Nano Energy 22 (2016) 548–557

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High-efficiency ramie fiber degumming and self-powered degummingwastewater 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 Statesb Key Laboratory of Science & Technology of Eco-Textile, Ministry of Education, College of Textiles, Donghua University, Shanghai 201620, Chinac School of Mathematics and Physics, Shanghai University of Electric Power, Shanghai 200090, Chinad Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China

a r t i c l e i n f o

Article history:Received 24 January 2016Received in revised form25 February 2016Accepted 1 March 2016Available online 2 March 2016

Keywords:TriboelectrificationSelf-poweredRamie fiberGreenDegumming

x.doi.org/10.1016/j.nanoen.2016.03.00255/& 2016 Elsevier Ltd. All rights reserved.

esponding authorresponding author at: School of Materials Scieof Technology, Atlanta, GA 30332-0245, Uni

ail addresses: yucw@dhu.edu.cn (C. Yu), zlwanese authors contributed equally to this work

a b s t r a c t

As one of the strongest and oldest natural fibers, ramie fiber has been widely used for fabric productionfor at least six thousand years. And degumming is a critical procedure that has been developed to holdthe 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 usingthe triboelectric effect. Resort to a water-driven triboelectric nanogenerator (WD-TENG), the ramie fibersdegumming efficiency was greatly enhanced with improved fiber quality, including both surface mor-phology 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 fromramie fibers. In addition, as a systematical study, the WD-TENG was further employed as a sustainablepower source to electrochemically degrade the degumming wastewater by recycling the kinetic energyfrom flowing wastewater in a self-powered manner. Under a fixed current output of 3.5 mA and voltageoutput of 10 V, the self-powered cleaning system was capable of cleaning up to 90% of the pollutants inthe wastewater in 120 min. Given the compelling features of being self-powered, environmentallyfriendly, extremely cost-effective, good stability, high degumming and degradation efficiency, the pre-sented work renders an innovative approach for natural fiber extraction, and could be widely adopted asa green and innovative technology in textile industry.

& 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Textile is critical to the development of human civilization andis everywhere in people’s daily life. Ramie, one of the most populartextile raw materials with distinctive characteristics, produces thestrongest and longest natural plant fibers with lustrous silky ap-pearance. This type of fiber possesses many excellent propertiessuch as high tensile strength, high moisture absorption, goodthermal conductivity, outstanding antibacterial function and fa-vorable air permeability [1–3]. The ramie fibers have been widelyused as an excellent textile material for clothing fabrics, industrialpackaging, car accessories, fiber reinforced composites, and so on.

nce and Engineering, Georgiated States.g@gatech.edu (Z.L. Wang)..

However, raw ramie is in the form of fiber bundles consisted ofmany individual fibers adhesive to each other. The gummy ornoncellulosic contents, such as pectin, lignin and hemicelluloses,are required to be degummed by placing in hot water or chemicalsolutions to free and extract the individual cellulose fibers, so as tofurther improve their downstream processing ability [4–6].

Considerable efforts have been committed to develop varioustechniques for natural fiber degumming and extraction, includingchemical, biological (enzymatic and microbic), ultrasonic or me-chanical methods [7]. However, widely adoption of these techni-ques may be shadowed by the limitations such as expensiveequipments, time-consuming procedures, high environmentalpollution, high energy consumption, high operating cost as well aslarge quantity of generated wastewater [8,9]. As a result, it ishighly meaningful and desirable to develop new approaches forramie degumming with improved fiber quality and less environ-mental contaminations.

Herein, in this work, we introduced a fundamentally new

Z. Li et al. / Nano Energy 22 (2016) 548–557 549

working principle in the field of ramie fiber degumming by re-porting a unique route that worked in a self-powered manner byharnessing the ambient energy using the triboelectric effect. Un-der the electric field provided by a water-driven triboelectric na-nogenerator (WD-TENG), the integrated electrochemical systemcan induce a large amount of OH- at the cathode, which willgreatly accelerate the degumming speed with less chemicalsconsumption. The surface morphologies and mechanical proper-ties of the degummed fibers exhibited greatly improved qualitycompared to the traditional approach. In addition, the WD-TENGwas also acting as a sustainable power source to electrochemicallydegrade the degumming wastewater by recycling the kinetic en-ergy from flowing wastewater. Under a fixed current output of3.5 mA and voltage output of 10 V, the self-powered cleaningsystem was capable of cleaning up to 90% of the pollutants in thewastewater in 120 min, in which the chromaticity, chemical oxy-gen demand (COD) and electrical conductivity were decreaseddistinctly.

