Current Progress in 2D Metal–Organic Frameworks for Electrocatalysis

The 2D nanosheets of metal–organic frameworks (MOFs) have recently emerged as a promising material that makes them valuable in widespread electrocatalytic fields due to their atomic‐level thickness, abundant active sites, and large surface area. Efficient electrocatalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting are highly desired with low overpotentials to promote the industrial applications of energy conversion and devices. 2D MOF nanostructures provide long‐term stability and high electrical conductivity to enhance catalyst activity and durability. This review briefly summarizes the synthesis and electrocatalytic applications of 2D MOF for HER/OER/water splitting. More attention is focused on the synthetic strategies of 2D MOF and their derivatives. The catalytic performance and superior properties of these materials are highlighted. The outperformance of these materials originates from the rational design, myriad of abundant active sites, and atomic‐level thickness. The current and future challenges in this field and the scientific perspectives to overcome these challenges are highlighted. It is suggested that the construction of 2D MOF nanostructures can develop a state‐of‐the‐art electrocatalyst in energy and environmental division.


Introduction
Preserving the environment while creating a globally sustainable energy system is one of the extremely critical challenges confronting humanity today. [1][2][3][4] Conferring to the International Energy Agency, the worldwide energy demand fell by 4% in 2020, the greatest ever decline due to COVID-19 since World War II. The worldwide energy demand in 2021 is established to increase by 4%, returning to pre-pandemic levels, the immense majority of which is a derivative of oil, coal, and gas. [5] According to the US Energy Information Administration, the energy demand will increase by 47% through 2050 with expanding industrialization and a growing world population, particularly in developing countries. The demand for liquid fuel will increase by 28% in the next 30 years compared to renewable energy sources at 27%. [6] Consequently, major apprehensions have been increased toward energy supply, mainly climate change associated with carbon dioxide emissions. Therefore, a stern incentive remains to diversify the energy source's resilience, specifically reducing our dependence on natural fossil fuels with the development of renewable energy such as hydroelectric power, wind, and solar. Figure 1 demonstrates the plausible energy sustainable strategies for producing chemicals and fuels, including ammonia, hydrogen, and oxygenates, by concerting with conventional energy sources. Earth's climate endows nitrogen, carbon dioxide, and water, which can be transformed into the aforementioned products through the development of electrocatalysis. For instance, hydrogen can be produced via a water-splitting reaction which evolves oxygen and hydrogen evolution half-reactions. [7,8] As an attractive energy source, hydrogen can be exploited to generate electricity in fuel cells, where oxygen reduction and hydrogen oxidation reactions transform chemical energy into electrical energy. [9,10] Ammonia, through the electroreduction of nitrogen, would allow its use in pharmaceuticals, agriculture, and chemicals. [11,12] Likewise, carbon dioxide (CO 2 ) captured from the atmosphere or point source is an iconic gas that can be a feedstock for precursors to polymers, fuels, and fine and commodity chemicals. [13,14] Thereby, the development of highly efficient electrocatalysts with appropriate selectivity toward chemical transformations is crucial to enable the vision aforementioned.
Myriad strategies play a critical role in designing an efficient electrocatalyst system. The most crucial approaches include abundant active sites on an electrode and supreme response of the intrinsic activity of individual active sites. [15][16][17] Platinum is the outperforming electrocatalyst for hydrogen evolution reaction (HER), which usually requires a negligible overpotential to outperform state-of-the-art reaction rates. However, the high cost and scarcity of Pt limit its extensive technological availability for clean energy applications. In this perspective, miscellaneous non-noble metal-based catalysts have been conceived due to their dependence of electrocatalytic properties on the morphology of tunable dimensionality of numerous nanostructures. 2D materials have received worldwide attention after discovering graphene. [18][19][20][21][22][23][24][25][26][27] The development of 2D materials reveals a great structural advantage on electrocatalysts compared to the 0D, 1D, and bulk counterparts. For instance, these nanostructures endow optical transparency, mechanical flexibility, large surface area, and numerous active sties, making them remarkable in electronics and electrocatalysis. [28][29][30] In addition, metal-organic frameworks (MOFs) are another class of star materials whose crystalline porous structure consists of organic ligands and metallic nodes. [31][32][33][34][35][36][37][38][39][40][41][42][43][44] The benchmarks of these materials, including ultrahigh porosity, a large surface area, and a huge possibility of variable structures, depict their significance as potential candidates in biomedicine, gas separation, sensors, and energy storage and conversion. [45][46][47][48][49] However, the poor ion diffusion ability and low metal utilization of MOFs are unsatisfactory, producing high interfacial resistance in electrocatalysis. [50] This has sparked a pursuit of functional non-noble metal-based catalysts -an exploration where the development of 2D MOFs provides an excellent family of porous materials with extraordinary physicochemical and topological properties.
In marked contrast, sheet-like 2D MOF morphology stimulates the properties of MOF and 2D materials. The large surface area of 2D MOF material provides ultrathin structures, many active sites, and enhanced conductivity to enhance electrocatalytic performance. [51] However, the preparation of 2D MOF nanostructures is challenging due to the growth of MOF in the vertical direction. [52] By considering the significant research, we have summarized a mini review on the state-of-the-art performance of 2D MOF for electrocatalysis.

