From energy to intelligence: MXenes transforming triboelectric nanogenerators

Abstract

As a 2D functional material, MXenes have attracted increasing interest. MXenes, with their remarkable features, have redefined the performance of energy harvesting by boosting stability, efficiency, and charge transfer. Triboelectric nanogenerators (TENGs) have become an appealing technology for energy harvesting and sensing applications due to the increasing demand for self-powered and sustainable energy solutions. However, owing to the inadequate output performance of TENGs, they are still a long way from being used commercially. The use of MXenes in TENG is associated with the terminal groups (such as =O, –OH, and –F), intrinsic metallic nature, and synthetic techniques of MXenes. In this review, the recent advancements in MXene synthesis, TENG operating mechanisms, and fabrication strategies are provided. The origin, advancement, and underlying mechanism of TENG are elucidated for new researchers in this field. Furthermore, the diverse roles of MXenes in TENGs and the integration of MXene TENGs in numerous applications, including biomedical wearables, environmental sensing, sports monitoring, and soft robots, are explored thoroughly. Finally, we delve into the obstacles and future outlook of MXene-TENG research, emphasizing avenues for further advancement in the field of multifunctional sensing and energy harvesting.

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Suresh Kumar Chittibabu

Suresh Kumar Chittibabu is currently pursuing his PhD at the Center for Nanotechnology Research, Vellore Institute of Technology, Vellore, India. He received his BEng degree in the stream of Electronics and Communication from Anna University, Chennai, India, and M.Tech degree in Nanotechnology from Vellore Institute of Technology, Vellore, India. His current research focuses on advanced functional materials, with particular emphasis on MXenes, metal–organic frameworks (MOFs), energy-harvesting, triboelectric nanogenerators, wearable physical sensors, and biomedical rehabilitation devices.

Arunkumar Chandrasekhar

Prof. Arunkumar Chandrasekhar is currently an Associate Professor in the Department of Sensors and Biomedical Technology, Vellore Institute of Technology, Vellore, India, and he is a recipient of the VIT SEED Grant research project. He worked as a postdoctoral researcher at the Nanomaterials and Systems Lab, South Korea. He received his PhD in Mechatronics Engineering from Jeju National University, Jeju, South Korea, where he was a scholarship recipient from the Korean Government Scholarship Program. He is interested in self-healing polymers, 2D materials, wearable triboelectric nanogenerators, battery-free electronic devices, energy storage devices, MEMS, and self-powered devices.

Krishnamoorthi Chintagumpala

Prof. Krishnamoorthi Chintagumpala received his PhD in Physics from the Indian Institute of Technology Madras, Chennai. After graduation, he worked as a Scientist at GE Global Research and Technology Center, Bengaluru, and as a Research Fellow at the National University of Singapore, Singapore. Currently, he is working as Professor of Physics at the Center for Nanotechnology Research, Vellore Institute of Technology, Vellore, India. He has worked on magnetoresistance, magnetocaloric, magnetic hyperthermia, dilute magnetic semiconductors, electrochemical, and physical sensors. His current research interest focuses on the development of various magnetic nanomaterials and wearable physical sensors for biomedical applications.

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1. Introduction

1.1. The quest for sustainable energy

The rapid growth of multipurpose electronic devices and energy sources has transformed daily life and professional protocols. Electricity and accessibility have been crucial in technological advances.¹ Today's society depends on brilliance and information technology, which require the systematic use of energy resources.² Based on power plants (hydropower and nuclear power), conventional energy supply themes are incompatible with the present expansion of distributed electronic devices.^3,4 Thus, to address the global energy scarcity and the existing state of environmental pollution, the quest for alternative energy resources with lower carbon footprints is essential for healthy evolution of human civilization.⁵ Abundant research has been conducted on harvesting energy from easily available sources such as human motions, ocean waves, air flow, chemical reactions, and rotational vibrations for powering several distributed portable IoT devices, such as implanted wearable biosensors,^6–8 gas sensors,^9,10 self-powered sensors,^11,12 and wireless sensor networks.^13,14 When necessary, the energy generated by these harvesters can be stored in supercapacitors or rechargeable batteries to power electronic devices.^15,16 In the past decade, sophisticated technology known as nanogenerators has been quickly and successfully created to transform mechanical energy into electric energy.^17,18

1.2. Why triboelectric nanogenerators? The intersection of innovation

A nanogenerator transforms mechanical and thermal energy into electricity. Based on their power-generating mechanisms, nanogenerators are divided into three categories: pyroelectric,^19,20 triboelectric,^21,22 and piezoelectric (PENGs).^23,24 Triboelectric nanogenerator (TENG), a promising green energy innovation which is sometimes referred to as Wang generator, generates electricity by harnessing mechanical motions that exist in the surrounding environment.^3,25,26 Ace. Zhong Lin Wang has discovered the TENG mechanism in 2012.²⁷ TENG functions based on the fusion of contact triboelectrification and electrostatic induction. Triboelectrification refers to the mechanism of generating electricity through the movement of electrons from one substance that has poor electron binding capacity (+vely charged) to another substance that has superior electron binding capacity (−vely charged).^28–30 This transfer occurs when a pair of non-identical materials come into contact and separate or are simply rubbed.³¹ TENG-based energy harvesting is considered next-generation energy harvesting technology due to its distinctive benefits, such as ease of fabrication, low cost, high voltage output, simple configuration, structural optimization, and versatile range of frictional materials.^32,33 When pristine polymers are used for the fabrication of TENGs, electrons can recombine with the molecules in the air or positive charges generated on the electrode, resulting in poor performance.^34,35 Thus, scientists are inclined to incorporate unique materials for the fabrication of TENGs.

1.3. 2D materials in energy harvesting: a revolutionary approach

2D materials are very appealing for use in a variety of applications, particularly energy harvesting, owing to their outstanding attributes, which include a high surface-to-volume ratio, exceptional mechanical strength, and excellent electrical and thermal conductivity.⁶⁴ Notable breakthroughs have seen using 2D materials, including graphene,⁶⁵ molybdenum disulfide (MoS₂), hexagonal boron nitride (h-BN),⁶⁶ metal–organic frameworks (MOFs),⁶⁷ covalent organic frameworks (COFs),⁶⁸ and black phosphorus,⁶⁹ enhancing the performance of TENGs.⁷⁰ Nevertheless, epoxy pastes or adhesives are always needed for adherence when integrating these 2D materials into TENGs,⁷¹ which impede the device fabrication and diminish the performance of TENGs.

1.4. MXenes: is this a future-forward material?

MXenes are two-dimensional (2D) metal carbides and nitrides comprised of two or more layers of transition metal (M) atoms, such as a honeycomb structure. The octahedral clusters within the nearest transition metal layers are occupied by layers of carbon (C) and/or nitrogen (N) (X atoms).^72,73 Ternary layered MAX phases are the precursors for synthesizing MXenes. MAX phases are represented as M_n+1AX_n (n = 1,2,3 or 4), where M indicates early transition metals, A is primarily group 13 to 16 elements, and X is carbon and/or nitrogen.^74,75 MXenes can be synthesized by selectively etching the A element from the MAX phase. After etching, the resultant MXene is represented as M_n+1X_nT_x, where M can be Ti, V, Cr, Nb, Ta, Mo, W, etc., X can be C or N, or CN, and T_x implies a surface termination group such as –F, –Cl, –OH, –O, and –Br.^76,77 Owing to their advantages such as superior electrical conductivity, mechanical robustness, rich surface chemistry, and electronegativity, MXenes play a vital role in enhancing the performance of TENGs.^78,79 The basic theory of TENG indicates that the charge density and dielectric constant associated with materials serve as essential parameters for its performance.⁸⁰ Due to the presence of surface functional groups and 2D structure in MXenes, they attract electrons and acquire charge amid interlayer sheets and polymer grids. Also, they increase the electric charge density and dielectric constant.^81,82 Consequently, MXene's distinct chemical and physical attributes are favourable for TENG performance.

Initially, this comprehensive review presents the origin of triboelectricity and how it has evolved from before the Common Era (BCE) to the present 21st century. The physical principle of TENG relies on the modified Maxwell's equation, and with six operating modes of TENG, it was elaborated to reach a wider audience. The dielectric-to-dielectric, metal-to-dielectric, and deformation modes were reported to establish novel functionalities and applications with diverse materials soon. Subsequently, researchers started to explore MXenes as negative and positive triboelectric layers in a series of triboelectric materials. The thematic framework of this review, highlighting the etching strategies, operating modes of TENG, its fabrication protocols, and innovative applications of MXene TENGs, is concisely illustrated in Fig. 1. The family of MXenes, from the preparation of MAX phases via etching to advanced protocols for the synthesis of MXenes with their compositional and structural assessment, is discussed in detail. The performance of TENG can be enhanced by embedding MXenes in TENGs as electrodes, and triboelectric layers (positive and negative) were analyzed. Novel applications of MXene TENGs, such as wound healing, food spoilage detection, intelligent robots, sports monitors, and smart homes, are discussed. Finally, the present obstacles in the synthesis, oxidation, cost, and exploration of novel double-transition MXenes are presented, and how these issues can be resolved and the future evolution of MXene in TENG are summarized concisely. This review differs from the previous reviews, as it highlights the operating modes, MAX and MXene synthesis, MXene multifunctionalities in TENGs, and their unique applications.

Fig. 1 Visual representation of the structure of this review. Etching strategies. Reprinted with permission from ref. 36. © 2011, John Wiley & Sons. Reprinted with permission from ref. 37. © 2022, American Chemical Society. Reprinted with permission from ref. 38. © 2018, John Wiley & Sons. Reprinted with permission from ref. 39. © 2020, Nature. Reprinted with permission from ref. 40. © 2020, Elsevier. Reprinted with permission from ref. 41. © 2014, Springer Nature. Reprinted with permission from ref. 42. © 2015, Royal Society of Chemistry. Reprinted with permission from ref. 43. © 2014, American Chemical Society. Reprinted with permission from ref. 44. © 2024, Elsevier. Reprinted with permission from ref. 45. © 2020, John Wiley & Sons. Operating modes: contact-separation mode, lateral-sliding mode, single-electrode mode, freestanding mode, rolling mode, and deformation mode. Fabrication approaches. Reprinted with permission from ref. 46. © 2021, Elsevier. Reprinted with permission from ref. 47. © 2023, Elsevier. Reprinted with permission from ref. 48. © 2019, Elsevier. Reprinted with permission from ref. 49. © 2023, John Wiley & Sons. Reprinted with permission from ref. 50. © 2023, American Chemical Society. Reprinted with permission from ref. 51. © 2023, American Chemical Society. Reprinted with permission from ref. 52. © 2023, Elsevier. Reprinted with permission from ref. 53. © 2024, John Wiley & Sons. Reprinted with permission from ref. 54. © 2022, Elsevier. Applications of MXene TENGs. Reprinted with permission from ref. 55. © 2024, John Wiley & Sons. Reprinted with permission from ref. 56. © 2024, Elsevier. Reprinted with permission from ref. 57. ©2024, AIP Advances. Reprinted with permission from ref. 58. © 2024, Elsevier. Reprinted with permission from ref. 59. © 2024, American Chemical Society. Reprinted with permission from ref. 60. © 2025, Elsevier. Reprinted with permission from ref. 61. © 2024, Elsevier. Reprinted with permission from ref. 62. © 2024, Elsevier. Reprinted with permission from ref. 63. © 2024, American Chemical Society.

2. Unveiling the triboelectric phenomenon

2.1. History and advancement of triboelectricity

Despite its ancient origins, triboelectricity, the generation of electric charge by friction, has recently become an important topic of study.⁸³ Around 600 BCE, the Greek philosopher Thales of Miletus became the first to identify this phenomenon. He noticed that amber attracted small items when it was rubbed with fur.^84,85 Although it was not properly investigated for centuries, this observation was one of the earliest steps toward understanding static electricity.

As scientists advanced their understanding of the basics of static electricity, the study of electricity gained momentum. However, triboelectricity did not receive concentrated scientific study until the late 20th century.^86,87 Researchers began to systematically study the triboelectric series to categorize materials based on their tendency to receive or lose electrons.⁸⁸ Triboelectricity has gained new attention in the twenty-first century due to the development of triboelectric nanogenerators (TENGs).^89,90 This development from a fascinating archaic observation to cutting-edge technology demonstrates the increasing importance of triboelectricity in modern research and engineering.

2.2. The physics behind TENGs: mechanism and their classification

Using basic physical principles, triboelectric nanogenerators (TENGs) transform mechanical energy from environmental sources into electrical energy, representing a novel approach to energy harvesting.^91,92 The triboelectric effect and electrostatic induction are two fundamental physical processes that TENGs rely on to function.⁹³ The underlying principle of TENG can be elaborated using modified Maxwell's equations, which are the basis of classical electromagnetism. To describe the current produced by the recurring contact and separation of triboelectric materials, the term “triboelectric displacement current” was established. This idea is essential to comprehend the electric field and the ensuing production of electrical power. According to the contact electrification and TENG principle, the output of TENG may be predicted and derived from Maxwell's equations as follows. The modified Maxwell's equation is presented as eqn (1), where is the displacement current density. The energy conversion is facilitated by the displacement current, which comes from the dielectric polarization caused by a transitory electric field. is the polarization term that emerges from the static electrostatic charges, ε₀ is the permittivity of free space, and is the electric field.^94,95

The induced current produced by the fluctuating electrical field is the first component and serves as the theoretical foundation for electromagnetic wave generation. The second component is the current produced by the polarization field due to electrostatic charges on the material surface, which serves as the nanogenerator's primary theoretical foundation and source.⁹⁶ Gaining knowledge of these concepts helps to understand how TENGs work and how they may be used in energy generation.

2.2.1. Effect of triboelectricity. Static electric charges are created on the surfaces of two materials when they come into contact, and then separate, a phenomenon called contact electrification or the triboelectric effect.⁹⁷ (i) Mechanism of charge transfer: electrons move from one material to another when two distinct materials come into contact. Higher electron affinity materials will acquire electrons and become negatively charged, whereas lower affinity materials will lose electrons and become positively charged.^98–100 (ii) Surface interaction: more contact points are produced by rougher surfaces with higher asperities, which improves the charge transfer. Similarly, higher contact area conditions increase the magnitude of charge generation.^101,102

2.2.2. Electrostatic induction. Once the materials are charged via the triboelectric effect, they create an electric field in their surroundings. Electrostatic induction is the process by which this electric field induces a redistribution of charges in nearby conductive materials.^103,104 (i) Induced current: when the charged materials in a TENG are separated after contact, the resulting electric field induces a potential difference across an external circuit connected to the TENG.¹⁰⁵ This potential difference drives the flow of electrons, creating an electric current.¹⁰⁶ (ii) AC power generation: the repetitive motion of bringing materials into contact, and then separating them generates alternating current (AC).¹⁰⁷

2.2.3. Modes of operation in TENGs. TENGs can be designed to operate in different modes, each leveraging the triboelectric effect and electrostatic induction in unique ways to optimize energy conversion for specific applications.
2.2.3.1 Vertical contact-separation mode. In this mode, two materials move perpendicularly to each other, coming into contact, and then separating.¹⁰⁸ This mode is further classified into two subdivisions of dielectric to dielectric and metal to dielectric.
2.2.3.1.1 Dielectric to dielectric. When two dielectric films with different electron affinities come into physical contact, their surfaces become oppositely charged. The electrodes placed on the top and bottom surfaces of the two dielectric films produce a potential decrease once the two sides are separated by a certain distance. To balance the electrostatic field, the free electrons from one electrode move to the other if the two electrodes are electrically connected by a load (Fig. 2(a)(i)). The induced electrons will return after the gap is closed and the potential decrease caused by the triboelectric charges vanishes. The induced electrons move back and forth between the two electrodes as a result of the periodic contact and separation of the two material, producing an electrical signal in the external circuit.^109,110

Fig. 2 Operating modes of a TENG. (a) Contact-separation mode. (i) Dielectric to dielectric. (ii) Metal to dielectric. (b) Lateral-sliding mode. (c) Single-electrode mode. (d) Freestanding mode. (e) Rolling mode. (f) Deformation mode.

2.2.3.1.2 Metal to dielectric. In this mode, the metal acts as the triboelectric material and electrode, pairing opposite with the positive or negative triboelectric material. Usually, the dielectric material is pre-charged with negative charges or acquires negative charges upon interacting with the environment. When the independent metal electrode and the dielectric material (negative) come into contact and separate, the independent electrode receives and separates positive and negative charges to attain a state of electrostatic equilibrium (Fig. 2(a)(ii)). In contrast, the dielectric material receives extra negative charges. The internal electric field intensities of the electrode should be zero. As a result, positive and negative charges are simultaneously present.^80,111 This simple yet effective configuration is ideal for applications where materials can be repeatedly pressed together, and then pulled apart, such as in footstep energy harvesting and pressure sensors.^112,113
2.2.3.2 Lateral sliding mode. When two triboelectric layers with opposite polarities slide horizontally, the two contact surfaces will produce an equal number of positive and negative charges. When the contact surface slides horizontally due to external mechanical action, polarization forms in that direction, and the induced charge is pushed to move, creating an induced potential difference (Fig. 2(b)). The free electrons in the circuit are pushed to neutralize the generated potential difference when an external load is connected, producing a current output. This mode is effective in scenarios involving relative sliding motions, such as in touchpads, wearable devices, and interfaces where surfaces are frequently in motion relative to each other.^114–116
2.2.3.3 Single-electrode mode. In this mode, there are no contact wires or electrodes in the free-flowing triboelectric layer. Only one electrode is connected, or it acts as a triboelectric layer on its own. Surface charge transfer takes place when the triboelectric layer and electrode are completely in contact, causing opposite charges to be induced on both surfaces without any electron flow. To balance the electric potential by exchanging charges with the ground, the induced charges on the surface of the lone electrode diminish when the two are separated (Fig. 2(c)). Electrons will move from the ground to the electrode to create equilibrium as they get closer to making contact again until both surfaces are completely in contact.^117,118 This design simplifies the system by reducing the number of electrodes needed and is useful for applications where one of the materials is stationary or grounded, such as in human motion detection and environmental monitoring.^119,120
2.2.3.4 Freestanding triboelectric-layer mode. A single triboelectric layer is attached to two electrodes with a tiny space between them. One triboelectric material functions as a distinct friction layer, while the other is affixed to the two electrodes as an induction layer. Both triboelectric materials are in contact and charged in the air. The electrons are driven by an induced potential difference between the two electrodes, which is created when the independent triboelectric layer moves and alters the charge (Fig. 2(d)). An AC output is produced when the independent triboelectric layer moves periodically, causing electron transfer to occur between the two electrodes.^121,122 The relative movement of the layer between the electrodes induces charge redistribution, making it suitable for energy harvesting from large-scale movements, such as ocean waves and wind energy.¹²³
2.2.3.5 Rolling mode. The rolling mode produces energy by using the rolling action between a triboelectric surface and a rolling element, such as a ball or cylinder. The rolling element repeatedly makes and breaks contact as it travels across the surface, resulting in cycles of charge transfer and separation. Electrons move via an external circuit due to a rapid change in electric potential (Fig. 2(e)). This model is beneficial because it converts mechanical energy from rolling motion into electrical energy with great efficiency and less wear and tear on the materials.^124–126
2.2.3.6 Deformation mode. This new mode was proposed in 2024. The two triboelectric layers are adhered together, and thus both layers are unable to slide or contact-separate. However, because the two layers have differing capacities for storing and adsorbing electrons, there is an electron balance between them. Furthermore, both layers feature excellent stretchability and flexibility. The adsorbing and storing capacities of the two layers must be altered if they deform (stretching, folding, etc.), which causes the redistribution and transfer of charge. Due to the disparity in their triboelectric characteristics, the change must be different. When the electron equilibrium is disturbed, electric signals are produced by electrons moving between layers (Fig. 2(f)). The silver wire that is attached to one layer gathers and outputs the electric signals.¹²⁷ Furthermore, TENG devices operating in this mode perform exceptionally well as self-powered flexible sensors.

3. MXenes: beyond their surface

3.1. The MXene family: configuration and composition

Following the discovery of stable 2D layered graphene, 2D MXenes have attracted considerable interest from various scientific domains due to their evident configurational advantages over their bulk counterparts.^128,129 The presentation of novel 2D transition metal nitride-, carbide-, and carbon nitride-based MXenes by Prof. Yury Gogotsi in 2011 marked an exciting new chapter in the ongoing hunt for more creative innovations and their prosperous application in the sciences.³⁶

MXenes can be divided into several categories based on their number of atomic layers. Based on the transition-metal composition of MXenes, they can be further classified into two unique types. Mono-transition-metal (Mono-M) MXenes are the first group, where only one type of transition metal is present in each of the M layers (Ti₃CN, Mo₂C, Ta₂C, etc.). The second group is double transition-metal (DTM) MXenes, where two different types of transition metals are present at the M sites (Mo₂TiC₂, (W₂Hf₂)C₃, etc.).^130–132 Based on their structural characteristics, DTM MXenes can also be divided into two groups of ordered MXenes and solid-solution MXenes. In-plane order and out-of-plane order are two further classifications of ordered MXenes.^133–136 The transition metals in the M sites of MXenes have a close-packed hexagonal crystal structure, while the X atoms are positioned in the octahedral sites between the M atomic planes. As mentioned above, carbon, nitrogen, or both can occupy the X sites.^137,138 Except for Ti₂NT_x and Ti₄N₃T_x, less research has been done on nitride MXenes owing to their difficult synthesis.¹³⁹ It has been proposed that the C and N atoms in carbonitride MXenes occupy the octahedral positions at random, regardless of the carbonitride stoichiometry.^140,141

The typical method for obtaining two-dimensional materials involves extracting one or a few atomic layers from a layered molecule, where the bonds between the layers are substantially weaker than that within the layers.^142,143 The precursor substance for synthesizing MXene is the MAX phase. High-binding ionic and covalent bonds constitute the M–X bonds in the MAX phase, whereas the M–A bonds are primarily metallic connections with a comparatively low binding strength.^144–146 Thus, employing the appropriate etchants, it is possible to remove the “A” layers from the MAX phases selectively. Selectively¹⁴⁷ etching the A element from the MAX phase results in the formation of exfoliated multilayer MXene flakes.¹⁴⁸ Single-layer or monolayer MXene can be obtained through intercalation and delaminating the multilayer MXene flakes.¹⁴⁹ The family of MXenes is subdivided based on their atomic layers. The reported MXenes (M₂X, M₃X, M₄X, and M₅X) with their atomic arrangement are depicted in Fig. 3. Adding more than 100 potential components to the MXene material, there are 18 M₂X (V₂C, Nb₂C, Cr_1.3C, (Mo_2/3Y_1/3)₂C, etc.), 12 M₃X₂ (Ti₃C₂, Zr₃C₂, (V₂Ti)C₂, (Cr₂Nb)C₂, etc.), 14 M₄X₃ (Ta₄C₃, (Mo₂Ta₂)C₃, (NbTi)₄C₃, (V₃Cr)C₃, etc.), and most recently 3 M₅X₄ ((Mo₄V)C_4, (Ti_2.5Ta_2.5)C₄, (Ti_2.765Nb_2.325)C₄) examples of MXene configurations reported to date.^150–152 The significance of this MXene family stems from the fact that M₅X₄ (n = 4) exhibits characteristics that are absent from other MXenes (n = 1–3). Their thermal stability was found to be the highest among MXenes to date (about 1000 °C compared to less than 800 °C for other MXenes).¹⁵³ More than 130 categories have been added to the MAX phase family thus far, where the M elements have been expanded to lanthanide rare earths ((Mo, R, Nb)₄C₃),¹⁵⁴ the A site has various new elements, which have lower d electrons, and more recently boron has been added to the X site (MBenes—Mo₂B, V₃B₄, and CrFeB₂).¹⁵⁵ Surface-terminating compounds that are not native to the parent MAX phase and come from the etchant aggressively functionalize the extremely reactive M-element surfaces during the MAX to MXene etching process.¹⁵⁶ Depending on the synthesis method and MXene composition, the surface of MXenes is covered with single or mixed terminations (T = F, Cl, Br, O, OH, NH, etc.).¹⁵⁷ The research on the surface of MXene is challenging due to several factors such as the presence of fragile elements such as F, O, and H on their surface, together with water and precursor impurities after etching, and changes in the experimental settings complicate their systematic evaluation.^153,158

Fig. 3 Atomic representations and compositions of M₂X, M₃X, M₄X, and M₅X MXenes: structural differences and reported materials.

