MXene-Carbon Nanotube Composites: Properties and Applications

Today, MXenes and their composites have shown attractive capabilities in numerous fields of electronics, co-catalysis/photocatalysis, sensing/imaging, batteries/supercapacitors, electromagnetic interference (EMI) shielding, tissue engineering/regenerative medicine, drug delivery, cancer theranostics, and soft robotics. In this aspect, MXene-carbon nanotube (CNT) composites have been widely constructed with improved environmental stability, excellent electrical conductivity, and robust mechanical properties, providing great opportunities for designing modern and intelligent systems with diagnostic/therapeutic, electronic, and environmental applications. MXenes with unique architectures, large specific surface areas, ease of functionalization, and high electrical conductivity have been employed for hybridization with CNTs with superb heat conductivity, electrical conductivity, and fascinating mechanical features. However, most of the studies have centered around their electronic, EMI shielding, catalytic, and sensing applications; thus, the need for research on biomedical and diagnostic/therapeutic applications of these materials ought to be given more attention. The photothermal conversion efficiency, selectivity/sensitivity, environmental stability/recyclability, biocompatibility/toxicity, long-term biosafety, stimuli-responsiveness features, and clinical translation studies are among the most crucial research aspects that still need to be comprehensively investigated. Although limited explorations have focused on MXene-CNT composites, future studies should be planned on the optimization of reaction/synthesis conditions, surface functionalization, and toxicological evaluations. Herein, most recent advancements pertaining to the applications of MXene-CNT composites in sensing, catalysis, supercapacitors/batteries, EMI shielding, water treatment/pollutants removal are highlighted, focusing on current trends, challenges, and future outlooks.


Introduction
MXenes as new celebrated materials with unique electronic, optical, thermal, mechanical, magnetic, and electrochemical characteristics have garnered considerable research interest, opening great opportunities for (photo)catalysis, lithium (Li)-sulfur (S), and sodium-ion batteries, electromagnetic interference (EMI) shielding, and supercapacitors, as well as tissue engineering, drug delivery, cancer (nano)theranostics, and regenerative medicine [1][2][3][4][5][6][7]; these materials have shown excellent optoelectronic properties as well as high metallic conductivity and optical transmittance, which make them promising candidates for photovoltaic systems [8,9]. MXenes with general formula of M n+1 X n T x (n = 1-3) have been synthesized using a wide variety of techniques, including etching processes [10], electrochemical synthesis [11], electrochemical fabrication [12], the urea glass route [13], glass route [13], microwave-assisted synthesis [14], chemical vapor deposition [15], hydrothermal production [16], ultrasonic-assisted synthesis [17], atomic layer deposition [18], among others. On the other hand, CNTs with suitable interfacial area, multifunctionality, and needle-like shapes have been fabricated using carbon arc discharge, spray pyrolysis, chemical vapor deposition, flame synthesis, laser-ablation, etc. [19,20]. The unique properties of CNTs such as good heat conduction, tensile strength, flexibility, hollow monolithic structures, suitable penetrability, and optico-electrical features make them promising candidates in environmental sciences and bio-and nanomedicine domains [21,22]. However, there are still important challenges regarding the flexibility, functionality, and stability of MXenes; thus, improvements are still required, particularly by applying hybridization techniques, suitable functionalization/modification, and optimization of synthesis/reaction conditions [23][24][25][26][27][28]. In this context, different types of MXene-based composites have been fabricated, such as MXene-polymer and MXene-oxide composites [29][30][31][32]. One of the recently introduced hybridized composites is the MXene-carbon nanotube (CNT) structure [33], with excellent electrochemical performances and unique mechanical properties ( Figure 1). These hybrid composites pave the way toward the synthesis of composites with different sensing, catalysis, water treatment, pollutant removal, EMI shielding, and supercapacitor/battery applications [1,[34][35][36][37][38]. The hybridization of MXenes with CNTs can improve their mechanical properties and alkaline resistance [39]. For instance, three-dimensional (3D) nano-fillers of MXene (Ti3C2Tx)/CNTs were constructed with high stability and enhanced interfacial adhesion between adjacent fiber yarns along with the increased tensile and flexural strength, showing excellent surface roughness and flexural strength even after 120 days of immersion in alkali solutions [39]. Moreover, interfacial adhesion could be enhanced in MXene-based membranes via the hybridization with CNTs, wherein the robust π-π interaction and van der Waals forces of CNTs impel the close-fitting of MXene nanosheets to improve anti-swelling features and interfacial bonding forces [40]. It has been revealed that CNTs with the effects of confined-space mass transfer and excellent mechanical strength could improve the performance of MXene-based membranes (~5 times) after hybridization, showing great potential in resource recovery and energy-saving [40]. The hybridization of MXenes with CNTs can improve their mechanical properties and alkaline resistance [39]. For instance, three-dimensional (3D) nano-fillers of MXene (Ti 3 C 2 T x )/CNTs were constructed with high stability and enhanced interfacial adhesion between adjacent fiber yarns along with the increased tensile and flexural strength, showing excellent surface roughness and flexural strength even after 120 days of immersion in alkali solutions [39]. Moreover, interfacial adhesion could be enhanced in MXene-based membranes via the hybridization with CNTs, wherein the robust π-π interaction and van der Waals forces of CNTs impel the close-fitting of MXene nanosheets to improve anti-swelling features and interfacial bonding forces [40]. It has been revealed that CNTs with the effects of confined-space mass transfer and excellent mechanical strength could improve the performance of MXene-based membranes (~5 times) after hybridization, showing great potential in resource recovery and energy-saving [40]. In addition, MXene-CNT (nano)composites exhibit enhanced electrochemical features and can be applied directly in designing electrodes in supercapacitors [41]. Li et al. [41] reported the preparation of MXene-CNT nanocomposite paper, which could be employed in Li ion batteries with improved cycling stability and high capacitance [41]. Herein, the most recent advancements regarding the properties and applications of MXene-CNT composites are cogitated, focusing on recent trends, crucial challenges, and future directions. The versatile applications of MXene-CNT hybrid composites in the field of catalysis/electrocatalysis, energy storage, sensors, EMI shielding, and pollutant removal/water treatment are discussed. Hopefully, this review will encourage researchers for further comprehensive studies on such composites with fascinating properties and multifunctionality. Other dimensions of the applications of such materials, especially in biomedicine, energy storage, soft robotics, electronic skins, and smart/wearable sensors still require multidisciplinary research for industrialization/commercialization.

