A Hydrocapacitor for Harvesting and Storing Energy from Water

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Energy, Environmental, and Catalysis Applications

A Hydrocapacitor for Harvesting and Storing Energy from Water Movement Ruhao Liu, Changhong Liu, and Shoushan Fan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12967 • Publication Date (Web): 21 Sep 2018 Downloaded from http://pubs.acs.org on September 22, 2018

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ACS Applied Materials & Interfaces

A Hydrocapacitor for Harvesting and Storing Energy from Water Movement

Ruhao Liu, Changhong Liu* and Shoushan Fan

Tsinghua-Foxconn Nanotechnology Research Center and Department of Physics, Tsinghua University, Beijing 100084, China Changhong Liu. Email: [email protected]

Keywords hydrocapacitor; nanogenerator; carbon nanotube; energy conversion; energy storage

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Abstract Based on porous carbon nanotube/polyaniline composite (CNT/PANI) and polyvinyl alcohol gel, we fabricated centimeter-sized hydrocapacitors with dual functions of energy conversion and storage with an efficient low-cost method. Owning to excellent hydrophily and large specific capacitance of CNT/PANI, the hydrocapacitors can easily convert energy from water movement induced by capillarity, gravity or air pressure difference into electricity and store the generated electricity. Especially, sandwich-like hydrocapacitors outputted large current of 1.65 mA through an external load of 100 Ω, and hydrocapacitors showed good extendibility by connecting in series. To explain the mechanism of hydrocapacitors in this work, a possible model based on capillarity and traditional streaming potential was proposed and discussed. Hydrocapacitors here also provide a reference for future integration of nanogenerators and energy storage parts.

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INTRODUCTION Harvesting energy from environment is promising, and widespread small-scale water movement attracted researchers’ attention long ago1-2. However, because hydroelectric generators can only be driven by large-scale water flows in dams, small-scale water movements had been neglected for long3-4. Since electricity generation of carbon nanotube (CNT) in flowing water was reported in 20035, nanogenerators, generating electricity from small-scale water movement, based on carbon materials were fabricated and studied6-8. Individual and bundles of CNTs were studied to generate electricity in flowing water, and a mechanism of coupling between CNTs and water diploes was proposed5, 9-11. A 30-cm-long one-dimensional fluidic nanogenerator based on aligned CNT sheets was reported to produce an output voltage of 650 mV and a current of 67 µA, where electric double layer was regarded to play an important role12. Similarly, nanogenerators based on graphene were researched13-15. Besides, nanogenerators working in moisture condition were found to generate electricity owing to the imbalance of oxygen-containing groups16-18. Moreover, electrochemical potential was found in graphene oxide-metal hybrid material19, and water-evaporation-induced electricity was found in nanostructured carbon black20. These nanogenerators are inspiring, but they can’t store the generated electricity. Containing continuous carbon network, hydrophilic carbon nanotube/polyaniline composite (CNT/PANI) shows good electric conductivity, whose porous structure is good for water moving. Herein, based on CNT/PANI and polyvinyl alcohol gel (PVA), we fabricated centimeter-sized hydrocapacitors with dual functions of energy 3



conversion and storage with an efficient low-cost method. Owning to excellent hydrophily and large specific capacitance of CNT/PANI, the hydrocapacitors can easily convert energy from water movement induced by capillarity, gravity or air pressure difference into electricity and store the generated electricity. Besides, the extendibility of hydrocapacitors was tested by connecting two hydrocapacitors in series. Meanwhile, a possible model for hydrocapacitors based on streaming potential and capillarity was proposed and discussed, explaining the mechanism of hydrocapacitors in this work. As is known, to effectively store the output electricity of nanogenerators like triboelectric nanogenerators, whose open-circuit voltage was of ~1200 V21, integration of nanogenerators and energy storage parts is promising and attractive6, 22-25, and therefore we hope hydrocapacitors here would provide a reference for that.

