Influences of Surface and Ionic Properties on Electricity Generation of

Letter pubs.acs.org/JPCL

Influences of Surface and Ionic Properties on Electricity Generation of an Active Transducer Driven by Water Motion Junwoo Park,† YoungJun Yang,† Soon-Hyung Kwon,†,‡ and Youn Sang Kim*,†,§ †

Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Seoul 151-744, Republic of Korea ‡ Flexible Display Research Center, Korea Electronics Technology Institute, Seongnam, Gyeonggi-do 463-816, Republic of Korea § Advanced Institutes of Convergence Technology, 864-1 Iui-dong, Yeongtong-gu, Suwon-si, Gyeonggi-do 443-270, Republic of Korea S Supporting Information *

ABSTRACT: In this Letter, we discuss the surface, ionic properties, and scaleup potential of an active transducer that generated electricity from natural water motion. When a liquid contacts a solid surface, an electrical double layer (EDL) is always formed at the solid/liquid interface. By modulating the EDL, the active transducer could generate a peak voltage of ∼3 V and a peak power of ∼5 μW. Interestingly, there were specific salinities of solution droplets that showed maximum performance and different characteristics according to the ions’ nature. Analyzing the results macroscopically, we tried to figure out the origins of the active transducing precipitated by ions dynamics. Also, we demonstrated the scale-up potential for practical usage by multiple electrode design.

I

Supporting Information. As the water droplets perpendicularly flowed on the horizontally patterned ITO electrodes with a 45°-tilted laid state, the electric signals were generated. When three 80 μL water (0.01 M NaCl) droplets that were consecutively ejected by syringe pump (Legato 200, KdScientific Inc.) dripped on the device, a peak voltage of ∼3 V (Figure 1b) and a peak power of ∼5 μW (Figure 1c) were generated. The generated electrical power was enough to turn on green LEDs. The origin of the electric signals generated by motion modulations between the solid and liquid could be assumed to be some factors, such as ion dynamics, the hygroscopic effect, and triboelectric effect. If the hygroscopic effect of a solid surface that attracts and holds water molecules was the dominant factor of transducing energy, the electric signals would be meaningfully generated by the flow of deionized water droplets. However, the hygroscopic effect was excluded on the ground that no signals were detected by the flow of deionized water droplets. In the triboelectric effect, if the electric signals were generated by a triboelectric effect between water droplets and the solid surface, the electric signals should have had positive and negative peaks (AC pulse) by the passing of a single water droplet. However, in this case, the active transducer generated only positive peaks (DC pulse) because in the whole energy-generating process, that is, while the circuit

on dynamics at the solid/liquid interface play an exceedingly important role in the various fields of studies, including surface science,1 electrochemistry,2 colloidal science,3 surface catalysis,4 biology,5 and geology.6 Recently, generating electricity from the ion dynamics, which is induced by motion modulations between solid and liquid, has received considerable attention. At an interval of a few years, various theories and models about the electric signals generated by the liquid flow surrounding a carbon nanotube7−10 and graphene11−14 have been suggested. Also, it was demonstrated to generate electric signals by motion modulations of water droplets on a hydrophobic surface.15−17 Kim and colleagues,18 eventually, succeeded in demonstrating practical devices that have a commercial potential, that is, the lighting on the LED just by a flowing motion of a single water droplet without any external sources or complicated conversion parts. In this study, we introduce some clues of the operation mechanism of an active transducer through the analyses of electrical signals depending on the variation of ionic properties or concentrations in water and the modification of the interface chemistry. Also, we demonstrate the scale-up potentials by multiple electrode design. A schematic image of the device, an active transducer, is shown in Figure 1a. Indium tin oxide (ITO) as an electrode (15 mm width, 2 mm gap) was patterned on the glass substrate and then, in serial order, poly4-vinylphenol (P4VP) and a silica gel film were spin-coated. The silica gel film was made by a sol−gel process using an ethanol solution containing tetraethoxysilane (TEOS) and perfluorooctyltriethoxysilane (POTS).19−21 Detail fabrications are introduced in experimental section in the © XXXX American Chemical Society

Received: December 10, 2014 Accepted: February 4, 2015

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DOI: 10.1021/jz502613s J. Phys. Chem. Lett. 2015, 6, 745−749

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Figure 1. (a) Schematic image of the active transducer. (b) The measured output voltage and c) power of active transducer as three droplets slid. The 2.0 MΩ road was used to measure power. A droplet (0.01 M NaCl solution) was ejected at the rate of 10 mL per minute by the syringe pump. The coated silica gel film had a 0.030 POTS/TEOS mass ratio. A silica gel film with a 0.030 POTS/TEOS mass ratio was used as a hydrophobic layer of the standard device because it stably maintained the contact angle after several numbers of droplets flowed on the substrate (Figure S3, Supporting Information). (d) The schematic cross-sectional image of the active transducer in the dynamic state. Cations, anions, and the water molecule (depicted by a circle surrounding an arrow) formed the electrical double layer (EDL).