MXene-CNT Composites
A variety of MXene-CNT composites has been designed with versatile applications in catalysis, electrocatalysis, EMI shielding, energy storage, water treatment, sensors, etc. (Table 1) [29][30][31][32]42]. These composites have been synthesized using mechanical mixing, self-assembly, co-dispersion, electrophoretic deposition, in-situ growth of CNTs on MXenes by chemical vapor deposition, thermal treatment, and microwave-assisted and hydrothermal processes. Among several MXene-CNT architectures introduced, aerogels and foams with 3D structures can be considered as attractive candidates with unique mechanical properties and excellent permeability for gas/liquid, offering great opportunities for various applications [33,43]. However, future studies ought to focus on systematic analysis of thermal, mechanical, porosity, and electrical/electronic features of MXene-CNT composites. It appears that the synergistic effects of MXenes and CNTs in hybrid structures can reduce/prevent the severe stacking issues of two-dimensional (2D) structures, thus improving the properties for different applications [33,44]. Li-based batteries and hybrid capacitors MXene (Ti 3 C 2 T x )-CNT composite High energy and power density [49]   The wide linear range was 0.1-50 µmol L −1 , the low limit of detection (LOD) was 0.06 µmol L −1 , the recovery rate was~84.00-125.60% [68] Electrochemical sensor for detection of ochratoxin A MXene (Ti 3 C 2 )-multi-walled CNT composite The concentration range was 0.09-10 µmol L −1 with LOD of 0.028 µmol L −1 [69] Microwave absorption High surface areas (55.6-103.7 m 2 g −1 ), moderate magnetism (19.8-24.6 emu g −1 ). Improved thermal oxidation stability (≥307 • C) [

Sensing
MXenes have shown fascinating electronic, optical, mechanical, and thermal features, which make them suitable candidates for hybridization with other materials to provide suitable (nano)composites with versatile diagnostic applications [3,29,[76][77][78][79]. Because of their excellent potential in electrochemical sensing, nanohybrids of MXene (Ti 3 C 2 T x ) nanoribbons/CNTs were constructed for the modification of glassy carbon electrode as sensing platforms, offering a new strategy without electrodeposition for specific detecting Hg 2+ [80].
MXenes have displayed excellent spontaneous adsorption and reduction potential towards Hg 2+ , causing the self-reduction of Hg 2+ to be pre-concentrated on the electrode surfaces. Notably, the electronic features of CNTs could help to provide an electrodeposition-free and sensitive sensor with robust sensing activities for analyzing Hg 2+ (the LOD was~5.2 nM, and the linearity range was~0.01-7.0 µM) [80].