Numerous experiments using first-principles computations have demonstrated the propensity for terminating at the bare MXene surface.¹⁵⁹ Upon surface termination, MXenes exhibit a notable increase in negative formation energy, signifying the establishment of robust bonds between the terminating species and their surface M atoms. Etching the MAX phases by immersion in HF or LiF/HCl aqueous acidic solutions results in the formation of MXenes with multiple surface terminations such as F, O, Cl, and OH at different ratios.¹⁶⁰ The synthesis of MXenes with invariable surface terminations was demanding until the development of molten salt etching with the ZnCl₂ precursor. These MXenes have chlorine surface terminations (Ti₃C₂Cl₂).^161,162 This strategy was expanded to provide a generic Lewis-acidic-melt etching technique that works with a variety of systems. The broad spectrum of components seen in Lewis-acidic melts suggests that it may be possible to modify the chemistry and surface terminations of MXenes. MXenes can be synthesized with a series of terminating groups such as Br and I.^163,164 The variety of MXene compositions can be further extended by exchanging these terminations with different functional groups through subsequent surface reactions.¹⁶⁵ The surface chemistry of MXenes, specifically the nature and density of surface terminations such as –O, –OH, –F, –Cl, and –Br groups, plays a pivotal role in determining their triboelectric performance in TENG devices. These terminations impact key parameters such as electron affinity, surface conductivity, hydrophilicity, and interfacial interactions, all of which influence the charge generation, transfer, and retention capabilities of MXenes in TENG applications.^166,167 By carefully tuning the surface terminations and surface roughness, MXenes can be optimized for high-efficiency TENG devices, offering great potential for energy harvesting technologies.

3.2. Triboelectric series: where do MXenes stand?

Two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, known as MXenes, are a new class of materials with special qualities such as durability, improved thermal conductivity, customizable surface chemistry, and high electrical conductivity.^78,168 MXenes are relatively new in the triboelectric series, which ranks materials according to their tendency to gain or lose electrons when they come into contact with another material. Their precise place in the series can change based on their particular chemical composition and surface functionalization.^169,170 Generally, MXenes are positioned towards the electronegative end of the series, indicating a propensity to gain electrons when in contact with many other materials.¹⁷¹

The distinct surface chemistry of MXenes enables them to display both positive and negative triboelectric properties, depending on their structure and the materials they come into contact with in triboelectric nanogenerators. The electron-donating or electron-accepting properties of MXene surfaces can be altered by the presence of different functional groups. MXenes function as a negative element when combined with materials that are more likely to give electrons, and vice versa (Fig. 4). Thus, by selecting appropriate functional groups and pairing materials, MXenes can be engineered to exhibit the desired triboelectric properties, making them versatile components in energy-harvesting devices.^48,172,173 The surface functional groups on MXenes have a significant impact on their triboelectric behavior. MXenes with hydroxyl (–OH) surface terminations typically have moderate electronegativity.^174,175 When in contact with less electronegative materials, they can attract electrons, but they are not as strongly electronegative as materials with other surface terminations.¹⁷⁶ MXenes terminated with oxygen (–O) exhibit higher electronegativity.¹⁷⁷ These MXenes are positioned lower in the triboelectric series, near materials such as polydimethylsiloxane (PDMS), which also tend to gain electrons readily.¹⁷⁸ This makes them more effective at becoming negatively charged when interacting with other materials. Fluorine-terminated MXenes are among the most electronegative in this class due to the strong electron-withdrawing nature of fluorine.^179,180 The properties and advantages of MXenes compared with conventional and advanced materials are presented in Table 1. These MXenes are positioned toward the bottom of the triboelectric series, similar to or slightly above highly electronegative materials such as polytetrafluoroethylene (PTFE).^181,182

Fig. 4 Categorization of positive (electron donors) and negative (electron acceptors) materials of the triboelectric spectrum: MXene as a dual-role contender.

Table 1 A comprehensive comparison of conventional vs. advanced vs. MXene materials

Criteria	Conventional materials	Advanced materials	MXenes
Examples	Polymers (PTFE, PDMS, PET, PVDF), metals (gold, silver, copper, aluminium), ceramics (BaTiO3, PLZT)	TMDs, MOFs, h-BN, graphene, perovskites (methylammonium lead halides), black phosphorus, quantum dots	Ti3C2, Ti2C, Nb2C, V2C, etc., with various surface terminations (–O, –F, –OH, –Cl, –Br)
Charge generation efficiency	Moderate	High	Very high under optimized conditions; influenced by surface terminations, MXene type, and the environment
Mechanical properties	Polymers: flexible but less strong	Highly flexible with elevated dielectric constant but often brittle	Excellent flexibility combined with high mechanical strength
	Metals: strong but not flexible
	Ceramics: brittle
Surface functionalization	Limited	Highly tunable	High tunable: the surface can be modified with functional groups (–O, –F, –OH) for specific triboelectric properties
Environmental stability	Polymers: prone to UV degradation	Highly stable but toxic and requires protective coatings	Moderate; MXenes are prone to oxidation
	Metals and ceramics: good stability but prone to oxidation
Durability	Moderate	Good	High potential durability; longevity depends on encapsulation and oxidation resistance
Cost	Low	High	Currently high due to complex synthesis
Scalability	Easy to produce on a large scale	Still in the development stage	Scalability is a challenge
Design flexibility	Limited	High	Very high

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MXenes are often used to form composites with polymers, metals, or other 2D materials to create hybrid structures with synergistic effects.^183,184 These composites can retain the advantageous qualities of the host material, while utilizing the high charge density, mechanical strength, and flexibility of MXenes.¹⁸⁵ For instance, high-performance TENGs with improved durability and efficiency have been created by mixing MXenes with polymers such as polydimethylsiloxane (PDMS) and polyvinylidene fluoride (PVDF).^186,187 TENGs based on MXene have demonstrated promise in capturing energy from various sources, such as wind, ocean waves, and human motion. MXene composites' high output performance makes them suitable for powering portable sensors and LEDs, among other small electronic equipment, offering a renewable and sustainable energy source.^188–190

3.3. Key indicators for TENG evaluation

Triboelectric nanogenerators (TENGs) are widely assessed based on a set of critical electrical parameters that represent the device efficiency and output capacity when subjected to mechanical stimuli. The critical performance indicators are short-circuit current (I_sc), open-circuit voltage (V_oc), output power (P), transferred charge (Q), and power density (W m⁻²). The maximum potential difference produced between the TENG electrodes in the absence of an external electrical load is known as the open-circuit voltage (V_oc).^3,89 Through contact electrification and electrostatic induction, triboelectric materials demonstrate innate capacity to produce an electric potential. When the electrodes are linked directly along the channel with zero resistance, the maximum instantaneous current that can flow is known as the short-circuit current (I_sc). The total electrical charge transferred between the electrodes throughout a full mechanical actuation cycle, which usually entails a total contact and separation operation, is measured by the transferred charge (Q).^191–193 To precisely monitor the low-current and high-impedance signals necessary for TENG performance evaluation, these parameters were continuously tracked using a high-sensitivity electrometer.

The maximum output power is measured by implementing an array of external resistors as a load and using the formula in eqn (2).

P = IV = I²R = V²/R (2)

Furthermore, power density (P/A) is a crucial metric for evaluating the appropriateness of TENG devices of various sizes for real-world energy harvesting applications. The power density can be calculated using the formula in eqn (3).

P/A = V²/(RA), (3)

where A is the effective contact area of the TENG. Reliable data interpretation requires the accurate assessment of this area. The power density is particularly essential for low-frequency mechanical inputs, which are frequently found in wearable electronics, biomedical devices, and Internet of Things applications that require small and effective energy solutions.^194–196 These performance indicators offer a thorough framework for evaluating and enhancing TENG devices.

3.4. MXene magic: smart synthesis strategies

3.4.1. From atoms to layers: crafting MAX phases for MXenes. Since their invention, numerous physical and chemical approaches have been thoroughly investigated for the synthesis of MXenes. Different synthesis procedures have been devised to create MXenes with rich configurations and distinctive properties by etching their MAX phase.¹⁹⁷ MAX phases serve as precursors to create MXene materials with higher efficacy in various applications. Therefore, creating high-quality MXenes requires the accurate synthesis of the MAX phase and associated characterization procedures.^198,199 The quality and properties of the generated MXene may be adversely affected by the presence of pollutants or phase impurities in the MAX phase precursor.²⁰⁰ The chemical constituents in the periodic table for the synthesis of MAX phases and MXenes are shown in Fig. 5(a). There are 342 MAX phases reported to date. Nearly half of the MAX phases were invented after 2018. MAX phases can be prepared through three techniques, which are subdivided into multiple types. These techniques include molten procedures (molten salt and infiltrating molten metal), physical vapor deposition (PVD), and solid/liquid state reactions. The synthesis of MAX phases is easy, but obtaining large yields can be challenging given that they interact with other phases such as ordered intermetallic alloys, carbides, and nitrides.^201,202 Although PVD techniques yield extremely pure MAX phases, they are not appropriate for producing powder samples. The precursor MAX phases for MXene synthesis are frequently created by pressure-less (atmospheric pressure) reactive sintering in an inert atmosphere (Fig. 5(b)). This method produces porous sintered billets and makes it easier to treat them further into etching powders.²⁰³ Forming MAX phases is most commonly accomplished by liquid/solid-state reaction. To prevent their oxidation, the temperature used for synthesis normally varies from 1100 °C to 1700 °C and is carried out in a protective environment.^204,205 Solid-state reactions are widely used to synthesize MAX phases. After evenly mixing the M, A, and X powders with ethanol, the particles are dried and pressed into a pellet. Then, the pellet is heated in a sealed atmosphere (>1200 °C) such as a vacuum hot press chamber for a specific period. This heating process produces an MAX phase with some intermetallic compounds. Microwave-supported heating is a unique way to achieve temperatures up to 1000 °C within minutes.^206–208 The atomistic pattern of the synthesized MAX (211) phase with M and X elements divided by the A element is shown in Fig. 5(c). The distinctive layered structure of the MAX phases originates from their distinct crystal structure (Fig. 5(d–f)). The SEM image of the Ti₂AlC MAX phase confirmed the formation of the MAX phase by producing a kink band, a feature of MAX phases, which is induced when the dislocations are restricted to their basal planes (Fig. 5(d)).²⁰⁹

Fig. 5 MAX phase—composition, synthesis, and characterization. (a) Elements in the periodic table for MAX phases and MXenes. M denotes the early transition metals (green), A indicates the group A elements (yellow), X indicates carbon, nitrogen, or carbon/nitrogen (purple), and T_x is the surface terminations of MXene after etching (rose). (b) The first step in the synthesis of the MAX phase, the usual MXene precursor, is pressure-less sintering under vacuum with the blend of M, A, and X powders. (c) Atomic arrangement of the synthesized 211 MAX phase. (d) SEM image of the synthesized 211 MAX phase. Scale 500 nm. Reprinted with permission from ref. 209. © 2020, John Wiley & Sons. (e) HR-STEM Z contrast image of the synthesized 211 MAX phase. (f) SAED pattern of the synthesized 211 MAX phase. Scale 1 nm. Reprinted with permission from ref. 210. © 2008, John Wiley & Sons.

To confirm the atomic-stacking sequence and phases present in the MAX precursor, high-resolution scanning transmission electron microscopy (HR-STEM) and selected area diffraction pattern (SAED) characterization were performed (Fig. 5(e and f), respectively). Fig. 5(e) shows the high-intensity Z contrast image of the 211 MAX phase (V_0.5Cr_0.5)₂AlC. The image shows that a black layer of Al separates each of the two light layers of (V and Cr). The atomic arrangement along the [0001] direction is described as ABABAB (the letters represent the Al atom), which is a typical 211-phase structure.²¹⁰ The electron beam in the SAED pattern is parallel to the 1210 direction. The SAED pattern of the 211 MAX phase confirmed its structure based on the differences in spacing for the 0001 reflections (Fig. 5(f)).²¹⁰ Besides the above-mentioned the solid-state processes, the synthesis of MAX phases has also been accomplished via a sol gel-based strategy that includes several gel-promoting chemicals and water-soluble metal precursors.²¹¹

3.4.2. Synthesis and structural assessment of MXenes. The scientific community has reported numerous synthesis processes for preparing MXene compounds based on their etchant, precursor material, and intended use. When transforming the MAX phase to MXene, the M element (transition metals) and A element are crucial in determining the etching technique.^212,213 In addition to being the first MAX phase to be etched, the triadic Ti₃AlC₂ is still the subject of extensive research, and numerous synthesis techniques have been developed to transform it into Ti₃C₂T_x. Firstly, in 2011, hydrofluoric acid (HF) was used for etching the Al element from the MAX phase (Ti₃AlC₂) for synthesizing MXene (Ti₃C₂T_x). It was performed by soaking the MAX phase (Ti₃AlC₂) in hydrofluoric acid for a couple of hours. The resultant suspension was washed several times with DI water until the pH became neutral.²¹⁴ After etching, the layers accommodated surface terminal groups such as –F, –OH, and –O. These surface terminal groups were annotated as T_x.²¹⁵ The process of the HF etching is illustrated through eqn (4)–(6), as follows:

Ti₃AlC₂ + 3HF = AlF₃ + 3/2H₂ + Ti₃C₂ (4)

Ti₃C₂ + 2H₂O = Ti₃C₂(OH)₂ + H₂ (5)

Ti₃C₂ + 2HF = Ti₃C₂F₂ + H₂ (6)

The etching time depends on the M atom. Intense etching is necessary for M atoms with more valence electrons. For instance, the stirring time for Nb₂AlC is longer than that for Ti₃AlC₂.²¹⁶ The MAX phase and MXene need to be characterized using multiple analytical techniques. Fig. 6(a) depicts the Raman spectroscopy analysis of Ti₃AlC₂ (MAX phase) and the etched MXene (Ti₃AlC₂). The Raman spectrum of the precursor Ti₃AlC₂ has prominent peaks in the range of 110 to 680 cm⁻¹, which indicate its distinctive molecular vibrations. In the spectrum of MXene, the peaks at around 204 and 718 cm⁻¹ are smooth and bent, respectively, which are comparable to the MAX phase, representing the proper removal of Al. The remaining peaks in the range of 228 to 470 cm⁻¹ and 525 to 745 cm⁻¹ are the vibrational modes associated with the surface functional groups (T_x) and carbon (C), respectively. Fig. 6(b) shows the XRD assessment of Ti₃AlC₂ (MAX phase) and the etched MXene (Ti₃C₂T_x). The distinctive 002 peak of the MAX phase shifted for MXene from the 2θ value to 9.7° to 6.8°, confirming an increase in the interlayer gap. The same peak 002 broadened and shifted, which were caused by the surface functional groups (T_x) replacing the Al layers owing to the appropriate exfoliation. Fig. 6(c) shows the UV-vis spectra of the MXene (Ti₃C₂T_x). The optical absorption of the MXene (Ti₃C₂T_x) with peaks at 328 and 720 nm is related to the interband conversion and plasmonic bands, respectively. These two peaks indicate the photothermal activity of MXene, particularly the second peak is due to the resonant activation at the surface of the MXene sheets.²¹⁷Fig. 6(d) and (e) show the scanning electron microscopy (SEM) images of the MAX phase (Ti₃AlC₂) and MXene (Ti₃C₂T_x). Fig. 6(d) depicts the morphological feature of the stacked structure of the MAX phase at the magnification of 1 μm. Fig. 6(e) indicates the morphology of the etched MXene with a layered framework and widened gaps between the interlayers.²¹⁸Fig. 6(f) shows the field emission scanning electron microscopy (FESEM) image of the MXene (Ti₃C₂T_x) after HF treatment. The image depicts that the MXene is etched via the nanosheets that are interleaved with each other at the magnification of 3 μm.²¹⁹Fig. 6(g)–(i) show the transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and SAED pattern, and the cross-sectional TEM images of the MXene (Ti₃C₂T_x), respectively. Fig. 6(g) shows the TEM image of the synthesized MXene, demonstrating its flawless surfaces and incredibly thin nature. Fig. 6(h) depicts the HRTEM image and SAED (inset) pattern of the MXene (magnification: 5 nm), showing its hexagonal crystal structure. Fig. 6(i) indicates that the MXene flakes are sufficiently thin to be transparent, in the range of nm. Also, the image shows that the MXene has visible 6 layers with a total thickness of 8 nm.²²⁰ Although HF etching offers multiple advantages such as absolute removal of the A element, superior morphology, and yield, the toxicity and corrosiveness of HF hinder its direct use, given that excess preventive measures and familiarity in handling HF are required. Thus, to reduce the usage of HF, various synthesis routes have been investigated.^221,222

Fig. 6 Characterization of MAX phase and MXene. (a) Raman spectroscopic analysis of MAX phase (Ti₃AlC₂) and MXene (Ti₃C₂T_x). (b) XRD analysis of MAX phase (Ti₃AlC₂) and MXene (Ti₃C₂T_x). (c) UV-vis spectroscopic analysis of MXene (Ti₃C₂T_x). Reprinted with permission from ref. 217. © 2022, American Chemical Society. SEM analysis images. (d) MAX phase (Ti₃AlC₂). (e) MXene (Ti₃C₂T_x). Scale 1 μm. Reprinted with permission from ref. 218. © 2022 MDPI. (f) FESEM analysis image of MXene (Ti₃C₂T_x). Scale 3 μm. Reprinted with permission from ref. 219. © 2018, Royal Society of Chemistry. TEM analysis. (g) TEM image of MXene (Ti₃C₂T_x). (h) HRTEM and SAED pattern of MXene (Ti₃C₂T_x). (i) Cross-sectional TEM image of MXene (Ti₃C₂T_x). Scale 5 nm. Reprinted with permission from ref. 220. © 2018, Royal Society of Chemistry.

In 2014, Michael Ghidiu et al. proposed a technique of incorporating hydrochloric acid + lithium fluoride (HCl/LiF) to remove the A elements to synthesize MXene (Ti₃C₂T_x).⁴¹ Lithium ions persistently penetrate the MXene layer, weakening the link between M and A, eventually removing the A element from the MAX phase. This procedure also generates flammable H₂ gas.²²³ Ammonium hydrogen bifluoride (NH₄HF₂) was introduced as a harmless etchant to etch the Al layers from Ti₃AlC₂.⁴³ The cations present in the bifluoride salts occupy the negatively charged sites of the MXene and expand the interlayer space.²²⁴ This approach is considered to be safe compared to HF etching given that the etching occurs at room temperature. Thus, this technique is recommended for etching the Ti₃AlC₂ MAX phase. However, whether it will be a suitable alternative MAX phase needs to be investigated.²²⁵ The bond between Al and N in nitride MAX phases is robust and the energy required for extraction is higher than that of Al and C. Therefore, etching the A layers from nitrides and carbonitrides is largely unsuccessful when using fluoride-based water solutions. To solve this issue, other fluoride salts such as sodium fluoride (NaF, 12%), potassium fluoride (KF, 59%), and LiF (29%) were also applied for the removal of A elements for the effective synthesis of MXene.^226,227 This molten-fluoride etching takes place at extremely high temperatures. It takes only 30 min for the etching process to be completed, making it an extremely efficient technique.²²⁸ In 2020, Varun Natu et al. synthesized MXene (Ti₃C₂T_z) without the use of water, in the presence of ammonium dihydrogen fluoride and a polar organic solvent with generous fluorine (–F) terminations.²²⁹ A comprehensive survey of MXene exfoliation techniques with their etchants, procedures, advantages, disadvantages, and safety and ecological implications is presented in Table 2.

