Effect of MXene Nanosheet Sticking on Supercapacitor Device Performance

Featured Application

The application involves exploring the impact of MXene nanosheet adhesion on the performance of supercapacitor devices for potential enhancements in energy storage and overall device functionality.

Abstract

Supercapacitors have garnered significant interest in recent years due to their high power density, rapid charge/discharge rates, and long cycle life. MXenes, a family of two-dimensional (2D) transition metal carbides/nitrides, have emerged as promising electrode materials for supercapacitors. However, one major challenge associated with incorporating MXenes in supercapacitor structures is the occurrence of sticking, wherein individual MXene flakes agglomerate, leading to reduced electrode performance. This review paper discusses various causes of sticking and approaches to preventing it, offering insights into the design and development of high-performance MXene-based supercapacitors. The morphology and size of MXene flakes, flake surface chemistry, thickness, surface area/volume ratio, electrode processing techniques (including solvent selection, additives incorporation, and deposition technology), and environmental factors were shown to be the basic factors resulting in sticking of MXene sheets. Among the strategies to mitigate this challenge, surface functionalization and passivation, integration with polymer matrices or carbon nanomaterials, and electrode processing optimization were considered. Possible paths for optimization and future directions of study, such as novel MXene compositions, understanding of interfaces and electrode–electrolyte interactions, development of advanced electrode architectures, and integration of energy storage systems, were assumed.

1. Introduction

1.1. Background

MXenes, a family of two-dimensional transition metal carbides, nitrides, and carbonitrides, have attracted considerable interest in various fields due to their exceptional properties [1,2,3]. When it comes to their application in supercapacitors, MXenes offer several advantages. Their high surface area, mechanical strength, and electrical conductivity make them promising materials for energy storage devices like supercapacitors and batteries. Additionally, MXenes are capable of accommodating large numbers of ions on their high surface area, enhancing their charge storage capacity [4,5,6]. This feature is essential for supercapacitors, where the storage of electrical energy in the form of ions at the electrode–electrolyte interface is crucial. As a result, MXenes have the potential to offer high energy and power density in supercapacitor applications. The exceptional mechanical strength of MXenes ensures that the supercapacitor electrodes maintain their structural integrity during charging and discharging cycles. This property is important for achieving long-term stability and durability, as it prevents electrode degradation and maintains the overall performance of the energy storage device over time. The excellent electrical conductivity of MXenes facilitates efficient charge transfer within the supercapacitor electrodes. This attribute enables rapid ion transport and electron mobility, leading to enhanced charge/discharge rates and overall electrochemical performance [7,8].

Despite these advantages, one of the primary challenges with MXenes lies in their tendency to stack together, potentially hindering ion accessibility and electron transport within the supercapacitor electrode structure. This issue can impact the overall performance of supercapacitors utilizing MXenes. When MXene nanosheets stick together or aggregate, it hinders the accessibility of the active surface area which is essential for the adsorption of electrolyte ions. This reduced accessible surface area can lead to diminished charge storage capacity and lower overall energy density of the supercapacitor. Sticking of MXene nanosheets can result in elevated charge transfer resistance within the electrode material. This resistance impedes the movement of ions during the charge and discharge cycles, leading to slower kinetics and reduced power performance of the supercapacitor. Aggregated MXene nanosheets can hinder the diffusion of the electrolyte throughout the electrode material, leading to non-uniform electrolyte distribution. This can limit the overall utilization of the active material, impacting the overall capacitance and energy storage capability of the supercapacitor and batteries [9,10] due to localized overcharging or discharging, leading to reduced life cycle and potential safety risks within the storage element.

Various approaches have been explored to mitigate these issues, such as intercalation technology and surface modifications with other nanomaterials. These strategies aim to improve the dispersion of MXene nanosheets and their adhesion to the supercapacitor electrodes, thereby enhancing the electrochemical performance and addressing the challenges associated with aggregation. Effectively tackling the obstacles related to nanosheet sticking and aggregation could pave the way for the widespread utilization of MXenes in supercapacitors, leading to the development of high-performance energy storage devices with improved efficiency and reliability.

1.2. Objectives of the Study

The objective of this review paper was to explore the reasons behind the sticking of MXene sheets and to compare approaches for preventing its effects on supercapacitor structure performance. The paper intends to provide comprehensive information on the various strategies and techniques aimed at mitigating the challenges associated with MXene nanosheets by systematically reviewing and analyzing the existing literature, research, and developments in this area to provide an in-depth understanding of the different approaches and their effectiveness in addressing the issue.

2. MXenes as Supercapacitor Electrodes

2.1. Overview of MXenes

The exploration of graphene, a groundbreaking two-dimensional (2D) material, marked the beginning of a new chapter in materials science. This significant achievement paved the way for the investigation of additional 2D materials with properties and potential applications similar to graphene’s. Graphene, with its single-layered structure, is renowned for its remarkable conductivity at room temperature and its ability to transmit 97.7% of light in the visible spectrum, positioning it as one of the most extensively researched 2D materials nowadays. The distinct properties and broad applications of 2D materials can be attributed to their larger specific surface area and quantum confinement effects.

Following the success of graphene, a wide variety of 2D materials have been explored, including transition metal dichalcogenides (TMDs), metal–organic frameworks (MOFs), metal-oxide compounds, hydroxides, hexagonal boron nitrides, germanene, phosphorene, silicene, and monoelemental 2D Xenes like graphdiyne, arsenene, borophene, bismuthene, antimonene, and tellurene [11,12,13]. The attractiveness of these materials stems from their atomically thin layers, distinct electronic structures, generous surface areas, high surface/volume ratios, adaptability, and resilience to mechanical stress, distinguishing them from their bulk equivalents [14].

However, the most groundbreaking development in the context of 2D materials lies in the emergence of MXenes. These materials, termed the “wonder materials” of the 2D world, have introduced a new paradigm in material science research [15]. MXenes are part of the 2D material family, hailing from layered transition metal carbides, nitrides, or carbonitrides. They are obtained by selective wet chemical etching processes, exfoliating them from their parent MAX phases [16]. The name “MXene” is reminiscent of graphene, a nod to their analogous properties [17]. The precursor of MXenes, known as the MAX phase, possesses a chemical formula of M_(n+1)AX_n (n = 1 to 4), where M stands for an early transition metal (e.g., Ti, V, Cr), A represents an element from groups 13 to 15 of the periodic table (e.g., Al, Ga, Si), and X includes carbon and/or nitrogen.

MAX phases are characterized by a hexagonal layered structure within a space group of D6h4-P63/mmc. This structure consists of alternating M and A layers, with X atoms occupying octahedral sites. Notably, the M–X bond displays metallic, covalent, or ionic characteristics, while the M–A bond is primarily metallic, yet slightly weaker than the M–X bond. This subtle weakening facilitates the chemical etching process for obtaining MXenes [16].

The selective chemical etching process leads to the removal of covalently bonded A layers from MAX phases, culminating in the formation of MXenes as M_(n+1)X_nT_x. The “T” in the formula signifies surface termination groups, such as –F, –Cl, –OH, or –O, and “x” represents the number of terminations. Generally, higher “n” values correspond to increased MXene stability. The surface termination groups, originating from the chemical etching process, impart hydrophilicity to MXene surfaces and influence their electronic and ion transport properties, ultimately impacting their conductivity [17].

The discovery of the first MXene material, Ti₃C₂T_x, in 2011 marked a watershed moment. It was etched from its parent MAX phase, Ti₃AlC₂, using hydrofluoric acid (HF) as the etchant [15]. Subsequent research led to the synthesis of various MXenes, including Ti₂CT_x, Zr₃C₂T_x, Nb₂CT_x, Nb₄C₃T_x, V₂CT_x, Ti₃CNT_x, Mo₂CT_x, Ti₄N₃T_x, Mo₄VC₄T_x, Mo₂ScC₂T_x, (Ti_0.5, Nb_0.5)₂CT_x, (Nb_0.8, Ti_0.2)₄C₃T_x, and (Nb_0.8, Zr_0.2)₄C₃T_x, among others. While over 100 MAX phases have been identified, only more than 30 MXenes have been synthesized and experimentally assessed, fueling ongoing research into the discovery of novel MXenes and their distinctive properties [18].

MXenes have become distinguished for their unique blend of metallic (transition metal atoms) and ceramic (carbon/nitrogen atoms) properties. This distinctive combination has resulted in exceptional attributes: high metallic conductivity reaching up to 6000–8000 S cm⁻¹, excellent thermal conductivity, remarkable mechanical stability, exceptional optical properties, impressive electric and magnetic properties, outstanding hydrophilicity, customizable surface functional groups, and intercalation capabilities [19,20]. A 3D VN/MXene composite structure was developed for aqueous zinc-ion batteries, enhancing storage capacity and service life [4]. By encapsulating VN microspheres in MXene nanosheets, the electrode demonstrated high reversible capacity, superior rate performance, and exceptional stability over 2200 cycles. This design strategy offers insights into advanced cathode materials for zinc-ion batteries. These exceptional properties have positioned MXenes as a versatile material suitable for a wide range of practical applications.

