Recent Advances and New Challenges: Two-Dimensional Metal – Organic Framework and Their Composites/Derivatives for Electrochemical Energy Conversion and Storage

. Metal – organic frameworks (MOFs), as a new generation of intrinsically porous extended crystalline materials formed by coordination bonding between the organic ligands and metal ions or clusters, have attracted considerable interest in many applications owing to their high porosity, diverse structures, and controllable chemical structure. Recently, 2D transition-metal-(TM) based MOFs have become a hot topic in this ﬁ eld because of their high aspect ratio derived from their large lateral size and small thickness, as well as the advantages of MOFs. Moreover, 2D TM-based MOFs can act as good precursors to construct heterostructures with high electrical conductivity and abundant active sites for a range of applications. This review comprehensively introduces the widely adopted synthesis strategies of 2D TM-based MOFs and their composites/derivatives. In addition, this paper summarizes and highlights the recent advances in energy conversion and storage, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO 2 reduction reaction, urea oxidation reaction, batteries, and supercapacitors. Finally, the challenges in developing these intriguing 2D layered materials and their composites/derivatives are examined, and the possible proposals for future directions to enhance the energy conversion and storage performance are reviewed.


Introduction
Crystalline porous materials are an attractive candidate to meet the needs of next-generation energy conversion and storage technologies. Metal-organic frameworks (MOFs) are the fascinating crystalline porous hybrid materials composed of infinite lattices of inorganic (metal ions or clusters) and organic ligands via coordination bonds [1][2][3][4][5][6]. This particular research area has gained momentum since the 1960s. Kitagawa et al., Robson et al., Yaghi et al., and Ferry et al. have attempted to relaunch the field with measurements of permanent porosity (95%), high surface area (over 7800 m 2 ·g -1 ), tunable pore size (ranging from micro-to macroscale), and surface functionalities. Thus far, thousands of MOFs have been reported, including Materials of Institutes Lavoisier (MIL), Universiteit I Oslo (UIO), Zeolitic imidazolate framework (ZIFs), and Hong Kong University of Science and Technology (HKUST). Although these catalysts exhibit remarkable electrochemical performance, mostly pristine MOFs have associated with drawbacks such as inadequate stability, low conductivity, and poor activity [3,4,[7][8][9][10][11][12]. Therefore, developing a MOF catalyst with excellent conductivity and cycling stability is still a challenging task. Hence, the rational design of two-dimensional-(2D-) based catalysts has been explored to overcome these drawbacks. As a typical advanced 2D material, Novoselov et al. explored graphene in 2004 [13]. Later, similar to graphene, phosphorene, and germanene, transition-metal dichalcogenides (TMDs) have attracted increasing attention [14,15]. Nowadays, 2D MOFs are gaining much research interest and as an emerging material of the 2D families, due to their layeredlike structure, diverse functionality, large surface area, high mechanical flexibility, and chemical stability [16]. Similar to other 2D materials, two different kinds of method can be applied for the synthesis of 2D layered MOF structure: top-down method and bottom-up method. A top-down approach involves the physical/chemical exfoliation of layer-structured MOFs, and the bottom approach refers to the direct synthesis of 2D MOF nanosheets from metal nodes and organic linker [17][18][19][20]. Meanwhile, 2D MOFs have been evolved for numerous applications in gas storage/ separation, catalysis, and energy storage [21][22][23]. Besides, 2D MOF bimetallic has received significant attention in the same field, because of their synergistic effects and carbon support, which causes superior charge and mass mobility, excellent electrochemical surface area, and faster movement of ions through interlayer spacing compared to than pristine MOFs. 2D MOF as an effective template matrix generates derivatives containing metal centers and carbon-based porous material through postthermal treatment. On other hand, 2D MOFs led to have emergence of stable MOF-based composites incorporating polymer, carbon, and metal oxides. This kind of preparation of electrocatalyst exhibits high catalytic performance. In addition, 2D conductive MOFs also exhibit π-d conjugation coordination and synergic properties, which provide them efficient delocalization of charge carriers within the plane and such frameworks exhibiting the excellent conductivities [24]. The ligand-dependent physicochemical properties would improve the stability characteristics [25]. Moreover, these features promote the rapid mass transportation of reactants and products and stabilize during the electrochemical process. The 2D MOFs and their derivatives/ composites are efficient for electrochemical-based energy conversion and storage application ( Figure 1). In fact, increased number of articles published so far showed their global recent trend in energy-related applications. Simultaneously, several reviews on 2D MOFs and conductive MOFs have also published in recent years, especially in the area of electrode and electrocatalysis for storage and conversion reaction. In this perspective, we systematically summarize the recent progress on 2D transition-metal-based (TM-Ni, Co, Fe, Ti, Mo, and V) MOFs and their composites/derivatives, which mainly focus on their synthesis strategies, catalytic properties, structure characteristics, lattice space, and strain engineering. In addition, challenges associated with electrochemical energy conversion (hydrogen revolution reaction (HER), oxygen evolution reaction (OER), ORR (oxygen reduction reaction), UOR (urea oxidation reaction) and CO 2 RR (CO 2 reduction reaction), and storage supercapacitors and batteries (ion and air batteries) with 2D MOFs are highlighted. It will also prove the knowledge about the design, functionalization, and synthesis strategy of various 2D MOFs for improving electrical conductivity, surface area, and strain engineering. It also highlights the intrinsic activity of 2D MOF electrochemical energy conversion and storage application and the challenges associated with the 2D MOFs. Finally, this review article includes the existing challenges and opportunities on the future aspects of 2D TM-based MOFs for various energy harvesting applications.

