MXenes: An Emerging Platform for Wearable Electronics and Looking Beyond

MXenes are potentially described as a “wonder material” in the class of 2D nanomaterials. Since their discovery in 2011, MXenes have been researched and developed for almost a decade. The synthesis methods are not only limited to HF etching; novel approaches, such as water-free etching and molten salts etching, have also been developed, thus endowing MXenes with multifunctional surface chemistry, structure, and coveted properties. Owing to their excellent electrochemical performance, MXenes can be used as promising materials for energy devices, especially flexible electronic devices. Here, the recent progress in the synthesis of MXenes and their application in flexible electronic devices are summarized. This will shed new light on industrial-scale synthesis of MXenes. Insights into the challenges, outlook, and future prospects with regard to MXene-based devices are discussed. MXenes (e.g., transition metal carbides, nitrides, and carbonitrides) have penetrated in diverse fields of research as wonder materials. The most attractive features of their applications show great promise in energy storage conversion, harvesting, and sensing. Here, the synthesis and novel fabrication techniques of MXenes are introduced, and conventional etching procedures are revisited. Tailoring hybrid mechanisms for MXenes to fine-tune their electronic and chemical properties are discussed in this review. Because of their unique layered structures and intriguing functional properties, MXenes serve as key components in a variety of flexible devices such as supercapacitors, triboelectric nanogenerators, and sensors. Finally, the challenges related to MXenes-based flexible electronics are presented, and viable solutions to overcome the shrouding issues are presented. This provides an insightful outlook for the future development of flexible and wearable applications of MXenes. MXenes (e.g., transition metal carbides, nitrides, and carbonitrides) have penetrated in diverse fields of research as wonder materials. The most attractive features of their applications show great promise in energy storage conversion, harvesting, and sensing. Here, the synthesis and novel fabrication techniques of MXenes are introduced, and conventional etching procedures are revisited. Tailoring hybrid mechanisms for MXenes to fine-tune their electronic and chemical properties are discussed in this review. Because of their unique layered structures and intriguing functional properties, MXenes serve as key components in a variety of flexible devices such as supercapacitors, triboelectric nanogenerators, and sensors. Finally, the challenges related to MXenes-based flexible electronics are presented, and viable solutions to overcome the shrouding issues are presented. This provides an insightful outlook for the future development of flexible and wearable applications of MXenes. In the past two decades, substantial progress has been achieved in the miniaturization of flexible wearable electronics devices and downscaling for human use (Figure 1). Wearable-based devices can be applied to almost every aspect of our life, including medical devices for monitoring and treatment, sports fitness, and communication.1Pyattaev A. Johnsson K. Andreev S. Koucheryavy Y. Communication challenges in high-density deployments of wearable wireless devices.IEEE Wirel. Commun. 2015; 22: 12-18Crossref Scopus (59) Google Scholar,2Son D. Lee J. Qiao S. Ghaffari R. Kim J. Lee J.E. Song C. Kim S.J. Lee D.J. Jun S.W. et al.Multifunctional wearable devices for diagnosis and therapy of movement disorders.Nat. Nanotechnol. 2014; 9: 397-404Crossref PubMed Scopus (856) Google Scholar However, the current progress in flexible wearable electronics is still at an early stage of development. Furthermore, the challenge is how to utilize the corresponding electronic equipment with robust, stable, and reliable performance. In comparison with traditional electronic devices, deficient reliability and stability have inhibited the advancement of flexible electronics to date. Among other important aspects in the construction of electronic circuitry, fabricating well-defined and controllable miniaturized flexible electronic devices is of crucial importance in this emerging field.3Bandodkar A.J. Jia W. Wang J. Tattoo-based wearable electrochemical devices: a review.Electroanalysis. 