2D Material‐Based Wearable Energy Harvesting Textiles: A Review

Wearable electronic textiles (e‐textiles) have emerged as a transformative technology revolutionizing healthcare monitoring and communication by seamlessly integrating with the human body. However, their practical application has been limited by the lack of compatible and sustainable power sources. Various energy sources, including solar, thermal, mechanical, and wind, have been explored for harvesting, leading to diverse energy harvesting technologies, such as photovoltaic, thermoelectric, piezoelectric, and triboelectric systems. Notably, 2D materials have gained significant attention as attractive candidates for energy harvesting and storage in e‐textiles due to their unique properties, such as high surface‐to‐volume ratio, mechanical strength, and electrical conductivity. Textile‐based energy harvesters employing 2D materials offer promising solutions for powering next‐generation smart and wearable devices integrated into clothing. This comprehensive review explores the utilization of 2D materials in textile‐based energy harvesters, covering their preparation, fabrication, and characterization strategies. Recent advancements are highlighted, focusing on the integration of 2D materials and their practical implementations, shedding light on the performance and effectiveness of 2D‐material‐based energy harvesters in e‐textiles, and highlighting their potential as a sustainable alternative to conventional power supplies in wearable technologies.


Introduction
[3][4][5] In this context, there is an increasing interest in capturing ambient energy and transforming it into usable electricity, which offers a viable path for powering interconnected wearable devices.8] The harnessing of these ambient energy resources has led to the emergence of various energy harvesting technologies, such as photovoltaic (PV), thermoelectric, piezoelectric, triboelectric, and more. [9,10][13][14][15] The invention of piezoelectric nanogenerator (PENG) and triboelectric nanogenerator (TENG) in 2006 and 2012, [16][17][18] respectively, marked the beginning of an era of advancement in mechanical energy harvesting.[27] 2D materials are those materials with an ultrathin crystalline structure, with thickness of approximately one or two atoms. [28,29][31][32][33][34] Researchers have been examining the future energy demand in an efficient and environmentally sustainable manner.There have been significant advancements in 2D materials, such as graphene, molybdenum oxide, molybdenum disulfide (MoS 2 ), tungsten disulfide (WS 2 ), hexagonal boron nitride (h-BN), and so on. [35,36]These 2D materials typically exhibit defect-free crystal structures, resulting in shorter distance for electrons transportation, leading to high mobility of charge carriers and electrical conductivity. [37,38]These characteristics make them an appealing candidates for the development of a wide is advantageous for applications such as photocatalysis, [39] PVs, [40] and supercapacitors, [41] where high surface area is critical for optimal device performance.Due to these innovative and unforeseen characteristics, 2D materials are being extensively researched for the conversion and storage of energy with high performance and promise. [42]here has been a significant focus on the development of new energy harvesting technologies employing 2D materials.These include the harvesting of ambient mechanical energy using PENG [43,44] and TENG, [45] as well as the harvesting of ambient heat using pyroelectric nanogenerator (PyNG) [46] and the thermoelectric nanogenerator (ThNG), [47] apart from that efficient utilization of enzymes, for example, harnessing the human body sweat employing catalytic nanogenerator, [48] by converting chemical energy to electrical energy, and the harvesting of the photons energy from sunlight utilizing the PV cells. [49]Figure 1 illustrates the broad concept of future textile garments, encompassing various types of energy harvesters and other wearables.
This comprehensive review article delves into a diverse array of techniques employed to harvest and utilize energy from ambient sources, such as solar radiation, waste heat from human body and surrounding, and mechanical forces.The focus is particularly placed on leveraging the unique properties of 2D materials in wearable textile-based energy harvesting applications.The review explores the practical applications of wearable energy harvesters utilizing these materials, shedding light on their performance and effectiveness as a viable alternative to conventional power supplies in the realm of e-textiles.Moreover, the review underscores the immense potential of these advancements in overcoming energy-related challenges encountered in the field of e-textiles.

Ambient Energy Harvesting Technologies
Ambient energy harvesters offer a promising solution to address the growing demand for environmentally friendly energy generation. [50]These devices harness natural resources such as sunlight, wind, ambient mechanical energy (e.g., human motion), and water to generate clean, renewable electrical energy without causing harm to the environment or contributing to climate change. [51]anotechnologies have revolutionized the development of various energy harvesting methods that are highly effective in tackling current and future energy crises.These methods include PVs, [52] piezoelectricity, [53] triboelectricity, [54] thermo/pyroelectricity, [55,56] electromagnetic, [57] and catalytic techniques. [58]

Solar Energy Harvesting
The power of the sun has been harnessed by human civilization since ancient times.However, it was not until the invention of PV SCs that the true potential of solar energy as a sustainable power source was unveiled. [59,60]Its abundance and cleanliness make it an ideal source of renewable energy.SCs are solar energy harvesting devices, having the capability to convert energy from sunlight directly to electrical energy. [61]They consist of two types of semiconductors, n-type and p-type, as shown in Figure 2a.In n-type materials, the majority of the carriers are electrons, while in p-type materials, the majority of carriers are holes.When these two types of materials are combined, electrons from the n-type layer migrate to fill the holes in the p-type layer, creating a depletion zone around the junction.Once all the holes in the depletion zone are filled, the p-type side of the junction becomes negatively charged, while the n-type side becomes positively charged.This separation of charge creates an internal electric field that prevents further electron-hole recombination. [62]When sunlight, in the form of photons, reaches the SC, electrons in the depletion zone are excited and leave their positions, creating holes in their wake.By connecting a metallic wire to both the n-and p-type materials, the electrons can flow through the depletion region, generating a current.This current can then be harnessed for various applications. [63]ith advancements in fabrication technologies and the discovery of new materials, SCs are typically categorized into three generations.First-generation SCs, such as crystalline silicon-and gallium arsenide (GaAs)-based SCs, represent mature PV technologies.These SCs are highly efficient and durable, capturing over 90% of the commercialized market.However, they are bulky in structure and require expensive and complex fabrication processes. [64]Second-generation SCs utilize thin film technology, including materials like cadmium telluride, amorphous silicon, copper indium gallium selenide, and cadmium sulfide.These SCs are lighter in weight and less expensive compared to first-generation SCs.However, they face challenges due to the toxic nature of their materials and their limited abundance in nature. [65]Third-generation SCs represent the latest PV technologies, offering the potential for integration into wearable devices.They can be further categorized based on material types, such as dye-sensitized SCs (DSSCs), perovskite SCs (PSCs), organic SCs (OSCs), and quantum dot SCs. [66,67]Although the durability of these SCs is still an area of improvement, they have gained significant interest in the last decade due to their ease of fabrication and compatibility with flexible and wearable substrates. [68]he use of 2D materials has received notable attention in improving the stability and performance of SCs, resulting in significant improvements.Graphene-based 2D materials have been widely used in SCs to perform a variety of functionalities.[71][72][73] Aside from graphene, 2D materials such as transition metal dichalcogenides have been extensively studied for possible use in SCs.[76][77][78] Furthermore, MXene, another class of 2D materials, has also been extensively studied for its potential applications in SCs.MXene exhibits a versatile nature, characterized by its modifiable bandgap and electrical properties, which make it suitable for various functions in SCs. [79][82][83] Given the significant advancements and rising interest in 2D materials over the last decade, they have become a very appealing study field for next-generation wearable PV applications.This is mostly owing to its enticing electrical, optical, and mechanical properties. [84]87][88][89]

Mechanical Energy Harvesting
Mechanical energy harvesting has emerged as a significant approach in the realm of renewable energy, offering the ability to convert ambient mechanical energy into usable electrical energy. [90][93] The growing importance of mechanical energy harvesting is driven by the need to power sensors and low-power electronic devices in remote and challenging environments, fueled by the rapid development of the IoT. [94]The ability of mechanical energy harvesting to extract energy from ordinary sources presents unique potential for providing sustainable and reliable power across a wide range of applications, positioning it as a vital technology in our pursuit of a more sustainable future. [95]2.1.Piezoelectric Nanogenerator Piezoelectric nanogenerators (PENGs) are mechanical energy harvesting devices introduced in 2006 that utilize piezoelectric materials to convert mechanical energy into electrical energy.[96] In piezoelectric materials, the application of mechanical deformation leads to a polarization of the atomic lattice structure, resulting in the separation of positive and negative charge carriers and the generation of an electrical potential across the material.Conversely, the application of an electric field causes a change in the polarization of the atomic lattice structure, leading to mechanical deformation.[97] Figure 2b illustrates the typical structure of a PENG device, comprising a layer of piezoelectric material sandwiched between two electrodes.Through the piezoelectric effect, when the device experiences mechanical stress or strain, such as vibrations, pressure, or human motion, a voltage is generated across the electrodes, thereby producing electrical energy.[98] The output voltage and power of a PENG are influenced by various factors, including the properties of the piezoelectric material, the geometry and structure of the device, and the magnitude of the external forces.[99] Significant advancements in nanotechnology have facilitated the development of highly efficient PENG devices with improved performance and stability. Consuently, PENGs have garnered considerable interest for diverse applications, such as self-powered sensors, wearable electronics, and biomedical devices.[100] 2D materials have shown promise for harvesting mechanical energy via PENG.Such materials are considered to be suitable for wearable e-textile applications due to their flexibility and strong mechanical characteristics.[101][102][103] While many 2D materials, such as graphene, do not possess inherent piezoelectric characteristics, researchers have investigated and engineered them to exhibit a piezoelectric effect.[104] Graphene-based 2D materials have been used as electrodes or in conjunction with piezoelectric materials to enhance the overall piezoelectric performance of wearable and textile-based PENGs.[105][106][107][108][109] Other 2D materials such as molybdenum diselenide (MoSe 2 ), WS 2 , MoS 2 , and MXene have been studied for their piezoelectric effects in wearable applications.[42,102,110] These investigations have emphasized the promise of 2D materials in wearable energy harvesting applications, notably directing toward the development of textile-based mechanical energy harvesting garments.

Triboelectric Nanogenerator
Triboelectric nanogenerators (TENGs) are energy harvesting devices that convert mechanical energy into electrical energy via the triboelectric effect and were first introduced in early 2012. [111]The triboelectric phenomenon happens when two dissimilar materials come into contact and are then separated, resulting in a charge transfer.This charge transfer produces a potential difference between the two materials, which might be utilized to produce an electrical current. [112,113]TENGs can operate in various modes, including vertical contact-separation mode (Figure 2c), lateral sliding mode, single-electrode mode, and freestanding mode, depending on the type of movement and materials involved. [114]TENGs are suitable for a variety of applications, including self-powered sensors, power source in portable electronics, and biomedical devices, due to their versatility, low cost, high-power density, and simplicity of fabrication. [115]TENGs are gaining traction as an intriguing technology for harnessing energy from ambient sources, such as human motion, wind, and ocean waves, paving the way for self-powered and sustainable systems. [3]he use of 2D materials in TENGs is well acknowledged for its capacity to considerably enhance conversion efficiency while also improving flexibility and durability.118] Numerous studies have indicated significant improvements in TENG performance with the use of various 2D materials. [33,119,120]123][124][125][126] These findings demonstrate that 2D materials are absolutely fascinating candidates for mechanical energy harvesting applications in wearable textiles.

