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\begin{document}

\title{Performance-enhanced eco-friendly triboelectric nanogenerator via
wettability manipulation of lignin}



\author[1]{Hongseok Jo}%
\author[2]{Dogun Park}%
\author[2]{Minkyeong Joo}%
\author[2]{Daekyu Choi}%
\author[3]{Jisong Kang}%
\author[3]{Jeong-Myeong Ha}%
\author[3]{Kwang Ho Kim}%
\author[4]{Ki Hyun Kim}%
\author[2]{Seongpil An}%
\affil[1]{ Sungkyunkwan University (SKKU), SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU)}%
\affil[2]{Sungkyunkwan University}%
\affil[3]{Korea Institute of Science and Technology}%
\affil[4]{Sungkyunkwan University (SKKU), School of Pharmacy}%


\vspace{-1em}



  
  \date{May 20, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Eco-friendly and sustainable energy harvests that can alleviate concerns
on the energy crisis and environmental pollution are in demand.
Exploiting nature-derived biomaterials is imperative to develop these
carbon-neutral energy harvesters. In this study, lignin/polycaprolactone
nanofiber (NF)-based triboelectric nanogenerators (TENGs) are fabricated
using an electrospinning technique. Nanotextured morphology of
electrospun lignin/polycaprolactone NFs and wettability modification of
lignin into hydrophilicity can significantly enhance electron transfer
between tribopositive and tribonegative materials, resulting in the
highest energy-harvesting efficiency in their class. The output voltage
of the lignin-based TENG exceeds 95 V despite relatively low tapping
force of 9 N and frequency of 9 Hz. Various mechanical and
physicochemical characterizations, including scanning electron
microscopy (SEM), nuclear magnetic resonance (NMR) spectroscopy, X-ray
diffraction (XRD) analysis, Fourier transform infrared (FTIR) analysis,
and atomic force microscopy (AFM), are performed, confirming the
mechanical durability, biocompatibility, and industrial viability of
lignin-based TENG developed here.%
\end{abstract}%



\sloppy


\textbf{Performance-enhanced eco-friendly triboelectric nanogenerator
via wettability manipulation of lignin}

\textbf{Hongseok Jo \textsuperscript{a,1}, Dogun
Park\textsuperscript{a,b,1}, Minkyeong Joo \textsuperscript{a,b,1},
Daekyu Choi \textsuperscript{a,b}, Jisong Kang \textsuperscript{c, d},
Jeong-Myeong Ha \textsuperscript{d,e}, Ki Hyun Kim\textsuperscript{f,*},
Kwang Ho Kim \textsuperscript{d,g,h,*}, and Seongpil An
\textsuperscript{a,b,i,j,*}}

\textsuperscript{a} SKKU Advanced Institute of Nanotechnology (SAINT),
Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea

\textsuperscript{b} Department of Nano Science and Technology,
Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea

\textsuperscript{c} Department of Chemical and Biological Engineering,
Korea University, Seoul 02841, Republic of Korea

\textsuperscript{d} Clean Energy Research Center, Korea Institute of
Science and Technology, Seoul 02792, South Korea

\textsuperscript{e} Division of Energy and Environment Technology, KIST
School, University of Science and Technology, Seoul 02792, South Korea

\textsuperscript{f} School of Pharmacy, Sungkyunkwan University (SKKU),
Suwon 16419, Republic of Korea

\textsuperscript{g} KIST-SKKU Carbon-Neutral Research Center,
Sungkyunkwan University (SKKU), Suwon, 16419, Republic of Korea

\textsuperscript{h} School of Chemical Engineering, Sungkyunkwan
University (SKKU), Suwon, 16419, Republic of Korea

\textsuperscript{i} Department of Nano Engineering, Sungkyunkwan
University (SKKU), Suwon 16419, Republic of Korea

\textsuperscript{j} Department of Semiconductor Convergence Engineering,
Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea

\textbf{Abstract}

Eco-friendly and sustainable energy harvests that can alleviate concerns
on the energy crisis and environmental pollution are in demand.
Exploiting nature-derived biomaterials is imperative to develop these
carbon-neutral energy harvesters. In this study, lignin/polycaprolactone
nanofiber (NF)-based triboelectric nanogenerators (TENGs) are fabricated
using an electrospinning technique. Nanotextured morphology of
electrospun lignin/polycaprolactone NFs and wettability modification of
lignin into hydrophilicity can significantly enhance electron transfer
between tribopositive and tribonegative materials, resulting in the
highest energy-harvesting efficiency in their class. The output voltage
of the lignin-based TENG exceeds 95 V despite relatively low tapping
force of 9 N and frequency of 9 Hz. Various mechanical and
physicochemical characterizations, including scanning electron
microscopy (SEM), nuclear magnetic resonance (NMR) spectroscopy, X-ray
diffraction (XRD) analysis, Fourier transform infrared (FTIR) analysis,
and atomic force microscopy (AFM), are performed, confirming the
mechanical durability, biocompatibility, and industrial viability of
lignin-based TENG developed here.

Keywords : eco-friendly triboelectric nanogenerator, energy-harvesting
technology, lignin, wettability manipulation, electrospinning

\textsuperscript{\textbf{1}} \textbf{Hongseok Jo, Dogun Park, and
Minkyeong Joo contributed equally to this work.}

\textbf{*Corresponding authors:} esan@skku.edu, kwanghokim@kist.re.kr,
khkim83@skku.edu

\textbf{Introduction}

The overdependence on fossil fuels has raised increasing concerns about
the energy crisis and environmental pollution. There have been
significant efforts to find green and sustainable resources that can
address these issues as new energy sources and materials. Nature-derived
biomaterials are increasingly being considered as alternatives to
existing petroleum-based materials.\textsuperscript{1} These
carbon-neutral biomaterials derived from plants and animals possess
advantages unattainable by petroleum-based materials. First, they are
non-toxic and biocompatible, making them suitable for energy
applications that require direct interaction with humans or nature.
Next, they are abundant in nature and thus easy to access, which can
reduce the reliance on existing limited resources. Third, the
biodegradability of nature-derived biomaterials can mitigate waste
accumulation and environmental pollution.