Table 2 A comparative study of MXene exfoliation techniques: etchants, procedures, benefits, key challenges, and safety and ecological implications^a

Exfoliation method	Etchants	Protocol synopsis	Favorable aspects	Key hindrances	Safety and ecological implications	References
a TMAOH: tetramethylammonium hydroxide, TBAOH: tetrabutylammonium hydroxide, Na2S: sodium sulfide, Br: bromide, LiTFSI: lithium bis(trifluoromethanesulfonyl)imide, TEGDME: tetraethylene glycol dimethyl ether, FeCl3: ferric chloride, BMIMPF6: 1-butyl-3-methylimidazolium hexafluoro-phosphate, EMIMBF4: 1-ethyl-3-methylimidazolium tetrafluoroborate.
Direct HF	Hydrofluoric acid (HF)	Direct HF etching removes A layer	Complete etching of A layer, stable, high yield	Presence of high fluorine contents, highly corrosive	Highly toxic, trained persons needed to handle	214
In situ HF	HCl/LiF, LiF/NaF	Etchants generate HF in situ, increasing interlayer spacing	High-quality MXene, direct delamination, high-surface area	More time to dry, interlayer spacing gets broken or reduced	Cautious stoichiometry of precursors and solvents used, H2 gas risks	41 and 223
Lewis acidic molten salt	CuCl2/TBAOH, ZnCl2, FeCl3, CuCl2/KCl/NaCl, CrCl3	Salts at high temperatures remove A layers	Binding strength, durability, stability	High temperature, undesired byproducts, extra purification	Expertise required for settings (etching time and temperature)	243 and 244
Electrochemical	LiCl/LiOH, KCl, NaCl, NH4Cl/TMAOH	Bias at the anode removes A layers in electrolyte solution	Safe and green synthesis, inexpensive, high lateral diameters	CDC formation, structural damage, workstation required	Electrical protection is needed, and less chemical waste	230 and 231
Algae extraction	Algae	When exposed to algae, MAX changes into an MXene	Low-cost, biocompatible, high-yield, and simple	Prolonged time for etching, controlling the purity is tedious	Algae species should be selected aptly and tested before etching	233
Photochemical	UV light	MAX with acid exposed to UV light, an MXene is generated	Toxic acids are not used, fine structures can be attained, and easy to reproduce	Expensive, multiple processes, the reaction is complicated and can cause impurities	Appropriate clothing and UV protective glass are needed, and UV intensity should be optimized	40
Thermal reduction	Inert gas (e.g., H2)	Treating the MAX with high temperature in an inert atmosphere	Precise structure, large-scale production, high purity, and simple	Low yield, restricted MXene types, high energy consumption	Inhalation hazard, handling of inert gases, high energy demand	245
Hydrothermal alkali	NaOH/Na2S, NaOH, TMAOH	High temperature initiates the MAX phase interaction with hydroxide ions	Fast, simple, high-purity multilayer MXenes, enhanced material stability	Autoclave might explode, oxidation, surface defects	Though greener, it requires waste management to reduce chemical waste	237 and 246
Alkali-based	NaOH/hydrazine, KOH, LiCl/TMAOH	The OH ions attack the A elements of the MAX, resulting in the removal of A element	Fluoride-free, less corrosive, control over etching	Low quality, incomplete etching, limited surface terminations	Caustic nature, heavy metal contamination, PPE equipment needed	247 and 248
Chemical vapor deposition	Metal foil (Ti, Zr), titanium chlorides, CH4, N2	Exposing the surface of the metal foil under the gas mixture diluted in Ar gas	High-quality films, precise thickness, uniform composition, control over surface terminations	High-cost, time-consuming, complex process, post-processing, defects	Toxic precursors, gas leaks, chemical emissions, waste generation	249 and 250
Scalable	CO2, NH4HF2	Etching the MAX phase with supercritical CO2 and NH4HF2 results in MXene (–OH)	Large-scale production, reduction in costs, high yield in a single step	Complex setup, non-commercial, usage of hazardous chemicals (HF), problematic reactor	Be cautious due to exothermic reaction; material disposal is crucial; gloves, face shields, and fume hoods are required	45
Microwave-treated	LiF/HCl, NaOH	Nucleation of etchant atoms into the MAX interlayer gap in microwave causes the A elements to be removed	Time-efficient, low etching time (30 min), less hazardous	Limited etching depth, non-uniform temperature, non-reproducibility	Microwave radiation exposure, impact on air quality, contaminants	208 and 251
Organic base	TMAOH, LiCl/TMAOH	TMAOH initiates the hydrolysis of Al elements, generates Al(OH)4^− ions, and leads to the formation of MXenes	Milder etching condition, compatible with other materials, high conductivity	Incomplete etching, surface chemistry variability, expensive	Inhalation risk, health effects with prolonged exposure, aquatic toxicity, flammability	252 and 253
Halogen + acid/halide	I2/HCl, IBr/ICl/I2/Br2/TBAX	Halogens lead to surface breakdown at the A element of the MAX phase, generating MXenes with halogen-terminated surfaces	High reactivity, rapid reaction rates, good selectivity, enhanced electrical attributes	Toxic, corrosion of equipment, high cost, limited etching depth	Explosion hazards, bioaccumulation, generates fumes, water disposal issues	241 and 254
Halide-based	NH4Br, KBr, NaBr, LiBr, HBr	Br ions are adsorbed, forming soluble metal halides, and the removal of A elements from the MAX	High degree of selectivity, faster etching, simple and cost-effective process	Risk of over-etching, only certain MXenes can be etched, low yield	Regulatory concerns, handling risks, soil and water contamination	255
Photo-Fenton	FeCl3/sodium oxalate	Acceleration of oxygen species weakens the M–A bonds, resulting in the formation of MXenes	Efficient etching, reduction of structural damage, versatility, fluoride-free	Light-dependent, iron ion contamination, pH sensitivity	Sustainability, harmful chemical release, risk to the skin and eyes	37
Gas-phase	TiCl3, CH4	The precursor (TiCl3) was loaded in the reactor at 770 °C and promptly sublimated for nucleation to produce MXene (Ti2CCl2)	Stable operation, large-production, accurate regulation for MXene synthesis	Complex reactor, difficult to maintain constant quality in large-scale production	Precautions required to operate the reactor, explosion risks (CH4), greenhouse gas emissions	44
Li intercalation–alloying–microexplosion	Li foil, LiTFSI, TEGDME	Lithium intercalated into the A layer and forms Al–Ti alloys, and etched into MXene sheets	Precise etching, single- or few-layer MXenes, high-purity, fast exfoliation	Prolonged processing, confined to selective MAX phases, non-uniform layer thickness	Highly reactive, ejection of particles, air pollution, demand of high energy	256
Ionic-liquid (IL)-based ionothermal	BMIMPF6, EMIMBF4, deep eutectic solvents	Fluorine present in the etchants operates as an anion and supports the removal of A element	Mild reaction, enduring stability, extensive tunability, enhanced dispersion, non-volatile	Cost of IL is high, viscosity issues, pH is much higher	Toxicity, spills or leaks can occur, reactive under high temperatures	221
DC magnetron sputtering	HF, NH4HF2	MAX phases were deposited on the substrate via sputtering, after which HF and NH4HF2 were used to etch the A element	Uniform films, large-scale production, high-purity, enhanced adhesion	Expensive, possibility of oxidation, heat generation, undesirable surface terminations	Safety precautions since it operates under high voltage, PPE is required, and metal leakage causes habitat destruction	43
Laser-based	CH4	A thin film of MXene grown over the substrate in a chamber filled with ionized CH4 which produces plasma	Enhanced layer quality, high electronic conductivity, rapid reaction kinetics	Expensive, complex equipment, time-consuming process, undesirable carbon formation	Harmful plasma generation, flammable methane gas, high release of carbon footprint	257

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All wet chemical approaches used to etch the MAX phases are intrinsically electrochemical, that is, electrons are transferred from the A elements of the MAX phases to other groups. During electrochemical etching, the anode and cathode are isolated, and the etching process is governed by the bias. Several electrolytes (NaCl and HCl) were used to etch the tightly packed MAX phases.²³⁰ Nevertheless, the MXenes formed through electrochemical etching are highly vulnerable to the excessive etching of the MAX phase. This is similar to the thorough HF etching of MXenes, which occurs by the removal of only the M element (Ti₂GeC) or both the M and A elements (carbide-derived carbons (CDC)) from the MAX phase.^231,232 For example, W. Sun et al. synthesized MXene (Ti₂CT_x) (fluorine-free) via electrochemical etching using an HCl electrolyte. In this process, Al was initially etched from the MAX phase (Ti₂AlC) with only –Cl terminations and the usual –O, and OH surface functional groups. Then, the outer layer of the MXene (Ti₂CT_x) was again etched to CDC.²³¹ The substantial issue of electrochemical etching is that the MXene (Ti₂AlC) is exposed to a high cathodic potential, resulting in the formation of CDC. Therefore, regulation of the etching voltage, electrode assembly, and etching time is vital to attain efficient MAX phase etching.²²³

The synthesis of MXenes through the abovementioned techniques tends to be expensive, clumsy, and difficult. Alternatively, Shah Zada et al. established a sustainable mechanism that uses algae extract to synthesize a vanadium carbide (V₂C) MXene from the parent V₂AlC MAX phase.²³³ The unique characteristic of the algae extract etching approach is that it is biocompatible in contrast to conventional chemical procedures. Additionally, the algae can be grown under sunlight and is reasonably priced.²³⁴ The process of etching the MAX phase to synthesize MXene is illustrated in eqn (7), as follows:

V₂AlC + algae extract = V₂C + Al − algae extract (7)

However, this etching process takes a long time and is sluggish. Furthermore, it is difficult to maintain consistency and purity during etching. Thus, this synthetic approach has rarely been used.²³³ To produce MXenes with a precise structure and specific morphology, Jun Mei et al. introduced a technique to synthesize a molybdenum carbide MXene (Mo₂C) using UV light. This UV-induced approach is unique and effective in etching the MAX phase (Mo₂Ga₂C). When the MAX phase is exposed to UV light with acid, an MXene is formed. The MXenes are terminated with –OH and –Cl.⁴⁰ The mechanism of etching is illustrated in eqn (8)–(10), as follows:

Mo₂Ga₂C + UV light + 6HCl = Mo₂C + 2GaCl₃ + 3H₂ (8)

Mo₂C + H₂O = Mo₂C(OH)₂ + H₂ (9)

Mo₂C + 2HCl = Mo₂C(Cl)₂ + H₂ (10)

According to reports, MXenes produced using this method show encouraging outcomes when utilized as an anode material for energy-storage applications.²³⁵ MXenes with superior grade and multilayers have not been synthesized using conventional acid techniques. This can be eliminated by incorporating hydrothermal alkali etching, which produces MXenes in a short period compared to the previous approaches.²³⁶ Chao Peng et al. initially synthesized MXenes (Ti₃C₂ and Nb₂C) using less toxic reagents (sodium borohydride (NaBH₄)/HCl) via hydrothermal treatment.²³⁷ Beyond that, Tengfei Li et al. proposed a hydrothermal method to produce a fluorine-free MXene (Ti₃C₂T_x) using alkali (NaOH) treatment. This delivered an MXene with superior purity (above 90%) and surface terminations of –O and –OH. The proposed method is environment-friendly without the use of harmful acids. In the hydrothermal process, the MAX phase reacts with water and hydroxide ions.²³⁸ The hydrothermal activity is depicted with eqn (11) and (12), as follows:

Ti₃AlC₂ + OH⁻ + 5H₂O = Ti₃C₂(OH)₂ + Al(OH)₄⁻ + 2.5H₂ (11)

Ti₃AlC₂ + OH⁻ + 5H₂O = Ti₃C₂O₂ + Al(OH)₄⁻ + 3.5H₂ (12)

This hydrothermal process may be risky given that the etching occurs at high pressure and temperature. Safety precautions should be taken before the reaction to guard against blasts.²³⁹ Although there is a shortage of experimental evidence, reports have indicated that the production of MXenes may be ramped up to an industrial scale. In 2022, Christopher E. Shuck and the team from A.J. Drexel Nanomaterials Institute schemed out a chemical reactor for the mass production of MXenes. In the early stages, they prepared MXenes in two classes (1 g and 50 g) for comparison. In terms of structural morphology, they projected no discernible differences between the two classes of materials.^45,78 Later, Ningjun Chen et al. synthesized large quantities (1 kg) of multiple MXenes using carbon dioxide as an etchant. The usage of carbon dioxide engages the etching of MAX phases with NH₄HF₂, and an –OH–terminated MXene was formed.²⁴⁰ This etching technique is demonstrated in eqn (13), as follows:

Ti₃AlC₂ + CO₂ + 3NH₄HF₂ = Ti₃C₂(OH)₂ + (NH₄)₃AlF₆ + 1.5H₂ (13)

Preventive measures must be taken before initiating the reaction because the etching procedure employed in this method is highly exothermic. All the reported methods are expensive and complex to handle. Huanhuan Shi et al. introduced an approach for etching the MAX phase (Ti₃AlC₂) using iodine. Firstly, the MAX phase (Ti₃AlC₂) is submerged in iodine-acetonitrile (CH₃CN), which forms Ti₃C₂I_x. Then, Ti₃C₂I_x is exposed to an HCl solution for delamination. After delamination, the resultant MXene is Ti₃C₂T_x (–OH terminated). The abovementioned process takes a longer time, and the availability of the precursor material is limited.²⁴¹ An MXene was also delaminated effectively using an ion-gelation process. Here, the MXene was exposed to ions (potassium hydroxide (KOH) surroundings) to convert it into microgels. The resultant monolayer MXene (Ti₃C₂T_x) film was collected through vacuum filtration. This delamination occurs when the NaOH ions introduced in Ti₃C₂T_x.²⁴² The process of delamination is shown in eqn (14), as follows:

Ti₃C₂T_x + NaOH_(aq) = monolayer MXenes (14)

MXenes have also been produced using several techniques, such as chemical vapour deposition (CVD), sputtering, and gas-phase (–Cl terminated) approach. Joseph Halim et al. prepared the Ti₃C₂T_x MXene through a magnetron sputtering procedure. Initially, a thin film (Ti₃AlC₂) was deposited employing a DC sputtering approach in an extreme vacuum apparatus. Then, the film was subjected to NH₄HF₂ for etching. After etching, an MXene (Ti₃C₂) film with an area of 1 × 1 cm² was obtained.⁴³ Sicong Liu et al. used the same sputtering technique to synthesize zirconium carbide (ZrC) above a fiber (D in shape) for the analysis of its optical attributes.²⁵⁸

To control the area and thickness of MXenes, Fan Zhang et al. synthesized a single crystalline Mo₂C film in a large area on a sapphire substrate. This team grew a thin film via plasma-enhanced pulsed laser deposition (700 °C).²⁵⁷ Acid-free etching was carried out by the team of Samantha Husmann et al. They particularly etched the Al component from the MAX phases (Ti₃AlC₂ and Ti₂AlC) adopting an ionic liquid. The ionic liquid intercalated between the layers of the MAX phases and served as an etchant. Even the interlayer spacing could be regulated through this ionic-liquid etching.²⁵⁹ Recently, Maoqiao Xiang and colleagues designed a reactor (fluid-based) for the synthesis of MXene (Ti₂CCl₂). They loaded the precursor (TiCl₃) in the reactor at 770 °C and it promptly sublimated for nucleation. Ar gas was used as the gas-phase precursor for the reactor.⁴⁴ The entire etching mechanism is depicted in eqn (15), as follows:

6TiCl₃ + CH₄ = Ti₂CCl₂ + 4TiCl₄ + 2H₂ (15)

Usually, MAX phases are etched to create MXenes. Di Wang et al. synthesized MXenes (Ti₂CCl₂ and Ti₂NCl₂) uniquely without using the parent MAX phases. They incorporated precursors such as TiCl₄ or TiCl₃, Ti, C or N, graphite, and methane (CH₄) or nitrogen (N₂) to synthesize Ti₂CCl₂ and Ti₂NCl₂ MXenes. The growth of MXenes was established by the CVD procedure (950 °C) with a knotty, unique morphology rather than a layered arrangement.²⁵⁰ Waterless, inexpensive, and harmless pathways for synthesizing MXene (Ti₃C₂) were proposed by Junbiao Wu et al. in 2020. The etching was accomplished using oxalic acid and choline chloride as the precursors, and a small amount of ammonium fluoride (NH₄F) was mixed in an autoclave with the MAX phase Ti₃AlC₂, followed by an ionothermal reaction. These were also termed “deep eutectic solvents”, which are simple to prepare and easy to handle in the process of etching at ambient temperature.²²¹ From 2011 to 2024, the MXene synthesis roadmap (Fig. 7) charts the significant developments in fabrication techniques, such as surface changes, precursor selection, etching procedures, and scalable production strategies.

Fig. 7 Evolution of MXene synthesis: a roadmap from 2011 to 2024. “HF etching” image: reprinted with permission from ref. 36. © 2011, John Wiley & Sons. “Intercalation (DMSO) and delamination image: reprinted with permission from ref. 260. © 2013, Springer Nature. “Bifluoride salt etching” image: reprinted with permission from ref. 43. © 2014, American Chemical Society. “HCl/LiF etching (clay technique)” image: reprinted with permission from ref. 41. © 2014, Springer Nature. “TBAOH delamination” image: reprinted with permission from ref. 42. © 2015, Royal Society of Chemistry. “Tape exfoliation” image: reprinted with permission from ref. 261. © 2015, Royal Society of Chemistry. “Molten fluoride salt etching” image: reprinted with permission from ref. 228. © 2016, Royal Society of Chemistry. “HCl/LiF etching (MILD) method” image: reprinted with permission from ref. 262. © 2016, The American Ceramic Society. “Electrochemical etching” image: reprinted with permission from ref. 38. © 2018, John Wiley & Sons. “Alkali etching” image: reprinted with permission from ref. 263. © 2018, John Wiley & Sons. “Fluoride-free molten salt etching” image: reprinted with permission from ref. 161. © 2019, American Chemical Society. “Lewis acid molten salt etching” image: reprinted with permission from ref. 39. © 2020, Nature. “UV-induced etching” image: reprinted with permission from ref. 40. © 2020, Elsevier. “Large-scale synthesis” image: reprinted with permission from ref. 45. © 2020, John Wiley & Sons. “Iodine-assisted etching” image: reprinted with permission from ref. 241. © 2021, John Wiley & Sons. “Photo-Fenton approach” image: reprinted with permission from ref. 37. © 2022, American Chemical Society. “Halide-based etching” image: reprinted with permission from ref. 255. © 2023, Elsevier. “Gas-phase synthesis” image: reprinted with permission from ref. 44. © 2024, Elsevier.

The hazardous chemicals utilized in the synthesis of MXene, particularly concentrated hydrofluoric acid (HF), which is extremely corrosive and presents major health and environmental risks, are intimately related to biosafety issues in research.²⁶⁴ HF exposure is inappropriate for large-scale or biomedical applications, wherein biosafety is a top concern given that it can result in significant tissue damage, systemic harm, and the need for stringent handling procedures. Thus, alternative etching techniques have been developed to overcome these restrictions. The LiF/HCl approach has been widely used for safer synthesis given that it produces HF in situ, greatly reducing the amount of concentrated HF that must be handled directly.^41,222 Fluoride-free techniques employing chemicals such as tetramethylammonium hydroxide (TMAOH) have surfaced recently, providing a safer and more environmentally friendly path with an equivalent etching efficiency.²⁴⁴ In addition to improving the lab safety, these developments satisfy the increasing need for biocompatible and ecologically friendly materials for wearable and biomedical applications. To guarantee the biosafety of MXenes in real-world applications, low-toxicity synthesis techniques must be developed and adopted further.

MXenes are highly desirable for the fabrication of triboelectric nanogenerators (TENGs) due to their excellent conductivity. However, the process of oxidation impairs their performance by changing their surface charge behavior and reducing their conductivity. Exposure to the environment (moisture, oxygen) is the primary cause of oxidation, particularly at the flake edges and inadequate defect areas. Degradation is initiated by surface terminations such as –OH and –F groups, which promote the production of TiO₂.^265,266 Atomic defects, surface terminations, constrained water, and synthesis techniques all have a major impact on the oxidation of MXenes. Particularly, at high temperatures, the constrained water molecules entrapped between the MXene layers promote oxidation, forming TiO₂ and amorphous carbon, which are more difficult to remove than surface-adsorbed water.^177,267 Given that the (010) and (100) planes are more reactive and allow the quicker diffusion of oxygen than the (001) plane, oxidation mostly starts in defect-prone locations, such as the low-coordination M atoms. With time, MXene degrades into titanium oxides (such as TiO₂) and disordered carbon, which results in black and white degradation byproducts that reduce their effectiveness, particularly under aqueous conditions. Similarly, it has been reported that MXenes based on Nb and Mo break down in water within days, impairing their electrical and dielectric qualities that are crucial for TENGs.^268,269

Therefore, methods to improve the stability of MXenes are essential. In this case, a popular technique is polymer encapsulation, which reliably maintains the triboelectric output for days by preventing oxygen and water intrusion through hydrophilic barriers such as PVA or hydrophobic layers such as PDMS.^270,271 Regulated synthesis and termination customization is another useful tactic. For instance, post-synthesis annealing can change unstable surface groups into more inert terminations, while softer etching chemicals such as LiF–HCl decrease the number of oxidation-prone sites in comparison to HF.²¹³ Furthermore, it has been demonstrated that low-temperature preservation (−20 to −80 °C in an inert atmosphere) and solvent-assisted storage (such as in ethanol or DMSO) significantly lower the oxidation rates of MXenes.^272,273 These strategies are essential for improving the long-term stability of MXenes in TENGs, where exposure to environmental conditions is unavoidable. They can be applied to a variety of MXenes beyond Ti₃C₂T_x. Thus, for MXenes to reach their full potential in practical energy applications, chemical, physical, and engineering solutions must be integrated.

4. Unique attributes of MXenes in TENGs

TENGs are the most effective option for capturing mechanical energy from widely accessible natural resources for harvesting electrical current. According to early TENG research, several polymers have been explored to fabricate TENGs, such as PVDF,²⁷⁴ PDMS,²⁷⁵ polytetrafluoroethylene (PTFE),²⁷⁶ poly(vinylidene fluoride-co-trifluoro ethylene) (PVDF-TrFE),²⁷⁷ polyurethane (PU),²⁷⁸ polyvinyl alcohol (PVA),²⁷⁹ and polyethylene phthalate (PET).²⁸⁰ The primary barrier to collecting current from TENGs is the insulating attribute of these polymers. Due to their thermal and chemical instability, an extra current-collecting material is always needed. As MXene was initially shown to be a negative triboelectric material in 2017, it has been used widely in TENGs.^182,281 This section presents the benefits of MXene (Fig. 8) over other 2D and polymer materials as a multipurpose material for the fabrication of next-generation TENGs.

Fig. 8 Unique attributes of MXenes in TENG.

4.1. Tailorable surface terminations

T_x in the MXene representation (M_n+1X_nT_x) denotes the surface terminal groups present after the etching process of the MAX phase. T_x can be –O, –OH, –F, –Cl, –Br, –I, –S, etc., depending on the etchant and technique used to etch the MAX phase. These generous surface terminations equip MXenes with exceptional electronegativity, tailorable surface chemistry, and facilitating interface with the molecular dipoles of the polymers.^177,282 The standard etchants (HF or LiF/HCl) used for etching the MAX phase establish the –O, –F, and –OH terminations on the MXene surface. When a high concentration of HF is used for etching, –F terminations are dominant in MXenes. In contrast, using light LiF/HCL etchants, the ratio of –O groups its higher than that of –F.²⁸³ Owing to these surface terminations, MXenes tend to act as either positive or negative triboelectric materials based on the paired material. Long Jin et al. controlled the surface terminations of an MXene (Ti₃C₂T_x) and analyzed the performance of TENG with an MXene and alkali-treated MXene. In the alkali-treated MXene, the fluorine terminations were reduced. Consequently, the TENG exhibited a superior performance with the MXene due to its rich fluorine groups supporting exceptional electron affinity.¹⁶⁹ The surface terminations of MXenes, mainly fluorine groups, play a key role in the array of triboelectric materials. Besides fluorine terminations, MXenes can also act as positive triboelectric materials by altering their surface terminations.