2015; 27: 562-572Crossref Scopus (165) Google Scholar Therefore, the emergence of smart, versatile, and multifunctional electronics has a pivotal role with stringent requirements with regard to the characteristics of devices, including energy storage devices, sensors, energy harvesters, health monitors, and so on. These requirements are mainly to achieve better performance, longer lifetime, and more mechanical resilience. To overcome these problems, systematic and through development to attain well-defined and flexible electronic materials is highly desirable. Thus, the central theme to address these challenges is to develop novel materials that can exhibit efficient energy storage properties, while simultaneously possessing good mechanical properties and high sensitivity to external stress/strain. However, these efforts have proven to be difficult to realize until the emergence of 2D materials. Several 2D materials have been extensively studied in recent years, such as transition metal dichalcogenides (TMDs), oxides (TMOs), hexagonal-boron nitride (h-BN), perovskites, metal organic frameworks, etc.4Peng X. Peng L. Wu C. Xie Y. Two dimensional nanomaterials for flexible supercapacitors.Chem. Soc. Rev. 2014; 43: 3303-3323Crossref PubMed Scopus (701) Google Scholar When they are processed into electrodes, these 2D materials are either electrochemically inert on account of low intrinsic electronic conductivity or mechanical unstable.5Zhang C. Ma Y. Zhang X. Abdolhosseinzadeh S. Sheng H. Lan W. Pakdel A. Heier J. Nüesch F. Two-dimensional transition metal carbides and nitrides (MXenes): synthesis, properties, and electrochemical energy storage applications.Energy Environ. Mater. 2020; 3: 29-55Crossref Google Scholar As a result, a considerable amount of electrode additives (e.g., conductive agents and polymeric binders) are required to maintain the conductive network and the integrity of the electrode, hence complicating the fabrication process and casting negative influences on the performances of electronic devices. In other words, advanced 2D materials that can provide abundant active sites for accelerating redox reactions and possess robust electronic conductivity and mechanical properties, are of critical importance for realizing next-generation wearable electronic devices. In 2011, layered 2D transition metal carbides or nitrides (known as MXenes) were discovered and since then, these intriguing materials have attracted substantial research attention.10Naguib M. Kurtoglu M. Presser V. Lu J. Niu J. Heon M. Hultman L. Gogotsi Y. Barsoum M.W. Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2.Adv. Mater. 2011; 23: 4248-4253Crossref PubMed Scopus (2824) Google Scholar In general, MXenes are prepared from MAX phases consisting of layered ternary carbides with the formula Mn+1AXn, where M, A, and X represent early d-block transition metals, main-group sp elements (prevailing groups 13 and 14), and carbon and/or nitrogen, respectively. The resulting MXenes have the general formula Mn+1XnTx, where Tx denotes abundant O, OH, and F groups on the surface of the MXenes.11Peng J. Chen X. Ong W.-J. Zhao X. Li N. Surface and heterointerface engineering of 2D MXenes and their nanocomposites: insights into electro- and photocatalysis.Chem. 2019; 5: 18-50Abstract Full Text Full Text PDF Scopus (0) Google Scholar Due to the presence of these groups, MXenes are hydrophilic and can be readily dispersed in aqueous medium without the assistance of any surfactants, resulting in thickness-dependent MXene nanomaterials.12Malaki M. Maleki A. Varma R.S. MXenes and ultrasonication.J. Mater. Chem. A. 2019; 7: 10843-10857Crossref Google Scholar Moreover, the transition metal oxide-like surface supports fast redox reactions on the highly conductive MXenes, indicating the potential of MXenes for energy storage13Zhang X. Zhang Z. Zhou Z. MXene-based materials for electrochemical energy storage.J. Energy Chem. 2018; 27: 73-85Crossref Scopus (219) Google Scholar,14Das P. Fu Q. Bao X. Wu Z.