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-1m-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.