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Enhancing performance of triboelectric nanogenerator by filling high dielectric nanoparticles into sponge PDMS film Jie Chen, Hengyu Guo, Xianming He, Guanlin Liu, Yi Xi, Haofei Shi, and Chenguo Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09907 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 20, 2015

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ACS Applied Materials & Interfaces

Enhancing performance of triboelectric nanogenerator by filling high dielectric nanoparticles into sponge PDMS film

Jie Chen,1 Hengyu Guo,1* Xianming He,1 Guanlin Liu,1 Yi Xi,1 Haofei Shi,2 Chenguo Hu1 * 1

Department of Applied Physics, Chongqing University, Chongqing 400044, PR China.

2

Chongqing Engineering Research Center of Graphene Film Manufacturing, Chongqing 401329, P. R.

China *Correspondence to: [email protected] (Chenguo Hu); [email protected] (Hengyu Guo)

ABSTRACT Understanding of the triboelectric charge accumulation from the view of materials plays a critical role in enhancing the output performance of triboelectric nanogenerator (TENG). In this paper, we have designed a feasible approach to modify the tribo-material of TENG by filling it with high permittivity nanoparticles and forming pores. The influence of dielectricity and porosity on the output performance is discussed experimentally and theoretically, which indicates that both the surface charge density and the charge transfer quantity have a close relationship with the relative permittivity and porosity of the tribo-material. A high output performance TENG based on a composite sponge PDMS film (CS-TENG) is fabricated by optimizing both the dielectric properties and the porosity of the tribo-material. By combining the enhancement of permittivity and production of pores in the PDMS film, the charge density of ~19 nC·cm-2, open-circuit voltage of 338 V, and power density of 6.47 W·m-2 are obtained at working frequency of 2.5 Hz with the optimized film consisting of 10% SrTiO3

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nanoparticles (~100 nm in size) and 15% pores in volume, which gives over 5-fold power enhancement compared with the nanogenerator based on the pure PDMS film. This work gives a better understanding of the triboelectricity produced by the TENG from the view of materials and provides a new and effective way to enhance the performance of TENG from the material itself, not only its surface modification. KEYWORDS: dielectricity, porosity, charge density, triboelectricity, nanogenerator

INTRODUCATION Harvesting mechanical energy from our living environment is an effective approach for sustainable, maintenance-free, and green power source for wireless, portable and wearable electronics.1-6 As a new energy-harvesting technology, triboelectric nanogenerator (TENG) based on the coupling of triboelectrification and electrostatic induction has promising applications in both small and large-scale energy generation due to its high efficiency, lowcost, environmental friendliness, and universal availability.7-20 Until now, the electric output of TENG has been improved up to ~500 W·m-2 by complicated design of device structure.21-24 However, the intrinsic properties of the triboelectric materials is critically important to the improvement of the TENGs’ output. With a contact-separation structure, the surface charge density of the tribo-material is the key to achieve a high output performance of TENG. Polydimethylsiloxane (PDMS) is the most commonly used as tribo-material due to its high electronegativity, flexibility, transparency, cost effectiveness and it can be easily made into composite films by mixing nanoparticles or other nanostructures.24-26 In addition, PDMS can be patterned on its surface or

inner

structures.27

In

most

previous

works,

micro-patterned

pyramid

array,

nanoporous/nanowire structure and chemical functional groups were applied to modify the surface of the tribo-material in order to obtain high surface charge density.15, 16, 29-33 Recently, 2 Environment ACS Paragon Plus

