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