-S. Recent advances in the preparation, characterization, and applications of two-dimensional heterostructures for energy storage and conversion.J. Mater. Chem. A. 2018; 6: 21747-21784Crossref Scopus (26) Google Scholar and as conductive electrodes.5Zhang C. Ma Y. Zhang X. Abdolhosseinzadeh S. Sheng H. Lan W. Pakdel A. Heier J. Nüesch F. Two-dimensional transition metal carbides and nitrides (MXenes): synthesis, properties, and electrochemical energy storage applications.Energy Environ. Mater. 2020; 3: 29-55Crossref Google Scholar Besides MXenes, the development of MXene-based composites has elicited tremendous attention to potentially enhance the performance of MXenes to some extent.15Gao L. Li C. Huang W. Mei S. Lin H. Ou Q. Zhang Y. Guo J. Zhang F. Xu S. et al.MXene/polymer membranes: synthesis, properties, and emerging applications.Chem. Mater. 2020; 32: 1703-1747Crossref Scopus (36) Google Scholar Thus, owing to their outstanding conductivity, abundant surface chemistry, high capacitance, and good mechanical properties, MXene-based materials are continuously triggering numerous breakthroughs in wearable electronic devices, especially wearable micro-supercapacitors (MSCs).16Zhou S. Yang X. Pei W. Liu N. Zhao J. Heterostructures of MXenes and N-doped graphene as highly active bifunctional electrocatalysts.Nanoscale. 2018; 10: 10876-10883Crossref PubMed Google Scholar, 17Yue Y. Liu N. Ma Y. Wang S. Liu W. Luo C. Zhang H. Cheng F. Rao J. Hu X. et al.Highly self-healable 3D microsupercapacitor with MXene–graphene composite aerogel.ACS Nano. 2018; 12: 4224-4232Crossref PubMed Scopus (0) Google Scholar, 18Li H. Li X. Liang J. Chen Y. Hydrous RuO2-decorated MXene coordinating with silver nanowire inks enabling fully printed micro-supercapacitors with extraordinary volumetric performance.Adv. Energy Mater. 2019; 9: 1803987Crossref Scopus (60) Google Scholar, 19Jiang Q. Kurra N. Maleski K. Lei Y. Liang H. Zhang Y. Gogotsi Y. Alshareef H.N. On-chip MXene microsupercapacitors for AC-line filtering applications.Adv. Energy Mater. 2019; 9: 1901061Crossref Scopus (25) Google Scholar, 20Yang Q. Wang Y. Li X. Li H. Wang Z. Tang Z. Ma L. Mo F. Zhi C. Recent progress of MXene-based nanomaterials in flexible energy storage and electronic devices.Energy Environ. Mater. 2018; 1: 183-195Crossref Google Scholar In comparison, the optical properties of MXenes and their corresponding applications have received relatively limited attention. With momentous breakthroughs on MXenes in optical-related research, the application of MXenes in ultrafast lasers21Ma C. Wang C. Gao B. Adams J. Wu G. Zhang H. Recent progress in ultrafast lasers based on 2D materials as a saturable absorber.Appl. Phys. Rev. 2019; 6: 041304Crossref Scopus (30) Google Scholar and photonics devices22Jiang X. Kuklin A.V. Baev A. Ge Y. Ågren H. Zhang H. Prasad P.N. Two-dimensional MXenes: from morphological to optical, electric, and magnetic properties and applications.Phys. Rep. 2020; 848: 1-58Crossref Scopus (82) Google Scholar has gradually taken shape. In this review, we present an overview of the current progress on the synthetic methodologies of MXenes, followed by several notable aspects on MXene applications for flexible and wearable electronics. Significantly, recent advances in novel systematic fabrication strategies for MXene-based MSCs are focused on lithography and printing technology. In addition, the application of MXene-based MSCs in flexible sensors is discussed in detail. In summary, in-depth outlook, challenges, and perspectives are presented to attract more breakthroughs on the applications of MXenes for wearable electronic devices. The advancement of synthesis routes of MXenes has had a tremendous influence on their electrical properties, physicochemical functionality, and diverse applications. In general, synthesis of MXenes can be classified into two categories: top-down and bottom-up approaches.23Verger L. Xu C. Natu V. Cheng H.