The tiny lateral size, atomic-scale thickness, and remarkable hydrophilic ability facilitate the creation of flexible, thin MXene layers through processes like vacuum filtration and spraying technology [21]. MXenes’ outstanding conductivity enables higher power density compared to metal-oxide semiconductors, ultimately contributing to fast device charging [22]. The tunable surface chemistry, a result of the HF acid etchant, makes MXenes an ideal material for efficient composite electrodes [23]. The swift electron transfer between MXene layers renders them an excellent substrate for catalysis. The characteristics and uses of MXenes demonstrate diversity based on elemental compositions, stoichiometry, synthesis techniques, interlayer distances, layer thickness, and lateral flake dimensions [24].

Traditionally, energy storage devices tend to be rigid and inflexible, limiting their application in emerging domains like wearable electronics, smart garments, and flexible displays [25]. This need has spurred research into devices that provide not just high energy storage capability but also the necessary flexibility and resilience for such uses. In this context, MXene materials have demonstrated significant potential in wearable electrochemical energy storage elements, thanks to their distinctive layered arrangement, abundance of surface terminations, superior electrical conductivity, hydrophilicity, and large specific surface area [26]. The myriad applications of MXenes in electrochemical energy storage devices inspire a comprehensive overview of their synthesis, microstructure, properties, and prospects, factors affecting their electrochemical behavior in supercapacitors, charge storage mechanisms, and state-of-the-art supercapacitors based on MXene composites.

2.2. Advantages of MXenes in Supercapacitors

Supercapacitors, also known as ultracapacitors or electric double-layer capacitors (EDLCs), are advanced energy storage devices that have gained increasing attention due to their high power density, rapid charge/discharge capabilities, and extended cycle life. Unlike traditional batteries, which store energy through chemical reactions, supercapacitors store energy electrostatically. This fundamental difference makes supercapacitors an appealing technology for applications requiring quick bursts of power, such as regenerative braking in electric vehicles, grid energy storage, and consumer electronics. However, despite their advantages, supercapacitors face challenges related to their energy density, which is typically lower than that of batteries.

2.2.1. MXenes with Various Materials: A Journey to Enhanced Supercapacitors

Research into supercapacitor technology has been characterized by a relentless pursuit of higher energy density, faster charging and discharging, and longer cycle life. The versatility of MXenes and their compatibility with various materials have opened new avenues for achieving these goals.

Zhao et al. [27] conducted pioneering research on MXene–Carbon Nanotube (CNT) composites, producing a sandwich-like structure by alternately filtering MXene and CNT dispersions. This innovative approach led to significantly improved volumetric capacitance, reaching 350 F cm⁻³ at 5 Ag⁻¹. By comparison, pure MXene or randomly mixed MXene–CNT papers fell short of this performance. These findings point to the potential of MXene–CNT composites in supercapacitor applications. Additionally, an MXene/rGO (reduced graphene oxide) composite demonstrated an exceptional volumetric capacitance of 435 F/cm³ with outstanding cycling stability [27]. These results suggest the versatility of MXenes, not only when combined with CNTs but also with other carbon-based materials.

Park et al. [28] contributed to the MXene supercapacitor field by constructing an asymmetric yarn supercapacitor. This innovative device utilized a 100 µm thick MXene–CNT anode and MnO₂/CNT cathode electrodes combined with a PVA/LiCl gel electrolyte, achieving an energy density of 100 mWh/cm². Such creative applications underscore the adaptability of MXenes in forming diverse supercapacitor designs.

A unique fabrication technique was employed by researchers who utilized a Ti₃C₂T_x MXene–CNT composite electrode formed by photolithography followed by vacuum filtration. This approach yielded an impressive areal capacitance of 61.38 mF cm⁻² at 0.5 mA cm⁻². This enhanced performance can be attributed to CNT intercalation between the MXene layers, which facilitates ion diffusion [29].

Furthermore, Ti₃C₂T_x MXene–multiwalled CNT (Ti₃C₂T_x MXene–MCNT) composite cathodes, in combination with polypyrrole-coated MCNT anodes, assembled with Na₂SO₄ electrolyte, created asymmetric supercapacitors exhibiting an areal capacitance of 0.94 F cm⁻² within a voltage window of 0–1.6 V [30]. These remarkable results underscore the potential for MXene-based supercapacitors to become a pivotal energy storage solution.

2.2.2. MXene–Carbon Allotropes: Pioneering Energy Storage Innovation

In the realm of supercapacitors, MXenes have not limited themselves to collaborating solely with CNTs but have also ventured into partnerships with various carbon allotropes. Li et al. [31] demonstrated that MXene combined with acid-activated carbon (AAC) could prevent the aggregation of 2D MXene sheets while increasing the interlamellar spacing. This breakthrough resulted in significantly improved electrochemical performance. An asymmetric supercapacitor was formed with MXene/AAC (2:1) as the positive electrode and AAC as the negative electrode, achieving a specific capacitance of 177 F g⁻¹ at 0.5 Ag⁻¹, with an impressive 97.4% retention over 10,000 cycles. This achievement stands as a testament to the potential of MXene–carbon allotrope composites in extending the life and performance of supercapacitors [31].

This progress has extended to micro supercapacitors, which are gaining prominence in various miniaturized energy storage applications. A micro supercapacitor incorporating MXene as the cathode and activated carbon as the anode, immersed in a neutral PVA/Na₂SO₄ electrolyte, showed an expanded potential window of 1.6 V. Even after 10,000 cycles, this supercapacitor maintained 91.4% of its capacitance, yielding an areal capacitance of 7.8 mF cm⁻², an energy density of 3.5 mWh cm⁻³, and a power density of 100 mW cm⁻³ [32].

A significant breakthrough occurred with the development of the Ti₃C₂T_x MXene–carbon quantum dots (MXene–CQD) composite electrode, which demonstrated an outstanding capacitance of 441.3 F g⁻¹ at 1 Ag⁻¹. Impressively, it retained 100% of its capacitance after 10,000 cycles at a current density of 10 Ag⁻¹ [33]. This notable increase in capacitance can be ascribed to the excellent conductivity and increased surface area of Ti₃C₂T_x MXene sheets, leading to a higher double-layer capacitance. By preventing layer restacking and activating surface functional groups with CQDs, the performance of the MXene–CQD composite electrode was further enhanced. An innovative approach was used to create a hybrid supercapacitor by spray-injecting Ti₃C₂T_x MXene onto a nickel–cobalt sulfide (NiCO₂S₄) material previously deposited on a carbon cloth (CC) substrate. This process resulted in the formation of a composite material known as Ti₃C₂T_x/NiCO₂S₄@CC, which is abbreviated as TNSC. TNSC showed a gravimetric capacitance of 2326 F g⁻¹ at 1 Ag⁻¹, with superior cyclic stability of 93.8% at a current density of 10 Ag⁻¹. This enhancement in the electrochemical behavior of the complex electrode, achieved by modulating the mass-loading of Ti₃C₂T_x MXene, contributes to electrical conductivity and manages ion permeability to the nickel–cobalt sulfide. The outcome was a semi-solid flexible supercapacitor capable of a power density of 800 W kg⁻¹ at an energy density of 57.5 Wh kg⁻¹, accompanied by a coulombic efficiency of 99.2% and 90.2% of its capacitance retained beyond 5000 cycles [34].

The advantages of hydrogel electrolytes has also come to the forefront. A zinc ion hybrid micro supercapacitor was constructed using battery-type vanadium pentoxide as the cathode and capacitor-type Ti₃C₂T_x MXene as the anode, set in a ZnSO₄ dissolved PAM hydrogel electrolyte. This micro supercapacitor displayed an areal capacitance of 129 mF cm⁻² at a current density of 0.34 mA cm⁻² and an energy density of 48.9 mWh cm⁻² at 673 mW cm⁻². Impressively, it retained 77% of its capacitance over 10,000 cycles. The adoption of PAM hydrogel electrolytes offered enhanced security and contributed to greater cycling stability for the supercapacitor electrodes when compared to organic electrolytes [35].

2.2.3. MXene–Polymer Composites: A Synergistic Approach to Enhanced Supercapacitors

While MXenes, in combination with various materials, exhibited extraordinary progress in supercapacitor technology, the stacking of MXene layers and diminishing electrochemical performance with increasing electrode thickness remained concerns. These limitations prompted researchers to explore composite structures of MXenes with polymers, offering flexibility and enhanced electrochemical performance.

One key consideration was the use of single-layered MXene sheets, which possess higher surface hydrophilicity and superior compatibility with polymers due to their high aspect ratio compared to multilayered MXene sheets [36]. Researchers embarked on a journey to unlock the potential of MXene–polymer composites to overcome these limitations.

