Revolutionizing energy storage: the emergence of MOF/MXene composites as promising supercapacitors

As the world becomes increasingly concerned with environmental preservation and the effects of fossil fuel consumption, it is essential to find new and innovative ways of providing energy. Supercapacitors are among the most promising devices for energy storage. Finding materials that can enhance their efficiency is still a major challenge. Research is currently underway to fabricate composite materials with specific properties that can improve the performance of supercapacitors. One class of materials that has shown great promise is MXenes, which are two-dimensional layers of carbides, nitrides, and carbonitrides of transition metals. These materials possess unique features such as high electrical conductivity, flexibility, and hydrophilic surfaces, which make them suitable for a range of electrochemical applications. Adding MXenes to metal–organic frameworks (MOFs) or MOF derivatives has been shown to enhance the output yield of supercapacitors. MOFs are widely used in various energy systems because of their adjustable porosity and high surface area. The addition of MXenes can prevent the stacking of MXene sheets on top of each other, leading to improved results due to the synergistic effect. In particular, MOF/MXene composites have shown significant promise for use in supercapacitor applications. This review provides a comprehensive overview of the recent advances in MOF/MXene composites, including their synthesis, properties, and potential applications. We also highlight the challenges and opportunities for future research in this field.


Introduction 1.General aspects of supercapacitors
Addressing environmental issues in the 21st century is crucial due to the extensive use of petroleum products and fossil fuels, leading to global warming.Renewable energy sources like sunlight and wind are practical solutions, but efficient energy storage and conversion systems are needed [1][2][3][4].Electrochemical energy storage and conversion systems (EESCs), such as supercapacitors, offer several advantages over conventional batteries, including high power density, enlarged capacitance, environmental compatibility, and longer lifespan [5][6][7].Supercapacitors can be used in vehicles and portable electronic devices, reducing environmental pollution and offering cost savings [1,8].Research is ongoing to improve electrode materials and structures for nextgeneration EESCs [8,9].Supercapacitors, as a reliable category of EESCs, can store and convert energy through ion transfer.Despite having lower energy density than chemical sources, advancements have been made in electrode materials and structural substances [10].Supercapacitors consist of two parallel layers with an electrolyte and separator in between.The device's properties depend on the characteristics of the electrodes, making extensive research on electrode selection and fabrication crucial for the development of next-generation EESCs.With their numerous advantages, supercapacitors find applications in portable electronics, wearable devices, and hybrid electric transportation.While improvements continue to be made, supercapacitors contribute to a greener future [11,12].

Inorganic electrode materials
Discovering higher-performance electrodes can be important for instruments such as supercapacitors [13].An electrode with the required pore size, surface area, and electrical conductivity could be used as an electrode.Conductive materials such as polymers and carbonaceous materials have become widespread in recent years, but using new inorganic compounds such as MOFs and MXenes is also a great option.This study provides a summary of the results obtained from the performance of these two substances in combination, so we will first present them in general terms.

Metal-organic frameworks as supercapacitors
Metal-organic frameworks are materials that consist of metal nodes and organic linkers and form frameworks with cavities and particular surface areas [14,15].One of the main advantages of these structures is the diversity in the selection of components as well as the variety of composite materials and their properties.Various designs are obtained from the strong bonds between different ligands and metal nodes with specific pore sizes.Each will have particular magnetic, electrical, optical, and catalytic properties and applications [16][17][18].
MOFs have a relatively short but impressive history in materials science.The first MOF was discovered in 1999 by researchers at the University of Michigan, who were exploring ways to store gases in porous materials [19][20][21].This first MOF, known as MOF-5, was made of zinc ions and 1,4-benzene dicarboxylic acid and had an incredibly high surface area.Its discovery opened up a new class of materials with immense potential for applications in gas storage, separation, catalysis, and sensing [22,23].Since then, thousands of MOFs have been synthesized with various metal ions and organic ligands, each with unique structures and properties.The field of MOFs has exploded with research into new synthesis methods, theoretical modeling, and applications [24][25][26][27].Today, MOFs have been explored for use in areas such as drug delivery, water purification, and carbon capture [28][29][30].As the field continues to grow and mature, there is no doubt that MOFs will continue to provide exciting opportunities for scientific discovery and technological advancement.
It is worth mentioning that MOFs have emerged as a promising class of materials for energy storage applications, including as supercapacitors due to their stability, excellent porosity, improved power density, and cycling durability [37][38][39].One advantage of MOFs over traditional carbon-based electrodes is their high specific capacitance, or the ability to store a large amount of charge per unit mass.This is due to their high surface area, which can range from hundreds to thousands of square meters per gram [30,40].In addition, MOFs have been shown to exhibit good stability and fast charge-discharge rates, making them promising candidates for high-performance supercapacitors.Several studies have reported the use of MOFs as electrodes in supercapacitors.For example, one study reported the synthesis of a MOF composed of cobalt ions and pyrazinebased ligands, which exhibited a specific capacitance of 356 F g −1 and good cycling stability over 5000 cycles [41].
One of the main challenges in using MOFs as supercapacitor electrodes is their poor conductivity, which can limit their charge-discharge rates and overall performance.To address this, researchers have developed strategies to enhance the conductivity of MOFs, such as through the incorporation of conductive materials like graphene and carbon nanotubes [42,43].Another challenge is the limited understanding of the charge storage mechanisms in MOFs [44].While it is clear that charge storage occurs via the electrostatic adsorption of ions at the surface of the MOF, the exact mechanisms involved are still not fully understood [45].More research is needed to elucidate these mechanisms and to optimize the design of MOFs for use in supercapacitors.Finally, MOFs can also suffer from stability issues in certain environments, such as exposure to moisture or acidic conditions.This can cause degradation of the MOF structure, leading to a decrease in performance over time.However, researchers are actively working on developing MOFs with improved stability in these conditions.
Overall, while MOFs have many advantages as supercapacitor materials, such as high surface area and good stability, they also have some limitations, such as poor conductivity and relatively low energy density [46].However, with continued research and development, these limitations can be addressed, and MOFs can become even more promising candidates for energy storage applications.Their high specific capacitance, tunable pore size, and good stability make them attractive candidates for use in a wide range of applications, from portable electronics to grid-scale energy storage [47].With continued research and development, MOFs have the potential to revolutionize the field of energy storage and help pave the way toward a more sustainable future.

