Structural and Chemical Modifications Towards High-Performance of Triboelectric Nanogenerators

Abstract

Harvesting abundant mechanical energy has been considered one of the promising technologies for developing autonomous self-powered active sensors, power units, and Internet-of-Things devices. Among various energy harvesting technologies, the triboelectric harvesters based on contact electrification have recently attracted much attention because of their advantages such as high performance, light weight, and simple design. Since the first triboelectric energy-harvesting device was reported, the continuous investigations for improving the output power have been carried out. This review article covers various methods proposed for the performance enhancement of triboelectric nanogenerators (TENGs), such as a triboelectric material selection, surface modification through the introduction of micro-/nano-patterns, and surface chemical functionalization, injecting charges, and their trapping. The main purpose of this work is to highlight and summarize recent advancements towards enhancing the TENG technology performance through implementing different approaches along with their potential applications. This paper presents a comprehensive review of the TENG technology and its factors affecting the output power as material selection, surface physical and chemical modification, charge injection, and trapping techniques.

Keywords: Chemical functionalization; Self-charging power systems; Surface micro-/nano-patterning; Triboelectric nanogenerator; Wearable electronics.

Fig. 1

a Schematic of triboelectric materials in series depending on their relative triboelectric charge attracting/receiving characteristics (Reproduced with permission from Ref [19].). b The atomic-scale-electron-cloud-potential-well model to describe the contact electrification process of a TENG (Reproduced with permission from Ref [21].)

Fig. 2

The four fundamental operation modes of TENG: a Vertical contact-separation mode (VCSTENG), b lateral sliding mode (LSTENG), c single-electrode mode (SETENG) and d freestanding triboelectric-layer mode (FSTENG)

Fig. 3

Tribo-positive polymers for the TENG fabrication: a the digital photographs of the AFR sample at various stages of preparation with (i) corresponding to as-prepared uncured sample deposited on an Al foil, (ii) corresponds to washed sample while (iii) and (iv) correspond to the dried and fully cured, hot-pressed AFR samples, respectively (Reproduced with permission from Ref. [31]), b a schematic diagram and a photo of the TENG using PEO and PDMS films as the positive and negative tribo-materials, respectively (Reproduced with permission from Ref. [33]), c a schematic diagram and a photo of three stacked PPy-based TENG (Reproduced with permission from Ref. [34]), d a schematic depiction of PPy NW preparation by electrochemical polymerization with AAO as the template and schematic diagram of the final TENG. (Reproduced with permission from Ref. [35])

Fig. 4

Fabrication of TENGs from eco-friendly materials: a CNF-based TENG. (Reproduced with permission from Ref. [37]); b a schematic of the fabrication process for CP biocomposite and interactions between the CA and PEI molecular chains. (Reproduced with permission from Ref. [38]); c a photo of the silk-based TENG and cross-section microscope image of its bottom layer. (Reproduced with permission from Ref. [39]); d the process for preparing IBTENG and the photos of friction layers and fabricated IBTENG. (Reproduced with permission from Ref. [40])

Fig. 5

TENGs based on regenerative natural substances: a lipid layers on natural regenerative surfaces. (Reproduced with permission from Ref. [44]); b a preparation of the biodegradable plant leaf and leaf powder-based TENGs. (Reproduced with permission from Ref. [45]); c TENG based on Chinese red rose. (Reproduced with permission from Ref. [46])

Fig. 6

Applications of 2D materials in TENG fabrication: a modified triboelectric series including 2D materials (Reproduced with permission from Ref. [48]); b a schematic of fabrication of 1L graphene-based TENG (Reproduced with permission from Ref. [52]); c a schematic of the roll-to-roll delamination of Cu and graphene onto EVA/PET substrate (Reproduced with permission from Ref. [53]); d a schematic of the conformal TENGs and images of the conformal TENGs attached on the palm (Reproduced with permission from Ref. [54])

Fig. 7

Applications of 2D materials in TENG fabrication: a a multilayer structure of the Al/PI/GO/PI/ITO/PET TENG (Reproduced with permission from Ref. [57]); b a structure of the designed S-TENG (Reproduced with permission from Ref. [58]); c a structure of the MXene-based TENG (Reproduced with permission from Ref. [60])

Fig. 8

Surface micro-/nano-patterning techniques: a a process flow of soft lithography (Reproduced with permission from Ref. [67]); b SE-TENG device schematic and fabrication (Reproduced with permission from Ref. [68]); c a process of the glass transition of the PS for obtaining nano-to-micro scale morphology (Reproduced with permission from Ref. [69]); d an illustration of the fabrication of the dome-shaped and pillar-shaped nanostructures by the replica molding process on the silicon wafers, and their surface taken by an atomic force microscope (Reproduced with permission from Ref. [70])

Fig. 9

Surface micro-/nano-patterning techniques: a a schematic fabrication of the overlapped microneedles arrayed PDMS and the assembly of Al/PDMS MN-TENG (Reproduced with permission from Ref. [71]); b SEM image of the patterned PDMS before plasma treated and the topography of the pattern PDMS film treated by plasma (Reproduced with permission from Ref. [72]); c diagrams showing TF-TENG with embedded AgNWs in the PEDOT:PSS layer and the enlarged cross-sectional view of PEDOT:PSS/AgNW layer on a substrate (Reproduced with permission from Ref. [73])