-M. Ren W. Barsoum M.W. Overview of the synthesis of MXenes and other ultrathin 2D transition metal carbides and nitrides.Curr. Opin. Solid State Mech. 2019; 23: 149-163Crossref Scopus (0) Google Scholar In essence, the first method primarily etches the A elements from the parent three-dimensional (3D) layered MAX phases that result in the layered structure of MXenes. To date, the top-down method is profiled as the main approach to synthesize MXenes (Figure 2). Unlike the top-down manufacturing methods, which typically require a large amount of precursors, bottom-up synthesis begins by carefully constructing the structure using small organic or inorganic molecules/atoms.24Huang K. Li Z. Lin J. Han G. Huang P. Two-dimensional transition metal carbides and nitrides (MXenes) for biomedical applications.Chem. Soc. Rev. 2018; 47: 5109-5124Crossref PubMed Google Scholar For example, a crystal growth method is feasible to assemble the precursors into a well-defined 2D ordered MXene structure. In this bottom-up synthesis, the advantages of the route offer an opportunity to precisely control the size distribution, morphology, and surface termination of MXenes in a well-ordered fashion. To date, the use of HF etching with a mixture of fluoride salts and HCl has been widely used. Alternatively, molten salt etching is also implemented to synthesize reliable MXenes. Several published reviews have introduced these classic methods in detail.25Wang H. Wu Y. Yuan X. Zeng G. Zhou J. Wang X. Chew J.W. Clay-inspired MXene-based electrochemical devices and photo-electrocatalyst: state-of-the-art progresses and challenges.Adv. Mater. 2018; 30: 1704561Crossref Scopus (198) Google Scholar, 26Chaudhari N.K. Jin H. Kim B. San Baek D. Joo S.H. Lee K. MXene: an emerging two-dimensional material for future energy conversion and storage applications.J. Mater. Chem. A. 2017; 5: 24564-24579Crossref Google Scholar, 27Verger L. Natu V. Carey M. Barsoum M.W. MXenes: an introduction of their synthesis, select properties, and applications.Trends Chem. 2019; 1: 656-669Abstract Full Text Full Text PDF Scopus (59) Google Scholar We have tabulated several reports of MXene synthesis methods reported in recent years in Table 1. We classify some state-of-the-art etching methods that currently have technological significance in MXene fabrication with intriguing physical properties.(1)Water-free etching. The mechanical and chemical stability of MXenes plays a critical role in terms of water-sensitive applications, such as energy storage,28Anasori B. Lukatskaya M.R. Gogotsi Y. 2D metal carbides and nitrides (MXenes) for energy storage.Nat. Rev. Mater. 2017; 2: 16098Crossref Scopus (2044) Google Scholar polymer composites,29Shahzad F. Alhabeb M. Hatter C.B. Anasori B. Man Hong S. Koo C.M. Gogotsi Y. Electromagnetic interference shielding with 2D transition metal carbides (MXenes).Science. 2016; 353: 1137Crossref PubMed Scopus (1417) Google Scholar and support for quantum dots.30Natu V. Pai R. Sokol M. Carey M. Kalra V. Barsoum M.W. 2D Ti3C2Tz MXene synthesized by water-free etching of Ti3AlC2 in polar organic solvents.Chem. 2020; 6: 616-630Abstract Full Text Full Text PDF Scopus (0) Google Scholar Structural instability be detrimental due to the presence of intercalating water, and -O/-OH functional groups originating from the water act as the main solvent in the preparation of MXenes. Therefore, a systematic exploration of a water-free etching method is of utmost significance to tackle the issue. Alternatively, a polar organic solvent was used in the presence of NH4HF2, resulting in Ti3C2Tx with a highly fluorinated structure and better water stability.30Natu V. Pai R. Sokol M. Carey M. Kalra V. Barsoum M.W. 2D Ti3C2Tz MXene synthesized by water-free etching of Ti3AlC2 in polar organic solvents.Chem. 2020; 6: 616-630Abstract Full Text Full Text PDF Scopus (0) Google Scholar,31Mashtalir O. Naguib M. Dyatkin B. Gogotsi Y. Barsoum M.W. Kinetics of aluminum extraction from Ti3AlC2 in hydrofluoric acid.Mater. Chem. Phys. 2013; 139: 147-152Crossref Scopus (171) Google Scholar In contrast to the aforementioned method, the use of this water-free solvent requires the entire synthetic procedure to be performed in a glove box. A thorough study is needed to understand the etching mechanism in detail, because this work is under the assumption that NH4HF2 subsequently separates into NH4F and HF when the precursors are dissolved in the polar solvents.