*3.1. Supercapacitors (SCs)*

Supercapacitors (SCs) are kinds of energy storage devices that contain negative electrodes, positive electrodes, separators, and electrolytes [58]. Normally, SCs possess significant advantages of fast charge/discharge rates, high power density, and long cycle stability due to their special energy storage mechanism. In general, SCs can be classified as electrical double-layer capacitance (EDLC), pseudocapacitors, and hybrid supercapacitors [59]. The EDLC mechanism is an electro-physical storage process that involves the charge accumulation and reversible adsorption/desorption of electrolyte ions on the interface between electrodes and electrolytes [60]. The commonly used electrode materials for EDLC are mainly porous carbonaceous materials. Pseudocapacitors, also called redox capacitors, store charges via the electrochemical processes of surface redox reactions with electron gain/loss or pseudocapacitive intercalation [61]. Hybrid supercapacitors (HSC) integrate EDLC-type and redox-type electrodes in the device and display an electrophysical and electrochemical mixed mechanism [62]. The schematic illustrations of different SCs are shown in Figure 3.

**Figure 3.** Schematic illustrations of (**a**) a EDLC with porous carbon materials for each electrode, (**b**) a pseudocapacitor with transition metal-related material for each electrode, and (**c**) asymmetric hybrid capacitors utilizing a porous carbon negative electrode and a transition metal oxide positive electrode.

The charge storage mechanism for MXene (e.g., Ti3C2Tx) is commonly identified as a Faraday redox processes, which involves transitions between various oxidation states of titanium during the intercalation/deintercalation processes of ions in the electrolytes [63]. Yury Gogotsi and co-workers demonstrated that the intercalation/deintercalation of cations (K<sup>+</sup> and NH4+) from alkaline electrolyte into Ti3C2 MXene layers in a range of −0.6–0 V are responsible for the electrochemical performance and capacitances of these materials, and the corresponding electrochemical reaction can be ascribed as follows [55,64]:

$$\text{Ti}\_3\text{C}\_2\text{T}\_\text{x} + \delta\text{K}^+ + \delta\text{e}^- \rightleftharpoons \text{K}\_\delta\text{Ti}\_3\text{C}\_2\text{T}\_\text{x} \tag{5}$$

The Faraday processes in a potential window of 0–0.6 V for the MXenes (e.g., Ti3C2) as the positive SC electrodes in alkaline electrolyte may involve the following reactions [65].

$$\mathrm{Ti\_3C\_2O\_x(OH)\_yF\_x + \delta OH^- \rightleftharpoons Ti\_3C\_2O\_x + \delta(OH)\_y^- \left\{F\_x + \delta H\_2O + \delta e^- \right\} \tag{6}$$

$$\text{Ti}\_3\text{C}\_2\text{O}\_3 + \delta\text{OH}^- \rightleftharpoons \text{Ti}\_3\text{C}\_2\text{O}\_3\text{(OH)}\_\text{\delta} + \delta\text{e}^- \tag{7}$$

MXene-based composites. In order to solve the restacking problem mentioned above and further enhance the charge storage performance of MXenes, Sun and co-workers adopted the Ti3C2 MXene and 1T-MoS2 to construct a 3D interpenetrated architecture of 2D/2D 1T-MoS2/Ti3C2 heterostructure by the magneto-hydrothermal method. Enhanced electrochemical performances are observed from this material, which result from the synergistically interplayed effect of the unique 3D interpenetrated heterogeneous networks (e.g., enlarged ion storage space between Ti3C2 and 1T-MoS2 for storing more electrolyte ions, boosted electron conductivity provided by MXene, and more active sites for redox reaction offered from 1T-MoS2 phase). As a result, the heterogeneous material exhibits a specific capacitance of 386.7 F g−<sup>1</sup> at 1 A g−<sup>1</sup> in a potential window of −0.3–0.2 V with outstanding rate performances (a high capacitance of 207.3 F g−<sup>1</sup> at 50 A g<sup>−</sup>1). They also reported that the total capacitance of this material comes from three parts, containing contributions originating from 1T-MoS2, MXene, and extra H+ storage in the interpenetrated space within the 3D heterostructure (Figure 4), while the 1T-MoS2/MXene sample without heterostructure lacks the extra H+ storage capacitance, indicating the existence of a strong coupled effect

between composition and structure for the 3D heterostructure. Furthermore, a symmetric SC based on the as-design material displayed a high areal capacitance of 347 mF cm−<sup>2</sup> at 2 mA cm−<sup>2</sup> with excellent cycle stability (91.1% capacitance retention after 20,000 cycles) (Figure 4) [66].