MOF-composites as supercapacitors
MOF composites are a promising avenue for improving the performance of MOFs as supercapacitor electrodes [42].Carbon nanotubes (CNTs), graphene and graphene derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO), conductive polymers like polyaniline (PANI), and polypyrrole (PP y ), metal nanoparticles like platinum (Pt), or gold (Au), metal oxides like manganese dioxide (MnO 2 ), and nickel oxide (NiO), conductive carbon materials like carbon black, carbon nanofibers, and carbon aerogels, MXenes, metal sulfides, carbon cloth or paper are redox-active materials that can be integrated with MOFs to improve supercapacitor performance.The choice of conductive or redox-active materials depends on the specific requirements and performance goals of the supercapacitor application.Combining MOFs with these materials can lead to enhanced energy storage capabilities, improved cycling stability, and faster charge/discharge rates [48][49][50].
One common approach for creating MOF composites is to incorporate conductive materials, such as carbon nanotubes, graphene, or metal nanoparticles, into the MOF structure.This can improve the conductivity of the composite, which can in turn improve its charge-discharge rates and overall energy storage performance [51].Another approach for creating MOF composites is to incorporate redox-active materials, such as metal ions or organic molecules, into the MOF structure [52].This can create a hybrid material with both electrical and chemical energy storage capabilities, which can result in improved energy storage performance compared to a purely electrochemical system [53][54][55].
Compared to pristine MOFs, MOF composites have several advantages as supercapacitor electrodes.The incorporation of conductive or redox-active materials can address the poor conductivity and relatively low energy density of pristine MOFs, resulting in improved performance [56].Additionally, MOF composites can exhibit improved stability in certain environments, such as exposure to moisture or acidic conditions, which can make them more practical for real-world applications.Overall, MOF composites have shown great promise as supercapacitor materials, with numerous studies demonstrating their improved performance compared to pristine MOFs.While there is still much research to be done to optimize their properties and address remaining challenges, such as scalability and cost-effectiveness, MOF composites represent a promising avenue for advancing the field of energy storage [57].
1.2.1.2.MOF-derived materials as supercapacitors MOF-derived materials, also known as MOF-derived carbons (MDCs), are obtained by the carbonization of MOFs, resulting in a porous carbon structure with high surface area and conductivity [58,59].These materials have been studied extensively as supercapacitor electrodes due to their excellent electrochemical performance.MDCs exhibit high specific capacitance, excellent cycling stability, and fast charge/discharge rates, making them ideal for use in supercapacitor applications.These properties make them suitable for use in supercapacitor applications [56].The performance of MDCs is dependent on the precursor MOFs and the carbonization conditions.The pore size and surface chemistry of the resulting carbon material can be tailored by selecting appropriate precursor MOFs and controlling the carbonization conditions, allowing for the optimization of their electrochemical performance.
Recent research has demonstrated the potential of MOF-derived materials in supercapacitor applications.For example, researchers have reported the synthesis of MDCs derived from various MOFs, such as MIL-100, UiO-66, and ZIF-8.These MDCs exhibit high specific capacitance, good cycling stability, and fast charge/ discharge rates [60].Researchers have also investigated the effect of different carbonization conditions on the electrochemical performance of MDCs.For example, varying the carbonization temperature, time, and atmosphere can alter the surface area and pore size of the resulting MDC, affecting its electrochemical performance.In addition to their excellent electrochemical performance, MOF-derived materials have other advantages, such as their low cost and easy synthesis.MOFs can be easily synthesized in large quantities and at low cost, making them an attractive option for the large-scale production of MDCs.Furthermore, the carbonization process is simple and can be performed at a relatively low temperature, making it energy-efficient and environmentally friendly.Overall, MOF-derived materials have shown great promise as supercapacitor electrodes, and research in this area is ongoing.With continued development, these materials have the potential to play a significant role in advancing energy storage technologies.