Yan et al. [37] significantly improved the specific capacitance of MXene by electrochemically depositing conducting polypyrrole (PPy) on MXene textile electrodes, resulting in a specific capacitance of 343.20 F g⁻¹ compared to the pristine MXene electrode with a specific capacitance of 182.70 F g⁻¹. A solid-state symmetrical supercapacitor fabricated with the MXene–PPy electrode showcased an energy density of 1.30 mWh g⁻¹. This innovation illustrated the promise of MXene–PPy composite materials in supercapacitor applications [38] with not only improved tensile strength but also resulting in a specific capacitance of 614 F g⁻¹ at 1 A g⁻¹. Crucially, this impressive performance retained 100% of its capacitance over 10,000 cycles. PPy nanoparticles and ionic liquid-based microemulsions were incorporated into Ti₃C₂T_x MXene layers, acting as dual spacers that elevated the electrochemical performance of supercapacitor electrodes. The PPy particles prevented MXene layer restacking, while the ionic microemulsions improve electrode wettability, enhancing the ion-accessible surface area. A symmetric supercapacitor produced with PPy–MXene–IL–mic composite showed an energy density of 31.2 Wh kg⁻¹ at room temperature. It maintained a capacitance retention rate of 91% over 2000 cycles and exhibited a coulombic efficiency of 91% over a temperature span of 4 °C to 50 °C [39].

Vahidmohammadi et al. [40] took a step further by synthesizing a Ti₃C₂T_x MXene composite through in situ polymerization of polyaniline (PANI) on the MXene surface. The composite exhibited volumetric and gravimetric capacitances of 1682 F cm⁻³ and 503 F g⁻¹, respectively. Moreover, the specific capacitance reached 336 F g⁻¹, with 98.3% capacitance retention after 10,000 cycles.

The scope of MXene–polymer composites extended to asymmetric supercapacitors, exemplified by an asymmetric supercapacitor composed of PANI0.7/MXene as the cathode and MXene as the anode in a 1M H₂SO₄ electrolyte. This configuration achieved an impressive energy density of 65.6 Wh L⁻¹ [41]. The performance of MXene/PANI nanoparticle composite symmetric supercapacitors was equally impressive, with an areal and volumetric energy density of 90.3 mWhcm⁻² and 20.9 Wh L⁻¹, respectively [41]. The heightened capacitance can be attributed to the effect of PANI nanoparticles, which intercalate between the MXene layers, preventing restacking and establishing an interconnected conductive channel that facilitates ion transport. The expansion of interlayer spacing by PANI nanoparticles enhanced the faradic charge storage mechanism, resulting in increased energy density for composite-electrode-based supercapacitors.

3. Factors Influencing Sticking in MXene Electrodes

3.1. Morphology and Size of MXene Flakes

The morphology of MXene flakes primarily refers to their overall shape, including their aspect ratio, edges, and surface roughness. It has been observed that MXene flakes with irregular shapes, jagged edges, or high surface roughness tend to exhibit a higher propensity for sticking. The irregularities on the surface of MXene flakes provide more sites for inter-flake adhesion, leading to higher agglomeration.

Ti₃C₂ was obtained after etching of Ti₃AlC₂ using hydrofluoric acid and lithium fluoride, and the interlayer gaps of the MXene nanosheets were investigated by transmission electron microscopy (TEM) [42]. Based on the image in Figure 1a, it is evident that the nanosheets exhibit nearly transparent properties, allowing for the visibility of individual layers (i.e., Ti and C layers) within a single Ti₃C₂ sheet. Upon closer inspection in the higher magnification image (Figure 1b), it is estimated that the thickness of a single layer Ti₃C₂ is approximately 1–2 nm. Given the observed layered morphological structure of MXene, it can be proposed as the “lamella” model.

A study on interdigital micro-supercapacitors has investigated how the electrochemical properties of MXene vary with flake size [43]. The authors’ findings reveal that nano-size MXene (200 nm) demonstrates greater ionic conductivity than its micron-size counterpart (1 μm). Conversely, larger flake sizes exhibit higher electron conductivities. Consequently, the capacitance of micron-size MXene surpasses that of nano-size MXene (200 nm) due to the dominance of electron conductivities.

3.1.1. Size of MXene Flakes

The size of MXene flakes also significantly influences the sticking behavior. Smaller flakes have a higher specific surface area compared to larger flakes, promoting stronger inter-particle interactions. As a result, smaller flakes have a tendency to clump together more easily, causing sticking. On the other hand, larger flakes, with a lower surface area/volume ratio, are less likely to stick to each other, reducing the tendency for adhesion between flakes and decreasing sticking.

So far, it has been found that the average particle sizes and conductivity of the sonication-derived layered MXene nanosheets decreased gradually from approximately 600 to 300 nm and from 667 to 2500 S cm⁻¹, respectively, as the sonication time increased. In contrast, the MXene nanosheets fabricated using the power-focused delamination (PFD) technique showed a much larger average diameter of approximately 4900 nm and an electrical conductivity of 8260 ± 130 S cm⁻¹ (film thickness: 11 µm), as can be seen on Figure 2 [44]. When comparing the electrical conductivity of the MXene materials with that of reported pure MXene films, it becomes apparent that the electrical conductivity of the PFD-synthesized MXene nanosheets was higher than that of the sonication-synthesized MXene nanosheets, highlighting the superiority of the PFD strategy. The LPFD-Ti₃C₂T_x MXene solution with large flakes (L) showed a 6.1 times higher proportion of the nanosheets compared to the small flakes (S) S-Ti₃C₂T_x, demonstrating the PFD strategy’s capability to produce exfoliated MXenes with high yields and larger sheet sizes. Furthermore, the surface area of LPFD-Ti₃C₂T_x (12.5 m²g⁻¹) was slightly higher than that of S-Ti₃C₂T_x (10.8 m²g⁻¹), potentially attributed to the compact stacking of MXene materials with smaller lateral size during the sonication process.

3.1.2. Flake Thickness

The thickness of MXene flakes is another important consideration. Thinner flakes possess higher flexibility and are more susceptible to conforming to the surface irregularities of neighboring flakes, resulting in enhanced sticking. On the other hand, thicker flakes tend to have lower conformability and exhibit reduced sticking due to reduced inter-particle contact.

In [45], a straightforward method to increase the yield of Ti₃C₂T_x few-layer flakes by reducing the precursor size has been proposed. By utilizing small 500 mesh Ti₃AlC₂ powders as the raw material, obtained through a facile ball milling process, a high yield of 65% has been successfully achieved. Furthermore, the resulting small flakes also demonstrated improved pseudocapacitor performance due to their excellent electrical conductivity, expanded interlayer space, and higher oxygen content on the surface. The Ti₃C₂T_x flakes in the TEM image (Figure 3) exhibit a transparent morphology, indicating that most of the received flakes are single-layer or few-layer.

At a scan rate of 2 mV/s, this electrode exhibits the highest gravimetric capacitance of 364 F/g. However, as the flake sizes increase, the capacitance of the electrodes noticeably decreases, with the lowest capacitance of 296 F/g achieved for an electrode prepared using 300 mesh MXene flakes. Additionally, the Ti₃C₂ 500 mesh electrode demonstrates stable cyclic performance, retaining 88% of its capacitance after 10,000 cycles.

It has been proved that a single-flake device eliminates the conductivity loss caused by the electronic transitions between stacked MXene sheets in layered films. Thus, single-flake devices are more favorable for obtaining high conductivities [46].

3.1.3. Flake Surface Chemistry

The surface chemistry of MXene flakes, such as the presence of functional groups or residual contaminants, can affect sticking behavior. Functionalization of MXene flakes with hydrophilic or hydrophobic moieties can alter the interfacial interactions, thereby influencing the propensity for sticking. Additionally, certain surface contaminants can act as adhesive agents, facilitating the agglomeration of MXene flakes.

When the initial MXene has been synthesized by immersion into highly concentrated HF, the “accordion” structure is considered indicative of successful synthesis [47]. Nonetheless, this accordion structure, which is synonymous with MXene, does not represent the appearance of all multilayer MXene. In Figure 4, SEM images of multilayer Ti₃C₂T_x synthesized under various conditions are depicted [48]. As the HF concentration decreases, the “accordion” structure becomes less prominent, and the multilayer MXene more closely resembles the typical MAX structure. Essentially, as the effective HF concentration increases, the production of H₂ becomes more rapid, resulting in an expanded structure. Conversely, at lower concentrations (and therefore slower kinetics), there is no morphological change in the particles.

3.1.4. Surface Area/Volume Ratio

The surface area/volume ratio of MXene flakes plays a crucial role in sticking. Higher surface area allows for more interaction sites, resulting in increased adhesion forces between flakes. As a result, MXene flakes with a higher surface area/volume ratio are more prone to sticking. However, Tao et al. have reported a porous MXene of nitrogen-doped Ti₃C₂, which is conditioned during electrospinning in such a way to promote self-stacking and facilitate electrolyte penetration [49]. The as-prepared N–MXene electrodes, exhibiting leaf-like shapes (Figure 5a), demonstrated specific capacitance about three times higher than MXene. Such a high capacitance can be ascribed to the combined contributions of the surface Faraday pseudocapacitance and electric double-layer capacitance that contribute equally in the case of a porous structure. As can be seen from Figure 5b, the application of melamine–formaldehyde (MF) microspheres as a template and nitrogen source results in the formation of a 3D porous design hindering the spontaneous stacking of MXene, which is not destroyed even after electrospinning on flexible substrate.

