High-efficiency ramie fiber degumming and self

Nano Energy 22 (2016) 548–557
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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: [email protected] (C. Yu), [email protected] (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
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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
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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
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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.
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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
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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.
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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.