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Environmental energy harvesting adapting to different weather conditions and self-powered vapor sensor based on humidity-responsive triboelectric nanogenerators Zewei Ren, Yafei Ding, Jinhui Nie, Fan Wang, Liang Xu, Shiquan Lin, Xiangyu Chen, and Zhonglin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21477 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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ACS Applied Materials & Interfaces

Environmental energy harvesting adapting to different weather conditions and self-powered vapor sensor based on humidity-responsive triboelectric nanogenerators

Zewei Ren1,2, Yafei Ding1,2, Jinhui Nie1,2,, Fan Wang1,2, Liang Xu1,2, Shiquan Lin1,2, Xiangyu Chen1,2* & Zhong Lin Wang1,2,3 1

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 100083, China. 2 School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing, 100049, China. 3 School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, USA. Correspondence and requests [email protected]).

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Keywords: triboelectric nanogenerator, humidity responsive materials, wind energy, rain drop, vapor sensor.

Abstract Triboelectric nanogenerators (TENGs) have been widely applied for energy harvesting and self-powered sensing, while smart deformable materials can be combined with TENG to acquire a more intelligent and self-adaptive system. Here, based on the vapor-driven actuation material of perfluorosulfonic acid ionomer (PFSA), we propose a type of humidity-responsive TENG. The integrated TENG array can automatically bend to the desired angles in responding to different humidity conditions and thus, it can effectively collect energy from both wind and rain drop, where the power density can reach 1.6 W m-2 at wind speed of 25 m s-1 and 230 mW m-2 under rainy condition. Meanwhile, this TENG array can fully lay down in dry weather, using reflective surface to reflect sunlight and heat radiation. The vapor absorption process of PSFA film can also result in the charge accumulation process. Accordingly, relying on the strong absorption capability of PFSA, a TENG-based vapor sensor with high sensitivity has been developed for monitoring chemical vapor leakage and humidity changing. This work opens up a promising approach for the application of the humidity responsive materials in the field of energy harvesting and self-powered sensors. It can also promote the development of TENG toward more intelligent direction.

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Introduction Fossil energy crisis and greenhouse effect caused by carbon emissions have become two of world’s most serious challenges, which also give rise to an urgent need for renewable energy technologies.1-4 In the past six years, Triboelectric nanogenerators (TENGs) that can collect energy from almost all kinds of ambient mechanical motions, have been intensively studied for meeting the energy demands of multiple scales.5-7 Comparing with other renewable energy techniques, TENG has the advantages of easy fabrication, easy scaling up, low cost, and high applicability for low-frequency resources,8-10 which can be applied for various potable energy package, self-powered sensors and environmental energy harvester.11-13 Wind and rain drop are two commonly existed energy source in the environment, while the TENG based energy harvesters specialize in collecting energy for this kind of low-frequency and irregular motions. Accordingly, a series of TENG prototypes have been developed to target at these two energy sources.14-20 However, a dual-mode TENG that can automatically harvest energy from both wind and rain drop is not easy to be realized. To target at different kind of mechanical motions, TENG devices have to modify its structure or change its appearance and a fully self-powered TENG should be able to automatically switch to different structures for different purposes. Hence, it is highly desirable to design a TENG device that can achieve the self-adaptive deformation in respect to different mechanical energy source. Recently, smart deformable materials that can provide actuation force under the drive of different external stimulus, such as electricity, heat, light, or humidity, have been intensively studied21-24 and various microrobotics, sensors and actuators based on these smart materials have be demonstrated,25-27 Vapor-driven soft actuator is one of these smart actuation devices and it can be designed to show different structures under different humidity condition.28,29 The vapor absorbing materials that are used to fabricate this kind of humidity responsive actuators usually have microporous or multilayer structures to enhance the transport of the vapor molecules,30,31 while the content change of vapor molecules inside these materials can generate internal stress to induce spontaneous deformation. In order to achieve a self-adaptive and self-deformable TENG, vapor-driven actuator is the ideal device to be coupled with TENG. When the weather condition changes from sunny to rainy, the increase of humidity can provide actuation force for TENG to automatically switch to different working modes. Hence, a humidity-responsive TENG based on vapor-actuating materials has the possibility to effectively harvest energy from both wind and rain drop. Meanwhile, the device can automatically switch between two optimized working structures without external intervention, indicating a fully self-powered smart system. On the other hand, TENG has many unique advantages, such as fast response speed and large output voltage, and therefore, the combination of TENG and humidity responsive materials may produce some interesting and practical applications for both fields. In this study, we have demonstrated the humidity-responsive TENG based on vapor-driven material of perfluorosulfonic acid ionomer (PFSA). The fabricated TENG array can generate self-adaptive deformation under different humidity conditions and three distinct functions can be realized: reflecting the sunlight in dry condition, harvesting wind energy in medium humidity and collecting energy from rain drop in rainy weather. The automatic switching among these different functions can be achieved based on the actuation force from PFSA. It has also been found that the PFSA membrane, even though its resistance is in the MΩ scale, can still work as the electrode materials for TENG. Accordingly, a highly sensitive self-powered vapor sensor based on