**Figure 4.** (**a**) Schematic illustrating the preparation process for the two types of the MoS2/Ti3C2 MXene 3D heterostructures. (**b**–**d**) SEM images of the prepared 1T-MoS2/Ti3C2 MXene sample. (**c**) Electrochemical performance of the prepared heterogeneous electrodes. (**e**–**h**) Electrochemical performance of the prepared samples. (**i**) Schematic diagram of the storage mechanism for the heterostructure [66].

A Ti3C2/CuS composite as the positive electrode in alkaline electrolyte (within a potential window of 0–0.6 V) delivers a capacity of 169.5 C g−<sup>1</sup> at 1 A g−<sup>1</sup> with a capacity retention of 90.5% at 5 A g−<sup>1</sup> after 5000 long-term cycles. An asymmetric SC fabricated by the Ti3C2/CuS positive electrode and Ti3C2 negative electrode at a voltage range of 0–1.1 V reveals a specific capacitance of 49.3 F g−<sup>1</sup> to 0–1.5 V with a maximum energy density of 15.4 Wh kg−<sup>1</sup> and a capacitance retention of 82.4% after 5000 cycles [65]. A MXene/nickelaluminum layered double hydroxide (MXene/LDH) composite in 6 M KOH aqueous electrolyte within a range of 0–0.6 V displays a specific capacitance of 1,061 F g−<sup>1</sup> at 1Ag−<sup>1</sup> with a retention of 70% after 4000 charge/discharge cycles [67]. A nickel sulfide/Ti3C2 (Ni–S/Ti3C2) nanohybrid in 6.0 M KOH aqueous electrolyte within a window of 0–0.6 V exhibits a capacity of 840.4 C g−<sup>1</sup> with enhanced rate performance (a capacity retention of 64.3% at 30 A g−1) and a long cycle life. An asymmetric SC assembled by the Ni–S/Ti3C2 positive electrode and Ti3C2 negative electrode delivers an energy density of 20.0 Wh kg−<sup>1</sup> at a voltage range of 0–1.8 V and a good cycling stability (a capacity retention of 71.4% after 10,000 cycles) [68]. Doping heteroatoms (e.g., nitrogen) into the MXene structures could modify their electronic structure, composition, and pseudocapacitance properties. In addition, heteroatom-doping can also lead to a remarkable increase of the interlayer spacing between MXene flakes. As Dai and co-workers found, the c-lattice parameter of the MXene layers could increase from 1.92 to 2.46 nm after the nitrogen-doping process, indicating the doped N atoms expand the interlayer spacing of the MXene sheets. Thereby, the resultant doped materials delivered much higher capacitances of 192 F g−<sup>1</sup> in 1MH2SO4 within a potential range of −0.2–0.35 V than those un-doped Ti3C2Tx materials [69].

Previous theoretical and experimental studies reveal that the chemical and physical characters of MXenes are heavily influenced by their surface functional groups. Yury Gogotsi et al. found that the MXene (e.g., Ti3C2Tx) with rich surface O-termination exhibited high surface activity and better pseudocapacitance performance [63,70]. Fan et al. enhance the electrochemical performance of MXene by using the ammonium persulfate as the weak oxidant and intercalation agent to partially remove the F terminations coupled with controllably oxidizing the Ti3C2Tx surface to produce rich O-terminations. Recently, electrochemical test results have shown that the modified Ti3C2Tx possess an enhanced capacitance of 303 F g−<sup>1</sup> and the capacitance retention could be 96.6% after 9000 cycles [71]. MXene can also form a composite with the N, O co-doped carbon materials to expand the interlayer spacing and avoid the re-stacking problems of the MXene sheets. Due to

