Hydrophobic Sponge StructureBased Triboelectric ... - NESEL

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Keun Young Lee, Jinsung Chun, Ju-Hyuck Lee, Kyeong Nam Kim, Na-Ri Kang, Ju-Young Kim, Myung Hwa Kim, Kyung-Sik Shin, Manoj Kumar Gupta, Jeong Min Baik,* and Sang-Woo Kim* Traditionally mechanical energy is converted into electricity, namely, by using piezoelectric,[1–10] electromagnetic,[11] or electrostatic effects.[12] Recently, a new type of power generating device, termed the triboelectric nanogenerator (TNG), based on triboelectric effects coupled with electrostatic effects, has been demonstrated.[13–27] In the triboelectric process, energy conversion is achieved by the periodic contact between two materials that differ in the polarity of triboelectricity yields surface charge transfer. TNGs have been also proven as a cost-effective, simple, and robust technique for energy harvesting. However, most of such TNGs required sophisticated pattern design, and a high degree of integration for high performance. Further, it has been reported that micro/nanostructured films produced large output power due to the increase in the contact area as compared with that of flat films.[13,14] Currently, various kinds of nanoparticles or nanowires are used to decorate triboelectric materials in order to increase the friction between two contact materials in the TNG. However, due to the soft and ductile behavior of nanoparticle/nanowires, the performance of TNGs is generally degraded under a large applied force, and therefore power stability is essential. Furthermore, the output performance of TNGs is also drastically affected by the humidity; maintaining a stable performance in moisture atmosphere is currently a challenge. Therefore, the realization of TNGs with high performance under a wide range

K. Y. Lee, K.-S. Shin, Dr. M. K. Gupta, Prof. S.-W. Kim School of Advanced Materials Science and Engineering Sungkyunkwan University (SKKU) Suwon 440–746, Republic of Korea E-mail: [email protected] J. Chun, K. N. Kim, N.-R. Kang, Prof. J.-Y. Kim, Prof. J. M. Baik School of Mechanical and Advanced Materials Engineering KIST-UNIST-Ulsan Center for Convergent Materials Ulsan National Institute of Science and Technology (UNIST) Ulsan 689–798, Republic of Korea E-mail: [email protected] J.-H. Lee, Prof. S.-W. Kim SKKU Advanced Institute of Nanotechnology (SAINT) Center for Human Interface Nanotechnology (HINT) Sungkyunkwan University (SKKU) Suwon 440–746, Republic of Korea Prof. M. H. Kim Department of Chemistry & Nano Science Ewha Womans University Seoul 120–745, Republic of Korea

DOI: 10.1002/adma.201401184

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Hydrophobic Sponge Structure-Based Triboelectric Nanogenerator

of humid conditions is highly desirable because the abundance of water molecules in moisture atmospheres results in antistatic materials with a “sweat layer” function, which results in a huge reduction of the triboelectric charging capacity in all materials.[28] Herein, we report a new type of sponge structure-based TNG (STNG) with stable power output performance under a wide range of humid conditions, which consists of a highly compressible inverse opal-structured polydimethyl-siloxane (PDMS) film and an aluminum (Al) film. The output voltage and current from the STNG increases up to a high value of 130 V and 0.10 mAcm−2, respectively, under cycled compressive force, which results in a 10-fold power enhancement, compared with the flat film-based TNG (FTNG) under the same mechanical force. The electrical output performance of the STNG was measured under several mechanical stresses. Various STNGs were also fabricated using different pore size diameters for the sponge structures. The output voltage and current density increases with the decrease in the pore size diameter of the sponge-structured film. Nanoindentation measurements were also conducted to evaluate the resistance to deformation by external force. The experiments showed that the elastic modulus decreased by over 30% in a sponge-structured film. Thus, the sponge-structured film becomes more flexible, and simultaneously the micro/ nanostructure contact area increases when external mechanical forces are applied. Under the same mechanical stress, a spongestructured film possesses a thinner structure than a flat film, increasing the relative capacitance by the increase in the effective (ε/d) value. In addition, we report a very stable and high output performance of the STNG under a wide range of humid conditions. We also show that a large number of light-emitting diodes (LEDs) can be powered by electrical output power generated by the STNG under application of mechanical strain, even in very humid conditions. A schematic diagram for the fabrication of the STNG is shown in Figure 1. The resulting free-standing PDMS film with an inverse opal structure, termed a sponge-structured film, is shown in Figure 1a (see the Experimental Section for fabrication details). It can be seen that the sponge-structured film has been uniformly formed, which also demonstrates the complete removal of polystyrene (PS) (Figure 1b). The corresponding morphologies of the sponge-structured film fabricated with different diameters of the PS spheres are shown in the Supporting Information Figure S1. To fabricate the STNG, a Kapton film was glued onto one side of sponge-structured PDMS films, followed by the attachment of Al electrode on the opposite side of Kapton film, which acts as the bottom electrode. Further, a spacer, made