was formed, the electron only flowed in one direction from the upper to lower electrode. During the process, the overlap area between the droplet and upper electrode got smaller, while the overlap area between the droplet and lower electrode got larger. Because the charges were induced on the interface of the overlap area and the variation of the overlap area had a directional change, the charges flowed in one direction. In this case, the one-direction peaks (DC pulse) mean that the origin of the electric signals generated by motion modulations between the solid and liquid is not dominantly caused by the triboelectric effect. Also, a reason why the electric signals in our previous work18 seemed like AC pulses is that the water droplets had been dripped onto the substrate very close to the gap between electrodes. At the moment that water droplets, which fell near the gap, started to flatten because the increasing of the overlap area between the water droplet and upper electrode was larger than that of the overlap area between the water droplet and lower electrode (Figure S2b, Supporting Information), the reverse electric signal was generated, and then, the forward electric signal was generated regarding directional flow of the water droplet to the lower electrode. Otherwise, in this study, we dripped the droplets onto the upper parts of the upper electrode to prevent an interference effect (Figure S2a, Supporting Information). To verify exactly the mechanism of electric signal generation, we made the conditions that the droplets solely slid by the gap.

For these reasons, a possible hypothesis could be then ion dynamics driven by electrical double layer (EDL) modulations, that is, the adsorption and desorption of ions onto the solid surface (Figure 1d). When the droplet is in the static state, macroscopically, ions and water molecules form an EDL so that the potential difference between two electrodes is simply zero. In the dynamic state, otherwise, because the droplets slide at a speed of v(t), ions are forced to undergo adsorption and desorption mechanically. The potential difference between two electrodes induced by the ions’ charges is expressed by VU − VL = d(QU/CU) − d(QL/CL),18 where CU and CL are the capacitances of upper electrode−droplet capacitor and lower electrode−droplet capacitor, respectively. VU and VL are voltages on CU and CL, respectively. Qi = CiVi (i = U,L). The equation indicates, again, that the potential difference is generated by the different overlap area variation. However, until now, fundamental principles governing the phenomena have not been clearly elucidated. Despite some researchers’ constant exertions,22−28 more generally, ion dynamics on the solid/liquid interface have not been comprehensively understood. Moreover, there are still discrepancies in understanding the air/liquid interface system,29,30 which is more fundamental than the solid/liquid interface. The difficulties of a fundamental understanding of ion dynamics on the interface are partially due to the difficulties of measuring ion dynamics experimentally. Now, we tried to give some clues of 746

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The Journal of Physical Chemistry Letters ion dynamics on the interface through analyses of electrical signals of the active transducer depending on the variation of ionic properties or concentrations in water droplets and the modification of interface chemical properties. The silica gel films were made by a sol−gel process with the mixture precursor of TEOS and POTS in ethanol under acidic conditions. The hydrolysis reactions of TEOS and POTS are expressed by the following equation, Si(OC2H5)4 + 4H2O → Si(OH)4 + 4C2H5OH and RfSi(OC2H5)3 + 3H2O → RfSi(OH)3 + 3C2H5OH, where Rf = CF3(CF2)5CH2CH2−. The hydrolyzed TEOS and POTS formed a cross-linked structure on P4VP by a condensation reaction (Figure S5, Supporting Information).19−21 The precursor composition is shown in Table 1. By modulating the mass ratio of TEOS and Table 1. Mass Composition of Precursor Solution with TEOS and POTSa

mass (g) a

POTS/TEOS ratio

TEOS

POTS

H2O

EtOH

HCl

x (0 < x < 1)

1

x (0 < x < 1)

1

7

0.01

The range of x is from 0 to 1 g.

POTS, x (=POTS/TEOS mass ratio), surface properties were modified. As shown in Figure 2, in the case of x > 0.04, the

Figure 3. (a) Measured contact angle and output peak voltage of the active transducer according to acidity. (b) With a chloride ion as an anion, various alkaline metal ions and a magnesium ion were used to investigate the effect of the atomic number of the cation. Falling height = 20 cm. POTS/TEOS mass ratio = 0.030. Measured voltages were normalized by the voltage using the 0.01 M NaCl solution droplets, which were used in Figure 1b.

the contact angle the in base solution region decreased. Difficulties in ions desorption of the base solution droplet also resulted in performance degradation. In the case of pure DI water (pH = 7), merely noise level electric signals were detected, which means that the absorption/desorption process in the active transducer is solely operated by charged ions not by the dipole moment of water molecules. The maximum peak voltages were measured at around 1.5 V at pH = 3 and at around 1.1 V at pH = 10. Up to 10−3 M concentration, the number of adsorbing and desorbing ions increased. After the maximum value, that is, EDL was saturated, the screening effect was dominant so that the performance decreased in concentration over 10−3 M.1,31 In this measurement, the falling height of the droplet was 20 cm. The silica gel film with a 0.030 POTS/TEOS mass ratio, x, was used. Similar graphical features were revealed in Figure 3b. While the chloride ion was fixed as an anion, various alkaline metal ions and the magnesium ion were tested as a cation. The measurement condition was the same with Figure 3a. The abscissa of Figure 3b is on the logarithmic scale of the concentration used from 10−6 to 10−1 M, and the maximum values were obtained at around a 10−2 M solution concentration. In addition, among the alkaline metal ions, the larger the atomic number of the cation generated, the less output voltage. Ions were specifically adsorbed, that is, ions were adsorbed depending on the ion’s own nature. In the case that halogen ions were varied as an anion with a fixed sodium

