The sustainable development of people’s lifestyle needs requires emerging green and renewable energy systems, which are the definite solution for global warming and demanding energy requirements [1]. Moreover, people approach the energy crisis with various personal and social needs, which leads to focus on different kinds of energy harvesting systems [2]. In the environment, mechanical energy is enormously available in the form of minor and major disturbances. This can be easily converted into useful energy to fulfill current energy requirements. Instead of using these energies, for the most part they are fully wasted or only partially used for energy conversion ^[3][4][3,4].

Nanogenerator types such as piezoelectric and triboelectric are developed to harvest energy from mechanical movements ^[5][6][5,6]. Here, easily accessible and fabricated materials of ceramics, polymers, their composites, and nanocomposites are used for various locations, ambiences, and applications [7]. Still now, the acquired performance of nanogenerators is comparatively low, which needs to be improved to obtain the highest energy conversion [8]. There is a big gap between practically gained performances due to the inadequate acquired performance of piezoelectric nanogenerators (PENGs). In addition to piezoelectric nanogenerators, there are also other types of nanogenerators such as triboelectric nanogenerators (TENGs), which generate electrical energy from friction and pyroelectric nanogenerators, which generate electrical energy from temperature changes ^[9][10][11][9,10,11]. Nanogenerators have a number of potential applications in areas such as energy harvesting, self-powered sensors, and wearable electronics [12]. They can be used to generate electrical energy from ambient vibrations, such as those caused by footsteps or wind, or from human movement such as walking or typing ^[13][14][13,14].

Recent studies have shown that MXenes have high mechanical strength, electrical conductivity, and thermal stability, making them ideal for energy harvesting applications ^[15][16][15,16]. In particular, MXene-based composites have been shown to exhibit improved energy conversion efficiency compared to traditional energy harvesting materials [17]. For example, MXene-based composites have been shown to be effective for harvesting mechanical energy from ambient vibrations and for generating electricity from temperature gradients ^[18][19][18,19]. In addition, MXenes have been explored for various energy harvesting methods, such as piezoelectric energy harvesting, electromagnetic wave harvesting, and energy storage applications ^{[20][21][22][23]}[20,21,22,23]. Researchers have found that incorporating MXenes into piezoelectric materials can improve the energy conversion efficiency and increase the power output [24]. The results of previous studies have demonstrated the potential of MXene-based energy harvesting for a variety of energy sources [25]. Further research and development in this field is expected to lead to even more efficient and effective energy harvesting technologies in the future ^[26][27][26,27].

The voltage output of MXene-based nanogenerators depends on several factors, including the specific MXene material used, the configuration of the device, and the mechanical energy input [28]. In general, MXene-based nanogenerators have demonstrated high voltage output, with some studies reporting voltage outputs of up to several volts under optimal conditions [29]. The high voltage output of MXene piezoelectric nanogenerators is due to the strong piezoelectric properties of MXene materials, which allow for efficient conversion of mechanical energy into electrical energy ^[24][30][24,30]. Additionally, the voltage output of MXene piezoelectric nanogenerators can be improved by optimizing the device configuration and by using MXene materials with higher piezoelectric coefficients ^[31][32][31,32]. It should be noted that the voltage output of MXene piezoelectric nanogenerators is typically small and is not enough to power most electronic devices on its own [33]. However, the small voltage output can be used to power low-power devices, such as sensors, or can be stored in a battery for later use [34]. MXene piezoelectric nanogenerators have demonstrated promising voltage output and have the potential to be used for applicable energy harvesting applications. There is ongoing research to further optimize the performance and stability of these devices for practical applications ^[18][35][36][18,35,36].

MXenes are a new class of two-dimensional (2D) materials composed of transition metal carbides, nitrides, or carbonitrides ^[37][106]. The term “MXene” is derived from the chemical formula of the precursor material, which typically consists of a transition metal (M), a group A element (such as Al or Si), and carbon/nitrogen (X) ^[38][107]. The resulting MXene material has a layered structure with a general formula of Mn + 1XnTx, where n represents the number of layers, T represents surface functional groups, and n + 1 is the number of metal layers ^[39][108]. MXene can be synthesized from layered ceramic precursors through a process known as etching ^[40][109]. MXene materials can be derived from a variety of precursor materials, and the resulting properties of the MXene depend on the choice of precursor and synthesis methods ^[41][110].

