Yanhong
Dong
ac,
Yange
Feng
*ab and
Daoai
Wang
*abc
aState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail: fengyange@licp.cas.cn; wangda@licp.cas.cn
bShandong Laboratory of Advanced Materials and Green Manufacturing at Yantai, Yantai 265503, China
cQingdao Center of Resource Chemistry and New Materials, Qingdao 266104, China
First published on 27th May 2024
Preparing nanostructured surfaces has been considered an effective method to improve the output of triboelectric nanogenerators (TENGs), but how to quickly prepare materials with a nanostructured surface for TENGs has always been a challenge. Here, polypropylene nanowires and electrospun nylon 11 nanofibers were successfully prepared through a simple and time-saving method with a high success rate. Compared with a flat TENG, the output performance of a dual nanostructured TENG is enhanced by more than 5 times. After 1H,1H,2H,2H-perfluorooctyl trichlorosilane was assembled on the surface of the polypropylene film, the dual nanostructured TENG achieved the maximum output with the short-circuit current, output voltage, and charge density of 63.3 μA, 1135 V and 161.5 μC m−2, respectively. Compared with a planar structured TENG, the short-circuit current and output voltage were enhanced by about 18 times, and the charge density was increased by about 36 times. In addition, the TENG showed good working stability with almost no decrease in output after continuous operation for 193000 cycles. The electricity generated by this TENG can successfully light up 1280 LEDs and continuously power a multi-functional electronic watch. Finally, the triboelectric signal generated by this TENG was used to control an optocoupler switch, indicating good application prospects in a remote control switching circuit.
To improve the applications of the triboelectric nanogenerators, researchers have proposed various methods to improve their output. A common method to improve the output of a triboelectric generator is to micro-nano-process the friction layer material to increase the contact area and thereby increase the amount of generated triboelectric charge.30,31 In addition, the generation of tribocharges can be improved by chemically modifying the frictional material surface to increase the triboelectric polarity gap between the two friction layer materials.32–36 Relying solely on nanotechnological treatment of the material surface or chemical modification of the surface of the material has made limited improvements to the performance of TENGs. Therefore, micro-nano processing is combined with chemical surface modification of the materials to effectively improve the output of the triboelectric nanogenerators.37 At present, the friction layer of a triboelectric generator is composed mainly of two plane films or a plane film and structured film. If both friction layers of the triboelectric generator can be made into nanostructures, the effective contact area may be further increased to improve the output. Current methods for micro-nano treatment of triboelectric material surfaces include inductively coupled plasma etching, template replication, and chemical etching.38–42 These methods can achieve micro-nano processing of some polymer materials such as PTFE, PI, PDMS and other materials. However, they also face some problems, such as inductively coupled plasma etching, which is less effective at processing thermoplastic materials and whose equipment is more expensive. There have been studies using anodic aluminum oxide (AAO) templates to obtain regular polypropylene nanowires through hot pressing, which can effectively improve the output of the triboelectric nanogenerators.43 However, the production process of an AAO template is cumbersome, cycle is long, cost is high, and film removal success rate is low, which limits its application. Therefore, it is very necessary to develop a simple and rapid method for preparing polymer nanowires that can be used in conjunction with surface chemical modification methods to improve the output of the triboelectric generators.
Herein, polypropylene (PP) nanowires with regular nanostructure were successfully prepared by a hot pressing method. Instead of AAO templates, a polycarbonate (PC) membrane is chosen as the template mainly because it is low in cost, easy to obtain, and has many aperture specifications. The most important thing is that the AAO template takes a long time to demold and generates a lot of heat, which reduces the success rate of demolding. Due to the characteristics of polypropylene being insoluble in chloroform and polycarbonate being soluble in chloroform, putting the hot-pressed mixed film into chloroform for about 30 seconds can completely remove the PC template, which greatly reduces the mold removal time and increases the success rate of mold removal. Assembling a polypropylene nanowire film and nylon (NY) nanofiber film into a dual nanostructure TENG can effectively improve the output of the triboelectric nanogenerator. Compared with flat PP and flat NY, its output is increased by more than 5 times. In addition, 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFTS) was successfully assembled on the surface of the polypropylene film using a vapor deposition method to further enhance the output of the triboelectric nanogenerator. Compared with a planar structure TENG, the short-circuit current (Isc) and output voltage (Vo) of the PFTS-enhanced dual nanostructure TENG were increased by about 18 times. The electricity generated by this TENG can successfully light up 1280 LEDs in real time and continuously supply power to a multi-functional electronic watch with a temperature measurement function, proving good application prospects in micro power supply devices. In addition, the nanowire film based TENG was integrated into a printed circuit board (PCB) circuit to control the ‘on’ and ‘off’ of an optocoupler switch, indicating good application prospects in intelligent switch control.