these reasons, a N, O co-doped carbon@MXene composite exhibited a higher specific capacitance of 250.6 F g−<sup>1</sup> at 1 A g−<sup>1</sup> compared with pristine Ti3C2 and retained 94% of its initial capacitance after 5000 cycles. Furthermore, a symmetric SC based on this composite electrode displayed an excellent electrochemical performance with an energy density of 10.8 Wh kg−<sup>1</sup> at 600 W kg−<sup>1</sup> and desirable cycling stability [28]. Zhenxing Li's group synthesized a dodecaborate/MXene highly conductive composite by surface modified with ammonium ion and inserted the dodecaborate ion into the inner surface of MXene via the electrostatic adsorption. The delocalized negative charge of dodecaborate ion facilitates the cations transfer, which efficiently boosted the ion transfer rate during the charge storage process due to its "lubricant" effect for electrolyte ions. Due to the above reasons, a high specific capacitance of 366 F·g−<sup>1</sup> can be obtained at 2 mV·s−<sup>1</sup> [72].

Among all other pseudocapacitive materials, MXenes offer ultrahigh rate capability, excellent conductivity, and volumetric capacitance in SC applications. Actually, MXenes and most MXenes-derived composites commonly are only served as negative electrode materials as their easy oxidation feature at positive potential (anodic oxidation process). To further enhance its antioxidant performance at positive potential, some works tried to improve the work function (WF) of the final product by means of compounding. For instance, Gogotsi and co-workers reported a PANI@MXene cathode material by evenly covering an oxidation resistive PANI layer on the surface of MXene network. The first principle calculation results manifested that the PANI with a large WF (WF = 3.46 eV, the WF for metallic MXene is 1.61 eV) effectively expanded the WF of the composite to 1.97 eV by forming a compact PANI@MXene heterostructure, which helps to enhance the electrochemical stability at a wider positive operating potential (0–0.6 V). Therefore, the as-formed PANI@MXene composite can be used as a positive electrode for steady energy storage with a high volumetric capacitance of 1632 F cm−<sup>3</sup> and a desirable rate capability. They also claimed that the asymmetric SC assembled by MXene anode and PANI@MXene cathode delivered a volumetric energy density of 50.6 Wh L−<sup>1</sup> with an ultrahigh volumetric power density of 127 kW L−1. This work also reasonably suggested that other materials (e.g., inorganic compounds or macromolecule materials) with higher WF may enhance the resistance of losing electrons at positive potentials and enlarge the operation working windows for MXenes via forming MXene-based composites [73].

Flexible MXene thin films or free-standing electrodes. It is well known that MXenes are simultaneously capable of conducting electrons and storing charges. In addition, they also possess the compatible structural properties of flexibility, self-assembly, and miniaturization. Meanwhile, the functionality of the MXene macroscopic films or assemblies could be further improved through further optimizing the MXene synthesis conditions, tuning the MXene interlayer spacing, altering the types or amount of surface terminations, controlling the size and thickness of MXene flakes, and compositing with other functional materials (e.g., metal organic framework), and therefore, it is reasonable to suppose that the Ti3C2Tx can be used as free-standing electrodes for SC devices with more functionality. Nicolosi and co-workers adopted a spin-casting method to produce highly aligned and transparent continuous Ti3C2Tx film, which displays remarkable optoelectronic performances, e.g., bulk-like conductivity, high transmittance, and desirable pseudocapacitance performances. Owing to the structural and electronic properties, the as-designed Ti3C2Tx electrodes could function as both active materials and transparent current collector for SCs. Meanwhile, they also found that the as-designed energy devices exhibit high areal/volumetric capacitances with superior energy/power densities and long cycle life [74], demonstrating the application potential for optoelectronics and flexible electronics of MXene materials.

