Electricity Generation from Capillary-Driven Ionic Solution Flow in a


Jan 11, 2019 - A constructed 3DG nanogenerator (3DGNG) with an effective size of 0.5 × 2 cm can produce a continuous voltage of ∼0.28 V and a ...
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Electricity Generation from Capillary-Driven Ionic Solution Flow in a Three-Dimensional Graphene Membrane Changzheng Li, Zhiqun Tian, Lizhe Liang, Shibin Yin, and Pei Kang Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16529 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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Electricity Generation from Capillary-Driven Ionic Solution Flow in a Three-Dimensional Graphene Membrane

Changzheng Lia,b, Zhiqun Tiana, Lizhe Lianga, Shibin Yina, and Pei Kang Shena,*

a

Collaborative Innovation Center of Sustainable Energy Materials, Guangxi Key Laboratory

of Electrochemical Energy Materials, State Key Laboratory of Processing for Nonferrous Metal and Featured Materials, School of Chemistry and Chemical Engineering, Guangxi University, Nanning, Guangxi 530004, PR China b

Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi

University, Nanning, Guangxi 530004, PR China

*Corresponding author. E-mail: [email protected] (P. K. Shen).

ABSTRACT: Harvesting energy from the ambient environment provides great promise in the applications of micro/nano devices and self-powered systems. Herein, we report a novel energy scavenging method that ionic solution infiltrating into a three-dimensional graphene (3DG) membrane can spontaneously generate electricity under ambient conditions. A constructed 3DG nanogenerator (3DGNG) with an effective size of 0.5×2 cm can produce a continuous voltage of ~0.28 V and a remarkable output current of ~62 μA. The voltage is higher than those generated from the interaction between water and carbon nanomaterials in previous researches typically in the 1

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range of microvolts to millivolts. Moreover, we demonstrate the potential application of the 3DGNG by illuminating a liquid crystal display (LCD) directly with ten 3DGNGs in-series. These results present a novel avenue for energy harvesting and show bright potential applications in small devices and self-powered systems. Keywords: 3D graphene; capillary-driven flow; streaming potential; energy harvesting; selfpowered system

1. Introduction Scavenging energy directly from the environment is an effective and promising approach for self-powered systems.1-4 In the past few decades, great efforts have been made to develop effective energy utilization techniques which possess compact structure, high power density, low cost, and long lifetime.5-14 In 2006, Wang et al. developed the first nanogenerator (NG) for energy conversion at nanoscale by using ZnO nanowires.15 Since then, various energy harvesting technologies for nanoscale energy scavenging have been demonstrated, such as piezoelectric NGs,16-18 triboelectric NGs,19-23 and pyroelectric NGs.24-25 However, these NGs usually rely on complicated nanofabrication methods such as lithography and soft templating,3 which require expensive scientific equipment and sophisticated processing procedure. Besides, these NGs output alternating current signals, usually need a processing system with electrical rectifier and energy storage units in practical application. These complicate the integral system and increase the energy loss of self-powered systems. It is highly desirable to develop new types of energy scavenging approaches based on different mechanisms to meet the demand of growing power requirement and diverse operation conditions. 2

