Flexible Sandwich-Structured Foldable Triboelectric Nanogenerator Based on Paper Substrate for Eco-Friendly Electronic Devices

Abstract

Recently, as the use of wearable devices and the demand for eco-friendly energy have increased, many studies have been conducted on triboelectric nanogenerators (TENGs), which can economically harvest energy. Paper is considered a promising substrate and frame material that can be used to manufacture self-powered TENGs, owing to its flexibility, low cost, and accessibility. Herein, we present a sandwich-structured foldable paper-based TENG (FP-TENG) that comprises flexible materials and uses paper as a substrate. The FP-TENG can generate up to 572 mW/m² of power via contact–separation of the triboelectric electrified body at the top and bottom. With more folds of the FP-TENG, the triboelectric cross-sectional area increases, and, thus, the electrical output increases. In addition, the proposed TENG exhibits excellent durability without signal degradation under 5000 cycles of repeated pushing motions. To demonstrate its practicality, the FP-TENG was manufactured in the form of a wristwatch Velcro and connected to an electronic watch panel to supply power. Various deformations are possible with origami, and they can drive wristwatches through external forces. Therefore, the FP-TENG is expected to be utilized as a sustainable and promising eco-friendly energy source for small electronic devices.

1. Introduction

Owing to the recent rapid development of portable electronic products and mobile devices, a technology capable of providing an efficient power supply, such as for batteries and wireless devices, is required. An inherent power supply provides portability and convenience, which can help humans considerably in their daily lives but affects the environment due to wasted thermal energy. In addition, with the depletion of finite energy resources, such as coal and natural gas, as well as increasing environmental issues, the market for eco-friendly electronic products is continuously growing. Therefore, energy-harvesting technology with natural energy sources is in the spotlight. Energy-harvesting technology produces electrical output from various mechanical energies, such as piezoelectric, electromagnetic, and triboelectric effects. Among these, the triboelectric nanogenerator (TENG), based on the combined effects of electrostatic induction and contact triboelectric charging, is a power source that can efficiently harvest energy via friction between two materials with different electron affinities [1,2]. TENGs are self-powered devices, independent of external power, that have attracted more attention than other generators due to their distinct characteristics, including their simple structure and fabrication process and high power generation efficiency. They can also be produced using a variety of materials. In addition, TENGs are cost-effective, flexible, lightweight, and environmentally friendly, making them suitable for a wide range of applications. However, most of them require complicated processes and expensive equipment, and their biodegradability and recyclability are limited.

To overcome this problem, a paper-based TENG that can be easily obtained and manufactured is being studied [3,4]. A paper-based TENG is a self-recyclable power supply that can supply sufficient power to portable electronic products through external pressure. Paper has distinct advantages, such as biocompatibility, flexibility, low cost, and easy disposal; therefore, it has been employed as a substrate for TENGs. Because paper is insulating, a conductive layer coating is required on the surface of the paper when it is employed as the substrate for a TENG. For this purpose, paper surfaces have been coated using metals such as Au or Ag, ink, and an airbrush, comprising metal nanoparticles that improve electrical conductivity [5,6,7,8,9,10]. In addition, acrylic, graphite, and PET have been used as triboelectric materials for paper-based TENGs [11,12,13,14,15]. Changsheng et al. have reported the fabrication of a TENG constituting a Cu-coated paper sheet with Kirigami patterns, harvesting energy from various types of motions owing to its shape-adaptive thin-film design [16]. Bhaskar et al. reported a wristband-structured TENG using recycled materials including plastic waste and paper wipes [17]. Paper is foldable; thus, it can be manufactured in various shapes. Further, the more the paper is folded, the larger the area is to which the external force is applied; thus, the frictional surface area increases. However, currently, folding is impossible because of the hard physical properties of the materials constituting most existing paper-based TENGs, or the method involving forming electrodes and electrification layers on each frictional layer after folding the paper [18,19,20,21,22,23,24]. This structure is weak in terms of durability to repeated external forces, and, thus, the TENG can be easily damaged, and energy generation is temporary. To compensate for this, a paper-based TENG should be developed that can be folded several times without deterioration in durability even under an external force.

In this paper, we propose a foldable paper-based TENG (FP-TENG) with a sandwich structure and improved durability that allows free deformation by combining flexible aluminum tape, polytetrafluoroethylene (PTFE), and Si rubber with paper. The FP-TENG was fabricated to have a sandwich structure with a high contact surface area to improve the total number of generated charges. The FP-TENG generates electric power by friction between two electrified objects and exhibits maximum power generation performance of 572 mW/m². In addition, the electrical characteristics and durability of the origami were analyzed, and the possibility of driving electronic devices for practical applications was confirmed. Based on these characteristics, the proposed TENG can be used as a power source for eco-friendly electronic devices and educational kits.

