Nano Energy 22 (2016) 548–557 Contents lists available at ScienceDirect Nano Energy journal homepage: www.elsevier.com/locate/nanoen High-efficiency ramie fiber degumming and self-powered degumming wastewater treatment using triboelectric nanogenerator Zhaoling Li a,b,1, Jun Chen a,1, Jiajia Zhou b, Li Zheng a,c, Ken C. Pradel a, Xing Fan a, Hengyu Guo a, Zhen Wen a, Min-Hsin Yeh a, Chongwen Yu b,n, Zhong Lin Wang a,d,nn a School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, United States Key Laboratory of Science & Technology of Eco-Textile, Ministry of Education, College of Textiles, Donghua University, Shanghai 201620, China c School of Mathematics and Physics, Shanghai University of Electric Power, Shanghai 200090, China d Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China b art ic l e i nf o a b s t r a c t Article history: Received 24 January 2016 Received in revised form 25 February 2016 Accepted 1 March 2016 Available online 2 March 2016 As one of the strongest and oldest natural fibers, ramie fiber has been widely used for fabric production for at least six thousand years. And degumming is a critical procedure that has been developed to hold the ramie fiber’s shape, reduce wrinkling, and introduce a silky luster to the fabric appearance. Herein, we introduce a fundamentally new working principle into the field of ramie fiber degumming by using the triboelectric effect. Resort to a water-driven triboelectric nanogenerator (WD-TENG), the ramie fibers degumming efficiency was greatly enhanced with improved fiber quality, including both surface morphology and mechanical properties. Furthermore, it saves the chemicals usage in the traditional method, which makes it a green and practical approach to fully remove the noncellulosic compositions from ramie fibers. In addition, as a systematical study, the WD-TENG was further employed as a sustainable power source to electrochemically degrade the degumming wastewater by recycling the kinetic energy from flowing wastewater in a self-powered manner. Under a fixed current output of 3.5 mA and voltage output of 10 V, the self-powered cleaning system was capable of cleaning up to 90% of the pollutants in the wastewater in 120 min. Given the compelling features of being self-powered, environmentally friendly, extremely cost-effective, good stability, high degumming and degradation efficiency, the presented work renders an innovative approach for natural fiber extraction, and could be widely adopted as a green and innovative technology in textile industry. & 2016 Elsevier Ltd. All rights reserved. Keywords: Triboelectrification Self-powered Ramie fiber Green Degumming 1. Introduction Textile is critical to the development of human civilization and is everywhere in people’s daily life. Ramie, one of the most popular textile raw materials with distinctive characteristics, produces the strongest and longest natural plant fibers with lustrous silky appearance. This type of fiber possesses many excellent properties such as high tensile strength, high moisture absorption, good thermal conductivity, outstanding antibacterial function and favorable air permeability [1–3]. The ramie fibers have been widely used as an excellent textile material for clothing fabrics, industrial packaging, car accessories, fiber reinforced composites, and so on. n Corresponding author Corresponding author at: School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, United States. E-mail addresses: yucw@dhu.edu.cn (C. Yu), zlwang@gatech.edu (Z.L. Wang). 1 These authors contributed equally to this work. nn http://dx.doi.org/10.1016/j.nanoen.2016.03.002 2211-2855/& 2016 Elsevier Ltd. All rights reserved. However, raw ramie is in the form of fiber bundles consisted of many individual fibers adhesive to each other. The gummy or noncellulosic contents, such as pectin, lignin and hemicelluloses, are required to be degummed by placing in hot water or chemical solutions to free and extract the individual cellulose fibers, so as to further improve their downstream processing ability [4–6]. Considerable efforts have been committed to develop various techniques for natural fiber degumming and extraction, including chemical, biological (enzymatic and microbic), ultrasonic or mechanical methods [7]. However, widely adoption of these techniques may be shadowed by the limitations such as expensive equipments, time-consuming procedures, high environmental pollution, high energy consumption, high operating cost as well as large quantity of generated wastewater [8,9]. As a result, it is highly meaningful and desirable to develop new approaches for ramie degumming with improved fiber quality and less environmental contaminations. Herein, in this work, we introduced a fundamentally new Z. Li et al. / Nano Energy 22 (2016) 548–557 working principle in the field of ramie fiber degumming by reporting a unique route that worked in a self-powered manner by harnessing the ambient energy using the triboelectric effect. Under the electric field provided by a water-driven triboelectric nanogenerator (WD-TENG), the integrated electrochemical system can induce a large amount of OH- at the cathode, which will greatly accelerate the degumming speed with less chemicals consumption. The surface morphologies and mechanical properties of the degummed fibers exhibited greatly improved quality compared to the traditional approach. In addition, the WD-TENG was also acting as a sustainable power source to electrochemically degrade the degumming wastewater by recycling the kinetic energy from flowing wastewater. Under a fixed current output of 3.5 mA and voltage output of 10 V, the self-powered cleaning system was capable of cleaning up to 90% of the pollutants in the wastewater in 120 min, in which the chromaticity, chemical oxygen demand (COD) and electrical conductivity were decreased distinctly. With a collection of compelling features, such as high ramie degumming efficiency and pollutants removal efficiency, cost-effectiveness, simplicity as well as stability, the reported self-powered approach based on triboelectric effect not only provides an efficient and green pathway for natural fiber extraction, but also promotes substantial advancement towards the practical applications of TENG and self-powered electrochemical systems. 