With a collection of compelling features, such as high ramiedegumming efficiency and pollutants removal efficiency, cost-ef-fectiveness, simplicity as well as stability, the reported self-pow-ered approach based on triboelectric effect not only provides anefficient and green pathway for natural fiber extraction, but alsopromotes substantial advancement towards the practical applica-tions 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 wascarried out to create the nanowires structure onto the fluorinatedethylene propylene (FEP) film. Typically, a 50 μm thick FEP thinfilm was cleaned with isopropyl alcohol and deionized water, andthen blown dry with nitrogen gas. Before etching process, Auparticles were deposited by sputtering as the mask to induce thenanowires structure later. Subsequently, Ar, O2, and CF4 gases wereintroduced into the ICP chamber, with flow rates of 10.0, 15.0, and30.0 sccm, respectively. The FEP film was etched for 10 s to obtainthe nanowires structure on the surface with a high density plasmagenerator (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 ro-tator. Stator: A square-shaped acrylic sheet was cut as a substratewith a dimension of 13 cm � 13 cm � 3 mm by using a lasercutter. Through-holes on edges of the substrate were drilled formounting it on a flat stage by screws. Fine trenches with com-plementary patterns were created on top of the substrate by lasercutting. A layer of Cu (200 nm) was deposited onto the substrateusing an electron-beam evaporator. After that, two lead wireswere connected respectively to the electrodes. A thin layer of FEP(50 μm) was finally laminated onto the electrode layer. Rotator: Adisc-shaped acrylic substrate was patterned and consisted of ra-dial-arrayed sectors by using a laser cutter. The rotator has a dia-meter of 10 cm and a thickness of 1.5 mm. A through-hole wasdrilled that has a D-profile at the centre of the rotator for a con-venient 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

surface. The electrical signals were acquired using a programmableelectrometer (Keithley Model 6514) and a low-noise current pre-amplifier (Stanford Research System Model SR570). The softwareplatform is constructed based on LabView, which is capable ofrealizing real-time data acquisition control and analysis. Tensileproperties of the fibers such as tenacity, breaking elongation werecarried out using instrument XQ-1A testing machine. The degreeof polymerization of all samples was determined by viscositymethod using an Ubbelohde capillary viscometer as the instru-ment and copper ethylene-diamine solution as the solvent. FT-IRspectroscopic analysis was performed on Nicolet 6700 Spectro-meter (Thermo Fisher, America). XPS measurement was conductedon a Kratos Axis Ultra spectrometer (ESCA LAB 250, Thermo FisherScientific) with monochromatic Al Kα X-ray source.

2.4. Self-powered integration electrochemical system for ramiedegumming

The WD-TENG was connected to the central shaft of a minia-ture water turbine. Normal tap water was directed into the turbineinlet through a plastic pipe. A Ti/PbO2 anode and a Ti cathode wereimmersed in the reaction pond. A power management circuit wasconnected to output end of the WD-TENG to convert the alter-nating current to direct current signals. And a pH controller wasemployed to monitor the pH values in the reaction solution. Thedegumming experiments were conducted in a 500 mL beaker fil-led with 10.0 g raw ramie fiber and 100 mL freshly prepared de-gumming solution, continuously mixed at 200 rpm with magneticstirred bar. The reaction temperature was raised to be 100 °C. Thenthe degumming reaction process was conducted and the reactiontime was recorded. The treated fibers were thoroughly washedwith deionized water. Finally they were squeezed and properlydried at oven (100 °C, 3 h) before a further surface characterizationand mechanical properties measurement.

