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Hydrophobic Sponge Structure-Based Triboelectric Nanogenerator

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 *

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 com-pressible inverse opal-structured polydimethyl-siloxane (PDMS) fi lm and an aluminum (Al) fi lm. The output voltage and cur-rent 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 fl at fi lm-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 fi lm. 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 fi lm. Thus, the sponge-structured fi lm becomes more fl exible, and simultaneously the micro/nanostructure contact area increases when external mechanical forces are applied. Under the same mechanical stress, a sponge-structured fi lm possesses a thinner structure than a fl at fi lm, increasing the relative capacitance by the increase in the effec-tive (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 gener-ated 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 fi lm with an inverse opal structure, termed a sponge-structured fi lm, is shown in Figure 1 a (see the Experimental Section for fabrica-tion details). It can be seen that the sponge-structured fi lm has been uniformly formed, which also demonstrates the complete removal of polystyrene (PS) (Figure 1 b). The corresponding mor-phologies of the sponge-structured fi lm fabricated with different diameters of the PS spheres are shown in the Supporting Infor-mation Figure S1. To fabricate the STNG, a Kapton fi lm was glued onto one side of sponge-structured PDMS fi lms, followed by the attachment of Al electrode on the opposite side of Kapton fi lm, which acts as the bottom electrode. Further, a spacer, made DOI: 10.1002/adma.201401184

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: kimsw1@skku.edu J. Chun, K. N. Kim, N.-R. Kang, Prof. J.-Y. Kim, Prof. J. M. Baik School of Mechanical and Advanced Materials EngineeringKIST-UNIST-Ulsan Center for Convergent Materials Ulsan National Institute of Science and Technology (UNIST) Ulsan 689–798 , Republic of Korea E-mail: jbaik@unist.ac.kr 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

Traditionally mechanical energy is converted into electricity, namely, by using piezoelectric, [ 1–10 ] electromagnetic, [ 11 ] or elec-trostatic 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 con-version is achieved by the periodic contact between two mate-rials 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 fi lms produced large output power due to the increase in the contact area as compared with that of fl at fi lms. [ 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. Fur-thermore, 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

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by insulating the polymer with double-sided adhesive, was staked on the sponge-structured fi lm. Finally, another Al elec-trode was used as top electrode, as well as triboelectric mate-rial, which has the opposite triboelectric polarity to PDMS. [ 15 ] To compare the output performance and confi rm the advantages of sponge-structured fi lms over fl at fi lms, a separate TNG was fab-ricated as well by using fl at PDMS fi lm of the same thickness.

Figure 2 a and 2 b present the output voltage and current den-sity, 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 2 a and 2 b that the output voltage and current density reaches a record value of 130 V and 0.10 mAcm −2 from the STNG; how-ever, 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 genera-tion of the STNG and FTNG under cycled compressive force can be understood from the coupling of the triboelectric effect and electrostatic induction. [ 17 ]

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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 fi lm.

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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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 fi lm, resulting in positive and negative tri-boelectric 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 negli-gible 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 fl ow 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 fl ow back from the top Al electrode to the bottom Al electrode through the external circuit, and the oppo-site 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 signifi cant increase of the total elec-trical 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 confi rm the mechanical stability/durability of the STNG (Figure S4).

On the basis of the above results, the electrical output per-formance can be effectively increased by introducing pores. In more detail, the dramatic increase in the electrical output per-formance of the STNG can be attributed to the increase of the surface area-to-volume ratio in the sponge-structured PDMS fi lms. In 0.5 µm sponge-structured fi lm, the surface area-to-volume ratio is increased up to several hundred times, com-pared to that of the fl at PDMS fi lm. The calculation of the sur-face 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 fi lms, 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 den-sity from the 1, 3, 5, and 10 µm pore diameter-based STNGs under the same compressive force. The electrical output perfor-mances from the increase of diameter of pore size are shown in Figure 2 d. The actual output voltage and current density data are given in the Supporting Information Figure S5. As the

pore size increases up to 10 µm, the output voltage and cur-rent density (68 V and 0.04 mAcm −2 , respectively) decreases signifi cantly, which is due to the decrease in the surface area-to-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 diameter-based STNG. Hence, the output voltage and current density decrease because of the reduction of contact area and electro-static 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 2 d, 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 sponge-structured fi lm over a fl at fi lm. The air with a lower dielectric constant ( ε air = 1.0) than PDMS ( ε PDMS = 3.0, measured in the air atmosphere) is mainly occupied in the pores. [ 29 ] Thus, in the original state, the capacitance of the fl at fi lm is larger than that of the sponge-structured fi lm. However, when an external force is applied, it is believed that the distance between the two electrodes in a sponge-structured fi lm is reduced signifi -cantly, and the dielectric constant also rapidly increases under the same mechanical force, compared with that of the fl at fi lm. This increases the capacitance in the sponge-structured fi lm; 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 fi lms and 10 µm sponge-structured fi lms, and the obtained curves are shown in the Supporting Informa-tion, Figure S6. It can be seen clearly from the fi gure that the 10 µm sponge structure exhibits a higher capacitance than the fl at PDMS fi lms of the same thickness. At the application of 40 mN force, the capacitance change for a 10 µm sponge struc-ture is over 2 times larger than that of the fl at PDMS fi lm. Such an enhancement is attributed to the higher fl exibility/com-pressibility of the sponge structure than that of the fl at fi lm, as shown in the displacement-force curves (see Figure S7 in the Supporting Information). The pores of the sponge struc-ture also play an important role in enhancing the fl exibility/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 fl at fi lm. This fl ex-ibility 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 fi ve 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 σ porous / σ bulk ≈ ( ρ porous / ρ bulk ) 3/2 and E porous / E bulk ≈ ( ρ porous / ρ bulk ) 2 , where ρ porous and ρ bulk are the relative density respectively and E porous and E bulk are the Young’s modulus of the porous and

