MXenes: Properties, Applications, and Potential in 3D Printing

Abstract

MXenes, a class of two-dimensional materials with appealing properties such as electrical conductivity, mechanical strength, and chemical stability, is rapidly gaining attention for potential applications in various fields, including energy storage, water treatment, biomedicine, and electromagnetic shielding. One of the most exciting developments is their integration with 3D printing technologies, which allows for precise control over material structure and composition. This combination has significantly expanded the scope of MXenes, particularly in electrochemical storage systems like supercapacitors and batteries, where 3D-printed MXene-based materials have demonstrated superior performance. This review article provides a detailed analysis of the synthesis, properties, and applications of MXenes, with a particular focus on their role in additive manufacturing. While the synergy between MXenes and 3D printing offers numerous advantages, challenges such as large-scale production, material stability, and refining processing techniques remain significant hurdles; all these issues are discussed in the present work. Future research directions are also highlighted that aim to enhance scalability, reduce costs, and explore new composite formulations to optimize the performance of MXenes across various applications.

1. Introduction

Two-dimensional (2D) materials have garnered significant attention due to their unique properties and outstanding potential for various scientific and technological applications. Unlike traditional bulk materials, which have a three-dimensional atomic arrangement, 2D materials are characterized by their atomic or molecular thickness, typically less than 5 nm. This ultra-thin structure results in exceptional mechanical, electronic, optical, and thermal properties, which are related to their reduced dimensionality and increased surface-to-volume ratio. These materials have demonstrated promising applications in energy storage and conversion, catalysis, sensing, biomedicine, and flexible electronics [1].

MXenes are typically produced by selectively etching the A layers from MAX phases (M_n+1AX_n), a group of layered ternary carbides and nitrides, through chemical processes that result in 2D structures with tunable surface functionalities (specific details will be provided in Section 2) [2].

Within the family of 2D materials, MXenes have emerged as a particularly interesting class of transition metal carbides and nitrides. MXenes show high electrical conductivity, excellent mechanical strength, good chemical stability, remarkable hydrophilicity, and tunable surface chemistry, making them promising candidates for a wide range of applications, including energy storage (e.g., supercapacitors and lithium-ion batteries), water treatment, electromagnetic shielding, and biomedicine (e.g., biosensing and cancer therapy due to their exceptional biocompatibility) [1,3,4].

In recent years, the interest in advanced manufacturing techniques has increased, and the integration of MXenes into 3D printing has emerged as a revolutionary approach for fabricating intricate architectures with improved functionalities. Three-dimensional (3D) printing, or additive manufacturing, enables precise structural control and customization, which, when combined with the exceptional properties of MXenes, paves the way for innovative applications. Recent studies have demonstrated that 3D-printed MXene-based materials exhibit superior performance in various fields and their adaptability in forming stable suspensions makes them highly versatile for a lot of printing technologies, expanding their practical implementation [1,4].

This review article explores the synthesis, properties, and diverse applications of MXenes, emphasizing their role in 3D printing technologies. By leveraging the synergy between MXenes and additive manufacturing, researchers can develop next-generation materials for energy storage, electronics, biomedicine, and more. The continuous advancement of scalable synthesis and printing methodologies will improve MXene accessibility, driving their industrial adoption and technological impact in the years to come.

2. Synthesis of MXenes: Methods and Approaches

Two-dimensional materials are distinguished by their high surface areas and their single- or few-layer nanosheet structures at the atomic scale. They are characterized by a layered configuration held together by weak van der Waals interactions, enabling their production through mechanical or liquid exfoliation from bulk 3D precursors, or via nano-chemical vapor deposition (nano-CVD) [5].

Since the discovery of graphene in 2004, researchers have shown significant interest on 2D materials due to their potentials in many fields, including electronics, biomedical, optoelectronics, and environmental remediation. This class of 2D materials took a groundbreaking turn with the discovery of MXene in 2011 [6].

MXene refers to transition metal carbides (TMC) and transition metal nitrides (TMN). The most common method used to synthesize this class of materials is the etching method from MAX phases, which serve as the 3D bulk precursor of MXenes (Figure 1). MAX phases belong to a group of ternary ceramics, specifically carbides or nitrides [7,8,9], where M represents the transition element, A is the main group element, and X denotes carbon and/or nitrogen [10]. These phases are divided into four categories (see also Figure 2 [11]), including more than 150 combinations [12] depending on the n value, and utilize 14 M and 16 A elements: M₂AX (n = 1), M₃AX₂ (n = 2), M₄AX₃ (n = 3), and M₅AX₄ (n = 4). To date, more than 50 different MXenes have been successfully synthesized [9]. Due to the weak van der Waals forces between the A and MX layers, the layer A can be easily etched to obtain the MX material. The result is a material with structural and functional properties (e.g., physical and electrochemical characteristics) similar to graphene. MXenes can be generally defined by the formula M_{n + 1}X_nT_x (n = 1–4), consisting of a transition metal layer (M) (e.g., Ti, Mo, Cr), carbon–nitrogen layer (X) (e.g., C), and a surface termination group (T) (e.g., -F, -OH, -O). The complete list of elements indicated in parentheses (M, X, and T) is provided in Figure 2C. These materials are typically categorized into three main families: M₂XT_x, M_3×2T_x, and M₄X₃T_x. Each family contains multiple materials: for example, the M₂XT_x type includes Ti₂CT_x and Mo₂CT_x; M₃X₂T_x includes Ti₃C₂T_x and Cr₃C₂T_x; M₄X₃T_x contains Nb₄C₃T_x and Ti₄N₃T_x [10].

2.1. Structure of MXenes

MXenes belong to the hexagonal crystal system and the P6₃/mmc space group, the same as MAX phases. In MAX phases, the transition metal (M) and carbon–nitrogen (X) atoms are covalently bonded to form the M₆X octahedra, with the X atoms positioned at the center [10]. The M atoms are hexagonally close-packed (HCP), while the A atoms occupy the interstitial spaces [12]. These A atoms have weaker bonding energy than M and X atoms, making them easier to remove chemically through selective etching. During this process, the covalent M-X bonds remain intact, but new surface terminations (-OH, -O, -F) are introduced, compressing the structure and resulting in a 2D HCP structure layered configuration [13]. Conventional studies often overlook surface defects and assume uniform distribution of T terminations [10]. M_n+1X_nT_x has three types of structure as shown in Figure 3 [14].

On a microscopic scale, MXenes resemble a multi-layered accordion-like structure, where strong van der Waals forces keep the layers interconnected. The abundance of surface terminations gives MXenes exceptional hydrophilicity, comparable to that of graphene oxide [15], as well as electrical conductivity and notable storage performance that make them unique among 2D materials [10].

The tunability of MXene surfaces, which play a critical role in defining their properties, has become a focal point for researchers aiming to develop a wide range of MXene types tailored for many applications [10].

MXenes are commonly synthetized by selective etching of the A layer from their MAX phases. There are two possible approaches: top-down and bottom-up. The former method is currently the most extensively studied, while the latter requires further exploration to achieve its full potential on a practical scale [6].

2.2. Top-Down Approach

The top-down approach involves a wet chemical etching process. In this method, the A layer in the MAX phase, which is chemically reactive and weakly bonded, is removed through etching to produce MXene [6].

2.2.1. HF Etching and Fluoride-Based Acid Etching

HF etching is the most effective method for synthesizing MXenes. The first successful demonstration occurred in 2011, when the Al layer was removed from the Ti₃AlC₂ MAX phase at room temperature using a 50% HF solution over 2 h. This process occurs in two distinct steps. In the first step, Ti₃C₂ with high surface activity is generated (Equation (1)). Subsequently, Ti₃C₂ reacts with the solution to form surface terminations, as outlined in Equations (2) and (3) [16].

Ti₃AlC₂ + 3HF = AlF₃ + ³⁄₂H₂ + Ti₃C₂

(1)

Ti₃C₂ + 2H₂O = Ti₃C₂(OH)₂ + H₂

(2)

Ti₃C₂ + 2HF = Ti₃C₂F₂ + H₂

(3)

The general formulas for MXenes etched through HF solutions are M_n+1X_n(OH)_xO_yF_z or M_n+1X_nT_x. The nature of the T_x functional groups depends on the etching method; in this case, the terminations are a mix of -OH, -O, and -F. The specific etching conditions vary depending on the precursor material being processed. Both the M element and the n value significantly influence the strength and duration of the etching process: lower M and n values result in a weaker and faster etching reaction. Additionally, the type of A element affects the M-A bond strength [16]. For example, Ti-Si bonds are stronger than Ti-Al bonds, necessitating the use of oxidant-assisted HF methods to produce the same type of MXene from different MAX phases [17]. HF etching is not the simplest and most versatile process and cannot be applied for mass production of MXenes due to its high toxicity, strong corrosiveness, and operational risks. Other softer and safer fluorine-containing salts (e.g., NaF, KF, NH₄F, FeF₃, LiF) can be used to etch the A layer. The etching results may vary depending on the chosen salt due to differences in their solubility and the adsorption energy of their positive ions. LiF is one of the hardest to dissolve in water; consequently, the concentration of Li in LiF-exfoliated MXene is high, conferring different adsorption capabilities, such as methane adsorption. Furthermore, an open or closed environment can influence the etching effects [16].

