Cylindrical Water Triboelectric Nanogenerator via Controlling

Sep 13, 2016 - We demonstrate a cylindrical water triboelectric nanogenerator (CW-TENG) that generates sustainable electrical output. The inner surfac...
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Cylindrical Water Triboelectric Nanogenerator via Controlling Geometrical Shape of Anodized Aluminum for Enhanced Electrostatic Induction Sukyung Lee,†,‡ Jihoon Chung,†,‡ Dae Yun Kim,‡ Jung-Yeul Jung,§ Seong Hyuk Lee,*,‡ and Sangmin Lee*,‡ ‡

School of Mechanical Engineering, Chung-Ang University, 84, Heukseuk-ro, Dongjack-gu, Seoul 156-756, Korea Technology Center for Offshore Plant Industries, Korea Research Institute of Ships and Ocean Engineering, KIOST, Daejeon 34103, Korea

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S Supporting Information *

ABSTRACT: We demonstrate a cylindrical water triboelectric nanogenerator (CW-TENG) that generates sustainable electrical output. The inner surface of the cylinder was patterned into superhydrophobic and hydrophilic parts to control water flow inside the packaged design of CW-TENG. Here, various thicknesses and roughnesses of the superhydrophobic surface, generated using aluminum oxide nanostructures for enhanced electrostatic induction, were measured to obtain the maximum output and superhydrophobicity. Also, we demonstrate the possibility of using a hydrophilic surface for energy harvesting and as a water reservoir in the packaged design. KEYWORDS: energy harvesting, triboelectric nanogenerator, wettability, nanostructure, anodized aluminum

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output of micro/nanostructured TENGs are highly affected by the geometrical dimensions of these structures,11,12 no study has shown the effect of surface nanostructure dimensions on these characteristics. Moreover, previous studies have only focused on creating a hydrophobic surface on water−solid TENGs for inducing separation, but creating hydrophilic surfaces for continuous contact-separation between water and the solid surface has not been studied. Therefore, a new design is necessary for using a hydrophilic surface along with the hydrophobic surface to control the water flow inside a packaged system for practical applications. Here, we demonstrate a cylindrical-type water TENG (CWTENG) for harvesting energy from rotation and water electrification. CW-TENG is a fully enclosed cylindrical design that provides a packaging solution for practical use of the water−solid contact TENGs. It utilizes both superhydrophobic

nergy harvesting from the environment is of great interest because of a rapid increase in energy consumption, with several new energy conversion methods being introduced.1,2 Among these, a triboelectric nanogenerator (TENG) has attracted attention because of its simple fabrication, high conversion efficiency, and cost effectiveness.3−5 A TENG generates electrical output through contact electrification, which is induced by friction between two dielectrics with different triboelectric polarity.4 However, their durability and stability needs to be improved for sustainable use because friction is the fundamental basis of TENGs.6 Recently, a liquid−solid contact TENG was introduced, which utilizes liquids as a dielectric material.7−9 These have not only significantly reduced the friction damage of dielectric materials but have also shown stable output regardless of humidity. All previous studies have designed TENGs with a hydrophobic surface, which was considered essential for the separation of liquid dielectrics, and used various nanostructured polymers8,10 or self-assembled monolayers (SAMs) on micro/nanostructured surfaces.7 Although both the wettability and electrical © 2016 American Chemical Society

Received: July 18, 2016 Accepted: September 13, 2016 Published: September 13, 2016 25014

DOI: 10.1021/acsami.6b08828 ACS Appl. Mater. Interfaces 2016, 8, 25014−25018

Letter

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration and working mechanism of CW-TENG. (a) Schematic illustration of CW-TENG. Working mechanism of (b) hydrophilic and (c) superhydrophobic surface TENG units.

