Protein-based contact electrification and its uses for mechanical energy harvesting and humidity detecting

Highlights: • The first protein-based contact electrification has been studied and applied to harvest mechanical energy. • The results show that the use of proteincan provide additional(mobile ions on the triboelectric layers. • By blocking the generation of mobile ions, the protein-based nanogenerator can become a self-powered humidity sensor with good stability and durability. • This work will inspire the development of self-powered protein sensors in the near future. In recent years, the use of triboelectric nanogenerators (TENGs) has become an effective technique to harvest mechanical energy based on contact electrification and the electrostatic effect. However, the electric output of TENGs decreases in environments with high relative humidity (RH), which restricts their application. The reason for this phenomenon is that charge transfer in most TENGs occurs through an electron pathway. By using gelatin/glycerol and polytetrafluoroethylene (PTFE) as triboelectric layers to construct a biocompatible TENG, we found that the electric output of the TENG can be maintained or is even enhanced at higher RH. The hydration of amino groups in gelatin resulted in additional charges (mobile ions) on the triboelectric layers during the contact electrification process, consequently increasing the electric output of the TENG. The electric output generated from the TENG increased 2-fold when the RH was increased from 20% to 60%. By using genipin to react with the amino groups in gelatin, the formation of mobile ions was blocked and the measured charge density on the triboelectric layers decreased as the RH gradually increased. We also demonstrated that the change of electric output generated from the TENG could be applied as a tool to detect RH, exhibiting the superior stability and durability of the TENG. This study also leads to a fundamental understanding of the mechanism of contact electrification when using protein as the triboelectric layer.