2.2.2. Molten Salt Method

Fluorine-based approaches are not suitable for synthesizing all types of MXenes. Nitrogen-based MXenes (Transition Metal Nitrides (TMNs)) can easily dissolve in HF etching solution due to their instability, which arises from their lower cohesive energy compared to carbide MXenes. In contrast, molten salt synthesis is a sustainable, simple, fast, and efficient alternative, enabling the preparation of fluorine-free MXenes. This method also opens new opportunities for tailoring surface terminations. The process involves mixing the reactants and salts in precise proportions, followed by heating the mixture in a controlled atmosphere, such as in vacuum or inert gas (Figure 4 [18]). Then, the reactants undergo chemical reactions within the molten salt environment, resulting in materials with unique morphological properties [10]. A final washing step is required to isolate pure MXenes. For optimal reaction efficiency, the process necessitates salts with large grain sizes and a significant temperature gap between their melting (T_m) and boiling (T_b) points [19]. MXenes derived from molten salt methods, particularly TMNs, show great potential for applications in supercapacitors [10].

2.2.3. Lewis Acidic Etching Methods

The Lewis acidic etching method has emerged as one of the most extensively studied synthesis techniques in recent years. It is also known as the element replacement approach due to its potentials in modifying terminal groups, enhancing MXene performances like energy storage, and enabling covalent modification. The Lewis acidic method is a green, safe, straightforward, and controllable method similar to the molten salt approach but with some differences in the etching mechanism [20].

Lewis acid salts, typically transitioned metal halides, such as CuCl₂, ZnCl₂, CuI, and ZnBr₂, are covalent compounds that cannot be involved in the traditional molten salt synthesis due to their narrow temperature range between melting (T_m) and boiling (T_b) points. The essence of the reaction lies in establishing a redox potential between the molten salt and the precursor phase, enabling the selective etching of the MAX phase through a Gibbs free energy exchange reaction at high temperatures [10].

2.2.4. Electrochemical Etching

Electrochemical etching is a sustainable and efficient method (Figure 5) [21]. The A layer is selectively etched from the MAX phase precursor through the interaction between the electron flow and Gibbs free energy. An electrochemical reaction took place through the application of a certain voltage in a circuit with an electrolyte and an electrode (the MAX phase). Electrolyte design, voltage control, and the duration of the reaction are critical factors in achieving a high-quality MXene [10]. Specifically, high voltage can cause the breaking of M-A bonds, removing the M layer after A layer etching. Other advantages of this process include the absence of toxic fluoride ions and the low energy consumption, which contribute to the environmental friendliness of this method. Avoiding the use of traditional etchants prevents issues like low purity and the introduction of fluorine-containing groups [22].

2.3. Bottom-Up Approach

The bottom-up approach, in contrast to the top-down approach, does not exfoliate MXenes from MAX phases but instead uses small organic and inorganic molecules or molecular materials as precursors to make MXene crystals grow. This approach is appealing due to its simplicity, faster functionalization, greater versatility, and more precise structural and morphological control than the top-down approach and easy surface manipulation. However, more research is still needed for mass production [3].

CVD Process

Chemical Vapor Deposition (CVD) is still in its early stages of study for MXene synthesis, but it has already shown a great promise [16]. In 2015, a strategy was proposed to grow 2D high quality α-Mo₂C crystals using methane as the carbon source and a Co/Mo foil as molybdenum source. At high temperature, methane decomposes and copper melts, then methane carbon atoms react with Mo diffusing on the surface. This process results in the formation of Mo₂C crystals with remarkable characteristics such as ultra-thin thickness (few nanometers), large lateral size (100 µm), low defect density, and excellent stability [23]. Subsequent studies have extended this approach to synthesize Mo₂N on Cu/Mo using ammonia as a N source [24] and WC and TaC by using, respectively, W and Ta foils instead of Mo ones [25].

The CVD process offers a precise control over the thickness, composition, and structure of the resulting film, enabling the fine-tuning of MXene properties for specific applications and structure with a lower amount of defects as compared to other synthesis methods. However, several challenges remain, including the requirement for tightly controlled gas environments, high processing temperatures, and extended reaction times, all of which can hinder scalability and affect the quality of the final material [26]. Additionally, the need for sophisticated equipment and highly skilled operators is a significant limitation [27]. Furthermore, the dependence on specific substrates may further constrain the versatility of this technique [26].

2.4. Other Synthesis Methods

Despite the extensive research already conducted on MXene synthesis methods, the suitability of these processes for large-scale production remains a key issue. Among the approaches previously discussed, top-down techniques such as electrochemical and molten salt etching have emerged as the most scalable alternatives, offering improved safety and control over traditional HF-based methods. In contrast, bottom-up strategies like CVD are still largely unexplored for mass production and currently appear impractical due to high cost and complexity [28].

Over the years, numerous studies and alternative approaches have been developed that seem to be promising for MXene synthesis. However, most of these methods require improvements and further research.

Table 1. Advantages and drawbacks of different types of MXene synthesis.

Synthesis Strategies	Reagents	Morphology	Merits	Demerits
HF etching [16,17]	HF	Accordion-like structure with abundant -F terminations and defects	1. Effective for most MAX 2. High yield	1. Dangerous operation 2. Cannot be peeled in situ
Fluoride-based acid etching [16,17]	LiF/NaF/KF + HCl NH4HF2	Clay-like MXene with large interlayer spacing and few -F terminations	1. Relatively safe 2. Direct ultrasonic peeling	1. Long etching time 2. Introducing fluoride salt impurities
Alkaline solution etching [13,29]	NaOH TMAOH	Accordion-like structure only with -O, -OH terminations (For TMAOH terminated with Al(OH)4−)	1. No risk of acid corrosion 2. fluorine-free functional group	1. For NaOH: severe etching conditions 2. For TMAOH: need HF pretreatment
Molten fluoride salt and Lewis acidic molten melts etching [17,18]	LiF + NaF + KF Lewis Acid	Accordion-like structure (For Lewis Acid terminated with various halogens)	1. For molten fluoride salt: can obtain nitride MXene 2. For Lewis acid: achieve precise control of surface functional groups	1. Introduce salt impurities 2. Severe etching conditions
Electrochemical etching [17,18,30]	NH4Cl + TMAOH	Single or few-lawyer structure without -F functional group	1. Safe etching environment 2. Fluorine-free functional group 3. Get stripped MXene directly	1. Strict etching conditions 2. Introduce salt impurities
Water-free etching [16,31]	NH4HF2 in organic solvents	Accordion-like structure with extremely high -F terminations	1. Conducive to the use of organic systems due to the absence of water 2. Can be delaminated directly by ultrasonication	1. Long etching time 2. Tedious washing steps
HCl-based hydrothermal etching [16,32]	HCl-hydrothermal	Layered structure with-Cl and -O terminations	1. Simple experiment operation 2. Fluorine-free functional group	1. Severe etching conditions 2. Rely on the prediction of precise reaction conditions by DFT
Halogenetching [33]	Br2, I2, ICl, IBr	Accordion-like structure terminated with various halogens	1. Mild etching environment 2. Precise control of surface functional groups	1. Tedious etching and purification steps 2. Strict etching condition
Chemical vapor deposition (CVD) process [26,27]	Methane and bimetal foil (Cu/Mo)	Ultra-thin and large-size flake	1. High purity 2. Accurate control of thickness	1. Low productivity 2. High synthesis temperature
In Situ electrochemical synthesis [15,34]	LiTFSI + Zn(OTF)2	In situ etching and stripping in the battery	1. Green synthetic environment friendly 2. Extremely convenient operation	1. Rely on expensive metal ion salts 2. Restricted to be used inside the battery
Lithiation-expansion micro explosion mechanism [15,35]	Lithium-ion	Single-layer or few-layer structure without -F functional group	1. Simple and safe synthesis environment 2. Fluorine-free functional group	1. Low productivity 2. Consume resources
Microwave-assisted etching [36]	Ti3AlC2 + HF	Layered MXene sheets	1. Very rapid (30 s) 2. High-quality sheets 3. Good capacitance 4. Scalable	1. Slight oxidation to TiO2

provides a summary of the previously described techniques and offers some examples of these recent promising methods. Water-free etching is a well-suited method for use in organic systems, where delamination is possible just through ultrasonication. However, this method has notable drawbacks, including long duration and multiple washing steps that require further optimization. Another reported top-down approach is alkaline solution etching, which, like water-free etching, produces accordion-like structures but without fluorine functional groups. Among bottom-up methods, in situ electrochemical etching stands out as an efficient, sustainable, and convenient process, though it is presently restricted to applications in the energy storage sector [16].

One significant challenge in achieving the mass production of MXene-based devices lies in reducing the synthesis process duration. Current methods, such as HF etching, require several hours to be completed, highlighting the need for advancements or innovative approaches to enhance efficiency and scalability.

