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Latest progress in MXenes

Time:2020/07/14 丨 source:Fast Silver 丨 visit count:

Latest progress in MXenes

Since the discovery of graphene, two-dimensional (2D) materials have become an important research direction in materials science. In recent years, a new family of two-dimensional materials has emerged, including transition metal carbides, nitrides, and carbonitrides, also known as MXenes. This is prepared by selectively etching the sp element layer from the corresponding three-dimensional (3D) MAX phase. The MAX phase is a layered ternary metal carbide, nitride or carbonitride, the general formula is Mn+1AXn (n=1, 2, 3).
So far, more than 70 MAX phases have been reported, but the established MXenes family includes only Ti3C2, Ti2C, (Ti0.5, Nb0.5) 2C, (V0.5, Cr0.5) 3C2, Ti3CN, Ta4C3 , Nb2C, V2C and Nb4C3. In the future, more MXenes materials are expected to be stripped from the MAX family. [1]

Since the discovery of MXenes, many of its special properties have been discovered, and therefore have been used in many fields such as energy storage, environment, catalysis and biology. Here, the author combs the typical work of MXenes materials in different fields. The principles of literature selection focus on the latest reports or the progress of the leading research groups.

1. Energy storage

(1) Lithium sulfur/selenium battery

Because of its high electronic conductivity and high energy density, selenium (Se) has attracted widespread attention as a cathode material for lithium/sodium secondary batteries in recent years. However, due to the severe shuttle effect of polyselenide, its cycle stability is poor, which hinders its practical application. Here, Professor Wang Guoxiu and the Hao Liu team [2] of Sydney University of Technology used ultra-thin (≈270 nm, 0.09 mg cm-2 loading) cetyl ammonium bromide (CTAB)/carbon nanotube (CNT)/Ti3C2Tx MXene hybrid modified polypropylene (PP) (CCNT/MXene/PP) separator realizes high stability lithium/sodium-selenium battery. Theoretical calculations and XPS indicate that the modified membrane can fix polyselenide through strong Lewis acid-base interaction between CTAB/MXene and polyselenide. The addition of carbon nanotubes helps to improve the permeability of the electrolyte and promote the migration of ions. An in-situ penetration experiment was conducted to intuitively study the diffusion behavior of polyselenide, prevent the shuttle effect, and protect the lithium anode from corrosion. Therefore, the lithium-selenium battery using CCNT/MXene/PP separator can achieve 500 stable cycles at 1 C, and the capacity decay rate of each cycle is only 0.05%. In addition, the modified separator also performs well in sodium-selenium batteries.

Figure 1 Schematic diagram of the preparation process of CCNT/MXene/PP membrane.

(2) Lithium ion battery

The growing demand for advanced lithium-ion batteries has greatly stimulated the demand for electrodes with high surface capacity. The preparation of thick electrodes with high-performance active materials can greatly increase the surface capacity. However, above the critical thickness, solution-treated electrode films often encounter electrical/mechanical problems, thereby limiting the surface capacity and rate performance of the electrode. The cooperation between Zhang Chuanfang, Valeria Nicolosi, Jonathan N. Coleman and Professor Yury Gogotsi of Drexel University [3] showed that two-dimensional titanium carbide or carbonitride nanosheets, MXenes, can be used as silicon electrodes. Conductive adhesive, without any other additives, to produce electrodes through a simple and scalable slurry coating process. The nanosheets form a continuous conductive network, capable of rapid charge transfer, and provide a good mechanical skeleton for thick electrodes (up to 450 µm). Therefore, the capacity of the prepared electrode surface is as high as 23.3 mAh cm-2.

Figure 2 Schematic diagram of composite electrode preparation.