2.5. Electrochemical degradation of wastewater solution

The electrochemical degradation of degumming wastewaterwas performed in a plastic box filled with 200 mL of wastewater atroom temperature. Due to the good chemical stability and highelectrocatalytic activity, Ti and Ti/PbO2 electrodes were placed intothe solution, acting as the cathode and anode, respectively. Apower management circuit was connected to the wave energyharvester to convert the alternating current to direct current sig-nals. The degradation processing of the wastewater was monitoredat fixed time intervals by measuring the chromaticity, COD valuesand electrical conductivity.

3. Results and discussion

The self-powered triboelectric effect enabled ramie treatmentmainly consists of two procedures, firstly, the WD-TENG assisted high-efficiency ramie degumming and secondly, degradation of the de-gumming wastewater. As demonstrated in Fig. 1a, the as-developedWD-TENG consists of mainly two parts: a rotator and a stator. The as-fabricated device’s dimension is 10 cm�10 cm�1.5 mm. The rotatoris a collection of radially arrayed sectors with a unit central angle of 6°.The stator comprises of three components laminated along the verticaldirection: a layer of fluorinated ethylene propylene (FEP) as an elec-trificationmaterial, a layer of electrodes with complementary patterns,and an underlying substrate. FEP nanowires arrays were created onthe exposed FEP surface by a top-down method through reactive ionetching. A scanning electron microscopy (SEM) image of the FEP na-nowires is displayed in Fig. 1b, which indicates an average diameter of100 nm and an average length of 500 nm. Detailed fabrication process

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) ASEM 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 outputof the WD-TENG via a power management circuit.

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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 wasdemonstrated in Fig. 1c. To operate, a relative rotation between therotator and the stator gives rise to alternating flow of electronsbetween electrodes. The electricity generation process of the WD-TENG is elaborated through a basic unit. We define the initial stateand the final state as the states when the rotator is aligned withelectrode A and electrode B, respectively. The intermediate staterepresents a transitional process in which the rotator spins fromthe initial position to the final position. Since the rotator and thestator are in direct contact, triboelectrification creates chargetransfer on contacting surfaces, with negative charges generatedon the FEP and positive ones on the metal. Due to the law ofcharge conservation, the density of positive charges on the rotatoris twice as much as that of negative ones on the stator because ofunequal contact surface area of the two objects. Free charges canredistribute between electrodes due to the electrostatic induction.At the initial state, induced charges accumulate on electrode A andelectrode B. As the rotation starts, free electrons keep flowing fromelectrode A to electrode B until the rotator reaches the final state

where the charge density on both electrodes is reversed in polaritycompared to the initial state. Therefore, alternating current isgenerated as a result of the periodically changing electric fieldacross the electrodes [17–32].

Experimentally, to demonstrate, the WD-TENG was driven by awater turbine. Normal tap water was directed into the turbineinlet through a plastic pipe. A photograph of the experimentalsetup was shown in Supporting Information Figure S1. It is noticedthat the WD-TENG output holds a high voltage but relatively lowcurrent, resulting in large output impedance and thus affecting itsapplicability as a power source. Besides, fluctuation in outputpower and the AC output current are also concerns for practicalapplications. These issues can be fully addressed by integrating theWD-TENG with a power management circuit to form a completepower-supplying system. Consisting of a transformer, a rectifier, avoltage regulator and capacitors, the power management circuit,as diagrammed in Supporting Information Figure S2, can deliver aDC 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 waterflow rate of 3 L min�1.

And a schematic diagram of the self-powered system for ramie

Fig. 2. Working principle of the triboelectrification enabled high-efficiency ramie fiber degumming and self-powered degumming wastewater treatment. (a)A schematicdiagram of the integrated self-powered ramie fiber treatment system. (b) Mechanism of the ramie fiber degumming and wastewater treatment without the assistance of anelectric field. (c) Proposed mechanism of the ramie fiber degumming and wastewater treatment with the assistance of electric field provided by the WD-TENG system.

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degumming is presented in Fig. 2a. It consisted of a WD-TENG forpower supply, a power management circuit and a chemical reac-tion 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 thereaction solution. The detailed fabrication and experimental setupof the self-powered integration system is presented in the Ex-perimental Section.