nanostructures were controlled through the anodizing and widening times.11,13 Superhydrophobic surfaces were created using a SAM of heptadeca-fluoro-1,1,2,2-tetrahydrodecyl trichlorosilane to lower the surface energy of the AAO nanostructure.7,14 The SAM-coated hydrophobic surface was then dipped in 0.3% polytetrafluoroethylene (PTFE) solution and dried at 25 °C. Superhydrophobic/hydrophilic AAO substrates were patterned inside a cylinder (Figure 1a) to control the water flow using different wettability. The rotating part of the CW-TENG was fabricated using an acrylic substrate, and PTFE film was placed on top of the protruding shape, which pumps up the water in the reservoir created by the hydrophilic surface and drives it up to the superhydrophobic surface (Figure S1), to ensure water separates easily from the rotating part. The working principle of the CW-TENG is based on the contact electrification between water and two surfaces with different wettability (Figure 1b, c). The fundamental principle modes of CW-TENG can be explained as a single-electrode mode.15 The water inside the cylinder is already positively charged from contact with the PTFE on the rotating component. Figure 1b-i shows the contact electrification between water and the hydrophilic surface, when they are charged with opposite triboelectric polarity, and then, reaches electrical equilibrium. Using the rotating component, water is splashed onto other surfaces, and EPD is established on the hydrophilic surface as the water volume changes. The electrons will flow from the Al electrode to ground to reach electrical equilibrium (Figure 1b-ii). When the rotating component completely overlapped with the hydrophilic surface (Figure 1biii), triboelectric charges with opposite polarities between AAO and the electrode are balanced. As the rotating component continues to move and slides to the superhydrophobic surface, the splashed water returns to the hydrophilic surface. With the volume change of the water, the electrons will flow back in order to maintain electrostatic equilibrium (Figure 1b-iv). During rotation, water is pushed onto the superhydrophobic surface and contact electrification occurs between the two surfaces by the rotating part (Figure 1c). The superhydrophobic surface is negatively charged when water contacts the surface (Figure 1c-i).7,8 EPD is formed between the superhydrophobic surface and the water surface. Thus, electrons flow from the ground to the Al electrode (Figure 1c-ii). When the rotating part reaches above the superhydrophobic surface, there is no contact surface area difference because the same amount of water remains between the rotating part and the superhydrophobic surface (Figure 1c-iii). As the relative rotation continues, water detaches from the superhydrophobic surface and goes to the hydrophilic surface. An EPD will again be established, causing electrons to flow to

Figure 2. Wettability and voltage measurements depending on AAO structure. (a) Contact angle of AAO and PTFE-coated AAO with widening time. (b) Initial voltage of hydrophobic surface TENG with anodization time. Continuous voltage of (c) 2.5 h anodization time.

and hydrophilic surfaces to control the flow of water and as an individual energy harvesting device. For the first time, the effect of the thickness and roughness of a superhydrophobic nanostructure on the wettability and electrical output was measured by analyzing the water contact angle (CA) and electrical output at different nanostructure roughnesses. Furthermore, the possibility of using a hydrophilic surface for both energy harvesting and as a water reservoir in a packaged design was verified through experiments and computational fluid dynamic (CFD) simulations. The CW-TENG is composed of a base and a rotating part (Figure 1a). On the base, superhydrophobic and hydrophilic surfaces were patterned using anodized aluminum oxide (AAO). The diameter and thickness of aluminum oxide 25015

DOI: 10.1021/acsami.6b08828 ACS Appl. Mater. Interfaces 2016, 8, 25014−25018

Letter

ACS Applied Materials & Interfaces

Figure 3. Hydrophilic generation unit of CW-TENG. (a) Schematic illustration of AAO and FE-SEM image of top and side view of AAO. (b) Opencircuit voltage (VOC) and (c) closed-circuit current (ICC) of hydrophilic surface TENG. (d) Simulation results of electrical potential field inside the electrode with respect to the volume fraction of moving water on the electrode.

the nanopore size by controlling the widening time from 0.5 to 3 h. To determine the diameter of nanopores based on the widening process, we obtained magnified images using a fieldemission scanning electron microscope, and the CA was measured using a 4 μL droplet at room temperature. The CA of the hydrophobic surface was analyzed according to widening time (Figure 2a). As the widening process progressed, the hydrophobic AAO surface becomes superhydrophobic (>150°) after a widening time of 2 h. However, the CA of AAO increases until 2.5 h, after which time, it starts decreasing. The diameter of the nanopores increased 40−50 nm/h as the widening process progressed (Figure S2). After 2 h, the nanopore structure starts collapsing, forming nanowires (Figure S2d, e). When the widening time was over 3 h, the nanostructured surface was etched excessively, exposing the aluminum substrate at the bottom (Figure S2f), and the roughness ratio decreases. In CW-TENG, a widening time of 2.5 h, which shows the maximum hydrophobicity, was used to measure the electrical power output. The open-circuit voltage (VOC) was measured to compare the electrical performance with the thickness of AAO. The voltage peaks generated by the first drop of deionized (DI) water gradually decreases depending on the thickness of AAO (Figure 2b). This is due to the electrostatic induction influenced by thickness of AAO, which leads to lower voltage peaks in the TENG. In addition, the dielectric losses and screening effect of AAO surface can decrease the voltage output of AAO surface.12 However, the surface with low AAO thickness has a low roughness ratio, and hence, a low

