The family of 2D transition metal carbides, carbonitrides and nitrides (collectively referred to as MXenes) has expanded rapidly since the discovery of Ti3C2 in 2011. The materials reported so far always have surface terminations, such as hydroxyl, oxygen or fluorine, which impart hydrophilicity to their surfaces. About 20 different MXenes have been synthesized, and the structures and properties of dozens more have been theoretically predicted. The availability of solid solutions, the control of surface terminations and a recent discovery of multi-transition-metal layered MXenes offer the potential for synthesis of many new structures. The versatile chemistry of MXenes allows the tuning of properties for applications including energy storage, electromagnetic interference shielding, reinforcement for composites, water purification, gas- and biosensors, lubrication, and photo-, electro- and chemical catalysis. Attractive electronic, optical, plasmonic and thermoelectric properties have also been shown. In this Review, we present the synthesis, structure and properties of MXenes, as well as their energy storage and related applications, and an outlook for future research.
2D materials have unusual electronic, mechanical and optical properties1,
Transition metal carbides, carbonitrides and nitrides (MXenes) are among the latest additions to the 2D world15,
Synthesis of MXenes
MXenes are made by selective etching of certain atomic layers from their layered precursors, such as MAX phases. MAX phases are a very large family of ternary carbides and nitrides with more than 70 reported so far, in addition to numerous solid solutions and ordered double transition metal structures22,31,
Aqueous fluoride-containing acidic solutions have been predominantly used to selectively etch the A-element layers from MAX phases to synthesize MXenes (Fig. 2), either by using aqueous hydrofluoric acid (HF)22 or by in situ formation of HF through the reaction of hydrochloric acid (HCl) and fluoride (for example, lithium fluoride (LiF))35. Ammonium hydrogen bifluoride (NH4HF2) and ammonium fluoride have also been successfully applied for Ti3C2 synthesis from Ti3AlC2 (Refs 36,
It is also possible to synthesize MXenes from non-MAX-phase precursors39,
MXenes can also be made by high-temperature etching of MAX phases, as was recently demonstrated by treating Ti4AlN3 in a molten fluoride salt mixture at 550 °C under an argon atmosphere to form Ti4N3 (Ref. 25). Before the discovery of MXenes, gaseous etchants (halides) were used to etch MAX phases at elevated temperatures, but their selectivity was not sufficient and removed both A and M elements, leading to the formation of carbide-derived carbon46,47. There have also been reports of high-temperature (>800 °C) removal of the A-element layers from MAX phases using molten salt48,49 and evaporating the A layer in a vacuum50. However, the resulting carbides were cubic, not 2D, owing to the specific treatment conditions (for example, the temperature and gas environment). For example, in 2011, a titanium carboxyfluoride with a cubic (rock salt) TiCx structure was prepared by heating Ti2AlC in molten LiF in air at 900 °C (Ref. 51). This is in agreement with the transition metal carbide non-stoichiometric phase diagrams, in which ordered non-stoichiometric carbides (for example, Ti2C and Ti3C2, which have similar formulae to MXenes) are stable below certain temperatures (∼800 °C) depending on the phase52. Therefore, it is reasonable to assume that the synthesis and annealing of MXenes must be performed below those temperatures and in a controlled atmosphere; otherwise, a cubic phase will form preferentially. These results and the recent synthesis of Ti4N3 MXene suggest that the molten-salt approach is effective for the synthesis of new MXenes.
Bottom-up synthesis methods, such as chemical vapour deposition (CVD), should also be possible for MXene synthesis21,53. In 2015, ultrathin (a few nanometres) α-Mo2C orthorhombic 2D crystals with up to 100-μm lateral size were produced by CVD from methane on a bilayer substrate of copper foil above a molybdenum foil53. Using the same method, other transition metals, such as tungsten and tantalum, were made into ultrathin WC and TaC crystals53. This method yields MXenes with a large lateral size and few defects, facilitating the study of their intrinsic properties21,53. The synthesis of MXene monolayers with this method is still to be demonstrated, and bottom-up synthesis options should be further explored.
In this Review, the focus is on wet etching, because it is the most widely used method to fabricate MXenes.
Etching with hydrofluoric acid. Various MXenes can be produced by HF etching (Fig. 2) at room temperature to 55 °C by controlling the reaction time and HF concentration22,54. HF is a highly selective etchant that is even capable of selectively removing different polytypes of SiC (Ref. 55).
The etching conditions for Al-containing MAX phases vary from one transition metal to another, depending on the structure, atomic bonding and particle size of the material. Etching conditions for every MXene synthesized so far are provided in the Supplementary information S3 (table). On the basis of experimental findings, increasing the atomic number of M requires a longer time and stronger etching. This can be related to M–Al bonding22: knowing that M–Al bonding is metallic, we speculate that a larger number of M valence electrons requires stronger etching.
