1. What Are MXenes and Where Does Nb₂CTx Fit In?

MXenes are a large and growing family of 2D materials made from transition metal carbides, nitrides, or carbonitrides. Their general formula is:

Mₙ₊₁XₙTₓ

• M = early transition metal (e.g., Ti, Nb, V)

• X = carbon and/or nitrogen

• Tₓ = surface terminations (–O, –OH, –F, –Cl, etc.)

• n = 1–3 (defines the number of layers in the metal–carbon/nitrogen framework)

MXenes are usually produced from a parent 3D layered material called a MAX phase, with formula:

Mₙ₊₁AXₙ

Here, A is typically an element like aluminum (Al) from groups 13–14 of the periodic table. During synthesis, the A layer is selectively removed (etched), leaving behind a layered M–X material: the MXene.

Why Nb₂CTx MXene is attracting so much attention

Within the MXene family, Nb₂CTx stands out for several reasons:

• High electrical conductivity

• 2D layered structure with accessible interlayers

• Tunable surface chemistry via different functional groups

• Excellent mechanical properties (strong yet flexible)

• Good electrochemical performance in energy storage

• Better oxidative stability than many Ti-based MXenes

Because of this combination, Nb₂CTx is being explored for:

• Supercapacitors and batteries

• Gas and biosensors

• Catalysis (e.g., hydrogen evolution, photocatalysis)

• Electromagnetic interference (EMI) shielding and microwave absorption

• Biomedical uses such as cancer therapy, bone regeneration, and antioxidant protection

The reviewed article focuses on recent developments specifically around Nb₂CTx-based MXenes, especially for energy storage, but also touches many other applications.

2. How Nb₂CTx MXene Is Synthesized

The way Nb₂CTx is made strongly affects its structure, stability, surface groups, and ultimately its performance. Two main “philosophies” of synthesis are discussed:

• Top-down methods – start from a layered MAX phase and etch away the A-layers

• Bottom-up methods – build the structure atom-by-atom or molecule-by-molecule (e.g., CVD, ALD – but for Nb₂CTx these are not yet practically used)

So far, Nb₂CTx is essentially prepared only by top-down methods, with several variations.

2.1. HF Etching (Traditional Wet Chemical Route)

The most common route uses hydrofluoric acid (HF) to selectively remove the A-layer (Al) from the MAX phase Nb₂AlC.

General idea:

1. Start with Nb₂AlC powder (the MAX phase).

2. Mix it with an HF-containing solution.

3. HF attacks and removes Al from the structure, while niobium and carbon form the layered Nb₂C framework.

4. Surface terminations (–F, –OH, –O) form naturally from the etching environment.

5. After washing and sometimes delamination (e.g., with tetramethylammonium hydroxide, TMAOH), you obtain few-layer or single-layer Nb₂CTx.

Many studies have used concentrated HF (around 40–50 wt%) at elevated temperatures (often ~55–60 °C) for several hours. The resulting product can be:

• Multilayer “accordion-like” stacks

• Exfoliated nanosheets (after intercalation + delamination steps)

Advantages:

• Relatively simple and well-established

• Scalable in principle

Disadvantages:

• HF is extremely corrosive and toxic

• Safety and environmental concerns limit industrial adoption

Because of HF hazards, researchers are actively exploring safer etching systems.

2.2. Mild or Alternative Etching Systems

To reduce HF exposure, some methods use in situ generated HF from a mixture of HCl + fluoride salts (e.g., LiF). This is well known for Ti₃C₂, and similar approaches are being adapted for niobium MXenes.

There are also more complex systems where Nb₂CTx is combined with other structures during or after etching, for example:

• Nb₂CTx with carbon nanotubes (CNTs)

• Nb₂CTx decorated with metals (Ni, Co, Pt)

• Nb₂CTx combined with semiconductors (MoS₂, CdS, ZnO, etc.)

These are usually produced via hydrothermal, solvothermal, self-assembly, or pyrolysis steps after the basic MXene is made.

