1. MAX and MXene: Another interpretation of the periodic table

The crystal structure of MAX phase is formed by alternating stacking Mn+1Xn structural unit and single atomic plane of A element, which combines many advantages of metals and ceramics. After the A-layer atoms of MAX phase are etched, two-dimensional nanomaterials with Mn+1XnTx(Tx is the surface group) are obtained, namely MXene. At present, the mechanism of many properties of such materials is still lacking a new interpretation. Huang Qing, a researcher from Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, introduced the recent research trends in the composition and structure, synthesis methods, properties and applications of MAX phase and MXene materials at home and abroad, and prospected the development direction of these novel materials in the next few years.

2. MXene Flexible material: tuxedo or sportswear?

With the development of wearable flexible electronic technology, the demand for flexible force sensors with high sensitivity and wide sensing range is gradually increasing. MXene material has become a potential conductive sensitive material due to its advantages of good conductivity, high flexibility, good hydrophilicity and controllable synthesis. But can such materials really be made into sensing tuxedos or sportswear? Sun Jing, a researcher from Shanghai Institute of Silicate, Chinese Academy of Sciences, summarized the research progress of MXene-based flexible force sensor in terms of types, microstructure design of sensitive materials, sensing performance and sensing mechanism.

3. Traditional phase diagram: Is this point thermally balanced?

The A-site of the traditional MAX phase may be replaced by more transitional metal elements to form A new MAX phase, and the properties of the MAX phase of different A-site elements are very different, but how to reveal their thermodynamic stability is urgent. Chang Ke Ke, a researcher at the Ningbo Institute of Materials, Chinese Academy of Sciences, investigated the thermodynamic stability of the new MAX phase at different temperatures. The thermodynamic database of the study system was established by CALPHAD method, and the ternary phase diagram containing the new MAX phase was obtained by coupling the enthalpy data obtained from first principles. They provide a systematic research method for determining the thermodynamic stability of the new MAX phase, which can be applied to guide the synthesis of more unknown MAX phase materials.

4. MAB phase ultra-high temperature ceramics: Can really ensure the roasting through the atmosphere?

Cr4AlB4 is a recently discovered ternary layered boride MAB phase ceramic. The material can form a protective oxide film, which has great application potential in the field of high-temperature structural materials, but the mechanism of its phase stability and mechanical behavior remains to be proved. The phase stability and mechanical behavior of Cr4AlB4 were studied by Professor Bai Yuelei from Harbin Institute of Technology based on the "linear optimization method" and "bond stiffness" theoretical models of first principles. They found that no virtual frequency appeared in the phonon spectrum, indicating that Cr4AlB4 was inherently stable. Compared with other Cr-Al-B systems, Cr4AlB4 has the lowest energy, indicating that it is also thermodynamically stable. Cr4AlB4 has high damage tolerance and fracture toughness similar to the MAX phase.

5. Electromagnetic shielding film: Play hide-and-seek with radar detection?

High-strength electromagnetic shielding film materials have wide application prospects in flexible devices, automotive electronics and aerospace, but more breakthroughs are needed to realize that warplanes and warships can hide and seek with the enemy. Inspired by the micro-nano structure of Mother of Pearl and its excellent mechanical properties, Professor Zhang Haobin of Beijing University of Chemical Technology mixed cellulose nanocrystals (CNC) and MXene and assembled them layer by layer to prepare high-performance MXene-based composite films by simple solution blending and vacuum filtration methods. The mechanical properties of the film have been significantly improved, while retaining the high electrical conductivity (104 S/m) and excellent electromagnetic shielding properties of the composite film, which can reach more than 40 dB at a thickness of 8 μm.

6. Ge/MXene composite material: energy storage battery one step

Lithium-ion batteries currently dominate the energy supply market for electronic devices, and are rapidly penetrating into power grids, automobiles and other fields, but the development of large-capacity, high-rate, long-life, low-cost electrode materials is still on the way. The Ge/MXene composite material was prepared by Kang Shuai of Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, and the surface of MXene was uniformly loaded with Germanium metal nanoparticles, and the electrode was prepared and assembled into a button battery. The charging and discharging performance of the battery was tested, and the specific capacity, rate and cycle stability of the battery were systematically analyzed. They first combined Ge metal with MXene 2D material and used it in the research of energy storage battery, which has important reference value for the preparation of metal /MXene composite material and its application in lithium-ion battery.

7. Intercalation strategy: a new trick for MXene sodium storage regulation

Sodium-ion battery (SIB) is rapidly developing into a substitute for lithium-ion battery due to its abundant sodium resources and low cost. However, because the radius of sodium ion is larger than that of lithium ion (0.102 nm versus 0.076 nm), the electrode material collapses during the charge and discharge process, resulting in the lack of suitable anode materials for SIB. Professor Song Li of the University of Science and Technology of China proposed a Mn2+ intercalation strategy to optimize the sodium storage performance of V2C MXene. The intercalation of Mn2+ not only expands the layer spacing of V2C MXene, but also forms V-O-Mn covalent bonds, which is conducive to stabilizing the structure of V2C and inhibiting the structural collapse caused by volume change during the process of Na+ deintercalation, which has broad potential application prospects.

8. Lattice mixing: electrochemical activity increased

Direct methanol fuel cells have good application prospects due to their advantages such as convenient operation, high conversion efficiency, low operating temperature, less pollution, and easy storage and transportation of liquid fuel. However, existing anode catalysts have disadvantages such as low catalytic activity and poor resistance to CO toxicity, which restrict their commercial application prospects. A series of PtRu/(Ti3C2Tx)0.5-(MWCNTs)0.5 anode catalyst materials with different Pt and Ru ratios were prepared by three-step method by Professor Zhang Jianfeng from Hohai University. Ru atoms were mixed with Pt atoms to form a platinum ruthenium bimetallic alloy with a particle size of about 3.6 nm. Their catalyst has the best electrochemical performance, with an electrochemically active area of 139.5 m2/g and a forward peak current density of 36.4 mA/cm2.

9. Two-dimensional MXene enzyme sensor: Food safety detection

Hydrogen peroxide is widely used as antibacterial agent, oxidant, reducing agent and bleach in many fields such as pharmaceutical, medical, textile, paper and food processing, but it is easy to form residue and difficult to detect. Professor Zhang Lingzhi from Ningbo University synthesized two-dimensional MXene material with vertical fence structure and connected with horseradish peroxidase to construct a hydrogen peroxide electrochemical enzyme sensor. The nano-fence has large specific surface area, excellent electron conduction property and good dispersion property in water. Horseradish peroxidase molecules fixed on the electrode show excellent catalytic effect of hydrogen peroxide, and have been successfully applied to the detection of hydrogen peroxide residues in food.

10. A Position switching reaction: natural selection between atoms

At present, more than 70 kinds of MAX phase materials have been synthesized, but the A-position elements have been limited to the main group ⅢA and ⅣA elements, such as Al, Si, Ga, etc., while the MAX phase with subgroup elements occupying the A-position is rarely reported. Huang Qing, A researcher at the Ningbo Institute of Materials Research, Chinese Academy of Sciences, proposed an element replacement strategy, that is, on the basis of maintaining the hexagonal layered crystal structure of MAX phase, using Al and Zn to form eutectic products at high temperatures to realize the migration of Zn atoms into the A-layer, and the presence of molten salt medium promotes the reaction kinetics. This method can avoid the formation of competing phase in the traditional synthesis of MAX phase, so it can be used to explore more unknown MAX phase materials.