A novel microwave-assisted method was developed for the rapid synthesis of Ti₃C₂T_x MXene, achieving complete etching of the Al layer from the Ti₃AlC₂ MAX phase in just 30 s. Using HF as the etchant and microwave irradiation at 600 W and 2.5 GHz, high-quality MXene sheets with an undistorted layered structure were obtained. Characterization techniques such as XRD, FTIR, Raman spectroscopy, and FESEM confirmed the successful removal of Al and the preservation of the layered morphology, along with a slight oxidation to anatase (TiO₂). The synthesized material exhibited a high specific capacitance of 282 F g⁻¹ at 5 A g⁻¹, reduced charge transfer resistance, and excellent cyclic stability, retaining 88% of its capacitance after 10,000 cycles. Furthermore, an asymmetric solid-state supercapacitor fabricated with the Ti₃C₂T_x electrode achieved an energy density of 23 Wh kg⁻¹ and a power density of 750 W kg⁻¹, demonstrating the potential of this rapid synthesis approach for large-scale energy storage applications [36].

3. Properties of MXenes

The layered structures and surface functional groups of MXenes endow them with exceptional properties, paving the way for a wide range of applications for these promising materials. The 3D bulk precursors of MXenes (MAX phases), along with delamination and synthesis methods, play a critical role in determining their properties [3]. The most interesting features of MXenes are outlined in the next sections.

3.1. Mechanical Properties

MXenes exhibit outstanding mechanical properties, including remarkable flexibility, high tensile strength, and significant flexural rigidity. The Young’s modulus of a Ti₃C₂T_x monolayer has been calculated to range between 0.33 TPa and 0.03 TPa, making it the highest among all 2D materials, including graphene oxide (GO). Notably, the flexibility of Ti₃C₂T_x is impressive: it has been reported that a Ti₃C₂T_x film can support up to 4000 times its weight without experiencing any shape distortion or structural damage [6].

The elastic behavior of MXenes is strongly influenced by their surface terminal groups. Specifically, the strength and length of surface bonds play a key role in determining their mechanical properties. Experimental studies indicate that MXenes with O-terminal groups are the most suitable candidates for applications such as supercapacitors and structural materials due to their exceptional flexibility [3]. The Ti-O bond is notably stronger than the Ti-OH and Ti-F terminations [37]. Density Functional Theory (DFT) simulations conducted on Ti₃C₂ and Ti₂C with different surface terminations have demonstrated that shorter bond lengths—such as those associated with oxygen groups—indicate stronger contacts, whereas F and OH bonds, being longer, result in weaker interactions [38].

Regarding Ti-C bonds, previous studies have shown that Ti₂C, Ti₃C₂, and Ti₄C₃ MXenes exhibit considerable rigidity. However, graphene remains a superior alternative due to its higher strength and stiffness [37]. Additionally, M-N and M-C bonds contribute to the tensile strength of MXenes, reaching 1050 GPa [39] and opening significant opportunities for their use as reinforcing materials in composite structures. While MXenes may demonstrate slightly lower performance compared to graphene, they offer excellent compatibility with polymeric phase to produce composites.

The thickness and number of layers also significantly influence the Young’s modulus of MXenes. When comparing carbide and nitride MXenes, the latter exhibit higher maximum values. In both cases, a decrease in the number of layers correlates with an increase in the Young’s modulus [37].

3.2. Chemical Stability

The oxidation state of MXenes depends on their surface terminal groups and is generally lower than that of the corresponding metal oxides. The stability of MXenes is heavily influenced by their surrounding environment [6]. For example, the stability of Ti₃C₂T_x colloidal solutions was tested under ambient conditions. It was observed that the solution degrades into TiO₂ within 15 days, identifying air, specifically oxygen, as the primary factor responsible for the oxidation of Ti₃C₂T_x [40,41].

Water also plays a significant role in the degradation of MXene solutions. Colloidal solutions of Ti₂CT_x and Ti₃C₂T_x degrade within 41 days in a water/argon environment. In contrast, when exposed to an IPA/oxygen environment, the degradation process is significantly slower, taking approximately 5.5 years to achieve similar results [42].

MXene oxidation can be mitigated through the use of antioxidants, such as sodium L-ascorbate, and by maintaining specific environmental conditions, which vary depending on the MXene type. For instance, V₂C_tx remains stable in an argon atmosphere at temperatures below 375 °C, whereas Ti₃C₂T_x can maintain its stability in argon up to 800 °C [40,41] (see Figure 6 [43]).

MXenes exhibit a remarkable hydrophilic nature, due to the presence of O and OH surface groups, which play a crucial role in enabling strong interactions with water. These groups enhance the ability of MXenes to attract and spread water across their surface. The ratio of surface terminations can be precisely controlled through synthesis methods, allowing for the optimization of their hydrophilic properties. This characteristic is particularly valuable in the production of inks for 3D printing of composite electrodes, where achieving a stable and uniform dispersion of materials in solvents is essential [44].

3.3. Optical Properties

MXenes are distinguished by their linear (e.g., absorption) and nonlinear (e.g., refractive index) optical properties, which are influenced by their energy structures, including energy band gaps (both direct and indirect) and topological insulator characteristics. Additionally, surface terminal groups play a critical role in shaping these optical properties. Notable optical attributes of MXenes include high transparency, strong plasmonic behavior, and a pronounced photo-thermal effect [37].

The optical performance of MXenes is also dependent on temperature. For instance, Nb-based MXene films show a reduction in optical performance as temperature increases, whereas Ti-based MXenes retain stable optical properties at ambient temperature [37]. MXenes demonstrate exceptional light-to-heat energy conversion with 100% efficiency, which opens promising applications in the biomedical field (e.g., photo-thermal therapies) and water evaporation systems [45].

MXenes exhibit absorption peaks spanning wavelengths from ultraviolet (UV) to near-infrared (NIR). Their strong UV-light absorption has led to extensive research into their use in photovoltaic and photocatalytic applications. Furthermore, their high flexibility, UV absorption, and remarkable transparency make MXenes ideal candidates for transparent conductive electrodes [37]. A Ti₃C₂T_x film with a thickness of 10 nm revealed a 91% transmittance [46], which can be further enhanced through ion intercalation (e.g., with tetramethyl ammonium hydroxide) or by reducing film thickness (see also Figure 7 [47]) [37]. Transmittance increases as the film thickness decreases [3].

Z-scan measurements revealed that Ti₃C₂T_x exhibits a high nonlinear optical absorption coefficient and a negative nonlinear refractive index, with values significantly surpassing those of graphene, transition metal dichalcogenides (TMDs), and black phosphorus (BP) [38].

3.4. Electronic and Electrical Properties

MXenes are highly valuable materials for electronic and electrical applications due to their exceptional properties and the ability to fine-tune these characteristics. One of their most notable features is their high electrical conductivity, which makes them ideal for energy storage, supercapacitors, and electromagnetic interference (EMI) shielding [44]. Among MXenes, Ti₃C₂T_x is the most studied for such applications [37], exhibiting a metallic conductivity of 2400 S/cm [48]. However, V₂C surpasses this with an impressive value of 3300 S/cm [48], paving the way for promising applications in wearable electronics [37]. Ti₃C₂T_x MXenes exhibit strong metallic and electrical conductivity, enhancing their electrocatalytic performance. Specifically, Ti₂COm MXenes show a carrier mobility of 1.03×10⁴ cm² V⁻¹ s⁻¹, which is significantly higher than that of graphene, with a carrier mobility ranging from 0.3 to 1.2 × 10³ cm² V⁻¹ s⁻¹. Carbon nanotubes (CNTs) and reduced graphene oxide (rGO), both of which have a conductivity of around 15,100 S/cm, also offer impressive electrical conductivity; however, the unique properties of MXenes allow them to perform better in electrocatalytic applications, revealing their potential advantages over these other 2D materials [3,49].

Moreover,

Table 2. Comparison of the electrochemical performance of different electrode 2D materials in aqueous supercapacitors [50].

Materials	Electrolyte	Capacitance (Scan Rate)	Stability (Retention/Cycles/Scan Rate)	Energy Density (W h kg−1)	Power Density (W h kg−1)	Ref.
Ti3C2Tx	3M H2SO4	210 F g−1 at 10 V s−1	-	-	-	[51]
Ti3C2Tx	1M H2SO4	429 F g−1 at 1 A g−1	89% after 5000 cycles at 10 A g−1	29.2 at 1 A g−1	320 at 1 A g−1	[52]
PPy/Graphene	1 M H2SO4	626 F g−1 at 0.22 A g−1	75.4% after 5000 cycles at 4 A g−1	21.7	110	[53]
MoS2	0.5 M Li2SO4	350 F g−1 at 5 mV s−1	88.0% after 10,000 cycles at 5 A g−1	1 50 at 1 A g−1	1000 at 1 A g−1	[54]

[50] presents the capacitance values of different 2D materials in aqueous electrolytes, highlighting the exceptional capacitance and rate capabilities of MXenes.

MXenes stand out as some of the most attractive materials for electronics, thanks to their tunable properties. By modifying surface functional groups, altering the ratio of M and X elements in the layered structure, or doping, it is possible to engineer a wide range of materials with tailored characteristics. Surface terminations significantly influence MXene properties (Figure 8 [55]); for example, replacing –F or −Cl groups with −OH can enhance electrical conductivity. Additionally, intercalating carbon or nitrogen, creating surface vacancies, or introducing dislocations can further optimize their performance. Doping multi-layer MXenes often yields superior results [37].

The electronic nature of MXenes can range from metallic (e.g., Ti₂C and Ti₃C₂, with high electron density near the Fermi level [38]) to semiconductive (e.g., Ti₃C₂(OH)₂ and Ti₃C₂F₂ monolayers [38]), or even insulating, depending on structural and functional group modifications [44]. Surface manipulation offers a powerful tool to tailor their semiconductive behavior [38].