2.3. Molten Salt / Lewis Acid Etching

Another interesting approach uses a molten salt mixture instead of aqueous HF.

• A mixture like CuCl₂/NaCl/KCl is heated to form a molten salt bath.

• The salt acts as a reaction medium and etchant (for example, CuCl₂ serves as a Lewis acid that helps remove the Al).

• The molten salt provides high ion mobility, good mixing, and can lower necessary reaction temperatures.

This “HF-free” method is promising because:

• It can offer different surface terminations

• It may improve stability and electrochemical behavior

• It avoids direct handling of concentrated HF solutions

However, it is still mainly at the research stage.

2.4. Summary of Synthesis Challenges

Right now, all practical Nb₂CTx syntheses are top-down, and most still involve HF directly or indirectly. For commercial use, open problems include:

• Making the process safer and greener (HF replacement or minimization)

• Reducing cost and improving yields

• Controlling surface terminations more precisely

• Avoiding aggregation and maintaining stable dispersions

3. Key Properties of Nb₂CTx MXene

The article then discusses the main physical, chemical, and electrochemical properties that make Nb₂CTx so attractive.

3.1. Mechanical Properties

Nb-based MXenes (like Nb₄C₃Tx and Nb₂CTx) have been tested using AFM nanoindentation. For example:

• Monolayer Nb₄C₃Tx shows a high Young’s modulus (hundreds of GPa) and strong breaking strength.

• Compared to other 2D materials such as graphene oxide or Ti₃C₂Tx, Nb₄C₃Tx can have even higher stiffness.

Although the highlighted data is for Nb₄C₃Tx, it suggests that niobium MXenes generally offer:

• High mechanical strength

• Good flexibility in thin-film form

These properties are valuable for:

• Flexible electrodes

• Structural composites

• Protective coatings

• Membranes and textiles

3.2. Magnetic and Superconducting Behavior

Many 2D materials are non-magnetic, but MXenes can show interesting magnetic and even superconducting behavior, depending on:

• Transition metal type

• Surface terminations

• Strain and defects

For niobium MXenes, theory and experiments indicate:

• Unusual Meissner effect (sign of superconductivity) in Nb₂CTx

• Superconducting transition temperature around 12.5 K in some samples

• Negative magnetic moments predicted by density functional theory (DFT), supporting diamagnetic or superconducting behavior

These properties are still mainly of fundamental interest, but they hint at future uses in spintronics or quantum devices.

3.3. Electrical Conductivity

One of the strongest advantages of MXenes is their metal-like conductivity.

• MXenes can reach conductivities around 10⁴ S/cm, much higher than many other solution-processed 2D materials like reduced graphene oxide.

• Niobium MXenes such as Nb₄C₃Tx can be much more conductive than Nb₂CTx, due to their thicker metal-carbon frameworks.

Conductivity can be tuned by:

• Heat treatment

• Surface chemistry

• Number of layers

• Degree of oxidation

High conductivity is crucial for:

• Fast electron transport in supercapacitors and batteries

• EMI shielding and microwave absorption

• Joule heating and wearable electronics

3.4. Oxidative Stability

A major weakness of many MXenes (especially Ti-based ones) is oxidation in water and air. They slowly convert to oxides, which changes both their structure and properties.

For niobium MXenes:

• Nb₂CTx and Nb₄C₃Tx both oxidize over time, but Nb₄C₃Tx is more stable.

• Inner layers in “thicker” MXenes (higher n) are better protected from oxygen.

• Oxidizable fraction can be reduced by:

  • Storing at low temperatures

  • Adding antioxidants like ascorbic acid

Improving oxidative stability is essential for long-term device performance, especially in aqueous electrolytes or biomedical environments.

3.5. Electrochemical Behavior

Nb₂CTx is particularly interesting for electrochemical energy storage, such as:

• Supercapacitors

• Batteries (Li-ion, Na-ion, K-ion, Al-ion, etc.)