An electrochemical sensor based on hierarchical porous MXene/amino CNT composites and the benefits of the molecularly imprinted method were synthesized with suitable stability for the specific detection of fisetin (the LOD was~1.0 nmol L −1 , and the linearity range was~0.003-20.0 µmol L −1 ) ( Figure 2). The extraction time, pH of the supporting electrolyte, incubation time, the ratio of functional monomer to template molecule, and the polymerization cycles are important factors to increase the sensitivity/selectivity of this type of sensor [81]. Similarly, an electrochemical biosensor was designed from MXene (Ti 3 C 2 T X ) and multi-walled CNTs to provide composite encompassing molecularly imprinted polymers for the sensitive detection of amyloid-β protein. The sensor with advantages of stability, repeatability, reproducibility, and reusability could successfully detect this protein with a linear range of 1.0-100.0 fg mL −1 and an LOD of 0.3 fg mL −1 [82]. Ni et al. [83] designed a dopamine electrochemical sensor with high sensitivity from MXene (Ti 3 C 2 ), graphitized multi-walled CNTs, and ZnO nanospheres. Accordingly, the sensor displayed significant sensitivity (16 A/M) with an LOD of~3.2 nM and a wide linear range (0.01-30 µM) under the optimal experimental conditions. The sensor exhibited improved stability and anti-interference potential as well as suitable accuracy in human serum samples, opening new opportunities for designing smart electrochemical sensors. Despite the importance of synthesis conditions and functionalization processes, the optimization of reaction and experimental conditions is also vital to obtain high yields and improved results [83]. Xia et al. [84] reported the design of molecularly imprinted polymer film through the electropolymerization of 1 H-pyrrole-3-carboxylicacid in the presence of diethylstilbestrol on CNTs/Cu 2 O nanoparticles/MXene (Ti 3 C 2 T x )-modified electrodes. The application of MXenes with accordion-like structures provided suitable electrical conductivity and enabled the immobilization of Cu 2 O nanoparticles. On the other hand, CNTs were deployed for improving the sensitivity of these electrochemical sensors with a wide linear response range (0.01-70 µM) and an LOD of 6 nM, providing suitable sensors with good stability for the reliable detection of diethylstilbestrol (a nonsteroidal estrogen medication) [84].
Designing intelligent sensors for skin and healthcare monitoring with enough flexibility is one the important fields of science in wearable electronics, helping to evaluate the physical signals from the human body. In this respect, challenges regarding their sensitivity, stability, and working range still persist [85]. For instance, to design sensors for wearable healthcare monitoring, CNT/Ti 3 C 2 T x /polydimethylsiloxane composites were constructed [86]. The sensor exhibited reliable responses at various frequencies and longterm cycling durability (>1000 cycles), as well as the benefits of excellent anti-interference to temperature alteration and water washing. This sensor with suitable applicability for monitoring human joint motions could be also deployed for real-time monitoring of the electrocardiogram (ECG) signals and joint movements [86]. In another study, a foam-shaped strain sensor was designed using MXene (Ti 3 C 2 T x ), multi-walled CNTs, and thermoplastic polyurethane [85]. Accordingly, this sensor displayed a wide working strain range of~100% with high sensitivity, showing excellent gas permeability with an appropriate elastic modulus close to that of skin, making it suitable to be applied as a wearable sensor. Subtle and large human movements could be detected, along with gesture recognition, offering a flexible and wearable sensor [85]. Cai et al. [87] introduced a flexible strain sensor constructed from MXene (Ti 3 C 2 T x ) and CNTs with excellent electric features consisting of 2D MXene nano-stacks and conductive/stretchable one-dimensional (1D) CNT crossing; the sensor exhibited significant stretchability (up to 130%) and sensitivity along with a tunable sensing range (30-130% strain), showing great reliability and stability (more than 5000 cycles). Such sensors can be employed for real-time monitoring and in situ analyzing of physiological signals with health and sporting purposes [87]. ticles/MXene (Ti3C2Tx)-modified electrodes. The application of MXenes with accordion-like structures provided suitable electrical conductivity and enabled the immobilization of Cu2O nanoparticles. On the other hand, CNTs were deployed for improving the sensitivity of these electrochemical sensors with a wide linear response range (0.01-70 μM) and an LOD of 6 nM, providing suitable sensors with good stability for the reliable detection of diethylstilbestrol (a nonsteroidal estrogen medication) [84].  With developments in wearable electronics, designing sensors with multifunctionality and high performance to monitor abilities is one the important challenges at the industrial scale. In one study, hybrid composites of multi-walled CNTs and MXenes were designed for lifetime health monitoring applications via a simple mixing and spray vacuum filtration technique ( Figure 3) [88]. The composite films exhibited good stability to respond to changes in the resin state; after 1000 cycles of fatigue evaluations, the hybrid film sensors maintained a good response and consistency in response to the external force for a long time. These film sensors with a long service time, significant response sensitivity, excellent synchronization to stretching, and solid real-time function could successfully monitor gel spots with various positions and thicknesses in real time. However, more explorations are still required using modern and cutting-edge technologies to adjust the bonding between MXene/CNT film sensors and composite laminates. Notably, the stretchability and flexibility of these film sensors can still be improved by optimization processes [88]. In addition, a wearable 3D porous polyurethane sponge sensor was designed deploying MXene/CNT composites constructed through a simple ultrasonic dip-coating technique [89]. The application of these composites could improve the measurement capability of the sensor from a widecompressive strain range (−80%) to a wide-stretching strain range (60%), providing a sensor with great electrical response and stability (>5000 cycles) to detect vocal vibrations, human movements, subtle expressions, etc. [89]. MXene-coated carboxylated CNTs/carboxymethyl chitosan aerogels were designed with superb electrical stability and repeatability for sensing piezo-resistive pressure ( Figure 4) [90]. The designed sensor exhibited a fast response time (62 ms) and a wide detection range (up to 80 kPa) owing to the synergism of the dual-conductive MXene-CNT network. It could be employed for the sensitive monitoring of human motions such as joint movements, walking, finger tapping, running, and pronunciation detection [90]. Chen et al. [91] reported the design of conductive films using dendriticlamellar MXene/carbon nanotube/polyvinylpyrrolidone electrodes, providing flexible composites for wearable tactile sensors and artificial skin. As a result, the LOD was ~0.69 MXene-coated carboxylated CNTs/carboxymethyl chitosan aerogels were designed with superb electrical stability and repeatability for sensing piezo-resistive pressure (Figure 4) [90]. The designed sensor exhibited a fast response time (62 ms) and a wide detection range (up to 80 kPa) owing to the synergism of the dual-conductive MXene-CNT network. It could be employed for the sensitive monitoring of human motions such as joint movements, walking, finger tapping, running, and pronunciation detection [90]. Chen et al. [91] reported the design of conductive films using dendritic-lamellar MXene/carbon nanotube/polyvinylpyrrolidone electrodes, providing flexible composites for wearable tactile sensors and artificial skin. As a result, the LOD was~0.69 Pa with a response time of~48 ms after the evaluation of the sensor for pulse measurement and voice recognition [91]. Pa with a response time of ∼48 ms after the evaluation of the sensor for pulse measurement and voice recognition [91].