Moreover, environmental moisture has been observed to enhance MXene electrical conductivity, opening new possibilities for sensing applications [37]. High electrical conductivity is crucial for achieving high power densities and can also reduce the need for additional conductive materials in the fabrication of electrodes and current collectors [44].

3.5. Surface Modification and Multifunctionalization

The most studied MXenes feature hydroxyl (-OH), oxygen (-O), or fluorine (-F) terminal groups, which provide these materials with high stability, a significant surface charge, and a hydrophilic nature. However, after delamination, MXenes often require enhancements to certain properties, making surface engineering a critical factor [25].

Surface modification is typically achieved through two main approaches: polymer-based strategies and inorganic particle surface chemistry. The former approach involves the immobilization of specific molecules or polymers onto the MXene surface via non-covalent or covalent interactions. For example, soybean phospholipids (SPs) have been utilized to modify the surface of Ta₄C₃T_x films, enhancing their stability in physiological environments [24]. An example of inorganic particle-based surface chemistry [24] is the preparation of AuNPs/Ti₃C₂T_x, achieved by immobilizing gold nanoparticles on the MXene surface to improve electrical conductivity [56].

The layered structure of MXenes, characterized by a high surface area and significant interlayer distances, facilitates ion intercalation, which can improve energy storage performance in applications such as batteries and supercapacitors [37]. Figure 9 illustrates the structure of Ti₃C₂T_x and provides a simplified scheme of the intercalation process, highlighting the increased interlayer spacing after ion insertion. Electrochemical intercalation of Ti₃C₂T_x cations, such as Al³⁺, is one example, with other ions like Li⁺, Na⁺, Mg²⁺, K⁺, and NH⁴⁺ also being widely studied [57].

However, the structure of MXenes still carries important challenges. Their tendency to reassemble and the lack of a controlled porosity are notable limitations [3]. In order to address these issues, a study proposed a method involving sequential bridging using hydrogen and covalent bonding agents to achieve optimal layer density and reduce vacancies in MXenes [24].

Table 3. Various synthesis techniques to obtain porous MXenes.

Samples	Methods	Structures
Porous Ti3C2Tx Aerogel shaped	Freeze drying	Mesoporus/macroporous
MXene lamellar-liquid-crystal	Mechanically shearing assisted freeze drying	Aligned vertically mesoporous/macroporous
Lamellar structured Ti3C2Tx/SiCnws foam	Bidirectional freeze-drying	Aligned vertically mesoporous/macroporous
Super-elastic MXene/PI aerogels	Freeze drying	Meso/macropore with wide size distribution
Fluffy-type MXene microspheres	Spray drying	Mesoporous/macroporous
3DMXenefilms	Hard-template method	Mesoporous/macroporous
Cellular-type MXene foam	Hydrazine reduction technique	Mesoporous/macroporous
hybrid 3D porous network of MXene-Sponge	Dip-coating and drying	Macroporous
MXene-rGO aerogel	Chemical-reduction	Mesoporous/macroporous
MXene-rGO aerogel	Freeze drying-calcination	Mesoporous/macroporous
Cellulose-MXene aerogel	Chemical crosslinking	Mesoporous/macroporous
TiO2/MXene, SnO2/MXene	Self-assembly	Mesoporous
FeNi-LDH-MXene	In situ growth	Mesoporous
Core–shell Ti3C2-mSiO2	Sol–gel	Mesoporous
MXene flakes	Oxidative-etching	Mesoporous
MXene with divacancy-ordering	Selective-etching	Microporous and mesoporous

lists additional approaches designed to control porosity and voids [3].

Nevertheless, many of the multifunctionalization techniques discussed above still require further research, especially considering that studies on graphene and graphene oxide (GO) are more advanced at present [25].

4. Applications

4.1. Biomedical Applications

Thanks to their interesting and highly tunable properties, such as hydrophilicity, large surface area, and exceptional conductivity, MXenes became promising materials for a wide range of biomedical applications (Figure 10 [24]) [25]. This section explores some of the most significant and recent studies in this field.

Inorganic MXene-based materials possess appealing properties, often surpassing those of organic materials, which typically offer exceptional biocompatibility but are frequently prone to significant chemical and thermal instability. MXenes are distinguished by their highly tunable structure and multifunctionalization capabilities [25], chemical affinity [11], remarkable physiological stability, and excellent biosafety [25]. For biomedical applications, toxicity and biocompatibility require careful consideration. MXene biosafety can be compromised by factors such as exfoliation, nanotoxicity/cell internalization [58], release of toxic ions, and generation of reactive oxygen species (ROS), which all carry potential hazards to cells, organs, and tissues [11]. Numerous studies have been conducted to assess the biosafety of MXenes, but further efforts are needed, particularly to certify their long-term stability [25]. Both in vivo (e.g., on mice [25]) and in vitro (e.g., using cell cultures [11]) studies have demonstrated low toxicity levels for MXenes. For instance, in an in vitro experiment, Ti₃C₂-SP and Ta₄C₃-SP films showed negligible effect on 4T1 cell viability. Similarly, in vivo tests on mice demonstrated normal behavior after administration of Nb₂C-PVP at a dosage of 20 mg/kg [25].

MXene can be incorporated in biomedical coatings to improve biocompatibility and make them electrically conductive, as recently reviewed by Anvari Hohestani et al. [59]. In this regard, an interesting example includes the study of Mayerberger et al. [60] who incorporated Ti₃C₂T_x MXene flakes into chitosan nanofibers for use in passive antibacterial wound dressings. Antibacterial tests against E. coli and S. aureus also showed a significant reduction in colony-forming units, with decreases of 95% and 62%, respectively [59].

Surface modification and the use of polymer composites are effective strategies to enhance biosafety and prevent the degradation of MXenes [11]. For instance, the incorporation of OTES-functionalized Ti₃C₂T_z MXene into poly(lactic acid) membranes has been shown to improve biocompatibility, promoting cell adhesion, proliferation, and osteogenic differentiation, while also enhancing mechanical properties [61].

4.1.1. Biosensing

MXene-based biosensors have been tested in the human body to detect and analyze biomolecules and their effects. These biosensors consist of three main components: a sensing element to identify the target, a transducer to convert biochemical signals into electrical signals, and a data interpreter for analysis [11]. The exceptional properties of MXenes, such as their large surface area [25] and high charge activity for efficient target capture [62], excellent photon absorption capability, rapid electron transfer fluorescence quenching [25], and remarkable sensitivity and selectivity [58], make them highly promising for a wide range of biosensing applications. Electrochemical biosensors, known for their rapid response, simplicity, and cost-effectiveness, have also demonstrated significant potential in applications such as pesticide detection and analysis in agriculture, specifically for paraoxon [62]. Furthermore, MXene-based biosensors have shown optimal performance in detecting glucose, circulating tumor DNA [11], dopamine [25], NADH, and uric acid [58]. For instance, an electrochemical biosensor using antibody-functionalized Ti₃C₂T_x MXene nanosheets was developed for point-of-care detection of vitamin D. The sensor achieved high sensitivity (LOD of 1 pg mL⁻¹) and a wide dynamic range (0.1–500 ng mL⁻¹), suitable for clinical screening of deficiency and toxicity levels. This MXene-based platform is low-cost, fast, and ideal for remote diagnostics [63].

4.1.2. Imaging

Quantum size effects, intrinsic photothermal properties, element-driven contrast enhancement, and efficient loading of functional contrast agents (CAs) make MXenes highly suitable for imaging applications. The most extensively studied imaging modalities include photoacoustic imaging (PAI), magnetic resonance imaging (MRI), X-ray computed tomography (CT), and luminescent imaging [64]. PAI is a non-invasive, non-ionizing technique that overcomes optical photon scattering to enable deep tissue imaging. It generates photoacoustic signals from the light absorption of tissue molecules, which are then converted into images via a transducer, effectively capturing biological information [24]. Among the explored materials, Nb₂C-MSNs-SNO composite nanosheets have exhibited exceptional performance as PA contrast agents [64], while Nb₂CT_x-PVP stands out for its remarkable stability and photothermal conversion efficiency [24]. Additionally, Ta₄C₃-SP nanosheets have shown promising contrast-enhancement capabilities [25]. MRI offers excellent spatial resolution, superior soft tissue contrast, and the advantage of avoiding ionizing radiation [64]. Ti₃C₂ MXenes have demonstrated significant potential in MRI applications due to their enhanced contrast properties [65]. CT imaging remains one of the most widely used methods due to its effectiveness, non-invasive nature, and ability to achieve deep tissue penetration. It generates 3D scans by segmentally scanning a body section with X-rays. Notably, MnOx/Ta₄C₃T_x-SP composite nanosheets have shown superior performance in CT imaging compared to iodine-based iopromide [66], a commonly used clinical agent, owing to tantalum’s high X-ray attenuation coefficient [24]. For luminescent imaging, the production of MXene quantum dots (MQDs) significantly enhances luminescent properties [67]. MQDs exhibit small dimensions, strong photoluminescence (PL) characteristics, remarkable stability, low toxicity, and tunable wavelengths [24]. Ti₃C₂ MQDs have demonstrated excitation-dependent photoluminescent spectra and impressive quantum yields (up to 9.9% [25]) [64]. Similarly, Nb₂CT_x QDs [48] have shown outstanding enzyme-responsive degradability, robust resistance to photobleaching, and remarkable stability [24].