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Water-enabled power generation has been demonstrated to be the most reliable and efficient approach for energy conversion in the past 100 years.26 However, traditional hydropower generation needs large dams and complex water wheels and usually targets at the energy needs in large scope. Recently, electricity generation from the interaction between water and carbon nanomaterials to satisfy the working needs of small size have attracted significant attention.27-29 Numerous theoretical and experimental studies on carbon nanomaterials have shown great potential for electricity generation.30-36 Different mechanisms have been proposed to explain the electricity-generating process, such as electron drag,37-38 electrokinetic effect,39 coupling effect,40 interface charging/discharging process,41-42 and triboelectrification.43 However, these mechanisms remain controversial. In addition, an obvious limit of the previous studies is the low output voltage, typically in the range of microvolts to millivolts, which is not sufficient to power up current devices. For example, Ghosh et al. first reported that an output voltage of 0.65 mV was produced when pure water flowed over bundles of single-walled CNTs with a velocity of 5×10-6 m s-1.38 Subsequently, Zhao et al. reported that a generated voltage of ~8 mV was observed in an individual water-filled single-walled CNT.44 In other works, Zhu et al. reported that a highest voltage of ~119.0 μV was measured when 0.6 M NaCl solution flowed along a 100-mesh graphene grids at a velocity of ~8.3 cm s-1.45 Yin et al. demonstrated that a few millivolts can be produced when a droplet of NaCl solution moved on a piece of monolayer graphene under ambient condition.42 Furthermore, previously reported devices always need external pressure or force to drive the fluid flow, which make the structure complex and consume energy. All of these hinder the practical application of this new type of nanogeneration technology. 3

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Recently, several reports demonstrated that evaporation-driven water flow within porous carbon nanomaterials can significantly improve the output voltage,46-48 open up a new prospect for this novel approach. This has been suggested to be a result of the streaming potential derived from electrokinetic effect.49 Three-dimensional graphene (3DG), as a novel carbon nanomaterial, has attracted much attention for its potential applications in supercapacitor and battery electrodes.50-52 The 3DG is integrated with two-dimensional graphene (2DG) sheets to form a self-supporting interconnected 3D porous morphology. It successfully prevents 2DG sheets from aggregating and thus can improve the solution permeability and shorten ion diffusion distance. Meanwhile, 3DG preserves the intrinsic merits of 2D graphene, such as superior mechanical property, high electrochemical stability, large specific surface areas and excellent ion selectivity.53-55 As the high permeability and ion selectivity of the capillaries in 3DG membrane can significantly improve the electricity output performance. Therefore, it would be an ideal candidate for making electrokinetic devices for power generation. In this work, we report a new phenomenon that capillary-driven ionic solution flow in a 3DG membrane can directly generate electricity without using any other auxiliary. We found that a constructed 3DGNG with an effective size of 0.5×2 cm can spontaneously produce a sustainable voltage up to ~0.28 V under ambient conditions. This value is much higher than those generated from the interaction between water and graphene in previous researches.32, 39, 41-43

The maximum short current of the 3DGNG was obtained to be ~62 μA cm-2. The output

performance of the 3DGNG can easily be scaled up and used to power low-power consumption electronic devices directly. We demonstrate that ten 3DGNGs in-series can directly be used to illuminate a liquid crystal display. Compared with other energy scavenging nanotechnologies, 4

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this novel approach exhibits high electric output and features compact structure and easy processing. More importantly, this new mothed do not need any electrical rectifier and energy storage units, showing great potential applications in environmental energy harvesting and selfpowered systems.

2. Results and discussion The 3DG membrane was fabricated by vacuum filtration with 200 mL as-prepared 3DG suspension solution, as shown in Figure 1a. After drying, we cut the obtained 3DG membrane into proper-sized rectangles and sandwiched with two pieces of 0.5×2 cm copper electrodes. Copper wires were connected to the electrodes for measurement. The morphology and microstructures of the original 3DG powders were characterized by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The unique interconnected network of the 3DG can be clearly observed from the SEM image in Figure 1b, which shows the 3DG is constructed by translucent few-layer graphene sheets with high porosity. This selfsupporting interconnected porous morphology provides robust mechanical property and a large number of channels for ion transportation. The thickness of the few-layer graphene walls of 3DG was further confirmed by high resolution TEM (Figure 1c). The outer edge of the walls with 2 layers, 3 layers and 6 layers of graphene can be clearly observed in Figure 1c. Raman spectroscopy was measured for further examination. There are three explicit peaks at the position about ~1350 cm-1, ~1580 cm-1 and ~2700 cm-1 in the Raman spectra (Figure S1), corresponding to the D band (disorder), G band (graphite) and 2D band (second-order harmonic), similar to those of few-layer graphene. The intensity ratio of the G band and D band 5