Etching is a kinetically controlled process, and each MXene needs a different etching time to achieve complete conversion. Usually MXenes with larger n in Mn + 1CnTx require stronger etching and/or a longer etching time. For example, Mo2Ti2AlC3 (n = 3) requires an etching time that is twice as long as its n = 2 counterpart (that is, Mo2TiAlC2) under the same etching conditions20,56 (Supplementary information S3 (table)). In general, every MXene can be made under different etching conditions, which lead to different quality (concentration of defects and surface chemistry), as is discussed in the following sections.
Etching in the presence of a fluoride salt. Instead of HF, a mixture of a strong acid and a fluoride salt can be used to synthesize MXenes35,40,57. HCl and LiF react to form HF in situ, which selectively etches the A atoms. Recently, using a mixture of HF and LiCl, similar etching results were achieved, suggesting that the presence of protons and fluoride ions is a necessary condition for etching and for MXene ‘clay’ formation58. Etching in the presence of a metal halide leads to the intercalation of cations (for example, Li+) and water, which increases the spacing between MXene layers and thus weakens their interaction. This is one of the advantages of this method over pure HF etching, because MXene can be delaminated with no additional step after washing to a pH value of about 6 to achieve single- or few-layer flakes (for example, Ti3C2Tx (Ref. 59)), as is discussed in the following section.
Delamination. In general, delamination of any 2D material is a necessary step in exploring its properties in the 2D state. Because multilayered MXenes have two- to sixfold stronger interlayer interactions than those in graphite and bulk MoS2 (Ref. 60), simple mechanical exfoliation provides a low yield of single layers. There are only two reports of Scotch tape exfoliation of multilayer MXene into single flakes61,62, and the remainder have been delaminated via intercalation (Fig. 2c). A complete list of MXene intercalants reported so far is presented in the Supplementary information S4 (box). MXenes can be intercalated with various polar organic molecules, such as hydrazine, urea and dimethyl sulfoxide (DMSO)63, isopropylamine64 or large organic base molecules, such as tetrabutylammonium hydroxide (TBAOH), choline hydroxide or n-butylamine65. MXene intercalation with these molecules, followed by mechanical vibration or sonication in water, leads to a colloidal solution of single- and few-layer MXenes. Filtering then results in freestanding MXene ‘paper’ (Refs 20,63,
MXenes (for example, Ti3C2Tx) can be intercalated with different metal cations, using aqueous solutions of ionic compounds, such as halide salts or metal hydroxides58,66. When etching with a fluoride salt mixed with an acid (for example, HCl and LiF), no additional molecule is needed, because etched MXene is intercalated with metal cations (Supplementary information S3 (table)). In situ delamination of MXenes can be achieved by raising the pH to almost neutral and with very mild mechanical vibration (for example, by hand shaking the solution)67,68. In general, the resulting aqueous colloidal MXene suspensions are stable (Fig. 2c) and do not aggregate owing to the negative zeta potential of the MXene flakes69.
Structure and properties
Structure of the MXene layer. Similar to their MAX precursors, M atoms in MXenes are arranged in a close-packed structure and X atoms fill the octahedral interstitial sites. Three packing arrangements are possible: BγA–AγB (M2X–M2X), BγAβC–CβAγB (M3X2–M3X2) and BαCβAγB–BγAβCαB (M4X3–M4X3). Here, the capital Roman and Greek letters correspond to the M and X positions, respectively. The lowercase Greek letters represent the X octahedral interstitial sites corresponding to their Roman letter counterpart positions (that is, α, β and γ correspond to A, B and C sites, respectively; Supplementary information S5 (figure)). The overall crystal of MXenes is a hexagonal close-packed structure. However, the ordering of M atoms changes from M2X to M3X2 and M4X3. In M2X, M atoms follow ABABAB ordering (hexagonal close-packed stacking), whereas in M3C2 and M4C3, M atoms have ABCABC ordering (face-centred cubic stacking). This atomic ordering (see Supplementary information S5 (figure)) becomes very important for the synthesis of MXenes based on transition metals with hexagonal close-packed structures in the bulk state, such as molybdenum and chromium carbides. For example, Mo2CTx is stable39,40, whereas Mo3C2Tx and Mo4C3Tx, where M would be in the ABCABC ordering, are unstable20. The latter two have been stabilized by inserting another M element (for example, titanium) in the structure to form an ordered double transition metal Mo2TiC2Tx and Mo2Ti2C3Tx (Ref. 20) (Fig. 1).
Surface terminations. MXenes that are synthesized using acidic-fluoride-containing solutions have a mixture of –OH, –O and –F terminations, with the chemical formula Mn + 1Xn(OH)xOyFz. For the sake of brevity, this is usually denoted as Mn + 1XnTx, where T represents the surface terminations.