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single-electrode TENG has been developed and this vapor sensor can monitor vapor leakage or humidity change in many application fields, such as hazardous gas storages, chemistry lab and so on. This work represents a different research approach of the humidity responsive material for energy harvesting and self-powered sensors, which can enrich its functionality and promote its commercialization. Meanwhile, the demonstrated TENG with self-deformable capability can stimulate the development of TENG toward more intelligent and functional directions.

Experimental Section Preparation of the single TENG blade. The PET substrate (thickness of 0.1 mm) is prepared with dimensions of 14 × 10 cm and the 300 nm thick Al is deposited on one side of the substrate as the contact electrode. Then, the FEP film (thickness of 125 μm, size of 8 × 10 cm) is attached on the other side of the substrate as the dielectric layer. The surface of FEP film is carry out with hydrophobic treatment with hydrophobic agent (combination of nano silicon dioxide, ethanol and organosilicon resin), while the triboelectrification capacity of the film is little changed. For the preparation of the PFSA membrane, the membrane can be directly purchased from company or acquired through the spin-coating process with PFSA solution (5%, Aldrich). For the spin-coating process, the PFSA solution is casted on the substrate (PET or glass plate) and dried under certain humidity condition, where the thickness of the membrane can be effectively controlled by the volume of the solution. The prepared PFSA membrane with two-dimensional size of 4 × 10 cm is attached on PET substrate, while the whole fabrication process should be carried out under fixed humidity condition. The detailed humidity for the fabrication process depends on the climate of various regions, where the TENG blade can be naturally straight under different humidity condition, guaranteeing the effectiveness for wind energy harvesting (here, the selected humidity in our city is around RH 50%). The supporting strength of the PFSA membrane can be regulated by adjusting its thickness and the thickness of PFSA membrane on our TENG blade is around 50 μm. Fabrication of the humidity-responsive TENG array. The TENG array consists of 8 blades, where the blades are inserted into the PMMA pedestal that prepared by Engraving machines laser (UNIVERSAL PLS6.75, US). Here, four pairs of contact-mode TENG unit are developed with 8 blades and they are connected in parallel by a full wave bridge, ultimately. To illustrate the application of the TENG array in wind and rain drop energy harvesting, the lead wires of the device are sealed in a PMMA chamber, as presented in Supplementary Figure 1b. Fabrication of the self-powered vapor sensor. Here, the PFSA solution is cast on a glass plate and dried under humidity condition of RH 50%. After curing, the acquired membrane is peeled off slowly from the glass substrate with a tweezer. Then, the PFSA membrane with size of 10 cm2 and thickness of around 50 μm is prepared as electrode of the vapor sensor based on single-electrode TENG. Characterization and measurements. Microstructure of the samples is acquired by using SEM (SU8020, Hitachi), while photographs are taken by a single-lens reflex camera (D7000, Nikon). The static contact angles of FEP film are measured by a contact angle measuring instrument (CA-100D). The bending angle of the TENG is acquired by a laser displacement sensor (KEYENCE IL-030). The electrical outputs of the TENG array are measured by Stanford Research Systems Kethiely 6514 and the oscilloscope (DSO2014A, KEYSIGHT).



