Assembled MXene Macrostructures for Multifunctional Polymer Nanocomposites

As a thriving family of 2D nanomaterials, early transition metal carbides and carbonitrides (MXenes) are regarded as promising candidates in various applications. To enhance the mechanical robustness and environmental stability of MXenes, research on their polymer‐based nanocomposites with improved oxidation and damage tolerance has attracted increasing attention. As a reinforcing and functional filler, MXenes can also endow polymer with desired thermal/electrical properties. Compared to the powder‐based direct dispersion, assembling MXene nanosheets into MXene macrostructures (MMs) is proven as an effective strategy to unite with polymer. In particular, performance of the nanocomposites can be readily optimized by manipulating the structure of MMs and their interactions with polymer. In this Review, recent research progress on the MMs/polymer nanocomposites is summarized to provide a systematic understanding of the relationships among structures, fabrication techniques, and performance. Beyond diverse exfoliation methods for synthesizing MXene flakes, special attention is given to the assembly routes to construct mechanically robust macrostructures and hybrid techniques to build various MMs/polymer nanocomposites. Then, their applications in several areas of fundamental research and practical application are discussed, including heat/electrical conduction, electromagnetic shielding, and flexible devices. Finally, the challenges and perspectives are summarized to guide the future exploration for multifunctional MMs/polymer nanocomposites.


Introduction
Polymers have been widely applied in various fields including additive manufacturing, electronic packaging, energy technology, and biological medicine, [1][2][3][4][5][6][7] due to their light weight, versatility, strong corrosion resistance, low-cost production, and ease of manufacture.However, the intrinsically inferior thermal and electrical conductivities [8,9] of most mechanically robust polymers severely limit their applications in advanced functional fields.Therefore, intrinsic modification and composite strategies have been proposed to overcome these shortcomings.[12][13] In particular, nanostructured materials with low densities, large specific surface areas, and high surface chemical activities [14][15][16] exhibit huge advantages to work as promising filler candidates to modify polymers mechanically and functionally.
[30] To date, more As a thriving family of 2D nanomaterials, early transition metal carbides and carbonitrides (MXenes) are regarded as promising candidates in various applications.To enhance the mechanical robustness and environmental stability of MXenes, research on their polymer-based nanocomposites with improved oxidation and damage tolerance has attracted increasing attention.As a reinforcing and functional filler, MXenes can also endow polymer with desired thermal/ electrical properties.Compared to the powder-based direct dispersion, assembling MXene nanosheets into MXene macrostructures (MMs) is proven as an effective strategy to unite with polymer.In particular, performance of the nanocomposites can be readily optimized by manipulating the structure of MMs and their interactions with polymer.In this Review, recent research progress on the MMs/polymer nanocomposites is summarized to provide a systematic understanding of the relationships among structures, fabrication techniques, and performance.Beyond diverse exfoliation methods for synthesizing MXene flakes, special attention is given to the assembly routes to construct mechanically robust macrostructures and hybrid techniques to build various MMs/polymer nanocomposites.Then, their applications in several areas of fundamental research and practical application are discussed, including heat/electrical conduction, electromagnetic shielding, and flexible devices.Finally, the challenges and perspectives are summarized to guide the future exploration for multifunctional MMs/polymer nanocomposites.33] Compared with other 2D nanomaterials, layered MXenes with abundant surface functional groups can be obtained by a simple chemical etching method, and particularly the resultant rich surface functionalities can provide a versatile platform to connect with polymers via covalent or hydrogen bonding, thus forming a good interface and constructing multifunctional features. [34,35]or MXene/polymer nanocomposites, the hybrid technique plays an important role in affecting the manufacturing quality and comprehensive properties of composites.Traditionally, MXene flakes are dispersed in polymer precursors through the direct mixing method, [36] which is easy to operate but normally results in a low MXene content level, [37,38] as its solubility in nonpolar or weak polar polymers remains challenging and excessive MXene is prone to nonuniform distribution and undesired agglomerations.Alternatively, the preconstruction of assembled MXene macrostructures (MMs) such as foams, [39] films, [40] fibers, and subsequent hybridization with polymers provides another ideal solution.Attributed to the interconnected robust 3D microstructure of MMs, [41] the agglomeration of MXene nanosheets in the polymer can be avoided effectively, leading to a dramatically increased filler content and construction of continuous conduction pathways inside the polymer.Accordingly, the contact resistance between MXene flakes could be largely reduced, and the carrier mobility inside nanocomposites can be enhanced.Furthermore, the shortcoming of MXene flakes being susceptible to oxidation could be mitigated to a great extent due to the existence of polymers as protecting layers.
Over the past ten years, MXene/polymer nanocomposites have impressed many fields remarkably as they exhibit excellent mechanical, electrical, thermal, and electromagnetic shielding performance.[44] Few systematic summaries focus on hybrid techniques especially for MMenhanced polymer composites.In addition, some pioneering works have been reported recently, such as the preparation of 3D MMs via 3D printing techniques and the microstructure modulation of 3D MMs via alignment manipulation routes.Therefore, it is imperative to provide a timely and comprehensive review of the latest progress of MMs for multifunctional polymer nanocomposites.This review summarizes the recent research progress of the MMs-enhanced polymer nanocomposites to provide a systematical understanding of the relationships among MMs structures, fabrication techniques, and final performance, as shown in Figure 1.First, the typical preparation methods of MXene flakes are briefly introduced, followed by assembly strategies to construct mechanically robust macrostructures and hybrid techniques to build various MMs/polymer nanocomposites.Then, their applications in several emerging application areas are discussed, including heat/electrical conduction, electromagnetic shielding, and flexible devices.Finally, the challenges and perspectives are summarized to guide future exploration of multifunctional MMs/polymer nanocomposites.

Fabrication Methods of MXene Nanosheets
Normally, MXenes are obtained by chemically etching the A-layer element from MAX phases, with the general formula M nþ1 X n T x , where M, A, X, and T represent early d-block transition metals, main-group sp elements (e.g., Al, Si and Ge), C, and/or N elements, and surface functional groups (e.g., -O, -OH, and -F), respectively.As shown in Figure 2, owing to the diverse selection of transition metals and accessible functionalization of surficial terminal groups, a growing number of emerging MXenes with tunable properties have been reported theoretically and experimentally, such as ordered double transition metal MXenes [32] and high-entropy MXenes (HE-MXenes), [45][46][47] greatly expanding their available properties and applications.This section briefly summarizes the preparation methods of MXene nanosheets.

HF Etching
Gogotsi and Barsoum's group found that the Al layers in Ti 3 AlC 2 can be selectively etched with 50% HF, based on the principle that the bonding strength between M--A in the MXA phase is much less than that of the bond between M--X. [48]ccordion-like multilayer MXenes can be obtained by HF etching, [29] with interlayers interacting by van der Waals (vdW) forces and interlayer hydrogen bonds, [49] and the layer spacing is enlarged compared to that of the original MAX phase.Subsequently, single/few layers of MXene sheets can be stripped by ultrasonication or by inserting intercalating agents (such as DMSO [50] or TMAOH [51] ).Normally, more -O terminal groups could be obtained by low-concentration HF etching, while more defects could be induced under a high-concentration etching environment. [52]HF etching is easy to operate and suitable for etching MAX containing Al.However, HF itself is dangerous and does not satisfy the reaction requirements of green and environmental protection.

Fluoride-Based Salt Etching
Fluoride-based salt etching refers to etching MAX with fluoride salt and acid.The fundamental principle is similar to HF etching, with high-activity F À reacting with Al in the MAX phase to form fluoride, H 2 and the target MXene.Importantly, metal cations in solution and produced H 2 can expand layer spacings, [31,53,54] generating large amounts of single/fewer-layer MXene without intercalators, as shown in Figure 3a.[57][58][59] Moreover, fluoride salts alone without the addition of acids, such as NH 4 HF 2 aqueous solution and molten fluoride salt, can also be employed as efficient etchants to successfully generate MXene. [60,61]Both HF and fluoride-based salt etching methods are widely used in the laboratory, endowing MXene surfaces with abundant -F, -O, -OH functional groups, thus facilitating their dispersion in polar solvents and connections with polymers.

Chemical Vapor Deposition (CVD)
Chemical vapor deposition (CVD) is a bottom-up method that employs gas-phase compounds or elemental substances to chemically react on the substrate to generate MXene thin films.For example, Xu et al. [62] prepared lamellar Mo 2 C with CH 4 as the carbon source and molybdenum foil as the Mo source.Uniform 2D MoN was also obtained within 5 min using Mo foil at 1100 °C and NH 3 (Figure 3b). [63]Although large-sized MXene with great controllability of surface states can be produced by CVD, [64] complex and extremely rigid preparation conditions such as atmosphere and equipment are required.More importantly, lack of rich surface functional groups for as-grown bare MXene presents great difficulty in its bonding with polymers via favorable interactions, resulting in a decrease in interfacial bonding strength, which makes CVD-produced MXene unsuitable for enhancing polymer composites.

Electrochemical Etching (E-etching)
[67][68][69] The essence of generating MXene via E-etching is modulating the efficient electron transfer from Al to other elements. [70]pecifically, Hao's group developed a universal strategy for HF-free facile and rapid synthesis of 2D MXenes (e.g., Ti 2 CT x , Cr 2 CT x , and V 2 CT x ) in HCl electrolyte (Figure 3c). [71]74][75][76] E-etching usually takes only a few hours to produce MXene, and the usage of hazardous F-containing agents is avoided, thus making this method environmentally friendly.However, the yield of the electrochemical process is relatively low, with only several milligrams of MXene being produced in each batch, impeding its self-assembly to macrostructures and nanocomposites.