Wastewater Treatment/Remediation and Pollutants Removal
MXenes (Ti3C2Tx) intercalated with CNTs were designed to provide thin-film nanocomposite membranes with a superb degree of cross-linking and roughness [92]. Compared to the composite membranes without interlayered structures, the prepared membranes exhibited a water flux four times higher and a lower specific salt flux. These membranes could be efficiently deployed for wastewater treatment with improved rejection of ammonia nitrogen [92]. An MXene (Ti3C2Tx)-CNT hybrid membrane was designed with robust potential for capturing precious metal ions from solutions ( Figure 5) [93]. As a result, gold (Au) ions (~99.8%) could be obtained from a solution with an extremely low concentration of 20 ppm. The considerable redox activity of C-Ti-OH could

Wastewater Treatment/Remediation and Pollutants Removal
MXenes (Ti 3 C 2 T x ) intercalated with CNTs were designed to provide thin-film nanocomposite membranes with a superb degree of cross-linking and roughness [92]. Compared to the composite membranes without interlayered structures, the prepared membranes exhibited a water flux four times higher and a lower specific salt flux. These membranes could be efficiently deployed for wastewater treatment with improved rejection of ammonia nitrogen [92]. An MXene (Ti 3 C 2 T x )-CNT hybrid membrane was designed with robust potential for capturing precious metal ions from solutions ( Figure 5) [93]. As a result, gold (Au) ions (~99.8%) could be obtained from a solution with an extremely low concentration of 20 ppm. The considerable redox activity of C-Ti-OH could be the reason for this excellent precious metal trapping potential, paving a suitable way for recovery/isolation of precious metal ions from wastewaters [93]. Nanomaterials 2023, 13, 345 10 of 23 be the reason for this excellent precious metal trapping potential, paving a suitable way for recovery/isolation of precious metal ions from wastewaters [93]. MXene/CNT/cotton fabric composites with water transport features, strong optical absorption, and light-to-thermal conversion have been prepared through a layer-by-layer assembly to be applied as solar steam formation for textile wastewater purification [94]. Under sun illumination, the evaporation rate reached 1.35 kg m −2 for water and >1.16 kg m −2 h −1 for textile wastewater, providing promising candidates for wastewater purification by solar-evaporation [94]. A variety of lamellar membranes has been designed using 2D nanomaterials with excellent potential for molecular separation with high efficiency [95]. In one study, high-performance hetero-structured membranes with fusiform transport channels were constructed using MXene layers and CNTs, showing significant water permeation (1270 L m −2 h −1 bar −1 ) [95]. Sun et al. [96] designed multidimensional MXene-CNT ultrathin membranes with high efficiency and stability owing to the van der Waals interactions (hydrogen bond) and repulsion forces between MXene and CNTs, showing improved permeability with a distinct suppressed swelling MXene/CNT/cotton fabric composites with water transport features, strong optical absorption, and light-to-thermal conversion have been prepared through a layer-by-layer assembly to be applied as solar steam formation for textile wastewater purification [94]. Under sun illumination, the evaporation rate reached 1.35 kg m −2 for water and >1.16 kg m −2 h −1 for textile wastewater, providing promising candidates for wastewater purification by solar-evaporation [94]. A variety of lamellar membranes has been designed using 2D nanomaterials with excellent potential for molecular separation with high efficiency [95]. In one study, high-performance hetero-structured membranes with fusiform transport channels were constructed using MXene layers and CNTs, showing significant water permeation (1270 L m −2 h −1 bar −1 ) [95]. Sun et al. [96] designed multidimensional MXene-CNT ultrathin membranes with high efficiency and stability owing to the van der Waals interactions (hydrogen bond) and repulsion forces between MXene and CNTs, showing improved permeability with a distinct suppressed swelling feature through a thermal cross-linking route. These high-performance membranes can be considered as promising candidates for water purification [96].