MXenes as supercapacitors
MXenes are a relatively new class of two-dimensional (2D) materials that have shown great potential for various applications in the field of materials science [61][62][63].They were first discovered in 2011 by researchers at Drexel University [64].MXenes are a family of 2D transition metal carbides, nitrides, and carbonitrides, which are synthesized by selectively etching the A element from the MAX phases (where M is a transition metal, A is an A-group element, and X is either carbon or nitrogen) [65].The resulting MXene has a layered structure with a thickness of a few atoms and a large surface area.Overall, MXenes are a highly versatile class of materials with many potential applications in various fields, and their research is still in its early stages.
One of the unique properties of MXenes is their high electrical conductivity and excellent mechanical strength, making them attractive for use in energy storage devices such as batteries and supercapacitors [66].For example, titanium carbide (Ti 3 C 2 T x ) MXene has been widely studied for supercapacitor applications due to its high electrical conductivity and large surface area.It has been reported to exhibit high capacitance and excellent cycling stability in aqueous electrolytes [67].Another MXene, vanadium carbide (V 2 CT x ), has also shown excellent electrochemical performance for supercapacitor applications, with high capacitance and good cycling stability.Other MXenes, such as niobium carbide (Nb 2 CT x ), molybdenum carbide (Mo 2 CT x ), and tantalum carbide (Ta 4 C 3 T x ), have also been studied for supercapacitor applications and have shown promising results [4,68].It's worth noting that the performance of MXenes as supercapacitor electrodes can also be enhanced by combining them with other materials, such as graphene or carbon nanotubes, to form hybrid materials.These hybrid materials can exhibit improved performance compared to individual MXenes or other materials alone [69].

MXenes-composites as supercapacitors
MXenes can be used in supercapacitors either as standalone electrodes or as part of composite materials with other conductive materials.MXene composites have been shown to have superior electrochemical properties, such as high capacitance, high-rate capability, and excellent cycling stability, making them attractive for use in supercapacitors.In MXene composite supercapacitors, MXenes are often combined with other conductive materials, such as carbon nanotubes, graphene, or metal oxides, to create a hybrid material with enhanced properties.The addition of other materials can improve the conductivity, increase the surface area, and enhance the electrochemical properties of the MXene composite.Many studies have reported promising results for MXene composite supercapacitors.However, challenges remain in terms of scaling up production and improving the long-term stability and performance of these devices.

MOF-MXene hybrid structures
So far, two important classes of inorganic materials, MXenes, and MOFs have been introduced in detail.The rapid growth of MOF/MXene structures is evident from the increasing trend of publications since 2017 (Scheme 1(a)).Further, Scheme 1(b) provides the timeline of MOF/MXene materials with initial stages of development a wide range of applications of MOF/MXene material.The MOF/MXene composite applied in supercapacitors offers several structural advantages.Firstly, the construction of the MOF/MXene composite helps prevent the restacking of MXene nanosheets, which is a common issue in their individual forms.This eliminates the loss of active surface area and ensures efficient ion transfer within the material.Secondly, the composite structure enhances the stability of the overall structure.The combination of MOFs and MXenes results in a synergistic effect, where the MOF component provides structural support and prevents the aggregation of MXenes.This stability improvement is crucial for maintaining the overall performance and longevity of the supercapacitor.Thirdly, the inclusion of MXenes in the MOF-based structure enhances the iontransfer and conductivity properties.MXenes have excellent electrical conductivity, and their integration with MOFs facilitates efficient charge transport throughout the composite material.This improved conductivity enhances the energy storage capacity and power characteristics of the supercapacitor.
In this regard, the construction of MOF/MXene composite eliminates restacking of MXene nanosheets, improves the stability of the structure, and enhances the ion-transfer and conductivity of MOF-based structures; as a result, the MOF/MXene composites and MOF-derived/MXene composites are promising electrode materials for energy storage applications.Overall, this review article provides a comprehensive overview of the recent advances and challenges in MXene/MOF composites and their applications in supercapacitors.They also highlight the potential opportunities for future research in this field (figure 1).We desire that our review will be a guideline for the researchers working in energy storage applications by using MOF/MXene-based chemistry.