Results and Discussion Based on the vapor-driven material PFSA, a humidity-responsive and self-adaptive TENG array has been developed to harvest energy from both wind and rain drop. Figure 1a shows the materials and detailed structure design of this humidity-responsive TENG, where the TENG can spontaneously switch to different deformations in response to changed humidity conditions (Movie 1, Supporting Information). Here, polyethylene terephthalate (PET) is chosen as backbone of the TENG blade, due to its good mechanical strength and flexibility. Aluminum (Al) coated surface has the dual functions as output electrode and positive tribo-layer of the TENG, while fluorinated ethylene propylene (FEP) has been selected as dielectric material and negative tribo-layer because of its decent electron affinity. The PFSA membrane attached on PET substrate has been developed as “active joint” of the TENG blade, which can provide bending force for the blade under different humidity conditions. PFSA is an ionic polymer, the chemical composition of which contains the hydrophobic tetrafluoroethylene backbone and hydrophilic sulfonic acid side chains. The chemical formula and microstructure of PFSA can be seen in Figure 1b and Figure S1a. Generally speaking, straight helical crystalline regions can be formed by the continuous hydrophobic tetrafluoroethylene chains inside the PFSA membrane, whereas an amorphous region would be developed owing to the branching hydrophobic tetrafluoroethylene chains.32,33 The sulfonic acid groups (-SO3) associated with these regions can develop randomly distributed cylindrical nanochannels inside the membrane,34,35 the schematic can be found in Figure S1b. The nanochannels, which are elongated inside the membrane with straight helical backbone segments, have the capacity of absorbing moisture or other vapor molecules.36 For PFSA membrane under a relatively high (or low) vapor concentration, the moisture molecules can be absorbed (or desorbed) by these nanochannels, while nanochannels gradually expand (or shrink) with the absorption (or desorption) process. The expansion (or shrink) of nanochannels can generate internal stress and drive the TENG blade to deform (as shown in Figure 1c), which is the basic working mechanism of this humidity responsive TENG. Photographs of the fabricated self-adaptive TENG array consisting of 8 blades are presented in Figure S1c. In addition, the surface of FEP has been modified by hydrophobic treatment for smoothly collecting energy from rain drop. Scanning electron microscopy (SEM) images of micromorphology of FEP film before and after hydrophobic treatment are shown in Figure 1d (i) and (ii), respectively. It’s suggested that high-density nano holes and nanorods are developed on the surface of FEP film with hydrophobic treatment (Figure 1d (ii)), which is helpful to reduce the actual contact area between liquid and the surface. Correspondingly, the static contact angles of FEP surface before and after hydrophobic treatment are measured to be 84.1°and 142.6°(Figure 1d I and II), which illustrates the good hydrophobicity of the TENG. Meanwhile, the Al electrode is also hydrophobic treated with the same method, while the triboelectrification of both FEP film and Al electrode is little changed (the comparison test can be found in Figure S2d and e). The fabricated TENG array can induce self-adaptive deformation under different humidity conditions and three functions can be realized in dry, windy and rainy weather, the schematic and photographs of which are presented in Figure 2. In strong sunlight day with low humidity (relative humidity (RH) < 20%), the TENG induces bending force along anti-clockwise direction spontaneously, with the reflective side faced up (schematic diagram and photograph in Figure 2 i). In this case, the strong sunlight can be reflected by the TENG array, which may help to reduce the temperature beneath the TENG array. Hence, this TENG array can be arranged on rooftop of