4.2. 2D layered configuration with a large surface area

After etching, MXenes are thin nanosheets with a lamellar structure. This 2D layered configuration enhances the contact area with the other side of the triboelectric material, enabling substantial electron transfer when the two triboelectric layers are in contact.²⁸⁴ Also, other metal oxides²⁸⁵ and 2D materials such as molybdenum disulfide (MoS₂),²⁸⁶ metal–organic frameworks (MOFs),²⁸⁷ graphene,²⁸⁸ and black phosphorus²⁸⁹ can be intercalated in the interlayer spacing of MXenes, preventing their oxidation, resulting in outstanding charge retention ability, and promoting superior electrical output. The efficiency of energy conversion also increases due to the combination of the conductivity and surface terminations of MXenes.²⁹⁰ Prolonged investigation of MXene intercalation, attention to unique surface terminations, and doping with other materials are envisioned to ignite new avenues for green energy harvesting.

4.3. Versatile fabrication and integration

MXenes show phenomenal flexibility in fabrication approaches and doping with other materials, making them ideal for multiple types of TENG designs. Due to their inherent attributes, MXenes can be prepared into numerous forms, such as films, composites, inks, and coatings.¹⁷⁸ The processing of MXene colloidal solutions is easy given that they are hydrophilic and readily dispersed in water or solvents, which can be used for mold casting, printing, and spraying on the desired surfaces. MXenes are suitable with both traditional and modern production techniques, facilitating the economical and scalable manufacturing of TENG.^291,292 The approaches for the fabrication of MXene-based TENGs (MX-TENGs) include molding,²⁹³ vacuum-filtration,⁴⁶ spraying,²⁹⁴ spinning (electrospinning, solution blow spinning, wet spinning),^295–297 chemical crosslinking,²⁹⁸ spin coating,²⁹⁹ printing (screen and 3D),^300,301 and fabric framework.³⁰² The approach for fabrication should be meticulously selected depending on the desired application and the challenge of the proposed TENG. Md Mehdi Hasan et al. introduced a thermal drawing approach for manufacturing MXene (Ti₃C₂T_x)/PVDF fibers with a length of 1 km in a single operation. Using textile looms, the fibers could be generated commercially and sufficiently adaptable to be knitted as fabric. Consequently, the fabric-based TENG was confirmed to show potential to capture the mechanical energy from human motions.³⁰³ In the context of (TENGs), the flexibility of MXenes in device integration is one of their notable features. MX-TENGs can be seamlessly integrated as wearable devices by integrating MXenes and polymers and simply placing them on the body to harvest energy from human activities.³⁰⁴ The triboelectric feature of MXene in TENG has a direct impact on the applied mechanical force. With this feature, TENG derived from MXene can be converted to a self-powered sensor for monitoring the vital signs of humans, vibrations in industries, humidity, and pressure in the environment.³⁰⁵ To harvest energy from numerous sources in the environment, MX-TENGs can be integrated with piezo and thermo-electric materials and function as hybrid devices.³⁰⁶ MX-TENGs have exceptional capability to continuously power low-energy IoT devices and wireless sensors in IoT networks for monitoring agriculture, and smart cities.¹⁹⁶ In this area, MX-TENGs harvest energy from the environment such as wind, water waves, and vibrations from the vehicles to power the IoT devices without an external power source.³⁰⁷ The flexibility of MX-TENGs in fabrication and integration in multiple ways makes them perfect for extensive energy harvesting, specifically in healthcare, autonomous systems, and environmental monitoring.

4.4. Augmentation of dielectric quality

Incorporating MXene as a filler in insulting polymers (PTFE, PDMS, PVA, PVDF, etc.) significantly increases the dielectric permittivity, enhancing the surface charge density in TENGs. This enhancement is caused by the polarizability and conductivity of MXenes, elevating the charge accumulation and retention, which boosts the efficacy of TENGs. The technique of mixing conductive or ceramic materials in polymers for elevating their dielectric constant is known as dielectric modulation.^308,309 Trilochan Bhatta et al. demonstrated the approach of compositing MXene into PVDF to enhance the dielectric quality of TENG for the first time in 2021. The nanofibers were prepared by electrospinning an MXene/PVDF blend to improve the dielectric constant. It was revealed that the dielectric constant of MXene/PVDF increased by 270% compared to that of the pristine PVDF nanofiber.¹⁹³ Blending MXenes in the polymer molecular chains initiated the development of surface dipoles within the hydrogen atoms of the polymers and F and O atoms of the MXenes. When exposed to an external electric field, the dipoles gravitate to the direction of the electric field, increasing the dielectric constant of the material.^303,310 When MXene fillers are closely spaced in the polymer chains, a fragile dielectric layer is formed in the intersection, generating a capacitor configuration. This capacitor model elevates the dielectric constant and smoothens the charge transfer in TENGs.^311,312 The content of the MXene as filler must be appropriately regulated because to accommodate the closeness of the fillers it should not be too low; on the other hand, an excessive content of MXene fillers can result in tunneling charge injection and reduced charge capture capability.³¹³ In contrast, R. L. Bulathsinghala and group incorporated a group of liquid fillers to analyze the inherent effect of the dielectric constant in TENGs. The results revealed that increasing the dielectric constant at a low load and decreasing the dielectric constant at a high load enhanced the electrical parameters of the TENG.³¹⁴ The dual perspectives are not mutually exclusive, MX-TENGs may not be able to handle high loads, but they boost the other effective parameters required for superior TENGs.

4.5. Bifunctional role

MXenes can be applied as both the triboelectric layer and electrode in TENGs. Owing to the electronegative nature of MXenes, they tend to be employed as superior triboelectric negative materials in TENGs. Several forms of MXene composites have been prepared using traditional and advanced manufacturing techniques such as surface-modified films, hydrogels, 3D frameworks, and fiber mats.¹⁹⁰ Mirza Mahmood Baig et al. analyzed the performance of a silica nanosphere-intercalated MXene (Ti₃C₂T_x) as a highly efficient tribonegative layer in TENGs. This intercalation eliminated the restacking of the MXene nanosheets and increased the interlayer spacing between the sheets. Also, the electronegative nature of the MXene increased through intercalation and the negative surface charges of the TENGs.³¹⁵ In contrast to the above-mentioned point of view, Viet Anh Cao et al. incorporated a chemically modified Ti₃C₂ MXene as both the tribopositive and tribonegative friction layer simultaneously, and analyzed the performance of the fabricated TENGs. In the positive and negative triboelectric layer, the surface of the Ti₃C₂ MXene was terminated with NH₂ and N groups using the precursor of ammonia solution for positive attributes and urea (NH₂CONH₂) for negative attributes. Then, the positive Ti₃C₂ modified with NH₂ and the negative Ti₃C₂ modified with N were composited with nylon and PVDF-TrFE for the fabrication of TENGs. The results revealed that the mechanical performance was enhanced with superior stability and durability in 14 400 cycles.³¹⁶ Likewise, Bernd Wicklein et al. demonstrated the relation of Ti₃C₂T_x and Ti₃CNT_x with sodium alginate biopolymers in TENGs. It was proposed that the MXenes can be used as either the positive or negative triboelectric material based on the other side of the contact material. They performed the analyses with different dielectric polymers and exhibited that the Ti₃CNT_x/sodium alginate composite acted as the positive electric layer with opposite fluorinated ethylene propylene negative layer. Also, they inferred that the Ti₃CNT_x composite delivered a superior triboelectric performance than the widely investigated Ti₃C₂T_x. The electrodes in the TENG are responsible for capturing the charges created during the contact electrification and transferring the charges to external devices.³¹⁷ Thus, the appropriate choice of electrodes and patterns is crucial in enhancing the flexibility and performance of TENGs.³¹⁸ Due to their attributes of superior conductivity and electrical parameters, MXenes have excelled as superior electrodes in TENGs. MXenes are widely applied as electrodes (positive or negative) in TENGs in the form of liquid, hydrogels, fabrics, and transparent materials.³¹⁹ Zichao Zhang et al. incorporated a unique approach called vacuum-filtration for constructing MXene electrode-based TENGs. Firstly, they prepared an MXene/PEDOT:PSS film through vacuum filtration using a PTFE membrane as a high-conductive electrode. PTFE is a membrane with a porous pattern that directly operates as a triboelectric layer. The entire TENG was fabricated in single-step, assuring close contact between the electrode and triboelectric layer, which enabled uninterrupted charge transfer between the layers.⁴⁶ Owing to their special blend of conductive and triboelectric qualities, MXenes can substantially improve TENG performance. The particular demands of the application, such as output performance, versatility, and architectural simplicity, will determine whether MXenes are used as an electrode, triboelectric layer, or both.

4.6. Intensifying robustness with exceptional conductivity

Maximizing the mechanical functionality and effective charge transfer is vital in TENG operation, as these devices are applied under frequent mechanical stress. Density functional theory (DFT) and molecular dynamics (MD) simulations revealed that the in-plane Young's modulus of MXenes was substantially greater than that of graphene with a similar thicknesses. According to DFT simulation, the Young's modulus of several carbide MXenes is larger than that of their parent MAX phases, ranging from 500 to 800 GPa.^320,321 Mixing even a tiny amount of MXenes in polymers enhances the crystallinity, Young's modulus, and tensile strength of the polymer matrix.³²² Kangshuai Li et al. applied an MXene to increase the mechanical performance of a TENG and used it to detect human motions and spot handwriting. Firstly, they prepared the hydrogel using hydroxypropyl methylcellulose and polyacrylamide. Then, the MXene was added to the hydrogel, which enhanced the hydrogen bonds due to the excessive hydrophilic atoms present in the MXene. The mechanical property analysis revealed that in the absence of the MXene, the hydrogel stretched to 1800% strain, whereas with the MXene-composited hydrogel, the network attained a maximum stretching of 2260% strain. It was inferred that incorporating MXenes increased the stability under multiple loading/unloading cycles, recovery rate, and strain of the hydrogel.³²³ The electrical conductivity of MXenes (10⁵ S m⁻¹) is superior to that of other materials such as metal oxides, graphene, MOFs, and conducting polymers in terms of charge transfer efficiency.³²⁴ The contact and separation of TENG devices result in the formation of charges by triboelectrification, and these charges flow through the electrode to reach the external connections. Thus, the conductivity of MXene ensures the fast and effective transfer of charges by reducing the charge decay, resistance, and energy loss. Moreover, the electron double layer is enhanced, resulting in long-term charge retention, enabling a maximum output in TENGs.^325,326

Owing to their customizable surface terminations, MXenes substantially enhance TENG performance by maximizing interfacial compatibility and charge transfer. Their 2D structure and high surface-to-volume ratio optimize the active contact regions for charge production. Furthermore, their ability to improve the dielectric characteristics increases the triboelectric efficiency, while the diverse production techniques for MXenes allow their smooth incorporation into a variety of systems. Additionally, MXenes with a bifunctional role enable the fabrication of robust and flexible TENG devices. MXenes offers a revolutionary framework for creating high-performance, enduring, and scalable TENG devices for cutting-edge energy-harvesting applications.

5. Embedding MXenes in TENG devices

5.1. MXene as a next-gen electrode

In TENG, the charge transfer occurs between the electrodes during sporadic contact based on the variations in their electronegativity. The oppositely charged electrodes are separate from each another, creating an electrical potential difference akin to that of a capacitor. The voltage generated from the contact between the electrodes and the current that can be extracted from the TENG is determined by the electrical conductivity of the electrodes.³²⁷ Therefore, to extract effective electrical power from a TENG, its electrodes must be electrically conductive. Triboelectrically negative electrical conductors are extremely rare; however, triboelectrically positive electrical conductors that are classified high in the triboelectric family exist.³²⁸ Considering MXene is an excellent electrical conductor, it can be utilized as the current collector-electrode in TENGs. MXene electrodes can boost the efficiency of TENGs by lowering their internal resistance.³²⁹

Beginning with the first analysis of an MXene in a TENG device, which was published in 2017, an MXene was incorporated as electrodes in both flexible and solid TENG devices to harvest productive energy in a two-electrode design. Firstly, the Ti₃C₂T_x MXene solution was coated on a glass substrate, which was used as the bottom electrode, as shown in Fig. 9(a). To introduce an air gap between the top and bottom electrode, 4 insulating spacers were fixed at the corners of the substrate. Then, an ITO-coated PET film was used as the top electrode. The entire illustration of the fabricated device is shown in Fig. 9(b). Fig. 9(c) shows that the fabricated TENG device was highly flexible. To evaluate its flexibility further, the TENG device was fixed at 30° in a horizontal plane and the force of 1 N was applied to its loose end through a mandrel (Fig. 9(d)). The fabricated TENG was flexible and merged with accessories to harvest electrical energy from the movements of the body.¹⁸² To attain the maximum electrical output in a cost-effective manner and hassle-free synthesis, Shoaid Anwer et al. proposed a TENG based on an MXene negative electrode with a unique structure and enhanced stability. Initially, the Ti₃C₂T_x MXene was spray-coated on warm Al foil to construct the negative tribo-electrode. Then, the MXene-coated Al film (15 μm) was wrapped with a PET film for shielding. In the case of the positive tribo-electrode, PVA/NaCl was coated on an Al film using the doctor-blade approach. A thick PET film was used to anchor the triboelectric layers and to maintain the distance of 8 mm between the electrodes. The entire configuration of the fabricated TENG is shown in Fig. 9(e). This device could power 500 LEDs and precisely observe the physiological motions of humans with a durable sensing performance.³³⁰ Maximizing the simplicity of the fabrication process of TENG further, Jiacheng Fan et al. incorporated MXene as an electrode layer with enhanced flexibility and durability. The cotton fabric was submerged in an MXene/cellulose nanofiber (CNF) solution. MXene/CNF was evenly distributed on the fabric due to electrostatic absorption and used as an electrode. The friction layer was prepared by molding an Ecoflex solution and spin-coated with AgNWs. The fabricated device could detect the mass of steel objects and sustain various deformations, indicating that it can be used as a wearable to detect human motions.³³¹ Detecting ions in humans requires pre-enzymes such as urease, DNAzymes, and glucose oxidase enzymes. These enzymes need to be modified specifically to detect the target of interest. When detecting glucose, the enzyme must be biocompatible, stable, and compatible with electrochemical systems.³³² Addressing the above-mentioned limitations of glucose detection, Wei Yang et al. designed a TENG device incorporating MXene/graphene oxide (GO) as an electrode for glucose detection. The MXene/GO film was prepared by mixing the exfoliated MXene and GO through an ultrasonic approach (Fig. 9(f)) and the mixture was filtered using a vacuum-filtration technique. After drying, the MXene/GO film easily peeled off. An Ecoflex solution was spin-coated on the glass substrate and Cu tape was fixed at the back of the Ecoflex film. MXene/GO was employed as the top electrode, Cu tape as the bottom electrode, and Ecoflex as the triboelectric layer (Fig. 9(g)). To detect glucose, glucose-dissolved water was dropped on the surface of the electrode and dried at room temperature.²⁸⁸

Fig. 9 MXene as electrodes in TENG. (a) MXene solution was coated on a glass surface. (b) Top PET-ITO electrode and bottom MXene-coated glass electrode were divided by spacers composed of insulating glass, which was about 1 mm thick. (c) Evaluation of the flexibility of MXene TENG by manual bending. (d) MXene TENG was held at a 30° angle and flexed by a mandrel at 1 N per second. Reprinted with permission from ref. 182. © 2018, Elsevier. (e) TENG device with a spray-coated MXene shielded by PET and PVA/NaCl coated on Al foil. Reprinted with permission from ref. 330. © 2023, Elsevier. (f) Approach for the fabrication of MXene/GO electrode. (g) Schematic of TENG device with MXene/GO film as the electrode. Reprinted with permission from ref. 288. © 2023, MDPI. (h) Diagrammatic representation of the fabrication procedure of MXene/CNF liquid electrode for TENG. Reprinted with permission from ref. 335. © 2020, John Wiley & Sons. (i) Layout dimensions and architecture of the MXene/PVA hydrogel electrode-based TENG. Reprinted with permission from ref. 298. © 2021, John Wiley & Sons. (j) Diagram showing the construction of MXene/PVA TENG with hydrophobic PVDF electrode. Reprinted with permission from ref. 47. © 2023, Elsevier. (k) Structure of chitin nanocrystals. (l) Fabrication of MXene/chitin nanocrystals through vacuum-filtration approach. Reprinted with permission from ref. 339. © 2024, Elsevier. (m) Copolymerization of MXene/CNF/PAS hydrogel. (n) Schematic of single-electrode TENG. Reprinted with permission from ref. 340. © 2024, Elsevier. (o) Fabrication of TENG skin patch based on MXene/gelatin hydrogel. Photographs of (p) TENG skin patch and (q) TENG skin patch applied to treat a mouse. Reprinted with permission from ref. 341. © 2024, Elsevier.

Skin-mounted electronics are more likely to use single-electrode TENGs due to their potential benefits of simple structure and portability. The appropriate electrode material is the most important element in ensuring that single-electrode TENGs operate normally. In this case, liquid electrodes are considered an excellent choice for constructing efficient single-electrode TENGs.^333,334 W. Cao et al. reported the fabrication of a single-electrode TENG based on an MXene composite as a deformable liquid electrode. Initially, the delaminated MXene Ti₃C₂T_x and naturally extracted CNF were mixed uniformly through stirring. Alternatively, liquid silicon was poured into plain PTFE and sandpaper-fixed PTFE molds separately. Then the two silicone rubbers were attached to form a capsule-like assembly. Finally, the MXene/CNF dispersion was injected into the capsule (Fig. 9(h)). A TENG was fabricated with MXene/CNF liquid as the electrode and silicone rubbers as the triboelectric and packaging layer. Employing this method, the application potential of single-electrode TENGs has been accelerated in harvesting energy and physiological signal monitoring with superior sensing properties.³³⁵ However, liquid electrodes necessitate a packaging layer to avoid leaks and are vulnerable to evaporation, non-biocompatible, less flexible, and pose chemical hazards, which can reduce their performance over time.³³⁶ Xiongxin Luo et al. fabricated a TENG device using an MXene/PVA hydrogel as an electrode to eliminate the above-mentioned drawbacks. MXene/PVA was prepared using PVA as the base solution and MXene as the crosslinking substance. Two Ecoflex layers were prepared with the same size and the prepared MXene/PVA hydrogel was encapsulated between them. The top and bottom Ecoflex layers were connected to prevent water leakage from the hydrogel (Fig. 9(i)). The MXene fillers facilitated the crosslinking of the PVA hydrogel and created microchannels within the hydrogel. This enhanced ion transport enabled the TENG to stretch about 200% from its original length.²⁹⁸ Triboelectric charges dissipate more quickly in hostile environments such as high humidity, biofouling, and elevated friction at the surface of the ocean, which will shorten the tribo-electrode service life. These factors significantly affect the stability and efficiency of energy harvesting and restrict the use of TENGs in the marine environment.^337,338

A novel TENG with enhanced triboelectric properties at 95% humidity and an effective sterilization rate of 98.9% was fabricated by Xiao Sun et al. using MXene/PVA as electrodes. The MXene/PVA electrode was prepared by mixing PVA and MXene solutions until their dispersion and pouring the dispersion into a PTFE mold, drying, and peeling it off. The alternate electrode was prepared using a 3D-printed stainless-steel substrate and PVDF nanowires to elevate the hydrophobicity of PVDF. Then, both the MXene/PVA electrode and PVDF electrode were assembled to fabricate a TENG device. As shown in Fig. 9(j), adding MXenes enhances the number of hydrogen bonds on the surface of PVA, which fixes more water molecules there. These water molecules act as an electropositive material and contribute to the friction process. This shows that the TENG can withstand water treatment with a long life in the oceanic environment.⁴⁷ Decreasing the electron affinity of MXenes enhances their charge-storing and charge-transfer capability in TENGs. Adding adhesives to MXene increases its electrical conductivity, which is highly required for its application as an electrode in TENG.³⁴² Yunqing He et al. proposed a single-electrode TENG based on an MXene/chitin nanocrystal composite as an electrode. Chitin nanocrystals were extracted from the disposed shell of shrimp through a hydrolysis approach (Fig. 9(k)). The composite was prepared with the synthesized MXene and chitin in different ratios through vacuum filtration (Fig. 9(l)). Then, the composite was packed using PDMS to avoid the oxidation of the MXene/chitin film. Due to electrostatic attraction and hydrogen bonding, chitin stuck to the surface of the MXene nanosheets when combined in a certain ratio (Fig. 9(l)). The negative charge on the surface of the MXene nanosheets was continually neutralized as the weight ratio of chitin to MXene increased. This reduced the electron affinity and increased the conductivity and robustness of the TENG.³³⁹

The high water content in hydrogels creates a thin barrier, which often keeps hydrogels from sticking directly to the skin. During repetitive deformations in TENG, efficient electrical signal detection and transmission depend on the excellent adherence of hydrogels.^343,344 A hydrogel with exceptional adhesive properties as a TENG electrode was proposed by Wei Zhang et al. using an MXene as the crosslinker, acrylic acid, 2-(methacryloyloxy)ethyldimethyl-(3-sulfopropyl)ammonium hydroxide, and acrylic acid N-hydroxysuccinimide ester were copolymerized and dissolved with CNF in water and potassium persulfate was added in a single process to create hydrogels (Fig. 9(m)). The MXene acted a multipurpose crosslinking agent that accelerated the gelation of the hydrogels, in addition to serving as the conductive filler. The TENG device was fabricated by encapsulating the MXene hydrogel electrode between two PU films (Fig. 9(n)). Even under moist conditions, the MXene hydrogel-based TENG device adapted to the surface of the skin and demonstrated advantageous self-adhesive qualities.³⁴⁰ Likewise, Bo-Yu Lai et al. used an MXene in a PVA/PEG hydrogel to further enhance the streaming vibration potential in TENGs. The fluorine-terminated MXenes in the hydrogel served as an ionic conductor (electrode), initiating an electrostatic mechanism, which in turn increased the effectiveness of the TENG device.³⁴⁵ Coupling photothermal materials with TENG devices, which combine electrical and thermal stimulation to fasten wound healing, is considered a favorable approach.³⁴⁶ A single-electrode mode TENG was fabricated based on an MXene/gelatin hydrogel as an electrode with strong wound-healing capability by Meiru Mao et al. The hydrogel was prepared by mixing an aqueous MXene solution and gelatin-dissolved water; afterwards, glycerol was added to the mixture (Fig. 9(o)). After vigorous stirring, the hydrogel was prepared. The TENG device was fabricated by sandwiching the hydrogel between the two Ecoflex layers and glued together with Ecoflex solution (Fig. 9(p)). The hydrogel-based TENG device was proven to accelerate wound healing in mice (Fig. 9(q)) by enhancing cell proliferation and has wide potential in other biomedical applications.³⁴¹

Attaining a dual-function sensor is the most demanding in electronic devices. In this case, fiber membranes have been widely used in both TENG and strain sensors. Fiber composite membranes are prepared via the electrospinning technique.^352,353 Qingsen Gao et al. fabricated a unique MXene composite membrane for both strain sensing and as an electrode in TENG for harvesting energy. First, a thermoplastic urethane conductive nanofiber film (TPU) was prepared with a TPU-dissolved solution by electrospinning. TPU was submerged in a polydopamine solution to enhance the binding force between the fiber and MXene. Then, the film was submerged in the prepared MXene/carbon black solution. After drying, a nanofibrous membrane with outstanding conductivity was obtained (Fig. 10(a)). At last, the fiber was used as a TENG electrode and strain sensor for harvesting energy and monitoring human activities.³⁴⁷ The nanostructure of an MXene was altered through the oxidation technique and used as a conductive filler in polyacrylamide for the preparation of a hydrogel. After etching, NaOH was added to the MXene solution to increase the number of hydroxyl groups in the MXene. The high amount of hydroxyl groups in MXene reacted with the Ti atoms, resulting in the production of TiO₂ nanowires and protecting the MXene from further oxidation (Fig. 10(b)). This oxidation treatment enhanced the dispersibility, sensitivity, and transparency of the MXene.³⁴⁸

Fig. 10 MXene as an electrode in TENG. (a) Schematic illustration of the electrospinning process to prepare the MXene/PDA/CB/SR/TPU composite fiber. Reprinted with permission from ref. 347. © 2024, John Wiley & Sons. (b) Schematic illustration of the preparation of oxidized MXene/polyacrylamide hydrogel. Reprinted with permission from ref. 348. © 2024, Elsevier. (c) Schematic illustration of the preparation of melamine/MXene/PDA/AgNW foam. (d) Single-electrode melamine/MXene/PDA/AgNW foam-based TENG. Reprinted with permission from ref. 349. © 2024, John Wiley & Sons. (e) Schematic illustration of the MXene/PMMA electrode TENG. (f) Preparation process of MXene/PMMA composite film through vacuum-filtration. Reprinted with permission from ref. 350. © 2023, John Wiley & Sons. (g) Electrohydrodynamic printing process of PAM/CNF/MXene conductive hydrogel. Single-electrode TENG based on conductive hydrogel. (h) Graphical representation. (i) Photographic image. Reprinted with permission from ref. 351. © 2025, American Chemical Society.