MOF/MXene composite-based electrodes for supercapacitors
Based on the studies conducted, Ni-MOF has several advantages over other MOFs for use in supercapacitors [70][71][72].Firstly, Ni-MOF has a high porosity, which provides a large surface area for the adsorption of electrolyte ions and improves the capacitive behavior of the electrode material.Secondly, Ni-MOF has a high electrical conductivity, which enables efficient charge transfer within the electrode material.Thirdly, Ni-MOF has high thermal stability and chemical resistance, which makes it suitable for use in harsh operating environments.Moreover, Ni-MOF has been shown to exhibit excellent electrochemical properties, including a high specific capacitance, good cycling stability, and fast charge-discharge rates, which are essential for high-performance supercapacitors [73,74].Overall, these advantages make Ni-MOF a promising material for use in highperformance supercapacitors, and the combination of Ni-MOF with MXene in this study further improved its performance.
In this regard, Zhang et al reported Ni-MOF/Ti 3 C 2 T x electrodes, which are designed using a facial reflux method (figure 2(a)).In this research, Ni-MOF/Ti 3 C 2 T x and active carbon were used as positive electrodes and negative electrodes for constructing asymmetric supercapacitors, respectively.In the design of Ni-MOF/Ti 3 C 2 T x , MXene is used as a structure-directing agent for the formation of micro belts morphology and prevents the accumulation of Ni-MOF nanosheets.The presence of MXene in the final composite can guarantee fast ion transfer and provide additional electric double-layer capacitances.Qu and co-workers reported another Ni-MOF/Ti 3 C 2 T x hybrid nanosheet electrode which was prepared through the ultrasonic method.The researchers employed an ultrasonic synthesis method to combine two different substances, nickel-based metal-organic frameworks, and two-dimensional titanium carbide nanosheets.The ultrasonic method involved using high-frequency sound waves to break down and mix the two materials, resulting in hybrid nanosheets with a high surface area and good conductivity.The researchers then conducted various tests on the hybrid nanosheets, which exhibited excellent electrochemical properties, including a high specific capacitance (867.3F g −1 at 1 A g −1 ) and good stability over repeated charge/discharge cycles (87.1% after 5000 cycles at 5 A g −1 ) [76].
According to conducted research, it can be deduced that nickel-based structures are suitable candidates for high-performance supercapacitors due to their high theoretical capacity and the phase transition between Ni(II) and Ni(III) complexes [77].The synthesis of pillared-layer nickel MOFs multiplies their advantages as electrode materials [78].Pillared-layer MOFs have emerged as a promising class of materials for supercapacitor applications due to their unique structural features and properties.One of the key advantages of pillared layer MOFs is their high surface area, which allows for the efficient storage of charge [79].Additionally, their porous structure facilitates the rapid transport of ions, leading to high power density and fast charge/discharge rates.Pillared-layer MOFs also exhibit exceptional stability and durability, as they are resistant to thermal and chemical degradation.Furthermore, their tunable nature enables the optimization of their properties for specific supercapacitor applications, making them a highly versatile and customizable material [80].Overall, the use of pillared layer MOFs in supercapacitor technology offers numerous benefits, making them a promising candidate for future energy storage systems.
In this direction, MXene@Ni-based MOF composite was reported by Shasha Zheng et al (figure 3(a)).The electrochemical performance of Ni-MOF, MXene, Ni-MOF+MXene, and MXene@Ni-MOF was studied in detail.figure 3(c) and (d) illustrated the cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) curves of as-prepared electrodes.MXene@Ni-MOF displays a higher peak current than other electrodes.As a result, the MXene@Ni-MOF electrode reaches the highest specific capacitance (979 F g −1 at 0.5 A g −1 ) compared to other electrodes.Consequently, figure 3(d) confirmed that the short ion diffusion distance and rapid electronic transmission cause the longest discharge time for MXene@Ni-MOF.Overall, the design of pillared-layer Ni-MOF on MXene compared with other electrode materials enhances the electrochemical capacitive property and improves cycling stability [81].
One more effective category in supercapacitors is cobalt-based metal-organic framework electrodes.In terms of electrochemical performance, similar to Ni-MOFs, Co-MOFs have been shown to have high surface areas and good capacitance, which makes them suitable for use as supercapacitor electrodes [82].However, Co- MOFs have been found to exhibit higher specific capacitance than Ni-MOFs, meaning that they can store more charge per unit of mass.Additionally, Co-MOFs have been shown to have better cycling stability than Ni-MOFs, meaning that they can maintain their performance over repeated charge/discharge cycles [83].
As discussed above, Ramachandran et al employed a novel Co-MOF/Ti 3 C 2 T x @Ni as a binder-free supercapacitor electrode material.In-situ growth of Co-MOF and titanium carbide on nickel foam improve the intercalation pseudocapacitance Co-MOF/Ti 3 C 2 T x @Ni electrode.The interlayer pores provide more active sites and enhance the electrochemical activity of the as-prepared electrode.As a result, the maximum gravimetric capacitance for Co-MOF@Ni and Co-MOF/Ti 3 C 2 T x @Ni was calculated at 2872.5 F g −1 and 3741 F g −1 in 3 M KOH electrolyte, respectively [84].
Mixing Co-MOF and Ni-MOF to create a mixed metal material is a promising approach for developing supercapacitor electrodes with improved electrochemical performance.One advantage of using a mixed metal Co/Ni-MOF material is that it can combine the unique properties of both Co-MOF and Ni-MOF [85].For example, Co-MOFs have been shown to exhibit high specific capacitance and good cycling stability, while Ni-MOFs are known for their high conductivity and low cost.By combining these properties, a hybrid Co/Ni-MOF material could potentially have even better electrochemical performance than either material on its own.Wang et al designed the binary NiCo-MOF (NCM) on nickel foam and then combined it with MXene nanosheets by electrodeposition method (figure 4(a)).The obtained M-NC@NCM/NF was utilized as a hybrid supercapacitor for further electrochemical tests (figure 4(b)).The unique hexagonal nanosheet morphology is the main reason for enhancing electrochemical performance.Figure 4(c) illustrated the CV curves of M-NC@NCM/NF, e-NC@NCM/NF, and NCM/NF electrodes at 5 mV s −1 .According to the results, M-NC@NCM/NF electrode reaches the highest specific capacitance (2137.5 F g −1 ) at 1 A/g.The large active sites of the as-prepared electrode and the unique design of MOF/MXene composite prevent the aggregation of nanoparticles are two main reasons for improving the specific capacitance, conductivity, and cycle performance (75.3% retention after 5000 cycles) [86].
Amino-functionalized MXenes have shown promise in supercapacitor applications due to their improved electrochemical performance.Yue and co-workers reported a hierarchical composite material consisting of Ni/ Co-MOF nanoparticles supported on aminated MXene nanosheets (figure 5(a)).The amination of MXene introduced NH 2 functional groups that served as active sites for the adsorption of electrolyte ions, improving the capacitance and energy storage capacity of the electrode material.The Ni/Co-MOF nanoparticles provided additional surface area and catalytic activity, enhancing the electrochemical performance of the composite material.The structural, morphological, and electrochemical properties of the composite material were characterized and its performance as a supercapacitor electrode was evaluated (figure 5(b)-(d).The Ni/Co-MOF@aminated MXene electrode exhibited excellent stability and cycling performance, with a high capacitance retention of over 90% after 5000 cycles.The improved performance was attributed to the synergistic effects of the individual components and the strong interaction between the aminated MXene and Ni/Co-MOF nanoparticles.The potential for further functionalization of the composite material by anchoring other functional molecules or nanoparticles onto the NH 2 groups was also demonstrated.In summary, the article reports a promising electrode material for high-performance supercapacitors based on a hierarchical composite of Ni/Co-MOF@aminated MXene, which offers enhanced stability, capacitance, and potential for further functionalization [87].
Iron-based metal-organic frameworks have recently emerged as a promising class of materials for supercapacitor applications due to their unique structural and electrochemical properties.Fe-MOFs are composed of metal ions coordinated with organic ligands to form a porous framework with high surface area and tunable porosity.In terms of structural properties, Fe-MOFs generally have a higher surface area and larger pore size compared to Co-MOFs and Ni-MOFs.This can result in higher capacitance and better ion diffusion, which can lead to improved electrochemical performance.In terms of electrochemical properties, Fe-MOFs typically exhibit higher redox activity compared to Co-MOFs and Ni-MOFs.This can provide additional active sites for energy storage and contribute to the overall capacitance of the material.Fe-MOFs have been integrated with MXenes to form composite materials with enhanced electrochemical performance.The strong interactions between the Fe-MOF and MXenes result in a synergistic effect that improves the capacitance and cycling stability of the composite material.
Jia et al describes a new MIL-100(Fe)/Ti 3 C 2 hybrid for high-performance supercapacitors.The material consists of a two-dimensional MXene electrode coupled with a Fe-MOF via an interfacial pillaring method.The interfacial pillaring method enabled strong interactions between the two components, improving the electrochemical performance of the composite material.The unique 3D porous structure of the Fe-MOF facilitated ion diffusion and provided additional active sites for energy storage, resulting in high energy density and good cycling stability.Consequently, the MIL-100(Fe)/Ti 3 C 2 T x MXene electrode exhibits exceptional energy density (85.53 Wh.kg −1 ), surpassing the energy densities of other known MXene-based electrodes.The study highlights the potential for new avenues for energy storage applications using the composite material based on 2D MXene electrodes and Fe-MOF.Overall, the findings presented in this research offer valuable insights into the development of high-performance supercapacitors and contribute to the ongoing efforts to explore new composite materials for energy storage applications [88].

