Recent progress in MXene layers materials for supercapacitors: High‐performance electrodes

Abstract

In 2011, Gogotsi et al. discovered a new type of two‐dimensional transition metal carbides and nitrides, called MXenes, which have become a dazzling new star in the energy storage industry. MXenes are endowed with a series of fascinating properties due to their unique structures and tunable surface chemical functional groups. The application of MXenes in electrochemical energy storage has attracted special attention, especially showing great potential in supercapacitor applications. Compared with other materials, MXenes have high mechanical flexibility, high energy density, and good electrochemical performance, so they are especially suitable as electrode materials for supercapacitors. However, similar to other 2D materials, due to the strong van der Waals forces, MXene layers inevitably undergo stacking agglomeration, resulting in severe loss of electrochemically active sites. If the self‐stacking of MXenes layers can be effectively suppressed, their electrochemical performance will be enhanced. Structural optimization of MXenes and composite doping of MXenes with other materials are two strategies with significant effects. This review summarizes recent advances in MXene synthesis, fundamental properties, and composite materials, focusing on the latest electrochemical performance of MXene‐based electrodes/devices, and puts forward the challenges and new opportunities that MXenes face in this emerging energy storage field.

INTRODUCTION

Today, energy storage devices (ESD) play a vital role in smart electronics, wearable textiles, and resource recycling. Rechargeable batteries (including lithium, sodium, potassium, and zinc ions) and supercapacitors are considered promising ESD for the sustainable development of smart electronics.^1–5 Although batteries are known for their high energy density, in most cases, their cycle life and charge/discharge rates are not ideal.^6,7 In contrast, supercapacitors (also known as electrochemical double-layer capacitors [EDLCs]) rely on the physical adsorption/desorption of ions by nanoporous carbon materials to store charge. This mechanism imparts high charge/discharge rates (indicative of high power density) but fundamentally limits the achievable energy density in supercapacitors due to the complex nature of porosity in electrodes.^8,9 There is a great need to find novel nanomaterials that can provide different charge storage mechanisms for supercapacitors.

MXene is composed of n + 1 (n = 1–3) layers of early transition metals (M) interwoven with n layers of carbon or nitrogen (X), and the general formula is M_n+1X_nT_x,^10–13 where T_x is introduced by liquid-phase etching. Surface functional groups, such as −OH, −F, −O, and so on, these fascinating surface groups can inherently provide a large number of active sites with great potential for surface modification and efficient loading of active materials.^14,15 Benefiting from their rich surface chemistry and tunable composition, 2D materials MXenes have excellent electrical conductivity, hydrophilicity, and ion intercalation, as well as high mechanical strength and volumetric specific capacity, which can be directly prepared binder-free electrode materials have attracted increasing interest in supercapacitor applications.^16–19 However, MXenes still have some problems to be solved.²⁰ For example, the preparation method of MXenes needs to be improved, and the traditional MXenes prepared by etching with hydrofluoric acid (HF) have a large number of defects.^21,22 In addition, MXene sheets are prone to close packing, which tends to restrict the transport and diffusion of electrolyte ions, especially organic electrolyte ions, thereby affecting the energy storage performance of MXenes in supercapacitors and limiting their excellent electrochemical performance.^23,24 On the other hand, MXenes are easily oxidized at higher anode potentials, reducing the cycle efficiency and lifetime.^25,26

To address the above problems, rational design and construction of MXene-based composite electrodes is considered an effective strategy. Over the past decade, MXene materials for supercapacitors have proliferated (Figure 1A). Notably, a total of 237 articles were published in 2021, of which 196 were related to MXene-based composites. These composite materials with MXene mainly include carbon nanomaterials, metal oxides, and conductive polymers. In particular, papers on carbon nanomaterials, metal oxides, and conducting polymers account for about 34.54%, 16.36%, and 38.18% of MXene-based composites (Figure 1B). The improved MXene-based composites can effectively weaken the MXene stacking phenomenon and improve the oxidation resistance, and have outstanding performance in improving cycle life and energy density.^27,28 Since 2013, MXene-based composites with excellent properties have been increasingly used in supercapacitors (Figure 2A–J).^{31–34,36,37} For example: Ti₃C₂T_x MXene mixed with charged polydiallyldimethyl ammonium chloride (PDDA) or electrically neutral polyvinyl alcohol (PVA) to produce Ti₃C₂T_x/polymer composites, this study is to explore the first, but also critical, step toward the potential of using MXenes in functional nanocomposites (Figure 2A).²⁹ Zhao et al.³⁰ fabricated freestanding and flexible interlayer-like MXene/carbon nanotube (CNT) papers using a simple filtration method, which exhibited high volumetric capacitance, good rate performance as electrodes for supercapacitors, and excellent cycling stability (Figure 2B). Wen's team³⁵ synthesized a novel nitrogen-doped two-dimensional MXene (N-Ti₃C₂T_x) by postetch annealing Ti₃C₂T_x (MXene) in ammonia gas, which is a promising supercapacitor electrode material (Figure 2E). With the increasingly widespread and irreplaceable role of MXene materials in the field of supercapacitors, there is an urgent need to systematically summarize the latest progress of MXene supercapacitors in a timely manner to provide possible development directions for the next generation of high-performance supercapacitors.

Figure 1. Increased demand for MXene supercapacitors over time. (A) Development of publications indexed by the keywords of “MXene supercapacitor” during the past decade. (B) A pie chart showing the proportion of various materials in MXene-base complexes. Data from Web of Science: http://www.webofscience.com/. Accessed August 24, 2021.

Figure 2. (A) Ti3C2Tx Mxene was mixed with charged polydiallyldimethyl ammonium chloride (PDDA) or electrically neutral polyvinyl alcohol (PVA) to prepare Ti3C2Tx/polymer composites. Reproduced with permission: Copyright 2014, Springer Nature.29 (B) MXene/carbon nanotube (CNT) paper electrode. Reproduced with permission: Copyright 2014, John Wiley and Sons.30 (C) MXene based SnO2 thin film nanosponge (SnO2-SPM) for stable solar energy driven simultaneous water purification and power generation. Reproduced with permission: Copyright 2015, Royal Society of Chemistry.31 (D) A rGO/Nb2CTx/Fe3O4 composite with a hierarchical architecture is prepared by integrating multilayered Nb2CTx, magnetic Fe3O4 nanoparticles and two-dimensional rGO sheets. Reproduced with permission: Copyright 2021, Elsevier.32 (E) An electrostatic assembly approach for fabricating 2D/1D/0D construction of Ti3C2Tx/carbon nanotubes/Co nanoparticles (Ti3C2Tx/CNTs/Co). Reproduced with permission: Copyright 2021, Springer Nature.27 (F) Flexible and fiber-based supercapacitors of Ti3C2Tx MXenes. Reproduced with permission: Copyright 2017, John Wiley and Sons.33 (G) The RuO2/Ti3C2Tx asymmetric pseudocapacitor. Reproduced with permission: Copyright 2018, John Wiley and Sons.34 (H) Schematic illustration of the Ti3C2Tx doped with nitrogen atoms, with terminal groups of −F and of −OH, respectively. Light gray color, Ti; dark gray color, C; red, O; white, H; cyan-blue, F; blue, N. Reproduced with permission: Copyright 2017, Elsevier.35 (I) Schematic representation of m-V2CTx MXene with various terminal functional groups. Reproduced with permission: Copyright 2020, American Chemical Society.36 (J) Braided coaxial FSC. Reproduced with permission: Copyright 2021, Springer Nature.37

Herein, a review on the use of MXene-based composites for supercapacitors is timely presented (Figure 3).^38,39 First, the synthesis, layering, and physicochemical properties of MXenes are summarized. Then the advantages and working principles of supercapacitors over other secondary batteries are presented. As the core part of this review, the design and electrode properties of MXene composites are fully discussed from four applications: symmetric supercapacitors, asymmetric supercapacitors, flexible supercapacitors, and other functional supercapacitors. Finally, the development trends, challenges, and opportunities of MXene composites for energy storage are summarized and prospected, to provide guidance for the design of MXene for energy storage and other related applications.

Figure 3. This illustration shows the importance of advanced MXene substrates in supercapacitors, including symmetrical supercapacitors, asymmetrical supercapacitors, and flexible supercapacitors. Reproduced with permission: Copyright 2020, Elsevier.38 Reproduced with permission: Copyright 2021, John Wiley and Sons.39

SYNTHESIS AND PROPERTIES OF MXene Synthesis of MXene

MXene is a kind of metal carbide and metal nitride materials with two-dimensional layered structure, which is a graphene-like structure obtained by MAX phase processing.^37,40 The specific molecular formula of the MAX phase is M_n+1AX_n (n = 1, 2, or 3), where M refers to a transition metal of the former group, such as Sc, Ti, Zr, V, Nb, Cr, or Mo; A usually represents III and IV Group chemical elements; X refers to C or N elements. Due to the strong bond energy of M–X and the more active chemical activity of A, A can be removed from the MAX phase by etching to obtain a graphene-like two-dimensional structure-MXene. Before etching, MAX has a compact bulk structure, and its properties are difficult to optimize, which limits the application range. After etching, a multilayer accordion-shaped MXene can be obtained, which can theoretically be applied in multiple fields. Finally, the etched MXene can be intercalated and layered to obtain single-layer or few-layer MXene, which will be more conducive to the modification of the material.