A transparent ionic liquid-based MXene–AgNW composite was also prepared on a large scale (20 cm wide roll) as a flexible transparent electrode for electromagnetic shielding and TENG. The composite exhibited excellent stability, and durability even at elevated temperatures.³⁵⁴ 3D foams exhibit enormous potential in TENGs for harvesting energy. Chenhui Xu et al. proposed a TENG device using MXene foam as the negative electrode. Melamine foam was dipped into a PDA solution under stirring, and then the melamine foam modified with PDA was dip-coated in an MXene/AgNW solution (Fig. 10(c)). The dried MXene/AgNW/melamine/PDA foam was used as the negative electrode and an Ecoflex layer was coated on the electrode as the friction layer (Fig. 10(d)) in a TENG for monitoring human activities.³⁴⁹ A versatile tile-structured MXene/polymethyl methacrylate (PMMA) film as an electrode (Fig. 10(e)) in TENG for monitoring vital signs was proposed by Huamin Chen et al. PMMA was added to the MXene solution and stirred until forming a dispersion. Using vacuum-filtration, the tile structure was assembled in the MXene/PMMA layer (Fig. 10(f)). It was shown that physiological signals in a variety of kinematic motions, including finger and elbow bending, belly breathing, and throat pronouncing, could be detected by a wireless monitoring system with the TENG.³⁵⁰ The conventional hydrogels are comprised of a polymer matrix with high structural voids. However, this polymer matrix exhibits low strength, resulting in inadequate mechanical attributes.³⁵⁵ The performance of MXene electrodes in TENGs with their triboelectric layers, fabrication approaches, electrical parameters, and corresponding applications is tabulated in Table 3.

Table 3 A brief outline of MXenes as high-performance electrodes in TENGs^a

Electrode (MXene)	Triboelectric layers	Fabrication approach	Mode	Voltage	Current	Power or power density	Stability (cycles)	Application	References
a TP: tribopositive electric layer, TN: tribonegative electric layer, CS: vertical contact-separation mode, SE: single-electrode mode.
Ti3C2Tx	PET	Spray coating	CS & SE	650 V	7.5 μA	0.65 mW	50 000	Energy harvesting from muscle movements	182
Ti3C2Tx	TP-PVA/NaCl	Spray coating	CS	390 V	96 μA	6.66 mW m^−2	15 000	Detect and monitor human motions	330
	TN-PET
Ti3C2Tx/CNF fabric	TN-silicone rubber	Dip coating	SE	400 V	680 nA	40 mW m^−2	1500	Monitor human body motions and handwriting	331
Ti3C2Tx/GO	TP-Ecoflex	Vacuum filtration	CS	258.8 V	2 μA	84.5 μW	2400	Non-invasive glucose detection	288
Ti3C2Tx/CNF liquid	TN-silicone rubber	Mold casting	SE	300 V	5.5 μA	505 mW m^−2	6000	Monitor human motions	335
Ti3C2Tx hydrogel	TP-Ecoflex	Chemical crosslinking	SE	230 V	270 nA	0.33 mW m^−2	—	Monitor body motions and handwriting	298
	TN-Kapton
Ti3C2Tx/PVA	TP-PVA	Solution blending	CS	1056 V	36.6 μA	—	50 000	Water treatment in an oceanic environment	47
	TN-PVDF
Ti3C2Tx/chitin	TP-glass	Vacuum filtration	SE	56 V	3.2 μA	99.5 mW m^−2	2000	Detecting joint bents and tactile sensing	339
	TN-PDMS
Ti3C2Tx/CNF/PAS hydrogel	TP-PU	Mold casting	SE	143 V	10 μA	—	3000	Detecting human motions	340
	TN-Kapton
Ti3C2Tx/PVA/PEG hydrogel	TP-silicone rubber	Chemical crosslinking	SE	212 V	1.1 μA	3 mW m^−2	—	Human motion monitoring	345
	TN-PTFE
Ti3C2Tx/gelatin hydrogel	TN-Ecoflex	Solution blending	SE	163.7 V	8.1 μA	—	6000	Wound healing	341
Ti3C2Tx/TPU/PDA/CB fiber	TN-silicone rubber	Electrospinning	SE	115 V	0.8 μA	68 mW m^−2	10 000	Detect human motions	347
Ti3C2Tx/PA hydrogel	TN-Ecoflex	Solution blending, oxidation treatment	CS & SE	243 V	31 μA	2.38 mW m^−2	850	Operate in vibrations, morse code recognition	348
	TP-PA
Ti3C2Tx/AgNWs foam	TP-Ecoflex	Vacuum-assisted dip coating	SE	115 V	1.7 μA	514.2 mW m^−2	10 000	EMI shielding, sport monitoring	349
	TN-PVDF
Ti3C2Tx/PMMA	TN-PDMS	Vacuum filtration	CS	37.8 V	1.8 μA	7.3 μW	10 000	Wireless human health monitor	350
Ti3C2Tx/CNF/PAM	TP-Ecoflex	Electrohydrodynamic printing	SE	67.5 V	0.13 μA	—	600	Monitor human motions and writing recognition	351
	TN-Kapton
Ti3C2Tx/GO/PAM/PDA	TP-butyronitrile	Chemical crosslinking	CS	105 V	0.72 μA	1 W cm^−2	—	E-skin and human–machine interface	356
	TN-PTFE

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Hongjie Zeng et al. prepared a conductive hydrogel with a dual matrix configuration, which eliminated the high gaps present in the single matrix configuration. Using polyacrylamide (PAM), MXene, and CNF, a conductive hydrogel was prepared via in situ photopolymerization aided by the electrohydrodynamic printing approach under the influence of UV light (Fig. 10(g)). Then, the hydrogel was encapsulated with Ecoflex as the negative triboelectric layer and a Kapton film was used as the positive triboelectric layer (Fig. 10(h)). A photograph of the fabricated TENG device is shown in Fig. 10(i). The fabricated TENG was not only used to detect human motions but also to transmit information.³⁵¹

Considering their many benefits compared to conventional materials such as metals, polymers, and other 2D materials such as graphene and MoS₂, MXenes have become promising electrode materials for TENGs.³⁵⁷ MXenes have metallic conductivity of over 20 000 S cm⁻¹, which makes charge transport more effective and improves the performance overall of TENG. Conversely, MWCNT/chitosan and graphene-based electrodes usually show conductivity in the range of 10–10 000 S cm⁻¹, whereas Ag/chitosan electrodes offer lower conductivity (∼6300 S cm⁻¹).³⁵⁸ Owing to their surface groups (–OH, –F, and –O), which improve charge trapping and adherence to dielectrics, MXenes have two-dimensional layered structures that improve charge transmission. In contrast to liquid metals, they are easily shaped and produced using low-cost, scalable techniques, and they provide exceptional mechanical strength and flexibility superior to CNTs and graphene.^331,359 As a result, when compared with conventional materials, MXenes offer a well-balanced combination of high conductivity, mechanical compatibility, and chemical activity, which significantly improve the performance of TENG devices.

5.2. MXene acting as an electrode and triboelectric layer concurrently

According to the working principle of TENGs, which combines contact electrification and electrostatic induction, the electrical conductivity of the electrode materials and the contact-generated voltage between the triboelectric layers determine the output performances of the resulting TENGs, which can impact the maximum energy extracted from these devices. Thus, the sensible selection of materials as the electrode and triboelectric pairs is essential in enhancing the output capabilities of TENGs. Numerous electronegative polymers such as PDMS, PI, PTFE, and FEP have been widely used as materials for the fabrication of TENGs.³²⁷ Nevertheless, their continued use as electrodes, where high electrical conductivity is required to extract the most energy from the devices, is hindered by the electrically insulating nature of the polymers or the low electrical conductivity of composites.³⁶⁰ Benefitting from the rich surface terminal groups, exceptional electronegativity and enhanced conductivity of MXenes, researchers have applied them as both the electrode and triboelectric layers simultaneously. This dual function of MXenes in TENGs simplifies the device structure, reduces interfacial losses, minimizes material consumption, and enhances charge transfer at a low cost.³⁶¹

A TENG device was fabricated by Yuzhang Du et al. utilizing the benefits of high conductivity and electronegativity of MXenes. An MXene-polymer composite was prepared using the mold (PTFE) casting technique and the device was fabricated by dropping a Ti₃C₂T_x MXene solution on an MXene-polymer mold. After drying, the other MXene-polymer composite was attached to the top (Fig. 11(a)). The pristine MXene electrode (50 μm) was sandwiched between the two MXene-polymer composites (triboelectric layers) and the device operated in single-electrode mode (Fig. 11(b)), which exhibited rich EMI shielding ability with outstanding self-healing capacity.⁵⁴ Reliable wearable electronics face a huge problem in developing flexible, affordable, and self-powered TENGs with a good output performance, while their potential applications are hampered by their low surface charge density.³⁶² Xu Yang et al. proposed a TENG device based on an MXene hybrid film, which was economically fabricated in a single step. Firstly, a PEDOT solution was mixed in an Nb₂CT_x MXene solution, and then the CNT was added to the MXene/PEDOT solution. The solution was ultrasonicated to attain complete dispersion and filtered through vacuum filtration using a PTFE membrane (Fig. 11(c)). Two membranes were prepared similarly and assembled. In the top film, the hybrid MXene composite acts as an electrode and PTFE as a tribolayer, and in the bottom film, the MXene hybrid acted as an electrode as well as a triboelectric layer (Fig. 11(d)), and the reported device has potential for wide application in biomedical wearables.³⁶³ Surface terminations in MXenes determine the performance of TENGs and the research on MXene in TENG mainly focuses on MXene composites and synthesis.³⁰⁴ The influence of surface terminal groups of MXenes when applied in TENGs was analyzed and compared by Xu Cai et al. by refining the MXene synthesis with different approaches. The comparison was between the synthesis of MXene by gentle etching with various times and temperatures, bare MXene, and HF etched MXene. As shown in Fig. 11(e), the TENG device exhibited a better response with the MXene synthesized via gentle etching (48 h at 40 °C) paired with an Ecoflex triboelectric layer.³⁶⁴ Preserving and protecting sensitive data as universal concerns have arisen with major consequences for the armed forces, the economy, and national security.³⁶⁵ As a luminescence emission method, alternating current electroluminescence (ACEL) is a significant substitute for luminous data encryption owing to its extremely long lifespan, consistent luminescence, and ease of construction.³⁶⁶ Nevertheless, the necessity of an external high-voltage AC power source significantly limits its use in portable electronics.³⁶⁷ In this case, ACEL devices can be directly powered by TENG. Copper wires and other weak electrical connections are still utilized to link two distinct TENG and ACEL devices, which still have significant issues with portability and integrity.³⁶⁸ A self-powered EL wearable device can be implemented by sharing the electrode in both the TENG and EL devices. Siyu Zhang et al. fabricated a wearable EL device by integrating a TENG and EL sharing s common single MXene electrode, which eliminated the requirements of external wires. The electrode was prepared by spin-coating a Ti₃C₂T_x MXene solution on a surface-modified PET film. The TENG device was fabricated by pouring the MXene/silicon solution on a mold and covering the mixture with half of the MXene electrode (Fig. 11(f)). After drying, the TENG device with a metal wire was peeled off and operated in single-electrode mode (Fig. 11(g)). The remaining half of the electrode was used by the EL device.⁴⁹

Fig. 11 MXene as an electrode and triboelectric layer in a TENG. (a) Fabrication of the TENG device by sandwiching the MXene electrode between two MXene/polymer elastomers. (b) Single-electrode operating mode of fabricated TENG. Reprinted with permission from ref. 54. © 2022, Elsevier. (c) Fabrication process of CNT/MXene/PEDOT hybrid film. (d) Simple architecture of TENG with two CNT/MXene/PEDOT hybrid films. Reprinted with permission from ref. 363. © 2022, Elsevier. (e) Graphical diagram of TENG with MXene as an electrode and triboelectric layer. Reprinted with permission from ref. 364. © 2023, John Wiley & Sons. (f) Fabrication procedure of TENG with MXene electrode and MXene/silicone elastomer as a triboelectric layer. (g) Device representation of single-electrode mode TENG. Reprinted with permission from ref. 49. © 2023, John Wiley & Sons. (h) Fabrication process of MXene/PEDOT:PSS hybrid film and simple architecture of TENG with two MXene/PEDOT:PSS hybrid films. (i) Operating procedure of TENG. Reprinted with permission from ref. 46. © 2021, Elsevier. (j) Optical image of the MXene/CNF film. (k) Device structure of MXene TENG. Reprinted with permission from ref. 370. © 2022, American Chemical Society. (l) Fabrication process of the MXene-leather coated LIG film. (m) Architecture of the TENG device. Reprinted with permission from ref. 51. © 2023, American Chemical Society. (n) Preparation process of the MXene/oxidized CNF film. (o) Device architecture with MXene/oxidized CNF film as an electrode and triboelectric layer. Reprinted with permission from ref. 371. © 2025, Elsevier. (p) Schematic diagram of TENG device driven by wind. Reprinted with permission from ref. 372. © 2024, Elsevier.

Finding a stable electrode-triboelectric layer interface for prompt and sustained triboelectric surface charge transfer is still very difficult.³⁶² A unique technique for fabricating a TENG combining the electrode and tribolayer in single-step without a hybrid membrane composite was first proposed by Zichao Zhang et al. The TENG was fabricated by mixing PEDOT/PSS in a Ti₃C₂T_x MXene solution under stirring. Then, the solution was filtered using a vacuum-filtration assembly with a PTFE membrane. Likewise, another film was prepared and assembled to fabricate the TENG device (Fig. 11(h)). As shown in Fig. 11(i), the TENG device operated in contact-separation mode, generating charges when the upper PTFE film came into contact with the lower MXene film (tribolayer and electrode).⁴⁶ Numerous researchers have incorporated MXenes as conductive fillers to increase the conductivity or dielectric permittivity of polymers. There are much less reports on high-content MXene composites with electronegative or even electropositive properties that function as efficient electrodes and tribolayer at the same time.³⁶⁹ A high-content MXene/CNF film with a novel spider-web architecture for attaining an enhanced performance in TENGs was proposed by Chenyang Xing et al. The MXene film was prepared by mixing a CNF solution with an MXene solution using an oscillator, and then the mixture was filtered using a vacuum-filtration approach, and a photograph of the film is shown in Fig. 11(j). The TENG device was fabricated by assembling the prepared MXene film with PTFE or nylon as the counter triboelectric layer (Fig. 11(k)). A high content denotes that the ratio of MXene starts from 20% to 80%.³⁷⁰

The addition of MXene to polymers not only enhances the flexibility of polymers but also increases the resistive property in the composites. Another approach to enhance the flexibility is by generating stretchable structures in the substrates.^373,374 The performance of MXene as an electrode and triboelectric layer simultaneously in TENGs with their triboelectric layers, fabrication approaches, electrical parameters, and corresponding applications is presented in Table 4. Several membranes have been used such as PTFE, and cellulose ester for preparing MXene films. A genuine and steady approach for maximizing the flexibility of MXene films using natural leather as a filter membrane was reported by Shaochun Zhang et al. The Ti₃C₂T_x MXene film was prepared through a vacuum-filtration approach using leather as a filter membrane. PI tape was fixed at the back of the leather and treated with a laser to attain a laser-induced graphene (LIG) electrode. Then, the film was dipped into an HAuCl₄ solution to coat Au on the LIG electrode (Fig. 11(l)). The TENG device was assembled by pairing the LIG electrode with triboelectric positive PU and operated in single-electrode mode (Fig. 11(m)).⁵¹ When natural cellulose was properly treated, CNF with better flexibility, stretchability, and biocompatibility was obtained. However, prior research has shown that surface modification with functional groups can influence the triboelectric charge density of CNFs.^375,376 To enhance the flexibility of CNFs further, Linlin Zhou et al. oxidized CNFs with carboxylate functions. The film was prepared by mixing an oxidized CNF and 50% MXene solution under stirring, and after that the solution was filtered using vacuum-filtration (Fig. 11(n)). The TENG was fabricated using the electronegative MXene-oxidized CNF composite paired with nylon as the positive triboelectric layer. The composite film was used as the bottom electrode and dielectric, while Cu was used as the top electrode (Fig. 11(o)).³⁷¹ Gaseous pollutants such as nitrogen dioxide (NO₂), ammonia (NH₃), and sulfur dioxide (SO₂) play a significant role in aggravating the problem of chemical gas pollution, which has become one of the main causes of sickness and fatalities. Thus, the requirement for chemical gas sensors for detecting chemical gases is high, leading to the necessity of energy consumption.^377,378 Dongyue Wang et al. found that the usage of MXene in interdigitated electrodes as a multi-gas sensing element powered by TENG will resolve the abovementioned issues. Four PVC sheets, each 1 mm thick, were initially attached to form the body of the TENG device. The cellulose acetate sheet was adhered to a sleek rod at one end. At last, the top and bottom of the frame were sprayed with Ti₃C₂T_x MXene. As shown in Fig. 11(p), the TENG was fabricated with MXene as the electrode and negative tribolayer and cellulose acetate as the positive tribolayer.³⁷²

Table 4 A brief outline of MXenes as high-performance electrodes and triboelectric layers concurrently in TENGs

Electrode (MXene)	Triboelectric layers	Fabrication approach	Mode	Voltage	Current	Power or power density	Stability (cycles)	Application	References
Ti3C2Tx	TN-MXene	Mold & drop casting	SE	245 V	29 μA	1150 mW m^−2	—	EMI shielding	54
Nb2CTx/CNT/PEDOT	TP-Nb2CTx/CNT/PEDOT	Vacuum filtration	CS	184.1 V	4.2 μA	420 μW	400	Detecting small human motions such as vocal cord movement and wrist bending	363
	TN-PTFE
Ti3C2Tx	TP-Ecoflex	Vacuum filtration	CS	281 V	9.7 μA	633.24 mW m^−2	4100	Wireless control over a computer game	364
	TN-Ti3C2Tx
Ti3C2Tx	TP-skin/nylon	Spin coating & mold casting	SE	290 V	24 μA	0.9 W m^−2	10 000	Decrypting and encrypting hidden information	49
	TN-MXene/silicone rubber
Ti3C2Tx	TP-MXene/PEDOT PSS, TN-PTFE	Vacuum filtration	CS & SE	29.56 V	0.07 nA	—	6000	Detecting human voices and facial expressions	46
Ti3C2Tx	TP-MXene/CNF	Vacuum filtration	CS	1120 V	24 μA	144 mW m^−2	7000	Gesture recognition	370
	TN-PTFE
Ti3C2Tx	TP-PU	Vacuum filtration & dip coating	SE	199.56 V	9.04 μA	0.469 mW cm^2	4000	Human–machine interaction	51
	TN-MXene
Ti3C2Tx/TOCNF	TP-nylon	Vacuum filtration	CS	210 V	0.84 μA	—	3600	Detecting human motions and transmission of information	371
	TN-MXene/TOCNF
Ti3C2Tx	TP-cellulose acetate	Spray coating	CS	269 V	13.2 μA	1.2 mW	12 500	Chemical gas detection	372
	TN-MXene

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TENGs that incorporate MXenes as both electrodes and tribolayers offer distinct benefits over conventional materials. In TENGs, MXenes can act as the tribolayer and electrode at the same time. This dual functionality simplifies the manufacturing process and device structure, which can lower costs and improve the device dependability.^285,379 Alternatively, traditional materials frequently require distinct tribolayers and electrodes, making the design and production procedures more difficult. MXenes have outstanding wear resistance and mechanical strength.⁵⁹ According to studies, MXene-based composites outperform other materials in durability tests due to their reduced friction and ultralow wear rates.³⁷⁰ Furthermore, to improve their triboelectric qualities, MXenes can be blended with a variety of compounds, including silicone, polyvinylidene fluoride (PVDF), and polydimethylsiloxane (PDMS).³⁸⁰ In summary, MXenes provide a special blend of strong electrical conductivity, superior mechanical strength, the ability to operate as both electrodes and tribolayers, high output performance, and material combination diversity.