The existence of surface terminations on the MXene sheets are beneficial to form strong bonds between MXenes and specific materials, which help to boost the electrical conductivity and structural/electrochemical stabilities of the active components and the final films. Huang and co-workers employed a vacuum-assisted filtration method to construct the MXene/metal-porphyrin frameworks (MPFs) hybrid flexible free-standing film via forming the interlayer hydrogen bond between the electronegative surface (results from the

functional groups of -O, -F, or -OH) of MXene and hydrogen atom of carboxy terminations (-COOH) in MPFs. The composite electrode with interconnected conductive networks and interlayer hydrogen bonds (e.g., F···H-O and O···H-O) eliminated the inferior conductivity and low structural stability problems of MPFs, showing favorable flexibility, high ionic/electronic transport rates, and durability. Meanwhile, the as-designed MXene/MPFs film can normally operate in a potential window of −0.3–0.3 V (vs. Ag/AgCl) and exhibits a specific capacitance of 326.1 F g−<sup>1</sup> and excellent durability. In addition, a flexible symmetric SC fabricated by the MXene/MPFs film delivers an areal capacitance of 408 mF cm−<sup>2</sup> and an areal energy density of 20.4 μWh cm−<sup>2</sup> with a long term stability of 7000 cycles (with a capacitance retention of 95.9%). This work also indicated that the formation of interlayer hydrogen bond between MXene and other components contributes to enhance the chemical stability by effectively avoiding the phase separation or structural collapse problems of the electrode materials during the energy storage processes (Figure 5) [75]. Wu et al. used the Buchwald-hartwig coupling reaction to prepare a decentralized conjugated polymer (PDT)/Ti3C2Tx hybrid flexible freestanding film via an electrophoretic deposition and the following spin coating processes. Similarly, they found that a stable chemical bond (e.g., hydrogen bonds) can be formed between the terminal groups of the Ti3C2Tx sheets and the decentralized chains of PDTs, which enables the final polymer matrix composite film to effectively relieve the volumetric swelling and shrinking problems of polymer chains during the charge/discharge processes [76].

**Figure 5.** Schematic illustration of the synthesis process for the MXene/MPFs film via interlayer hydrogen bond [75].

The delaminated MXene flakes with monatomic or few-atom-layer thicknesses have shown their capabilities to assemble free-standing and flexible films with outstanding electronic conductivity and excellent electrochemical performance due to their 2D lamellar structures, abundant surface functional groups, and remarkable charge transfer capabilities. Nevertheless, the propensity for "face to face" horizontal stacking with less open tunnels reduces the interlayer space between isolating MXene flakes and suppresses electrolyte ions transport or intercalation, which limits their potential in electrochemical storage applications. Therefore, opportunities to control the MXene nanosheets alignment, especially through designing unique architectures to tune the porosity, expanding the interlayer spacing, and manipulating the surface functional groups, need to be further explored. In order to alleviate the stacking problems of MXenes, Zheng et al. employed the vacuum filtration method with the assistance of entwined metal mesh to control the alignment of MXene sheets and fabricate an "anti-T-shape" MXene film electrode. They claimed that the unique "magazine-bending" structure facilitated the vertical electrolyte ion transport and enhanced the kinetics of the electrochemical process which enable the "anti-T-shape" electrodes in a symmetric SC to display a low interfacial impedance, a high pseudocapacity of 194 F g−1, superior energy/power densities of 11.27 Wh kg−1/699.9 W kg−1, and a favorable capacitance retention of 70.3% after 10,000 cycles [31].

Zhang's group utilized the vacancy ordered Mo1.33C MXene and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) to fabricate the flexible aligned Mo1.33C MXene/PEDOT:PSS composite films by a vacuum filtration process. The electrochemical test results show that the MXene-based films with the optimal ingredients ratio (the mass ratio between Mo1.33C MXene and PEDOT:PSS is 10:1) display a high conductivity of 29,674 S m−<sup>1</sup> and a maximum volumetric capacitance of 1,310 F cm−<sup>3</sup> in the potential range from −0.35 to 0.3 V in 1 M H2SO4 aqueous electrolyte, which may result from the synergistic effect of expanded interlayer spacing of MXene sheets caused by the PEDOT component and the extra redox activity of the PEDOT. Due to these reasons, a maximum volumetric capacitance of 568 F cm−<sup>3</sup> and energy density of 33.2 mWh cm−<sup>3</sup> can be obtained for the film-based flexible all-solid-state SC [77].

