell. [22]ergy Environ.Mater.2023, 6, e12521 9 of 17 supercapacitor electrodes can improve the charge transfer between electrolyte and electrode to improve discharge capacitance. [71]For example, nickel-cobalt ions have synergistic effects, and nickel-cobalt bimetallic compounds have higher specific capacitance and better reversibility than their respective mono-metal compounds. [72]Xu's group fabricated Ni/Co-MOF nanosheets as well as Ni-MOF nanosheets by solvothermal method to compare their performance in supercapacitors.Ni/Co-MOF nanosheets have a high specific capacitance of 530.4 F g À1 at 0.5 A g À1 in 1 M LiOH aqueous solution, which is significantly higher than that of 306.8 F g À1 for Ni-MOF nanosheets.Moreover, the capacity retention of Ni/Co-MOF nanosheets remains 99.75% after 2000 cycles.It indicated that the synergistic effect of different metal ligands has a certain impact on electrochemical energy storage performance, which provided an example for the design of 2D MOFs with adjustable structure in the future and laid a foundation for the realization of more efficient energy storage research. [73]2D nanosheets NiCo-MOF grew on the surface of rGO by precipitation at room temperature was reported by Liu's group in 2019.The resulting NiCo-MOF/rGO heterostructures were used as electrode materials for supercapacitors.In this hybrid structure, 2D MOF ultra-thin nanosheets offered a large surface area, providing rich channels for rapid mass transport of ions.Meanwhile, the conductivity and physical support of rGO could provide rapid electron transport.Under the synergistic advantage of reduced GO and NiCo-MOF nanosheets, the material presented an excellent specific capacitance of 1553 F g À1 at the current density of 1 A g À1 by constantly adjusting the ratio of reduced GO.In addition, the material exhibited remarkable cycle stability (1553 F g À1 after 5000 cycles at the current density of 1 A g À1 ), which indicated that the 2D MOF nanosheet/rGO heterostructure could be a potential candidate electrode material for energy storage and provided guideline for the synthesis of the next generation of supercapacitor materials. [74]The excellent electrochemical performance of bimetals 2D nickel-cobalt may be related to the ultra-thin thickness, which provides abundant active sites and large electrochemical contact surface area for redox reactions.The improved performance could be related to the increase of charge transfer during electrochemical processes as well, the strong coupling effect existing in Ni and Co can lead to a high oxidation state of Ni 2+ , which improves the pseudocapacitance reactivity. [41]o improve the application performance of 2D MOFs in capacitors, it is necessary to modify the properties of materials.Moreover, carrying out theoretical simulation calculation and combining both the theoretical simulation results with experimental observation and analysis are in urgent need.Feng and colleagues investigated the basic EDL charge storage mechanism and charging kinetics of three 2D conductive MOFs (Cu-HITP, Cu-THQ, and Cu-HITN) using potentiostatic molecular dynamics simulations. [75]The results of the strategy are in good consistence with the experimental performance of macro electrochemical double layer capacitor (EDL) device.These EES devices based on 2D conductive MOFs exhibit large specific capacitance, truncated battery resistance, significant energy density, and power density.This work integrates theoretical simulations with experimental observations and provides relevant molecular insights into the development of 2D MOFs with better electrochemical performance.In addition, this work offers guideline for the future construction of 2D MOFs as electrode materials for energy storage devices. [76]In future, it is believed that better performance of electrochemical energy storage device materials can be achieved by integrating theoretical calculation with experimental results.

Application of 2D-MOFs in Batteries
2D MOFs can not only be wielded as electrode materials for supercapacitors but also be adopted in various battery systems, containing lithiumion batteries, lithium-sulfur batteries, and other battery systems.

