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Electricity generation from water droplets via capillary infiltrating
Nano Energy 48 (2018) 211–216
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Nano Energy journal homepage: www.elsevier.com/locate/nanoen
Communication
Electricity generation from water droplets via capillary infiltrating a,1
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Jia Li , Kang Liu , Guobin Xue , Tianpeng Ding , Peihua Yang , Qian Chen , Yue Shen , ⁎ Song Lic, Guang Fengc, Aiguo Shend, Ming Xub, Jun Zhoua, a
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China d College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Energy conversion Capillary Nanofluidics Self-powered system
Electricity generation from the interaction between water and carbon materials is a new approach in the pursuit of high-efficiency energy conversion. Here, we report that a water droplet dropping onto a piece of porous carbon film can spontaneously generate electricity under ambient conditions. Twelve 5-μL water droplets can generate a voltage up to 5.2 V and illuminate a liquid crystal display. The voltage can be controlled by modulating the direction of the droplet infiltration, the zeta potential of the porous carbon, and the ion concentration. The results demonstrate that a hydrophilic porous carbon film with water droplets may function as a cost-effective electricity generator for harvesting energy from natural resources.
1. Introduction
for ionic solution flows outside [16–18] of and inside of CNT [19], respectively. While, the generated voltage is ~ 50 μV to 85 mV for water or ionic solution flows over graphene [20–25]. Furthermore, as previously reported, external pressure or force is necessary to drive the fluid flow, i.e., the energy harvesting process consumes energy. Recently, we have demonstrated that natural evaporation can drive the capillary water flow within porous carbon sheet, thus can produce significant voltage up to 1.0 V with lifetime over hundreds of hours under ambient conditions [41,42]. To deeply understand to underlying mechanism, we systematically studied electricity generation through the transformation of water droplets into water flow via the capillary force produced by a hydrophilic porous carbon film (PCF). A water droplet with volume of 1 μL can reliably generate a voltage of up to 0.8 V during the spontaneous filtration process of the water in the PCF. On the basis of the dependence of the induced voltage on the direction of the droplet infiltration, zeta potential of the porous carbon and ion concentrations, the phenomenon may be attributed to the electrokinetic effect [29-32]. In addition, a self-powered sensor for in situ detection of water-droplet delivery and positioning on a hydrophilic surface was also demonstrated. These results demonstrate a novel means to generate electricity from a natural process under ambient conditions, and the developed approach has potential applications in self-powered devices and systems.
Harvesting energy from the environment offers great promise in the application of self-powered devices and systems [1]. In the past few decades, intensive studies in this field have inspired an explosive growth of environmental energy utilization approaches [2–13]. Ideally, highly effective energy conversion processes should be environmentally friendly and spontaneous while not requiring any external energy input, such as mechanical movement, concentration difference or temperature variation. Electricity generation from the interaction between water flow and a carbon nanomaterial is a new approach in the pursuit of high-efficiency energy conversion and represents an interesting topic from both basic research and practical application viewpoints [14]. In 2001, Král and Shapiro first theoretically predicted that metallic carbon nanotubes (CNTs) in a flowing ionic liquid could generate a net potential difference and an associated electric current [15]. Since then, numerous researchers have reported voltage generation phenomena in which the detailed results differ with the carbon nanomaterials [16–25], the ion content of the flowing water, and the device configuration [26–28]. However, the reported voltage is typically in the range of several of microvolts to tens of millivolts, which is not adequate for modern devices. For example, the generated voltage is ~ 9 μV–30 mV and ~ 8 mV
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Corresponding author. E-mail address: [email protected] (J. Zhou). J.L. and K.L. contributed equally to this work.
https://doi.org/10.1016/j.nanoen.2018.02.061 Received 12 February 2018; Received in revised form 28 February 2018; Accepted 28 February 2018 Available online 05 March 2018 2211-2855/ © 2018 Published by Elsevier Ltd.
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2. Experiment
(FTIR, Bruker Vertex 70). Contact angles were measured using a contact angle meter (Kino SL200B). The current-voltage characteristics of the device were measured using a Keithley 2400 source measurement unit. The Voc and Isc as a function of time were record using a Keithley 2000 multimeter and a low-noise current preamplifier (SRS model SR570), respectively. The environmental temperature and humidity were simultaneously recorded by a Center 310 RS-232 humidity and temperature meter.
2.1. Materials Multi-walled carbon nanotubes (MWCNTs) were purchased from XF Nano, Nanjing, China. Sulfuric acid (H2SO4), nitric acid (HNO3), sodium hydroxide (NaOH), ethanol, ethyl cellulose, acetic acid, glutaric dialdehyde (25 wt% aqueous solution), terpineol and toluene were purchased from Sinopharm. 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (PFDTS) was purchased from Sigma-Aldrich. PEI (MW ≈ 600) was purchased from Aladdin Industrial Corporation. Alumina strips were purchased from EMS Corp., Kunshan, China.