The performances and surface chemistries of the MXene film electrode can be further tuned by compositing with various functional nanomaterials or in different types of electrolytes. Xingbin Yan's group adopted the MXene (Ti3C2) as charge-transfer pathways and graphite carbon nitride (g-C3N4) as the ion-accessible channels to construct a 2D heterogeneous nanospace for dual confining the FeOOH quantum dots via a vacuum filtration to fabricate a freestanding FQDs/CNTC film electrode, which shows superior pseudocapacitive performances with favorable kinetics in a high-voltage ionic liquid (IL) electrolyte (1-ethyl-3-methylimidazolium tetrafluoroborate, abbr., EMIMBF4). The test results indicated that the surface functional groups of Ti2C3 and active sites (e.g., N defects) of the g-C3N4 in the FQDs/CNTC electrode are crucial for offering sufficient redox reaction by efficiently forming the strong adsorption between the electrolyte ions and the electrode interface (Figure 6a). Furthermore, a 3 V flexible SC was constructed in the IL electrolyte, which exhibited volume energy/power densities of 77.12 mWh cm<sup>−</sup>3/6000 mW cm−3, and desirable flexible cycling performance (Figure 6b,c) [78]. The as-obtained FQDs/CNTC film based flexible SC can easily power portable electronics under complex motion states (Figure 6d) or store solar energy (Figure 6e), showing a new insight into construction of MXene-based composite film electrode and flexible SC.

Mahiar M. Hamedi's group prepared MXene-based freestanding films via a vacuum filtration by using the carboxymethylated cellulose nanofibrils (CNFs) as functional additive to strengthen the mechanical properties of the films. A hybrid film with a 10% CNF loading displays a high Young's modulus (41.9 GPa), a high mechanical strength (154 MPa), a high electric conductivity (690 S cm<sup>−</sup>1), and a high specific capacitance (325 F g−1), which shows great potential for flexible functional electronics. They claimed that the superior properties root in the strong interfacial interactions between Ti3C2Tx flakes and CNF (Figure 7) [79].

**Figure 6.** (**a**) Schematic illustration of the charge-discharge processes for FQDs/CNTC film electrode. (**b**) Ragone plot of the as-designed flexible SC. (**c**) Cycling performance of the flexible SC under the bent state. (**d**) Digital photos of the flexible SCs power diverse portable electronics. (**e**) Schematic of the flexible SC charged by harvesting solar energy [78].

**Figure 7.** (**a**) Schematics of Ti3C2Tx/CNF hybrid dispersion. (**b**) The synthesis method and digital photographs of Ti3C2Tx/CNF nanopapers. (**c**) Photos of a Ti3C2Tx/CNF nanopaper. (**d**) Schematic of the three-electrode system in this work. (**e**) CV curves of MXene with different ratio of CNF. (**f**) CV curve of Ti3C2Tx–5% CNF electrode at various scan rates [79].

Given the interconnected networks and high tensile strength, bacterial cellulose (BC) was confirmed as excellent spacers for loading MXene to construct robust film electrodes with strong mechanical strength. In this regard, Guohui Yuan's group prepared a 3D self-supporting film electrode comprising interconnected MXene and BC networks. Due to excellent electron conductivity, large ion-accessible active sites, effective ion diffusion, and robust mechanical properties of the MXene/BC network, the as-obtained film electrode exhibits a high capacitance (416 F g−1, 2084 mF cm−2) coupled with excellent mechanical performance. Moreover, an ultrahigh energy density of 252 μWh cm−<sup>2</sup> can be obtained in an asymmetric SC, which provided a simple route for fabricating high performance film electrodes in energy storage fields [80].