Similar to most contact-separation mode triboelectric nanogenerators (CS-TENGs), the working principle of a PP@NY triboelectric nanogenerator is based on the coupling effect of triboelectrification and induction electrification. The working mechanism of a PP-NY based TENG is illustrated in Fig. 2. As shown in Fig. 2a, before PP contacts NY, no charge transfer occurs. When the PP and NY layers are pressed to contact, positive charges are generated on NY, and negative charges are generated on the PP surface (Fig. 2b). Due to the electrostatic induction effect, the Ag conducting layer generates opposite charges to the PP friction layer, resulting in the formation of a current flow from the NY electrode to the PP electrode during the release process (Fig. 2c). Once the charges reach the balanced state, no current flows in the circuit (Fig. 2d). Similarly, during the pressing process, a reverse current is detected from the PP electrode to the NY electrode (Fig. 2e). Thus, by coupling the contact electrification with electrostatic induction effects, alternating electricity is generated while the TENG experiences a contact-separation process.
Fig. 3a–c shows the surface morphology SEM images and cross-sectional SEM image of the PP nanowires fabricated through a PC template with a 400 nm pore size. According to the images, uniform nanowires were successfully fabricated with about 470 ± 45 nm diameter and 8.9 μm length. Fig. 3d–f shows the surface morphology SEM images and cross-sectional SEM image of the PP nanowires fabricated through a PC template with a 200 nm pore size. According to the images, the uniform nanowires were successfully fabricated with about 209 ± 25 nm diameter and 8.3 μm length. To facilitate data comparison, we abbreviate the above two PP nanowires to 400 PP and 200 PP, respectively. Fig. 3g–i shows the surface morphology SEM images and cross-sectional SEM image of the NY nanofibers fabricated through electrospinning. It can be seen from figures that the nylon nanofibers are very uniform with about 426 ± 121 nm diameter. The thickness of the nylon nanofibers film is about 17.5 μm, according to Fig. 3i. The statistical results of the diameters of the polypropylene nanowires and nylon nanofibers can be seen in Fig. S2.†
To study the effect of nanostructure on the output of a triboelectric nanogenerator, friction layer materials with a planar structure and nanowire structure were combined, as shown in Fig. 4a–d. This combination includes main four different groups: flat PP@flat NY, PP nanowire@flat NY, flat PP@NY nanowire and PP nanowire@NY nanowire. Since there are two types of the polypropylene nanowire, 200 PP and 400 PP, a total of six triboelectric nanogenerator outputs with different friction layer materials were tested, and the short-circuit current (Isc), output voltage (Vo) and charge density results are shown in Fig. 4e–g. According to the results, Isc, Vo and charge density follow the same trend: that is, the output of the planar structured TENG is smaller than that of the nanostructured TENG. It was demonstrated that nanostructures can increase the contact area and thereby increase the amount of triboelectric charge generated. When the two friction layer materials are flat structures, the output of Flat PP@Flat NY TENG is the smallest, and the corresponding Isc, Vo, and charge density are 3.0 μA, 62.5 V and 4.4 μC m−2, respectively. When both the friction layer materials are nanostructured, the output of the 200 PP@Nano NY TENG is a maximum, and the corresponding Isc, Vo, and charge density are 16.0 μA, 391 V and 27.6 μC m−2, respectively. Even with the same dual nanostructure, the output of the 400 PP@Nano NY TENG is lower than that of the 200 PP@Nano NY TENG. As can be seen from Fig. S1(a–d),† the number of nanopores per unit area of a polycarbonate filter membrane with a pore size of 200 nm is much greater than that of a polycarbonate filter membrane with a pore size of 400 nm. Therefore, the specific surface area of the prepared 200 PP nanofilm is much larger than that of the 400 PP nanofilm, which results in the 200 PP@Nano NY TENG being able to generate more triboelectric charges and obtain a higher output. In summary, the polypropylene nanowires can effectively increase the output of the triboelectric nanogenerators, and higher-density, relatively smaller-diameter nanowire structure can more effectively increase the output of the triboelectric generators.