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(IG/ID) indicates the degree of graphitization. The high IG/ID ratio of 2.01 indicates the 3DG is well graphitized. Furthermore, the sharp peak at ~2700 cm−1 corresponding to the 2D band indicates that the 3DG powders consist of “few-layer” graphene,50, 56 which is consistent with above carried out HRTEM observations. The chemical structure of the 3DG was further studied by EDS elemental mapping, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The elemental mapping images show that the 3DG consists of carbon (C) and oxygen (O) with uniform distribution (Figure S2). The atomic ratio of C and O are obtained to be 91.21% and 8.79%, respectively, indicating low amount of oxygen-containing functional groups of the 3DG. Figure S3 shows the XPS spectrum of the 3DG. The corresponding C 1s peak and O 1s peak reveal the atomic ratio of C and O are 92.45% and 7.55%, agreeing well with the EDS mapping results. The high-resolution C 1s spectra was further analyzed by means of peak fitting (Figure S3b). The fitting result shows a sharp peak of sp2 (C-C/C=C, ~284.4 eV) and a nonobvious peak of epoxy/hydroxyl (C-O, ~286.6 eV), meaning good graphitization and few oxygen-containing functional groups of the 3DG. A broadened diffraction peak at about 24 degree was observed in the XRD spectra (Figure S4), which is similar to those of 3DG from other works.50-51 The porosity of the 3DG powders was measured to be about 59.23% with a pore size distribution mainly ranging from 0.5 to 2 μm (Figure S5). The average pore diameter is obtained to be 1.17 μm. Figure 1d is the digital image of the 3DG membrane, exhibiting a dark black-colored surface. The SEM images of the 3DG membrane show the 3DG powders are well adhered with each other, meanwhile maintain interconnected channels for ions and solution diffusion (Figure S6). The cross-section view of the 3DG membrane in Figure S6c shows the membrane is about 6

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35 µm in thickness. It is worth noting that the pristine 3DG powder is hydrophobic for few oxygen-containing groups on the surface. However, the fabricated 3DG membrane behaves hydrophilic feature after adding with 10 wt% PTFE preparation, meaning that the PTFE preparation has changed the surface characteristic of the 3DG membrane. The contact angle of the 3DG membrane is measured to be about 55.2° (Figure 1e). The 3DG membrane can highly be stable in water for a long time, as illustrated in Figure S7. It did not exhibit any observable peel-off after being immersed in water for 3 months. Figure 1f is the digital image of a typical 3DGNG, showing a simple and compact structure of the 3DGNG.

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Figure 1. (a) Schematic diagram of the fabrication procedure of the 3DGNG. (b) The SEM and (c) TEM images of the 3DG powders. It confirms the few-layer graphene walls of 3DG. (d) The digital image of the 3DG membrane. Scale bar, 2 cm. (e) The 3DG membrane is macroscopically hydrophilic and the contact angle is about 55.2°. (f) The digital image of a typical 3DGNG, showing a simple and compact structure. Scale bar, 2 cm.

Figure 2a shows the schematic of the device for experimental measurement. The 3DGNG is suspended on a glass substrate. The short-circuit current (Isc) and the open-circuit voltage (Voc) of the 3DGNG are measured with Keithley 2450 source measurement unit and Keithley 2700 multimeter connected to a computer. At the beginning, there is no obvious Isc and Voc observed. When a droplet of 0.1 M NaCl solution (30 μL in volume) is dropped onto the right end of the 3DGNG, it infiltrates into the 3DG membrane and quickly moves to the left end. During this process, surprisingly, a maximum Isc of ~62 μA is produced (Figure 2c and Video S1). The Isc decreases gradually as the flow velocity slows down when reaching capillary equilibrium, indicating that the solution flow in 3DG membrane dominates the electricity generation. Unlike Isc, the Voc sharply increases to ~0.23 V within around 1~2 s when the NaCl solution infiltrates into the 3DG membrane, and then gradually increases to a saturated value of ~0.28 V (Figure 2d). This voltage maintains nearly unchanged under a stable environmental condition. This phenomenon can be highly reproducible and the characteristics of Isc and Voc can be well reproduced (Figure S8).