Electromagnetic Generator
Electromagnetic induction has long been recognized as a significant and efficient method for converting mechanical energy into electrical energy, based on Faraday's law of electromagnetic induction.According to this law, [127] a changing magnetic field induces an electric current in a conductor.This principle forms the basis of electromagnetic generators.In a basic structure, an electromagnetic generator comprises a coil of wire and a magnet.Mechanical oscillations or motions generate a fluctuating magnetic field, thereby producing an electrical current in the coil (Figure 2d). [57,128]Recent advances in materials science and wearable technology have led to the development of innovative wearable energy harvesting devices that utilize electromagnetic induction, including flexible coils and magnetic materials. [129,130]hese devices hold great promise as sustainable and autonomous power sources for portable and wearable electronics, offering an alternative to conventional batteries and other energy storage systems. [131]With further advancements in design and technology, electromagnetic energy harvesting has the potential to revolutionize the field of ubiquitous electronics and beyond.It enables more efficient and environmentally friendly power generation solutions.By harnessing ambient mechanical energy through electromagnetic induction, these devices offer a pathway toward a greener and more sustainable future for the electronics industry. [132][135] However, there is still ample room for researchers to explore novel materials possessing both electromagnetic characteristics and flexibility, which are essential for wearable and textile-based energy harvesting applications.In addition, while significant advancements have been made in the field of 2D materials, the absence of a family of 2D magnets remains a notable gap, constraining the progress in developing 2D material-based wearable electromagnetic generators. [136]

Thermal Energy Harvesting
Thermal energy harvesting involves the conversion of ambient heat from various sources, such as human body temperature and industrial boilers, and so on, into electrical energy. [137]his approach relies on two primary mechanisms: thermoelectricity and pyroelectricity.Thermoelectricity is based on the Seebeck effect, which converts temperature differences into electricity.The Seebeck effect occurs when a voltage is generated across materials that have a temperature gradient. [138]This phenomenon enables the direct conversion of heat energy into electrical energy.Pyroelectricity, on the other hand, utilizes polarization changes in materials induced by temperature fluctuations to generate electrical energy.When the temperature of a pyroelectric material changes, its internal polarization is altered, leading to the generation of an electric charge. [139]igure 2e,f illustrates the general schematics of thermoelectric generator and pyroelectric generator, respectively.
Thermal energy harvesting has gained significant attention in recent years due to its potential applications in wearable and portable devices, wireless sensor networks, and various other powerharvesting technologies. [140]The ability to convert waste heat or ambient heat into usable electrical energy offers great promise for energy-efficient and self-powered systems.In wearable technologies, thermal energy harvesting holds particular significance as it can tap into the body heat of individuals, enabling selfpowered health monitoring devices and wearable electronics.Additionally, in industrial settings, the recovery of waste heat from boilers and other sources can provide an efficient and sustainable means of generating electricity.Continued advancements in materials such as 2D materials having high Seebeck coefficient, i.e., monolayer MoS 2 , device design, and system integration, are crucial for enhancing the efficiency and practicality of thermal energy harvesting. [47,141]Further research in this field will contribute to the development of energy-efficient and environmentally friendly solutions for a wide range of applications.

Catalytic Energy Harvesting
Catalytic energy harvesters, also referred to as biofuel cells, transform chemical energy into electrical energy by using enzymes as a catalyst. [142]This form of energy harvester generates electrical energy by electrochemically oxidizing bioenzymes such as glucose and lactate at the anode of a biofuel cell and reducing oxygen at the cathode (Figure 2g). [143]One of the most significant benefits of biofuel cells is their compatibility with living environment and their capacity to generate energy from biological components.However, owing to the device's tiny size, the outputs are often recorded at the microscale. [144]Biofuel cell research has received a lot of interest as a possible source for self-powered wearable devices that can collect energy from the high concentration of lactate dehydrogenase in human sweat, making them a viable technology for the future. [23,145][151] These application highlights the considerable potential of 2D materials as viable candidates for constructing wearable biosensors with self-powering capabilities.

Osmotic Energy Harvesting
Osmotic energy harvesting (OEH) is the method of transforming osmotic pressure differences into useful energy.Osmotic pressure occurs when two solutions of differing concentrations are separated by a semipermeable membrane (Figure 2h).The solvent molecules favor to flow from the area of lower solute concentration to the area of greater solute concentration, resulting in a pressure differential across the membrane.This pressure difference is harnessed to drive and, in turn, produce electricity. [152][155] Recently, there have been arisen interest in building wearable osmotic energy harvesters and have determined potential for sensing applications such as muscle activity and so on. [154,156]2D materials have been utilized for building OEHs; [157][158][159] however, the output of these energy harvesters is quite small but it opens the door to an alternative approach for self-powered wearable biomedical sensing applications.

2D Materials and Characteristics for Energy Harvesting
2D materials are a family of materials with a few atoms or a single-layer thickness that display unique and often configurable physical, electrical, and optical characteristics. [160]Graphene the most well-known 2D material, has sparked tremendous interest in energy harvesting devices because of its superior electrical conductivity, large surface area, and mechanical flexibility. [161]urthermore, owing to their inherent piezoelectric and thermoelectric characteristics, 2D materials such as transition metal dichalcogenides (TMDCs) and black phosphorus (BP) have showed remarkable promise for energy harvesting applications. [33]Layer-by-layer stacking of 2D materials may result in the production of heterostructures with customized bandgaps, bringing up new possibilities for the design of high-performance energy harvesting devices. [162]Furthermore, because of the unique features and flexibility of 2D materials, they have been integrated into flexible and wearable energy harvesting devices, opening the way for the development of self-powered and portable electronics. [85]Table 1 summarizes the potential properties of these types of 2D materials for energy harvesting applications, highlighting their unique characteristics and potential suitability for various energy conversion processes.

Graphene
Graphene is a unique material composed of a single layer of carbon atoms arranged in a 2D hexagonal lattice and was discovered in 2004. [163]These carbon atoms are bonded together through sp 2 hybridized π bonds, resulting in a strong and stable structure. [25]he average bond length between these carbon atoms is approximately 1.24 Å, making the bonds very strong and difficult to break (Figure 3a). [164,165][168][169] This is due to its high charge carrier mobility (>104 cm 2 V À1 s) and lower production costs. [170,171]Furthermore, graphene's robust configuration confers exceptional transparency, placing it as an effective rival to replace transparent conductive oxides (TCO) in third-generation SCs. [172,173]Graphene also has the potential to improve charge carrier transfer and exciton dissociation in SCs. [174,175]Its smooth and uniform surface results in minimal contact resistance, lowering potential drop and avoiding leakage currents at p-and n-type material interfaces in SCs. [176,177][180] These characteristics make them a good candidate for mechanical energy harvesting via the PENG and TENG method. [181,182][185] This property is critical for effective thermal energy harvesting, making graphene-based materials ideal for such applications. [186,187]able 1 summarizes some of the basic properties of graphenebased materials that make them appropriate for energy harvesting applications.

Transition Metal Dichalcogenide
Transition metal dichalcogenides (TMDCs) are a class of 2D materials that consists of a transition metal atom bonded to a chalcogen atom (such as sulfur, selenium, or tellurium).Each layer of TMDCs is made up of a hexagonal lattice of transition metal atoms sandwiched between two layers of chalcogen atoms (Figure 3b). [188,189]These nanomaterials, denoted as MXs in general (M represent transitional metals and X for S, Se, or Te), can exist in both monolayer and multilayer configurations. [190]he interaction of s-P orbitals during the exfoliation of multilayers causes an increase in the bandgap in single-layer TMDCs. [189,191]97] Furthermore, some TMDCs, such as ultrathin 2D nanomaterials made of MoS 2 , have highly appealing properties, such as an outstanding density of electron and hole states and a notably high Seebeck coefficient. [198,199]Therefore, these materials have received a lot of attention for their potential in thermoelectricity output. [200,201]Table 1 summarizes the key properties of 2D TMDCs applicable to various energy harvesting applications.[204][205] As a result, they appear as extremely interesting candidates for use in flexible and wearable electronics, where they provide improved mechanical efficiency.

MXene
MXene belongs to a recently discovered class of 2D materials that encompasses various transition metal carbides, nitrides, and carbonitrides, (Figure 3c).Its standard chemical formula is M nþ1 X n , where M represents transition metals such as Ti, Nb, V, Cr, and Ta, and X denotes carbon or nitrogen.[208][209] MXene can be synthesized and studied using various methods.Pristine MXene, for instance, displays metallic characteristics and can serve as an electrode in a range of devices. [79,210]Moreover, MXene can be functionalized to achieve semiconductor properties with tuneable bandgaps ranging from 0.25 to 2.0 eV, [211][212][213][214] which makes MXene highly versatile for energy storage and harvesting applications.][217][218][219] The diverse utility and notable features of MXene in energy harvesting applications are summarized in Table 1.Its unique composition, tenable properties, and compatibility with various device architectures make MXene an attractive option for advancing energy harvesting technologies.

Black Phosphorus
Black phosphorus (BP), commonly known as phosphorene, was introduced in 2014 for the first time. [220]BP is a single-layer 2D materials with covalent bonds between each phosphorus atom and its three neighboring atoms.The bond length between these atoms is %2.18Å, while the distance between phosphorus atoms in neighboring layers is a %5 Å. [221,222] The structure of BP can be visualized as a puckered hexagonal lattice (Figure 3d).[228] BP has been used in SCs, demonstrating its usefulness as absorption additives, electron-transport layers (ETLs), and hole-transport layers, and has shown potential for improving SC efficiency. [225,229,230]In addition, due of its noncentrosymmetric Table 1.Characteristics, functionalization, and stability of some common 2D materials in energy harvesting perspectives.

Electrical conductivity
233] Aside from its piezoelectric capabilities, BP has been shown to have considerable thermal anisotropy, making it a suitable material for thermoelectric applications. [234,235]Table 1 summarizes BP's diverse properties and capabilities for numerous energy harvesting applications.