2. Experimental Details

As shown in Figure 1a, the FP-TENG has a sandwich-type structure, comprising 0.03-mm-thick paper as the top and bottom substrates and four flexible materials with sizes of 0.04 ± 0.02 mm. The insulating coating was attached to the paper using flexible aluminum tape. The aluminum (charge affinity of +10~30 nC/J) tape of the upper paper was regarded as an insulating material and a positive triboelectric material, with a length and width of 10 cm and a thickness of 0.03 mm. A copper wire was used as the electrode of the positive triboelectric layer. Si rubber and PTFE (charge affinities of −72 and −190 nC/J, respectively) of the lower paper were considered as the negative triboelectric materials. Si rubber is a negative triboelectric material with excellent elasticity and flexibility that induces electron flow and improves surface charge density. Therefore, to improve the output performance, contact area, and durability of the FP-TENG, the surface was coated with aluminum tape with a thickness of 0.03 mm. To coat the Si rubber, aluminum tape was attached to the paper and coated with the Si rubber solution. The electrode of the negative triboelectric layer was the aluminum tape attached to the paper. When the Si rubber hardened to a certain thickness, PTFE was attached in a striped pattern with uniform spacing. Because Si rubber and PTFE have a high negative charge affinity according to the triboelectric series, they can generate a large amount of triboelectric charge in the material, thereby improving the power generation performance of the TENG. Figure 1b,c show the FP-TENG fabricated with dimensions of 10 cm × 10 cm and a constant thickness of 0.2 ± 0.05 mm. Because the FP-TENG comprises flexible materials including paper, it can be deformed and folded freely.

The proposed FP-TENG was fabricated with a basic sandwich structure to improve the contact area. Figure 2 depicts the working mechanism of the FP-TENG when it is operated in the vertical contact–separation mode. Initially, no charge is present between the electrode and contact surface. When an external force is applied to the FP-TENG, the aluminum at the top and PTFE/Si rubber at the bottom come into contact and are charged positively and negatively, respectively (Figure 2i). When the force is released, the two triboelectric materials separate and return to their original shapes, and the opposite charges in each material are rapidly separated by voids that form a dipole moment. Electrons flow from the bottom to the top electrode until a potential difference occurs between the two electrodes, and the charges accumulate (Figure 2ii). When they are completely separated and reach an equilibrium state, there is no movement of electrons between the two substances (Figure 2iii). When an external force is applied to the top of the FP-TENG again, the two separated triboelectric materials come into contact with each other and the dipole moment decreases (Figure 2iv). Therefore, electrons flow in the reverse direction from the top to the bottom electrode. More specifically, the contact–separation process between aluminum and PTFE/Si rubber generates an instantaneous alternating current via an external load.

3. Results and Discussion

As shown in Figure 3, the electrical performance of the FP-TENG (area of 100 cm²) was evaluated. The output voltage was measured using an oscilloscope (MSO9104A) with an internal impedance of 1 MΩ, and the output current was measured using a precision source/measurement device (B2911A). Because the FP-TENG comprises a flexible material, it can be folded, as depicted in Figure 3a. The more it is folded, the wider is the cross-sectional area to which the force is applied. As shown in Figure 3b,c, when a force of approximately 1 kgf was applied, the open-circuit voltage (V_OC) was 386 V and the module short-circuit current (I_SC) was 60 µA. As shown in Figure 3d,e, in the same experimental environment, the double-folded FP-TENG had a voltage of 456 V and a current of 75.8 µA. In addition, as shown in

Table 1. Comparison of existing triboelectric nanogenerators using paper.

Ref.	Electrode	Triboelectric	Typical Performance
			ISC (μA)	VOC (V)
[8]	Conductive ink	PTFE tape	72	218
[9]	Conductive ink	Cardboard, PTFE	43.6	292.5
[11]	Copper	Crepe cellulose, NCM	45	103.2
[13]	Copper wire	Graphite, PET	75.6	69.8
[15]	Copper foil	B1 powder, Teflon	46.3	340
[21]	Copper	FEP	0.00264	7.32
[25]	Copper	Graphite, Teflon	3.75	85
This work	Copper, aluminum	Aluminum, PTFE/Si rubber	75.8	456

, the proposed FP-TENG exhibited better output performance than the previously reported paper-based TENG. As the frictional surface area increased when the paper was folded, the double-folded FP-TENG exhibited more than 1.5 times the output performance. Because PTFE and Si rubber are negative triboelectric materials, the output was compared according to the number of PTFE patterns to obtain the optimal output performance.

