13-4 Review Natreview Gogotsi---2d Metal Carbides and Nitrides (Mxenes) for Energy Storage

• Introduction
• Applications of MXenes
• Conclusions

Introduction

Two-dimensional (second) materials have been the focus of significant research due to their unique electronic, optical, and mechanical properties. Since the discovery of the unusual physical backdrop of graphene in 2004, research groups effectually the world have focused on the discovery, evolution, and applications of 2D materials. Post-obit the evolution of graphene, a wide variety of other 2D families have been explored, including recently discovered unmarried elemental structures (phosphorene (902896), stanene, silicene, germanene (906026), etc.), and known binary compounds (BN (901349, 901410), transition metal dichalcogenides (TMDs) (901867, 901187, 902462, 903841), oxides, etc.), or more circuitous compositions, such every bit clays. In 2011, a family of conductive 2D carbides, nitrides, and carbonitrides, known as MXenes, was discovered.^ane

MXenes are potentially the largest grade of second materials known today, with more than than xxx different types reported (Figure 1), and hundreds computationally studiedin-silico. Moreover, at that place exists the potential for thousands of additional members of this family if solid solutions are included.^two MXenes have the general construction M_northward+iTen_nT_x, where Thousand is an early transition element (Ti, V, Nb, etc.) and X is C and/or N, with northward ranging from 1–4.³ T_x represents the surface terminations
(typically -O, -OH, and -F), with n+i layers of M covering n layers of X in the arrangement of [MX]_northwardM. These conductive, hydrophilic 2D ceramics are synthesized via top-down selective etching of their precursor materials MAX phases (910740, 910767, 910775, 910759, 910821, 910708), typically in the course of Grand_northward+1AX_n, but some MXenes are synthesized from different precursors, including G₂A₂X and M_n+1A₁₀X_n+x besides. The synthesis of MXenes has been shown to be readily scalable, with no change in the properties equally the batch size is increased.^four In these cases, A is primarily Al, merely Si and Ga have besides been used.² MXenes come in multiple forms: single metal element structures (Ti₃C₂T_x, Ti₂CT₁₀, 5_twoCT_x, etc.), ordered double transition metal MXenes (i.e., Mo₂Ti_twoC₃T_x, Mo₂TiC₂T_x, Cr_iiTiC_twoT_ten, etc.), solidsolution MXenes (i.east., Ti_two-_yV_yCT₁₀, Mo_4-y5_yC_iiiT_ten, Ti_2-yNb_yCT_ten), and ordered divacancy MXenes (Mo_one.33CT_x, W_one.33CT_x, etc)_. ² The MXene family unit is quite diverse, with the materials that can exist tuned through a variety of approaches, including modification of the number of atomic layers (due north), changing the M or X elements, adjusting the surface chemistry (T_x) through post-handling or during synthesis, size selection of MXenes, and intercalation of unlike species into the structure, amidst multiple other approaches.²

Effigy 1. MXene compositions reported to date. MXenes have the general construction Mn+1XnTx where M is an early on transition metal (Ti, Five, Nb, etc.), 10 is C and/or North, Tx are the surface terminations (typically -O, -OH, -Cl and -F), and n = 1–4. MXenes discovered to engagement include mono-M MXenes, ordered double-transition metal MXenes, solid-solution MXenes, and ordered divacancy MXenes.³

MXenes typically come in two forms, multilayer (ML) powder or delaminated single flakes. To synthesize ML MXene pulverization, typically fluoride-containing etchants (HF or HF/HCl) are used to selectively remove the A (in nigh cases Al) layer. To convert the ML powder into a unmarried-flake colloidal solution, an intercalant (i.e., LiCl, tetramethylammonium hydroxide (TMAOH), dimethyl sulfoxide (DMSO), etc.) is used.⁵ Another arroyo where the precursor is simultaneously etched and delaminated using anin-situ HF formation approach (HCl+LiF, NH₄HF₂ (455830), etc.) can be used.5 Because MXenes are primarily synthesized via a topochemical process in an aqueous environment, they maintain a hydrophilic nature. Due to this hydrophilicity, MXenes can be processed using standard solution-based techniques (primarily water-based), including vacuum filtration, spray-coating, dip-blanket, spin-coating, etc. In add-on to aqueous solvents, MXenes form stable colloidal solutions in polar organic solvents, including dimethylformamide (DMF), Northward-methyl-2-pyrrolidone (NMP), propylene carbonate (PC), and ethanol.⁶ Due to their all-encompassing customizability (e.thousand., composition, surface terminations, thickness, etc.) and processability, MXenes have already found use in multiple diverse fields, including energy storage devices, biomedical applications, composite materials, electrochromic devices, and endless other applications (Figure 2).