A further step was taken to investigate the working mechanismof the ramie degumming and the following wastewater treatmentunder assistance of the self-provided electric field. In the de-gumming process, the coated gummy materials which have lowerdegree of polymerization (DP) will break away from the cellulosicpart of bast fibers and easily be dissolved in hot alkaline solutionunder the effect of hydroxyl ions, while the cellulose is compara-tively resistant to such attacks.

Gummy materials mainly include hemicelluloses, lignin, pectinand so on. Hemicelluloses are a mixture of various kinds of poly-saccharide. Each polysaccharide is made up of one or several formsof monosaccharide [33]. These polysaccharide primarily includethe components of galactoglucomannan, glucomannan, and xylan,which are hard to be dissolved in the solution. However, under theeffect of hydroxyl ions, these polysaccharides can be degraded intomonosaccharide components, such as glucose, mannose, galactoseand xylose, which can be finally dissolved in the degumming so-lution and easily removed from the raw ramie.

Pectins, also known as pectic polysaccharides, are rich in ga-lacturonic acid and is insoluble in water [34]. When immersed inhot alkaline solution, the protopectin loses some of its branchingand chain length and goes into solution. As a consequence, thepectin components in ramie fiber dissociate and separate from thefibers as well after chemical processing.

After extraction treatment, the following degumming solutioncontains a category of sugar polymers including the six-carbonsugars, such as mannose, galactose, and glucose, and the five-carbon sugars, such as xylose, arabinose, and rhamnose. Theresugar residues are formed in the degumming liquid. The sugarcomponents in the degumming solution can be measured by a gaschromatography (GC) analysis method. With the increase of thetreatment time, the content of sugar components increase, andthus the effluent of the degumming system contains more andmore organic pollutant. It is worth noting that the amount ofhydroxyl ions keep reducing all the time during the degummingprocess. And correspondingly the pH values of the aqueous solu-tion is decreasing continually. Eventually, most alkali has beenconsumed and the effluent of the reaction system contains lessamount of hydroxyl ions. It is worth noting that alkali is a must forthe degumming process of ramie fiber. Without it, the natural fibercan not be extracted.

The self-provided electric filed can promote the directed mi-gration of the ions, which is called electrophoresis, in the in-tegrated electrochemical system. The cations move toward the

Fig. 3. Performance characterization of the self-powered ramie fiber degumming system. (a) Comparison of the degumming efficiency of the ramie fiber with and withoutthe 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 ofthe 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 beforeand after degumming.

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cathode, while the anions move toward the anode under the in-fluence of the applied electric field. The self-provided electric filedcan boost the migration of hydroxyl ions as well as the electrolysiseffect (Fig. 2c). The later induced a generation of large amount ofOH- at the cathode in the degumming solution. In the meanwhile,a great deal of hydroxyl free radicals would generate at the cath-ode as well, and the gummy materials such as hemicelluloses,lignin and pectin would be electrocatalytically oxidized by suchhydroxyl radicals, which led to a greatly enhanced degummingefficiency of the treated fibers.

To evaluate the performance of the self-powered integration sys-tem for gummy components removal from raw ramie, the WD-TENGwas driven at a fixed water flow rate of 3 L min�1. Control experi-ments were carried out to compare the degumming efficiency of ra-mie fibers with or without an assistance of the WD-TENG. As de-monstrated in Fig. 3a and Figure S3, under both the 10% and 15% NaOHwithout the assistance of WD-TENG, the residual gum percentagedemonstrated a decrease trend throughout the time window of ob-servation. However, the removal percentage and removal efficiencywere much lower even though in the presence of high concentrationof hydroxide ions. Comparatively, under the provided electric field ofthe WD-TENG, the degumming speed of ramie fiber was greatlyboosted over time. The generated electric field had largely improvedthe gummy components removal performance in terms of both re-quired time and removal percentage. Besides, the proposed approachcould also save the chemicals consumptions.