(2)Lewis acidic etching. Notably, the mild production environment of the preparation of MXene has drawn immense attention for large-scale production and potential commercial applications. Lately, a series of novel MAX phases (Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC) have been synthesized by elemental replacement in the A atomic plane of traditional MAX phases in ZnCl2 molten salts.32Li M. Lu J. Luo K. Li Y. Chang K. Chen K. Zhou J. Rosen J. Hultman L. Eklund P. et al.Element replacement approach by reaction with Lewis acidic molten salts to synthesize nanolaminated MAX phases and MXenes.J. Am. Chem. Soc. 2019; 141: 4730-4737Crossref PubMed Scopus (107) Google Scholar When the proportion of ZnCl2 increases, MXenes (Ti3C2Cl2 and Ti2CCl2) with Cl functional groups are obtained by direct redox coupling between the A element and the cation of the Lewis acid molten salt.32Li M. Lu J. Luo K. Li Y. Chang K. Chen K. Zhou J. Rosen J. Hultman L. Eklund P. et al.Element replacement approach by reaction with Lewis acidic molten salts to synthesize nanolaminated MAX phases and MXenes.J. Am. Chem. Soc. 2019; 141: 4730-4737Crossref PubMed Scopus (107) Google Scholar In addition, this method can also be used to obtain MXene from MAX phases with A = Ga and Si through CuCl2 molten salt.33Li Y. Shao H. Lin Z. Lu J. Liu L. Duployer B. Persson P.O.Å. Eklund P. Hultman L. Li M. et al.A general Lewis acidic etching route for preparing MXenes with enhanced electrochemical performance in non-aqueous electrolyte.Nat. Mater. 2020; https://doi.org/10.1038/s41563-020-0657-0Crossref Scopus (49) Google Scholar The moderate molten salt environment and fluorine-free conditions guarantee a viable and green chemistry for the fabrication of MXenes that paves the way for their scale up and even commercial applications. Significantly, this is the first time that Cl-terminated MXenes have been obtained exclusively through non-fluorine chemistry. The Cl-terminated MXenes are expected to be more stable than the F-terminated MXenes. Moreover, through this method, other functional groups such as -NH, -S, -Cl, -Se, -Br, -Te can be also added as the terminations on the MXene surface, which has a profound influence on the control of the surface chemistry, structure, and properties of MXenes.34Kamysbayev V. Filatov A.S. Hu H. Rui X. Lagunas F. Wang D. Klie R.F. Talapin D.V. Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes.Science. 2020; : eaba8311https://doi.org/10.1126/science.aba8311Crossref Scopus (35) Google Scholar(3)Large-scale production. In future research, mass production is predicted to be important in terms of various applications. Nevertheless, to date, there have been no studies on large-scale production of MXenes even though it is feasible theoretically. Recently, a report by Gogotsi et al.35Shuck C.E. Sarycheva A. Anayee M. Levitt A. Zhu Y. Uzun S. Balitskiy V. Zahorodna V. Gogotsi O. Gogotsi Y. Scalable synthesis of Ti3C2Tx MXene.Adv. Eng. Mater. 2020; 22: 1901241Crossref Scopus (24) Google Scholar compared different outputs of Ti3C2Tx preparation in large versus small batches to confirm the feasibility of scaling synthesis. Various characterizations of scaling MXene production confirmed that there was no significant structural difference in the as-synthesized materials. In other words, the scaling effort of MXenes toward industrial quantities is feasible. However, a challenge probably remains at larger scale in translating the laboratory framework to pilot plant levels. Thus, the quest continues to probe effective methods and approaches for mass production.(4)Current research trend on MXenes is still at an early stage. Although numerous reports on MXene fabrication have been published within this decade, some theoretical aspects remain largely unexplored. The majority of new MXene materials are currently in the early stages of development, implying that their exact properties are likely to be underestimated due to non-optimal synthesis conditions. Emerging new types of MXenes include Mo1.33C and W1.33C; the former exhibit outstanding electrochemical performance and the latter show excellent hydrogen evolution reaction activity.36Ahmed B. Ghazaly A.E. Rosen J. i-MXenes for energy storage and catalysis.Adv. Funct. Mater. 