2. Experimental section 2.1. Growth of FEP nanowires on FEP film Inductively coupled plasma (ICP) reactive-ion etching was carried out to create the nanowires structure onto the fluorinated ethylene propylene (FEP) film. Typically, a 50 μm thick FEP thin film was cleaned with isopropyl alcohol and deionized water, and then blown dry with nitrogen gas. Before etching process, Au particles were deposited by sputtering as the mask to induce the nanowires structure later. Subsequently, Ar, O2, and CF4 gases were introduced into the ICP chamber, with flow rates of 10.0, 15.0, and 30.0 sccm, respectively. The FEP film was etched for 10 s to obtain the nanowires structure on the surface with a high density plasma generator (400 W) and plasma-ion acceleration (100 W). 2.2. Fabrication a WD-TENG The WD-TENG mainly consists of two parts: a stator and a rotator. Stator: A square-shaped acrylic sheet was cut as a substrate with a dimension of 13 cm 13 cm 3 mm by using a laser cutter. Through-holes on edges of the substrate were drilled for mounting it on a flat stage by screws. Fine trenches with complementary patterns were created on top of the substrate by laser cutting. A layer of Cu (200 nm) was deposited onto the substrate using an electron-beam evaporator. After that, two lead wires were connected respectively to the electrodes. A thin layer of FEP (50 μm) was finally laminated onto the electrode layer. Rotator: A disc-shaped acrylic substrate was patterned and consisted of radial-arrayed sectors by using a laser cutter. The rotator has a diameter of 10 cm and a thickness of 1.5 mm. A through-hole was drilled that has a D-profile at the centre of the rotator for a convenient connection to the water turbine. Finally, a layer of Cu (200 nm) on the rotator was deposited using a DC sputter. 2.3. Characterization and measurement A Hitachi SU-8010 field emission scanning electron microscope, operated at 5 kV and 10 mA, was used to characterize the FEP 549 surface. The electrical signals were acquired using a programmable electrometer (Keithley Model 6514) and a low-noise current preamplifier (Stanford Research System Model SR570). The software platform is constructed based on LabView, which is capable of realizing real-time data acquisition control and analysis. Tensile properties of the fibers such as tenacity, breaking elongation were carried out using instrument XQ-1A testing machine. The degree of polymerization of all samples was determined by viscosity method using an Ubbelohde capillary viscometer as the instrument and copper ethylene-diamine solution as the solvent. FT-IR spectroscopic analysis was performed on Nicolet 6700 Spectrometer (Thermo Fisher, America). XPS measurement was conducted on a Kratos Axis Ultra spectrometer (ESCA LAB 250, Thermo Fisher Scientific) with monochromatic Al Kα X-ray source. 2.4. Self-powered integration electrochemical system for ramie degumming The WD-TENG was connected to the central shaft of a miniature water turbine. Normal tap water was directed into the turbine inlet through a plastic pipe. A Ti/PbO2 anode and a Ti cathode were immersed in the reaction pond. A power management circuit was connected to output end of the WD-TENG to convert the alternating current to direct current signals. And a pH controller was employed to monitor the pH values in the reaction solution. The degumming experiments were conducted in a 500 mL beaker filled with 10.0 g raw ramie fiber and 100 mL freshly prepared degumming solution, continuously mixed at 200 rpm with magnetic stirred bar. The reaction temperature was raised to be 100 °C. Then the degumming reaction process was conducted and the reaction time was recorded. The treated fibers were thoroughly washed with deionized water. Finally they were squeezed and properly dried at oven (100 °C, 3 h) before a further surface characterization and mechanical properties measurement. 2.5. Electrochemical degradation of wastewater solution The electrochemical degradation of degumming wastewater was performed in a plastic box filled with 200 mL of wastewater at room temperature. Due to the good chemical stability and high electrocatalytic activity, Ti and Ti/PbO2 electrodes were placed into the solution, acting as the cathode and anode, respectively. A power management circuit was connected to the wave energy harvester to convert the alternating current to direct current signals. The degradation processing of the wastewater was monitored at fixed time intervals by measuring the chromaticity, COD values and electrical conductivity. 