MXene inks and printable microsupercapacitor (MSC). To date, various construction methodologies to manufacture versatile micro-power systems have been developed. Among them, printing (e.g., laser printing, direct writing, screen printing, and extrusion printing) has been regarded as one of the most revolutionary techniques to construct functional energy storage devices with designed or desirable patterning. MXenes can also be used as attractive printing materials (e.g., conductive inks), which are essential for realizing the fabrication of geometric flexible, printable, and free-standing electrochemical devices. Thus, many works focus on the design of certain formulations of MXene conductive inks for achieving higher compatibility with various patterning methods. In addition, many simple and affordable strategies to obtain the MXene-based coplanar interdigital electrodes for further assembling into printable or direct-write SC devices are progressively developed, which shows extra advances in their versatility, high resolution patterning, simplicity, and desirable performances. Many previous works also indicated that some of the existing micro fabrication techniques often require sophisticated instrumentation and tedious multistep manufacturing processing, and therefore, MXenes inks or MXene-based coplanar electrodes and the corresponding printing techniques exhibit exciting potential for manufacturing of low-cost and easily processing printable electronics.

Yury Gogotsi and co-workers adopted simple and additive-free formulations to prepare solution processable MXene-in-water (e.g., Ti3C2) inks, which can be loaded in pens to direct write conductive MXene electrical circuits and desired shape either by manual drawing or automatic drawing tools on a variety of substrates (e.g., printer paper and polypropylene membranes) for functional energy storage devices. The developed MXene versatile inks are also compatible with other patterns, such as stamping, printing, and painting. Additionally, the as-written MXene collector-free MSC as power sources display an areal capacitance of 5 mF cm−2, and a tandem device can drive a LED to operate normally, demonstrating the practical application of this technique (Figure 8) [81].

**Figure 8.** (**a**–**e**) Written MXene on various substrates via various ways, (**f**,**g**) using the automatic drawing tool of AxiDraw for patterning by using the Ti3C2 inks, (**h**,**i**) versatile patterning using AxiDraw, and (**j**) a demonstration of MXene inks drawn on an apple [81].

Screen printing and extrusion printing are two kinds of versatile direct printing techniques to fabricate the SC electrodes or MSC devices with geometric flexibility and desirable architectures. Screen printing is widely used to build up 2D flexible or wearable electronics due to its scalable and facile property, while extrusion printing could implement customized 3D printing through digital processing. In a manner, the rheological properties of the employed inks are pivotal for realizing the different printing modes. Commonly, extrusion printing requires more viscous inks compared to screen printing for keeping the 3D architectures from collapsing [56]. MXenes or their derivates could develop certain functional conductive inks for printed energy storage devices due to their intrinsic favorable hydrophilicity, mechanical flexibility, and modifiability [82]. Sun and co-workers used the melamine formaldehyde as the template to develop the crumpled heteroatom nitrogendoped MXene flakes (MXene-N). They also found that the nitrogen doping effectively boosts the conductivity and electrochemical activity of the MXene flakes. Accordingly, two kinds of MXene-N inks (e.g., binder-additive aqueous ink and binder-free hybrid ink) are developed via optimizing the viscosity for screen printing and extrusion printing, respectively. The obtained screen printed flexible MXene-N based MSCs display an areal capacitance of 70.1 mF cm−2. The as-fabricated 3D extrusion printed SC using the hybrid ink that includes the N-doping MXene, AC, CNT, and GO, delivers an areal capacitance of 8.2 F cm−<sup>2</sup> with an areal energy density of 0.42 mWh cm−2. This work also proposed a versatile solution to relieve the restacking issues of MXene flakes within the printed electrodes for further facilitating the ion/electrolyte transport (Figure 9) [56].

**Figure 9.** (**a**) Schematic illustration of the synthetic procedure for the crumpled MXene-N and the direct MXene-N ink printing routes of screen printing and extrusion printing. (**b**) Digital photographs of the MSC printed by screen printing. (**c**) Schematic showing of the preparation process of MXene-N based hybrid ink and the extrusion printed 3D architecture [56].