Other 2D Materials
[247][248] The triangular and hexagonal crystal structures of 2D perovskites offer additional advantages, enabling tuneable photoluminescent properties and broadening their optoelectronic characteristics. [239,249]BN is a 2D material that shares a similar structural basis with graphene (Figure 3d).It consists of interconnected honeycomb lattices formed by boron and nitrogen atoms, held together by sp 2 bonds. [250,251]With its wide bandgap, excellent thermal conductivity, and high insulating properties, h-BN exhibits potential for various energy conversion mechanisms, including mechanical energy harvesting through piezoelectric and triboelectric effects. [252,253]MOFs are porous crystalline 2D materials in which metal ions or clusters are linked together by organic ligands that act as multidirectional connectors within the network architecture (Figure 3d).[256] MOFs have been investigated in numerous energy harvesting applications owing to mentioned characteristics including TENGs, [257] PENGs, [258] and PVs. [259]COFs are organic pores crystalline materials with building blocks featuring light elements such as C, H, O, N, and so on and linked by a covalent bond in two or three dimensions (Figure 3d). [260]COFs are gaining recognition as a viable material for energy harvesting due to their large surface area, tuneable pore size, and great chemical stability. [261,262][269] LDHs are crystalline inorganic solids with a lamellar structure similar to hydrotalcite (Figure 3d).The replacement of trivalent cations for divalent cations results in the creation of a positive charge sheet inside this structure, which is balanced by anions situated in the interlayer regions. [270]LDHs differentiate themselves from other 2D materials due to intrinsic properties such as high surface positive charge densities, inherent hydrophilicity, and the existence of interior nanochannels or nanopores inside LDH flakes. [271]These characteristics, together with the material's great sensitivity to adsorbed substances because of its considerable charge densities and the anion concentrations in its interlamination regions, make LDH extremely adaptable in applications linked to hydrovoltaic energy harvesting. [272,273][276] Because of its unique features, LDH is a major material in the field of energy harvesting, pushing developments in sustainable energy technology.Its unique properties position LDH as a leading material in the realm of energy harvesting, driving advancements in sustainable energy technologies.

Preparation of 2D Materials
For 2D material preparation, there are two major approaches known as top-down and bottom-up approaches. [277]In the top-down approach, larger bulk materials are mechanically or chemically processed to obtain thin layers with a thickness of a few atomic layers.One of the widely used techniques in topdown approach is mechanical exfoliation, where layers are peeled off using adhesive tape.By using mechanical exfoliation method, monolayer graphene was first successfully isolated in 2004, [278] and has been used to isolate other 2D materials including MXene, [279] hBN, [250] and TMDCs. [280,281]Mechanical exfoliation can achieve high-quality of 2D materials; however, it has limited control over the thickness and size of exfoliated 2D flakes.Additionally, it is a time-consuming and labor-intensive process, which hinders its capability for mass production. [282]mong various top-down approach to exfoliate 2D materials, the liquid phase exfoliation (LPE) is very popular as it is a simple, scalable, and cost-effective method.Most importantly, this versatile method is ideal for graphene exfoliation to formulate conductive inks, pastes, thin films, composites, and coatings.Typically, in this method, the application of ultrasonic or shear energy breaks the intersheet forces in the presence of a stabilizing liquid, either in a nonaqueous solution or an aqueous solution with surfactant.][285] Researchers have demonstrated [286] industrially viable water-based exfoliation method that can produce high-quality graphene with 5 wt% concentration with a production rate of 82%170 g h À1 .The quality of exfoliated graphene flakes is generally characterized by their Raman shift, binding energy, flake thickness, and flake size (Figure 4b-g).The typical honeycomb lattice with long-range periodicity is visible in the atomic-resolution scanning transmission electron microscopy (STEM) image, demonstrating that graphene's crystal structure has been successfully preserved despite partial oxidation and shear exfoliation.As mentioned in another study, [287] high shear mixing of graphite in suitable stabilizing liquids results in largescale exfoliation to produce dispersions of graphene nanosheets (Figure 4, Inset of c, d, and f, respectively).Industrially scalable methods to produce large quantities of defect-free graphene are required to advance the technology from laboratory to  [286] Copyright 2018, Springer Nature.The presence of monolayers of graphene was confirmed by shear mixing (inset of (c), (d), and (f ), respectively).Adapted with permission. [287]Copyright 2014, Springer Nature.Spectra, XPS (NMP-exfoliated samples) measured on thin films, AFM on a surfactant-exfoliated sample.h) AFM images of MoS2 nanosheets; i) layer number distribution of MoS 2 nanosheets; j) thickness plot of selected MoS 2 nanosheets in images (h); [294] k) SEM image of MX-H delaminated layered surface morphology.Reproduced with permission. [295]Copyright 2021, Elsevier.l) SEM images of exfoliated BN.Reproduced with permission. [295]Copyright 2018, IOP Publishing Ltd.commercial applications.[293] It has been found in a study that MoS 2 nanosheets with a few layers are clearly present in both the layer number distribution graph (Figure 4i) and the atomic force microscopy (AFM) image in Figure 4h.The majority of these nanosheets exhibit a similar apparent AFM height of 2-4 nm (Figure 4j), with a step height of 0.8-1 nm, which is consistent with the monolayer height of chemically exfoliated MoS 2 . [294]The high shear method created significantly expanded/delaminated MXene (Ti 3 C 2 Tx) nanosheets, according to scanning electron microscopy (SEM) images from a study.In Figure 4k, layered morphology illustrates the expanded/delaminated MXene's existence. [295]In addition to graphene and MoS 2 , h-BN was also exfoliated (as shown the SEM image in Figure 4l) using simply the high-shear mixing of 2D stacked materials and pulping waste liquid.This approach was not only straightforward and effective, but also resource-and environmentally friendly.In that investigation, the exfoliated BN showed a substantially lower size (0.5-7 μm). [296]LPEs are commonly processed as solvent-assisted, surfactant-assisted, and electrochemical exfoliation methods.
[299] Ultrasonication, whether with a sonication tip or in a bath, is an effective LPE approach. [300]avitation bubbles induced by high-frequency ultrasonic waves agitate the liquid, and the accumulative effect of the millions of imploding bubbles creates ultrasonic wave compression and rarefaction cycles, which leads to shock waves and turbulence to exfoliate the bulk material into flakes.This technique generates enough shear force, compressive stress, and tensile stress to overcome van der Waals bonding between layers and enable exfoliation of 2D materials into the solvent. [301,302][305] Despite the costefficiency and simplicity of solvent-assisted LPE, [306,307] achieving precise control over sonication and shear forces in solvents has presented significant challenges.As a result, the yield of precise thickness of 2D materials has remained a challenge, [308] and the process itself is time-consuming, further limiting the versatility and scalability of this method for 2D material preparation in bulk production.
[310] This LPE approach includes inserting ions into the interlayer voids of bulk materials, such as cations (e.g., Na þ , Li þ , and K þ ), anions (e.g., SO 4 2À ), or radicals (e.g., OH and O radicals). [311]Consequently, sonication or thermal shock is used to stimulate exfoliation of the stacked layers.These surfactants lower interfacial tension and prevent flakes from reaggregating, producing stable dispersion with control over the thickness of the resultant flakes.314][315][316][317][318] Electrochemical exfoliation is a controlled and fast synthesis approach where an electric field or current is applied to the bulk material submerged in a suitable electrolyte. [319,320]This electric field causes ion intercalation in the material, resulting in exfoliation.The resultant nanosheets are collected and disseminated in a liquid media.[323] It has been used to successfully synthesize a wide range of 2D materials, including graphene, TMDCs, MXene, BP, and others. [321,322,324,325]he bottom-up approach involves the controlled growth or synthesis of 2D materials from atomic or molecular precursors.Chemical vapor deposition (CVD) is the commonly employed technique in bottom-up approach. [326,327]In CVD, vapor-phase precursors are introduced into a reaction chamber, where they undergo chemical reactions and deposit as thin layers on a substrate.This method allows for the large-scale synthesis of 2D materials with precise control over their thickness and quality. [281,328]Other bottom-up techniques include atomic layer deposition (ALD), molecular beam epitaxy, and solution-based methods such as hydrothermal or solvothermal synthesis. [329,330]oth top-down and bottom-up approaches offer unique advantages and limitations in the preparation of 2D materials.The choice of method depends on factors such as desired material properties, scalability, cost, control over layer thickness, and quality.Continued research and development in both approaches are essential for advancing the synthesis and application of 2D materials in various fields.

Fabrication Techniques for Energy Harvesting Textiles
Following selection of materials and device design, the fabrication processes for energy harvesters play an essential role in device development.An appropriate fabrication processes for developing a wearable energy harvesting textile may vary depending on the specific application and performance required.However there are some common fabrication processes employed for energy harvesting textiles.This section provides a through discussion on commonly used fabrication process to produce for various wearable energy harvesters.

Coatings
Coating techniques offer a simple and versatile approach for depositing functional materials onto substrates offering flexibility in terms of substrate compatibility. [289]Some of the common coating techniques employed are spray coating, dip coating, and spin coating.

Spray Coating
Spray coating involves small droplets of dispersion applied onto the substrate with a spray nozzle (Figure 5a). [331]Benefits of spray coating are fast throughput, scalability, and homogeneity of coating.It is frequently employed in fabrication of energy harvesters for large-area deposition of active materials. [332,333]A spray coating approach has recently been used for developing OSCs on a textile substrate. [334]An interface material was applied to the textile substrate to reduce surface roughness, following application of functional layers of the OSCs using a spray coating technique (Figure 5b). Figure 5c compares the electrical output of developed textile-based OSCs to that of other substrates.This comparison emphasizes spray coating's adaptability for energy harvesters on flexible and stiff substrates, demonstrating its versatility for many applications.A fabric-based PENG with high output performance [335] was developed using spray-coated lead-free piezoceramic layer onto glass fabric, providing an all-inorganic-state solution.In corresponding developed fabric PENG, the spray coating of 0.5(Ba 0.7 Ca 0.3 )TiO 3 -0.5Ba(Zr0.2 Ti 0.8 )O 3 (BCTZ) sol-gel solution proved to be a simple and cost-effective approach, particularly ideal for the textile substrates. [335]Additionally, silver nanowire (AgNW) solution was also spray-coated on both sides of the fabric for long-lasting  and c) the corresponding electrical performance in comparison with other substrate such as FTO-coated glass and bare glass, which determined the versatility and compatibility of spray coating techniques for various substrate.Adapted with permission. [334]opyright 2018, IEEE.d) Dip coating setup.e) Image of fiber-based flexible DSSC, where dip coating is utilized for deposition of functional layer TiO 2 , also includes the SEM image of two and six dip-coated layers of TiO 2 , and f ) the corresponding I-V curves in comparison with the dip-coated layers; the increasing number of dip coating cycles enhanced the performance but after fourth cycle, further coating leads to poor response.Adapted with permission. [354]Copyright 2021, Elsevier B.V. g) Spin coating system.h) Schematic and image of 2D MXene-based wearable spin-coated TENG, attached to a human hand for harnessing human hand motion.i) The open-circuit voltage generated curve of the corresponding TENG.Reproduced with permission. [364]Copyright 2022, Wiley-VCH GmbH.
flexible electrodes and the resultant all-inorganic-state fabric PENG has generated a voltage of around 3 V and a current of around 110 nA. [335]The compatibility and flexibility of 2D materials make them well suited for various solution processing fabrication techniques. [336,337]Therefore, these materials have been widely employed in energy harvesting applications utilizing spray coating techniques fully or partially.[340] As evidenced by current literature and research progress, spray coating, either entirely or partially, has developed as a key fabrication process used in energy harvesting devices [332,341,342] such as TENG, [332,341,343,344] ThNG, [345,346] and so on, which determined spray coating as a key technique in the fabrication of 2D material-based wearable energy harvesting textiles.