Synthetic Strategies for 2D MOF
The nanostructures with high aspect ratios allow dopants to approach abundant and exposed active sites, making them an attractive choice for electronics, gas separation, energy conversion, and storage applications. [53][54][55][56] The tunable dimensional growth strategy is vital to achieving the desired nanomaterials. Today, various growth mechanisms, including chemical vapor deposition, liquid exfoliation, wet chemical method, and mechanical cleavage, have been developed to prepare lowdimensional materials. [57][58][59][60][61][62][63][64][65][66] Specifically, the construction of 2D MOF is categorized into top-down and bottom-up strategies. [67][68][69][70][71][72][73] A top-down approach refers to a series of methods that synthesize the 2D nanostructures in which MOFs with lamellar morphologies are used as a starting material. Analogous to the synthesis of traditional 2D materials, numerous MOFs can be transformed into atomically thin and highly crystalline nanosheets using exfoliation techniques. In this regard, exfoliation is the most efficient method widely used to exfoliate MOF nanosheets from their bulk counterparts. Layered bulk MOFs combined with weak interactions like hydrogen bonds, and van der Waals forces can be separated into 2D MOF nanosheets/nanoribbons of required size via the exfoliation process, including physical exfoliation and chemical/electrochemical exfoliation. The key step for synthesizing 2D MOF nanostructures is to destroy the relatively weak interactions between the adjacent layers of bulk MOFs by the external forces or to increase the distance between layers by inserting molecules. The singlelayered 2D MOF nanomaterials can be obtained by various physical exfoliation methods, including ultrasonication, ball milling, shaking, micromechanical force, and freeze-thaw exfoliation. [51,74,75] Zhou et al. reported the pioneered work to chemically exfoliate single-layer Zn 2 (PdTCPP)-MOFs from  tetrakis (4-carboxyphenyl) porphyrin (TCPP) by intercalating trimethyl phosphine and 4,4 0 -dipyridyl disulfide. [72] These synthetic methodologies generated highly crystalline and uniformly distributed 2D MOFs using harsh chemical or mechanical exfoliation conditions. Moreover, these exfoliation mechanisms demand precise control and extensive optimization to prepare ultrathin 2D MOFs, resulting in an extremely low yield of nanomaterials.
In contrast with the top-down mechanism, the bottom-up strategy is a simple and more efficient pathway to design and construct ultrathin 2D MOF nanomaterials by choosing appropriate precursors. The assembly of organic linkers and metal ions through coordination interactions in a bottom-up approach is more convenient for synthesizing 2D MOF nanosheets. In the bottom-up method, modulators or surfactants such as hexadecyltrimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP), sorbitol-alkylamine, triethylamine (TEA), and/or sonication are mostly utilized during the growth process by controlling the kinetic effects or limiting the stacked MOF layers. [76][77][78][79][80] The surfactants can react with the ligands/solvents to synchronize with the metal nodes and tune the growth of 2D MOFs. [81] For example, ultrathin nanosheets of 2D NiCo MOFs can be synthesized by mixing TEA and benzenedicarboxylic acid under sonication. The surfactant molecules of TEA bind metal ions or organic ligands for the directional growth of MOFs to generate atomically thin nanosheets. [82] This approach is intensively deployed to control the directional growth, homogeneity, and thickness of 2D MOFs. [83] This methodology includes interfacial synthesis, [84][85][86] template method, [87,88] surfactant-assisted method, [89][90][91] sonication synthesis, [92,93] and competitive coordination method. [94] Compared to interfacial and template synthesis mechanisms, which mainly construct relatively thick (>10 nm) with low yield (<10%), the surfactant-assisted growth is a captivating strategy to prepare 2D MOFs with a low thickness (<10 nm) and high yield. The merits of surfactant-assisted growth are the selectivity and directional growth during the synthesis of MOF, which suppress the undesired lattice growth and form 2D MOF nanosheets. Lee et al. synthesized a series of nickel-based MOF nanosheets with a thickness of %2-3 nm on Ni foam by using urea and poly(vinyl alcohol) (PVA). [90] The chelation of PVA constructed the ultrathin 2D MOF nanosheets with nickel ions to generate PVA-Ni 2þ , which acted as a surfactant agent to create gaps among the nanosheets. Other surfactants, for instance, sodium lauryl sulfate and CTAB, have been demonstrated to grow 2D MOF nanosheets effectively. [95][96][97] Undoubtedly, this as-obtained thickness and homogeneity of prepared 2D MOF nanosheets are advantageous compared to the interfacial method. However, the conventional cleaning procedure could make detachment of surfactants from the 2D MOF surface relatively hard. Consequently, most of the active sites of 2D MOF nanosheets would be blocked by the surfactants, hindering their energy storage and electrocatalysis performance.
Legion efforts have been input to synthesize 2D MOF nanosheets to achieve desirable milestones, as shown in Figure 2.
Mainly, stacked precursors are essential for the top-down approach, and layers exfoliated from MOF via chemical or physical routes contain anisotropic morphologies and thicknesses. In addition, the constructed 2D MOF nanosheets contain several surface defects that may arise due to the smashing of interlayer bonds and provide active sites for their applications in sensors and electrocatalysis. Conversely, the lateral size and thickness of 2D MOF nanosheets can be controlled by modifying the reaction mechanism with a bottom-up strategy. The grown nanostructures possess a directional growth, controllable thickness, and high quality and yield of synthesized 2D MOF nanosheets. The downside of this mechanism is the structural instability which obstructs their role for surface-sensitive applications. In conjunction with synthetic deliberates of 2D MOF nanosheets, www.advancedsciencenews.com www.small-structures.com a vigilant selection of organic linkers is vital to coordinate with the metallic ions. The self-assembly of metallic nodes and organic ligands permits the construction of MOFs with stacked structures during the processing of the solution. The design strategy for fully functionalized 2D MOF nanosheets is concerned with coordination geometry, coordination number, coordination environment, structural transformation, and metal-ligand ratio. Regardless of available synthetic methodologies, constructing large-scale, highly crystalline 2D MOF nanosheets is still challenging, and a facile growth mechanism is highly demanded to achieve the well-ordered nanostructures.