Zhang and co-workers employ the MXene sediments (usually thrown away) of unetched or multilayered MXene to prepare the additive free inks for scalable high-resolution screen printing. By designing different modes, various printed patterns (e.g., conductive tracks, letters, MSCs, and integrated circuits) can be quickly printed by using the as-formulated ink (Figure 10). They found that a low proportion of delaminated MXene flakes in the ink is crucial for realizing a desirable conductivity and excellent mechanical flexibility of the final printed circuitries or the electrodes. They also demonstrated an excellent electrochemical performance of the as-obtained printed MSCs with a high areal capacitance of 158 mF cm−<sup>2</sup> and a high energy density of 1.64 μWh cm−2. This ink formulation strategy of using MXene etching trash would effectively reduce the cost and waste of MXene-based printed techniques into consideration [83].

Liang et al. developed a thixotropic electrode ink that consisted of RuO2-decorated Ti3C2Tx MXene nanosheets (RuO2·xH2O@MXene) and conductive silver nanowires to fabricate the flexible printable MSC via the screen-printing method (Figure 11). They also found that the in situ growth of RuO2 nanoparticles effectively relieves the agglomeration and restacking problems of the MXene flakes while maintains a suitable rheological nature for printing. Due to the synergistically interplayed effect of ingredients, the as-designed MSCs possess micrometer-scale resolution and desirable electrochemical properties, including a high volumetric capacitance of 864.2 F cm−3, satisfactory energy/power densities of 13.5 mWh cm−3/48.5 W cm−3, a long-term cycling life with a retention of 90% after 10,000 cycles, and high mechanical stability [64].

**Figure 10.** (**a**) Schematic illustrating of the screen printing using the MXene sediment ink. (**b**) Various screen-printed patterns. (**c**) Optical photo of scalable produced screen-printed MXene-based MSC [83].

The traditional micro-fabrication technologies that are based on the conventional silicon-based strategies, such as laser scribing, have also demonstrated feasibility for the fabrication of MXene-based MSC devices. For example, Hua's group employed the laser printing techniques with the assistance of vacuum deposition and physical sputtering to fabricate the MXene-based coplanar interdigital electrodes and planar symmetric on-chip MSC. They also reported that the as-developed fabrication method allows the deposition of fully delaminated MXene sheets with tunable thickness. Due to the good alignment of the MXene flakes, high electrical conductivity, and the unique layered porous structure of the as-designed MXene-based coplanar electrodes, the as-designed all-solid-state MSC displays excellent high-rate capacity, a high areal capacitance of 27.29 mF cm−<sup>2</sup> and competitive volumetric energy densities of (5.48–6.1 mW h cm<sup>−</sup>3) [84]. This work also demonstrated that

the MXene active materials have great potential for the laser printing process technology for patterning thick coplanar interdigital electrodes or planar SC.

**Figure 11.** Schematic illustrating of (**a**) the preparation of MXene, (**b**) the synthesis process of the RuO2·xH2O@MXene nanohybrid, and (**c**) the screen-printing process for the fabrication of the flexible MXene-based MSCs [64].

Flexible MXene-based yarn or fiber electrodes. Fiber-shaped SCs are attractive for powering miniaturized or portable wearable electronics due to their compatibility of integrating with textiles. In general, effective and scalable strategies to produce fiber electrodes with desirable electrical storage performances fall within two categories. The first category of deposition technique is based on covering of electrochemical active or conductive materials onto fiber matrix [85]. For instance, Yang and co-workers employed the MXene-polymer nanofibers as the coating layer on the polyester (PET) yarns to construct the PET@MXene nanofiber-based yarn via a modified electrospinning technique, from which the complex yarns display favorable flexibility and certain mechanical strength during the weaving, braiding, or knitting processes, indicating that the obtained yarns can be used as flexible yarn electrodes for wearable SCs. They also reported that two parallel PET@MXene yarn based SC devices delivered an areal capacitance of 18.39 mF cm−<sup>2</sup> with desirable energy/power densities of 0.38 μWh cm<sup>−</sup>2/0.39 mW cm−<sup>2</sup> and long term stability (a retention percentage of 98.2% after 6000 cycles) [86].

