Water–Solid Surface Contact Electrification and its Use for Harvesting Liquid‐Wave Energy

Electrical wave mechanics: A newly designed triboelectric nanogenerator is based on the contact electrification between a patterned polydimethylsiloxane pyramid array and water. Cost-effective and simple, the prototype triboelectric nanogenerator shows the potential to harvest energy from liquid waves and serve as chemical and temperature sensors. Contact electrification, also called triboelectrification, is an old but well-known phenomenon in which surface charge transfer occurs when two materials are brought into contact. Although some of the fundamental mechanisms about triboelectrification are still under discussion, such as what subjects (electrons, ions, or small amounts of material) are transferred during the contact and separation process to produce the charged surface,1, 2 and why surface charge transfer occurs even between identical materials,3 triboelectrification does exist and it has some practical applications together with many negative consequences. Recently, contact electrification has been demonstrated for some potential applications, such as energy harvesting,4 chemical sensors,5, 6 electrostatic charge patterning,7, 8 metal-ion reduction,9–11 and laser printing.12 The triboelectric nanogenerator (TENG),4 which is the first invention utilizing contact electrification to efficiently convert mechanical energy into electricity, has been systematically studied to instantaneously drive hundreds of light-emitting diodes (LEDs)13 and charge a lithium-ion battery for powering a wireless sensor and a commercial cell phone.14 Recently, the research has been broadened to collect energy from environment, such as wind15 and human motion,16 under which the TENG works in relatively dry conditions, because the surface triboelectrification would be greatly decreased if not totally eliminated by the presence of water. However, water vapor and liquid water are abundant and the most obvious example is ocean waves and tides that have huge amounts of mechanical energy, which is inexhaustible and not largely dictated by daytime, season, weather and climate, in contrast to solar energy.17 Until now, TENG is designed to work between solid materials and works best under dry conditions.4, 13–16 However, tribolelectricity does exist when liquids are flowing through insulating tubes.18–21 For example, a voltage variation rising up to 300 mV is observed when deionized water flows through a 1 m-long rubber tube.20 Or a surface charge density of 4.5 μC m−2 is measured on each water droplet pipetted from a polytetrafluoroethylene (PTFE) tip.21 Therefore, herein we explore the opportunity to use water contact as one type of "material" choice for TENG. We demonstrate that the contact electrification between water and insulating polymer films can also be useful for TENG, which can derive a new application of TENG especially in liquid environments for sensing. Polydimethylsiloxane (PDMS) and PTFE are chosen in this study for their hydrophobic properties and high negativity in the triboelectric series.22, 23 To further investigate this effect, TENGs using tap water, deionized water, and deionized water with a high concentration of NaCl are also compared. Under periodic contacting deionized water by a linear motor, PDMS film with patterned pyramid array can provide an open-circuit voltage (Voc) of 52 V and short-circuit current density (Jsc) exceeding 2.45 mA m−2 with a peak power density of nearly 0.13 W m−2, which is large enough to light up 60 commercial LEDs. The incubation shaker and platform rocker are used to stimulate different wave motions in the environment and the water–TENG successfully harvests these types of mechanical energy into electricity. Moreover, the water–TENG also has the capability to act as a chemical and temperature sensor. Figure 1 shows the fabrication process of the TENG and how the water contact electrification is included in the action unit. The TENG fabrication starts from the design of a PDMS film with patterned pyramid array (Figure 1 a). The Si wafer mold was made by photolithography and then etched by a dry etching process. Liquid PDMS elastomer and cross-linker were mixed, degassed, and uniformly spin-coated on the Si wafer mold. After thermal incubation, a uniform PDMS film with a patterned pyramid array was formed. For the other part of the construct, thin films of Cu (100 nm) were deposited on two poly(methyl methacrylate) (PMMA) substrates by a RF magnetron sputtering deposition system. PMMA is selected as the substrate material because it provides a flat surface, light weight, and high strength. The PDMS film with the pyramid array pattern was peeled off the Si wafer mold and then placed on one of the Cu thin-film-deposited PMMA substrates with uncured PDMS mixture on top. Finally, the structure was incubated again to obtain a fully developed device for use in the next step. a) Fabrication process of patterned PDMS pyramid array and related device. b) Schematic diagram of the water-based TENG. c) SEM image of patterned PDMS pyramid array. Inset is an SEM