HF

Initially, Ti₃AlC₂ is etched with HF solution to remove the Al layer and obtain an accordion shape, this is the HF etching method.⁴¹ When Ti₃AlC₂ is immersed in HF solution, the simplified reaction principle is shown in the below equations. [Image Omitted. See PDF] [Image Omitted. See PDF] [Image Omitted. See PDF]

It can be seen from Equation (1) that the Al layer is successfully removed and that there is an exfoliated layer of Ti₃C₂ containing −OH or −F in Equations (2) and (3). Ghidiu et al.⁴² added 2 g Ti₃AlC₂ (400 mesh) to 400 ml HF solution, stirred at 200 r/min for 24 h at 25°C, centrifuged at 3500 r/min for 5 min, washed the suspension to pH 6, and finally dried at 35°C for 24 h to complete the etching step. The dried sample was then sonicated in deoxygenated water for 4 h for delamination and then centrifuged at 3500 r/min for 20 min to obtain Ti₃C₂ nanosheets. A typical HF solution etching method with the advantages of simple steps and high productivity, the operating, and the structure diagram were shown in Figure 4A. The preparation of MXene by HF etching of the MAX phase is currently the most widely used method. The MXene prepared by this method has the characteristics of clear sheet layer structure and uniform interlayer spacing. However, the HF reaction conditions are violent and there are often certain hole defects in the prepared MXene structures,^46,47 which will adversely affect the structural stability and transmission rate of the subsequently prepared electrode materials. Therefore, optimization of the reaction conditions such as HF concentration, etching time, and etching temperature is crucial.

Figure 4. (A) Schematic of the exfoliation process for Ti3AlC2. Ti3AlC2 structure. Al atoms are replaced by OH after reaction with HF. Breakage of the hydrogen bonds and separation of nanosheets after sonication in methanol. The 3D physical structure of the MAX phase and the corresponding MXene 3D physical structure. Reproduced with permission: Copyright 2011, John Wiley and Sons.41 (B) Chematic of MXene clay synthesis and electrode preparation. Reproduced with permission: Copyright 2013, Springer Nature.43 (C) MXene film synthesized by epitaxy. Schematic diagram of magnet sputtering for depositing Ti3AlC2 phase on a supporting substrate. The atomic structure of the Ti3C2 MXene film after selective removal of the Al layer. Indicates that the colors of Ti, C, O, and H are yellow, black, red, and white, respectively. Reproduced with permission: Copyright 2014, American Chemical Society.44 (D) Schematic of Pt/e-TAC catalyst formation, location of metals in the structure, and HRTEM of Pt/e-TAC. Reproduced with permission: Copyright 2014, RSC Pub.45

LiF + HCl

Gogotsi et al.⁴³ devised a one-step etching and intercalation method based on the nature of MXene's preference for cations when intercalating, with the flow shown in Figure 4B. Ti₃AlC₂ was slowly added to 6 mol/L HCl mixed with LiF and mixed with stirring at 40°C for 45 h. This process occurs in Equation (4), producing HF which is used as etching agent. The resulting precipitate was then washed to remove the reaction product and centrifuged to raise its pH to obtain a monolayer of MXene. The clay paste can be shaped according to requirements and can be used for the production of electrodes and supercapacitors. Furthermore, Soundiraraju and George⁴⁸ used a mixture of HCl + KF to etch the MAX phase of Ti₂AlN. The K⁺ and H₂O molecules in the mixed solution can intercalate the Ti₂N nanosheets, resulting in well-spaced and uniform Ti₂N phase MXene. The one-step process to obtain monolayers of MXene is simpler, gentler, and safer. [Image Omitted. See PDF]

NH₄HF₂

Both HF etching and hydrogen fluoride (NH₄HF₂) etching were investigated by Halim et al.⁴⁴ In the study, the same Ti₃AlC₂ samples were etched by two different chemical solutions and the Ti₃C₂ obtained were both successful and the presence of −OH and −F functional groups was observed. Some of the etching intercalation Equations with NH₄HF₂ are below equations. [Image Omitted. See PDF] [Image Omitted. See PDF] As can be seen from the equations above, the NH₄HF₂ etching process differs from HF in that (NH₄)₃AlF₆ is generated during the reaction and the NH₄HF₂ solution is chosen to allow direct intercalation, and both NH₃ and NH⁴⁺ are intercalated into the Ti₃C₂T_x interlayer (Figure 4C). Halim et al.⁴⁴ used mild NH₄HF₂ to slowly etch Ti₃AlC₂ at room temperature. As the Al atomic layer in the MAX phase was gradually etched, NH₃ and NH₄⁺ in solution were equally embedded in the Ti₃C₂ sheet layer, and finally, MXene with uniform atomic structure distribution was obtained. Further etching and delamination with NH₄HF₂ solution to obtain monolayer MXene is simpler than the composition of the etching method using LiF and HCl solutions and results in a few layers of MXene with richly linked moieties.

Anhydrous etching with polar organic solvents

Barsoum et al.⁴⁹ added 1 g of Ti₃AlC₂ to 10 ml of propylene carbonate (LPC) and stirred it at 500 r/min for 196 h at 35°C under argon to obtain Ti₃C₂T_x and compared the difference between etching in organic solvents such as acetonitrile (ACN), N-methylmethacrylate (NMP) and N,N-dimethylformamide (DMF) and HF alone. Finally, it was found that the electrode capacity of Ti₃AlC₂ obtained in the LPC environment was almost twice that in the HF environment, and that the low melting point of ACN and the simple recovery and purification after etching provided a new idea for large-scale industrial production. In 2021, Shi et al.¹⁴ reported a fluorine-free etching method using iodine in anhydrous acetonitrile with subsequent exfoliation in a hydrochloric acid solution. The etching reaction was kept at 100°C to ensure the production of Ti₃C₂T_x, which could be further converted to Ti₃C₂T_x nanosheets with suitable dimensions and high oxygen content.

Molten fluoride salt etching

Ghidiu et al.⁴² mixed Ti₄AlN₃ powder with a fluoride salt mixture (59 wt%) potassium fluoride (KF), 29 wt% lithium fluoride (LiF), and 12 wt% sodium fluoride (NaF) in a 1:1 mass ratio and then heated the mixture at 550°C for 30 min at a rate of 10°C/min to prepare Ti₄N₃T_x for the first time. The innovative use of molten salt etching to obtain Ti₄N₃T_x provides a new idea for the preparation of MXene. However, the MXene structure will contain large amounts of fluoride salt impurities and the high-temperature molten salt method will be strictly limited by decomposition conditions such as temperature and thickness of the raw material. Li et al.¹⁸ used Lewis acid (ZnCl₂) instead of HF to obtain a new MXene (M_n+1XnCl₂) with a Cl surface. In addition, further exfoliation of Ti₂AlC, Ti₃AlC₂, Ti₃AlCN, Nb₂AlC, Ta₂AlC, Ti₂ZnC, and Ti₃ZnC₂ MAX phases using a variety of molten salts (CdCl₂, FeCl₂, CoCl₂, CuCl₂, AgCl, NiCl₂) confirmed the commonality of molten salt particles to obtain MXene.

NaOH + H₂SO₄

Xie et al.⁴⁵ first immersed large pieces of Ti₃AlC₂ in NaOH solution for surface treatment, followed by H₂SO₄ hydrothermal treatment to obtain Ti₃C₂ layers with the Al layer and the −OH-containing end removed. The conventional carbon carrier will degrade the Pt/C cathode catalyst, resulting in reduced catalyst activity and stability, while the Pt/e-TAC layered structure obtained by loading Pt onto the Ti₃C₂ layer will produce strong interactions causing changes in the electronic structure, resulting in higher catalytic activity and stability, indicating that the NaOH + H₂SO₄ etching method is more suitable for use in harsh environments (Figure 4D). [Image Omitted. See PDF]

Bayer etching method

In 1887, Bayer refined bauxite for the aluminum industry, using concentrated NaOH solutions at high temperature and pressure to erode bauxite and extract aluminum, lithium, and so on. Inspired by this, it was recognized that higher temperatures and higher alkali concentrations would help dissolve aluminum hydroxide. After gradient experiments, Yang et al.⁵⁰ found that treatment in an aqueous 27.5 mol/L NaOH solution at 270°C under an argon atmosphere gave Ti₃C₂T_x (Figure 5A) with high purity (92 wt%) and no −F contamination to obtain samples containing −OH and −O terminals (Figure 5B,C). This provides new ideas and synthetic diversity options for the preparation of MXene and is more suitable for use in large-scale preparations.

Figure 5. (A) Schematic of the etching and delamination process. Cross-sectional HRTEM images (B) and SEM images (C) of Ti3C2Tx. Reproduced with permission: Copyright 2018, John Wiley and Sons.50 (D) Schematic preparation process of few-layered Ti3C2Tx MXene powders. (E–G) SEM images of Ti3C2Tx MXenes. (H) AFM results of Ti3C2Tx MXenes. (I,J) TEM images of Ti3C2Tx MXenes. Reproduced with permission: Copyright 2020, American Chemical Society.51

Solution phase flocculation etching

The solution phase flocculation method (NH₄⁺ method and modified NH₄⁺ method) was used by Zhang et al.⁵¹ to prepare monolayer or less L amount of layered Ti₃C₂T_x powder on a large scale (Figure 5D), which fundamentally solved the agglomeration problem of MXene nanosheets. NH₄⁺ combined with a small amount of MXene nanosheets reduced the negative surface charge of MXene, and the MXene nanosheets finally completed classical flocculation, and after freeze-drying, annealed to produce TiC₂T_x nanosheet powder. A variety of layered MXene powders such as V₂CT_x, Nb₂CT_x, Nb₄C₃T_x, and Ti₃C₂T_x could also be successfully prepared from silica-based MAX using this fast NH₄⁺ method, demonstrating the advantages of the universality and mass production of the method.

Other methods of etching

The synthesis of carbide-derived MXene is also possible by passing the more corrosive chlorine gas into the carbon-based MAX phase by gas phase etching. In addition, alkali solution etching has also become a new option for the synthesis of MXene. Yang et al.⁵² used electrochemical etching to prepare fluorine-free Ti₃C₂ phase MXene materials in yields of over 90% in alkaline solutions above 270°C and with a large number of hydroxyl functional groups in the structure.

