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Ultra‐Stable Electret Nanogenerator to Scavenge High‐Speed Rotational Energy for Self‐Powered Electronics

DOI: 10.1002/admt.201600233 ambient, high-speed rotational energy to sustainably power some small electronics/sensors. Figure S1a in the Supporting Information illustrates a schematic diagram of a typical TENG with multilayered structures, where the device consists of a rotator and a stator. The rotator consists of radially arrayed Cu sectors on a polytetrafluoroethylene (PTFE) substrate (Figure S1b). The stator includes a layer of fluorinated ethylene propylene (FEP) film as the triboelectric material, a layer of electrode-protection film, and a layer of two complementary-patterned Cu electrode networks on a printed circuit board (PCB) substrate, as depicted in Figure S1c. The TENG works under the relative rotational motion between the rotator and the stator. Figure S1d displays the output stability of the TENG with increasing continuous working time under different rotational speeds. Under a rotational speed of 1000 rpm, the output voltage of the TENG increased at the beginning from about 130 V to about 350 V in about 12 min, which is due to the contact-charging process. After the TENG was continuously working for about 20.5 min, a dramatic decrease of the output voltage was observed due to the triboelectric charging-induced heat, where the FEP film was seriously damaged and melted, as presented in Figure S1e. The TENG could only survive for about 22 min under the rotational speed of 1000 rpm. The lifetime of the TENG decreased rapidly with increasing rotational speeds, where the lifetime of the TENG was only about 6 min under a rotational speed of 4000 rpm. More serious structure damage and material melting can be observed in Figure S1f. These results indicate that this type of TENG cannot be utilized to scavenge high-speed rotational energy because the triboelectric effect-induced heat results both in a decrease of the output performance and in damaging of the triboelectric materials in the device. As displayed in Figure 1a, negative charges can be injected into the surface of the electret film by corona charging. Figure 1b illustrates a scanning electron microscopy (SEM) image of the electret film surface, showing that a large number of cracks with a width of less than 50 nm can be observed on the surface. The cracks are beneficial for the preservation of the negative charges, where the surface charge density of the electret film can be up to −0.3 mC m−2. Figure 1c shows a cross-sectional SEM image of the electret film, indicating that the electret film consists of multilayered polypropylene films with a total thickness of less than 100 μm, where each layer has a thickness of a few micrometers, as depicted in Figure 1d. Figure 1e displays that radially-arrayed electret film sectors can be achieved by using a laser engraving technique. As exhibited in Figure 2a, the fabricated ENG includes a rotator and a stator. The rotator consists of the radially-arrayed electret film sectors on an acrylic substrate, as shown in Figure 2b. Figure 2c Triboelectric nanogenerators (TENGs) have been the focus of the nano-energy scavenging community because of their unprecedented enhancement of the output power density from 3.67 mW m−2 to over 300 W m−2 in about 3 years.[1,2] By coupling contact charging with an electrostatic induction through the periodic contact/separation between two triboelectric materials with different triboelectric polarities, TENGs have been utilized to scavenge energy from various ambient mechanical motions, such as sliding motions, vibrations, and rotations.[3–9] Compared to conventional electromagnetic generators or piezoelectric nanogenerators,[10–13] TENGs exhibit several obvious advantages of small volume, light weight, low cost, and high output power, whereby the rotation-based TENGs have the largest output powers due to high working frequencies.[14–16] However, the output instability of rotation-based TENGs can largely hamper the real applications of these devices, resulting in that TENGs usually cannot survive for a long time, especially under high-speed rotation conditions, due to the triboelectric charging-induced heat. Currently, effective solutions to this issue are still lacking due to the difficulty of avoiding heat generation in the contact-charging process. These studies suggest that non-contact charging might be a possible way to solve the stability issue of these devices. Electrets are a kind of material that can contain some charges on its surface with a theoretical lifetime of hundreds of years.[17] These charges on the surfaces of the electret film can be created by using a noncontact charging process, which may be used for scavenging high-speed rotational energy. Here, we report an electret nanogenerator (ENG) that can be utilized to scavenge high-speed rotational energy by using electrostatic induction on the electret film. The fabricated ENGs exhibit an excellent stability of the output voltage/current over a continuous working period of 10 h or intermittent working period of 100 days at a high rotation speed of 4000 rpm. By using a power management circuit, the ENG can deliver a constant output voltage of 5 V and a pulsed output current of about 160 mA. The produced energy by the ENG can be utilized to directly power some small electronics or charge capacitors. Moreover, a self-powered flow speed or temperature-humidity sensor is demonstrated by using the ENG to scavenge the flow-induced mechanical energy. This research provides a new route for effectively harvesting