11495 FACTA UNIVERSITATIS Series: Electronics and Energetics Vol. 36, No 3, September 2023, pp. 411-426 https://doi.org/10.2298/FUEE2303411A © 2023 by University of Niš, Serbia | Creative Commons License: CC BY-NC-ND Original scientific paper A CRITICAL REVIEW ON THE MATERIAL ASPECTS OF TRIBOELECTRIC NANOGENERATORS (TENG) Deepak Anand, Ashish Singh Sambyal, Rakesh Vaid Department of Electronics, University of Jammu, Jammu 180006, India Abstract. Triboelectric nanogenerators (TENG) take the advantage of coupling effect for harvesting energy in the area of electronics for various self-powered applications. These nanogenerators are capable of converting energy in our surroundings into electrical energy by using the process of electrostatic induction and contact electrification. Triboelectric layers of a TENG are formed basically with the use of various polymers, metals and other inorganic materials like PTFE (Poly tetra fluoro ethylene), PDMS (polydimethyl siloxane), FEP (Fluorinated ethylene propylene) and Kapton. Selection of different materials for the device fabrication is very important since it contribute towards the triboelectric effect and also forms the fundamental structure for the proposed TENG device. In this review article, we emphasis mainly on various triboelectric materials considering factors such as stability, flexibility, power density etc., to improve upon the electrical output of the devices for different applications. Key words: TENG (Triboelectric Nanogenerator), PTFE (Poly tetra fluoro Ethylene), FEP (Fluorinated ethylene propylene), PDMS (Poly dimethyl siloxane), TET (triboelectric textile), PMMA (polymethyl methacrylate), Energy harvesting 1. INTRODUCTION AND BACKGROUND With the transport of electrons from one surface to another, Triboelectric nanogenerators (TENG) can be termed as significant devices for generating electrical output from ambient mechanical energy. Triboelectricity can be seen in our daily actions including walking, running, hand-to-hand contact, etc. [1-3] when charge separation on the surfaces that come into contact produces electrical potential and can be stored in energy storage units [4]. The TENG based devices can work in four modes such as vertical contact-separation [5-7], sliding [8-9], single electron [10-11], and free standing mode [12-13]. These devices use stretched materials as spacers [14–17] and also have arc-shaped structures [18] and are based on various models reported in the literature such as the band structure model [19], Received January 24, 2023; revised April 01, 2023; accepted April 09, 2023 Corresponding author: Rakesh Vaid Department of Electronics, University of Jammu, Jammu 180006, India E-mail: rakeshvaid@ieee.org 412 D. ANAND, A. SINGH SAMBYAL, R. VAID molecular orbital model [20], and electron-cloud potential well model [21]. According to the band structure model, the movement of electrons is dependent on the energy disparity between the Fermi level of the metal and the valance band of the dielectric. This model does not work well when triboelectrification occurs between two dielectric materials which do not have well-developed band structures. The electron cloud potential well model states that the electrons have distinct energy levels where the electrons with a higher energy level tend to transfer from the other substance to the contacting atom. The molecular orbital model uses LUMO (lowest unoccupied molecular orbital) and HOMO (highest occupied molecular orbital) instead of using the conduction and valance bands as in case of semiconductor materials. Here, the electrons move as a result of the disparity in neutral levels between the surfaces of two dielectrics that are in touch. The method of electron transfer during the creation of tribolelectricity is still under investigation and depends on the selection of triboelectric materials according to various theories proposed so far. In this review paper, we have presented various material strategies to increase triboelectric power, triboelectric- series along with an analysis of the material selections for the different TENG structures investigated during the last decade. 