image with higher magnification. To investigate the contact electrification between water and the PDMS film, the second Cu thin-film-deposited PMMA substrate was placed on the bottom of an insulating tank (Figure 1 b), acting as the conducting electrode for the water. The dimensions of the tank were 11 cm×7 cm. After the tank was filled with water, the device consisting of the patterned PDMS pyramid array was controlled by a linear motor to periodically make contact with the water and then separate. The depth of water layer is 2 cm. During the contact electrification process, the PDMS film only made contact with the water surface and did not touch the Cu thin-film-deposited PMMA substrate on the bottom of the tank. Figure 1 c displays scanning electron microscopy (SEM) images of the patterned PDMS pyramid array, showing uniform microstructures distributed across the whole area. The thickness of PDMS film is nearly 140 μm (Figure S1 in the Supporting Information). These microstructures not only act as steady gaps between two contact materials in a strain-free condition, but also enable the contact area of two materials to be enlarged, which will increase the electrical output of the water–TENG. High-magnification SEM image (inset of Figure 1 c) further reveals that the size of single pyramid is 10 μm, and each pyramid has a perfect geometric structure and a sharp tip. Figure 2 illustrates the contact electrification between water and PDMS and the working mechanism of the water–TENG, which can be explained as a result of contact electrification and electrostatic induction. Before PDMS contacting with water (Figure 2 a), no charge transfer occurred. When PDMS is forced into contact with water (Figure 2 b), the ionization of the surface groups on the PDMS will cause the PDMS to be negatively charged21, 24 and create a positively charged electrical double layer (EDL) on the surface of the water to maintain electrical neutrality.25 As the PDMS moves off the water, the positive charges in the EDL can be carried away with the water and the negative charges can remain on the surface of PDMS, then an electric potential difference will be established between PDMS and water. In the ideal situation, the PDMS can be completely separated from water leaving no residual water on the PDMS, generating the maximum electricity output. This is why we choose hydrophobic polymer materials, especially with the design of surface micro-patterns. In short-circuit case, the electrons will transfer from Cu electrode 2 to Cu electrode 1 through the external load to balance the potential difference between water and PDMS (Figure 2 c). This contributes to the instantaneously positive current. The inducted charge density (σ′), which determines the magnitude of the generated current, has been shown to be highly dependent on the separated distance (d′).26 The maximum value of σ′ is observed when PDMS returning to its original position (d′=d3) (Figure 2 d). Once PDMS is pressed again toward the water surface, the electric potential difference starts to decrease as the PDMS gets closer to the water (Figure 2 e). Therefore, the electrons will flow from Cu electrode 1 to Cu electrode 2 through the external load, until achieving a new equilibrium (Figure 2 b). This process corresponds to an instantaneous negative current. Once the PDMS is periodically making contacting with the water (Figure 2 b–d), a continuous output will be obtained. Working mechanism of the water–TENG. a) Initial status without any external force applied. b) External force brings the PDMS layer into contact with water. c) Removing the PDMS layer from the water surface. d) PDMS layer returning back to the original position. e) External force applied makes the PDMS layer contact with water again. For simplification, both PMMA substrates are not shown. We first measured the electrical output of the water–TENG through the contact electrification between the patterned PDMS pyramid array and deionized water. The TENG was mechanically triggered by a linear motor that provides dynamic impact with controlled force at a frequency of 2 Hz. Voc and Jsc were measured separately to characterize the electrical performance of the water–TENG. From Figure 3 a, the Voc of the water-TENG surged from 0 to 82 V upon removing the patterned PDMS pyramid array from the deionized water surface to a separation distance of 1.5 cm. The generated voltage held as a plateau because in an open circuit the electrons cannot flow to screen the electric potential difference between patterned PDMS pyramid array and the deionized water (inset of Figure 3 a). When the PDMS pyramid array moving back into contact with deionized water again, the Voc fell back to 0. a) Voc and b) Jsc of the water-TENG under a constant external force with a frequency of 2 Hz. Measurements operated both at the forward connection, shown at the left-hand side, and the reversed connection, shown at the right-hand side. c) The AC output transformed into a pulse output in the same direction by a full-wave rectifying bridge. d) The rectified output used to charge a capacitor. The insets are the magnified