Lignin, the most dominant aromatic polymer in nature, is found in
terrestrial biomasses in the range of 15 -- 40\%
weight.\textsuperscript{2} it provides structural support to plants,
contributing to their biomechanical strength. Lignin also plays
important roles in water conduction from roots to leaves, and often
serves as a defense system against harmful microbial invasion and
various environmental stimuli. A large amount of lignin is produced as a
by-product in pulp and paper making industries. Considering its
attractive physicochemical properties, including thermal stability,
durability, redox activity, and antioxidant property, lignin is viewed
as a promising alternative to petroleum-based
materials.\textsuperscript{3} In addition, lignin has unique structural
and chemical features that make it an excellent building block for
manufacturing functional materials for energy applications. For example,
the aromatic backbone of lignin provides lignin-derived materials with
high thermal stability and structural
rigidity.\textsuperscript{4}Furthermore, the aromatic rings form \selectlanguage{greek}π--π
\selectlanguage{english}conjugated system that can facilitate producing various kinds of
precursors.\textsuperscript{5}

Because of these features, lignin has gained significant interest as a
carbon-neutral and sustainable biomaterial. In addition, the ease of
incorporation of lignin into various existing manufacturing processes is
also advantageous for the development of industrially viable and
scalable energy materials and devices.\textsuperscript{6} Therefore,
recent research and development efforts are focused on harnessing the
potential of lignin to contribute to advancing renewable energy
technologies, including energy-harvesting technologies. The
triboelectric nanogenerator (TENG) is a representative energy-harvesting
technology that can transduce mechanical energy, such as motion or
vibration, into electrical energy based on the triboelectric
effect.\textsuperscript{7-9} An \emph{et al.} reported eco-friendly
triboelectric nanogenerators (eco-TENGs) using lignin-based nanofibers
(NFs) with a solution-blowing technique.\textsuperscript{10} In their
study, the lignin NF mat was employed as the tribopositive material,
while a polyamide tape served as the tribonegative materials. Although
they for the first time reported the lignin NF-based TENG, a low output
voltage of \textless{} 1 V was observed at the energy-harvesting tests.
Similarly, Wang \emph{et al.} recently reported eco-TENGs composed of
lignin-based electro spun NFs and a Teflon film.\textsuperscript{11}They
could achieve a high output voltage of \textgreater{} 100 V under a high
applied force of 40 N and a frequency of 10 Hz.

As demonstrated in the aforementioned studies, the combination of lignin
and NF techniques is increasingly being preferred in the energy-related
fields. Because the NF fabrication techniques, such as electrospinning
and solution blowing, are facile, industrially
scalable.\textsuperscript{12, 13}, and most importantly, capable of
exploiting bare lignin powder directly without additional thermal or
physicochemical treatments during their fabrication process. In
addition, the nanotextured surface morphology of NF mat (or film) can
maximize the effective friction area regardless of its projected contact
surface area, thus can significantly increase the effect of contact
electrification (or triboelectricity).\textsuperscript{14} In this
study, kraft lignin, wettability-manipulated hydrophilic, and
hydrophobic lignins were prepared and electrospun, and then their
energy-harvesting performance as tribopositive materials were explored
(\textbf{Figure 1} ). More sophisticated electrospinning techniques and
the use of the utmost tribonegative material, Teflon film, allowed the
bare kraft lignin to yield a higher output voltage of \textgreater{} 25
V compared to the values observed from the work of An \emph{et al} .
despite similar test conditions.\textsuperscript{10} Moreover, the
wettability manipulation of kraft lignin to hydrophilicity could enhance
its surface energy, thus leading to a remarkable increase in the output
voltage value over 90 V even with a lower applied force of 9 N than that
of the work of Wang \emph{et al.} \textsuperscript{11} Accordingly, the
utilization of lignin, the second most abundant biomaterial among
natural biomaterials, combined with surface wettability design methods
demonstrated here, holds significant promise as an industrially viable
and sustainable energy material and technique.

\textbf{2. Results and discussion}

2.1. Fabrication of lignin/polycaprolactone nanofiber-based
triboelectric nanogenerator

Lignin, a major component of lignocellulosic biomass, was employed to
fabricate biodegradable and sustainable nanofiber (NF) mats via
electrospinning. The structure of lignin was modified to improve its
processability. The electrospinning process is commonly employed to
produce nonwoven NF mats. However, electrospinning with low molecular
weight (\emph{M} \textsubscript{w}) lignin is challenging due to its
insufficient viscoelasticity, which is essential for self-sustainable
electrospinning (\textbf{Table 1} ).\textsuperscript{15-17} Therefore,
polycaprolactone (PCL), which has a relatively high molecular weight
(\emph{M} \textsubscript{w}) of 80,000 kDa and thus can enhance the
electrospinnability of lignin, was incorporated with lignin. To evaluate
the influence of lignin wettability on its energy-harvesting performance
(cf. \textbf{Section 2.3} ), we prepared a bare PCL solution and three
kinds of different lignin/PCL solutions. For lignin/PCL solutions, a
lignin/PCL ratio was the same as 1:1 (w/w) while lignins with different
wettabilities were used. The three variants of lignins, namely kraft,
hydrophilic, and hydrophobic lignins, were prepared, then mixed with PCL
before electrospinning (cf. \textbf{Sections S1.2} and \textbf{S1.3} ).
Henceforth, these different lignin/PCL combinations are denoted as KLP,
HILP, and HOLP, respectively.