2020; : 2000894https://doi.org/10.1002/adfm.202000894Crossref Scopus (6) Google Scholar Moreover, some unobtrusive MXenes (Ta3C2, Ti2N, Cr2Ti2C3, and so on) were proven to have potential in a magnetic field, which has received relatively little attention from researchers.22Jiang X. Kuklin A.V. Baev A. Ge Y. Ågren H. Zhang H. Prasad P.N. Two-dimensional MXenes: from morphological to optical, electric, and magnetic properties and applications.Phys. Rep. 2020; 848: 1-58Crossref Scopus (82) Google Scholar The emergence of methods and types of MXenes has create large interest across multidisciplinary topics. Based on the available methods of MXene preparation, we could find several potential prospects for the realization of flexible devices based on MXenes.Table 1Summary of the Etching Methods, Products, and Experimental Conditions of Typical Synthetic Routes of MXenesMethodPrecursorMXenesEtchantTemperature (°C)Etching TimeFunctional Groups or BreakthroughReferenceHF-containing etchingMo2Ga2CMo2CHF503 hMo-based MXene for the first timeMeshkian et al.41Meshkian R. Näslund L.-Å. Halim J. Lu J. Barsoum M.W. Rosen J. Synthesis of two-dimensional molybdenum carbide, Mo2C, from the gallium based atomic laminate Mo2Ga2C.Scripta Mater. 2015; 108: 147-150Crossref Scopus (0) Google ScholarTi3AlC2Ti3C2LiF + HCl4045 h-O, -FGhidiu et al.37Ghidiu M. Lukatskaya M.R. Zhao M.-Q. Gogotsi Y. Barsoum M.W. Conductive two-dimensional titanium carbide ‘clay’ with high volumetric capacitance.Nature. 2014; 516: 78Crossref PubMed Scopus (1812) Google ScholarTi3AlC2Ti3C2NH4HF2300<1 h-O, -H, -F (intercalation of NH3 and NH4+)Halim et al.42Halim J. Lukatskaya M.R. Cook K.M. Lu J. Smith C.R. Näslund L.-Å. May S.J. Hultman L. Gogotsi Y. Eklund P. et al.Transparent conductive two-dimensional titanium carbide epitaxial thin films.Chem. Mater. 2014; 26: 2374-2381Crossref PubMed Scopus (465) Google ScholarAlkaline solution etchingTi3AlC2Ti3C2NaOH270N/AF-free for the first timeLi et al.43Li T. Yao L. Liu Q. Gu J. Luo R. Li J. Yan X. Wang W. Liu P. Chen B. et al.Fluorine-free synthesis of high-purity Ti3C2Tx (T=OH, O) via alkali treatment.Angew. Chem. Int. Ed. 2018; 57: 6115-6119Crossref PubMed Scopus (136) Google ScholarElectrochemical etchingTi2AlCTi2CHClN/A5 days-Cl, -O, -OHSun et al.38Sun W. Shah S.A. Chen Y. Tan Z. Gao H. Habib T. Radovic M. Green M.J. Electrochemical etching of Ti2AlC to Ti2CTx (MXene) in low-concentration hydrochloric acid solution.J. Mater. Chem. A. 2017; 5: 21663-21668Crossref Google ScholarMolten salt etchingTi4AlN3Ti4N3KF + LiF + NaF5500.5 h-F, -O, or -OH (the first Ti4N3-based MXene)Urbankowski et al.44Urbankowski P. Anasori B. Makaryan T. Er D. Kota S. Walsh P.L. Zhao M. Shenoy V.B. Barsoum M.W. Gogotsi Y. Synthesis of two-dimensional titanium nitride Ti4N3 (MXene).Nanoscale. 2016; 8: 11385-11391Crossref PubMed Google ScholarTi3ZnC2Ti3C2ZnCl25505 h-Cl -O (for the first time)Li et al.32Li M. Lu J. Luo K. Li Y. Chang K. Chen K. Zhou J. Rosen J. Hultman L. Eklund P. et al.Element replacement approach by reaction with Lewis acidic molten salts to synthesize nanolaminated MAX phases and MXenes.J. Am. Chem. Soc. 2019; 141: 4730-4737Crossref PubMed Scopus (107) Google ScholarTi3AlC2Ti3C2CdCl2/CdBr26106-Cl/-BrKamysbayev et al.34Kamysbayev V. Filatov A.S. Hu H. Rui X. Lagunas F. Wang D. Klie R.F. Talapin D.V. Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes.Science. 2020; : eaba8311https://doi.org/10.1126/science.aba8311Crossref Scopus (35) Google Scholar Open table in a new tab The clay-like MXene made from the mixing fluoride salts and HCl can be shaped to form a moldable conductive solid or rolled into thin sheets, which are promising candidates for flexible electronic devices.37Ghidiu M. Lukatskaya M.R. Zhao M.-Q. Gogotsi Y. Barsoum M.W. Conductive two-dimensional titanium carbide ‘clay’ with high volumetric capacitance.Nature. 2014; 516: 78Crossref PubMed Scopus (1812) Google Scholar The Cl-terminated MXenes are expected to enhance the electrochemical properties and are potentially promising for energy storage because this MXene is more stable than the F-terminated MXenes.38Sun W. Shah S.A. Chen Y. Tan Z. Gao H. Habib T. Radovic M. Green M.J. Electrochemical etching of Ti2AlC to Ti2CTx (MXene) in low-concentration hydrochloric acid solution.J. Mater. Chem. A. 2017; 5: 21663-21668Crossref Google Scholar,39Kajiyama S. Szabova L. Iinuma H. Sugahara A. Gotoh K. Sodeyama K. Tateyama Y. Okubo M. Yamada A. Enhanced Li-ion accessibility in MXene titanium carbide by steric chloride termination.Adv. Energy Mater. 