Dip Coating
Dip coating is a flexible approach in material fabrication that involves dipping a substrate in a solution or suspension to deposit thin layer of materials onto substrate (Figure 5d).It offers homogeneous coating on complex-structured substrates, making it appropriate for wearable energy harvesters.[349][350][351][352][353] Previously researchers have demonstrated the use of dip coating method to deposit titanium dioxide (TiO 2 ) layer for DSSC application (Figure 5e). [354]In their research, the investigators fabricated DSSCs using one to six dip-coated layers of TiO 2 .Their observations revealed that the device's performance improved linearly up to four layers, after which a plateau was reached.This phenomenon can be attributed to the optimal condition achieved by the four layers, providing the maximum zone for dye molecule diffusion within the porous semiconductor.Consequently, the inner zone of the coated layer decreased, leading to the observed saturation effect (Figure 5f ).Another research work on textilebased TENG shown dip coating method to deposit corresponding tribo layer which has demonstrated a power density of 0.30 W m À2 under a load of 100 MΩ while converting mechanical energy from a tap. [347]The recent survey revealed that dip coating is the preferred choice for textile substrates over spray coating, primarily due to its self-soaking nature. [38]Dip coating has been employed to deposit 2D materials for wearable textile-based energy storage and harvesting application. [355]For instance, dip coating was employed for graphene-based electrode in a DSSC; [356] MoS 2 is dip-coated while building a TENG device. [357]hile dip coating is a basic fabrication technique, it comes with certain disadvantages, such as material waste, challenges in attaining perfect patterning, and limited control over layer thickness.These limitations limit its use in sensitive device fabrication.

Spin Coating
The spin coating process is a low-cost and simple method for depositing thin, homogeneous film layers on flat surfaces.In spin coating, a hand syringe or a dispensing unit is used to inject required amount of solution onto the spinner.The solution is distributed onto the substrate by centrifugal force created by rapid spinning (up to 10 000 revolutions min À1 ).This fast rotation allows the fluid to distribute uniformly across the substrate (Figure 5g). [358,359][362] For instance, a research team recently fabricated a 9 cm Â 9 cm array of OSCs on a glass substrate employing spin coating only, which demonstrated a high efficiency of %14%. [363]Additionally, this method has been proved to be a suitable fabrication method for 2D materials on both stiff and flexible substrates.Researchers have demonstrated use of spin coating to deposit a composite of carbon nanofibers (CNFs) and 2D MXene, to produce flexible TENG to harness energy from human movement.Figure 5h-i shows the schematic and original image of developed flexible TENG, respectively.The output voltage of the device was observed by generating 55 V for continued stress and release at a frequency of 3 Hz (Figure 5j). [364]Here, CNFs/MXene composite showed enhanced piezoresistive response in strain sensing, determined the usefulness of 2D materials for self-powered applications.Although spin coating has gained significant attention for producing smooth deposit layers of controllable thickness, [365] there are several limitations to consider for textile-based energy harvester application, such as inappropriate for large-scale manufacturing, [366] material waste, [367] and challenge, to create smooth layers on rough substrates like textiles. [368]However, researchers have used an interface layer to smooth the surface of the textile, [334] but this disrupts the elasticity of the textile.

Printing
Printing, in its fundamental concept, involves transferring a pattern, such as an image, text, or any other design, to concern substrate such as a book page.In the field of nanotechnology, printing is utilized to deposit layers of functional materials in predecided pattern onto substrates.The ability to print these functional elements onto textiles offers advantages in terms of flexibility, conformability, and scalability, opening up new possibilities for the integration of energy harvesting capabilities into everyday fabrics. [29,32,288,369,370]Two commonly used printing methods in the manufacturing of solution-processed energy harvesters are screen printing and inkjet printing.In the context of 2D material-based energy harvesting textiles, printing methods play a crucial role in the fabrication and integration of these materials into functional textile structures.These techniques offer advantages in terms of scalability, precision, and versatility for creating energy harvesting functionalities on textiles.

Screen Printing
Screen printing is a dynamic and popular method for fabricating devices, especially when depositing functional materials on surfaces.In screen printing, the substrate is covered with a stencil or mesh screen with open regions that match the intended design.The required material is then distributed onto the screen as a paste form.The design is transferred to the substrate placed below the screen by forcing the paste with a squeegee through the open mesh apertures (Figure 6a). [371][374] Screen printing has benefits including scalability, affordability, and compatibility with a variety of substrates and materials. [375]Screen printing has been used to  and c) the corresponding I-V curve for two different textile DSSCs, bare textile without any surface treatment, and polyamide printed textile.Adapted with permission. [376]Copyright 2019, IEEE.As shown, the surface smoothening enhanced the electrical performance in comparison with bare rough textile.d) Image of a screen-printed flexible ThNG, and e) the maximum power and open-circuit voltage response to temperature gradient.Adapted with permission. [377]Copyright 2020, Elsevier B.V. f ) Schematic of inkjet printing system.g) An image of inkjet-printed transparent conductive polymer (TCP) layer of PSC, [391] and h) the corresponding I-V curve of the PSC along with inset SEM image of the TCP layer.Adapted with permission. [391]Copyright 2020, Wiley-VCH.i) Photograph of a 5 X 5 TENG array printed using inkjet printer and utilized for tactile sensing.Figure also includes the proposed pattern and developed device in a human hand.j) The corresponding output voltage of the inkjet-printed TENGs with respect to different value of forces.Reproduced with permission. [392]Copyright 2020, Elsevier Ltd.
deposit the silver bottom electrode and the TiO 2 layer as the ETL in the development of textile DSSCs (Figure 6b). [376]The current density curves of DSSCs directly developed on bare textile and textile with interface polyamide layer are shown in Figure 6c to demonstrate the effect of surface smoothness in improving performance of screen-printed textile SCs.A ThNG has been developed on a flexible substrate for low-temperature heat harvesting applications utilizing screen printing technique (Figure 6d).The ThNG was developed by screen printing Bi-Sb-Te (p-type) and Bi-Se-Te (n-type) film and sintering at 345 °C under a pressure of 25 MPa.At room temperature, these films demonstrated thermoelectric power factors of 14.3 and 8.4 μW (cm K) À2 (Figure 6e). [377][380][381][382] In previous work, [288,[383][384][385] screen printing was employed for fabricating graphene-based sensors such as for electrocardiogram (ECG), activity monitoring, and as textile-based supercapacitors.These results highlight the potential of screen printing to be extended for various 2D material-based energy harvesting applications.

Inkjet Printing
Inkjet printing is a precise, direct printing technology used in device development to deposit functional materials onto substrates.It ejects 3-10 pL droplets of ink through a piezoelectric printhead nozzle onto substrate (Figure 6f ). [386]This noncontact printing approach offers high-resolution patterning and is appropriate for applications in flexible electronics, displays, and sensors. [387,388]Scalability, compatibility with variety of materials, and ability to generate complicated high-resolution patterns are all advantages of inkjet printing. [389]Its digitally controlled technique enables easy customization and quick prototyping in device production, such as SCs. [390]A PSC device with an all-inkjet-printed absorber and extraction layer was recently developed. [391]The inkjet-printed PSCs attained an efficiency greater than 17%.A photograph of the inkjet-printed TCO layer with corresponding cross-sectional electrical output curves and inset SEM images of the inkjet-printed TCO layer is shown in Figure 6g,h.Using direct ink writing technique, a nanogenerator with micropatterned friction silicone layers is previously developed. [392]The silicone layers sandwich and protect wrinkled silver electrodes developed through inkjet printing.The outcomes of the nanogenerator with 100% prestrain reach a maximum output voltage of 44.16 V at a frequency of 5 Hz and an external force of 10 N. The developed TENG was also employed to harness the biomechanical energy from human motion and swiftly power 20 commercial LEDs.Photo and pattern of the developed TENGs array and the output voltage curve with respect to 5, 10, and 15 N applied force of the fabricated device are shown in Figure 6i,j, respectively.[395] Inkjet printers have been utilized in the fabrication of wearable and textile-based electronic devices. [396,397]The findings and outcomes from various research groups have demonstrated the considerable potential of inkjet printing in a wide range of applications, including wearable and textile-based sensing, energy storage, and energy harvesting. [398]Although inkjet printing has shown promise for a variety of applications in wearable electronics, it does have certain limits.These include the necessity to manufacture ink with extremely low viscosity and precise surface tensions, as well as the need for rough textile surfaces to be presurface treated to achieve a smoother substrate.However, the additional materials and techniques needed in these surface treatment methods degrade the uniqueness of the cloth and raise total costs. [397]

2D Material-Based Wearable Energy Harvesters
This section discusses the efficacy of 2D materials in energy harvesting applications, such as solar, thermal, and mechanical energy harvesting.The bandgap, carrier mobility, thermal conductivity, and mechanical strength of 2D materials have a significant impact on their energy harvesting performance. [399,400]The use of 2D materials in energy harvesting can provide a sustainable and dependable energy source for wearable devices, thereby tackling the energy challenges associated with wearable e-textile technologies. [401]