2D MOFs for Efficient Electrocatalysts
The rising global population and climate change cause a shortage of fossil fuels, demanding a rapid development of sustainable energy with high efficiency. In this regard, one possibility is to develop a highly efficient electrochemical conversion process. Substantial attempts have been given to advance the highly efficient catalyst. Among these, the unique structure and electronic features of 2D MOFs are of particular interest, which provide an excellent intrinsic activity compared to conventional electrocatalysts. The association of 2D materials with traditional MOFs facilitates their physicochemical characteristics. [67,70] In contrast with their bulk materials, MOF nanosheets integrated with 2D materials contain unique electronic, structural, and chemical features that can be tuned through synthesis procedures and rational designs, as highlighted in Figure 3. These strategies boost flexibility, active sites, and conductivity, improving their performance in water-splitting applications. [8,94,[98][99][100] This section discusses several key challenges in demonstrating the rational design of 2D MOF electrocatalysts.

Highly Exposed Surface Area
The activity of an efficient electrocatalyst is directly linked with the elemental composition and geometry of nanostructures. The crystal structure of 2D MOF nanosheets provides highly exposed surface area and active sites at the atomic level. These consequences shorten the diffusion lengths of the products and reactants to improve the performance of an electrocatalyst significantly. [101][102][103][104][105] As demonstrated by Liu et al., the ultrathin 2D Fe-doped NiS/MoS 2 nanosheets exhibited significantly improved catalytic performance, attributed to the plenty of exposed active sites on ultrathin nanosheets. [104] Furthermore, various compositions in 2D MOF nanosheets, made of transition metal nodes, ligands substitution, and doping of extra atoms, [106] have provided an understanding of reaction mechanisms to achieve highly efficient electrocatalysts. Additionally, stability is a crucial challenge for electrocatalytic applications of MOFs, and it might be anticipated to be more problematic for ultrathin 2D MOF nanosheets. For instance, if the organic ligands in 2D MOFs catalysts are unstable, then the coordinatively unsaturated metal nodes would be produced to harm the long-term stability. Recently, 2D MOF composed of hexaiminohexaazatrinaphthalene (HAHATN)-based organic ligand on fabricating 2D Ni 3 (Ni 3 . HAHATN) 2 nanosheets displayed an electrocatalytic overpotential of 115 mV at 10 mA cm À2 and poor stability of 5 h. [107] Given the poor electrocatalytic stability of MOF, an impressive strategy to overcome these challenges is required to derive MOF through post-treatment methods (like pyrolysis) with or without the involvement of metal composites. [108,109] Li et al. synthesized 2D C-Co nanostructures via in situ growth of ZIF-67 on CoAl-based layered double hydroxide (LDH). [110] In addition to the high electrocatalytic activity in alkaline solution, the catalyst represented long-term stability for 20000s of continuous operation. Therefore, 2D MOF nanosheets should be considered carefully for practical industrial applications.

Conductive 2D MOFs
The electrical conductivity of designed 2D MOF nanosheets is extremely important for state-of-the-art electrocatalysts. [111][112][113] The catalysis performance is greatly accelerated due to the fast charge transfer rendered by the improved conductivity of electrocatalysts. However, the insulating nature of organic linkers makes most of the MOFs behave like an insulator and enhances the energy barrier for transferring charge carriers. [113] Therefore, advancing the synthetic methodologies and design of 2D MOF electrocatalysts is highly demanded to explore large conductivities. Recently, numerous conductive and semiconducting 2D MOF materials have successfully been synthesized. [114] It has been reported by Zhao et al. that the integration of conductive materials can enhance the conductivity of 2D MOFs. [115] In their work, MXene (Ti 3 C 2 T x ) was introduced to 2D cobalt 1,4-benzenedicarboxylate (CoBDC) to generate MXene/MOF hybrid nanosheets through interdiffusion reaction. The distinct interface between the two materials permitted the fast ion and charge transfer with low resistance (118 Ω of 2D CoBDC to 86 Ω of the MXene/MOF) during the electrochemical processes.  The conductivity of MXene significantly improved the reaction kinetics by modifying the oxygen evolution from a limited charge-transfer process to a limited-reaction process. In addition, the hydrophilic nature of MXene prevented the aggregation of porous MOF layers to exploit the electrolyte contact with the catalyst surface for excellent oxygen evolution reaction (OER) electrocatalytic efficiency. Therefore, scientists have primarily focused on developing conductive materials to reduce the challenges related to the insulating appearance of MOF. Several design strategies, mainly the formation of 2D conjugated structures to make the 2D MOF conductive, have been employed in the literature. [91,106,116]

Application of 2D MOFs in Electrocatalysis
The electrocatalytic reactions are fundamentally related to the surface of the materials, and the performance of these reactions is strongly dependent on their surface properties. 2D MOF materials provide an opportunity to construct an efficient electrode with the advantages of flexible surface and active sites for electrocatalysis. Although several challenges, including instability, low conductivity, and aggregation, are associated with 2D MOFs, limiting their performance for practical applications. However, abundant active sites, ability to electrode adhesion, and controllable structure of different derivatives of 2D MOFs represent the unique advantages of electrocatalysis. Therefore, the ultrathin 2D MOF nanosheets and their derivatives possess high-effective catalysts due to superior charge transfer, rapid mass transport, abundant active sites, and tunable atomic surfaces. This review summarizes the recent progress of 2D MOF electrocatalysts for HER, OER, and overall water-splitting applications. Table 1 summarizes the electrochemical performance of 2D MOF-derived materials in recent literature.