output curve in one cycle (a–c) and a diagram of the corresponding connection polarity (a–d). e) Photograph of a setup in which the water-TENG acts as a direct power source for 60 commercial green LED lamps and f) when the patterned PDMS pyramid array is in contact with the water, the LEDs light up. In the short-circuit case, the charges transferred between Cu electrode 1 and Cu electrode 2 (Figure 2) in the external circuit during the contact and separation between the PDMS film and the water surface instantaneously generated positive and negative currents. The peak value of the Jsc reaches 1.05 mA m−2 (Figure 3 b). The integration of positive and negative current peaks represents the transferred charges in releasing and pressing movements (Figure S2). Both the generated Voc and Jsc exhibit corresponding reversed signs when the connection polarity to the electrometer is switched, indicating that the measured signals were generated by the water-TENG. The patterned pyramid array is effective for enhancing the output. Without the microstructures, the Jsc of PDMS film decreases to 0.3 mA m−2 (Figure S3), which is even smaller than that of a commercial PTFE sheet (Figure S4). Furthermore, the AC output could be transformed to a pulse output in the same direction simply by using full-wave rectifying bridge (Figure 3 c) and stored in a capacitor of 33 μF (Figure 3 d). With such a power output generated from water-contact electrification, 60 commercial LEDs can be instantaneously driven by a single water–TENG device (Figure 3 e, Figure 3 f, and Supporting Information Video S1). As this model differs from the solid-material-based TENGs, we need to consider how the wave surface of water disturbs the contact areas with the PDMS, which consequently affects the generated output current. Actually, the movement of the PDMS film contacting and separating from water will generate a water wave. By changing the contact frequency of the linear motor, water waves with different wavelengths are created. The size of the tank could be critical. Therefore, the dependence between generated output and contact frequency was studied. Figure 4 a displays that the generated Jsc decreased from 1.05 mA m−2 to 0.46 mA m−2 first as the contact frequency changing from 2 Hz to 3 Hz, then increased to 1.34 mA m−2 and 2.45 mA m−2 when the contact frequency altering from 4 Hz to 5 Hz, respectively. We found the dependence between Jsc and contact frequency is unlike that in solid-material-based TENGs,27, 28 in which, the generated Jsc keeps increasing with the increasing contact frequency, because the contact areas are fixed even with the change of contact frequency. Increasing the contact frequency only causes a higher flow rate of charges in the external circuit, but the total amount of charges transferred remains constantly. However, in the case of the water–TENG, the movement of the PDMS film controlled by the linear motor will create waves on the water surface, which in turn will disturb the contact area between the PDMS film and water. Hence the Jsc generated will not be solely dependent on the contact frequency. This can be further explained by the generated Voc (Figure 4 b), which is determined by the triboelectric charge density in the open-circuit condition.4 The Voc of a solid-material-based TENGs is not affected by the contact frequency.14 But in the case of the water–TENG, the generated Voc decreased from 82 V to 23 V, 45 V, and 52 V as the contact frequency changing from 2 Hz to 3 Hz, 4 Hz, and 5 Hz, respectively. The tendency explicitly shows that the triboelectric charge density in the water-TENG is influenced by the motions of water wave. a), b) Influence of contact frequency between patterned PDMS pyramid array and deionized water on the a) Jsc and b) Voc generated by the water-TENG. c),d) Dependence of output voltage (c), output current density (c), and output power density (d) of the water–TENG on the resistance of the external load. e),f) Comparison of e) Jsc and f) charge density generated from the contact electrification between different types of water and the patterned PDMS pyramid array. A different contact frequency was applied to evaluate the electrical output. We measured the electrical output of the water–TENG upon connecting directly to loads of different resistances at a contact frequency of 5 Hz. As shown in Figure 4 c, when the resistance is below 0.1 MΩ, the voltage remains close to 0 and the current density has only little changes. When the resistance is raised from 0.1 MΩ to 1 GΩ, the voltage through the load will generally increase, but the current across the load will decrease. Consequently, the instantaneous power density on the load (Figure 4 d) remains small with the resistance below 0.1 MΩ and reaches the maximum value of nearly 50 mW m−2 at a resistance of 88 MΩ. This is the maximum output power density on this water–TENG. To demonstrate the concept of the water–TENG which can be used to harvest energy from the environment, tap water and deionized water with high concentration of NaCl are also evaluated (Figure 4 e,f, and S5). The concentration of NaCl (0.6 M) tested