Its layered structure allows:

• Ion intercalation between sheets

• Fast redox reactions at the surface

• Good reversibility

In aqueous electrolytes, Nb₂CTx can show:

• High volumetric capacitance

• High operating voltage windows (depending on the system)

• Fast charge–discharge behavior

Because it combines electric double layer behavior with redox activity, Nb₂CTx often shows pseudocapacitive characteristics – meaning it stores more charge than a purely capacitive carbon surface would.

3.6. Hydrophilicity and Interaction with Water

Unlike many hydrophobic carbon materials, MXenes are usually strongly hydrophilic.

• Surface –O, –OH, and –F groups attract water molecules.

• This helps ion transport in aqueous systems and facilitates dispersion in water.

The article states that Nb₂CTx is very hydrophilic and interacts strongly with water, which:

• Benefits energy storage (good ion accessibility)

• Makes Nb₂CTx attractive for water treatment, since it can adsorb ions and pollutants

(Some wording in the source text confuses hydrophilic/hydrophobic terms, but the key idea is that Nb₂CTx interacts well with water.)

3.7. Thermal Properties

Nb₂CTx MXene has:

• High thermal stability, surviving high temperatures in inert atmospheres

• Good thermal conductivity and diffusivity, much better than many oxide ceramics like alumina

Thin, single-layer Nb₂CTx tends to respond more sensitively to temperature changes than thicker films. Doping or combining Nb₂CTx with other elements can tune its thermal behavior.

These properties are useful for:

• Heat spreaders

• Thermal management in electronics

• Photothermal applications (heating under light)

3.8. Optical Properties

Optical properties are important for:

• Photocatalysis

• Photothermal therapy

• Optoelectronics

Nb₂CTx absorbs strongly across broad wavelength ranges, especially when combined with other semiconductors or metals. Surface terminations and doping can:

• Change its absorption spectrum

• Improve light harvesting in the UV, visible, or near-infrared range

This makes Nb₂CTx a good partner for photocatalysts or photothermal agents.

4. Applications of Nb₂CTx MXene

Now we come to the more applied side of the review. Nb₂CTx and its composites have been tested in a wide variety of fields.

4.1. Gas Sensing

Nb₂CTx has:

• High surface area

• Plenty of surface functional groups

• High conductivity

This combination is ideal for chemoresistive gas sensors (sensors that work by measuring changes in electrical resistance in the presence of a gas).

An example highlighted in the article:

• Nb₂CTx was functionalized with (3-aminopropyl)triethoxysilane (APTES).

• This created an Nb₂CTx–APTES sensor with significantly improved sensitivity and selectivity toward NO₂, a toxic gas.

The sensing mechanism involves:

• Adsorption of NO₂ on amine and MXene surface sites

• Charge transfer that changes the depletion region and resistance of the material

Because these sensors operate at relatively low temperatures and can detect low concentrations, Nb₂CTx is a strong candidate for environmental monitoring and air quality sensing.

4.2. Supercapacitors

MXenes are already famous for their supercapacitor performance, especially Ti₃C₂Tx. Nb₂CTx is now joining that group.

Some reported results:

• Pristine Nb₂CTx coated on Ni foam showed specific capacitance in the hundreds of F/g range.

• Ni-decorated Nb₂CTx showed even higher capacitance (well above 600 F/g at low scan rates) and good cycling stability (around 81% retention after 10,000 cycles).

In other work:

• Nb₂CTx combined with multi-walled carbon nanotubes (MWCNTs) increased conductivity and improved performance.

• Asymmetric supercapacitors using Nb₂CTx/CNT as the negative electrode and activated carbon as the positive electrode achieved high energy and power densities.

The takeaway:

Nb₂CTx-based MXenes can deliver high capacitance, fast charging, and good durability, making them highly promising for next-generation supercapacitors.