With the increasing variety of raw material nitrides and carbides and the significant increase in demand for MXene, more and more preparation methods have been developed,^{16,17,19,53–57} such as chemical meteorological deposition (CVD) and plasma enhanced pulsed laser deposition (PE-PLD).⁵⁸ These methods are still in the exploratory stage and prepare MXene for a smaller range of applications.⁵⁹

In LiF + HCl, NH₄⁺ etchants, both Li⁺ and NH₄⁺ have the ability to intercalate in MXene, thus allowing mechanical sonication to obtain a single layer of MXene, while etching steps where intercalation is not possible (e.g., HF) require additional intercalation to obtain a single layer of MXene, with the option of mechanical layering and solvent intercalation layering methods.^15,60–66 Due to the strong interactions between multiple layers of MXene, most organic compounds are currently chosen as intercalators for delamination,^67–72 for example, hydrazine hydrate, urea, isopropylamine, tetrabutylammonium hydroxide or dimethylsulfoxide after HF etching, the latter two being considered as excellent intercalators for use.^73–80

In summary, it can be seen that the various preparation schemes for MXene have shortcomings.^26,81–88 The structure of HF acid etching is clear and complete, but monolayer MXene cannot be obtained in one step; the structure of MXene obtained by the LiF + HCl method is more ambiguous and the small monolayer pieces are easy to fall off, which is suitable for the preparation of monolayer MXene; the NH₄HF₂ product is hierarchical but the by-products are difficult to remove; the product obtained by molten fluoride salt is incomplete in structure and fluoride impurities have to be removed, so when no new perfect preparation method appears, it is necessary to choose a suitable MXene preparation scheme according to the content of each study. Table 1^89–99 lists the preparation conditions and parameters of some MXenes.

Table 1 Experimental parameters of MXene synthesis

MXene	MAX phase	Etching solution	Etching time (h)	Temperature reflex	References
Ti3C2Tx	Ti3AlC2	50% HF	2	RT	[41]
	Ti3AlC2	40% HF	20	RT	[10]
	Ti3AlC2	NH4HF2	11	RT	[44]
	Ti3AlC2	KHF2	24	60°C	[89]
	Ti3AlC2	3 mol/L LiF + 6 mol/L HCl	24	35°C	[90]
	Ti3AlC2	27.5 mol/L NaOH	12	270°C	[91]
	Ti3SiC2	1.17 mol/L H2O2 + 15 mol/L HCl	45	40°C	[92]
Ti2CTx	Ti2AlC	10% HF	10	RT	[10]
Ti3CNTx	Ti3AlCN	30% HF	18	RT	[10]
TiNbCTx	TiNbAlC	50% HF	28	RT	[10]
Ta4C3Tx	Ta4AlC3	50% HF	72	RT	[10]
(V0.5Cr0.5)3C2Tx	(V0.5Cr0.5)3AlC2	50% HF	69	RT	[10]
Mo2CTx	Mo2Ga2C	50% HF	3	50°C	[93]
	Mo2Ga2C	3 mol/L LiF + 12 mol/L HCl	384	35°C	[94]
	Mo2Ga2C	14 mol/L HF	160	55°C	[94]
Mo2TiC2Tx	Mo2TiAlC2	50% HF	48	RT	[95]
Mo2Ti2C3Tx	Mo2Ti2AlC3	50% HF	90	55°C	[95]
Nb4C3Tx	Nb4AlC3	48%–51% HF	96	RT	[42]
Nb2CTx	Nb2AlC	50% HF	90	RT	[96]
(Nb0.8Ti0.2)4C3Tx	(Nb0.8Ti0.2)4AlC3	50% HF	96	50°C	[97]
(Nb0.8Zr0.2)4C3Tx	(Nb0.8Zr0.2)4AlC3	LiF + 12 mol/L HCl	168	50°C	[97]
V2CTx	V2AlC	50% HF	90	RT	[96]
Zr3C2Tx	Zr3Al3C2	50% HF	72	RT	[98]
Hf3C2Tx	Hf3Al(Si)4C6	35% HF	60	RT	[99]

Properties of MXene

As M in MXene is a transition metal (e.g., Ti, Mo, Cr, V, etc.) and the surface functional groups vary,¹⁰⁰ MXenes of different compositions exhibit different properties.^101,102 The various properties and applications of MXene are shown in Figure 6.

Figure 6. Schematic showing the properties of MXenes (conductivity, mechanical flexibility, hydrophilicity, thermal stability) and applications (energy storage, sensors, electromagnetic shielding, biodetection).

Electrical properties

By first-principles calculations, MXenes exhibit similar conductivity to metals when there are no functional groups on the surface, but it is still a challenge to prepare synthetic MXenes without surface functional groups.¹⁰³ The conductivity of Ti₂CT_x, Ti₃C₂T_x, Mo₂CT_x, Mo₂TiC₂T_x, and Mo₂Ti₂C₃T_x has been obtained by experiments.^53,104 It has been shown that the conductivity of MXenes is closely related to the type of M-layer atoms, for example, titanium-based MXenes have metal-like conductivity, while molybdenum-based MXenes exhibit semiconducting properties.¹⁰⁵ Surface functional groups also have a great influence on the electrical conductivity of MXenes. Taking TiCT_x, the most typical representative of MXenes, as an example, when T_x is a fluorine group (Ti₂F_x), its Fermi level is located at the d-bond of the M-layer atom, which has metal-like conductivity, while when T_x is an oxygen functional group (Ti₂CO_x), whose d-bonds are above the Fermi level, exhibiting semiconducting properties.¹⁰⁶ Another factor that affects the conductivity of MXene is the state of the MXene sheet.^107,108 The fewer defects and the larger the lamellar scale, the higher the electrical conductivity of MXenes. For example, Ti₃C₂T_x etched by HF has a large number of defects, and the conductivity after cold pressing is lower than 1000 S/cm,¹⁰⁶ while the Ti₃C₂T_x etched by LiF and HCl mixed system reached 4600 and 15,100 S/cm after vacuum filtration and dirty coating, respectively.^104,109

Mechanical properties

Through first-principles and molecular dynamics simulations, the elastic modulus and flexural strength when stretched along the MXene plane are higher than those of multilayer graphene in the same case, with favorable mechanical properties.¹¹⁰ MXenes can be vacuum filtered to produce films with excellent mechanical strength.^108,111 Both the M atom and the surface group T have significant effects on the properties of MXenes. In general, oxygen-functionalized MXenes have smaller lattice parameters and higher mechanical strength than fluorine- and hydroxyl-functionalized MXenes, and the fracture strains of 2D Ti₂Co₂ due to strong covalent bonds between Ti atoms and surface end groups, respectively increased by 20%, 28%, and 26.5%.^112,113 In other words, the bonding strength is a key factor in maintaining the elastic stiffness of the nanosheets. On the other hand, the mechanical properties of Ti₃C₂T_x films can be improved by incorporating polymers such as chitosan and polyethylene.¹¹⁴ The introduction of chitosan can amplify the displacement of Ti₃C₂T_x nanosheets, increasing the tensile strength of the film from 8.2 to 43.5 MPa.¹¹⁴ By incorporating PVA into the MXene matrix, the mechanical strength of the composites can be greatly enhanced. For example, Ling et al.²⁹ embedded PVA into Ti₃C₂T_x thin films, which showed good electronic conductivity and excellent mechanical properties, which could withstand 5000 times its own weight. This study presents a huge opportunity to form conducting polymer/MXene composites, that is, reinforced composites, for various applications.

Hydrophilic

The surface of MXene prepared by liquid phase etching contains a large number of −OH, −O, −F, and other energy groups, which makes its surface negatively charged. When dispersed in water, MXene will maintain a stable dispersion in an aqueous solution due to the classical repulsion between MXene layers.^115,116 The hydrophilic nature of MXenes not only allows them to be used for the preparation of MXene inks, films, and sprayed devices but also facilitates the liquid phase synthesis of MXene-based nanocomposites.^117,118

Thermal stability

Density functional theory calculations show that MXene has negative lattice energy, its structure does not change at room temperature and atmospheric pressure, and it is relatively stable.^{115,119–121} The chemical expression of MXene is M_nT_x, and the stability of MXene increases with the value of n. On the contrary, the binding energy of MXene decreases with the increase of n value, and the binding energy of carbon atom MXene is smaller than that of nitrogen atom MXene, indicating that the stability of transition metal carbides is better than that of transition metal nitrides. Combined with thermogravimetric and mass spectrometry analysis demonstrated that the thermal stability of MXenes strongly depends on their chemical composition and environment.^108,122 However, MXenes with exposed metal atoms on the surface are generally thermodynamically metastable, possess high surface energies, and usually oxidize spontaneously in air.¹²³ For Ti₂C MXenes, the unsaturated Ti 3D orbitals on the pristine Ti₂C surface strongly interact with the approaching O₂ molecules, leading to efficient O₂ dissociation. Therefore, the adsorbed O on Ti₂C impairs the thermodynamic stability of the latter.¹¹² In addition, the excellent thermal conductivity of MXene also facilitates the heat dissipation of electronic devices and improves the reliability of the devices.^124,125

Other properties

MXene film and its complex have a certain transparency.^66,67,126 It is found that the 1 nm thick MXene can penetrate 97% of visible light, and its photoelectronic properties can be adjusted by chemical or electrochemical methods to embed cations, indicating that MXene has a certain prospect in the field of transparent conductive coatings and photoelectronic devices.¹⁵

According to theoretical calculations, MXenes without functional groups on the surface exhibit a certain degree of magnetism,^127–129 but once functional groups are present, the magnetism decreases or even disappears, but there are special cases where, for example, chromium-based MXenes exhibit magnetism even in the presence of functional groups.⁶⁴