HER
The HER is a vital reaction that has gained widespread attention to produce high purity and large quantities of hydrogen. At present, a fascinating method to generate hydrogen is splitting water into hydrogen and oxygen, and electrochemical routes are believed to be the most efficient and feasible methods to accomplish this transformation. In HER, the electrocatalytic performance is associated with the catalyst on the electrode. Thereby, it is essential to advance the electrocatalysts to accelerate the reaction kinetics for efficient HER. Today, 2D MOFs have attracted extensive attention for HER electrocatalysts due to their small thickness and large surface area for abundant active sites to enhance the performance of electrocatalysis. Yan et al. reported the large-scale synthesis of N-doped hollow carbon nanotubes (CNTs)/graphene oxide (GO) heterostructure film (HNCGHF) Ni-MOF. [117] The schematic illustration of Ni@N-HCGHF is shown in Figure 4a.
Initially, a conventional solvothermal route prepared the MOF consisting of hollow spheres of Ni-BTC. Typically, trimesic acid (H 3 BTC), nickel nitrate (Ni (NO 3 ) 2 ·6H 2 O), and PVP were mixed in a solution, and a hydrothermal process was performed in a Teflon-lined autoclave at 150°C for 6 h. The morphology of hollow Ni-BTC spheres is shown in Figure 4b. The filtration process helped achieve a continuous film after vigorous stirring and sonication of Ni-BTC and GO solution. The thermal  [146] treatment of prepared film with dicyandiamide supported the synthesis of Ni@N-HCGHF, as shown in Figure 4c. N-doped CNTs were stitched using reduced GO nanosheets (N-CNT@rGO), and a porous 3D carbon matrix network was made. The MOF-derived N-CNT hollow structure, represented in Figure 4d, offered a large surface to generate abundant active sites for catalysis. Inspired by the anticipated composition and special structure of the as-synthesized catalyst, a standard threeelectrode system performed the electrochemical catalytic property in 1 M potassium hydroxide (KOH). The linear sweep voltammetry (LSV) curves in Figure 4e exhibited a low overpotential of 95 mV at 10 mA cm À2 . However, a weak instability of electrocatalyst (Ni@N-HCGHF) was examined after 2000 cyclic voltammetry cycles, as shown in Figure 4f. The thiocyanate ions (SCN À ), which are metal active site poisoners, were immersed in the 1 M KOH to understand outperform activity of the material. It was observed that the performance of the electrocatalyst was still better than most of the non-nobel-metal catalysts, which depicted that the catalyst has supreme catalytic activity.
Gayathri et al. have designed and developed a unique tunable morphology of 2D Co-derived leaf-like zeolitic imidazolate framework and named ZIF-Co-L controlled by GO. [118] The growth of 2D ZIF-Co-L@GO was initiated by preparing three solutions containing GO dispersion, Co(NO 3 ) 2 ·6H 2 O, and 2-methylimidazole (mlm) in an aqueous solution. The contents of GO dispersions were controlled by tuning the wright ratio of GO to Co (NO 3 ) 2 .6H 2 O (10%-80%). Later, all the three solutions were mixed under continuous stirring for 3 h at room temperature. The 2D ZIF-Co-L@GO was attained after the centrifuge process. Furthermore, they prepared 2D Co-driven nitrogendoped carbon@rGO (Co-NC@rGO) through the pyrolysis of ZIF-Co-L@rGo at 900°C for 120 min under an Ar environment. The morphological transition from leaf-shaped to hexagonalshaped ZIF-Co-L was elucidated with the help of bond and crystallographic. It is indicated that atypical ZIF-Co-L consists of four mlm ligands, two types of Co sites, and one free Hmlm molecule. The tetrahedral symmetry has occurred in Co 2þ ions, where Co1 ions coordinate with N16 (2) and N26 (2) μ2-bridging nitrogen of mlm. The Co 2 ions coordinate with  the N45 atom of a monodentate mlm and N15, N25, and N35 atoms of three mlm, as shown in Figure 5a. Furthermore, six μ2-bridging mIm ligands are interconnected with two Co1 ions, and four Co2 ions make a hexagon. Four μ2-bridging mIm ligands bonded with two of each Co1 and Co2 ions to form a parallelogram. The hexagons and parallelograms bind each other to construct a 2D network along the ab-direction, whereas the layers are stacked along the c-plane, as shown in Figure 5b. The increase in the interlayer distance of GO, from 7.4 Å (ZIF-L@10%GO) to 10 Å (ZIF-L@80%GO), confirmed the trap-water phenomenon between GO layers and can be depicted in Figure 5c. Therefore, it was estimated that GO has the ability to vary the morphology of ZIF-Co-L during the growth process. The detailed synthesis process is represented in Figure 5d. Their methodology transformed the typical leaf-shaped ZIF-Co-L (10% GO) morphology into elongated hexagonal-shaped ZIF-Co-L (40% GO). This transition was attributed to the water molecules trapped in stacked GO. The growth of crystal structure is restricted along the a-axis of ZIF-Co-L and results in the construction of hexagonal shapes ZIF-Co-L. Furthermore, the electrocatalytic performance of MOF-derived structures was estimated at 1 M KOH. The LSV curves of the catalysts were obtained at a scan rate of 5 mV s À1 , as shown in Figure 5e. The Co-NC@10rGO-leaf showed the lowest overpotential 220 mV@10 mA cm À2 compared to other synthesized electrocatalysts. The outperform electrocatalyst response was accredited to the appropriate weight percentage (%3%) of GO.