2. VARIOUS MATERIALS TO ENHANCE TRIBOELECTRIC POWER The progress in materials for the four representative component layers has been discussed in this section. The various layers are: the charge generating layer, the charge trapping layer, the charge collecting layer, and the charge storage layer as shown in Figure 1. Fig. 1 Components of a TENG affecting triboelectric power generation [22] 2.1. Charge Generating Layer The charge density of the contact surface and TENG performance are directly proportional to one another [23] and can be increased to boost the electric output power. The primary factor used to determine the figure of merit is the square of charge density [24]. Several strategies have been reported from the perspective of materials [25] and in each of these methods; the contact area is enhanced through the production of nanostructures to raise the surface charge density, or through the doping of surfaces to improve the triboelectric effect, or through the use of molecular synthesis to produce novel materials. A simple way to boost the overall charge created is to increase the contact surface area. Several techniques, such as force assembled colloidal arrays [26], lithography [27], anodic aluminum oxide [28], block- copolymer assembly [29], and surface nano material production [30–31], have been reported A Critical Review on the Material Aspects of Triboelectric Nanogenerators (Teng) 413 for fabricating the microstructures. According to a recent study by Lee et al. [32], an increase in adhesion energy enhances the formation of surface charges, as can be seen in Figure 2 (a). PDMS (Poly dimethyl siloxane) is known to be a silicon-based organic polymer with the chemical formula CH3 [Si (CH3)2O]n Si (CH3)3, having many chains which can be clearly seen from the FESEM images, where n is the number of repeating monomers [SiO (CH3)2] units. To prepare a solution of PDMS, the sylgard 184 elastomer and its curing agent are mixed thoroughly in 10:1 proportion and further to make the solution more dilute, 1 ml of methyl chloride can be added in the prepared solution and then is kept in vacuum for about 30 minutes to remove air bubbles. PDMS layer can be deposited on the surface of a substrate by spin coating where layers of different thickness can be deposited by varying rpm, time and acceleration. On the substrate, micro pillars of PDMS are created, and a PDMS film is employed to cover their top. The PDMS thin film stores the applied mechanical energy during the contact and separation processes due to its significant elastic deformation. As a result, more charge can be generated on adhesive surfaces than on the non-adhesive surfaces. Triboelectricity can be produced when two surfaces come into contact with different potentials, which supports the idea that one of the main components of sticky surfaces is fictionalization of the material surface. (a) (b) (c) Fig. 2 Power enhancement of devices using charge generating layer (a) Increased output using pillar based structure [32]; (b) Ionized-air injection [33]; (c) Ferroelectric polarization and dielectric properties for boosting power [34] The main techniques for altering the surfaces in contact are plasma treatment and ion- doping radical injection, and both these techniques have a considerable impact on charge creation [35–38]. The flow of electrons from aluminum surface to FEP [fluorinated ethylene propylene] surface increases when FEP is exposed to negatively ionized air [33] [see Figure 2 (b)]. A change in the intrinsic material properties, such as dielectric constant, polarity, work- function, etc., can be sought through the processes of nanocomposite formation, electrical poling, chemical doping, and material synthesis. Polyvinylidene fluoride-co-trifluoroethylene, one of the well-known ferroelectric polymers, can also be polarized utilizing an electrical poling procedure [39]. The positive triboelectric series suggested by Lee et al., [40] indicates that PVDF-TrFE after electrical poling outperformed on human skin. On electrical poling, the charge generation has been increased by about 18 times with PVDF-TrFE and high K barium titanate (BaTiO3) nanoparticles as compared to pure PVDF-TrFE [41] as can be seen from Figure 2(c). 414 D. ANAND, A. SINGH SAMBYAL, R. VAID 2.2. Charge Trapping Layer In this section, we will present current developments in the materials to block the charge. Combination charge traps are produced by a variety of physical and chemical flaws, including dangling bonds, cross-linking sites, amorphous free volumes, and functional groups in the polymer chains [42–43]. Since their chains contain aromatic rings, several polymers, including polystyrene (PS) and polyimide (PI) have a lot of trapping sites [44]. When an aromatic interlayer is added between the PVDF layer and the collector electrode, the triboelectric output increases by about 7-9 times when compared to the device without an interlayer [45] as shown in Figure 3 (a). On the other hand, PDMS contains larger and deeper charge traps than aromatic polymers as can be seen