3. Results and discussion A schematic of the device for measuring the droplet-induced voltage is shown in Fig. 1a. First, two carbon nanotube (CNT) electrodes were coated onto the ends of a well-cleaned Al2O3 ceramic plate (details in Experimental Section). Next, carbon slurry composed of toluene carbon black, terpineol, ethyl cellulose and ethanol was coated using a blade such that the slurry crossed the CNT electrodes. Afterwards, the device was sintered at 350 °C for 150 min to obtain a porous carbon film (PCF). The PCF was then treated with air plasma for 60 s to ensure superhydrophilic. A scanning electron microscope (SEM) image (Fig. 1b) shows that the PCF has a thickness of ~ 5 µm and consists of interconnected carbon nanoparticles with a mean diameter of tens of nanometers, and with most of the pore sizes ranging from several to tens of nanometers (Fig. S1). After the device was wired and encapsulated by epoxy in the exposed electrode regions, selected regions of the device were modified with 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTS) to make it superhydrophobic. The right inset of Fig. 1a shows a photograph of a typical device with two ends modified by PFDTS. The Fourier transform infrared (FTIR) spectrum of PFDTS@PCF (red line in Fig. 1c) shows obvious additional absorption peaks at 1256 cm−1 and 1155 cm−1 compared to the spectrum of pristine PCF (black line); these new peaks correspond to C–F and C–O–Si bonds, respectively [33]. The pristine PCF and PFDTS@PCF exhibit different degrees of wettability, with water contact angles of ~ 0° and ~ 152° (insets of Fig. 1c), respectively. The prepared device shows robust ohmic contact, with a resistance of ~ 23.6 MΩ (Fig. S2). When deionized water (DI water, conductivity of ~ 1.6 × 10−4 S m−1, 1 μL) was dropped onto the PFDTS@PCF/PCF interface, water at the bottom of the droplet infiltrated into the porous carbon film, driven by capillary force. Because of the superhydrophobicity of the PFDTS@PCF region, water inside the PCF infiltrated only toward the hydrophilic region; infiltration in the opposite direction was disfavored. During this process, surprisingly, a sustainable voltage up to ~ 0.3 V was generated between the two CNT electrodes (Fig. 1d). The voltage remained nearly unchanged during the entire process and decreased quickly to zero when the droplet was completely vaporized. Such a phenomenon was highly reproducible. When five water droplets with the same size were dropped at the interface in sequence, similar open-circuit voltage signals were detected for each droplet (Fig. 1d). The short-circuit current was demonstrated to exhibit the same response (Fig. S3). On the contrary, water droplets moving on surface of graphene or aligned single-walled nanotubes only generated pulsed-electric signals [34–36]. The duration of the voltage was observed to be dependent on the size of the water droplets, as shown in Fig. 1e. The voltages induced by the water droplets with volumes of 1 μL, 2 μL, 5 μL and 10 μL were observed to persist for periods of ~ 73.7 s, ~ 195 s, ~ 610 s and ~ 1177 s, respectively, under ambient conditions, and nearly identical corresponding voltages (~ 0.31 V, ~ 0.31 V, ~ 0.33 V, ~ 0.33 V and ~ 0.33 V, respectively) were generated (Fig. 1e). The whole phenomenon appears to be due to the PCF functioning as a unique electricity generator. To explore the behavior of the water-droplet-induced voltage, we used a device with dimensions of ~ 50 × 7 mm2 with two ends and the central section modified with PFDTS. In the measurements, the right end was taken as positive and the left end was taken as negative
2.2. Synthesis of soluble CNT ink 3 g of MWCNTs were dispersed in 130 mL of mixed HNO3 and H2SO4 (HNO3/H2SO4 = 1/3 v/v) solution by sonication for 10 min. The mixture was then refluxed at 90 °C in an oil bath for 2.5 h under constant magnetic stirring. Afterwards, the mixture was cooled to room temperature. The mixture was subsequently washed with distilled water until the supernatant was neutral and was then re-dispersed in deionized water to form a 20 mg mL−1 MWCNT ink. 2.3. Synthesis of carbon slurry Toluene carbon black powder was first prepared by a methylbenzene flame synthesis method. One gram of toluene carbon black, 1 g of ethyl cellulose and 3 mL of terpineol were subsequently mixed in 50 mL of ethanol. The resulting mixture was probe-sonicated (Kesheng Sonics Vibra Cell, 550 F) for 20 min. Finally, the homogeneous slurry was evaporated to a total volume of 10 mL in an oil bath at 60 °C under magnetic stirring. 2.4. Fabrication of the device Alumina strips were sequentially ultra-sonicated in acetone, alcohol and DI water and then dried at 70 °C in an oven. Next, CNT ink was printed into the alumina strip with required patterns using a floating knife coater. The carbon slurry was next blade-coated across the electrodes. After the electrodes were sintered at 350 °C for 150 min and cooled to room temperature, the obtained porous carbon film (PCF) was treated with air plasma (pressure 100 Pa, RF power 130 W) for 60 s in a plasma cleaner (Mingcheng, PDC-MG). The device was subsequently wired and encapsulated with epoxy in the exposed electrodes regions. 2.5. PFDTS modification PFDTS was diluted with ethanol (1:10 v/v), and 1 wt% acetic acid was added to the solution and well shaken before use. The desired PCF regions were modified by being dip-coated in pre-diluted PFDTS ethanol solution at 70 °C and then heated for 30 min at 100 °C. 2.6. PEI modification A 0.1 wt% PEI-600 aqueous solution was carefully dripped onto half of the PCF region and then heated to 70 °C for 30 min; after being washed with water and ethanol, the PEI-treated PCF area was then dipped in 0.1% glutaric dialdehyde aqueous solution heated to 70 °C in a water bath. Finally, the PEI-modified PCF was thoroughly washed with 70 °C DI water to remove residual reactants. 2.7. Characterization The morphology, structure and functional groups of the PCFs were characterized by scanning electron microscopy (SEM, FEI Nova Nano450) and FTIR spectroscopy coupled with infrared microscopy 212
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Fig. 1. Water-droplet-induced voltage. (a) Schematic of the porous carbon film device with two ends modified with PFDTS. The right inset shows a photograph of a typical device with dimensions of 50 × 7 mm2. The areas enclosed with red dashed lines represent PFDTS-modified regions, and the areas enclosed with green dashed lines represent epoxy-encapsulated regions. (b) Cross-sectional and magnified SEM images of the porous carbon film. (c) FTIR spectra of the pristine carbon film (black curve) and PFDTS-modified carbon film (red curve). Insets show the corresponding contact-angle images of a pristine carbon film (~ 0°) and a PFDTS-modified carbon film (~ 152°), respectively. (d) Open-circuit voltage obtained by repeatedly dropping 1 μL water droplets at the PFDTS@PCF/ PCF interface under ambient conditions (~ 23.5 °C and relative humidity ~ 71.7%). (e) Measured Voc vs. time of the device when water droplets with volumes of 1 μL, 2 μL, 5 μL and 10 μL were dropped onto the PFDTS@PCF/PCF interface.