These properties have made MXene widely studied in various fields. For example, in the field of energy storage,^{40,76,130–136} Zhang et al.¹³⁷ used C-MoS₂/CNTs-Ti₃C₂ composite negative electrodes to achieve excellent performance sodium-ion batteries. Sikdar et al.¹³⁸ proposed a pure room temperature casting-based method with zinc particles to induce MXene and graphene 3D self-assembly for high-performance pseudocapacitors. Wu et al.¹³⁹ used a facile strategy of multiscale structural engineering to fabricate high-performance MXene hydrogel supercapacitor electrodes. Lin et al.¹⁴⁰ reported a method to synthesize MXene by preparing titanium Al carbide using elemental precursors and subsequent in situ etching in molten salt tanks and explored its excellent lithium-ion energy storage properties. Yao et al.¹⁴¹ selfassembled MXene + ZIF derived composite electrodes for improving the multiplicative performance of Al-ion batteries. Wang et al.¹⁴² used the topology of MXene films to design high-performance lithium batteries. In the field of catalysis,^44,143,144 MXene can be used for high-temperature catalysis.³⁶ MXene powers photocatalytic synthesis of NH₃.¹⁴⁵ In the field of electromagnetic shielding,^18,50,146 Xiao et al.¹⁴⁷ demonstrated a polymer-polyacrylic acid latex (PAL)-based MXene aqueous coating (MWP), which not only has powerful terahertz EMI shielding/absorption efficiency but also can be easily adhered to various substrates commonly used in the terahertz band. Miao et al.¹⁴⁸ successfully prepared Ag nanowires by laminating them with MXene to produce flexible electromagnetic shielding films. In the field of sensing,^149–154 Ko et al.¹⁵⁵ demonstrated a 3D multivacancy structure based on MXene(Ti₃C₂T_x)/PVDF with high sensitivity and wide sensing range for self-supplied voltage electronic skin. Wang et al.¹⁵⁶ reported a self-fed NH₃ sensor prepared from accordion-like Ti₃C₂T_x MXene and metal-organic backbone derived CuO. In the field of biology,^91,92 Cao et al.¹⁵⁷ drove MXene-based sodium/potassium ion storage by adding microorganisms and MXene-based surface Raman high-sensitivity detection as a new method for the detection of neo-coronavirus S proteins.¹⁵⁸ In recent years, MXene has been more widely used, for example, MXene membranes to fuel chemical potential discharges,¹⁵⁹ MXene quantum dots,²⁴ MXene membranes to anchor wastewater treatment,¹⁶⁰ MXene based preparation of sensitive humidity driven brakes¹⁶¹ and high-efficiency MXene based organic light-emitting diodes.¹⁶² In particular, MXene's excellent electrical conductivity, hydrophilicity, abundance of surface functional groups, and high density have led to the most extensive research in the field of flexible energy storage (flexible batteries,¹⁶³ flexible supercapacitors¹⁶⁴).

ADVANTAGES AND PRINCIPLES OF SUPERCAPACITORS Features of supercapacitors

Supercapacitors have higher energy density and power density than other ESD. Supercapacitors are capable of storing thousands or even tens of thousands of times more power than ordinary capacitors,^61,63 which is the reason for the “super,” by generating a large capacity through a large specific surface area or redox reaction.^165,166

The performance of supercapacitors is compared with that of flat plate capacitors and conventional batteries in Table 2.¹⁶⁷ From the table, it can be seen that supercapacitors show the ability to store energy in conventional batteries, but also have the advantages of ordinary capacitors. Specifically, supercapacitors have the following characteristics.^{103,166,168,169}

• (1)

  Super-high electric capacity:^170–173 Supercapacitors have a large capacity (6000 F), thousands of times larger than those of flat capacitors of the same volume.^114–119

• (2)

  Super-high specific power: The specific power of supercapacitors is tens to hundreds of times higher than that of batteries.^89,174–176 It can release high currents ranging from hundreds to thousands of amperes in a very short period of time. Therefore, supercapacitors are more suitable than other ESD for high output power and short charge/discharge times.^89,122,177

• (3)

  Superfast charging speed: There are two modes of supercapacitor charging, a physical process (charging and discharging of the double electric layer) and a chemical process (the reversible and fast electrochemical process of electrodes adsorbing substances on their surface).^178–180 The synergy of the two engineering processes allows supercapacitors to be charged and discharged using high current density, enabling them to be charged and discharged in a very short time with very little loss of capacity.^177,181

• (4)

  Extra-long service life: In the process of charging and discharging, a supercapacitor occurs with a good reversible electrochemical reaction, and it is not easy to have the phenomenon of active material crystal transformation and shedding, so the life of a supercapacitor is very long.^182,183 In theory, the life of a supercapacitor can be infinite, but in practice, it can reach more than 100,000 times, but it is also hundreds of times of the battery.^175,184

• (5)

  Excellent low-temperature performance: When supercapacitors work, most of the charge transfer process takes place on the surface of the electrode active material, so it is not affected by temperature. Unlike batteries, whose capacity is affected by electrochemical thermodynamics, batteries can lose more than 70% of their capacity at low temperatures.^185,186

Table 2 Comparison of supercapacitors, capacitors, and batteries

Parameters	Supercapacitor	Capacitor	Battery
Energy storage	Watt-second energy	Watt-second energy	Watt-hours of energy
Charging method	Voltage between terminals	Voltage between terminals	Current and voltage
Power supplied	Fast discharge, linear or exponential voltage decay	Fast discharge, linear or exponential voltage decay	Constant voltage over a long period of time
Charging/discharging time	Milliseconds to seconds	Picosecond to millisecond	1–10 h
Dimension	Small	Small to large	Large
Weight (g)	1–2	1–10	1 to >10
Energy density (Wh/kg)	1–5	0.01–0.05	8–600
Power density (W/kg)	High, >4000	High, >5000	Low, >100–3000
Operating voltage	2.3–2.75 V per section	6–800 V	1.2–4.2 V per section
Lifetime	>100 k cycles	>100 k cycles	20–1500 cycles
Operating temperature (°C)	−40 to 85	−20 to 100	−20 to –35

Note: Reproduced with permission: Copyright 2018, American Chemical Society.¹⁶⁷ Energy storage principle of supercapacitor

Depending on the energy storage mechanism, supercapacitors are divided into two categories, one is the double layer supercapacitor and the other is the Faraday pseudo capacitor.^187–191 As an ESD, the size of its energy storage is the size of its capacitance.^19,192–195

• (1)

  Electric double-layer capacitors (EDLCs)

  The energy storage principle of a double-layer capacitor is simply that the energy is stored through the double layer formed at the interface between the electrode and the electrolyte, using the high specific surface area of the electrode material.^138,139 The detailed explanation is that when the electrode is charged, under the action of intermolecular forces, a stable double layer of charges of opposite sign will appear on the contact surface of the electrode and the electrolyte,^123,140,141 and because there are barriers at the interface, the double layer of charges will not be neutralized, thus forming a tight double layer on the surface of the electrode, called the interface double layer.¹⁴² The principle is shown in Figure 7A.

• (2)

  Faraday pseudo capacitor

  For Faraday pseudocapacitance, the stored charge contains not only the charge stored on the double potential, but also the charge stored by the electrolyte ions in the active material through redox reactions.¹⁴³ The charging and discharging principle of the pseudo capacitor is shown in Figure 7B. The electrode is charged under the action of an applied electric field,^196,197 and the ions in the electrolyte move from the solution to the electrode–solution interface, followed by an electrochemical reaction at the interface, and then the charge enters the electrode active material, and a large amount of charge is stored in the electrode.¹⁴⁶ During discharge, the stored charge is released in the form of current through an external circuit, while the electrolyte ions that have entered into the active substance reenter the electrolyte solution.¹⁴⁷

Figure 7. Schematics of charge-storage mechanisms for (A) electrochemical double-layer capacitors and (B) different types of pseudocapacitive electrodes: underpotential deposition, redox pseudocapacitor, and ion intercalation pseudocapacitor. Reproduced with permission: Copyright 2018, American Chemical Society.167

Classification of supercapacitors

As there are more criteria for classifying supercapacitors, this paper mainly introduces two classifications, the first one according to the different energy storage mechanisms of electrode materials and the second one according to the different electrolytes.

• (1)

  Classification according to different energy storage mechanisms

  According to the different energy storage mechanisms, supercapacitors can be classified into symmetrical supercapacitors, asymmetrical supercapacitors, and hybrid supercapacitors, and the performance of the three types of supercapacitors is shown in Table 3.¹⁶⁷

• (2)

  Classification according to different electrolytes

  According to the type of electrolyte, it can be conventionally classified into the water-based electrolyte and organic electrolyte, of which water-based electrolyte includes three types, one is an acidic electrolyte, mostly using 36% H₂SO₄ aqueous solution as an electrolyte, the second is an alkaline electrolyte, usually using strong bases such as KOH and NaOH as electrolyte and water as a solvent, and the last is a neutral electrolyte, usually using KCl and NaCl as electrolyte and water as solvent. The last category is neutral electrolytes, usually using salts such as KCl and NaCl as the electrolyte and water as the solvent, mostly used in the electrolyte of manganese oxide with age materials. Organic electrolytes usually use LiClO₄ as a typical representative of lithium salts, Tetraethylammonium tetrafluoroborate (TEABF4) as a typical representative of quaternary amine salts, and so forth as electrolytes, and organic solvents such as polycarbonate (PC), acetonitrile (ACN), gamma-butyrolactone (GBL), trehalase (THL), and so on as solvents, where the electrolyte is close to saturation solubility in the solvent. Also included are solid-state electrolytes, which have become a research hotspot in the field of electrolytes for supercapacitors as solid-state electrolytes for lithium-ion batteries continue to break through.