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Figure 1. Schematic illustration of the STNG. (a) Structure and fabrication process of the STNG. (b) FE-SEM images of the sponge-structured film.

by insulating the polymer with double-sided adhesive, was staked on the sponge-structured film. Finally, another Al electrode was used as top electrode, as well as triboelectric material, which has the opposite triboelectric polarity to PDMS.[15] To compare the output performance and confirm the advantages of sponge-structured films over flat films, a separate TNG was fabricated as well by using flat PDMS film of the same thickness. Figure 2a and 2b present the output voltage and current density, generated by the FTNG and STNG (based on diameter of pore size ∼ 0.5 µm), under a cycled compressive force of 90 N

at an applied frequency of 10 Hz. It is obvious from Figure 2a and 2b that the output voltage and current density reaches a record value of 130 V and 0.10 mAcm−2 from the STNG; however, only 50 V and 0.02 mAcm−2 were obtained from the FTNG under the same mechanical force. The electrical output results show that both the FTNG and STNG generate AC-type signals under cycled compressive force. In general, the charge generation of the STNG and FTNG under cycled compressive force can be understood from the coupling of the triboelectric effect and electrostatic induction.[17]

Figure 2. Electrical output performances of the FTNG and STNG. (a) Output voltage, and (b) current density, of the FTNG and STNG (0.5 µm), (c) with an additional peak for inner pore releasing. (d) Variation of the total output voltage of the FTNG and STNG (0.5, 1, 3 and 10 µm), with the magnitude of applied force ranging from 30 to 90 N.

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pore size increases up to 10 µm, the output voltage and current density (68 V and 0.04 mAcm−2, respectively) decreases significantly, which is due to the decrease in the surface areato-volume with the increase of pore diameter. For the case of 10 µm diameter-based STNG, the surface area-to-volume ratio decreases by over 10 times, compared to the 0.5 µm diameterbased STNG. Hence, the output voltage and current density decrease because of the reduction of contact area and electrostatic induction. To investigate the relationship between the electrical output performance of the STNG with the contact force, a systematic measurement was also performed under different compressive forces. As shown in Figure 2d, under a contact force of 30 N, the maximum output voltage generated by the STNG (0.5 µm) is 15 V. As the force increases from 30 to 90 N, the output voltage also increases, and reached a maximum value of 130 V, under a compressive force of 90 N. The enhancement in the electrical power generation by the mechanical force in STNG can be explained by the higher compressibility of a spongestructured film over a flat film. The air with a lower dielectric constant (εair = 1.0) than PDMS (εPDMS = 3.0, measured in the air atmosphere) is mainly occupied in the pores.[29] Thus, in the original state, the capacitance of the flat film is larger than that of the sponge-structured film. However, when an external force is applied, it is believed that the distance between the two electrodes in a sponge-structured film is reduced significantly, and the dielectric constant also rapidly increases under the same mechanical force, compared with that of the flat film. This increases the capacitance in the sponge-structured film; thereby, the STNG produces an almost 10-fold improvement in power generation, compared with the FTNG. We have also measured the force-response capacitance curves for PDMS films and 10 µm sponge-structured films, and the obtained curves are shown in the Supporting Information, Figure S6. It can be seen clearly from the figure that the 10 µm sponge structure exhibits a higher capacitance than the flat PDMS films of the same thickness. At the application of 40 mN force, the capacitance change for a 10 µm sponge structure is over 2 times larger than that of the flat PDMS film. Such an enhancement is attributed to the higher flexibility/compressibility of the sponge structure than that of the flat film, as shown in the displacement-force curves (see Figure S7 in the Supporting Information). The pores of the sponge structure also play an important role in enhancing the flexibility/ compressibility of the sponge structure, and a nearly four times larger displacement is obtained from a sponge structure with a normal force of 7 mN, compared to the flat film. This flexibility of the sponge structure is also observed in the results of nanoindentation tests. Nanoindentation tests were also conducted on the samples in order to evaluate the resistance to deformation by external force. Figure 3 shows force-indentation depth curves for five samples with no pores, and pore sizes of 0.5, 1, 3, and 10 µm. Compared to solid materials with no pores, the strength (or hardness) and elastic modulus of porous materials can be described by mathematical relations with the relative density of σporous/σbulk ≈ (ρporous/ρbulk)3/2 and Eporous/Ebulk ≈ (ρporous/ρbulk)2, where ρporous and ρbulk are the relative density respectively and Eporous and Ebulk are the Young’s modulus of the porous and