(Fig. 2a). When a 2 μL water droplet was dropped onto the PFDTS modified region, the shape of the water droplet was maintained for nearly 1 h without infiltration because of the surface hydrophobicity, and no voltage was detected between two electrodes (Fig. S4). However, when a 2 μL-water droplet was dropped onto the PFDTS@PCF/ PCF interface with the pristine PCF region at the right (position A and position D in Fig. 2a), water infiltration occurred from left to right (Fig. 2b), while the infiltration from right to the left was blocked because of superhydrophobicity in the left region. Simultaneously, a
voltage of ~0.3 V was detected (Fig. 2c and video S1). When the water was completely vaporized, the voltage signals decreased to the initial zero point. When a 2 μL water droplet was dropped onto the PFDTS@ PCF/PCF interface with the pristine PCF region at the left (marked as position B and position C in Fig. 2a), water infiltrated from right to left (Fig. 2b), while the infiltration from left to the right was blocked because of superhydrophobicity in the right region. A corresponding voltage of ~ − 0.3 V was detected. By comparison, we also dropped 2 μL water droplets onto the center of the PCF region (marked as Fig. 2. Wetting dependence of the induced voltage. (a) Schematic of the Voc measurement and the water-droplet position. (b) Snapshots of water infiltration in the porous carbon film and (c) the corresponding Voc when a 2 μL water droplet was dropped onto positions A, B, C, D, E and F, as shown in 2a. (d) Normalized Voc of the device when 2 μL water droplets were dropped onto position A, with incline angles of 0°, 30°, 60°, 90°, 120°, 150° and 180°.
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curve in Fig. 3c, Fig. 3d I). When modified with polyethylenimine (PEI) (Fig. S5), the zeta potential of the carbon surface (PEI@PCF) changed to ~ 25 mV (red line in Fig. 3b). The voltage generated by a droplet, as shown in Fig. 3d II, became negative at the downstream end, with a larger value of ~ 0.6 V (red curve in Fig. 3c). Thus, we concluded that the sign of the voltage generated by the droplet was also related to the zeta potential of the PCF. Here, a notable phenomenon is that, when a droplet was dropped onto a PEI@PCF/PCF interface (Fig. 3d III), the generated voltage was approximately equal to the sum of the voltages (~ 0.8 V, blue curve in Fig. 3d) at the PFDTS@PCF/PCF and the PFDTS@PCF/PEI@PCF interfaces. Based on the aforementioned results above, we propose that a possible mechanism of the induced voltage in this study is the streaming potential, which arises when pressure-driven fluids flow through a narrow channel with surface charges on its walls [37]. Because streaming current is generated within the electric double layer, the phenomenon is confined to the thickness range of the layer, i.e., the Debye length (λD), which can be calculated as [39]
position E and position F in Fig. 2a). Water infiltration in both directions (Fig. 2b) was observed, without any significant and stable voltage signal output (Fig. 2c). These results reveal the following key features: i) the voltage could only be induced when directional water infiltration occurred on the carbon film; ii) no apparent correlation was observed between the induced voltage and the location of water droplets; iii) the sign of the induced voltage was determined by the water infiltration direction. Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.nanoen.2018.02.061. The dependence of the induced voltage on the tilting angle of carbon films was also investigated. When a 2 μL water droplet was dropped onto position A of the device with an incline angle of 0°, 30°, 60°, 90°, 120°, 150° and 180°, the generated voltages remained at a similar value of ~ 0.3 V (Fig. 2d). These results confirm that the voltage is related to the water infiltration process because the capillary force provided by the porous carbon is much larger than the gravity of the water droplet. To further elucidate the mechanism of the induced voltage, we investigated the dependence of the voltage on the ionic concentration and surface property of the PCF. In this case, droplets of NaCl solutions with different concentrations were used for measurements. As shown in Fig. 3a, the voltage decreased slightly from ~ 0.38 V to ~ 0.34 V as the NaCl concentration was increased from 10−7 mol L−1 to 10−4 mol L−1 and then decreased sharply to ~ 0.035 V at a concentration of 1 mol L−1. Such a trend is similar to the dependence of streaming potential on the ionic concentration [31], as will be discussed later. In the case of the solid/liquid interface, the zeta potential is a key indicator of surface property for solid materials in fluidic analysis [37,38]. Thus we investigated the effect of the zeta potential of the PCF surface on the induced voltage. As shown in Fig. 3b, the pristine PCF (black line) has a zeta potential of approximately − 30.5 mV and produces a voltage of approximately + 0.3 V at the downstream end (black
λD =
ԑε0 kB T/2nbulk z 2e 2
(1)
where ԑ is the dielectric constant of water, ԑ0 is the permittivity of vacuum, kB is the Boltzmann constant, T is the absolute temperature, nbulk is the bulk ion concentration, z is the valence of the ions, and e is the charge of an electron. Taking the ion concentration of DI water in our study as 10−7 mol L−1, we estimated λD to be ~ 1 µm, which is much larger than the pore size of the PCF (Fig. S1). Thus, directional water filtration in the PCF can induce a flow of counter charges in the tortile channels and subsequently induce a streaming potential [39–42]. An increase of the ionic concentration increases the number of charges in the liquid but also causes a dramatic decrease of λD according to Eq. (1). Hence, the induced potential will decrease with increasing ionic concentration, consistent with the results in Fig. 3a.