(3) Sodium/potassium ion battery

Because of its low cost and similar energy storage mechanism to lithium-ion batteries, potassium-ion batteries have received increasing attention. In response to the large size of K+ (1.38 Å), poor structural stability, and slow kinetics of the electrochemical redox reaction, the team of Chengxiang Wang and Longwei Yin of Shandong University [4] adopted the method of electrostatic attraction self-assembly and carefully designed a new type of PDDA- NPCN/Ti3C2 hybrid is used as the negative electrode of potassium ion battery. The PDDA-NPCN/Ti3C2 composite has a stacked structure and a large specific surface area, which can ensure the close contact between Ti3C2 and NPCNs, effectively use the two components, and obtain active sites more easily. The mixture provides greater layer spacing and a unique three-dimensional interconnected conductive network to accelerate the ion/electron transmission rate. At the same time, the mixture can guarantee good stability during charging/discharging. DFT calculations further show that the PDDA-NPCN/Ti3C2 hybrid effectively reduces the adsorption energy of K+ and accelerates the reaction kinetics. The mixture has a significant synergistic effect. At a current density of 0.1 A g-1, after 300 cycles, a reversible capacity of 358.4 mAh g-1 can be obtained. This work provides inspiration for the application of self-assembled mixtures in the field of energy storage

Figure 3 Schematic diagram of PDDA-NPCN/Ti3C2 mixture preparation process.

(4) Capacitor

The direct printing of functional inks is essential for applications in different fields such as electrochemical energy storage, smart electronics, and healthcare. However, the existing printable ink formulations are far from ideal. Generally, the use of surfactants/additives or lower ink concentrations increases the complexity of manufacturing and reduces print resolution. Ireland’s Trinity College Zhang Chuanfang, Valeria Nicolosi and Drexel University Yury Gogotsi [5] demonstrated two types of two-dimensional titanium carbide (Ti3C2Tx) MXene inks: water and organic. In the absence of any additives or binary solvents, it is used for extrusion printing and inkjet printing, respectively. The authors demonstrated full MXenes printed structures, such as micro-supercapacitors, conductive tracks and ohmic resistors on untreated plastic and paper substrates, with high print resolution and spatial uniformity. The volume capacitance and energy density of all MXenes printed micro-supercapacitors are an order of magnitude greater than existing inkjet/extrusion printed active materials. The universal direct ink printing technology highlights the prospect of additive-free MXenes inks and can be used to manufacture electronic components that are easy to integrate.

Figure 4 Schematic diagram of direct printing of Mxenes ink.

2. Catalytic applications

(1) HER reaction

Monoatomic catalysts achieve the purpose of economical and efficient catalysis with the least precious metals. However, during the experiment, preparing and maintaining the stability of single atoms is still a challenge. Academician Li Yadong of Tsinghua University, Professor Yury Gogotsi of Drexel University, and Professor Wang Guoxiu of Sydney University of Technology [6] reported that the electrochemical peeling method was used to synthesize a double transition metal MXene nanosheet-Mo2TiC2Tx with a large number of exposed surfaces and molybdenum vacancies. Through the interaction of protons with the functional groups on the surface of Mo2TiC2Tx, the formed Mo vacancies are used to fix a single Pt atom, improving the catalytic activity of MXenes to the hydrogen evolution reaction. The developed catalyst has a high catalytic capacity, the low overpotential at 10 and 100 mA·cm-2 is 30 and 77 mV, and the mass activity is about 40 times that of the commercial platinum-carbon catalyst. The strong covalent interaction between the positively charged Pt single atom and MXenes makes it have excellent catalytic performance and stability.

Figure 5 Synthesis mechanism of Mo2TiC2O2-PtSA during hydrogen production.

(2) ORR/OER reaction

The research group of Professor Qiao Shizhang [7] at the University of Adelaide in Australia prepared a self-supporting flexible film composed of two-dimensional graphite phase carbon nitride and titanium carbide MXene phase nanosheets. The composite film exhibits excellent activity and stability in catalyzing the oxygen evolution reaction in an alkaline water system, which stems from the porous structure with Ti–Nx sites as the catalytic core and highly hydrophilic surface. Its excellent electrocatalytic ability is comparable to the most advanced precious metal/transition metal catalysts, and better than most self-supporting membranes reported so far, so it can be directly used as a high-efficiency positive electrode for rechargeable zinc-air batteries. The research results of this paper show that the reasonable interaction between different two-dimensional materials can significantly promote oxygen electrochemistry, thereby promoting the development of the entire clean energy system.