The fabrication of lignin/PCL NF-based triboelectric nanogenerators
(LP-TENGs) involves several steps. First, electrospun LP NF mats
(\emph{i.e.} , KLP, HILP, and PCL NF mats) and tribonegative Teflon tape
were transferred to a Cu substrate that served as the electrode. Second,
they were affixed to a polyvinyl chloride (PVC) film, which provided
structural support and resistance to pressure (\textbf{Figure S1a} ).
The illustration at the bottom of \textbf{Figure 1} demonstrates the
operation mechanism of the LP-TENG, which relies on the contact-induced
triboelectric effect. Although the exact underlying principles of the
triboelectric effect remain unclear, it is widely recognized that this
phenomenon arises from electron transfers occurring at the interface of
dissimilar materials owing to the overlapping of the electron clouds of
different atoms (or molecules).\textsuperscript{18} When materials
exhibiting distinct electrical polarization come into contact or
separate from each other under the influence of external forces, the
interaction at the interface induces friction, resulting in the
generation of positive and negative charges on their respective
surfaces. This charge imbalance establishes a potential difference
between the materials, thereby facilitating the movement of electrons
through an external circuit.\textsuperscript{19}

2.2. Mechanical properties of lignin/polycaprolactone nanofibers

Photos and scanning electron microscopy (SEM) images of the KLP, HILP,
and HOLP NF mats are shown in\textbf{Figure 2}. Lignins, the main
component of LP-TNEGs, retained their inherent coloration, and these
colors revealed their identities when the lignins were combined with PCL
\textbf{(Figure S1b)}. The introduction of different lignins into the
PCL resulted in a noticeable reduction in the brown color of the
resulting NF mats (\textbf{Figures 2a}, \textbf{2b}, and \textbf{2c}).
The degree of color change in the lignin/PCL NF mats was found to be
correlated with the specific lignin powder employed, with KL, HIL, and
HOL powders resulting in a lighter shade in order. In addition,
discernible changes were observed in the morphology and mechanical
properties of the lignin/PCL NF mats. The average diameter
(\emph{D}\textsubscript{avg}) of the lignin/PCL NF varied depending on
the wettability of lignin, despite using a constant lignin/PCL ratio and
the same electrospinning conditions. Compared to the pristine PCL NFs
with \emph{D}\textsubscript{avg} =103 nm (\textbf{Figures S2a} and
\textbf{Sb}), the lignin/PCL NFs showed an increase in
\emph{D}\textsubscript{avg} for all lignin cases. Among the different
lignin/PCL NF cases, the HOLP NFs exhibited a
smaller\emph{D}\textsubscript{avg} of 136 nm than those of KLP and HIL
NFs, which had \emph{D}\textsubscript{avg} of 218 and 273 nm,
respectively (\textbf{Figure 2d}).

The wettability manipulation of lignin also had a significant influence
on the mechanical properties of the resulting lignin/PCL NF
mats.\textbf{Figure 2e} presents the results of the tensile tests
conducted on the KLP, HILP, and HOLP NF mats. The values of Young's
modulus (\emph{E} ) and yield strength (\emph{Y} ) were obtained by
using the elastic--plastic Green equation {[}\selectlanguage{greek}\emph{σ} \selectlanguage{english}=
\emph{Y}tanh(\selectlanguage{greek}\emph{Eε} /\selectlanguage{english}\emph{Y} ), where \selectlanguage{greek}\emph{σ} \selectlanguage{english}and \selectlanguage{greek}\emph{ε} \selectlanguage{english}are the
tensile stress and strain measured
experimentally{]}.\textsuperscript{20} The values of \emph{E} and
\emph{Y} for the pristine PCL NF mat were 6.16 \selectlanguage{ngerman}± 0.02 and 0.05 ± 0.02
MPa, respectively (\textbf{Figure S2c} ). The incorporation of KL, HIL,
and HOL into PCL led to improvement in mechanical properties. The HILP
NF mat exhibited the highest \emph{E}value of 15.95 ± 4.32 MPa among all
the NF mats, which is seemingly attributed to its highest value in
\emph{D} . On the other hand, the\emph{Y} values of the KLP, HILP, and
HOLP NF mats were 2.66 ± 0.69, 2.93 ± 0.57, and 7.31 ± 0.12 MPa,
respectively, which shows that blending higher \emph{M}
\textsubscript{w} lignins into PCL resulted in higher \emph{Y} values
(\textbf{Table 1} and \textbf{Figure 2e} ). The higher \emph{M}
\textsubscript{w} lignins possess longer polymeric chains, which can
enhance intermolecular interactions and entanglement inside of
individual NFs.\textsuperscript{21, 22} Thus, these enhanced
intermolecular behaviors improved the mechanical strength of the NF
mats.

2.3. Physicochemical properties of lignin/polycaprolactone nanofibers

To examine molecular structures, presence of ordered domains, and
chemical features of KL, HIL, and HOL, their powders were investigated
using \textsuperscript{1}H nuclear magnetic resonance (NMR)
spectroscopy, X-ray diffraction (XRD), and Fourier transform infrared
(FTIR) spectroscopy. \textbf{Figure 3a} shows the \textsuperscript{1}H
NMR spectra observed from KL, HIL, and HOL powders. The characteristic
peaks of KL observed at \selectlanguage{greek}\emph{δ} \selectlanguage{english}= 2.57, 3.41, and 3.87 ppm are
attributed the protons of methylene and methoxy
groups,\textsuperscript{23-27} and the broad peak between \selectlanguage{greek}\emph{δ} \selectlanguage{english}=
6.79 and 6.97 ppm represents the proton of the phenyl ring in
KL.\textsuperscript{26, 28, 29} With the chemical treatment of KL to
HIL, the new peak was detected at \selectlanguage{greek}\emph{δ} \selectlanguage{english}= 3.14 ppm, which corresponds
to the proton of methanimine group with
N-CH\textsubscript{3}bond.\textsuperscript{30, 31} In the spectra of
HIL, the characteristics peaks associated with KL were found at \selectlanguage{greek}\emph{δ}
\selectlanguage{english}= 3.87 and 6.79 to 6.97 ppm, indicating that the lignin backbone
structure retained even after chemical modification. In addition, the
prominent peak observed at\selectlanguage{greek}\emph{δ} \selectlanguage{english}= 4.70 ppm is attributed to the
absorption of H\textsubscript{2}O, thereby indicating that the
hydrophilic surface exhibits the capability to adsorb water molecules in
air through the formation of hydrogen bonds with hydroxyl groups in
HIL.\textsuperscript{32-34}