Non-terminated MXenes are yet to be synthesized. In most recent computational studies, surface terminations have been considered, in addition to the evaluation of the properties of bare Mn + 1Xn layers23,27,70. Many studies have focused on a specific surface termination (for example, pure –OH, –O or –F) and predicted the properties of the MXene18,24,71,
The understanding of surface terminations is developing37,86,
The surface terminations and flake stacking of Ti3C2Tx and V2CTx, among all MXenes, were recently studied by electron energy-loss spectroscopy in transmission electron microscopy37,86, neutron scattering87 and NMR spectroscopy93,94. These studies confirmed that there is a random distribution of terminations on MXene surfaces, rather than regions terminated by a certain kind of atom or group87,94, with atomic stacking in agreement with DFT predictions, as discussed earlier; OH and F are directly bonded to the surface of MXene flakes and water is hydrogen bonded to the OH groups93. Also, there are no neighbouring –OH terminations94. These studies provided a realistic map of surface terminations on Ti3C2 sheets that can be used for predicting their properties by DFT.
Based on neutron scattering measurements on Ti3C2Tx, it was suggested that interactions between the layers can be described by hydrogen bonding between O or F atoms of one surface with the OH surface groups of the opposing Ti3C2Tx sheet and by van der Waals bonding of O and/or F atoms between the sheets. The extent of interlayer hydrogen bonding depends not only on the orientation of the OH groups relative to the layers, but also on the amount and distribution of –OH relative to the –O and –F moieties positioned on the opposing surface87. When water is present between the layers, it hydrogen bonds strongly with O or OH terminations88,95. Moreover, the intercalation of cations can lead to easy sliding of the Ti3C2Tx sheets relative to each other86, altering their rheological properties and leading to clay-like behaviour.
Effect of synthesis conditions on MXene quality and terminations. As-synthesized MXene flakes contain intrinsic defects, such as atomic vacancies and adatoms37,67. Etching and delamination conditions affect the quality, overall crystallinity, defects and surface functionalization of MXene flakes, as well as their delamination efficiency. In general, milder etching and delamination conditions produce larger MXene flakes with lower defect concentrations40,59,67,87,96 (Fig. 3a,b). Single metal vacancies or vacancy clusters were observed in Ti3C2Tx flakes synthesized even under very mild conditions (Fig. 3c–e), and their concentration is strongly dependent on the HF concentration used during etching of the carbide precursor67 (Fig. 3f). Ti3C2Tx flakes of 3–6 μm lateral size with minimal defects have been produced via mild delamination59,68,97 (Fig. 3b). Sonication of these flakes in water reduces their lateral size to less than 1 μm and increases the concentration of defects59,97 (Fig. 3a). The etching temperature and time are also important. For example, when synthesizing Ti3C2Tx in HCl and LiF, a higher etching temperature and longer etching time can lead to complete conversion to MXene, but can also reduce the delamination yield and quality of the flakes owing to a higher concentration of defects35.
Scanning electron microscopy images of 50% HF-etched Ti3C2Tx reveal accordion-like particles (Supplementary information S6 (figure, panel a)), whereas etching in HCl and LiF produces densely packed particles (Supplementary information S6 (figure, panel b)) owing to milder etching accompanied by Li+ intercalation94. In agreement with this observation, it was shown that intercalation of metal cations can change the HF-etched Ti3C2Tx accordion-like morphology to thicker multilayer lamellas with fewer interstack gaps66. However, it is important to note that this is a result of the morphology of the particle and does not imply that 50% HF-etched MXene atomic layers are more separated. In fact, the distance between MXene layers of the as-etched MXene in HCl and LiF (with intercalated cations and water) is about 2.8 Å larger than that of the 50% HF-etched MXene; this is because of the presence of a layer of water with cations between the MXene layers in the former case58,66,94. In general, MXene interlayer spacing depends on the number of intercalated water molecules between the MXene layers, as described in the Supplementary information S6,S7 (figure, panel c; box).
V-, Nb-, Ta- and Mo-containing MXenes require more aggressive etching conditions. So far, they have primarily been etched using ∼50% HF and delaminated either by DMSO or TBAOH40,56,65. However, all MXenes can be produced by using acid with a metal fluoride salt mixture (for example, HCl–LiF), when the etching conditions are optimized. For example, Mo2CTx etched with 50% HF and delaminated using TBAOH can also be etched and delaminated using HCl–LiF (Refs 40,57). Although the resulting MXene colloidal solution concentration in the former method is much higher than the latter, the MXene flakes are larger with fewer defects for the HCl–LiF method owing to the milder etching conditions40. Reducing the etching time and bubbling with argon during the process are important measures for preventing oxidation, over-etching and the formation of defects.
Surface terminations depend on the etching and delamination conditions, the type of M element, and post-synthesis treatment and storage88,94. Different concentrations of HF give different surface termination ratios87. In general, lower HF concentrations result in a larger oxygen to fluorine ratio. For example, Ti3C2Tx, when etched using less concentrated HF solutions (that is, 10% HF (Ref. 87) and HCl–LiF (Ref. 94)), has more oxygen and less fluorine compared with MXene samples synthesized using 50% HF (Supplementary information S6 (figure, panel d)). Moreover, different etching solutions affect the surface chemistry of MXenes; for example, etching Ti3AlC2 with NH4HF2 and HCl–LiF results in the intercalation of NH4+ and Li+ cations, respectively36,58.