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At the initial state, i.e., before the contact of the materials (in the present case, Al and PDMS), there is no charge transfer, and thus no electric potential. When the two materials are brought into surface contact by compressive force, friction occurs, which results in electrons being transferred from the Al to the PDMS film, resulting in positive and negative triboelectric charges on the surface of Al and PDMS, which are produced due to the triboelectric effect. At this stage, the TNG remains in electrostatic equilibrium state because of a negligible dipole moment. As the TNG is released, a strong dipole moment is formed due to electrostatic effects, which results in an electrical potential difference between the bottom and top Al electrodes being developed. Because the top Al electrode has a higher potential than the bottom electrode, electrons start to flow from the bottom electrode to the top Al electrode through the external circuit to neutralize the positive triboelectric charges in the top electrode, which results in an electric signal being observed from the TNG. Furthermore, when a pressure is applied to the TNG, the former electrostatic equilibrium is broken, and electrons flow back from the top Al electrode to the bottom Al electrode through the external circuit, and the opposite electric signal is detected. Moreover, we assume that in the case of the STNG, some additional charges on the surface of the inner pores are generated, due to the restoration of pores to their original shape and associated electrostatic effects after release, which results in a significant increase of the total electrical output. The details of charge generation inside the pore under the application and release of vertical compressive forces are schematically presented in the Supporting Information, Figure S2. A switching polarity test was conducted in order to verify that the signals are generated from the STNGs rather than from the measurement systems (Supporting Information Figure S3). Further, a durability test (over 1000 cycles) was also conducted to confirm the mechanical stability/durability of the STNG (Figure S4). On the basis of the above results, the electrical output performance can be effectively increased by introducing pores. In more detail, the dramatic increase in the electrical output performance of the STNG can be attributed to the increase of the surface area-to-volume ratio in the sponge-structured PDMS films. In 0.5 µm sponge-structured film, the surface area-tovolume ratio is increased up to several hundred times, compared to that of the flat PDMS film. The calculation of the surface area-to-volume ratio is given in the Supporting Information Table S1. Therefore, the larger ratio increases the contact area, which larger area further increases the surface charges during the contact of the films, enhancing the total output power in the STNG. Furthermore, although the Al electrode and the inner pores in the sponge are not in contact during application of the cycled compressive force, we assumed that additional negative charges can also be generated on the surface of inner pores by electrostatic induction, which enhances the electrical output performance of STNG. We further measured the output voltage and current density from the 1, 3, 5, and 10 µm pore diameter-based STNGs under the same compressive force. The electrical output performances from the increase of diameter of pore size are shown in Figure 2d. The actual output voltage and current density data are given in the Supporting Information Figure S5. As the