Figure ii. Applications and properties of MXenes explored to date. The center pie nautical chart shows the ratio of publications in each awarding/ property of MXenes with respect to the total number of publications on the "MXene" topic from 2011 to Feb 2019 based on Web of Science. The eye pie chart ring, with similar color to the middle one, shows the starting year for exploration of each application/holding of MXenes. There might be one or two papers published earlier some of the mentioned years; we considered a yr with a few important publications as the starting year for each slice. The outer band shows the ratio of publications but on Ti3C2Tx MXene versus the publications on all MXene compositions (M2XTx, M3X2Tx, M4X3Tx), and M5X4Tx).^ii,seven

Applications of MXenes

The showtime awarding of MXenes was every bit free energy storage materials. Ti₃C_iiT_x, for example, was shown to take a volumetric capacitance of up to ane,500 F cm-three.^viii Considering this property, information technology is expected that thinner (M₂XT_x) MXenes can take a college theoretical gravimetric capacity. Significant work has focused on utilization of various MXenes equally free energy storage materials.⁹ Inside energy storage devices, MXenes accept been used as electrodes in electrochemical capacitors, micro-supercapacitors, and batteries, utilizing Li-, Na-, Mg-, Al-, and other chemistries, with a wide variety of electrolytes being used, including aqueous (eastward.thousand, H_twoAnd then₄, Li₂And then₄, KOH, etc.) and non-aqueous organic (e.1000., DMSO, PC, acetonitrile (ACN), etc.)-based solutions (Figure 3), in addition to ionic liquids.^{2, nine-11} Depending on the electrolyte system used, the usable voltage range can extend from 0.v to 3.0 V.2 Due to the large number of inherent chemistries bachelor to MXenes, significant work has been done to examine the charge storage mechanisms of diverse MXenes, also every bit how to further optimize the materials. MXenes typically have a pseudocapacitive storage machinery; the ions penetrate between MXene sheets, interacting with the surface terminations on the basal airplane. Depending on the ions used, the MXene interlayer spacing can exist varied, all-around the size of the intercalating ion.¹² Finally, MXenes have fantabulous cyclability, with no change in the capacitance recorded after x,000 cycles for Ti₃C₂T_x in aqueous electrolytes.¹¹ The apply of metallically-conductive MXenes as binders and electric current collectors in energy storage devices is also very promising. While a significant portion of MXene research has focused on energy storage capabilities, a wide variety of other applications have also been explored.

Figure 3. Macroporous Ti3C2Tx electrode with 1 M LiTFSI in DMSO, ACN and PC organic electrolytes. A), CV curves. The OCVs (marked past arrows) are −0.xiii V (blackness), −0.32 V (bluish) and −0.12 V (red) versus AgCI for DMSO, ACN and PC-based electrolytes, respectively. B), Chronoamperometry data collected at the applied maximum potentials. C), EIS data collected at the OCV. D), EIS data collected at the maximum negative potential versus AgCI. The insets in C and D show the magnified curves in the high-frequency range; they utilize the same units as in C and D. E), Schematic of a supercapacitor using 2D MXene (pink, Ti; cyan, C; red, O) as negative electrode with solvated or desolvated states. Legend for the electrolyte: green, cation; orange, anion; xanthous, solvent molecule.^ten