Further insight into surface terminations was provided by X-ray photoelectron spectroscopy (XPS), which revealed that Ti3C2Tx oxidizes upon storage in air and that fluorine is slowly replaced by oxygen; these findings were in agreement with theoretical calculations71. This is noteworthy because oxygen-terminated MXenes are predicted to have a higher capacity in lithium-ion and other batteries71, which is discussed later. However, NMR studies show that the termination groups remained the same after drying at 200 °C in vacuum and that only the content of intercalated water was reduced. Therefore, there are discrepancies between the content of each termination measured with these two techniques (for example, the hydroxyl content measured by NMR is very small94, whereas it is relatively large according to the XPS results88). These discrepancies could stem from a multitude of factors, including NMR providing averaged data on terminations in contrast to the surface-sensitive XPS, or possible variations in sample preparation and storage. It is important that this is clarified both experimentally and computationally.
Stability. Single MXene flakes are not indefinitely stable in environments with oxygen and water present98. However, they are relatively stable in oxygen-free degassed water or in dry air. Also, exposure to light can accelerate the oxidation of colloidal MXene solutions. Therefore, it is recommended to refrigerate MXene colloids in an oxygen-free dark environment for storage. In general, the oxidation of MXene flakes starts from the edges, leading to the formation of metal oxide nanocrystals (for example, TiO2) decorating the flake edges, and then develops through nucleation and growth throughout the entire surface99,
The understanding of high-temperature stability of MXenes is still developing. Phase diagrams of nonstoichiometric transition metal carbides may help to predict the phase stability of MXenes52. The high-temperature stability of MXenes depends on their composition and the environment. Consequently, different studies have reported different high-temperature behaviour. Recently, it was shown that Ti3C2Tx is stable at 500 °C in an argon atmosphere, but some TiO2 crystals formed, decorating the edges of the particles99. Moreover, the Ti3C2 structure was preserved even at 1,200 °C under argon, and defect annealing occurred. However, MXene phase transformation was indicated by strong XRD peaks of cubic TiCx (Ref. 99), which is the most stable phase at 1,200 °C in the non-stoichiometric TiC phase diagram52. Temperature-programmed desorption with mass spectroscopy showed substantial weight loss of Ti3C2Tx above 800 °C in a helium atmosphere95, indicating a phase transformation, which is in agreement with vacuum calcination treatment at the same temperature102 and in situ TEM observations103. Ti2CTx is confirmed to be stable at 250 °C under different inert atmospheres101. However, in a different study, Ti2CTx (T = O) was shown to be stable at 1,100 °C under argon and hydrogen62, which is above the phase stability limit of non-stoichiometric Ti2C. Although the characterization results are convincing, XRD or detailed Raman analysis is needed to confirm whether any new phases are formed. Zr3C2Tx was shown to have good thermal stability and retain its 2D nature at temperatures of up to 1,000 °C under vacuum, in contrast to Ti3C2Tx, which transforms to cubic carbide. The better thermal stability of Zr3C2 can be explained by its structure being more energetically favourable than bulk ZrC, in contrast to Ti3C2, which is metastable relative to bulk cubic TiC (Ref. 41). Owing to its higher stability, Zr3C2Tx may be useful for high-temperature applications.
Attempts to synthesize Cr2C from Cr2AlC have so far been unsuccessful104. One possible explanation is the lower stability of chromium carbide owing to its lower cohesive energy than other carbides31. During etching, Cr2C possibly forms in the aqueous solutions and quickly transforms to other phases, such as chromium oxides.
Physical and mechanical properties. The rich transition metal chemistry of MXenes (Fig. 1) led to several computational studies investigating the effect of M, X and T, the number of M layers and the lattice strain on the electronic, thermal and mechanical properties of MXenes18,23,24,26,27,29,30,79,89,105,
Both DFT23 and molecular dynamics (MD)113 predict that M2X MXenes are stiffer and stronger than their M3X2 and M4X3 counterparts (Fig. 4a). However, experimental mechanical testing has only been conducted for MXene films and not for single-layer MXenes (Fig. 4b–d). A cylinder with walls made of a 5-μm-thick Ti3C2Tx paper can support ∼4,000 times its own weight. These films can be further strengthened by creating a Ti3C2Tx composite with 10-wt% polyvinyl alcohol (PVA) to hold ∼15,000 times their own weight117 (Fig. 4d).
Many MXene–polymer composites have been developed — for example, with PVA117, polypyrrole118, polyethylene119 or polydimethylsiloxane (PDMS)120 — with enhanced mechanical, thermal and electrochemical properties, and wear resistance. In a recent study, pyrrole monomer was mixed with Ti3C2Tx colloidal solution and, owing to the acidic properties of the MXene, polymerization initiated without the need for an oxidant to form a MXene–polypyrrole composite with high electrochemical capacitance118. In addition to polymers, MXene/carbon-nanotube (CNT) hybrids have been prepared, mostly for electrochemical applications64,121,
Thin films of MXenes and their composites are transparent. Ti3C2 transmits >97% of visible light per nanometre thickness124,125 (Fig. 4e,f), and its optoelectronic properties can be tuned by the chemical and electrochemical intercalation of cations125. This suggests applications of MXene films in transparent conductive coatings and optoelectronics.