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Figure 2. Electricity generation from the 3DGNG. (a) Schematic of the device for experimental measurements. The 3DGNG is suspended on a glass substrate. A NaCl droplet is dropped onto the right end of the 3DGNG for power generation. (b) Schematic diagram of the flow-induced streaming current and streaming potential in the 3DG membrane. (c) Measured Isc and (d) Voc vs. time of the 3DGNG when a 0.1 M NaCl droplet (30 μL in volume) infiltrates into the 3DG membrane driven by capillary force. Scale bars, 0.5 cm.

Unlike previous reports where electricity derived from the flow direction,42,

46, 49

the

generated electricity of our work is perpendicular to the flow direction. The mechanism of this interesting phenomenon can be explained with electrokinetic effect.57-59 When the 3DG membrane is contacted with the NaCl solution, an electric double layer (EDL) forms at the 3DG-solution interface. The zeta potential of the 3DG aqueous suspension was measured to be -19.2 mV (Figure S9), indicating a negative surface of the 3DG membrane. Thus, when capillary-driven NaCl solution flow in the 3DGNG, the sodium ions (Na+) will be absorbed to the 3DG membrane and the chloride ions (Cl-) repelled to the microfilter substrate (Figure 2b). 9

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A streaming current is formed in the membrane from microfilter substrate to the 3DG membrane. The Isc of external circuit is out from the electrode attached to 3DG and back to the electrode attached to microfilter substrate. We denote the Isc sign of this direction is positive as shown in Figure 2c. Besides, this process will result in a high potential at the 3DG membrane and a low potential at the microfilter, corresponding to a positive Voc (Figure 2d). The velocity of capillary-induced NaCl solution flowing along the 3DG membrane will determine the separation speed of ions thus affect the output performance. When the NaCl solution contacts with the 3DG membrane at first, the flow velocity is maximum, corresponding to the maximum Isc of ~62 μA. The Isc gradually decreases as the velocity decreasing with capillary equilibrium. The final stable Isc gradually decreases to zero when the solution is completely vaporized (Figure S10). For the Voc, because there is no electricity dissipation, separated Cl- and Na+ gradually accumulate near two electrodes and the Voc increases until reaching saturation. A set of experiment using inert electrodes (Au) was conducted for further study to verify the equilibrium electricity output. The thin layer Au (~60 nm) was deposited directly on the two sides of the membrane to work as electrodes. The experimental results are shown in Figure S11. The equilibrium Isc of ~0.15 μA and the Voc of ~0.23 V were observed, lower than that of 3DGNG with copper electrodes. These indicate the final stable electricity output may contribute to the electrochemical reaction of the copper electrodes60 and the water-evaporation-induced solution flow.46-47 To verify the stability and lifetime of the 3DGNG, we put the right end of the 3DGNG immersing into a compensation reservoir (20×20×15 mm3, filled with 0.1 M NaCl) for continuous solution supplement (Figure 3a and 3b). A piece of tissue paper (20×20 mm2) was 10