Lithium-ion Batteries
[79][80] However, in the application process, there are many problems such as large electrode volume expansion, low specific capacity, and poor rate performance.In order to obtain more active sites for lithium-ion reaction and further improve specific capacity, 2D MOFs with ultrathin nanosheet structure have appeared.2D MOFs have attracted increasing attention as electrode materials for lithium-ion batteries owing to the high porosity, large specific surface area, sufficient redox-active metal sites, and short ion diffusion distance.The reversible capacity, satisfactory rate performance, and stable cycle stability are improved by optimizing metal centers and organic connectors and developing advanced morphologies.
Chen et al. prepared 2D Cu-THQ MOF material through solvothermal method and assembled the lithium-ion battery (Cu-THQ MOF as positive electrode) (Figure 9).A reversible capacity of 387 mAh g À1 and enormous specific energy density of 775 Wh kg À1 were obtained at 50 mA g À1 during the second cycle.Cu-THQ MOF exhibited stable cycling property and remained at capacity of 340 mAh g À1 after 100 cycles. [81]After been analyzed through comprehensive spectral measurement, the high capacity originated from the special lithium storage mechanism: each coordination unit can carry out a threeelectron redox reaction, and each copper ion can bring a one-electron redox reaction, both the metal center and the organic connector can be utilized to store charges.This clear mechanism provided feasible guideline for the synthesis of high-performance 2D MOF-based cathode materials, manifesting the importance and necessity of comprehensive energy storage mechanism for advanced energy storage performance.
Silicon is considered to be a strong candidate as anode for LIBs, with advantages of low discharge potential, high theoretical capacity, and eco-friendliness.However, during repeated lithium/dilithium processes, Si would undergo huge volume changes.In the process of dilithium, the active particles of the electrode were mechanically crushed and the transport path of electron ions was interrupted, and the solid electrolyte interphase would continue to form on the cracked anode surface, endangering the degradation of the electrochemical performance of the silicon electrode.Therefore, it is very important to control the particle size to nanometer scale, which can solve the limitation of particle crushing of silicon material and improve the performance and life of anode material in lithium-ion batteries.Wu's group in situ grew Si@Cu 3 (HITP) 2 composites around Si nanoparticles (SiNPs) on a Cu-centered 2D conductive MOF Cu 3 (HITP) 2 at 27 °C.Cu-MOFcoated Si nanoparticles can produce a buffer that inhibits the huge volume expansion and provide efficient conductivity channels for SiNPs.The material has good structural stability along with low electrochemical degradation.The initial reversible capacity of SiNPs electrode coated with 5% Cu-MOF reaches 2511 mAh g À1 at 0.1 C, an initial coulomb efficiency of 78.5% can be delivered, and the capacity could still maintain 2483 mAh g À1 after 100 cycles.At rates 5 C, 10 C, and 20 C, the reversible capacities are 1303, 785, and 404 mAh g À1 , respectively.In full battery (Cu-MOF-coated Si as anode and LiCoO 2 as cathode), the Energy Environ.Mater.2023, 6, e12521 reversible discharge capacities were 1267 and 1105 mAh g À1 at 0.5 C and 1 C, respectively.A remarkable specific capacity of 1038 mAh g À1 can be achieved after 50 cycles at 0.1 C for Si@Cu 3 (HITP) 2 .These results indicated that the composite materials fabricated by selecting conductive Cu-MOF coating on the surface of Si/C rod have great potential as anodes for LIBs in the future. [82] 2018, Nishihara and colleagues synthesized a 2D metal conducting bis (diimino) nickel frame (NiDI) for the first time, which had a variety of redox states and can be oxidized and reduced (Figure 10a-e).Both negative ions (PF 6 À ) and cations (Li + ) were carriers of electrons in MOF and acted as electron carriers to charge and discharge energy.The specific capacitance was one of the highest cathode .f) Charge/discharge profiles of 2D Cu-THQ MOF electrodes at 50 mA g À1 in the initial three cycles.g) Rate performance at various current densities of 2D Cu-THQ MOF electrodes.h) Cycling performance of 2D Cu-THQ MOF. [81]ergy Environ.Mater.2023, 6, e12521 materials of MOF base, with stable cycle performance up to 300 times, comparable to other commercial lithium battery cathode materials, which laid a foundation for further study of other potential 2D conductive MOFs as cathode materials of lithium-ion batteries. [83]n 2021, Li and Bin's research group reported Cu-HHTQ, a 2D conductive MOF material, which combined nitrogen-rich aromatic molecule tricyclic quinazoline (TQ) with redox CuO 4 unit, demonstrating application potential as a lithium storage material (Figure 10f-i).The material could deliver a high specific capacity of 657.6 mAh g À1 at 600 mA g À1 , demonstrating high-rate performance and cycling performance.The success of this work showed that it was feasible to combine multiple redox-active groups with conductive MOFs for the promotion of energy storage electrode materials, which provided inspiration for the future research of new materials. [84]n 2019, Zboril's group prepared 2D nickel-based MOF nanosheets through the template strategy with Ni(OH) 2 , squaric acid, and polyvinylpyrrolidone (PVP).PVP acted as a structural guiding agent.It can also effectively prevent the agglomeration of MOF nanosheets.The Ni 7 S 6 /graphene nanosheet (GNS) composite was prepared by in situ  [83] f) Synthesis of Cu-HHTQ.g, h) Rate performance and Cycling performance of Cu-HHTQ.i) Structure process of TQ-Li pathway in LIBs. [84]ergy Environ.Mater.2023, 6, e12521 curing method.The reversible capacity of Ni 7 S 6 /GNS composite could reach 1010 mAh g À1 at 0.12 A g À1 , which was superior than other metal disulfide matrix composites.This strategy opened up original direction for the design of novel functional heterostructures in the future and can be feasible to hybridize 2D MOF nanosheets with other 2D nanomaterials for electrochemical energy storage. [85]2.Lithium-Sulfur Batteries Lithium-sulfur batteries are popular candidates for high-energy rechargeable batteries due to the merits of environmental friendliness, theoretical energy density of up to 2600 Wh kg À1 , ultra-high theoretical capacity (1675 mAh g À1 ), and relatively low price.