Furthermore, the influence of the current output on the de-gumming performance of ramie fiber was also studied at a fixed10% NaOH. As demonstrated in Fig. 3b, to reach a same residual

gum percentage, a shorter degumming time is needed with alarger current output. Likewise, given a fixed degumming timeinterval, a larger current output will contribute to a larger gumremoval percentage from the raw ramie. However, the residualcontent of the gummy components is independent of the appliedcurrent. And it remains almost the same in the ramie fibers after acontinuous removal process of 300 min. Actually, the gummycomponents are very difficult to be completely removed from theraw ramie. The residual gum percentage will keep relatively stableafter a certain period of degumming time, even though in thepresence of high concentration of hydroxide ions and highstrength of applied current.

It is worth noting that the mechanical properties of degummedfibers with WD-TENG have been improved a lot compared with thedegummed fibers without aWD-TENG. As shown in Figs. 3c and d, thetenacity increased from 4.34 cN/dtex to 6.53 cN/dtex, while thebreaking elongation increased from 1.89% to 3.25%. Likewise, the de-gree of polymerization increased from 1780 to 2251, while the fine-ness increased from 1352 Nm to 1851 Nm. It indicated that the self-powered integrated system not only enhanced the degumming effi-ciency, but also served the role of improving the mechanical perfor-mance of the degummed fibers. The reason could be that with theassistance of WD-TENG, most of gummy components could be re-moved successfully from ramie fibers and a higher purity of cellulosecontents contributed to a larger mechanical properties.

Surface morphologies of the treated fibers with and without WD-TENG were also studied and compared via scanning electron micro-scopy (SEM). As shown in Fig. 4a, the untreated raw fibers exhibit arough and coarse surface due to the coating heavily with noncellulosic

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 are1.5 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 oframie 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.

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components. Fig. 4b is the SEM image showing the degummed ramiefibers without the assistance of WD-TENG, onwhich a certain amountof gummy substances still covered on the surface to prevent the fibersfrom fully separating each other. The poor degumming efficiency aswell as the bad surface morphologies is mainly attributed to the in-adequate removal reaction. To contrast, Fig. 4c shows the surface of thetreated fibers under the assistance of WD-TENG, which appeared mostclean and smooth surface. This revealed that vast majority of gummycomponents were effectively removed and the bundle fibers wereseparated thoroughly with each other owning to the applied electricfiled by the WD-TENG. In addition, the photographs of the raw ramieand 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 degummedfibers were depicted and compared visually in Fig. 5a. All the threecurves presented major crystalline peaks for 2θ ranging between 22°and 23°, which corresponded to the (002) crystallographic plane fa-mily of cellulose I. The other peaks for 2θ presented between 14.8° and16.7° corresponding to the (101) crystallographic plane family of cel-lulose II. Fig. 5b demonstrated the crystallinity index (CrI) of ramiefibers. The CrI of raw ramie was the lowest value of 68.20%, which isattributed to a high content of amorphous region resulting from theresidual gums. The value of CrI increases with the increase of crys-talline cellulose content in the treated fibers. Specifically, degummedfibers withWD-TENG possessed the highest CrI value of 86.96%, owingto a more adequately removal of amorphous noncellulosic com-pounds. Meanwhile, the absorption peaks reflected stronger intensitycompared to the untreated fibers. For degummed fibers without WD-TENG, a value of 77.61% CrI was observed, which indicated an in-sufficient degumming reaction leading to an existence of gummycomponents within the treated fibers.

To further identify the change of chemical compositions in un-treated and treated fibers, FTIR spectroscopy analysis [37] was alsocarried out, as the results shown in Fig. 5c. The value ranging from3000 cm�1 to 3600 cm�1 corresponded to the -OH stretching vibra-tions, which were mainly attributed to the large amount of hydroxylgroups in the cellulose fibers. After degumming with the assistance ofWD-TENG, the relative intensities in the range of 3600–3000 cm�1

increased, which suggested the treatments have removed the mostgummy components and the purity of cellulose increased accordingly.The treated fibers with WD-TENG exhibited strong absorption in-tensities, such as C–H stretching around 1630–1640 cm�1, CH2 sym-metric bending around 1320 cm�1, and C–O–C stretching around1064 cm�1. These corresponding intensities in the spectrum werehigher as compared with the raw ramie or treated fibers without theassistance of WD-TENG. From the spectra analysis we can safely drawconclusion that most gummy substances were successfully removedfrom ramie under the assistance of WD-TENG.