Theoretical studies have shown that the electronic properties of different MXenes range from metallic to semiconducting, depending on the nature of M, X and the surface termination15,18,28,
In addition to the composition, the electrical conductivity of MXenes depends on the sample preparation method. In general, a low concentration of defects and large flake size result in higher conductivity. This can be achieved through milder etching and sonication-free delamination68, good contact between individual flakes by coplanar alignment124 and drying to remove intercalated species between the layers102. As a result, the conductivity of Ti3C2Tx ranges from less than 1,000 S cm−1 for cold-pressed discs made of highly defective HF-etched powder16,102, to 4,600 S cm−1 and 6,500 S cm−1 for milder etched and delaminated vacuum filtered68 and spin-cast films124, respectively; these values exceed those of other solution-processed materials, including graphene129,130.
Ferromagnetic and antiferromagnetic properties have been predicted for some termination-free MXenes, although magnetism disappears in the presence of surface terminations18,27,108,131. Of all MXenes, two — Cr2CTx and Cr2NTx — have been predicted to possess a magnetic moment, even with surface terminations18,108; however, their magnetic nature is not yet clear. In one study, terminated Cr-MXenes were predicted to be ferromagnetic18. By contrast, when antiferromagnetic configuration was considered, terminated Cr2CTx and Cr2NTx were determined as antiferromagnetic108,132, except Cr2NO2, which remained ferromagnetic132. These Cr-MXenes are yet to be produced experimentally. In 2015, the first chromium-containing MXene, Cr2TiC2Tx (Ref. 20), was synthesized; however, the magnetic properties of this MXene are yet to be characterized.
Energy storage applications of 2D carbides
MXenes in batteries. MXenes have wide chemical and structural variety, which makes them competitive with other 2D materials133,134. For this reason, theoretical studies can help to define the most promising candidates for energy storage applications. For example, it was found that in terms of theoretical gravimetric capacity (that is, the amount of charge that can be stored per gram of material), MXenes with low formula weights, such as Ti2C, Nb2C, V2C and Sc2C, are the most promising81. Therefore, M2X electrodes are expected to show higher gravimetric capacities than their M3X2 and M4X3 counterparts. Because the bonds between M and X are too strong to be broken easily, it is reasonable to assume that ions penetrate only between the MXene sheets. This is supported by all available experimental data. For example, by comparing Ti2C and Ti3C2, both of which have the same surface chemistry, Ti2C should have ∼50% higher gravimetric capacitance than Ti3C2 because Ti3C2 has one inactive TiC layer. This was confirmed experimentally: the gravimetric capacity of Ti2CTx for Li+ uptake is ∼1.5 times higher than that of Ti3C2Tx prepared in the same way63,135.
It is important to note that the capacity is not completely defined by the formula weight. For example, V2CTx shows the highest Li+ capacity of all MXenes tested under similar conditions (280 mAh g−1 at 1C and 125 mAh g−1 at 10C cycling rates)17. Moreover, although niobium atoms are heavier than titanium, the gravimetric capacity of Nb2CTx is higher than that for Ti2CTx at the same cycling rate (180 mAh g−1 for Nb2CTx versus 110 mAh g−1 of Ti2CTx at 1C)17. In part, this can be explained by the complex nature of ion storage. The surface terminations are one particular factor than can affect the performance, as demonstrated by theoretical investigation71,81,136. For example, oxygen terminations are considered most favourable, whereas hydroxyls and fluorines result in lower capacity as well as impeded lithium-ion transport71,76.
The key features of ion intercalation into MXenes from organic electrolytes have been revealed theoretically and confirmed experimentally (Fig. 6a). When the mechanism of lithium-ion charge storage in Ti3C2Tx was studied using in situ X-ray absorption spectroscopy (XAS), it was shown that there is a continuous change in the transition metal (that is, titanium) oxidation state during charge and discharge (Fig. 6b) up to 0.5 V versus Li/Li+ (Ref. 71). Interestingly, a further decrease in potential does not translate into a change in oxidation state. Instead, owing to the 2D nature and conductivity of MXenes, lithium atoms can reversibly form an additional layer (as shown in Fig. 6a). This provides a twofold boost of the capacity, and this mechanism is also expected to be applicable to other MXenes136,137. A further increase in capacity was achieved by optimization of the electrode architecture, hybridizing porous MXene flakes with CNTs, resulting in a lithium-ion capacity in excess of 750 mAh g−1 (Ref. 138).
MXenes can accommodate ions of various sizes between 2D layers of Mn + 1XnTx. This makes them suitable for non-lithium-ion batteries (NLiBs), for which the current selection of electrode materials is limited. Theoretical capacities for some oxygen-terminated MXenes in Na-, K-, Mg-, Ca- and Al-ion batteries136 are shown in Fig. 6c. Note that the formation of an additional metal layer was predicted for Na+ and other ions, which would result in a doubling of the capacity. Moreover, owing to chemical and structural variability and surface chemistry tunability, different MXenes can provide a range of working potentials, which makes some of them suitable as either anodes139 or cathodes140 (Fig. 6d).