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our group reported a conductive nanomaterial composite PDMS film based TENG with 2.6 times of power enhancement, and its possible mechanism of the capacitance effect on surface charge density.34 However, the effects of dielectric properties of the tribo-material on the output performance need systematical investigation to deepen the understanding of the charge accumulation and transfer on the surface.35 In this work, combining the high dielectricity and porosity, we present a composite sponge PDMS film based triboelectric nanogenerator (CS-TENG) by a simple and feasible filling and removing process. The influence of the dielectricity and porosity on the output performance is discussed experimentally and theoretically, which indicates that both the surface charge density and the charge transfer quantity have a close relationship to the relative permittivity and porosity of the tribo-material. By optimizing the permittivity and pore ratio of PDMS film, the film consisting of 10% SrTiO3 nanoparticles (NPs) and 15% pores in volume achieves the best performance, where the output open-circuit voltage, short-current density and transfer charge increase up to 338 V, 9.06 µA·cm−2 and 19 nC·cm-2, respectively. The maximum instantaneous output power is 6.47 W·m-2 under periodic compressive force at frequency of 2.5 Hz, which is over 5-fold power enhancement compared with the pure PDMS film based TENG (P-TENG). The mechanism to maximize surface charge density by changing relative permittivity and porosity is proposed. This work provides a new way by both increasing the permittivity and reducing the effective thickness of the tribo-material for enhancing the performance of TENG rather than by only modifying its surface properties. EXPERIMENT SECTION Fabrication of dielectric material@PDMS composite film. Figure 1a shows the schematic diagram of the process of fabricating the dielectric material@PDMS composite film. In this experiment, the PDMS solution (Sylgard 184, Dow 3 Environment ACS Paragon Plus


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Corning) contains both the elastomer and the curing agent in a mass ratio of 10:1. To achieve uniform PDMS composite films, first, the SiO2 (εr=3), TiO2 (εr=80), BaTiO3 (εr=150) and SrTiO3 (εr=300) nanoparticles (from Sigma-Aldrich) were respectively dispersed in hexane, and then mixed into elastomer by magnetic stirring for 2 h until the hexane evaporated completely. Secondly, the curing agent was put into the well-prepared mixture by magnetic stirring for 15 min to yield a uniformly-mixed suspension. Thirdly, the suspension was cast into a film shape in an acrylic template and then kept it in the oven at 335 K for 3 h to get cured. Finally, PDMS composite film was peeled off from the template. Fabrication of sponge/composite sponge PDMS films. The sponge or composite sponge PDMS films were fabricated by repeating the abovementioned processes. Briefly, NaCl (~10 µm) or NaCl and SrTiO3 nanoparticles (~ 100 nm) were used as fillers mixed into PDMS solution in the first step. The volume ratio of NaCl particles to PDMS was adjusted to create different pore ratios. The films were immersed in water solution and then treated by mechanical stirring for 4 h to completely remove the NaCl particles. After washing, the films were dried in atmosphere at 335 K and the sponge or composite sponge PDMS films were obtained. CS-TENG design and testing. The morphology and structure of the samples were characterized by a field emission scanning electron microscope (Nova 400 Nano SEM). Relative permittivity and light transmittance characterizations of the PDMS films were performed by the broadband dielectric spectrometer (Germany NOVOCONTROL, Concept 40) and UV3600, respectively. To fabricate the device, two pieces of cast acrylic glass (3 cm × 3 cm × 1.5 mm) were prepared as substrates. Four half-thorough holes were drilled at corners for spring installation. Two pieces of copper foils (2 cm × 2 cm) were prepared as electrodes attached to the acrylic 4 Environment ACS Paragon Plus