4.3. Batteries

Nb₂CTx has been explored as an electrode in several types of batteries:

4.3.1. Aluminum-Ion Batteries

Aluminum-ion batteries are attractive because Al is:

• Cheap

• Abundant

• Multivalent (can store more charge per ion)

Using Nb₂CTx as the cathode, paired with an aluminum anode and an AlCl₃-based ionic liquid electrolyte:

• Calcined Nb₂CTx achieved specific capacities of ~80–108 mAh/g at different current densities.

• Cycling stability improved when the MXene was carefully heat-treated, which influenced surface terminations and structure.

4.3.2. Sodium-Ion Batteries

Sodium-ion batteries are seen as a more sustainable alternative to Li-ion for grid or large-scale storage.

A 3D hybrid Nb₂CTx@MoS₂@C was made where:

• Nb₂CTx provides conductive backbone and ion pathways.

• MoS₂ stores sodium.

• Carbon improves conductivity and stability.

This composite showed:

• High reversible capacity over many cycles

• Good rate capability (still significant capacity at very high current densities)

4.3.3. Lithium–Sulfur Batteries and Others

Nb₂C (related niobium MXene) has been combined with MoS₂ as a cathode host for Li–S batteries:

• Enhanced sulfur utilization

• Reduced shuttle effect of polysulfides

• High capacity retention after many cycles

Similar niobium MXenes have also been applied in:

• Lithium-ion batteries

• Potassium-ion batteries

• Hybrid Li-ion capacitors

The general advantage comes from MXenes’ layered structure, conductivity, and surface chemistry, which boost ion transport and electron transfer.

4.4. Catalysis and Photocatalysis

Nb₂CTx and Nb₄C₃Tx have been used as:

• Electrocatalysts for the hydrogen evolution reaction (HER)

• Photocatalyst supports for water splitting and CO₂ reduction

Examples include:

• Hybrids like Nb₂O₅/C/Nb₂C, where Nb₂O₅ nanoparticles decorate Nb₂C. These hybrids show higher H₂ production rates than pure Nb₂O₅ because Nb₂C helps separate charges and conduct electrons.

• Nb₂CTx supporting metal nanoparticles (e.g., Ni) for photothermal CO₂ conversion, where the MXene absorbs light, heats locally, and accelerates catalytic reactions.

• Composites such as Bi₂WO₆/Nb₂CTx that degrade organic pollutants (dyes, antibiotics) more efficiently than the oxide alone.

In short, Nb₂CTx acts as:

• A conductive support

• A light absorber

• Occasionally, an active catalytic site

This makes it versatile in both electrocatalysis and photocatalysis.

4.5. Biomedical Applications

One of the most fascinating sections of the article deals with biomedical uses of Nb₂CTx.

Key reasons Nb₂CTx is interesting in biomedicine:

• Strong photothermal response in the near-infrared (NIR-I and NIR-II) windows

• Ability to scavenge reactive oxygen species (ROS)

• Biodegradability and tunable surface chemistry

• High X-ray attenuation and compatibility with imaging/therapy combinations

Some reported uses:

4.5.1. Photothermal Cancer Therapy and Bone Regeneration

• Nb₂CTx nanosheets have been used to kill cancer cells via photothermal therapy, where NIR light is converted into heat that destroys tumor tissue.

• By embedding Nb₂CTx into 3D-printed bone scaffolds, researchers created implants that can:

  • Ablate osteosarcoma (bone cancer)

  • Support bone regeneration

  • Promote blood vessel growth and tissue repair

4.5.2. ROS Scavenging and Anti-Inflammatory Effects

Nb₂CTx can:

• Absorb excessive ROS (like H₂O₂, ·OH, O₂⁻·)

• Reduce inflammatory cytokine production

• Inhibit osteoclast-driven bone loss in models of osteolytic disease

This suggests potential roles in:

• Protecting bone around implants

• Treating diseases linked to oxidative stress

4.5.3. Radioprotection

Nb₂CTx coated with polyvinylpyrrolidone (PVP) has been shown to:

• Scavenge radiation-induced ROS

• Protect tissues and blood systems in mice from radiation damage

• Be gradually cleared from the body without obvious toxicity

4.5.4. Smart Implants and Infection Control

Nb₂CTx has also been applied as a coating on titanium implants to:

• Kill bacteria via photothermal heating

• Reduce biofilm formation

• Support tissue healing by reducing inflammation and ROS levels

These multifunctional implants could be useful in orthopedic or dental applications where infection and poor healing are major problems.