The composition of the surface of the friction material is another crucial factor that impacts the performance of a TENG. Normally, for a contact-type triboelectric generator, to increase the output of the triboelectric generator, it is necessary to increase the difference in triboelectric polarity of the two friction layer materials as much as possible. It has been demonstrated that fluoropolymers, such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), are currently among the most appropriate choices as triboelectric negative friction materials for a TENG. This is primarily because fluorine is the most electronegative element among all the elements. To achieve this, we have implemented a surface chemical modification technique to fine-tune the surface composition of the friction material for a TENG. Here, since PFTS has been proven to have very high triboelectric negative polarity, PFTS was chosen as a modifier to chemically modify the polypropylene film surface. Fig. 5a shows the entire process of physical vapor deposition for depositing and assembling PFTS on the surface of the polypropylene film. Specific experimental steps can be found in the Experimental section. Compared with the surface morphology SEM image before surface modification in Fig. 3b and e, the assembly of PFTS has little effect on the surface morphology of planar and nanostructured films, as shown in Fig. 5b–d. To facilitate data comparison, we abbreviate the above three PFTS modified PP films to flat FPP, 400 FPP and 200 FPP, respectively. To prove that PFTS has been successfully assembled on the surface of the flat PP and PP films with nanostructure, XPS was used to analyze the surface composition of PFTS-modified materials. According to Fig. 5e, there is no F1s characteristic peak in the flat PP film or PP film with nanostructure before PFTS modification. After the physical vapor deposition process, obvious F1s characteristic peaks were detected on the surfaces of flat FPP, 400 FPP and 200 FPP, proving that PFTS had been successfully assembled. In addition, the C1s peak transformed into two peaks, –CF3 and C–C, after chemical modification, which proved that PFTS had been successfully assembled on the surface of the PP film. In addition, EDS was used to conduct surface element analysis on three chemically modified polypropylene films. As shown in Fig. S3,† EDS surface scanning results show that the F element is distributed very evenly on the surface of the polypropylene films, which also proves that PFTS had been successfully assembled on the surface of the PP films. The water contact angle measurement was also used to verify functionalized PP surfaces. As shown in Fig. S4,† when a water droplet dropped on the flat PP surface, a small contact angle of about 87.9° was observed. After fabrication of 400 nm PP nanowire and 200 nm PP nanowire, water contact angles increased to 145.9° and 145.3°, indicating that the nanostructure is an important factor in creating a hydrophobic or super-hydrophobic surface. After assembling PFTS, the water contact angles of flat PP, 400 PP and 200 PP were further increased to 111.7°, 153.2°, and 152.2°, respectively. This is mainly due to the fact that the non-polar (–CF3) end groups in PFTS reduce the surface energy of PP and thereby increase its water contact angle.