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substrates. Then, conducting wires were connected to the two copper foils for subsequent electric measurement. PDMS film was adhered to the bottom electrode, as tribo-layer. Finally, four springs were installed in the holes to connect the two substrates together, leaving a space (10 mm) between the top electrode and triboelectric layer. A Stanford low-noise current preamplifier (Model SR570) and a Keithley voltage preamplifier (Model 6514) were used for electrical measurement. Figure S1 schematically shows the measuring system. RESULTS AND DISCUSSION Schematic diagrams for the fabrication process of the high dielectric PDMS composite film and the PDMS composite-sponge film based TENG (CS-TENG) are shown in Figure 1 Typical steps include the filling with dielectric or/and NaCl particles into PDMS matrix, film forming, cutting and CS-TENG assembling. The morphologies and characterizations of the filling materials are given in the Supporting Information Figure S2 Field emission scanning electron microscopy (FE-SEM) image in Figure 1b shows the composite PDMS film with 20% SrTiO3 NPs by volume, from which we can see that SrTiO3 NPs are well dispersed in PDMS matrix. Figure 1c shows a cross-section FE-SEM image of the PDMS sponge film, revealing that the pores are well formed in the film, which demonstrates the complete removal of NaCl particles. The diameter of the pores is about 10 µm. In order to investigate the influence of dielectric property on the output performance of TENG, the PDMS films filled with different permittivity materials are prepared. The digital photographs and light transmittance of the films with different filling volume ratios are presented in Figure S3. From both the digital photographs and measured light transmittance, it is clear that the transparency of the composite films decreases with the increase in filling volume ratio. Combining the above two process, the CS-PDMS film (20% SrTiO3 NPs by volume, 15% porosity by volume and thickness in 0.55 mm) is fabricated and its digital photograph is shown in Figure 1d, which has excellent flexibility but non-transparency. Figure 1e presents the fabrication 5 Environment ACS Paragon Plus


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process and digital photograph of CS-TENG. The CS-PDMS film with a Cu layer deposited on one side (as the bottom electrode) adheres to an acrylic panel with four springs fixed at the corners. The Cu layer on another acrylic panel acts as the opposite tribo-material and top electrode. Working mechanism of TENG with capacitor structure. Figure 2 schematically presents the basic working mechanism of the TENG with pure PDMS film under the vertical compressive force. In the initial state, before the contact of the tribo-material and the top Cu electrode, there is no charge transfer, and thus no electric potential. When the two tribo-materials contact and rub, a number of charges exchanges between Cu and PDMS to bring the Fermi levels of the two materials into coincidence according to the surface state model for metal-polymer contact charging36, which forms triboelectric charges with density (σ0) on the PDMS film (bound charges). During the periodical contact-separation process, the triboelectric charges on the PDMS induce a periodical movement of the free electrons on the top and bottom electrodes to generate electron flows in external circuit. In detail, when an external force is applied to the TENG, the electrostatic equilibrium is broken and electrons transfer from the top electrode to the bottom electrode to neutralize the positive charges on the bottom electrode through the external circuit, generating the positive current signal. When the top electrode contacts PDMS film, the electrons on the bottom electrode transfer completely to the top electrode due to the electrostatic induction (driven by electrical potential difference). When the external force is gone, the two surfaces separate automatically by the elastic force. The negative charges on the PDMS surface induce positive charges on the bottom electrode, thus, electrons on the bottom electrode move to the top electrode through external circuit to neutralize the positive charges in the top electrode, generating the negative current signal. The details of charge distribution under the application and release of vertical compressive force are schematically presented in 6 Environment ACS Paragon Plus

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Figure 2a, and the output current in a contact-separate cycle is depicted in Figure 2b. The cross-section of the structure of the TENG is given in Figure 2c. From previous work,37 the output voltage V can be expressed by

V=

(σ 0 − ∆σ ) ⋅ x(t ) − ∆σ ⋅ d PDMS ε0

(1)

ε 0ε r

where ε0, εr, σ0 and ∆σ are the vacuum permittivity, relative permittivity of the PDMS, triboelectric charge density on the PDMS, transferred charge density on the Cu electrode in a stage, and x(t), dPDMS and t are the interlayer distance, thickness of PDMS film and time, respectively. At open-circuit (OC) condition, there is no charge transfer, which means that ∆σ is 0. Therefore, the open-circuit voltage VOC is given by37

VOC =

σ 0 ⋅ x(t ) ε0

(2)

indicating that the open-circuit voltage is obtained at a maximum gap between the top electrode and PDMS surface. From Eqn (2), it seems that the VOC does not depend on the PDMS film. However, the σ0 is depend on the capacitance of the device for a contact-mode generator, because the generator acts as both an energy storage and energy output device.33 The maximum capacitance (Cmax) of the device is determined by

C max = ε 0 S ⋅

εr d PDMS

(3)

Therefore, σ0 is proportional to Cmax, that is, proportional to εr/dPDMS. According to this understanding, the VOC increases with an increase in the relative permittivity εr or with a decrease in the thickness of PDMS film or with both.