Theoretical studies show low diffusion barriers for Li+ (Ref. 71) and other ions73,86,136,141 in MXenes. This is in agreement with the remarkably high-rate performance experimentally observed for several MXenes17,63,139. Typical capacities of MXene-based electrodes at rates beyond 10C (that is, a 6-minute charging time) fall in the range of 50–200 mAh g−1. As a result, the MXenes in metal-ion batteries do not display a plateau region in the galvanostatic charge–discharge profiles (Fig. 6b), which resembles the behaviour of supercapacitors.
MXene-based composite electrodes hold particular promise for high-performance, high-rate batteries. For example, Ti2CTx or Ti3C2Tx have been used as conductive sulfur hosts in Li–S batteries, resulting in dramatically improved cyclability and stability owing to the strong interaction of polysulfide species with MXene functional groups142,143 (Fig. 6e). Similarly, encapsulation of tin nanoparticles between layers of Ti3C2Tx results in stable performance with a superior volumetric capacity approaching 2,000 mAh g−1 (Ref. 144). A similar approach of co-integration (hybridization) with MXenes can be applied to substantially improve the cycle life and rate capability of other high-capacity electrode materials that experience a significant volume change upon intercalation. In this approach, MXenes provide a conductive matrix that accommodates expansion and contraction of particles while maintaining structural and electrical connectivity.
MXene-based electrochemical capacitors. MXenes can be spontaneously intercalated by polar organic molecules63,
Ti3C2Tx is the most studied MXene for electrochemical capacitors. The volumetric capacitance of freestanding Ti3C2Tx paper electrodes in neutral and basic electrolytes has been demonstrated to be 300–400 F cm−3; these outstanding values exceed the best all-carbon electrical double-layer capacitors35,145 and are comparable to recently reported activated graphene-based electrodes (350 F cm−3)148. Although the shape of the cyclic voltammetry curves slightly differs depending on the cation145,146 (Fig. 7b), there are no pronounced peaks and the cyclic voltammetry profiles look ‘capacitor-like’. Volumetric capacitance exceeding 900 F cm−3 was obtained in 1 M H2SO4 for a rolled pure Ti3C2Tx clay electrode, presumably because protons are the smallest cations and can thus access the largest number of the electrochemically active sites, especially when Li+ ions with water molecules intercalated during the synthesis prevent restacking of MXene sheets. In addition, as can be seen in Fig. 7c, perfect capacitive behaviour is observed for Ti3C2Tx even at fairly high charge and discharge rates35. This is in contrast to the slow intercalation of ions usually observed in other layered materials used in battery applications, such as graphite. Interdigitated thin MXene electrodes on flexible substrates demonstrate even better rate performance97,149. MXenes also exhibit excellent cyclability, with no change in capacitance reported after 10,000 cycles for Ti3C2Tx electrodes35.
The mechanism of high volumetric capacitance of MXene was not immediately clear. Cyclic voltammetry profiles of MXenes have no pronounced redox peaks and resemble those of carbon-based double-layer capacitors. However, the accessible specific surface area is not sufficient to explain the performance35,145. To check whether the mechanism of charge storage is pseudocapacitive (that is, it involves changes in the oxidation state of the transition metal), electrochemical in situ XAS measurement was carried out for Ti3C2Tx (HCl–LiF etched) in 1 M H2SO4 (Ref. 150). Similar to the case for lithium-ion batteries described earlier71, changes in the titanium oxidation state were detected through XAS during cycling, which were consistent with the experimental values of the capacitance of the material. Therefore, it can be concluded that the mechanism of electrochemical storage of Ti3C2Tx MXene in sulfuric acid is predominantly pseudocapacitive and not diffusion limited, at least up to scan rates of 20 mV s−1 (Ref. 35).
There are several important factors that affect the volumetric capacitance of MXenes. First, the density of the electrode serves as a conversion factor of gravimetric to volumetric performance. Because carbides are much denser than carbon, freestanding additive-free MXene electrodes typically possess densities of 3–4 g cm−3, whereas MXene-based composite electrodes with polymer binders and conductive additives have lower densities ranging from 1 to 2.5 g cm−3.
Another important factor that affects the gravimetric and consequently volumetric capacitance is the MXene surface chemistry. For example, replacement of fluorinecontaining functional groups with oxygen-containing groups results in a substantial increase in capacitance. When HF-produced Ti3C2Tx was chemically modified using KOH (Ref. 151), N2H4 (Ref. 95) or DMSO136,145 solutions, the gravimetric capacitance values improved by a factor of two to seven, depending on the electrolyte being used, with the most dramatic improvements demonstrated in acidic electrolytes. Interestingly, it was also shown that Ti3C2Tx produced using a HCl–LiF mixture (instead of HF) possesses predominantly oxygen-containing functionalities94. Owing to these functionalities, outstanding volumetric capacitances of up to 900 F cm−3 were demonstrated. The scalability of the process is notable: electrodes with thicknesses of up to 75 μm were readily produced by rolling the as-synthesized Ti3C2Tx (HCl–LiF etched) clay sample and showed capacitance of 350 F cm−3 (Ref 35) (Fig. 7d). It is important to mention that MXenes other than Ti3C2Tx also demonstrate much promise for supercapacitors. For example, Mo2CTx (Ref. 40) and Mo2TiC2Tx (Ref. 20) showed high volumetric capacitance with rectangular cyclic voltammetry profiles.