In the analyzed range, the distribution ratio reveals that 62.5% of the nanofibers have a size falling between 200 and 300 nm, with an average diameter of approximately 230 nm.

Understanding the factors influencing sticking in MXene electrodes, particularly the morphology and size of MXene flakes, is essential for developing effective strategies to prevent sticking and enhance the performance of MXene-based supercapacitors. By controlling these factors, one can minimize agglomeration and improve dispersion and overall electrode stability.

3.2. Surface Chemistry of MXene Flakes

The surface chemistry and hydrophobicity of MXene flakes are vital factors that influence the sticking behavior in MXene electrodes. In this paper, we provide a closer look at their impact.

3.2.1. Surface Chemistry

The surface chemistry of MXene flakes refers to the composition and functional groups present on their surface. MXenes typically exhibit functional groups such as –OH, –O, –F, or –Cl, resulting from the etching process used to synthesize them. The presence of these functional groups affects the electrostatic and van der Waals forces between adjacent flakes, thus influencing sticking behavior.

Surface functional groups can alter the surface charge of the MXene flakes, influencing inter-particle interactions. For instance, hydrophilic functional groups like –OH promote stronger inter-particle adhesion due to increased hydrogen bonding and dipole–dipole interactions. These interactions can facilitate the agglomeration of MXene flakes, resulting in sticking. On the other hand, hydrophobic functional groups can weaken attractive forces between the flakes, reducing the tendency to stick.

MXenes display a broad spectrum of surface terminations except –OH, including –O, –Cl, or –F (as depicted in Figure 6), attributable to the etching process during their synthesis. These terminations affect the surface chemistry, stability, and interactions of MXenes with other materials, emphasizing their versatility across various applications. Diverse techniques are employed for the synthesis and delamination of MXenes, resulting in varied outcomes related to surface chemistry, interlayer spacing, flake dimensions, and defect density. The choice of etching method significantly influences the surface chemistry (Figure 6a,b). The synthesis of MXenes in acidic solutions containing F-ions generally results in limited control over terminations. The dominant surface terminations consist of –F and –O/–OH, with the modulation of –F terminations achievable by adjusting acid concentrations. Additionally, the complete replacement of –F terminations with –O/–OH terminations can be accomplished through treatment with a base such as NaOH or TBAOH [50]. In molten salt environments, the cations undergo a Lewis acid–base reaction with the A layer, initially replaced by Zn. Subsequent removal of Zn results in MXenes with exclusive –Cl terminations (Figure 6c). Manipulation of the molten salt composition for MAX phase etching allows for the attachment of various halogen terminations in desired stoichiometries to the MXene surface. Beyond halogens (–Cl, –Br, and –I), a diverse range of terminations such as –S, –Se, –Te, –P, and –Sb can be uniformly applied to MXenes.

The attachment of elements to these terminations is an effective approach to prevent the self-stacking of the MXene layers and thus to maximize ion conduction and improve energy storage capabilities of MXene-based supercapacitors. The predisposition of certain atomic-thickness MXene nanosheets to undergo progressive layering is affected by the robust van der Waals interaction between the sheets. This layering process leads to the creation of a relatively large inactive volume, resulting in a reduced active surface area for ion storage and impeding optimal ionic transport pathways [52,53]. In addition to the incorporation of elements in the terminated sites between the sheets, enhanced ion transport can be achieved through the vertical orientation of the MXene sheets as opposed to the traditional horizontal alignment, as illustrated in Figure 7, offering a more accessible transport mechanism.

3.2.2. Hydrophobicity

Hydrophobicity, which is the tendency of a material to repel water, is another critical aspect of the surface chemistry of MXene flakes. MXenes are generally hydrophilic, meaning they have an affinity for water due to the presence of oxygen-containing functional groups. The hydrophilicity of MXene surfaces can contribute to sticking by facilitating water-mediated inter-particle interactions and promoting agglomeration.

To reduce sticking, researchers have focused on modifying the surface chemistry of MXene flakes to enhance their hydrophobicity. The introduction of hydrophobic functional groups, such as alkyl chains, can reduce the affinity of MXene surfaces towards water. This hydrophobic modification reduces water-mediated interactions and disfavors agglomeration, mitigating sticking issues in MXene electrodes.

The authors in [54] have reported functionalization of Ti₃C₂ with silylation reagents ((3-aminopropyl)triethoxysilane (APTES)) to tailor its hydrophilicity. The stability of Ti₃C₂ was improved by APTES functionalization by blocking the contact between water and MXenes. This improvement was also achieved for hexadecyltrimethoxysilane and 1H,1H,2H,2H–perfluorodecyltriethoxysilane (FOTS) (Figure 8). The results indicated the change in surface properties from hydrophilic to hydrophobic. Two superhydrophobic self-assembled monolayers (SAMs) [(3-chloropropyl)trimethoxysilane (CPTMS)] and FOTS were used for surface modification of MXenes via formation of covalent bonds between –OH and different surface functional groups. The water contact angle findings revealed a superhydrophobicity of 156° and 96° (not shown) for the fluorine-containing (Ti₃C₂–F) and chlorine-containing (Ti₃C₂–Cl) films, respectively, whereas the pristine Ti₃C₂ is inherently hydrophilic due to the presence of –O and –OH functional groups (exhibiting a water contact angle of 33°). Therefore, modifying Ti₃C₂ with superhydrophobic agents like FOTS and CPTMS could serve as an effective strategy to impede the degradation of MXene materials.

Recently, hydrophilic MXenes were transformed into hydrophobic forms through low-energy N⁺ ion irradiation at varying fluences [56]. The characterization techniques used confirmed the removal of surface functional groups after irradiation. Additionally, DFT simulations validated the significant impact of –OH and –O–OH groups on the hydrophilic properties of pristine MXene. Analysis of adsorption energy data indicated that the adsorption of water molecules on surface-terminal-group-free MXenes was energetically unfavorable, suggesting a hydrophobic nature. Ion irradiation was employed to selectively eliminate surface-terminating functional groups, thereby rendering MXenes hydrophobic. Furthermore, an impressive enhancement in electrical conductivity was observed with increasing ion fluence, which can be attributed to the preferential sputtering of functional groups and structural changes in MXene. Following ion irradiation at varying significant alterations in both morphology (Figure 9) and composition were observed. As the ion fluence increased, MXene layers gradually fused together, culminating in the formation of a densely interconnected structure at higher fluences. Analysis of the energy-dispersive X-ray spectroscopy (EDS) spectrum for irradiated samples at an ion fluence of 5 × 10¹⁶ ions cm⁻² revealed a reduction in the relative peak intensity for fluorine. Moreover, a post-irradiation decrease in the atomic percentage of oxygen was noted, indicating the rejection of oxygen groups from the sample surface.

It is worth noting that achieving a balance in surface chemistry is crucial. Extreme hydrophobicity can lead to poor dispersion and reduced accessibility of active sites, adversely affecting the electrode’s performance.

Understanding the influence of surface chemistry and hydrophobicity on sticking in MXene electrodes aids in designing effective surface modification strategies that promote better dispersion and prevent agglomeration. By tailoring the surface properties, researchers can enhance the stability and performance of MXene-based supercapacitor electrodes.

3.3. Electrode Processing Techniques

The choice of electrode processing techniques can significantly impact the sticking behavior in MXene electrodes, as well as the overall performance of MXene-based supercapacitors. Below are some key details:

3.3.1. Dispersion Method

The dispersion method used for MXene flakes is a crucial factor in preventing sticking. Traditional methods include sonication, mechanical stirring, or high-shear mixing to disperse MXene flakes in a solvent. The intensity and duration of sonication or mixing play a role in achieving optimal dispersion. Over-sonication or excessive mixing can lead to increased flake damage and agglomeration, resulting in sticking issues. Hence, careful optimization of the dispersion process is necessary to achieve a homogeneous and stable MXene dispersion.

3.3.2. Solvent Selection

The choice of solvent affects the dispersion stability of MXene flakes and consequently influences sticking. Solvents with high polarity, such as water or polar organic solvents, can promote strong interactions between MXene flakes, leading to increased agglomeration and sticking. Conversely, non-polar solvents or solvents with lower polarity can weaken inter-flake interactions and enhance dispersion stability, thereby reducing sticking. Thus, in solvent selection, one should consider the surface chemistry of MXene flakes and compatible solvents that minimize inter-particle adhesion.

It has been demonstrated that Ti₃C₂T_x can be dispersed in many polar organic solvents, but the best dispersions are achieved in dimethylformamide, propylene carbonate, and ethanol. By employing solvent analysis, various organic solvents can be identified as capable of dispersing Ti₃C₂T_x [57]. This broadens the possibilities for processing techniques, including blending MXenes with other nanomaterials or polymers to create solution-processable composites, developing inks for printing, and facilitating deposition like spraying, for example. As well, the organic solvents prevent oxidation and enhance wetting ability. MXenes with hydrophilic surfaces have allowed for favorable dispersion in polar solvents [58]. However, MXenes show intensive aggregation in the highly polar organic solvents such as alcohols. Some authors have reported isolation of MXene flakes by intercalation with the solvent dimethyl sulfoxide (DMSO). As a result, Li-ion storage was achieved [59]. Figure 10 shows the formation of monolayers in few-layered non-stacked MXene nanosheets having a mean lateral size 1–2.1 mm, which is suitable for inkjet printing.