Overview and Synthesis Approach of 2D TM-Based MOFs
2D structures include sheets, plates, rings, membranes, rings, walls, discs, films, and flakes, which are characterized by their unique geometry, well-expanded surface area, and high surface-to-volume ratios [26]. These unique properties allow easy interactions between the reactant molecules and the active sites, thereby improving the catalytic activity. In this regard, MOF is a type of coordination polymer with high surface area and permanent porosity. MOFs are generally constructed using metal nodes and organic linkers. In (0D-3D) MOFs, dimensions are dependent on the coordination geometry of their building blocks [27]. The structures of 2D MOFs are predicted from the building blocks attached along the x-axis and y-axis directions that form single layers. These layers can overlap on each other by weak intermolecular forces, such as π-π stacking, hydrogen bonding, and van der Waals force, forming a multilayer structure [28]. Commonly, the preparation of 2D MOFs is based on top-down and bottom-up approaches. As the name signifies, the topdown approach involves the intercalate and exfoliate of the bulk layered to form single or few layered 2D MOFs, for example, lithium intercalation, sonication, mechanical exfoliation, and physical and chemical exfoliation [29][30][31]. Solvents play a crucial role in sonication exfoliation of layered MOFs and to stabilize the nanosheets in the solvent. To achieve the exfoliation and stability simultaneously, good wettability, solubility parameter, surface tension intercalation, and strong interaction are highly preferable. Till to date, the role of solvent is not clear to determining extent of exfoliation rate [32], but surface energy of the solvent is believed to play an important factor in determining the exfoliation of layered materials. For example, Cristina et al.
reported the exfoliation of MOF Cu(m-pym 2 S 2 ) (m-Cl)].nMeOH (PymS 2 = dipyrimidindisulfide) [33]. The large surface area flasks were obtained by strong sonicating bulk crystal of Cu(m-pym 2 S 2 ) (m-Cl)].nMeOH in water at ambient temperature. Further, they prepared free-standing Cu(m-pym 2 S 2 ) (m-Cl)].nMeOH flakes from dip coating in suspension of water/polymer over Si/SiO 2 substrate at 55°C. After solvent evaporation, capillary forces tend to collapse the structures and weaken the bond strength to obtain 2D layered structure. The AFM and optical images reveal that the thickness of Cu(m-pym 2 S 2 ) (m-Cl)].nMeOH is 2 to 30 nm (Figures 2(a) and 2(b)). Li et al. reported synthesis of layered MOFs via top-down delamination approach [34]. In their study, acetone was used as solvent for breaking the bulk MOF crystal into few layer MOF nanosheets. Presence of hydrogen bonds between layers facilitates easy delamination to MOF-2 nanosheets with thickness of about 0.75 nm. Similarly, ethanol, methanol, propanol, and water are used as solvent for exfoliation process [35][36][37]59]. Among [38]. The atomic force microscopy exhibits the thickness of 2D Zn DMS which is about 10 nm (Figures 2(c) and 2(d)). Besides N,N-dimethylformamide, tetrahydrofuran, and hexane are also used as solvent for exfoliation of layered MOFs, but these chemicals are harsh and not efficient enough for single or few layered exfoliations [39,40]. Therefore, ecofriendly approaches and efficient exfoliation rate are highly recommended.    6 (m-OH) 4 (BBDC-tert-butyl-1,3-benzenedicarboxylicacid(H 2 bbdc) denoted as MAMS-1 using deep eutectic solvent (DES) [41]. In the exfoliation process, first were DES (choline chloride and ethylene glycol) (1 : 2) solution was heated at 80°C. Then, MAMS-1 crystal was dispersed in DES solvent with different surfactants (PVP, SDS, and CTAB) by strong sonication, and bulk MAMS-l crystal is deliberately broken into 2D layer by green manner. This green technique helps in maintaining the surface tension of the solvent and hydrophobic interaction between solvent and surface ligand of MAMS-1 and stabilization of surfactants. The thickness of the obtained MAMS-1 nanosheets was about 4 nm, and the lateral dimension was 5 × 1 μm 2 . Moreover, the solvent is essential in stabilizing the exfoliated nanosheets and prohibiting their restacking and aggregating. The choice of solvent and external force plays a crucial role in breaking the weak van der Waals interactions between the layers to form 2D MOF, and solvent helps to minimize the energy and increase the exfoliation rate. In our opinion, abovementioned solvent and exfoliation process to 2D MOF is more useful and mostly provides uniform and stable 2D MOF structures, but do not give much yield; so, the choice of solvent is more important for the further improvement in exfoliation efficiency. For example, N-methyl-2-pyrrolidone and mixed solved water/ IPA are the few solvents which are generally used in exfoliation methods to development uniform nanosheets with large size sheets or flask [42].
Exfoliation has been divided into two types: physical and chemical exfoliation [42,43]. A physical exfoliation is a facile approach for preparation of 2D MOFs. Ultrasonication, wet ball milling, and shear mixing are general techniques which are used to exfoliate 2D MOFs. This method relies on external force to agitate solvent with high speed, thus resulting in strong shear force in the solvent to exfoliate the MOF nanosheets. For example, Yang et al. have prepared 2D Zn 2 (bim) 4 (bim = benzimidazole) by combined wet balling and ultrasonication process [44]. At the first step, pristine Zn 2 (bim) 4 bulk crystal was wet ball milled at 60 rpm, followed by sonication exfoliation in volatile solvent (Figures 2(e) and 2(f)). The mixture of methanol and propanol is the best solvent for this exfoliation process. The wet balling process facilitated the trapping of small methanol molecules in to the interlayer space of MOF crystal, and npropanol acts as stabilized agent of exfoliated nanosheets by attaching onto the surface of nanosheets with hydrophobic alkane chains. In single Zn 2 (bim) 4 layer, Zn ions coordinate with four benzimidazole ligands in distorted tetrahedral geometry. These 2D layers further stacked normal to c-axis through van der Waals interaction to form final MOF crystal with interlayer distance of 0.988 nm and thickness of 1.12 nm and lateral size in micrometers.
Yuxia et al. reported the micromechanical exfoliation of layered vdW MOF-2 using the scotch tape method (mechanical exfoliation). In this method, followed by a dual phase system, the upper phase consisting of hexane with TEA and bottom phase consisting diethylformamide (DEF) were solved with metal precursors (M = Zn and Cu) and H 2 BDC as organic ligand with pyridine [37]. Then, the two-phase system was sealed in glass vial and kept at ambient temperature for 24 h. Finally, highly crystalline vdW MOF nanosheet was formed. The Fe-SEM images confirm the lamellar sheet-like structures of vdW crystal (~10 nm). Obviously, this work provides novel strategy and improves properties of 2D MOFs. Later on, another author reported on synthesis and investigated in mechanical exfoliation. Zhao et al. reported the Ni 8 (5-BBDC) 6 (m-OH) 4 (labeled as MAMS-1 and BBDC = 5 − tert − butyl, 1, 3 − benzene dicarboxylic acid) MOF nanosheets using freeze-thaw exfoliation [45]. Bulk MAMS-1 crystal first was dispersed in hexane (Figures 2(g) and 2(h)). Then, the fluids were frozen in liquid nitrogen and have a temperature followed by thawing in hot water at 80°C. The shear force induced by volumetric change of hexane between solid state and liquid state is applied on the suspended MAMS-1 crystals, causes their exfoliation, and helped in the preparing freestanding nanosheets. The thickness and average lateral size of prepared nanosheets were 4 nm and 10.7 nm, respectively.
Recently, Xia et al. developed a surface acoustic wave exfoliation method to prepare Zn 2 (bim) 4 nanosheets. In their work, bulk Zn 2 (bim) 4 crystal was dispersed into mixed solvent of MeOH and n-propanol [46]. They first fabricate the lithium niobate (LiNbO 3 , LN) wafer and polydimethylsiloxane (PDMS) reservoir. The mixture of solution was added into PDMS reservoir via surface acoustic wave (SAW) exfoliation devices. The bulk Zn 2 (bim) 4 crystal exfoliates at frequency of 9.7 MHz at 15 V with time duration 1 to 90 min. Due to the action of electric field, the solution was ionized, and H + ions were impregnated into the layer of 3D MOF, which expands the interlaminar distance and reduces the interlayer bond strength. This led to the separation of layers and also release of the 2D flakes into the solution. Zn 2 (bim) 4 nanosheets with thickness of 5 nm and 66% monolayer were achieved at 90 min, and the maximum lateral dimension up to 3.5 mm was achieved.
Chemical/electrochemical methods were also reported for the exfoliation. Electrochemical intercalation of electrolyte ions and exfoliation materials played a crucial role in fabricating 2D MOF nanosheets. During electrochemical reaction, H 2 and O 2 molecules were produced on the anode or cathode layered surface and helped in formation of 2D MOFs. As a result, a low uniformity has been obtained in process. To obtain the higher uniformity in 2D MOFs, the electrolyte and applied time and potential are the key parameters. Moreover, the chemical/electrochemical method has strong oxidative ability and can break coordination bonds between the pillared layer and release them to form 2D nanosheets at various conditions. Zhou et al. reported a synthesis of layer structured MOF of (Zn 2 (PdTCPP)-(TCPP= tetrakis(4-carboxyphenyl)-porphyrin) by using the chemical exfoliation and ligand intercalation approach [47]. In this work, 4,4-dipyridyl disulfide as organic ligand was used as intercalated process instead of Li ions. They first synthesize bulk Zn 2 (PdTCPP) MOF. Later, Zn 2 (PdTCPP) crystal coordinate with 4,4-dipyridyl disulfide reduces the interlayer spacing of bulk MOFs. After selective cleavage of the disulfide bond, the free standing nanosheets with thickness of 1.65 nm were formed. Meanwhile, the layered MOF 4 International Journal of Energy Research crystal structure was still maintained after chemical exfoliation. Conclusively, the chemical exfoliation and ligand intercalation approaches are useful in reducing the interlayer spacing of bulk counterpart, causing the formation of 2D nanosheets. Instead of Li intercalation process, chemical exfoliation is a more efficient approach to construct the 2D MOF. Zhang et al. developed an electrochemical/chemical exfoliation to prepare 2D Co 6 O(dhbdc) 2 MOF denoted as MCF-13 nanosheets [48]. Herein, Co metal and 2,3-dihydroxy-1,4 benzene dicarboxylic acid pillar ligand were used as precursor to form nanosheets (Figures 2(i)-2(j)). In the electrolysis process, 3D-Co MCF-13 crystal was converted into 2D MOF nanosheet, but the oxidative ability during electrolysis process was slower and reduces the conversion rate of crystal. Researcher study the mechanism of electrochemical exfoliation of MCF-13 crystal. In the oxidation reactions, original pillars of layered ligand were bent into bridging angle by weakening the coordination bond and help to release the layers and results into the formation of MFM-13 nanosheets. AFM characterization confirms the formation of ultrathin sheets, having the thickness of 2 nm.
Qin et al. reported a synthesis of 2D Hemin-MOF using an electrochemical assembly technique [49]. Herein, GCE or ITO was used as working electrode, and Pt and KCl saturated calomel as counter and reference electrode, respectively. They first applied potential between the electrodes. Hemin coordinate with zinc metal was deposited into ITO substrate by deprotonation. Further researcher investigated potential-dependent and time-dependent formation of materials and also analyzed morphologies of the MOFs. At the increasing time between 150 and 1200 s, hemin-based MOF particles can be converted into nanosheets. The thickness of nanosheet is about 1.3 to 4.3 mm.
Moreover, top-down approach is simple and promising to synthesize highly crystalline 2D MOF nanosheets. But despite of their simple preparation strategies, the top-down method still has many challenges such as yield, reproducibility, and retention of the geometric parameters. In particular, preventing aggregation/restacking of 2D MOFs is also a challenging task. Therefore, systematic studies will be always conducted to explore the suitable solvent for the exfoliation and stabilization of 2D MOFs.
Bottom-up approach is also efficient and more flexible for the preparation of 2D layered MOFs [50] which involve direct synthesis of single or few layer 2D MOFs from metal node and organic ligand via interfacial synthesis, modulated synthesis, three-layer synthesis, and surfactant-assisted synthesis techniques [51][52][53]. The challenging aspects of bottom-up approach are the vertical growth along x and y directions, which inhibit the stacking of the layer along the respective direction. To overcome these challenges and for synthesizing the 2D layered MOF structure, surface modification by chemical route and regulating reaction condition are most preferable [51].
Monomers are coordinate by interface region, which led to growth crystal at horizontal direction, thereby resulting into formed 2D nanosheets via interfacing synthesis. The liquid/liquid interface, liquid /air interface, and liquid/solid interface are used for tuning the reaction conditions for the preparation of 2D layered MOF [52]. In liquid/liquid interfacial synthesis, the flat space between two immiscible liquids (e.g., water/dichloromethane and water/ethyl acetate) can provide the possibility to overcome the limited diffusion of monomer. Simultaneously, interface acts as a template for 2D-confined polymerization. Huang et al. reported π-conjugated Cu-BHT MOF (BHT = benzenehexathiol) nanosheets at interface of water/dichloromethane (Figure 3(a)). BHT was immersed into the dichloromethane, and water was put over organic phase to form oil/water interface condition [53]. Later, aqueous Cu salt solution was gently added into dichloromethane solution of BHT, and Cu-BHT nanosheets were obtained (Figure 3(b)). The Marinescu group fabricated π-conjugated-Co-THT (THT-2,3,6,7,10,11-triphenylenehexathiolate) MOF nanosheets at interface at water/ethyl acetate [54]. In this work, first ethyl acetate was added into surface of aqueous metal salt solution to create liquid/liquid interphase. Then, THT suspension was gently mixed into an ethyl acetate layer, and jar was tightly closed and allowed to stand till overnight to form the Co-THT film. Further, AFM confirmed the thickness of Co-THT film of around 100 and 200 nm.
Additionally, combination of Langmuir-Blodgett and liquid phase is also efficient in synthesis of 2D MOFs [56]. Kitagawa and coworkers developed a combination of Langmiur-Boldgett method and interfacial route with different synthesis conditions .61 . This technique reveled that oil layer containing organic ligand attached on the surface of an aqueous metal ion solution led to diffusion metal ion and coordinates with organic linker and resulting formed 2D MOF.
The liquid/air interface and liquid/solid interface are also good techniques to synthesis 2DMOFs [57].The liquid/air interface is usually employed with water/air interface. The evaporation of the organic solvent leaves the monomer on the aqueous surface, and air/water interface was formed. The polymerization reaction takes place at aqueous surface, which control the nucleation and growth of MOFs. The combination of Langmuir-Blodgett (LB)/liquid air interface method is more convenient for the synthesis and controlling the composition and structure of 2D materials.
For example, Xu et al. presented a combination of LB and liquid/air interface method to synthesis CoTCPP-py-Cu nanosheets (NAFS-1). Initially, CoTCPP and pyridine molecules dissolve into the mixed solvent of chloroform and methanol [58]. After mixing, the solution was added to Cu metal ion solution at LB trough, both the CoTCPP-py and Cu metal ions can quickly move to the liquid/air interface. During the continuous evaporation of the mixed solution, there are opportunities for the monomer and Cu metal ions to interact with each other and form CoTCPPpy-Cu nanosheets. The surface stress the at liquid/air interface provides a smooth space and control over thickness of nanosheets. The thickness of 20-layer MOF nanosheet was about 20 nm. Similarly, Feng et al. designed 2D mixed organic linker-based Co-THTA MOF (THT-2,3,6,7,10,11triphenylenehexathiol) and triphenylene hexamine (THA) by liquid/air interface [58]. The LB method was applied over Co-THTA to achieve the single layered nanosheets and obtained thickness that was about 0.8 to 0.1 nm (Figure 3(e)). The surface chemistry of 2D MOF at liquid/ solid interface is the interaction of molecular building units at the surface of growing nanosheets in solution. The solvent plays a key role in formation in self-assembled monolayer (SAM) through solvent-substrate and molecule-solvent interactions. In addition, concentration and temperature are also important for controlling the monolayer structures. For example, Sakaida et al. fabricated Fe(py) 2 [Pt (CN) 4 ] (pypyridine) MOF thin film through a combination of LB and layer-by-layer growth (LbL) technique at liquid/solid interface which is shown in Figures 3(g) and 3(h) [59] .In the fabrication, first Au/Cr/Si substrate protected with self-assembled monolayer using 4-merpatopyrindine. Then, substrate was washed with ethanol and dried at N 2 atmosphere. Afterward, the substrate was soaked into a metal ion (Fe and Pt) solution at room temperature for 30 cycles. In each cycle, one layer of 2D MOF nanosheets was formed, and thickness nanosheets were controlled by number of cycles (e.g., 16 nm~30 cycles and 30 nm~60 cycles). Interestingly, layer by layer (LbL) growth technique at liquid/solid interface supports the formation of 2D MOFs, which is promising for versatile functional materials.
Surfactant also plays a crucial role in controlling the growth and nucleation of MOF and anisotropic growth along in 2D direction [60]. Usually, MOF nanosheets have a tendency to restack or aggregate once formed due their high surface energy and high aspect ratio. The surfactant can be selectively attached to the MOF surface and control their growth by reducing the interlayer interaction and control stabilization of 2D MOFs. Polyvinylpyrrolidone (PVP) and hexadecyltrimethylammonium bromide (CTAB) are commonly used as surfactant for the stabilization of MOF nanosheets. The Zhang group introduced formation of Zn-TCPP nanosheets by using surfactant [61]. In this work, PVP was used as surfactant for the favorable formation of ultrathin MOF nanosheet and stabilized nanosheets and preventing from restacking/aggregation. In the rection, Zn 2+ ions coordinate with TCPP ligand in the layered stacked AB pattern. PVP molecules can easily attach on surface of MOFs, which controlled growth of crystal and support the formation of ultrathin MOF nanosheets (Figure 4(c)). AFM and Fe-SEM images represent the lateral thickness of MOF sheets and found to be 7.6 nm. Interestingly, they observed the amount of PVP to control the morphologies and prevent the aggregation. This surfactant-assisted strategy provides uniform morphology and improved yield of 2D MOFs. Bao et al. reported a synthesis of single crystalline nanosheet Cu-HHB MOFs through anion surfactant [62]. In this work, sodium dodecyl sulfate (SDS) was used as surfactant to form uniform ultrathin 2D c-MOF nanosheets. During preparation of Cu-HHB MOF, Cu metal was mixed in SDS aqueous solution. Then, tetrahydroxy-1,4-quinone  (Figure 4(a)). The AFM images demonstrated the smooth surface of Cu-HHB, and thickness of nanosheets was found to be 4.2 nm. Additionally, three layered techniques are also widely used technique to control growth rate and reduce the diffusion of metal ion during crystal growth [63]. During the preparation, metal ions and organic linker are dissolved into a top and bottom layer. The diffusion of metal ions and organic linker into middle layer led to buffering of precursors, which supports the formation of thin MOF nanosheets.
Modulators are also reacted with monomers or solvent and acts as a modified organic ligand which coordinate with metal node and regulate the anisotropic grow of 2D MOFs [64]. For example, Minh et al. reported the synthesis of two distinct linker-based 2D MOFs Cu 2 (NDC) 2 (DABCO)] n (NDC-1,4-naphthalene dicarboxylate and DABCO-1,4-diazabicyclo [2.2.2]-octane) by using modulator such as acetic acid and pyridine (Figure 4(b)) [65]. The modulator concentration was simulated for controlling morphology of Cu 2 (NDC) 2 (DABCO) n MOF and nanosheets obtained from 1.5 M concentration of pyridine (Figure 4(d)). The pyridine can efficiently block the coordination between copperdabco mode. Besides, acetic acid played a crucial role in modulating the copper-ndc mode, leading to formation of nanocubes and nanorod, respectively. 2D FeCo bimetallic MOF nanosheets can be prepared by mixing metal precursor, 1,4-benzene dicarboxylic acid, and triethylamine (TEA) in DMF/H 2 O/EtOH solvent together under stirring at ambient temperature [66]. The modulator TEA deprotonates the H 2 BDC and accelerated the coordination of H 2 BDC with metal ions and also generated an OHin water, which stabilize the edges of 2D MOFs and prevent from aggregation. Similarly, 2D Ni/Co MOF and Ni/Fe MOF have been synthesized.
The template-assisted method is the preparation of template, followed by the fabrication of the desired material using template and removal of the template, if necessary [67][68][69]. This technique provides the appropriate to control over morphology of 2D MOFs. Hard template (carbon, silica, Ni foam), soft template (surfactant, flexible organic molecules), and colloidal template (synergetic of both soft and hard template) are support to form the MOF nanosheets. For example, Ir-doped Ni (BDC) 2 TED nanosheets were grown on Ni foam by using the solvothermal condition [70].
In addition, combined bottom up and top-down strategy creates a new route to design 2D MOF [71]. Tang et al. reported this combined synthesis strategies of NiCo bimetallic MOF nanosheets using Ni and Co metal ions and H 2 BDC as ligand [72]. Initially, metal ions and H 2 BDC dissolved into DMF solvent and kept under stirring to obtain colloidal solution of MOF materials, which were metal ions coordinate with H 2 BDC, viz., bottom-up approach. Subsequently, obtained MOF was kept at sonication to formed nanosheets. This combined approach provides uniqueness and explores   [73][74][75][76]. Thus far, CO and Ni MOF is considered very important in transition-metal-based metalorganic frameworks owing to their low-cost, high corrosion resistance, good strength, high elemental abundance, good heat conduction, and high electrical conductivity [77][78][79] .Recently, 2D MOFs of Co and Ni are studied by many research groups due to their topologically diverse and fascinating morphological structures. In addition, synergistic effects between bimetallic and the adjacent heteroatoms provide better surface adsorption properties, improving the electrocatalytic properties of the resulting materials. Further, Co MOF and Ni MOF-based derivatives and composites have become more popular than pristine 2D MOFs. This section discusses the synthesis route, morphology, and the properties of single-layer structures, bimetallic structures, and their composites/derivatives.  [81]. The as-synthesized product was in good agreement with the single-crystal data reported. The X-ray diffraction (XRD) peaks showed that the nickel center is bonded to three BTC units. The peak assignable to the NiO phase indicates the formation of NiO from Ni-MOF. This layered Ni MOF possesses a large specific surface area (436.06 m 2 ·g -1 ) with honeycomb pores. The high specific surface areas of 2D Ni MOF are beneficial to electrolyte-electrode interface study, and the porous nature accelerates the ion diffusion rate. Wei et al. fabricated a 2D Ni single-layered MOF using an intercalation and sonication exfoliation approach [82]. In this study, [Ni 2 (bdc)(dabco)].guest (H 2 bdc = 1, 4 − benzenedicarboxylic acid, dabco = 1, 4 − diazabicyclo − ½2:2:2 octane) as a 3D pillared-layer MOF was used as a precursor for preparing single-layered 2D MOF [Ni 2 (bdc) (H 2 O) 2 ]. guest layer through the synergistic action of host-guest interactions and external forces. The removal of the dabco pillar in 3D Ni was accomplished by solvent substitution and capping agent (H 2 O, MeOH, and EtOH), which resulted in an intrinsic 2D layered sheet. The solution was subjected to ultrasonic waves to strengthen the exfoliation effect and inhibit the well-ordered restacking of the resulting mass, generating the single layer Ni nanosheet MOF with uniform thickness. The 2D Ni nanosheets were distributed homogenously over the glass substrate, as confirmed by atomic force    9 International Journal of Energy Research microscopy (AFM) and scanning electron microscopy (SEM). The corresponding thickness was~0.9 nm, which matched the thickness of a single 2D coordination layer. The SEM image clearly showed that bulk 3D Ni MOF transformed into 2D MOF nanosheets with a wrinkled morphology. This work nicely illustrated about the synergetic action of host-guest interaction and external force and opened a new direction of intercalation approach for synthesis 2D MOFs. The same year, Xiang Wu et al. designed ultrathin 2D Ni (Im) 2 nanosheets from pristine Ni 2+ and a zeolitic imidazolate framework (Ni (Im) 2 .ZIF) by liquid exfoliation [83]. AFM revealed a thickness of 5 to 140 nm. The nanosheets are separated with different thicknesses owing to the centripetal forces of ultrasound exfoliation. Further coinciding with the transmission electron microscopy (TEM) results, the 2D Ni (Im) 2 nanosheets were dispersed homogenously in water with stronger Tyndall effects than bulk Ni (Im) 2 . These beneficial thin nanosheets are well-suited electrocatalysis with reduction diffusion kinetics and superior electron transfer properties to its bulk counterpart. Ran et al. reported that ultrathin carboxylate carbon nanotubes interpenetrated (Ni-MOF/C-CNTs) nanosheets [84]. These nanohybrids were prepared using a solvothermal reaction and solvent exchange step. The deprotonated H 3 BTC ligands were coordinated to the Ni 2+ ions, and the bridged ordered chain selfassembled into layer-by-layer 2D Ni MOF nanosheets and a C-CNT skeleton ( Figure 6(e)). Hence, well-interconnected nanosheets can deliver rapid ion diffusion/transportation, assessable surface area, and electroactive sites, as well as enhanced reaction kinetics for hybrid energy-storage devices. Interestingly, this work provides Ni-MOF/C-CNTs nanosheets which exhibit an outstanding specific capacity of 680 Cg -1 at 1 A·g -1 and better capacity retention.
The Co-based layered structure inherited the characteristics of the pristine MOFs and possessed highly accessible active sites and more exposed metal atoms on their large surface. The coordinative unsaturated metal sites with dangling bonds are suitable for practical applications, such as gas separation, storage, electrocatalysis, and sensors [86][87][88][89][90]. A single and few layers of Co-based MOF are mostly prepared by the exfoliation route. Hui Luo et al. reported an ultra-forceassisted liquid exfoliation method for the preparation of single layer 2D Co-based MOF [91]. A layered bulk Co (CNS) 2 (pyz) 2 as a precursor was exfoliated in the ethanolic solution to obtain an ultrathin 2D nanosheet with a lateral area using a top-down approach. During the exfoliation process, the ethanolic solvent was intercalated into the interlayer space with the help of an ultrasonic force, and single-layered nanosheets were then shielded from the bulk precursors on the absorbing surface. These aforementioned 2D nanosheets were characterized by AFM and TEM. The AFM topological image revealed a thickness of approximately 1.0 nm. Further, a highresolution TEM image revealed the random stacked giant ultrathin sheets with unambiguous outlines and curling edges. The results indicated the flexibility and easy stacking of these ultrathin nanosheets. Moreover, 2D nanosheets have excellent platforms for intermolecular interactions and the visual detection of toxic anions, cations, or other harmful chemicals.
Ding et al. implemented a MOF on MOF twodimensional heterostructure with improved electroactivity [48]. The Co 2+ ion coordinated with 4,4′-biphenyl dicarboxylate in the DMSO solvent to form Co-BPDC film-like nanostructure. The Co 2+ ions were then coordinated with 1,4-benzene dicarboxylic acid in a DMF solution to obtain the Co-BDC nanosheets. Finally, the Co-BPDC/Co BDC heterostructure was constructed using a mild two-step solution route (Figure 6(b)). The as-obtained nanostructures were characterized by Fourier transform infrared (FTIR) spectroscopy and XRD. In the spectrum of Co-BDC and Co-BPDC, a sharp peak at 3600 cm -  at 1579 and 1360 cm -1 were assigned to the symmetric and asymmetric stretching vibrations of the carboxylate group, which represent the BDC and BPDC present in the framework. The XRD peaks suggest that Co BDC probably grows on the surface of Co BPDC. This growth model enhanced the Co-BPDC/Co-BDC heterojunction and improved thermal stability and Brunauer-Emmett-Teller (BET) surface area. These unique advantages of Co-BPDC/Co-BDC have endowed electrochemical activities and a faster conversion rate than Co BTC and Co-BPDC. Zha et al. reported 2D hierarchically layered cobalt MOF via MOF-mediated growth [92]. In the synthesis route, hl-MOF and urea-assisted hl-U-MOF were grown directly on well-cleaned Ni foam using a hydrothermal method. The nanosheets were constructed on the hl-U-MOF via a MOFmediated approach, and the hl-U-MOF was completely covered with Co (OH) 2 NRSs. Finally, interconnected u-hl-MOF/nanorod and nanosheets were formed. The u-hl-MOF morphology was confirmed by FE-SEM imaging. The hl-MOF completely covered the surface of the Ni foam, and well-ordered hl-MOF exhibited a layered nanosheet-like structure with a thickness of 30-50 nm. Their interlayer spacing ranged from 50 to 500 nm. The addition of urea (labeled as hl-u-MOF) reduced the interspacing and resulted in a packed layered structure. The nanorods were grown simultaneously on the hl-u-MOF surface. The thickness of the nanosheets was 30-80 nm, and that of the nanorods was 80-10 nm. Thus, Co (OH) 2 NRSs easily penetrated the hl-u-MOF -nanosheets without causing significant damage, resulting in excellent interconnectivity between the nanorod and nanosheets. The morphology of hl-u-MOF-NRSs improved the electrochemical conductivity of the pseudo-supercapacitor.