Table 3 Performance indicators for different types of supercapacitors

Projects	Symmetrical supercapacitors	Asymmetric supercapacitors	Hybrid supercapacitors
Main mechanisms	Double electric layer	Double electric layer + pseudocapacitor	Double electric layer + Faraday
Energy density (Wh/kg)	5	30	100
Power density (kW/kg)	9	5	4
Operating temperature (°C)	−40 to 80	−25 to 60	−40 to 60
Typical electrodes	Carbon material	Carbon materials metal oxides conductive polymers	Carbon materials de-embeddable materials, and so on
Typical electrolyte	Organic system	Aqueous system	Organic systems
Maturity	Already commercialized	Material research	Process research Early commercialization
Advantages	High power density	Higher power density higher energy density	High energy density
Disadvantages	Low energy density	High price short lifetime	Low power density

Note: Reproduced with permission: Copyright 2018, American Chemical Society.¹⁶⁷ MODIFICATION OF MXENE FOR SUPERCAPACITOR Structural optimization of MXenes

The problem of heavy stacking of 2D MXene materials greatly limits the electrochemical performance of their electrode materials.¹⁹⁷ Typical strategies such as the insertion of nanomaterials and 3D structure design are expected to reduce the advantages of MXene materials and diminish the advantages of their electrodes over other electrode materials. Tang et al.¹⁹⁸ proposed a novel sulfuric acid oxidation method for the restacking of link MXene layers with a small amount of electrochemically active negative products (Figure 8A,B). The MXene obtained by the optimized pathway still has a very high surface-specific capacitance and is of great practical application.

Figure 8. (A) Schematic illustrations of the etching process of Ti3C2Tx nanosheets and the obtained hierarchical nanoporous structure. (B) The schematic illustration of the ion pathway optimization in S-etched Ti3C2Tx film compared with the l-pristine film. Reproduced with permission: Copyright 2020, John Wiley and Sons.198 (C) Schematic of the synthesis of few-layer Ti3C2Tx. (D) Top-view and (E) cross-sectional SEM image of Ti3C2Tx freestanding film. Reproduced with permission: Copyright 2020, Elsevier.38 (F) Schematic diagram of the strategy for constructing the bioinspired MXene pearl layer. (G) FE-SEM image of MXene@SnS2 heterostructure. Reproduced with permission: Copyright 2021, Elsevier.199

Zhang et al.³⁸ performed low-temperature annealing (below 400°C) of MXene under argon conditions to improve the capacitive properties of the MXene material (Figure 8C). This was due to the formation of more active C–Ti–O sites and larger interlayer voids (Figure 8D). This MXene can exhibit high capacitive energy (429 F/g) in a sulfuric acid electrolyte with an energy density of 29.2Wh/kg and the capacitance is maintained at 89% after 5000 cycles, achieving the importance of maintaining the original structure of MXene while improving its electrochemical properties.

Inspired by the mortar and brick structure of the pearl layer, Cai et al.¹⁹⁹ developed a stand-alone MXene pearl layer through a layer-by-layer column and effectively willed the stacking of MXene nanosheets (Figure 8F). The MXene “pearl layer” (Figure 8G) exhibits excellent mechanical strength (78.3 MPa) without sacrificing flexibility, and good specific capacitance (190 F/g in 1 mol/L H₂SO₄ at 10 mV/s scan rate), and high capacitance retention (87.4% after 5000 cycles). The all-solid-state supercapacitor exhibits 6.7 μWh/cm², 91.5% capacitance retention, and superb cycling performance (over 90% capacitance retention after 500 cycles of folding/unfolding) after 4000 cycles. This study provides a new idea for further applications of structural flexible ESD.

MXene-based composites for supercapacitors Symmetrical supercapacitors

Sun et al.²² prepared layered MXene nanosheets on cobalt sulfide nickel/carbon cloth (CC) by spraying MXene on the surface of cobalt sulfide nickel (Figure 9A). The nanosheets (Figure 9B) not only obtained excellent specific capacitance at high current densities but also had better cycling stability. The electrodes were optimized to achieve a maximum specific capacitance of 2326 F/g at a current density of 1 A/g and a cycling stability of 93.8% at a current density of 10 A/g. The results show that proper surface coating of MXene can simultaneously improve the conductivity and ion penetration of cobalt-nickel sulfide, resulting in excellent electrochemical performance. Thus, the excellent electrochemical properties of this hybrid electrode make it a prominent candidate for high-performance, flexible ESD.

Figure 9. (A) Schematic illustration of the assembly process of Ti3C2Tx/NiCo2S4@CC (TNSC) by hydrothermal and spray coating methods. (B) SEM image of TNSC. Reproduced with permission: Copyright 2021, Springer Nature.7 (C) Galvanostatic charge–discharge (GCD) of xCoOx-NiO/Ti3C2Tx and Ti3C2Tx. (D) X-ray photoelectron spectroscopy (XPS) survey spectra of xCoOx-NiO/Ti3C2Tx and Ti3C2Tx. (E) Cyclest ability of xCoOx-NiO/Ti3C2Tx at a current density of 5 A/g. Reproduced with permission: Copyright 2021, Elsevier.200 (F) The represent SEM image of PPy-MXene coated textile. (G) The comparison of GCD curves between MXene-based and PPy-MXene coated textile electrodes under a current density of 2 mA/cm2 (H) GCD curves of PPy-MXene coated textile electrodes at various current densities. Reproduced with permission: Copyright 2021, American Chemical Society.201

Polymers are another promising additive that can be combined with PEDOT to greatly improve the mechanical properties of materials, particularly conductive polymers, which can optimize mechanical strength without sacrificing conductivity. Liu and coworkers²⁰² successfully prepared Ti₃C₂T_x@PEDOT composites with significantly improved electrical properties for use as supercapacitor electrode materials by loading a large number of nanoscale PEDOT particles on the surface of Ti₃C₂T_x via chemical oxidation. Compared with pure Ti₃C₂T_x, Ti₃C₂T_x@PEDOT has improved capacitive performance and good electrochemical stability (Figure 9C). One of the Ti₃C₂T_x@PEDOT (1:10) composites showed a maximum capacitance value of 564 F/g at a current density of 1 A/g and capacitance retention of 96.5% after 10,000 charge/discharge cycles (Figure 9E), which can be attributed to the synergistic effect between Ti₃C₂T_x and PEDOT. PEDOT with high conductivity can improve the interlayer conductivity and charge transport of Ti₃C₂T_x, effectively inhibiting the restacking of multilayer Ti₃C₂T_x, where the sulfur in PEDOT can further improve the hydrophilicity and ion exchange rate of the composite. Yan et al.²⁰¹ prepared selfsupporting PPy-Ti₃C₂T_x MXene film materials (Figure 9F) by electrochemical deposition of PPy and Ti₃C₂T_x MXene on the fabric surface and assembled all-solid-state flexible capacitors, which also exhibited good electrochemical properties (Figure 9G).²⁰⁰

In recent years, cobalt and nickel oxides have become typical pseudocapacitive materials due to the availability of low cost, low toxicity, and high theoretical ratio capacitance. Cobalt–nickel bimetallic oxides have attracted much attention due to their enhanced charge transfer and surface redox reactions that increase their specific capacitance. Zhang et al.²⁰⁰ successfully deposited cobalt–nickel bimetallic oxides on MXene nanolayers using atomic layer deposition techniques and used them as pseudocapacitive materials for supercapacitors. The uniform distribution of metal oxide particles on the MXene nanosheets enabled the pseudocapacitor material to have more active sites, and the synergistic effect between cobalt oxide and nickel oxide greatly improved the electrochemical performance. Using the number of times the metal oxide was deposited as a variable parameter, the material exhibited an optimum electrochemical capacitance of 1960 F/g at a current density of 1 A/g as the number of depositions reached 90, and still preserved 90.2% capacitance after 8000 cycles, demonstrating excellent cycling stability.

Ti₃C₂T_x, as a type of the MXene family, has ultrahigh electrical conductivity, good flexibility, and high electrochemical activity, but the inevitable Ti₃C₂T_x film self-stacking problem seriously affects the specific area of the film and hinders ion transport in the film. Zhang et al.¹³¹ introduced layered porous carbon (HPC) into Ti₃C₂T_x films (Figure 10A), which not only, can act as a columnar structure for adjacent Ti₃C₂T_x nanosheets (Figure 10B), effectively preventing their typical self-stacking and accelerating ion transport in the lateral direction, but also ensuring fast ion transport in the longitudinal direction by introducing abundant macroscopic mesopores (Figure 10C). The quasi-solid-state symmetric supercapacitor prepared by introducing 60% HPC exhibits good stability at different bending angles with a capacitance of 211 mF/cm² and an energy density of 4.68 μWh/cm² at 19.91 μW/cm², and capacitance retention at 10,000. The capacitance retention after charge/discharge cycles was 86%. By constructing transverse and longitudinal channels for rapid ion transport, a foundation is laid for alleviating the self-stacking phenomenon in Ti₃C₂T_x films.

Figure 10. (A) Schematic illustration for preparing the MXene/layered porous carbon (HPC) hybrid film by vacuum-assisted filtration. (B) Large number of macroscopic holes in Ti3C2Tx nanosheets. (C) Longitudinal channels. Reproduced with permission: Copyright 2021, Elsevier.131 (D) Digital photographs of Ti3C2Tx MXene colloid and flexible films. (E) Schematic illustration of the fabrication process for the Ti3C2Tx MXene negative electrode (left section), IDT@rGO heterojunction positive electrode (right section), and the all-solid-state asymmetric supercapacitor device. Reproduced with permission: Copyright 2021, American Chemical Society.203 (F) Schematic representation illustrating the fabrication process of asymmetric supercapacitor. (G) Low magnification SEM image showing uniform coating of Ti3C2Tx on carbon fibers. (H) Wrapped Ti3C2Tx flakes over an individual fiber of the carbon fabric. Reproduced with permission: Copyright 2021, Royal Society of Chemistry.39

Asymmetric supercapacitors

The voltage window for the symmetrical MXene supercapacitor is only about 0.6 V because oxidation occurs at high anode potentials. The assembly of MXene electrodes and other pseudocapacitive materials into asymmetric supercapacitors makes for an effective strategy to extend the voltage window.