RESULTS AND DISCUSSION Fabrication of hydrocapacitors. A sandwich-like hydrocapacitor is shown in Figure S1b (working area of 1 × 3 cm2), and measurement for its output voltage (VR) on an external resistor (RL; 100 Ω) is shown in Figure 1a and 1b. To fabricate hydrocapacitors, CNT/PANI and PVA should be prepared first. To be brief, superaligned carbon nanotubes (SACNTs)26 were oxidized after being heated at 500 oC for 20 min in air, and active-SACNTs with excellent hydrophily and electric conductivity were obtained27. Later, active-SACNTs were combined with PANI to get uniform CNT/PANI films (average mass density of 1.3 mg cm-2, thickness of 67 ±8 µm; Figure S1a) with an in situ polymerization method28-29. Then, CNT/PANI films were

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cut into pieces with the same shape (CNT/PANIslic; 4.5 cm × 2 cm; Figure S1c and Figure 2a) by laser. Owing to containing continuous active-SACNT networks, these CNT/PANI slices had porous microstructure (Figure S2a), excellent hydrophily and electric conductivity (resistance of 58 ±7 Ω; Figure 2a). Meanwhile, PVA-H2SO4 gel electrolyte (PVA-HS) was prepared by resolving polyvinyl alcohol into 1 M H2SO4 aqueous solution, and PVA-H2O gel electrolyte (PVA-HO) was prepared by resolving polyvinyl alcohol into deionized water. When coated onto one side of CNT/PANI slices, PVA would soak into porous network naturally and integrate with CNT/PANI well28-29 (Figure S2b), which was good for water to move through CNT/PANI and PVA. By pressing two random CNT/PANI slices coated with PVA-HS together with PVA-coating side to PVA-coating side, a sandwich-like hydrocapacitor with symmetric structure was fabricated (Figure S1b). Moreover, planar hydrocapacitors (Figure S1d) were fabricated by coating PVA-HS or PVA-HO on the gap of cutting-off CNT/PANIslic (CNT/PANIslic-brek; Figure 2b). For convenience, sandwich-like hydrocapacitor with PVA-HS was labeled as CNT/PANIsand, planar hydrocapacitor with PVA-HO as CNT/PANIslic-HO (Figure 2c), and planar hydrocapacitor with PVA-HS as CNT/PANIslic-HS (Figure 2d). After fabrication, for the sake of accurate measurements, gold films were used to act as measuring electrodes, which were connected with CNT/PANI electrodes of hydrocapacitors using silver paint. Meanwhile, all hydrocapacitors were horizontally placed on clean glass plates during measurements (room temperature of 20 - 23 oC; relative humidity of 40 - 45 %). Besides, before each

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measurement, the two electrodes of hydrocapacitors were connected together to do pre-discharge.

Electricity generation and storage of hydrocapacitor. Dropping water onto hydrocapacitors could induce electricity generation and storage repeatedly. As Figure 1c shows, at the beginning of a measurement for output voltage of a CNT/PANIsand with switch 1 and switch 2 on, VR was about zero because of the pre-discharge. When 20 µL of deionized water was dropped to the bottom CNT/PANI electrode of CNT/PANIsand (Figure 1b) at the time of T1, VR rose to 17.9 mV, and then showed approximately exponential decline with time, which looked like a discharge process in a simple resistor-capacitor circuit30. Owing to pseudocapacitance of CNT/PANI, CNT/PANIsand could act as supercapacitor (internal resistance of 64 ±10 Ω, specific capacitance of 56 ± 4 mF cm-2; Figure S1f), so above measurement implied that dropping water onto CNT/PANI electrode could induce current and charge the CNT/PANIsand. When another 20 µL of deionized water was dropped to the bottom CNT/PANI electrode at T2, VR rose and declined again, which meant dropping water to the bottom electrode would induce current and charge CNT/PANIsand to store generated electric energy repeatedly. Since the thickness of hydrocapacitors here was below 0.2 mm and there was no heater in our study, thermovoltage was avoided, which could be induced by thermal gradient across ion electrolyte via thermodiffusion (Soret effect)31. Therefore, induced currents in hydrocapacitors might be caused by water movements from dropping water.