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bulk fi lm, respectively. [ 30 ] Figure 3 a 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 loading-unloading curves for sponge-structured fi lms in Figure 3 a suggest their repeatable elastic deformation in uniaxial com-pression. As shown in Figure 2 , the absence of degradation in output voltage and current density under cycled compressive force also supports this presumption.

Figure 3 b shows that the elastic moduli of sponge-structured fi lms are lower than that of fl at fi lm. In a sponge-structured fi lm with lower elastic modulus than fl at fi lm, 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 effi ciency in power generators. (It is noted that the fl exibility ( f ) is govern by f = L / EA , where L is the thickness of sponge-struc-tured fi lm, A is the effective area and E is the elastic modulus of the sponge-structured fi lm.) Further COMSOL Multiphysics software simulation is carried out in order to examine the dis-placement profi le of fl at fi lm and sponge-structured fi lm with 10 µm pore size under different normal forces from 0 to 7 mN. The simulation results, as shown in the Supporting Informa-tion Figure S7, confi rm that as force increases, the displace-ment also increases for fl at fi lm and sponge-structured fi lm; however, the sponge-structured fi lm shows higher displace-ment over that of fl at fi lm at every applied force, which reveals the advantage of sponge-structured fi lm, compared to fl at fi lm.

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 signifi cantly 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 4 a. The experimental set up for measuring

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Figure 3. Nanoindentation measurements of the fl at fi lm and sponge-structured fi lms. (a) Force-displacement curves. (b) Hardness and elastic modulus for fi lm and sponge structures with pore size of 0.5, 1, 3, and 10 µm.

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 fl at fi lm and sponge-structured fi lms 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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electrical output performances under a controlled humidity condition is shown in the Supporting Information Figure S8. In the experiment, humid air from a humidifi er 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 signifi -cantly 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 fi lms, 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 gen-erated 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 fl at PDMS fl at fi lm and 1 µm sponge-structured PDMS fi lm were measured with pure deionized (DI) water (see Figure S9 in the Supporting Information). It has been observed that the contact angles of a fl at fi lm and 1 µm sponge-structured fi lm 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 signifi cantly enhanced. A schematic image of the net force exerted between the sponge fi lm 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 instanta-neously lit up only 25 LEDs under the same compressive force and RH (Figure 4 b). The above results confi rm that the surface modifi cation of PDMS from the fl at fi lm to sponge-structured fi lm 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 fi lms 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 confi rm the advantages of sponge-struc-tured fi lms, which was evaluated by the resistance to deforma-tion by external force. The nanoindentation results show that

the elastic module decreases by over 30% in a sponge-struc-tured fi lm (0.5 µm pores). The results depict that the sponge-structured fi lm is more compressible than the fl at fi lm; 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 step-ping stone for high-performance and stable TNG studies and will also inspire major developments of TNGs towards self-powered electronics in the near future.

Experimental Section Fabrication of the Sponge-structured Film (Free-standing PDMS Film with

an Inverse Opal Structure) : In the experiment, fi rst an aqueous suspension of PS spheres (2.6 wt%, Polysciences, Warrington) was used to fabricate the PDMS inverse opal structured fi lm. 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 SiO 2 /Si substrate in order to synthesize sponge structured fi lms of different diameter/pore size, and for their respective STNG fabrication (Figure 1 a). Then, the PDMS solution was poured into periodically arranged PS spheres, and was allowed to solidify into an amorphous free-standing fi lm 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 fi lm, which subsequently was soaked in acetone for 24 h to remove the PS spheres. The effective area and thickness of both fl at and sponge-structure fi lms were 1 cm × 1 cm and 300 µm, respectively. The spacer was made of an insulating polymer fi lm with double-sided adhesives with a thickness of 0.03 mm.

Characterization and Measurements : The morphologies of sponge-structured PDMS fi lms were further characterized by fi eld emission-scanning 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 preamplifi er (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 = ε 0 ε r A / d , where ε 0 , ε r , 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 cm 2 ), 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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