Xu et al.²⁰³ took the breakthrough of expanding the voltage window and maximizing capacitance by rationally coupling a multielectron redox reversible and structurally stable indolone π backbone with a reduced graphene oxide (GO) backbone in 2021 to form idt@rGo molecular heterojunction to fabricate a flexible asymmetric supercapacitor based on PVA/sulfuric acid hydrogel electrolyte, this flexible supercapacitor is based on idt@rGo heterojunction positive electrode and Ti₃C₂T_x MXene (Figure 10D) negative electrode, steps as shown Figure 10E,²⁰³ due to the rational construction of graphene and hetero molecules and the selection of MXene film as the paired negative electrode, which increases the electrode capacity and widens the assembled asymmetric supercapacitor, can output 1.6 V and high capacitance of 60 F/g for the whole device, reaching 17 Wh/kg at a power density of up to 8 kW/kg, and also has excellent multiplicity performance. The device also offers excellent multiplier performance, cycle stability, and mechanical flexibility. These outstanding performances offer brighter prospects for the future of smart and wearable devices.

Jiang et al.³⁴ designed a fully pseudocapacitive asymmetric supercapacitor (Figure 10F) by combining MXene in an acidic electrolyte with a ruthenium oxide cathode, extending the operating window of the device to 1.5 V, approximately twice that of a symmetric MXene supercapacitor. Ti₃C₂T_x MXene sheets are uniformly coated with carbon fibers (Figure 10G,H).³⁹ The complementary operating site windows of MXene and ruthenium oxide and the proton-induced pseudocapacitance improved the performance of the device. After 20,000 charge/discharge cycles, the capacitance remains at around 86% with very high cycle stability.

Liang et al.³⁹ assembled asymmetric supercapacitors by wrapping MXene material on CNTs as the negative electrode and polybrene on CNT as the positive electrode, resulting in devices with voltages up to 1.6 V. The good material performance at high active masses was associated with the application of multifunctional chelating dispersants for the codispersion of MXene and CNT and highly water-insoluble hydrophilic binders. The high-capacitance polybrene cladding on CNT obtained in the complementary site range first had an excellent performance as a positive electrode, achieving higher capacitance at a lower resistance.

The surface modification of MXene was also carried out by Ma et al.²⁰⁴ in response to the build-up problem. The P-Π conjugated structure of the lignosulfonate imparts a strong chemical activity and local positive potential to α and β carbonyl groups (Figure 11A), which can modify the surface of MXene and avoid the problem of MXene restacking. On this basis, lignosulfonate-modified MXene reduced GO 3D porous aerogels were synthesized, exhibiting excellent electrochemical properties and being lightweight (Figure 11B). The energy density of this asymmetric supercapacitor was 142 Wh/m² at a power density of 4900 μW/cm², and the capacitance retention was 96.3% after 10,000 charge/discharge cycles (Figure 11C), achieving the dual utilization of the high chemical reactivity and pseudocapacitance properties of lignosulfonates on the positive and negative electrodes of the asymmetric supercapacitor.

Figure 11. (A) Illustration of the fabrication process of the LS modified-MXene (Ti3C2Tx)-reduced graphene oxide (rGO) 3D porous aerogel (MLSG) aerogel negative electrodes and LSG aerogel positive electrodes. (B) Digital images of MLSG-6 aerogel standing on a flower. (C) Cycling stability and coulombic efficiency of the asymmetric device over 10,000 cycles in 3 mol/L H2SO4 electrolyte at a current density of 8 A/g. Reproduced with permission: Copyright 2021, Elsevier.196 (D) Schematic showing the synthesis of materials and their application in SCs. (E) Sub-50 nm MnO2 NWRs, and (F) MXene flakes incorporated with MnO2 NWRs. Reproduced with permission: Copyright 2021, American Chemical Society.203 (G) Schematic diagram of the preparation process of the asymmetric supercapacitor. (H) GCD curves of the device at different current densities. (I) CV curves of the flexible device bent at 0°, 90°, and 180° at the scan rate of 50 mV/s. Reproduced with permission: Copyright 2020, John Wiley and Sons.205

Transition metal oxides can be used in electrodes for supercapacitors due to their excellent pseudocapacitive properties, but their poor electrical conductivity has hindered the study of high specific capacitance.^206,207 In 2021, Mahmood et al.²⁰⁸ reported a new MnO₂/MXene composite to overcome the bottleneck of MXene stacking and poor metal oxide conductivity by introducing sub-50 nm thick MnO₂ nanowires (NWR) into the interior of MXene (Figure 11D), effectively organizing the MXene stacking and the new super. The surface area of the capacitors was also increased, with specific requirements for the thickness of the NWR, which was to ensure that the introduction of MXene could be adjusted in a controlled manner. A comparison of individual MXene (527.8 F/g), MnO₂ (337.5 F/g) and MnO₂ (NWR)/MXene electrochemical plots show that the MnO₂ (NWR)/MXene composite (Figure 11E) is the best choice for supercapacitor electrodes, with the MnO₂/MXene composite (Figure 11F) being observed to be about 611.5 F/g. The calculated specific capacity of the MnO₂/MXene composite was about 489.5 C/g at 1 F/g. The electrode material synthesized from the composite exhibited excellent capacitance retention of about 96% over 1000 cycles (Figure 11C). The specific capacity of the MnO₂/MXene composite was about 489.5 C/g at 1 A/g. Chen et al.²⁰⁹ used electrochemical deposition of MnO₂ and MXene on activated CC to synthesize free-bonded MnO₂/MXene/CC composite electrodes (Figure 11G). Taking advantage of the excellent electrical conductivity of MXene and the excellent pseudocapacitance property of MnO₂, the prepared composite electrode could obtain a high specific capacitance (411.5 F/g) at a current density of 1 A/g. The MnO₂/MXene/CC composite was used as the positive electrode and the MXene/CC composite as the negative electrode to successfully assemble an all-solid-state flexible asymmetric supercapacitor (Figure 11I). The assembled supercapacitor can reach a voltage of 1.7 V, a power density of 850 W/kg, and an energy density of 24.3 Wh/kg, making a great contribution to the preparation of composite electrodes and all-solid-state asymmetric supercapacitors with high electrochemical performance.

Conductive polymers are considered as a class of organic materials with unique advantages,²⁰¹ including low cost,²⁰⁴ easy processing,²⁰⁶ compatibility,²⁰⁷ and tunable intrinsic properties (electronics, optics, conductivity, and stability).^210–212 In recent years, the synthesis of many different conducting polymers has been developed under mild conditions, which greatly expands the possibilities of advancing them into the fabrication process of ESD.^213–215 For instance, recognizing that combining positive materials with negative MXene to design asymmetric devices can widen the operating voltage window, Wang et al.¹¹ embedded small-sized PANI nanoparticles into the MXene interlayer to prepare dense PANI/MXene thin-film electrodes (Figure 12A,D,E) and developed functional PANI/MXene inks to prepare composite films on a large scale. The prepared electrodes have a combination of MXene nanosheet dispersion (Figure 12B), binding, conductivity, and a flexible substrate for PANI nanoparticles (Figure 12C). Meanwhile, PANI nanoparticles can be used not only as high pseudocapacitive materials but also as interlayer conductive column assemblies to reduce MXene stacking and achieve electron and ion transport, resulting in a good synergistic effect. In addition, the assembled asymmetric devices consist of fully pseudocapacitive dense film materials, providing an incredible energy density of up to 65.6 Wh/L (1687.3 W/L).

Figure 12. (A) Schematic representation of designed all pseudocapacitive asymmetric device in which PANI0.7/MXene and MXene electrodes serve as the cathode and anode, respectively. (B,C) PANI0.7/MXene film (1.0 mg/cm2). (D) Schematic illustration of the fabrication process of PANI/MXene inks. (E) Schematic diagram of ion and electron transport pathway in the PANI/MXene film electrode. Reproduced with permission: Copyright 2021, Elsevier.11 (F) Schematic illustration for the synthesis of the Ti3C2/FeOOH hybrid films. (G) HRTEM image of the Ti3C2/Fe-15% hybrid nanosheet. (H) Cycling stability of Ti3C2/Fe-15% electrode at 4 mA/cm2, the inset shows the GCD curves before and after 5000 cycles. Reproduced with permission: Copyright 2019, Elsevier.216

Zhao et al.²¹⁶ designed Ti₃C₂/FeOOH quantum dots (QDs) hybrid films by a simple electrostatic selfassembly method, in which amorphous FeOOH QDs not only act as a barrier to Ti₃C₂ nanosheet repacking (Figure 12F), but also provide considerable capacitance as an active material. The surface capacitance of Ti₃C₂/FeOOH QDs hybrid films (485 mF/cm²) was 2.3 times higher than that of the pure Ti₃C₂ film (Figure 12G), with good cycling stability in a neutral electrolyte (94.8% capacitance over 5000 cycles) (Figure 12H). And the asymmetric device with an output potential difference of 1.6 V was made by using the hybrid film as the negative electrode and MnO₂ on CC as the positive electrode. The maximum energy density of the device was 40 µWh/cm² and the maximum power density was 8.2 mW/cm², and the capacitance retention after 3000 cycles was 82%.

Flexibility supercapacitors

The preparation of active electrode materials for flexible SCs using MXene materials as substrates has become a hot research topic in recent years. Yu et al.²¹⁷ reported a strategy for using two-dimensional Ti₃C₂T_x MXene as a flexible, conductive, and electrochemically active binder in a one-step process to prepare MXene bound activated carbon as a flexible electrode for supercapacitors in organic electrolytes (Figure 13A). In this electrode, activated carbon particles are encapsulated between MXene layers without the need for an insulating polymer binder. MXene plays a multifunctional role in the electrode, including as a binder, flexible backbone, conductive additive, and additional active material. The synergistic effect of MXene and activated carbon forms a three-dimensional conductive network, increasing the MXene layer spacing and greatly improving the electrode capacitance and multiplicity capacity. The results show that the specific capacity of the MXene bonded flexible activated carbon electrode is up to 126 F/g in 0.1 and 100 A/g organic electrolytes, with a retention rate of 57.9% in 100 A/g organic electrolytes (Figure 13B), which is necessary for the development of high-performance flexible supercapacitors.