Fig. 3. Mechanism for the water-droplet-induced voltage. (a) Water-droplet-induced voltage as a function of NaCl concentration. (b) Zeta potential of a pristine PCF (black curve) and a PEI@PCF (red curve). (c) Water-droplet-induced Voc as a function of time for devices based on PCF (black curve), PEI@PCF (red curve) and PEI@ PCF/PCF junction (blue curve). (d) Schematic of devices based on (I) PCF, (II) PEI@PCF and (III) PEI@PCF/PCF junction. (e) Schematic of the surface charge effects in the nanochannels of (I) PCF, (II) PEI@PCF and (III) PEI@PCF-PCF junction. 214
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Fig. 4. Application demonstration of the water-dropletinduced voltage. (a) Measured Voc of three devices connected in series with a sequence of twelve 5 μL water droplets dropped onto the regions marked with black arrows in b, c under ambient conditions (~ 24.7 °C and ~ 67% RH). The dimensions of each device were ~ 120 × 7 mm2, and each device included five PFDTSmodified zones for series connection. The bottom-right inset is a schematic of the devices and measurement configuration. The stepped increase of Voc corresponds to the dropping of each water droplet. The top-left inset shows a magnified plot. (b, c) Photographs of a 32 × 43 mm2 LCD (e) before and (f) after being powered by the devices. (d, e) Schematic and photograph, respectively, of the self-powered water droplet delivery and position sensor. (f) Voltage responses of V1 and V2 when 1 μL water droplets were dropped onto positions A, B, C and D, as illustrated in (d, e).
http://dx.doi.org/10.1016/j.nanoen.2018.02.061. On the basis of the unique properties of the device, we also fabricated a self-powered sensor for in situ detection of water-droplet delivery and positioning on a hydrophilic surface, as demonstrated in Fig. 4d. We employed a square device with dimensions of ~ 32 × 32 mm2, the edge region of which was modified with PFDTS. The center region, with dimensions ~ 16 × 16 mm2 (dashed-white-squareenclosed area in Fig. 4e), was hydrophilic. Two pairs of electrodes were patterned on the four sides of the device and covered with epoxy, as shown in Fig. 4e. On the basis of the voltages detected between the electrodes, the water droplet delivery time and position on the surface were easily addressed simultaneously. As shown in Fig. 4f, voltage V1 sensed the vertical flow, whereas voltage V2 sensed the horizontal flow. The detection of electric voltage indicated delivery of a droplet. The position of the droplet was determined from the sign of V1 and V2. For example, positive V1 and positive V2 indicated a water droplet on the top-right site (A-position), whereas positive V1 and negative V2 indicated a droplet dropped on the top-left site (B-position).
The voltage generation of the three types of devices in Fig. 3d can be well explained by the streaming potential theory [29–32]. In the case of the pristine PCF, the surface of the PCF is negatively charged (zeta potential is negative); hence, the hydroxide ions in the water are repelled when water infiltrates the nanochannels in the PCF [41–45]. Excess hydronium ions flow and induce a higher potential at the downstream end (Fig. 3e I). In the case of the device based on the PEI@ PCF, the surface of the PEI@PCF is positively charged (zeta potential is positive), and water filtration carries excess hydroxide ions and results in lower potential at the downstream end (Fig. 3e II). For the device with a PEI@PCF/PCF junction, water droplets dropped onto the junction infiltrate symmetrically in the two directions. Water flow in the PEI@PCF induces a lower potential at the downstream end (left side), whereas flow in the PCF causes a higher potential at the downstream end (right side) (Fig. 3e III). Hence, the device can be considered as two cells connected in series. The divergence between the voltages generated by the pristine and the modified PCFs may be caused by the differences in sample resistance or differences in the ionic mobility of hydronium and hydroxide ions. To demonstrate the application of the device, we scaled up the voltage by connecting 3 devices (each with dimensions of ~ 120 × 7 mm2 and with individual resistances of 41.1 MΩ, 49.5 MΩ and 44.0 MΩ) in series (Fig. 4a). Each device has 5 PFDTS-modified areas noted as red zones in the bottom-right inset of Fig. 4a, forming an inseries connection in one device. When twelve 5-μL water droplets were dropped at the black arrow marked position in sequence (Fig. 4b and c), the total open-circuit voltage (Voc) increased from 0 V to ~ 5.2 V under ambient conditions. The stepped increase of Voc corresponds to the dropping of each water droplet (top-left inset of Fig. 4a and Video S2). The generated voltage is sufficient to power a 32 × 43 mm2 liquid crystalline display (LCD), as shown in Fig. 4c. Supplementary material related to this article can be found online at
4. Conclusion In summary, we demonstrated a novel electrical generation strategy based on water droplets infiltrating PCFs. The merits of such an electricity generation strategy are as follows: 1. simple fabrication, thereby eliminating the complexity of micro/nanofabrication; 2. easy scale-up for large-scale energy harvesting and sensing; 3. spontaneously driven by a natural process without an external energy input. Our study highlights the possibility of utilizing water in the environment as an energy supply. Because of the widespread and rich water resources in nature and the low cost of the materials involved (carbon films), the device presented here exhibits potential for use as an energy-harvesting device and self-powered sensor. 215
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Acknowledgments [14] [15] [16] [17] [18] [19]
This work was financially supported by the National Natural Science Foundation of China (51672097, 51606082, 51322210, 61434001), the National Program for Support of Top-Notch Young Professionals, the program for HUST Academic Frontier Youth Team, the China Postdoctoral Science Foundation (2015M570639, 2017M610471) and Director Fund of WNLO. The authors thank to Prof. Wang for his valuable discussion. The authors thank to the facility support of the Center for Nanoscale Characterization & Devices, WNLO-HUST and the Analysis and Testing Center of Huazhong University of Science and Technology.