MXene materials have several unique properties which are promising for use in a variety of applications, including energy storage and conversion, electronic devices, and sensors ^[42][111]. There are key properties of MXene materials that are attractive for energy-related applications, including their high electrical conductivity, high surface area, and good thermal stability ^[43][112]. These properties make MXene materials ideal for use in energy storage devices such as supercapacitors, as well as energy conversion devices such as thermoelectric generators, solar cells, and batteries ^[15][44][15,113]. In addition to energy applications, MXene materials have also been shown to have potential for use in other areas, including water purification, electromagnetic interference (EMI) shielding, and as catalysts in chemical reactions ^[45][46][114,115]. Despite their promising potential, there are also several challenges associated with the use of MXene materials in practical applications ^[47][116]. For example, the synthesis of high-quality MXene materials is still a relatively new field, and further research is needed to fully understand their properties and optimize their performance ^[48][117]. Additionally, the relatively high cost of synthesis and processing of MXene materials remains a barrier to widespread commercialization ^[49][118]. Conclusively, MXene materials are a promising new class of materials with potential for use in a wide range of energy-related and other applications ^[50][119]. Further research and development in this field is likely to lead to new and innovative applications for MXene materials in the future ^[51][120].

MXenes are typically synthesized by selectively etching the A-layer from MAX phases, which are ternary carbides or nitrides with the general formula Mn + 1AXn, where M is a transition metal, A is an A-group element, X is carbon or nitrogen, and n = 1, 2, or 3 ^[52][121]. MXenes can have different shapes depending on the synthesis method used. MXenes are characteristically synthesized as thin nanosheets that are a few nanometers thick and several micrometers wide. These nanosheets have a large surface area and are often used in applications such as energy storage, catalysis, and biosensing ^[40][109]. On the other hand, MXenes can also be synthesized as hollow nanotubes with a diameter of a few nanometers to several micrometers. These nanotubes have unique electronic and optical properties and are being explored for applications in electronics, photonics, and biomedicine ^[53][122]. MXenes as microspheres with a diameter of a few micrometers to several millimeters are also synthesized. These microspheres have a large surface area and are being explored for applications in energy storage and catalysis ^[54][123]. MXenes can be formed as thin films by depositing MXene nanosheets on a substrate. These films have excellent electrical conductivity and are being explored for applications in electronics and energy storage ^[55][124]. Based on the structures, aerogels, foams, and sponges are grouped as 3D structured MXenes. These 3D structures have a high surface area and are being explored for applications in energy storage, catalysis, and water treatment ^[56][125].

The synthesis of MXenes typically involves these preparations of the precursor: exfoliation of the precursor and surface functionalization steps ^[55][124]. The precursor is usually a layered transition metal carbide or nitride compound, such as Ti₃C₂ or Ti₂C, that can be synthesized using various methods, including solid-state reactions and chemical vapor deposition ^[57][58][126,127]. The precursor material is exfoliated to obtain individual 2D layers by using a variety of methods including mechanical exfoliation, liquid exfoliation, and electrochemical exfoliation ^[59][128]. The surface of the MXene is functionalized with different functional groups, such as carboxyl or hydroxyl groups, to modify its properties and enhance its performance in various applications ^[60][129]. The synthesis of MXenes requires a combination of materials science and chemistry knowledge, and the conditions used can significantly impact the quality and properties of the final product. Hence, the careful control of the synthesis conditions and optimization of the process are critical for the successful synthesis of MXenes ^[49][61][62][118,130,131]. 