In addition, the performance of Co-NC@40rGO-hexagonal structures was lower as compared to Co-NC@10rGO-leaf, which was attributed to the Co-to-carbon/rGo ratio. The Tafel slope was utilized to understand the catalysts' HER activity and reaction mechanism, as shown in Figure 5f. The Co-NC@10rGO-leaf exhibited a lower slope than others, indicating fast reaction kinetics. Tafel slopes ranged 120-162 mV dec À1 indicating a Volmer-Heyrovsky mechanism. Hexagonal-and leaf-shaped Co-derived N-doped carbon catalysts were constructed with GO-modified 2D ZIF-Co-L and outperformed HER in alkaline media.
Recent studies demonstrate that MOF-derived Ru-based nanostructures are promising for HER electrocatalysis over a wide pH range. The bond energy for Ru-H is comparable to that of Pt-H. However, the Ru price is merely 5% of Pt. Thus, Ru-based catalysts are considered a promising alternative to their Pt-based counterparts. For instance, Zhang et al. prepared a self-supported MOF-derived Ru doped cobalt-nickel oxide nanosheet arrays on Ni foam (NF) as an efficient HER electrocatalyst. [119] The lowtemperature thermal annealing process was adopted to fabricate Ru-Co 3 O 4 -NiO-NF ( Figure 6a) catalysts with nanosheet morphology. Leaf-shaped nanosheet arrays with a single sheet thickness of 200-300 nm and a growth height of 9-10 μm provide abundant active sites and fast mass or charge transport to improve HER activity significantly. Notably, the Ru-Co 3 O 4 -NiO-NF catalyst exhibited high HER activity in an alkaline medium (Figure 6b), featuring an overpotential (η 10, HER ) of 44 mV much lower than those of other fabricated catalysts and comparable to the benchmark Pt-C-NF (η 10, HER ¼ 32 mV). Interestingly, Ru-Co 3 O 4 -NiO-NF catalyst displays an extremely low overpotential at a relatively high current density, i.e., (η 100, HER ) of 115 mV, which even surpasses that of Pt-C-NF (143 mV). To eliminate the mass loading effect of catalysts, mass-normalized results were obtained (Figure 6c   These results signify that the combined effect of morphology engineering and electronic structure regulations can remarkably improve the HER kinetics. In another work, Huang et al. reported bimetallic sited conductive MOF with in-plane mesoporous structures for enhanced HER reaction kinetics. [107] The authors provide experimental and computational analyses to investigate metallic, active sites in fabricated conductive MOFs. HAHATN was designed as an organic ligand to fabricate bimetallic conductive MOF. Ni 3 (Ni 3 . HAHATN) 2 was obtained from a conjugated ligand (Ni 3 . HAHATN) via a synthetic step and two consecutive coordination reactions (Figure 7a). Petaloid morphology comprising thin-layered nanosheets with abundant wrinkles was grown over a large area (Figure 7b). High-resolution transmission electron microscope (HRTEM) image corresponds smooth silk layered structure with a 1.6 nm thickness of fabricated nanosheets (Figure 7c). These abundant nanosheets bring a hierarchical porous structure that can significantly increase electroactivity due to exposed active sites and rapid mass diffusion between nanosheets. The electrochemical results showed a smaller overpotential (115 mV at η 10 ) for Ni 3 (Ni 3 . HAHATN) 2 (Figure 7d). The obtained overpotential was much smaller than Ni 3 (HITP) 2 , indicating that Ni-N 2 moiety plays a vital role in improved HER activity of fabricated catalyst. Various Ni 3 (M1 3 . HAHATN) 2 catalysts were electrochemically tested to study the effect of the M1-N2 site. The obtained electrocatalytic activity trend showed gradual enhancement of HER activity, indicating the active discrepancy of metal ionic species: Cu-N 2 < Co-N 2 < Ni-N 2 (Figure 7e). Furthermore, Tafel slope was also obtained for all fabricated catalysts; a smaller value of 45.6 mV dec À1 was obtained for Ni 3 (Ni 3 . HAHATN) 2 , which is far less than other catalysts indicating a faster reaction kinetics toward HER electrocatalysis. Hence, benefiting from improved electrical conductivity and unique porous morphology, Ni 3 (Ni 3 . HAHATN) 2 facilitates the faster mass transfer and remarkably improves the HER activity (Figure 7f ).

OER
The OER, a four-electron transfer process, is an important reaction to generate molecular oxygen via electrocatalysis of water into hydrogen and oxygen. The state-of-the-art OER electrocatalyst is essential for renewable energy technologies. However, the high activation energy barrier and the sluggish reaction kinetics due to the transfer of multistep electrons strongly require durable and efficient OER electrocatalysts. Although IrO 2 and RuO 2 outperform OER activities, their high cost and scarcity limit large-scale industrial applications.