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buildings to alleviate hot temperature caused by solar radiation in summer season, which may reduce the energy consumption of air-condition equipment. The shading performance of the TENG blade is significant (Figure S2) and a simulating experiment for demonstrating the heat reflection capability of this TENG device has been designed, as can be seen in Supplementary Movie 2. In windy weather with normal humidity (RH 20%-80%), the TENG blade naturally stands up and points to vertical direction (Figure 2 ii). When wind blows across the TENG array, the wind energy can be effectively harvested by the tribo-electrification due to the swing of free-standing TENG blades. In the rainy days (RH > 80%) , the TENG array fully bends to clockwise direction with the tribo-surface facing up and collects energy from rain drops, as indicated in Figure 2 iii. In this case, a multifunctional TENG system is developed, while the mode switching among different functions can be automatically achieved according to different weather conditions. In order to clarify the deformation characteristics of this humidity-responsive TENG in response to different humidity, physical and geometric analysis of the PFSA membrane have been carried out. Figure 3a illustrates the size change of PFSA membrane and the bending behavior of TENG blade with humidity increasing. The blade performs a continuous bending deformation, when the humidity increases from RH 45% to around RH 90%. Accordingly, the change of two-dimensional size of a piece of PFSA membrane under moisture triggering as humidity increases is recorded by optical microscope, as shown in Figure 3a and Figure S3. With an original size of 163×190 μm under the condition of RH 45% (Figure 3a (i)), the two-dimensional size of PFSA membrane increases by about 54.3% with ultimate size of 200×239 μm, when humidity increases to RH 90% (Figure 3a (iii)). The deformation degree of the TENG is determined by the amount of absorbed/desorbed moisture inside PFSA membrane, since the generated stress inside the membrane depends on the volume change. Figure 3b demonstrates the weight - time dependence plot of the PFSA membrane (original weight is 336.5mg) in different humidity conditions, where the weight of the membrane increases by 10.5% as the RH changes from 25% to 95%. Figure 3c and Figure S4b elaborates the deformation force of the TENG blade under changing humidity (the schematic of measurement can be found in Figure S4b). Based on Figure 3a, b and c, it can be found that geometric change of the PFSA membrane and the deformation degree of the TENG is stabilized under certain humidity. That’s mainly because the moisture concentration under the certain humidity condition is nearly constant, and thereby vapor absorption/desorption by PFSA membrane under this circumstances is quantifiable, which can establish a close relevance between deformation degree and humidity. Meanwhile, the mechanical cycling performance of PFSA membrane has been proved systematically studied,23 where the actuation process of the membrane appeared robust with no apparent fatigue induced after several few thousand times cycle. To evaluate the bending degree of the TENG blade with different humidity, we consider the deformation angle θ is zero when the blade stands in the vertical direction (Figure S5). Figure 3d and e illustrate deformation angle of the TENG in response to increasing humidity, while the height of PET substrate changes from 8 to 16 cm and the thickness changes from 50 to 150 μm, respectively. The relationship between deformation angle of TENG blade and length of the attached PFSA membrane can be seen in Figure 3f. The changes in Figure 3d, e and f indicate that the maximum deformation angle of the TENG can be adjusted by change geometric characteristics of PET substrate and PFSA membrane, suggesting a good applicability and design flexibility of this TENG array adapting to different environments. It also suggests that



the deformation degree of the single blade can be quite repeatable under the certain humidity condition, which may be applied for humidity detecting. The experiments for demonstrating the capability of this humidity-responsive TENG array for both wind and rain drop energy harvesting have been systematically studied, as presented in Figure 4. For wind energy harvesting, the working principle of this TENG array is relying on the contact-separation motion among TENG blades. As presented in Figure S6, owing to the opposite triboelectric polarities of Al and FEP, the contact electrification effect can induce charges with opposite polarities on the surface of two films.37 Then, the electrostatic induction effect can change the charges distribution on Al electrode, once the two TENG blades are brought into contact. Accordingly, an electrical potential difference is established and thus the free electrons are driven flow across the electrodes alternatively with the continuous contact-separation motions, resulting in a generate alternating current in the external circuit. With natural wind blow, the swing of TENG blades leads to reciprocating contact and separation, converting wind energy into electricity. During the operation, wind velocity is the key parameter related to the output performance of the TENG array, since the swing amplitude of blade mainly depends on the intensity of the wind. Figure 4a and 4b illustrate the dependence of open-circuit voltage (Voc) and short-circuit current (Isc) of the TENG array, respectively, while the transferred charges (Qsc) in the external circuit is shown in Figure 4c (with the wind velocity ranging from 5 to 25 ms−1). It is obvious that the generated electrical output is higher with stronger wind, resulting from the higher vibration frequency and larger separation distance of the TENG blades. The amplitude of vibration frequency induced by different wind velocity can be found in Figure S7a, where the maximum frequency can reach to 146 Hz with wind speed of 25 m s−1. Meanwhile, the instantaneous power density of this TENG array in respect to its contact surface can reach 1.6 Wm−2 (P = UI / S contact −1 surface) with a load resistance of around 10 MΩ, under the wind blow of 25 ms (as shown in Figure S7b and c). Furthermore, the output performance influenced by the deformation angle of the TENG blade has been studied (Figure 4e), where three representative angles of 0°, 45°and 90° are selected for the test (the schematic can be found in Figure S8). It can be found that the electrical outputs (Voc, Isc and Qsc) decrease with the increasing of deformation degree of the TENG array and the maximum output is obtained at the deformation angle of 0°. The separation distance among the blades would be decreased with larger bending deformation (as presented in Figure S8) and thus the contact-separation motion among the blades is not sufficient. Meanwhile, the dependence of the output on wind direction is investigated, the schematic of which is shown in Figure S9a. As suggested in Figure S9b and c, wind energy from arbitrary wind direction can be harvested by the TENG array, even though the output decreases when the wind direction is not perpendicular to the surface of the blade. A display board consisting of serially connected light-emitting diodes (LEDs) has been connected to the TENG array, where the LEDs can be continuously illuminated by the wind energy, as elaborated in Figure 4d and Movie 3. The TENG array is suitable to be applied in practice, taking Beijing city as an example, the average velocity of wind is around scale 4 in the last month and the wind speed of which is around 8 m/s. In this case, this kind of TENG arrays can be paved on the rooftop of many buildings and the output energy can meet the electricity consumption of some small equipment. Furthermore, as explained in Figure 2i, this humidity-responsive TENG array on rooftop can fully lay down in dry weather and use its reflective surface to reflect sunlight, which can also help to alleviate hot temperature caused by solar radiation. Hence, this self-adaptive TENG system can serve as a useful constituent