Lewis Acid Molten Salt Etching
The molten salt method takes advantage of the strong Lewis acidity and electron-accepting capability of transitional metal halides in their molten states.Thermodynamically, Lewis cations with higher redox potentials can effectively oxidize the A-layer atoms with lower redox potentials in the MAX phase, as shown in Figure 3d. [77]For example, Li et al. [78] synthesized novel MAX phases such as Ti 3 ZnC 2 by substituting Al sites, which could be further transformed to Cl-terminal MXenes Ti 3 C 2 Cl 2 using excess ZnCl 2 .As the redox potential differences between the cations from Lewis acid molten salt and A-species from MAX play an important role in determining the reactivity and efficiency of the etching reactions, appropriate selection of Lewis acidic molten salts is critical to converting different types of MAX precursors (such as Al, Si, Zn, Ga) to the corresponding MXenes.In addition to the commonly used ZnCl 2 , CuCl 2 was also used as an efficient etchant to produce Ti 3 C 2 T x from Ti 3 SiC 2 or Ti 3 AlC 2 . [79]Furthermore, the surface functional groups are greatly affected by the anionic type of Lewis salt, and halogen terminals, for example, -Cl, -Br, -I, -BrI, and -ClBrI, can be easily achieved. [80]

Other Synthesis Methods
Apart from the aforementioned methods, other techniques may also be employed to produce MXene nanosheets.For example, when the concentration and temperature of the alkali reach certain levels, it will react with MAX.As shown in Figure 3e, Li et al. [81] prepared multilayer Ti 3 C 2 T x with a purity of %92 wt% using the alkali-assisted hydrothermal method with a concentration [82] of 27.5 M NaOH at 270 °C.Wang et al. [82] calculated the possibility of etching different MAXs by HCl through density functional theory, which was further experimentally verified by the successful preparation of Mo 2 CT x in 12 M HCl at 140 °C.In addition, a novel molten salt-assisted E-etching method for the synthesis of Ti 3 C 2 Cl 2 without fluoride and metal impurities was reported. [83]Combining the advantages of applied voltage and increased temperature, -Oand/or -S-terminated Ti 3 C 2 T x could be obtained with greatly shortened steps of modifying the terminal groups.
Overall, MXenes prepared by different methods possess differentiated surface chemistry features, which will affect the hydrophilicity and the diffusion or adsorption of surface ions, along with tunable electronic properties such as band structure and conductivity, leading to debatable feasibility in different applications. [84,85]Particularly, for designing multifunctional MXene/polymer nanocomposites with favorable interfaces, abundant surface functional groups are preferentially desired to form strong interactions and good compatibility with polymers.In addition, the complexity, yield and cost of the fabrication process need to be taken into consideration in terms of the actual dimensions and application scenarios of polymer nanocomposites.

Assembly Methods of MXene Macrostructures
Although direct mixing of polymer precursors with MXene in the form of powder or dispersion is a facile and common technique for filler-enhanced polymer composites, the loading of MXene in polymers is normally limited due to the unsatisfactory dispersion of MXene in polymers and the growing tendency of agglomeration with increased filler contents.Consequently, the contact resistance between MXene sheets is high due to the lack of an effective conduction pathway, which seriously limits the strengthening effect and functional attributes of MXene on polymers.Alternatively, MMs with inherent 3D interconnected networks could essentially solve the high MXene content-induced local aggregation and performance degradation.The strategy is similar to the utilization of robust graphene foams as 3D-reinforcing and conductive carbon-based fillers to modify polymers, [14,86] and it could provide MXene architecture with a well-preserved large specific surface area, high porosity, and short ion transport distance.Moreover, the crosslinking structure of the polymer will not be damaged, and a stable conduction pathway can be constructed inside the nanocomposite, exhibiting huge advantages over the traditional direct mixing method.In this section, various assembly methods to construct MMs have been reviewed, as schematically summarized in Figure 4.

Vacuum Filtration
Owing to the unique 2D anisotropic lamellar feature, macroscale MXene thin films could be obtained through layer-by-layer assembly from stably dispersed precursor solutions based on the pressure difference to effectively remove solvents.The quality of the membrane is highly dependent on the concentration of MXene solutions, as well as the inherent lateral dimension, thickness, and surface chemistry of the exfoliated MXene nanosheets.For instance, a film composed of large-area MXene sheets synthesized via a power-focused delamination strategy has more remarkable mechanical properties and electrical conductivity than a membrane assembled by small-sized MXene platelets. [87]urthermore, introducing extra additives may improve the mutual effect of MXene flakes during the cofiltration process (Figure 5a). [88]In particular, external ions or small molecules could be employed to assist the filtration process, as expanded nanochannels and enhanced interlayer interactions between negatively charged MXene layers could be created by hydrogen bonding or electrostatic interactions.For example, Ding et al. [89] selected a positively charged Fe(OH) 3 colloidal solution to intercalate MXene, thus producing a larger transport channel and rougher surface morphology.In addition, Wan et al. [90] introduced carboxy-methyl cellulose to form hydrogen bond connections among MXene nanosheets and optimize the toughness of the film while expanding the interplanar spacing.As a state-of-the-art laboratory-scale fabrication method, vacuum filtration is easily operated with low demand for equipment, but it is highly time-consuming to fabricate film-type MMs with low yield and compact size.

Blade Coating
Scalable manufacturing of large-area MXene films could be achieved by the commonly used blade coating method in industrial production, effectively overcoming the limitation of filtration setup dimensions and performance variation among different batches by a lab-scale vacuum filtration approach.Normally, MXene flakes with favorable aspect ratios and mechanical characteristics are suspended in an appropriate solvent to produce uniform dispersion with desirable rheological behavior, which is subsequently blade coated on the substrate under the applied shear force and then peeled off to produce a free-standing film.For example, Zhang et al. [91] prepared a strong 1 m-long and Reproduced with permission. [88]Copyright 2020, National Academy of Science.b) Illustration of blade coating and digital photograph for 1 m-long MXene film.Reproduced with permission. [91]Copyright 2020, Wiley-VCH.c) Preparation process for gelation and freeze drying of MXene and GO, and the SEM image of composite aerogel.Reproduced with permission. [96]Copyright 2022, American Chemical Society.
10 cm-wide Ti 3 C 2 T x film by blade coating (Figure 5b) and optimized the tensile strength and electrical conductivity of the thick film by precisely adjusting the blade speed, height, and dispersion state.In addition, heteroatom doping and extra nanomaterial functionalization of the MXene dispersions before blade coating provide alternative modification approaches to further enhance the mechanics and stability of the as-cast MXene films.For example, Liao et al. [92] used S-, N-doped MXene and reduced graphene oxide to generate a hybrid film with mechanical strength as high as 45 MPa and capacitance cycling stability up to 98%.

Gelation and Freeze Drying
Apart from 2D film-shaped MMs, 3D MMs, for example, aerogels with preserved hierarchically interconnected porous structures, are highly desired.Due to the intrinsic features of MXene including their superior hydrophilicity, relatively small flake size, and negative zeta potential on the surface, the direct assembly of individual MXene flakes into a stable 3D network is still challenging even at high dispersion concentrations.Various gelation agents have been developed to functionalize MXenes, thus improving their gelation capability and facilitating their assembly into interlinked macrostructures, such as the metal ions ethylenediamine [93] and hydrochloric acid and smallmolecule polymers. [94]Recently, Zhou et al. [95] investigated the influence of various metal ions on the positive ion-assisted MXene gelation process and found that trivalent Fe-linked MXene aerogels exhibit enlarged pore sizes and enhanced dielectric properties, owing to the effective destruction of electrostatic repulsion between MXene nanosheets and their preferential linkage by strong M--OH bonding with metal ions as joining sites.In addition, as another widely studied 2D nanomaterial with abundant surface functional groups, graphene oxides were also reported to facilitate the formation of 3D MXene with macroscopic architecture (Figure 5c). [96]These gelation agents enhance the coactions of MXene flakes and facilitate their connections into 3D crosslinked porous structures by freeze drying.It is worth noting that the alignment of MXene plays an important role in determining the microstructure of 3D MMs with diversified pore distributions and orientations, thus affecting the electrical and thermal conduction pathways and corresponding performance.Particularly, an in situ self-sacrificial ice template has been adopted to construct MMs via a directional freezing approach, thus forming an anisotropic microstructure and enhanced electromagnetic coupling. [97]

Dip Drying
The mechanical and conductive performance of 3D MMs prepared by freeze drying mainly depends on the structures of MXene nanosheets.Typically, large amounts of MXene flakes are required to form a solid structure.Instead, the existing 3D skeleton immersed in MXene solution is an effective strategy to reduce the dosage of MXene.Especially, MXene sheets are attached to the 3D skeleton surface by bonding or electrostatic adsorption, thus composing a connected network, which can not only improve the strength but also retain high conductivity.
For instance, Yue et al. [98] fabricated an MXene-sponge sensor by multiple dipping-drying processes (Figure 6a) and maintained excellent durability over 10 000 cycles.The toughness and conductivity of the MXene layer can be controlled by varying the number of immersions and the concentration of the MXene solution, and such a 3D porous structure shows good mechanical strength under fold and twist states.More importantly, these 3D skeletons, for example, metal foams and porous polymers, can be removed by acid etching or heating, creating hollow networks.Song et al. [99] prepared a hollow-structured MXene-polydimethylsiloxane (PDMS) pressure sensor by etching away the Ni foam encased in MXene and PDMS.Even without the support of Ni foam, MXene still retains a dense conducting path.The hollow and porous structure greatly enhances the flexibility and interfacial activity of MM hybrid composites, which has prominent advantages in stimulus-response, microwave loss, and catalysis.