SCs
The performance of 2D materials in SCs has been extensively studied due to their excellent electrical and optical properties.The large surface area and high carrier mobility of 2D materials such as graphene, MoS 2 , and WSe 2 make them promising candidates for high-efficiency SCs. [311,401]Researchers have reported impressive power conversion efficiencies (PCEs) using 2D material-based SCs, with some studies reporting efficiencies exceeding 29%. [402]This potential has led to continued research and development in the field of 2D material-based SCs, with the aim of developing more efficient and cost-effective SC technologies. [12]Extensive research has been carried out to exploring the properties of graphene as a semimetal, which has revealed its remarkable characteristics such as exceptional electrical conductivity, optical transparency, and outstanding flexibility. [403]raphene and its derivatives have been extensively utilized in SCs, demonstrating impressive results across various functionalities.428] Graphene a highly flexible and versatile material, has emerged as a key component in the development of wearable and textilebased SCs.In a recent study, [429] researchers successfully fabricated ultrathin and flexible PSCs by utilizing a transparent graphene electrode.The device structure consisted of a 20 μm thick polyethylene terephthalate (PET) substrate with PET/graphene/ P 3 HT/CH 3 NH 3 PbI 3 /PC 71 BM/Ag layers, achieved through a lowtemperature solution process.These flexible PSCs demonstrated an impressive power conversion efficiency (PCE) of %11.5%, while maintaining exceptional durability and air stability.In another study, [429] graphene materials were employed to replace the conventional indium tin oxide (ITO) electrode in inverted PSCs, resulting in an efficiency of %14.2%.This highlights the potential of graphene as a promising material for flexible PSCs, surpassing the efficiency of traditional ITO-based devices.Reduced GO (rGO) electrodes, along with a PET substrate, were recently utilized in the fabrication of flexible OSCs.The resulting device exhibited remarkable robustness, withstanding a tensile strain of 2.9% over a thousand bending cycles. [405]Additionally, textile-based DSSCs have been developed using a cotton fabric coated with graphene as a CE, achieving a remarkable PCE of 6.93% with the use of a polymer electrolyte. [430]Furthermore, a lightweight and wearable polymer solar textile (OSC) was developed using transparent electronic fabrics (e-fabrics) (Figure 7a-c). [431]These e-fabrics, consisting of a polyester/ AgNWs/graphene core-shell structure, served as both anodes and transparent substrates for the solar textiles.By blade coating  [431] b) optical and cross-sectional SEM images of the fabricated device, and c) the corresponding I-V curve in both dark and illumination.These results determined the potential of graphene as transparent electrode in solar energy harvesting.Adapted with permission. [431]Copyright 2016, Elsevier Ltd. d) Another schematic of flexible PSC, where both graphene and CNTs are utilized as electrode, e) The corresponding current density curve of fabricated PSC, and f ) the results variation in normalized PCE in comparison with silver, only CNTs, graphene (one layer)/CNTs (eight layers), and graphene (four layers)/CNTs (eight layers) based electrodes, where the graphene (one layer)/CNTs (eight layers) showed better performance in comparison and highlight the potential to replace noble metals.Adapted with permission. [432]Copyright 2020, American Chemical Society.g) The schematic of flexible textile DSSC where the MoS 2 /TiC/CNFs (h: optical and SEM images) are employed as CE by replacing the Pt and TCO, and i) determining the potential results I-V curve and the inset is an image of final fabricated textile DSSC.This integration of 2D TMDCs represent the potential for solar energy harvesting applications.Adapted with permission. [442]Copyright 2019, Elsevier Ltd.
the anode buffer layer and bulk heterojunction layer onto the efabrics, a textile-based OSC was achieved with a PCE of %2.27%.The textile exhibited outstanding mechanical deformation resistance and high compatibility with clothing.In another study, [432] a research group harnessed the potential of graphene and developed printable, free-standing hybrid graphene/carbon nanotube (CNTs) films as multifunctional electrodes for highly stable PSC (Figure 7d,e).The researchers examined the influence of graphene layer thickness in combination with a fixed eight layers of CNTs and observed that the PCE exhibited increasing enhancement up to four layers of graphene, beyond which further increases resulted in a reduction in PCE (Figure 7f ).Additionally, when compared to silver-based devices, the graphene/CNTs electrodes demonstrated superior stability and durability, underscoring the potential of graphene for solar energy harvesting applications.These advancements demonstrate the significant potential of graphen-based materials in enabling flexible and wearable SCs.The integration of graphene into textile structures provides a promising avenue for the development of lightweight and versatile solar textiles that can be seamlessly integrated into various applications.
Graphene-based composite materials have shown promise in various functional layers of SCs.When combined with poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as a hole-transport layer, graphene has demonstrated an enhanced response compared to PEDOT:PSS alone. [433,434]earable PSC devices have recently been developed using either PEDOT:PSS or PEDOT:PSS/graphene quantum dots (GQDs). [435]The PCE significantly improved from %12.77% to %16.15% with the inclusion of graphene.Graphene has also been utilized as an ETL, enhancing the stability of SCs.For instance, lithium-neutralized GO was recently employed as an ETL in PSCs, resulting in significantly increased device stability. [436]Additionally, GQDs have been used in combination with tin oxide (SnO 2 ) and achieved a PCE of % 21.1%, surpassing the PCE of 18.6% obtained with SnO 2 alone in a PSC. [437]These advancements highlight the potential of graphene-based materials in enhancing the performance and stability of SCs, paving the way for improved efficiency and durability in future wearable energy harvesting technologies.
In addition to graphene, other 2D materials, such as TMDCs, have emerged as a major focus for wearable solar energy harvesting.TMDCs, characterized by their tuneable bandgap and strong light-matter interaction, hold great promise as photoactive layers in PV applications.A single layer of 2D TMDCs offers excellent light absorption capabilities, capturing 5%-10% of incoming sunlight, which exceeds the absorption of traditional materials like silicon and GaAs. [438,439]The unique electrical structure and high photon absorption of 2D TMDCs have been reported to enhance the external quantum efficiency by over 30%. [440]For instance, Sibased SCs have employed a type-II heterojunction of MoS 2 /p-Si.By introducing a monolayer of MoS 2 , an electric field was introduced into the device, resulting in improved carrier separation.This led to a PCE of %5.23% for the SC. [441]Researchers have also developed textile-based DSSCs by fabricating a flexible MoS 2 /TiC/C nanofibrous film, which serves as a platinum (Pt)-free and TCO-free CE for the textile DSSC (Figure 7g,h).The unique mechanical properties of the MoS 2 /TiC/C nanofibrous film eliminate the need for a separate CE substrate in the device.Impressively, the textile-based DSSC exhibited a high PCE of %5.08%, surpassing that of Pt-based devices (Figure 7i). [442]he tuneable properties, exceptional light absorption capabilities, and unique device architectures 2D materials, such as TMDCs, will contribute to improved SC performance and efficiency.Table 2 summarizes some recent reported work on the application of 2D materials in SCs.The continued exploration and development of 2D materials in SC technologies hold promise for advancing the field of wearable solar energy harvesting textiles, enabling the creation of efficient, lightweight, and flexible energy generation systems for various wearable and portable applications.

PENGs
The utilization of 2D materials in piezoelectric-based energy harvesting applications has recently gained significant attention due to their remarkable mechanical properties, ultrathin size, and inherent piezoelectricity. [102]Unlike graphene, [443] other 2D materials, such as WS 2 , [444] MoS 2 , [445] and MoSe 2 , [446] exhibit strong piezoelectric properties.However, researchers have explored the use of graphene with piezoelectric materials to enhance the performance of PENGs and achieve significantly improved output results. [447,448]For example, rGO was incorporated into piezoelectric polyvinylidene fluoride (PVDF) to fabricate nanofiber mats using electrospinning and successfully developed a PENG device and achieved a high short-circuit current (I sc ) of 700 nA and an open-circuit voltage of 16 V.The inclusion of rGO not only enhanced the conductivity but also facilitated efficient transfer of the generated induced charge by PVDF, by enhancing the overall performance of the device. [449]Graphene has been employed previously as surface modifier while developing a trilayer flexible PENG device (Figure 8a). [448]The fabricated flexible PENG device was Reproduced with permission. [448]Copyright 2017, Elsevier B.V. d) Images of a fabricated MoS 2 -based PENG fixed in a shoe sole, along with a schematic where orange-colored rectangles represent the polarized MoS 2 ; e,f ) display the electrical performance outputs: the open-circuit voltage in response to walking and running, as well as the charging curves of three different capacitors.These results demonstrate the significance of 2D TMDCs in harvesting ambient mechanical energy by utilizing the piezoelectric effect.Reproduced with permission. [453]Copyright 2022, Elsevier Ltd. g) The schematic and image of a MoS 2 monolayer-based self-powered sensor.h) The output voltage responses of two different MoS 2 orientation (inset: schematic of zigzag and armchair MoS 2 orientations), while i) is the respiratory response of the device which demonstrates the potential of 2D materials in self-powered sensing applications.Reproduced with permission. [446]Copyright 2020, American Chemical Society.
composed of barium titanate (BTO), PVDF, and surface-modifier n-type graphene (n-graphene).Under an applied force of 2 N, the developed PENG device has a highest output voltage of 10 V and a current of 2.5 μA (Figure 8b,c).Even after 1000 pressingreleasing cycles, it demonstrates excellent stability and holds promise for future advancements in piezoelectric energy harvesting technologies. [448]D materials based on TMDCs (e.g., MoS 2 ) have garnered significant interest among energy harvesting researchers due to their impressive characteristics, including high carrier mobility and a direct bandgap. [450,451]MoS 2 made its debut in PENGs back in 2014, demonstrating remarkable performance, exhibited an output voltage reaching up to 15 mV and a power density of 2 mW m À2 , despite only experiencing a strain of 0.53%. [452]ecently, a research group demonstrated a flexible PENG utilizing a 3D structure based on MoS 2 . [110]The structure combines MoS 2 monolayers with vertically grown hollow MoS 2 nanoflakes (v-MoS 2 NF).The developed flexible PENG was integrated to commercial flexible shoes, and a maximum output voltage of 18 V and a corresponding power density of 1.62 mW m À2 were obtained during walking.In another study, [453] a PENG device was fabricated using a layer of flexible porous piezoelectric aerogel made by mixing TEMPO-oxidized cellulose nanofibrils (TOCN) and MoS 2 nanosheets.The PENG prototypes were installed in sport shoes and tested the output response their paces during running and walking (Figure 8d,e).An open-circuit voltage of 42 V, I sc of 1.1 μA, and maximum area power density of 1.29 μW cm À2 were obtained for PENG device with a TOCN/ MoS 2 aerogel film comprising 6 wt% MoS 2 .The fabricated device was further tested to charge three different commercial capacitors having values of 0.1, 0.33, and 1 μF (Figure 8f ). [453] team of researchers have utilized a monolayer of MoS 2 to develop a self-powered sensor based on the piezoelectric effect (Figure 8g). [446]They employed two types of monolayer MoS 2 orientations, namely, Zigzag and Armchair.The voltage response of the Armchair orientation was significantly superior to that of the Zigzag orientation (Figure 8h).The developed MoS 2 -based sensor demonstrated great potential for respiratory measurements when tested in both calm and after-running states (Figure 8i).Apart from MoS 2 , other 2D-TMDC materials have been significantly investigated for energy harvesting applications.For instance, a flexible PENG was developed based on a PVDF-WS 2 composite. [454]The developed wearable PENG device achieved an impressive output voltage of approximately 116 V, in response to a stress/force of 105 kPa.The piezoelectric energy conversion efficiency reached %25.6%, which is the highest reported value among PVDF-2D material-based PENG.Notably, the fabricated PENG demonstrated efficient energy harvesting from various daily human activities, including finger tapping, writing on paper, mouse clicking, and body movement. [454]hese applications of 2D TMDCs materials as piezoelectric enhancer showcase a promising avenue for the development of self-powered wearable e-textile.
Other 2D materials (e.g., MXene and h-BN) have shown potential in mechanical energy harvesting. [101,253,455]High mechanical flexibility and piezoelectric coefficient make them ideal candidates for the efficient energy conversion. [456,457]In addition, the large bandgap of hBN ensures reduced leakage current and improved energy harvesting capabilities in piezoelectric devices. [253]A flexible hBN/cotton hybrid PENG was recently fabricated, which showed the ability to pick up tiny signals like heartbeat and exhaled air pressure.The developed PENG device demonstrated a peak-to-peak voltage output of about 1.5 V for each heartbeat when it was tested for real-time motion detection.Additionally, the as-fabricated device demonstrated a high electrical output of up to 10 V when a pressure of 3 MPa was applied.Furthermore, the device was fixed on different spots of the human body and the output response was observed with different motions. [458]Table 3 summarizes the recently reported 2D material-based PENGs.