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Appendix A. Supporting information
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Contents lists available at ScienceDirect
Nano Energy journal homepage: www.elsevier.com/locate/nanoen
Communication
Electricity generation from water droplets via capillary infiltrating a,1
a,1
a
a
a
a
T b
Jia Li , Kang Liu , Guobin Xue , Tianpeng Ding , Peihua Yang , Qian Chen , Yue Shen , ⁎ Song Lic, Guang Fengc, Aiguo Shend, Ming Xub, Jun Zhoua, a
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China d College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Energy conversion Capillary Nanofluidics Self-powered system
Electricity generation from the interaction between water and carbon materials is a new approach in the pursuit of high-efficiency energy conversion. Here, we report that a water droplet dropping onto a piece of porous carbon film can spontaneously generate electricity under ambient conditions. Twelve 5-μL water droplets can generate a voltage up to 5.2 V and illuminate a liquid crystal display. The voltage can be controlled by modulating the direction of the droplet infiltration, the zeta potential of the porous carbon, and the ion concentration. The results demonstrate that a hydrophilic porous carbon film with water droplets may function as a cost-effective electricity generator for harvesting energy from natural resources.
1. Introduction
for ionic solution flows outside [16–18] of and inside of CNT [19], respectively. While, the generated voltage is ~ 50 μV to 85 mV for water or ionic solution flows over graphene [20–25]. Furthermore, as previously reported, external pressure or force is necessary to drive the fluid flow, i.e., the energy harvesting process consumes energy. Recently, we have demonstrated that natural evaporation can drive the capillary water flow within porous carbon sheet, thus can produce significant voltage up to 1.0 V with lifetime over hundreds of hours under ambient conditions [41,42]. To deeply understand to underlying mechanism, we systematically studied electricity generation through the transformation of water droplets into water flow via the capillary force produced by a hydrophilic porous carbon film (PCF). A water droplet with volume of 1 μL can reliably generate a voltage of up to 0.8 V during the spontaneous filtration process of the water in the PCF. On the basis of the dependence of the induced voltage on the direction of the droplet infiltration, zeta potential of the porous carbon and ion concentrations, the phenomenon may be attributed to the electrokinetic effect [29-32]. In addition, a self-powered sensor for in situ detection of water-droplet delivery and positioning on a hydrophilic surface was also demonstrated. These results demonstrate a novel means to generate electricity from a natural process under ambient conditions, and the developed approach has potential applications in self-powered devices and systems.
Harvesting energy from the environment offers great promise in the application of self-powered devices and systems [1]. In the past few decades, intensive studies in this field have inspired an explosive growth of environmental energy utilization approaches [2–13]. Ideally, highly effective energy conversion processes should be environmentally friendly and spontaneous while not requiring any external energy input, such as mechanical movement, concentration difference or temperature variation. Electricity generation from the interaction between water flow and a carbon nanomaterial is a new approach in the pursuit of high-efficiency energy conversion and represents an interesting topic from both basic research and practical application viewpoints [14]. In 2001, Král and Shapiro first theoretically predicted that metallic carbon nanotubes (CNTs) in a flowing ionic liquid could generate a net potential difference and an associated electric current [15]. Since then, numerous researchers have reported voltage generation phenomena in which the detailed results differ with the carbon nanomaterials [16–25], the ion content of the flowing water, and the device configuration [26–28]. However, the reported voltage is typically in the range of several of microvolts to tens of millivolts, which is not adequate for modern devices. For example, the generated voltage is ~ 9 μV–30 mV and ~ 8 mV
⁎
1
Corresponding author. E-mail address: [email protected] (J. Zhou). J.L. and K.L. contributed equally to this work.
https://doi.org/10.1016/j.nanoen.2018.02.061 Received 12 February 2018; Received in revised form 28 February 2018; Accepted 28 February 2018 Available online 05 March 2018 2211-2855/ © 2018 Published by Elsevier Ltd.
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2. Experiment
(FTIR, Bruker Vertex 70). Contact angles were measured using a contact angle meter (Kino SL200B). The current-voltage characteristics of the device were measured using a Keithley 2400 source measurement unit. The Voc and Isc as a function of time were record using a Keithley 2000 multimeter and a low-noise current preamplifier (SRS model SR570), respectively. The environmental temperature and humidity were simultaneously recorded by a Center 310 RS-232 humidity and temperature meter.
2.1. Materials Multi-walled carbon nanotubes (MWCNTs) were purchased from XF Nano, Nanjing, China. Sulfuric acid (H2SO4), nitric acid (HNO3), sodium hydroxide (NaOH), ethanol, ethyl cellulose, acetic acid, glutaric dialdehyde (25 wt% aqueous solution), terpineol and toluene were purchased from Sinopharm. 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (PFDTS) was purchased from Sigma-Aldrich. PEI (MW ≈ 600) was purchased from Aladdin Industrial Corporation. Alumina strips were purchased from EMS Corp., Kunshan, China.