3. Results and discussion The self-powered triboelectric effect enabled ramie treatment mainly consists of two procedures, firstly, the WD-TENG assisted highefficiency ramie degumming and secondly, degradation of the degumming wastewater. As demonstrated in Fig. 1a, the as-developed WD-TENG consists of mainly two parts: a rotator and a stator. The asfabricated device’s dimension is 10 cm 10 cm 1.5 mm. The rotator is a collection of radially arrayed sectors with a unit central angle of 6°. The stator comprises of three components laminated along the vertical direction: a layer of fluorinated ethylene propylene (FEP) as an electrification material, a layer of electrodes with complementary patterns, and an underlying substrate. FEP nanowires arrays were created on the exposed FEP surface by a top-down method through reactive ion etching. A scanning electron microscopy (SEM) image of the FEP nanowires is displayed in Fig. 1b, which indicates an average diameter of 100 nm and an average length of 500 nm. Detailed fabrication process 550 Z. Li et al. / Nano Energy 22 (2016) 548–557 Fig. 1. The employed water-driven triboelectric nanogenerator (WD-TENG) for self-powered ramie fiber treatment. (a) Structural design of the water-driven disk TENG. (b) A SEM image of the FEP polymer nanowires. The sale bar is 500 nm. (c) Schematic illustration of the operating principle of the WD-TENG. (d) The current and (e) voltage output of the WD-TENG via a power management circuit. of the WD-TENG was presented in the Experimental Section. Relying on a coupling of triboelectrification and electrostatic induction [10–16], the working principle of the WD-TENG was demonstrated in Fig. 1c. To operate, a relative rotation between the rotator and the stator gives rise to alternating flow of electrons between electrodes. The electricity generation process of the WDTENG is elaborated through a basic unit. We define the initial state and the final state as the states when the rotator is aligned with electrode A and electrode B, respectively. The intermediate state represents a transitional process in which the rotator spins from the initial position to the final position. Since the rotator and the stator are in direct contact, triboelectrification creates charge transfer on contacting surfaces, with negative charges generated on the FEP and positive ones on the metal. Due to the law of charge conservation, the density of positive charges on the rotator is twice as much as that of negative ones on the stator because of unequal contact surface area of the two objects. Free charges can redistribute between electrodes due to the electrostatic induction. At the initial state, induced charges accumulate on electrode A and electrode B. As the rotation starts, free electrons keep flowing from electrode A to electrode B until the rotator reaches the final state where the charge density on both electrodes is reversed in polarity compared to the initial state. Therefore, alternating current is generated as a result of the periodically changing electric field across the electrodes [17–32]. Experimentally, to demonstrate, the WD-TENG was driven by a water turbine. Normal tap water was directed into the turbine inlet through a plastic pipe. A photograph of the experimental setup was shown in Supporting Information Figure S1. It is noticed that the WD-TENG output holds a high voltage but relatively low current, resulting in large output impedance and thus affecting its applicability as a power source. Besides, fluctuation in output power and the AC output current are also concerns for practical applications. These issues can be fully addressed by integrating the WD-TENG with a power management circuit to form a complete power-supplying system. Consisting of a transformer, a rectifier, a voltage regulator and capacitors, the power management circuit, as diagrammed in Supporting Information Figure S2, can deliver a DC output at a constant current of 3.5 mA (Fig. 1d) and voltage (Fig. 1e) of 10 V when the WD-TENG was driven at a fixed water flow rate of 3 L min 1. And a schematic diagram of the self-powered system for ramie Z. Li et al. / Nano Energy 22 (2016) 548–557 551 Fig. 2. Working principle of the triboelectrification enabled high-efficiency ramie fiber degumming and self-powered degumming wastewater treatment. (a)A schematic diagram of the integrated self-powered ramie fiber treatment system. (b) Mechanism of the ramie fiber degumming and wastewater treatment without the assistance of an electric field. (c) Proposed mechanism of the ramie fiber degumming and wastewater treatment with the assistance of electric field provided by the WD-TENG system. degumming is presented in Fig. 2a. It consisted of a WD-TENG for power supply, a power management circuit and a chemical reaction pond with freshly prepared degumming solution. A Ti/PbO2 anode and a Ti cathode were vertically fixed in the reaction pond. And a pH controller was employed to monitor the pH values in the reaction solution. The detailed fabrication and experimental setup of the self-powered integration system is presented in the Experimental Section. A further step was taken to investigate the working mechanism of the ramie degumming and the following wastewater treatment under assistance of the self-provided electric field. In the degumming process, the coated gummy materials which have lower degree of polymerization (DP) will break away from the cellulosic part of bast fibers and easily be dissolved in hot alkaline solution under the effect of hydroxyl ions, while the cellulose is comparatively resistant to such attacks. Gummy materials mainly include hemicelluloses, lignin, pectin and so on. Hemicelluloses are a mixture of various kinds of polysaccharide. Each polysaccharide is made up of one or several forms of monosaccharide [33]. These polysaccharide primarily include the components of galactoglucomannan, glucomannan, and xylan, which are hard to be dissolved in the solution. However, under the effect of hydroxyl ions, these polysaccharides can be degraded into monosaccharide components, such as