4.6. EMI Shielding and Microwave Absorption

Because Nb₂CTx is:

• Highly conductive

• Layered with internal interfaces

• Able to form hybrids with magnetic materials

it works very well as an electromagnetic interference (EMI) shielding material and a microwave absorber.

Examples:

• Nb₂CTx mixed with wax can show shielding effectiveness of tens of dB at GHz frequencies.

• Composites like rGO/Nb₂CTx/Fe₃O₄ show very strong microwave absorption (reflection loss < –59 dB), thanks to:

  • Multiple loss mechanisms (dielectric, magnetic, conduction loss)

  • Good impedance matching

Such materials are attractive for:

• Protecting electronics from interference

• Stealth and radar absorption

• Wireless communication systems

5. Commercial Prospects and Remaining Challenges

The review ends by discussing where Nb₂CTx stands in terms of real-world use.

Promising Aspects

Nb₂CTx-based materials show strong potential in:

• Energy storage (batteries, supercapacitors)

• Catalysis and photocatalysis

• Sensors (gas, strain, biosensors)

• Biomedical therapies and implants

• EMI shielding and microwave absorption

Their combination of:

• Redox activity

• High conductivity

• Layered morphology

• Tunable surface groups

makes them uniquely flexible.

Main Obstacles

However, several key issues must be addressed:

1. Synthesis safety and scalability

   • Heavy reliance on HF (dangerous, corrosive).

   • Need greener, less toxic etchants and processes.

2. Cost and precursors

   • Nb₂AlC and related MAX phases can be expensive.

   • Large-scale synthesis must reduce material and processing costs.

3. Stability and aggregation

   • Nb₂CTx layers can oxidize and aggregate, reducing surface area and performance.

   • Better packaging, antioxidants, and surface engineering are needed to ensure long lifetimes.

4. Control of surface terminations

   • Current methods produce mixed –O, –OH, –F, etc., in a non-uniform way.

   • Many properties depend strongly on which functional groups are present and where.

   • More precise control would allow “designer” MXenes for specific applications.

5. Deeper understanding of mechanisms

   • Energy storage, catalysis, and biological interactions are complex and depend on nanoscale details.

   • More combined experimental and theoretical work is needed to link structure → property → performance.

6. Expanding the range of applications

   • Many potential uses (e.g., flexible electronics, tribology, nanofiltration, nuclear waste management) are still barely tested.

   • Systematic studies are required to move from lab-scale demonstrations to robust devices.

6. Final Thoughts

Nb₂CTx MXene is a next-generation 2D material with an impressive list of abilities:

• It can store energy quickly and efficiently.

• It can catalyze chemical reactions and support photocatalysts.

• It can sense gases and mechanical changes.

• It can absorb microwaves and shield electronics from interference.

• It can help kill cancer cells, protect tissues from damage, and support bone healing.

At the same time, it faces real-world challenges related to synthesis safety, stability, cost, and precise control of structure and chemistry.

The reviewed article paints a clear picture: Nb₂CTx isn’t just an interesting laboratory curiosity anymore. It is moving toward practical roles in energy, environment, health, and advanced electronics. With continued progress in safer synthesis and deeper understanding of its behavior, Nb₂CTx-based MXenes may soon become key materials in many technologies that shape our daily lives.

If you’re working on energy storage, sensors, biomedical materials, or advanced coatings, Nb₂CTx is definitely a material worth keeping an eye on.