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Influence of dielectric material filled into PDMS. In order to increase the relative permittivity εr of the PDMS film, we fabricated the high permittivity composite PDMS films by filling dielectric NPs. Here, the SiO2 (εr=3, the same as that of PDMS), TiO2 (εr=80), BaTiO3 (εr=150), SrTiO3 (εr=300) NPs are used to increase the permittivity of the PDMS film. Of course, other nano-materials with high permittivity can also be used as a filling to modify the dielectricity of the PDMS film. Figure 3a and b show the open-circuit voltage and short-current density of the TENGs using the different dielectric NP filling films of the same size (2 × 2 cm2) and thickness (~ 0.55 mm) under a periodic compressive force at frequency of 2.5 Hz. Obviously, both the open-circuit voltage and shortcurrent density are greatly enhanced after the PDMS are filled with higher dielectric NPs, and they reach a peak value of 305 V and 7.18 µA·cm−2 for the composite PDMS film containing 10 vol % SrTiO3 NPs, while only 172 V and 2.98 µA·cm−2 are obtained for the pure PDMS. The peak voltage and current density for the composite PDMS film are 1.8 and 2.4 times as much as that of P-TENG. It is worth noting that the output signals increase first and then decrease with an increase in filling volume ratio, while they remain the same within a certain error range with filling the SiO2 NPs in volume ratio from 0% to 25% owing to the same permittivity as that of PDMS. With the same filling volume ratio, the intensity of the output signals increases with the increase in the filling permittivity. The open-circuit voltage and short-circuit current density versus time for the composite PDMS films with different filling ratios are shown in Figure S4 and S5, respectively. Transfer charge quantity obtained from current integration in half cycle has the same tendency as that of current density, and reaches a peak value of 16 nC·cm-2 for the composite PDMS film containing 10 vol % SrTiO3 NPs (Figure 3c). Therefore, filling high permittivity materials into the PDMS can enhance the output performance of the PDMS based TENG.


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To understand the phenomenon that the output do not increase linearly with an increase in filling volume ratio, the relative permittivity of the PDMS film with filling different high permittivity NPs in the filling volume ratio from 0-25% is measured. Figure 3d (symbols) depicts the influence of dielectric properties of the PDMS film by filling the high permittivity NPs, which indicates that the permittivity of the composite PDMS film increases almost linearly with the increase in filling volume ratio of the filling NPs except SiO2 NPs. There are multiple models to calculate the permittivity of the composite film with filling high dielectric particles into a matrix, such as the Maxwell-Garnett formula38, 39 and the Landau-LifshitzLooyenga model.40, 41 As the filling particles in this case have irregular geometry (Figure 1b), the Lichtenecker logarithmic mixture formula42 is selected to model the properties of the effective medium, which is valid for homogenized dielectric mixtures in which the shapes and orientations of the components are randomly distributed. This formula is given in Eqn (4), where εn and fv,n are the complex permittivity and volumetric fraction of the nth constituent medium, respectively, and εeff is the effective permittivity of the overall mixture. N

ε eff = ∏n =1 ε n f v,n

(4)

The schematic diagram of the composite film is given in Figure 3e, according to which we deform eqn (4) to a suitable eqn (5)

ε eff = ε a f a + ε b f b

(5)