glucose, mannose, galactose and xylose, which can be finally dissolved in the degumming solution and easily removed from the raw ramie. Pectins, also known as pectic polysaccharides, are rich in galacturonic acid and is insoluble in water [34]. When immersed in hot alkaline solution, the protopectin loses some of its branching and chain length and goes into solution. As a consequence, the pectin components in ramie fiber dissociate and separate from the fibers as well after chemical processing. After extraction treatment, the following degumming solution contains a category of sugar polymers including the six-carbon sugars, such as mannose, galactose, and glucose, and the fivecarbon sugars, such as xylose, arabinose, and rhamnose. There sugar residues are formed in the degumming liquid. The sugar components in the degumming solution can be measured by a gas chromatography (GC) analysis method. With the increase of the treatment time, the content of sugar components increase, and thus the effluent of the degumming system contains more and more organic pollutant. It is worth noting that the amount of hydroxyl ions keep reducing all the time during the degumming process. And correspondingly the pH values of the aqueous solution is decreasing continually. Eventually, most alkali has been consumed and the effluent of the reaction system contains less amount of hydroxyl ions. It is worth noting that alkali is a must for the degumming process of ramie fiber. Without it, the natural fiber can not be extracted. The self-provided electric filed can promote the directed migration of the ions, which is called electrophoresis, in the integrated electrochemical system. The cations move toward the 552 Z. Li et al. / Nano Energy 22 (2016) 548–557 Fig. 3. Performance characterization of the self-powered ramie fiber degumming system. (a) Comparison of the degumming efficiency of the ramie fiber with and without the applied electric field provided by the WD-TENG. (b) A study of the output current of the WD-TENG on the degumming efficiencies of the ramie fiber. (c) Comparison of the tenacity and breaking elongation of the ramie fibers before and after degumming. (d) Comparison of the degree of polymerization and fineness of the ramie fibers before and after degumming. cathode, while the anions move toward the anode under the influence of the applied electric field. The self-provided electric filed can boost the migration of hydroxyl ions as well as the electrolysis effect (Fig. 2c). The later induced a generation of large amount of OH- at the cathode in the degumming solution. In the meanwhile, a great deal of hydroxyl free radicals would generate at the cathode as well, and the gummy materials such as hemicelluloses, lignin and pectin would be electrocatalytically oxidized by such hydroxyl radicals, which led to a greatly enhanced degumming efficiency of the treated fibers. To evaluate the performance of the self-powered integration system for gummy components removal from raw ramie, the WD-TENG was driven at a fixed water flow rate of 3 L min 1. Control experiments were carried out to compare the degumming efficiency of ramie fibers with or without an assistance of the WD-TENG. As demonstrated in Fig. 3a and Figure S3, under both the 10% and 15% NaOH without the assistance of WD-TENG, the residual gum percentage demonstrated a decrease trend throughout the time window of observation. However, the removal percentage and removal efficiency were much lower even though in the presence of high concentration of hydroxide ions. Comparatively, under the provided electric field of the WD-TENG, the degumming speed of ramie fiber was greatly boosted over time. The generated electric field had largely improved the gummy components removal performance in terms of both required time and removal percentage. Besides, the proposed approach could also save the chemicals consumptions. Furthermore, the influence of the current output on the degumming performance of ramie fiber was also studied at a fixed 10% NaOH. As demonstrated in Fig. 3b, to reach a same residual gum percentage, a shorter degumming time is needed with a larger current output. Likewise, given a fixed degumming time interval, a larger current output will contribute to a larger gum removal percentage from the raw ramie. However, the residual content of the gummy components is independent of the applied current. And it remains almost the same in the ramie fibers after a continuous removal process of 300 min. Actually, the gummy components are very difficult to be completely removed from the raw ramie. The residual gum percentage will keep relatively stable after a certain period of degumming time, even though in the presence of high concentration of hydroxide ions and high strength of applied current. It is worth noting that the mechanical properties of degummed fibers with WD-TENG have been improved a lot compared with the degummed fibers without a WD-TENG. As shown in Figs. 3c and d, the tenacity increased from 4.34 cN/dtex to 6.53 cN/dtex, while the breaking elongation increased from 1.89% to 3.25%. Likewise, the degree of polymerization increased from 1780 to 2251, while the fineness increased from 1352 Nm to 1851 Nm. It indicated that the selfpowered integrated system not only enhanced the degumming efficiency, but also served the role of improving the mechanical performance of the degummed fibers. The reason could be that with the assistance of WD-TENG, most of gummy components could be removed successfully from ramie fibers and a higher purity of cellulose contents contributed to a larger mechanical properties. Surface morphologies of the treated fibers