Exploring the Diverse World of MXene Materials: Applications and Innovation

Since MXene first appeared in scientific literature researchers have rapidly focused on this class of two-dimensional transition metal carbides/nitrides because of their exceptional structural properties like high conductivity and flexibility combined with large specific surface area and modifiable surface chemical characteristics. Ti₃C₂Tx possesses a conductivity level of 15,000 S/cm which greatly surpasses that of traditional materials. The preparation of MXene requires selective etching of the Al atomic layer from MAX phases like Ti₃AlC₂ to create diverse surface functional groups that enable broad performance regulation.

MXene Synthesis and Performance Optimization

MXene preparation requires removing the A atomic layer (like Al or Si) from the MAX phase through chemical etching. The conventional technique involves etching with hydrofluoric acid (HF) solutions to transform Ti₃AlC₂ into Ti₃C₂Tx. Because HF exhibits high levels of corrosiveness and toxicity its use in industry encounters significant restrictions. Researchers have developed multiple environmentally friendly synthesis methods to address these concerns.

Fluorine-free etching method: Researchers use ammonium bifluoride (NH₄HF₂) or molten salts like zinc chloride (ZnCl₂) instead of hydrofluoric acid (HF) to reduce toxicity while producing high-purity MXene materials.

Electrochemical anodic corrosion: Controlling both voltage and electrolyte composition allows for selective Al layer extraction under gentle conditions.

In-situ chemical conversion: Direct synthesis of functional group-free MXene is achieved through deoxidization and carbonization of the precursor which helps minimize the need for further processing steps.

Surface functionalization and heterostructure design present ways to enhance MXene performance.

Surface functionalization: Surface functional groups (-O, -F, etc.) directly affect its chemical activity and stability. Functionalization with -O groups increases MXene's catalytic effectiveness, but the presence of -F groups might prevent some reactions from occurring.

Control over the types and proportions of surface groups are achievable through precise manipulation of etching variables including pH level and etchant type.

Heterostructure design: When MXene is combined with graphene which has high conductivity together with MXene's catalytic activity results in a significant improvement of supercapacitors' energy density. The electrocatalytic nitrogen reduction reaction (NRR) becomes more efficient when MXene/MAX heterojunctions facilitate interfacial electron transfer which moves electrons from the MAX phase toward the MXene. Conductive polymers mixed with MXene enhance both film formation capabilities and flexibility which broadens MXene's use in flexible electronics.

MXene Composites: Structural Innovation and Application Expansion

As a new two-dimensional material MXene exhibits significant potential within composite materials because it combines high electrical conductivity with modifiable surface functional groups and outstanding mechanical strength.

Carbon fiber composites

The mechanical and functional properties of carbon fiber (CF) composites depend primarily on their interfacial characteristics. Research reveals that MXene integration on CF surface leads to an increase in active groups like -OH and -O. The introduction of MXene to the CF surface results in increased active groups which leads to better interfacial bonding and enhanced mechanical strength and fracture toughness for the composite material. Researchers at Beijing University of Chemical Technology decreased interfacial residual stress and enhanced composite material mechanical properties through the creation of a graphene transition layer at the CF interface. MXene's conductivity works in conjunction with CF to boost electromagnetic shielding performance. Shielding effectiveness for MXene/CF composites achieves values above 35 dB particularly for low-frequency (<1 GHz) scenarios because MXene modification of CF increases magnetic permeability to optimize the shielding performance.

MXene gives CF composites multifunctionality. MXene/CF electrode materials demonstrate superior surface capacitance exceeding 200 mF/cm² and energy density above 10 μWh/cm² in supercapacitors because of MXene's high conductivity and CF's stable structural framework. MXene and CF composites serve as effective materials for absorbing stealth technologies. The microwave absorption efficiency reaches over 90% with interface polarization control methods like zinc oxide nanowires and mechanical strength remains intact.

Multifunctional composite materials

The combination of MXene with graphene and carbon nanotubes (CNT) resolves the self-stacking challenge faced by MXene. MXene/CNT composite films employ carbon nanotubes as spacers to expand MXene interlayer spaces which results in enhanced ion transmission efficiency and a supercapacitor specific capacity increase of over 30%. Graphene together with MXene creates an effective electric and thermal network which improves the composite material's thermal management capabilities. The composite material demonstrates enhanced mechanical properties with a 20.3% increase in tensile strength and a 17.2% improvement in elongation at break when gelatin-treated CNT is combined with MXene.