Figure 6 Preparation of porous g-C3N4 and Ti3C2 composite films.

3. Other applications

(1) Superconductivity

The multifunctional chemical transformation of the functional groups on the surface of two-dimensional transition metal carbides (MXenes) has opened up new design space for such functional materials. The Dmitri V. Talapin team [8] from the University of Chicago and Argonne National Laboratory introduced a general strategy for the addition and removal of MXenes surface groups by performing substitution and elimination reactions in molten inorganic salts. The experiments successfully synthesized MXenes with O, NH, S, Cl, Se, Br and Te surface functional groups, as well as pure MXenes (without surface functional groups). These MXenes have unique structural and electronic properties. For example, surface groups control the distance between atoms in the MXenes lattice. Compared with the bulk TiC lattice, Tin+1Cn modified with Te2− ligand (n=1, 2 ) MXenes exhibit huge (>18%) in-plane lattice expansion. Nb2C MXenes have superconductivity determined by surface groups.

Figure 7 The surface reaction of MXenes in inorganic molten salt.

(2) MXenes fiber

Ti3C2Tx Mxenes is a new class of two-dimensional nanomaterials with excellent electrical conductivity and electrochemical performance, and has broad application prospects in the preparation of multifunctional macro materials and nanomaterials. Based on this, the Tae Hee Han team [9] of Hanyang University developed a simple, continuously controlled, additive-free/adhesive method to prepare pure MXenes fibers through a large-scale wet spinning approach. The resulting MXenes sheet (average lateral dimension is 5.11 μm2) has a high concentration in water and does not form agglomeration or phase separation. Introducing ammonium ions during the solidification process can successfully assemble the MXenes sheet into soft, meter-length fibers with extremely high electrical conductivity (7713 S cm−1). The prepared MXenes fiber has wide application potential in electrical equipment. The wet spinning strategy proposed by the author provides a way for the continuous mass production of MXenes fibers for high-performance, wearable electronic devices.

Figure 8 Schematic diagram of wet spinning MXenes fiber

(3) Electromagnetic shielding

The miniaturization of electronic products requires shielding of electromagnetic interference at the nanometer scale. Sang Ouk Kim of the Korean Academy of Science and Technology, Chong Min Koo of the Korean Academy of Science and Technology and Professor Yury Gogotsi of Drexel University [10] systematically reported the electromagnetic interference shielding behavior of the two-dimensional Ti3C2Tx MXene assembled film in different film thickness ranges . The author established a theoretical model to explain the shielding mechanism. Below the skin depth, multiple reflections become significant, as well as surface reflections and bulk absorption accompanied by electromagnetic radiation. The single-layer assembled film can provide ≈20% electromagnetic wave shielding, while the 24-layer film with a thickness of ≈55 nm shows 99% shielding (20 dB), showing a very large absolute shielding effectiveness (3.89×106 dB cm2 g−1) . This work exemplifies the excellent electromagnetic interference shielding performance of Ti3C2Tx MXene, and will help to achieve a lightweight, portable, and flexible next-generation electronic product protection mode change.

Figure 9 Comparison of EMI SET with different thickness of different materials.