is similar to that in sea water. The Jsc generated decreases first and then increases as the contact frequency is varied from 2 Hz to 5 Hz, which is the same tendency as that of deionized water (Figure 4 e). The maximum generated Jsc from tap water and 0.6 M NaCl solution are 1.85 mA m−2 and 0.73 mA m−2 at a contact frequency of 5 Hz, respectively, which are 76 % and 30 % of that for the deionized water–TENG. By integrating the peak of Jsc, we can obtain the transferred charge density (Δσ) in each type of water (Figure 4 f). The results show the deionized water can provide the largest Δσ of 31.3 μQ m−2. The Δσ of tap water and 0.6 M NaCl solution TENGs are 15.02 μQ m−2 and 5.7 μQ m−2, respectively, smaller than that of deionized water–TENG. The results indicate the output of the water-TENG will be affected by the electrolytes in water. This is because the PDMS film cannot be completely eliminated from the adhesion of water droplets after it is separated from the water. Once there are electrolytes in water, more positive charges including dissolved ions will remain on the films, resulting in a partial screening of the tribo-charges on the PDMS film.21, 24 Therefore, smaller electrical outputs than that in deionized water will be obtained. Although at the present time the electrical output of the water-TENG will be affected when electrolytes are present in water, this could be overcome in the future by designing the polymer surface to be superhydrophobic and/or modifying the polymer with specific functional group to eliminate the ions with opposite charges adsorbed onto the contacted surfaces. As a potential application of water–TENG, we used the fluctuating wave on surface to generate a wave controlled contact and separation process between the water surface and PDMS film for harvesting wave energy (Figure S6 and S7). Finally, we demonstrated that the water-TENG can also be used as chemical and temperature sensors. The generated Jsc of the water-TENG decreased from 1.05 mA m−2 to 0.32 mA m−2 as the ethanol percentage by volume increase to 15 % (Figure 5 a). The decrease in Jsc probably comes from the mixing with ethanol decreasing the water polarity. Previous study has verified that the dielectric constant and water polarity will decrease in the presence of less polar solvent, such as ethanol.29 Therefore increasing the ethanol concentration in the mixture of water and ethanol will enhance the interaction with hydrophobic PDMS film, and consequently reducing the triboelectric charge density and generated output during the separation process. To clarify this point, the Jsc values of the water-TENG operating at different water temperatures were compared. Temperature has been studied as other key factor to affect the dielectric constant and polarity of water.30 A similar tendency between water temperature and generated output was observed (Figure 5 b). From 25 °C to 75 °C, the Jsc value decreased from 1.05 mA m−2 to 0.28 mA m−2. These two results indicate that the water–TENG can be applied to detect substances that will change the dielectric constant and/or polarity of water. In the future, the sensing system can be improved by growing nanomaterials or functionalizing specific molecules onto the surface of polymer films, which will serve as probes to selectively detect the targets in the solution samples. Once the probes catch the targets, the electrical output generated from the contact electrification with water will be different. This will be a better strategy in a water–TENG-based sensing system because it can directly change the triboelectric charge density upon detecting targets. a) Jsc generated by the water–TENG in the presence of different percentage ethanol by volume. b) Jsc generated by the water–TENG upon working at different water temperatures. The measurements were conducted under a constant external force with a frequency of 2 Hz. In summary, we have demonstrated a newly designed TENG based on the contact electrification between a patterned PDMS pyramid array and water. This new prototype water–TENG provided an open-circuit voltage of 52 V and a short-circuit current density of 2.45 mA m−2 with a peak power density of nearly 0.13 W m−2, which is able to drive 60 LEDs simultaneously. The dependence of the electrical outputs on the contact frequency and motions of water waves has been systematically studied. Tap water and deionized water with a similar ion concentration to sea water were also evaluated and showed the potential for harvesting water-related energy from the environment. Compared with traditional TENGs that are designed for the contact of solid materials, this study opens the possibility in utilizing liquid movements and extends its application scope. Furthermore, we believe the electrical output of the water–TENG could be enhanced in the future by using superhydrophobic nanostructures as the contact materials or functionalizing material surface with specific groups. This work will also inspire the development of TENG toward directly sensing metal ions and biomolecules in solution samples. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.