MXene-based hybrid materials can provide enhanced electrochemical and mechanical performance. For example, it was found that the integration of 5–10 wt% CNTs (Fig. 7e), graphene or onion-like carbon results in improved rate performance owing to better ion accessibility in aqueous121 and organic electrolytes64 (Fig. 7d). Composites with polymers represent a very important and promising direction for the development of electrode architectures. It was found that polymers with polar functional groups in their chains, such as PVA and poly(diallyldimethylammonium chloride) (PDDA), can intercalate between Ti3C2Tx MXene layers, preventing them from restacking and substantially improving the mechanical properties of the MXene papers without compromising their electrochemical perfromance117. If redox-active polymers are used instead of electrochemically inert polymers, enhanced electrochemical performance can be achieved. For example, a 13-μm-thick Ti3C2Tx–polypyrrole composite demonstrated almost doubled gravimetric capacitance118 in comparison with pure Ti3C2Tx (Ref. 35) films (420 F g−1 versus 240 F g−1 at 2 mV s−1), whereas the increase in its volumetric capacitance to ∼1,000 F cm−3 compared with pure Ti3C2Tx (Ref. 35) films was less dramatic because of the lower density of the composite (2.4 g cm−3 versus 3.8 g cm−3).
Importantly, MXenes demonstrate excellent performance when cathodic (negative) potentials are applied; however, when the material is subjected to a positive potential in aqueous electrolyte (more than ∼0.2–0.4 V above the open circuit potential depending on the MXene composition) irreversible oxidation may start145, leading to an increase in resistance and a loss in capacitance. Therefore, asymmetrical cell design with another electrochemically stable material of high capacitance (for example, nitrogen-doped graphene in acidic electrolyte or MnO2 in basic or neutral electrolyte) should be implemented as the positive electrode for building durable MXene-based energy storage devices.
As an alternative to the aqueous systems mentioned above, organic electrolytes can be used. The extended voltage window of organic electrolytes results in a larger energy density (E = 0.5CV2), where E is energy density, C is the capacitance and V is the voltage window) for given specific capacitances when compared with aqueous electrolytes. However, it is important to remember that the behaviour in aqueous electrolytes cannot be directly translated to performance in organic electrolytes. Thus, it is more appropriate to use the performance of different MXenes in lithium-ion batteries as a guideline, because similar electrolytes are used in lithium-ion capacitors (the difference is that the counter electrode is activated carbon in this case). Also, because organic electrolytes have a lower conductivity and can feature large organic ions, such as the tetrabutylammonium cation, the introduction of conductive spacers, such as CNTs, is important for improving the accessibility to ions and, therefore, the capacitive and rate performance. For example, Nb2CTx–CNT paper (Fig. 7d) electrodes showed a high volumetric capacitance of 325 F cm−3 when tested in a lithium-ion capacitor configuration64, and a Ti3C2Tx–CNTsample demonstrated a capacitance of 245 F cm−3 in organic electrolyte123. Moreover, promising performance of MXenes in an ionic-liquid gel electrolyte within a 3-V potential window has recently been demonstrated152.
The mechanical response of MXenes can be tuned by cation selection, as shown by in situ electrochemical atomic force microscopy experiments146. This study also showed that deformations are highly dependent on the cation nature: in particular, the charge-to-size ratio. Specifically, highly charged small cations do not change the interlayer spacing of 2D Ti3C2Tx electrodes, whereas larger cations with smaller charges expand the interlayer spaces. This potentially provides a route to creating energy storage devices with a close to zero volume change upon charging and discharging, which is a key criterion for achieving a long lifetime and minimizing energy dissipation.
Applications other than energy storage
Rich chemistry and a range of MXene structures make them promising candidates for many applications. Energy storage has been the first and most studied application of MXenes. However, there are potentially other applications in which MXenes can outperform other materials. For example, flexible films of Ti3C2Tx showed the highest ever reported electromagnetic interference shielding for a synthetic material of comparable thickness, higher than that of graphene and other carbons68. Other applications have also been studied, such as reinforcement for composites117, water purification153, catalysts in the chemical industry154, gas- and biosensors85,155, lubricants156, photocatalysts98, electrocatalysts57 and photothermal therapy157 (Table 1).
2D carbides, carbonitrides and nitrides were produced in the past 5 years by selective etching and exfoliation of layered ternary precursors forming a large family of 2D materials named MXenes. The synthesis process adds hydroxyl, oxygen and fluorine terminations, leading to hydrophilic MXene surfaces. Hydrophilicity and high surface charge (negative zeta potential exceeding −30 mV) lead to stable water-based colloidal solutions that do not require surfactants for stabilization. This makes the processing of MXenes easy, facilitating applications such as device printing and the manufacture of coatings and films. As a result, both freestanding films and coatings on various substrates have been produced by a range of techniques, from spray and spin coating to vacuum-assisted filtration.