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Dropping water continuously or adding wind onto hydrocapacitors could raise the output voltage, and hydrocapacitors showed good electricity storage performance. As Figure 1d shows, after the pre-discharge, switch 2 was turned off at T1 while switch 1 kept on, deionized water was added to the bottom electrode at a speed of 20 µL min-1, and VR rose continuously to 60 mV. At T2, 50 µL of deionized water was added to bottom electrode, and VR rose higher. To study the relation between induced currents and water movements caused by adding water to CNT/PANI electrodes, a grazing wind simulated by compressed air flow was added to the surface of the top CNT/PANI electrode at T3, and then the velocity of wind was increased at T4. Meanwhile, deionized water was added to the bottom electrode at a speed of 10 µL min-1 during T3-T5. As Figure 1d shows, VR rose to 90 mV in the wind. On one hand, according to Bernoulli Equation32 (Equation S1), when wind was on, air pressure on the top electrode (pt) would reduce and become lower than the air pressure on the bottom electrode (pb), causing water moving from the bottom electrode to the top electrode because of the air pressure difference (∆p). On the other hand, wind would accelerate water evaporating from the top electrode, causing water moving from the bottom electrode to the top electrode owing to capillarity20 (Equation S2-S4). The performance of the CNT/PANIsand during T1-T5 suggested that water movements in CNT/PANIsand caused by adding water or blowing wind would induce current and charge CNT/PANIsand. When switch 2 was turned on at T5, CNT/PANIsand discharged to release stored electric energy, and VR declined again. Comparing to reported nanogenerators without energy storage parts, hydrocapacitors here could store

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generated electric energy effectively and release the stored electricity when external load was added, whose coulombic efficiency was 86.7 – 98.1 % (Figure S1e and S1f).

Figure 1. Induced voltages in sandwich-like hydrocapacitors. (a - b) Schematic diagrams of a measurement for output voltage VR of the sandwich-like hydrocapacitor; (c - d) Typical measurement results of VR based on a sandwich-like hydrocapacitor, where 20 µL water means adding 20 µL of deionized water, switch 2 on/off means turning switch 2 on or off, wind on/off means adding wind to or removing wind off top CNT/PANI electrode, and wind larger means increasing the velocity of wind.

Discussion on the mechanism of hydrocapacitors here. Although above measurements showed the CNT/PANIsand could easily convert energy from water movement into electricity and store the generated electric energy, it didn’t identify where the voltage occurred. As introduced at the beginning that coupling between carbon materials and water dipoles might also induce voltage on carbon materials, a

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measurement for output voltage of CNT/PANIslic was done (Figure 2a) to identify whether induced voltages occurred at the CNT/PANI electrode. As Figure 2g shows, when tiny fluctuations of VR caused by environment and testing instrument itself were ignored, no matter dropping water, dropping 0.1 M H2SO4 aqueous solution or blowing wind in any direction onto the CNT/PANIslic, VR kept at zero. Similarly, potential induced by water evaporation was just about 10 µV in multiwalled carbon nanotube sample20, also much lower than induced voltages in this work. Therefore, induced voltages observed in hydrocapacitors, which were on the order of millivolts, didn’t occur at the single CNT/PANI electrode, and coupling between carbon materials and water dipoles5, 9-15 could be neglected here.

To identify whether induced voltages occurred between the two CNT/PANI electrodes and study how that happened, more measurements were done on CNT/PANIslic-brek and planar hydrocapacitors (Figure 2b-2d). As Figure 2e shows, when water moved between Bzone and Tzone of CNT/PANIslic-brek, there would be a water bridge at the gap. Without adding more water, owing to surface tension of water33, the water bridge would narrow and disappear as water at the gap drained away. As Figure 2h shows, when 50 µL of deionized water was dropped onto Bzone at T1, water moved to Tzone through the gap of CNT/PANIslic-brek, and VR decreased to -1.37 mV and then rose; when 50 µL of deionized water was dropped onto Tzone at T2, water moved to Bzone, and VR rose to 0.85 mV and then decreased; when 50 µL of 0.1 M H2SO4 aqueous solution was dropped onto Bzone at T3, solution moved to Tzone, and VR decreased to -11.1 mV and then rose; when 50 µL of 0.1 M H2SO4 aqueous 9