[86][87][88] However, in practical applications, the poor cycle life caused by the shuttle of polysulfide in high-sulfur load cells is a limitation of the wide application.2D MOFs can be utilized as the main materials to carry sulfur in lithium-sulfur batteries, and also as a barrier to block the migration of Li 2 S x (4 ≤ 9 ≤ 8, LiPS) intermediates to the anode side.The adjustable pore structure and high porosity enable 2D MOFs to hold large amounts of sulfur in the pores and allow the bulk expansion of sulfur while limiting the leaching of intermediate LiPS.[89] MOFs with large pore size are beneficial for sulfur storage, while MOFs with smaller pore sizes can effectively reduce LiPS shuttle, but sulfur loading and transport are limited.Therefore, researchers are developing appropriate structures to inhibit LiPS dissolution and exploring MOFs with other functional components.[90] The idea of implementing a 2D conductive MOF on Li-S batteries was first proposed by Zhao and his colleagues with the assistance of first-principle calculations.[92] Firstly, Cu-BHT was selected as the ideal sulfur packaging material in Li-S battery to study its potential.It was found that Cu-BHT exhibited a very high conductivity of about 1580 S cm À1 at room temperature. The excllent electrical conductivity of Cu-BHT held the key to ensure the maximum utilization of sulfur.So far, Ni 3 (HITP) 2 (HITP =2,3,6,7,10,11-heximinotriphenyl) film has been recognized with strong electrical conductivity.The highly conductive Ni 3 (HITP) 2 film can form a 2D structure similar to graphene, which is an ideal material for the sulfur host in Li-S batteries.In 2019, Ni 3 (HITP) 2 as a sulfur host material to capture and transform polysulfide in lithium-sulfur batteries was reported by Han's group (Figure 11a-e).The results illustrated that S@Ni 3 (HITP) 2 cathode containing 65.5 wt% sulfur exhibited excellent sulfur utilization rate.The initial capacity of the cathode could reach 1302.9 mAh g À1 , and the capacity can remain 848.9 mAh g À1 at 0.2 C after 100 cycles.The reversible The rate performance and cycling performance of S@Ni 3 (HITP) 2 -CNT.[91] f) Zr-Fc MOF/CNT membrane and Li-S cell structures as effective interlayers for immobilizing polysulfide and accelerating polysulfide redox kinetics.g) The CV curves of the coupling between the actual Li-S cells and Zr-Fc MOF/CNT interlayer, raw CNT interlayer, and bare CNT/S cathode at a scanning rate of 0.2 mV s À1 within the range of 1.5-3.0V. h) The active sulfur utilization rate based on the rate performance of Zr-Fc MOF/CNT interlayer corresponds to areal sulfur loading of 1.56 mg cm À2 .[21] Energy Environ.Mater.2023, 6, e12521 discharge capacities of 807.4 mAh g À1 and 629.6 mAh g À1 after 150 and 300 cycles can be obtained at 0.5 C and 1 C, respectively, indicating that the cyclic durability of the material is very promising.By virtue of high conductivity of the material, it can effectively solve the disadvantage of limited contact between sulfur and electrons due to the insulation.It was expected to possess a broad application prospect in high-performance lithium-sulfur batteries in the future.[91] Fang et al. used 2D conductive MOF Ni 3 (HITP) 2 to grow directly on the diaphragm through a novel interfacial induction growth process and prepared a large-area crack-free crystalline microporous conductive film, which was used to improve the performance of lithium-sulfur batteries.The membrane can effectively inhibit the transport of polysulfide, which proved that the crystalline microporous membrane was a favorable barrier layer for the structure of high-performance lithium-sulfur batteries for the first time.The membrane can significantly improve the capacity and deliver the best sulfur utilization rate and cycle stability in lithium-sulfur batteries with similar sulfur load, which provided a possible direction for the subsequent research on lithium-sulfur batteries.[93] In 2021, Ye et al. proposed a multifunctional interlayer using ferrocene-based 2D metal-organic framework (Zr-Fc MOF) nanosheets and carbon nanotubes (CNT) woven through simple vacuum filtration to achieve high-performance lithium-sulfur batteries (Figure 11f-h). As anovel polysulfide inhibitor, Zr-FC MOF can inhibit the transport of polysulfide by electrostatic attraction and chemical anchoring capability.Carbon nanotubes penetrating Zr-Fc MOF nanosheets promoted electronic conductivity and exposed the active site of Zr-Fc MOF, which can improve the rate performance and cycling performance of lithium-sulfur batteries.When the sulfur load was 4.11 mg cm À2 , the battery capacity attenuation rate was only 0.027% after 1500 cycles at 1 C.This work provided a possible research direction for obtaining lithium-sulfur batteries with high energy density and long life span.[21]