The initial progress on the synthesis of various MXenes is highly appreciable as there were a lot of complications involved in that, including the processing of precursors, reaction conditions, and post-processing. Ti₃AlCl₂ was derived from Ti₂Al Cl through a balling process for utilization as a precursor material in the exfoliation process with a hydrogen fluoride (HF) solution and successfully demonstrated the synthesis of two-dimensional Ti₃C₂ nanosheets, multilayer structures, and conical scrolls ^[63][133]. The low-cost synthesis process of Ti₃C₂T_z MXene from low-cost precursors such as recycled carbon recovered from waste tires, recycled aluminum scrap, and titanium oxide was demonstrated through the “Evaporated-Nitrogen” Minimally Intensive Layer Delamination (EN-MILD) synthesis approach, and the final MXenes showed better electronic conductivity ^[64][65][134,135]. Including titanium, various transition metal carbides have been attempted through the selective etching process along with different delamination methods, demonstrating successful synthesis of a two-dimensional structure with proper surface termination ^[66][136]. The chemical vapor deposition (CVD) process was also attempted for the development of transition metal carbides, and a successful demonstration for the synthesis of molybdenum carbide (α-Mo₂C) has also been reported ^[67][68][137,138]. Salt–acid etching followed by a sonication process were demonstrated for the successful synthesis of a vanadium nitride MXene ^[69][139]. A high-energy ultrasonic cell-crushing extraction method to successfully prepare Ti₃C₂T_x MXenes from Si-based MAX using a single low-concentration etchant was demonstrated with a fast processing time ^[70][140]. Various etching processes including HF etching, in situ HF-forming etching, electrochemical etching, alkali etching, and molten salt etching, along with delamination strategies with proper demonstration, were reported ^[71][141]. Scalable synthesis, a fundamental process involved in synthesis, is the role of various metal ions in the synthesis of MXene, and safety guidelines to reduce the risk during the synthesis process of MXene have also been reported ^[72][73][74][142,143,144]. With this knowledge, further processes are still ongoing to reduce the cost, time, and risk.

The basic building block of MXenes is a metal–carbon or metal–nitrogen layer, where the metal atoms are surrounded by carbide or nitride anions. The metal–carbon or metal–nitrogen layer is sandwiched between two graphene-like layers. The graphene-like layers are composed of hexagonal arrays of metal and anion atoms ^[75][76][77][145,146,147]. The metal atoms in the MXene structure have coordination numbers ranging from 4 to 6, and they bond covalently with the anion atoms ^[78][148]. The interlayer spacing between the metal–carbon or metal–nitrogen layers is very small, typically less than 0.5 nm ^[79][149]. This gives MXenes their unique combination of high mechanical strength and electrical conductivity ^[80][150]. The overall structure of MXenes is highly ordered, with well-defined layers that can be easily exfoliated to produce single- or few-layer materials ^[81][151]. This makes MXenes highly suitable for different applications including energy storage, catalysis, and electronics ^[82][152].

MXenes have several unique properties that make them highly attractive for use in a wide range of applications, including energy storage and conversion, electronic devices, and sensors ^[83][153]. MXenes have high electrical conductivity, making them ideal for use in applications such as energy storage devices and electronics ^[84][154]. Moreover, they have a high surface-area-to-volume ratio, which is important for applications such as energy storage devices, catalysts, and sensors ^[85][155]. MXenes have good thermal stability, which is important for applications such as energy storage and conversion devices where they are subjected to high temperatures ^[86][156]. The mechanical properties of the devices should be appropriate after the fabrication. MXenes have excellent mechanical properties, including high strength, flexibility, and durability, and that is why they are attractive for use in flexible electronics and wearable devices ^[87][157]. MXenes have good chemical stability, which is important for applications such as water purification and catalysts ^[88][158]. MXenes have been shown to have good biocompatibility, which is highly suitable for use in biomedical applications such as drug delivery and tissue engineering ^[89][159]. MXenes have been shown to have high hydrogen storage capacity for use in hydrogen fuel cells ^[90][160]. Conclusively, unlike other 2D materials such as graphene, MXenes are possible to synthesize using a relatively low-cost process ^[52][121]. The combination of these properties gives special attention to MXenes for use in a wide range of energy-related and other applications ^[91][161].

MXenes have several mechanical characteristics, which is highly suitable for energy harvesting applications ^[92][162]. MXenes have a high Young’s modulus, which indicates their ability to resist deformation under stress with high tensile strength, which resembles those properties resisting fracture under tensile condition ^[93][163]. Moreover, MXenes possesses high fracture toughness, which is important for resisting crack propagation ^[94][164]. Based on the tribological behavior, MXenes have a low coefficient of friction and good wear resistance, which contribute more in tribological applications ^[95][165]. Apart from this, some MXenes such as Ti₃C₂Tx can be fabricated into flexible films, which results in using the flexible films in flexible electronics and energy storage devices ^[96][166]. The mechanical properties of MXenes are to be fine-tuned by controlling their composition, synthesis method, and processing conditions ^[97][167].

The structure of MXenes typically consists of a transition metal layer sandwiched between two surface functional groups such as hydroxyl or fluorine ^[98][168]. These surface functional groups allow for easy exfoliation of the MXene layers and also provide opportunities for chemical modification ^[48][117]. The interlayer spacing in MXenes can be controlled by varying the size and nature of the surface functional groups ^[99][169].