Thirumal et al. [97] reported the synthesis of MXene-CNTs by applying a catalytic chemical vapor deposition technique. These composites with significant photocatalytic performance were deployed for the efficient elimination of rhodamine B dye pollutant. Compared to the pure MXenes for photocatalytic dye degradation (the efficiency was 60%), MXene-CNT hybrids exhibited improved efficiency of~75%. It appears that the introduced chemical vapor deposition technique can be applied for designing novel MXene-CNT hybrid structures with a high yield of production after the optimization process, providing suitable composites with good stability (recyclability) for the removal of hazardous pollutants [97]. For the removal of pharmaceutical wastes, MXene-CNT composites were fabricated through the pyrolysis of homologous metal-organic framework precursors, showing catalytic performance for the degradation of antibiotic tetracycline hydrochloride. Notably, these composites exhibited a high specific surface area with improved adsorption capacity (~35.9%) and degradation rate (0.26 min −1 ) [98].

EMI Shielding Performance
MXenes have shown attractive hydrophilicity and electrically conductive transition, making them suitable for electrically conductive and EMI shielding purposes [99]. However, it is difficult to construct compressible three-dimensional (3D) architectures with significant conductivity from MXenes due to the weak interaction among MXene nanosheets. In one study, MXene (Ti 3 C 2 T x )/acidified CNT anisotropic aerogels with high stability were designed, inspired by the plant (Parthenocissus tricuspidata), showing super-elasticity and considerable thermal insulation ( Figure 6A) [99]. The robust conductivity (447.2 S m −1 ) and ultralow density (9.1 mg cm −3 ) demonstrated a robust EMI shielding efficiency of 51 dB at an ultralow filler content of 0.3 vol %; by enhancing the density of composites to 18.2 mg cm −3 , the EMI shielding effectiveness could reach 90 dB [99]. Weng et al. [100] constructed MXene (Ti 3 C 2 T x )-CNT composite films with high stability, flexibility, and conductivity using a spin spray layer-by-layer technique for EMI shielding purposes. These films exhibited significant conductivity (up to 130 S cm −1 ) with high specific shielding effectiveness (up to 58 187 dB cm 2 g −1 ), paving a way for next-generation EMI shielding with improved absorption and electrical conductivity [100]. In addition, MXene (Ti 3 C 2 T x )-CNT hybrid composites were combined with a waterborne polyurethane matrix for EMI shielding and sheet heater purposes [35]. The composite films exhibited significant electrical conductivity with suitable mechanical flexibility, providing EMI shielding activity (~20-70 dB), along with heat dissipation and excellent Joule heating functions. Such composites with superb EMI shielding and thermal control capabilities can be considered as promising materials in designing wearable electronic devices [35]. In addition, bark-shaped CNT/MXene (Ti 3 C 2 T x ) composite films were constructed using a roll-to-roll layer-by-layer assembly tactic ( Figure 6B) [101]. After that, these films were decorated on the surfaces of fibers to obtain composites with suitable air permeability, electrical conductivity, and flexibility. These composites with improved electro-thermal activity and multi-interface scattering influences exhibited EMI shielding performance, along with significant sensitivity as the flexible piezoresistive sensors to monitor the human movements/motions, paving the way for designing EMI shielding materials and smart wearable electronics [101]. Moreover, the functionalization could increase the shielding by 15.4%, wherein MXene-CNT composites were designed for EMI shielding purposes; the composites could block 99.99% of the electromagnetic radiation. They had improved thermal stability along with a maximum electric conductivity of~12.5 Scm −1 [102]. In an impressive study, 3D porous hybrid aerogels were constructed from MXene (Ti3C2Tx) and CNTs by applying a freezing technique, showing suitable applicability for lightweight EMI shielding [103]. As was indicated, the synergistic effects from the lamellar and porous structures of the aerogels could extensively contribute to their fascinating electrical conductivity (9.43 S cm -1 ) along with the excellent value of EMI effec- In an impressive study, 3D porous hybrid aerogels were constructed from MXene (Ti 3 C 2 T x ) and CNTs by applying a freezing technique, showing suitable applicability for lightweight EMI shielding [103]. As was indicated, the synergistic effects from the lamellar and porous structures of the aerogels could extensively contribute to their fascinating electrical conductivity (9.43 S cm −1 ) along with the excellent value of EMI effectiveness (~103.9 dB at 3 mm thickness) at the X-band frequency. The incorporation of CNTs in these hybrid aerogels could significantly enhance the mechanical robustness and improve the compressional modulus compared to the pristine MXene aerogels [103]. For both EMI shielding and solar-thermal conversion applications, polymer-based nacre-like conductive films were constructed from hybrid MXene-CNT structures, wherein single-walled CNTs with unique electrical conductivity along with the fire/heat resistance and MXenes with lamellar and highly oriented structures were deployed to provide a significant electrical conductivity of~1851.9 S cm −1 [104]. These flexible composite films with long-term environmental stability and structural integrity provided superb EMI shielding effectiveness (~78.9 dB) and considerable specific shielding effectiveness (~15,263.1 dB cm 2 g −1 ); they could be applied for solar-thermal energy conversion with high performance [104].