MOF-derived-materials and MXene composite-based electrodes for supercapacitors
MOF-derived materials have gained significant attention in recent years due to their tunable chemical and physical properties, which can be tailored for various applications [89][90][91].MOFs are a class of porous materials consisting of metal ions or clusters coordinated to organic ligands, forming a three-dimensional network structure with nanoscale cavities.By carefully manipulating the composition and synthesis of MOFs, researchers can create novel materials with unique properties that are not possible with traditional materials [92].The synthesis of MOF-derived materials is a complex and multi-step process that involves the pyrolysis of MOFs under controlled conditions.There are several synthesis methods available for MOF-derived materials, including direct pyrolysis, template-assisted pyrolysis, sol-gel conversion, hydrothermal synthesis, and microwave-assisted synthesis [93].MOF-derived materials have opened up new avenues for materials design and engineering.MOF-derived porous carbon and MOF-derived metal oxides are two categories of materials that have shown great potential for energy storage applications [94,95].By further optimizing their synthesis and properties, these materials have the potential to revolutionize fields such as supercapacitors.
The combination of MOF-derived materials with MXenes can result in hybrid materials with improved properties, such as enhanced electrical conductivity, increased surface area, and improved ion transport properties.The composition of MOF-derived materials with MXenes can be achieved by using MXenes as a template for the synthesis of MOF-derived materials.In this approach, MXenes are used as a sacrificial template, and the MOF precursors are deposited onto the MXene surface.After the MOF precursors are deposited, the MXenes are removed via an etching process, leaving behind MOF-derived materials with a porous structure.The resulting MOF-derived/MXene hybrid materials exhibit improved performance for energy storage applications due to their unique properties.For example, the combination of MOF-derived materials with MXenes can result in materials with high electrical conductivity and high surface area, leading to improved charge storage capacity and faster charge/discharge rates.The tunable pore size distribution of MOF-derived materials can also be used to optimize ion transport properties, leading to further improvements in energy density and power density.
Up to now, many researchers have investigated various materials for energy storage devices.MOFs and MOF-derived materials are highly superior porous materials as supercapacitors but also suffer from low conductivity.With the composition of MOFs and MXenes and enhancement of synergetic effect and convenient ion transfer between them, they have overcome low stability and conductivity problems.Therefore, MOF/ MXene structures have been extensively appreciated for supercapacitor application [43].