solution was dropped onto Tzone at T4, solution moved to Bzone, and VR rose to 7.1 mV and then decrease. However, if both of Bzone and Tzone of CNT/PANIslic-brek were covered by water, there was no output voltage, which suggested that induced voltages in hydrocapacitors were indeed caused by water movement. Moreover, when water bridge disappeared (blue dotted line in Figure 2h), VR went to zero immediately in above four tests. Above tests suggested that voltages induced by water movement in hydrocapacitors didn’t occur at single CNT/PANI electrode but between the two CNT/PANI electrodes separated by insulators like air gap or PVA, similar to the reported mechanism where voltages induced by water evaporation relied on water moving through narrow gaps between nanoparticles of carbon black20. When there were insulators like air gap or PVA between two CNT/PANI electrodes, ions in water could pass through the insulators, but the current induced by water movement would not be counteracted due to the insulators, so insulators were necessary for hydrocapacitors in this work. Besides, results showed in Figure 2h were informative: (1) the sign of VR was related to the direction of water movement, and the potential of the upstream electrode was higher than the downstream one. Because only free-moving ions like hydrogen ions (H+) and hydroxyl ion (OH-)/sulfate ion (SO42-) can carry current in PVA, the relation between the sign of VR and the direction of water movement suggested that there occurred negative net charge flow from upstream to downstream in hydrocapacitor; (2) the same volume of 0.1 M H2SO4 would induce larger voltage than deionized water, which probably resulted from higher concentration of ions in the H2SO4 aqueous solution.

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Figure 2. Induced voltages in planar hydrocapacitors. (a – d) Schematic diagrams of measurements for VR of CNT/PANIslic, CNT/PANIslic-brek, CNT/PANIslic-HO and CNT/PANIslic-HS separately; (e) Schematic diagram of water on Bzone moving to Tzone through the gap of CNT/PANIslic-brek; (f) Schematic diagram of water on Tzone moving to Bzone through the PVA of CNT/PANIslic-HS or CNT/PANIslic-HO; (g – j) Typical measurement results of VR based on a CNT/PANIslic, CNT/PANIslic-brek, CNT/PANIslic-HO and CNT/PANIslic-HS separately, where water means adding deionized water, H2SO4 means adding H2SO4, and T→B/ B→T means water moving from Tzone to Bzone or vice versa.

Since water moving through the gap of CNT/PANIslic-brek could induce voltage, the role of PVA in hydrocapacitors should be researched. As Figure 2i shows, 50 µL of deionized water was dropped onto Bzone of a CNT/PANIslic-HO at T1, which would 11



cause water moving through PVA-HO from Bzone to Tzone, and then VR became negative, which was consistent with the results in CNT/PANIslic-brek; 50 µL of 0.1 M H2SO4 was dropped onto Tzone at T2, which would cause water moving through PVA-HO from Tzone to Bzone, and then VR became positive; later, 50 µL of 0.1 M H2SO4 was dropped onto Bzone at T3 and onto Tzone at T4 successively. VR during T2-T4 showed more smooth curves and longer rise/fall time than that during T1-T2 in Figure 2i, meaning that higher concentration of ions in PVA could improve the charging/discharging performances of CNT/PANIslic-HO. As Table 1 and Figure S1e show, CNT/PANIslic-HS had smaller internal resistance but larger capacitance than CNT/PANIslic-HO. According to above results, PVA acted an important role in hydrocapacitors: (1) owing to excellent stickiness, PVA could stick the two CNT/PANI electrodes together well and form self-supporting device; (2) PVA could separate the two electrodes as air gap did in CNT/PANIslic-brek, and as discussed above, such a separation is necessary for water movement to induce currents in hydrocapacitors; (3) owning to hydrophily, PVA could act as a bridge for water to move between the two CNT/PANI electrodes, making it possible for hydrocapacitors to fast respond to water dropping and convert energy from water movement into electricity, because it took time and needed enough water for CNT/PANIslic-brek to establish a water bridge at its gap. To verify above results, 50 µL of deionized water was dropped onto Tzone and onto Bzone of CNT/PANIslic-HS successively, and the VR of CNT/PANIslic-HS behaved as expected (Figure 2j), whose sign was also related to the direction of water movement. What’s more, comparing Figure 2i and Figure 2j,

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CNT/PANIslic-HS performed much better than CNT/PANIslic-HO, which suggested that PVA in low pH solution could promote energy conversion and storage in hydrocapacitors. Table 1. Internal resistance and capacitance of CNT/PANIslic-HO and CNT/PANIslic-HS in galvanostatic discharge-charge tests (charging/discharging current of 20 µA, voltage range of 0-0.25V) and width of their gaps.