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of the intelligent home. In rainy days, the TENG array can automatically lay down with the tribo-surface facing up to collect energy from rain drop. A group of 16 LEDs has been simultaneously lighted by the TENG array, as illustrated in Figure 4f and Movie 4. The working principle of the TENG array for rain-drop energy harvesting is consisted of two kinds of electrification processes. That is, the contact electrification between rain drop and FEP surface and the contact electrification between different TENG blades induced by the impact force of rain drop. The interaction between rain drop and FEP surface has already been systemically illustrated in our previous research19 (Figure S10a and b). Generally speaking, the existence of rain drops on the FEP surface can change the localized potential distribution, which can induce charges on the electrode beneath the FEP film. Then, the motion and the detachment of the rain drops can also induce the change of electrostatic field, which can be utilized for energy generation by TENG system. Hence, the hydrophobic treatment for the FEP is quite necessary, which can help the rain drop to smoothly slide off. Meanwhile, since the TENG blade is elastic material, the impact force carried by rain drops can result in the vibration of the TENG array, which changes the contact positions of the blade array. Thus, tiny contact-separation motions and the triboelectricity can be induced between adjacent blades. The schematic of the process is illustrated in Figure S10c, which is similar to that of wind energy harvesting (as has been demonstrated in Figure S6). To further investigate this composite working principle, the relationship between power output and number of TENG blade is studied, as shown in Figure 4g and Figure S10d. With the same water dropping rate (2 mL s-1) and dripping area, the output of single TENG blade is much smaller than TENG array with multiple blades (2, 4, 6 and 8) and a sudden increase of output signal between single blade and double blades can be observed, which confirms the composite working principle of the TENG array. Moreover, the water drop has been replaced by dry silicone ball (no electrification with FEP film) to demonstrate the relation between output of the TENG array and impact force, as shown in Figure S11. It’s suggested that contact electrification induced by impact force contributes more electrical output than that resulting from the contact electrification between water drops and FEP film. In addition, under windy/rainy condition, both wind and rain drop energy can be harvested by the TENG array simultaneously. The working principles of TENG array are relying on the contact-separation motion among TENG blades and the contact electrification between rain drop and FEP surface, as have been demonstrated in Figure S6 and Figure S10. However, for the experimental testing, it is really hard to isolate the contribution of rain drop in the windy/rainy conditions, since wind can also increase the speed of droplets. Figure 4h and 4i show the output of the TENG array generated from the water drops, where dropping rate of the water increases from 0.5 to 3.5 mL s-1. It’s noted that the outputs (Voc, Isc and Qsc) increase with water dropping rate initially, as the amount of triboelectric charges and impact force brought by water drops are both enhanced. However, the output is weaken with excessive water dropping rate (>2.3mL s-1), this phenomenon can be analyzed from the composite working principle in rain-drop case. For the electrification process happened on the interface between FEP surface and rain drops, the continuous output is induced by the periodical contact-separation process (contacting with FEP and sliding off the FEP surface) of the water drops. Nevertheless, with excessive water dropping, the water drops on the FEP surface easily join together and form water film (schematic in Figure S12a). Under the circumstances, the density of the water drops on the FEP surface of the TENG array is little changed, resulting in the suppression of the electrical