Synthesis of Mixed
Metal-based 2D MOFs. In recent years, the good electrochemical properties of the 2D bimetallic metal-organic framework have attracted increasing attention [93]. Bimetallic MOFs have two different metal clusters in single organic nodes. They exhibit a synergistic effect and improved properties compared to their monometallic counterparts. They could serve as outstanding templates/precursors for synthesizing functional nanomaterials and porous carbon with controlled morphologies and structures [94]. For example, Fu et al. synthesized NiFe bimetalorganic framework nanosheets from a mixed solution of Ni 2+ , Fe 2+ , and benzenedicarboxylic acid (BDC) and confirmed by X-ray photoelectron spectroscopy (XPS) [95]. The composition was determined to be Ni 0.75 Fe 0.25 BDC using various characterization techniques. The highresolution XPS spectra of the Ni 2p and Fe 2p peak located at 855.8 eV and 712.9 eV reveal an oxidation state of +3. The as-synthesized 2D bimetallic MOF could be assigned to ultrathickness, high concentration of exposed active sites for electrochemical reactions.
Similarly, Liu et al. reported the synthesis of NiFe-2DMOF by layered double hydroxide (LDH) approach. In this work, NiFe-LDH and 1,4-H 2 BDC was dispersed into DMF/H 2 O solvent and transferred to autoclave and heated at 150°C. During reaction, carboxyl organic ligand coordinated NiFe-LDH, and proton transfer mediator endows free standing 2D MOF nanosheets. The Fe SEM image confirms the morphology of NiFe-2DMOF [69].
Jin et al. reported a two-dimensional (2D) few layer black phosphorus (BP)/Ni Co MOF hybrid [96]. The carboxylate group in BDC 2could not coordinate with metal ions but was bonded to BP, resulting in a stable 2D hybrid structure. The formation of (BP)/Ni-Co MOF hybrid was detected by XRD and TEM. The XRD pattern of BP/Ni-Co MOF hybrid matched with the JCPDS 985792 card, space group of C 2 /m. The crystallographic planes (010), (001), and (200) show that Co and Ni are coordinated octahedrally to the six O atoms. These atoms share the edge/coroner to form a 2D layer separated by the BDC ligands. The other planes (002) and (004) correspond to black phosphorus. Hence, Ni-Co MOF is hybridized with few-layer BP. TEM confirmed that the (BP)/Ni-Co MOF hybrid has a nanosheet-like morphology with~6 nm thickness. This BP/ Ni-Co MOF hybrid provided the pathways for an innovative electrocatalyst to enhance the electrochemical performance. Wen et al. prepared two-dimensional bimetallic MOF, which was used as an electrode material for tyrosinase biosensors [97]. In the solvothermal reaction, 1,4-H 2 BDC was made to react with Ni (OAc) 2 .4H 2 O and Zn SO 4 .7H 2 O in a DMAC solution to form Ni Zn MOF nanosheets using a nucleation process. Herein, the Zn ions do not affect the crystal structure of Ni MOF, as confirmed by XRD and TEM images. It also showed that flower-like clusters assembled by ultra-nanosheets and edges tended to curl and lose flexibility. The flexibility of NiZn MOF nanosheets could render rapid mass transport and charge transfer and sufficient active sites accessible to a catalytic reaction. In addition, Huang et al. examined the in situ synthesis of Fe-doped Ni MOF on nickel foam [98]. In this process, the high nearsurface concentration of Ni 2+ and Fe 3+ ions react with NH 2 -H 2 BDC in a mixed solution and Ni foam substrate to form a NiFe nanosheet via solvothermal reaction. The AFM phase image clearly showed the morphology of a few layers of Fe-Ni MOF nanosheets, and the corresponding thickness was 10 nm. Thus, element mapping of the nanosheets showed that iron and nickel atoms are distributed uniformly on the nanosheets, indicating the well-dispersed catalytic active sites. Interestingly, this work offered a binder-free electrode for energy conversion and storage application.
Li et al. reported 2D bimetallic Co x -Fe-MOF nanosheets for electrocatalytic application. The NH 2 -BDC ligand was treated with Co (NO 3 ) 2 .6H 2 O and FeCl 3 .6H 2 O in a mixed solution (DMF, ethanol, and water), whereby Co x -Fe-MOF was formed [99] .The as-synthesized heterometallic MOF was examined by XRD and TEM. The structural data of Co x -Fe-MOF exactly matched the cambridge crystallographic data center No. 985792. Co x -Fe-MOF belongs to the space group C2/m. Thus, the surface of Co or Fe is partially coordination bonded to NH 2 -BDC ligands because of the formation of unsaturated metal sites ( Figure 6(d)). TEM showed a predominant nanosheet-like morphology with distinct fringes of 0.247 nm. Hence, the well-dispersed Co Fe MOF nanosheets provide more metal ion sites for strong intermolar interactions. In addition, the synergistic effect of Co x -Fe-MOF exposed the fast charge transport for 11 International Journal of Energy Research electrochemical performance. Overall, this approach provides new insight to the preparation of 2D bimetallic MOF catalyst together with ideal efficiency and appreciable electrocatalytic activities.
A conductive bimetallic MOF Co x Ni y -CAT was prepared by Hajin et al. by ball milling and hydrothermal reaction [100]. The obtained Co x Ni y -CAT was later used for the electrochemical oxygen reduction activity, and the relevant properties were studied. The hydrothermal reaction between 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and Co 2+ or Ni 2+ led to form the 2D layered hexagonal structure of M-CAT. The HRTEM images also confirm the hexagonal pattern in the ab plane and also, CoxNiy-CAT has well-developed pore nature through the c-axis. The lattice fringes were clear observed in HRTEM images, and their corresponding d -spacing is 1.91 nm and 2.20 nm. Energy dispersive X-ray spectroscopy (EDX) of the sample revealed cobalt and nickel incorporated as metal nodes within the sample. In the ORR, bimetallic CoxNiy-CAT achieved excellent performance compared to monometallic Ni-CAT or Co-CAT because it exhibited high onset potential and diffusion-limiting current density.
With the similar research idea to improve the properties of materials, Gao et al. fabricated 2D Co-based bimetallic MOFs (Co-Fe/Ni @ HPA-MOF nanosheets) via an in situ hydrothermal reaction strategy employing Co (OAC) 2 .4H 2 O, Fe (ClO 4 ) 2 .xH 2 O, Ni (NO 3 ) 2.6 H 2 O, and hypoxanthine (HPA) as a precursor. The products were characterized by SEM and XPS [101]. SEM images revealed that Co@HPA-MOF, Co-Ni@HPA-MOF, and Co-Fe@HPA-MOF are stacked with each other having a very sharp and defined edge, indicating its ultrathickness. XPS revealed the presence of Co, Ni, Fe, and N on the surface of Co-Ni @HPA-MOF. These 2D bimetallic MOFs exhibited a synergistic effect of the mixed metals, and the active sites showed increased charge transfer at the catalytic interface.
2.1.3. Composites/Derivatives of 2DMOFs. The 2D Ni MOF composites and their derivatives help broaden the application scope of Ni MOFs from gas separation, absorption, storage, and catalysis to other fields, such as drug delivery, optical application, and sensors [102][103][104]. For example, NiCo-MOF/reduced graphene oxide heterostructure designed by Liu et al. has been used as electrode materials [105]. Following this, in a facile and simple one-step process, an aqueous solution containing Co(NO 3 ) 2 , Ni(NO 3 ) 2 , and rGO was added directly to an aqueous solution of 2-methylimidazole with stirring at 28°C to obtain a novel Ni-Co MOF. The mixture turned slowly to opaque. The final product was obtained after 24 h. In this approach, rGO served as a spacer between Ni-Co MOF sheets and reduced the agglomeration of the MOF precursors. The synergic effects have led to enhanced electrical conductivity in hybrid materials. Moreover, NiCo-MOF/rGO hybrid demonstrated high power and energy density efficiency of supercapacitors and excellent cycling stability.
Wen et al. reported similar materials like Ni-Co-MOF with exfoliated graphene from the electrochemical oxidation process at ambient condition [105]. The exfoliated graphene (EG) provided better electrical conductivity than rGO. The metal node (Ni/Co) and organic ligand (1,4-BDC) attach easily to the EG surface through electrochemical interactions and formed 2D Ni/CO-MOF/EG composites. The thickness of composite structure was 3.2 to 3.5 nm, determined by AFM analysis. The obtained Ni-CO MOF/EG composite provided a higher surface area and showed an excellent glucose oxidation reaction. This work represented an interesting study of exfoliated graphene and electrochemical interaction between EG and Ni/CO MOF, which causes to improve the properties of 2D MOF.
Liu et al. prepared NiO nanosheets from Ni-MOF: 4 ]. X H 2 O) as a precursor via simple pyrolysis [106]. The obtained NiO nanosheets were confirmed by XRD and TEM. All XRD peaks corresponded to the orthorhombic system and exactly matched the Hofmann-type Ni MOF with a 2D layered structure. Then, the Ni MOFs were transformed to NiO nanosheets by simple annealing in air, as confirmed by TEM. Hence, the sheet-like morphology would be maintained after pyrolysis, and the nanosheet thickness was approximately 27 nm. Moreover, the NiO nanosheets showed a high surface area and large void space, which allowed the accessibility of ions through the electrolyte to the active materials. The NiO nanosheets from Ni-MOF are promising electrode materials for energy storage applications. Sun et al. have reported the controllable synthesis of 2D Ni-based MOF nanoplates [107]. Subsequently, 2D Ni-based MOF nanoplates were used as precursors to form the freestanding nitrogen-doped Ni-Ni 3 S 2 @carbon nanoplate by pyrolysis. As a bottom-up approach, the solvothermal reaction of 4,4 ′ -bipyridine and NiSO 4. 6H 2 O in an aqueous solution formed 2D Ni-based MOF nanoplates. The MOF was then converted to freestanding N-doped Ni Ni 3 S 2 @carbon nanoplates (denoted as Py-1) by a self-sulfidation process. The counter anion SO 4 2was the internal sulfur source, while the Ncontained 4,4 ′ -pyridine provided a nitrogen source for doping. AFM showed that the thickness of Py-1 ranged from 10 to 20 nm. XPS revealed Ni 2p ½ and Ni 2p 3/2 in Ni 3 S 2 and C-N bond, suggesting the N-doped on the carbon matrix. This study opened up new avenues for the rational bottom-up synthesis of layered MOF nanomaterials using pyridine as an inhibitor, resulting in a low dimensional carbon-decorated metal/metal sulfide nanocomposite. Lin et al. stated the synthesis of porous NiCoSe nanosheets from 2D MOFs in Ni form using the hydrothermal method [108]. The ZIF-67 (Co)/Ni foam precursor was immersed in ethanol solution, which would promote H + and OH + during hydrolysis. These H + and OH + led to etching and produced Ni 2+ and Co 2+ ions to form NiCo layered double hydroxide (NiCo LDH). Subsequently, NiCo LDH nanosheets were treated with Se powder at 180°C to obtain a NiCoSe nanosheet via ion exchange reaction ( Figure 6(c)). SEM image and EDX spectrum indicate that Ni, Co, and Se elements are evenly distributed in the nanosheets, and thickness is 230 nm (Figure 6(c)). The composites/derivatives of 2D Co MOF play a better role in electrocatalysts and electrodes. For example, Lee et al. prepared a 2D Co/N-C hybrid structure from a 2D layered Co-based MOF by optimizing the annealing conditions in a N 2 atmosphere [109]. The International Journal of Energy Research obtained hybrid structure was confirmed by XRD, AFM, and HRTEM. The XRD peaks corresponded to the presence of carbon and metallic cobalt atom. The thickness of the 2D Co/N-C hybrid was 2-3 nm according to AFM. High-resolution TEM confirmed the composite structure for the 2D Co/N-C hybrid. Simultaneously, the lattice fringe with an interplanar distance of ∼0.205 nm was assigned to the (111) crystal planes of Co. This confirmed the formation of zerovalent Co nanoparticles and the uniform distribution on the nitrogen-doped carbon matrix. During the annealing process, Co 2+ in the layered Co-based MOF was converted to Co nanoparticles. The nitrogen-rich hexamethylenetetramine molecules formed the N-doped graphitic carbon by a simple pyrolysis process. In addition, the mapping results revealed the distribution of Co, O, and N, suggesting graphitic-structured carbon and uniform dispersed metallic cobalt nanoparticles mixed to form an excellent configuration for electrochemical performance.
Wang et al. carbonized the Co-based layered 2D MOF (Co(TPT)-(fma)(H 2 O).3H 2 O) obtained by solvothermal conditions to obtain N-doped carbon-coated cobalt nanoparticle. Li et al. used layered 2D MOF as templates to construct metal oxide/carbon (MO x/C, M=Co, Ni, and Cu) nanosheets arrays for the electrocatalytic OER [110]. The obtained product was confirmed by TEM. The highly dispersed Co 3 O 4 nanoparticles on the 2D nanosheet had a smaller size (~10nm) than the 3D MOF-derived Co 3 O 4 (~100 nm). This 2D MOF template strategy enabled the resulting 2D MOx/C arrays with an open void arrayed 2D structured and strong interactions between MO and the carbon matrix. This unique structure contributed to the significant catalytic active sites and rapid mass transfer; the latter afforded enhanced conductivity and structural integration. Zhang et al. synthesized nanolayered cobalt @carbon hybrids using Co MOF nanosheets via a simple pyrolysis route [111]. The Co MOF nanosheets were prepared by a surface-assistant method using PVP as the surfactant to control the crystal growth of pristine Co-MOF. Subsequently, after pyrolysis, Co-MOF nanosheet was converted to amorphous carbon matrix embedded with Co nanoparticles. The pyrolysis temperature played an important role in tuning the microstructure and performance of the nanolayered Co@C hybrid. The optimized Co@C 800 hybrid exhibited a large BET surface area of 110.9 m 2 ·g -1 than the other hybrids. AFM images of the hybrid showed an average thickness of~3.5 nm and a lateral size of ∼300 nm. This work provided a new avenue for the design and construction of high-performance microwave absorbers by modifying the morphology of pure MOF precursors. The texture and high degree graphitization of the carbon supports have potential significance on performance of the composites. The carbon that supports with large surface area and well-dispersed porosity can ensure zero diffusion of substrates. This can improve the electrical conductivity, as well as the chemical and thermal stability of graphitized carbon. Lei et al. reported the first aqueous solution route for the preparation of bimetallic zinc and cobalt MOF nanosheets and produced a series of porous zinc cobalt sulfide nanosheet arrays on Ni foam (Zn-Co-S/NF) using this Zn/Co MOF as a precursor via sulfurization-thermolysis ( Figure 6(f)). The backbone of the imidazole ions played a significant role in the bottom-up approach of MOFs and the preparation of the derivatives [112] .After simple sulfurization with thioacetamide (TAA), the Zn and Co species were transformed to Zn-Co-S/NF. The as-obtained product was employed as supercapacitor electrode. When Zn-Co-S/NF was used as a positive electrode and activated carbon was employed as a negative electrode for ASC devices, a high energy density (31.9 Wh·kg -1 ) was obtained. Moreover, bimetallic metal sulfides on the current collector can be simply expanded to the robust design of many other electrodes for electrochemical energy conversion and storage applications.

Synthesis of Iron and
Titanium-Based 2D MOFs. 2D Fe MOF-based materials exhibit considerable catalytic activity and are promising candidates for electrocatalyst and electrode materials. Fe MOFs have many metal sites, often coordinated with organic ligands, which result in insufficient exposure of the active sites and low activity [113][114][115]. An efficient method to modify 2D Fe MOFs and ameliorate the electrocatalytic activity is to expose more metal active sites or enhance the intrinsic activity of each site. In addition, introducing foreign metal atoms into 2D Fe MOF, such as Ni, Co, Mo, and W led to the formation of bi or ternary 2D MOFs with their synergistic effects. These effects expose more metal sites and improve the bond strength between the metal sites and organic linkers and achieve better catalytic performance. In addition, Ti is one of the most essential metals in photocatalytic systems. In particular, Ti-based metal-organic frameworks have great potential for photocatalytic reactions, including CO 2 reduction, N 2 fixation, H 2 O 2 production, H 2 generation, environmental remediation of organic and inorganic pollutants, and organic transformation reactions, owing to their low toxicity, high abundance in the earth's crust, excellent optical properties, and unique structures [116]. 3D Ti MOFs have been synthesized and characterized. On the other hand, there are fewer 2D-based Ti MOFs than other 2D MOFs, e.g., Ni, Co, and Fe 2D-MOFs. The reason behind this is the complexity of titanium chemistry in solution that impedes the design and synthesis of 2D Ti-based MOFs. This section presents a few 2D Ti and Fe-MOFs, bimetallic, composites, and derivatives. In addition, the properties and applications to energy storage, conversion, sensor, and chemical separation are given.  [117]. Subsequently, the film thickness was controlled from 20 μm to several mm by reaction time (Figure 7(a)). The Fe 3 (THT) 2 (NH 4 ) 3 MOF film was confirmed by SEM. The 2D network exhibited a honeycomb structure with a pore size of~1.9 nm. The Fe metal and THT organic ligand are bonded together in the monolayer. The Fe 3 (THT) 2 (NH 4 ) 3 MOF film exhibited a significant photodetection performance  14 International Journal of Energy Research at low temperatures. The same research group reported π-dconjugated semiconducting multilayered Fe 3 (THT) 2 (NH 4 ) 3 MOF films prepared using the interfacial method [117]. Elemental analysis confirmed the chemical composition of Fe 3 (THT) 2 (NH 4 ) 3 MOF film. In the film, Fe ions are coordinated to trigonal organic ligands of the thiol group. It exhibited a high surface area of around 526 m 2 ·g -1 and a bandgap 0.25 eV. This multilayered 2D MOF provided plenty of room for development in this exciting research field.
The Li group demonstrated that 2D magnetic MOFs with Sharstry-Sutherland lattice, TM (Fe, Co, and Mn)-PBP (PBP-bis (4-pyridyl) (2,20-biprimidine) by first principle of density functional theory (DFT) [114,118]. Further, electronic and strain engineering are discussed. The electronic modification is greatly induced by strain effects of MOFs. The electronic structure calculation estimated that TM (Fe, Co, and Mn)-MOFs of band gap ranges from 0.12 eV to 0.85 eV, which can be regulated by various routes, such as strain or hole/electron doping. The 95% compressive strain level caused Mn-PBP to transit from semiconductor to metal; as a result, it demonstrates that Mn-PBP framework can modulate from half semiconductor to half metal. Additionally, it is observed that due to their electronic properties and strain level, Fe-PBP and Co-PBP are more promising material than Mn-PBP.
Zhang et al. research team introduced a surfactant molecule as PVP (polyvinyl pyrrolidine) to selectively attach on MOF surface, modulating the anisotropic growth Co-TCPP (Fe) via surfactant-assisted approach. This led to produce of ultrathin 2D Co-TCPP (Fe) nanosheets with a subthickness -10 nm. 2D M-TCPP (Fe) nanosheet-based film was used as a novel electrochemical platform for detecting H 2 O 2, which exhibits higher sensitivity than natural heme proteins [119]. Owing to its high synergistic effect, excellent stability, moderate surface area, and ideal thermal structure, the 2D Co-TCPP (Fe) nanosheet presented appreciable catalytic activities for detecting H 2 O 2 with a wide linear dynamic range, lower detection limit, high selectivity, and sensitivity. Besides, the solid-liquid interface and liquidliquid interface provide a beneficial platform to assemble 2D MOF nanosheets. Kitagawa et al. fabricated Fe(py) 2 [Pt (CN) 4 ] (py = pyridine) nanosheets on a metal substrate by layer-by-layer growth at the liquid/solid interface. The Fe(py) 2 [Pt(CN) 4 ] 2D layered MOF was formed by the connection of [Pt(CN) 4 ] and Fe 2+ ions with each other by pillar of pyridine molecules [120]. In the layer-by-layer growth method at the liquid/solid interface, the Au/Cr/Si substrate is first covered with functionalized self-assembled monolayer (SAM) using 4-mercaptopyridine. The substrate is then immersed into an ethanol solution containing Fe 2+ ions and [Pt (CN) 4 ] 2at ambient temperature for 30 cycles. In each cycle, one layer of 2D MOF nanosheet is produced, and the thickness is controlled during each cycle (e.g., 30 nm for 60 cycles). The obtained 2D layers were stacked uniformly along the c direction with a p-π interaction between the adjacent pyridine molecules.