5.3. MXenes as electron donor (positive) tribolayer

According to research, the triboelectric characteristics of MXenes are greatly influenced by their synthesis techniques, etchants, intercalating chemicals, and processes.¹⁷² Etching the MAX phase (Ti₃AlC₂) through a minimally intensive layer delamination (MILD) approach using LiF/HCl etchant results in an MXene with positive polarity. The Ti₃C₂T_x MXene synthesized through MILD etching was incorporated as the positive triboelectric layer and paired with PTFE as the negative tribolayer in a TENG (Fig. 12(a)). The TENG device operating in single-electrode mode could be used to detect infusion and clinical discharge procedures in real-time, lowering the workload of medical personnel.⁴⁶ Extending the investigation of the MXene on positive polarity, Lin Shi et al. synthesized MXenes (Ti₃C₂T_x, Nb₂CT_x, and V₂CT_x) via mild etching and paired them with 9 different polymers to determine their surface potential. The MXene film was prepared via the vacuum-filtration technique using a PVDF membrane. The Ti₃C₂T_x, Nb₂CT_x, and V₂CT_x MXenes exhibited positive triboelectric characteristics when paired with PVC as triboelectric layers (Fig. 12(c)). Particularly, Nb₂CT_x showed an extremely elevated power density compared with the other two MXenes. Using Kelvin probe force microscopy (KPFM), the contact potential difference was depicted to confirm the polarity of the MXenes. Based on the contact potential difference, Nb₂CT_x (Fig. 12(b)(iii)) possessed a higher positive voltage compared to Ti₃C₂T_x. V₂CT_x (Fig. 12(b)(i)) exhibited a lower voltage than both the Ti₃C₂T_x (Fig. 12(b)(ii)) and Nb₂CT_x MXenes. However, all MXenes examined in this study are oriented toward the positive end of the triboelectric series, as seen in Fig. 12(d).¹⁷³

Fig. 12 MXene as a positive tribolayer in TENG. (a) Schematic diagram of water droplets splashing on a single-electrode TENG. Reprinted with permission from ref. 46. © 2021, Elsevier. (b) Kelvin probe force microscopic images of MXenes. (i) V₂CT_x, (ii) Ti₃C₂T_x and (iii) Nb₂CT_x. (c) Schematic representation of the MXene/PVA composite TENGs. (d) Transformed triboelectric sequence of materials of the V₂CT_x, Ti₃C₂T_x, and Nb₂CT_x MXenes. Reprinted with permission from ref. 173. © 2024, John Wiley & Sons. (e) Schematic diagram of the MXene/silk fibroin composite-based TENG. Reprinted with permission from ref. 63. © 2024, American Chemical Society. (f) Preparation process of MXene/TPU composite film. Operating mechanism of TENG in (g) vertical contact-separation mode and (h) novel deformation mode. Reprinted with permission from ref. 127. © 2024, Elsevier. (i) Schematic diagram of kirigami spider fibroin-based microneedle patch. (j) TENG device design with kirigami spider fibroin-based microneedle patch. Reprinted with permission from ref. 61. © 2024, Elsevier. (k) Schematic illustration of the fabrication of 3D MXene/PU gel. (l) Architecture of TENG device in operation. Reprinted with permission from ref. 381. © 2024, American Chemical Society. (m) Optical image of a silkworm cocoon. (n) Fabrication procedure of 3D silk fibroin/MXene composite aerogel. (o) Fabricated TENG device with 3D silk fibroin/MXene composite aerogel as a positive triboelectric layer. Reprinted with permission from ref. 50. © 2023, American Chemical Society.

Recently, a 3D architecture biocompatible MXene Ti₃C₂T_x–silk fibroin composite as a positive triboelectric layer in TENG was proposed by Xueqiang Tan et al. The TENG was fabricated by assembling the vacuum-filtered MXene–silk fibroin composite (positive) and PDMS (negative) with ITO and Al as electrodes (Fig. 12(e)). This device was applied to detect insomnia and the quality of sleep for humans.⁶³ Flexible sensors, particularly for the detection of significant deformation, require exceptional flexibility and stretchability. Consequently, the lack of stretchability in TENGs limits their use as self-powered sensors.³⁶² A novel approach for increasing the flexibility of the TENG device by detaching the electrodes with Ti₃C₂T_x MXene/TPU as the positive tribolayer and silicon rubber as the negative tribolayer was unveiled by Jiacheng Fan et al. MXene/TPU was prepared using a mold-casting approach (Fig. 12(f)). This technique introduced a new mode in TENG known as the deformation mode. In deformation mode (Fig. 12(h)), MXene/TPU attached as the positive and silicone rubber as the negative triboelectric layers generated voltages based on stretching. However, the TENG device operated both in contact separation (Fig. 12(g)) and deformation mode.¹²⁷ One of the most common causes of disability is joint injuries. Currently, their treatment focuses on oral medications and surgical procedures, which cause patients needless and serious problems. In this case, to achieve effective joint management, intelligent patches with stretchability and cognitive drug control-release capability are highly desirable.^382,383 Drawing inspiration from kirigami structures, Shuhuan Li et al. created a novel TENG patch that can be applied for joint care. The kirigami-based MXene@spider fibroin patch was prepared by laser engraving on a stretched Ecoflex mold, followed by coating with a Ti₃C₂T_x MXene solution. Then, the prepared spider fibroin solution was added dropwise to the stretch-free mold (Fig. 12(i)). As shown in Fig. 12(j), the TENG device was fabricated by using the MXene composite as the positive and PTFE film as the negative triboelectric layer pasted with Cu foil on the top and bottom.⁶¹

The performance of MXene as a positive triboelectric layer in TENGs with their electrodes and pairing negative triboelectric layers, fabrication approaches, electrical parameters, and corresponding applications is presented in Table 5. Achieving enhanced sensitivity, a wide working range and exceptional stability is vital in pressure sensors and triboelectric nanogenerators.³⁸⁴ Thus, to achieve this, 3D conductive polymer composites with MXene/PU gel were prepared using an iron template approach. The 3D composite gel was prepared by mixing the Ti₃C₂T_x MXene and polyurethane solution and the solution was poured into a flask with the iron foam, followed by pressing until the gel was formed. Then, the 3D gel was immersed in an H₂SO₄ solution to remove the iron foam (Fig. 12(k)). The MXene/PU gel was washed to neutralize the pH value. The MXene/PU gel was used as the positive and PDMS as the negative triboelectric layer for the fabrication of a TENG device (Fig. 12(l)).³⁸¹ The advancement of self-powered bioelectronics is aided by the rising demand for triboelectric layers composed of biocompatible and biodegradable materials for reuse and combining with biomedical electronics.³⁸⁵

Table 5 A brief outline of MXenes as positive triboelectric layers in TENGs

Electrode	Triboelectric layers	Fabrication approach	Mode	Voltage	Current	Power or power density	Stability (cycles)	Application	References
Al-positive	TP-Ti3C2Tx/silk fibroin, TN-PDMS	Vacuum filtration	CS	418 V	11.6 μA	9.92 W m^2	—	Detecting insomnia and quality of sleep	63
ITO-negative
No electrode	TP-Ti3C2Tx/TPU	Mold casting	Deformation	7.1 V	—	—	—	Detecting human motions	127
	TN-silicone rubber
Copper foils	TP-Ti3C2Tx/spider fibroin, TN-PTFE	Laser engraving & mold casting	CS	4 V	—	—	26	Joint treatment	61
Copper foils	TP-Ti3C2Tx/PU	Iron foam template technique	CS	8 V	31 nA	—	7800	Human step frequency monitoring	381
	TN-PDMS
Al-positive	TP-Ti3C2Tx/silk fibroin, TN-PDMS	Freeze-drying	CS	545 V	16.13 μA	13.25 W m^2	—	Detecting asthma symptoms	50
ITO-negative
Copper tapes	TP-Ti3C2Tx/gel/sodium lignosulphonate TN-PDMS	Scalable one-pot	CS	230 V	28 μA	2.9 W m^−2	1000	EMI shielding and sensing human motions	386

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A Ti₃C₂T_x MXene–silk fibroin aerogel-based biocompatible TENG was proposed for monitoring the respiratory rate of humans. The silk fibroin was extracted from the natural silkworm cocoons (Fig. 12(m)). The composite was prepared by intercalating the silk fibroin into the MXene sheets, resulting in the formation of a 3D bionic configuration. Then, the 3D MXene@silk fibroin was freeze-dried to form an aerogel (Fig. 12(n)). As shown in Fig. 12(o), using the aerogel as the positive tribolayer and the PDMS sponge as the negative tribolayer with Al and ITO as electrodes, a TENG is fabricated.⁵⁰

To develop superior triboelectric materials for TENGs with improved performances, MXenes have been blended with other materials such as poly(vinyl alcohol) (PVA), PDMS, Ecoflex (silicone rubber), PTFE, PVDF, poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS), and poly(lactic acid).³²⁹ Two recent studies have shown that Ti₃C₂T_x films are more triboelectrically negative than PTFE,¹⁸² even when they are used as the triboelectric layer in TENGs. The second study showed that Ti₃C₂T_x films are more positive than PTFE.⁴⁶ Subsequent investigation showed that the triboelectric characteristics of MXenes are strongly influenced by their synthesis techniques, etchants, intercalating chemicals, and processes.¹⁷³ Similar to fluorine in FEP, the fluorine atoms on the MXene flakes at or just below the contact surface are probably responsible for removing electrons from the tribopositive material.³¹⁷ In contrast to their typical application as tribonegative electrodes or fillers in TENGs, it can be deduced that these MXenes can act as both tribopositive and tribonegative materials.

5.4. MXene as an electron acceptor (negative) tribolayer

MXenes are potential negative triboelectric materials due to the presence of negative surface terminations –OH and –F. According to reports, MXenes are more electronegative compared to PTFE.³⁸⁷ In TENGs based on MXenes, their open-circuit voltage is often high owing to the strong electronegativity of MXenes, which attract electrons during triboelectrification processes. Thus, researchers have embedded or composited MXenes with polymers to enhance TENG performance. Dielectric permittivity and percolation are two impacts of MXenes in polymer films that must be balanced by the MXene proportion to achieve the optimal performance.^34,167 MXene (Ti₃C₂T_x) as a negative friction layer was first incorporated in TENG in 2019. To enhance the flexibility, the MXene was mixed with the PVA to create nanofibers through electrospinning (Fig. 13(a)). Silk fibroin nanofiber was used as the positive friction layer. With Al foils as electrodes, a TENG was fabricated with the prepared positive and negative friction layers (Fig. 13(b)).⁴⁸

Fig. 13 MXene as a negative tribolayer in TENG. (a) Preparation process of MXene/PVA nanofibers. (b) TENG device structure with MXene/PVA nanofibers. Reprinted with permission from ref. 48. © 2019, Elsevier. 3D Graphical illustration of (c) the preparation process of MXene/SRP composite through hot pressing and (d) TENG device structure with MXene/SRP composite. Reprinted with permission from ref. 53. © 2024, John Wiley & Sons. (e) Preparation process of the MXene/MWCNT/Ecoflex composite film. (f) Schematic of TENG with MXene/MWCNT/Ecoflex operating in single-electrode mode. Reprinted with permission from ref. 390. © 2025, Elsevier. (g) 3D design of the TENG device in spring design. (h) Digital image of the fabricated TENG in spring design. (i) Working structure of TENG device with MXene/PVDF composite. Reprinted with permission from ref. 391. © 2024, American Chemical Society. (j) Schematic of TENG device with MXene/PVDF-HFP nanofibers. Reprinted with permission from ref. 392. © 2024, Elsevier. (k) Peony flower for surface modification of triboelectric layer. (l) Process of preparing the surface-modified MXene/PDMS film. (m) Schematic of TENG device assembly with gelatin and MXene/PDMS films. Reprinted with permission from ref. 393. © 2024, Springer Nature.

After this, numerous approaches such as hydrogels, aerogels, and 3D networks were proposed employing MXenes with different polymers (PDMS, PTFE, PVDF, etc.). Fluorine has the highest electron negativity in the Periodic Table, meaning that fluoropolymers can effectively remove electrons from other materials to produce a large negative surface charge density. Nevertheless, fluoropolymers cause considerable environmental damage by leaking poly-fluoroalkyl compounds into the surroundings, leading to serious medical conditions.^388,389

Thus, to alleviate this environmental issue, a sulfur-based polymer with minimal MXene (Ti₃C₂T_x) as a filler was used to fabricate a green and sustainable TENG. The composite was prepared by immersing the sulfur-based polymer (SRP) powder into an MXene solution under mechanical stirring. After drying, the MXene-coated SRP powder was hot-pressed with Al foil on one side (Fig. 13(c)). On the other end, Al foil was thermally coated on the substrate as a conductor and positive friction layer and assembled to create a TENG (Fig. 13(d)).⁵³ The distinctive benefits of noncontact human–machine interfaces (HMI) of high accuracy, rigidity, and hygiene have recently attracted increasing attention. These advantages hold considerable potential for smart robots and augmented or virtual medical facilities.^394,395 A novel TENG was developed using MWCNT and MXene (Ti₃C₂T_x) as a filler in an Ecoflex dielectric to generate an electrical signal without contact for wireless robots. The negative friction layer was prepared by blending the MXene and MWCNT into the Ecoflex solution and streamed into a glass, followed by curing in an oven (Fig. 13(e)). The array-based TENG was constructed using 8 × 8 composite matrices adhered to acrylic substrates and Cu as electrodes (Fig. 13(f)).³⁹⁰ MXenes, in particular, Ti₃C₂T_x have been the focus of much research on TENGs due to their remarkable electrical conductivity. Reports suggest that the niobium carbide (Nb₂CT_x) MXene possesses enhanced electrical conductivity and large surface area compared to Ti₃C₂T_x, but reports on its use in TENG is not common.³⁹⁶ Yang Tao et al. fabricated a TENG device using an Nb₂CT_x MXene and PVDF composite as the negative friction layer. Multiple Al electrodes were laminated in 3D to fabricate the device as a spring assembly, as shown in Fig. 13(g). The composite was prepared by pouring PVDF solution on a glass substrate and drying in an oven, and then an Nb₂CT_x solution was sprayed on the surface of PVDF using a spray gun. A photograph of the device is shown in Fig. 13(h). The TENG possessed 16 components for generating electricity, with polypropylene and Nb₂CT_x/PVDF as the positive and negative friction layers (Fig. 13(i)).³⁹¹ To enhance the dielectric constant and reduce charge loss, Omar Faruk et al. incorporated the V₂CT_x MXene into poly(vinylidene-fluoride-co-hexafluoropropylene) (PVDF-HFP) to create nanofibers as the negative friction layer in TENGs. PEO nanofibers were used as the positive friction layer in TENG with fabric as the electrodes (Fig. 13(j)). Both the MXene/PVDF-HFP and PEO nanofibers were prepared through electrospinning.³⁹² The development of TENG technology greatly benefits from the usage of eco-friendly friction materials.³⁹⁷ Zekun Wang et al. proposed a TENG device with MXene/PDMS and gelatin as the negative and positive friction layers, respectively, and Cu foil as electrodes (Fig. 13(m)). Both friction layers were surface-modified using peony petals (Fig. 13(k)) as the substrate (Fig. 13(l)). In addition to its exceptional electrical performance, gelatin is a green and eco-friendly material.³⁹³ For biosensing and antimicrobial activity, the TENG device requires the side that comes into touch with the human skin to be hydrophilic to absorb sweat. Additionally, for better self-cleaning and longer use, the exterior surface should be hydrophobic.^398,399 Shanguo Zhang et al. proposed a TENG device based on bilayer MXene Ti₃C₂T_x composites for antibacterial activity and avoiding bacterial septicity. The negative friction bilayer was prepared by casting MXene/Ag/PVDF/cellulose acetate as the bottom layer, followed by casting the MXene/PVDF top layer on an Al plate (Fig. 14(a)). Nylon was used as the positive friction layer and conductive tape as the electrode (Fig. 14(b)). The top layer enhanced the performance of the TENG, while the bottom layer exhibited a competent shield against E. coli and S. aureus bacteria.⁴⁰⁰ The performance of MXene as a negative triboelectric layer in TENGs with their electrodes and pairing positive triboelectric layer, fabrication approaches, electrical parameters, and corresponding applications is presented in Table 6.

Fig. 14 MXene as a negative tribolayer in TENG. (a) Schematic of the preparation of MXene/Ag/PVDF/cellulose acetate composite. (b) Schematic illustration of TENG device with MXene/Ag/PVDF/cellulose acetate composite. Reprinted with permission from ref. 400. © 2024, Elsevier. (c) 3D illustration of the TENG device. (d) Schematic of TENG device with bilayer MXene/silicone composite. Reprinted with permission from ref. 401. © 2024, Elsevier. (e) Schematic of the preparation of alternating ABAB-layered MXene composite. (f) Structure of TENG device with alternating ABAB-layered MXene composite. Reprinted with permission from ref. 380. © 2021, Springer Nature. (g) Schematic of TENG device with MXene/Ecoflex composite. Reprinted with permission from ref. 404. © 2025, Springer Nature. (h) Schematic of MXene/Ecoflex composite with pyramid structure attached to fabric. (i) Photograph of the composite with a pyramid structure. (j) Schematic of the toroidal TENG device. Reprinted with permission from ref. 52. © 2023, Elsevier. (k) Unique intercalation strategy of MXene with modified silica nanospheres. Reprinted with permission from ref. 315. © 2024, John Wiley & Sons.

Table 6 A brief outline of MXenes as negative triboelectric layers in TENGs

Electrode	Triboelectric layers	Fabrication approach	Mode	Voltage	Current	Power or power density	Stability (cycles)	Application	References
Al foils	TN-Ti3C2Tx/PVA	Electrospinning	CS	117.7 V	—	1087.6 mW m^−2	124 000	Power electrowetting on a dielectric device	48
	TP-silk fibroin
Al foils	TN-Ti3C2Tx/SRP	Hot-pressing	CS	1717.7 V	129 μA	3.80 W m^−2	7200	Power commercial electronics	53
	TP-Al foil
Al foils	TN-Nb2CTx/PVDF	Spraying	CS	420 V	33.45 μA	619 mW m^−2	—	Water level monitoring	391
	TP-polypropylene
Conductive fabric	TN-V2CTx/PVDF-HFP, TP-PEO	Electrospinning	CS	1240 V	119 μA	18.20 W m^−2	15 000	Human activity and posture monitoring	392
Copper foils	TN-Ti3C2Tx/PDMS	Drop-casting	CS	417.39 V	12.01 μA	170 μW cm^−2	10 000	Foot status monitoring	393
	TP-gelatin
Conductive tape	TN (bilayer)-top-Ti3C2Tx/PVDF, bottom-Ti3C2Tx/Ag/PVDF/cellulose acetate	Combined crystallization and diffusion	SE	1236 V	70 μA	18.55 W m^−2	400 000	Protect against bacterial infections	400
	TP-nylon
Copper foils	TN (bilayer)-top-silicone, Bottom-Ti3C2Tx/silicone	3D printed template	CS	1160 V	—	—	1500	Ethanol sensing and alarm	401
Al foils	TN-Ti3C2Tx/CeO2/CNF, TP-silicone	Electrospinning	CS	160 V	—	3485 mW m^−2	16 000	Ammonia and biomarker detection	402
Al foils	TN-Ti3C2Tx/PVC	Electrospinning	CS	51 V	29.79 μA	4.94 mW cm^−2	1600	Human motion monitoring	403
	TP-Al foil
Copper foils	TN-Nb2CTx/Ti3C2Tx	Vacuum filtration	CS	34.63 V	8.06 μA	100 μW cm^−2	—	Harvest energy from human motions	380
	TP-Nylon
Cu–Ni polyester	TN-Ti3C2Tx/Ecoflex	Roll-casting	CS	350 V	42 μA	7.8 W m^−2	—	Energy storage	404
	TP-PDMS
Conductive fabric	TN-Ti3C2Tx/Ecoflex	3D printed template	SE	19.91 V	1.15 μA	—	50 000	Smart glove	52
	TP-human skin
Al	TN-Ti3C2Tx/modified silica nanospheres/PDMS TP-Al	Mold casting	CS	461 V	16 μA	691.1 μW cm^−2	>2800	Energy harvesting	315
Copper	TN (bilayer)-top-PDMS, bottom-Ti3C2Tx/PDMS	Spin coating	SE	2016 V	86 μA	11.27 m W	30 000	Volleyball training monitoring	405
	TP-Nylon
Al	TN-Ti3C2Tx/PVDF	Electrospinning	CS	305 V	10.6 μA	10.91 Wm^−2	10 000	Smart home	406
	TP-ZIF-67/PAN

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Although TENG is a powerful energy harvesting technique, it still has the drawback of poor output performance when used to create a self-powered sensing system.⁴⁰⁷ Thus, to mitigate the issue of charge dissipation in the dielectric layers with ease, a TENG device with negative dual dielectric layers was proposed by Shasha He et al. The dual dielectric layers of silicone rubber (top) and MXene (Ti₃C₂T_x)/silicone composite (bottom) were prepared by 3D mold casting. As shown in Fig. 14(c), employing the dual layer as the negative and nylon as the positive friction layers with copper foil as the electrodes and Kapton as substrate, a TENG was constructed and the device operated in CS mode (Fig. 14(d)).⁴⁰¹

The agglomeration of MXenes limits the access of their –F group to nanochannels, which impairs their performance. Ti₃C₂T_x MXene nanosheets with a porous framework and interlayer spacing were demonstrated to fully exploit their electrochemical characteristics. The output performance of TENG has also been greatly enhanced by the addition of interlayer spacers to MXene.^408,409 This layer-stacked configuration was developed to construct alternate-layered MXene films with rich –F groups and a homogeneous intrinsic microstructure. An alternating-layered composite (negative friction layer) was prepared by assembling delaminated Ti₃C₂T_x and Nb₂CT_x MXenes in an ABAB configuration via the vacuum-filtration technique (Fig. 14(e)). Adopting nylon as the positive friction layer and Nb₂CT_x/Ti₃C₂T_x as the negative friction layer with copper electrodes, a TENG device was fabricated (Fig. 14(f)).³⁸⁰ Determining the percolation level of MXenes is critical in attaining a stable dielectric constant for the smooth operation of TENG.⁴¹⁰ A TENG device with a Ti₃C₂T_x MXene/Ecoflex composite as the negative friction layer was prepared to determine the dielectric constant of the composite. The MXene fillers enhanced the dielectric constant of Ecoflex to 19.5 from 11.2, which started to decrease when the content of MXene exceeded 3 wt%. As shown in Fig. 14(g), applying MXene/Ecoflex as the negative and PDMS as the positive friction layers, together with a Cu–Ni polyester electrode, a TENG device was fabricated.⁴⁰⁴ Even with the advancements, the field of gesture sensors can still be improved. This is because gesture sensors are invariably made smaller to fit comfortably in the finger; however, this results in a poor triboelectric output due to the size and the CS mode operation.^411,412 Considering these problems and difficulties, a triboelectric toroidal device with a pyramid surface MXene/Ecoflex composite was proposed. The composite was prepared by casting an MXene/Ecoflex suspension into a 3D mold and fixing with conductive fabric. After curing, the composite was peeled off from the mold (Fig. 14(h)). A photograph of the prepared composite with a pyramid surface is shown in Fig. 14(i). A glove was fabricated by 3D printing all the components separately and assembled with copper wire connections. The TENG device (Fig. 14(j)) with MXene/Ecoflex as the negative friction layer with fabric electrode operated in SE mode.⁵² Despite its unique potential, MXene still has significant problems with restacking and charge blockage because of the van der Waals interaction, which makes it difficult to restrict charge transport paths. Body-penetrated charges have a high chance of recombining with opposing charges that are induced on the triboelectric electrodes.^413,414 Intercalating the Ti₃C₂T_x MXene with altered silica nanospheres was proposed as an effective solution to the above-mentioned drawback. The modified silica nanospheres were synthesized using disposed bagasse ash from a sugar factory. The silica nanospheres were intercalated into the MXene nanosheets by mixing in water, under stirring for 2 h and the nanosheets were intercalated with silica nanospheres through hydrogen bonding and electrostatic interaction (Fig. 14(k)). Then, the resultant composite was mixed with PDMS in different concentrations, which was applied as the negative friction layer in a TENG paired with Al as the top electrode as well as tribopositive material.³¹⁵ To retain the surface charges, given that a portion of the surface charges is lost during the triboelectrification process,⁴¹⁵ a dual-layer negative friction material comprised of a sodium niobate/PDMS composite at the bottom and Ti₃C₂T_x MXene/PVDF composite at the top was proposed. The performance of TENG devices is greatly enhanced by this method given that it produces noticeably larger surface charges and prolongs their retention.⁴¹⁶

As an electron acceptor material with superior triboelectric qualities, PTFE has been recognized in numerous series presented to date, which explains its widespread use in TENG applications.^417,418 Research conducted in 2017 revealed that MXene behaves similarly to PTFE while resolving some of the issues with the use of PTFE, such as low conductivity.¹⁸² The combination of high electrical output, mechanical toughness, and long-term stability provided by the incorporation of MXenes as negative triboelectric layers in TENGs makes them potential materials for sophisticated energy harvesting applications.