Spinning
To date, spinning has become one of the most valuable methods to prepare nanofiber materials due to its advantages such as less equipment requirement, low cost, and controllable processing.MXene nanofibers can be synthesized by spinning and assembled into MM felt.Normally, the liquid squirted by the needle must have a certain viscosity to form nanofibers. Due to the weak gelation of MXene nanosheets, it is difficult to comprise pure inorganic MXene nanofibers.Fortunately, the polymers can help improve the gelation between MXene sheets and manipulate the viscosity of the solution.Therefore, a MXene/polyurethane (PU) fiber is produced by the wet-spinning technique.The Young's modulus of the fiber increased to %5997 MPa when the MXene loading reached 80%, much higher than that of pure MXene fiber (%2750 MPa). [100]Thus, adding polymers can greatly improve the mechanical properties of MXene.Similarly, Liu et al. [101] used aramid nanofibers (ANFs) to obtain core-shell ANF@MXene nanofibers (Figure 6b).Due to the protection of the ANF shell, the composite nanofibers showed a high strength of %502.9MPa and excellent chemical stability under malconditions.The highly oriented MM core inside retains a high conductivity of 3.0 Â 10 5 S m À1 .Furthermore, the compact film formed by crisscross overlap of nanofibers has more conspicuous use.Based on electrospinning, Jiang et al. [102] integrated MXene flakes with polyvinyl alcohol (PVA) for nanofiber films, which exhibited excellent triboelectric performance and may offer many promising applications in self-powered electronics.

3D Printing
As an advanced preparation technology, additive manufacturing is capable of printing 3D objects with complex structures in a controllable manner, therefore offering an opportunity to construct 3D MMs via a cost-effective and versatile route.Normally, extrusion-based direct ink writing is used to deposit MXene through a nozzle in a layer-by-layer fashion, and the colloidal MXene ink directly determines the processability and final quality of the printed architectures.The viscoelastic characteristics and shear-thinning behavior of gel-type inks need to be optimized to enable favorable fluidity, great adhesion, and well-retained shape.For instance, Yang et al. [103] reported 3D-printed Ti 3 C 2 T x scaffolds using aqueous inks in an ideal rheological state consisting of atomically thin (1-3 nm) 2D MXene nanosheets with a lateral size of %8 μm (Figure 6c).Sometimes extra additives are required to guarantee the desired printability and endow enhanced mechanical robustness and multifunctionality to the as-formed 3D MMs.For example, a mixture of nitrogen-doped Ti 3 C 2 T x , carbon nanotubes, and graphite oxide were formulated by Fan et al. [104] to fabricate customized hybrid architectures with excellent energy storage capabilities.

Hybrid Techniques for MMs/Polymer Nanocomposites
Apart from the inherent compositional and structural features of MMs reviewed in the previous section, MM hybrid with polymers also plays an essential role in determining the final performance of nanocomposites.In this section, corresponding to respective material characteristics and application scenarios, several hybrid techniques for MMs and polymers are summarized to obtain MXene/polymer composites with inherited 3D structures, stable conduction pathways, and excellent comprehensive performance.

MM Membrane and Polymer Composite
As a basic form of MM, free-standing MXene membranes present excellent flexibility, electrical conductivity, thermal stability, and electromagnetic performance, which have been extensively investigated in sensors, wearable devices, and electromagnetic interference (EMI) shielding fields.Correspondingly, various MXene membrane/polymer composites have also been designed and different fabrication techniques have been adopted.

Layer-By-Layer Assembly Method
Due to the favorable film-forming features of both MXene and polymers, individual films could be prepared, followed by alternating layer-by-layer assembly to construct multilayered composites.By this method, the thickness of the MXene and polymer layers can be easily regulated.Moreover, when mechanical deformation occurs, the polymer can prevent cracks in the MXene layer from growing into the whole membrane, which improves the mechanical strength of the composite membrane.Taking the combination of MXene and PVA as an example, alternating multilayer films of PVA/MXene were obtained by repeated casting operations (Figure 7a). [105]The thermal and electron conduction is highly dependent on the MXene layer, which enhances the thermal conductivity and EMI shielding performance.Simultaneously, the multilayer film exhibits conspicuous antidripping performance due to the oxidation of MXene to TiO 2 during combustion.
However, weak interlayer bonding occurs in simple multilayer casting, so vacuum-assisted filtration (VAF) is subsequently used to increase the connection between layers.Zhou et al. [106] prepared a closely combined alternating multilayered cellulose nanofiber (CNF)/MXene film by the VAF method (Figure 7b).The highly compact layered structure enables the hybrid film to exhibit excellent flexibility and higher mechanical properties compared to both freestanding MXene film and homogeneous CNF/MXene film.Furthermore, in addition to the polymer, introducing other high-performance inorganic materials to the MMs membrane is the key to enhancing the composite film.As shown in Figure 7c, Ma et al. [107] obtained double-layer ANF-MXene/AgNWs composite films via VAF and hot pressing.The exerted pressure and temperature are beneficial for the removal of solvent to obtain a dry film within a short time, but also strengthen the film in the meantime and greatly improve the binding effect between multiple materials.In the same way, MXene/polymer can also be combined with CuNWs, [108] CNTs, and other inorganic materials that are not easily combined to form stable, high-performance films themselves.

Direct Embedded Method
In general, the composite film obtained by the layer-by-layer assembly method will have part of the fragile MXene layers exposed to the surrounding environment, which can be easily oxidized and suffer serious performance degradation.Completely coating MXenes with stable polymers is an efficient strategy to protect them from being damaged.The embedded MXene/polymer composite can be combined by simply immersing the MMs membrane into the polymer precursor solutions.For instance, Xu et al. [109] directly embedded the MXene/CNT film into PDMS matrix and the final three-component composite was formed after fully cured (Figure 7d).The strongly embedded structure ensured the high stability and sensitivity of the sensors.More typically, Shi et al. [110] evenly wrapped PDMS on a multi-interface assembled MXene/HCFG/AgNW(p-LMHA) film, effectively protecting the internal MXene conduction layer and thus facilitating its utilization in terrible conditions.
Even though direct embedding can perform as a simple method with commercial PDMS as a universal and compatible wrapping layer, there is usually a mismatch between internal and external mechanical properties.When subjected to large  [105] Copyright 2019, Elsevier.b) The VAF process and SEM of alternating CNF/MXene film.Reproduced with permission. [106]Copyright 2020, American Chemical Society.c) Schematic illustration of double-layer ANF-MXene/AgNWs composite film by VAF and hot pressing.Reproduced with permission. [107]Copyright 2020, American Chemical Society.d) The embedding process of the MXene/ CNTs/PDMS composite film.Reproduced with permission. [109]Copyright 2021, Springer.mechanical deformation (stretching or bending), due to the fragility of the MXene film, it will deform and crack prior to the polymer shell, resulting in the failure of the conduction pathway.Hence, how to further improve the performance of MXene is one of the main focuses of current research.

MM Networks and Polymer Composites
As another basic existence form of MM, free-standing MXene network possesses a stable conduction pathway, high porosity, and large loading capacity.Moreover, the crosslinking structure of the polymer will not be damaged during composite process.Compared with 2D-structured MMs films, 3D networks exhibit similar surface functional characteristics inherited from dispersed MXene flakes but with significantly increased surface area and mechanical robustness, especially largely improved compressibility and damage tolerance.Therefore, the MMs network could function as a promising reinforcement and functional filler, and present more compounding possibilities with polymers in versatile application scenarios.

Presupport Molding and Impregnation
Typically, MXene nanosheets are constructed into 3D macrostructures as porous preforms via the abovementioned assembly strategies, [111][112][113] which are subsequently filled with polymer through ambient immersion, vacuum impregnation, or VAF, followed by final polymer solidification and composite formation.The unique 3D MXene skeleton forms a perfect internal continuous path, and the polymer is wrapped around the MXene skeleton to improve mechanical robustness and provide good protection for MXene with enhanced stability.
For instance, Wang et al. [114] prepared a loose and porous 3D Ti 3 C 2 T x skeleton directly by adhesive-free directional freeze drying (Figure 8a).MXene sheets were interconnected in the horizontal direction and stacked vertically, and the diameter of the as-formed aerogel decreased significantly with increasing MXene concentration due to stronger interlayer interactions.PDMS was filled into the MXene skeleton by vacuum impregnation, and scanning electron microscopy (SEM) images show that the micropores were completely filled with PDMS without obvious interface cracks.Similarly, Yang et al. [115] obtained a highly ordered 3D porous MXene aerogel with an average pore size of %45 μm by directional freeze drying and constructed 3D MXene/ epoxy composites by vacuum impregnation.The X-ray microcomputed tomography (CT) scan indicates no air-filled holes inside, confirming the good permeability of the epoxy in the aerogel.Overall, 3D MMs as preforms could be prepared easily with a relatively uniform porous structure and rich surface functionalities, thus forming good interfacial contact with polymers.Importantly, the conductivity of the directly mixed sample is several orders of magnitude lower than that of the presupport sample, indicating the crucial role of the effective connection and complete pathway between macrostructures and polymers.
Apart from the pure MXene skeleton with a fragile structure due to its weak gelation ability, hybrid macrostructures combining the advantages of 2D MXene flakes and other nanostructures have been designed and fabricated.1D nanomaterials, as common reinforcers, can enhance the gelation of 2D nanosheets.Particularly, MXene flakes with abundant functional groups on the surface could form hydrogen bonds with predesigned nanowires easily, thus improving the stability of the entire network structure.For example, Wang et al. [116] prepared CNF/Ti 3 C 2 T x MXene aerogels by freeze drying and then composited them with epoxy by vacuum impregnation (Figure 8b).With the support of CNF, the aerogel exhibits excellent mechanical properties via a bonding join, which ensures that it is still intact even after the process of impregnation.The storage modulus increased by 62% under the crosslinking of aerogel and epoxy.Actually, beyond CNF, various nanowires with desired composition, morphology, surface functionality, and inherent mechanical/electrical/thermal properties could be designed to adapt to feasible polymer substrates and application scenarios.For example, Liu et al. [117] combined MXenes with highly conductive AgNWs under the assistance of sodium alginate (SA).The MXene/AgNWs/epoxy nanocomposites with outstanding heat and electric transferrin performance were obtained by freeze drying and subsequent vacuum impregnation.The SA with abundant oxygen-containing functional groups compensates for the weak gelation of AgNWs and MXene by forming strong hydrogen bonds.The addition of small molecules or 1D materials has been shown to strengthen the gelation of MXene, which not only results in a remarkable MXene network but also paves the way for impregnation processes of polymers.