Synthesis of Mixed-Metal-Based MOFs.
Recently, 2D bimetallic Fe MOFs have been prepared with both noble metals (Pt, Pd, Ru, Ag, Au, and Ir) and transition (Ni, Co, and Cu) to promote catalytic activity and enhance electrical conductivity [122]. In this regard, Liao et al. reported the design and synthesis of 2D bimetallic MOF using 1,4,5,8naphthalenetetracarboxylic anhydride as an organic ligand and Fe 2+ and Zn 2+ as a metal node [123]. They used a self- assembly synthesis strategy to construct the highly porous 2D bimetallic MOFs. The XRD pattern of 2D-Fe/Zn MOF suggested that the doped Fe ions were either incorporated into the Zn MOF or atomically dispersed in the precursor (Figure 7(e)). In this work, incorporating a second metal was beneficial for exposing active sites and improving the charge and mass transport. Zhang [125]. 2D Bimetallic MOF was synthesized by a direct solvothermal method and utilized as heterogeneous Fenton-like catalysts. Incorporating Mn into bimetallic Fe MOF could promote the catalytic process, while Co exhibited no favorable behavior. The introduction of Ni had an inhibitory impact. Compounds of this new series were supported unambiguously by infrared spectroscopy, X-ray diffraction, and EDS measurement. Ni-based metals have commendable electronic properties similar to Pt and Pd. Mai et al. developed the low crystalline bimetallic Fe/Ni MOF for OER applications [126]. In this case, 3D Ni Fe hydroxide was used as a precursor or semisacrificial template to obtain structural host growth of ultrathin NiFe-MOF nanosheets with a thickness of 1.5 nm. These 2D bimetallic nanosheet was formed by tuning the reaction period and solvent ratio. The assynthesized Ni-Fe MOFs were tightly inlaid into the NiFe hydroxide precursor, effectively avoiding the possible stacking and activity loss issues. Thus, the resulting material exhibited a low over potential of 240 mV at 10 mA·c -2 for the OER. Li et al. [127] reported the modulated synthesis of bimetallic 2D Fe/Co MOF nanosheets. First, Fe/Co BDC was prepared by bonding Fe and Co metal ions and 1,4phthalic acid linker (Figure 7(c)). Triethylamine was used a modulator to regulate MOF crystal and prevent from aggregation. Then, intercalation of BDC ligand between 2D layers led to the formation of 2D Fe/Co-BDC nanosheets. The obtained nanosheets were applied into dye removal treatment. Fe/Co-BDC nanosheets showed an efficient oxidation of organic substance in water. This system achieved a high kinetic constant (1.066 min -1 ) for chromium and BPA, respectively.
Wang et al. synthesized a 2D Ti MOF (NTU-9 NS) using 2,5-dihydroxy terephthalic acid (DOBDC) and Ti (OiPr) 4 . In this work, NTU-9 nanosheets were prepared by the solvothermal method. Metal ion and organic ligand were dissolved in acetic acid and heated at 150°C for two days, whereby red hexagonal prismatic crystals were formed [128]. The bulk NTU-9 was delaminated by an ultrasonication treatment in isopropanol at room temperature. AFM and TEM confirmed the delamination NTU-P-NS and appropriate thickness of 15 to 50 nm. Luminescent sensing experiments on NTU-9-NS were performed with various concentration that were achieved in aqueous solution containing Ag + , Co 2+ , Ni 2+ , Cu 2+ , Mg 2+ , Hg 2+ , Cu 2+ , Cr 3+ , Zn 2+ , and Fe 3+ . Among these metal ions, the Fe 3+ showed a rapid response and the best detection limit.

Composites/Derivatives of Fe and
Ti-Based MOFs. The 2D Fe MOFs composites and their derivatives with a high surface area and enhanced electrical conductivity have been widely investigated for electrocatalysis [129,130]. MOFs are used as a precursor to produce desirable platforms for synthesizing various functionalized materials with various morphologies and compositions.
Zhang et al. reported a simple approach to synthesize porous Fe 2 O 3 by calcining MOF sheets in air [131]. Porous Fe 2 O 3 with a controllable morphology selectively oxidized H 2 S to sulfur. The Fe-based MIL-53 (Fe) structure called (Fe III (OH).[O 2 C-C 6 H 4 -CO 2 ].H 2 O) with different shapes was synthesized using a solvothermal method using acetic acids as the growth modulator. Pyrolysis of the parent MOF produced a series of porous Fe 2 O 3 with an inherited morphology. The pyrolysis products denoted as Fe 2 O 3 -xH (x = 0, 3, 5, 10; x represents the volume of introduced acetic acid (HAc)) were characterized by TEM and XPS. The products exhibited high exposure to surface-active sites favorable for the catalytic reaction. The particle diameters of Fe 2 O 3 -xH (x = 0, 3, 5, 10) were approximately 100, 40, 50, and 80.0 nm, respectively, and XPS revealed a higher Fe 3+ content. The Fe 2 O 3 with a spindle morphology showed excellent performance, achieving a yield of 97% of sulfur at 120°C.
Zhan et al. constructed two-dimensional N-doped carbon nanosheets coupled with Co-Fe-P-Se from 3D Fe-Co MOF [132]. Fe and Co metal ions coordinated in situ with the hexamethylenetetramine linker to form Co-Fe-HMT nanorods. The bimetallic Co-Fe-HMT nanorods would be a promising precursor to form the 2D nitrogen-doped carbon matrix that prevents the aggregation of metallic nanoparticles and improves the electron transfer rate. In addition, the catalytic activities were improved by introducing phosphorous and selenium during pyrolysis to form phosphides and selenides with a metal particle/carbon matrix. Hence, the doping of Se and P in the carbon matrix could expose more defect lattice structures to accelerate the 16 International Journal of Energy Research electrochemical reaction by modulating the surface of the electronic structure and producing more active sites. These defects were revealed by Raman spectroscopy using the ID/ IG ratio.  [134]. Due to the coordination, the A atomic layer connects with BTC to form octahedral structure of MIL-100 (Fe) and subsequently formed the composite with MXene which are shown in Figure 7(f). The MIL-100 (Fe)/Ti 3 C 2 T x composite was used as an electrode material. It exhibits a superior energy density of 85.53 Whkg -1 . The synergistic effects between MIL-100 (Fe) and Ti 3 C 2 Tx improve energy density of hybrid electrode. Zhao et al. reported 2D novel MOF-MXene composites, which were prepared using an interdiffusion reaction strategy [135]. In this reaction, Co 2+ metal ions and 1,4-benzene dicarboxylic were interdiffused in a mixed solvent layer. Subsequently, the Ti 3 C 2 T x nanosheets were coated with Co BDC. Further, XRD analysis confirmed the formation of layered structure of Co MOF/Ti 3 C 2 T x nanosheets. A strong XRD peak at 8.3°was assigned to the (002) plane of Ti 3 C 2 T x , and other peaks at 8.8, 14.1, 15.7, and 17.8°were attributed to the (001), (201), and (400) planes of the layered triclinic structure Co BDC, respectively. The Co 2+ ions were deposited on the functionalized surface of Ti 3 C 2 T x by an electrostatic interaction. Simultaneously, Co 2+ was coordinated with BDC molecules by an interdiffusion reaction. This process allows the efficient and easy fabrication of composites without a surfactant. Zhao et al. developed a titanium nanosheets array/Co MOF composite using a simple solvothermal reaction [135].
Similar, Xia et al. demonstrated liquid-phase deposition and pyrolysis of Ti 3 C 2 T X MXene/CoNi MOFs to form a CoNi bimetallic nanoparticle composite [136]. First, Ti 3 C 2 T X MXene/CoMOF powder was poured into Ni metal aqueous solution at strong stirring, and the (Ti 3 C 2 Tx/CoNi-MOFs) system was obtained. Afterwards, it was pyrolyzed at 800°C in N 2 atom, and Ti 3 C 2 Tx/CoNi-MOFs was converted into Ti 3 C 2 Tx/CNF/TiO 2 /CoNi (TCTCN) composite (Figure 7(d)). HR-TEM reveals the lattice spacing of Ti 3 C 2 Tx/CNF/TiO 2 /CoNi (TCTCN) composite, showed spacing of 0.35 nm, 0.24 nm, and 0.20nm, and corresponds to (101) and (103) plane of TiO 2 and (111) of CoNi alloy, respectively. This composite was applied for a microwave absorption application. Owing to magnetic-dielectric synergistic effect, the microwave absorption performance was enhanced.

Synthesis of Molybdenum and Vanadium-Based 2D
MOFs. Mo and V-based low dimensional MOFs are attractive candidates for efficient electrochemical energy conversion and storage system applications owing to their unique thermal, mechanical properties, electrical conductivity, and stability [137]. The properties of MOFs can be improved by incorporation with 2D transition-metal dichalcogenides (MoS 2 ) or graphene. Thus, layered structure MoS 2 or graphene covalently linked to Ni, Co, and Zn MOF can improve the novel properties for a myriad of applications. This section discusses the potential applications of 2D-based Mo and V MOFs (single-layer structure, bimetallic, composites, and derivatives) in energy storage, conversion, sensors, and chemical separation.  [139]. Inspired by this, Lahiri et al. synthesized a 2D Mo-based MOF. They found it to be efficient as a catalyst in the electrochemical reduction of N 2 to NH 3 under ambient conditions with high stability and catalytic efficiency [140]. The obtained 2D structure of Mo-based MOF led to more exposed active sites. Michael et al. reported a conductive 2D MOF from (H 2 NMe 2 ) 2 Nb 2 (Cl 2 dhbq) 3 and Mo 2 (Cl 2 dhbq). The MOFs contained Nb and Mo as metal centers and 6dichloro-2,5-dihydroxybenzoquinone (H 2 cl 2 dhbq) as the coordinating ligands [141]. The reaction between MoCl 2 / NbCl 2 with chloranilic acid (H 2 cl 2 dhbq) yielded few-layer hexagonal nanosheets with oxidized molybdenum and niobium and a reduced ligand. The product was characterized by X-ray absorption and vibrational spectroscopy. A local trigonal prismatic Mo and Nb coordination geometry was suggested, which is consistent with their increased covalency relative to first-row transition-metal compounds. Vibrational spectroscopy revealed the electronic structure of the secondrow transition MOF. Moreover, the results showed that covalency of the second-row transitions metals increased due to the improved charge transport in the metal-organic materials. Barthelet et al. described a new type of architecture, i.e., three-dimensional vanadium (III) dicarboxylate (MIL-71 (V)) that was constructed from [V III O 2 (OH) 2 F 2 ] octahedra as metal clusters and 1,4-benzene dicarboxylic acid [142]. The vanadium octahedra were bridged by -OH groups 17 International Journal of Energy Research and F atoms along the b and c axes, respectively, to produce 2D layers of octahedra perpendicular to a axis. The 2D inorganic layers interconnected by 1,4-benzene dicarboxylic acid ligands formed the 3D pillar-layered structure of MIL-71(V).
Wu et al. designed a novel 2D vanadium MOF from MXene under solvothermal conditions. In this work, the V 2 AlC max phase was added slowly to the acid-salt mixture and heated at 35°C for 24 hr. The multilayered V 2 CTx was delaminated with tetramethylammonium hydroxide (TMAOH) for 4 hr to form the few and monolayer V 2 CTx. Later, few-layered V 2 CT x MXene and tetrakis (4-carboxyphenyl) porphyrin (TCPP) were used as the metal node and organic linker to produce a 2D-V-PMOF. In addition, they developed MIL-68 from V 2 CT x MXene as metal cluster and H 2 BDC as an organic ligand, respectively. From this, V-PMOF exhibited appealing proton conductivity, and this thin sheet-like MOF has many electronic and sensing applications [143]. Although there are few reports on 2D-based vanadium MOF, 2D V-based MOF has great potential as electrocatalysts for energy conversion. This has triggered scientists to explore 2D-based vanadium MOF for various applications.