Fig. 10

Schematic representations of surface functionalized negative and positive PETs with adopted molecules. The PET/ITO substrates were treated by O₂ plasma to form hydroxyl (–OH) groups on the PET surface for strong hydrogen bonding with target molecules. a PET surfaces were functionalized with halogen (Br, F, and Cl)-terminated phenyl derivatives for negatively charged surfaces. b Aminated molecules, such as linear polyethyleneimine (PEI(l)), hexyltrimethoxysilane (HTMS), poly-L-lysine (PLL), 3-aminopropyltrimethoxysilane (APTES), and branched polyethylenimine (PEI(b)), were functionalized on the O₂ plasma treated PET surfaces (Reproduced with permission from Ref. [77])

Fig. 11

Chemical functionalization of TENG surface layer: a schematic representation of surface fluorinated modification of WRP by FOTS (Reproduced with permission from Ref. [78]); b schematics illustrations of the reactions of fluorination on a PET surface (Reproduced with permission from Ref. [79]); c a schematic of TENG based on the fluorinated polymers with different kinds of fluorine units (Reproduced with permission from Ref. [80])

Fig. 12

Chemical functionalization of TENG surface layer. a Fabrication process of TENGs from the thiol-SAM functionalized Au films (i) and from SAM surface functionalized SiO2 (ii) (Reproduced with permission from Ref. [82]). b Synthesis of PVDF-Gn graft copolymers and schematic diagram of the fabrication process for the PVDF-Gn-based TENG (Reproduced with permission from Ref. [83]). c Schematic diagram of polydopamine modification of paper (Reproduced with permission from Ref. [84]). d Schematic illustration of surface treatment of polydimethylsiloxane (PDMS) polymer film and a TENG device (Reproduced with permission from Ref. [85])

Fig. 13

Chemical functionalization of TENG surface layer. a Schematic depiction of the surface modification with fluorinated compounds of the PP nanowires prepared by hot processing technique (Reproduced with permission from Ref. [86]). b i—Scheme of functional high permittivity liquid doping PDMS composite; ii—Fabrication of high permittivity liquid doping PDMS composite and fluorine functionalization (Reproduced with permission from Ref. [87]). c FTIR spectrum and molecular structure of pristine CNF (top row), nitro-CNF (middle row), and methyl-CNF (bottom row). Photos of the transparent films fabricated from these three cellulosic materials (Reproduced with permission from Ref. [63])

Fig. 14

Charge injection techniques: a a process of the ion injection on the FEP film (Reproduced with permission from Ref. [89]); b a structure of the high-performance TENG and schematic image of the equipment used for charge injection (Reproduced with permission from Ref. [90])

Fig. 15

Different approaches applied for charge trapping. a) Illustrations of electrons’ drift in G-TENG and the schematic diagram of electrons’ escape from PDMS to the gold (Reproduced with permission from Ref. [91]). b Distribution of GO on a nanofiber and stored charge on the surface of a GO sheet (Reproduced with permission from Ref. [94]). c Illustration of the vertical contact-separation mode TENG with a MoS₂-monolayer film and a schematic diagram of the electron transfer from the PI layer to the MoS₂ monolayer (Reproduced with permission from Ref. [97]). d Schematic of the transport process of triboelectric electrons in the negative friction layer of a TENG and improvement effects of different composite friction layer structure (Reproduced with permission from Ref. [98]). e Illustration of the vertical contact-separation mode TENG with a PI:rGO film and a schematic diagram of electron transfer from the PI layer to the rGO sheets (Reproduced with permission from Ref. [99])

Fig. 16

Hybrid devices based on TENG. a Schematic illustration of the hybrid generator (Reproduced with permission from Ref. [105]). b Schematic diagram of the fabricated hybridized EM-TENG (Reproduced with permission from Ref. [106]). c Schematic illustration of the H-P/TENGs mounted in the custom frame and the enlarged structure of single H-P/TENG (Reproduced with permission from Ref. [107]). d Schematic illustration of a typical TENG structure and working mechanism of TENG (Reproduced with permission from Ref. [108]). e Schematic diagram of the TPENG and working principle of the triboelectric, and pyroelectric generators during interaction with a hot water droplet (Reproduced with permission from Ref. [109])

Fig. 17

Applications of TENGs for mechanical energy harvesting: a a flexible TENG harvesting small mechanical energy from different parts of the body (Reproduced with permission from Ref. [113]); b schematic diagram of the integrated hybridized NGs on the roof of a house model (Reproduced with permission from Ref. [114]); c TENG for harvesting environmental wind energy (Reproduced with permission from Ref. [115]); d a self-powered backpack on the basis of a TENG with integrated rhombic gridding for harvesting the vibration energy from natural human walking (Reproduced with permission from Ref. [116])

Fig. 18

Different applications of TENG: a a sensor array for self-powered static and dynamic pressure detection and tactile imaging (Reproduced with permission from Ref. [118]); b a self-powered ammonia nanosensor developed from conducting polyaniline nanofibers (Reproduced with permission from Ref. [119]); c self-powered detection sensor and electrochemical degradation of phenol utilizing β-cyclodextrin enhanced triboelectrification (Reproduced with permission from Ref. [120]); d a self-powered endocardial pressure sensor implanted into a swine’s heart (Reproduced with permission from Ref. [121]); e a diagram of an in vivo self-powered prototype of a pacemaker from the breath of a living rat (Reproduced with permission from Ref. [122]); f a pulse sensor based on SETENG (Reproduced with permission from Ref. [123])