Where εa and fa are the relative permittivity and volumetric fraction of filling particles; εb and fb are the relative permittivity and volumetric fraction of PDMS matrix. According to eqn (5), we have calculated the effective permittivity εeff, as is shown in Figure 3d (solid lines), indicating that the εeff of the composite films increases with an increase in the filling particle volume ratio. The results show reasonable agreement with the 9 Environment ACS Paragon Plus


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measured values, with discrepancy potentially due to agglomeration of filling particles that compromises the homogenization of the dielectric. It can be seen that the relative permittivity can increase from 3 to 9.68 by increasing the volume ratio of SrTiO3 NPs from 0% to 25%. According to eqn (2-3), VOC should be proportional to εr. However, the VOC, short-circuit current and charge transfer of composite PDMS film based TENG (C-TENG) versus the relative permittivity εeff present a nonlinear character (Figure 3a-c), which can be divided into two parts. First, the output signals increase with an increase in the filling content of the higher permittivity NPs in the PDMS matrix, which agrees with the theoretical analysis. Second, the output signals decrease after they reach the peaks in a higher filling content of the higher permittivity NPs, which do not comply with the expectation. To consider the influence of high permittivity NPs filling in PDMS on the output performance of C-TENG, we should consider two factors, permittivity (εr) and surface area of PDMS (S) if the dPDMS is a constant. The increase in the permittivity (εr) by filling the higher permittivity particles enhances the capacitance of the C-TENG according to Eqn. (3). Although the increase in the capacitance of the C-TENG due to higher εr would enhance the surface charge density on the PDMS by the repeated friction,34 the effective friction area of PDMS (high negative electricity) reduces because the filling particles appear on the surface at higher filling content. Therefore, the competition of these two reactions results in the decrease in the output signals at higher filling content (larger than 10%). In Figure 3a-3c, the output peaks of the C-TENG by filling TiO2 NPs appear at smaller 8 vol% than that at 10 vol% by filling other fillers might be a result of the smaller particle size of TiO2 (Figure S1b), as smaller particles might cover more surface area of the PDMS. Similar phenomenon also appears in light transmittance (Figure S3b). To verify the decrease in the effective friction area of PDMS at higher filling content, the surface morphology of the composite PDMS film with various volume ratios is characterized in Figure 3f. Obviously, the SrTiO3 NPs on the surface of PDMS increase with an increase in the


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filling content, demonstrating a smaller effective friction area of PDMS at higher filling volume ratio.

Influence of pores formed in PDMS. According to Eqn. (3), another way to increase the capacitance of the TENG is to reduce the dPDMS. However, unlimited reduction of the dPDMS is forbidden as a result of short circuit between top and bottom electrodes. The pores forming in/on the sponge PDMS film can effectively reduce its thickness and increase its surface area. Therefore, the sponge PDMS film is fabricated at a constant apparent thickness by first filling the PDMS with NaCl particles (~10 µm in size) in different volume ratios of NaCl to PDMS from 0% to 45% and then removing the NaCl particles. Figure 4a-c present the open-circuit voltage, short-current density and transfer charge of the sponge PDMS film based TENG (S-TENG), revealing that all the signals nonlinearly change with the increase in the pores (first increase and then decrease). The open-circuit voltage and short-circuit current density versus time for the sponge PDMS films with different pore ratios are shown in Figure S6. Among the various films, the S-TENG with 15 vol% pore ratio has the best performance, and the maximum opencircuit voltage, short-current density and transfer charge are approximately 257 V, 6.6 µA·cm2

and 13.2 nC·cm-2, which is 1.5, 2.2 and 1.7 times as much as that of the plate PDMS film. Figure 4c and 4d give the schematic diagrams of the pores forming in the PDMS film.