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used as an absorbent pad putting at the left end of the 3DGNG. Water evaporation on the tissue paper drives the NaCl solution flowing in the 3DG membrane. The experiments were conducted in the laboratory environment with the temperature fluctuating between 25.6 °C and 26.2 °C and relative humidity (RH) between 40% and 46%. The produced Voc at the beginning when the right end of the 3DGNG contacting the NaCl solution is about 0.25 V, and then it gradually increases to a saturated value of ~0.28 V, acting the same exhibition as the droplet experiments (Figure 3c). This saturated Voc can reliably be maintained at around 0.28 V during the whole test for more than 3 hours. The enlarged insets are the measured curves between 0 and 0.3 h, 1.3 and 1.5 h, and 3.0 and 3.2 h. The slightly fluctuations in the measured results can be explained by the fluctuating laboratory temperature and humidity during the test. The capillaryinduced electricity is also found in the NG with a graphene oxide (GO) membrane. The preparation of the GO is carried out by modified Hummers’ method.61 The obtain saturated Voc of the graphene oxide NG (GONG) in the same experiment condition is about 0.14 V, only half of the 3DGNG (Figure S12). This arises from the circuitous twists and turns of 2D nanochannels between closely spaced nanosheets in the GO membrane,39 which is hard for water permeability and ion transportation. There is no obvious generated voltage can be observed in the NG with a bare micro-filter (blue curve in Figure 3c), indicating that the 3DG plays a very important role in the electricity generation process.

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Figure 3. (a) Schematic of device and (b) experimental set-up for continuous electricity generation. The right end of the 3DGNG is immersed into a compensation reservoir (20×20×15 mm3, filled with 0.1 M NaCl) for continuous solution supplement. Scale bar, 1 cm. (c) Voc generated from the 3DGNG under ambient laboratory conditions with the temperature fluctuating between 25.6 °C and 26.2 °C and relative humidity (RH) between 40% and 46%. The enlarged insets are the measured curves between 0 and 0.3 h, 1.3 and 1.5 h, and 3.0 and 3.2 h.

To further elucidate the mechanism of the induced voltage, we investigated the dependence of the voltage on the NaCl concentration and solution flow velocity of the 3DGNG. As shown in Figure 4a and Figure S13, the Voc decreases as the NaCl concentration increases from 10−6 M to 1 M. This concentration dependency is similar to those of reported streaming potential deriving from electrokinetic effect,62 which can be explained by the Debye length changing of the of the EDL in 3DG membrane. In dense NaCl solution, the Debye length of the EDL in 3DG surface is dramatic decreased and the screening effect is significant enhanced,

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resulting in a weak ion selective ability.57 Hence, the induced Voc will decrease with NaCl concentration increasing. Besides, the copper electrodes may be oxidized in high concentration of NaCl solution and cause electrode potentials changing. This may be another reason for the Voc decreasing in high concentration NaCl solution. More experiments need to be done for further confirmation. From the above analysis we know that the 3DGNG has a high Voc when using low concentration solution. Thus, we can use deionized water (regarded as infinite dilution solution) for high-performance power generation. The Voc up to ~0.35 V was observed for lasting more than 3 hours (Figure S14). In order to investigate the voltage dependence on the solution flow velocity, we enhance the solution evaporation of the tissue paper by increasing its temperature with a micro heater. As the temperature rising, the flow velocity at capillary equilibrium will be broken and increased. This will enhance the generated voltage. In our experiment, when the evaporation temperature is raised from ~22.5 to 57.3 °C, the saturated Voc increases significantly from ∼0.28 to ∼0.42 V, as shown in Figure 4b. The output performance of the 3DGNG is also investigated by connecting with different load resistances (RL). Figure 4c shows the load resistance dependence of the output voltage and the output current. When the load resistance is increased from 0.51 KΩ to 46.8 kΩ, the output voltage increases from 0.01 to 0.19 V, whereas the output current decrease from 17.8 to 4.0 μA. The corresponding output power is observed to increase for small RL and then decreased for large RL (Figure 4d), acting similar with those of standard electrical sources. The maximum output power is evaluated to be ~0.9 μW at the position where RL= Rin (Rin is internal resistance of the 3DGNG, calculated to be ~9.92 kΩ). It is worth noting that this output electricity is achieved from one 3DGNG with an effective area of 1 cm2. This value can be easily scaled up by 13

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fabricating large-sized 3DGNG with a bigger piece of 3DG membrane or connecting multiple 3DGNGs with serial/parallel packages.

