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130 Y. Xie et al. energy from ambient environment [1–3]. Among various energy sources, mechanical energy is one of the most effort-attracting candidates because it universally exists in our living environment, but usually goes to waste. In this regard, nanogenerator (NG), which utilize nanostructures for converting environmental mechanical energy into electricity, has been developed as a very effective and practically-applicable technology since 2006 [4–6]. Recently, along the progressive track of this area, triboelectric nanogenerator (TENG) has emerged as a promising sub- category based on the conjunction of triboelectrification and electrostatic induction [7–10]. With the numerous advantages such as simple fabrication, low cost, light weight, small volume and high efficiency, TENGs are not only becoming unprecedented technology for mechanical energy conversion, but also showing a great potential as self-powered active sensors [11–14]. Until now, two funda- mental working modes have been established for TENGs – vertical contact-separation mode and in-plane sliding mode with distinct characteristics and different application areas [15–18]. Compared with the vertical contact-separation mode, the in-plane sliding mode enables more compact structures, and is more effective for triboelectric charge generation and transfer, thus more efficient for charging energy storage units. Moreover, the electricity generation efficiency based on this mode can be easily enhanced by introducing segmental or grating design to achieve multi- cycles of charge transfer in one sliding motion. As a typical structure, disk-based TENG was demonstrated to be a promising structure for harvesting rotation energy [19]. Since most types of irregular motions can be transformed into rotations by certain mechanisms, such disk TENGs are applicable for scavenging different forms of mechanical energy, similar to the electromagnetic-induction-based turbine engines. However, in the first design of the disk TENG, only one pair of triboelectric layers were driven to slide by rotation motion, which is inefficient for fully utilizing the input energy. Thus, it is highly desirable to realize multi-layered integration [20] on the disk TENGs, which requires two critical issues resolved in the structure design in order to reach a multiplied output performance: intimate contacts between any two adjacent triboelectric layers maintained during the rotation, under a minimized pressing for a small resistance; strictly synchronized rota- tion of all the segmentally-structured disk pairs so that the output from each pair can be perfectly added-up. Here in this work, we developed an effective strategy for the multi-layer integration of the disk TENGs, with the above two issues resolved. In this design, a D-shape shaft was introduced to coaxially transmit the rotation motion onto each rotor (rotating part in each pair of triboelectric layers), with the segmental phase synchronized. On the other hand, to maintain intimate contact of the tribo- surfaces, light weight and low stiffness springs were adopted to fix the stators at the four corners and provide a gentle and adjustable pressing force. Through a parallel connection of four integrated units into one multi-layered disk TENG, the device is capable of generating an enhanced short-circuit current density (Jsc) of 90.6 mA/m2 with a peak power density of 42.6 W/m2 (2.68 kW/m3), under an input rotating speed of 1000 rpm. Furthermore, when the device is coupled with a water turbine, the multi-layered TENG can effectively harvest water flow energy, producing a max- imum short-circuit density of 26.3 mA/m2 by scavenging the water flowing from an ordinary household faucet, which can be used as a direct power source for instantaneously lighting up hundreds of serially connected light-emitting diodes (LEDs). This work provides a significant progress for the power improvement and applicability of triboelectric nanogenerators for harvesting large scale mechanical energy, such as hydroelectric power and wind power. Results and discussion The multi-layered disk TENG is mainly composed of two groups: rotors with Al film as the attached triboelectric layers, and stators with the purposely chosen polytetra- fluoroethylene (PTFE) film as the other triboelectric layers. They are coaxial with the input rotation motion, as sche- matically depicted in Figure 1a. For achieving flat surfaces, acrylic sheets were used as template substrates for both of them. At first, they were processed by laser cutting to form eight-sector structures with through-holes in the center for the shaft connection, which were of the same shape as the attached triboelectric layers. For the rotor templates, the center-hole is of D-shape, strictly matching the D-shaft we used, so that they can be led by the shaft to rotate with little angular variations (Figure 1b). Both sides of a rotor substrate were deposited with Al films by e-beam evapora- tion, which serve as one side in a disk TENG unit. As for the stators, the center-hole is in round-shape, with the dia- meter a bit larger than the D-shape shaft. In this way, the stators will not rotate with the spinning shaft. On their surfaces, 50-mm-thick PTFE films with aluminum electrodes on the back side were securely attached. For the four-layer integration as depicted in Figure 1, the stator placed in the middle was double-side coated with PTFE. The multiple- layered structure was obtained by stacking three stators and two rotors in an alternating sequence, with the D-shape shaft going through all the center-holes. Each pair of PTFE film and aluminum film next to each other constitutes a disk-TENG unit. The stators were fixed together by 4 screws going through the holes in their four corners, with one screw fastened to a bracket to ensure their stationary during operation. In order to maintain intimate surface contact between adjacent tribo-layers, low-stiffness springs were installed on these screws to apply a well-controlled gentle pressing force between the plates. When the D-shape shaft is connected to a rotating object, e.g. a rotary motor in the experimental measurement, all the Al layers on the rotors will be driven to rotate with the same phase, while the PTFE layers on the stators will stay still. In this manner, the relative rotation between the adjacent layers enables periodic contact-separation cycles of the opposite tribo- electric charges, and thus generates electricity in each layer. When all of the four layers of the multi-layered disk TENG are connected in parallel, the current with the same phase can be added up, contributing to a multiplied power output. It is worth noting that, in order to enhance the surface roughness and promote the triboelectrification intensity, the aluminum films on the rotor part were coated with silver nanoparticles [21]. Figure 1c shows the SEM images of the assembled silver nanoparticles on Al film,PDF Image | Multi-layered disk triboelectric nanogenerator for harvesting hydropower
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