Figure 13. (A) Schematic diagram for the fabrication of MXene-bonded AC films, including mixing MXene flakes and AC particles in water, vacuum-assistant filtration, peeling off, and drying. (B) Cyclic performance at 10 A/g. Reproduced with permission: Copyright 2018, American Chemical Society.217 (C,D) Cross-section SEM images of 3D porous MXene-rGO-20 film. (E) Long-term cycle performance at 100 A/g of MXene-rGO-20 with the charge–discharge curves at different cycles in the inset. Reproduced with permission: Copyright 2021, Elsevier.218

Miao et al.²¹⁸ prepared a flexible 3D porous MXene film by introducing GO into the MXene film followed by self-propagation reduction (Figure 13C). The self-propagation process is simple and effective, taking only 1.25 s to complete, and forms a three-dimensional porous skeleton through the instantaneous release of secondary gases. The MXene-rGO films (Figure 13D) exhibit excellent capacitive and multiplicative properties as the three-dimensional porous structure provide a large number of ion-accessible active centers, facilitating rapid ion transport. At 20% rGO content, the MXene-rGO-20 film has a capacitance of 329.9 F/g at 5 mV/s in a 3-mol/L H₂SO₄ electrolyte and 260.1 F/g at 1000 mV/s and has good flexibility. In addition, the initial capacitance remained above 90% after 40,000 cycles at 100 A/g (Figure 13E), demonstrating good cycling stability. This study not only provides high-performance flexible electrodes for SCs, but the self-propagation reduction method offers an efficient and time-saving strategy for building 3D structures using 2D materials.

Wang and colleagues²¹⁹ successfully prepared HHK-CC@Ti₃C₂T_x (HHK-CC = activated CC, TX = −F, =O or −OH) electrode materials by a drop coating method (Figure 14A,B) and studied the effect of loading on the electrical properties. At a loading of 3 mg/cm³, HHK-CC@Ti₃C₂T_x had the best electrical properties and the capacitance retention could still reach 97.6%. The flexible solid electrode was assembled and had a capacitance value of 413 mF/cm² at a current density of 0.5 mA/cm² and the capacitance value did not change significantly in 180° bending conditions, demonstrating the stability and flexibility of the electrode material (Figure 14C). The capacitance retention rate was 94.2% and an energy density of 0.0045 mWh/cm² was obtained at a power density of 0.779 W/cm². In the same year, Li et al.²²⁰ presented a unique composite material consisting of uniformly grown multiwalled carbon nanotubes (MWCNTs) on CC supported MXene sheets (noted as MWCNTs-MXene@CC). MWCNTs-MXene@CC exhibited a synergistic combination of exfoliated large specific surface area and excellent electrical conductivity. The specific capacitance of the electrodes was 114.58 mF/cm² at a discharge current of 1 mA/cm², while high retention was maintained after 1.6 × 10⁴ cycles at 10 mA/cm². This high-performance composite structure is attributed to the good interlayer and intergranular conductivity of the grown MWCNTs, which also act as interlayer pillars for the MWCNTs, thus preventing the spontaneous collapse of the MWCNTs.

Figure 14. (A) SEM image of HHK-CC. (B) Synthesis schematic representation of the HHK-CC@Ti3C2Tx-X (X¼ 1, 2, 3, and 4). (C) Pictures of the prepared symmetric supercapacitor under bending or twisting. Reproduced with permission: Copyright 2020, John Wiley and Sons.220 (D) MnO2/MXene/CC composite. (E) Ragone diagram of the supercapacitor and some other devices from previous literature for comparison. (F) Cycle performance of the device for 10,000 cycles. Reproduced with permission: Copyright 2019, Elsevier.221

Zhou et al.²²¹ designed a highly flexible self-supporting electrode (MnO₂/MXene/CC electrode [Figure 14D]) based on a CC composite of MnO₂ nanorods and MXene and compared with MnO₂ nanopins,²²² the electrochemical performance of the new electrode was improved with excellent rate performance (Figure 14E) (when the current was increased from 1 to 5 A/g, 60.3% of the capacitance is still retained), good cycling stability (after 10,000 cycles, the capacitance only drops to 83%) (Figure 14F), and the supercapacitor prepared from the new electrode also has good flexibility (bending at 180°, the performance remains largely unchanged).

In 2020, Zheng et al.²²³ used a novel and facile vapor phase polymerization (VPP) and spraying strategy for the surface of fibers on the construction of laminates containing PEDOT films and Ti₃C₂T_x MXene sheets (Figure 15A–K). The prepared PEDOT/MXene decorated cotton fabrics exhibit excellent electrochemical properties, Joule heating properties, good electromagnetic interference (EMI) shielding, and strain sensing properties. This provides a new strategy for the structural design of multifunctional textiles and lays the foundation for the development of multifunctional wearable electronics.

Figure 15. Strain sensing performance of PEDOT/MXene decorated fabrics (PMF as the strain sensors to detect the human motion. Finger bending and recovering process (A) and (B), wrist bending and recovering process (C) and (D), elbow swing process (E) and (F), knee bending and recovering process (G) and (H). (I) Preparation of Ti3C2Tx MXene. (J) Fabrication of PMFs. (K) Multiple applications of PMFs on energy storage, strain sensing, EMI shielding, and joule heating. Reproduced with permission: Copyright 2021, RSC Pub.223

The selfstacking of MXene nanosheets seriously affects their rate performance and mass loading. To solve this problem, Fan et al.²²⁴ doped Fe(OH)₃ nanoparticles with diameters of 3–5 nm into MXene films and then dissolved Fe(OH)₃ nanoparticles, followed by low-temperature calcination at 200°C to obtain highly interconnected nanopore channels, thus without affecting their ultrahigh density effectively facilitating ion transport, resulting in the preparation of a free-standing, flexible modified nanoporous MXene film. The results show that the modified nanoporous MXene film has good bulk capacitance (1142 F/cm³ at 0.5 A/g) and good multiplicative properties (828 F/cm³ at 20 A/g). Furthermore, it even exhibits a high bulk capacitance of 749 F/cm³ and good flexibility under a high-quality load of 11.2 mg/cm². Thus, this flexible, self-supporting nanoporous MXene film is a promising electrode material for portable, compact and flexible memory devices. This study provides an effective material design for flexible ESD with high bulk capacitance and high multiplicity performance even under high-quality loads. Some typical MXene-based flexibility supercapacitors are summarized in Table 4.^225–236

Table 4 Summary of main performances of MXene-based flexibility supercapacitors

Materials	Rate	Specific capacitance	Cycle number	Capacity retention (%)	Flexibility	References
rGO/Ti3C2Tx	1 A/g	370 F/cm3	10,000	100	Bending	[172]
MXene/rGO (20%)	1000 mV/s	260.1 F/g	40,000	90	Bending/twisting	[218]
MXene/CNTs	5 mV/s	74.1 F/g	10,000	86.3	Bending/rolling/folding	[225]
CNTs/d-Ti3C2	5 mV/s	393 F/g	10,000	≈100	Bending	[226]
MXene/carbon cloth	1 mA/cm2	312 mF/cm2	8000	97	Folding	[227]
HHK-CC@Ti3C2Tx	0.5 mA/cm2	413 mF/cm2	1000	94.2	Bending	[219]
MXene/MnO2/CC	5 A/g	511.2 F/g	10,000	83	Bending/rolling/folding	[221]
MXene/MnO2(NWs)	10 mV/s	216.2 mF/cm2	10,000	85	Stretching/bending	[228]
MXene/MnO2(NWR)	1 A/g	489.5 F/g	1000	96	Folding/bending	[208]
MXene/CoOx-NiO	1 A/g	1960 F/g	8000	90.2	Bending/folding/twisting	[201]
Ti3C2Tx@ PEDOT (1:10)	1 A/g	564 F/g	10,000	96.5	Twisting	[229]
MXene/PSS: PEDOT	5 mV/s	614.5 F/cm2	10,000	95	Stretching	[230]
MXene/PPy/PVA	1 A/g	614 F/g	1000	83	Stretching/knotting/twisting/bending	[231]
PPy/MXene@Cotton	1 A/g	506.6%	10,000	87	Bending/stretching/folding	[232]
Ti3C2/V2O5	1 mA/cm2	477 mF/cm2	6000	93	Bending/twisting/folding	[233]
MXene/Ni-Fe	0.2 mA/cm2	328.35 mF/cm2	10,000	90	Bending/folding	[234]

Other supercapacitors

Sun et al.²²⁶ assembled multidimensional MXene CNT ultrathin films by loading MXene inserted into CNT onto tubular ceramic membranes. Benefiting from the interaction forces between MXene and CNT, the CNT was able to disperse the insert well into the MXene nanosheets, forming a mesh-like labyrinth-like three-dimensional channel (Figure 16A). This three-dimensional continuous MXene-CNT nanostructure (Figure 16B) exploits the hydrophilicity of MXene to improve water permeability and separation, which is a new attempt and breakthrough in the field of water purification (Figure 16C).²³⁵

Figure 16. (A) Fabrication process. (B) Schematic diagram of the synthesis process for the PDA-modified α-Al2O3 support. (C) Rhodamine B permeances and rejections of MCNM-180 under feed pressures from 1 to 5 bar. Reproduced with permission: Copyright 2021, Elsevier.226 (D) Photographs of pr-MXene dispersion and the aged samples in water (wa-MXene) and tartaric acid (ta-MXene) and UV-vis spectra for pr-MXene dispersion, the aged dispersions in water (wa-MXene), and tartaric acid (ta-MXene). (E) Structurally stabilized Ti3C2Tx/PEDOT:PSS composite coating. Reproduced with permission: Copyright 2021, Elsevier.229

Zhang et al.²²⁹ reported the benefits of tartaric acid, a natural source, as a nonharmless additive in MXene composites. In water, most importantly, it inhibits the oxidation of Ti₃C₂T_x and firmly binds the components of the Ti₃C₂T_x/poly (3,4-ethylenedioxythiophene): polystyrene sulfonate (Ti₃C₂T_x/PEDOT:PSS) composite together (Figure 16E); equally importantly, it increases the electronic conductivity of the composite by a factor of four. At the same time, they fabricated a tartaric acid-treated, water stable MXene/PEDOT:PSS conductive coating for use as electrodes in ultrafast SCs, demonstrating excellent electrochemical performance and cycling stability.