Other Batteries
96][97][98] Exploiting various 2D MOFs as advanced materials applied in the configuration of SIBs sheds light to promote the further development of SIBs.Most of the previously reported 2D MOFs adopted copper (II) and nickel (II) as metal nodes, since these ions prefer D 4h coordination symmetry when interacting with strong field ligands, and can generate (2,3) cellular 2D lattices when connecting with hexagonal ligands with D 3h symmetry.As the consequence of effective d-p orbital overlap between metal center and ligand, highly conjugated system can be formed.Generally, limited by the tough synthetic condition induced by the less inclined D 4h coordination symmetry of Co (II), bulk synthesis of conductive Co-MOFs has rarely been realized.Notably, Bao et al. found that Co-HAB, a 2D conductive metal-organic framework based on cobalt with weaker electronic affinity, can rapidly and stably store sodium ions.Before adding the linker, HAB and Co (II) salts were mixed in water to form a complexation and could avoid the formation of hydroxide impurities.A stable and dense active site of high-energy energy storage device was formed by conjugation coordination between hexaaminobenzene (HAB) and cobalt center through redoxactive linker.The synthesis of Co-HAB successfully proved the reversible three-electron redox reaction of each HAB, providing a new electrode material for sodium-ion storage.The bulk conductivity of the material was as high as 1.57 s cm À1 , which could be utilized to manifest excellent rate performance.It can provide 214 mAh g À1 in 7 min and 152 mAh g À1 under 45 s.When the effective mass load increased to 9.6 mg cm À2 , the area capacity increased almost linearly.Co-HAB was the first conductive MOF to prove sodium storage, which contributed to the research of cobalt-based 2D conductive materials. [99]iao and colleagues proposed a novel mechanism for polysulfide confinement based on 2D MOF.In combination with in situ synchronous X-ray diffraction, electrochemical measurements, and theoretical calculations, the dynamic electronic state of the Ni center of 2D MOF enabled the interaction between polysulfide and MOF, which can be regulated during charge/discharge processes (Figure 12a,b).In metal sulfur batteries, where the sulfur cathode can be combined with a series of metal anodes, sodium is cheaper than other metals.Restricted by the high reactivity of sodium, the shuttle of polysulfide in sulfur cathode will seriously hinder the practical application.Excessive adsorption strength of polysulfide results in decomposition of polysulfide and blockage of adsorption site.The proper adsorption of polysulfide and the kinetics of promoting the conversion are largely determined by the local electronic states of sulfur cathode materials.However, the transformation process of polysulfide is complex, and systematic studies on the connection between the local electronic states of polysulfide and the adsorption behavior are still very limited.Based on a 2D metal-organic framework, Qiao's group discovered the relationship between the local electronic states of sulfur cathode materials and the kinetics of polysulfide conversion.By adjusting the electronic states of polysulfide materials, high-stability sulfur cathode materials have been designed for metal sulfur batteries.This study strengthened the exploration and in-depth understanding of the confinement mechanism of polysulfide and provided ideas for the realization of high stability of metal sulfur batteries. [100]ater zinc battery is also a research hotspot in the field of energy storage in recent years because of the relatively low zinc cost, low toxicity, non-flammable, and other advantages.Unlike lithium-ion batteries, water-based zinc ion batteries (ZIBs) in aqueous solution allow multivalent ion charge transport carriers, which can produce higher power and energy densities. [102]Instead of traditional organic electrolytes, aqueous electrolytes are safer, more environmentally friendly, and better in terms of ionic conductivity.The layered morphology of 2D MOFs allows Zn 2+ ions to be inserted/de-inserted in porous channels or interlamellar space, which is expected to develop ZIBs with high capacity, high coulomb efficiency, and long cycle life.Stoddart and colleagues used Cu 3 (HHTP) 2 as the cathode of a zinc battery, and this 2D conductive MOF had a high reversible capacity of 228 mAh g À1 at 50 mA g À1 (Figure 12c-g).The high diffusion rate of zinc ions and the low interfacial resistance caused by the intercalation of zinc ions made Cu 3 (HHTP) 2 cathode material follow the pseudocapacitance mechanism.XPS measurements and DFT calculations showed that Cu 3 (HHTP) 2 used copper and quinone structure as the redox-active site, thus increasing the specific capacity of the material.This study provided an idea for further designing 2D conductive MOF, improving the redox potential, and obtaining high-performance cathode materials for zinc batteries. [101]