In a conventional TENG, a spacer is formed between the positive and negative triboelectric materials to improve the output performance. In this study, PTFE was fabricated in a striped pattern without an additional spacer to improve the flexibility; thus, the TENG thickness (0.2 ± 0.05 mm) could be reduced. Because of its high negative charge affinity, PTFE generates a large amount of electrostatic charge when in contact with friction materials. Figure 3f,g show the output voltage and current values of the FP-TENG fabricated by varying the number of PTFE stripe patterns, respectively. The output voltage and current for two, four, six, and seven patterns of PTFE were measured, and it is apparent that the output increased as the number of patterns increased. Therefore, the FP-TENG was fabricated with seven patterns of PTFE. As shown in Figure 3h, to evaluate the detailed output performance of the proposed FP-TENG, the voltage and current according to the contact–separation operation were measured at loads of 10–10¹⁵ Ω. As the load resistance increased, the output voltage of the FP-TENG increased and became saturated after 10¹⁰ Ω. On the contrary, the output voltage reduced by Ohm’s law. The output power density was calculated using P = I²R. The maximum output power density can be obtained when the load resistance is equal to the internal impedance of the FP-TENG. As shown in Figure 3i, a maximum power density of 572 mW/m² was observed at a load resistance of 10⁶ Ω.

The output value of the FP-TENG was not constant due to the shape deformation of the friction material (PTFE stripe pattern/Si rubber) when cut. However, by adjusting its durability, as paper is easily torn, and by changing the ratio of Si rubber, the FP-TENG can be expected to generate a sufficiently stable electrical output.

Because paper can be relatively easily wrinkled or torn, it is important to ensure its durability and stability when it is used as a frictional material or substrate for TENGs. The FP-TENG generated a maximum voltage of 386 V and a current of 60 µA when an external force was applied; however, it exhibited an error of up to 50 V and 15 µA depending on human motion. As shown in Figure 4a, the output voltage and current values from repetitive contact–separation motions in the FP-TENG were measured using a pushing tester. As can be seen in Figure 4b, by applying a force of approximately 0.1 kgf for 5000 cycles, the output voltage exhibited an error of up to 1.6 V. In addition, from Figure 4c, the output current had a low error of up to 0.7 µA in the same experimental environment. Thus, the FP-TENG exhibited excellent mechanical durability and stability as it had a constant signal output without significant degradation of the electrical output.

As depicted in Figure 5, the applications of FP-TENG were demonstrated by operating a wristwatch and turning on LEDs. As can be seen in Figure 5a, the FP-TENG was manufactured as a watchband and connected to an electronic watch panel such that it could be worn on the wrist. The FP-TENG watchband could continuously drive the watch when an external force was applied. It supplied power to the electronic watch face via hand tapping. In particular, the watch was operated using a 2.2 µF capacitor and bridge circuit. The alternating current output by hand tapping was rectified into a direct current (DC) by the bridge circuit. In addition, as shown in Figure 5b, a pear shape was realized via origami owing to the foldable characteristics of the FP-TENG, and voltage was generated by an external force. Furthermore, the origami was achieved as shown in Figure 5c, and LEDs connected in series were turned on by folding and unfolding motions. Finally, as shown in Figure 5d, the FP-TENG operated up to 96 LEDs under an external force. Thus, we observe that the FP-TENG generates sufficient power from external forces to drive electronic clocks and LEDs, and its flexible characteristics make inherent origami possible. Therefore, it can considered a promising power source for eco-friendly wearables and portable devices and can also be used as an educational tool for demonstrating the power generation principle of triboelectric devices [26].

4. Conclusions

In summary, we fabricated a sandwich-structured FP-TENG that used paper as the substrate, PTFE/Si rubber as the negative triboelectric layer, and aluminum as the positive triboelectric layer. The FP-TENG generated up to 572 mW/m² of power, and, owing to its flexibility, the frictional surface area increased when it was folded, resulting in an increase in the output by a factor of 1.5. The results proved its excellent durability without a degradation in the output during 5000 cycles of pushing motion. To evaluate the performance of the FP-TENG, a wristwatch and 96 LEDs were operated using the generated power, and the electrical output performance using origami was demonstrated. The new structure and practical application potential of this environmentally friendly TENG using paper, a natural material, were demonstrated.