MXene combined with conductive polymers such as polyaniline and polypyrrole achieves both flexible properties and superior capacitance performance. The composite material composed of MXene and polyaniline creates an intercalation structure by means of hydrogen bonding together with charge interaction which prevents MXene from stacking. The material achieves a surface capacitance of 450 mF/cm² with more than 95% capacitance retention after 500 bends. The application of polypyrrole through in-situ assembly on MXene surfaces improves electromagnetic shielding effectiveness beyond 50 dB without sacrificing the material's lightweight and flexible properties.

Revolutionary Application of MXene in Supercapacitors

Electrode material innovation

MXene is an ideal choice for supercapacitor electrode materials due to its high conductivity (about 8000 S/cm) and rich surface functional groups (such as -OH, -O, etc.). However, the easy stacking of its nanosheets limits the exposure of active sites. In recent years, the performance has been significantly improved through composite structure design:

High volume capacitance: The composite electrode of MXene and graphene retains the high conductivity of MXene through a synergistic effect, while using the porous structure of graphene to inhibit stacking. For example, the MXene/graphene composite electrode achieves a surface capacitance of 1347 mF/cm² at 1 mA/cm², a volume capacitance of up to 1040 F/cm³, and a capacity retention rate of more than 80% after 5000 cycles. In addition, through surface modification and structural optimization, a flexible self-supporting MXene electrode with a volume capacitance of 770 F/cm³ was developed, which has excellent mechanical flexibility and cycle stability.

Flexible electrode: The mechanical flexibility of MXene makes it shine in wearable devices. For example, supercapacitors based on MXene/carbon nanotube composite fibers can withstand repeated bending and stretching, with an energy density of 10.6 Wh/kg and a power density of up to 12,780 W/kg, suitable for devices such as smart watches. By enhancing the flexibility of MXene films through intercalation molecules (such as polyethyleneimine) or cellulose, the prepared symmetric supercapacitors still maintain a capacitance of 93.9 mF/cm² when bent at 90°.

Breakthrough in voltage window and energy density

The energy density of MXene supercapacitors is limited by a narrow voltage window (usually<1 V) and low specific capacitance, but important progress has been made through electrolyte optimization and hybrid design:

Polarization failure mechanism and electrolyte optimization: MXene is prone to hydrogen evolution reaction in aqueous electrolytes, resulting in a limited voltage window. The use of organic electrolytes (such as EMIMBF₄) or ionic liquids can extend the voltage window to more than 3 V, while suppressing side reactions through surface end group regulation (such as the introduction of -F groups).

Hybrid supercapacitor design: Combining the fast double-layer capacitance of MXene with the pseudocapacitive properties of metal oxides (such as RuO₂, MnO₂) can break through the performance bottleneck of a single material. For example, the hybrid supercapacitor of MXene/V₂O₅ hybrid film has an energy density of 32.6 Wh/L, and a capacity retention rate of 70.4% after 5,000 cycles. Through mechanically induced nanostructure design, CoO@polypyrrole is compounded with MXene to achieve a volume energy density of 27.2 Wh/L, which is one of the highest levels currently.

Microsupercapacitor technology

MXene's film-forming and patternable properties have promoted the development of miniaturized energy storage devices:

Chip-level energy storage: All-MXene solid-state microsupercapacitors (MSCs) are only micrometers thick and can be directly integrated into chips. The MXene MSC prepared by laser etching or inkjet printing technology has an area capacitance of 38.5 mWh/cm², and its performance is stable when connected in series and parallel, making it suitable for IoT sensors.

Transparency and self-healing function: The transparency of MXene (transmittance>80%) makes it suitable for transparent electronic devices. For example, the transparent MSC composited with MXene/silver nanowires has a transmittance of 75% in the visible light region and a capacitance retention rate of more than 90%. In addition, MXene/graphene aerogel has self-healing properties. After damage, it can restore 95% of the initial capacitance through hydrogen bond recombination, which is suitable for wearable devices in extreme environments.

Environmental protection and sustainability: Innovative preparation methods further reduce costs. For example, using waste denim as a substrate, MXene is loaded through needle punching and carbonization processes, and the prepared supercapacitor has a specific capacitance of 577.5 mF/cm², and the capacity retention rate is 94% after 15,000 cycles.