with and without WDTENG were also studied and compared via scanning electron microscopy (SEM). As shown in Fig. 4a, the untreated raw fibers exhibit a rough and coarse surface due to the coating heavily with noncellulosic Z. Li et al. / Nano Energy 22 (2016) 548–557 553 Fig. 4. Surface morphology of the ramie fiber. SEM images of (a) raw ramie fibers and (b) degummed ramie fibers without TENG and (c) degummed ramie fibers with TENG. The scale bars are 20 μm. Photographs of (d) raw ramie fiber and (e) degummed ramie fiber without TENG and (f) degummed ramie fiber with TENG. The scale bars are 1.5 cm. Fig. 5. Physical characterization of the ramie fiber. (a) Comparisons of X-ray diffraction pattern of the raw fibers and degummed fibers. (b) The percent crystallinity index of ramie fibers. (c) Comparisons of FT-IR spectra of the raw fibers and degummed fibers. (d) Comparisons of XPS spectra of the raw fibers and degummed fibers. 554 Z. Li et al. / Nano Energy 22 (2016) 548–557 components. Fig. 4b is the SEM image showing the degummed ramie fibers without the assistance of WD-TENG, on which a certain amount of gummy substances still covered on the surface to prevent the fibers from fully separating each other. The poor degumming efficiency as well as the bad surface morphologies is mainly attributed to the inadequate removal reaction. To contrast, Fig. 4c shows the surface of the treated fibers under the assistance of WD-TENG, which appeared most clean and smooth surface. This revealed that vast majority of gummy components were effectively removed and the bundle fibers were separated thoroughly with each other owning to the applied electric filed by the WD-TENG. In addition, the photographs of the raw ramie and treated fibers were also compared and presented in Figs. 4d–f, which also shows a greatly improved fiber surface after degumming. X-ray diffraction patterns [35,36] obtained for raw and degummed fibers were depicted and compared visually in Fig. 5a. All the three curves presented major crystalline peaks for 2θ ranging between 22° and 23°, which corresponded to the (002) crystallographic plane family of cellulose I. The other peaks for 2θ presented between 14.8° and 16.7° corresponding to the (101) crystallographic plane family of cellulose II. Fig. 5b demonstrated the crystallinity index (CrI) of ramie fibers. The CrI of raw ramie was the lowest value of 68.20%, which is attributed to a high content of amorphous region resulting from the residual gums. The value of CrI increases with the increase of crystalline cellulose content in the treated fibers. Specifically, degummed fibers with WD-TENG possessed the highest CrI value of 86.96%, owing to a more adequately removal of amorphous noncellulosic compounds. Meanwhile, the absorption peaks reflected stronger intensity compared to the untreated fibers. For degummed fibers without WDTENG, a value of 77.61% CrI was observed, which indicated an insufficient degumming reaction leading to an existence of gummy components within the treated fibers. To further identify the change of chemical compositions in untreated and treated fibers, FTIR spectroscopy analysis [37] was also carried out, as the results shown in Fig. 5c. The value ranging from 3000 cm 1 to 3600 cm 1 corresponded to the -OH stretching vibrations, which were mainly attributed to the large amount of hydroxyl groups in the cellulose fibers. After degumming with the assistance of WD-TENG, the relative intensities in the range of 3600–3000 cm 1 increased, which suggested the treatments have removed the most gummy components and the purity of cellulose increased accordingly. The treated fibers with WD-TENG exhibited strong absorption intensities, such as C–H stretching around 1630–1640 cm 1, CH2 symmetric bending around 1320 cm 1, and C–O–C stretching around 1064 cm 1. These corresponding intensities in the spectrum were higher as compared with the raw ramie or treated fibers without the assistance of WD-TENG. From the spectra analysis we can safely draw conclusion that most gummy substances were successfully removed from ramie under the assistance of WD-TENG. In this study, X-ray photoelectron spectroscopy (XPS) was performed to trace and compare the structure differences between raw ramie and treated cellulose fibers. The raw and degummed fiber samples were firstly scanned in a low-resolution mode performed on the Kratos Axis Ultra spectrometer, as the results shown in Fig. 5d. The raw and degummed fibers exhibited very simple spectra containing two characteristic peaks of carbon, with a binding energy of 284.5–288.9 eV and oxygen with a binding energy of 532.3–533.3 eV, while some weaker peaks associated with the existence of element N and S. The high-resolution C1s peak in the XPS spectra gives detailed information of surface chemistry. Figure S4 demonstrated the peak assignment of C moieties in the ramie fibers. The chemical shifts for carbon (Cls) in cellulose fibers can be deconvoluted into four categories C1. These four C moieties exhibited corresponding peaks at 284.5, 286.5, 287.8 and 288.9 eV, respectively. It can be seen that the peak densities of C moieties experienced considerable change after degumming with the assistance of WD-TENG. The peak densities of C1 and C2 appeared a decrease trend while the peak densities of C3 and C4 showed an increase trend. Table S1 summarized the peak areas and relative atomic percentage of C moieties in raw and degummed fibers under different conditions. In terms of O1s peaks, O1 (C-OH, C–O– C) and O2 (C ¼ O, COOR) were generally involved. These two oxygen moieties respectively exhibited corresponding peaks at 532.3 eV and 