from Figure 3 (b). The inclusion of a PDMS thin film beneath the charge-generating layer can boost the power density of TENG devices by more than 100 times. The addition of a PDMS layer further improves the device stretchability [46]. It has also been shown that the performance of a TENG [47] and its function as charge reservoir [48] can be significantly improved by the addition of Titanium Oxide (TiOx) and Indium Zinc Oxide (IZO) interlayers. When compared to thin film interlayers, more charge trapping can be achieved by using nanomaterials with greater surface-to-volume ratio. The power density of TENG devices can be increased by the monolayer of MoS2 by acting as an electron acceptor [49]. Similar to this, composites made up of reduced Graphene Oxide nanosheets can effectively trap electrons by improving output performance [50]. In order to create the high performance textile TENG as depicted in Figure 3 (c), paintable and coatable (a) (b) (c) (d) Fig. 3 Power enhancement using charge trapping layer (a) Trapping of Triboelectric charges with/without the PS dielectric interlayer [45]; (b) Mechanism of trapping charge with a PDMS layer [46]; (c) Trapping of electrons using black phosphorous layer coated with hydrophobic nanoparticles [51]; (d) Increasing of charge trapping and charge induction by using silver nanowires [52] A Critical Review on the Material Aspects of Triboelectric Nanogenerators (Teng) 415 composites of black phosphorous (BP) ornamented or coated with hydrophobic cellulose oleoyl ester nanoparticles are being used as an electron trapping ingredient. In case of PVDF nanofibers, formation of a β-Crystalline phase in the PVDF chains takes place by the addition of silver nanowires (AgNW’s) resulted in the induction of charge trapped at the metal- dielectric interface. This process increases the output performance of TENG devices [52] as demonstrated in the Figure 3 (d). Because of synergetic effects by incorporating nanocrystals of metal organic frameworks (MOFs) [53] and Titania monolayer (Ti0-87O2) [54] into the charge generating layer, there can be an increase in the total output power of a TENG device. 2.3. Charge Collecting Layer When two dielectric surfaces come into contact with one another, the generated charges are propelled electrostatically to flow from one electrode to the other. When an electrode makes contact with a dielectric, it simultaneously acts as a charge-generating layer and a charge-collection layer [35]. Metals [55] Graphene [56] and ITO [57] are just a few examples of the stiff and flexible electrodes that have been used during the fabrication process. The methods used to create deformable electrodes and their applications in stretchy TENG are covered in this section. Most stretchy devices [58–62] use buckling electrodes and in-plane serpentine metal extensively. Such wavy metal electrode can’t be used in the fabrication of TENG devices due to the enormous dimensions and application of significant mechanical force. Alternative to these electrodes are the conductive elastic nanocomposites because they have a very low cost of printing and highly stable under repeated mechanical impacts and frictions [63-65]. To fabricate deformable TENG devices [66-67], composites made up of metal (Au) nanosheets are embedded in a PDMS matrix thus used as a charge collecting layer in TENG structure (refer Figure 4 (a) [68]). As shown in Figure 4 (b), the output voltage has (a) (b) (c) (d) Fig. 4 Charge collection using stretchable materials (a) Stretchable electrode made by embedded gold nanosheets in the PDMS matrix; (b) Voltage output by applying cyclic elongation strain [68]; (c) Working of device using a liquid metal electrode; (d) A high stretchable device [72] 416 D. ANAND, A. SINGH SAMBYAL, R. VAID been maintained constant for elongation strain (30%) across 10,000 iterative cycles. Liquid metals can also be used for very stretchy electrodes [69–71] due to the fluidic nature of elastomeric matrix and can be injected into a tiny channel created in it at room temperature as shown in Figure 4 (c). A single electrode TENG [72] as depicted in Figure 4 (d) is highly stretchable and malleable without rupturing the electrode. Stretchable charge collectors for a TENG has been created by combining (PEDOT:PSS), or Poly (3.4-ethylene dioxythiophene): Poly (styrene sulfonate), with physiological saline, one of the liquid state conducting polymers [73-74]. 