CNT/PANIslic-HO

CNT/PANIslic-HS

internal resistance [kΩ]

2.1 (±0.1)

0.3 (±0.1)

capacitance [mF]

4.5 (±0.3)

62.1 (±4)

width of gap [mm]

0.32 (±0.11)

0.35 (±0.09)

According to above results and discussions, on one hand, oxidized CNTs were ultrasonically mixed to get uniform CNT films, so imbalance of oxygen-containing groups16-18 contributed little to electricity generation; on the other hand, separation like air gap or PVA and water movement were necessary for hydrocapacitors to generate electricity, similar to the situation of water-evaporation-induced electricity in carbon black20, where traditional streaming potential was thought to play a major role in electricity generation. Therefore, to explain the mechanism of electricity generation in hydrocapacitors here, we proposed a possible model (Figure 3a) based on streaming potential. With lone pairs of electron at their alcoholic hydroxyls, PVA chains could attract H+ but repulse negative ions, and there would be an electric double layer on the surface of PVA chains. According to streaming potential theory34-36, when water moved through narrow channels in PVA between the two CNT/PANI electrodes of a hydrocapacitor, more H+ than negative ions would be absorbed onto the surface of PVA chains, and there would appear net positive charges in the PVA gel, which would 13



allow negative ions pass but block positive ions like H+. Therefore, when water was moving, H+ would concentrate at the upstream electrode, while negative ions would concentrate at the downstream electrode, leading to negative net charge flow from upstream electrode to downstream and higher potential at the upstream electrode. These theoretical conclusions were consistent with above experimental results. Besides, results during T3-T5 in Figure 1d suggested that wind could raise the output voltage, which could also be explained by the streaming potential model (Equation S5 and S6), where larger air pressure difference (∆p) would lead to larger voltage.

Therefore, based on above streaming potential model showed in Figure 3a, a whole model for hydrocapacitors was proposed (Figure 3b), where resistor Rin, capacitor C and current source I represented internal resistance, capacitance and induced current in hydrocapacitors separately. Moreover, if there was tiny or no water movement, current resource I in the whole model would be too weak and could be regarded as open circuit. To verify the whole model, more discussions are necessary. According to the whole model, if deionized water is added to the bottom electrode of CNT/PANIsand, transient fast water movement through PVA from bottom electrode to top electrode caused by capillarity (Figure 3e) will induce large streaming current and charge the capacitor C to store generated electric energy, consistent with the experimental results showed in Figure 1 and Figure 2. A wind on top electrode would increase the air pressure difference between two CNT/PANI electrodes and accelerate water evaporating from top electrode, causing stronger water movement and larger streaming current, consistent with the results during T3-T5 in Figure 1d. Experimental 14


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results in planar hydrocapacitors (Figure 2i and 2j) could be discussed in similar ways. By reversing whole devices consisting of sandwich-like hydrocapacitors and connecting lines (Figure 1a and 1b), positions of the two electrodes were exchanged, then: as Figure 3c shows, switch 1 and switch 2 were on at the beginning, and pre-discharge was done; switch 2 was turned off and on at T1 and T4 separately, deionized water was continuously added to new bottom electrode during T1-T4, and wind was added and removed onto new top electrode at T2 and T3 separately; the VR of reversed CNT/PANIsand behaved similarly to former CNT/PANIsand.

Robustness of the whole model and extendibility of hydrocapacitors. Although theoretical conclusions from the whole model were consistent with above experimental results, more measurements for the CNT/PANIsand (Figure 1a and 1b) were done to test the scope of application and robustness of the model. As Figure 3d shows, switch 1 and switch 2 being on at the beginning, when 20 µL of deionized water was dropped onto the top electrode, VR decreased to -41.1 mV first, rose to 6.4 mV and then declined slowly towards zero, different from the results in Figure 1c where deionized water was dropped onto the bottom electrode; when wind was added onto top electrode at T2, VR rose to 10.8 mV first and then declined slowly towards zero; later, when the wind was removed at T3, VR decreased to -8.8 mV and then rose slowly towards zero; when deionized water was continuously added to top electrode from T4, VR declined; when switch 2 was turned off at T5, VR became smaller than -170 mV; when switch 2 was turned on again at T6, VR rose towards zero. During above measurements, maximum absolute value of VR was 165 mV, meaning maximum output 15