6. MXene TENGs in action: real-time applications

6.1. Biomedical wearables

Over the past 30 years, biomedical devices have been crucial in enhancing healthcare outcomes; however, their continuous and active biomedical sensing function in daily life has been hindered by wearability constraints and limited power source access. In this case, triboelectric nanogenerators have been proven to be incredibly versatile and capable of producing wear-optimized and self-powered biomedical sensors.^419,420

6.1.1. Wound healing. The skin is a diversified microbial habitat and the body's main barrier against external touch. Accordingly, if electronic equipment is not bactericidal, it may introduce bacteria and viruses when it comes into close contact with the skin.⁴²¹ Thus, the antimicrobial attributes of the bilayer triboelectric MXene/Ag/PVDF/cellulose (bottom)-MXene/PVDF layer (top) layer, PVDF/cellulose acetate, and bilayer without Ag were examined against bacteria (E. coli and S. aureus). The findings (Fig. 15(a)) indicate that the bilayer without PVDF eliminated limited bacteria, while the bilayer with PVDF completely eliminated the bacteria. By the eighth hour, neither the E. coli nor S. aureus colonies could be identified on the agar plates given that their numbers drastically declined over time (Fig. 15(b)). Microscopy images confirmed (Fig. 15(c)) that when the bilayer composite was applied to the skin on the back of a mouse for 24 h, its skin remained healthy, demonstrating the great safety and biocompatibility of the tribolayer.⁴⁰⁰ The difference in trans-epithelial potential (TEP) between the injured area and healthy skin typically promotes wound healing. To aid the healing process, the endogenous TEP produces an electric current when a wound arises, which pushes the charged Na⁺ and Cl⁻ ions toward the center of the injured area. Alternatively, a poor TEP lengthens the healing period for chronic wounds.^422,423 As shown in Fig. 15(d), styrene-butadiene-styrene-coated silver nanoparticle nanofibers were used to deliver induced energy to the injured location, and a TENG was made using a PET substrate, which allowed free tapping on the rat skin when it moved. The TENG was directly mounted on the injured rats in an in vivo experiment, with its AC and DC electrical performance analyzed (Fig. 15(e)) to demonstrate this. During AC and DC simulations, the positive and negative charges were transferred to the wounded area. As illustrated in Fig. 15(f), on days 3, 7, 10, and 14, pictures were taken to track the condition of the wound. The rats in the control sample were not given any treatment. The electrode was patched to contact the wound edges, allowing symmetrical wound recovery. On day 14, the reported wound closure ratio had already surpassed 95% (Fig. 15(g)).⁵⁵

Fig. 15 MXene TENG for wound healing. (a) Comparison of E. coli and S. aureus bacterial survival rates with different composites. (b) Survival rates of bacteria at different time intervals. (c) Photograph and microscopy images of the rat skin after 24-hour implantation of bilayer MXene composite. Reprinted with permission from ref. 400. © 2024, Elsevier. (d) Schematic representation of the electrical stimulation of TENG during the healing process of wounds. (e) Illustration showing the procedure of the AC and DC electrical stimulation on the injured area. (f) Rat wound skin photos were taken at different time points. (g) Recovery rate of the wound following 3, 7, 10, and 14 days of therapy. Reprinted with permission from ref. 55. © 2024, John Wiley and Sons. (h) Schematic illustration of wound healing with MXene TENG electronic skin patch that responds to NIR light. (i) Thermal images showing temperature variations in mice treated with or without a TENG patch following a 120 s exposure to NIR light. (j) Recovery rates of wound healing with multiple groups of therapy. (k) Photothermal heating curves of wounds both with and without TENG skin patch. Reprinted with permission from ref. 341. © 2024, Elsevier.

Furthermore, photothermal techniques can effectively speed up wound healing by producing heat. By transforming light into thermal energy, photothermal materials heat the wound site locally, promoting cellular metabolism and preventing the growth of microorganisms.^424,425 A single-electrode TENG as a skin patch was proposed based on an MXene/gelatin photothermal hydrogel. The hydrogel was directly applied to a rat epidermal wound and served as an electrode. To facilitate wound healing, the TENG device gathered biophysical energy, created an electric field surrounding the injured tissues, and worked in tandem with the near-infrared photothermal effect (Fig. 15(h)). The temperature barely rose when near-infrared light was applied to the wound; however, when the TENG patch with NIR was applied, the temperature increased from 32 °C to 45 °C in just two minutes (Fig. 15(i and j)). This showed that the key to converting light energy into thermal energy in TENG is the photothermal property of MXene. After ten days, it was evident that the TENG device treatment with near-infrared light had the greatest healing impact among the groups (Fig. 15(k)).³⁴¹ Incorporating MXenes into TENG enhances wound healing, utilizing both electrical stimulation and photothermal heating.

6.1.2. Health diagnostics. An essential component of the human knee joint that keeps it stable is the anterior cruciate ligament (ACL). Using wearable technology to check for early ACL injuries lowers the risk of long-term consequences and allows prompt intervention.^426,427 An MXene/PVDF-HFP-based TENG integrated with peripheral devices was proposed to diagnose early ACL injuries in mobile applications, as presented in Fig. 16(a). The peripheral device was comprised of a hardware component (filters and microcontroller) and a signal process unit, as shown in Fig. 16(b). The TENG device was positioned on the front part of the patient's wounded leg to confirm the injury. Then, the output voltage of the TENG was measured using an Anterior Drawer Test (ADT). After that, the same test was conducted with the TENG in the same location on the patient's normal leg. The comparison of signals denoted that the output voltage of the injured leg was higher than the normal leg (Fig. 16(c)). Based on the confusion matrix results, the TENG device exhibited 100% accuracy in detecting ACL injuries.⁵⁶ One common sleep problem that has a significant detrimental impact on daily functioning, sleep quality, and mental and physical health is insomnia disorder.⁴²⁸ A TENG-based eye mask was proposed to identify the signs of insomnia and track sleep quality in real-time. Using MXene–silk fibroin and PDMS as the positive and negative friction layers, with ITO and Al electrodes, a TENG device was tightly sealed into an economical eye mask (Fig. 16(d)). As illustrated in Fig. 16(e), the TENG device offered wired and wireless (gadgets) sensing of the quality of the human sleep pattern. Based on the electroencephalography (EEG)⁴²⁹ technique of frequency bands, the sleep pattern of a healthy individual and insomnia patient was recorded. When recording the electrical impulses of the normal individual, the δ (0.5–4 Hz, deep sleep) wave generated an output voltage of 108 mV and the θ (4–8 Hz, entire sleep) wave produced a significant voltage of 196 mV (Fig. 16(f)). No δ wave was visible when recording the signals of the insomnia patients; only the α (8–3 Hz, half-sleep) wave generated a voltage of 76 mV and the θ wave was present (Fig. 16(g)). By comparing the frequencies of both signals, it was evident that the normal person attained deeper sleep, while the insomnia patient exhibited a disturbed sleep pattern.⁶³ Nowadays, the main focus of joint management is using medication and surgery to slow the progression of this condition. However, regular drug use has the danger of causing acute renal impairment and gastrointestinal system damage, and injections may cause edema.^430,431 These problems were addressed by the proposal of a TENG drug delivery system and microneedle patches based on kirigami spider fibroin. Employing kirigami structured-MXene/spider fibroin microneedles and PTFE as the positive and negative triboelectric layers, respectively, with copper electrodes, a TENG was fabricated (Fig. 16(h)). To assess the capacity of the TENG patch to speed up the joint treatment, 0.1 mL of drug was injected into various rats and compared with the paw thickness of the untreated rat. The microneedle-TENG-treated rat had the best healing efficacy because electrical stimulation accelerated medication release, and after six days, paw swelling virtually vanished. This indicates that the reported microneedle-TENG can be used in joint treatment.⁶¹

Fig. 16 MXene TENG for health diagnostics. (a) Schematic of TENG device for ligament injury diagnosis in mobile application. (b) Hardware assembly of the diagnosis system. (c) TENG device fixed on a volunteer to diagnose ligament injury. Reprinted with permission from ref. 56. © 2024, Elsevier. (d) Optical image of a portable MXene TENG mask for the eyes. (e) Schematic illustration of the MXene TENG mask for detecting the status of human sleep. TENG mask's output circuit voltage (V_oc). (f) Detection of normal sleep patterns. (g) Detection of insomnia signs. Reprinted with permission from ref. 63. © 2024, American Chemical Society. (h) Joint treatment through spider-fibroin microneedle-based TENG patch. Reprinted with permission from ref. 61. © 2024, Elsevier.

An important metric for assessing the physiological well-being of the human body is the blood glucose level. However, the development of viable glucose sensors for diabetes patients is hampered by the unreliability of enzymatic glucose biosensors.⁴³² In this case, an MXene/GO-based TENG was fabricated to detect glucose levels without the support of glucose enzymes. Common and significant chemicals, such as glucose and Na⁺, were examined to assess the MXene/GO TENG biosensor potential. The TENG was shielded before the analysis to prevent contamination and 0.1 mL of glucose solution was dropped on its electrode. The output voltage of the TENG with multiple glucose concentrations was recorded and the output voltage developed to 426 V for 500 μmol concentration. Thus, this MXene/GO electrode in TENG opens the door for innovative biosensors and non-invasive health monitoring.²⁸⁸

6.1.3. Vital sign assessment and gait analysis. In everyday life, the ongoing monitoring of vital signs can help detect patient decline early and make the transition to home care easier. In in-patient situations, wearable sensors for constant vital sign monitoring are frequently assessed utilizing patches, mattress sensors, or more comprehensive sensors worn on the arm.^433,434 For monitoring vital signs, TENGs can function as active wearable sensors that provide real-time electrical signals without an external power source.⁴¹⁸ An MXene (Ti₃C₂T_x)/PVC film was prepared through electrospinning and applied as a negative friction layer in a TENG for monitoring the respiratory rate, heart rate, and gait analysis pattern (Fig. 17(a)). The TENG was fastened to a face mask for respiratory monitoring (Fig. 17(b)). It generated recurring voltage signals that represent the respiratory rate of the user. The inset demonstrates the possibility of real-time breath monitoring, which presents additional details about the single breathing cycle. Fig. 17(c) shows the heartbeat tracking before and following exercise, with noticeably stronger signals following exercise because of the elevated cardiac activity. When analyzing the gait pattern (Fig. 17(d)), running produced signals with higher amplitudes because of the increased force and frequency of motion.⁴⁰³ The advancement of self-powered bioelectronics is supported by the rising demand for triboelectric layers composed of biocompatible and biodegradable materials for reuse and integrating with biomedical electronics.⁴³⁵ A TENG mask based on an MXene (Ti₃C₂T_x)/silk fibroin aerogel composite and PDMS sponge as the positive and negative friction layers, respectively, was proposed to detect the respiratory rate and asthma. Air permeability is crucial in fabricating TENG to integrate with fabrics and masks. Fig. 17(e) illustrates that the MXene/silk fibroin aerogel offers an air permeability of 389 mm s⁻¹, which is greater than the disposable medical mask and lab coat. The entire porous TENG device was integrated within a disposable medical mask owing to its exceptional air permeability (Fig. 17(f)). Fig. 17(g) shows that the output voltage was 1.3 V at 0.2 Hz and 1.9 V at 1 Hz, confirming that the entire TENG device was frequency dependent. The output voltage frequency by the TENG (Fig. 17(h)) for normal breath was much less than the frequency of asthma-related breath. Despite the lack of noticeable changes in the output voltage, it is a trustworthy indicator for detecting asthma.⁵⁰ Most battery-powered wearable sensors have a short lifespan, making them unsuitable for sustainable use. Thus, it is essential to develop wireless self-powered systems for monitoring vital signs.⁴³⁶ Employing MXene (Ti₃C₂T_x)/PMMA and PDMS as the positive and negative tribolayers, respectively, for wireless breath monitoring was proposed. A tiny chip, a TENG device, a lithium-ion battery, and a mobile application were integrated for wireless transmission. The TENG was fixed to the abdomen (Fig. 17(i)) to monitor the shallow and deep breathing patterns based on the frequency and amplitude of the signal (Fig. 17(j)).⁴³⁷ In wearable and implanted devices, photodetectors and photoplethysmography (PPG), a non-invasive technique for health monitoring, are essential for measuring vital signs.⁴³⁸

Fig. 17 MXene TENG for vital sign assessment and gait analysis. (a) MXene/PVC TENG for monitoring vital signs and gait pattern. (b) TENG device was adhered to a face mask to monitor the respiratory rate. (c) Monitoring of heartbeat before and after physical activity. (d) Monitoring of gait pattern. Reprinted with permission from ref. 403. © 2025, AIP Publishing. (e) Air permeability test of usual fabrics and fabricated TENG device. (f) Optical image of a face mask integrated with a TENG. (g) Output voltage of the TENG face mask at various breathing frequencies. (h) Output voltages of the TENG device detecting usual breathing patterns and asthma signs. Reprinted with permission from ref. 50. © 2023, American Chemical Society. (i) MXene/PMMA TENG device fixed on the abdomen of a human to detect breathing patterns. (j) Output voltage of the TENG monitoring breath rate. Reprinted with permission from ref. 437. © 2023, John Wiley & Sons. (k) A wristband comprising TENG, photodetector, and LED. (l) Current versus time graph revealing the heart rate. Reprinted with permission from ref. 58. © 2024, Elsevier. (m) TENG device attached to the front and back foot for gait analysis. (n) Output voltages in all directions during walking. (o) Output voltages in all directions during running. Reprinted with permission from ref. 393. © 2024, Springer Nature. (p) Output voltages of TENG based on MXene/PAM hydrogel for detecting walking and running. Reprinted with permission from ref. 348. © 2024, Elsevier.

An MoS₂-based photodetector powered by an MXene (Ti₃C₂T_x)/PDMS TENG device was proposed to track the heart rate and UV illumination (Fig. 17(k)). The systolic and diastolic phases of the heart are represented by the current peaks in the I–t diagram caused by LED light (660 nm) reflected off the fingertip onto the active area of the photodetector. When tapping the TENG device at the frequency of 1 Hz, it charged the capacitor of 220 μF in less than 2 min. The photodetector received the DC voltage that is stored in the capacitor when the tapping stopped. As shown in Fig. 17(l), heartbeats were obtained in DC mode by plotting the current versus time of the photodetector.⁵⁸ GAIT ability is a crucial component in assessing the quality of life of a person and reflects their status as a healthy individual. GAIT is a complicated, unrealized motor pattern that is essential to living, enabling people to function in the human world and engage in daily activities.^439,440 A TENG device based on MXene (Ti₃C₂T_x)/PDMS and gelatin as the negative and positive friction layers, respectively, was fabricated simply and economically. Its surface was modified using a flower petal and only 0.03 wt% of MXene was added to PDMS. The TENG device was fixed on the front part and heel of the foot to record the gait pattern (Fig. 17(m)). The output voltage of the TENG device fixed on the front foot had 3 peaks, and the negative voltage was 40 V (Fig. 17(n)), which was higher than the negative voltage of the heel-fixed TENG. The positive voltage was 60 V in running from the heel-fixed TENG (Fig. 17(o)), which was higher than the forefoot TENG positive voltage. These variations in the voltage are due to the diverse movements of the foot.³⁹³ Similarly, a hydrogel based on MXene/PAM was applied in a TENG to monitor the walking and running of humans. To enhance the dispersibility of the MXene in the hydrogel, the MXene was oxidized to alter its nanostructure. The single-electrode TENG was attached to the foot sole to track human steps. The recorded voltage and number of peaks corresponding to walking were less than running, as illustrated in Fig. 17(p).³⁴⁸ As a result, MXene TENGs enhance the applicability of monitoring vital signs and gait patterns in real-time.

MXene TENGs have demonstrated great potential in biomedical applications such gait analysis, vital sign monitoring, wound healing, and health diagnostics because of their remarkable electrical conductivity, mechanical flexibility, and biocompatibility. Their antibacterial qualities and potential for medication delivery further improve the therapeutic efficacy, and their capacity to produce localized electrical stimulation and photothermal impacts speeds up tissue regeneration in wound healing.^341,441 MXene TENGs provide self-powered, high-sensitivity physiological signal detection, such as breathing and pulse, for health diagnostics and vital sign assessment.^440,442 Their versatile form factors allow continuous, wearable monitoring. Because of their tremendous electrical output, they offer precise, real-time motion monitoring in gait analysis.³⁹³ However, issues including long-term biocompatibility, mechanical durability, environmental stability, and the requirement for scalable production must be resolved.

6.2. Gas sensing

Gas sensors transform complicated ambient biochemical data into signal data, enabling the real-time quantitative monitoring of certain gaseous targets, including hazardous components. Due to the materials and design of sensors, the majority of detecting sensors on the market are expensive and power-hungry. Therefore, creating gas detectors with a detection limit that are affordable and readily available is essential for environmental monitoring.^443,444 Self-powered gas sensors rely on energy harvesting techniques to produce electrical power for gas detection, allowing them to function without an external power source.⁴⁴⁵ Integrating gas sensors with TENGs to create a revolutionary self-powered gas sensing systems was suggested based on the “impedance matching effect”.⁴⁴⁶ MXenes are becoming progressively known as exciting materials in the field of gas sensing because of their excellent ion-transmission properties and wide range of surface functional group modifications.⁴⁴⁷ A TENG device incorporating a dual-layer tungsten trioxide (WO₃)/MXene–carbon quantum dots as the negative friction layer was fabricated to detect triethylamine for tracking rotten fish meat. As shown in Fig. 18(a), the response of the MXene/WO₃–carbon quantum dots increased when the concentration of triethylamine increased, exhibiting the maximum response of 53%. The response rate of triethylamine with other gases is shown in Fig. 18(b). The output voltages of the fish meat followed different decay durations, and through time, the output voltage increased (Fig. 18(c)). This denotes that triethylamine reacts with the composite, exhibiting high sensitivity.⁴⁴⁸ Ammonia (NH₃) monitoring is a reliable indicator of dangerous gas leaks or formation during fires because unlike the more common gases such as CO₂, NH₃ is not usually found in everyday situations.⁴⁴⁹ A self-powered gas sensor based on MXene/MoS₂/cellulose diacetate/tetraethyl orthosilicate and (PVDF HFP)/BaTiO₃ as the negative and positive friction layers, respectively, was fabricated to sense ammonia (Fig. 18(d)). The response and recovery time of the TENG for NH₃ (86.1%) was 2 s and 3 s at the rate of 10 ppm, respectively (Fig. 18(e)). Through FTIR analysis, the NH₃ gas was sensed, which was intentionally leaked by burning hair (Fig. 18(f)). The TENG device responded 28 s earlier than other NH₃ detectors.⁶² Gaseous pollutants such as sulfur dioxide (SO₂), nitrogen dioxide (NO₂), and ammonia (NH₃) are aggravating the problem of chemical gas pollution, which has become one of the main causes of disease and death. Although MXene improves gas sensor responsiveness, cross-sensitivity is still a problem when using these sensors to detect multi-component chemical gases.^450,451 A multi-gas sensing array powered by a TENG integrated with a convolution neural network-gated recurrent unit neural network mode was proposed to detect the composition and forecast the concentration of 3 chemical gases (Fig. 18(g)). The TENG was driven by the wind. The effective identification of the composition of NH₃–SO₂–NO₂ chemical gas mixtures and concentration prediction offer a general approach for more intricate multi-component gas mixture detection.³⁷² In vegetable greenhouses, small quantities of ammonia (NH₃) generated during the breakdown of organic fertilizers, urea, and ammonium bicarbonate may reach plants through their stomata, burning their tender shoots, and even causing the entire plant to perish.^452,453 An Nb₂AlC/Ta₂O₅ composite-based sensor powered by a wind-driven TENG was proposed to detect trace NH₃ in greenhouses. The reported sensor could detect the presence and concentration of NH₃ with alarms. This paves the way for modern agriculture.⁴⁵⁴ Regular inspection of fruit and vegetable storage warehouses is necessary to prevent the degradation of fruits and vegetables. Excessive ethanol accumulation can cause deterioration in quality, faster aging, unpleasant flavours, and a major decrease in storability.^455,456 A TiO₂/CuO/MXene-based sensor driven by a TENG for ethanol detection in warehouses was reported. The assembled ethanol sensor can be mounted on a moving inspection robot, and the TENG is anticipated to capture the mechanical energy produced by the robot. The electrical output of the TENG array is stored in a management circuit, which serves as the operational power source of the sensor for the ethanol concentration warning and online warehouse ethanol level monitoring.⁴⁰¹

Fig. 18 MXene TENG for gas sensing and sports monitoring. (a) Response rate of triethylamine sensing (different concentrations) using self-powered TENG-based on MXene/carbon quantum dots/WO₃ composite. (b) Selectivity test of the TENG sensor for multiple gases. (c) Optical image and voltage signal of 30 g of fish meat deteriorating after various storage times. Reprinted with permission from ref. 448. © 2025, American Chemical Society. (d) Release of ammonia gas from the environment harms human health. (e) Response and recovery time of the TENG device for multiple NH₃ concentrations. (f) FTIR spectrum of burnt hair (releases ammonia). Reprinted with permission from ref. 62. © 2024, Elsevier. (g) TENG array driven by wind powering the gas sensor array for detecting multiple elements of gases. Reprinted with permission from ref. 372. © 2024, Elsevier. (h) MXene composite-based TENG was fixed at the different parts of the human body for scientific basketball training. (i) Output voltages of the TENG device when lifting the arm and jumping. Reprinted with permission from ref. 457. © 2024, AIP Publishing. (j) Bilayer MXene/PDMS TENG was fixed at different parts of the body to assist volleyball training. (k) Output voltages of the TENG device when bending the wrist and knee during play. Reprinted with permission from ref. 405. © 2024, Elsevier. (l) Schematic of a boxer throwing punches at the target. (m) Optical image of the MXene/PDMS TENG for monitoring boxing movements. (n) Output voltages of the TENG for straight fist and hook fist punches. Reprinted with permission from ref. 57. © 2024, AIP Publishing. (o) MXene/PI-based TENG was attached to a tennis bat to track batting activities. (p) Current signals of the TENG during the table tennis practice. Reprinted with permission from ref. 458. © 2025, AIP Publishing.