TENG
The long-term retention of surface charge density has made 2D materials viable alternatives for TENG applications. [311,459,460]heir unique features, including as high surface-to-volume ratio and outstanding charge trapping capabilities, make them ideal for TENG systems.Furthermore, 2D materials enable effective charge transfer and accumulation during mechanical motion, resulting in improved energy harvesting efficiency. [119]Among other 2D materials, graphene and its derivatives have been widely reported by several researchers for flexible and wearable TENG applications. [125,460,462]A fabric-based TENG has been developed by combining conductive fabric with a polydimethylsiloxane (PDMS) layer containing GO (Figure 9a,b). [463]The developed TENG device performed excellently, with an opencircuit voltage of 140.4 V, I sc of 2.57μA, and maximum power production of 130.5 μW with a load resistance of 50 MΩ (Figure 9c,d).Furthermore, the device was linked to clothes and the body, and distinct moments were detected, demonstrating the potential applicability of the fabric-based TENG.In another study, [464] researchers developed graphene textile-based TENGs using a sequential and scalable technique that included spinning, dip coating, multiaxial winding, and machine knitting (Figure 9e,f ).These graphene textile-based TENGs showed outstanding properties such as high flexibility, form modification, structural integrity, cycle washability, and great mechanical stability.Using a 3D cardigan stitch knitting technique, the graphene textile TENG reached a tremendous peak output of 3.6μW, demonstrating its potential to power portable electronic devices (Figure 9g,h).In certain applications, surface roughness is typically viewed as a disadvantage.However, in the case of graphene, increasing its surface roughness has been observed to yield high performance in TENG.For example, [465] a research group used gold chloride (AuCl 3 ) nanoparticles to increase the surface roughness of graphene by 300%, resulting in an output voltage of 80.6 V and current of 11.9 μA cm À2 , which was 20 times more than the planer graphene.Apart from triboelectrification, highly conductive graphene has been employed as electrode while fabricating TENGs. [466,467]For instance, [468] aluminum adhesive electrodes were replaced with laser-induced graphene (LIG), and when comparing TENG output, LIG electrode-based TENGs deliver 150% more power than metalbased electrode in TENGs.
2D TMDCs have demonstrated considerable potential as active materials for TENG. [119]Such as in a developed TENG, [469] MoS 2 was employed as an electron acceptor due to its ability to efficiently trap electrons and prevent their recombination with positively charged molecules on the surface.The addition of MoS 2 dramatically improved TENG performance, improving it 120 times over TENGs without MoS 2 .Additionally, a textile-based TENG was fabricated employing heterojunction of MoS 2 and tantalum carbide, [470] and showed promising performance, such as an output voltage of around 3.3 V and a I sc of about 75 μA.Furthermore, the device was highly durable, enduring 43 000 cycles.In addition to MoS 2 , various other 2D materials have been studied.In one study, [471] a group of researchers investigated four different kind of 2D TMDCs: WS 2 , MoS 2 , WSe 2 , and MoSe 2 .Surprisingly, they observed that WSe 2 had the greatest output voltage, reaching up to 286 V, outperforming the other materials. [472]A flexible and wearable single-electrode-based TENG device was composed of cellulose paper, and a polyvinyl alcohol (PVA) membrane substrate has recently been developed.Various triboelectric counter layers, including Al and PTFE, were used to examine triboelectric functioning.In comparison to all other CE systems, the Au-WS 2 -PTFE-based TENG excelled by providing the greatest output voltage of 7.9 V.
Other 2D materials such as BP, MXene, and hBN have also been key materials for energy harvesting applications.BP, for example, has been used by developing a textile-based TENG that incorporates BP wrapped with hydrophobic cellulose oleoyl ester nanoparticles (Figure 9i,j). [473]Despite extensive deformations, rigorous washing, and lengthy environmental exposure, this TENG proved long-term durability and kept excellent triboelectricity.It produced significant output voltage up to 880 V, with current density of 1.1 μA cm À2 (Figure 9k,l).These excellent results were obtained by mild hand contact at a low frequency of roughly 4 Hz and a little force of around 5 N. A TENG based on MXene material had an impressive response.MXene nanosheets were electrospun onto a flexible Al foil after being dispersed in a PVA solution. [474]The TENG achieved an incredible peak output of 1087.6 mW m À2 and proved the capacity to power numerous LEDs at the same time.In summary, the keen research interest in energy harvesting and the rising popularity of 2D materials in TENG research indicates their potential to revolutionize energy harvesting techniques and pave the path for sustainable power generation. [125,475]

ThNG
2D materials possess a captivating appeal compared to their bulk counterparts.Their unique density of states (DOS) correlates strongly with the Seebeck coefficient. [476]DOS refers to the number of available energy levels for electrons or charge carriers in a material.It influences the material's electrical and thermal transport properties, including the ability to generate an electrical voltage in response to a temperature difference. [47,477,478]Despite its great electrical and thermal conductivity, pure graphene is not a good thermoelectric material. [479]However, several attempts have been undertaken by researchers to change its band structure in order to increase the Seebeck coefficient and decrease thermal conductivity.[482] The integration of GQDs into unique composite films resulted in a considerable increase in the thermoelectric performance of PEDOT:PSS.The electrical conductivity of PEDOT:PSS/GQDs was measured to be 7172 S m À1 , with the corresponding Seebeck coefficient of 14.6 μV K À1 .Furthermore, the optimized PEDOT:PSS/GQDs composite had a power factor that was 550% greater than PEDOT: PSS alone. [483]A textile-based wearable ThNG was developed employing reduced rGO and PEDOT:PSS.The developed technology gathers energy efficiently from low-grade  [490] b) images of fabricated textile ThNG attached to a human hand as wrist band, and c) the corresponding output voltage, showing the engaged and disengaged after reaching the maximum voltage on a wrist band, where it determined a continued output voltage as far as it engaged to wrist, showing the potential to be a constant source for powering wearables.d) The thermal power factor of MoS2 alone and Au-MoS 2 hybrid composites.Adapted with permission. [490]Copyright 2018, American Chemical Society.e) The schematic of Cu doped-MoS 2 -based fabric ThNG; [491] f ) images of the fabricated fabric ThNGs connected in series and attached to human wrist for harvesting human body temperature.Electrical performance as function of temperature gradient, g) output voltage, and h) output power and power density.Reproduced with permission. [491]Copyright 2022, Elsevier Inc. i,j) Another schematic and optical image of an all-MXene-based flexible ThNG, [492] and k) the output voltage of the ThNG under various temperature gradient.l) The thermoelectric performance comparison of two types of Mo 2 TiC 2 T x and Nb 2 CT x MXenes.The results obtained by various researchers have significantly opened up possibilities for utilizing 2D materials in efficiently harvesting thermal energy through textile-based ThNG.Reproduced with permission. [492]Copyright 2022, Elsevier Ltd. human body heat.The ThNG performed well, with Seebeck coefficient ranging from 25 to 150 μV K À1 and a power factor ranging from 2.5 to 60 μWm À1 K À2 . [186]D TMDCs with monolayer have recently gained attraction for thermoelectric applications due to having low thermal conductance comparative to graphene, which make them suitable for Seebeck effect. [484,485]The thermoelectric performance of 2D TMDCs like MoS 2 , however, is constrained by their limited electrical conductivity.However, this constraint may be overcome by implementing a number of factors, such as the composition of other conductive materials like graphene or the decoration of conductive particles like Cu, Au, etc. [486][487][488] Recently, a flexible PET substrate was utilized to fabricate a ThNG based on a combination of MoS 2 and graphene.The study focused on evaluating and comparing the performance of three different device configurations: pristine graphene, standalone MoS 2 , and graphene/ MoS 2 composite.The results demonstrated remarkable enhancements in the ThNG device when utilizing the graphene/MoS 2 composite configuration by generating 0.24 mV at temperature difference of 16 K. [489]In a recent study, [490] 2D MoS 2 was decorated with Au nanoparticles by in situ growth as a potential way to improving thermoelectric performance, (Figure 10a,b).The resultant Au-decorated MoS 2 -assembled heterojunction system has a distinct decoupling behavior, with an enhanced Seebeck coefficient and conductivity.At room temperature, these composite flexible films have a high power factor of 166.3 W m À1 K À2 , indicating the enormous potential for harvesting human body heat (Figure 10c,d). [490]Similarly, [491] Cu-doped 2D layered MoS 2 nanosheets were hydrothermally developed on carbon fabric (Figure 10e,f ).The temperature-dependent analysis demonstrated that the electrical conductivity, Seebeck coefficient, and power factor of Cu-doped MoS 2 increased as the temperature ascended (Figure 10g,h).The greatest Seebeck coefficient was reported in a MoS 2 sample doped with 4 at% Cu, reaching around 10 μV K À1 at 303 K and over 13 μV K À1 at 373 K. To exploit these properties, a thermoelectric device was built employing four pairs of thermoelectric materials, with Cu-doped MoS 2 acting as the p-type material and Cu wire serving as the n-type material.These materials were linked electrically in series and thermally in parallel, resulting in a voltage of 190.7 μV over an 8 k temperature differential. [491]][494][495][496][497][498] A flexible ThNG was previously developed by combining a highly ordered arrangement of Bi 2 Te 3 nanocrystals with a network of single-walled CNTs (SWCNTs).The resultant device has a significant power factor of 6. 3μW m À1 K À2 , making it one of the most efficient flexible nanocomposite films in terms of thermoelectric performance. [499]Researchers recently developed composites of Bi 0.5 Sb 1.5 Te 3 /PEDOT:PSS and developed ThNG device.The resultant ThNG device performed well, with a peak power factor of 312 μW m À1 K À2 at ambient temperature.Furthermore, when the wearable ThNG was put on a human wrist, it generated 7.7 mV of thermomotive.These findings show that the invented gadget has the potential for practical applications in collecting energy from the body heat. [500]Two types of MXene materials were employed by a research team [492] to develop a flexible ThNG device composed of all MXene materials, (Figure 10i,j).Through organic molecule intercalation and heat treatment, the performance of Mo 2 TiC 2 T x and Nb 2 CT x MXenes was  When exposed to a temperature differential of 30 °C, the ThNG device, made solely of MXene materials, produced an output voltage of 35.3 mV and a power of 33.9 nW.The output voltage in response to different temperature gradients was observed to increase with higher ΔT values (Figure 10k).Additionally, a comparison of the thermal power factors between the two types of MXenes revealed that Mo 2 TiC 2 T x exhibited better performance for high-temperature gradients (Figure 10l).These findings emphasize MXenes' potential for effective thermoelectric energy harvesting applications. [492]Table 5 summarizes the recent advancement in 2D material-based wearable textile ThNGs.