Upon subjecting KL to hydrophobic chemical treatment, the characteristic
peaks of KL were found at \selectlanguage{greek}\emph{δ} \selectlanguage{english}= 2.57, 3.84, and 6.79 to 6.97 ppm,
confirming the preservation of the macromolecular lignin backbone.
Furthermore, the resulting NMR spectrum of HOL revealed characteristic
peaks associated with palmitic groups. The peaks corresponding to
different functional groups within the palmitic structure, namely
CH\textsubscript{3} (methyl), aliphatic CH\textsubscript{2} (methylene),
-CH\textsubscript{2}-CH\textsubscript{2}-C(=O)-, and
-CH\textsubscript{2}-(C=O)-, were observed at \selectlanguage{greek}\emph{δ} \selectlanguage{english}= 0.91, 1.27,
1.63--1.77, and 7.26 ppm, respectively.\textsuperscript{35, 36} The
emergence of these new peaks, specifically those associated with the
methyl (CH\textsubscript{3}) and methylene (CH\textsubscript{2}) groups,
can be attributed to the increased hydrophobicity of
HOL.\textsuperscript{37, 38}

Chemical distinctions between KL, HIL, and HOL were also identified in
the FTIR spectra (\textbf{Figure S3a} ). The FTIR band assignment
obtained from KL were presented in \textbf{Table S1}
.\textsuperscript{11, 39-42} Chemical modification of KL induced a
change in the chemical structures of both HIL and HOL. In the FTIR
spectrum of HIL, the weak signal of band at 1708 cm\textsuperscript{-1},
which is observed in the KL spectrum, indicated the presence of
unconjugated carbonyl groups after KL
modification.\textsuperscript{43}On the other hand, in the FTIR spectrum
of HOL, the absence of broad band from 3600 to 3000
cm\textsuperscript{-1} was attributed to substitute palmitic groups
(C\textsubscript{15}H\textsubscript{31}) for O--H group. The prominent
emergence of band at 2915 cm\textsuperscript{-1} and strong signal of
band at 2848 cm\textsuperscript{-1} in the HOL spectrum, corresponding
to the asymmetric and symmetric C--H stretching vibrations of aliphatic
methylene (--CH\textsubscript{2}--),\textsuperscript{44-46} signify the
transformation of kraft lignin into hydrophobic lignin. Furthermore, the
strong signal of band at 1708 cm\textsuperscript{-1} was ascribed to the
presence of C=O bond as substituting palmitic groups
(C\textsubscript{15}H\textsubscript{31}) for O--H group.

The XRD patterns revealed significant variations in the phase
characteristics of lignin due to hydrophilic and hydrophobic treatments.
As presented in \textbf{Figure S3b} , two broad peaks corresponding to
the ordered domain and amorphous region of biomass were observed in the
XRD spectra of KL at 2\selectlanguage{greek}\emph{θ} \selectlanguage{english}= 22.2\selectlanguage{ngerman}° and 40°,
respectively.\textsuperscript{44} With the functionalization of KL to
HIL, the position of the peak associated with the ordered domain of
lignin shifted from its original position at 2\selectlanguage{greek}\emph{θ} \selectlanguage{english}= 22.2\selectlanguage{ngerman}° to a
lower angle of 2\selectlanguage{greek}\emph{θ} \selectlanguage{english}= 19.1\selectlanguage{ngerman}°. This shift was attributed to the
presence of a uniformly tensioned ordered domain within the lignin
structure, achieved through the attachment of trimethylamine groups to
the lignin framework.\textsuperscript{47, 48} However, the XRD pattern
of HOL exhibited significant changes compared to those of KL and HIL.
These changes were attributed to the incorporation of the palmitic group
in the KL molecule. The peaks observed in the XRD pattern of HOL at
2\selectlanguage{greek}\emph{θ} \selectlanguage{english}= 21.3\selectlanguage{ngerman}° and 23.9° were identified as the presence of fatty
acid crystals corresponding to the palmitic groups.\textsuperscript{49,
50} In addition, the HOL peak associated with the ordered domain of
lignin shifted toward 2\selectlanguage{greek}\emph{θ} \selectlanguage{english}= 20.3\selectlanguage{ngerman}° owing to the uniform tensile
stress exerted on the ordered domain. This resulted in the attachment of
the palmitic groups onto the lignin framework.

The \emph{M} \textsubscript{w} of KL, HIL, and HOL were determined using
gel permeation chromatography (GPC). An alternative approach combining
elemental analysis and degree of substitution was employed to analyze
the \emph{M} \textsubscript{w} of HIL, which was insoluble in the GPC
mobile phase. The results presented in \textbf{Table 1} reveal that KL,
HIL, and HOL had \emph{M} \textsubscript{w} values of 4.826, 6.654, and
13.430 g mol\textsuperscript{-1}, respectively. In comparison to\emph{M}
\textsubscript{w} of KL, the mass percentages of HIL and HOL
significantly increased by 37.8\% and 178.3\%, respectively, which was
attributed to the presence of functional groups in HIL and HOL.