Catalysis
MXenes have been employed to develop a variety of (nano)catalysts with environmental and biomedical applications [6,36,[105][106][107][108]. For instance, MXene@Pt/single-walled CNTs were constructed as nanocatalyst hydrogen evolution reactions. As a result, these catalysts exhibited ultrahigh stability with a high-volume current density of up to 230 mA cm −3 at −50 mV versus a reversible hydrogen electrode and a low over-potential of −62 mV versus a reversible hydrogen electrode at the current density of −10 mA cm −2 [109]. On the other hand, to obtain a continuous power supply and to generate appropriate chemicals (e.g., carbonates), the electrochemical carbon dioxide conversion at ambient temperature can be considered as a promising strategy [110]. With this purpose, parallel-aligned tubular MXene (Ti 3 C 2 T x )/CNT composites were designed via a self-sacrificial templating technique with significant catalytic activity and high stability, promoting the adsorption of CO 2 and accelerating the decomposition of lithium carbonate (Figure 7) [110]. In addition, MXene (Ti 3 C 2 T x )-CNT composites were designed with remarkable electrical conductivity and corrosion resistance as supporting materials for Pt [36]. Compared to the commercial Pt on carbon catalysts, the introduced catalyst could enhance the durability and improve the oxygen reduction reaction performance. The composite could be applied as a cathode catalyst for a single cell and stack, and the maximum power density of the stack reached 138 W [36].
Cobalt (Co)-tipped CNT/MXene (Ti 3 C 2 ) composites were constructed using a metalorganic-framework-engaged tactic for oxygen reduction reaction applications [111]. These composites exhibited suitable oxygen reduction reaction due to the abundant Co-N/C active sites and rationally significant graphitization of carbon, along with high surface areas. In addition, superb stability could be obtained compared to the commercial platinum (Pt)based electrocatalysts, showing excellent potential for renewable conversion and storage applications [111]. In another study, MXenes (Ti 3 C 2 T x ) were hybridized with Co/N-CNTs to provide bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions. The robust interfacial coupling and electron transfer could be detected, facilitating the electrocatalytic performances of these composites towards these reactions in alkaline solution [112]. Yang et al. [113] introduced multiwall CNT-based composites loaded with MoS 2 and MXene (Ti 3 C 2 T x ) quantum dots with suitable electrocatalytic activities for oxygen reduction and methanol oxidation reactions in alkaline solution. The catalysts demonstrated superb oxygen reduction reaction performance along with electro-oxidation activity for methanol in alkaline solution, providing a maximum methanol oxidation current density at 2.2 V of 160 A g −1 [113]. Moreover, NiCoFe-layered double hydroxide/MXene (Ti 3 C 2 )/Ndoped CNT structures were designed with optimum nitrogen content, strong electronic interactions, high surface area, plentiful active sites, and improved electrical conductivity, showing significant electrocatalytic performance towards oxygen evolution and oxygen reduction reactions. These composites can be considered as high-performance bifunctional catalysts for oxygen electrocatalytic reactions in metal-air batteries [114]. Cobalt (Co)-tipped CNT/MXene (Ti3C2) composites were constructed using a metal-organic-framework-engaged tactic for oxygen reduction reaction applications [111]. These composites exhibited suitable oxygen reduction reaction due to the abundant Co-N/C active sites and rationally significant graphitization of carbon, along with high surface areas. In addition, superb stability could be obtained compared to the commercial platinum (Pt)-based electrocatalysts, showing excellent potential for renewable conversion and storage applications [111]. In another study, MXenes (Ti3C2Tx) were hybridized with Co/N-CNTs to provide bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions. The robust interfacial coupling and electron transfer could be detected, facilitating the electrocatalytic performances of these composites towards these reactions in alkaline solution [112]. Yang et al. [113] introduced multiwall CNT-based composites loaded with MoS2 and MXene (Ti3C2Tx) quantum dots with suitable electrocatalytic activities for oxygen reduction and methanol oxidation reactions in alkaline solution. The catalysts demonstrated superb oxygen reduction reaction performance along with electro-oxidation activity for methanol in alkaline solution, providing a maximum methanol oxidation current density at 2.2 V of 160 A g −1 [113]. Moreover, Ni-

Supercapacitors and Batteries