PyNGs
PyNGs have been highlighted as an important and alternative technique of harvesting ambient heat.There has been a lot of research done, notably on harnessing the ambient heat in wearable applications. [501,502]For example, [503] a flexible nanogenerator based on both piezoelectric and pyroelectric effects was developed (Figure 11a,b).Their approach entailed combining a PDMS-CNT (PDMS-CNT) composite with graphene nanosheets as the top electrode.Graphene was chosen because of its strong thermal conductivity, which allows for quick temperature changes across the device.This hybrid nanogenerator produced an optimal output voltage of 1.4 V, with compress release 1.0 V and the pyroelectric output voltage response was around 0.4 V to temperature gradient (Figure 11c). [503]In another research, [46] a sunlight-triggered PyNG has been developed, where the device utilizes a polarized PVDF film, with a rGO and polyethyleneimine (PEI) layer deposited in a sandwiched architecture between Ag electrodes (Figure 11d).Remarkably, the fabricated device gained maximum output power of 21.3 mW m À2 , making it capable of powering fitness monitoring electronic devices (Figure 11e,f ).In addition to its ability to harvest heat from solar radiation, PyNG has also been employed to measure and detect the level of infrared radiation. [504]A photodetector based on tin sulfide nanoflakes was developed and demonstrated a broadband photoresponse from UV (365 nm) to near-infrared (850 nm) wavelengths.Using the pyro-photo potential, the device's photocurrent density grew from 100 to 470 μA cm 2 at a wavelength of 760 nm and an intensity of 7 mW cm À2 .Furthermore, an excellent photoresponsivity of 13 mA W À1 and a high photo detectivity of 3 Â 10 14 Jones at 760 nm with an intensity of 7 mW cm À2 were recorded, reflecting 340% and 3960% increases with the pyroelectric potential, respectively. [505]

Outlook and Future Direction
This in-depth literature explores current research advances in 2D material-based wearable energy harvesting devices, notably in the nanogenerator, [503] where graphene nanosheet was used as flexible electrode.c) The corresponding output voltage during cool and heating cycles.Adapted with permission. [503]Copyright 2013, Wiley-VCH.d) Optical cross-sectional microscopic image of another wearable PyNG device-based PEI chemically modified rGO and PVDF. [46]e) Image of the fabricated wearable PyNG attached to human hand and powering a smart heart rate monitoring watch, while f ) is the output voltage response during static and moving hand while harvesting heat energy (inset: image of the fabricated PyNG device).Reproduced with permission. [46]Copyright 2020, Elsevier Ltd.
realms of solar, mechanical, and thermal energy.Since the discovery of 2D materials, researchers have been focusing on investigation and understanding properties of these novel materials, refining the preparation process, and applying them for multiple applications via various fabrication processes.2D materials have the capacity to tune both electrical and optical properties, as well as have great flexibility and mechanical resilience, which makes them ideal candidate for wearable energy harvesting devices.The structural composition and chemistry of 2D materials, such as thickness, phases, and surface functional groups, have a considerable impact on their mechanical, electrical, optical, and thermal properties.It is possible to construct a desirable 2D material appropriate for certain purposes by carefully managing the preparation of 2D materials using either a top-down or bottom-up approach.Other methods for modifying the characteristics of 2D materials include functionalization, vacancy engineering, doping with anions or cations, and surface defect engineering.Consequently, 2D materials offer a promising avenue for building wearable, and flexible energy harvesters and smart sensors.While significant contributions have been made in energy harvesting applications, there is still ample room for further research and optimization in this field.As illustrated in Figure 12, the global market trend for wearables, specifically solar powered smartwatches, [506] combined with growing investment in 2D materials from 2022 to 2031, [507] and the increasing popularity and investment market of smart clothing and e-textiles [508] has encouraged research and development authorities to intensify their efforts in enhancing and directing their focus toward wearable textile-based energy harvesting technologies utilizing 2D materials.In light of this, this review further emphasizes on several research challenges and opportunities for exploration and advancement in 2D material-based wearable textile-based energy harvesters.
Figure 12.Market demands: trend revolves around global investments in solar-powered watches, [506] 2D materials, [507] and smart clothing e-textiles. [508]erformance optimization for potential energy harvesting device in wearable garments: the schematic shift toward optimizing energy harvesting based on 2D materials, aiming to create self-powered textile garments equipped with essential functionalities like monitoring, communication wearable devices, power storage, and power generation capabilities with a focus of negligible hazards.

Electrical Performance
Despite receiving a lot of attention and making significant advances, energy harvesters have still relatively small power generation capacities, often in the range of μW cm À2 to mW cm À2 , which is still quite low to power up wearable devices. [509]In other words, there is significant research potential for optimizing these energy harvesters to maximize power densities and PCEs.One focus area is the exploration of novel 2D materials and the optimization of device structures (Figure 12).By linking the physical and chemical properties of 2D materials with optimized device designs, it is possible to achieve high-performance energy harvesters.Extensive studies have been conducted to understand the physical and chemical properties of 2D materials, such as the variation of bandgap with size. [510]However, a comprehensive understanding of the underlying science of 2D materials is still lacking.More study into the science underpinning 2D materials will provide researchers more opportunity to examine, use, and optimize these materials for specific energy harvesting applications.In addition to material considerations, the output power of energy harvesters may be maximized by combining two or more types of energy harvesters by developing a hybrid energy harvester.For example, PENGs are more likely to produce higher current, while TENGs are high voltage generators; combining these two mechanical energy harvesters would maximize power density when compared to using TENG/PENG alone. [3]Similarly for utilizing both thermal and mechanical energy from human body at the same time, a hybrid energy harvester having capability of harvesting both mechanical (PENG/TENG) and thermal energy (ThNG/PyNG) harvesters may be employed to attain maximum power output.In addition, utilizing both photon energy and thermal energy from sunlight employing PV and thermoelectric effect concurrently is another sustainable option to increase the power generation capabilities.Anyhow there are still many challenges and opportunities to tackle and optimize the energy conversion efficiency and power densities.

Maximizing Stability and Durability
Wearable energy harvesters have shown great potential for powering wearable devices but the durability and stability of these devices remain a challenge. [511]Wearable energy harvesters are considerably in contact with body; the endurance and stability are always a big concern due to continued stress/release of body movements.2D materials have gained attention for their impressive mechanical strength, making them a prominent choice in the development of 2D wearable energy harvesters.Nonetheless, these materials face significant stability challenges, especially when exposed to variations in temperature and openair environments.For instance, materials like MoS 2 and BP demonstrate instability under such conditions, raising concerns about their long-term performance and reliability in wearable devices.Researchers are actively working to address these issues and enhance the stability of 2D materials for sustainable and efficient energy harvesting applications. [512]Furthermore, the capacity of wearable devices to resist multiple washing and drying cycles is critical for their use as everyday wear.Wearable e-textiles are designed to retain their look and shape over time, which is critical for their intended use.Researchers have been investigating to improve the washability of wearable electronics while retaining the performance, such as recently textile SCs shown excellent resistivity in performance degradation to both hand and machine washing cycles. [513]However, the incorporation of energy harvesters into fabric adds a sensitivity to cleaning chemicals and is still a barrier for the longevity of the wearable devices.

Toxicity and Biocompatibility
The constant interaction between 2D material-based devices and the human body raises serious concerns about possible harm from hazardous substances.For instance, GO has been found to be hazardous in biological systems due to the formation of reactive oxygen species and the possible damage to cellular membranes. [514,515]Furthermore, inhaling nanoparticles from 2D materials is hazardous to one's health because these particles may penetrate deep into the respiratory system, potentially causing lung inflammation and other negative consequences. [516,517]t is worth noting that biocompatibility investigations for 2D materials other than graphene are currently limited.To address these issues, researchers are diligently developing safer synthesis processes, investigating encapsulating approaches, and expanding their understanding of toxicity mechanisms.Furthermore, the strictly controlled preparation process and processstructure-properties interaction of 2D materials are important in defining biocompatibility. [518,519]Understanding how these materials interact with living organs is critical for understanding how they will affect human health in the long run.Although the biocompatibility and nanotoxicity standards for wearable energy harvesting are not as strict as those for medication administration or treatment, there is still a lack of understanding about the biodegradation and bio persistence of 2D materials in interaction with human tissues.To acquire a better knowledge of the biocompatibility and cytotoxicity of 2D materials, a comprehensive library of these materials that includes both in vivo and in vitro investigations is required.In sum up, there is plenty of work required to investigate the corresponding toxicity and biocompatibility of 2D materials.These initiatives will assure the proper use of 2D materials in wearable energy harvesters, with a focus on user safety and risk minimization (Figure 12).

Scalability and Integrability
The development of large-area, high-quality 2D materials at a reasonable cost and with fine control over shape and dimensions offers substantial potential for wearable energy harvesting.However, 2D material production on a large scale and at a reasonable cost while retaining precise control remains challenging.For instance, 2D materials synthesis methods such as exfoliation and solvothermal are comparatively cost-effective but the control over precise size and thickness are strong barriers.On the other hand, other techniques such as physical vapor deposition and CVD are considerably controlled process but expensive.A comprehensive approach that combines both basic understandings and technological capabilities is necessary to overcome severe production difficulties in 2D material-based wearable energy harvesting.This strategy should prioritize tackling major difficulties such as successfully reducing defects during process scaling as well as finding nonlinear scaling connections for possible mass manufacturing.By overcoming these challenges, breakthroughs in the fabrication of 2D materials may be attained, leading to higher scalability, increased production effectiveness, and broader application in wearable energy harvesting devices.
Smart wearable e-textiles have made great development over the last decade, due to breakthroughs in electronics miniaturization, nanotechnology, and the digital revolution.This incredible advancement in flexible and wearable technology has cleared the path for the development of personalized wearable.These e-textiles may interface with the human body in a natural way, continually monitor vital signs, collect data, and permit exchange of varied physiological information. [369,370]Over the last decade, extensive research has been conducted to explore the sensing and monitoring capabilities of wearable devices.However, integrating energy harvesting devices with other components poses several challenges.For instance, the current fiber SCs are only a few centimeters in scale, making them impractical for powering wearable devices. [520]Some other challenges involve incorporating the necessary circuitry and power regulation devices to ensure seamless functionality.Additionally, the integration of energy storage devices is crucial for enabling the operation of wearable devices during periods when energy harvesting is in inactive mode.Researchers have investigated the practicality of supercapacitors as a solution for wearable energy storage, focusing on 2D material-based fiber supercapacitors. [521,522]owever, the power density of wearable supercapacitors is currently limited, necessitating further research to optimize their capacity and charging capabilities in integration mode with wearable harvesting devices.Addressing these challenges will be vital in realizing the full potential of wearable energy harvesting devices in full e-textile wearable garment (Figure 12).In the pursuit of scalable and prototype-ready e-textiles featuring comprehensive functionalities like self-powering and monitoring capabilities, the major hurdles lie in the diverse structures and fabrication processes of individual components.The critical decision-making factor involves selecting materials that can serve multiple purposes effectively, such as common electrode materials for energy harvesting, sensors, and energy storage.Even if the materials differ, maintaining consistency in the processing methods, like the fabrication process, is pivotal.Addressing these concerns and leveraging modern fabrication techniques such as electrospinning, yarn fabrication, and controlled digital printing can pave the way for the seamless production of fully functional e-textile garments. [523]It is critical to reconsider structures that provide numerous capabilities at the same time.For example, there is a focus on producing yarns that can harness and store energy, enabling novel alternatives for multifunctional applications.
Iftikhar Ali is a Ph.D. student at the Centre for Print Research (CFPR), University of the West of England (UWE) Bristol, UK.He has been researching textile-based energy solutions for wearable electronic textiles.His research focuses on the application of graphene and other 2D materials in the development of textile-based wearable energy harvesters such as tribo/piezoelectric nanogenerators and solar cells.Prior to these, he has hands-on experience with nanofabrication technologies and a strong interest in researching nanoelectronics such as memristors and sensors.
Marzia Dulal is a Commonwealth scholar and a Ph.D. student at the CFPR, UWE Bristol, UK.She has been researching smart wearable electronic textiles for personalized healthcare with a sustainable approach to maximize their potential while minimizing their detrimental environmental effects.She previously achieved the Prime Minister Gold Medal for academic excellence and obtained her B.Sc. and M.Sc. in textile engineering from Bangladesh University of Textiles, Dhaka, Bangladesh.She has about 10 years of experience in textile supply chain management, consumer behavior, and smart composites areas from both academia and industry.
Nazmul Karim is a professor of advanced textiles at the Nottingham Trent University, UK.His research interests lie in the area of new materials and sustainable technologies for developing next-generation wearable e-textiles, sustainable natural fiber-reinforced composites, and high-performance functional clothing.Prior to that, Karim was an associate professor at UWE Bristol (UK) and knowledge exchange fellow (graphene) at the National Graphene Institute of University of Manchester (UK).He has %15 years of industry and academic experiences in new materials and textile-related technologies, and a passion for getting research out of the lab and into real-world applications.Shaila Afroj is a senior research fellow at the CFPR, UWE Bristol (UK), where her research group investigates graphene and other 2D material-based technologies aimed at developing sustainable wearable electronics textiles and functional clothing for future.Prior to that, she worked as a research associate at National Graphene Institute (NGI), University of Manchester, after completing her Ph.D. from the same university.She has %15 years of industry (including multinational companies like C&A and Intertek) and academic experiences related to sustainable e-textiles, advanced materials, printing, and fashion clothing.