Like unmodified PP films, a total of six groups of the triboelectric nanogenerators are composed according to different friction layer materials. The short-circuit current, output voltage and charge density results are shown in Fig. 5f–h. Among the six TENGs, the outputs of Flat FPP@Flat NY, 400 FPP@Flat NY, and 200 FPP@Flat NY show a gradually increasing trend, which is the same as that without fluorination modification. Similarly, the outputs of flat FPP@Nano NY, 400 FPP@Nano NY, and 200 FPP@Nano NY show a gradually increasing trend, which is the same as the trend without PFTS modification. However, unlike the output without PFTS modification, the output of the 200 FPP@Flat NY TENG is higher than that of the flat FPP@Nano NY TENG. According to the frictional electrification sequence of the material, the ability of PFTS to be negatively charged during friction is much greater than that of PP, while nylon is very easily positively charged during friction. When the PP film is not modified with PFTS, the ability of polypropylene to generate triboelectric charges cannot be greatly improved simply by nano-treatment of the polypropylene surface. Since NY has a very strong triboelectrically charged ability, the output of a TENG can be improved more by nano-processing NY than by nano-processing polypropylene. Therefore, without fluorination, the output of the 200 PP@Flat NY TENG is smaller than that of the flat PP@Nano NY TENG. When PP is fluorinated, PFTS plays a leading role in improving the output of the TENG, which results in the output of the 200 FPP@Flat NY TENG being higher than that of the Flat FPP@Nano NY TENG. Compared with the output of the planar structure TENG without fluoropolymer modification, after PFTS modification and nano-processing of the nylon material, Isc increased from 3.0 μA to 63.3 μA, Vo increased from 62.5 V to 1135 V and charge density increased from 4.4 μC m−2 to 161.5 μC m−2. Isc and Vo increased by more than 18 times, while charge density was increased by about 36 times. It can be seen from the above results that modifying PFTS can greatly increase the amount of tribocharges generated by the triboelectric generator, and PFTS plays a dominant role in improving the output performance of the TENG.
To prove that PFTS significantly promotes the output of the TENG, the surface potentials of the PP, PFTS-PP and NY films were tested using KPFM. In Fig. 6a, b, d and e, it can be seen that the surface potentials of PFTS-PP and PP are negative, and the absolute value of the surface potential of PFTS-PP is significantly improved compared to that of PP. The mean potential value decreased from −12.4 V (PP) to −72.9 V (PFTS-PP), proving that the electron-gathering ability of the PFTS-modified PP film is increased nearly 6 times. Fig. 6c and f show the KPFM results of the NY film. The mean surface potential value of nylon 11 is 25.0 V, which proves that the electron-losing ability of nylon is stronger than the electron-gaining ability of PP and weaker than the electron-gaining ability of PFTS-PP. The calculated electrostatic surface potential (ESP) maps in Fig. 6g–i also prove that PFTS has a more negative potential than PP and nylon has a more positive potential. The output performance of a TENG is proportional to the surface charge density, which essentially depends on the charge transfer capability driven by contact electrification. Previous studies have reported that triboelectric positive materials lose electrons from the HOMO, while triboelectric negative materials gain these electrons from the LUMO.47,48 Therefore, reducing the energy difference between the HOMO and LUMO of the two friction layer materials can effectively enhance the output of the triboelectric generator. According to the calculated HOMO and LUMO energy and diagrams in Fig. 6j and S5,† the energy difference between the LUMO of PP and HOMO of NY (ΔE1 = 7.33 eV) is larger than the energy difference between the LUMO of PFTS and HOMO of NY (ΔE2 = 5.42 eV). According to energy potential well models in Fig. 6k and l, when PP and PFTS are in contact with and separated from nylon, respectively, electrons are more easily transferred from the HOMO of nylon to the LUMO of PFTS, resulting in higher output performance.49–51 Therefore, it has been experimentally and theoretically proven that self-assembled PFTS can effectively improve the output of polypropylene-based triboelectric nanogenerators.