Composites/Derivatives of Mo and V-Based MOFs.
In view of the synergistic combination of MOF and other active components, new electrocatalysts and electrode materials have been fabricated for various applications. For example, Cheng et al. reported composites and derivatives of Cobased MOF through direct carbonization for application in sodium-ion batteries [144]. The derivatives were prepared using Co MOF as a precursor to form an N-doped carbon nanowall array through simple pyrolysis in an Ar/H 2 atmosphere. MoS 2 was then grown on the surface of an N-doped carbon array to form CC@CN@MoS 2 composite by the hydrothermal method. In this case, the Co-based MOF played multiple roles in improving the catalytic performance and electron-ion transport of its carbon derivatives. SEM of the CC@CN@MoS 2 composite revealed 2D vertical structures coated uniformly on the carbon fibers. The carbon atoms from the pristine Co-based MOFs remained in the resulting N-doped carbon array/MoS 2 as large interlayer spacers, which promote ionic diffusion paths to favor Na + insertion/extraction. Zhu et al. synthesized 2D Co-BDC/MoS 2 hybrid nanosheets using a sonication-assisted solution strategy [145]. Li-ion intercalation/exfoliation can form few-layered MoS 2 nanosheets easily from bulk MoS 2 . Co 2+ and BDC were then deposited on the MoS 2 nanosheets, which induced partial transfer from the 2H phase to the IT Phase of MoS 2 . It could be more favorable for electrochemical performance. The assynthesized product was confirmed by XRD. The pattern showed two broadened peaks at (200) and (400) [146]. The Ni 2+ metal ions bonded to the nitrogen atom of 4,4-bipyridine to form the Ni-MOF nanosheet. The Mo atoms were then incorporated into the pores of the frame structures via a self-assembly route. The morphology of Mo/Ni-MOFs showed a large nanosheet structure with the accumulation of small nanorods. This significant feature of the nanosheets improved the charge delivery, which accelerated the charge storage and enhanced the electrical conductivity of the electrode materials. The resulting supercapacitors exhibited a superior power density of 802 W·kg -1 , an energy density of 59 Wh·kg -1 , and a long cycle retention of 93% after 20,000 cycles in an alkaline medium.
2.4. Synthesis of 2D-Conductive MOFs. Conductive 2D metal-organic frameworks as emerging class multifunctional catalysts have attracted increasing interest because of their predictable and unique structures, intrinsic permanent porosity, high charge mobility, and good electrical conductivity. Such favorable characteristics render them promising excellent platforms for advanced energy conversion and storage applications. In conductive MOFs, both metallic nodes and organic ligands could serve as the sources of charge carriers [147,148]. In particular, organic ligands with either stable radicals or redox-active molecules together with metal centers have unpaired electrons that can be employed to ensure high charge delocalization through continuous conjugation in the MOF. In 2009, Shinya et al. showed the first-time synthesis of conductive MOFs, Cu [Cu (pdt) 2 ] (pdt = 2, 3 − pyrazinedithiolate). Its electrical conductivity was 6 × 10 −4 Scm − 1, and it exhibited a thermal activation energy of 0.193 eV. In Cu [Cu-(pdt) 2 ], Cu 2+ ions bridged by pdt linker via an N atom formed square two-dimensional (2D) sheets and were confirmed by single-crystal XRD and magnetic susceptibility. A Cu [Cu-(pdt)] 2 composed of a divalent Cu atom and pdt organic ligand with well-defined geometric exhibited relatively high electrical conductivity [149]. Dinca et al. fabricated conductive MOF Ni 3 (HITP) 2 (HITP = 2, 3, 6, 7, 10, 11 − heamionotriphenylene) via a simple ammonia-assisted solvothermal technique. The hexagonal Ni 3 (HITP) 2 structure is shown in Figure 8(a). The AFM images confirm that thickness of Ni 3 (HITP) 2 film is about 500 nm (Figure8(a)). The obtained Ni 3 (HITP) 2 was used as an active electrode material for the EDL supercapacitors without conductive additives or other binders. This conductive-based MOF exhibited a bulk electrical conductivity (5,000 S·m -1 ) that was greater than activated carbon and holey graphite (~1,000 S·m -1 ). The gravimetric capacitance of Ni 3 (HITP) 2 was nearly about 111 F/g with only 10% loss in the capacitance over 10000 cycles in stability test which was higher than carbon based capacitor. These materials showed outstanding electronic and host-guest properties [150]. Similarly, Lian et al. synthesized Co 3 (HITP) 2 and mixed-metal CoxNi 3 -x(HITP) 2 MOF under ammoniaassisted solvothermal conditions. They obtained micromorphologies and fine structures [151]. On the other hand, the crystallinity of the Co 3 (HITP) 2    3-x ) (HITP), reproduced with permission [150][151][152][153]. may be at least partially responsible for the relativity low bulk conductivity of 8 × 10 −4 S/cm, which is several orders of magnitude lower than that Ni 3 (HITP) 2 . By contrast, the mixed metal Co/Ni MOFs showed increasing conductivities with increasing Ni content. The material exhibited different electron counts, structural distortions, layers stacking patterns, or other factors. Similarly, Lian et al. designed π-d-conjugated-Co 3 (HITP) 2 MOF and applied into electrocatalyst for oxygen evolution reaction. Co 3 (HITP) 2 MOF has followed the same ammonia synthesis route [151]. The electrical conductivity of Co 3 (HITP) 2 MOF was reached at 1150 Sm -1 , due to the high loading amount of Co metal (23.44 wt%) and Co-N 4 sites. The Fe-SEM and EDX mapping confirm the fine structures and element distribution in Co 3 (HITP) 2 MOF (Figure 8(b)). During electrochemical reaction, Co 2+ converts into Co 3+ ion in Co 3 (HITP) MOF, which enhance catalytic activity and provide low overpotential of 254 mV at 10 mA·cm -2 .
Campbell et al. reported the synthesis of Cu 3 (HITP) 2 using ammonia-assisted solvothermal technique and applied into the chemiresistive sensor device. In this work, CuSO 4 and 2,3,6,7,10,11-hexamainotriphenylene hexahydrochloride were dissolved in ammonia solution and kept for strong stirring at ambient temperature [152]. The resulting black precipitation was formed, and XRD pattern revealed that hexagonal with slipped-parallel stacking of the 2D sheets was formed (Figure8(c)). The electrical conductivity was measured by two probe equipment. The conductivity was reached to 0.2 S·cm -1 , which was slightly lower than Ni 3 (HIPP) 2 (2 S·cm -1 ). Ni 3 (HIPP) 2 MOF did not response to chemiresistive sensor device. Then, Cu 3 (HIPP) 2 was analyzed into sensor device. Cu 3 (HIPP) 2 device detecting NH 3 gas at sub-ppm detection limits (0.5 ppm) at applied potential of 100 mV at ambient temperature. Sensing performance was constant in the presence relative humidity (60%). MOFbased sensing devices can be possibly controlled by the selection of metallic nodes. The same group established a synthesis of binary alloy (M x M ′ 3-x ) (HITP) (MM ′ = CuNi, CoNi, and CoCu)-based conductive MOF [153]. HR-TEM microscopy, X-ray photoelectron spectroscopy, and X-ray absorption fine structure (XAFS) spectra were analyzed to confirm the binary alloy 2D c-MOF (Figure 8(d)). Similar to the pristine modulated (Ni 3 (HIPP) 2 , Co 3 (HIPP) 2 , and Cu (HIPP) 2 ) MOF, binary alloy has also allowed to turned the optical and electric band gaps. The electrical conductivity varied from 5:8 × 10 −3 to 55.4 S/cm in 4 orders of magnitude.
The smaller size π-d conjugation ligand, such as hexaminobenzene, has excellent conductivity compared to larger ligands, such as benzenehexathiol and 2,3,6,7,10,11-hexaminotripheylene [154]. In a foundational work in 2017, an M (acetylacetonate) (M-Ni, Cu, and Co) motif was incorporated into a π-conjugation material M (HAB) (HAB = hexaminobenzene) framework. This framework was synthesized using bottom-up techniques [140]. The as-synthesized material, which was carried out a liquid-liquid and air-liquid interfacial reaction, showed a thick (~1-2 μm) and thin (<10 nm) framework. AFM, SEM, and TEM confirmed the thin and relieved the nature of the framework. M (HAB) has a smooth and flat surface of the nanosheets. The lattice spacing was found to be~0.5 nm for M (HAB) (M-Ni,Co, and Cu) MOFs. Ni (HAB), Co-HAB, and Cu-HAB MOF's TEM images show the lattice spacing in the range of (0 .5 nm) with a hexagonal diffraction pattern. This nanosheet complex form was applied to devices and electrically characterized by electron-beam lithography. The resulting Ni analog was mildly conducting compared to the other complex. Park et al. reported the stable and dense active sites of Co-HAB in 2018. Here, Co-HAB was synthesized under systematic conditions [155]. Conductive MOFs were proposed based on the deprotonation and subsequent partial oxidation of the linker. HAB and metal salts were first mixed in water to avoid hydroxide formation. An ammonia hydroxide solution was subsequently added in air, and the reaction was stirred until dark navy-colored precipitates of Co-HAB were formed. The as-synthesized product showed an increase in particle size and crystallinity. In addition, the Co-HAB exhibited a bulk electrical conductivity of 1.57 S·cm -1 and outstanding chemical stability in both aqueous and organic media.
Cui et al. produced 2D mixed-metal MOF(Ni/Co) (HAB) sheets for electrocatalytic overall water splitting and methanol oxidation reaction [156]. The researcher followed the same synthesis route. HRTEM of CoxNi 3 -x(HAB) 2 MOF showed nanosheet-like structures with lattice fringes of around 0.21 nm corresponding to the (111) plane. The dosage of Ni 2+ ions has affected the structures, which could have increased the amorphous nature of the framework. As the resultant active sites of CoxNi 3 -x(HAB) 2 MOF was increased. Lie et al. reported mixed metal MOF (Co/Ni) (HAB) by a hydrothermal treatment. In their synthesis, they optimized the Co/Ni ratio and hydrothermal treatment time (2 h, 6 h, 12 h, and 16 h). During the process, HAB drew OHaway from Co (OH) 2 , releasing Co 2+ coordinated with HAB. The Ni 2+ ions assembled with HAB to form the CoNi-HAB structure [157]. At 16 hours, CoNi-HAB started to aggregate, and electrochemical performance to this catalyst exhibited slowed electrode kinetics.

Applications of 2D TM-Based MOF Materials in Energy Conversion and Storage devices
3.1. Electrocatalysis. In the past few years, research on 2D matrix-supported MOF for an advanced electrochemical energy conversion reaction has increased dramatically. Electrocatalysts play important roles in the next generation of energy devices [158][159][160][161]. Pt and other noble metals (Pd, Ir, and Ru) are standard electrocatalysts for energy conversion reactions because of their high intrinsic catalytic potential and stability. Nevertheless, the prohibitive cost and natural scarcity limit their widespread commercial application. Electrocatalysis is a surface reaction, in which the catalytic performance of the catalyst depends on their surface properties to a large extent. In this regard, 2D pristine MOFs and their composites are the most prominent electrocatalysts with favorable 20 International Journal of Energy Research properties, such as ultrahigh thickness, enhanced conductivity, high surface to volume atom ratio, tunable oxidation state, and exposed active sites which are shown in Figure 9 [162][163][164]. This section presents advanced 2D MOF-based electrocatalysts for the HER, OER, ORR, CO 2 RR, and UOR.

Hydrogen Evolution Reaction.
Electrochemical hydrogen evolution is the most efficient cathodic reaction and sustainable process to produce the high-energy carrier, molecular hydrogen [165][166][167]. Hydrogen evolution is a 2etransfer process involving a multistep reaction consisting of absorption, reduction, and desorption. The initial step is the generation of absorbed hydrogen (H * ) on the surface electrode (Volmer step). A proton bonds directly with (H * ) to generate H 2 (Heyrvosky step), and a combination of 2H * produces H 2 (Tafel step) [168,169]. The equations for the reaction in acid and alkaline media are shown below.
(1) Acid media The evaluation parameters of the HER include the onset potential, current density, overpotential (thermodynamic barrier HER catalysis), turnover frequency, Tafel slope (rate-determining step), and stability [170][171][172]. The ideal electrocatalyst for HER should generate a large exchange current density and a small Tafel slope. The volcano plots show that precious metals (Pt and Pd) are highly efficient electrocatalysts [173]. On the other hand, alternatives are needed owing to their high price and low abundance.
2D transition-metal-based organic frameworks have been utilized (as alternatives for precious metals electrocatalysts) for catalyzing HER with a lower overpotential and increased faradic efficiency. In this context, Downess et al. prepared Co-benzenehexthiolate (BHT) films for use as electrocatalysts for the hydrogen evolution reaction. This study also reported the effects of the film thickness on the HER activity [174]. The optimized Co-BHT film with 244 nm exhibited a low overpotential of 185 mV, Tafel slope of 62 mV dec -1 , and a current density of 10 mA.cm -2 . In addition, the authors investigated the efficiency of BHT films containing various metal centers as electrocatalysts for the HER in a basic medium, and the order was Co − BHT > Ni − BHT > Fe − BHT. Clough et al. fabricated two metalorganic surfaces (MOS) to evaluate their activity as electrocatalysts for hydrogen evolution [55]. The cobalt MOS [Co 3 (BHT) 2 ] [3] was prepared by the liquid-liquid interface. The other MOS [Co 3 (THT) 2 ] 3in which the length of the framework was extended was prepared by a reaction of triphenenylene-2,3,6,7,10,11-hexathiolate (THT) linkers with cobalt dithiolene, (Figure 10(a)). The average thickness of these films is 360 nm, as confirmed by the SEM image. Both MOS films served as excellent cathode materials for hydrogen generation at an aqueous solution. Dong et al. fabricated three immobilized metal sites, viz., metal bis (diamine) (MN 4 ), metal bis (dithiolene) (MS 4 ), and metal dithiolenediamine (MA 2 N 2 , M=Co, and Ni) incorporated into 2D MOF-based Co-THAT from simultaneously coupling with two building blocks, i.e., triphenylene hexathiol (THT) and

22
International Journal of Energy Research triphenylene hexamine (THA) [175] . The electrocatalytic two dimensional MOFs followed the order, MS 2 N 2 > MN 4 > MS 4 . Among this series, Co-THAT integrated with MS 2 N 2 showed higher electrochemical activity with a moderate overpotential of 283 mV at a current density of 10 mA·cm -2 in an acid medium. The authors demonstrated the theoretical calculation for proton reduction. The M-N site was more active than the other possible active sites (e.g., N-N, N-S, -Co-S, and S-S). The coordination with the S atom-boosted H 2 adsorption ability of the atomic Co led to an improved HER catalytic activity of the CoS 2 N 2 compared to CoN 4 and CoS 4. Wei et al. employed a one-pot hydrothermal procedure to anchor 2D Co Ni bimetallic MOF onto a copper film and characterized this material using AFM and TEM images [176]. The images showed that the nanoplates had sharp angles and clear edges of 400 × 600 nm 2 . AFM showed that the average thickness of the nanoplate was 5.3 nm. The 2D Co Ni MOF nanoplates were examined for their electrochemical performance in the HER. The samples showed 99.0% faradaic efficiency of H 2 with a low overpotential of 120 mV. The nanoplates demonstrated high durability above 5000 cycles. Louie et al. synthesized ultrathin NS array of NiFe-based MOFs, which demonstrated excellent performance towards the HER, OER, and water splitting [177]. The substrate and metal salts (nickel acetate and iron nitrate) were mixed in an aqueous solution, and 2,6naphthalenedicarboxylic acid was then introduced. The organic linker bridged with two metal atoms to form a 2D Ni Fe MOF nanosheet array via a dissolution-crystallization mechanism. The product exhibited HER activity with a low overpotential of 134 mV at 10 mA·cm -2 and robust stability in 0.1 M KOH. The superior HER activity was attributed to the optimal structural characteristics of the electrocatalysts, including high exposed molecular metal active sites resulting from MOF nanosheets, enhanced electrical conductivity, and favorable kinetics through a low dimension nanostructure and a combination of hierarchical pore sizes. Studies on the HER performance of metal phosphides, nitrides, and carbides have emerged in recent years. Many MOFderived carbon or functional materials phosphides have been chosen as active catalysts for HER because of their favorable features, such as hydrogen adsorption free energy and excellent electrical conductivity [178][179][180].
Monamag et al. fabricated Co-P@NC from a 2D MOF as HER electrocatalysts, and the origin of the HER activity of the material was also studied [181]. A Co-based porphyrin paddlewheel framework (PPF-3) was treated with NaH 2-PO 2 .H 2 O and then carbonized under an Ar atmosphere to obtain Co-P@NC (Figure 10(b)).The materials pyrolyzed at different temperatures are denoted as Co-P@NC-600, Co-P@NC-700, Co-P@NC-800, and Co-P@NC-900. The HER measurement was carried out on both alkaline (1.0 M KOH) and acidic electrolytes (0.5 M H 2 SO 4 ). In the HER performance, Co-P@NC-800 was the best electrocatalyst with a good Tafel slope of 74 mV dec -1 and a low overpotential of 98 mV. Co-P@NC-800 was also studied for the cycling stability test using a chronopotentiometry technique at a constant current density of 10 mA cm -2 in alkaline and acid electrolytes and showed a superior durability for 12 hr. Co-P@NC-800 was confirmed by XRD and Raman spectroscopy. The results showed a higher degree of graphitization   [182]. The Ni MOF@Pt was leading to smaller Tafel slope and overpotential than commercial Pt/C (Figures 10(c) and 10(d)).
Wang et al. synthesized novel bifunctional porous Ni 2 P nanosheets derived from NiO-MOF-74 for catalytic applications [183]. Well-defined nanosheets with numerous porous structures could enhance the conductivity and allow the catalyst to have a large surface area and porosity. Such a structure suggested that the linker could infiltrate the pores of MOF-74. The NiO species was beneficial in generating H + ions, which caused the dissolution of NiO to Ni 2+ and produced long-term stability. The porous Ni 2 P nanosheets exhibited a small Tafel slope of 63 mV dec -1 and an overpotential 168 mV at 10 mA cm -2 . Moreover, the porous channels provided a new route for electron conduction and thus improved electron transfer and accelerated the high bubble (H 2 and O 2 ) diffusion on the electrode surface.

Challenges for 2D
Metal Organic Framework as Acidic HER catalyst. As discussed in the previous section, the HER performance of 2D MOF could be improved considerably by enhancing the active sites and intrinsic conductivity, which can be achieved with the help of various strategies. Although sufficient advancement has been accomplished in preparing 2D MOF for HER applications, there is an opportunity to design and synthesize many 2D MOFs to face energy conversion challenges in the near future. Most MOF template precursors have been shown to afford various porous nanostructured materials with good catalytic performance. Despite this, less electrical conductivity of a few 2D MOFderived nanocomposites is unfavorable for electrocatalysis. In this regard, graphene oxides or carbon matrices have attracted unprecedented scholarly attention because of their distinctive advantages. In addition, 2D MOF-derived nanocomposites or 2D MOF hybrid materials might effectively exert their synergistic effect. For example, FeNi 3 -Fe 3 O 4 nanoparticles can be cointercalated into 2D MOF sheets and CNT matrix via facile hydrothermal process to generate FeNi 3 -Fe 3 O 4 NPs/MOF-CNT, which can be decomposed by pyrolysis to obtain a low-cost Ni/Fe/C precursor. The carbon nanotube-2D MOF matrix can be employed as a bifunctional electrocatalyst with superior electrical conductivity for HER performance. It delivered a low Tafel slope of 37 mV/dec -1 and an overpotential of 108 mV at 10 mA cm -2 [184]. Gao et al. synthesized Hofmann type MOF trimetallic (Co 2+ /Fe 2+ /Ni 2+ ) carbon nanoflower electrocatalysts for HER applications [185]. The Hofmann MOF, formed by dozens of 2D nanosheets, exhibited a nanoflower morphology with smooth surfaces. The thickness of the nanosheets was 20-60 nm. After pyrolysis, the nanoflower morphology became rough, while the nanosheets embedded with loosely packed CNTs and NPs turned porous. The diameter of trimetallic carbon nanoflower was approximately 20 nm, as confirmed by SEM. This unique morphology increased the catalyst/electrolyte contact area. The synergism between the trimetallic carbon nanoflowers with additional oxygen vacancies and a higher degree of graphitization of carbon afforded Co 0.2 Fe 0.8 Ni-OCNF with superior HER performance with a small Tafel slope of 36.1 mV dec −1 and an overpotential of 1.65 V at 10 mA cm -2 in two-electrode water electrolysis in an alkaline medium. In an acidic medium, the 2D MOF-based materials degraded during the HER operation because of the reduction and subsequent dissolution in the electrolyte. In addition to the numerous works on 2D MOF towards alkaline HER discussed in the previous section, their applications in acidic HER have also been explored. Because 2D MOF derivatives or the carbon support has extraordinary electrical conductivity, they can produce a platform for exploring the acidic HER mechanism.