In this study, X-ray photoelectron spectroscopy (XPS) wasperformed to trace and compare the structure differences betweenraw ramie and treated cellulose fibers. The raw and degummedfiber samples were firstly scanned in a low-resolution mode per-formed on the Kratos Axis Ultra spectrometer, as the results shownin Fig. 5d. The raw and degummed fibers exhibited very simplespectra containing two characteristic peaks of carbon, with abinding energy of 284.5–288.9 eV and oxygen with a binding en-ergy of 532.3–533.3 eV, while some weaker peaks associated withthe existence of element N and S. The high-resolution C1s peak inthe XPS spectra gives detailed information of surface chemistry.Figure S4 demonstrated the peak assignment of C moieties in theramie fibers. The chemical shifts for carbon (Cls) in cellulose fiberscan be deconvoluted into four categories C1. These four C moietiesexhibited corresponding peaks at 284.5, 286.5, 287.8 and 288.9 eV,respectively. It can be seen that the peak densities of C moietiesexperienced considerable change after degumming with the as-sistance of WD-TENG. The peak densities of C1 and C2 appeared a

decrease trend while the peak densities of C3 and C4 showed anincrease trend. Table S1 summarized the peak areas and relativeatomic percentage of C moieties in raw and degummed fibersunder different conditions. In terms of O1s peaks, O1 (C-OH, C–O–C) and O2 (C¼O, COOR) were generally involved. These two oxy-gen moieties respectively exhibited corresponding peaks at532.3 eV and 533.3 eV. Figure S5 illustrated the high resolutionXPS spectra of O1s peaks in raw and degummed fibers. As de-monstrated, the peak area, peak width and peak height were ap-parently changed with the increase of cellulose content after beingdegummed with the assistance of WD-TENG. The relative atomicpercentage 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 theO2/O1 ratio had a significant increase from 0.691 in raw fiber to0.771 in degummed fibers with the assistance of the WD-TENG.

For a systematical investigation of degumming ramie fibers, theambient triboelectric effect was further utilized to develop theWD-TENG as a sustainable power source [38–45] to electro-chemically degrade the pollutants in the degumming wastewaterby recycling the kinetic energy from flowing wastewater. Experi-mentally, to demonstrate, the WD-TENG was connected to thecentral shaft of a miniature water turbine. Normal tap water wasdirected into the turbine inlet through a plastic pipe. Under a fixedwater flow rate of 3 L min�1, the short-circuit current (Isc) has acontinuous AC output with an average amplitude of 3.5 mAthrough the power management circuit. And the open-circuitvoltage (Voc) oscillates at the same frequency as that of Isc with apeak-to-peak value of 10 V. A Ti/PbO2 anode and a Ti cathode,vertically fixed in a degumming wastewater container, were con-nected to the WD-TENG system.

In electrochemical oxidation process, the degummed pollutantscan not only be mineralized by the hydroxyl radicals on the anodesurface, but also can be directly oxidized and degraded on thesurface of anodes, which were eventually mineralized into CO2

and H2O. This process can largely relieve the pollution and im-prove the water quality. In environmental chemistry, the chemicaloxygen demand (COD) test is commonly used to determine theamount of organic compounds found in wastewater. Electricalconductivity (EC) estimates the amount of total dissolved salts(TDS), or the total amount of dissolved ions in the water. Chro-maticity is an objective specification of the quality of a color re-gardless of its luminance. In this study, the degradation efficiencyof the degumming wastewater was characterized in terms ofchromaticity, COD values and electrical conductivity.

The self-powered cleaning system for wastewater treatmentfrom ramie degumming was demonstrated in Fig. 6. As shown inFig. 6a, with the increasing of degradation time, the chromaticityin the degumming wastewater decreased evidently, indicating theeffectiveness of the route for self-powered pollutants electro-chemical degradation. The visual color in the solution also ex-perienced obvious change from the initial heavy color to lastlylight color during the degradation time. Besides, after a continuousdegradation of 120 min, the COD values decreased from 4589 mg/Lto 420 mg/L while the electrical conductivity decreased from32200 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 to90% of the pollutants in the wastewater in 120 min.