3.3.3. Additive Incorporation

Incorporating additives during electrode processing is an effective approach to reduce sticking in MXene electrodes. Additives can include polymers, surfactants, dispersants, or binding agents. These additives alter the interfacial properties between MXene flakes, hindering their agglomeration and reducing sticking. Surfactants or dispersants can enhance the electrostatic or steric repulsion between flakes, improving their dispersion stability. Meanwhile, polymers or binding agents can act as a matrix to prevent direct flake-to-flake contact, thereby minimizing sticking [61].

Intercalation technology is widely acknowledged for its ability to prevent the stacking of MXene nanosheets. The interaction between intercalators and MXene, relying on van der Waals forces, holds significant importance for MXene-based composite development. Xin et al. achieved enhanced mechanical properties in MXene/cellulose nanofiber composite membranes by intercalating silver ions. Additionally, simple deposition methods have been employed to load metal oxides onto MXene nanosheets, serving as superior hosts through intermolecular forces. This not only prevents the accumulation of MXene layers but also facilitates electron transport. Solution casting, a method for mixing polymer matrices and fillers in solvents, results in well-dispersed composite materials after solvent evaporation and mold pouring. Thorough stirring during the solution casting process is essential for achieving a homogeneous mixture. MXene and PVA were successfully mixed under ultrasonic and stirring conditions, resulting in a composite material with excellent dispersibility. Similarly, PVA/MXene nanocomposites have been obtained using the same approach, resulting in MXene with a homogeneous dispersion. This method was also utilized to fabricate Ti₃C₂/TPU composites with exceptional thermal conductivity and mechanical properties [62,63,64,65]. The described approaches are illustrated in Figure 11.

3.3.4. Deposition Techniques

Another widely employed technique involves the use of additive manufacturing or 3D printing for electrode fabrication. The versatility of additive manufacturing offers precise control over the electrode architecture and porosity, providing avenues for tailoring MXene-based electrode designs to meet specific performance requirements. By modulating the printing parameters and material compositions, researchers can optimize the porosity and interconnectivity of the electrode network, thereby influencing the ion transport kinetics and electrolyte accessibility within the electrode structure.

Electrodes made from 3D printing with nanocarbon that has undergone surface modification using Ti₃C₂ have been demonstrated. This modification occurred through electrochemically activating the 3D-printed nanocarbon electrodes (3DnCE) material and then functionalizing it with Ti₃C₂. Testing showed that Ti₃C₂@3DnCEs outperformed bare 3DnCEs in terms of electrochemical capacitance (Figure 12). The MXene material’s capability to undergo diffusion-controlled redox processes alongside surface capacitive processes, as well as improved graphene interconnection facilitated by the MXene material, were proven [66].

Vacuum filtration stands as a notable electrode processing method for MXene-based electrodes. This technique involves the filtration of a well-dispersed MXene suspension through a porous membrane, resulting in the formation of a compact and densely packed MXene film on the membrane surface. The filtration process governs the film thickness, porosity, and inter-particle interactions, crucial parameters that influence the electrochemical performance of the resulting electrode. Controlling the filtration parameters can enable the customization of the film morphology and microstructure, thereby tailoring the electrode’s transport properties and electrochemical performance. The filtration process has been utilized in [67], as is shown in Figure 13. A filter membrane is utilized to obtain a filter “cake” from the MXene dispersion. This filter “cake” was subsequently rapidly frozen in liquid nitrogen and then freeze-dried to produce a 3D macroporous MXene film with plentiful interlayers (as depicted in Figure 13a). The rapid freezing process triggers significant expansion of the interlayers within the MXene films due to the rapid growth of ice crystals (Figure 13b). This 3D macroporous MXene film demonstrates outstanding specific capacitance and rate capability. Furthermore, the symmetric supercapacitor assembled from this material retains 87% capacitance after 4000 cycles (Figure 13c).

Electrodeposition has emerged as an effective technique for the fabrication of MXene-based electrodes. By employing electrodeposition, researchers can directly grow MXene coatings onto conductive substrates, imparting precise control over the film thickness and porosity. The ability to modulate deposition parameters such as voltage, current density, and deposition time allows for the fine-tuning of the electrode morphology and electrochemical properties. This technique offers the advantage of producing dense and adherent MXene coatings with enhanced interfacial contact, contributing to improved charge storage and transport characteristics within the electrode [68].

3.3.5. Drying Techniques

The drying process during or after electrode fabrication is another critical step that can influence sticking behavior. Improper drying conditions, such as excessive heat or prolonged drying times, can result in the formation of strong inter-particle bonds, causing MXene flakes to stick together. Optimizing the drying process to maintain suitable temperature and time conditions is crucial for preventing sticking and preserving the desired electrode structure. For example, as illustrated in Figure 14, when a droplet of a well-wetting ink is placed on a substrate, the irregular evaporation of the solvent from the sides, due to the higher surface area/volume ratio, leads to an outward flow from the center of the droplet to the edges, particularly in low viscosity inks. This flow carries and accumulates the particles on the edge of the droplet, resulting in the formation of a ring-like structure known as a “coffee-ring” [69]. Addressing this issue can be achieved using multicomponent carrier solvents for ink formulation. The differential evaporation rates of various components in the inks give rise to surface tension and compositional gradients within the droplet, thus creating inward Marangoni flows, which facilitate a more regular distribution of the particles.

3.4. Environmental Factors

The environmental conditions in which MXene electrodes are processed and utilized can impact their sticking behavior and overall performance. The key environmental factors to consider are temperature and humidity.

3.4.1. Temperature

The temperature during electrode processing and device operation can significantly influence sticking in MXene electrodes. Higher temperatures can promote the mobility of MXene flakes and increase the likelihood of their agglomeration. This enhanced mobility enables the flakes to come into close contact and form strong inter-particle adhesion, leading to sticking. Conversely, lower temperatures tend to reduce the mobility of the flakes, minimizing agglomeration and sticking. Therefore, careful control of the processing and operating temperature is crucial to prevent sticking and maintain optimal electrode performance.

For example, the authors in [42] have found that the differences in the sheet length, width, and thickness across samples at temperatures ranging from 20 °C to 60 °C were less than 2 Å, which aligns with the uncertainty range of experimental errors. These findings indicate that the structure of the Ti₃C₂ nanosheets is not significantly impacted within this temperature range. Nonetheless, the study presents valuable insights into the lamellar architecture present in the sample sets. The Ti₃C₂ nanosheets are characterized by a multilayer lamellar morphology, comprising layers of transition metal carbides with interstack gaps. As such, the Small-Angle Neutron Scattering (SANS) data, suggesting a hard block thickness or crystalline thickness of 11.4–11.8 Å (equivalent to 1.14–1.18 nm), indicates the thickness of a single layer of Ti₃C₂. Additionally, the soft block thickness of 20.3–21.5 Å (approximately 2.03–2.15 nm) can be interpreted as interstacking layer gaps within the Ti₃C₂ samples, which is evidence for a lack of sticking.

3.4.2. Humidity

Humidity, or the moisture content in the surrounding environment, is another influential factor in sticking behavior. High humidity can facilitate the absorption of moisture by MXene flakes, causing them to swell or become sticky. This increased stickiness promotes the agglomeration of flakes, leading to sticking. Conversely, low humidity conditions can reduce flake moisture absorption and mitigate sticking. It is important to note that the surface chemistry of MXene flakes, such as the presence of hydrophilic or hydrophobic functional groups, can affect their affinity for moisture absorption and thus impact sticking behavior in humid environments.

3.4.3. Other Environmental Factors

Besides temperature and humidity, other environmental factors may also have some influence on sticking behavior. These factors include exposure to light, air quality, and atmospheric gases. Light exposure, particularly in the presence of certain sensitizers, can induce photochemical reactions that alter the surface chemistry of MXene flakes, potentially affecting sticking. Air quality and atmospheric gases, such as oxygen or pollutants, can also interact with MXene flakes and impact their surface chemistry, thereby contributing to sticking issues. For example, it was reported that the aging process of aerated aqueous MXene dispersions has led to the oxidation of MXene flakes, initially commencing from the edges where the formation of TiO₂ crystals resembling branches was observed [71].

In Figure 15a, a non-oxidized MXene flake is shown. The TEM image in Figure 15b illustrates evident signs of oxidation on the MXene flakes. The image in the inset of Figure 15b further confirms the presence of TiO₂ crystals growing in a branch-like manner at the edges. During this phase, the MXene flakes maintained their 2D morphology, indicating partial oxidation reactions. However, continued aging of the MXene dispersion resulted in the agglomeration of C atoms, leading to the formation of amorphous carbon and TiO₂-rich regions, representing a fully oxidized MXene dispersion (Figure 15c). At this stage, the sheet-like morphology of MXene flakes underwent a transformation into agglomerated TiO₂ nanocrystals.