(4) Sensor

The development of wearable electronic products, instant detection and soft robot technology requires that strain sensors have a high degree of sensitivity, scalability, adherence to any complex surface compatibility, and preferably self-repair. Conductive hydrogels have broad application prospects as sensing materials. However, their sensitivity is generally low, and due to their viscoelastic properties, there will be signal lag and fluctuations, which affects their sensing performance. The Husam N. Alshareef team [11] of King Abdullah University of Science and Technology proposed MXene (Ti3C2Tx) hydrogel composite material as a strain sensor, and its performance is superior to all reported hydrogels. The prepared composite hydrogel has excellent tensile strain sensitivity, and its strain coefficient (GF) is 25, which is 10 times that of the original hydrogel. In addition, the MXenes hydrogel has significant stretchability exceeding 3400%, instantaneous self-healing ability, excellent conformability, and adhesion to various surfaces including human skin. MXenes hydrogel composites exhibit higher sensitivity under compressive strain (GF 80) than under tensile strain. We use this asymmetric strain sensitivity combined with viscous deformation (self-recovery residual deformation) to add a new dimension to the hydrogel's sensing capabilities. Therefore, the direction and speed of the hydrogel surface movement can be easily detected. Based on this effect, MXenes hydrogels exhibit superior sensing performance in advanced sensing applications. Therefore, traditionally, the adverse effects brought about by the viscoelastic properties of hydrogels can be converted into the advantages of hydrogel sensors, which provides a broad prospect for the development of hydrogel sensors.

Figure 10 MXenes hydrogel characterization.

4. Summary

Over the past decade, two-dimensional transition metal carbides, nitrides and carbonitrides have attracted the attention of the scientific community for their superior mechanical strength and flexibility, physical/chemical properties and a variety of exciting functions. Despite the great success in the research of the stability, mechanical properties and various functions of MXenes, there are still some key issues that need to be solved, such as preparation costs, scalability of production methods and sample durability. Therefore, research in this area still has broad prospects. [12].

[1]     Lei J-C, Zhang X, Zhou Z. Recent advances in MXene: Preparation, properties, and applications. Frontiers of Physics, 2015, 10(3): 276-286.
[2]    Zhang F, Guo X, Xiong P, et al. Interface Engineering of MXene Composite Separator for High‐Performance Li–Se and Na–Se Batteries. Advanced Energy Materials, 2020, 10(20): 2000446.
[3]    Zhang C J, Park S H, Seral-Ascaso A, et al. High capacity silicon anodes enabled by MXene viscous aqueous ink. Nat Commun, 2019, 10(1): 849.
[4]    Zhao R, Di H, Hui X, et al. Self-assembled Ti3C2 MXene and N-rich porous carbon hybrids as superior anodes for high-performance potassium-ion batteries. Energy & Environmental Science, 2020, 13(1): 246-257.
[5]    Zhang C J, Mckeon L, Kremer M P, et al. Additive-free MXene inks and direct printing of micro-supercapacitors. Nat Commun, 2019, 10(1): 1795.
[6]    Zhang J, Zhao Y, Guo X, et al. Single platinum atoms immobilized on an MXene as an efficient catalyst for the hydrogen evolution reaction. Nature Catalysis, 2018, 1(12): 985-992.
[7]    Ma T Y, Cao J L, Jaroniec M, et al. Interacting Carbon Nitride and Titanium Carbide Nanosheets for High-Performance Oxygen Evolution. Angew Chem Int Ed Engl, 2016, 55(3): 1138-1142.
[8]    Kamysbayev V, Filatov A S, Hu H, et al. Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes. Science, 2020
[9]    Eom W, Shin H, Ambade R B, et al. Large-scale wet-spinning of highly electroconductive MXene fibers. Nat Commun, 2020, 11(1): 2825.
[10]    Yun T, Kim H, Iqbal A, et al. Electromagnetic Shielding of Monolayer MXene Assemblies. Adv Mater, 2020, 32(9): 1906769.
[11]    Zhang Y Z, Lee K H, Anjum D H, et al. MXenes stretch hydrogel sensor performance to new limits. Science Advances, 2018, 4(6): eaat0098.
[12]    Fu Z, Wang N, Legut D, et al. Rational Design of Flexible Two-Dimensional MXenes with Multiple Functionalities. Chem Rev, 2019, 119(23): 11980-12031.

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