MXenes have unique optical backdrop and applications, such equally electrochromic devices, transparent conductors, electron transport layers, etc. Both, the thickness (north) and chemistry impact the optical absorption spectra, pregnant a different MXene can be used based on specific optical needs. For instance, Ti₃CNT₁₀ was shown to exhibit thickness-dependent nonlinear saturable absorption at high light fluences, making it useful for fashion-locking in fiber-based femtosecond lasers.¹³ Furthermore, a photodiode based on Ti₃C₂T_x was shown to intermission time-reversal symmetry, achieving nonreciprocal transmission of nanosecond light amplification by stimulated emission of radiation pulses.¹³ Recently, it was shown that MXenes are tunable electrochromic materials; depending on the bias used, the optical absorption elevation of Ti₃C₂T_ten can be reversibly and controllably shifted past more than 100 nm.¹⁴ Additionally, MXenes have been used for surface-enhanced Raman spectroscopy, with calculated enhancement factors reaching 106, leading to the possibility of enhanced biochemical molecular sensing.^fifteen Due to the plasmonic nature of MXene optical properties, it is probable that they will soon observe diverse optical applications spanning a broad wavelength range.

Recently, significant attention has been focused on the electronic and electromagnetic properties of MXenes due to their high conductivity and processability. For case, Ti_iiiC₂T_x was recently shown to have the highest electromagnetic interference shielding effectiveness of all constructed materials with comparable thickness, a 45 μm film corresponds to 92 decibels (dB), a 2.five μm motion-picture show gives >fifty dB, and a l nm motion picture gives 20 dB.¹⁶ It was also shown that MXenes tin exist used equally spray-on flexible antennas and radiofrequency identification (RFID) tags with only ~100 nm flick thickness required.¹⁷ Some double-M MXenes (Mo, West, Ti, Zr, and Hf-based) have been proposed every bit topological insulators owing to the ability to use light and heavy elements simultaneously to tune the construction.^xviii Furthermore, a number of MXenes (Cr-, Mn-, V-, and Ti-based) are expected to exhibit ferromagnetic or antiferromagnetic backdrop, depending on the specific chemical science and surface terminations.¹⁹ Due to the loftier conductivity and surface functionalization of MXenes, it is possible to use them every bit gas sensors with a very loftier bespeak-tonoise ratio, and very depression detection limits. For example, the limit of detection for Ti₃C_twoT_x was calculated to exist 0.011 and 0.thirteen ppb for acetone and ammonia, respectively, which are some of the lowest values ever reported.² It was institute that these MXene based gas sensors outperformed other 2d materials in terms of bespeak-to-noise ratios.²

The elastic properties of Ti₃C_iiT₁₀ were studied and the Young's modulus of a single Ti₃C₂T_x layer was found to be 0.33 TPa, which is the highest of whatever solution-processed 2D material (Effigy 4).²⁰ Due to these extreme mechanical properties and coupled with their desirable optical/electronic backdrop, MXenes have been widely utilized in composites. Recently, ceramic and metal matrix composites take been reported. In improver, because MXenes are h2o processable, polymeric composites are fabricated in polar solvents; research groups accept used a multifariousness of polymers, including polyvinyl alcohol (PVA) (363065, 563900, 341584), polyacrylamide (PAM) (92560 and 749222), polyethyleneimine (PEI) (904759, 181978), polyethylene glycol (PEG) (MM Cat. No. 8.18892), and other polymer systems.²¹ These composites accept been used for a variety of applications, including electromagnetic interference shielding, electrocatalysts, electrochemical energy storage systems, and many others. In addition to polymeric composites, heterostructures take been synthesized in conjunction with other second and 1D materials, including graphene (900561, 763705, 777676), carbon nanotubes (901046, 901082, 901056), transition metallic oxides (TMOs), transition metal dichalcogenides (TMDs), and others.²² In addition to composites fabricated with polymers or other 2D materials, MXenes have also been utilized in smart fabrics. The hydrophilicity of MXenes allows them to easily coat natural fibers, such equally cotton fiber and wool, synthetics, such as polyester, and to be combined with other materials for spinning into fibers.² These composites have been used equally knitted flexible supercapacitors for energy storage, as conductive fibers, and equally wearable smart textiles.^two Thus, by combining MXenes with other materials to course composites, the advantages of both materials can be realized. The large diversity within the MXene family unit, coupled with the diverse families of materials with which they can be composited, leads to a massive library of possible composite systems with the ability to choose specific properties of involvement.

