Catalyst-free synthesis of bio-based thermosets with self-healing ability for triboelectric nanogenerators

Triboelectric nanogenerators (TENGs) represent a smart approach to harvest energy effectively and sustainably. Fabrication of bio-based TENGs offers an opportunity to enhance the green credential of the devices even further. In this work, a catalyst-free synthesis of bio-based thermosets from epoxidized soybean oil (ESO) and succinic anhydride (SUC) was developed. Glycerol (GLY) was employed as a co-curing agent as its hydroxyl groups can react with SUC, inducing ring-opening to generate a carboxylic acid, which subsequently reacts with the epoxides to produce a β-hydroxyester linkage, facilitating catalyst-free curing. Moreover, intrinsic hydroxyls present in glycerol promote the transesterification between ester functionalities and residual hydroxyls, instigating the self-healing properties to the materials. Self-healing ability is a great benefit to TENGs as it aids in repairing the damage that might occur from the friction motions during the energy collecting. ESO:SUC:GLY thermosets with a 1:1.5:0.25 molar ratio demonstrated self-healing properties at 180 °C. The biobased thermosets exhibited excellent thermal stability (352 °C), high gel content (>90 %), and great hydrophobicity (water uptake <1.8 %, contact angles >105°). The sample achieved an electrical output voltage of 65 V and the current of 6 μA. The optimization of the crosslinker loading revealed that an increase in the anhydride content significantly affected the self-healing and TENGs performance. The sample with an ESO:SUC:GLY ratio of 1:3:0.25 showed an improved self-healing at a relatively lower temperature (150 °C) and the electrical output of 70 V with the current of 6 μA, marking a promising material for the manufacture of sustainable bio-TENGs.