Understanding the impact of environmental factors on sticking behavior is essential for designing robust MXene-based supercapacitor electrodes. By controlling and optimizing the temperature and humidity conditions during electrode processing, as well as considering other relevant environmental factors, researchers can reduce sticking, enhance dispersion stability, and improve the overall performance and reliability of MXene electrodes.

4. Approaches for Preventing Sticking of MXenes

4.1. Surface Functionalization and Passivation

Recent study has examined the effective functionalization of acid-treated carbon fiber (CF) using Ti₃C₂T_X, leading to a 186% improvement in interfacial shear strength [72]. Additionally, the modified CF’s surface topography changed, enhancing the mechanical engagement effect during debonding. To incorporate short CF-functionalized MXene molecules into an epoxy resin (ER) matrix, the CF was immersed in a Ti₃C₂T_x dispersion. After coating, they were mixed with the ER by stirring under sonication. In this case, composites with a 2 wt% load exhibited the best behavior, with tensile strength 100% higher than ER and 13% higher than a composite with non-coated CF. The MXene-coated CF formed a stronger interfacial interaction with the epoxy matrix by physically interlocking and forming strong hydrogen bonds.

Applying amino functionalities to carbon fibers (CF) through aminopropyl triethoxysilane (APTES) grafting following acid treatment is a standard method to enhance the interphase and prevent self-sticking. The resulting CF-NH₂/ER composite showed increased tensile strength and flexural strength by 40.8% and 45.9%, respectively, when compared to CF-NH₂/ER without MXene. The inclusion of Ti₃C₂T_x nanosheets improved the interface connection between the fiber and ER, reducing interlaminar stress concentration, as these nanosheets exhibit superior mechanical properties compared to the ER (Figure 16). An alternative approach was explored by Ding et al. [73], involving grafting APTES to Ti₂C nanosheets, followed by their covalent bonding to the carboxyl groups of acid-treated CF through amide bonds. The remaining amine groups of the MXene also participate in a chemical linkage with the ER, playing a pivotal role in impeding agglomeration. This mechanism also effectively prevents crack propagation along the interface when the fiber breaks into shorter units.

4.2. Integration with Polymer Matrices or Carbon Nanomaterials

One effective approach to prevent sticking in MXene electrodes is to integrate MXenes within polymer matrices or carbon nanomaterials. This strategy provides physical separation and dispersion stability for MXene flakes, thereby reducing their tendency to agglomerate.

4.2.1. Polymer Matrices

Integrating MXenes within polymer matrices offers several benefits for preventing sticking and enhancing overall electrode performance. Polymers act as a binding medium, encapsulating the MXene flakes and providing a dense network. This polymer matrix prevents direct flake-to-flake contact, minimizing sticking and preserving the dispersion of MXenes. The selection of suitable polymers is crucial to ensure compatibility with MXene flakes. Polymers with high elasticity, such as hydrogels or elastomers, can offer flexibility and conformability, reducing stress-induced flake agglomeration. Furthermore, polymers with good adhesion to MXene surfaces can create strong interfacial interactions, improving the stability and preventing sticking.

Ti₃C₂T_x MXene has been suspended in dimethylformamide (DMF) and different quantities of poly(vinylidene fluoride-trifluoro-ethylene-chlorofluoroehylene) or P(VDF-TrFE-CFE) (61.5/30.2/8.3 mol %) have been added [76]. By ultrasonication, flake sizes of 4–5 μm (denoted as large MXene flakes (L-MXene)) and 1–2 μm (denoted as small MXene flakes (S-MXene)) were obtained. Figure 17 indicates that the MXene flakes are evenly dispersed throughout the polymer matrix without forming aggregates, or the polymer prevents sticking of the flakes.

In addition, the MXene/P(VDF-TrFE-CFE) composite with large MXene flakes was found to reach a dielectric permittivity of 10⁻⁵. In contrast, the composite with small MXene flakes reached a dielectric permittivity of 10⁻⁴. These results are attributed to the accumulation of charges at the interfaces between the two-dimensional MXene flakes and the polymer matrix, where the switchable microscopic dipoles are located.

4.2.2. Carbon Nanomaterials

Integration of MXenes with carbon nanomaterials, such as carbon nanotubes or graphene, can also prevent sticking in MXene electrodes. Carbon nanomaterials possess excellent electrical conductivity, high aspect ratios, and large surface areas, making them ideal candidates for dispersing MXene flakes. Carbon nanomaterials effectively act as scaffolds, providing a three-dimensional network structure that facilitates the dispersion and stabilization of MXenes. The high surface area and strong interfacial interactions between MXenes and carbon nanomaterials hinder the agglomeration of MXene flakes, mitigating sticking.

MXene foams with varying pore structures have been synthesized in [77] using polystyrene (PS) spheres with different diameters of 80, 310, and 570 nm (Figure 18a,d,g). These foams exhibit uniform and interconnected pores that offer ample active sites and a strong electrical connection for electron transfer. Of particular interest is the finding that when the sizes of the MXene flakes and templates (310 nm) match, the resulting foam demonstrates the highest gravimetric capacitance of 474 ± 12 F g⁻¹ at 2 mV s⁻¹, outperforming the others. Furthermore, the ratio of MXene to PS mass influences the packing density of the foams and the sticking trend of the nanosheets (Figure 18b,c,e,f,h,i), which in turn affects the inner resistance of the foam electrodes. Introducing carbon nanotubes further enhances the electrical conductivity of the foams, resulting in a capacitance of 462 ± 8 F g⁻¹ at 2 mV s⁻¹ and retaining 205 ± 10 F g⁻¹ at 1000 mV s⁻¹. This research holds promise for energy storage applications and provides valuable guidance for the design of porous electrodes based on 2D nanomaterials for supercapacitors without sticking.

4.2.3. Hybrid Composites

Combining polymer matrices and carbon nanomaterials with MXenes can create hybrid composites that offer synergistic effects to prevent sticking. In these composites, the MXenes are dispersed within a polymer matrix, while carbon nanomaterials are incorporated to enhance electrical conductivity, mechanical strength, and stability.

The hybrid composite approach leverages the complementary properties of polymers, carbon nanomaterials, and MXenes. The polymer matrix provides encapsulation and dispersion stability, while carbon nanomaterials enhance the conduction pathways and mechanical integrity. Together, this integrated approach effectively prevents sticking and improves the overall performance of MXene-based supercapacitor electrodes.

By integrating MXenes with polymer matrices or carbon nanomaterials, researchers can create composite structures that alleviate sticking issues, promote better dispersion, and enhance the stability and performance of MXene electrodes in supercapacitor devices. The choice of suitable polymers and carbon nanomaterials must be carefully considered to achieve optimal results.

Heterostructures of MXene including transition metal dichalcogenides, such as MoS₂ for example, have been examined for enhanced stability. It has demonstrated good prevention of restacking of delaminated MXene nanosheets due to the favorable layered nature and tendency to easily form heterostructures with the MXene materials, thus decreasing the probability of their oxidation and restacking [78,79].

4.3. Electrode Processing Optimization

Apart from the composition of the MXene itself, the interactions of colloidal particles are influenced by their surrounding medium. Instead of solely specifying the solvent used, it is essential to consider the ionic strength, determined by salt concentration, as it impacts the Debye screening length, which is directly linked to the strength of percolating networks, influencing rheology. Additionally, the solvent’s acidity (or pH in the case of water) plays a role in determining the ratio of protonated and deprotonated alcohol termination groups (–OH/–O–). This parameter affects the thermodynamic stability of termination group mixtures and influences the preference for parallel or perpendicular flake stacking [80]. The hydrophilic nature of Ti₃C₂T_x layers due to the presence of polar and terminal groups enables easy dispersion in various polar solvents, forming dilute colloids (Figure 19). At higher concentrations, the non-uniform distribution of charges on the 2D monolayers (positively charged at the edges and negatively charged on the basal plane) leads to clay-like swelling as the flakes assemble.

In the case of Ti₃C₂T_x MXene, it has been determined that the optimal parameters of the selected solvent involve surface tensions ranging from 35 to 42 mN/m and a ratio of polar/dispersive components between 0.61 and 0.87. For example, solvents such as propylene carbonate, NMP, DMF, and DMSO serve as effective carrier solvents for this MXene type. It has also been demonstrated that the synthesis route and the types of surface groups of MXene significantly influence its dispersibility, thereby enabling the use of various solvents for ink formulation by modifying the MXene’s surface chemistry. This approach even offers the possibility of converting hydrophilic MXene to hydrophobic and dispersing it in organic solvents.

Since MXenes are primarily synthesized using solution-based methods, it is preferable to employ liquid-phase fabrication techniques such as printing or coating for further processing, including device fabrication. These techniques involve processing stable MXene dispersions into continuous films with good adhesion to the target substrate. Such a dispersion is typically referred to as an ink and typically contains additives (e.g., binders, surfactants) to improve processability and film formation behavior [82]. In most cases, these additives degrade the electronic properties of the films/structures and should be removed at the end of the fabrication process.

