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.