4.1.3. Theranostic Applications

Theranostic applications combine therapeutic and diagnostic functions within a single platform [11]. Photothermal therapy (PTT) is a prominent approach in cancer and tumor treatment, leveraging the exceptional properties of MXenes (Figure 11 [68]). PTT employs photothermal agents accumulated within tumors to act as internal energy absorbers, converting NIR light energy into heat, leading to necrosis and/or apoptosis of cancer cells [24]. High photothermal performance has been observed in various studies, including in vivo and in vitro experiments, with materials such as Ti₃C₂, which is distinguished by its rapid light-to-heat conversion, Ta₄C₃, known for its excellent biocompatibility, high conversion efficiency (44.7%), and strong absorption band, and Nb₂C, the latest addition, which shows high performance in both NIR-I and NIR-II biowindows. Moreover, Ti₃C₂ has been utilized in a synergistic therapy combining photothermal ablation with DOX-based chemotherapy, demonstrating outstanding stability, complete eradication of 4T1 cells without recurrence, and a drug encapsulation capacity of 84.2% [25]. Immunotherapy works by enhancing or activating the immune system to prevent or eliminate tumor cells. A 3D-printed Nb₂CT_x scaffold for breast cancer treatment showed promising results, but immunotherapy still requires additional therapeutic strategies (e.g., PTT and PDT) to achieve optimal outcomes [24].

4.1.4. Tissue Engineering

Tissue engineering aims to design and develop biological tissues to restore or regenerate functions lost due to disease or injury [69]. MXenes have been explored in various tissue engineering fields, including stem cell engineering [69], skin tissue engineering, nerve tissue engineering, and myocardial tissue engineering [64]. Research has shown that Ti₃C₂T_x enhances cell proliferation and osteogenic differentiation in PLA scaffolds and promotes the production of cell spheroids [70], with both in vitro and in vivo tests highlighting their excellent biocompatibility [64]. Additionally, MXene/PLLA-PHA electrospun nanofibers [71] were found to improve the activity, growth, and osteogenic differentiation of BMSCs [70]. Ti₃C₂T_x has shown great promise in tissue engineering, particularly due to its functionalized conductive surface, which supports stem cell differentiation and enables precise photothermal stimulation of dorsal root ganglion neurons, facilitating the development of advanced neural therapy models [72]. Furthermore, Ti₂C was used to develop bio-functional engineered cardiac patches (ECP) [73] that can repair myocardial infarction (MI) when applied to the natural heart. These patches demonstrated remarkable effectiveness in enhancing heart function, reducing infarct size, and alleviating inflammation [64]. Additionally, an MXene-reinforced PVA cryogel scaffold was developed for neural tissue engineering, successfully promoting PC-12 cell proliferation, neurite outgrowth, and reducing oxidative stress, showing strong potential for neural regeneration applications [74].

The suitability of MXenes for bone tissue engineering applications has also been investigated; a comprehensive overview of this specific topic can be found elsewhere [75].

4.1.5. Drug Delivery

The distinct 2D architecture and tunable physicochemical features of MXenes make them attractive candidates for targeted drug delivery. Among them, Ti₃C₂T_x stands out due to its ultrathin morphology, strong near-infrared responsiveness, effective photothermal properties, and adaptable surface chemistry. Despite these advantages, Ti₃C₂T_x-based carriers often exhibit limited drug-loading capacity and poor retention at target sites, leading to premature clearance and potential tissue damage. Similarly to other 2D inorganic materials, MXene-based systems also face stability issues under physiological conditions, complicating controlled drug release [70]. In order to overcome these challenges, integrating magnetic nanoparticles into MXene structures has been proposed, enabling external magnetic fields to guide and retain the drug carriers at the desired location, with drug release triggered by specific internal or external stimuli [76].

Recent studies have demonstrated the use of MXene-based systems, particularly Ti₃C₂T_x, for efficient drug delivery. For instance, a dual therapeutic agent (DOXjade), formed by linking DOX to a deferasirox derivative, was successfully loaded onto Ti₃C₂-PVP MXene, achieving high loading capacity of 210%, pH-triggered drug release of 5.3 with release time in a range from 0 to 72 h [77]. Other strategies involved functionalizing Ti₃C₂T_x with polymers like PEG or chitosan, or forming composites with Co nanowires (CoNWs) to enhance drug loading and ensure pH-responsive release, favoring drug liberation in acidic tumor environments, such as breast cancer, reaching loading and release efficiencies, respectively, up to 225% and 77%, with a release time of 24 h [78].

4.2. Electronic Applications

The remarkable and tunable electrical and electronic properties of MXenes position them as strong candidates for a wide range of electronic applications. This section highlights the most extensively studied and promising examples in this context.

4.2.1. Supercapacitors

Many features of MXenes make them promising candidates for the design of micro-supercapacitors (MSCs) and for use in high-precision integrated circuits (Figure 12 [79]). The morphology of MXenes plays a crucial role in determining their electrochemical performance. For instance, after etching the 3D bulk precursor, the material capacitance can be enhanced by the intercalation of water molecules within the layered structure, facilitated by the 2D slits. Additionally, water dielectric constant is key for shielding external electric fields. The small thickness of MXenes contributes to superior ion diffusion and further enhances capacitance. The functional groups of MXenes, along with their tunability, endow these materials with excellent energy density, acting as active sites that promote ion insertion and removal. As a result, MXene-based MSCs stand out for their excellent flexibility, high power density and voltage, compact dimensions, and remarkable durability [10]. To further enhance the performance of MXene-based supercapacitors, such as ion transfer and accessibility, composite materials have been developed by combining MXenes with organic polymers (also called conductive polymers or ICPs), metal oxides [43], metal hydroxides, and carbon materials [80]. The intercalation of these elements expands the interlayer spacing and minimizes MXene rearrangements [43]. Ti₃C₂Ti_x/ZnMnNi LDH van der Waals heterostructured electrodes [81] were produced through the combination of MXene and layered double hydroxides that showed a great capacitance of 2065 F/g at 5 mV/s and a capacitance retention rate of 99.8% after 100,000 cycles at 1 A/g [81]. Another notable example is the combination of graphene and MXene in a modified MXene/graphene oxide (MX-rHGO) film [79], which has demonstrated impressive volumetric capacitance (1445 F/cm³) at a scan rate of 2 mV/s, excellent mass loading, and outstanding durability, retaining 93% of its capacitance after 10,000 cycles at 5 A/g [81].

4.2.2. Perovskite Solar Cells

Perovskite solar cells (PSCs) have shown great potential in the field of photovoltaic technology due to their low production cost and high efficiency [82]. Since the first studies on the application of MXenes in PSCs, dating back to 2018 [83], power conversion efficiencies (PCEs) of these devices have reached values comparable to those of the most widely used silicon solar cells, up to 26%. Considering that this performance has been achieved in silicon cells over 70 years of research, perovskite solar cells are supposed to match and surpass them. PSCs are distinguished by their high-quality structure, which can be further enhanced through MXene additives, offering improved conductivity, reduced diffusion resistance, and tunable properties [82]. For example, the optoelectronic performance of (BA)₂(MA)₄Pb₅I₁₆ films has been improved by adding Ti₃C₂T_x nanosheets, achieving a PCE of 15.71% and a short-circuit current density of 20.87 mA/cm² [84]. Other MXenes, such as Nb₂CT_x [85], V₂CT_x [62], and Mo₂CT_x [86], have also been studied for PSC applications, and further research is needed to identify additional effective additives [82]. Moreover, it is essential to optimize MXene production by developing more sustainable methods than HF etching, aiming to support mass production. Another aspect that warrants further investigation is the most efficient dosage of MXenes and the concentration of functional groups to maximize PSC performance, particularly in terms of stability, chemical compatibility, and durability [82].

4.2.3. Batteries

In the realm of energy storage, lithium–sulfur batteries (LSBs) stand out as highly effective devices, thanks to their exceptional specific capacities, impressive energy densities [87], and superior safety standards, surpassing even lithium-ion batteries (LIBs) [38]. MXene-based materials have proven to significantly enhance LSB performance due to their remarkable electrochemical properties. MXenes are extensively utilized in cathodes, interlayers, and separators, where they improve durability, enhance catalytic processes, support lithium polysulfide and Li₂S kinetics, increase sulfur loading, and facilitate lithium anode hosting [87]. For example, Ti₃C₂T_x has been employed to coat separators, achieving an impressive capacity of 850.9 mAh/g with a sulfur loading of 2.8 mg/cm² and maintaining a capacity retention of 89.66% [88].

Furthermore, Ti₃C₂T_x MXene has been applied in aqueous zinc metal batteries (ZMBs) as an electrolyte additive [89], effectively inhibiting Zn dendrite nucleation and growth—common issues that compromise battery efficiency or cause internal short circuits [38]. Additionally, high-purity V₂CT_x MXene, used as an anode in LIBs [90], has exhibited outstanding electrochemical performance (Figure 13) [91].