Attractive electronic, optical, plasmonic and thermoelectric properties have been predicted for many MXenes. High metallic conductivity has been reported for Ti3C2Tx even in the presence of surface functional groups and with water molecules between the layers. By contrast, Mo2CTx and Mo2TiC2Tx show semiconductor-like behaviour. A bandgap of about 0.9 eV was calculated for oxygen-terminated Ti2C, and Dirac cones are expected for the fluorine-terminated Ti2C; however, those and many other property predictions are still waiting for experimental verification. In addition to these properties, MXene films and polymer-bonded MXenes show good mechanical properties.
Cations, including multivalent ones, and polar organic molecules intercalate MXenes, allowing for control of interlayer spacing and enabling the use of MXenes in energy-storage applications, as well as water desalination and purification. The combination of high electronic conductivity and an oxide- or hydroxide-like surface comprising exposed redox-active transition metal atoms makes MXenes very attractive for the fabrication of electrodes. Their energy-storage applications already cover areas from Li-ion and Na-ion batteries and capacitors, to Li–S batteries and aqueous supercapacitors. Extremely high values of volumetric capacitance have been reached in sulfuric acid electrolytes, and promising capacitive performance has been demonstrated in aqueous neutral and basic electrolytes, as well as in organic and ionic liquid electrolytes. Moreover, a continuous change in the oxidation state of titanium in Ti3C2-based electrodes on charging–discharging has been identified, demonstrating a dominant contribution of the pseudocapacitance to charge storage. This not only explains how high capacitance can be demonstrated by a material with a moderate surface area, but also opens new avenues for achieving high energy-density storage at high charging rates.
Gaps in the current knowledge
With the first MXene discovered only in 2011, there are many open questions that need to be addressed before the 2D metallic and semiconducting properties of MXenes can be used to the fullest extent.
Over one hundred distinct compositions have been predicted when considering surface terminations and multi-element MXenes, and many more layered ternary metal carbides and nitrides are waiting to be transformed to MXenes. Any layered transition metal carbide or nitride, in which Mn + 1Xn layers are separated with A-group metals or A-group carbides and nitrides, can potentially be selectively etched and exfoliated to form MXenes. Computational studies have been performed on many MXenes for which ternary carbide precursors have not yet been produced, such as Sc2C, Hf2C or W2C. Thus, the synthesis of new MAX phases and other layered carbide and nitride precursors becomes an important research direction.
For many MXenes that have already been produced or that can be made from available precursors, predictions of the electrical, thermoelectric, magnetic and other properties should be verified experimentally. In this context, computational studies should further guide the synthesis of MXenes by attracting the attention of experimentalists to the synthesis of the most promising compounds.
Characterization of the surface chemistry of various MXenes after synthesis, drying and ageing is important, but it is even more important to develop methods for achieving uniform terminations with the same kind of surface moieties (for example, hydroxyl, fluorine, oxygen or hydrogen) for different applications. MXenes with no surface functional groups have yet to be produced; these must be synthesized by physical or CVD methods in vacuum.
In the energy-storage applications of MXenes, minimization of the first cycle irreversibility observed after Li- and Na-ion intercalation is of great practical importance. Better understanding of intercalation of multivalent (for example, Al3+, Mg2+ or Zn2+) and large organic ions would provide important guidelines for the development of electrode materials for the next generation of batteries and supercapacitors.
Understanding and controlling ion dynamics between MXene sheets can enable the development of high-power supercapacitors and may even allow low-resistance MXene-based devices to replace electrolytic capacitors. An understanding of catalytic and electrocatalytic properties of MXene electrocatalysts will open new avenues in energy conversion. Conversely, suppression of electrocatalytic processes on the surface of MXenes would increase the voltage window and, therefore, the amount of energy stored, as well as the cycle life, in battery and supercapacitor applications.
Finally, mechanical characterization of single MXene flakes should be conducted to determine their intrinsic mechanical properties, and structural and multifunctional composites should be produced by covalently attaching polymer chains to strong and rigid MXene layers.
The authors worked with M. W. Barsoum (Drexel University) and P. Simon (Paul Sabatier University) on MXene synthesis and energy storage, respectively. Y.G. thanks numerous graduate students and post-docs, as well as collaborators at Drexel and elsewhere, who helped in the exploration of MXenes. Research on MXenes was supported by the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science and Office of Basic Energy Sciences. B.A. was supported by King Abdullah University of Science and Technology under the KAUST-Drexel University Competitive Research Grant.
Schematics of M2AX, M3AX2, M4AX3 crystal structures.
Known M2AX, M3AX2, M4AX3 MAX phases to date1,2.
MXene synthesis conditions
Delamination via intercalation
MXene crystal structures showing atomic ordering of M, X and T elements.
Effect of etching conditions on MXenes.
XRD patterns during MXene synthesis