Figure 2. Measured contact angle and normalized peak voltage of the active transducer according to the POTS/TEOS mass ratio. A 0.01 M NaCl solution was used. Measured voltages were normalized by the voltage using a silica gel film that had a 0.4 POTS/TEOS mass ratio.

contact angle started to saturate at ∼105°. As the surface energy is also related to the contact angle, the high contact angle values mean that the solid surface desorbs well the adsorbed ions. Considering the suggested hypothesis, the desorption capability of the adsorbed ions on the solid surface has an effect on electric power generation of the active transducer. Hence, the output peak voltages in flowing motion followed the contact angle variation tendency. To investigate the effect of ion properties on the electric power generation of the active transducer, output voltages depending on the various ion concentrations were measured. In Figure 3a, the variation of output voltages and contact angles depending on the pH of the water droplet are shown. The acid and base solutions were obtained by dissolving HCl and NaOH in deionized water, respectively. Being relatively stable in contact with acid solution, the silica gel film in the acid solution region held the contact angle to around 105°. Because the silica gel film was damaged by the hydroxide ion, on the other hand, 747

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Figure 4. Normalized peak currents of the active transducer according to the (a) falling height (with 40 μL volume) and (b) droplet volume (with 10 cm falling height) were measured using 0.01 M NaCl solution droplets. Measured values were normalized with the criteria of a (a) 20 cm falling height and (b) 80 μL droplet volume. (c) Schematic top view image of the active transducer with a comb-like electrode. (d) The output voltage of the active transducer was measured using a 40 μL volume droplet (falling height of 10 cm).

ion as a cation, however, few explicit propensities were shown (Figure S4, Supporting Information). On the basis of the experimental results, it is estimated that the cation was the adsorbed ion, while the anion screened the surface charges.32,33 Therefore, it is considered that the output voltage obtained by magnesium chloride solution droplets that had two chloride ions per one cation was lower than the output voltage obtained by the sodium chloride solution droplet. The performance of the active transducer depends on the amount of charges transfer by adsorbed ions and the screening effect of counterions. To understand the exact ion dynamics, further studies are needed, such as the quantitative measurements of transferred charges and the screening effect, the investigation for specific concentration values showing maximum performance, and chemicophysical reactions between ions and the solid surface. The physical variables, such as falling height and droplet volume, also have effects on the electric signal generation of the active transducer. The output current I(t) is linear to the droplet velocity.32 Because the terminal velocity v is proportional to the square root of the falling distance, the output current according to the height of falling matched well with the square root fit (Figure 4a). In addition, the output current increased as the droplet volume increased (Figure 4b). Multiple electric signals could be obtained by patterning the electrodes comb-likely (Figure 4c). During the sliding, the droplet passed six electrodes and five gaps. Thus, serial 5 DC pulses were generated by a flowing of a single droplet. Depending on the

relative position, three positive peaks for odd numbers of gaps and two negative peaks for even numbers of gaps were generated (Figure 4d). These results showed that the active transducers have a good potential to scale-up power generation. In summary, we have investigated the performance of an active transducer driven by water motion like raindrops according to surface, ionic, and physical properties. The active transducer that is coated by P4VP and silica gel film successfully generated a peak voltage of ∼3 V and a peak power of ∼5 μW. Macroscopically, the positive correlation between surface hydrophobicity and output electric power was observed. Also, the phenomena that occurred at the solid/liquid interface of an active transducer were governed by not only surface properties but microscopic ionic properties. As for ionic properties, there were two peculiarities. First, a specific concentration showing the maximum performance existed. Second, the performance tendency had correlation with the atomic number of the cation. Moreover, the output performance was fitted quite well with respect to the physical variables. Toward scale-up power generation, serial pulse electric signals were demonstrated by modified electrode design. To get the entire picture, the ion dynamics of the active transducer on the molecular level should be more severely studied, but we believe that this study will give good inspiration for the fundamental understanding of the ion dynamics for electricity generation. 748

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ASSOCIATED CONTENT

S Supporting Information *

Details of the experiment. Figures including (1) the SEM image of the active transducer and (2) captured images of high-speed camera video. (3) With sodium ion as the cation, various halogen ions were used to investigate the effect of the atomic number of the anion. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Research Program (No.2011-0018113) funded by the National Research Foundation of Korea and the Center for Advanced SoftElectronics as the Global Frontier Project (2013M3A6A5073177) funded by the Ministry of Science, ICT, and Future Planning of Korea.

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