Conclusion
2D MOFs possess nanometer thickness and large transverse dimensions, which can fully expose the reactive site, adjust the band gap Energy Environ.Mater.2023, 6, e12521 energy, and reduce the energy barrier of Faraday redox reaction.To improve the electrochemical performance of 2D MOFs in energy storage systems, it is of necessity to synthesize 2D MOFs with uniform morphology and high yield output.This review introduces strategies for synthesizing 2D MOFs, including top-down and bottom-up methods.Ultrasonic stripping and mechanical stripping are the most commonly employed top-down methods.The top-down method only breaks the weak interaction within adjacent layers, and can barely deliver the threat of destroying the covalent bonds in the individual layer of MOF structure.The choice of solvent has an important effect on the stripping rate and stability.Using the top-down method, high yield products can be obtained through simple methods, but the thicknesses of the products can hardly be controlled, and the structures are obtained with inconsistency.The bottom-up method is more widely used.Although the operation is complex, the structure and function of the product can be directly regulated with lower cost by changing the structure and symmetry of the organic ligand, as well as the type and proportion of the metal center.The integration of topdown method and bottom-up method can combine the advantages and synthesize more ideal products with high yield, low cost, and high quality.
From the perspective of energy storage application, 2D MOFs can be applied to supercapacitors, lithium-ion batteries, lithium-sulfur batteries, sodium-ion batteries, and other batteries.Since the 2D structure can be conducive to ion transfer in the electrolyte, more active sites could be offered to promote the redox reaction of transition metal ions and ligand units, which can obtain better electrochemical performance.Although 2D MOFs have accomplished great achievements in the field of energy storage and exhibited great application potential, tough issue of poor conductivity urgently needs to be solved in practical application.Moreover, inactive conductive agents have been added to improve the conductivity of the material, which would reduce the relative load per unit mass and volume of the active component, thus limiting the total energy density of the electrode.In addition, current researches of 2D conductive MOFs are mostly fabricated based on high-cost organic ligands, strategies to cut down the cost of synthetic materials possess the key to mass and commercialization, and it is of prime importance to prepare new conductive active electrode materials.Due to the fact that the development of 2D conductive coordination polymers is still in the infancy state, there are few reports on their structure, and the complex and unbalanced interaction forces within the materials during coordination crystallization limit the further research and application.In  [100] c) Schematic plot of the rechargeable Zn-2D MOF cell.d) Structure of Cu 3 (HHTP) 2 .e) Expected redox process in coordination unit of Cu 3 (HHTP) 2 .f) Cyclic voltammetry of Cu 3 (HHTP) 2 recorded at various scanning rates.g) capacitive contribution of Cu 3 (HHTP) 2 at 0.5 mV s À1 . [101]ergy Environ.Mater.2023, 6, e12521 the future, in-depth theoretical guidelines on constructing 2D MOFs are needed to offer comprehensively understanding about the charge storage mechanism, dedicating to develop advanced materials with high energy density, excellent cycle, and rate performance.