Furthermore, the influence of the current output of the WD-TENG on the pollutant degradation performance was also studied.As demonstrated in Fig. 6c, to reach a same COD value, a shortertime is needed with a larger current output. Likewise, given a fixeddegradation time interval, a larger current output will contributeto a smaller COD value of the wastewater. A further test wasperformed to evaluate the electrical conductivity, and it shares a

Fig. 6. Investigation of the self-powered system for wastewater treatment from ramie degumming. (a) Dependence of the chromaticity in the degumming wastewater on thedegradation time. Inset shows the color change of the degumming wastewater with the increase of degradation time. (b) Dependence of the COD and electrical conductivityof 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 thedegumming wastewater.

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similar trend with that of the COD, as demonstrated in Fig. 6d. Thisclearly indicated the effectiveness of the reported route for self-powered electrochemically degrading degumming wastewater.We further investigated the durability of the self-powered elec-trochemical system. The experimental results are shown in FigureS6. No observable degradation of the surface polymer nanowiresof the as-fabricated WD-TENG after a continuous operation of36 h, as indicated by the SEM image in Figure S6a and S6b. Andalso, both the measured output current (Figure S6c and S6d) andvoltage (Figure S6e and S6f) of the as-fabricated WD-TENG areconstant after long-term device operation. These indicate a gooddurability of the integrated system for ramie fiber degumming andthe following wastewater treatment.

4. Conclusions

In this work, we paved a new avenue in the field of ramie fiberdegumming and the following wastewater treatment by reporting aself-powered route based on triboelectric effect. By harvesting the ki-netic energy from ambient water flow, the generated electric field fromthe WD-TENG can largely boost the migration of hydroxyl ions andenhance the electrolysis effect, which can greatly improve the de-gumming speed and degumming efficiency. The surface morphologyand mechanical properties of degummed fibers exhibited much moreimprovement compared to the treated fibers without WD-TENG.Moreover, the power generated by the WD-TENG can also act as asustainable power source to electrochemically degrade the pollutantsin the degumming wastewater. Under a fixed current output of 3.5 mAand voltage output of 10 V, the self-powered cleaning system wascapable of cleaning up to 90% of the pollutants in the wastewater in120min. And experimental results indicate that the chromaticity, COD

and electric conductivity in treated wastewater have been decreaseddistinctly with the assistance of WD-TENG. The reported WD-TENGassisted electrochemical system not only provides an efficient pathwayand new alternative to environmentally friendly extraction of naturalfiber, but also promotes substantial advancement toward the practicalapplications of TENG based self-powered electrochemical systems.

Acknowledgments

Z. L. and J. C. contributed equally to this work. The research wassupported by the Hightower Chair foundation, and the “thousandstalents” 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 TechnologyResearch System, Ministry of Agriculture of China (CARS-19-E25).

Appendix A. Supplementary material

Supplementary data associated with this article can be found in theonline version at http://dx.doi.org/10.1016/j.nanoen.2016.03.002.

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Zhaoling Li is a Ph.D. candidate in the College of Tex-tiles in Donghua University, China. He is currently avisiting student in the School of Materials Science andEngineering at Georgia Institute of Technology underthe supervision of Prof. Zhong Lin (Z. L.) Wang. Hisresearch mainly focuses on triboelectric nanogenera-tors as sustainable power sources and self-poweredactive sensing.

Dr. Jun Chen received his Ph.D degree in MaterialsScience and Engineering at Georgia Institute of Tech-nology under the supervision of Prof. Zhong Lin Wangin 2016. His doctoral research focuses primarily onnanomaterial-based energy harvesting, energy storage,active sensing and self-powered micro-/nano-systems.He has already published 60 papers in total and 31 ofthem as first-author in prestigious scientific journals,such as Nature Communications, ACS Nano, AdvancedMaterials, Nano Letters, and so on. And still, he filed8 US patents and 15 Chinese patents. His research ontriboelectric 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 2015Materials Research Society Graduate Student Award, and the 2015 Chinese Gov-ernment Award for Outstanding Students Abroad. His current H-index is 30.