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Figure 3. Nanoindentation measurements of the flat film and spongestructured films. (a) Force-displacement curves. (b) Hardness and elastic modulus for film and sponge structures with pore size of 0.5, 1, 3, and 10 µm.

bulk film, respectively.[30] Figure 3a shows that the loading and unloading curves for all samples are almost identical. This means that a plastic collapse does not occur, even in sponge

structures, and elastic deformation and recovery occur during nanoindentation loading and unloading. Nanoindentation using the Berkovich indenter induces a much higher local strain than uniaxial compression; thus, the elastic loadingunloading curves for sponge-structured films in Figure 3a suggest their repeatable elastic deformation in uniaxial compression. As shown in Figure 2, the absence of degradation in output voltage and current density under cycled compressive force also supports this presumption. Figure 3b shows that the elastic moduli of sponge-structured films are lower than that of flat film. In a sponge-structured film with lower elastic modulus than flat film, lower mechanical energy is necessary to attain a certain compressive force, which is very helpful and effective in decreasing the mechanical input energy, contributing to an enhancement in energy conversion efficiency in power generators. (It is noted that the flexibility (f) is govern by f = L/EA, where L is the thickness of sponge-structured film, A is the effective area and E is the elastic modulus of the sponge-structured film.) Further COMSOL Multiphysics software simulation is carried out in order to examine the displacement profile of flat film and sponge-structured film with 10 µm pore size under different normal forces from 0 to 7 mN. The simulation results, as shown in the Supporting Information Figure S7, confirm that as force increases, the displacement also increases for flat film and sponge-structured film; however, the sponge-structured film shows higher displacement over that of flat film at every applied force, which reveals the advantage of sponge-structured film, compared to flat film. In triboelectrics, the surface of the insulating material, such as PDMS, is electrically charged during contact; and it has the ability to hold charges for a longer time. However, the surface charge significantly depends on the ambient relative humidity, and generally degrades with an increase of humidity in the atmosphere. Therefore, maintaining the electric charges, and a stable and even electrical output in a high moisture atmosphere is a challenging task. The electrical output performance of the FTNG and STNG based on different pore size were measured and plotted under various relative humidity conditions, as shown in Figure 4a. The experimental set up for measuring

Figure 4. Performance characterization of the FTNG and STNG under different humid atmosphere. (a) The change in output voltage with the relative humidity change for flat film and sponge-structured films with pore size of 0.5, 1, 3, and 10 µm. (b) Snapshots of 75 commercial LEDs connected in series with relative humidity 75% RH.

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the elastic module decreases by over 30% in a sponge-structured film (0.5 µm pores). The results depict that the spongestructured film is more compressible than the flat film; thereby, increasing the contact area and the capacitance by the increase in effective (ε/d) value under the same force, gresulting in an STNG with high electrical output. Finally, we also achieved a stable output performance from the STNG in a very humid environment. It is believed that this work will serve as a stepping stone for high-performance and stable TNG studies and will also inspire major developments of TNGs towards selfpowered electronics in the near future.