The physicochemical difference depending on lignins was also evident in
the lignin/PCL NF mats. \textbf{Figures 3b} and \textbf{3c} present the
FTIR spectra and XRD patterns of the pristine PCL, KLP, HILP, and HOPL
NF mats. The FTIR spectra of all NF mats exhibited the characteristic
bands of the PCL NF mat at 2947, 2866, and 1720 cm\selectlanguage{english}\textsuperscript{-1},
corresponding to asymmetric elongation of the methylene-oxygen
(CH\textsubscript{2}\selectlanguage{english}-O), symmetric methylene groups
(CH\textsubscript{2}\selectlanguage{english}-), and vibration of --C=O bonds,
respectively.\textsuperscript{51, 52} This indicates the presence of PCL
within all NF mats. The FTIR spectra of the KLP and HILP NF mats
revealed the same bands observed from KL and HIL powders; O--H
stretching (3600\textasciitilde{}3000 cm\selectlanguage{english}\textsuperscript{-1}) and
aromatic C=C (1600 and 1515 cm\selectlanguage{english}\textsuperscript{-1)}. However,
discernible changes in the FTIR spectrum of the HOLP NF mats were
observed, \emph{i.e.} , the elimination and the reduction of the
characteristic bands of O--H and aromatic C=C. Meanwhile, new bands of
C--H bonds (2915 and 2848 cm\selectlanguage{english}\textsuperscript{-1}) and C=O ester (1708
cm\selectlanguage{english}\textsuperscript{-1}), which are related to the hydrophobic feature
and observed from HOL powder, were also observed in the FTIR spectrum of
HOLP NF mat (\textbf{Figure 3b} ).

Contrary to the analyses above, no discernible difference between the NF
mats was observed in the XRD result (\textbf{Figure 3c} ). For the
pristine PCL NF mat, the XRD pattern exhibited two intense peaks at
2\selectlanguage{greek}\emph{θ} \selectlanguage{english}= 21.4\selectlanguage{ngerman}° and 23.6°, indicative of the (110) and (200) PCL
lattice phases, respectively.\textsuperscript{53} A broad peak in the
range of 10--20° attributed to the semicrystalline phase was also
observed. The dominant (110) and (200) lattice planes in the PCL NF mat
were also observed in both the KLP and HILP NF mats. In case of the HOLP
NF mat, a peak at 2\selectlanguage{greek}\emph{θ} \selectlanguage{english}= 23.9\selectlanguage{ngerman}°, which corresponds to the presence
of fatty acid crystal, was also found.

The thermal stability and degradation behavior of the HILP and HOLP NF
mats, compared to the KLP NF mat, were evaluated through
thermogravimetric analysis (TGA). \textbf{Figure 3d} shows the TG curves
of the specimens. The thermal degradation process of the HILP NF mat was
observed to occur in four stages, whereas that of the KLP and HOLP NF
mats occurred in two stages. The two thermal degradations observed in
the KLP and HOLP NF mats correspond to the decomposition of phenolic
groups in the lignin (150--320\selectlanguage{english} degC) and the decomposition of PCL and
lignin (320--500\selectlanguage{english} degC), respectively.\textsuperscript{54, 55} A
significant decomposition occurred around 250 degC of the HOLP NF mat is
attributed to the thermal degradation of palmitic groups. The first
thermal degradation of the HILP NF mat between 30 and 100 degC, presenting
a weight loss of 2\%, resulted from the loss of water initially absorbed
from ambient moisture because of the hydrophilicity of HILP. This result
agrees well with the \textsuperscript{1}H NMR and FTIR analyses. The
second degradation occurred between 130 and 300 degC, corresponding to a
weight loss of 12\%, is attributed to the decomposition of the cationic
group substituted from the hydroxyl group in KL. The third degradation
observed between 300 and 350 degC ascribed to the cleavage of inter-unit
linkages of lignin.\textsuperscript{56, 57} The fourth degradation
between 350 and 500 degC corresponds to the comprehensive decomposition of
PCL and lignin.

The modified functional groups in HIL and HOL resulted in changes in the
wettability of the corresponding NF mats (\textbf{Figure 3e} ). The
water contact angle (WCA) of the PCL NF mat was 134deg, predominantly
because of the hydrophobic properties of the C--H bonds in the methylene
group. The introduction of KL into PCL led to a reduction in WCA to
108deg. The hydrophilic O--H groups from KL is responsible for this
wettability change. The HILP NF mat, where the C--H bonds of the
methylene and methoxy groups were eliminated through the chemical
treatment, exhibited a significant further decrease in WCA to 28deg. The
HOLP NF mat showed the highest WCA of 138deg. The additional C--H bonds
present in HOLP contributed to its enhancement in hydrophobic nature.

The modified functional groups resulting from the chemical treatment of
KL to HIL and HOL conversely influenced the electrostatic potential
states of the resulting NF mats. This was evaluated through surface
potential (\selectlanguage{greek}\emph{φ} \selectlanguage{english}) measurements (\textbf{Figure 3f} ). For HILP NF
mat,\selectlanguage{greek}\emph{φ} \selectlanguage{english}was measured to be 248 mV, indicating the presence of a
positive charge on its surface. The positive charge is attributed to the
cationic site of the quaternary ammonium substituents
(--N\textsuperscript{+}(CH\textsubscript{3})\textsubscript{3})
introduced during the modification process to transform KL into
HIL.\textsuperscript{58, 59} On the other hand, the HOLP NF mat
exhibited \selectlanguage{greek}\emph{φ} \selectlanguage{english}of 82 mV, implying the degree of negative surface
charge is higher than the HILP NF mat. This is due to the protonation of
the carboxyl group in HOLP.\textsuperscript{59, 60} The presence of
positive or negative surface charges can affect the charge transfer and
accumulation during the triboelectric process. Indeed, the chemical
treatments of KL to HIL and HOL significantly impacted the
energy-harvesting performance of the resulting NF mats, which is further
discussed in the forthcoming section.