The large surface area, pore structure, controllable morphologies, and variety of metal ion selection make MOFs a promising candidate for OER. [120] Recently, the challenges related to the low electrical conductivity of MOF tried to be resolved by the  pyrolysis procedure; however, this strategy is accomplished at a high temperature which deteriorates the porosity of the MOF structure. [121,122] The catalytic properties of MOFs have widely been improved with non-metal elements including S, N, and P at relatively low temperatures to maintain the crystal structures. [123][124][125][126] The ability of non-metal elements to create atomic vacancies and defects on carbon surfaces generates additional active sites for numerous energy conversion reactions, including OER. [127] Despite non-metal elements, point defects have recently been introduced in transition metal-based catalysts through halogen elements to enhance catalytic performance. Kou et al. reported chlorine (Cl) doped cobalt hydroxide (Co (OH) 2 ) nanosheets for OER and ORR. [128] Wang et al. reported chlorine Cl-doped lithium cobalt oxide (LiCoO x ) nanostructures for OER. [129] The leaching of specific elements in the catalyst generates abundant active sites for outperforming the OER process. Park et al. recently synthesized Cl-and metalloid (Te)-doped NiFe MOFs (Te, Cl-NiFe MOF) through a two-step synthesis process. [130] The synthesis of NiFe MOF was initiated by a mixture of Ni (NO 3 ) 2 ·6H 2 O and Fe (NO 3 ) 3 ·9H 2 O precursors in methanol, followed by a dropwise mixing of C 4 H 6 N 2 solution. Afterward, nickel foam was immersed into the prepared mixture for the hydrothermal procedure, performed at 180°C for 6 h. To construct the Te, Cl-NiFe MOF, the synthesized NiFe MOFs were incorporated with TeCl 4 , and calcination occurred in a chemical vapor process (CVD) at 350°C. The growth procedure is explained in Figure 8a. The OER performance of Te, Cl-NiFe MOF on nickel foam was investigated in 1 M KOH, pH 13.5. The LSV curves from the electrocatalyst represented in Figure 8b exhibited an overpotential of 224 mV at 30 mA cm À2 . The reaction kinetics of OER obtained from the Tafel slopes of Te, Cl-NiFe MOF represented the low value of 37.6 mV dec À1 (Figure 8c). It was attributed that the incorporation of Te in NiFe MOF helps to enhance the charge transfer and enables adsorption of oxygen on the surface. The Cl helped to induce more active sites on the NiFe MOF surface by a leaching procedure during the electrochemical reaction. It is mentioned that the synergistic effects exhibited in Te, Cl-NiFe MOF support to enhance the electrocatalytic activity of OER.
Recently, Zheng et al. have reported a facile in situ synthesis strategy for 2D FeNi layered double hydroxide (FeNi LDH) nanosheets coupled with MOF. [86] The authors adopt a two-step synthesis method: first, FeNi LDH nanosheets were grown on the carbon cloth (CC) through a hydrothermal approach (Figure 9a). In the second step, in situ phase reconstruction was carried out using 2,5-dihydroxyterephthalic acid, and finally, CC supported 2DFeNi LDH nanosheets decorated with 1D sword-like FeNi-MOF crystals were obtained. The scanning electron microscope (SEM) image presented FeNi LDH having uniform 2D  nanosheets morphology (Figure 9b). To examine morphology evolution during the second step, time-dependent experiments were conducted. By prolonging the reaction time, 1D sword-like structure was initiated, and nanosheets became thinner. The observed diameter of the 1D structure was around 250 nm, and the length was up to 1.3 μm (Figure 9c). On further increasing the coordination reaction time to two days, it was observed that 1D sword-like structure fully covered the carbon cloth surface and made nanosheets almost invisible (Figure 9d). All the fabricated catalysts were then examined for OER in a 1 M KOH solution. All the MOF incorporated catalysts showed lower activity than FeNi LDH and the commercial RuO 2 , representing that MOF-74 is promising for OER activity. Among MOF-derived catalysts, the 2D/1D FeNi LDH/MOF displays the best activity with η100 ¼ 272 mV (Figure 9e) and the smallest Tafel slope of 34.1 mV dec À1 (Figure 9d). With variation in the reaction time, the activity of the fabricated sample showed a volcanic-type trend (Figure 9e). These results suggest that the improved catalytic activity of fabricated samples depends not only on the formation of MOMOF-derived catalyst but also on morphological evolution. Thin nanosheets accelerate mass transport and provide abundant edge sites that facilitate the OER reaction kinetics. The Co-N 4 is an effective OER catalyst due to its fast charge transfer during the electrochemical reaction. [131][132][133] However, the active sites in single-atom-based Co-N 4 catalyst are small and many research groups devoted to enhancing the active sites to improve the efficiency of the electrocatalyst for OER.
Considering it as a challenge, Xing et al. synthesized 2D Co 3 (HITP) 2 (HITP ¼ 2,3,6,7,10,11-hexaiminotriphenylene) with well-defined porosity and abundant active sites. [134] The detailed growth methodology of Co 3 (HITP) 2 was accomplished by a costeffective and simple method. The detailed schematic representation is shown in Figure 10a. During their growth process, an aqueous solution of CoCl 2 was mixed with HITP.6HCL and ammonia under the stirring process. The microstructure and morphology of the prepared material were investigated with TEM, which is represented in Figure 10b. It was examined that the synthesized Co 3 (HITP) 2 contained a sheet-like shape which indicated a 2D structure. The crystal structure of 2D MOF was further confirmed by HRTEM with the indication of arranged honeycomb pattern (Figure 10c,d). The OER performance of Co 3 (HITP) 2 was investigated in 1 M KOH using LSV curves and compared with RuO 2 and IrO 2 (Figure 10e). The overpotential of 254 mV at 10 mA cm À2 of Co 3 (HITP) 2 was lower than IrO 2 and RuO 2 . However, HITP did not display distinguishable activity. Tafel slope of Co 3 (HITP) 2 was much better than HITP and comparable with RuO 2 and IrO 2 (Figure 10f ). Based on the aforementioned figure of merits, it was attributed that 2D MOF can be considered an efficient OER catalyst. Reproduced with permission. [130] Copyright 2022, Elsevier.