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electrical output performances under a controlled humidity condition is shown in the Supporting Information Figure S8. In the experiment, humid air from a humidifier balanced by dry air was introduced into the chamber under 1 atm of dry air background. By controlling the valve between the humid/ air sources, the humidity inside the chamber can be easily increased or decreased, and measured by the humidity sensor. The FTNG and STNG were then placed in the chamber, and the electrical output performance was measured at each level of relative humidity (RH). It is clearly seen that the output voltage is maintained up to 70% RH although the output voltage starts to decrease significantly from 80% RH in both FTNG and STNG. This may be ascribed to the dissipation of the charge build-up due to the formation of a water layer on the surface of the PDMS films, which is a well-known effect.[31,32] Interestingly, it is clearly seen that the STNG shows a higher electrical output performance, even at 80% RH, than the FTNG, which is attributed to the increase in the hydrophobicity due to the upward force generated by the air inside the pores, lifting water droplets. It is observed that the electrical output performance dropped rapidly at about 80% RH for the FTNG. The actual contact angles of a sessile drop on flat PDMS flat film and 1 µm sponge-structured PDMS film were measured with pure deionized (DI) water (see Figure S9 in the Supporting Information). It has been observed that the contact angles of a flat film and 1 µm spongestructured film are enhanced up to 93° and 102°, respectively, because the sponge-structure-like overhang structure plays a key role in the enhancement of the hydrophobicity.[33] Thus, the hydrophobicity on the sponge structure surface is significantly enhanced. A schematic image of the net force exerted between the sponge film and DI water, and the build contact angle is shown in the Supporting Information Figure S9. To demonstrate the effect of humidity on the performance of the STNG, we directly operated series-connected LEDs from the STNG under varying humid conditions. In the case of 75% RH, the output power from the STNG (0.5 µm diameter) was able to light up 75 LEDs instantaneously and simultaneously. On the other hand, the output power from the FTNG instantaneously lit up only 25 LEDs under the same compressive force and RH (Figure 4b). The above results confirm that the surface modification of PDMS from the flat film to sponge-structured film can dramatically enhance the performance of TNGs over a wide range of humid conditions, and is therefore less sensitive to humidity and produces stable electrical output performance. In conclusion, we have demonstrated a new type of STNG with a stable electrical output performance over a wide range of humid conditions. Sponge-type micro/nanostructured films with various diameters were fabricated. A very large output voltage and current of up to 130 V and 0.10 mAcm−2, respectively, could be obtained from the STNG in comparison to the electrical output obtained from the FTNG (50 V and 0.02 mAcm−2, respectively), under the same mechanical force. The electrical output performance of the STNG increases with a decrease in the pore size diameter, which is attributed to the rapidly increased contact area. Nanoindentation experiments were also carried out to confirm the advantages of sponge-structured films, which was evaluated by the resistance to deformation by external force. The nanoindentation results show that

Experimental Section Fabrication of the Sponge-structured Film (Free-standing PDMS Film with an Inverse Opal Structure): In the experiment, first an aqueous suspension of PS spheres (2.6 wt%, Polysciences, Warrington) was used to fabricate the PDMS inverse opal structured film. Many layers of PS spheres with diameters of 0.5, 1, 3, and 10 µm were stacked in a face-centered cubic structure mode onto a SiO2/Si substrate in order to synthesize sponge structured films of different diameter/pore size, and for their respective STNG fabrication (Figure 1a). Then, the PDMS solution was poured into periodically arranged PS spheres, and was allowed to solidify into an amorphous free-standing film by heating on a hot plate at 90 °C. Next, the PDMS matrix was detached from the substrate to obtain the PDMS inverse opal structured film, which subsequently was soaked in acetone for 24 h to remove the PS spheres. The effective area and thickness of both flat and sponge-structure films were 1 cm × 1 cm and 300 µm, respectively. The spacer was made of an insulating polymer film with double-sided adhesives with a thickness of 0.03 mm. Characterization and Measurements: The morphologies of spongestructured PDMS films were further characterized by field emissionscanning electron microscope (FE-SEM). A pushing tester (Labworks Inc., model no. ET-126–4) was used to create a vertical compressive strain in the nanogenerator. A Tektronix DPO 3052 Digital Phosphor Oscilloscope and a low-noise current preamplifier (model no. SR570, Stanford Research Systems, Inc.) were used for electrical measurements. Nanoindentation tests were carried out at a constant indentation strain rate of 0.05 s−1, with a maximum indentation depth of 20 µm with Berkovich indenter, using the DCM II module in Nanoindenter G200 made by Agilent Corporation. Capacitance in force-response curves were calculated using equation C = ε0εrA/d, where ε0, εr, A, and d are the permittivity of free space (8.854 × 10−12 Fm−1), relative static permittivity (1.0–3.0), contact area (1 cm2), and the distance (300 µm) between top and bottom electrodes, respectively.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements K.Y.L. and J.C. contributed equally to this work. This work was supported by the Basic Research Program (2012R1A2A1A01002787, 2009–0083540) and the Pioneer Research Center Program (2013M3C1A3063602) of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT & Future Planning (MSIP).


Received: March 16, 2014 Revised: April 10, 2014 Published online: May 22, 2014


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