Oxygen Evolution Reaction.
The OER is the critical reaction for energy-related applications, such as electrochemical water splitting and rechargeable metal-air batteries [159,186,187]. This follows the four-step, proton-coupled process with slow kinetics necessitating a high overpotential to produce O 2 . The issue can be overcome by developing robust and efficient catalysts that cross the energy barrier. An efficient electrocatalyst should have neither too strong nor too weak affinity to the oxide, hydroxide, and oxyhydroxide. RuO 2 and IrO 2 are considered state-of-the-art catalysts and are used widely as electrocatalysts for the OER owing to their specific properties [188]. On the other hand, both were made of precious metals, and their poor reversibility and high price are unsuitable for large-scale production. Therefore, it is essential to develop efficient alternative materials based on inexpensive elements. Recently, 2D matrixbased metal organic frameworks have attracted attention as candidates for the OER. The performance of these compounds can be evaluated by analyzing the turnover frequency (TOF), overpotential, and Tafel slope. The equations associated with the OER in acid and alkaline electrolytes are as follows: Alkaline : Huang et al. reported an electrochemical-chemical exfoliation strategy to synthesize 2D-based Co MOFs [49]. First, a pillar-type layer MOF was synthesized by the solvothermal reaction of cobalt (II) acetate and 2,3-dihydroxy-1,4-benezenedicarboxylic acid (H 4 dhbdc) in an EtOH/H 2 O mixture at 140°C. Once formed, the pillar might be oxidized to lower their coordination ability and then removed selectively, keeping the 2D layer intact, called Co-NS. Single crystal synchrotron diffraction measurements confirmed the 3D pillar-layer structure with the 1-42 m space group. The unique structure enhanced the thermodynamic and kinetic stabilities of the materials. 3D Co can be grown directly on Ni foam to obtain 3D-Co@Ni, which could be transformed to 2D-Co-NS@ Ni 24 International Journal of Energy Research by electrolysis at 10 mA·cm -2 for 0.5 hr. The ultrathin thickness,~2 nm 2D-Co-NS@ Ni NS metal-organic layers, showed good OER activity (Figure 11(a)). Subsequently, electrodes modified by the Fe dopant with 2D-Co-MOF (Fe: 2D-Co-NS@Ni) exhibited an ultralow overpotential of 211 mV and a Tafel slope of 46 mV·dec -1 at 10 mA·cm -2 . The faradic efficiency of O 2 was 99%. Moreover, the long-term stability was maintained for 96 h at 10 mA·cm -2 (Figure 11(c)). Interestingly, Xu et al. reported a ligand-assisted strategy to synthesize 2D-based MOFs [189]. 2D bimetallic Co-Fe was synthesized by a solvothermal reaction using Co-Felayered double hydroxide (LDHs) as the template and terephthalic acid as the linker in DMF (Figure 11(b)). The obtained product was coated on a glassy carbon electrode for testing the electrochemical performance of the OER in an alkaline medium. It exhibited a low overpotential of 274 mV, Tafel slope of 46.7 mV·dec -1 , and long-term electrolysis for 74 h at 10 mA·cm -2 (Figure 11(d)). The 2D Co Fe bi-metallic MOF showed better OER performance because of the coupling effects of Co and Fe.

International Journal of Energy Research
Zhao et al. reported a simplified surfactant-assisted strategy to prepare ultrathin 2D Co-MOF nanosheets [190]. Typically, Co 2+ metal ions were reacted with phthalic acid in DMF/EtOH to form Co-MOF NS under solvothermal conditions. The vertical growth and stability of 2D Co MOF were promoted by adding an anion surfactant (PVP) to the mixture. The product exhibited an acceptable catalytic activity for the OER with an overpotential and Tafel slope of 263 mV and 74 mV·dec -1 , respectively, in 1 M KOH. The physical and structural properties did not change considerably, even after 12000 sec. The enhanced OER activity was attributed to the unsaturated Co (II) active center on the surface of ultrathin 2D Co-MOF nanosheets.
Zheng et al. demonstrated an ultrasonic-assisted strategy to synthesize bimetallic 2D MOF, in which BDC organic ligands bridge with metal ions (Co and Fe) by the ultrasonic force to form micro-Co-Fe MOF [191]. The micro-Co Fe MOF was converted to ultrathin Co Fe-MOF nanosheets by a solvothermal treatment. The as-synthesized materials were confirmed by low magnification TEM and AFM. Under ultrasonic conditions, the nanosheets exhibited a smooth morphology, and no mesoporous network was observed. The as-synthesized Co-Fe MOF under solvothermal conditions displayed much thicker layers, and a mesoporous network was observed in the layered nanosheets. AFM revealed a thickness of 1.3 nm. The bimetallic MOF contained catalytically active MO 6 octahedra and more surface sites and enabled facile mass transport, which enhanced the activity for OER significantly. Furthermore, after Co introduction, synergistic effects between Co and Fe resulted in a considerable increase in activity. A hierarchical 2D MOF exhibited an excellent OER and only required an overpotential of 277 mV to drive a current density of 10 mA·cm -2 in a 1 M KOH electrolyte with a small Tafel slope of 31 mV·dec -1 . These materials demonstrated electrochemical stability of more than 2000 sec. Similar to 2D Co-Fe MOF nanosheets, some other bimetallic MOFs, such as Ni-Co-UMOFNs, Fe-Ni MOFs, and Co-FeNi@HPA MOFs, were also shown to exhibit outstanding catalytic performance towards OER [192][193][194]. The derivatives of the Co-, Fe, and Ni-based 2D MOF nanosheets are considered as high-performance electrocatalysts for the OER because of their high concentration of metal ions or atoms disturbed evenly on the surface of the electrocatalysts. Du et al. fabricated a series of Co 1x Fe x P nanoparticles embedded in carbon nanosheets from 2D MOF precursors and evaluated their electrochemical activity towards the OER [195]. The morphology was confirmed by AFM, which showed that the nanosheets are disturbed uniformly and have a smooth surface. The thickness of the nanosheets was 50-30 nm. The experimental results showed that moderate iron doping could preserve the catalytically active sites and increase the oxidation state of the surface CoP species. Therefore, the Co 0.7 Fe 0.3 P/C exhibited a superior OER performance with an overpotential of 270 mV and ultrasmall Tafel slope value of 27 mV·dec -1 in a 1 M KOH electrolyte. The Co 0.7 Fe 0.3 P/C electrocatalyst delivered good stability of above 1000 cycles. The improved OER performance was attributed to the large surface area and size of the materials.
Wen et al. grew a Ni-doped ZIF-67 NS template on the Ti 3 C 2 Tx MXene and was converted directly to NiCoS/ Ti 3 C 2 Tx through a pyrolysis treatment in the presence of sufficient sulfur powder [196]. The NiCoS/Ti 3 C 2 T X exhibited an overpotential 364 mV at 10 mA·cm -2 was much better than that IrO 2 (400 mV). The Tafel slope of 58.2 mV·dec -1 was much smaller than IrO 2 . These positive effects from unique structures and strong interfacial interaction between NiCoS and Ti 3 C 2 Tx enhance the electrical conductivity and active sites, resulting in excellent OER performance.
Cheng et al. designed a Ni form supported by NiMn/ MOF nanosheets through hydrothermal technique. To further improve the activity and stability of NiMn/MOF, the researcher modulated Ni/Mn-MOF with multiple lattice strains (edge dislocation, oxygen vacancy, and lattice distortion), viz., electrochemical method [197]. The obtained product was Ni/Mn-MOF@Co (OH) 2 and NiMn-MOF@-CoOOH nanosheets. The lattice fringe is clear to Ni/Mn-MOF@Co (OH) 2 , and strain was not observed by TEM. In the NiMn-MOF@CoOOH, lattice is affected by edge dislocation, due to the compressed strain. So, the interspacing between the plane (012) changes from 0.231 nm to 0.228 nm, viz., 1.3% compressed strain. XRD analysis confirms the existence of lattice strain, and the crystalline plane (012) of the NiMn-MOF@CoOOH has been shifted downwards. The HR-TEM and geometric phase analysis (GPA) also reveals surface strain of NiMn-MOF@CoOOH nanosheet. The stain distribution of NiMn-MOF@CoOOH which belongs to (011) plane and lattice strain is -13.7%, respectively. Through the HRTEM and XRD characterization, lattice strain regulated structure into 2D@2D form is determined which causes more interface and as well as channels for electron transportation to accelerate redox kinetics. After modulated lattice strain of NiMn-MOF@CoOOH, nanosheet delivered a low overpotential 220 mV @ 10 mA·cm -2 and maintain stability over 50 h.
Xia et al. synthesized Co/CoO X nanoparticle and CNT from 2D Co-BDC-MOF through H 2 annealing and applied into OER and HER application [198]. The graphitic carbon (CNT) encapsulation with high valance of Co 2+ ions enables efficiency of reactant and product for charge transfer. Strain engineering and morphologies of Co-BDC/H 2 -MOF were confirmed by HR-TEM and GPA analyses. The lattice fringes of Co-BDC/H 2 -MOF belong to (111) plane, and corresponding d-spacing is 0.24 nm. The high valance Co 2+ was introduced by lattice strain, which led to modified catalytic activity. The tensile strain was generated into Co-BDC/H 2 -MOF and results into their increased HER and OER activity. Tensile strain was an effective approach for reducing absorbate, stabilizing intermediate and promoting the release of H 2 and O 2 gas. The average tensile strain was 5% of Co-BDC/H 2 -MOF, which led to high electroactivity with low overpotential 310 mV @ 10 mA·cm -2 of Co-BDC/H 2 -MOF.
3.3.1. Challenges for 2D Metal Organic Framework as Acidic OER Aatalyst. Electrocatalysts suitable for the OER in alkaline electrolytes are used in alkaline electrolyzers and metal-air batteries. They have also entered into the commercial production of hydrogen fuel cells. Nevertheless, alkaline 26 International Journal of Energy Research electrolyzers face limited current density, low operating pressure, and limited options for loading. On the other hand, water electrolyzers utilizing polymer electrolyte membrane (PEM) such as Nafion could offer high proton conductivity, compact design, low gas crossover, high operating pressure, and high current density [199][200][201]. Integrated with PEMFC systems, PEM water electrolyzers can be used in future storage devices. On the other hand, a high applied overpotential (2 V) and harsh acidic environment (pH) are the major challenges for PEM water electrolyzers. Thus far, only a handful of rare and expensive metals (such as Ir and Ru)-based catalysts have shown appropriate OER activity in acidic environments. In addition, hybrid organic-inorganic MOF materials make stability an even more significant challenge under harsh reaction conditions because the organic linker in MOF could be oxidized in an oxidative environment and collapse the framework. Compared to 1D or 3D MOF and their derivatives, layered MOF-derived porous composites have attracted considerable interest in the field of OER. For example, Zhou et al. synthesized 2D MOx/C (M=Co, Ni, and Cu) array nanosheet structures with 30 nm thickness via pyrolysis treatment of 2D M-MOF (M= Co, Ni, and Cu) [202]. After pyrolysis, the morphology was maintained with a reduction in particle size, i.e., Co 3 O 4 nanoparticles on the 2D sheet exhibited smaller size (~10 nm), compared to the 3D MOF-derived Co 3 O 4 (~100 nm). This unique 2D MOx/C has a high surface area and abundant hierarchical pore structures that expose more active sites, allow electrolyte penetration, and release gas bubbles for rapid redox reaction. Thus, in the OER, they showed excellent catalytic activity and good durability under harsh conditions. This synthesis strategy can led to many unique 2D MOF derivatives or 2D MOF materials for stable OER electrocatalysis in acid media.

Oxygen Reduction Reaction.
In metal-air batteries and fuel cells, the ORR is one of the most critical reactions occurring at the cathode surface, and the Pt-based system is a benchmark ORR electrocatalyst. The electrochemical ORR involves both two-electron and four-electron pathways and operates in acid and alkaline media. The four-electron pathway is highly efficient, but it is a relatively slow process. On the other hand, the two-electron reduction pathway generates H 2 O 2 , which causes degradation of fuel cells. Hence, the electron transfer number is a key parameter to evaluate the ORR performance of catalysts. This number can be calculated using the Koutecky-Levich (K-L) equation from linear sweep voltammetry [203][204][205][206][207][208]. The ORR mechanism depends on the propensity of the catalysts to cleave the O-O bond. Heteroatom-doped 2D carbons are efficient electrocatalysts for the ORR [209]. In addition, 2D-based MOF materials and their derivatives are suitable candidates for ORR applications. Zhong et al. reported a facile strategy to synthesize copper phthalocyanine-based 2D-conjugated MOF for electrocatalysts in the ORR [210]. The 2D-conjugated Pc Cu- Wei et al. prepared a 2D dual metallic (Ni/Co)-based MOF nanoflakes as a catalyst for ORR [211]. The Ni/Co MOF nanoflakes showed higher electrochemical performance than Ni/MOF and pristine ZIF-67 MO. This enhancement was attributed to the synergistic effect of dual metallic (Ni 3+ /Ni 2+ and Co 3+ /Co 2+ ) ions in Ni/Co MOF that increased the active sites during the ORR performance, maximizing the electron charge transport. The Ni/Co MOF nanoflakes exhibited the highest onset potential (0.76 V) and current density. The electron number transferred in the O 2 reduction process calculated by the K-L equation was about 3.7 over the potential range 0.05 to~0.3 V. Quasi 4-e process is the dominant pathway for the ORR at the Ni/ Co MOF nanoflake electrode.
Xia et al. reported MOF/GO/MOF sandwich-like nanosheets synthesized by the seed-mediated deposition of ZIF-67, GO, and ZIF-8 [212]. The compound was examined for its electrochemical performance towards ORR. Graphene oxide sheets were deposited with ZIF-8 seeds on both sides directly through the strong interaction between Co 2+ ions from ZIF-8 and O 2 from GO to obtain a ZIF-8/GO/ZIF-8 sandwich-like structure. In another way, ZIF-67 crystals were deposited on the surface ZIF-8/GO to obtain coreshell ZIF-8/GO/ZIF-67 nanosheets. Both nanosheets exhibited a good current density and onset potential. The ORR activity was improved further by converting ZIF/GO/ZIF to cobalt nanoparticle/N-doped porous carbon nanosheets via carbonization/etching, which showed an onset potential of~0.93 V vs. RHE and a current density of~101 mA·mg -1 at a potential of 0.80 V. The electron number in the oxygen reduction process was calculated to be 3.9-4.0, indicating dominate four-electron pathway for the ORR. Other than 2D MOF, heteroatom-doped 2D carbons were efficient electrocatalysts for the ORR.
Zhang et al. synthesized graphene-based nitrogen-doped carbon sheets (GNPCSs-800) from pristine ZIF-8 and tested its catalytic activity for the ORR [213]. The GNPCS-800 sheets exhibited reasonable onset potential, higher current density, and excellent tolerance to methanol than the commercial Pt/C. Therefore, the synergistic effects of graphene and NPC led to increased active sites and boosted the catalytic activity. This beneficial effect was used in direct methanol fuel cells with a higher open-circuit voltage and power 27 International Journal of Energy Research density. Huang et al. reported the synthesis of a 2D Co-N carbon-based electrocatalyst from the direct carbonization of NaCl-caged 2D Co-MIM MOF [214]. The electron transfer number of the optimized sample of CoN-CNS-800°C was estimated to be 3.9, and the H 2 O 2 yield was less than 5%. This performance was comparable to that of the Pt/C catalyst. The stability of CoN-CNS-800°C and Pt/C was examined through chronoamperometric measurements at 0.70 V. CoN-CNS-800°C displayed a low current density with 98.5% retention over 36000 s, whereas Pt/C showed a negligible current density with 84.5% retention.
Cheng et al. designed a Ni foam supported NiFe bimetallic MOF nanosheet array through low-temperature hydrothermal technique. The lattice parameter of NiFe-MOF nanosheets was calculated to be 11.6 Å along (110) plane, viz., HR-TEM images [215]. The interlayer spacing of NiFe-MOF nanosheets from 11.6 to 11.8, 12.0, and 12.1 Å has to be gradually increased, after ultraviolet treatment for different time interval. As resulting of lattice strain, produced lattice expansion ratio of MOF is 1.7%, 3.6%, and 4.3%, respectively (Figure 12(a)). The researcher further confirms lattice strain and diffraction peak through the XRD analysis. The characteristic peaks at~7.6°, which is related to the (220) plane, significantly shifted down to 7.48, 7.36 ,and 7.30°for the~1.7%,~3.6%, and~4.3% compressed strain MOF, respectively. This reduction in 2θ corresponding to the increased interlayer spacing of 11.8, 12.0, and 12.1 Å of (200) plane, respectively. Inorganic metal

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International Journal of Energy Research oxide layer (NiO 6 and FeO 6 octahedron units) and organic ligand in crystal structure of NiFe-MOFs were recognized to alternate along the (110) direction with theoretical lattice spacing of 11.6 Å between any two adjacent metal oxide layer. However, the unstable, unreactive, and P-conjugated naphthalene-based acid molecules between the two-metal oxide layers were selectively dissociated under ultraviolet treatment. Treated lattice strain of NiFe-MOF exhibitd a fast and efficient pathway for electrocatalysis which boosts intermediate superoxide * OOH species on high valance Ni 4+ active site. The onset potential of NiFe-MOF is~0.92 V, and RHE is nearly to commercial Pt/C (Figure 12(b)).