2.4. Energy-harvesting performance of lignin/polycaprolactone
nanofiber-based triboelectric nanogenerator

\textbf{Figures 4a--4d} present the output voltage (\emph{V} ) signals
of the LP-TENGs composed of the KLP, HILP, and HOLP NF mats under
different tapping conditions of applied force (\emph{F} ) and frequency
(\emph{f} ). Teflon tape was selected as the tribonegative material
because of its low surface energy. The KLP- and HILP-TENGs exhibited
increases in\emph{V} signals with increasing \emph{F} (\textbf{Figures
4a} and\textbf{4b} ). On the other hand, no discernible effect on
\emph{V}signals was observed for the HOLP-TENG. The maximum output
positive voltage (\emph{V} \textsubscript{max}) of the HILP-TENG
increased from 22 to 60 V and further to 96 V as the \emph{F} increased
from 3 to 6 and 9 N, respectively. However, those of the HOLP-TENG were
just 12, 16, and 15 V when the \emph{F} were 3, 6, and 9 N,
respectively. Similarly, the\emph{V} signals of all LP-TENGs increased
as the \emph{f} increased from 3 to 9 Hz (\textbf{Figures 4c} and
\textbf{4d} ). This is because of a decrease in contact time (\emph{t}
), corresponding to an increase in the tapping speed, thus leading to an
increase in \emph{V} (= \emph{RI}= \emph{Rq} /\emph{t} , where \emph{R}
is an external load and \emph{q} is the surface
charge).\textsuperscript{61} Note that this behavior can be explained
because the LP-TENGs were evaluated at the short-circuit condition.

The superior energy-harvesting performance of the HILP-TENG was
attributed to an increase in electron exchange between tribo-positive
and -negative materials because of the increased distinction in their
electrostatic potential. When tribo-positive and -negative materials
with different electrostatic potential come into contact, electrons are
hopped from the high-electrostatic-potential material to the
low-electrostatic-potential one.\textsuperscript{8, 62} Because of the
electron transfer, a potential difference between two dissimilar
materials is generated. The higher surface electrostatic potential of
tribopositive HILP NF mat led to a further increase in the surface
electrostatic potential difference against the tribonegative Teflon
film, thus resulting in a greater potential difference, thereby
exhibiting higher \emph{V} signals.

Cyclic tests by repetitive tapping on the KLP-, HILP-, and HOLP-TENGs
were conducted (\textbf{Figure 4e} ). 100,000 tapping cycles were
performed with fixed \emph{F} of 9 N and \emph{f} of 9 Hz. The KLP-TENG
exhibited a slight increase in \emph{V} \textsubscript{max} from
\emph{V}= \textasciitilde{} 15 to \textasciitilde{} 30 V during the test
while the HILP-TENG showed a significant increase in\emph{V}
\textsubscript{max} from \emph{V} = \textasciitilde{} 30 to
\textasciitilde{} 80 V (\textbf{Figure S4a} ). This gradual increase
in\emph{V} over hundreds of thousands of cycles originates from the
unique morphological structure of the nonwoven NF mat, as discussed in
other previously-reported studies.\textsuperscript{63} The NFs are
continuously compressed during the repetitive tapping, which contributes
to an increase in the actual contact area between NFs, thus facilitating
the gradual rise in \emph{V.}

The energy-harvesting efficiency \selectlanguage{greek}\emph{η} , \selectlanguage{english}defined as \selectlanguage{greek}\emph{η}
\selectlanguage{english}=\emph{V} max \emph{F} \textsuperscript{-1}\emph{S}
\textsuperscript{-1}\emph{,} where \emph{S} is the projected contact
surface area,\textsuperscript{10} of the HILP-TENGs and
previously-reported tribopositive biopolymer-based TENGs are compared
in\textbf{Figure 4f} and \textbf{Table 2} . Note that other
tribopositive biopolymer-based TENGs that did not provide specific
\emph{V} max,\emph{F} , or \emph{S} values are not described here. The
HILP-TENG in this study exhibited an outstanding \selectlanguage{greek}\emph{η} \selectlanguage{english}value of
4133.33 V N\textsuperscript{-1} m\textsuperscript{-2} despite a
relatively small\emph{S} (see the red star in \textbf{Figure 4f} ),
demonstrating its great potential as an eco-friendly and sustainable
high performance energy harvester.

\textbf{Figure 5} shows the industrial potential of HILP-TENG as a
kinetic energy harvester. The output performance of the HILP-TENG was
evaluated by exploring the variations in the peak-to-peak voltage
(\emph{V} \textsubscript{ptop}) and current (\emph{I}
\textsubscript{ptop}) as varying the external load (\emph{R} ) from
10\textsuperscript{5} to 10\textsuperscript{9} \selectlanguage{greek}Ω \selectlanguage{english}(\textbf{Figures 5a}
and \textbf{5b} ). Repetitive tapping was conducted under tapping
conditions of \emph{F} = 9 N and \emph{f} = 9 Hz. As the
\emph{R}increased, the \emph{V} \textsubscript{ptop} increased up to 75
V while the \emph{I} \textsubscript{ptop} decreased to 0.3 \selectlanguage{greek}μ\selectlanguage{english}A, which
aligns with the principles outlined in Ohm's law. The resultant maximum
output power (\emph{P} ) and output power density (\emph{PS}
\textsuperscript{-1}) at\emph{R} = 2\selectlanguage{ngerman}×10\textsuperscript{7} \selectlanguage{greek}Ω, \selectlanguage{english}were 392
\selectlanguage{greek}μ\selectlanguage{english}W and 157 mW m\textsuperscript{-2}, respectively.

\textbf{Figures 5c} and \textbf{S4b} depict the current (\emph{I} )
signals obtained from HILP- and HOLP-TENGs under tapping conditions
of\emph{F} = 9 N and \emph{f} = 9 Hz, allowing to quantify of the
surface charge accumulation occurring in the HILP- and HOLP-TENGs. For
the HILP-TENG, the magnitudes of the positive and negative surface
charges during a single tapping (\emph{q} \textsubscript{pos}
and\emph{q} \textsubscript{neg}, respectively) were almost the same with
showing \emph{q} \textsubscript{pos} = 2.42\selectlanguage{ngerman}×10\textsuperscript{--8} C
and\emph{q} \textsubscript{neg} = -- 2.42×10\textsuperscript{--8} C. The
similarity in the magnitudes of \emph{q} \textsubscript{pos} and\emph{q}
\textsubscript{neg} indicates that the electrons engaged in the pressing
motion were equally involved in the subsequent release motion during
repetitive tapping of the HILP-TENG. For the HOLP-TENG, it exhibited
lower magnitudes in both \emph{q} \textsubscript{pos} and\emph{q}
\textsubscript{neg} values measured to be 1.93×10\textsuperscript{--8}
and 1.92×10\textsuperscript{--8} C, respectively.