Dip-Coating and Capsulation Method
The dip-coating method is similar to the aforementioned presupport molding and impregnation one with the polymer being filled into the 3D porous skeleton, but herein, the MXene layer is assembled on the preformed 3D structure by immersing the sponge into the MXene solution with adjustable thickness by regulating the precursor concentration.Jia et al. [118] obtained MXene@polyaniline/m-polypropylene (MXene@PANI/mPP) foam beads as building blocks by immersing PANI/mPP in dispersed MXene solution (Figure 8c), which were then assembled into a 3D construct followed by the encapsulation of PDMS.Due to the interaction between MXene and PANI and the encapsulation of PDMS, the MXene layer would not shed even after suffering repetitive compression cycles.More significantly, the thickness of the MXene shell can be simply adjusted by the number of dips, thus to realize the asymmetric MXene network.
Nonetheless, electrostatic adsorption is a weak interaction.When MXene forms a bond with the preformed framework, the connection will be more stable.Particularly, polydopamine (PDA) has strong adhesion to nanomaterials without causing extra surface defects and could be utilized as a promising interfacial transition layer.Luo et al. [119] utilized PDA to modify the fabric fiber and immersed it into a Ti 3 C 2 T x dispersion followed by PDMS coating (Figure 8d).The interfacial interaction between the MXene and fiber is strengthened through hydrogen bonding under the assistance of PDA as a connecting bridge.Meanwhile, the external PDMS layer could be stably bound with MXene and provide protection for the interior structure as well.
In summary, dip coating could be regarded as an extension of presupport molding method.Normally, a large amount of MXenes is required to directly construct 3D MMs, and their mechanical properties are not sufficiently strong along with large fluctuations compared to the commercial 3D skeleton used by the dip-coating method.Essentially, the partial MXene skeleton has been replaced by accessible and preformed commercial macrostructures including foams or fibers, reducing the required MXene loading while maintaining the continuous conductive pathway via interconnected MXene assembly.The dip-coating method is very simple and cost-effective.Owing to the diverse surface functional groups and favorable defective sites, the MXene coating functions as a good connection bridge between multiple polymers while forming a complete network structure, endowing polymers with enhanced multifunctionalities.

Template-Sacrificing and Encapsulation Method
The template-sacrificing method refers to that the existing 3D skeleton is immersed in MXene solution and then externally MXene/PDMS by presupport molding and impregnation.Reproduced with permission. [114]Copyright 2019, Elsevier.b) The illustration of CNF/Ti 3 C 2 T x /epoxy and the SEM for pore size of aerogel at different contents.Reproduced under the terms of CC By license. [116]Copyright 2020, The Authors, published by AAAS.c) Dip-coating process of MXene on PAIN/mPP.Reproduced with permission. [118]Copyright 2020, Elsevier.d) The preparation process and structure of textile/MXene/PDMS.Reproduced with permission. [119]Copyright 2020, Elsevier.e) The metal foam as a template for the sacrifice process of MXene/PDMS.Reproduced with permission. [99]Copyright 2019, Elsevier.f ) The polymer as a template for 3D porous PANI@MXene.Reproduced with permission. [121]opyright 2019, Wiley-VCH.encapsulated with polymer.After molding, the inner 3D skeleton is removed by chemical etching or heating to prepare hollow MXene/polymer composites.The hollow structure possesses low density, large specific surface area, and high load capacity.The hollow 3D MXene skeleton is substantially interconnected and thus maintains excellent electrical conductivity and flexibility.
Commercially available metal foams have been extensively explored as templates to construct hollow 3D MXene skeletons due to their lightweight and porous structure.As shown in Figure 8e, Song et al. [99] immersed Ni foam in the MXene dispersion, penetrated PDMS, and finally removed the Ni foam by HCl.The coated PDMS maintained the 3D hollow structure of the MXene layer.The coating parameters were carefully optimized, as with increasing coating times and MXene concentrations, MXene sheets would keep accumulating to an undesired level where the holes of the Ni foam were blocked and the stable combination between PDMS and MXene was hindered.Similarly, Nguyen et al. [120] deposited graphene and Fe 3 O 4 @Ti 3 C 2 T x on Ni foam successively, and the surface was encapsulated with PDMS and then etched away from the Ni foam.This process results in porous and hollow structures, which exhibit excellent electromagnetic loss performance and potential energy storage efficiency due to the existence of abundant interfaces.
Apart from metal foams, various low-cost polymer spheres, for example, polystyrene (PS), are also promising template candidates, which can be packed to form ordered structures and subsequently removed by complete decomposition at high temperatures.Li et al. [121] cofiltered MXene and PS spheres and annealed the mixture at 450 °C to decompose PS, followed by the PANI solution being dropped on the surface (Figure 8f ).The obtained 3D porous PANI@MXene films possess excellent conductivity, flexibility, and bending stability.However, the porous structure might be damaged during stretching, which causes a great challenge to the application of MXene-based devices.In another report, Chen et al. [122] prepared a unique wrinkled PDMS/Ti 3 C 2 T x composite based on the thermal shrinkage of PS to solve the abovementioned problem.The folded structure can change the direction of crack growth to prevent crack expansion and effectively relieve stress concentration, and a high stretchability up to 100% and strain-invariant conductivity in a strain range of 0-100% could be achieved consequently.
In summary, unlike dispersed MXene flakes directly modified polymer composites, assembled MMs (e.g., aerogels, coatings, films) could be employed to enhance polymers via various compositing techniques and present significantly improved carrier transport characteristics due to the formation of interconnected networks and favorable interfacial bonding.High MXene loadings could be achieved without the concern of agglomeration-induced local stress concentration, performance degradation, and heterogeneity.At present, the interaction between the polymer and MXene, as well as its enhancement mechanism and corresponding interface engineering strategy, needs to be further explored due to the important role it plays in the efficient stress and electron transfer process.In addition, the microstructure design, particularly the orientation of MXene flakes in the macrostructure skeleton, and the optimal loadings and compositions, such as the introduction of other 0D/1D/2D nanostructures, also affects the final composition, structure, and performance of the MMs/polymer nanocomposites.Considering their different application scenarios and specific requirements in accordance with differentiated material characteristics, it is necessary to choose appropriate hybrid techniques benefiting from their respective advantages.

Applications of MM/Polymer Nanocomposites
Theoretical calculations show that MXenes with bare surfaces exhibit high elastic modulus [123] and electrical conductivity. [124]n particular, Young's modulus is significantly higher than that of graphene with a similar thickness. [125]More importantly, the electrical transport and magnetic conductance characteristics of MXene could be adjusted effectively and facilely by adjusting their surface terminal groups, therefore exhibiting tunable semiconductor/magnetic properties. [126]At present, it has been measured experimentally that the Ti 3 C 2 T x sheet possesses a high Young's modulus of 330 GPa and tensile strength of 17.3 GPa. [127]Some MXenes containing transitional metal species such as Cr, V, and Mn also possess magnetic properties, [128,129] and a robust intrinsic ferromagnetic response with a saturation magnetic moment of 0.013 emu g À1 has been observed on V 2 C MXene nanosheets with small-angle twisting. [130]Apart from unique mechanical, electrical, and magnetic properties, MXene also shows great potential to interact with electromagnetic waves via dipolar polarizations, interfacial reflections, and conductive loss, thus exhibiting excellent EMI shielding effectiveness. [131]Combined with the great processing and interface connection characteristics, MXenes are promising candidates to modify polymers to enhance their mechanical robustness with targeting multifunctionalities. Currently, MMenhanced polymer nanocomposites have been extensively investigated in various fields and will be introduced in detail in the following section.