533.3 eV. Figure S5 illustrated the high resolution XPS spectra of O1s peaks in raw and degummed fibers. As demonstrated, the peak area, peak width and peak height were apparently changed with the increase of cellulose content after being degummed with the assistance of WD-TENG. The relative atomic percentage of oxygen was given in Table S2. The intensity of O1 peak showed a decrease trend as the cellulose content increases, while the intensity of O2 peak shows a reversed trend. And the O2/O1 ratio had a significant increase from 0.691 in raw fiber to 0.771 in degummed fibers with the assistance of the WD-TENG. For a systematical investigation of degumming ramie fibers, the ambient triboelectric effect was further utilized to develop the WD-TENG as a sustainable power source [38–45] to electrochemically degrade the pollutants in the degumming wastewater by recycling the kinetic energy from flowing wastewater. Experimentally, to demonstrate, the WD-TENG was connected to the central shaft of a miniature water turbine. Normal tap water was directed into the turbine inlet through a plastic pipe. Under a fixed water flow rate of 3 L min 1, the short-circuit current (Isc) has a continuous AC output with an average amplitude of 3.5 mA through the power management circuit. And the open-circuit voltage (Voc) oscillates at the same frequency as that of Isc with a peak-to-peak value of 10 V. A Ti/PbO2 anode and a Ti cathode, vertically fixed in a degumming wastewater container, were connected to the WD-TENG system. In electrochemical oxidation process, the degummed pollutants can not only be mineralized by the hydroxyl radicals on the anode surface, but also can be directly oxidized and degraded on the surface of anodes, which were eventually mineralized into CO2 and H2O. This process can largely relieve the pollution and improve the water quality. In environmental chemistry, the chemical oxygen demand (COD) test is commonly used to determine the amount of organic compounds found in wastewater. Electrical conductivity (EC) estimates the amount of total dissolved salts (TDS), or the total amount of dissolved ions in the water. Chromaticity is an objective specification of the quality of a color regardless of its luminance. In this study, the degradation efficiency of the degumming wastewater was characterized in terms of chromaticity, COD values and electrical conductivity. The self-powered cleaning system for wastewater treatment from ramie degumming was demonstrated in Fig. 6. As shown in Fig. 6a, with the increasing of degradation time, the chromaticity in the degumming wastewater decreased evidently, indicating the effectiveness of the route for self-powered pollutants electrochemical degradation. The visual color in the solution also experienced obvious change from the initial heavy color to lastly light color during the degradation time. Besides, after a continuous degradation of 120 min, the COD values decreased from 4589 mg/L to 420 mg/L while the electrical conductivity decreased from 32200 mS/cm to 3100 mS/cm, as shown in Fig. 6b. Remarkably, under a fixed current output of 3.5 mA and voltage output of 10 V, the self-powered cleaning system was capable of cleaning up to 90% of the pollutants in the wastewater in 120 min. Furthermore, the influence of the current output of the WDTENG on the pollutant degradation performance was also studied. As demonstrated in Fig. 6c, to reach a same COD value, a shorter time is needed with a larger current output. Likewise, given a fixed degradation time interval, a larger current output will contribute to a smaller COD value of the wastewater. A further test was performed to evaluate the electrical conductivity, and it shares a Z. Li et al. / Nano Energy 22 (2016) 548–557 555 Fig. 6. Investigation of the self-powered system for wastewater treatment from ramie degumming. (a) Dependence of the chromaticity in the degumming wastewater on the degradation time. Inset shows the color change of the degumming wastewater with the increase of degradation time. (b) Dependence of the COD and electrical conductivity of the degumming wastewater on the degumming time. Influence of the WD-TENG output current on (c) the COD change and (d) the electrical conductivity change in the degumming wastewater. similar trend with that of the COD, as demonstrated in Fig. 6d. This clearly indicated the effectiveness of the reported route for selfpowered electrochemically degrading degumming wastewater. We further investigated the durability of the self-powered electrochemical system. The experimental results are shown in Figure S6. No observable degradation of the surface polymer nanowires of the as-fabricated WD-TENG after a continuous operation of 36 h, as indicated by the SEM image in Figure S6a and S6b. And also, both the measured output current (Figure S6c and S6d) and voltage (Figure S6e and S6f) of the as-fabricated WD-TENG are constant after long-term device operation. These indicate a good durability of the integrated system for ramie fiber degumming and the following wastewater treatment. 