TENGs based on MXene have shown great promise for self-powered gas sensing applications because of their distinct electrical and structural characteristics. In gas detection applications, the 2D structure of MXenes improves the sensitivity by facilitating improved contact with gas molecules.^363,459 The selectivity of sensors can be improved by customizing the surface functional groups of MXenes to interact with particular gas molecules.⁴⁴⁷ When incorporated in TENGs, MXenes help create completely self-powered devices that do not require external power, which makes them perfect for wearable technology or remote environmental monitoring.^453,454 Their mechanical adaptability also facilitates their incorporation into stretchable and flexible systems.⁴⁴⁴ However, there are still issues that must be resolved for dependable long-term use, namely fabrication scalability, surface area reduction from sheet aggregation, and environmental instability caused by oxidation.

6.3. Sports monitoring

In sports, wearable technology has emerged as a crucial instrument that is transforming how athletes prepare, perform, and rehabilitate. These electronic devices offer real-time data on important performance indicators including heart rate, pace, distance, and muscle activity. They range from smartwatches and fitness trackers to sophisticated sensors built into clothes.^460,461 Basketball stands out among the various sports as a very competitive sport where the focus is on winning and losing. Thus, the development of an athlete's competitive skills is necessary to win basketball games.⁴⁶² Scientific training is the foundation for all these criteria, and a crucial part of scientific training is scientific training monitoring.⁴⁶³ An N-isopropylacrylamide/PDA/MXene hydrogel-based TENG was fabricated to monitor the posture of a basketballer. The device was attached to the elbows and joints of the volunteer to monitor human motions such as arm lifting and jumping (Fig. 18(h)). It was demonstrated through lifting and jumping that the TENG could distinguish between different pressure levels. The corresponding output voltages of arm lifting and jumping were 8.9 V and 17.16 V, respectively (Fig. 18(i)). Thus, the TENG device can monitor diverse human motion in sports.⁴⁵⁷ Similarly, a TENG device based on dual-layer PDMS–MXene/PDMS was reported for monitoring volleyball guidance. Monitoring different body joints is crucial to aiding training because volleyball players move their bodies in a variety of ways during practice, as illustrated in Fig. 18(j). The device was placed on the wrist and knee to monitor the motions and based on the output voltage, which could distinguish wrist and knee motions (Fig. 18(k)).⁴⁰⁵

Research on the physical demands of boxing has only been conducted on amateur fighters; studies on professional fighters have only looked at injury risk or weight loss. This implies that it is unknown what demands a professional boxer will face.⁴⁶⁴ Thus, a TENG device for developing a database comprised of the objects of professional boxers was fabricated based on MXene/PDMS film. A representation of a boxer throwing punches at a target is shown in Fig. 18(l). This graphic portrayal makes it easier to understand the experimental apparatus used to record the triboelectric signals. An actual boxing action is shown in Fig. 18(m), where a glove hits the TENG sensor, highlighting the usefulness of this device in a real-world situation. Plotting the voltage output against time (Fig. 18(n)) is done for the jab, straight fist, and hook fist punches. In contrast to the straight and hook fists, which produce noticeably greater voltages because of the increased impact force and surface area contact, the jab produces comparatively smaller voltage peaks.⁵⁷ Different athlete motion abilities and task objectives result in varying table tennis motion durations, which can potentially lead to issues with motion recognition.⁴⁶⁵ Incorporating MXene/PI film, a TENG sensor was reported to track the batting activities of tennis. The TENG sensor was integrated into a tennis bat to track the batting activities (Fig. 18(o)). The current signals were produced in real-time throughout table tennis practice (Fig. 18(p)). The data showed clear peaks that correspond to each ball strike, giving specific details about how frequently and how hard the hits occur.⁴⁵⁸ Thus, TENG-based self-powered sensors can be applied in intelligent sports monitoring.

MXene triboelectric nanogenerators (TENGs) are becoming increasingly popular as cutting-edge self-powered wearable sensor components in the constantly evolving environment.⁴⁶⁶ These digital devices provide up-to-date information on critical performance metrics such as muscle activity, heart rate, pace, and distance.⁴⁶⁷ Additionally, MXene TENGs have excellent mechanical durability, continuing to function for thousands of cycles, which makes them perfect for continuous, real-time sports monitoring.^403,457 Besides, their ability to create wearables that are strong, lightweight, and function effectively might completely change how athletes track their performance and avoid injuries.

6.4. Smart homes

Automation based on the Internet of Things (IoT) has been popular for years. Automation is being incorporated into every part of daily life, from consumer homes to industrial machinery.⁴⁶⁸ Smart homes have attracted significant interest because of their potential to increase the quality of life of people. Environmentally friendly, highly stable, and high-performing electrical devices are desperately needed as a key part of smart homes. Triboelectric nanogenerators (TENGs), which can transform mechanical energy into electrical outputs at low frequencies, are considered the most promising way to deal with this problem.^469,470 A highly sensitive MXene/silicone film that operates in single-electrode was used for security purposes. An Arduino board was connected to an aluminium electrode attached to this triboelectric layer. Through contact electrification, a voltage pulse is produced when a human hand touches the triboelectric layer. The Arduino board then records this voltage signal, which causes the Arduino's built-in Wi-Fi module to send a security warning to a mobile device. Every time there is unwanted access, the mobile phone receives emails and SMS. Additionally, the time and date of the unauthorized access are included in email notifications (Fig. 19(a)).⁵⁹

Fig. 19 MXene TENG for smart homes, EMI shielding, and soft robotics. (a) Schematic of the single-electrode MXene/silicone TENG device as an IoT touch sensor for security applications. Reprinted with permission from ref. 59. © 2024, American Chemical Society. (b) Schematic of the MXene/PVDF IoT TENG device for wireless control of multiple home appliances. Reprinted with permission from ref. 473. © 2024, American Chemical Society. (c) Schematic of the MXene/PVDF nanofiber-based TENG device attached to the multiple areas of a door for energy harvesting. Reprinted with permission from ref. 406. © 2023, John Wiley & Sons. (d) Schematic illustration of EMI shielding using an MXene/gelatin/sodium lignosulphonate composite. (e) EMI shielding performance after applying the composite with different contents of MXene. (f) EMI shielding effectiveness of the composite before and after 1000 bending cycles (MXene 50 wt%). Reprinted with permission from ref. 386. © 2025, Elsevier. (g) Schematic illustration of EMI shielding using 3D printed MXene/chitosan aerogel. (h) MXene/chitosan composite's EMI shielding effectiveness (8–12 GHz). Reprinted with permission from ref. 477. © 2025, Elsevier. (i) Schematic illustration of EMI shielding using carboxylated styrene-butadiene rubber/polydopamine-altered carbon nanotube/MXene composite. (j) EMI shielding effectiveness of the composite with multiple contents of MXene/CNT/PDA. Reprinted with permission from ref. 478. © 2025, Elsevier. (k) Schematic representation of the HMI system with TENG, Arduino board, transceivers, and robotic arm. (l) Output voltage signals of the TENG captured by the microcontroller circuits. (m) Robot arm performs 3 digital motions according to the control instructions. Reprinted with permission from ref. 390. © 2025, Elsevier. (n) Schematic of constructed intelligent gripper with multi-modal recognition. (o) Schematic of constructed intelligent gripper by coupling photo-thermoelectric and triboelectric actuators. (p) Optical images of the intelligent gripper operation under NIR light. (q) Temperature variations, PTE voltage signals, and triboelectric voltage signals of the intelligent gripper as it comes into touch with six materials exposed to NIR light. Reprinted with permission from ref. 60. © 2025, Elsevier. (r) TENG-powered MXene/CuO/TiO₂ sensor as an inspection robot for ethanol alarms in vegetable warehouses. Reprinted with permission from ref. 401. © 2024, American Chemical Society.

The primary objective of power management is to efficiently manage the load profiles of its clients. Disruptions are prevented to maintain the stability of the main power system and lower the likelihood of issues. Reducing energy consumption and electricity costs is essential.⁴⁷¹ Controlling home appliances remotely is a tedious task and it prevents the waste of power.⁴⁷² A smart home system operating in a dual mode based on an MXene porous TENG was revealed to control multiple appliances in the home (Fig. 19(b)). The output voltages of the TENG varied based on the applied pressure. In both modes, the system compared the voltages in multiple thresholds and sent commands. Mode 1 can operate only three appliances given that only three thresholds were selected. In mode 2, the signals are transformed into digital form through a microcontroller, which enables the system to control multiple appliances.⁴⁷³ Global warming and other serious ecological and environmental issues are increasing the demand for alternative energy, in addition to renewable energy sources. Therefore, the development of clean, green energy collecting and storage technologies is imperative.^474,475 Employing MOF/PAN and MXene/PVDF as positive and negative friction layers, respectively, a TENG device was fabricated to harvest electricity from waste mechanical motions. The TENG was attached to several places in a smart door and produced electricity when the door was opened, knocked, or tapped. After fixing the TENG on the door, the device generated a voltage of 28 V and 18 V when opening and knocking on the door (Fig. 19(c)).⁴⁰⁶ These findings show that the manufactured TENG gadget might be used in a smart home or office to regulate various appliances to save power, harvest electricity from waste physical motions, and provide security features.

MXene TENGs are showing great promise as parts of security and smart home systems because of their remarkable mechanical, electrical, and environmental characteristics.^53,286 These exceptional properties make it feasible to incorporate self-powered, robust, and extremely responsive sensors into motion detection, tamper warning, and environmental monitoring systems.⁴⁷⁶ They are perfect for powering security devices without the need for external sources because of their capacity to capture mechanical energy from the surrounding environment.^308,473 However, in actual application, issues including environmental sensitivity, fabrication scalability, and interaction with current systems must be resolved.

6.5. EMI shielding

Electronic waste and electromagnetic radiation are becoming increasingly dangerous, endangering public health and electronic device functionality. The growth of portable electronics and the introduction of 5G mobile network communication technologies have increased the demand for robust, portable materials with exceptional electromagnetic interference (EMI) shielding qualities.⁴⁷⁹ An MXene/gelatin/sodium lignosulphonate film-based TENG was used as an effective EMI shield with a vector network analyzer (Fig. 19(d)). The effectiveness of EMI shielding with diverse MXene contents was analyzed. As shown in Fig. 19(e), the EMI shielding effectiveness dramatically improved with an increase in the MXene (Ti₃C₂T_x) nanosheet content, peaking at 49 dB with 50% content. Even after 1000 bending cycles, the MXene composite film maintained exceptional EMI shielding stability and dependability because of its outstanding mechanical flexibility and stability (Fig. 19(f)).³⁸⁶ Porous conductive polymer nanocomposite foams and aerogels have been shown in numerous experiments to efficiently attenuate electromagnetic radiation by absorption. These thin composites act as sound-absorbing and thermal-management materials, in addition to protecting against electromagnetic radiation.^480,481 An MXene (Ti₃C₂T_x) was solidified into a 3D-printed chitosan aerogel through a direct-ink writing approach and prepared for EMI shielding (Fig. 19(g)) and energy harvesting. The EMI shielding efficiency of the aerogels is shown in Fig. 19(h). The findings showed that increased MXene concentrations enhanced the ability of the printed aerogels to shield against electromagnetic interference. In particular, the printed aerogel with 1% MXene had a shielding effectiveness of around 10 dB, but when the MXene concentration was increased to 10%, its shielding effectiveness significantly increased to about 27 dB.⁴⁷⁷ In addition to improving their EMI shielding capabilities, composites made of a polymer matrix should be considered owing to the need for multifunctionality under challenging application circumstances.⁴⁸² A multifunctional carboxylated styrene-butadiene rubber/polydopamine-altered carbon nanotube/MXene film (Fig. 19(i)) was prepared for energy harvesting, EMI shielding, and thermal management. The effectiveness of EMI shielding with different MXene contents in the composite was analyzed. When the MXene content was 50%, the EMI shielding effectiveness reached 58.8 dB from 24.2 dB (Fig. 19(j)). This composite showed certain potential applications, such as Joule heating and TENG, in addition to its good EMI shielding capability.⁴⁷⁸ Likewise, melamine/PDA/MXene/AgNW foam with multifunctional abilities was prepared. This composite exhibited excellent EMI shielding effectiveness of 48.3 dB, in addition to thermal management and energy harvesting.³⁴⁹ These multipurpose composites will provide scientific guidance and motivation for creating the next generation of wearable electronics that can satisfy the demands of various applications in challenging settings.

MXene films surpass many conventional materials, with EMI SE values exceeding 20 dB at thicknesses as low as approximately 40 nm.⁴⁸³ Composites with improved mechanical strength and EMI shielding qualities can be created by combining MXenes with polymers, carbon nanotubes, and other materials.^478,484 In recent years, MXenes, promising absorbing agents, have attracted significant interest for the creation of EMI shielding or absorption materials.⁵⁴ However, their incorporation in commercial EMI shielding solutions will require addressing issues associated with their stability, environmental sensitivity, and manufacturing scalability.

6.6. Intelligent soft robots

Robots are widely used in many different industries because of their uniqueness and stability. In the past, these robots were made of rigid parts that effectively completed repeated duties. Several sensor modules were used for motion direction and sensing. However, the function of robots has changed over time, moving from basic mechanical tasks to intricate interactive behaviors.^485,486 The design and manufacturing of soft sensors have proven essential in unlocking complicated functionality and reducing rigid components. In the course of steady rigid-hybrid-soft evolution, soft robots have arisen with high degrees of freedom and intricate shape creations.⁴⁸⁷ Robots with TENG-based sensors are promising substitutes for traditional rigid sensors and will expand the capabilities of robotics. TENGs also encourage the creation of novel flexible structures and the inclusion of robotics.^488,489 A TENG device based on a single-layer MXene/MWCNT/Ecoflex film was developed for remote robotic control without human touch (noncontact). An HMI system was assembled with a TENG, an Arduino microcontroller for generating instructions and signal processing, a couple of wireless transceiver modules for commands, and a robot arm for tracking and executing the commands (Fig. 19(k)). The robot arm, which could execute various digital gestures that correspond to various microcontroller channels, was controlled by the output voltages of the TENG units. The analog output signals were recorded and detected by the microcontroller, as seen in Fig. 19(l), depending on the TENG unit. The microcontroller then translated the identified impulses into control commands to operate the robot arm, enabling it to precisely perform the matching digital motions (e.g., “1” and “2”) (Fig. 19(m)).³⁹⁰ Most soft robots must include external power and sensing modules to obtain stimulus-response information because traditional light-driven actuators cannot perceive on their own. This compromises their compact size and remote actuation sensitivity.^490,491 A dual-layer MXene/chitin nanofibers/PET/PEDOT:PSS film-based intelligent gripper (without external power) was created to achieve the combined benefits of light-driven actuator and multi-modal (temperature/material) recognition by concisely merging the photo-thermoelectric and the triboelectric actuator (Fig. 19(n)). Initially, the material recognition was evaluated with diverse materials and the TENG generated a significant change in the output voltage. An intelligent gripper is constructed by coupling photo-thermoelectric and triboelectric actuators (Fig. 19(o)). Under the influence of NIR light, multiple materials were adhered to a 3D-printed cube and touched by the intelligent gripper (Fig. 19(p)). The triboelectric and photo-thermoelectric actuator generated a different voltages and temperatures (Fig. 19(q)) for different materials contacted by the intelligent gripper. Based on the triboelectric and photo-thermoelectric signals, the intelligent gripper recognized the light-driven information and material without interference.⁶⁰ Intelligent gas monitoring depends on installing massive sensors, which can result in significant energy waste and installation expenses. Thus, novel energy harvesting techniques to lower power consumption are needed.^492,493 An MXene/TiO₂/CuO-based ethanol sensor powered by a TENG array was developed as an inspection robot in warehouses to impede the spoilage of fruits and vegetables. The constructed ethanol sensor was mounted on a moving inspection robot, and the TENG was anticipated to capture the mechanical energy produced by the robot. A management circuit stored the electrical output produced by the TENG array, which served as the operational power source of the sensor to enable the concentration of ethanol alarm and online warehouse ethanol level monitoring (Fig. 19(r)).⁴⁰¹

MXene-based triboelectric nanogenerators (TENGs) have rich surface functional groups, remarkable electrical conductivity, and mechanical flexibility, making them extremely promising for intelligent soft robots.^52,293 Because of these characteristics, MXenes can operate as sensors, actuators, and energy harvesters all at once, enabling self-sufficient, multipurpose robotic systems.⁴⁹⁴ Their low-voltage actuation and high sensitivity improve energy efficiency and real-time responsiveness.³⁹⁰ Their incorporation into next-generation robotics is further enhanced by their interoperability with wearable human–machine interfaces.

7. Obstacles and future outlook

2D MXenes unquestionably have unique benefits and various features that are not to be undervalued. As a result, they offer numerous innovative ideas for triboelectric nanogenerators. A thorough grasp of MXenes enables the identification of areas with substantial development potential as well as the establishment of a theoretical framework for maximizing their functioning and structure. MXenes have become potential materials for enhancing TENG performance. TENG efficiency is significantly boosted by its special qualities, which include mechanical stability, tailorable surface chemistry, and high electrical conductivity. MXenes enhance the surface charge density through their surface functional groups (–OH, –F, –Cl, –O, etc.) and promote the interactions with the polymer network. These developments make MXene TENGs a viable option for self-powered systems and efficient energy harvesting. However, despite the significant progress, there are many issues that need to be resolved in the emerging field of MXenes in TENG research.

MXenes are extremely hydrophilic due to the presence of surface functional groups (–OH, –F, –O). Their high affinity for water speeds up their oxidation and hydrolysis, which eventually causes structural deterioration. Oxidation transforms MXenes into insulating metal oxides (TiO₂, V₂O₅, and Nb₂O₅) and reduces the charge transfer ability in TENGs. In this case, encapsulating MXenes with polymers or depositing oxide layers prevents their oxidation by serving as a barrier against moisture. The production of MXene entails laborious procedures, including the use of extremely toxic HF solution and centrifugation to create MXene nanosheets of the proper size. The most common etchant in MXene synthesis is HF, which presents significant safety hazards for large-scale production. Thankfully, various fluorine-free etching substitutes are now available, and it has been demonstrated that large-batch synthesis preserves the product quality comparable to smaller batches. Although MXenes are classified as carbides, nitrides, or carbonitrides, only carbide MXenes (Ti₃C₂T_x, V₂CT_x, and Nb₂CT_x) have been widely explored in TENG. Investigations on double-transition MXenes (Mo₂TiC₂T_x and Nb₂TiC₂T_x), high-entropy MXenes (TiVNbMoC₃ and TiVCrMoC₃), and nitride MXenes (V₂N and Mo₂N) in TENGs can increase their stability and charge retention. The key developments and the future outlook on MXene TENGs are shown in Fig. 20. The presence of multi-metals in MXenes improves their durability and charge transfer efficiency, while preventing their oxidation. The restacking ability of MXenes has been found to significantly impair their stability; this issue can be resolved by creating various MXene heterostructures. This can increase their stability and assist in resolving restacking concerns.

Fig. 20 Significant advancements and prospects for MXene TENGs.

The long-term sonication used for delamination produces MXenes with a large number of surface defects, which may alter the properties of MXenes. Moreover, the delamination and intercalation processes use organic solvents, which are hazardous to the environment. Alternatively, MXenes can be delaminated using electrolyte solutions (LiCl and TMAOH), amino acids, natural polymers, and electrochemical intercalation. These green and sustainable approaches make MXenes more appropriate for energy harvesting applications. Understanding the dimensions and structure–property relationships of MXenes is crucial for their large-scale synthesis and for comprehending the intricate mechanism at the interface between the MXene and the deposited materials, but very few papers have paid significant attention to this topic. To fully comprehend and gain a thorough grasp of the reaction process between MXenes and foreign elements, a mix of in situ synthesis and simulation studies should be performed. Finding the ideal MXene compositions with optimum triboelectric characteristics is one of the main hurdles in MXene-based TENGs; nevertheless, Artificial Intelligence (AI)/machine learning (ML) can forecast high-performance MXenes before their synthesis. AI and ML in MXene TENGs have a bright future, allowing improvements in real-time energy management, device optimization, and material discovery. First-principles computations (discrete Fourier transform (DFT) and molecular dynamics (MD) simulations) can be processed by AI algorithms to forecast the triboelectric and bandgap characteristics of novel MXene compositions. The effects of various functional groups (–OH, –F, –Cl, and –O) on the triboelectric performance can be predicted by ML. Real-time IoT connectivity, self-optimizing storage integration, and adaptive power management can all be made possible by intelligent AI-based energy harvesting systems. These developments in AI and ML will transform MXene-TENGs into scalable, intelligent, and effective energy harvesting systems.

8. Conclusions

Triboelectric nanogenerators based on MXene have attracted significant attention because of their exceptional mechanical flexibility, structural adaptability, and electrical conductivity, making them essential components for energy harvesting in the future. In addition to describing the numerous TENG operating processes and tracing the genesis and evolution of triboelectricity, this review attempted to enhance the knowledge on the recently developed deformation mode, which presents new functional possibilities. The tunability of MXenes for optimal triboelectric performances was highlighted by examining their location in the triboelectric series, their varying compositions, and their synthesis developments between 2011 and 2024. Their versatility in various TENG configurations was highlighted by their integration as electrodes, tribolayers, and multifunctional materials. The design scope of MXene TENGs was further expanded by their capacity to operate as both positive and negative tribolayers. Their potential in wearable and smart sensing technologies was demonstrated by real-world applications such as wound treatment, sports monitoring, food spoilage detection, intelligent robotics, gait analysis, and EMI shielding. Despite the current obstacles, improving their performance will require developments in large-scale fabrication, surface engineering, and material stability. Material optimization powered by AI and ML may hasten their actual implementation. With ongoing research, MXene-based TENGs have the potential to revolutionize intelligent environmental sensing, healthcare monitoring, and self-powered electronics, spurring innovation in sustainable energy solutions.

Data availability

No primary research results, software, or code have been included, and no new data were generated or analyzed as part of this review. All figures and tables were reproduced from references with permission.

Author contributions

Conceptualization, S. K. C., A. C.; methodology, S. K. C., A. C.; formal analysis, S. K. C., A. C.; visualization, S. K. C., A. C.; investigation, A. C.; validation, A. C.; writing – original draft, S. K. C., writing – review & editing, S. K. C., A. C.; supervision, A. C., K. C.; project administration, A. C., K. C.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

No acknowledgement is necessary for this work.

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