MXenes with robust electrical conductivity have attracted special attention as electrode materials in designing supercapacitors [115]. In one study, MXene (Ti 3 C 2 T x )/CNT composite films were designed with outstanding functions as supercapacitor electrodes, showing significant capacitance of 300 F g −1 at 1 A g −1 with a superior rate of performance of 199 F g −1 , even at 500 A g −1 , along with high stability (~92% capacitance retention after 10,000 cycles at 20 A g −1 ). These MXene/CNT composites can be employed for designing flexible, portable, and highly integrated supercapacitors [115]. In addition, CNTs were employed for constructing highly conductive net structures, tightly anchoring porous carbon on MXene flakes, and causing fast electron delivery by enhancing the contact area between MXene and porous carbon (Figure 8) [116]. Accordingly, MXene (Ti 3 C 2 T x )/CNT/porous carbon films with suitable flexibility were designed to provide high areal specific capacitance of 364.8 mF cm −2 at 0.5 mA cm −2 (>80% even at a high current density of 50 mA cm −2 ). In addition, the designed supercapacitor exhibited a large areal energy density of 10.5 µ Wh cm −2 at 29.8 µ W cm −2 , showing excellent potential of MXene-CNT hybrid composites for designing supercapacitors with significant rate capa-bilities and large charge storage capacities [116]. Cai et al. [117] designed nanocomposites of CNTs/polyaniline and hybridized them with MXenes (Ti 3 C 2 T x ) to obtain a composite electrode with superior gravimetric capacity and cyclic stability (>93% retention after 10,000 cycles). Studies on the mechanisms revealed that the surface capacitance storage was the major reason for its great rate potential, providing great opportunities for designing high-performance electrodes [117]. Li et al. [118] introduced N-doped CNT/MXene (Ti3C2Tx)/polyacrylonitrile nanocomposite films as electrodes of supercapacitors, which were fabricated through vacuum filtration and electrospinning processes. These film electrodes with a high area-specific capacitance of 669.27 mF cm -2 and a high mass-specific capacitance of 446.18 F g -1 at 5 mV s -1 displayed suitable cycling stability with the retention of 90.9% after 4000 cycles, showing small capacity loss and good flexibility [118]. Despite the fascinating advantages of MXenes, such as significant volumetric capacitance, electrical conductivity, and hydrophilic properties, MXene-based electrodes may suffer from poor rate potential due to sheet restacking, particularly when the loading level is excessive and solid-state gels are employed as electrolytes [119]. Thus, MXenes were hybridized with CNTs to obtain fibers with helical structures, offering open spaces for rapid ion diffusion and obtaining fast electron transport. Accordingly, the designed fibers were applied in designing solid-state supercapacitors with gel electrolyte coating to show a volumetric capacitance of 22.7 F cm −3 at 0.1 A cm −3 with capacitance retention of 84% at a current density of 1.0 A cm −3 (19.1 F cm −3 ); these supercapacitors with mechanical robustness exhibited an enhanced volumetric energy density of 2.55 mWh cm −3 at a power density of 45.9 mW cm −3 [119]. Yang et al. [120] constructed MXene (Ti3C2)-CNT films and deposited them onto graphite paper through an electrophoretic deposition technique for super- Li et al. [118] introduced N-doped CNT/MXene (Ti 3 C 2 T x )/polyacrylonitrile nanocomposite films as electrodes of supercapacitors, which were fabricated through vacuum filtration and electrospinning processes. These film electrodes with a high area-specific capacitance of 669.27 mF cm −2 and a high mass-specific capacitance of 446.18 F g −1 at 5 mV s −1 displayed suitable cycling stability with the retention of 90.9% after 4000 cycles, showing small capacity loss and good flexibility [118]. Despite the fascinating advantages of MXenes, such as significant volumetric capacitance, electrical conductivity, and hydrophilic properties, MXene-based electrodes may suffer from poor rate potential due to sheet restacking, particularly when the loading level is excessive and solid-state gels are employed as electrolytes [119]. Thus, MXenes were hybridized with CNTs to obtain fibers with helical structures, offering open spaces for rapid ion diffusion and obtaining fast electron transport. Accordingly, the designed fibers were applied in designing solid-state supercapacitors with gel electrolyte coating to show a volumetric capacitance of 22.7 F cm −3 at 0.1 A cm −3 with capacitance retention of 84% at a current density of 1.0 A cm −3 (19.1 F cm −3 ); these supercapacitors with mechanical robustness exhibited an enhanced volumetric energy density of 2.55 mWh cm −3 at a power density of 45.9 mW cm −3 [119]. Yang et al. [120] constructed MXene (Ti 3 C 2 )-CNT films and deposited them onto graphite paper through an electrophoretic deposition technique for supercapacitor electrodes. These electrodes with improved specific capacitance exhibited cycling stability (~10,000 cycles), thus displaying enhanced electrochemical performance [120].