Figure 4. (a) Generated voltage as a function of NaCl concentration. The Voc decreases as the NaCl concentration increases from 10−6 M to 1M. (b) Voc variation as a function of evaporation temperature. (c) Output voltage (red curve) and current (blue curve) of the 3DGNG measured with different load resistances increased from 0.51 KΩ to 46.8 kΩ. (d) Output power of the 3DGNG measured for different load resistances.

To demonstrate the application of the 3DGNG, we scale up the voltage output and current output with series and parallel connections. As shown in Figure 5a, a single 3DGNG presents an open voltage of ~0.28 V at capillary equilibrium under ambient conditions, and series connection with two 3DGNGs can provide an open voltage of ~0.56 V, equal to 2 times of the

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single 3DGNG output. Three 3DGNGs in-series can provide an open voltage of ~0.84 V, equal to 3 times of the single 3DGNG output. This behaving is similar to a practical power source. On the other hand, the current performance of the parallel connections is demonstrated (Figure 5b). The short current of a single 3DGNG is about 5 μA. The enhanced parallel current almost equals to the sum of each single 3DGNG. When three 3DGNG are connected in parallel, the flow-induced Isc can reach ∼15 μA. These results indicate that the 3DGNG can be used for lowpower electronics or sensor powering. Figure 5c is the schematic of ten 3DGNGs in-series for powering a commercial liquid crystal display (LCD). The total voltage of ten 3DGNGs is sufficient to power the LCD directly by showing the number “8” (Figure 5d). It is worth to note that if inert electrodes (like Au) are used, more numbers of the 3DGNGs are needed to illuminate the LCD because of the low output electricity as discussed above. Besides, the 3DGNGs features compact structure, easy processing and do not need any other auxiliaries such as electrical rectifier or energy storage units, demonstrating promising potential in practice applications.

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Figure 5. Application demonstration of the 3DGNG as a practical power source in serial/parallel packages. (a) Voltage output of a serial package containing one, two and three 3DGNGs. (b) Current output of a parallel package containing one, two and three 3DGNGs. (c) The circuit schematic diagram of ten 3DGNGs in-series for powering a commercial liquid crystal display (LCD). (d) Photograph of ten 3DGNGs in-series illuminates the LCD directly. Scale bar, 2 cm.

3. Conclusion In summary, we have demonstrated that electricity can generate from capillary-driven ionic solution flow in a 3DG membrane. The short-circuit current and open-circuit voltage were experimentally analyzed and found to be dependent on the ionic concentration and the solution flow velocity. Based on the experimental observation, we proposed this electricity generating phenomenon resulting from the directional movement of the counterions on the interface of 3DG membrane and the microfilter substrate due to electrokinetic effect. A constructed 3DGNG with an effective area of 1 cm2 can produce a continuous voltage of ~0.28 V and a 16

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maximum current of ~62 μA. The output performance of the 3DGNG can be easily scaled up to power small electronic devices, such as a commercial LCD we have demonstrated in the paper. This novel energy scavenging nanotechnology features compact structure, no auxiliary, easy scale-up, long lifetime, high voltage output and low-cost characteristics, showing great potential in the applications of micro/nano devices and self-powered systems.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx Experimental section, Raman spectra of the 3DG powders, elemental mapping images of the 3DG powders, XPS spectrum of the 3DG, XRD spectra of the 3DG, pore size distribution of the 3DG powders, SEM images of the 3DG membrane, stability test of the 3DG membrane immersed in water, reproducibility of Voc, Zeta potential distribution of the 3DG membrane, measured Isc vs. time of the 3DGNG as the ionic solution vaporized, measured Isc and Voc of 3DGNG with Au electrodes, measured Voc of 3DGNG and GONG in the same experiment conditions, concentration dependence of Voc, measured Voc of 3DGNG with deionized water. (PDF) Short-circuit current of the 3DGNG when a 0.1 M NaCl droplet infiltrates into the 3DG membrane. (AVI) AUTHOR INFORMATION Corresponding Author 17

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*E-mail: [email protected] (P. K. Shen) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the Major International (Regional) Joint Research Project (No. U1705252), the Guangxi Science and Technology Project (No. AA17204083 and AB16380030), the China Postdoctoral Science Foundation (No. 2017M622928), and the Guangxi University doctor startup funding (No. XBZT161047).