3. Results and discussion A schematic of the device for measuring the droplet-induced voltage is shown in Fig. 1a. First, two carbon nanotube (CNT) electrodes were coated onto the ends of a well-cleaned Al2O3 ceramic plate (details in Experimental Section). Next, carbon slurry composed of toluene carbon black, terpineol, ethyl cellulose and ethanol was coated using a blade such that the slurry crossed the CNT electrodes. Afterwards, the device was sintered at 350 °C for 150 min to obtain a porous carbon film (PCF). The PCF was then treated with air plasma for 60 s to ensure superhydrophilic. A scanning electron microscope (SEM) image (Fig. 1b) shows that the PCF has a thickness of ~ 5 µm and consists of interconnected carbon nanoparticles with a mean diameter of tens of nanometers, and with most of the pore sizes ranging from several to tens of nanometers (Fig. S1). After the device was wired and encapsulated by epoxy in the exposed electrode regions, selected regions of the device were modified with 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTS) to make it superhydrophobic. The right inset of Fig. 1a shows a photograph of a typical device with two ends modified by PFDTS. The Fourier transform infrared (FTIR) spectrum of PFDTS@PCF (red line in Fig. 1c) shows obvious additional absorption peaks at 1256 cm−1 and 1155 cm−1 compared to the spectrum of pristine PCF (black line); these new peaks correspond to C–F and C–O–Si bonds, respectively [33]. The pristine PCF and PFDTS@PCF exhibit different degrees of wettability, with water contact angles of ~ 0° and ~ 152° (insets of Fig. 1c), respectively. The prepared device shows robust ohmic contact, with a resistance of ~ 23.6 MΩ (Fig. S2). When deionized water (DI water, conductivity of ~ 1.6 × 10−4 S m−1, 1 μL) was dropped onto the PFDTS@PCF/PCF interface, water at the bottom of the droplet infiltrated into the porous carbon film, driven by capillary force. Because of the superhydrophobicity of the PFDTS@PCF region, water inside the PCF infiltrated only toward the hydrophilic region; infiltration in the opposite direction was disfavored. During this process, surprisingly, a sustainable voltage up to ~ 0.3 V was generated between the two CNT electrodes (Fig. 1d). The voltage remained nearly unchanged during the entire process and decreased quickly to zero when the droplet was completely vaporized. Such a phenomenon was highly reproducible. When five water droplets with the same size were dropped at the interface in sequence, similar open-circuit voltage signals were detected for each droplet (Fig. 1d). The short-circuit current was demonstrated to exhibit the same response (Fig. S3). On the contrary, water droplets moving on surface of graphene or aligned single-walled nanotubes only generated pulsed-electric signals [34–36]. The duration of the voltage was observed to be dependent on the size of the water droplets, as shown in Fig. 1e. The voltages induced by the water droplets with volumes of 1 μL, 2 μL, 5 μL and 10 μL were observed to persist for periods of ~ 73.7 s, ~ 195 s, ~ 610 s and ~ 1177 s, respectively, under ambient conditions, and nearly identical corresponding voltages (~ 0.31 V, ~ 0.31 V, ~ 0.33 V, ~ 0.33 V and ~ 0.33 V, respectively) were generated (Fig. 1e). The whole phenomenon appears to be due to the PCF functioning as a unique electricity generator. To explore the behavior of the water-droplet-induced voltage, we used a device with dimensions of ~ 50 × 7 mm2 with two ends and the central section modified with PFDTS. In the measurements, the right end was taken as positive and the left end was taken as negative
2.2. Synthesis of soluble CNT ink 3 g of MWCNTs were dispersed in 130 mL of mixed HNO3 and H2SO4 (HNO3/H2SO4 = 1/3 v/v) solution by sonication for 10 min. The mixture was then refluxed at 90 °C in an oil bath for 2.5 h under constant magnetic stirring. Afterwards, the mixture was cooled to room temperature. The mixture was subsequently washed with distilled water until the supernatant was neutral and was then re-dispersed in deionized water to form a 20 mg mL−1 MWCNT ink. 2.3. Synthesis of carbon slurry Toluene carbon black powder was first prepared by a methylbenzene flame synthesis method. One gram of toluene carbon black, 1 g of ethyl cellulose and 3 mL of terpineol were subsequently mixed in 50 mL of ethanol. The resulting mixture was probe-sonicated (Kesheng Sonics Vibra Cell, 550 F) for 20 min. Finally, the homogeneous slurry was evaporated to a total volume of 10 mL in an oil bath at 60 °C under magnetic stirring. 2.4. Fabrication of the device Alumina strips were sequentially ultra-sonicated in acetone, alcohol and DI water and then dried at 70 °C in an oven. Next, CNT ink was printed into the alumina strip with required patterns using a floating knife coater. The carbon slurry was next blade-coated across the electrodes. After the electrodes were sintered at 350 °C for 150 min and cooled to room temperature, the obtained porous carbon film (PCF) was treated with air plasma (pressure 100 Pa, RF power 130 W) for 60 s in a plasma cleaner (Mingcheng, PDC-MG). The device was subsequently wired and encapsulated with epoxy in the exposed electrodes regions. 2.5. PFDTS modification PFDTS was diluted with ethanol (1:10 v/v), and 1 wt% acetic acid was added to the solution and well shaken before use. The desired PCF regions were modified by being dip-coated in pre-diluted PFDTS ethanol solution at 70 °C and then heated for 30 min at 100 °C. 2.6. PEI modification A 0.1 wt% PEI-600 aqueous solution was carefully dripped onto half of the PCF region and then heated to 70 °C for 30 min; after being washed with water and ethanol, the PEI-treated PCF area was then dipped in 0.1% glutaric dialdehyde aqueous solution heated to 70 °C in a water bath. Finally, the PEI-modified PCF was thoroughly washed with 70 °C DI water to remove residual reactants. 2.7. Characterization The morphology, structure and functional groups of the PCFs were characterized by scanning electron microscopy (SEM, FEI Nova Nano450) and FTIR spectroscopy coupled with infrared microscopy 212
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Fig. 1. Water-droplet-induced voltage. (a) Schematic of the porous carbon film device with two ends modified with PFDTS. The right inset shows a photograph of a typical device with dimensions of 50 × 7 mm2. The areas enclosed with red dashed lines represent PFDTS-modified regions, and the areas enclosed with green dashed lines represent epoxy-encapsulated regions. (b) Cross-sectional and magnified SEM images of the porous carbon film. (c) FTIR spectra of the pristine carbon film (black curve) and PFDTS-modified carbon film (red curve). Insets show the corresponding contact-angle images of a pristine carbon film (~ 0°) and a PFDTS-modified carbon film (~ 152°), respectively. (d) Open-circuit voltage obtained by repeatedly dropping 1 μL water droplets at the PFDTS@PCF/ PCF interface under ambient conditions (~ 23.5 °C and relative humidity ~ 71.7%). (e) Measured Voc vs. time of the device when water droplets with volumes of 1 μL, 2 μL, 5 μL and 10 μL were dropped onto the PFDTS@PCF/PCF interface.