Heat Dissipation
Polymers are widely used in electronic packaging due to their light weight, low cost, and easy processing.However, the inherent low thermal conductivity of traditional polymers makes it difficult to dissipate produced heat promptly and satisfy the current needs of rapidly growing electronic industries.Improving the thermal conductivity of polymers is of scientific and practical significance.However, based on the commonly accepted phonon heat conduction mechanism, in long-range disordered polymers, the anharmonic motion of the lattice and defects will cause phonon scattering, thus affecting the thermal conductivity of the polymer. [8]At present, composites of polymers and materials with high thermal conductivity (e.g., metal, graphene, MXene) are a general and effective solution. [132]Nevertheless, dispersing MXene into the polymer precursor will lead to the disconnection of the thermally conductive region, and more composite interfaces will increase the interfacial loss of phonon transmission, failing to improve thermal conductivity of the composite.Alternatively, it turned out to be effective through taking advantage of MMs inside the polymer and therefore constructing a continuous thermal conduction pathway.As shown in Figure 9a, heat tends to transfer along MXene sheets due to MXene showing higher thermal conductivity than PDMS, and interconnected MXene could reduce the interfacial thermal resistance of composites. [114]When the MXene content is 2.5 vol%, the thermal conductivity can reach up to 0.576 W (m K) À1 , which is 220% of PDMS.However, excessive MXene may affect the orientation of the 3D skeleton and reduce the pore size, resulting in difficulty in completely filling PDMS and affecting the thermal conductivity.To further improve the performance, it is necessary to introduce extra materials as binders to construct a more stable and high-performance multicomponent composite.
As excellent intrinsic thermally conductive materials, thermal fillers such as Ag are normally utilized to enhance thermal conductivity.For example, Ag nanoparticles (NPs) are deposited on the surface of MXene to obtain MXene/AgNPs aerogels by directional freeze drying, and then epoxy is infused with vacuumassisted impregnation.The 3D MXene/AgNPs aerogel aligned in the plane orientation functions as the heat transfer framework for epoxy nanocomposite.At an MXene/AgNPs loading of 15.1 vol%, the in-plane thermal conductivity reaches 2.65 W (m K) À1 , which is 1225% higher than that of pristine epoxy. [133]The 0D NPs can only be embedded in the MXene sheets, while the 1D materials can shuttle between the 3D skeleton, which not only enhances the bonding between MXene sheets but also enriches the conduction pathway inside, potentially improving the thermal conductivity of the polymer nanocomposite.An ordered MXene/AgNWs aerogel was prepared by a directional freezing-drying method, followed by subsequent compositing with epoxy by vacuum impregnation. [117]The thermal conductivity of the composite with low filler content (8.2 wt%, MXene: AgNWs = 1:1) can reach 2.34 W (m K) À1 , which is 1014% higher than that of epoxy (Figure 9b).
In addition to the selection of appropriate thermally conductive fillers from the materials perspective, the precise regulation of the microstructure by tuning the 3D skeleton density and connection states, along with favorable pore size and distribution, also plays a critical role in determining the final thermal conductive performance.Guo et al. [134] prepared 3D carbon fiber Figure 9. Thermal conductivity of MM-enhanced polymer composites.a) The through-plane heat transfer performance and enhanced thermal conductivity of 3D MXene/PDMS.Reproduced with permission. [114]Copyright 2019, Elsevier.b) Schematic of MXene/AgNWs/epoxy and the thermal conductivity of different samples.Reproduced with permission. [117]Copyright 2021, Elsevier.c) Thermal conductivity and cycling stability of carbon fiber/MXene/ epoxy at different contents.Reproduced with permission. [134]Copyright 2020, Elsevier.
(CF)/MXene foam with CF functioning as a directional conduction skeleton and MXene as a connecting bridge.The thermal conductivity of the CF/MXene/epoxy composites was elevated to a high level (9.68 W (m K) À1 ) when the ratio of mixed packing reached 30.2 wt% and MXene accounted for 20% of mixed packing, a 4509% improvement over epoxy (Figure 9c).When the proportion of mixed fillers decreases, the thermal conductivity decreases greatly.This might be related to the enlarged pores between the CF skeletons, where MXene is no longer able to bridge individual CFs, thus aggravating the phonon loss in the polymer and reducing the thermal conductivity.
Importantly, the architecture of thermal conductive fillers plays a dominant role in affecting the final performance of polymer composites, which makes the construction of nanofiller into macrostructure more attractive.Apart from aforementioned Ag as extra heat conductor, both graphene and BN exhibit higher intrinsic thermal conductivities.When mixed directly with polymer in a muddled order, these materials fail to deliver the desired performance.Normally, more fillers are needed to obviously hoist the phonon transmission.For example, the disordered sample requires a much higher filler content (e.g., 40 wt% hBN of hBN/RGO@Ni(OH) 2 /epoxy [22] and 50 wt% f-BN of PI/f-BN/glycidyl methacrylate-grafted graphene [135] ) than the ordered sample (8.2 wt% MXene/AgNWs) to achieve similar thermal conductivity.The contrast illustrates that the orderly arranged 3D crosslinked skeleton will heighten the thermal conductivity of the polymer more evidently than the disordered skeleton.The 3D MM network provides a complete thermal conductivity path and reduces the thermal contact resistance.In addition, the rich surface functional groups of MXene can spontaneously form hydrogen bonds with polymers.Without complex surface modification, the difficulty of recombination is greatly reduced.

Conducting Polymer Composites
According to theoretical calculations, MXenes exhibit electrical conductivity comparable to that of metals.Experimentally, the conductivity of clay-like MXene prepared by LiF and HCl as etchants reaches 1500 S cm À1 , [55] and the conductivity of a selfassembled 214 nm-thick MXene film is %15 100 S cm À1 . [91]dditionally, the superior single-layer MXene sheet shows ultrahigh conductive property.Zeraati et al. [136] synthesized Ti 3 C 2 T x flakes via the so-called "wvaporated nitrogen minimally intensive layer delamination" route with a conductivity as high as 24 000 S cm À1 .As shown in Figure 10, the electroconductibility of Ti 3 C 2 T x nanosheets fabricated by a power-focused delamination strategy up to 25 000 S cmÀ1 [87] is the highest reported value for Ti 3 C 2 T x to date.Due to its outstanding electrical conductivity mentioned above, MXene has been chosen as a functional filler to enhance the electrical characteristics of polymers.Compared with traditional metal fillers, MXenes possess a lower density and stronger interface bonding with the polymer matrix.In addition, compared to 0D and 1D conductive fillers, 2D fillers normally exhibit higher aspect ratios and lower contact resistance, which make them readily accessible to construct 3D conductive networks with continuous carrier transfer pathways. [137] the one hand, the free-standing MXene membrane could be utilized as a functioning layer to modify electrically insulating polymer films with significantly improved electrical conductivity.For example, PVA is a degradable polymer material with low price and film-forming ability.However, the high insulativity of PVA severely hinders its potential application in membrane electrodes, electromagnetism, batteries, etc.To improve the conductivity of PVA, Jin et al. [105] prepared multilayer composite films with alternating PVA/MXene by the layering assembly method.As the content of MXene increased from 7.5 to 19.5 wt%, the electrical conductivity increased from 17 to 716 S m À1 (Figure 11a).The continuous MXene layer provides a complete conductive pathway, which significantly improves the electrical conductivity of the PVA/MXene film.The thickness of MXene layer grows with mass fraction, which benefits the expansion of the conductive route.
For the same purpose, the multilayer CNF/MXene film is composed in a similar way. [106]Depending on the number of layers of MXene, the conductivity of the composite film can reach 82À621 S m À1 (Figure 11b), much higher than that of the mixed CNF/MXene film (2 S m À1 ).Although the insulating CNF layers reveal improved mechanical strength, they hamper carrier transport between MXene layers, which makes the performance of the composite films in the in-plane and cross-plane directions express a great difference.The in-plane conductivity is nearly 9 orders of magnitude higher than the crossplane conductivity.Therefore, Ma et al. [107] prepared a double-layered ANF-MXene/ AgNWs composite film.The bistratal structure relieves the problem of poor crossplane conductivity and retains the remarkable tensile strength of ANF.The conductivity increases from 4.2 to 3725.6 S cm À1 as the content of MXene/AgNWs increases from 5 to 80 wt%, which is much higher than the conductivity of ANF/MXene/AgNWs film (0.1-98.9 S cm À1 ) obtained by homogeneous mixing (Figure 11c).
On the other hand, constructing 3D structured fillers from 2D material assembly shows more potential to enhance the conductivity of polymers, as previous studies have shown that the carrier transport efficiency usually follows the trend of 3D > 2D > 1D > 0D fillers.Indeed, higher electrical conductivity could be achieved by 3D architecture construction via presupport molding and impregnation strategies.Yang et al. [115] reported an improved electrical conductivity of 2.1 S cm À1 for the MXene/ epoxy composite with 10 wt% MXene addition, far higher than the value of less than 10 À3 S cm À1 for the directly mixed sample with the same MXene content.
Wang et al. [138] obtained a porous 3D Ti 3 C 2 T x /C hybrid foam by sol-gel and thermal reduction and corresponding MXene/C/ epoxy nanocomposites by subsequent vacuum-assisted impregnation and curing processes.The synergistic effect between MXene and the carbon skeleton facilitates efficient electron transfer with conductivity increasing to 184 S m À1 with 1.64 wt % MXene and 2.61 wt% carbon (Figure 11d).However, the orientation of MXene nanosheets is chaotic here, which could be aligned regularly to further provide a shortened electron transfer pathway.For example, the orderly oriented CNF/Ti 3 C 2 T x aerogel/epoxy composite with an MXene content of 1.38 vol% was finally obtained with electrical conductivity reaching 1672 S m À1 along the axial direction and 1283 S m À1 along the radial direction, which is much higher than that of the aerogel with a random structure. [116]part from orientation, the intrinsic conductivity of the 3D MXene skeleton could be further enhanced with optimized MXene loading and microstructure modulation.Wu et al. [139] prepared a preferentially aligned Ti 3 C 2 T x aerogel by directional freezing-drying with the assistance of SA, followed by immersion in PDMS solution to obtain a porous MXene/PDMS composite.Along with abundant and uniformly distributed SA conducive to promoting ion transport and PMDS providing a thin coating layer, an ultrahigh conductivity of 2211 S m À1 was obtained for the composite aerogel when the content of MXene reached 95% (Figure 11e).
In contrast, when MXene nanosheets are directly mixed into epoxy, the penetration threshold is 0.85 wt% with very limited improvement to the electrical conductivity of the composite to only 4.52 Â 10 À4 S m À1 , [140] far less than the value achieved by the construction of an inherent 3D MXene skeleton.This can be mainly attributed to the island-shape distribution of high-conductivity MXene in the insulating polymer, in which case the internal contact resistance is extremely high due to the lack of a continuous conductive network.For the MMs/polymer nanocomposite, the completed pathway is formed and electrons are transmitted mainly through the grid structure, where the contact resistance is largely reduced and the electrical conductivity is mainly affected by the filler architecture and its intrinsic characteristics.Particularly, assembled MXenes with an oriented arrangement facilitate the formation of conductive pathways with the smallest contact resistance and the shortest migration distance along the arrangement direction, superior to those with a random structure.