MOF-derived nanoporous carbons
MOF-derived porous carbon, a form of material derived from MOFs, can be obtained by pyrolyzing the organic components of MOFs to high temperatures.This process leads to the formation of a carbon structure that has pores and a large surface area, and the size distribution of these pores can be adjusted [96].The abundant surface area of MOF-derived porous carbon allows for the presence of more active sites that can participate in electrochemical reactions, thereby increasing its capacitance.Additionally, the porous structure facilitates the interaction between the electrolyte ions and the carbon surface, enabling faster diffusion of ions and consequently enhancing the speed of charging and discharging [97].Furthermore, the good electrical conductivity of MOF-derived porous carbon ensures efficient electron transfer, further enhancing the device's performance.The high surface area of MOF-derived porous carbon allows for more electrochemically active sites, resulting in a higher capacitance.The porous structure also provides a large surface area for electrolyte ions to interact with, leading to faster ion diffusion and higher charge/discharge rates.Additionally, the good electrical conductivity of MOF-derived porous carbon ensures efficient electron transfer, further enhancing the device's performance [55].
The first MOF-derived carbon/MXene hybrid, NC-Ti 3 C 2 T x , which was used as a supercapacitor, was reported in 2018 by the Cheng Zhang group.Designing ZIF-8 as a MOF template prevents restacking of MXene sheets so that the electrochemical capability of NC-Ti 3 C 2 T x hybrid will be improved.The obtained NC-Ti 3 C 2 T x hybrid nanosheets reflected a specific capacity of 82.8 F g −1 at 1 A g −1 .The device also exhibited cycling stability associated with 100% of the initial capacitance retention over 5000 cycles at 1 A g −1 .The improved electrochemical property of NC-Ti 3 C 2 T x was credited with the nanosheet-like structure, enhanced electronic conductivity, increased charge/discharge rate, and established synergy between the hybrid electroactive components [98].
Further, the synthesis method of the high-performance asymmetric supercapacitor based on nickel MOF anchored MXene/NPC/RGO was described by Ganiyat Olatoye et al The nickel MOF was synthesized by a hydrothermal method, and then MXene/NPC/RGO composite was prepared using a two-step process.The first step involved the preparation of the MXene/NPC composite by a hydrothermal method, followed by the addition of graphene oxide (GO) and subsequent reduction to obtain the MXene/NPC/RGO composite.Finally, the nickel MOF was anchored onto the MXene/NPC/RGO composite via a coordination reaction to form the final electrode.The resulting electrode exhibited excellent electrochemical performance.The high specific capacitance of the NZ-R-2-200/K-Ar-MXene electrode was found to be 557 C g −1 at a current density of 0.5 A/g.The cycling stability of the electrode was also evaluated, and after 5000 cycles, the specific capacitance remained at 66% of the initial value.The researchers attributed the superior electrochemical performance of the electrode to the synergistic effect of the different materials used in the electrode, which provided a high surface area and facilitated ion transport [99].