current through RL was 1.65 mA, which corresponded to a current density of 5.5 A m-2. Meanwhile, comparing to the energy harvester based on carbon nanotube20, whose evaporation-induced potential was on the order of 10 µV, hydrocapacitor based on carbon nanotube composite here could output much larger potential. To explain above experimental results, capillarity and streaming potential theories were used. As Figure 3f shows, owing to the equilibrium between gravity and capillarity before T1, there were height differences (h) between upper surfaces of water in narrow channels of PVA and water in bottom electrode; when some water was dropped onto top electrode at T1, previous equilibrium would be broken, and water would move downwards driven by gravity first. Before new equilibrium between gravity and capillarity was formed, upper surfaces of water in channels of PVA would vibrate around new equilibrium position, which behaved similarly to a spring hanging a mass37. Therefore, VR vibrated around zero during T1-T2. As Figure 3g shows, when wind was added onto top electrode at T2, because pt became lower than pb and water evaporating from top electrode was intensified, upper surfaces of water in narrow channels of PVA would move upwards; when wind was removed, upper surfaces of water in channels would move downwards driven by gravity to equilibrium position. During T2-T4, the motion of upper surfaces of water in channels meant water movement through channels, causing the variation of VR. As Figure 3h shows, when water was continuously added to top electrode, water on top electrode would move to bottom electrode and VR would decline. As to the behavior of VR after T5, when switch 2 was turned off, there became open circuit on RL, and induced current charged the capacitor C continuously. After these discussions, the

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whole model for hydrocapacitors could give reasonable explanations to above results and was robust. What’s more, above experiments were conducted again on hydrocapacitors after one week, and they showed as good performances as earlier. To test the durability of hydrocapacitors, some hydrocapacitors were totally exposed to ambient environment and were tested after four months. Although their performances indeed reduced and their peak output voltage reduced from 40 mV to 12 mV, they showed the similar electric behavior as they were just fabricated. (Figure S4).

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Figure 3. Model based on capillarity and traditional streaming potential for hydrocapacitors here. (a) Schematic diagram for possible mechanism of electricity generation in hydrocapacitors; (b) Whole model for hydrocapacitors; (c - d) Typical measurement results of VR based on a reversed sandwich-like hydrocapacitor and on a not reversed CNT/PANIsand separately, where switch 2 on/off means turning switch 2 on or off, wind on/off means adding wind or removing wind, 20 µL 18


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water means adding 20 µL of deionized water, and water more means adding deionized water continuously; (e - f) Schematic diagrams of dropping some water onto the bottom electrode and onto the top electrode of CNT/PANIsand separately; (g) Schematic diagram of adding wind to and removing wind off the top electrode of CNT/PANIsand; (h) Schematic diagrams of adding water onto the top electrode of CNT/PANIsand continuously.

As shown in above results, output voltages of single hydrocapacitor were on the order of millivolts. If higher voltage is needed, combining hydrocapacitors in series might be an interesting idea. To verify the extendibility of hydrocapacitors, two CNT/PANIsand were connected in series as shown in Figure S3a. 20 µL of deionized water was added to the bottom electrodes of CNT/PANIsand-1, CNT/PANIsand-2 and CNT/PANIsand-1 successively, and as Figure S3b shows, adding water to two CNT/PANIsand in series could output larger voltages than to one. Meanwhile, related performances of output voltage were included in Video S1.

Besides, tap water was tried at the beginning of our study, and it did not show obvious difference from deionized water. In this work, we focus on the feasibility of fabricating hydrocapacitor. Therefore, to avoid influence from unknown chemical substances and do more accurate analysis, not regular water, like rainwater and seawater, but deionized water was used in the experiments. For a wider application, regular water might also be used.

CONCLUSIONS In summary, in this work, based on the CNT/PANI and PVA, we demonstrated a dual-function device named hydrocapacitor, including sandwich-like hydrocapacitor

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and planar hydrocapacitor, which could easily convert energy from water movement induced by capillarity, gravity and air pressure difference into electricity and store the generated electric energy. Meanwhile, to explain the mechanism of electricity generation in hydrocapacitor, a possible model based on capillarity and traditional streaming potential was proposed. After repeated various experiments and careful discussions, we found that conclusions from the model were consistent with experimental results in this work. On one hand, hydrocapacitors here have showed fast response to water movement, good reproducibility and extendibility, which are promising in harvesting energy from small-scale water movement. On the other hand, the whole model for hydrocapacitors here might inform the design of future hydrocapacitors and the integration of nanogenerators and energy storage parts. Since we focused on the feasibility of hydrocapacitors in this work, further researches are needed to improve the performances of hydrocapacitors, including impacts from thickness of the PVA layer, mass density of CNT/PANI, large change of temperature or humidity of environment, and so on.