2017; 7: 1601873Crossref Scopus (78) Google Scholar Furthermore, an ion-intercalated Ti3C2Tx has been confirmed to possess a higher volumetric capacitance,40Lukatskaya M.R. Mashtalir O. Ren C.E. Dall’Agnese Y. Rozier P. Taberna P.L. Naguib M. Simon P. Barsoum M.W. Gogotsi Y. Cation intercalation and high volumetric capacitance of two-dimensional titanium carbide.Science. 2013; 341: 1502Crossref PubMed Scopus (1770) Google Scholar and thus it can be synthesized using many approaches, for example, using LiF and HCl as an etchant agent to etch the MAX phase. Owing to their higher electronic conductivities than carbide MXenes, transition metal nitrides MXenes are therefore suitable candidates for electrodes in electrochemical capacitors. In terms of large-scale applications, alkaline solution etching methods have emerged with multiple advantages over HF-containing etching, including higher production yield, lower cost, and safer and easier to scale up. With regard to energy storage, supercapacitors are important electronic devices and are urgently needed for future technologies. Combining the advantages of characteristics such as high power density, long cycle life, and fast charge and discharge, supercapacitors have become the primary power source for smart electronics, in particular for miniaturized devices.45Wang G. Zhang L. Zhang J. A review of electrode materials for electrochemical supercapacitors.Chem. Soc. Rev. 2012; 41: 797-828Crossref PubMed Google Scholar The use of high-capacitance materials (high surface area or pseudo-active species) is a key factor to ensure high energy density for reliable electronic devices. At present, the developments with graphene are promoting research on 2D nanomaterials in the field of supercapacitors, but the low energy density and capacitance of graphene is hindering its further development.46Cranford S.W. Buehler M.J. Packing efficiency and accessible surface area of crumpled graphene.Phys. Rev. B. 2011; 84: 205451Crossref Scopus (84) Google Scholar On the other hands, 2D layered MXene nanomaterials can facilitate the intercalation of water and electrolyte ions between adjacent layers. MXenes can effectively store charges through cations an adsorption mechanism and promote swollen MXene galleries facilitated by rapid layer expansion and contraction.47Lukatskaya M.R. Kota S. Lin Z. Zhao M.-Q. Shpigel N. Levi M.D. Halim J. Taberna P.-L. Barsoum M.W. Simon P. et al.Ultra-high-rate pseudocapacitive energy storage in two-dimensional transition metal carbides.Nat. Energy. 2017; 2: 17105Crossref Scopus (544) Google Scholar For instance, a stable specific volumetric capacitance close to 1,500 F cm−3 was reported for Ti3C2Tx MXenes,47Lukatskaya M.R. Kota S. Lin Z. Zhao M.-Q. Shpigel N. Levi M.D. Halim J. Taberna P.-L. Barsoum M.W. Simon P. et al.Ultra-high-rate pseudocapacitive energy storage in two-dimensional transition metal carbides.Nat. Energy. 2017; 2: 17105Crossref Scopus (544) Google Scholar exceeding all carbon-based supercapacitors.48Yan J. Wang Q. Wei T. Jiang L. Zhang M. Jing X. Fan Z. Template-assisted low temperature synthesis of functionalized graphene for ultrahigh volumetric performance supercapacitors.ACS Nano. 2014; 8: 4720-4729Crossref PubMed Scopus (319) Google Scholar,49Jung N. Kwon S. Lee D. Yoon D.-M. Park Y.M. Benayad A. Choi J.-Y. Park J.S. Synthesis of chemically bonded graphene/carbon nanotube composites and their application in large volumetric capacitance supercapacitors.Adv. Mater. 2013; 25: 6854-6858Crossref PubMed Scopus (190) Google Scholar Moreover, most MXenes are metallic conductors with an electrical conductivity up to 15,000 S cm−1,50Zhang J. Kong N. Uzun S. Levitt A. Seyedin S. Lynch P.A. Qin S. Han M. Yang W. Liu J. et al.Scalable manufacturing of free-standing, strong Ti3C2Tx MXene films with outstanding conductivity.Adv. Mater. 2020; 32: 2001093Crossref Scopus (31) Google Scholar enabling fast transfer and high current charge. Therefore, all these benefits endow great potential for MXene-based supercapacitor electrodes and have led to great attention from researchers concerning MXene-based supercapacitors. Flexible e