As the treatment time increases, it is understood that continual removal of Al leads to increased delamination, causing the stacked sheets to thin. HF is used to exfoliate the MAX phase and produce Ti₃C₂ MXene nanosheets. Developing MoS₂/MO₂TiC₂T_x heterostructures results in an open architecture formed of 2D sheets, compared to pure MO₂TiC₂T_x, created by filtering of a colloidal solution and involving non-stacked 2D sheets. Figure 20 demonstrates that the interlayer distance of this compound was about 9.4 angstroms, which is close to that of bulk MoS₂ (~6.2 angstroms). The interaction of two layers of MoS₂ with layers of MO₂TiC₂T_x leads to the creation of MoS₂-on-MXene heterostructures. Examination of the transmission electron micrograph of MoS₂/MO₂TiC₂T_x reveals a larger quantity of MoS₂ layers, which hinders the stacking of MXene sheets. MoS₂ is usually prepared by mechanical or electrochemical exfoliation, hydrothermal, or microwave synthesis. Ti₃C₂ MXene/MoS₂ is prepared by hydrothermal synthesis [83].

A biothermochemistry technique for synthesis of Ti-N and S-S chemical bonds and creating a 3D crosslinked Ti₃C₂T_x network has been applied, thereby addressing issues of oxidative degradation and self-stacking. This approach involves the use of different biological reagents—sodium citrate, glutathione, tannic acid, sodium alginate, and phytic acid—to construct 3D network structures (Figure 21). Upon crosslinking, the interlayer distance of Ti₃C₂T_x expands, and the modified Ti₃C₂T_x-based supercapacitors exhibit almost no attenuation, displaying a capacitance of 265 F g⁻¹ at a current density of 1 A g⁻¹. For comparison, the crosslink-free Ti₃C₂T_x experiences a 19% decrease in capacitance after 30 days. This 3D crosslinking effect serves to expand the interlayer distance and reduce the self-stacking problem, while the bioagents effectively minimize the number of exposed Ti atoms, preventing the oxidation of Ti₃C₂T_x MXene. This study effectively resolves the challenges of self-stacking and oxidative degradation of MXenes, offering a promising approach for studying future applications and extended protection, particularly of Ti₃C₂T_x.

A one-step amination process was utilized to introduce aromatic diamines of different molecular sizes as pillars between MXene interlayers. This method successfully inhibited self-stacking and achieved diverse expanded interlayer spacing. X-ray diffraction results showed higher interlayer spacing of MXene from 1.23 to 1.40 nm. The p-phenylenediamine-intercalated MXene (PDA-MXene) demonstrated improved interlayer spacing (1.38 nm) and pore structure, leading to enhanced electrolyte-accessible surface area, improved charge-transport properties, and facilitated storage of ions. Consequently, a supercapacitor utilizing PDA-MXene as the cathode displayed a higher specific capacitance of 124.4 F g⁻¹ at 0.2 A g⁻¹ [86].

To summarize and compare the various strategies employed for enhancing supercapacitor performance, including MXene-based approaches,

Table 1. Comparative analysis of strategies employed for enhancing supercapacitor performance.

Strategy	Performance Metrics	Merits	Demerits	Refs.
MXene coating	High specific capacitance	Improved electrical conductivity; enhanced ion accessibility	Potential issues with scalability; stability concerns	[87,88]
Doping	Fast charge/discharge rates	Increased specific capacitance; improved cycling stability	Dependent on choice of dopant; cost implications	[89,90]
Nanostructuring	High energy density	Enhanced surface area; improved electrolyte penetration	Complexity in fabrication; potential loss of mechanical strength	[91,92]
Hybrid materials	Long-term cycling stability	Combined benefits of different materials; complementary properties	Synthesis challenges; compatibility issues	[93,94]

,

Table 2. Comparison of stackability and nanosheet cohesion and their impact on the supercapacitor performance.

Material	Stackability Characteristics	Factors Influencing Nanosheet Cohesion	Impact on Supercapacitor Performance	Refs.
MXene coating	Potential issues with stackability due to strong inter-particle bonds	Coating stability, uniformity, and adhesion to substrates	Influence on capacitance, electrical properties, and overall stability	[95]
Doping	Reliability of nanosheet stacking dependent on dopant choice	Integration of dopants, impact on nanosheet interactions	Effects on capacitance, cycling stability, and cost implications	[96]
Nanostructuring	Challenges in stacking during fabrication; potential impact on strength	Complexity of fabrication process, nanosheet alignment	Relationship to energy density, surface area enhancement, and electrolyte penetration	[97]
Hybrid materials	Stackability influenced by material compatibility and synthesis issues	Compatibility between materials, stacking interactions	Impact on long-term stability, leveraging diverse material benefits	[98]

,

Table 3. Comparison of the advantages and disadvantages of the basic MXene strategies for enhancing storage device performance.

MXene Strategy for Enhancing Storage Device Performance	Performance Metrics	Advantages	Disadvantages	Refs.
Surface functionalization	Enhanced specific capacitance	Improved wettability; increased ion accessibility	Potential process complexity; stability concerns	[99,100]
Nanocomposites	Higher energy density	Synergistic effects with additives; enhanced structural stability	Challenges in achieving homogeneous dispersion; synthesis complexity	[101]
Templated growth	Improved rate capability	Controlled morphology; facilitates ion transport pathways	Limited scalability; template removal process	[102]
Surface modification	Enhanced cycling stability	Improved conductivity; resistance to electrolyte-induced degradation	Reversibility issues; potential impact on mechanical properties	[103]

,

Table 4. Comparison of the effect of measures for sticking prevention on supercapacitor metrics.

Sticking Prevention Approach	Effects on Supercapacitor Metrics	Merits	Demerits	Refs.
Surface coating	Improved specific capacitance	Enhanced stability; prevention of restacking	Potential impact on electrical conductivity; additional processing steps	[104]
Functional spacer molecules	Enhanced energy density	Effective prevention of restacking; promotes ion diffusion	Complexity in molecule design; cost implications	[105,106]
Intercalation	Increased rate capability	Facilitates ion transport; enhances cycling stability	Dependency on guest species; potential structural distortions	[107]
Surface roughening	Improved electrolyte accessibility	Enhanced active sites; prevents restacking issues	Need for precise control in surface modification; potential structural changes	[108]
Nanostructure design	Increased surface area; improved electrical conductivity	Enhanced ion accessibility; structural stability	Fabrication complexity; potential performance variability	[109]

,

Table 5. Comparison of the approaches for prevention of MXenes sticking, taking into account their synthesis complexity, ecological impact, performance stability, and cost.

Sticking Prevention Approach	Synthesis Complexity	Environmental Impact	Long-Term Stability	Cost Analysis
Surface coating	Moderate	Low impact	Good stability observed	Medium
Functional spacer molecules	Moderate	Low impact	Stable cycling behavior	High
Intercalation	High	Moderate impact	Enhanced life cycle	High
Surface roughening	High	Moderate impact	Impact on long-term performance	Medium
Nanostructure design	High	High impact	Improved durability	High

and

Table 6. Comparative table of supercapacitor parameters enhanced by the application of MXene composites, in particular with polymers.