When the S-TENG is pressed by an external force, the film will shrink into minimum thickness, which makes the output signals of the S-TENG increase due to the enlarged capacitance. Although the pores forming in the PDMS film can reduce its effective thickness, the effective permittivity and surface area are not a constant in this case. The surface area S is enlarged with an increase in the pores on and near the top surface of the PDMS film. Meanwhile, the pores in the PDMS film can be regarded just as the filling particles with the



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permittivity of air εr=1 (lower than that of PDMS). The lower dielectric particles filling in the PDMS result in reduction in effective permittivity according to Eqn (5). The effective permittivity of the sponge PDMS film drops from 3 to 1.83 with the decrease in volume fraction from 0% to 45%, as is shown in Figure 4f, which agrees with the theoretical values, with a small discrepancy possibly due to non-uniform distribution of the pores. Therefore, the competition among three factors, effective thickness, top surface area and permittivity of the sponge PDMS film, leads to the initial increase in the output signals of the S-TENG in a lower volume fraction of pores (< 15%) and the final decrease in a higher volume fraction of pores (>15%). The insets in Figure 4f show surface SEM images of sponge films with different volume fractions of pores, indicating that the pores increase with the increase in the volume fraction of NaCl particles in the first stage. To see the detailed parameter changes of the thickness, relative permittivity (εr), transfer charge (Q) for the C-TENG with filling the SiO2, TiO2, BaTiO3 and SrTiO3 NPs are listed in Table 1. As expected, the samples exhibit an increase in relative permittivity with higher filling content for certain filling and with higher permittivity filling for certain content. For the SiO2 filling PDMS composite films, the measured relative permittivity remains unchanged due to fact that the permittivity of SiO2 NPs is the same as PDMS, while the transfer charge slightly reduces with an increase in the filling volume ratio demonstrates the decrease in the surface area of the PDMS. For TiO2\BaTiO3\SrTiO3 filling PDMS composite films, the measured relative permittivity increases with an increase in the filling volume ratio, the largest relative permittivity is achieved in the sample filled with 25% by volume, which is 137%, 178%, and 224% compared to that of the pure PDMS. The filled samples exhibit an increase first then a decrease in transfer charge (∆Q) in a half cycle. The largest increase in ∆Q is achieved with the sample that is filled with 8% TiO2, 10% BaTiO3 or 10% SrTiO3 NPs

by volume, with an increase of 63%, 89% or 115% as much as that of pure PDMS, 12 Environment ACS Paragon Plus

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respectively. Note that the non-linear trend in the transfer charge of the filled PDMS can be ascribed to the competition of relative permittivity (increase) and effective friction area of PDMS (decrease), leading to the same non-linear tendency in output signals. Moreover, the peak value of the sample with TiO2 fillings at different volume ratio (8%) is a result of its smaller particle size, which makes the PDMS effective surface area drop faster than others versus the filling volume ratio. Based on the above results, it is believed that the permittivity and friction area influence the capacitance of the device, and the capacitance affects the surface charge density on the high negative/positive charge dielectrics.34 Furthermore, the increase in surface charge density can directly enhance the output performance of TENG.

Optimization of both dielectric properties and porosity. As the increase in εr, S and the decrease in dPDMS can increase the capacitance of the TENG to maximize the surface charge density of the dielectric, we think of a strategy to increase the relative permittivity εr and decrease dPDMS by combining the filling higher dielectric NPs and forming pores in the PDMS film simultaneously. From the investigations above, we choose the optimized conditions of 10 vol% SrTiO3 NPs and 15 vol% pores in the PDMS matrix and fabricate the composite and sponge PDMS film. Electrical measurement of the optimized CSTENG is presented in Figure 5, where the short-current density and open-circuit voltage of CS-TENG reach up to 338 V and 9.06 µA·cm-2. It is obvious that both the short-current density and open-circuit voltage of the CS-TENG are further improved compared with the composite or sponge PDMS film based TENG, which are 11% and 39% as much as the best output of the C-TENG. And the inset shows the transferred charge quantity of 19 nC·cm-2 in a half cycle, which is higher than that of the C-TENG (16 nC·cm-2) and S-TENG (13.2 nC·cm2