From the above study, it can be concluded that the 200 FPP@Nano NY TENG with a dual nanowire structure obtained the highest output, and its corresponding Isc, Vo, and charge density are 63.3 μA, 1135 V, 161.5 μC m−2, respectively. The transferred charge density can more intuitively reflect the ability of the friction layer of the triboelectric generator to generate tribocharges. Compared with relevant research results in recent years that use structural design and surface chemical modification to improve the output of contact-separation mode triboelectric nanogenerators, it was found that by combining the design of double nanostructures and chemical modification methods, the charge density obtained in this work can achieve a relatively higher value, as shown in Fig. 7a and Table S1.†,43,52–56 As an energy conversion device, stability during operation is crucial for practical application. Herein, a durability test was also conducted under 5 Hz contact frequency for the 200 FPP@Nano NY TENG, and the results are shown in Fig. 7b. It can be seen from the test results that the output did not decrease during the entire 193000 cycles of 200 FPP@Nano NY TENG operation, proving that the 200 FPP@Nano NY TENG can maintain a stable output for a long time in practical applications. The output performance of the 200 FPP@Nano NY TENG under different humidities was also tested via a sealed acrylic box, as shown in Fig. S6.† As shown in Fig. S7a and S7b,† the output of the TENG shows a decreasing trend as the humidity increases. To more intuitively reflect the changing trend in output with humidity, the relationship between the maximum value of the output and humidity is constructed into a curve, as shown in Fig. S7c and S7d.† As can be seen from Fig. S7c,† as the humidity increases, the maximum value of Isc shows an almost linear decreasing trend with the increase in humidity. The decrease in Isc is relatively slow when the humidity is below 50%, and the rate of decrease increases significantly when the humidity is over 50%. This trend looks more obvious with Vo. Overall, the 200 FPP@Nano NY TENG, like most TENGs, does not have anti-humidity characteristics. However, it can maintain a relatively high output with an output of 52.8 μA and 1076 V when used in an environment with humidity below 50%. Fig. 7c shows the Isc values of the 200 FPP@Nano NY TENG under different load resistances from 0.1 MΩ to 1000 MΩ, and its corresponding power. According to the test results, current decreases with the increase in loading resistance, while the calculated output power of the device first rises and later drops with increasing resistance. The maximum value of the output power is about 17.0 mW under a loading resistance of 20 MΩ. As a power supply, the 200 FPP@Nano NY TENG has also been rectified for charging capacitors with different capacities. As shown in Fig. 7d, it took the 200 FPP@Nano NY TENG 33 s to charge 4.7 μF, 10 μF and 47 μF capacitors to 10 V, 4.7 V and 1.0 V, respectively. Furthermore, to verify that the generated electricity can indeed be used as an energy supply, the 200 FPP@Nano NY TENG was rectified to power 1280 LEDs. As shown in Fig. 7e and Video S1,† driven by a linear motor, the 200 FPP@Nano NY TENG can instantly light up 1280 commercial LEDs. Besides, the AC current generated by the 200 FPP@Nano NY TENG can be converted to DC current via an LTC3588-1 module and stored in integrated capacitors as a power supply for a multifunctional electronic watch. Fig. 7f shows the circuit connection diagram of the LTC3588-1 module, TENG and a multi-functional electronic watch. According to Fig. 7g, when the TENG runs for about 150 s, the output voltage of the LTC3588-1 module stabilizes at 1.8 V. To ensure that the electronic watch can run for a long time, it should continue to run for a period of time to charge the capacitor on the module, and the electronic power should not be turned off before 300 s. It can also be seen that the voltage curve can remain stable without a sufficient power supply. As shown in Fig. 7h and Video S2,† after 5 minutes of charging, the LTC3588-1 module can continuously supply power to a multi-functional electronic watch with a temperature measurement function and ensure the normal operation of the electronic watch. This also proves that the 200 FPP@Nano NY TENG can be used for energy storage and to power small electronic devices for a long time.
In view of the long-term durability and high frictional electrification performance of the 200 FPP film, it was integrated into an PCB circuit to control the ‘on’ and ‘off’ of an optocoupler switch. Fig. 8a and b show schematic images of the 200 FPP TENG as a trigger signal for controlling an optocoupler switch and PCB schematic diagram of the TENG triggering an optocoupler switch. As shown in Fig. 8c and Video S3†, eight 200 FPP films were pasted on the PCB using double-sided conductive tape to form a single-electrode TENG, which was used to generate pulse signals that excited the optocoupler switch. Each TENG was used to control a key of an electronic piano from C4 to C5. When a finger touches the FPP film, the triboelectric signal generated can activate the corresponding optocoupler switch and emit the corresponding key sound. By pressing different TENGs, a complete piece of music can be played. It can be seen from Fig. 8d that after processing, the TENG signal changes from an AC signal to a more regular trigger signal. When the optocoupler switch is triggered, the resistance at the load end also changes from 400 kΩ to 0 Ω in Fig. 8e, which proves that the TENG signal can normally turn on the optocoupler switch. Therefore, the 200 FPP@Nano NY TENG with a PFTS-modified double nanowire structure prepared by a simple method has very high output performance and great application prospects in practical use.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4sc01432d |
This journal is © The Royal Society of Chemistry 2024 |