www.advancedsciencenews.com www.small-structures.com In another report, Shrestha et al. have reported a simple and scalable synthesis strategy for Cu-Fe-NH 2 -based MOF thin film electrode for efficient OER activity. [135] The thin film was composed of 2D nanosheets of MOF, which were deposited either by a few drops of MOF ink obtained from solvothermal synthesis of bulk MOF onto NF or by dipping substrate tips into the ink. It was found that both deposition methods yield uniform deposition of MOF films. However, the authors found that dip coating can be used for large-scale synthesis by simply immersing the substrate in ink. Polarization curves were recorded to evaluate the OER activity of fabricated electrodes at a scan rate of 5 mV s À1 . It was found that the NF electrode coated with Cu-Fe-NH 2 MOF exhibits the highest catalytic activity with a low overpotential of 270 and 390 mV at current densities of 100 and 1000 mA cm À2 , respectively. Owing to the fact that durability is an important parameter in electrocatalysis, continuous bulk electrolysis was performed for 72 h. Interestingly, after bulk electrolysis of the Cu-Fe-NH 2 MOF electrode, its OER performance was even better than the fresh sample. This result suggests that electrocatalyst activation was continued during the long-term stability test. The remarkable activity of MOF film was attributed to the intimate contact between film and Nickel foam and the synergistic effects between Cu and Fe metal redox centers.

Overall Water Splitting
Overall water splitting is one of the most attractive approaches for storing clean and renewable energy. It consists of two halfreactions that take place simultaneously, in which HER occurred at the cathode and OER happened at the anode. Unfortunately, achieving efficient overall water splitting is extremely critical due to different reaction mechanisms and conditions between OER and HER processes. However, recently 2D MOFs have gained worldwide attention for overall water-splitting applications. Sun et al. synthesized ultrathin metallic 2D MOF architectures including unary, binary, ternary, quaternary, and quinary composites on MOFs using the dissolution-recrystallization method. [136] The synthesized samples exhibited fast reaction kinetics, excellent charge transport, and electrocatalytic activity for HER and OER, as shown in Figure 11. The strong synergetic effect between their unique 2D MOFs demonstrated high current densities, small overpotentials, and high-water splitting efficiency. Further, DFT calculations were conducted to investigate the reaction mechanisms, which depict that the excellent performance of HER and OER is due to their unique 2D MOF structure, which has effective mass transport and abundant active sites.
Chhetri et al. have investigated mesoporous 2D MOF-derived CoS 2 integrated with FeS 2 -MoS 2 as a vital electrode for overall   Figure 12a-h. The LSV curves related to HER measured in 1 M KOH are exposed in Figure 12i. The catalyst performed an overpotential of 92 mV at a current density of 10 mA cm À2 . The OER LSV curves measured in 1 M KOH are shown in Figure 12j. The prepared electrocatalyst demonstrates remarkable OER performance with an improved overpotential of 211 mV at 20 mA cm À2 . The electrocatalysts represented lower overpotential and Tafel slope values than IrO 2 at 20 mA cm À2 and resulted in a higher current. These results predicted that the synthesized mesoporous material outperforms the OER catalyst. Benefitted from excellent HER and OER performance, the electrocatalyst further experimented for overall water splitting, as shown in Figure 12k. The electrolyzer achieved the lowest voltage of 1.51 V with excellent stability for 30 h at a current density of 10 mA cm À2 . The superior electrocatalyst response is attributed to the improved conductivity and more exposed active sites of FeS 2 -MoS 2 @CoS 2 -MOF.
Recently, Wang et al. have reported a template-based synthesis strategy for in-situ growth of 2D MOF nanosheets grown on Ni foam (Figure 13a). [138] During synthesis, first, they prepared vertically oriented CoO nanowalls on NF via the facile hydrothermal method, and subsequently, pyrolysis was performed to convert 2D Co-MOF nanosheets into Ni@CoO@Co-MOFC. The thickness of the fabricated electrode (%0.312 nm) was almost the same as that of Ni foam (%0.30 nm). SEM images confirmed the vertically grown CoO nanowall arrays on Ni foam with a thickness of about 20-50 nm (Figure 13b,c).
Afterward, the CoO nanowalls acted as a template for the growth of 2D Co-MOF nanosheet arrays with a thickness of Reproduced with permission. [134] Copyright 2020, Elsevier. 30-60 nm (Figure 13d). The electrocatalytic performance of Ni@CoO@CoO-MOFC was studied toward HER and OER at a scan rate of 0.5 mV S À1 in 1 M KOH solution. HER's polarization curves exhibit the highest activity for Ni@CoO@CoO-MOFC with an overpotential of 138 mV (Figure 13e) at a current density of 10 mA cm À2 , superior to other fabricated samples. In addition, the LSV curves obtained for OER also showed the lowest overpotential of 247 mV (Figure 13f ) at a current density of 10 mA cm À2 , which outperforms other fabricated electrodes. Inspired by remarkable HER and OER activity, Ni@CoO@CoO-MOFC was assembled in a two-electrode system for the overall water-splitting process. It was observed that with a low cell voltage of 1.61 V, the prepared sample showed better overall water-splitting performance than Pt/C and RuO 2 on Ni foam (Figure 13g). Overall, benefiting from the combined advantage of composition and structural superiorities, fabricated selfsupported structure demonstrates significant electrochemical properties. There was a noteworthy increase in catalytic performance after an increase in current density beyond 76 mA cm À2 , indicating good activity at large current densities.