). The output power of the CS-TENG is also measured with external loads from 0.1 MΩ to

520 MΩ, as is shown in Figure 4c. It can clearly be seen that the output current density drops with an increase in the resistance, while the output voltage follows an increasing trend and 13 Environment ACS Paragon Plus


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finally goes to saturation. Consequently, the maximum power density is 6.47 W·m-2 at an external load of 11 MΩ, as is shown in Figure 4d. The CS-TENG can be used to light at least 44 green LEDs in series (Figure 4e). Moreover, in order to prove the CS-TENG stability and durability, a test is carried out for 15000 cycles (Figure 4f), which does not have any obvious change in the output current.

CONCLUSIONS In summary, we have designed a feasible approach by filling the tribo-material of PDMS with high permittivity NPs and pores. It is found that its output performance has a close relationship with permittivity, thickness and surface area of the tribo-material, as the TENG is an energy storage and conversion device whose capacitance determines the surface charge density of the tribo-material. The relative permittivity of the composite PDMS is proportional to the permittivity of filling NPs. The open-circuit voltage and short-current density reach up to 305 V and 7.18 µA·cm −2 by filling the SrTiO3 NPs with 10 vol% in PDMS, approximately 1.8 and 2.4 times as much as that of P-TENG (172 V and 2.98 µA·cm−2). The pores formation in the PDMS film can reduce its effective thickness. The largest output open-circuit voltage and short-current density is 257 V and 6.6 µA·cm-2, respectively, with 15 vol% of pores. The electrical output performance increases with an increase in the porosity within a certain range, which is attributed to the increase in the ratio of the effective friction area to the thickness. However, the output performance of TENG is not proportional to the volume ratio of the filling high permittivity NPs or the pores to the PDMS in a higher filling content or pore ratio due to side effect of the reduction in the surface area of PDMS by dielectric particles or reduction in the permittivity of PDMS by pores. By combining the filling high permittivity NPs and forming pores in the PDMS simultaneously with optimized condition, the output voltage, current density and power density are achieved to be 338 V, 9.06 µA·cm-2 and 6.47 W·m-2 respectively, by the CS14 Environment ACS Paragon Plus

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TENG with 10 vol% SrTiO3 NPs and 15 vol% pores in the PDMS film, giving over 5-fold power enhancement, compared with the P-TENG. The CS-TENG has been used to light up at least 44 green LEDs in series. This work gives a better understand of the triboelectricity produced by TENG from the view of materials and also presents a novel strategy to enhance the performance of TENG by modifying the trio-material itself rather than by modifying its surface.

ASSOCIATED CONTENT

Supporting Information Available: (Figure S1) Schematic diagram of the experiment setup and measurement system for testing the output performance of the CS-TENG; (Figure S2) Morphology characterization of the fillings; (Figure S3) The optical characters of the composite film; (Figure S4) Open-circuit voltage of each as-fabricated composite film based TENG (f =2.5 Hz); (Figure S5) Shortcurrent density of each as-fabricated composite film based TENG (f =2.5 Hz); (Figure S6) The light transmittance Open-circuit voltage and current density of the sponge PDMS films; (Table S1) The thickness of each sponge PDMS films, including standard deviations These materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author *Tel.: +86 23 65678362. Fax: +86 23 65678362. E-mail address: [email protected] (CG Hu), [email protected] (HY Guo).

Notes



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The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by NSFC (51572040, 11204388), the National High Technology Research and Development Program (863 program) of China (2015AA034801), the Fundamental Research Funds for the Central Universities (CQDXWL-2014-001 and CQDXWL-2013-012), and the large-scale equipment sharing fund of Chongqing University.