To further investigate the effect of the surface charge of HILP and HOLP
on the surface potential distribution of the different triboelectric
pairs, we also performed COMSOL Multiphysics simulations (\textbf{Figure
5d} ). The surface charge density (\selectlanguage{greek}\emph{σ} \selectlanguage{english}\textsubscript{0}) was
calculated as:

, (1)

where \emph{q} and \emph{S} are the charge resulting from the
accumulation of current (\textbf{Figures 5c} and \textbf{S4b} ) and the
size of the TENG, respectively. The transferred charge density (\selectlanguage{greek}\emph{σ}
\selectlanguage{english}') upon the contact of the tribo-positive and -negative materials can be
expressed as a function of \selectlanguage{greek}\emph{σ}
\selectlanguage{english}\textsubscript{0}as:\textsuperscript{64}

, (2)

where \emph{d} \textsubscript{gap}, \emph{d} \textsubscript{PTFE},
and\selectlanguage{greek}\emph{ε} \selectlanguage{english}\textsubscript{PTFE} are the gap distance, thickness, and
dielectric constant of the PTFE (Teflon) film, respectively. Then, the
electric potential (\emph{V} \textsubscript{p}) generated by the TENG
can be obtained as:

, (3)

where \selectlanguage{greek}\emph{ε} \selectlanguage{english}is the vacuum permittivity
(8.85 \selectlanguage{ngerman}×\selectlanguage{english} 10\textsuperscript{-12}~F m\selectlanguage{english}\textsuperscript{-1}). As presented
in \textbf{Eq. (3)} , \emph{V} \textsubscript{p} is proportional
to\emph{q} . Based on the results obtained from COMSOL with the
presented parameters (\textbf{Table S2} ), it can be predicted that the
HILP-TENGs demonstrate a higher potential difference between the top and
bottom layers than the HOLP-TENGs, which agrees well with the
experimental results.

The utilization of a full-wave bridge rectifier circuit along with
capacitors enabled the accumulation of output voltage (\textbf{Figure
5e} ). Given tapping conditions of \emph{F} = 9 N and \emph{f} = 9 Hz,
the electrical energy could be effectively stored in various capacitors
having capacitances of 0.22, 2 and 22 \selectlanguage{greek}μ\selectlanguage{english}F. The 0.22, 2, and 22 \selectlanguage{greek}μ\selectlanguage{english}F
capacitors were charged to 0.28, 0.25 and 0.01 V, respectively. The
HILP-TENGs also demonstrated the capability of running 34 individual
LEDs by repetitive tapping (\textbf{Figure 5F} and \textbf{Movie S1} ),
showing the potential of the HILP-TENG for self-powered electronic
devices.

To demonstrate the development of TENGs without petroleum-based
materials, we also fabricated the HILP-HOLP-TENG. In contrast to the
HILP-TENG, the HILP-HOLP-TENG employed the HOLP NF mat as a
tribonegative material instead of using Teflon film (cf. \textbf{Figures
1} and \textbf{S1a} ). Then, its energy-harvesting performance was
evaluated (\textbf{Figures S4c} , \textbf{5G} , and \textbf{Movie S2} ).
For the cyclic tapping test with tapping conditions of \emph{F} = 9 N
and \emph{f} = 9 Hz, the \emph{V} \textsubscript{max} gradually
increased from 15 to 35 V (\textbf{Figure S4c} ), showing a reasonable
energy-harvesting performance even without using Teflon film.
Furthermore, we also demonstrated that when a user engages in activities
such as walking or running, the HIILP-HOLP-TENG can produce
\emph{V}signals. This remarkable capability of the HIILP-HOLP-TENG
demonstrates their potential for practical energy generation from
everyday human motion.

\textbf{3. Conclusions}

Kraft, hydrophilic, and hydrophobic lignins were prepared, followed by
an electrospinning process. The electrospun nonwoven kraft, hydrophilic,
and hydrophobic lignin nanofibers (NFs) were evaluated as tribopositive
materials of triboelectric nanogenerators (TENGs). The enhancement in
the surface energy of hydrophilic lignin NFs enabled the TENG to exhibit
an outstanding output voltage over 95 V at a tapping force and frequency
of 9 N and 9 Hz, respectively. The energy-harvesting efficiency, which
considered the output voltage with the size and applied tapping
conditions of the tested TENG, was 4133 V
N\textsuperscript{-1}m\textsuperscript{-1}, showing the highest value
among the other previously-reported biomaterial-based TENGs.
Furthermore, a TENG entirely based on both tribo-positive and -negative
lignin NFs was demonstrated, which also exhibited a reasonable
energy-harvesting performance at user-engaged activities, such as
walking and running.

\textbf{Acknowledgements}

This work was partly supported by a National Research Foundation of
Korea (NRF) grant funded by the Korea Government (MSIT)
RS-2023-00211303, Korea Institute for Advancement of Technology (KIAT)
grant funded by the Korea Government (MOTIE) (P0023521, HRD Program for
Industrial Innovation), and the Postdoctoral Research Program of
Sungkyunkwan University (2023)



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\textbf{Table 1.} \emph{M} \textsubscript{w} of kraft, hydrophilic, and
hydrophobic lignins.\selectlanguage{english}
\begin{longtable}[]{@{}ll@{}}
\toprule
\textbf{Lignin} & \emph{M}\textsubscript{\textbf{w}}
\textbf{(Da)}\tabularnewline
\midrule
\endhead
Kraft lignin & 4,826\tabularnewline
Hydrophilic lignin & 6,654\textsuperscript{a}\tabularnewline
Hydrophobic lignin & 13,430\tabularnewline
\bottomrule
\end{longtable}

\textsuperscript{a}Determined by elemental analysis and the degree of
substitution due to the insolubility of hydrophilic lignin in the mobile
phase (\emph{i.e.} , tetrahydrofuran) of GPC.