2.4. Charge Storage Layer Electricity generated by a TENG can be used in the form of a power supply by employing a capacitor and a bridge rectifier circuit for converting AC into DC [75] and by installing a rechargeable battery at its output terminals as demonstrated by Nun et al. where high conversion efficiency has been reported for a TENG with the use of a Lithium ion battery (LIB) [76] as shown in Figure 5 (a). For triboelectric energy charging, various materials have been utilized which includes Li3V2(PO4)3 [76], LiFePO4 (LFP) [77-78], LiMn2O4 (LMO) [79] and LiCoO2 (LCO) [80] and provide a very high efficiency of the order of ~ 80%. Li-ion battery requires a current in milli-amperes (mA) range for charging and discharging [80] and need a coil transformer with high amplitude output AC current. However, the direct charging of a battery is not possible due to very low current density to overcome the charging energy barrier. Thus the energy output of a TENG can be stored in capacitors and can be fabricated by inserting a dielectric layer between two electrodes. The dielectric capacitance is expressed as: dKC /0=  (1) where, K = Dielectric constant, ε0 = Vacuum permittivity, A = Electrode area and d = Dielectric thickness. (a) (b) Fig. 5 Storing charge in TENG: (a) Combination of TENG and LIBs based on Li3V2(PO4)3 /C nanocomposites [76]; (b) Dielectric with a PVDF-TrFE used for storing and releasing the electrostatic charges [84] A Critical Review on the Material Aspects of Triboelectric Nanogenerators (Teng) 417 The performance of a capacitor is characterized by its dimensions and inherent permittivity. To achieve a high-power density; fast charging and discharging, inorganic dielectric materials can be utilized. Since these inorganic dielectric film capacitors have very poor flexibility; the growth of dielectric materials on metal foils as well as on flexible organic substrates has been investigated [81-82]. Liang et al. has grown a film of ITO on Fluorophlogopite (FMica) and can be used as a bendable and a high temperature resistant to grow a BZT [Barium Zinc Titanate] layer of dielectric on ITO [83]. A capacitor integrated TENG was reported by Chung et al. in which PVDF-TrFE was used for storing electrostatic charge released due to Leyden jar effect [84] as shown in Fig. 5 (b) which clearly indicates that the charging of dielectric layer (green) takes place through the triboelectric layer (blue) in between a TENG electrode (gray) and a capacitor electrode (light gray). A large current (~4.3mA) was generated when the metal comes in contact with TENG electrode and the metal to metal contact released the stored charges. 3. TRIBOELECTRIC SERIES After going through a triboelectrification process using a reference material, the surface charge density of the materials determines their triboelectric effects. Triboelectrification can be based on sliding or contact separation processes. Same materials with different triboelectrification have different surface charge densities. PTFE is the most frequently used triboelectric material in current TENG experiments due to its high electron-accepting capabilities. Based on the affinity to electrons, various materials are arranged in a series in which the materials are having different charge densities after the triboelectrification process. The first triboelectric series was published in 1757 by Swedish physicist Johan Carl [85]. At the Georgia Institute of Technology, Wang's team in 2019 created a novel technique [86] that could measure the triboelectric charge densities (TECD) of various materials using mercury- based triboelectrification in the contact separation mode. Based on TECDs of materials, a new series was developed which is more reliable because of precise control over electrification process. With quantified charge densities, another triboelectric series [87] was developed based on sliding mode. One of the most important parameters that influences the output of a TENG is the displacement current which is totally different from the displacement current represented by Maxwell equations. The electric displacement vector in Maxwell equations is represented as, D = ε0E + P where E = Electric field and P = Polarization vector. The surface charges that are generated in a TENG are independent of the external electric field [88]. This in case of TENG a new term needs to be added and hence the displacement vector can be rewritten as: 0 sPD = ++ (2) The expression for displacement current density in case of TENG is written as: D 0J εs sP PD E T T T T    = + = +     (3) In case of a short circuit the current density can be written as: [89] 418 D. ANAND, A. SINGH SAMBYAL, R. VAID 0 0 1 2 2 20 0 0 01 2 1 2 1 2 1 2 1 2 1 1 ( ) ( ) T D T ddH J d d H dt dt d d Z d d Z              = + + + + + + (4) where