1 Hongseok Jo, Dogun Park, and Minkyeong Joo
contributed equally to this work.
*Corresponding authors: esan@skku.edu, kwanghokim@kist.re.kr,
khkim83@skku.edu
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.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.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.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.4Furthermore, the aromatic rings form π–π conjugated system that can
facilitate producing various kinds of precursors.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.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.7-9 An et al. reported eco-friendly
triboelectric nanogenerators (eco-TENGs) using lignin-based nanofibers
(NFs) with a solution-blowing technique.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 < 1 V was observed at the energy-harvesting tests.
Similarly, Wang et al. recently reported eco-TENGs composed of
lignin-based electro spun NFs and a Teflon film.11They could achieve a high output voltage of > 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.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).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
(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 > 25
V compared to the values observed from the work of An et al .
despite similar test conditions.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 et al. 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.