REFERENCES (1) Wang, Z. L. Self-Powered Nanosensors and Nanosystems. Adv. Mater. 2012, 24, 280-285. (2) Mahmud, M. A. P.; Huda, N.; Farjana, S. H.; Asadnia, M.; Lang, C. Recent Advances in Nanogenerator-Driven Self-Powered Implantable Biomedical Devices. Adv. Energy Mater. 2018, 8, 1701210. (3) Fan, F. R.; Tang, W.; Wang, Z. L. Flexible Nanogenerators for Energy Harvesting and SelfPowered Electronics. Adv. Mater. 2016, 28, 4283. (4) Xu, S.; Qin, Y.; Xu, C.; Wei, Y.; Yang, R.; Wang, Z. L. Self-Powered Nanowire Devices. Nat. Nanotechnol. 2010, 5, 366-373. (5) Choi, D.; Yoo, D.; Kim, D. S. One-Step Fabrication of Transparent and Flexible Nanotopographical-Triboelectric

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Figure 1. (a) Schematic diagram of the fabrication procedure of the 3DGNG. (b) The SEM and (c) TEM images of the 3DG powders. It confirms the few-layer graphene walls of 3DG. (d) The digital image of the 3DG membrane. Scale bar, 2 cm. (e) The 3DG membrane is macroscopically hydrophilic and the contact angle is about 55.2°. (f) The digital image of a typical 3DGNG, showing a simple and compact structure. Scale bar, 2 cm. 107x150mm (300 x 300 DPI)

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Figure 2. Electricity generation from the 3DGNG. (a) Schematic of the device for experimental measurements. The 3DGNG is suspended on a glass substrate. A NaCl droplet is dropped onto the right end of the 3DGNG for power generation. (b) Schematic diagram of the flow-induced streaming current and streaming potential in the 3DG membrane. (c) Measured Isc and (d) Voc vs. time of the 3DGNG when a 0.1 M NaCl droplet (30 μL in volume) infiltrates into the 3DG membrane driven by capillary force. Scale bars, 0.5 cm. 108x69mm (300 x 300 DPI)

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Figure 3. (a) Schematic of device and (b) experimental set-up for continuous electricity generation. The right end of the 3DGNG is immersed into a compensation reservoir (20×20×15 mm3, filled with 0.1 M NaCl) for continuous solution supplement. Scale bar, 1 cm. (c) Voc generated from the 3DGNG under ambient laboratory conditions with the temperature fluctuating between 25.6 °C and 26.2 °C and relative humidity (RH) between 40% and 46%. The enlarged insets are the measured curves between 0 and 0.3 h, 1.3 and 1.5 h, and 3.0 and 3.2 h. 146x85mm (300 x 300 DPI)

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Figure 4. (a) Generated voltage as a function of NaCl concentration. The Voc decreases as the NaCl concentration increases from 10−6 M to 1M. (b) Voc variation as a function of evaporation temperature. (c) Output voltage (red curve) and current (blue curve) of the 3DGNG measured with different load resistances increased from 0.51 KΩ to 46.8 kΩ. (d) Output power of the 3DGNG measured for different load resistances. 108x83mm (300 x 300 DPI)

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Figure 5. Application demonstration of the 3DGNG as a practical power source in serial/parallel packages. (a) Voltage output of a serial package containing one, two and three 3DGNGs. (b) Current output of a parallel package containing one, two and three 3DGNGs. (c) The circuit schematic diagram of ten 3DGNGs in-series for powering a commercial liquid crystal display (LCD). (d) Photograph of ten 3DGNGs in-series illuminates the LCD directly. Scale bar, 2 cm. 122x76mm (300 x 300 DPI)

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