A hollow MoSe₂@MXene composite was synthesized via a hydrothermal method by growing MoSe₂ nanoflakes on hollow MXene spheres. This 3D structure, used as an anode in sodium-ion batteries, demonstrated excellent cycling stability and rate performance, delivering 350.7 mAh g⁻¹ after 1000 cycles at 1 A g⁻¹ with 93.7% retention. Its high performance is due to the synergistic effect between MoSe₂ and MXene, which prevents material aggregation and improves sodium-ion transport [92]. A Sn₄P₃@MXene anode with a protective MXene shell enables inward expansion and stable cycling, delivering 302.1 mAh/g at 10 A/g and 94.9% retention over 1500 cycles. The full cell achieves 287.8 Wh/kg and 390.5 mAh/g after 500 cycles, highlighting its practical potential [93].

A PTCDA-based potassium-ion battery anode using MXene as an electrochemically active binder achieved 462 mAh/g at 50 mA/g, 116.3 mAh/g at 2000 mA/g, and excellent stability over 3000 cycles with only ~0.0033% capacity loss per cycle. The MXene/Super-P matrix improved conductivity suppressed PTCDA solubility, and enhanced K⁺ adsorption. The full cell reached 179.5 Wh/kg energy density, highlighting MXene’s role in enabling stable, high-performance organic PIBs [94]. The CNS@Ti₃C₂ composite, created by encapsulating Ti₃C₂ MXene with carbon nanospheres from carbon quantum dots, enhances potassium storage in PIBs. It delivers 229 mAh/g at 100 mA/g after 200 cycles and 205 mAh/g at 500 mA/g after 1000 cycles, with >99% coulombic efficiency, offering a promising approach for high-performance potassium-ion batteries [95].

4.3. Separation and Filtration Applications

MXenes offer significant potential in the design of membranes for separation applications thanks to their remarkable properties such as hydrophilicity, conductivity, and high flexibility (Figure 14) [96]. This section discusses some of the most valuable examples and provides a future outlook in this field.

4.3.1. Water Treatment

Over the years, MXenes have been engineered as functional membranes for water treatment and ion sieving due to their remarkable hydrophilicity and their overall negative charge. The surface functionalization of MXenes enhances their layered structure, improving permeability, enabling the separation of ions of different dimensions. The size-exclusion mechanism promotes the selection of ions smaller than the channels, while the Donnan exclusion method sieves negative ions, allowing only positive charges to permeate through electrochemical interactions with the net charge of MXenes [96]. An optimal MXene-based membrane should exhibit high water flux, excellent selectivity, and remarkable mechanical and chemical stability, particularly in resisting fouling [97]. In order to enhance fouling resistance, Ag nanoparticle-modified Ti₃C₂T_x (Ag@MXene) membranes have been developed [98], achieving impressive water flux (approximately 420 L m⁻² h⁻¹ bar⁻¹), high rejection rates for bovine serum albumin (100%), methyl green (92.3%), and rhodamine B (RhB, 79.9%) [96]. Additionally, the combination of graphene oxide (GO) and Ti₃C₂T_x in a 90 nm thick membrane [99] has demonstrated significant performance in rejecting positively charged dye molecules with radii greater than 5 Å [100]. MXenes have gained attention as promising materials for seawater desalination and pollutant removal to address the global demand for freshwater [101]. A Ti₃C₂T_x-based membrane has achieved nearly 100% efficiency in NaCl separation, with water permeance reaching 85 L m⁻² h⁻¹ bar⁻¹ at 65 °C [100]. Furthermore, Ti₃C₂ MXenes have been employed in photothermal water desalination under solar irradiation, leveraging their high light-to-heat conversion efficiency [102]. Regarding the antibacterial properties of MXene-based membranes, fresh Ti₃C₂T_x nanosheets [103] have shown a 73% antibacterial rate against Bacillus subtilis and 67% against Escherichia coli through direct contact damage. Upon air oxidation, these nanosheets can inhibit bacterial growth by up to 99% [96].

4.3.2. Gas Separation

Gas separation technologies are crucial for a wide range of industrial and environmental applications, such as H₂ purification and CO₂ capture [104], and are known for their low energy consumption, operational simplicity, and scalability [105]. Two-dimensional materials like MXenes are emerging as excellent alternatives to traditional polymer-based membranes due to their superior mechanical strength, layered structure, and tunable properties. The interlayer spacing and channel length of MXenes are key factors in determining their selectivity and performance in gas separation applications. In general, wider channels lead to improved permeability and selectivity [105]. Research has shown that Pd intercalation in MXene-based membranes [106] can enhance H₂ permeability and H₂/CO₂ selectivity [105]. Additionally, an MXene/PEG mixed matrix membrane [107] was designed to enhance CO₂ permeability for CO₂/N₂ and CO₂/CH₄ separation [105]. The presence of functional groups and surface reactivity also plays a significant role in membrane performance: CO₂, due to its quadrupole moment, can be easily absorbed by MXene interlayers, which facilitates effective H₂/CO₂ separation [104]. A recent study tested the permeability of He, H₂, CO₂, N₂, and CH₄ in C_{1_80} at room temperature and under different pressures, revealing that larger molecules like N₂ and CO₂ exhibit lower permeability compared to smaller ones such as He and H₂, as illustrated in Figure 15 [104].

The effectiveness of gas separation can also be temperature-dependent. MXene-based membranes were tested for H₂ selectivity at high temperatures (320 °C) over 200 h of continuous testing, with no significant changes in performance or structure. However, as the temperature increased, both permeability and selectivity decreased significantly, highlighting an area that requires further investigation [108].

5. Perspectives in 3D Printing

Additive manufacturing has revolutionized the production of advanced materials, improving their performance and unlocking new application opportunities [109,110]. Among emerging material classes, MXenes stand out for their exceptional properties. This section explores the latest advancements in MXene ink formulation and highlights various applications of 3D-printed MXene-based materials across different fields, providing insights into future developments.

5.1. Key Techniques in 3D Printing

3D printing marks a turning point in the vast array of manufacturing methods, offering numerous advantages in fabrication. Additive manufacturing is cost-effective, more sustainable due to reduced material waste, and highly scalable. Its versatility and flexibility allow for the design of complex shapes and structures without compromising product quality [1]. Additionally, it simplifies the production process through the use of design software like CAD, eliminating the need for highly skilled personnel or specialized facilities [111]. Three-dimensional printing can be divided in different techniques; the following is a brief overview of the most commonly used methods, highlighting their advantages and drawbacks.

Stereolithography (SLA) was the first 3D printing technique to be developed, dating back to the 1980s. It utilizes liquid photopolymer resins that are solidified layer by layer through photopolymerization induced by ultraviolet (UV) lasers, resulting in three-dimensional structures with high resolution [1], approximately up to 10 µm [112]. SLA is distinguished by its exceptional precision and rapid printing speeds [113], which can be further optimized. Ti₃C₂T_x MXene has been employed in an advanced SLA technology known as Continuous Liquid Interface Production (CLIP) as a photo-blocker. This application reduces light scattering in hydrogel-based aqueous inks, thereby enhancing printing accuracy [4]. However, SLA suffers from certain limitations, including high cost of the equipment compared to other printers [1] and poor mechanical properties in the final products, such as low specific stiffness and brittleness, which can compromise durability [114]. Despite these drawbacks, SLA supports a broad range of materials, including polymers, composites, ceramics, and metals [1].

Digital Light Processing (DLP) is the latest evolution of SLA and employs a digital light projector to cure liquid photopolymer resins in a vat, building objects layer by layer until fully solidified [4]. DLP is characterized by high accuracy and precision. However, printed objects often require additional cleaning processes to achieve optimal surface quality [4], and the technology faces limitations in producing large-scale structures [112]. Incorporating Ti₃C₂T_x MXenes into composite inks for DLP [115] has demonstrated improvements in both structural stability and electrical conductivity [4].

Fused Deposition Modeling (FDM) is one of the most widely used 3D printing techniques. It involves the extrusion of molten thermoplastic filaments through a heated nozzle, which deposits material layer by layer [113] following a CAD-designed model [1]. A diverse array of thermoplastics, such as PET and PLA, can be utilized, that can often serve as a polymer matrix for composite materials, including those based on MXenes [1]. FDM is renowned for its versatility [113], user-friendliness, and cost-effectiveness [1], making it suitable for various applications ranging from prototype development [113] to the production of standard components [1]. Incorporating MXenes has been shown to significantly enhance the mechanical properties and EMI shielding performance of FDM-printed composites [4], such as recycled carbon fiber (rCF)-reinforced PLA [116]. Nonetheless, FDM has certain disadvantages, including the need for chemical cross-linkers and substantial heat energy input [114].

Selective laser sintering (SLS) is a technique that fabricates solid structures by sintering powdered materials using a high-energy laser [4], such as a carbon dioxide laser [114]. The laser melts and solidify the powder particles layer by layer, allowing for the creation of complex geometries with high resolution. However, post-processing treatments are often required to improve surface roughness [112]. SLS is compatible with a variety of thermoplastic polymers and metals, which can attain excellent mechanical strength [4] as well as glasses/ceramics [117]. This technology has been used to fabricate composite of Ti₃C₂ MXene and polyamide 12 (PA12) [118], resulting in a material with outstanding flame-retardant properties, including reductions in heat release, peak heat release, and total smoke emission by 18.5%, 26.1%, and 28.1%, respectively, along with improved tensile strength and durability [4].