Challenges for 2D MOF as an Acidic ORR Catalyst.
Several MOFs and carbon-based electrocatalysts have been tested in alkaline electrolytes. New 2D MOFs and their composites/derivatives prepared by efficient strategies are promising electrocatalysts, whose activities are comparable to those of commercial Pt/C. Nevertheless, fuel cells operating under acidic conditions, particularly PEMFCs, based on advanced structures, have achieved commercial applicability [217]. At the same time, carbon-based electrocatalysts have been tested for acidic ORRs. The MOF-derived porous carbons have lower activity in the ORR because of the low graphitization degree and weaker electron transferability. Graphene with a large surface area and good electrical conductivity has been reported to be an active support. Recently, graphene-supported 2D MOF and 2D MOF-derived M-N-C complexes have been demonstrated to be highly active for the ORR in both acidic and alkaline media. For example, Xia et al. performed the ORR using N-doped graphene nanomesh (NGM) produced from Zn-containing nanoleaves (Z-ZIF-L) via thermal exfoliation [218]. NGM exhibited unprecedented catalytic activity for the ORR in acid media owing to its structure and composition. Furthermore, numerous nanopores were observed on the surface of NGM nanosheets because of the intercalation of Li ions in Zn-ZIF-L, which might etch the carbon layer to form large mesopores (4.0 nm). The optimized NGM prepared at 800°C has shown ultrathin nanosheet morphology. The thickness of the NGM nanosheets was approximately 1.3 nm, according to AFM. The electron transfer number in the oxygen reduction process was calculated to be approximately 3.85, indicating a dominant four-electron pathway for the ORR. The peroxide yield was below 7.6% over a potential range of 0.2-0.8 V. This novel 2D MOF-derived functional carbon material has been shown to improve the performance in the ORR, compared to traditional 3D and 1D materials. The same research group developed Ceria@ Co, N-doped leaf-like porous carbon nanosheets (Ce-HPCNs) by carbonizing 2D-hexagonal-leaf-like ZIF lamella [216]. Air-stable Ce 3+ host precursors were added to ZIF lamella during the crystalline nucleation process to obtain Ce-ZIF-L, which might generate oxygen and facilitate O 2 adsorption. The direct formation of Ce-HPCN from Ce-ZIF-L retained the overall morphology and hierarchical structure of the ZIF precursor Fe-SEM and element mapping of Ce-HPCNs (Figure 12(c)). The Ce-HPCN exhibited a current density of 3.946 mA·cm -2 and a positive half potential of 0.831 V ( Figure 12(d)). These results suggest that Ce helps increase O 2 adsorption, and its synergistic effect with the Co-Nx active sites enhances the ORR activity. Interestingly, the ORR is pH-dependent, and dramatic changes have been observed in alkaline and acid media using carbon-based materials as the electrocatalyst. From a theoretical viewpoint, the ORR follows the same pathways in alkaline and acid media because the adsorption properties of the reaction intermediates remain the same. In an experimental viewpoint, the ORR does not follow the same pathway in alkaline and acid media [219]. The first explanation is that the ORR can generally follow a dissociative or an associative pathway. In the dissociative pathway, O 2 molecules dissociate into two oxygen atoms to various sites on the electrocatalyst surface via a hydrogenation process. This platform is favorable when the kinetics of O 2 dissociation is fast, comparable to an energy barrier of 0.8 V. Subsequently, the associative pathway occurs if the dissociative barrier is high and kinetically prohibited. Thus, the same materials might follow different ORR pathways in acidic and alkaline media. The second explanation is based on ions. In acidic solutions, H + is absorbed on the electrocatalyst surface, while OHanions are absorbed in an alkaline solution. These two absorbates might cause changes in the Fermi level of the substrate and affect the absorption of the reaction intermediate, leading to changes in the overall thermodynamics of the reaction. The third explanation is based on the ions solvated in the solution, i.e., Na + and K + in alkaline media and Cland SO 4 -2 in acid media. This phenomenon can affect the oxygen dissociative barrier and strength of the adsorption reaction intermediates during the ORR. The fourth explanation considers the physicochemical properties of as-synthesized samples, their morphologies, testing conditions, and time, which can affect the mass transport and reaction pathways.
3.5. CO 2 RR and UOR. Electrochemical CO 2 reduction is an ecofriendly process to convert CO 2 to fuels. The CO 2 reduction reaction involves three steps [220]. In the first step, active sites of the catalyst absorb CO 2 molecules. The second step involves the transfer of electrons or protons to cleave the C-O bonds or C-H bonds. In the third step, configuration rearrangement of the product occurs, followed by desorption from the catalyst surface. In the presence of metals, metal oxides, and carbon-based materials, CO 2 is converted to carbon monooxide or alcohols [152]. Recently, nonprecious 2D-based MOF matrices have shown great potential for the CO 2 RR because of their unique advantages, such as ultrathin morphology, functionalized modification of organic linker in framework, actives sites, and geometric structures [221].
For example, Yang et al. adopted the atomic layer deposition method to form a cobalt-porphyrin MOF and Al 2 (OH) 2 TCCP-Co thin film on a conductive substrate [222]. During the electrochemical process, the reduction of Co (II) ions to Co (I) has been confirmed by in situ spectro-electrochemical measurements. Using an MOF as an electrocatalyst has provided more advantages, such as improved active sites and facile electron and charge 29 International Journal of Energy Research transport. The synthesized cobalt-porphyrin MOF, Al 2 (OH) 2 TCCP-Co, exhibited excellent electrocatalytic activity towards the CO 2 RR. The catalyst exhibited good selectivity with a low overpotential. The faradic efficiency of CO was 76%, and the Tafel slope was 165 mV·dec -1 . The catalyst exhibited long-term CO 2 reduction up to 7 h, suggesting its high durability.
Gunnoe synthesized 2D-based porphyrin MOF as an electrocatalyst for efficient CO 2 RR to increase the faradic efficiency [223]. Using tetra (4-pyridyl) porphyrin Co (II) as the building block, they formed RTPyP-Co and STPyPCo, whose morphologies were nanorods and nanosheets, respectively. Ultrathin (3.8 nm) STPyP-Co nanosheets showed excellent performance in the experimental and theoretical methods. The STPyP-Co catalyst achieved 96% faradic efficiency of CO at a potential of 1.6 V vs. RHE pH 7.2. Thus, the STPyP-Co catalyst showed excellent performance with high stability towards the CO 2 RR. Commonly, when MOFbased electrocatalysts are used for the CO 2 RR, the main product is CO; formic acid, methane, ethanol, and others are rarely reported. On the other hand, Shen et al. reported a molecular catalyst of Co protoporphyrin (CoPP) nanosheets to reduce CO 2 to CO [224]. The catalysts produced CH 4 and a small amount of formic acid and methanol. The authors also proposed a mechanism for the reduction reaction. They suggested that a moderate acid environment was favorable for the further reduction of CO, owing to weak binding with Co (I). Functionally modified linkers in 2D MOF-based molecular catalysts could enhance the selectivity towards hydrocarbon or alcohols by strengthening the absorption of the intermediates, methane, and methanol, during the CO 2 RR in an aqueous medium. Aoi et al. fabricated Cu (II)/adeninato/carboxylate metal-biomolecule frameworks (Cu (II)/ade-MOF nanosheets) and employed the electrochemical CO 2 RR [225]. The Cu 2+ ions coordinated with the biolinker of adenine and acetic acid and produced Cu-ade MOF nanosheets. This biomediated framework had abundant pores and contained nitrogen functional groups, which contributed to the electrochemical performance. The main product of reducing CO 2 is C 2 H 4 and H 2 . The faradic efficiency of hydrocarbons increased with more negative potentials and achieved the highest value of above 73% at 1.6 V vs. RHE. Furthermore, XPS confirmed N1s in the cathodized Cu-MOF, which enhanced the N-Cu proportion and decreased the N-C proportion. This would activate protons, stabilize the intermediates, and further promote hydrogenation to yield hydrocarbons during CO 2 RR. In addition, the urea oxidation reaction is an ideal substitute in the anodic reaction (OER) of water splitting. In this regard, Qiao et al. reported 2D Ni-MOF nanosheets synthesized from a simple sonochemical method using Ni 2+ ions and 1,4-benzene dicarboxylic acid [226]. The 2D Ni-MOF proved efficient for the UOR. It exhibited a small Tafel slope of 23 mV·dec -1 at 10 mA·cm -2 and a low potential of 1.36 V vs. RHE, indicating rapid kinetics.
3.5.1. Challenges for 2D MOFs as Electrochemical CO 2 RR. Two-dimensional MOF, including Cu MOF, Al MOF, and Co MOF, has been extensively explored for electrochemical CO 2 reduction reactions [227][228][229]. Nevertheless, this field of research is in its infancy. More efforts are needed to exploit graphene and CNT matrix in designing 2D MOFs and their derivatives for the CO 2 RR. In an electrochemical system, the mechanism of the CO 2 RR was primarily hypothesized without direct intermediate detection. To develop a rational material design, the theoretical insights of the CO 2 RR are necessary. This would help in tuning the efficiency and selectivity in the CO 2 RR. The poor selectivity of the 2D MOF led to the generation of various gaseous and liquid products. Furthermore, stability is still a significant issue and needs to be improved.
3.6. Electrode Materials. Smart electronics, such as electric vehicles, portable electronic devices, and grid-scale storage, are prevalent in almost every facet of human activities in modern society. These electronic devices contain advanced electrochemical energy storage systems with a high energy density, excellent flexibility, and long lifespan [230,231]. Recently, lithium-ion batteries and supercapacitors are considered well-established storage devices [232]. On the other hand, the application of these systems requires costeffective materials, and their performance characteristics are limited. Therefore, researchers are constantly trying to enhance their performance by optimizing various aspects, including the cost of the electrodes and the composition of the electrolyte blend. Therefore, research interest in layered structures has flourished, and 2D materials, such as graphene, MXene, and 2D MOF layered-structured materials, have been developed. Among these, 2D MOFs have attracted considerable attention as electrochemical energy storage systems, owing to their exposure to more metal atoms, high electrical conductivity, suitable porosity, and ultrathin thickness. This section discusses the recent studies on 2D MOF and their composite/derivative electrode materials as supercapacitors and batteries.
3.7. Supercapacitors. Supercapacitors (SCs), also called as ultracapacitors, which are electrochemical energy devices that combine the high-power density of conventional capacitors with the high energy density of traditional batteries [233]. These supercapacitors shows the rapid charge/discharge rates, high-power density, and longer lifespan than conventional dielectric capacitors and batteries. The rapid charge/discharge rates, high-power density, and longer lifespan than conventional dielectric capacitors and batteries. Supercapacitors are promising for various applications, such as AC line filtering, uninterrupted power supplies, and hybrid electric vehicles (HEVs) [234][235][236]. Generally, in electrochemical energy storage devices, electrical energy can be stored in two fundamentals ways: electrical doublelayer capacitors (EDLC) and pseudocapacitors. In an EDLC, charge separation accumulated at electrode/electrolyte interface in the nonfaradic region. Capacitance of EDCL is calculated by following equation: International Journal of Energy Research where ε r is the dielectric constant of electrolyte, ε 0 is the vacuum permittivity, d is the thickness of the EDLC, and A is the specific surface area of the electrode. Pore structure, electrical conductivity, and specific surface area are some of the factors on which electrochemical performance of EDLC electrodes depends on. For example, activated and porous carbon materials exhibit high capacitance. The second mechanism is pseudocapacitors with fast and revisable faradic redox reaction to store the charge. The theoretical value of specific capacitance of a pseudocapacitor can be obtained through the following equation: whereas n is the number of electrons involved in the redox reaction (mole), Q is the potential window (V), F is the Faraday constant, and M is the molar mass of the active materials (gmol -1 ). Conventional electrochemical materials for pseudocapacitor include metal oxides, metal sulphides, metal hydroxides, metal nitrides, tellurides, and conducting polymers (polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTh)). Faradic as well as nonfaradic mechanisms can usually occur at the same time in the hybrid capacitor for energy storage [237][238][239][240][241]. Developing supercapacitors with greater power and energy density as well as excellent cycling stability is still a big challenge for practical application.
A unique strategy to increase the energy density/power density and stability of SCs is utilizing electrolytes with large potential windows and a high-capacitance electrode. Twodimensional transition-metal-(TM-) based MOFs are an excellent choice for SC applications because of their structural metrics, such as easy ion accessibility on the sheet surface, superior ion transport channel, and high in-out plane electronic conductivity [242]. Feng et al. reported hexaminobenzene-(HAB-) derived 2D MOFs as the electrode materials for SCs. In their research, ultrasmall HAB was used as organic linkers, which led to sub-nanometer pores and provided a dense framework [243]. The tritopic HAB coordinated with square planar Ni 2+ and Cu 2+ to form a 2D honeycomb-layered structure (Ni/Cu-HAB framework). The honeycomb 2D layered structures were confirmed by grazing-incidence XRD (GIXD). The diffraction pattern showed in-plane (qz̴≈0) (100), and (200) reflection peaks, indicating that the film consisted mainly of 2D sheets. The semirectangular cyclic voltammetry curves of Cu-HAB and Ni-HAB at 1-100 mVs -1 exhibited a capacitance of 215 Fg -1 and 420 Fg -1 indicating a highly conjugated system dominating the redox charge-storage mechanism. Galvanostatic charge and discharge curves of Cu-HAB and Ni-HAB were performed. The semitriangular shape corresponded to pseudocapacitance and highly reversible redox behavior. High volumetric capacitance and areal capacitance of 760 F·cm -3 and 20 F·cm -2 , respectively, were achieved at a current density of 10 Ag -1 , and 90% capacitance was retained after 12,000 cycles.
Wang et al. synthesized cobalt-based layered MOF from (Hmt-hexamethylenetetramine and H 2 tfbdc = 2, 3, 5, 6 − tetrafluorotephtlic acid) as mixed organic ligands and Co 2+ as the metal ion and evaluated them as electrode materials for ultrahigh supercapacitors [244]. In this work, the 2D layers were linked through hydrogen bonding interactions, forming a 3D supramolecular framework. The lattice H 2 O molecules at the interlayer space favored the diffusion path and storage of electrolyte ions. The cumulative pore volume and surface area of Co-LMOF were 0.168 cm [3]g -1 nm -1 and 12.12 m 2 g -1 , respectively. SEM provided structural information on the Co-LMOF morphology and size of nanoparticles (35-250 nm). The Co-LMOF nanoparticles delivered a high specific capacitance of 2474 Fg -1 at a current density of 1 Ag -1 . Approximately 94% capacity was retained after 2000 cycles. The 2D MOF-derived or composite electrode materials can improve the performance of supercapacitors because of the synergistic effects of the two materials, high redox-active capacitance, and high electrical conductivity. Cao et al. synthesized a zeolitic imidazolate framework nanosheet MOF by introducing building blocks of 2methylimidazole (Hmim) via a novel bottom-up approach and labeled the MOF materials as ZIF-67 [245]. The ZIF-67 electrode showed a capacitance of 94.9 Fg -1 at a current density of 1-20 Ag -1 in an alkaline electrolyte. When ZIF-67 made from Co 2+ ions coordinated by four imidazole rings was used as the precursor, the resulting Co 3 O 4 ultrathin nanomeshes provided active sites for the reversible faradic reaction. The Co 3 O 4 ultrathin nanomeshes achieved a specific capacitance of 1216 Fg -1 at a current density of 1 Ag -1 . Furthermore, this material of asymmetric supercapacitors (Co 3 O 4 /activated carbon) exhibited an energy density of 46.5 Wh·kg -1 at 790.7 Wk·g -1 . The morphology features of the MOF precursors were tuned during pyrolysis, and the SC performance was improved by optimizing the morphology.
Xiao et al. reported the synthesis of UPC-9 nanosheets, which yielded hierarchical Co 3 O 4 hexagonal nanosheets upon further carbonization. UPC-9 exhibited layered arrangement structures with Co 4 (μ 3 -OH) 2 (COO) 6 and 3,5,6-tetramethyl-1,4-disophthalate organic ligands as the pillars in between. The simultaneous hydrolysis-(KOH) converted UPC-9 to hexagonal Co (OH) 2 nanosheets and subsequent calcination formed the hierarchical structures of the Co 3 O 4 nanosheets (Figure 13(a)) [246]. SEM images of Co 3 O 4 showed a thickness of 3.5 nm. The nanovoids were attributed to etching of the pore structure by OHions. This unique structure was beneficial for improving the specific surface area and ion migration efficiency. The electrochemical performance of the SCs tested with 6 M KOH aqueous electrolyte and Co 3 O 4 nanosheets as positive electrode and activated carbon as an electrode, resulted in a specific capacitance of 1121 Fg -1 at 1 Ag -1 . Approximately 98% capacity was retained after 6000 cycles (Figure 13(c)). In addition, synergic combination materials contributed mainly to enhancing the supercapacitor performance. Li [246,247]. International Journal of Energy Research 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) was used as an organic linker and allowed to react with copper acetate monohydrate to form Cu 3 (HHTP) 2 nanosheets (Figure 14(e)). The morphology of the Cu 3 (HHTP) 2 nanosheets was studied by HR-TEM (Figure 14(f)). It contained ultrathin nanosheets with sizes of 20-40 nm. The Cu 3 (HHTP) 2 nanosheet cathode exhibited improved Li storage. The specific capacity was 95.61 mAh·g -1 , and it exhibited a stable redox cycling perfor-mance at a current rate above 20 C (Figures 14(g) and  14(h)). The enhanced lithium storage performance of the Cu 3 (HHTP) 2 nanosheets assured high interfacial contact area between the electrode and electrolyte, and open void structures provided the transfer and release of lithium ions. Similar to Cu 3 (HHTP) 2 nanosheets, woven net-like 2D Cu 3 (HITP) 2 as an anode was fabricated using 2,3,6,7,10,11-hexaaminotriphynylene as an organic ligand and CuSO 4 ·5H 2 O as metal salt.