Jiajia Zhou received her B.S. degree in Textile En-gineering from Qingdao University, China in 2014. Sheis currently a master studnet in the College of Textilesin Donghua University under the supervison of Prof.Chongwen Yu. Her research interests include naturalfiber extration and ramie fiber degumming.

Dr. Li Zheng is a visiting scholar in School of MaterialsScience and Engineering at Georgia Institute of Tech-nology and an assistant professor in Shanghai Uni-versity of Electric Power. She received her B.S. in phy-sics from Ocean University of China in 2002, M.S. inoptics from Shanghai Institute of Optics and Fine Me-chanics, 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, nanogen-erator, and self-powered nanosensors.

Dr. Ken C. Pradel received his B.S. and M.S. in MaterialsScience and Engineering from the Robert R. McCormickSchool of Engineering at Northwestern University in2010 and 2011, respectively. He is currently a Ph. D.student in the School of Materials Science and En-gineering at the Georgia Institute of Technology,working for Dr. Zhong Lin Wang. His research focusesprimarily on the synthesis and characterization of na-noma-terials for piezotronic applications.

Dr. Xing Fan received his Ph.D. degree from PekingUniversity in 2009. He then joined the College ofChemistry and Chemical Engineering of ChongqingUniversity. Now he is a visiting scholar at Georgia In-stitute of Technology through the program of ChinaScholarship Council. His current research interests in-clude electrochemistry and nano energy.

Z. Li et al. / Nano Energy 22 (2016) 548–557 557

Hengyu Guo received his B.S. degree in Applied Physicsfrom Chongqing University, China in 2012. He is a Ph.D.candidate with the research focus on Condensed Mat-ter Physics, Chongqing University. And now he is avisiting Ph.D. student at Georgia Institute of Technologythrough the program of China Scholarship Council. Hiscurrent research interest is energy harvesting for self-powered systems.

Zhen Wen received his B.S. degree in Materials Scienceand Engineering from China University of Mining andTechnology (CUMT) in 2011. He started to pursue hisPh.D. degree at Zhejiang University after that. Now he isa visiting Ph.D. student at Georgia Institute of Tech-nology through the program of China ScholarshipCouncil (CSC). His research interests mainly focus onnano-materials and nano-energy.

Dr. Min-Hsin Yeh received his Ph.D. degree in ChemicalEngineering from National Taiwan University (NTU) in2013 under the supervision of Prof. Kuo-Chuan Ho.Now he is a visiting scholar at Prof. Zhong Lin Wang’sgroup in the school of Materials Science and En-gineering, Georgia Institute of Technology. His researchinterests mainly focus on triboelectric nanogenerator,self-powered electrochemical systems, electro-chemistry, sensitized solar cells, and other energymaterials.

Dr. Chongwen Yu is a professor in the College of Tex-tiles at Donghua University, China. He winned his M.S.in Textile Engineering from China Textile University in1986, and his Ph.D. in Textile Engineering from ChinaTextile University in 1994. He was once a visitingscholar in the College of Textiles, North Carolina StateUniversity supported by national government funding.His research interests include bast fiber degumming,fiber property analysis and new spinning technology.

Dr. Zhong LinWang is a Hightower Chair and Regents’sProfessor at Georgia Tech. He is also the Chief scientistand Director for the Beijing Institute of Nanoenergy andNanosystems, Chinese Academy of Sciences. His dis-covery and breakthroughs in developing nanogenera-tors establish the principle and technological roadmapfor harvesting mechanical energy from environmentaland biological systems for powering personal electro-nics. His research on self-powered nanosystems hasinspired the worldwide effort in academia and industryfor studying energy for micro-nano-systems, which isnow a distinct disciplinary in energy research and fu-

ture sensor networks. He coined and pioneered the

field of piezotronics and piezo-phototronics by introducing piezoelectric potentialgated charge transport process in fabricating new electronic and optoelectronicdevices. This historical breakthrough by redesigning CMOS transistor has importantapplications in smart MEMS/NEMS, nanorobotics, human-electronics interface andsensors.