Direct Ink Writing (DIW) is an extrusion-based technique that uses a nozzle or pen-like tip to deposit materials layer by layer. It accommodates a wide range of materials, particularly hydrogels and their composites, which are commonly used in applications such as tissue engineering, soft robotics, electrodes, and sensors [1,111]. DIW has also been used to process MXene-containing composites for example, Ti₃C₂T_x MXenes/PEDOT:PSS inks have been developed for DIW-based 3D printing of micro-supercapacitors (MSCs) [119]. While DIW offers the advantage of fabricating complex geometries with tunable mechanical properties and varying viscosities [1], it also presents challenges related to high manufacturing costs and extended production times [111].

A summary of additive manufacturing technologies is reported in Figure 16.

5.2. MXene-Based Inks for Additive Manufacturing

Ink is commonly defined as a liquid mixture containing colorants, such as pigments or dyes, used for writing, drawing, or printing. The rheological properties of inks play a crucial role in determining their suitability for various printing techniques. Each printing method requires specific ink characteristics to achieve the desired final products. Consequently, designing appropriate ink formulations and tailoring their properties represent the main challenges in these techniques [1].

Table 4. Reference ranges of values for different printing methods (values taken from [122]).

Printing Technology	Resolution [µm]	Viscosity [cP]	Thickness [µm]	Speed
3D printing	≈10–100	≈105–108	>50	≈4 (m min⁻1)
Screen printing	≈30–100	≈100–107	≈10–100	≈70 (m min⁻1)
Inkjet printing	≈10–50	≈1–100	≈0.5–5	≈1 (m min⁻1)

presents the reference ranges of key parameters for the most employed additive manufacturing techniques used with MXene-based inks, highlighting their potentially achievable resolutions; more details can be found in [122]. Inks typically include both organic and inorganic solvents to ensure optimal printability by adjusting viscosity, controlling moisture levels, and enhancing mechanical properties [1]. Solvents such as dimethyl sulfoxide (DMSO), N-dimethylformamide (DMF), isopropyl alcohol, N, and N-methyl-2-pyrrolidone (NMP) are commonly used to facilitate the proper dispersion of MXene and to limit oxidation, which can compromise the ink stability and durability [111]. Additionally, small amounts of additives are often incorporated to enhance specific properties; for example, alkalis in water-based inks are used to adjust pH levels and improve durability [1]. However, the aim is to develop additive-free inks to simplify the printing process. In this regard, aqueous-based inks with varying concentrations of Ti₃C₂T_x thin layers, characterized by large lateral dimensions, have been formulated [123]. These inks exhibit optimal rheological properties, making them suitable for extrusion printing through a 250 µm diameter nozzle without the need for additives [111].

In recent years, MXenes and 2D materials have gained much attention in the field of 3D printing due to their exceptional properties, including high superficial conductivity, hydrophilicity, and outstanding rheological characteristics [1]. Additive manufacturing offers new opportunities for producing MXene-based structures with a high degree of precision and excellent performance in terms of flexibility, electrical conductivity, and mechanical robustness, capabilities that are difficult to achieve using traditional manufacturing methods. However, further research efforts are required to optimize MXenes for large-scale production through 3D printing techniques [1].

The remarkable hydrophilic nature of MXene nanosheets makes them highly suitable for dispersion in aqueous solvents. The main challenges associated with these solutions are oxidation and long-term stability [1]. This issue is worse at high temperatures, with prolongated light exposition, and in colloidal suspensions of single-layer or few-layer MXenes due to their high surface-to-volume ratio. Oxidation is commonly mitigated through refrigeration, storing MXene suspensions at 4 °C or lower, and by avoiding light exposure. Nevertheless, storage in organic solvents, such as ethanol, propylene carbonate (PC), and dimethyl sulfoxide (DMSO), has proven to be more effective in enhancing stability [124]. On the other hand, organic solvents implementation during ink formulation generates problems in terms of process sustainability. Aqueous-based inks represent a greener alternative, as their preparation does not involve the evaporation of toxic compounds (i.e., volatile organic compounds (VOC)) that can cause health and environmental risks, and they are more readily accessible [111]. In order to improve durability, antioxidant additives such as sodium L-ascorbate or polyanionic salts can be incorporated [97]. Furthermore, it has been observed that the concentration of MXenes in liquid-based inks significantly affects their stability, with more stable inks achieved at concentrations not exceeding 45 mg/mL [125].

The size of MXene sheets are another critical factor to consider when formulating inks, as many printing techniques have strict requirements regarding the maximum lateral size. This parameter plays a key role in determining the properties of MXenes: larger flakes exhibit higher electrical conductivity and greater UV-visible light absorption compared to smaller ones [125]. These characteristics are particularly important for applications in electronics and electromagnetic shielding.

As previously mentioned, the rheological properties of MXene-based inks are crucial for achieving high-quality printing and ensuring compatibility with specific techniques. Ink performance is influenced by the relationship between viscosity (η) and shear rate, as well as the G′/G″ ratio, where G′ represents the elastic modulus and G″ the viscous modulus (Figure 17) [126].

MXene-based inks with a concentration of approximately 300 mg/mL, an elastic modulus of ~104 Pa, and a shear yield stress of ~102 Pa have been identified as ideal candidates for FDM and DIW techniques. For instance, an MXene-based scaffold was successfully printed using the DIW method [127], achieving outstanding electrical conductivity (1013 S/m) and excellent EMI shielding performance. This was accomplished with an MXene water-based ink formulated with graphene oxide microgels [1].

In general, liquids exhibiting shear-thinning non-Newtonian (pseudoplastic) behavior are highly suitable for extrusion-based printing techniques [128]. Appropriate pseudoplastic and thixotropic properties enable inks to flow smoothly through printer nozzles while rapidly recovering their original viscosity, thus preserving the designed pattern. It has been demonstrated that increasing the concentration of MXenes leads to a corresponding increase in viscosity. Additionally, a high elastic modulus is essential to maintain the structural integrity of printed objects [124]. A study investigated the addition of 1.5 wt% 2D MXene to alumina ink for Direct Ink Writing (DIW). The MXene significantly improved the ink’s rheological properties, enhancing extrudability and shape fidelity by reducing the storage modulus and yield stress. The composite ink exhibited strong shear-thinning behavior and better viscosity recovery after high shear, enabling smoother extrusion, improved dimensional accuracy, and stable filament deposition during 3D printing [129]. Figure 18 illustrates the different rheological behaviors of single-layer and multi-layer MXenes, providing theoretical support for selecting the appropriate Ti₃C₂T_x concentration for various printing techniques [126].

Table 5. Comparison between printable MXene inks and traditional functional inks (Sc, supercapacitors; MSc, micro-supercapacitors).

Material	Fabrication Technique	Concentration	Solvent	Viscosity	Application	Ref.
Ti3C2Tx	Stamping	22 mg mL⁻1	Water	1.37 Pa.s	MSc	[131]
MXene (Ti3C2Tx)	Extrusion 3D printing	36 mg mL⁻1	Water	0.71 Pa.s	MSc	[132]
MXene (Ti3C2Tx)	Inkjet printing	12.5 mg mL⁻1	NMP	13.8 m Pa.s	MSc	[132]
MXene (Ti3C2Tx)	Inkjet printing	2.1 mg mL⁻1	DMSO	12.8 m Pa.s	MSc	[132]
MXene (Ti3C2Tx)	Inkjet printing	0.8 mg mL⁻1	ethanol	7.3 m Pa.s	MSc	[132]
N-doped MXene	Screen printing	-	Water	>104 Pa.s	Sc	[133]
AC/CNT/MXene-N/GO	Extrusion 3D printing	-	Water	>104 Pa.s	Sc	[133]
Pure MXene	Screen printing	-	Water	288.2 Pa.s	MSc	[134]
M-A	Screen printing	-	Water	371.7 Pa.s	MSc	[134]
R-M-A	Screen printing	-	Water	234.4 Pa.s	MSc	[134]
R-M-A0.75:1	Screen printing	-	Water	179 Pa.s	MSc	[134]
2D Ti3C2Tx	Extrusion-based 3D printing	15 mg mL⁻1	Water	>103 Pa.s	Sc	[135]
2D Ti3C2Tx	Extrusion-based 3D printing	30 mg mL⁻1	Water	>103 Pa.s	Sc	[135]
2D Ti3C2Tx	Extrusion-based 3D printing	50 mg mL⁻1	Water	>10⁴ Pa.s	Sc	[135]
RuO2/PEDOT: PSS/Graphene	Screen printing	-	Water	>103 cP	Sc	[136]
V2O5/GO and G- VNQDs/GO	Extrusion 3D printing	50 mg mL⁻1	-	>10⁴ Pa.s	MSc	[137]
GO LFP/GO and LTO/GO	Extrusion 3D printing	-	Water	102 to 103 Pa.s	Batteries	[138]
B-phosphorous	Inkjet printing	≈5 gL⁻1	IPA/2-butanol	≈2 mPa.s	Optoelectronics and photonics	[139]
Graphene MoS2 WS2 and hexagonal boron nitride (h-BN)	Inkjet printing	-	Water	Between 1.38 and 1.27 mPa.s	Biocompatibility (cytotoxicity studies)	[140]
MoS2	Inkjet printing	0.1 mg mL⁻1	Terpineol	≈40 cP	-	[141]

shows a selection of rheological parameters pertaining to both MXene-based inks and conventional ink formulations employed in additive manufacturing processes, along with an indication of their respective application domains; specific details can also be found in [130]. Among the various 3D printing methods, extrusion and inkjet techniques emerge as the most commonly employed for processing MXene-based inks.