34
International Journal of Energy Research The material exhibited a specific capacity of 1039mAh·g -1 after 1000 cycles at 1 C. To improve the Li-ion storage capacity further, Cu 3 (HITP) 2 was coated on Si nanoparticles, and an inner void space was formed to accommodate a large volume expansion. This boosted the diffusion efficiency of Li ions, resulting in an extremely high specific capacity of 2511 mAh·g -1 after 100 cycles at 0.1 C. Advanced carbon material with a desirable morphology has crucial role in energy storage applications. Liu et al. investigated the LIB and NIB performance on 2D Ga 3 C 6 N 6 (c-MOF) by first principle density functional theory [266] .Ga 3 C 6 N 6 (c-MOF) was constructed by gallium metal node and hexaminobenzene (HAB) ligand. Through studies of energy band structure and density of state, Ga 3 C 6 N 6 monolayer exhibits an intrinsic metallic characteristic with several band crossing the fermi level, thus confirming the electronic conductivity without structural deformation (Figures 14(a) and 14(b)). Ga 3 C 6 N 6 monolayer affords the strong adsorption energies (Li-3.06 eV, Na: -2.09 eV), which prevent from the metallic insulation. The low diffusion barrier (Li: 1.12 eV, Na: 0.61 eV) provides the fast charging and discharging rate for LIB and NIBs (Figures 14(c) and 14(d)). The theoretical specific capacities (330 mAh/g for LIB and NIBs) confirm a good storage capacity.
Shanqing reported a redox copper-based 2D conductive MOF for cathode material in lithium-ion batteries. A facile solvothermal reaction between copper cluster and redox active organic bridging ligand benzenehexathiolate (C 6 S 6 H 6 ) formed 2D Cu-BHT with Kagome lattice architecture [267]. The formula of Cu-BHT was found to be [Cu 3 (C 6 S 6 )] n since both Cu (II) ions and benzenehexathiolate ligand of charge are evenly distributed, which led to generate 2D π-d-conjugated plane. This plane allows them into more stable redox building block and enhance number of delocalized electrons, as per in density functional theory and cyclic voltammetry results. An initial specific capacity of 175 mAh·g -1 was observed, which was near to theoretical capacity of (236 mAh·g -1 ). The loss of capacity deterioration (0.048% per cycle) upon 500 cycles at 300 mA·g -1 infers that Cu-BHT exhibits a high intrinsically conductivity and provides an ideal way for fast redox activity and high energy density.
The Zhen group assembled 2D UT-Zn(bim)(OAc) nanosheets by a gluconate-assisted method and obtained 2D carbon nanosheet-UT-CNSs by subsequent pyrolysis and acid treatment [268]. When used as an anode material for LIB, the 2D carbon nanosheets delivered high reversible capacity (553 mAh·g -1 at 10 Ag -1 ), 100% Coulombic efficiency, and good cycling stability. This performance was attributed mainly to the ultrathin sheet morphology of UT-CNSs and superior conductivity, enhancing the stability and accelerating the transport of Li + . Similar to Li rechargeable batteries, Li-S batteries consist of a Li transition-metal oxide cathode and elemental sulfur anode. They deliver a higher theoretical energy density than LIB. Kim et al. reported the facile fabrication of 3D CoS/Co9S8@NCCNSs from 2D ZIF nanosheet as a precursor for high capacity LiS application [269]. CoS/Co 9 S 8 @NCCNSs exhibited a high discharge capacity of 911 mAh·g -1 at 0.2 C owing to the 2D ZIF nanosheet nature with an anisotropic morphology (nanometer thickness and few microscales lateral sizes) providing active sites and enhanced lithium polysulfide adsorption and lithium-sulfur redox kinetics. Moreover, this retained long-term cyclability of 600 mAh·g -1 at 1 C after 500 cycles. A MOF is commonly used as the host material for sulfur in Li-S batteries.
In this regard, Zhao et al. reported the synthesis of MOF-808@S/GCE composites [270]. The material MOF-808 was prepared under solvothermal conditions using ZrOCl 2 ·8 H 2 O as the metal precursor and 1,3,5-benzenetricarboxylic acid. MOF-808@S was obtained by the subsequent loading with sulfur and diffusion at 155°C. The MOF was grown on a graphene template with ethyl cellulose and preheated at 80°C to obtain the MOF-88@S/GCE composite. The MOF-808@S/GCE composite showed a capacity of 685 ± 18 mAh · g −1 after 100 cycles. Sodium-ion batteries have recently attracted increased attention because of their wider resource abundance, higher system safety, and low cost. It has a chemistry similar to lithium. Huaihe et al. obtained 2D nanosheet NiSe 2 /N-rich carbon nanocomposites from Ni-Hexamine frameworks and applied them to high sodium ion storage [271]. During carbonization/selenization of Ni-HMT, NiSe 2 /NC was obtained at 400°C under an H 2 /Ar atmosphere. The NiSe 2 /NC nanocomposite retained the same morphology with a diameter of 11 nm, which was evaluated by HR-TEM. In addition, the lattice spacing suggested that graphitic carbon matrix and metallic Ni nanoparticles are crucial for enhancing the electrical conductivity and specific capacity. NiSe 2 /NC nanocomposite electrode showed a high reversible capacity of 410 mAh·g -1 at 1 Ag -1 . Large capacity retention was obtained after 1000 cycles at 240 mA·hg -1 . These materials improved the sodiation-desodiation kinetics because of the uniformly nitrogen-doped carbon-coated porous structure and NiSe 2 . A twodimensional MOF-derived bimetallic hybrid has attracted considerable attention because of their improved electrical properties, high surface area, and compositional elasticities.
Baumann et al. prepared a 2D c-MOF, labeled as Co-HAB (HAB-hexaminobenzene) with combining of electric conductivity and redox active sites on metal node (Co 2 (AC) 4 ) and organic ligand [272]. This 2D c-MOF shown a specific capacity of 228 mA·hg -1 , which close to its theoretical capacity of 312 mAh·g -1 . The redox and cycle capability test was carried which composed of 90% as active material half-cell vs. sodium metal. It delivered a reversible specific capacity of 2.8Na + -291 mAg -1 , which is very close to its theoretical store of 3 Na +~3 12 mAh g -1 . As results suggest that HAB was utilization of redox activity, it suffers from viable capacity~34 mA·hg -1 after 40 cycles which was primarily due to irreversible redox process (i.e., destruction of the framework).
Liu et al. prepared 2D porous hybrid bimetallic Co 3 O 4 / ZnO nanosheets using Co Zn(n)-MOFs as a template [273]. High-resolution TEM and SEM confirmed that the uniform and nanosheets contained both macropores and mesopores, and the diameter was in the range of 10-20 nm. These bimetallic hybrid nanosheets possessed a rich oxygen vacancy, high surface area, and large void space, 35 International Journal of Energy Research which enhanced their performance. The hybrid Co 3 O 4 /ZnO materials delivered an improved capacity of 242 mA·hg -1 at 2 Ag -1 and outstanding cycling performance with capacity retention of 91% after 1000 cycles than ZnO and Co 3 O 4 . The significant electrochemical performance was attributed to the synergistic effects between ZnO and Co 3 O 4 and the harmonious electrochemical behavior. In addition, potassium ion batteries also follow the same electrochemical principle as lithium. Chen et al. fabricated layered K 2 [(VO) 2 (H-PO 4 ) 2 (C 2 O)] as a KVPC-S framework as the cathode materials for KIBs [155]. The obtained KVPC-S plate has large interplanar lattice spacing with a thickness of 100-300 nm using an optimized hydrothermal method. The KVPC-S plate, as a negative electrode, exhibited a reversible capacity of 81 mAh·g -1 at 4.0 V and good cycling stability with 83% capacity retention after 200 cycles.
The lithium air battery (LAB) system is one of a such advanced battery system that is regarded as promising next generation electrochemical technology for sustainable energy storage because of its relatively high theoretical energy density above~3500 Wh.Kg -1 , in comparison with LIBs. The Li-O batteries are composed of lithium metal anode coupled with air cathode input, and the energy storage mechanism of Li-O batteries is based on the electrochemical reaction between oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). An O 2 atmosphere can increase concentration of dissolved oxygen in the electrolyte and reduce the working pressure of the cathode catalyst [155,274]. However, their lesser chemical stability of electrode/electrolyte interface, poor capability rate, low round-trip efficiency, and capacity degradation is critical issues hindering commercialization of LABs. To overcome these issues by employing more stable electrode and catalyst to improve the electrochemical performance, reduce the overpotential, and accelerate ORR during discharge process and OER during charge process, respectively, 2D MOF is a promising catalyst for improving activity of LABs.
Hyung-Kyu et al. successfully synthesized a bifunctional 2D-Ni III -NCF from Ni II -NCF (MOF) (2,3,6,7,10,11-hexahydrotriphenylene (HHTP) as organic ligand) and applied it in Li-O 2 batteries as an excellent cathode [275]. The spin-state regulation enables the improved Ni-O covalency in Ni III -NCF, which are beneficial for the electron transfer between Ni sites and oxygen absorbates and accelerate the oxygen redox kinetics. Furthermore, well-aligned nanowire array structures on carbon fiber (Ni III -NCF) exhibit special catalytic performance. The Ni III -NCF provides a strong adsorption and nucleation sites, which can prominently accelerate breakdown of the production of Li 2 O 2 nanosheets. Taking advantages of spin-state modulation catalyst, when tested in LABs, the Ni III -NCF catalyst achieves high specific capacities of up to 16800 mA·hg -1 at a rate of 0.5 Ag -1 and longterm stability over 200 cycles. Similar to LABs, 2D MOFs and their derivatives/composites have received great attention and have been utilized as effective catalyst in zinc air batteries.
Lv et al. also used 2D conductive MOF (Cu metal node and 2,3,6,7,10,11-hexahydroxytriphenylene as organic ligand) and applied it as bifunctional electrocatalyst appropriate for both OER and ORR processes [276]. When working as the air cathode in rechargeable ZAB with aqueous as the electrolyte, high capacity of 228 mAhg -1 at 50 mAg -1 can be achieved. The related charge-storage mechanism of Cu 3 (HHTP) 2 is discussed. In mechanism, charge storage occurs by intercalation/ deintercalation of cation in active materials (bulk), and diffusion of cation is not limited in kinetics reaction, which led to achieve high capacitance and supercapacitor in one integrated system via pseudocapacitance charge-storage system. Cu 3 (HHTP) 2 exhibits good operation durability for long-term operation.
Similarly, Zhong et al. also investigated the storage mechanism of ZABs in [Ni 6 /Ru x (HHTP) 3 (H 2 O)x] framework. Further, they investigated the doping effects of Ru in [Ni 6 /(HHTP) 3 (H 2 O) x] and applied in to ZIBs [210]. In this study, researcher reports that 1 : 3 ratio of [Ni 5.7 Ru 0.3 (HHTP) 3 (H 2 O) x] n demonstrated significantly improve active sites. And because of this enhanced active site, the resulting Ni/Ru-HHTP bifunctional exhibited good electrochemical performance with prominent specific capacitance as 654 mAhg -1 at 5 mA·cm -2 , and no degradation was observed after 200 cycles.
The Xuerong et al., prepared a bimetallic Co/Ni-MOF nanosheets by the Langmuir-Schafer Method [277]. The rGO was incorportaed in Co/Ni-MOF to improve activity of nanosheets. The support catalyst of rGO offered high electric conductivity and large specific area. The highly porous structure and active sites of Co/Ni-MOFs enable the fast ion transfer and low overpotential and boost the kinetic reaction between electrode/electrolyte. The bimetallic composite nanosheets achieve a high specific capacitance and energy density (i.e 711 mAh g -1 and 934 Wh kg -1 ) and which also posses good durability.

Challenges for 2D Metal Organic Framework as
Electrochemical Energy Storage. Although many unique opportunities exist for 2D MOF electrode materials, this field also has many challenges. First, some 2D MOFs have a limited life span because of their poor chemical-electrochemical reactions. For example, Mn-LCP nanosheets can be destructive under an anodic potential in aqueous electrolytes. 2D materials have a large volume expansion owing to their interlayer spacing and interactions between intercalating ions, leading to decreased structural stability [2,278]. Thus, improving the intrinsic stability of these 2D MOFs by developing novel electrolytes with a suitable electrode-electrolyte interface needs to be explored. Second, the relationship between the structure and properties of many 2DMOF materials has not been studied in detail. Therefore, more computational and experimental studies will be needed to achieve better low-cost engineering 2D materials as electrochemical energy storage devices [279][280][281]. Third, the energy density capacity has to be improved. In this regard, 2D MOFs deliver a higher energy density than other 2D materials [282,283]. Electrodes with a suitable composition and functionalities are needed to enhance the energy density by introducing novel 2D MOFs.

Outlook and Opportunities
Highly active and stable 2D materials are of great interest for a greener environment and sustainable energy conversion and storage applications. 2D MOFs have grown rapidly and established a new benchmark in developing electrocatalysts/electrode in the past few years, because of their tunable structure, availability of active metal/ligand centers, high surface area, and porosity. 2D MOFs are more beneficial for many electrochemical applications than 1D and 3D MOFs. In particular, TM-based 2D MOFs enhance the electric field during the electrochemical performance, which will enhance the accessible active sites and led to the design of efficient electrocatalysts-electrodes. This review systematically summarizes and discusses the recent progress on 2D-TM-based MOFs, their interesting properties, synthesis methods, composites, and derivatives related to advanced electrochemical energy applications, such as the HER, OER, ORR, CO 2 RR, UOR, supercapacitors, and batteries.
Despite the significant progress achieved in the recent decade, few challenges remain before 2D MOFs can reach their full potential in electrochemical applications.
Although 2D TM-MOFs have shown great potential as electrocatalysts-electrodes, there is still a long way to meeting scalable fabrication with high purity and low cost. In particular, Mo-and V-based 2D MOFs have not been much explored. These are highly active in electrochemical applications. In addition, theoretical calculations of Mo-and Vbased 2D MOFs suggest these would be supportive catalysts for future energy-related applications. The 2D Ni-, Co-, and Fe-based MOFs are well documented, achieving a better yield. Compared to other 2D MOFs, 2D Ti-based MOFs are in the infant stage. Furthermore, theoretical calculations and experimental attempts are needed to develop a novel synthesis method for Ti, Mo, and V-2D MOFs with controllable morphology and composition. 2D TM-based MOFs have attracted considerable attention for electrochemical water splitting (HER/OER). The transfer of electrons and protons is facilitated by their exposed active sites. In addition, transition metals (Ni, Co, Fe, and Cu), organic ligands, and guest molecules influence the electrochemical performance. Instability is a major drawback of 2D MOFs, and it is a great hindrance for the application point of view. The strong coordination bonds between the metal and ligands with lattice stain improve the stability of 2D TM-based MOFs. The formation composites with polymer, carbon, ceramics, and other 2D-based porous materials improve the structural stability and properties, leading to good performance and enhance stability cycles. Further, 2D MOFbased composite/derivatives prevent agglomeration and offer a greater structural stability along with more active sites and as well as enhance mass transfer rate. Thus far, expensive organic ligands have been used to construct 2D TM-based MOFs. Therefore, it is essential to explore low-cost ligands to form 2D TM-based MOFs with high performance. Furthermore, 2D MOF-derived metal oxides or porous carbon ha many intriguing properties different from or better than their bulk counterparts. Graphenesupported 2D MOF and 2D MOF-derived M-N-C com-plexes have become fascinating for improving the stability of the electrochemical performance. Moreover, applying energy conversion in 2D MOFs and their composites could allow unique properties, providing a platform to design the high-performing and stable electrocatalyst. In addition, the use of different synthesis strategies of 2D MOF and their composites/derivatives can provide information, such as the effects of interlayer space, doping, synergic effects, and defect generation.
The 2D TM-based MOFs and their composites/derivatives have great potential towards electrochemical CO 2 reduction and ORR applications, metal ion batteries, and fuels cells, and the UOR. Various 2D TM-based MOFs and their composites are more efficient than commercial catalysts, such as Pt/C and IrO 2 . Despite these predictions and substantial progress in 2D TM-based MOFs, challenges remain. Therefore, more experimental and theoretical efforts are needed to utilize 2D TM-based MOFs for energy-related applications. Moreover, stability issues need to be solved. Techniques such as XAFS and HAXPES analyses are powerful tools for understanding the relationship between electrocatalytic/electrochemical performance and the porosity of 2D MOFs and have been suggested for better characterization of the ORR and OER. In addition, 2D MOFs and their composites are promising electrode materials in advanced energy storage applications with maximum capacitances. On the other hand, the main drawbacks of 2D MOFs are their limited life span. The high valence metal ions such as Al and Zr can improve cyclic stability in 2D MOFs. Charging and discharging of 2D MOFs can also be improved by introducing a specific functional group such as flouride and amine due to change in steric barrier and electronic structure. Overall, improvement in the electrochemical properties of 2D TM-based MOFs and their composites/ derivatives can open new avenues for developing novel 2D TM-based MOFs and state-of-the-art electrocatalysts-electrodes for advanced energy-related materials. This article would serve as a useful reference for researchers interested in 2D TM-based MOFs and electrocatalysis. With the desired features for electrocatalysis, more 2D MOF-derived M-N-C complexes or porous carbon and newly 2D-TMbased MOFs are expected in the future.