output. Moreover, when the water drops falls on the FEP surface, the impact force can press the blade to deform downside and this vibration can generate electrostatic energy in the TENG array. However, when the water dropping rate is excessive, the total impact force brought by the water drops is larger and close to constant, and thus the elastic recovery of the blades is suppressed by the uninterrupted impact force (schematic in Figure S12b). In this case, the elastic vibration of adjacent blades is suppressed, leading to a smaller output of the TENG array. With the water dropping rate of around 2 mL s-1, the outputs (Voc and Isc) changing with increasing load resistance have been demonstrated (Figure S13a and b). At a load resistance of 10 MΩ, the maximum output power density is around 230 mW m −2, illustrating the effectiveness of the TENG array in rain drop energy harvesting. On the other hand, the output performance of the TENG array influenced by the incident angle in rainy weather has been investigated. Here, α is defined as the angle between water dropping direction and the surface of bending TENG (Figure S14a). The output performance depending on different incident angles can be found in Figure 4j and Figure S14b. The amplitude of the electrical output increases firstly and then decreases when α is set from 10°to 80°. With a small incident angle, as the TENG is nearly horizontal, the fallen water drops aren’t able to slid off timely and would gather together on the FEP surface, resulting in a weaken output. When α is large (> 60°), the amount of water drops collected by TENG surface decreases since the effective contact area on the TENG array is smaller, leading to a low output. In view of the influence of α on output performance of the TENG array above, the incident angle ranging from 20°to 60°should be more suitable for practical application. In order to prevent the leakage of electricity of exposed Al electrode in rainy days during the water drops harvesting, the surface of Al electrode is treated with hydrophobic agent, while the triboelectrification of Al electrode is little changed (the comparison test is shown in Figure S1 d and e).With the hydrophobic treatment, the falling water drop is difficult to stay on the film and will slip away rapidly, preventing the leakage of electricity. It has also been found that the resistance of PFSA membrane is in MΩ scale and thus, it’s difficult to work as the electrode materials for common electronics devices. In fact, PFSA has been widely employed as separating membrane for lithium-ion battery partially due to its low conductivity. However, the matching resistance for TENG device is also in the scale of MΩ, suggesting that PFSA can work as the electrode material for TENG. The resistance of the PFSA membrane in different humidity condition is presented in Figure 5a, which demonstrates that the conductivity of this material is increased with increasing humidity. To investigate the electrification capability of PFSA electrode, several common materials have been selected to work with it (contact-separation mode) and the results can be found in Figure 5b and Figure S15a. The PFSA film can induce the highest output contacting with FEP material, while the electrification with Kapton or Al is rather weak. As a moisture/vapor driven material, we further compare its triboelectric characteristic with Al under different humidity conditions, as shown in Figure 5c, Figure S15b and c (the schematic and details are presented in Table S1 ). The triboelectric performance of the PFSA film is close to Al, while slight difference exists under high humidity condition when the film is used as dielectric material. In comparison with other moisture/vapor-driven materials possessing microporous structure, the PFSA membrane with nano channel structure has properties of rapid mass transfer owing to the nanofluidic channels and a larger functionalized area that is favorable for rapid vapor adsorption.30,38 Meanwhile, we have found that as an electrode material of TENG, potential change on PFSA membrane would be