With the advantages of high-power density, fast charging and discharging, and long lifetime, micro-supercapacitors have received a lot of attention for their potential applications in the field of microelectronics and have made great progress in recent years. Huang et al.²³⁶ utilized a low-cost Ti₃C₂T_x MXene substrate on a process-cut-coat (PCC) fabrication platform for the rapid preparation of MXene-based electrodes by cold laser cutting and Xuan coating on a Kapton masked Si/SiO₂ substrate (Figure 17A–C). The material has an area and volume capacitance of 472 and 21.4 F/cm³ and has excellent cycling stability with capacitance retention of more than 87.6% after 10,000 cycles. This study successfully addresses the challenges of weakly connected interfaces and expensive fabrication processes for silicon-based micro-supercapacitors.

Figure 17. (A) The detailed manufacturing process of on-chip Micro-supercapacitor (MSC). Kapton tape was used as a mask. (B) Cross-section SEM image of the MXene electrodes. (C) AFM image of MSC-10. Reproduced with permission: Copyright 2020, Elsevier.236 SEM images showing the surface (D) and (E) cross-section morphologies of the BMX yarn containing 97.4 wt% MXene. (F) High-magnification colored SEM image of the marked section in (E) with enhanced contrast illustrating how the MXene sheets are trapped within carbon nanotube yarn corridors. (G) SEM image showing a BMX yarn containing 90 wt% MXene tied into a reef knot. (H–J) Photographs showing the freestanding YSC device under different bending angles. Reproduced with permission: Copyright 2018, John Wiley and Sons.59

MXene is well known for its metallic conductivity and high bulk capacitance in acidic electrolytes. By incorporating MXene into fibers, a variety of functional fiber structures (coated, double rolled, laminated, and integrated) can be created.^75,118 The integration of these fiber sensors into the textile makes the garment “smart”-capable of storing charge (Figure 17E), harvesting energy, heating, sensing, and communicating with nearby electronics (Figure 17F).⁵⁹ Driven by these excellent properties, there are numerous ways to integrate MXene into textile-based supercapacitors (TSCs) that significantly improve the area and bulk capacitance. For example, asymmetric TSCs were fabricated by pairing MXene/CNT double-roll yarns with RuO/CNT double-roll yarns. In this way, the operating window in the H₂SO₄/PVA gel electrolyte was extended from 0.6 to 1.6 V, and the maximum energy and power densities obtained were 61.6 MWh·cm (168 and 8.4 μWh·cm) and 5428 mW·cm (14.8 mW·cm and 741 μW·cm) and excellent flexibility (0°, 90°, 180° [Figure 17H–J]).

CONCLUSIONS AND PERSPECTIVES

We review recent advances in modified MXenes and MXene-based materials engineering for supercapacitor electrodes, starting with four applications including symmetric supercapacitors, asymmetric supercapacitors, flexible supercapacitors, and other functional supercapacitors. First, the different synthetic methods, hierarchical processing, and rich properties of MXenes are introduced. The advantages of supercapacitors over other secondary batteries and how they work are also discussed. The focus of this paper is that MXenes are expected to become active materials by exploiting their advantages (e.g., metallic conductivity, excellent hydrophilicity, rich surface chemistry, etc.) and further process design and doping for better performance, conductive additives, hosts or substrates, and other multifunctional substrates. Researchers are more interested when MXenes are designed into several layers or flakes, especially for the fabrication of MXene-based materials as stand-alone electrodes, modified layers, and ESD. Besides attractive electrochemical and mechanical properties, several aspects contribute to this trend: (i) graphene and its derivatives have been widely assembled into graphene-based films for energy storage before 2D MXenes, paved the way for the exploration of MXenes; (ii) in the past few years, great progress has been made in the modified synthesis and solution processing of MXenes, and theories on how to improve their stability and mechanical properties have also been formed; (iii) various techniques have emerged for the designable, cost-effective, and large-scale fabrication of MXenes to enhance device performance. At present, MXenes have been widely used in catalysts, ion sieving, photothermal conversion, field effect transistors, topological insulators, and hydrogen evolution reactions (Figure 18). Despite the significant achievements of MXene materials in supercapacitor applications to date, there are still a number of challenges and opportunities for the further development of MXene improvements and designs and their corresponding devices, some of which are highlighted below.

• (1)

  Materials and film preparation (Figure 19A): Although nearly 30 MXene species have been experimentally synthesized to date and at least 100 stoichiometric MXene compositions have not been fully investigated, only a limited number of MXene have been investigated as building blocks for MXenes to date, with more than 70% of them concentrated in the first discovery of Ti₃C₂T_x. More attempts should be made with new physical or chemical structured species and compositions. In addition, most of the MXene synthesis routes employed involve fluorinated compounds in aqueous environments, and it can be a challenging task to achieve MXenes with controlled and uniform surface terminations. However, these terminations are critical for optimizing the solution process, hybridization, and stability of MXene nanosheets as well as the performance of the obtained MXene. Moreover, degradation/oxidation of MXene under humid conditions during the solution process remains a limitation for practical applications. Controlling and modifying the MXene surface under mild conditions during MXene synthesis and film preparation may be beneficial to obtaining monolayer or few-layer MXenes with desirable properties. Further studies are needed to control the size, number of layers, and atomic defects of MXenes to fine-tune the microstructure and chemical properties of the films. On the other hand, most thin film preparation methods are not fine enough to precisely control the pore size or interlayer spacing within MXenes, especially when large-scale preparation is required. Therefore, the trade-off between fine control and large-scale fabrication should be well handled. Improved processes or new methods with controllable parameters may help to achieve this. For example, smart and digital printing techniques, emerging as a versatile, reproducible, scalable, and cost-effective patterning method, show great promise for the preparation of MXene for functional devices.

• (2)

  Material improvement design (Figure 19B): Serious problems such as tortuous and inefficient interlayer nanochannels and reduced active surfaces due to the selfstacking of MXene nanosheets remain to be solved. Several emerging solutions show great potential to overcome these limitations, including reduction of sheet size and film thickness, cultivation of porous structures on MXene surfaces or in thin films, and insertion of functional spacers. So far, MXene intercalation has been performed based on a variety of materials, such as CNT, graphene/rGO, polymers, and QDs. Sheet size tuning, in-plane pore engineering, and the introduction of three-dimensional interconnected networks in multiscale fibers in laminar structures are relatively effective but need to be further developed. In addition, finely designed pores with high density and subprecision can create favorable structural and chemical microenvironments, improve ion transport, and bring more exposed active sites for fast redox reactions, which deserve further investigation. In addition, ultrathin films and simulations based on a single or few layers of MXene can be used to experimentally and theoretically investigate the effect of structure on ion diffusion within MXene-based materials.

• (3)

  Performance improvement and practical applications: In terms of implementing practical applications, several specific issues need to be addressed. First, how to select high-performance guest materials and good applicability of film preparation methods. Currently, most high-capacity options are randomly incorporated into MXenes (Figure 19C), leading to inhomogeneous distribution and ambiguous heterojunction interactions. The construction of fine nanocomposite films by reliable and low-cost methods is extremely challenging and requires further exploration of the relationships and interactions between MXenes and guest materials. Second, how to balance mechanical and electrochemical properties. Most of the reported ones are proposed for flexible devices in future applications of portable and wearable electronic devices (Figure 19D). Therefore, it is important to improve the electrochemical properties of materials while maintaining their mechanical properties such as flexibility, strength and toughness. Finally, MXenes with high weight capacity/capacitance fabricated and tested in the laboratory typically have low mass loading (mg/cm²) and/or low vibronic density (g/cm³), resulting in mediocre area and/or bulk properties. When targeting miniaturized ESD, a rigorous and systematic study of this aspect would be highly desirable.

Figure 18. The growth of the MXenes tree, the absorption of nutrients by the roots (graphene research, MXene improvement, designability, low sink costs, and large scale), the tree is full of fruits (catalysts, ion sieves, topological insulators, and hydrogen evolution reactions).

Figure 19. Challenges and prospects of MXenes materials in future energy storage include (A) preparation and precise thin film synthesis of MXenes materials. Reproduced with permission: Copyright 2020, American Chemical Society.45 (B) Improved optimization of materials including intercalation, doping, and so on. Reproduced with permission: Copyright 2020, American Chemical Society.208 (C) Energy storage of large capacity natural resources, and (D) flexible and wearable electronics.

Due to their unique physicochemical properties and typical structural features, MXenes offer a large number of new possible applications. To date, various approaches to overcome current problems with MXene itself continue to be reported in the energy storage field that will advance the MXene layer improvement design process and enable further potential applications related to water purification, electromagnetic shielding, sensors, and wearable ESD. It can be expected that the continued rapid development of fundamental understanding and technical processing associated with MXenes will open the door to even more exciting discoveries.

ACKNOWLEDGMENTS

This study was supported by the National Natural Science Foundation of China (No. 11375136).

CONFLICTS OF INTEREST

The authors declare no conflicts of interest.