Figure 2 .
Figure 2. a) The histogram of the number of published papers about 2D MOFs.b) Pie chart of the application of 2D MOFs in various fields.c) Pie chart of the application of 2D MOFs in energy storage field.All the results are obtained from web of science with the key words of 2D MOFs.

Figure 4 .
Figure 4. a-c) The photograph, FE-SEM image, SAED pattern (left), and HR-TEM image (right) of the interface between water and CH 2 Cl 2 .[38]d) Hydrogen bond mediated assembly of the layered MOF and ultrasonication induced liquid-phase exfoliation of Cd-TPA.e, f) Fluorescence emission spectra of Cd-TPA colloidal solutions and relative fluorescence intensities in various solvents.[42]g) The exfoliation of bulk 2D c-MOF Ni 2 [CuPc(NH) 8 ] into nanosheets.[37]h) Solvent-assisted interaction fully exfoliated a coordination polymer.[31]i) Intercalation and chemical exfoliation approach to produce 2D MOF nanosheets.[32]j1, k3) The SEM and TEM image of MOF-La in bulk state, nanosheets made by ultrasonic for 30 minutes and nanosheets prepared by the method of Li-intercalation respectively.[44]

Figure 7 .
Figure 7. a) Synthesis scheme of Ni-pPD.b) CD curves of the asymmetric supercapacitors.c) Comparison of the actual available energy and volume available energy of asymmetric supercapacitors with those made of other materials.d, e) Potential and theoretical window match for the asymmetric supercapacitors.f, g) The height profile and FTIR of Ni-pPD.[20]h, i) Cyclic voltammetry profiles for Cu-HAB and Ni-HAB electrodes gathered at different scan rates.j) Weight and volume-specific properties of Ni-HAB particle electrodes with different area densities (9, 35, and 65 mg cm À2 ).k) Surface properties of Ni-HAB electrode under different weight loads.l) Capacitance retention data gathered by galvanostatic charge-discharge at 10 A g À1 .m) Comparison of volume and area capacitance of Additive-free Ni-HAB electrodes with other materials.[64]

Figure 8 .
Figure 8. a) Constant current charge-discharge curves of Cu-CAT NWA-based supercapacitor at different current densities.b) Rate-dependent specific capacitance of Cu-CAT NWAs electrode in a three-electrode cell.c) Performance comparison of Cu-CAT NWAs and carbon materials with symmetric solidstate supercapacitors.d) Comparison of the specific capacitances of the electrodes prepared by MOFs.[69] e) Rate capability curves of the areal and volumetric capacitances of Cu-DBC symmetric solid-state cell.f) Ragone plot of the Cu-DBC and other reported materials symmetric solid-state cell.[22]

Figure 9 .
Figure 9. a) Galvanostatic charge/discharge curves of 2D Cu-THQ MOF electrode at 50 mA g À1 .b) The evolution of electronic states of the repeating coordination unit of 2D Cu-THQ MOF during the charge/discharge process.c) Representation structure.d) Unit-cell structure.e) CV curve of 2D Cu-THQ MOF electrodes at 0.1 mV s À1. f) Charge/discharge profiles of 2D Cu-THQ MOF electrodes at 50 mA g À1 in the initial three cycles.g) Rate performance at various current densities of 2D Cu-THQ MOF electrodes.h) Cycling performance of 2D Cu-THQ MOF.[81]

Figure 10 .
Figure 10.a) Chemical structure and the redox reactions of Ni(L isq ) 2 .b) Structure of NiDI.c) The redox reactions of NiDI with respective counter ions.d)Charge-discharge curves.e) Cycling test and coulombic efficiency.[83]f) Synthesis of Cu-HHTQ.g, h) Rate performance and Cycling performance of Cu-HHTQ.i) Structure process of TQ-Li pathway in LIBs.[84]

Figure 11 .
Figure 11.a) The fabrication and purification processes for Ni 3 (HITP) 2 .b) The schematic diagram of the synergistic effects of Ni 3 (HITP) 2 and CNTs on electrochemical performance.c) Li-S adsorption tests.d, e)The rate performance and cycling performance of S@Ni 3 (HITP) 2 -CNT.[91]f) Zr-Fc MOF/CNT membrane and Li-S cell structures as effective interlayers for immobilizing polysulfide and accelerating polysulfide redox kinetics.g) The CV curves of the coupling between the actual Li-S cells and Zr-Fc MOF/CNT interlayer, raw CNT interlayer, and bare CNT/S cathode at a scanning rate of 0.2 mV s À1 within the range of 1.5-3.0V. h) The active sulfur utilization rate based on the rate performance of Zr-Fc MOF/CNT interlayer corresponds to areal sulfur loading of 1.56 mg cm À2 .[21]