MXene Composite	Application/Enhancement	Key Findings
MXene–Carbon Nanotube (CNT) and MXene–Reduced Graphene Oxide (rGO)	Improved volumetric capacitance	Sandwich-like structure improved volumetric capacitance up to 350 F cm−3 at 5 A g−1; MXene–rGO composite demonstrated volumetric capacitance of 435 F cm3 with cycling stability [27].
Asymmetric yarn supercapacitor	Achieved high energy density	Utilized MXene–CNT anode, MnO2/CNT cathode, and achieved an energy density of 100 mWh cm2 [28].
Ti3C2Tx MXene–CNT composite	High areal capacitance	Fabricated through photolithography and vacuum filtration, achieving areal capacitance of 61.38 mF cm−2 at 0.5 mA cm−2 [29].
Ti3C2Tx MXene–Multiwalled CNT (MCNT) composite	Demonstrated high areal capacitance	Asymmetric supercapacitors exhibited areal capacitance of 0.94 F cm−2 with Na2SO4 electrolyte [30].
MXene–carbon allotropes (AAC)	Preventing aggregation, enhanced electrochemical performance	Asymmetric supercapacitor with MXene–AAC (2:1) achieved specific capacitance of 177 F g−1 at 0.5 A g−1 with 97.4% retention over 10,000 cycles [31]; micro supercapacitor with MXene as cathode and activated carbon as anode showed areal capacitance of 7.8 mF cm−2, energy density of 3.5 mWh cm−3, and power density of 100 mW cm−3 [32].
MXene–carbon quantum dots (CQD)	Superior capacitance	Ti3C2Tx MXene–CQD composite electrode demonstrated capacitance of 441.3 F g−1 at 1 A g−1 with 100% retention after 10,000 cycles [33].
Ti3C2Tx/NiCO2S4 @ CC composite (TNSC)	Enhanced gravimetric capacitance, cyclic stability	Gravimetric capacitance of 2326 F g−1 at 1 A g−1 with cyclic stability of 93.8% at 10 A g−1, resulting in quasi-solid-state flexible supercapacitor [34].
Zinc ion hybrid micro supercapacitors	Utilizing hydrogel electrolytes for improved performance	Constructed with V2O5 cathode and Ti3C2Tx MXene anode in PAM hydrogel electrolyte, offering 129 mF cm−2 areal capacitance and 48.9 mWh cm−2 energy density with 77% capacitance retention over 10,000 cycles [35].
MXene–Polymer Composites	Application/Enhancement	Key Findings
MXene–polypyrrole (PPy) composite	Enhanced specific capacitance	Electrodeposited composite (PPy) on MXene textile electrodes resulted in specific capacitance of 343.20 F g−1 compared to pristine MXene electrodes [37].
MXene–PPy composite	Improved tensile strength, specific capacitance	Specific capacitance of 614 F g−1 at 1 A g−1, retaining 100% capacitance over 10,000 cycles [38].
PPy–MXene–IL–mic composite	Enhanced energy density, capacitance retention	Energy density of 31.2 Wh kg−1, capacitance retention of 91% over 2000 cycles and 91% coulombic efficiency across temperature range of 4 °C to 50 °C [39].
MXene–Polyaniline (PANI) composite	Volumetric and gravimetric capacitance improvement	Synthesized composite exhibited volumetric capacitance of 1682 F cm−3 and gravimetric capacitance of 503 F g−1, with specific capacitance reaching 336 F g−1 with 98.3% capacitance retention over 10,000 cycles [40].
Functionalization with silylation reagents	Tailoring surface wetting properties	Functionalization with silylation agents can modify MXene flake hydrophobicity, hindering water contact and promoting hydrophobic characteristics [54].
Ion irradiation for hydrophobic transformation	Altering surface chemistry to hydrophobic nature	N+ ion irradiation method selectively eliminates surface-terminating groups, leading to a hydrophobic nature in MXenes, enhancing electrical conductivity [58].
Additive manufacturing for electrode fabrication	Precise control over electrode architecture	Electrochemically activating 3D-printed nanocarbon electrodes and functionalizing with Ti3C2 demonstrated enhanced electrochemical capacitance in Ti3C2@3DnCEs [68].
Surface functionalization and passivation	Enhancement of interfacial shear strength in carbon fibers; Improved mechanical engagement in epoxy resin matrix	Functionalization of acid-treated carbon fiber using Ti3C2TX led to a 186% increase in interfacial shear strength. Modified carbon fibers enhanced mechanical engagement effect during debonding, showing improved tensile strength compared to non-coated fibers [74].
Integration with polymer matrices	Prevention of sticking, enhancement of dispersion stability, improved electrode performance	Integration of MXenes within P(VDF-TrFE-CFE) polymer matrices prevented sticking, provided dispersion stability, and improved electrode performance. MXene flakes were evenly dispersed throughout the polymer matrix without forming aggregates, showing improved dielectric permittivity [80].

were prepared.

5. Conclusions

5.1. Emerging Strategies for Sticking Prevention

Future perspectives for MXenes related to emerging strategies for sticking prevention could encompass several areas of development. Some potential directions for advancing the use of MXenes while addressing sticking prevention might include the following:

Advanced intercalation techniques: enhancing and refining methods for intercalating compounds between MXene layers to prevent self-stacking. This could involve exploring a wider range of intercalants, including organic molecules with carefully tailored properties to effectively space MXene layers and prevent sticking.

Tailoring surface functionalities: investigating and developing new approaches to modify MXene surface functionalities to mitigate the tendency to self-stack. By engineering the MXene surface at a molecular level, it may be possible to reduce the interlayer interaction forces, thereby preventing stacking without significantly compromising other desirable properties.

Nanocomposite design: exploring the integration of MXenes into nanocomposite materials with specific structures that inherently prevent sticking. This could involve creating hybrid materials with MXenes dispersed within a matrix in a way that inhibits natural stacking tendencies.

Machine learning and computational design: leveraging computational techniques and machine learning algorithms to predict and design effective sticking prevention strategies for specific MXene compositions. This approach could help accelerate the discovery of novel intercalation agents or surface modifications that minimize self-stacking.

Innovative electrochemical energy storage device architectures: exploring the application of MXenes in novel device architectures that inherently minimize sticking. By integrating MXenes into innovative structures for energy storage devices, it may be possible to design systems that inherently prevent stacking while maximizing performance.

5.2. Challenges and Opportunities

In addition to the stacked sheets reducing the storage performance, some of the other challenges that MXenes still face in supercapacitor applications include the following:

Scalability and cost: While the properties of MXenes make them promising for supercapacitor development, challenges related to large-scale production and cost-effective synthesis still need to be addressed. Finding scalable methods that can produce high-quality MXene materials at a reasonable cost is essential for practical commercialization.

Stability: Ensuring long-term stability and robust performance under various operating conditions remains a significant challenge. MXene materials need to maintain their electrochemical properties over numerous charge/discharge cycles and under different environmental conditions to be reliable for commercial supercapacitor use.

High areal capacitance: Although MXenes exhibit high specific capacitance values, increasing the areal capacitance—how much charge a supercapacitor can store per unit of surface area—remains an ongoing challenge. Enhancing the areal capacitance while maintaining other desirable properties is critical for practical device implementation.

Electrode processing: Developing effective, scalable methods for integrating MXenes into electrode structures with a controlled morphology is crucial. Ensuring efficient charge transfer and accessibility of active sites within the MXene material is essential for optimizing supercapacitor performance.

Compatibility with electrolytes: MXenes must be compatible with a wide range of electrolytes to maximize their potential in various supercapacitor designs. Solving challenges related to maintaining stability and performance across different electrolyte chemistries is key for broadening the application scope of MXene-based supercapacitors.

Binder and separator compatibility: Ensuring that MXene-based electrodes are compatible with a range of binders and separators presents a challenge. The interactions between MXene, binders, and separators can significantly impact overall device performance, requiring careful optimization.

5.3. Prospect of MXene-based Supercapacitors

The prospects of MXene-based supercapacitors are highly promising, offering several key advantages that make them attractive for energy storage applications:

High specific capacitance: MXenes have demonstrated high specific capacitance, allowing for the storage of significant amounts of electrical charge per unit mass, making them well-suited for high-performance supercapacitors.

Rapid charge/discharge rates: MXenes exhibit excellent charge/discharge kinetics, enabling rapid energy storage and release. This property is advantageous for applications requiring quick bursts of power, such as in regenerative braking systems or energy harvesting devices.

Wide potential windows: MXenes have the potential for high operating voltages, which allows for a wide potential window. This characteristic can contribute to increased energy density and overall device performance.

Good electrical conductivity: the metallic conductivity of many MXene compositions facilitates efficient charge transport within the material, contributing to low internal resistance and high power density in supercapacitor devices.

Tunability and versatility: MXenes’ tunable surface chemistry and compositional flexibility offer the potential for tailoring properties such as wettability, interlayer spacing, and surface functional groups, allowing for customized electrode designs to optimize supercapacitor performance.

Excellent mechanical properties: some MXenes exhibit exceptional mechanical strength and flexibility, which can be leveraged for the development of mechanically robust and flexible supercapacitor devices.

5.4. Recommendations for Future Research

Once the above-described obstacles are overcome, future research endeavors in the field of application of MXenes in supercapacitors could focus on the following areas:

Novel MXene compositions: Exploration of novel MXene compositions with tailored properties to further enhance specific capacitance, rate capability, and stability. This may involve synthesizing new MXene derivatives or composites that exhibit improved electrochemical performance.

Scalable synthesis methods: continued development of scalable, cost-effective synthesis methods for producing high-quality MXene materials suitable for large-scale supercapacitor applications, with a focus on energy and cost efficiency.

Understanding interfaces and electrode-electrolyte interactions: in-depth investigation of MXene–electrolyte interactions and interfaces to improve charge storage mechanisms, electrochemical stability, and ion transport within supercapacitor devices.

Advanced electrode architectures: development of advanced electrode architectures and scalable manufacturing techniques to optimize the utilization of MXene materials in supercapacitors, potentially enhancing energy density and load-bearing capabilities.

Flexible and wearable supercapacitors: research into flexible and wearable supercapacitors based on MXenes, considering the design of conformal, stretchable, and lightweight energy storage devices for emerging electronic and wearable technologies.

Environmental impact assessment: investigation into the environmental impact and sustainable practices related to MXene synthesis, processing, and end-of-life recycling, aimed at developing environmentally friendly strategies for MXene-based supercapacitors.

Integrated energy storage systems: research efforts could focus on integrating MXene-based supercapacitors with complementary energy harvesting systems, such as piezoelectric, thermoelectric, or photoelectric systems, to create multifunctional energy storage solutions.

Real-world performance and durability studies: comprehensive performance and durability studies under real-world operating conditions to assess long-term stability, cycle life, and reliability of MXene-based supercapacitors for various end-use applications.