Electromagnetic Interference Shielding
With the advent of the 5G era and rapid development of various electronic devices, EMI problems have become progressively worse for both human beings and electronic devices, [141,142] and exploration of effective EMI shielding materials has attracted  [87] Copyright 2022, Wiley-VCH.[145] Although metal-based materials are widely adopted due to their excellent electrical conductivity and EMI shielding properties, [146,147] their high density and poor chemical resistance limit their applications in many situations.Polymers, which are lightweight, corrosion resistant, and easy to process, can compensate for the shortcomings of metals and they are able to be combined with other functional materials to obtain equivalent performance. [148,149]As introduced in the previous section, 2D MXenes exhibit good electrical conductivity.Accordingly, the 40 μm-thick Ti 3 C 2 T x MXene film reveals a fascinating SE T of 84 dB. [150]The outstanding EMI SE of MXenes could make them promising candidates to enhance the shielding performance of polymer matrices.In addition, abundant polar functional groups and defect sites on the surface of MXene are beneficial to induce dipolar polarization loss, as well as multiple reflections caused by stacking MXene sheets and MXene/polymer interfaces, enabling MXene composites to possess excellent EMI shielding performance. [151]ased on the excellent EMI shielding efficiency (SE) of MXene, the binary MXene/polymer normally shows decent performance.Liu et al. [117] produced 3D MXene/epoxy composite by presupport molding and impregnation, and an EMI SE of 42.5 dB with a thickness of 2 mm was obtained with 4.3 wt% Ti 3 C 2 T x being introduced.Yuan et al. [152] developed an MXene/melamine sponge switch by dip coating.As shown in Figure 12a, the switch (thickness % 10 mm) presents adjustable EMI SE from 0 to 43.1 dB in the S-band, which synchronously satisfies the demand of shielding and transmission.At similar thicknesses, improving the conductivity is the key to enhancing the EMI SE.Annealing has been shown to reduce the termination groups of MXenes and improve electrical conductivity. [153]ccordingly, the 2 mm-thick Ti 3 C 2 T x /C/epoxy expresses a maximum EMI SE of 46 dB with 1.64 wt% MXene addition (Figure 12b). [138]By annealing, the composites can achieve higher EMI SE with less MXene content.
Furthermore, increasing the reflection and scattering times inside the material is also conducive to advancing the EMI shielding performance. [154]For example, Song et al. [155] constructed epoxy-based composite with rGO as a support frame, an Al 2 O 3 honeycomb plate as a template, and assembled MXene via electrostatic adsorption as a booster.The multiple boundaries and synergistic effects of rGO/MXene/epoxy greatly enhance the EMI SE value of %55 dB with 1.2 wt% rGO and 3.3 wt% MXene, which is 5 times that of rGO/epoxy (11 dB) (Figure 12c).In addition, the aperture of the aerogel can be tuned by simply controlling the amount of MXene to achieve multiple reflection losses.Wang et al. [116] manipulated the cell size of CNF/MXene aerogel to decrease from 21.3 to 10.2 μm by increasing the content of MXene per unit volume.With the decrease in pore size, the EMI SE of 2 mm-thick CNF/MXene/epoxy increases from 22 to 74 dB, due to the increase in electrical conductivity and scattering times.Moreover, the composites Reproduced with permission. [105]Copyright 2019, Elsevier.b) The in-plane and through-plane conductivities of CNF/MXene films.Reproduced with permission. [106]Copyright 2020, American Chemical Society.c) The double-layered ANF-MXene/AgNWs with outstanding electrical conductivity and stability.Reproduced with permission. [107]Copyright 2020, American Chemical Society.d) Schematic illustration of the fabrication process and electrical conductivity for Ti 3 C 2 T x /C hybrid foam.Reproduced with permission. [138]Copyright 2019, Elsevier.e) The fabrication of the MXene/PDMS composite and its electrical conductivity.Reproduced with permission. [139]Copyright 2019, Elsevier.
fabricated by the template-sacrificing method normally exhibit unique hollow characteristics with low density and abundant reflection interfaces, and the multicomponent system also has an advantage over the unit one due to increased interfacial loss, thus to show better EMI SE.The hollow porous structure of Fe 3 O 4 @Ti 3 C 2 T x /graphene/PDMS has a porosity of 47%.Under a thickness of 1 mm, the average EMI SE reaches 80 dB in the X-band (8.2-12.4GHz) and 77 dB at the Ka-band (26.5-40GHz), respectively (Figure 12d). [120]The crosslinked MMs inside the hybrid systems and complete conductive pathways partially reflected EM waves, and the waves almost dissipated in multiple internal scatters, which generated exceptional efficiency.
The 3D MM network prominently heightens the EMI SE of the polymer.Recently, flexible, ultrathin EMI shielding materials have attracted tremendous attention in the military and civil fields.Free-standing MMs membranes are stacked by a large number of MXene sheets, which exhibit ultrahigh flexibility and electrical conductivity.In addition, the film/polymer is similar to 3D MMs network that can dissipate EM waves through multiple reflections, which also expresses outstanding EMI SE.For instance, the multilayer structure CNF/MXene film with a thickness of 35 μm shows the EMI SE of 40 dB in the X-band and K-band, and a high specific shielding effectiveness up to 7029 dB cm 2 g À1 (Figure 13a). [106]On this basis, more potential enhancement can be stimulated by gradient design and interface control of multilayer structures to further improve the EMI shielding effectiveness.Jia et al. [105] constructed a triple-layered copper nanowire/Ti 3 C 2 T x /ANFs hybrid film with a sandwich structure by designing gradient.Under CuNWs/Ti 3 C 2 T x content of 60 wt%, the EMI SE of the triple-layered hybrid film reached 46.67 dB and specific shielding effectiveness of 9120.6 dB cm 2 g À1 at a thickness of 43 μm (Figure 13b), which was due to the high reflection, multiple scattering loss, and polar loss of the three-layer gradient structure.Furthermore, the p-LMHA films were assembled by gradient design and an interface control strategy. [110]The EMI SE of p-LMHA can be regulated by changing the content of graphene oxide-wrapped hollow carbon fiber, revealing the contribution of porous microstructure to improve the ohmic loss of EM waves.The hollow carbon fiber at the surface greatly improves the surface/air impedance matching, which allows the EM waves to enter the interior to achieve the absorption loss-based shielding performance.As shown in Figure 13c, benefiting from the efficient electron conduction pathway and optimized design of hierarchical microstructures, the p-LMHA film demonstrated an EMI SE Reproduced with permission. [152]Copyright 2021, Elsevier.b) The SE T , SE A , and SE R for Ti 3 C 2 T x /C/epoxy with a thickness of 2 mm.Reproduced with permission. [138]Copyright 2019, Elsevier.c) Schematic for the fabrication of rGO/MXene/epoxy and its EMI SE.Reproduced with permission. [155]Copyright 2020, Elsevier.d) The EMI shielding mechanism in Fe 3 O 4 @Ti 3 C 2 T x /graphene/PDMS and its EMI SE in the X-band and Ka-band.Reproduced with permission. [120]Copyright 2020, Elsevier. of 73.2 dB and a specific shielding effectiveness of 31150.1 dB cm 2 g À1 at a thickness of 11 μm.As summarized in Table 1, more MMs/polymer composites exhibit excellent EMI shielding performance.
On the whole, owing to the intrinsic advantages of MXenes including low density, good electrical conductivity, and excellent EMI shielding effectiveness, and the unique features of MMs with a 3D interconnected microstructure providing a stable and efficient transmission path for electrons, MMs/polymer nanocomposites present huge potential as lightweight EMI shielding materials with controllable mechanical deformation capabilities.