Another fabrication approach for fiber production is wet-spinning method, which is more viable for industrial manufacture and applications [85]. Holistically, when considering the "spinnablility" for this approach to construct long continuous fibers, the high conductivity and available electrochemical activity provided by the composite formulations are the critical barrier hindering the electrochemical performances of the fiber electrodes. In addition, the homogeneous dispersion and the suitable spinning behaviors of the composite formulations also have an impact on the ultimate fiber properties (e.g., durability, flexibility, resilient stretching, bending, and twisting). In view of the hydrophilic properties and the excellent conductivity of MXene, which are suitable for wetspinning processing, Seyedin and co-workers adopted the MXene (Ti3C2Tx) and poly(3,4 ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) to successfully fabricate an MXene/PEDOT:PSS hybrid fiber electrode. By combining with the flexible PEDOT:PSS, the as-designed optimized Ti3C2Tx-based fiber electrode (containing 70% MXene and 30% PEDOT:PSS, referred to as M7P3) exhibited a record conductivity of 1.489 S cm−1, a high volumetric capacitance of 614.5 F cm−<sup>3</sup> at 5 mV s<sup>−</sup>1, and an excellent rate performance of 375.2 F cm−<sup>3</sup> at 1000 mV s−<sup>1</sup> in a potential range of −0.8–0.2 V with a three-electrode setup in1MH2SO4 electrolyte. The as-designed fiber SC device in an operated voltage of 0.6 V could deliver maximum volumetric energy/power densities of 7.13 Wh cm−3/8.249 mW cm−3. Moreover, a stretchable and elastic fiber SC prototype assembled by prestretched silicon rubber and M7P3 wrapping with a PVA/H2SO4 electrolyte maintains capacitance retention of ≈98% and ≈96% after cyclically stretched under different stretched strains [85].

Flexible stretchability MXene-based electrodes. The unique properties of MXene (e.g., metallic conductivity, strong interactions among large flakes, hydrophilic surface groups, and high volumetric capacitance (up to 1500 F cm−3)) enable them particularly attractive for stretchable energy storage devices or electronics. Nevertheless, the fragility and high mechanical modulus (3–75 GPa) at dry state for MXene would increase the device resistance when subjected to larger tensile strains and further limit their stable electrochemical outputs [30]. To address these challenges and constraints, several methods have been developed for constructing robust SC electrodes with high stretchability, superior electrochemical performance, and high chemical stability. Chen and co-workers used a sequential patterning approach to prepare high dimensional MXene nanocoatings with mechanically stable architectures, which can be program crumpling and unfolding. After transferring on the elastomer, MXene/elastomer SC electrodes were obtained, which show high stretchability for reversibly folded/unfolded. A stretchable asymmetric SC fabricated by using the as-obtained electrodes is capable of delivering stable electrochemical output under various mechanical deformations (e.g., bend or stretch), such as favorable capacitances of 395, 390, and 362 F cm−<sup>3</sup> under 0%, 50%, and 80% strains, efficient energy density of 5.5 Wh kg−1, and long term cycling stability under stretching/bending conditions [87].

Some works also revealed that the combination of Ti3C2Tx with other high mechanical robustness nanomaterials (e.g., reduced graphene oxide (RGO)) would generate flexible and stretchable free-standing electrodes with superior electrochemical performance. Cao and co-workers adopted the MXene (Ti3C2Tx in this case) and RGO to fabricate a robust and stretchable electrode (Ti3C2Tx/RGO composite) for flexible SC devices. The test results show that the final composite with 50 wt% RGO would alleviate cracks that result from large strains. Due to strong nanosheet interactions between larger nanoflakes, mechanical flexibility, and excellent electrochemical properties of each component, the as-obtained electrode delivered a capacitance of 49 mF cm−<sup>2</sup> (~490 F cm−<sup>3</sup> and ~140 F g−1) with high mechanical stability after various cyclic strains (e.g., uniaxial and biaxial strains). An all-solid-state stretchable symmetric SC fabricated by these electrodes displayed a high capacitance of 18.6 mF cm−<sup>2</sup> (~90 F cm−<sup>3</sup> and ~29 F g−1) with high mechanical stretchability [30,87–90]. The comparison of the electrochemical performance of various MXene-based materials for SCs are listed in Table 1.