EXPERIMENTAL SECTION Chemicals and reagents. Superaligned carbon nanotubes (SACNTs) were synthesized on silicon wafers by the previously reported method, where iron acted as catalyst and acetylene acted as precursor in a low-pressure chemical vapor deposition system (LP-CVD)26. Aniline (purity of 99.5+%) was purchased from Tianjin YongDa Chemical Reagent Development Center. Ammonium persulfate

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((NH4)2S2O8; purity of 98+%) and Sulfuric acid (H2SO4; purity of 95-98%) were purchased from Modern Oriental (Beijing) Technology Development Co., Ltd. Polyvinyl alcohol (PVA 1750±50; purity of 99+%) was purchased from Sinopharm Chemical Reagent CO., Ltd. Hydrochloric acid (HCl; purity of 36-38%) was purchased from Beijing Tong Guang Fine Chemicals Company. Silver paint (PELCO® Colloidal Silver, Prod NO:16304) was purchased from TED PELLA INC. Ethanol (purity of 99.7+%) was purchased from Beijing Chemical Works. Preparation

of

carbon

nanotube/polyaniline

composite

slices.

First,

active-SACNTs were obtained by heating SACNTs at 500 oC for 20 min in air. Then, active-SACNT was ultrasonically mixed with ethanol, and was filtered to get CNT films. After being dried, CNT films (average mass density of 0.8 mg cm-2) were soaked in aniline/HCl aqueous solution (0.05 M aniline, 1 M HCl) at 0 oC. 30 min later, the same volume of 0.05 M (NH4)2S2O8 aqueous solution was added into above solution drop by drop. Subsequently, the mixture was kept at 0 oC for 24 h to get CNT/PANI films (Figure S1a) through in-situ polymerization reaction. After being cleaned by acetone, ethanol and deionized water separately, CNT/PANI films were dried and then were cut into slices with the same shape (Figure S1c) by laser, which would be used as electrodes of hydrocapacitors. Preparation of PVA gel. PVA-H2SO4 gel electrolyte (PVA-HS) was prepared by resolving polyvinyl alcohol (8 g) into 1 M H2SO4 aqueous solution (50 mL). PVA-H2O gel electrolyte (PVA-HO) was prepared by resolving polyvinyl alcohol (8 g) into deionized water (50 mL).

21



Fabrication of sandwich-like hydrocapacitors and planar hydrocapacitors. PVA-HS was coated onto one side of CNT/PANI slices, and then they were placed onto Teflon plates to dry in air. 2 h later, by pressing two random CNT/PANI slices together with PVA-HS side to PVA-HS side, a sandwich-like hydrocapacitor with symmetric structure was fabricated (Figure 1b, Figure S1b). Planar hydrocapacitors (Figure S1d) were fabricated by coating PVA (PVA-HO or PVA-HS) on the gap of cutting-off CNT/PANIslic (Figure 2b). Material characterizations. The morphology and structure of CNT/PANI and CNT/PANI with PVA-HS were characterized by scanning electron microscope (SEM, EI Sirion 200). Device characterizations. Environmental temperature and humidity were recorded by UNI-T UT333 Mini Temperature Humidity Meter. Voltage-time (V-t) curves of CNT/PANIslic, CNT/PANIslic-brek and hydrocapacitors were recorded by Keithley 2410 SourceMeter and Fluke101 basic digital multimeter. Resistances of CNT/PANIslic were recorded by Keithley 2410 SourceMeter. Galvanostatic discharge-charge tests were conducted using Land Battery Testing System CT2001A. Video was recorded by Nikon CoolPix P80 Digital Camera.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website.

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Photos

of

CNT/PANI

composite

and

hydrocapacitors,

diagrams

of

galvanostatic discharge-charge tests, SEM characterization, photo and performance of two hydrocapacitors in series, durability test data, and related equations.

AUTHOR INFORMATION Corresponding author *Email: [email protected] ORCID Changhong Liu: 0000-0002-8041-9455 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Research was sponsored by the Natural Science Foundation of China (51572146) and

the

National

Key

Research

&

Development

Program

of

China

(2018YFA0208401).

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Table of Contents graphic:

A Hydrocapacitor for Harvesting and Storing Energy from Water Movement

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A Hydrocapacitor for Harvesting and Storing Energy from Water

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