αT = Surface charge density, H = Function of time, dH / dT = Contact-separation of two media in TENG, d1, d2 = Thickness of two media, ε0 = Vacuum permittivity, ε1 and ε2 = Permittivity of two media, Z = Gap between two triboelectric layers. When the surface charge density built up after few cycles the second term in equation (4) becomes negligibly small and hence can be neglected. This equation (4) can be written as: 0 0 1 2 20 01 2 1 2 1 2 1 ( ) D T dH J d d dt d d Z          + + + (5) The output current “I” of a TENG can be represented as: / , .D DJ J=      (6) 0 0 1 2 20 01 2 1 2 1 2 1 . ( ) T dH d d dt d d Z            + + + (7) 4. CHOICE OF MATERIALS FOR A TENG Triboelectrification depends on the movement of electrons between two materials that are in close proximity to one another. The ability of two materials to transport electrons between one another is determined by their electron affinities. Higher electron-affinity material will draw electrons and thus act as an electron donor. The material is electron donor or, electron acceptor depends upon its position in the triboelectric series. In this section, summary of various materials has been mentioned on random basis. Figure 6 shows various materials acting as electron acceptors and electron donors respectively. A total of fourteen different materials acting as electron acceptor and a total of 20 different materials acts as electron donor. The most popular materials for creating triboelectric layers and electrodes are Al, Cu, Ag, and Au. Figure 7 depicts the networking that occurs when electron acceptors and donors from various articles are paired. 14 donors are partnered with PTFE, 10 donors with PDMS, 6 donors with FEP, and 14 donors with Kapton. The materials used as 4, 3, and 2 donors are PET, silicone, polystyrene (PS), and cellulose respectively. The most extensively utilized triboelectric materials are PTFE, PDMS, and FEP because they have a high electron attracting affinity and therefore more negative materials. In contrast, Kapton, PET, and silicone are less common since they have a low electron attracting affinity. Recently cellulose materials have drawn attention due to their capacity for mass production and sustainability. TENG have been made using a variety of cellulose materials, including nano and micro cellulose [89–90], regenerated cellulose, and modified cellulose [91]. Because they have a more transparent mechanism than other polymers, several inorganic materials like graphene [92], MoS2 [93], and WS2 [94] have also been employed in the fabrication of TENG. Specific material selections based on low coefficients of friction and high electron acceptivity, PTFE and FEP are the materials that are most frequently employed A Critical Review on the Material Aspects of Triboelectric Nanogenerators (Teng) 419 for lateral sliding mode and freestanding triboelectric layer mode. Many other pairs such as PTFE – nylon [95], PTFE-Al [96-97], FEP-Cu [98 -99] and PTFE-skin [100] are used for lateral sliding mode and freestanding triboelectric layer because of their low co-efficient of friction. The commercialized application of TENG involves building of ultrasensitive sensors, micro-electromechanical devices, self powered systems, wearable electronics etc. to meet the high energy requirements. 4.1. Material Choice Evaluation for TENG One of the most important factors for selecting a pair of triboelectric material is the difference in their charge affinity which ultimately depends upon the separation of materials in the triboelectric series i.e., how far the two materials are from each other. If this principle is followed then PTFE-Nylon is one of the best pairs for TENG fabrication. There are also many other factors that influence the output performance of TENG devices. This can be explained clearly from the equation as under: 1 2 0 0 1 23 20 01 2 1 2 1 2 1 ( ) TP dH P dt P d d d d Z         = = = + + + (8) The first Part P1 of equation (8) is the surface charge density αT which depends upon various factors such as materials chemical composition, elasticity in case of contact separation mode and co-efficient of friction in case of sliding and freestanding triboelectric layer mode. Part P2 represents dH / dT which describes the rate/ speed of contact-separation and sliding in case of these two modes of operation. Electrostatic induction is the third part represented by P3. In this part, the main impact is of permittivity on the induced charge on the back electrodes. Fig. 6 Fraction (%) of the electron acceptor and donor materials 420 D. ANAND, A. SINGH SAMBYAL, R. VAID Fig. 7 