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Table

Filler SiO2

TiO2

BaTiO3

SrTiO3

Content in

Thickness

volume ratio

(mm)

5%

ɛeff

ɛtheory

Q (nC/cm2)

0.52±0.02

2.970±0.019

3

7.972

10%

0.55±0.01

3.035±0.032

3

7.586

15%

0.59±0.03

2.983±0.045

3

7.352

20%

0.54±0.01

2.929±0.044

3

7.262

25%

0.57±0.01

3.294±0.070

3

7.089

5%

0.53±0.03

3.586±0.091

3.535

11.389

8%

0.52±0.02

3.694±0.029

3.629

11.923

10%

0.54±0.02

4.267±0.024

4.166

11.718

15%

0.53±0.01

4.996±0.022

4.909

10.270

20%

0.50±0.01

5.921±0.034

5.785

8.602

25%

0.49±0.02

7.108±0.024

6.817

7.171

5%

0.52±0.03

3.649±0.031

3.648

12.367

10%

0.54±0.04

4.577±0.017

4.436

14.071

15%

0.55±0.05

5.602±0.073

5.395

12.895

20%

0.57±0.02

6.829±0.081

6.56

10.583

25%

0.56±0.01

8.343±0.030

7.977

8.482

5%

0.57±0.03

4.165±0.031

3.919

13.684

10%

0.60±0.02

4.967±0.061

4.755

16.012

15%

0.62±0.04

6.484±0.049

5.986

15.229

20%

0.63±0.02

7.823±0.067

7.536

12.996

25%

0.62±0.04

9.713±0.023

9.487

11.075

Table 1. Thickness, relative permittivity (ɛexp, ɛtheory), transfer charge (Q) of the PDMS film filled with the SiO2, TiO2, BaTiO3 and SrTiO3 particles with various volume ratio, including standard deviations.


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Figures

Figure 1. Experimental design of the Composite sponge PDMS based TENG (CS-TENG). (a) Schematic diagrams of the fabrication process for dielectric material/PDMS composite film, sponge PDMS. (b) and (c) Cross-sectional SEM images of the composite film and sponge film. The zoom-in illustration (the insets in b and c shows the detailed particle and the pore. (d) Digital photograph of the composite sponge film. (e) The fabrication process of CSTENG.



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Figure 2. The schematic diagram of the TENG. (a) The electricity generation process in a full cycle of the CS-TENG under external force. (b) The output current generated by the TENG with special space of 10 mm. (c) The cross-section of the capacitance structure of the TENG.


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Figure 3. Electrical measurements of each as-fabricated film based TENG (f =2.5 Hz). (a) Open-circuit

voltage,

(b)

short-current

density and

(c)

transfer

charge

of

the

SiO2/TiO2/BaTiO3/SrTiO3-filled samples with various volume ratios, respectively. (d) Comparison of the measured results with effective medium theoretical calculations. (e) The schematic diagram of the composite film. (f) Relative permittivity changes as a function of SrTiO3 content from 0 to 25 vol%. The insets show SEM images of composite films at various volume ratios. The scale bars are 1 µm.



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Figure 4. Electrical measurements of sponge film based TENG (f=2.5 Hz). (a) Open-circuit voltage, (b) short-current density and (c) transfer charge of the sponge PDMS film with various pore ratios. (d) The schematic diagrams of the pores forming in the PDMS film with maximum separate distance and (e) fully compressed. (f) Comparison of the measured results with the effective medium theoretical values of the sponge PDMS film. The insets show SEM images of sponge films at various pore ratios. The scale bars are 1 mm.


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Figure 5. Electrical measurements of the optimized composite-sponge film based TENG (f =2.5 Hz). (a) Current density and (b) open-circuit voltage curve of the CS-TENG. The inset shows the transferred charge quantity (Q) in a half cycle. (c) Maximum output current, voltage and (d) power density under different external loads. (e) Digital photographs of green LEDs lit by the CS-TENG driven by a hand, (f) the stability and durability test of the CSTENG for 15000 cycles.



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Table of Contents


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Enhancing Performance of Triboelectric ... - ACS Publications

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