For improving the energy density of batteries, researchers have focused on the construction of cathodes with electroactive materials. However, irreversible Li + consumption in full-cell configurations and poor structural integrity, along with low electronic/ionic transport are crucial challenges in this way. In one study, 3D MXene (Ti 3 C 2 T x )-CNT-cellulose-LiFePO 4 cathodes were designed with improved LiFePO 4 loading (120 mg cm −2 ) and enhanced electronic/ionic transport, providing better electrochemical performance, a significant capacity of 0.86 mAh cm −2 at 5 C, and a high retaining capacity of 1.45 mAh cm −2 after 500 cycles at 1 C (Figure 9) [37]. These cathodes exhibited an ultrahigh areal capacity of 19.2 mAh cm −2 , and the introduced 3D-MXene-CNTs-cellulose-LiFePO 4 /SnO 2 full-cell had a high areal capacity of 6.3 mAh cm −2 at 1.6 mA cm −2 , offering MXene/CNT hybrid composites as promising materials in designing Li-ion batteries with high performance [37]. In addition, hydroxyl-functionalized Mo 2 C-based MXene nanosheets were fabricated through the simple removal of the Sn layer of Mo 2 SnC [121]. The prepared MXene with its hydroxylfunctionalized surface could suppress the shuttle effect of lithium polysulfides via robust interactions between Mo atoms on the surface of MXene and lithium polysulfides. For enlarging the specific surface area of final composites, CNTs were utilized into the Mo 2 C phase, improving the electronic conductivity and alleviating the volume change during charging/discharging processes. The robust surface-bound S in the hierarchical Mo 2 C-CNT host could lead to excellent electrochemical activity in Li-S batteries, providing a large reversible capacity (≈925 mAh g −1 ), even after 250 cycles at a current density of 0.1 C [121]. For improving the energy density of batteries, researchers have focused on the construction of cathodes with electroactive materials. However, irreversible Li + consumption in full-cell configurations and poor structural integrity, along with low electronic/ionic transport are crucial challenges in this way. In one study, 3D MXene (Ti3C2Tx)-CNT-cellulose-LiFePO4 cathodes were designed with improved LiFePO4 loading (120 mg cm −2 ) and enhanced electronic/ionic transport, providing better electrochemical performance, a significant capacity of 0.86 mAh cm −2 at 5 C, and a high retaining capacity of 1.45 mAh cm −2 after 500 cycles at 1 C (Figure 9) [37]. These cathodes exhibited an ultrahigh areal capacity of 19.2 mAh cm −2 , and the introduced 3D-MXene-CNTs-cellulose-LiFePO4/SnO2 full-cell had a high areal capacity of 6.3 mAh cm −2 at 1.6 mA cm −2 , offering MXene/CNT hybrid composites as promising materials in designing Li-ion batteries with high performance [37]. In addition, hydroxyl-functionalized Mo2C-based MXene nanosheets were fabricated through the simple removal of the Sn layer of Mo2SnC [121]. The prepared MXene with its hydroxyl-functionalized surface could suppress the shuttle effect of lithium polysulfides via robust interactions between Mo atoms on the surface of MXene and lithium polysulfides. For enlarging the specific surface area of final composites, CNTs were utilized into the Mo2C phase, improving the electronic conductivity and alleviating the volume change during charging/discharging processes. The robust surface-bound S in the hierarchical Mo2C-CNT host could lead to excellent electrochemical activity in Li-S batteries, providing a large reversible capacity (≈925 mAh g −1 ), even after 250 cycles at a current density of 0.1 C [121].

Conclusions and Perspectives
A wide variety of hybrid composites has been designed using MXenes and their derivatives because of their reducibility and hydrophilicity along with their large surface area, ease of functionalization, and unique mechanical/electrical properties. MXenes

Conclusions and Perspectives
A wide variety of hybrid composites has been designed using MXenes and their derivatives because of their reducibility and hydrophilicity along with their large surface area, ease of functionalization, and unique mechanical/electrical properties. MXenes with their layered structures and chemical compositions have been hybridized with CNTs to obtain composites for catalysis, Li-ion batteries/supercapacitors, electrochemical capacitors, sensing, water treatment/pollutants removal, and EMI shielding applications. However, their electrocatalytic and biomedical potentials have been relatively less explored by researchers. MXene-CNT composites with significant electrical conductivity and strong mechanical features can be considered as attractive candidates for EMI shielding in electronic devices. The need for additional explorations on wearable sensors, high-rate electrochemical energy storage, soft robotics, EMI shielding films, and catalytic reactions is felt more than ever. Future explorations ought to focus on developing wearable and stretchable strain sensors with ultrahigh sensitivity and an adjustable sensing range using MXene-CNT composites. MXenes with good conductivity and electrochemical properties as well as abundant surface terminations and a large surface area are attractive candidates for designing novel composites with applicability in electronics, energy storage, catalysts/electrocatalysts, and pharmaceutics/biomedicine.
The density, specific surface area, and pore volume as well as the mechanical, electrical/electronic (e.g., electrical conductivity), and thermal (e.g., thermal conductivity, photothermal, and electrothermal) properties of MXene-CNT hybrids need more specific evaluations. The development of novel approaches of functionalization and hybridization along with comprehensive investigations for the industrial/commercial production of MXene-CNT composites should be considered. Transferring from the laboratory phase to industrial production as well as production adhering to green chemistry tenets and environmentally benign principles are very important in this field. Although promising results have been reported, their reproducibility along with the ability to be industrialized is equally crucial. Moreover, challenges regarding the oxidation of MXenes during production and their stability still need comprehensive evaluation. Furthermore, both experimental and theoretical studies can help to overcome these challenges, thus improving the properties of these composites.