Figure 1 .
Figure 1.Schematic of a self-powered textile garment, having capabilities of harvesting different types of ambient energy and utilizing them for storing and powering different wearable and portable electronic.
type to type such as imine-based COFs are found very stable in water but other side COFs with reversible boroxine bonds are sensitive to water.Found stable up to 300 °C under air by TGA analysis.

Figure 4 .
Figure 4. 2D material preparation.a) Schematic of 2D material exfoliation by LPE; Quality of the exfoliated graphene flakes b) atomic-resolution STEM image of a graphene flake (Inset shows the corresponding magnified image with perfect graphene lattice, c) Raman spectra of graphene, showing an I D /I G ratio of 0.23, d) X-ray photoelectrons spectroscopy (XPS) C 1s spectrum of graphene, and e) wide-field AFM image of graphene flakes and f ) the corresponding thickness and g) lateral size histograms.Scale bar: a) 2 nm; d) 5 μm.Adapted with permission.[286]Copyright 2018, Springer Nature.The presence of monolayers of graphene was confirmed by shear mixing (inset of (c), (d), and (f ), respectively).Adapted with permission.[287]Copyright 2014, Springer Nature.Spectra, XPS (NMP-exfoliated samples) measured on thin films, AFM on a surfactant-exfoliated sample.h) AFM images of MoS2 nanosheets; i) layer number distribution of MoS 2 nanosheets; j) thickness plot of selected MoS 2 nanosheets in images (h);[294] k) SEM image of MX-H delaminated layered surface morphology.Reproduced with permission.[295]Copyright 2021, Elsevier.l) SEM images of exfoliated BN.Reproduced with permission.[295]Copyright 2018, IOP Publishing Ltd.

Figure 5 .
Figure 5.Some common coating techniques: a) Schematic of spray coating system.b) Front and back images of textile-based OSC, where spray coating is employed for different functional layers, and c) the corresponding electrical performance in comparison with other substrate such as FTO-coated glass and bare glass, which determined the versatility and compatibility of spray coating techniques for various substrate.Adapted with permission.[334]Copyright 2018, IEEE.d) Dip coating setup.e) Image of fiber-based flexible DSSC, where dip coating is utilized for deposition of functional layer TiO 2 , also includes the SEM image of two and six dip-coated layers of TiO 2 , and f ) the corresponding I-V curves in comparison with the dip-coated layers; the increasing number of dip coating cycles enhanced the performance but after fourth cycle, further coating leads to poor response.Adapted with permission.[354]Copyright 2021, Elsevier B.V. g) Spin coating system.h) Schematic and image of 2D MXene-based wearable spin-coated TENG, attached to a human hand for harnessing human hand motion.i) The open-circuit voltage generated curve of the corresponding TENG.Reproduced with permission.[364]Copyright 2022, Wiley-VCH GmbH.

Figure 6 .
Figure 6.Common printing techniques: a) screen printing, b) image of screen-printed-based textile DSSC,and c) the corresponding I-V curve for two different textile DSSCs, bare textile without any surface treatment, and polyamide printed textile.Adapted with permission.[376]Copyright 2019, IEEE.As shown, the surface smoothening enhanced the electrical performance in comparison with bare rough textile.d) Image of a screen-printed flexible ThNG, and e) the maximum power and open-circuit voltage response to temperature gradient.Adapted with permission.[377]Copyright 2020, Elsevier B.V. f ) Schematic of inkjet printing system.g) An image of inkjet-printed transparent conductive polymer (TCP) layer of PSC,[391] and h) the corresponding I-V curve of the PSC along with inset SEM image of the TCP layer.Adapted with permission.[391]Copyright 2020, Wiley-VCH.i) Photograph of a 5 X 5 TENG array printed using inkjet printer and utilized for tactile sensing.Figure also includes the proposed pattern and developed device in a human hand.j) The corresponding output voltage of the inkjet-printed TENGs with respect to different value of forces.Reproduced with permission.[392]Copyright 2020, Elsevier Ltd.

Figure 7 .
Figure 7. 2D material-based flexible and textile SCs: a) schematic of a fabric-based OSC where polyester/AgNW/graphene structure was employed as transparent anode,[431] b) optical and cross-sectional SEM images of the fabricated device, and c) the corresponding I-V curve in both dark and illumination.These results determined the potential of graphene as transparent electrode in solar energy harvesting.Adapted with permission.[431]Copyright 2016, Elsevier Ltd. d) Another schematic of flexible PSC, where both graphene and CNTs are utilized as electrode, e) The corresponding current density curve of fabricated PSC, and f ) the results variation in normalized PCE in comparison with silver, only CNTs, graphene (one layer)/CNTs (eight layers), and graphene (four layers)/CNTs (eight layers) based electrodes, where the graphene (one layer)/CNTs (eight layers) showed better performance in comparison and highlight the potential to replace noble metals.Adapted with permission.[432]Copyright 2020, American Chemical Society.g) The schematic of flexible textile DSSC where the MoS 2 /TiC/CNFs (h: optical and SEM images) are employed as CE by replacing the Pt and TCO, and i) determining the potential results I-V curve and the inset is an image of final fabricated textile DSSC.This integration of 2D TMDCs represent the potential for solar energy harvesting applications.Adapted with permission.[442]Copyright 2019, Elsevier Ltd.

Figure 8 .
Figure 8. Performance of 2D material-based wearable PENGs: a) image of a flexible PENG device where graphene was employed with piezo materials to enhance the piezoelectric performance; b,c) are the output voltage and current density curves, respectively.Reproduced with permission.[448]Copyright 2017, Elsevier B.V. d) Images of a fabricated MoS 2 -based PENG fixed in a shoe sole, along with a schematic where orange-colored rectangles represent the polarized MoS 2 ; e,f ) display the electrical performance outputs: the open-circuit voltage in response to walking and running, as well as the charging curves of three different capacitors.These results demonstrate the significance of 2D TMDCs in harvesting ambient mechanical energy by utilizing the piezoelectric effect.Reproduced with permission.[453]Copyright 2022, Elsevier Ltd. g) The schematic and image of a MoS 2 monolayer-based self-powered sensor.h) The output voltage responses of two different MoS 2 orientation (inset: schematic of zigzag and armchair MoS 2 orientations), while i) is the respiratory response of the device which demonstrates the potential of 2D materials in self-powered sensing applications.Reproduced with permission.[446]Copyright 2020, American Chemical Society.

Figure 10 .
Figure10.Performance of 2D material-based wearable and textile ThNGs: a) schematic of a proposed Au-decorated MoS 2 film-based textile ThNG,[490] b) images of fabricated textile ThNG attached to a human hand as wrist band, and c) the corresponding output voltage, showing the engaged and disengaged after reaching the maximum voltage on a wrist band, where it determined a continued output voltage as far as it engaged to wrist, showing the potential to be a constant source for powering wearables.d) The thermal power factor of MoS2 alone and Au-MoS 2 hybrid composites.Adapted with permission.[490]Copyright 2018, American Chemical Society.e) The schematic of Cu doped-MoS 2 -based fabric ThNG;[491] f ) images of the fabricated fabric ThNGs connected in series and attached to human wrist for harvesting human body temperature.Electrical performance as function of temperature gradient, g) output voltage, and h) output power and power density.Reproduced with permission.[491]Copyright 2022, Elsevier Inc. i,j) Another schematic and optical image of an all-MXene-based flexible ThNG,[492] and k) the output voltage of the ThNG under various temperature gradient.l) The thermoelectric performance comparison of two types of Mo 2 TiC 2 T x and Nb 2 CT x MXenes.The results obtained by various researchers have significantly opened up possibilities for utilizing 2D materials in efficiently harvesting thermal energy through textile-based ThNG.Reproduced with permission.[492]Copyright 2022, Elsevier Ltd.
carefully regulated to produce n-and p-type properties, respectively.At room temperature, the optimized Mo 2 TiC 2 T x and Nb 2 CT x MXenes demonstrated outstanding thermoelectric power factors of 13.26 and 11.06 μW m À1 K À2 , respectively.

Figure 11 .
Figure 11.PyNG performance: a,b) schematic and images (attached to a knee and shoulder) of highly stretchable hybrid (piezo/pyroelectric) nanogenerator,[503] where graphene nanosheet was used as flexible electrode.c) The corresponding output voltage during cool and heating cycles.Adapted with permission.[503]Copyright 2013, Wiley-VCH.d) Optical cross-sectional microscopic image of another wearable PyNG device-based PEI chemically modified rGO and PVDF.[46] e) Image of the fabricated wearable PyNG attached to human hand and powering a smart heart rate monitoring watch, while f ) is the output voltage response during static and moving hand while harvesting heat energy (inset: image of the fabricated PyNG device).Reproduced with permission.[46]Copyright 2020, Elsevier Ltd.

Table 4
summarizes recently reported works on 2D material-based wearable TENGs.