(Fig. 2a). When a 2 μL water droplet was dropped onto the PFDTS modified region, the shape of the water droplet was maintained for nearly 1 h without infiltration because of the surface hydrophobicity, and no voltage was detected between two electrodes (Fig. S4). However, when a 2 μL-water droplet was dropped onto the PFDTS@PCF/ PCF interface with the pristine PCF region at the right (position A and position D in Fig. 2a), water infiltration occurred from left to right (Fig. 2b), while the infiltration from right to the left was blocked because of superhydrophobicity in the left region. Simultaneously, a
voltage of ~0.3 V was detected (Fig. 2c and video S1). When the water was completely vaporized, the voltage signals decreased to the initial zero point. When a 2 μL water droplet was dropped onto the PFDTS@ PCF/PCF interface with the pristine PCF region at the left (marked as position B and position C in Fig. 2a), water infiltrated from right to left (Fig. 2b), while the infiltration from left to the right was blocked because of superhydrophobicity in the right region. A corresponding voltage of ~ − 0.3 V was detected. By comparison, we also dropped 2 μL water droplets onto the center of the PCF region (marked as Fig. 2. Wetting dependence of the induced voltage. (a) Schematic of the Voc measurement and the water-droplet position. (b) Snapshots of water infiltration in the porous carbon film and (c) the corresponding Voc when a 2 μL water droplet was dropped onto positions A, B, C, D, E and F, as shown in 2a. (d) Normalized Voc of the device when 2 μL water droplets were dropped onto position A, with incline angles of 0°, 30°, 60°, 90°, 120°, 150° and 180°.
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curve in Fig. 3c, Fig. 3d I). When modified with polyethylenimine (PEI) (Fig. S5), the zeta potential of the carbon surface (PEI@PCF) changed to ~ 25 mV (red line in Fig. 3b). The voltage generated by a droplet, as shown in Fig. 3d II, became negative at the downstream end, with a larger value of ~ 0.6 V (red curve in Fig. 3c). Thus, we concluded that the sign of the voltage generated by the droplet was also related to the zeta potential of the PCF. Here, a notable phenomenon is that, when a droplet was dropped onto a PEI@PCF/PCF interface (Fig. 3d III), the generated voltage was approximately equal to the sum of the voltages (~ 0.8 V, blue curve in Fig. 3d) at the PFDTS@PCF/PCF and the PFDTS@PCF/PEI@PCF interfaces. Based on the aforementioned results above, we propose that a possible mechanism of the induced voltage in this study is the streaming potential, which arises when pressure-driven fluids flow through a narrow channel with surface charges on its walls [37]. Because streaming current is generated within the electric double layer, the phenomenon is confined to the thickness range of the layer, i.e., the Debye length (λD), which can be calculated as [39]
position E and position F in Fig. 2a). Water infiltration in both directions (Fig. 2b) was observed, without any significant and stable voltage signal output (Fig. 2c). These results reveal the following key features: i) the voltage could only be induced when directional water infiltration occurred on the carbon film; ii) no apparent correlation was observed between the induced voltage and the location of water droplets; iii) the sign of the induced voltage was determined by the water infiltration direction. Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.nanoen.2018.02.061. The dependence of the induced voltage on the tilting angle of carbon films was also investigated. When a 2 μL water droplet was dropped onto position A of the device with an incline angle of 0°, 30°, 60°, 90°, 120°, 150° and 180°, the generated voltages remained at a similar value of ~ 0.3 V (Fig. 2d). These results confirm that the voltage is related to the water infiltration process because the capillary force provided by the porous carbon is much larger than the gravity of the water droplet. To further elucidate the mechanism of the induced voltage, we investigated the dependence of the voltage on the ionic concentration and surface property of the PCF. In this case, droplets of NaCl solutions with different concentrations were used for measurements. As shown in Fig. 3a, the voltage decreased slightly from ~ 0.38 V to ~ 0.34 V as the NaCl concentration was increased from 10−7 mol L−1 to 10−4 mol L−1 and then decreased sharply to ~ 0.035 V at a concentration of 1 mol L−1. Such a trend is similar to the dependence of streaming potential on the ionic concentration [31], as will be discussed later. In the case of the solid/liquid interface, the zeta potential is a key indicator of surface property for solid materials in fluidic analysis [37,38]. Thus we investigated the effect of the zeta potential of the PCF surface on the induced voltage. As shown in Fig. 3b, the pristine PCF (black line) has a zeta potential of approximately − 30.5 mV and produces a voltage of approximately + 0.3 V at the downstream end (black
λD =
ԑε0 kB T/2nbulk z 2e 2
(1)
where ԑ is the dielectric constant of water, ԑ0 is the permittivity of vacuum, kB is the Boltzmann constant, T is the absolute temperature, nbulk is the bulk ion concentration, z is the valence of the ions, and e is the charge of an electron. Taking the ion concentration of DI water in our study as 10−7 mol L−1, we estimated λD to be ~ 1 µm, which is much larger than the pore size of the PCF (Fig. S1). Thus, directional water filtration in the PCF can induce a flow of counter charges in the tortile channels and subsequently induce a streaming potential [39–42]. An increase of the ionic concentration increases the number of charges in the liquid but also causes a dramatic decrease of λD according to Eq. (1). Hence, the induced potential will decrease with increasing ionic concentration, consistent with the results in Fig. 3a.