Opportunities and Challenges
Designing a cost-effective, highly active, and stable electrocatalyst is crucial for industrial applications and has become a key feature in electrochemistry. An ideal catalyst can be constructed by considering several aspects, including large surface area, optimized porosity, wide availability, and abundant exposed active sites. The eminent non-noble metal-based catalysts comprise metal oxides, metal-decorated hard and soft substrates, macrocyclic materials, and graphene. The MOF-based catalysts have been constructed recently with promising catalytic features for vital electrochemical reactions. The adaptable design of MOF provides the opportunity to link with the materials mentioned earlier. MOF precursors are ideal for constructing 2D materials, CNTs, metal oxide nanostructures, metal-free carbon, and carbon/metal composites. MOF is the next-generation material due to its structural ability for atomically thin nanostructures by controlling the reaction conditions. MOF is the next-generation material due to its structural ability for atomically thin nanostructures by controlling the reaction conditions. There are several challenges Reproduced with permission. [136] Copyright 2021, Elsevier. associated with the development of 2D MOF nanosheets. 1) Despite several reported routes, the role of various kinds of solvents and their interactions with MOF during the growth process is unclear and needs to be described.
2) The selectivity and directional growth of MOF are challenging for constructing homogeneous nanosheets.
3) The high-temperature pyrolysis has been reported for the construction of conductive MOF and MOF-derived nanosheets. However, there is a great possibility that the extremely high temperature can collapse the porosity of the MOF. Therefore, it is highly recommended to design conductive 2D MOF nanosheets under an ambient environment. 4) The reported conductive MOFs are prepared with extremely expensive ligands and void practical applications for energy storage and conversion. Therefore, it is necessary to explore the environmentally friendly and low-cost ligands to form these materials. 5) The detailed growth methodology for forming ultrathin 2D MOF nanosheets is inaccessible. Therefore, a facile method to synthesize 2D MOF nanosheets for high crystallinity, better stability, high yield, and large lateral size is still invulnerable.
During the past decade, 2D MOF has become a state-of-the-art material and emerged as a promising candidate in electrochemistry due to its captivating physicochemical features, including controllable structure, abundant unsaturated atoms, outperforming electrical conductivity, and large surface area. The development of 2D MOF catalysts is still immature in their particular field of photocatalysis, electrocatalysis, and thermocatalysis. Therefore, more efforts are required to explain the working mechanism and establish an efficient 2D MOF catalyst with large conductivity and low overpotential. These ultrathin materials represent sluggish long-term stability under strong alkaline or acidic conditions. Therefore, 2D MOF nanosheets should be considered carefully for practical industrial applications. Though a significant development has been achieved in the last decade, various challenges are still to overcome.

Scientific Outlooks
Despite having myriad challenges associated with the growth of 2D MOF and the construction of efficient catalysis, contemporary research can resolve a few of the challenges mentioned earlier.
The combined growth methodology, such as chemical exfoliation and freeze-thaw methods, has been developed to increase the yield of the material. The atomically thin 2D MOF nanosheets can be synthesized through liquid interfacial or surfactant-assisted methods. The critical component toward 2D MOF stability is to enhance the interactions between organic ligands and metallic nodes. The stabilizers or suitable solvents with specific surface energies can increase the stability of the ultrathin 2D MOF. The guidance for constructing 2D MOF nanosheets can be obtained from the theoretical analysis and a piece of profound knowledge about the growth mechanism. The possibility for the formation of 2D MOF with organic ligands, metallic nodes, and their stability in the stabilizers can be investigated theoretically before the experiments. In addition, several in situ and state-ofthe-art characterization tools, including spectroscopy and X-ray absorption spectra (XAS), can be employed to understand the growth methodology of 2D MOF. These methodologies can reduce the chances of failure to construct ultrathin 2D MOF nanosheets. The reaction mechanism of the state-of-the-art 2D MOFbased catalyst can be investigated with the aid of operando studies and in-situ characterizations, including spectroscopy, XAS, and TEM. These techniques provide direct evidence for the reaction mechanism and catalyst's evolution in the catalytic process.

Conclusions
This review summarizes recent developments on the growth of 2D MOF and their applications in electrocatalysis. Although various methods have been developed to grow ultrathin 2D MOF nanosheets, there are still myriad challenges to explaining the large-scale and controllable growth of these materials with high quality, high production rate, good stability, and high yield. The irregular shape of MOF contains ambiguities for structural investigations and catalyst performance. The fully developed 2D MOF or MOF-derived nanosheets will exhibit uniform shapes, controlled directional growth, and large lateral sizes for catalytic applications. For practical energy storage and device applications, efficient electrocatalysts require extremely low overpotential and long-term stability. The scientific research on 2D MOF has delivered a plethora of promising electrocatalysts to promote the research progress in such applications. We expect this mini-review to be a valuable citation for scientists working in the field of 2D MOF and energy and conversion devices. It is estimated that a rational designed 2D MOF will provide the desired electrocatalyst for HER, OER, and overall water-splitting applications.
Qichun Zhang completed his Ph.D. in chemistry at the University of California Riverside. Then, he worked as a Postdoctoral Fellow at Northwestern University. In January 2009, he joined Nanyang Technological University (Singapore). In 2020, he moved to Department of Materials Science and Engineering at the City University of Hong Kong as a full professor. From 2018 to 2021, he has been recognized as one of the highly cited researchers (top 1%) in cross-field in Clarivate Analytics. He is a fellow of the Royal Society of Chemistry. Till now, he has published > 460 papers and 5 patents (H-index: 100).