\textbf{Table 2.} Comparison of materials, methods, and performances of
the previously-reported tribopositive biomaterial-based TENGs.\selectlanguage{english}
\begin{longtable}[]{@{}lllllllllllllll@{}}
\toprule
\begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{Cases in Fig. 4f}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{Year}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{Tribopositive biomaterial}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{Tribonegative material}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{Type}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{Fabrication} \textbf{method}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{External force}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{External force}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{External force}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{Maximum} \textbf{output} \textbf{voltage} \textbf{{[}V{]}}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{Efficiency}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{Efficiency}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{Efficiency}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{Cycles}\strut
\end{minipage} & \begin{minipage}[b]{0.06\columnwidth}\raggedright\strut
\textbf{Ref.}\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\textbf{Type}\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\textbf{Applied} \textbf{force {[}N{]}}\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\textbf{Applied} \textbf{frequency} \textbf{{[}Hz{]}}\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\textbf{V N\textsuperscript{-1}}\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\textbf{V m\textsuperscript{-2}}\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\textbf{V N\textsuperscript{-1} m\textsuperscript{-2}}\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
a\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
2016\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Silk\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Polyethylene terephthalate (PET)\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Film\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Coating\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Tapping\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
40\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
50,000\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
18,000\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
\textsuperscript{65}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
b\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
2016\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Silk\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Polymide (PI)\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Film\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Electrospinning\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Tapping\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
44.5\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
10\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
26\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
0.58\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
9,290\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
209\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
25,000\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
\textsuperscript{66}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
c\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
2017\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Cu\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Bacterial nanocellulose from \emph{Acetobacter xylinum}\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Film\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Casting\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Tapping\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
51.2\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
10\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
23\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
0.45\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
9,200\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
180\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
\textsuperscript{67}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
d\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
2018\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Soy protein\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Polyimide (PI)\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Film\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Solution blowing\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Tapping\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
21.5\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
10\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
5\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
0.23\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
5,710\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
266\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
96,000\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
\textsuperscript{10}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
e\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
2019\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Starch from potato\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Polytetrafluoroethylene (Teflon)\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Film\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Casting\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Tapping\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
10\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
8\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
2\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
0.3\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
5,000\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
500\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
\textsuperscript{68}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
f\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
2019\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Pullulan\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Polyimide (PI)\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Film\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Casting\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Tapping\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
10\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
79\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
12,340\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
\textsuperscript{69}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
g\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
2020\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Carboxymethyl cellulose sodium\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Carboxymethyl chitosan\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Film\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Electrospinning\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Tapping\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
2\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
3\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
3,330\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
3000\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
\textsuperscript{70}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
h\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
2020\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Polyamide (PA)/Lignin\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Polydimethylsiloxane (PDMS), Polyethylene (PE)\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Film\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Fused deposition modeling\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Tapping\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
100\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
30\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
308\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
3.08\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
25,700\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
256\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
-\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\textsuperscript{71}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
i\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
2022\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\emph{Ulmus davidiana} var. \emph{japonica} root bark/PCL\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Polytetrafluoroethylene (Teflon)\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Film\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Electrospinning\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Tapping\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
20\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
8\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
80\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
4\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
32,000\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
1,600\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
100,000\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\textsuperscript{72}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
j\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
2022\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Sulfonated lignin/Polyvinyl alcohol (PVA)\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Polytetrafluoroethylene (Teflon)\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Film\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Electrospinning\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
Tapping\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
40\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
10\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
116\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
2.9\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
72,500\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
1,812.5\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.06\columnwidth}\raggedright\strut
\textsuperscript{11}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
2023\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Lignin/PCL\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Polytetrafluoroethylene (Teflon)\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
film\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Electrospinning\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
Tapping\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
9\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
9\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
93\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
10.33\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
37,200\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
4133.33\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.04\columnwidth}\raggedright\strut
This work\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 1.} Schematic of the process employed to transform KL
into HIL and HOL and the operating mechanism of LP-TENG as an energy
harvester.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 2.} Photos and SEM images of electrospun NF mats of (a)
KLP, (b) HILP, and (c) HOLP cases. (d) Fiber size distributions and (e)
stress-strain curves for KLP, HILP, and HOLP NF mats.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 3.} (a) \textsuperscript{1}H NMR spectra of KL, HIL, and
HOP powders. (b) FTIR spectra and (c) XRD patterns of PCL, KLP, HILP,
and HOLP NF mats. (d) TG curves and (e) water contact angles of KLP,
HILP, and HOLP NF mats. (f) Surface potential (\selectlanguage{greek}\emph{φ} \selectlanguage{english}) of HILP and
HOLP NF mats.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Figure 4.} \emph{V} signals of the KLP, HILP, and HOLP-TENGs
with (a, b) varying \emph{F} from 3 to 9 N with a fixed \emph{f} of 9 Hz
and (c, d) varying \emph{f} from 3 to 9 Hz with a fixed \emph{F} of 9 N.
(e) Cyclic tapping tests of the KLP-, HILP-, and HOLP-TENGs for 100,000
cycles. (f) \selectlanguage{greek}\emph{η} \selectlanguage{english}\emph{vs} . \emph{S} for the previously-reported
tribopositive biopolymer-based TENGs.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

\textbf{Figure 5.} (a) \emph{V} \textsubscript{ptop} and \emph{I}
\textsubscript{ptop} as a function of \emph{R} ranging from
10\textsuperscript{5} to 10\textsuperscript{9} \selectlanguage{greek}Ω \selectlanguage{english}and (b) the
corresponding \emph{P} and\emph{PS} \textsuperscript{-1}. (c) A
short-circuit \emph{I} signal of HILP-TENG for electric charge
measurement. (d) Simulation results obtained using COMSOL. (e)
Accumulated \emph{V} signals for different capacitors. (f) LED operation
using the HILP-TENG. (g) \emph{V} signals obtained from the
HILP-HOLP-TENG during walking and running.

\selectlanguage{english}
\FloatBarrier
\end{document}

