Unprecedented Triboelectric Effect of Lignin on Enhancing the Electrical Outputs of Natural-Rubber-Based Triboelectric Nanogenerators (TENGs)

Because of a lack of electron donor/acceptor groups, TENGs fabricated from natural rubber (NR) usually produce low electrical outputs. The present study demonstrated that the electrical outputs of NR TENGs could be enhanced by the presence of lignin. Lignin with an unanticipated triboelectric property became a triboelectric active material upon subjecting to ultrasound in a basic solution. Physically, lignin irradiated with ultrasound would undergo particle size reduction from tens of micrometers down to a micrometer in diameter, leading to an increase of interfacial area for triboelectric charge generation. Accompanying the size reduction was a partial depolymerization of lignin arising from the chemical bond cleavage via the nucleophilic attack of hydroxide ions. This event caused an increase of polar functional groups on lignin that benefited the charge generation and transfer. TENGs from the NR/lignin composite generated a power output density up to 500% larger than that produced from NR TENG. However, NR/lignin TENGs suffered from the aggregation of lignin leading to a compromise of the electrical outputs. Therefore, NR was grafted with polyacrylamide (NR-g-PAM) to inhibit lignin aggregation as well as to promote an even distribution of lignin particles in the NR matrix. As a result, NR-g-PAM/lignin TENG was capable of enhancing the power output density to 260 and 1300% compared with that of NR/lignin and NR TENGs, respectively