The TENG can operate in four different working modes with a wide range of material availability. The figure of merits (FOM) was proposed in 2015 by Zi et al. to create a common standard for the comparison of different TENGs [28]. The figure of merits and factors affecting the TENG output are summarized in Figure 1.6, where the performance figure of merit (FOMP) for TENG comprises of a material FOM (FOMM) and structural FOM (FOMS). The FOMP can be considered as a standard to evaluate the performance of the TENG. The relationship of FOMS for different working mode is contact freestanding triboelectric (CFT) > CS > sliding FT (SFT) > LS
> SE contact mode (SEC) [28]. The only material related parameter is surface charge density (σ).
The dimensionless FOMS is given as
𝐹𝑂𝑀
𝑠=
2∈0𝜎2𝐴 𝐸𝑚
𝑥𝑚𝑎𝑥 (5)
Where A is the area of triboelectrification in TENG and
∈
0 is the permittivity of the vacuum. The material properties and device size are not considered in the FOMS. The FOMP is given by
𝐹𝑂𝑀
𝑝= 𝐹𝑂𝑀
𝑠. 𝜎
2(6)
The σ2 is the FOMM. To quantify the triboelectric performance of a material, a dimensionless material FOM (FOMDM) was also proposed. For the purpose, the charge density of the material under test can be measured with a liquid metal-like Galinstan. The FOMDM is given as
11
𝐹𝑂𝑀
𝐷𝑀= ( 𝜎
𝑁)
2=
𝜎𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙
𝑙𝑖𝑞𝑢𝑖𝑑 𝑚𝑒𝑡𝑎𝑙
⁄ 2
𝜎𝐹𝐸𝑃
𝑙𝑖𝑞𝑢𝑖𝑑 𝑚𝑒𝑡𝑎𝑙
⁄
2 (7)
Where FEP is the reference material. The negative
𝜎
𝑁 signifies that the material is more tribo positive considering the liquid metal and𝜎
𝑁>1 denotes that the material is more negative compared to FEP [10, 28]. The in-between value stands for more positive than FEP and more negative than liquid metal. The development of FOM can be treated as standards for the TENG performance, which can also act as a foundation for the industrialization of TENG based systems.Figure 1.6. The illustration of factors affecting the output of TENG and figure of merits for TENG.
12 1.6 Materials for TENG
Figure 1.7. Triboelectric series developed by AlphaLab (© 2009).
13 Almost all materials can be used for triboelectrification, ranging from metal, polymers, textile to wood, nanomaterials, etc. The various materials are arranged in the triboelectric series, depending on their tendency to lose or gain electrons. The triboelectric series thus can be used as a reference for the development of TENG. Recent efforts were made by John Wilcke and Harper to arrange the materials in the triboelectric series. Figure 1.7 shows the triboelectric series developed by AlphaLab (© 2009). The data was acquired at 22◦C and 35 % relative humidity (RH) by using a surface voltmeter (AlphaLab) [18, 29]. The materials at the opposite end or far apart in triboelectric series can be used for high performance device. The triboelectric series thus can be used as a reference for the development of TENG. In 2018, Seol et al. arranged the 2D layered materials MoSe2, MoS2, WSe2, WS2, graphene, and graphene oxide in the triboelectric series [30].
The triboelectric order was supported by the work function values of the materials. The triboelectric series of 2D materials is shown in Figure 1.8.
In 2019, Zou et al. proposed a standard method for the quantification of the triboelectric series. The triboelectric series of polymers were developed using the liquid metal as opposite contact under well-defined and controlled experimental conditions. The intrinsic ability of a polymer to either gain or lose electrons was derived by using the normalized triboelectric charge density (TECD) [31]. Figure 1.9 shows the quantified triboelectric series developed by Zou et al.
The factors like surface roughness, material functionalization, etc. also play a key role in the output of the TENG [32-34]. Various morphologies like nanowire, pyramid, square, and other micro-nano patterns are well reported to enhance the output of TENG. The temperature and humidity are other critical parameters that influence the output of TENG [34, 35].
14 Figure 1.8. Triboelectric series of 2D layered materials. Reproduced with permission [30].
Copyright John Wiley & Sons.
15 Figure 1.9. The quantified triboelectric series developed by Zou et al. Open Access [31].
16 1.7 Research progress of TENG
Since its invention in 2012, TENG is explored for a vast range of applications. In the past years, the TENG is incorporated with a more significant number of features, device designs, and materials, etc. The TENG, which was invented in 2012, operates in vertical contact-separation mode with two dielectric layers [3]. Transparent TENG device was also proposed in the same year, which was of great significance in the field of optoelectronics [36]. The research proceeds with the development of a single electrode and lateral sliding mode in the year 2013 [19, 37-41].
Moreover, liquid-solid contact, human skin as one layer also emerge for different applications [42].
During the same time, the TENG was combined with other energy harvesters (piezoelectric, electromagnetic, etc.) for the development of hybrid devices with the advantages of both the devices [43]. Later on, in 2014, freestanding mode TENG was proposed [27, 44, 45]. The four modes of TENG were explored experimentally and theoretically at that time. One of the exciting areas that were developed is tribotronics, i.e., the use of TENG to control the FET [46]. The research was followed by the development of textile-based and implantable devices [47, 48]. The TENG produces AC output, which needs to be converted to a DC signal for practical use. The issue was solved by the development of DC-TENG, which eliminate the use of a rectifier [49]. In the year 2015, shape-memory alloy based self-healable TENG was demonstrated to enhance the durability and life of the device [50]. A biodegradable TENG for in-vivo energy harvesting was developed in 2016 [51]. The progress was continuing with the concept of bionic TENG and wireless power delivery in 2018, which further extends the application of TENG [52, 53].
Harnessing water wave energy, i.e., blue energy becomes a growing area for TENG. In the year 2018, the triboelectric series of 2D layer material was developed [4, 30]. The significant development in the current year starts with quantified triboelectric series of non-metallic materials.
17 During the development, the power density of the TENG was significantly improved to 2.67 kW/m2 from 3.67 W/m2. The research roadmap of TENG is summarized in Figure 1.10.
Figure 1.10. The roadmap summarizing the progress in the TENG.