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 (M w) lignin is challenging due to its
insufficient viscoelasticity, which is essential for self-sustainable
electrospinning (Table 1 ).15-17 Therefore,
polycaprolactone (PCL), which has a relatively high molecular weight
(M 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. 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. Sections S1.2 and 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
(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 (Figure S1a ). The
illustration at the bottom of 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).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.19