4. Conclusions In this work, we paved a new avenue in the field of ramie fiber degumming and the following wastewater treatment by reporting a self-powered route based on triboelectric effect. By harvesting the kinetic energy from ambient water flow, the generated electric field from the WD-TENG can largely boost the migration of hydroxyl ions and enhance the electrolysis effect, which can greatly improve the degumming speed and degumming efficiency. The surface morphology and mechanical properties of degummed fibers exhibited much more improvement compared to the treated fibers without WD-TENG. Moreover, the power generated by the WD-TENG can also act as a sustainable power source to electrochemically degrade the pollutants in the degumming wastewater. Under a fixed current output of 3.5 mA and voltage output of 10 V, the self-powered cleaning system was capable of cleaning up to 90% of the pollutants in the wastewater in 120 min. And experimental results indicate that the chromaticity, COD and electric conductivity in treated wastewater have been decreased distinctly with the assistance of WD-TENG. The reported WD-TENG assisted electrochemical system not only provides an efficient pathway and new alternative to environmentally friendly extraction of natural fiber, but also promotes substantial advancement toward the practical applications of TENG based self-powered electrochemical systems. Acknowledgments Z. L. and J. C. contributed equally to this work. The research was supported by the Hightower Chair foundation, and the “thousands talents” program for pioneer researcher and his innovation team, China, National Natural Science Foundation of China (Grant No. 51432005), the earmarked fund for Modern Agro-industry Technology Research System, Ministry of Agriculture of China (CARS-19-E25). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.nanoen.2016.03.002. References [1] L. Rebenfeld, J. Text. I. 92 (2001) 1–9. [2] O. Faruk, A.K. Bledzki, H.-P. Fink, M. Sain, Prog. Polym. Sci. 37 (2012) 1552–1596. [3] S. Nam, A.N. Netravali, Fiber. Polym. 7 (2006) 372–379. [4] Z. Li, C. Yu, J. Text. I. 106 (2015) 1251–1261. [5] X.-S. Fan, Z.-W. Liu, Z.-T. Liu, J. Lu, Text. Res. J. 80 (2010) 2046–2051. [6] Z. Li, C. Yu, Fiber. Polym. 15 (2014) 2105–2111. 556 [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] Z. Li et al. / Nano Energy 22 (2016) 548–557 H.K. Shin, J.P. Jeun, H.B. Kim, P.H. Kang, Radiat. Phys. Chem. 81 (2012) 936–940. Z. Li, C. Meng, C. Yu, Text. Res. J. 85 (2015) 2125–2135. Q. Zhang, S. Yan, J. Text. I. 104 (2013) 78–83. G. Zhu, J. Chen, T. Zhang, Q. Jing, Z.L. Wang, Nat. Commun. 5 (2014) 3426. W. Yang, J. Chen, G. Zhu, J. Yang, P. Bai, Y. Su, Q. Jing, Z.L. Wang, ACS Nano 7 (2013) 11317–11324. Z.L. Wang, J. Chen, L. Lin, Energy Environ. Sci. 8 (2015) 2250–2282. J. Yang, J. Chen, Y. Yang, H. Zhang, W. Yang, P. Bai, Y. Su, Z.L. Wang, Adv. Energy Mater. 4 (2014) 1301322. H. Zhang, Y. Yang, Y. Su, J. Chen, C. Hu, Z. Wu, Y. Liu, C.P. Wong, Y. Bando, Z. L. Wang, Nano Energy 2 (2013) 693–701. J. Chen, G. Zhu, J. Yang, Q. Jing, P. Bai, W. Yang, X. Qi, Y. Su, Z.L. Wang, ACS Nano 9 (2015) 105–116. T.C. Hou, Y. Yang, H. Zhang, J. Chen, L.J. Chen, Z.L. Wang, Nano Energy 2 (2013) 856–862. G. Zhu, J. Chen, Y. Liu, P. Bai, Y. Zhou, Q. Jing, C. Pan, Z.L. Wang, Nano Lett. 13 (2013) 2282–2289. Z. Li, J. Chen, J. Yang, Y. Su, X. Fan, Y. Wu, C. Yu, Z.L. Wang, Energy Environ. Sci. 8 (2015) 887–896. J. Chen, J. Yang, Z. Li, X. Fan, Y. Zi, Q. Jing, H. Guo, Z. Wen, K.C. Pradel, S. Niu, Z. L. Wang, ACS Nano 9 (2015) 3324–3331. J. Yang, J. Chen, Y. Su, Q. Jing, Z. Li, F. Yi, X. Wen, Z. Wang, Z.L. Wang, Adv. Mater. 27 (2015) 1316–1326. X. Fan, J. Chen, J. Yang, P. Bai, Z. Li, Z.L. Wang, ACS Nano 9 (2015) 4236–4243. Y. Yang, H. Zhang, Y. Liu, Z.-H. Lin, S. Lee, Z. Lin, C.P. Wong, Z.L. Wang, ACS Nano 7 (2013) 2808–2813. H. Guo, J. Chen, M.-H. Yeh, X. Fan, Z. Wen, Z. Li, L. Lin, C. Hu, Z.L. Wang, ACS Nano 9 (2015) 5577–5584. J. Wang, X. Li, Y. Zi, S. Wang, Z. Li, L. Zheng, F. Yi, S. Li, Z.L. Wang, Adv. Mater. 27 (2015) 4830–4836. Z. Wen, J. Chen, M.-H. Yeh, H. Guo, Z. Li, X. Fan, T. Zhang, L. Zhu, Z.L. Wang, Nano Energy 16 (2015) 38–46. W. Yang, J. Chen, G. Zhu, X. Wen, P. Bai, Y. Su, Y. Lin, Z.L. Wang, Nano Res. 6 (2013) 880–886. J. Chen, J. Yang, H. Guo, Z. Li, L. Zheng, Z. Wen, X. Fan, Z.L. Wang, ACS Nano 9 (2015) 12334–12343. G. Zhu, Y. Su, P. Bai, J. Chen, Q. Jing, W. Yang, Z.L. Wang, ACS Nano 8 (2014) 6031–6037. Z.-H. Lin, G. Cheng, L. Lin, S. Lee, Z.L. Wang, Angew. Chem. Int. Ed. 52 (2013) 1–6. Y. Su, G. Zhu, W. Yang, J. Yang, J. Chen, Q. Jing, Z. Wu, Y. Jiang, Z.L. Wang, ACS Nano 8 (2013) 3843–3850. Z.H. Lin, G. Zhu, Y.S. Zhou, Y. Yang, P. Bai, J. Chen, Z.L. Wang, Angew. Chem. Int. Ed. 52 (2013) 1–6. W. Yang, J. Chen, X. Wen, Q. Jing, J. Yang, Y. Su, G. Zhu, W. Wu, Z.L. Wang, ACS Appl. Mater. Interfaces 6 (2014) 7479–7484. S. Perez, M. Rodriguez-Carvajal, T. Doco, Biochimie 285 (2003) 109–121. P. Sriamornsak, Silpakorn Univ. Int. J. 3 (2003) 206–226. L.M. Matuana, J.J. Balatinecz, R.N.S. Sodhi, C.B. Park, Wood Sci. Tech. 35 (2001) 191–201. M.N. Belgacem, G. Czeremuszkin, S. Sapieha, Cellulose 2 (1995) 145–157. N.E. Zafeiropoulos, P.E. Vickers, C.A. Baillie, J. Mater. Sci. 38 (2003) 3903–3914. W. Yang, J. Chen, Q. Jing, J. Yang, X. Wen, Y. Su, G. Zhu, P. Bai, Z.L. Wang, Adv. Funct. Mater. 24 (2014) 4090–4096. M.-H. Yeh, H. Guo, L. Lin, Z. Wen, Z. Li, C. Hu, Z.L. Wang, Adv. Funct. Mater. 26 (2016) 1054–1062. J. Chen, G. Zhu, W. Yang, Q. Jing, P. Bai, Y. Yang, T.C. Hou, Z.L. Wang, Adv. Mater. 25 (2013) 6094–6099. J. Yang, J. Chen, Y. Liu, W. Yang, Y. Su, Z.L. Wang, ACS Nano 8 (2014) 2649–2657. G. Zhu, P. Bai, J. Chen, Z.L. Wang, Nano Energy 2 (2013) 688–692. Y. Yang, H. Zhang, S. Lee, D. Kim, W. Hwang, Z.L. Wang, Nano Lett. 13 (2013) 803–808. Y. Yang, H. Zhang, Z.H. Lin, Y. Liu, J. Chen, Z. Lin, Y. Zhou, C.P. Wong, Z.L. Wang, Energy Environ. Sci. 6 (2013) 2429–2434. H. Zhang, Y. Yang, Y. Su, J. Chen, K. Adams, S. Lee, C. Hu, Z.L. Wang, Adv. Funct. Mater. 