Flexible Sensing
In recent years, flexible sensors have aroused increasingly more attention [156][157][158] due to their application prospects in wearable devices, [159] electronic skin, memristors, [160] and other intelligent fields. [161]The sensors are mainly composed of flexible substrates, flexional sensing materials, and electrodes, and performance of the sensor is predominantly determined by the sensing materials.MXene emerges as one of the most promising materials for the fabrication of flexible sensors due to its flexibility, excellent conductivity, and large specific surface area. [162,163]iezoresistive sensors are discovered to sense the change in the current reaction in response to external pressure situations. [164,165]Yue et al. [98] attached a layer of MXene to the sponge surface as a sensor.The detection line of the composite sensor is as low as 9 Pa with the MXene solution of 1 mg mL À1 .As shown in Figure 14a, the MXene/sponge sensor also possesses high sensitivity with a wide range (147 kPa À1 for the less than 5.37 kPa region and 442 kPa À1 for the 5.37-18.56kPa region) and maintains outstanding durability over 10 000 cycles.In addition, the fabric is another representative flexible porous substrate to manufacture sensing devices.However, the MXene sheets that simply adsorb on the surface of the fabric will easily fall off after repeated bending cycles.The interface interaction between the MXene and fabric is strengthened through hydrogen bonding of PDA. [119]The stable interface consolidated the conductive path and strain sensitivity, which resulted in ΔR/R 0 reaching 0.68, 1.36, 2.15, and 3.50 at strains of 10%, 30%, 50%, and 70%, respectively.Under the condition of 10% strain and 1500 mm min À1 , the response time is 242 ms, and the recovery time is 219 ms.The rapid response and recovery ensured the accurate detection of continuous movements.More importantly, the MXene-based textile shows a high-resistance temperature coefficient of resistance (TCR = À0.7% °CÀ1 % À1.8% °CÀ1 ) in the temperature range of 20-50 °C and can be used as an excellent temperature sensor to monitor the body or environment (Figure 14b).
Apart from attaching MXene flakes to sponges and fabrics, the MXene film can be used as a flexible substrate directly because of its remarkable flexibility and strain response properties. [166,167]or example, the conductivity of MXene/PDMS film with a pleated structure is relatively stable under tensile strain (0-100%). [122]As a pressure sensor, the MXene-based film exhibited a high sensitivity of 66.3 nF kPa À1 , excellent cycle stability of 1000 cycles, and subtle pressure-monitoring capability at 50% tensile strain.However, MXene films are prone to cracking during the tensile process.Adding a suitable binder is a feasible Reproduced with permission. [106]Copyright 2020, American Chemical Society.b) The structure and EMI SE of CuNWs/MXene/ANFs film with different CuNWs/MXene contents.Reproduced with permission. [108]Copyright 2022, Elsevier.c) Illustration of EMI shielding mechanisms for p-LMHA film and the EMI SE under diverse thicknesses and structures.Reproduced with permission. [110]Copyright 2018, Elsevier.b) Strain and temperature-sensing performance of the MXene/PDMS textile.Reproduced with permission. [119]Copyright 2020, Elsevier.c) The sensing properties of the CNT/MXene/PDMS sensor.Reproduced with permission. [109]opyright 2021, Springer.way to ensure connectivity among flakes.Yang et al. [109] fabricated high-performance CNTs/MXene/PDMS strain sensors that showed ultrastable durability of over 1000 cycles (Figure 14c), and the relative resistance changes of the strain sensor were less than 8.3% and 9.5% when the temperature increased from À20 °C to 80 °C and ultrasonic treatment for 120 min, respectively.In addition, the sensor also exhibited good resistance sensitivity (gauge factor is 13.3) in the sensing range of 60.3%, which could be applied to wearable healthcare monitoring.Similarly, Yang et al. [168] obtained MXene/graphene composite film by VAF and then embedded the film in PDMS to construct MXene/graphene/PDMS composite sensor with a layered structure.The interconnections of individual 2D Ti 3 C 2 T x and graphene nanosheets actively promote the formation of a transitional layer and ensure the integrity and durability of the entire device.As shown in Figure 15, sensor based on the Ti 3 C 2 T x /graphene film exhibited ultrahigh gauge factors of 190.8 and 1148.2 at wide sensing ranges of 0-52.6% and 52.6-74.1%,respectively.More importantly, the detection limit is as low as 0.025%, and superb cycling stabilities with 5000 cycles under the strain of 40% were achieved, which can accurately monitor arbitrary human movements.
In summary, MXenes have been increasingly studied in the field of flexible sensor components due to their excellent conductivity, flexibility, and ease of control.Compared with other sensing materials, such as carbon materials (graphene, CNTs) [169,170] and metals (metal nanowires, NPs), [171] MXene possesses the advantage of being hydrophilic with abundant surface functional groups.Hydrophilicity allows MXene to be dispersed and recombined under more benign conditions.The abundant surface functional groups help the accessible construction of stable composites of MXene with other materials or flexible substrates, which enables MXene-based flexible sensors with good interfacial connections and excellent sensing performance and durability.However, the oxidation susceptibility and high cost of MXene also limit the large-scale application of MXene flexible sensors, and there is still a great gap to fill between laboratory research and commercial applications.

Energy and Environment
MMs-enhanced polymer composites normally inherit the unique conductive network from assembled MMs structure.[174][175] For example, the MXene/PVA-KOH film offered an impressive volumetric capacitance of %530 F cm À3 at 2 mV s À1 . [176]However, one-fold MXene capacitor unsatisfied the need for next-generation flexible energy storage devices, and elastic capacitors hold great potential for emerging applications. [177]Feng et al. [178] obtained an MXene/acrylic elastomer membrane, which showed an ultrahigh tensile strain of 800%.Moreover, it exhibited a supernal specific capacitance of 470 mF cm À2 and retained over 90% capacitance after 1000 cycles of stretching (Figure 16a).Nevertheless, pure MXenes are easily oxidized at the anodic potential.To construct a high-performance device, Li et al. prepared a 3D porous MXene/PANI film as positive electrode.The film exhibited a high volumetric capacitance of 1632 F cm À3 and rate capability of 827 F cm À3 at 5000 mV s À1 . [121]ore importantly, MXene can also be used as active component and conductive agent for battery electrodes, leading to an enhanced electrode capacity and excellent cycle performance. [179]ong et al. [180] demonstrated the a-Ti 3 C 2 -S/PP electrode for Li-S batteries.Copyright 2019, Elsevier.
Particularly, due to the synergy of the MXene layers, the rate capacity reached 288 mAh g À1 at 10C, and the cycling stability was over 200 times at 2C, which showed more fascinating performance than the conventional Al electrode.Meanwhile, Shi et al. [181] constructed MXene/MF as an elastic scaffold for dendrite-free, high-areal-capacity alkali anodes (Figure 16b).For the Li anode, MXene/MF achieves a high current density of 50 mA cm À2 , areal capacity of 50 mAh cm À2 , coulombic efficiency of 99% and long lifetime of 3800 h, which demonstrate its potential for alkali-metal batteries.
In addition to the burgeoning energy applications, MXene also displays great power in environmental protection (such as organic molecular sieving, [182] gradient energy harvesting, [183,184] and sewage disposal [185] ).In particular, MXene shows great advantages in the field of photothermal conversion (the internal conversion efficiency is 100%). [186]Solar water generation by photothermal performance is attractive to alleviate the scarcity of freshwater. [187]Lei et al. [188] designed a honeycomb-like MXene/fabric solar evaporator, which showed a high solar efficiency of 93.5% and an evaporation rate of 1.62 kg m À2 h À1 under 1 kW m À2 .Furthermore, regulating structures and heterogeneous connections could enhance the performance of the evaporator.Wang et al. [189] synthesized a MoS 2 -Mo 5 N 6 /MF evaporator with a porous structure and nitrogen-rich nitride heterostructures, which exhibited an outstanding evaporation rate of 2.31 kg m À2 h À1 (Figure 16c) and a conversion efficiency of 106.6%, providing a high-quality device for solar energy harvesting.

Other Applications
Owing to their excellent electromagnetic properties, MXenes not only shine in the field of electromagnetic shielding, but also in the field of electromagnetic wave absorption. [97,190]Qing et al. [191] directly mixed Ti 3 C 2 T x and epoxy, and the reflection loss (RL min ) was measured to be only À11 dB at a thickness of 1.4 mm of the composite, making it difficult to satisfy the requirements of practical use.Nevertheless, composite materials with 3D porous structures can generate multiple interfaces.Hence, further studies are needed by adjusting impedance matching, increasing scattering paths, and improving microwave absorption performance.For example, a 3D MXene foam-modified epoxy composite exhibited an RL min of À41.46 dB and an effective bandwidth of 4.67 GHz (10.58-15.25 GHz), which is significantly superior to that prepared via direct mixing. [192]Copyright 2021, American Chemical Society.b) The fabrication process of MXene/MF scaffold, and corresponding coulombic efficiency and long-life cycling of MXene-MF-Li electrode.Reproduced with permission. [181]Copyright 2020, American Chemical Society.c) The surface temperature and evaporation cycle performance of MF, MoS 2 /MF, and MoS 2 -Mo 5 N 6 /MF solar evaporator under 1 kW m À2 solar irradiation.Reproduced with permission. [189]Copyright 2022, Elsevier.