Fig. 3. Mechanism for the water-droplet-induced voltage. (a) Water-droplet-induced voltage as a function of NaCl concentration. (b) Zeta potential of a pristine PCF (black curve) and a PEI@PCF (red curve). (c) Water-droplet-induced Voc as a function of time for devices based on PCF (black curve), PEI@PCF (red curve) and PEI@ PCF/PCF junction (blue curve). (d) Schematic of devices based on (I) PCF, (II) PEI@PCF and (III) PEI@PCF/PCF junction. (e) Schematic of the surface charge effects in the nanochannels of (I) PCF, (II) PEI@PCF and (III) PEI@PCF-PCF junction. 214
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Fig. 4. Application demonstration of the water-dropletinduced voltage. (a) Measured Voc of three devices connected in series with a sequence of twelve 5 μL water droplets dropped onto the regions marked with black arrows in b, c under ambient conditions (~ 24.7 °C and ~ 67% RH). The dimensions of each device were ~ 120 × 7 mm2, and each device included five PFDTSmodified zones for series connection. The bottom-right inset is a schematic of the devices and measurement configuration. The stepped increase of Voc corresponds to the dropping of each water droplet. The top-left inset shows a magnified plot. (b, c) Photographs of a 32 × 43 mm2 LCD (e) before and (f) after being powered by the devices. (d, e) Schematic and photograph, respectively, of the self-powered water droplet delivery and position sensor. (f) Voltage responses of V1 and V2 when 1 μL water droplets were dropped onto positions A, B, C and D, as illustrated in (d, e).
http://dx.doi.org/10.1016/j.nanoen.2018.02.061. On the basis of the unique properties of the device, we also fabricated a self-powered sensor for in situ detection of water-droplet delivery and positioning on a hydrophilic surface, as demonstrated in Fig. 4d. We employed a square device with dimensions of ~ 32 × 32 mm2, the edge region of which was modified with PFDTS. The center region, with dimensions ~ 16 × 16 mm2 (dashed-white-squareenclosed area in Fig. 4e), was hydrophilic. Two pairs of electrodes were patterned on the four sides of the device and covered with epoxy, as shown in Fig. 4e. On the basis of the voltages detected between the electrodes, the water droplet delivery time and position on the surface were easily addressed simultaneously. As shown in Fig. 4f, voltage V1 sensed the vertical flow, whereas voltage V2 sensed the horizontal flow. The detection of electric voltage indicated delivery of a droplet. The position of the droplet was determined from the sign of V1 and V2. For example, positive V1 and positive V2 indicated a water droplet on the top-right site (A-position), whereas positive V1 and negative V2 indicated a droplet dropped on the top-left site (B-position).
The voltage generation of the three types of devices in Fig. 3d can be well explained by the streaming potential theory [29–32]. In the case of the pristine PCF, the surface of the PCF is negatively charged (zeta potential is negative); hence, the hydroxide ions in the water are repelled when water infiltrates the nanochannels in the PCF [41–45]. Excess hydronium ions flow and induce a higher potential at the downstream end (Fig. 3e I). In the case of the device based on the PEI@ PCF, the surface of the PEI@PCF is positively charged (zeta potential is positive), and water filtration carries excess hydroxide ions and results in lower potential at the downstream end (Fig. 3e II). For the device with a PEI@PCF/PCF junction, water droplets dropped onto the junction infiltrate symmetrically in the two directions. Water flow in the PEI@PCF induces a lower potential at the downstream end (left side), whereas flow in the PCF causes a higher potential at the downstream end (right side) (Fig. 3e III). Hence, the device can be considered as two cells connected in series. The divergence between the voltages generated by the pristine and the modified PCFs may be caused by the differences in sample resistance or differences in the ionic mobility of hydronium and hydroxide ions. To demonstrate the application of the device, we scaled up the voltage by connecting 3 devices (each with dimensions of ~ 120 × 7 mm2 and with individual resistances of 41.1 MΩ, 49.5 MΩ and 44.0 MΩ) in series (Fig. 4a). Each device has 5 PFDTS-modified areas noted as red zones in the bottom-right inset of Fig. 4a, forming an inseries connection in one device. When twelve 5-μL water droplets were dropped at the black arrow marked position in sequence (Fig. 4b and c), the total open-circuit voltage (Voc) increased from 0 V to ~ 5.2 V under ambient conditions. The stepped increase of Voc corresponds to the dropping of each water droplet (top-left inset of Fig. 4a and Video S2). The generated voltage is sufficient to power a 32 × 43 mm2 liquid crystalline display (LCD), as shown in Fig. 4c. Supplementary material related to this article can be found online at
4. Conclusion In summary, we demonstrated a novel electrical generation strategy based on water droplets infiltrating PCFs. The merits of such an electricity generation strategy are as follows: 1. simple fabrication, thereby eliminating the complexity of micro/nanofabrication; 2. easy scale-up for large-scale energy harvesting and sensing; 3. spontaneously driven by a natural process without an external energy input. Our study highlights the possibility of utilizing water in the environment as an energy supply. Because of the widespread and rich water resources in nature and the low cost of the materials involved (carbon films), the device presented here exhibits potential for use as an energy-harvesting device and self-powered sensor. 215
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Acknowledgments [14] [15] [16] [17] [18] [19]
This work was financially supported by the National Natural Science Foundation of China (51672097, 51606082, 51322210, 61434001), the National Program for Support of Top-Notch Young Professionals, the program for HUST Academic Frontier Youth Team, the China Postdoctoral Science Foundation (2015M570639, 2017M610471) and Director Fund of WNLO. The authors thank to Prof. Wang for his valuable discussion. The authors thank to the facility support of the Center for Nanoscale Characterization & Devices, WNLO-HUST and the Analysis and Testing Center of Huazhong University of Science and Technology.
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Appendix A. Supporting information
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