the ground from the Al electrode (Figure 1c-iv). Therefore, alternating current is generated in both hydrophilic and superhydrophobic surface. As the CW-TENG uses both the hydrophobic and hydrophilic surface to harvest energy, it is necessary to create appropriate nanostructures. The nanostructured hydrophobic surface could influence wetting properties because of increased surface roughness and trapped air according to the Cassie− Baxter equation16 cos θ = rf f cos θY + f − 1

where θ is the contact angle, rf is the roughness ratio of the wet surface area, f is the fraction of solid surface area wet by the liquid, and θY is the ideal Young’s contact angle on a smooth surface. The roughness ratio can be explained as the ratio of the true area to the apparent surface area of the solid surface, which can be increased by increasing the diameter and depth of the AAO nanopores. On the hydrophilic surface, water continuously stays on the AAO surface. Therefore, it is important for the nanostructure to block direct contact of water and the electrode to avoid a short circuit. The nanostructures on the hydrophilic surface are designed to have sufficient thickness to prevent this. Water is repelled by a hydrophobic surface, and so the depth of the nanostructure influences the electrostatic induction for triboelectric effect.12 Therefore, an analysis of the relation between wettability and electrostatic induction based on the dimensional parameters of AAO is necessary. For a hydrophobic surface, specimens were prepared with a nanopore depth of ∼10 μm to analyze wettability according to 25016

DOI: 10.1021/acsami.6b08828 ACS Appl. Mater. Interfaces 2016, 8, 25014−25018

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ACS Applied Materials & Interfaces

consistent output with a high voltage is 12.5 μm, which produced an average output of 8 V (Figure 2c). Until now, a hydrophilic surface was considered as incompatible with TENG because water is not completely removed from the surface.17 CW-TENG has a hydrophilic surface patterned with a superhydrophobic surface to perform multiple functions. One is to act as a water reservoir for continuous water flow and another is an additional generating device for water energy harvesting. The hydrophilic surface is fabricated with a porous AAO surface, with a depth of 5 μm, and an average diameter of 50 nm (Figure 3a). Water (1 mL) was filled in the base cylinder, and high-speed photographs of water behavior were obtained in an enclosed system at an angular velocity of 220 rpm to observe the water behavior (Figure S4). Water is first present on the hydrophilic surface, and then, pumped up to the superhydrophobic surface by the rotating component. Moreover, the water pumped by the rotating part generates an output on generating unit located on the top and the right side (Figure S4-iii, iv. The electrical output of the lower hydrophilic surface was measured at 400 rpm. The measured VOC peak is about 2 V (Figure 3b), and the measured closed-circuit current (ICC) is about 60 nA (Figure 3c). To demonstrate this, we conducted CFD simulation for the electrical potential generated by the movement of water with time. The method for simulation is further explained in the Supporting Information. Figure 3d shows the electrical potential field inside the electrode with respect to the volume fraction of moving water on the superhydrophobic surface. In the simulation, the electrical potential difference between the water and the electrode as well as the volume fraction of moving water increases. At 90% volume fraction, the maximum potential difference is estimated to be 1.3 V at 90 ms after movement of water. This shows that the volume change of water is related to EPD and the electrical output is influenced by the water behavior. The measured output between the splashed water by rotation at 400 rpm and the superhydrophobic surface is a VOC of 7 V and an ICC of 50 nA (Figure S5-a,b). In addition, the electrical output of the hydrophobic surface was analyzed using CFD simulation, performed while the droplet falls from the hydrophobic surface having a CA of 150° (Figure S6). An electrical potential is generated inside the electrode owing to the running droplet. Additionally, to show the performance of multiple patterned CW-TENG, the performance of other superhydrophobic and hydrophilic surfaces of CW-TENG was measured from the upper hydrophilic surface and the superhydrophobic surface on the right side. The upper hydrophilic surface produced an output with a voltage of up to ∼5.5 V and the superhydrophobic surface produced an output of up to ∼4 V (Figure S5-c,e), and the output current was measured as Figure S5-d,f. CW-TENG generates multiple outputs in a single rotation through the patterned surface. With more patterned electrodes inside the device, the total power output generated by the device can be multiplied. As CW-TENG has a fully-packaged design, property of water inside can be critical to output of CW-TENG. Therefore, the effect of salinity and PH value on output of CW-TENG is measured. As shown as Figure 4a, output of super-hydrophobic surface in CW-TENG were measured depending on DI water and 4 different concentration of NaCl Solution. The output of CW-TENG is shown to decrease because amount of triboelectric charges decrease with increment in the ion concentration.18,19 In Figure 4b, output of same surface was measured