Conclusions and Outlook
Since 2011, MXene has gone through a decade of glory.For MXene/polymer nanocomposites, MXene is as an enhancing and functional filler to improve the mechanical, electrical, thermal, and electromagnetic performance of polymers, while polymers protect MXene from oxidation and destruction.In this article, MMs, involving synthetic routes and assembly strategies, and their composites with polymers concerning hybrid techniques and multifunctional applications, have been reviewed.Although various methods have been reported to synthesize MXene and corresponding macrostructures with abundant terminal groups, an environmentally friendly technique in a scalable and repeatable manner is highly desired.In addition, detailed information about the MXene/polymer interface is still lacking concerning the interfacial atomic arrangement and bonding mechanism.Advanced characterization techniques and simulation methods are necessary to further explore the fundamental science.Moreover, the relationship between the choice of MMs, the processing techniques, and the final performance of the nanocomposite is still not clear at present, and a series of systematic in-depth investigations are required to provide clear guidelines toward future material design and performance optimizations.Overall, due to the intrinsic advantages of MXene and the unique 3D interconnected microstructure of MMs, MMs/polymer composites with favorable interfacial bonding show great potential in various applications, such as electrical and thermal enhancement, flexible sensor, and EMI shielding.However, compared with the long development history of traditional materials, MXene is still in its infancy.Especially in the field of MMs-enhanced polymer composites, there are still some scientific and technological problems to be solved.
First, to satisfy the needs of practical applications, the preparation of MMs should be controllable with excellent batch repeatability and ecofriendly technical flow.However, quantitative and precise manipulation of key factors such as surface functional groups, dimensions, density, and pore size cannot be fully achieved using the current techniques.This is mainly related to the hard-to-control assembly process from single-layer MXene nanosheets, where both distributions of surface terminal groups determined by the etching step and pore size and wall thickness of MMs affected by the experimental conditions are crucial prerequisites to acquiring predesigned well-constructed 3D MXene skeletons.Furthermore, a unified standard should be established to evaluate the MMs/polymer composites regarding their manufacturing quality and service performance due to the boom in this area, thus providing better comparability among various studies.
Second, although it has been widely claimed that the formation of a stable interfacial connection between MXene and polymer is mainly attributed to the bonding between the functional groups on the surface of MXene and the polymer chains, which will further enhance the composite properties, including mechanical robustness and multifunctionalities, as well as improved oxidation resistance and stability of MXene, the mechanism of atom arrangement and electron transfer at the composite interface has not been deeply studied.A more explicit bonding mechanism based on defined experimental results and quantitatively determined binding degree between MMs and polymers is not clearly understood.As the hybrid mechanism is the basis of studying the composite processing strategies and guiding the preparation techniques, it is necessary to further explore the interfacial states of MMs and polymers through theoretical simulation and newly emerged characterization methods to provide theoretical guidance for the hybrid strategies of MXene and polymers.
Another challenge hindering the practical application of MMs/polymer composites is that the quantitative relationship between the composition-processing-property of the multicomponent materials is still unclear.Most of the current studies only remain in the initial stage of composite preparation and performance testing, without deeply exploring the interrelated effects of composition design and processing strategy on the composite performance.In addition, the microcosmic synergy between multiple components still needs to be studied, and the explanation of the mechanism in the previous work is relatively shallow, which makes it difficult to guide the design and performance optimization of composites.Therefore, theoretical models and experimental results need to be explored simultaneously to deeply understand the structure-effect relationship and thus to point out the direction for material design and optimization.
Finally, most current researches mainly focus on the performance enhancement effect of MMs on polymers, but ignore the synergistic effect of polymers themselves, leading to certain limitations in the selection of composite polymer types.This is not only detrimental to the understanding of complicated hybrid mechanisms, but also limits the application directions for MMs/polymer composites.In the future, more different types of polymers with adjustable linkage groups, rheological behaviors, and functional characteristics might be explored to enhance MMs and construct composites with excellent comprehensive properties.
From a long-term perspective, the large-scale production of MMs with controllable surface terminal functional groups and adjustable pore size and morphology distributions in an ecofriendly and beyond laboratory manner is the first priority to potentially achieve the wide applications of MMs/polymer nanocomposites.In addition, instead of conducting similarly superficial studies, comprehensive and systematic in-depth investigations are highly desirable to clarify the interface synergy effect and the internal relationship among composition-processproperty, thus guiding future material designs and optimizations.Undoubtedly, the results concerning MMs/polymer composites obtained in just a few years are exciting enough.With the progress of preparation technology and the in-depth study of the underlying mechanisms, it is expected that composites with excellent comprehensive properties will be prepared sophisticatedly and controllably and further applied in a wide range of application scenarios with designable and achievable multifunctionalities.

Figure 4 .
Figure 4. Schematic of the assembly routes for MMs.

Figure 5 .
Figure 5. Assembly methods for manufacturing MMs.a) The course for vacuum filtration of MXene nanosheets and the structure of as-obtained MXene film.Reproduced with permission.[88]Copyright 2020, National Academy of Science.b) Illustration of blade coating and digital photograph for 1 m-long MXene film.Reproduced with permission.[91]Copyright 2020, Wiley-VCH.c) Preparation process for gelation and freeze drying of MXene and GO, and the SEM image of composite aerogel.Reproduced with permission.[96]Copyright 2022, American Chemical Society.

Figure 7 .
Figure 7. Schematic for hybrid techniques of MMs membrane and polymer.a) The multiple casting processes and corresponding SEM image of alternating multilayered PVA/MXene.Reproduced with permission.[105]Copyright 2019, Elsevier.b) The VAF process and SEM of alternating CNF/MXene film.Reproduced with permission.[106]Copyright 2020, American Chemical Society.c) Schematic illustration of double-layer ANF-MXene/AgNWs composite film by VAF and hot pressing.Reproduced with permission.[107]Copyright 2020, American Chemical Society.d) The embedding process of the MXene/ CNTs/PDMS composite film.Reproduced with permission.[109]Copyright 2021, Springer.

Figure 8 .
Figure 8. Schematic for hybrid techniques of 3D MMs network and polymer.a) The fabrication process and SEM images of the 3D MXene aerogel and 3DMXene/PDMS by presupport molding and impregnation.Reproduced with permission.[114]Copyright 2019, Elsevier.b) The illustration of CNF/Ti 3 C 2 T x /epoxy and the SEM for pore size of aerogel at different contents.Reproduced under the terms of CC By license.[116]Copyright 2020, The Authors, published by AAAS.c) Dip-coating process of MXene on PAIN/mPP.Reproduced with permission.[118]Copyright 2020, Elsevier.d) The preparation process and structure of textile/MXene/PDMS.Reproduced with permission.[119]Copyright 2020, Elsevier.e) The metal foam as a template for the sacrifice process of MXene/PDMS.Reproduced with permission.[99]Copyright 2019, Elsevier.f ) The polymer as a template for 3D porous PANI@MXene.Reproduced with permission.[121]Copyright 2019, Wiley-VCH.

Figure 10 .
Figure 10.The reported electrical conductivity of large-sized MXene nanosheets.a,b) The measurement principle for electrical conductivity of a single nanosheet.c) The sheet resistivity of a large MXene at V G = 0. d) Conductivity comparison of Ti 3 C 2 T x prepared by different methods.Reproduced with permission.[87]Copyright 2022, Wiley-VCH.

Figure 11 .
Figure 11.Electrical conductivity of MM-enhanced polymer composites.a) Electrical conductivity of multilayered MXene/PVA films with different MXene contents.Reproduced with permission.[105]Copyright 2019, Elsevier.b) The in-plane and through-plane conductivities of CNF/MXene films.Reproduced with permission.[106]Copyright 2020, American Chemical Society.c) The double-layered ANF-MXene/AgNWs with outstanding electrical conductivity and stability.Reproduced with permission.[107]Copyright 2020, American Chemical Society.d) Schematic illustration of the fabrication process and electrical conductivity for Ti 3 C 2 T x /C hybrid foam.Reproduced with permission.[138]Copyright 2019, Elsevier.e) The fabrication of the MXene/PDMS composite and its electrical conductivity.Reproduced with permission.[139]Copyright 2019, Elsevier.

Figure 12 .
Figure 12.EMI SE of 3D MM-enhanced polymer composites.a) EMI shielding switch control of the MXene/melamine sponge.Reproduced with permission.[152]Copyright 2021, Elsevier.b) The SE T , SE A , and SE R for Ti 3 C 2 T x /C/epoxy with a thickness of 2 mm.Reproduced with permission.[138]Copyright 2019, Elsevier.c) Schematic for the fabrication of rGO/MXene/epoxy and its EMI SE.Reproduced with permission.[155]Copyright 2020, Elsevier.d) The EMI shielding mechanism in Fe 3 O 4 @Ti 3 C 2 T x /graphene/PDMS and its EMI SE in the X-band and Ka-band.Reproduced with permission.[120]Copyright 2020, Elsevier.

Figure 13 .
Figure 13.EMI SE of 2D-structured MMs films-enhanced polymer composites.a) EMI shielding performance at the X-band and K-band of CNF/MXene film.Reproduced with permission.[106]Copyright 2020, American Chemical Society.b) The structure and EMI SE of CuNWs/MXene/ANFs film with different CuNWs/MXene contents.Reproduced with permission.[108]Copyright 2022, Elsevier.c) Illustration of EMI shielding mechanisms for p-LMHA film and the EMI SE under diverse thicknesses and structures.Reproduced with permission.[110]Copyright 2022, American Chemical Society.

Figure 14 .
Figure 14.MM-enhanced polymer composites as flexible sensors.a) The low detection line, high sensitivity, and durability of the MXene/sponge pressure sensor.Reproduced with permission.[98]Copyright 2018, Elsevier.b) Strain and temperature-sensing performance of the MXene/PDMS textile.Reproduced with permission.[119]Copyright 2020, Elsevier.c) The sensing properties of the CNT/MXene/PDMS sensor.Reproduced with permission.[109]Copyright 2021, Springer.

Figure 15 .
Figure 15.The sensing properties of the MXene/graphene/PDMS composite sensor.a) Various stretching states.b) Resistance variation-strain curve.c) Cycling stability under the strain of 40%.d) Resistance-strain curves of stretch/release cycles.e) The current signals of different movements under the monitoring of the composite sensor.Reproduced with permission.[168]Copyright 2019, Elsevier.

Figure 16 .
Figure16.Energy and environment-related applications of MMs-enhanced polymer composite.a) The C-V curves and capacitance retention of MXene/ acrylic elastomer with different area strains.Reproduced with permission.[178]Copyright 2021, American Chemical Society.b) The fabrication process of MXene/MF scaffold, and corresponding coulombic efficiency and long-life cycling of MXene-MF-Li electrode.Reproduced with permission.[181]Copyright 2020, American Chemical Society.c) The surface temperature and evaporation cycle performance of MF, MoS 2 /MF, and MoS 2 -Mo 5 N 6 /MF solar evaporator under 1 kW m À2 solar irradiation.Reproduced with permission.[189]Copyright 2022, Elsevier.

Table 1 .
Comparison of EMI shielding performance for MMs/polymer composites.