MOF-derived metal oxides
Another type of MOF-derived material is MOF-derived metal oxides.The process of deriving metal oxides from MOFs involves heating the MOF in an oxygen-rich environment, causing the organic ligands to decompose and leave behind a highly porous metal oxide with a large surface area [100][101][102].The resulting metal oxide materials have several desirable properties for supercapacitor applications, including high capacitance, fast charge/ discharge rates, and excellent cycling stability.Additionally, the porous structure of MOF-derived metal oxides allows for the efficient transport of electrolyte ions, which is critical for the performance of a supercapacitor [103].
Recent research has explored the use of MOF-derived metal oxides in combination with MXenes as electrode materials in supercapacitors.This approach combines the high surface area and porosity of MOFderived metal oxides with the excellent conductivity and mechanical properties of MXenes, resulting in a hybrid material with superior electrochemical performance.Ramachandran's group systematically reported NiO/Ti 3 C 2 T x hybrid as supercapacitor electrode materials.In the mentioned NiO/Ti 3 C 2 T x , enhancing the synergetic effect between the 3D porous Ni-MOF network and Ti 3 C 2 T x causes easy access of electrolyte ions to the surface of the electrode; hence electrochemical application has improved.The high surface area of the NiO/Ti 3 C 2 T x hybrid (72.019 m 2 g −1 ) leads to an enhancement in capacity performance.The mentioned electrode has gained a maximum specific capacity of 630.9 C g −1 at a current density of 1 A g −1 which was 1.6 times higher than that of pure NiO (376.8C g −1 ) [104].
In another research, Xie et al reported Co-Fe oxide porous nanorod derived from MOF and incorporated it into MXene film to construct a high-performance flexible supercapacitor electrode.Due to the composition of MOF-derived bimetallic oxide and MXene sheets, Co-Fe oxide/Ti 3 C 2 T X has new structural benefits.First, the presence of MXene sheets acts as a conductive binder to coat Co-Fe oxide and simplify the charge transfer alongside the preservation flexibility of the film electrode.Second, the placement of CoFe 2 O 4 between titanium carbide layers expands interlayer distance and improves electrode conductivity (figure 6(a)).Subsequently, CoFe 2 O 4/ MXene composite also showed impressive volumetric capacitance of 2467.6 F.cm −3 in a 1 M LiCl electrolyte.Figure 6(b) demonstrated the stability of the structure under repeated mechanical deformations (bending angles of 30°, 60°, 90°, and 120°).As a result, the Co-Fe oxide/Ti 3 C 2 T X = 8% device (8% device means that the mass ratio Co-Fe oxide/ Ti 3 C 2 T X (electrode active material) on the membrane is 8%.)still illustrates 101.5, 97.7, 96.8, and 92.2% capacitance for 30°, 60°, 90°, and 120°bending angles, respectively.After 10000 charge/discharge cycles, the flexible supercapacitor illustrates high capacitance retention as well as excellent cycling performance.Furthermore, since connecting the single flexible device in series with 2, 3, and 4 devices, the potential window expanded from 0.8 V to 1.6, 2.4, and 3.2 V, respectively (figure 6(c)) [105].
To understand the effect of phosphorization on MOF/MXene electrodes, the Zhang group reported NPO derived from Ni-MOF/MXene.Hierarchically porous nickel phosphate nanospheres on the MXene prevent the agglomeration of MXene nanosheets and enhance the fast electron transportation among NPO and MXene.figure 7(a)-(d) illustrated the CV curves, galvanostatic charge-discharge tests, and cycling stability tests of MXene-NPO, respectively.MXene-NPO performs an outstanding specific capacity of about 639 C g −1 at 0.5 A g −1 (figure 7(b)).Also, owing to the support of nickel phosphate into MXene layers, it reached excellent cycling stability near 10,000 cycles with 85% capacity retention (figure 7(c)) [106].