Figure 4. Output performance of super-hydrophobic surface in CWTENG. (a) VOC output of CW-TENG depending on DI water and 4 different concentration of NaCl solution, and (b) 3 different pH buffer solution. (d) The charging capacitor through CW-TENG during rotating motion. (d) VOC output of super-hydrophobic surface measured for 24 hours.

hydrophobicity.13 In the device, if the surface has insufficient hydrophobicity, water remains on the surface of CW-TENG after it first comes in contact with the hydrophobic surface. Hence, we measured the open-circuit voltage continuously to obtain the appropriate condition for superhydrophobicity and clear electrostatic induction. The voltage output of each hydrophobic AAO samples were measured to compare their performance by placing DI water 5 times with a delay of 5 s (Figure S3). A specimen anodized for 0.5 h produced an average voltage peak of 40 V the first time, but the next output rapidly decreases since there is water remaining from after the first contact. However, when the thickness of AAO increased, the generated output becomes lower but sustainable. The minimum thickness of the anodized specimen that shows 25017

DOI: 10.1021/acsami.6b08828 ACS Appl. Mater. Interfaces 2016, 8, 25014−25018

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ACS Applied Materials & Interfaces

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depending on pH concentration. Different pH buffer solutions were used in this experiment. As shown as the plot, the output voltage slightly increases depending on the pH value. This indicates that lower concentration of positive hydrogen ion (H +) assists generation of triboelectric charge. To demonstrate possibility of CW-TENG as stand-alone generating device, we charged a 0.1μF capacitor using rectifying circuit (Figure 4c). The capacitor is charged in 400 rpm rotating condition. In addition, to demonstrate the robustness and sustainable output generation of CW-TENG, VOC output of super-hydrophobic surface is measured for 24 hours while the specimen is exposed in flow of tap water (Figure 4d). The VOC output in 0, 6, 20, 24 hours is shown at Figure S7. This study shows the potential of the CW-TENG as a self-powered device for robust, sustainable electronic application. In summary, we have successfully developed a novel packaged design of water-solid contact TENG using patterned superhydrophobic and hydrophilic surfaces. The effect of the superhydrophobic surface for sustainable electrical output and hydrophobicity is analyzed to balance the hydrophobicity and electrostatic induction. We found appropriate surface conditions based on the CA analysis and the measured output. We also found the possibility of using the hydrophilic surface as an energy harvesting unit and as a water reservoir in the packaged design. On the basis of the optimized condition, we developed a cylindrical TENG with a patterned surface. The patterned surface on the base showed consistent contact and separation during rotation of the rotating part and sustainably produced electricity. Furthermore, with this packaged design, the CWTENG is expected to be an effective energy harvester using water flow and provides a highly promising platform as a selfpowered system using surfaces with different wettability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08828. Figures S1−S9 and simulation methods (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] Homepage: http://slee.cau.ac.kr. *E-mail: [email protected] Homepage: http://cau.ac.kr/ ~mtel/. Author Contributions †

S.L. and J.C. contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Chung-Ang University Graduate Research Scholarship in 2015, the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2014R1A1A2058621), and the Korea Research Institute of Ships & Ocean Engineering (KRISO) Endowment-Grant (PES2180).

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REFERENCES

(1) Wang, Z. L. Triboelectric Nanogenerators as New Energy Technology for Self-Powered Systems and as Active Mechanical and Chemical Sensors. ACS Nano 2013, 7 (11), 9533−9557. 25018

DOI: 10.1021/acsami.6b08828 ACS Appl. Mater. Interfaces 2016, 8, 25014−25018