Pairing of electron acceptor and donors 4.2. Donor of Electrons 4.2.1. Metals In case of metals, the term d2(0/2) is Zero because d2 = 0, that’s why metals are widely used for fabricating TENG devices. Thus the current density JD can be reduced as: 0 0 1 2 201 2 1 1 1 ( ) D T dH J d d dt d Z        + + (9) and, equation (9) can be rewritten as: ' 0 0 1 23 201 2 1 1 1 ( ) P d d d Z      = + + (10) Equation (9) still works even if we assume that there is a thin layer of triboelectric material on the surface of metal. It is because the metals have very high relative permittivity as in case of copper the value of relative permittivity is as high as > 250,000 with such a high value the term d2(0/2) = 0, thus equation (10) is still valid. When different metals are used as electron donors, the plots of the value of P'3 in equation (10) for various materials versus the gap distance are shown in Figure 9 (a), whereas Figure 9 (b) shows the plots of the value in equation (8) versus the gap distance for various material pairs using PTFE as the electron acceptor. According to the above plotted data, those metals are ideally utilized as electron donors in pairs of triboelectric materials based on electrostatic induction. However, as depicted in Figure 8, electrostatic induction is the only one component of current density, and it is yet unknown about how big contribution it makes. A Critical Review on the Material Aspects of Triboelectric Nanogenerators (Teng) 421 (a) (b) Fig. 9 (a) Variations of P’3 for different metals used as electron donors, 9 (b) shows the variations of P3 using PTEE as the electron acceptor 4.2.2. Acceptors of Electrons The popularly used materials that act as electron acceptors are PTFE [101], FEP [102] and PDMS [103]. Many studies done on polymers suggests that their output performance is very low because they show very less triboelectric effects against most commonly used electron donors. Triboelectric effect of such polymers is not strong against Mercury used TECD. PET has an extremely low TECD value of 101.48 µc/m2 against Mercury, which is close to PTFE's -113.06 µc/m2. Another test indicated that PET's charge density was 1.09 µc/m2, which is significantly lower than PTFE as -27.5 µc/m2 where PET was laterally dragged across a copper board. According to Fig. 8, P1 stands for the surface charge density T, which causes the triboelectric effect and depends on a number of factors, including the affinity of the charge, interaction between the several triboelectric layers, mode of operation or interaction, mechanical qualities, surface geometry, etc. The various materials used for TENG fabrication along with their output parameters are shown below in Table 1. Table 1 Variations of various parameters with different triboelectric materials [104] Positively Charged Materials Negatively Charged Materials Working Mode Output Power (mW/cm2) Charge Density (µC/m2) PET Kapton Vertical-contact 0.00036 - PET PDMS Vertical-contact 0.00234 - Al PDMS Vertical-contact 3.56 - Au PDMS Vertical-contact 31.3 594.2 Nylon PTFE Lateral-sliding 0.53 59 Al FEP Vertical-contact 31.5 240 Al PVDF Vertical-contact 0.26 360.2 Cu PTFE Freestanding 50 323 Al ZnSnO3-PVDF (composites) Vertical-contact 3 101.3 Cu Kapton Lateral-sliding 13.2 - Al PDMS Vertical-contact 46.8 270 Al PVDF-Gn Vertical-contact 2.6 23 422 D. ANAND, A. SINGH SAMBYAL, R. VAID 5. CONCLUSION The usage of various materials in the fabrication of TENG devices is due to the phenomenon of triboelectrification that occurs at the interface when two triboelectric materials come into physical contact with each other. A number of materials have been tested and used in TENG fabrication amongst which Fluoro-polymers such as PTFE and FEP are the most widely used materials utilizing the free standing triboelectric and lateral sliding modes. These materials are the most commonly used electron acceptors whereas the most commonly used electron donors are Aluminum and Copper and has been explained quantitatively by using the recently developed equations which clearly indicates an increase in the electrostatic induction with the use of metals. Although, the selection of various materials for a particular TENG device has been investigated still many materials remained unexplored such as various green and functionalized materials, inorganic and composite materials. This review paper summarized the various material choices along with different types of triboelectric pairs that could be employed to contribute optimized displacement current density JD for TENG structures. 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