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induced, when moisture/vapor molecules are absorbed (or desorbed) by the membrane. The reason for this phenomenon can be possibly attributed to the tribo-charges carried by the vapor molecules. The triboelectric charges on moisture/vapor molecules can be generated owing to the contact electrification between the molecule and air or drifting particles. Hence, during the vapor absorption, the charges carried by vapor molecules can be accumulated on the PFSA membrane, which results in the potential change of the PFSA electrode. Based on the absorption behavior and electrode performance of the membrane, a highly sensitive vapor sensor using single-electrode TENG has been developed, the schematic can be seen in Figure 5d. The TENG with PFSA-electrode is placed in a sealed chamber, where the amounts of moisture generation or humidity can be controlled by applying vapor flow to the chamber. Meanwhile, the change of vapor concentration in the camber leads to the absorption/desorption process of PFSA, which can change the electrical potential on the PFSA electrode. Accordingly, the vapor flow as well as humidity change can be detected by the output (potential) change of the TENG. The output change of the sensor with the water vapor is illustrated in Figure 5e, where the vapor is generated by compressed air atomizer. Then, the PFSA material is replaced by an Al electrode with the same size, the output change of which under the same vapor condition has also been recorded (Figure 5e). Since Al foil is common electrode material with poor capacity of vapor absorption, the output change of the Al-electrode TENG is much smaller than that of PFSA electrode (Figure 5e (i)). Moreover, the potential increase induced by vapor molecule on Al electrode vanishes quickly when vapor is off (Figure 5e (ii)), which is due to the fast desorption process happened on Al electrode. This comparing result suggests that the vapor-responsive materials, such as PFSA, are ideal electrode materials for TENG-based vapor detector, where both high sensitivity and good stability can be achieved. Figure 5f shows the output change of the sensor with hot water (the schematic is presented in Figure S16a), the curve is different from that of vapor generated by air atomizer in Figure 5e. As shown in the insert of Figure 5f, two kinds of increasing slope (K1 and K2) of output signal can be approximately observed during the first 30 seconds, while the temperature decrease of the hot water at different time is also recorded. With the temperature decrease, the kinetic energy of the escaping vapor molecule from hot water also decreases gradually, which weakens the vapor molecules’ collision with air and thus reduce the amount of generated triboelectric charges. Meanwhile, generation rate of vapor molecules from hot water also decreases with heat loss. Accordingly, the increase rate of output signal gradually slows down during the whole process. Finally, the vapor evaporation stops after about 150 seconds and the voltage from TENG also reaches saturated value. In contrast, the slope change of the curve is nearly constant in Figure 5e, since the generated vapor flow from compressed air atomizer is stable and hardly influenced by outside interference. For the vapor monitoring, the vapor flow is through into the chamber with a hole on the chamber (schematic in Figure 5d), the diffusion of vapor inside the chamber requires a process with some relaxation time. The diffusion process of vapor then leads to the slight continued change of the output voltage of the sensor, while the output signal is no longer changed until the diffusion of vapor is completed. The temperature change of the PFSA electrode in the chamber with hot water of 70℃can be found in Figure S16b. Figure 5g and Figure S17 show the output response of the vapor sensor exposed to water vapor at different concentrations, where the RH in the sealed chamber is 40%, 60% and 85%. The potential change increases gradually with increasing moisture content, and the valid concentration of moisture can be detected by the PFSA electrode (size of 10cm2) ranges from 50 to 18000 PPM.