5.3. Emerging Applications of 3D-Printed MXene-Based Materials

In this section, the applications of 3D-printed MXene-based materials will be discussed, highlighting their potential in various fields. This overview focuses on the most significant uses and the advantages they offer in each specific area with several examples and an outlook on future perspectives.

5.3.1. Biomedical Applications

The large surface area, conductivity, exceptional biocompatibility, and antibacterial properties of MXenes open promising perspectives for mimicking natural tissues and accelerating the regeneration of bone, neural, and cardiac tissues through the use of 3D printing, which opens new horizons in the field of biomaterials [142]. For instance, in vivo studies on nude mice and Sprague Dawley rats have demonstrated that integrating 2D Ti₃C₂ MXene with 3D-printed bioactive glass (BG) scaffolds effectively promotes tissue regeneration and induces bone tumor ablation via NIR irradiation. This is attributed to the exceptional photothermal conversion capability of MXene and the bone-regenerating properties of BG. As a result, the developed TBGS represents a promising approach for the postoperative treatment of osteosarcoma [143]. In vivo and in vitro studies of Ti₃C₂ MXene composite scaffolds produced through 3D printing extrusion techniques with remarkable biomimetic and mechanical properties, confirm promising performances in bone tissue regeneration, exhibiting high osteogenic and osteoblastic differentiation of BMSCs [144]. Furthermore, a bioink has been designed for 3D printing, combining human mesenchymal stem cell-laden hydrogels with MXene nanoparticles (GelMA/HAMA-MXene bioink) to facilitate bone regeneration, mimicking the natural extracellular matrix. This bioink exhibits outstanding physicochemical properties, including enhanced osteogenic activity and excellent cell adhesion, improving cell proliferation, and ultimately accelerating tissue regeneration [145].

The exceptional properties of MXenes have been utilized in the 3D and 4D printing of wearable biosensors. By combining the excellent conductivity and mechanical properties of MXenes with the versatility of additive manufacturing, these devices achieve enhanced performance in terms of sensitivity and responsiveness to external stimuli. For example, 3D-printed MXene-based biosensors are used for real-time monitoring of heartbeat, respiration, and pressure [4]. A wearable sensor was fabricated by printing an aqueous-based ink containing Ti₃C₂T_X MXene onto a thin styrene–ethylene–butylene styrene (SEBS) substrate [146], resulting in a highly efficient triboelectric material. This sensor was applied for artery pressure detection, demonstrating high sensitivity (6 kPa⁻¹) and requiring no external power source, as it was fully powered by human motion (Figure 19) [4].

Further research is needed to enhance processes scalability and to ensure enduring biocompatibility [142].

5.3.2. Electronic Applications

In recent years, the demand for micro-supercapacitors (MSCs) with high flexibility, energy density, and durability has steadily increased. Traditional manufacturing techniques are often impractical for large-scale production, and additive manufacturing is the perfect candidate to overcome this issue and further improve customization and the intrinsic properties of the final product [147]. Zhang et al. [131] developed an MSC by printing Ti₃C₂T_x aqueous ink onto porous hydrophilic paper, achieving an exceptional areal capacitance of 61 mF/cm² at 25 µA/cm² and 50 mF/cm² at 800 µA/cm², highlighting its potential for mass production via pad and cylindrical stamping [148]. Zhou et al. [149] successfully fabricated an MXene/CNF ink for energy storage and conversion applications. The incorporation of CNFs enhanced ink printability and rheological properties, mitigating MXene restacking and enabling the production of freestanding conductive structures. This ink was utilized for 3D printing a solid-state interdigitated SSC device, which demonstrated excellent volumetric capacitance and impressive rate performance, achieving, respectively, 2.02 F/cm² at 1 mA/cm² and 1.14 F/cm² at 20 mA/cm². Furthermore, it maintained 85% of its initial capacitance after 5000 cycles, indicating outstanding stability [149]. Optimizing ink rheology is essential for high-quality 3D-printed MSCs. While aqueous-based MXene inks and Ti₃C₂T_x organic inks, such as Ti₃C₂T_x-ethanol, are widely used in extrusion and inkjet printing for MSCs and ohmic resistors on flexible substrates, additive-free formulations remain highly desirable for simpler, more cost-effective, and efficient production (Figure 20) [147].

The field of portable electronics has been steadily expanding, driving an increasing demand for high-performance batteries to serve as reliable power sources. The primary goal is to develop batteries with outstanding mechanical properties, such as flexibility, alongside superior electrochemical characteristics, including high capacitance, remarkable energy densities, and minimal self-discharge rates [4]. MXenes have emerged as promising candidates for energy storage applications due to their unique properties. The integration of additive manufacturing and MXenes offers a cost-effective, scalable, and straightforward approach, such as 3D printing, to fabricate complex structures with customizable properties [124]. Among energy storage technologies, lithium-ion capacitors and batteries are the most extensively studied [4]. Zhang et al. [150] formulated Si/Ti₃C₂T_x (MX-C) and Ti₃CNT_x (MX-N) inks to fabricate composite electrodes for lithium-ion batteries. By coating a copper sheet with a mixture of nanoscale silicon powders (nSi), graphene-wrapped silicon (Gr-Si), and MXene, they achieved excellent rheological properties, reaching thicknesses of 650 µm for nSi and 2100 µm for Gr-Si, enabling high mass-loading. It was observed that a 30% MXene concentration in the silicon powder significantly enhanced conductivity. Additionally, the nSi/MX-C anode demonstrated stable charge/discharge performance over 280 cycles, whereas the nSi/MX-N anode exhibited inferior stability. Further research is required to enhance scalability and mitigate lithium dendrite formation, which can negatively impact battery performance [124].

5.3.3. Electromagnetic Shielding Applications

The increase in high-frequency circuits and wireless technologies has increased electromagnetic radiation, raising concerns about its interference with devices and effects on health [1]. Electromagnetic waves, capable of propagating through free space without a physical medium, facilitate long-distance communication by carrying electromagnetic energy [151]. However, they can cause signal disruptions, data loss, and hardware damage in electronic systems [1]. To eliminate these effects, high performance shielding materials are required to reflect, absorb, or dissipate electromagnetic energy [151]. MXenes have emerged as highly promising candidates for EMI shielding due to their exceptional electrical conductivity, mechanical flexibility, and durability making them an efficient solution for reducing electromagnetic radiation [1]. Additive manufacturing is an opportunity to improve MXenes’ performances through a simple and cost-effective method. Generally, 3D EMI shielding structures are printed via layered deposition techniques, fabricating high precision, porous, and internally isolated materials [151]. For instance, Liu et al. [152] developed a Ti₃C₂-MXene functionalized PEDOT:PSS aqueous-based ink optimized for additive manufacturing. This hydrogel exhibited excellent printability, high structural precision, and superior mechanical properties, including reversible stretchability and robustness. However, its most valuable features were its exceptional durability and conductivity. Combined with the porous structure and high water content of the hydrogel, these properties conferred remarkable EMI shielding capabilities even at reduced thicknesses. Potential applications for this material include energy storage, wearable miniaturized electronics, and water treatment technologies (Figure 20) [152]. Another notable example is a highly printable MXene/AlOOH ink designed for EMI shielding and EMI-thermochromic applications. Structures fabricated using DIW technology demonstrated excellent conductivity of 5323 S/m, tunable electromagnetic shielding effectiveness ranging from 25 dB to 80 dB in the X-band, and outstanding mechanical robustness, enabling them to support loads up to 3000 times their own weight. Additionally, thermochromic properties were achieved by integrating printed MG patterns with a thermochromic PDMS layer, allowing the structure to change color by converting electromagnetic energy into heat. These unique features open new possibilities for applications in warning labels and equipment for high-intensity EMI radiation environments (Figure 21) [151].

6. Conclusions

The advancements in 2D materials, particularly MXenes, have opened new avenues for scientific and technological progress. Their unique combination of electrical conductivity, mechanical strength, chemical stability, and surface tunability has positioned them as highly promising materials across various fields. From energy storage and water treatment to biomedical applications and electromagnetic shielding, MXenes have demonstrated exceptional versatility and functionality. A key development in this field is the integration of MXenes into 3D printing technologies. The ability to precisely control structure and composition through additive manufacturing has significantly expanded the potential applications of these materials. 3D-printed MXene-based materials exhibit enhanced performance in supercapacitors, batteries, and other electrochemical storage systems. This thesis has provided an in-depth analysis of the synthesis, properties, and applications of MXenes, with a particular focus on their role in 3D printing. The synergy between these advanced materials and additive manufacturing has the potential to revolutionize industries. However, challenges such as optimizing large-scale production, improving material stability, and, generally, refining processing techniques remain areas of active research. Future studies should aim to enhance the scalability and cost-effectiveness of MXene synthesis while exploring novel composite formulations to further improve their performance in different applications. The continuous evolution of MXene research, combined with advancements in 3D printing technologies, will undoubtedly drive their industrial adoption and contribute to the development of innovative, high-performance materials for energy, healthcare, and beyond.