24 (2014) 1401–1407. Zhaoling Li is a Ph.D. candidate in the College of Textiles in Donghua University, China. He is currently a visiting student in the School of Materials Science and Engineering at Georgia Institute of Technology under the supervision of Prof. Zhong Lin (Z. L.) Wang. His research mainly focuses on triboelectric nanogenerators as sustainable power sources and self-powered active sensing. Dr. Jun Chen received his Ph.D degree in Materials Science and Engineering at Georgia Institute of Technology under the supervision of Prof. Zhong Lin Wang in 2016. His doctoral research focuses primarily on nanomaterial-based energy harvesting, energy storage, active sensing and self-powered micro-/nano-systems. He has already published 60 papers in total and 31 of them as first-author in prestigious scientific journals, such as Nature Communications, ACS Nano, Advanced Materials, Nano Letters, and so on. And still, he filed 8 US patents and 15 Chinese patents. His research on triboelectric nanogenerators has been reported by worldwide mainstream media over 400 times in total, including Nature, PBS, The Wall Street Journal, Washington Times, Scientific American, NewScientist, Phys.org, ScienceDaily, Newsweek, and so on. Jun also received the 2015 Materials Research Society Graduate Student Award, and the 2015 Chinese Government Award for Outstanding Students Abroad. His current H-index is 30. Jiajia Zhou received her B.S. degree in Textile Engineering from Qingdao University, China in 2014. She is currently a master studnet in the College of Textiles in Donghua University under the supervison of Prof. Chongwen Yu. Her research interests include natural fiber extration and ramie fiber degumming. Dr. Li Zheng is a visiting scholar in School of Materials Science and Engineering at Georgia Institute of Technology and an assistant professor in Shanghai University of Electric Power. She received her B.S. in physics from Ocean University of China in 2002, M.S. in optics from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences in 2006 and Ph. D. in Optics from Shanghai Jiao Tong University in 2009. Her current research interests include nanowire lasers, nanostructure-based optoelectronic devices, nanogenerator, and self-powered nanosensors. Dr. Ken C. Pradel received his B.S. and M.S. in Materials Science and Engineering from the Robert R. McCormick School of Engineering at Northwestern University in 2010 and 2011, respectively. He is currently a Ph. D. student in the School of Materials Science and Engineering at the Georgia Institute of Technology, working for Dr. Zhong Lin Wang. His research focuses primarily on the synthesis and characterization of nanoma-terials for piezotronic applications. Dr. Xing Fan received his Ph.D. degree from Peking University in 2009. He then joined the College of Chemistry and Chemical Engineering of Chongqing University. Now he is a visiting scholar at Georgia Institute of Technology through the program of China Scholarship Council. His current research interests include electrochemistry and nano energy. Z. Li et al. / Nano Energy 22 (2016) 548–557 557 Hengyu Guo received his B.S. degree in Applied Physics from Chongqing University, China in 2012. He is a Ph.D. candidate with the research focus on Condensed Matter Physics, Chongqing University. And now he is a visiting Ph.D. student at Georgia Institute of Technology through the program of China Scholarship Council. His current research interest is energy harvesting for selfpowered systems. Dr. Chongwen Yu is a professor in the College of Textiles at Donghua University, China. He winned his M.S. in Textile Engineering from China Textile University in 1986, and his Ph.D. in Textile Engineering from China Textile University in 1994. He was once a visiting scholar in the College of Textiles, North Carolina State University supported by national government funding. His research interests include bast fiber degumming, fiber property analysis and new spinning technology. Zhen Wen received his B.S. degree in Materials Science and Engineering from China University of Mining and Technology (CUMT) in 2011. He started to pursue his Ph.D. degree at Zhejiang University after that. Now he is a visiting Ph.D. student at Georgia Institute of Technology through the program of China Scholarship Council (CSC). His research interests mainly focus on nano-materials and nano-energy. Dr. Zhong LinWang is a Hightower Chair and Regents’s Professor at Georgia Tech. He is also the Chief scientist and Director for the Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. His discovery and breakthroughs in developing nanogenerators establish the principle and technological roadmap for harvesting mechanical energy from environmental and biological systems for powering personal electronics. His research on self-powered nanosystems has inspired the worldwide effort in academia and industry for studying energy for micro-nano-systems, which is now a distinct disciplinary in energy research and future sensor networks. He coined and pioneered the field of piezotronics and piezo-phototronics by introducing piezoelectric potential gated charge transport process in fabricating new electronic and optoelectronic devices. This historical breakthrough by redesigning CMOS transistor has important applications in smart MEMS/NEMS, nanorobotics, human-electronics interface and sensors. Dr. Min-Hsin Yeh received his Ph.D. degree in Chemical Engineering from National Taiwan University (NTU) in 2013 under the supervision of Prof. Kuo-Chuan Ho. Now he is a visiting scholar at Prof. Zhong Lin Wang’s group in the school of Materials Science and Engineering, Georgia Institute of Technology. His research interests mainly focus on triboelectric nanogenerator, self-powered electrochemical systems, electrochemistry, sensitized solar cells, and other energy materials.