Moreover, the dryer environment can improve the sensitivity of the triboelectric-based vapor sensor, which may be caused by the distinct diffusion rate of vapor molecule and varied absorption capability of PFSA electrode under different humidity conditions. The detecting response of PFSA electrode in different size (ranging from 5 to 50 cm2) has also been studied, as presented in Figure 5h. With the same vaporized volume (175 µL), both response speed and changing amplitude of the induced voltage signal increase with the size of PFSA electrode, which implies that the sensitivity of the vapor sensor can be improved with the PFSA electrode in larger size. In comparison with previously reported vapor sensors,39-41 this TENG-based vapor sensor has the advantages of simple fabrication with commercial available materials, little power consumption and fast response speed with high sensitivity, which may provide a new approach for monitoring vapor or chemical leakage. Figure 6a elaborates the schematic of the self-powered sensor system based on this PFSA-TENG sensor for detecting possible leakage in vapor pipeline, which can be integrated with computer and information processing system to serve for the Internet of Things (IoT). Moreover, the leakage detector is also quite necessary for the safe storage of volatile chemicals and this self-powered vapor sensor may also work for this purposes, the promising applications of which can be developed in many fields such as hazardous gas storages, chemistry lab and so on (Figure 6 b and c). Accordingly, adsorption experiments based on this self-powered vapor sensor are carried out on some conventional volatile organic vapors and the corresponding output signals are presented in Figure 6d, where various vapors (methanol, ethanol, 1-propanol, diethyl ether, n-hexane and acetone) are generated by the same compressed air atomizer. Here, ethanol can induce the strongest signal change with this PFSA-TENG sensor, while the output signal cannot show stable increase with acetone vapor. The real-time potential change is decided by two factors, the volatilization rate of each solvent and the tribo-induced charge amount on vapor molecule. In order to clarify these results, we have studied the volatilization rate (generated from the compressed air atomizer) of each solvent, as can be seen in Figure 6e. The volatilization rate of ethanol is about 200% higher than that of moisture (water). Meanwhile, the saturated voltage signals of each solvent with the same amount of vapor injection (500 μL) are also summarized (Figure 6f), where the moisture can induce the highest output signal, implying water molecule can carry the largest amount of tribo-charges. However, the low volatilization rate of water may slow down the signal change in real-time measurements, which is consistent with the result in Figure 6d. It has also been found that the PFSA electrode show slight reaction to the vapor of Diethyl ether, N-Hexane and especially acetone. This distinct performance may be determined by the various chemical functional groups and ionic properties of these vapor molecules. On the other hand, based on the results in Figure 6f, the PFSA electrode may have the possibility to distinguish different volatile solvents by checking the induced output signal with fixed volatilized volume. The TENG-based vapor sensor there is difficult to tell which liquid medicine has leaked in some a complex scenario such as pharmaceutical kitchens, since the sensing mechanism of this sensor is based on potential change on PFSA electrode. In order to distinguish different liquid medicines, it is better to employ some chemical sensor device. However, this TENG-based vapor sensor has the advantages of simple fabrication with commercially available materials, little power consumption and fast response speed with high sensitivity, which may provide a new approach for vapor or leakage monitoring.

Conclusion


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In summary, a humidity-responsive TENG that can spontaneously switch to optimized working structures with different humidity conditions has been demonstrated, opening up a new application of smart actuation material in field of energy harvesting and self-powered sensory. By coupling moisture/vapor-driven PFSA material with TENG, three distinct working modes corresponding to different weather conditions have been realized by the designed self-adaptive TENG array. In dry weather, the TENG array can lie down and reflect the sunlight to reduce the heat. Then, the TENG array stands in the vertical direction with the medium humidity to fully collect the wind energy. Finally, in rainy days, the TENG array can bend to 180ºwith the tribo-surface facing up and collect energy from rain drop. The power density of this device in respect to its surface area can reach 1.6 W m-2 with a wind speed of 25 m s-1 for wind energy collection and 230 mW m-2 at water dropping rate of 2 mL s-1 under rainy condition. Moreover, we have found that the vapor absorption process of PSFA film can also result in the charge accumulation on the PSFA film and these accumulated charges can be fully utilized by TENG devices, since the matching resistance for TENG is quite similar to the resistance of PFSA membrane. Based on single-electrode TENG mode, a highly sensitive vapor sensor by using PFSA electrode is developed, which can detect the vapor flow ranging from 50 to 18000 PPM. This self-powered sensor can be applied for identifying the concentration change of many types of vapors and used to monitor the vapor leakage in many fields. The combination of vapor-responsive material and TENG is a new research approach for both fields, while it can guide the development of self-powered technique toward more intelligent and functional direction. The demonstrated concept and prototype device in this work can inspire many future studies in the field of intelligent energy system, vapor and chemical detecting, human–environment interactions, etc.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Figures and tables of detailed date, working mechanisms and schematic illustrations related to the TENG devices. (PDF) Movie 1: Deformation of the TENG in response to changed humidity conditions. (AVI) Movie 2: The shading performance of the TENG. (AVI) Movie 3: Wind energy harvesting by the TENG array. (AVI) Movie 4: Rain drop energy harvesting by the TENG array. (AVI)

Acknowledgements This work is supported by the National Key R & D Project from Minister of Science and Technology (2016YFA0202704), NSFC Key Program（no. 21237003）, Beijing Municipal Science & Technology Commission (Z171100000317001, Z171100002017017, Y3993113DF), National Natural Science Foundation of China (grant nos. 51775049, 51432005, 11674215, 5151101243, 51561145021) and Young Top-Notch Talents Program of Beijing Excellent Talents Funding (2017000021223ZK03).

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