Other MOF-derived materials
Yang and co-workers utilized a method to grow CoSe 2 /Ni 3 Se 4 nanosheets on MXene nanosheets in situ, forming a MOF-derivative.Synthesis of the CoSe 2 /Ni 3 Se 4 -MXene hybrid material was carried out via an in situ hydrothermal method.The researchers used a two-step approach to synthesize the hybrid material.First, the MXene nanosheets were prepared by selectively etching the aluminum layer from the MAX phase (Ti 3 AlC 2 ) using hydrofluoric acid.Then, a hydrothermal reaction was conducted in the presence of cobalt and nickel sources, which resulted in the growth of CoSe 2 and Ni 3 Se 4 nanosheets on the surface of the MXene nanosheets.The electrochemical performance of the CoSe 2 /Ni 3 Se 4 -MXene hybrid material was evaluated using cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy.The hybrid material showed excellent electrochemical performance, with a specific capacitance of 1019 C g −1 at a current density of 1 A g −1 , which was much higher than that of the individual components.The hybrid material also exhibited good rate capability and cycling stability, with a capacitance retention of 80% after 5000 cycles.The excellent electrochemical performance of the hybrid material was attributed to the synergistic effect between the MXene nanosheets and the CoSe 2 /Ni 3 Se 4 nanosheets, which provided a high surface area, fast ion diffusion, and efficient charge transfer [107].
Further, Guo et al reported a sandwich-like porous MXene@Ni 3 S 4 /CuS material derived from Ni-MOF.The p-MXene@Ni 3 S 4 /CuS composite was synthesized through a two-step approach involving the synthesis of Ni-MOF followed by thermal decomposition.The Ni-MOF was synthesized using a solvothermal method, and the resulting precursor was then subjected to thermal decomposition to obtain the final composite material (figure 8(a)).The resulting MXene@Ni 3 S 4 /CuS composite exhibited a sandwich-like porous structure with a high specific surface area, which facilitated efficient ion transport and enhanced electrochemical performance.The electrochemical performance of the MXene@Ni 3 S 4 /CuS was evaluated using various techniques (figure 8(b)).The researchers found that the composite material had a specific capacitance of 1917 F g −1 at a current density of 1 A/g.The composite also illustrated excellent rate capability and cycling stability, with a capacitance retention of 92.4% after 30000 cycles (figure 8(c)).The results suggest that the MXene@Ni 3 S 4 /CuS composite has great potential for use in high-performance energy storage devices [108].
A summary of MOF/MXene and MOF-derived/MXene nanostructures used as electrodes in supercapacitors is provided in table 1.In this way, various MOF/MXene composites were synthesized through different methods.The unique structural stability, suitable porosity, and excellent conductivity opened new opportunities and healthy competition for this category of structures for further innovations.

Conclusion and outlook
The increasing concern over the negative environmental impact of fossil fuels has led to a focus on clean energy utilization and effective energy storage.Supercapacitors have emerged as crucial energy storage devices, and enhancing their efficiency through suitable materials is a key area of research.The utilization of porous materials like MOFs in combination with conductive materials such as MXenes sheets as electrodes has shown promise.
In summary, this article reviews MOF-MXenes supercapacitors.The synthesis of these composites is categorized into three parts: MOF-MXenes, MOF derived-MXenes, and MOF-MXenes derived.Further investigation reveals that MOF-derived-MXenes composites are more practical for energy storage, demonstrating improved conductivity and capacity.These composites are further classified into subcategories such as MOF-derived metal oxides, phosphides, selenides, sulfides, and carbon, and relevant articles are reviewed accordingly.The high capacity and cycle stability of these composites indicate a promising future in the field of electrochemistry.
However, despite the advantages offered by these composites, there are several challenges and areas for improvement: 1. Two-dimensional MOFs exhibit high surface activity, and adding more active groups to the surface can enhance their performance.Using two-dimensional MOFs for synthesizing MOF/MXene composites can eliminate the issue of MXene sheets sticking together.The use of both 3D and 2D MOFs seems promising in various applications.
2. Exploring modified materials and synthetic methods can yield improved electrochemical characteristics.Modifying MOFs through ligand exchange, metal exchange, or doping techniques can enhance conductivity and other key features for energy storage applications.For example, the conductivity of the material which is a key feature in the field of energy storage will be enhanced by converting transition metal hydroxides to metal selenides, phosphides, and sulfides.3.In spite of the fact that a variety of MXenes have been reported so far, unfortunately, a small number of them have been used in MOF-MXene composites, therefore, it is necessary to study and select other appropriate MXenes.
4. MOF-MXene composites have shown potential in various applications, including photocatalysis, catalysts, sensors, and wave absorption.Electrocatalytic applications have received significant attention, but subcategories such as ORR, HER, and NRR require more investigation and exploration despite their promising characteristics.15 5. Despite extensive research on MOFs in recent years and their proven properties in various applications, their use is primarily limited to the laboratory scale.The industrial application of recombinant MOF-MXene composites, including supercapacitors, is still in its early stages.
Overall, the results obtained with composites were all inferior to either component alone due to the synergistic effects of the properties, as described in various MOF-MXene publications.In recent years, the use of these composites in supercapacitors has evolved steadily and is used in most models in this field.The use of these composites in supercapacitors has gained traction in recent years, but further research is required to translate laboratory findings into commercial products.

Scheme 1 .
Scheme 1.(a) Bar graph data of MOF/MXene synthesis publications for various applications, and (b) The timeline of the synthesis and applications of MOF/MXene composites.

Figure 2 .
Figure 2. (a) Schematic illustration of the synthetic process of Ni-MOF and Ti 3 C 2 T x /Ni-MOF, (b) GCD profiles at a current density of 1 A g −1 , (c) stability test at a current density of 10 A g −1 [75].Reprinted from [75], © 2021 Elsevier Inc.All rights reserved.