Figure 4. Mechanical measurements of monolayer and bilayer Ti3C2Tx flakes. A) Noncontact AFM image of a monolayer Ti3C2Tx flake placed over an array of microwells in a Si/SiO2 substrate. B,C) Height profiles forth the B) dashed blue and C) red lines shown in A). D) Scheme of nanoindentation of a suspended Ti3C2Tx membrane with an AFM tip. E) Force–deflection curves of a bilayer (2L) Ti3C2Tx chip at different loads. The inset shows an AFM image of the fractured membrane. F) Comparing of loading curves for monolayer (1L) and bilayer (2L) Ti3C2Tx membranes. Hole diameter is 820 nm. G) Histogram of elastic stiffness for 1L and 2L membranes. Solid lines stand for Gaussian fits to the data. H) Comparison of experimental force–deflection curves for monolayer graphene and Ti3C2Tx membranes. I) Comparing of effective Young's moduli for several 2D materials: Go, rGO, MoS2, h-BN, and graphene. The nautical chart compares values produced on membranes of monolayer 2d materials in similar nanoindentation experiments.²⁰

Many research groups have begun investigating the ecology applications of MXenes, including gas separation and water purification. Due to the stacked layer morphology of MXene films, it is possible to achieve >2200 Barrer H₂ permeability with a H_ii/CO₂ selectivity >160.23 It was besides shown that cations (K⁺, Na⁺, Li⁺, Ni²⁺, Caⁱⁱ⁺, Mg^two+, and Al3+) can exist selectively filtered from water with loftier permeability by varying the interlayer spacing of MXene films.24 Mo_1.33CT_x was studied for brackish and seawater desalination, showing that ion removal in seawater concentrations (600 mM NaCl) could exist achieved with depression energy consumption (17 kT), which is in absence of whatever ion substitution membranes.²⁵ Information technology was likewise shown that Ti₃C_twoT_x is viable for the removal of various heavy metallic ions, including Cr, Atomic number 82, Cu, and others at relatively loftier rates.²⁶ While research into this direction is relatively recent, MXenes have demonstrated potential to play a major role in future environmental efforts.

MXenes take found promise in biomedical applications, including biosensors, antibacterial materials, in bioimaging, as therapeutics, and in theranostics (Figure five).²⁷ The majority of biomedical applications take focused on Ti-, Nb-, and Ta-based MXenes due to their non-toxic nature. No cytotoxicity has been observed for Ti_threeC₂T_x so far. A variety of biosensors accept been fabricated, focusing on detection of small molecules, including NH_iii, H₂O_ii, glucose, and even heavy metals. Ti₃C₂T₁₀ has also been institute to be feasible for type-I photodynamic therapylike cell killing. MXenes can likewise inhibit bacterial growth by imposing oxidative stress on the bacterial cell membranes. Due to the optical properties inherent in MXenes (i.due east., the specific absorption spectra depend on the MXene chemical science), they can be used for bioimaging (both for photoacoustic or luminescence imaging) or every bit contrasting agents for computed tomography. MXenes take also been used equally therapeutics for reactive oxygen species generation, every bit photothermal materials, and for drug cargo loading for synergistic therapy (theranostics). 2 other important medical applications of MXenes include sorbents for removal urea and other toxins form blood,²⁸ too as implantable and skin electrodes.^29-30 While the applications of MXenes in the biomedical field are nevertheless relatively new, the breadth of research already conducted illustrates the variability and hope of this family for future work.

Effigy v. Schematic illustration of the fabrication process of Ta4C3-IONP-SPs blended nanosheets and their unique functionality for dual-modal dissimilarity-enhanced MRI/CT imaging-guided photothermal ablation of chest cancer.³¹

Conclusions

The unique electronic, optical, chemical and mechanical properties of MXenes coupled with the ease of processing have helped to demonstrate their tremendous hope for revolutionizing many fields. While the initial focus of MXene research has been for free energy storage, their expanse of potential applications has since broadened to include fields ranging from smart textiles to medicine, communication, gas sensors and electrochromic devices, and in environmental remediation. Considering the growing number of discovered MXenes available, further control over the surface chemistry, ameliorate primal understanding of properties, and advances in MXene processing, it is probable that numerous novel applications of MXenes will likewise exist discovered.

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