First-principles investigations on the contact electrification mechanism between metal and amorphous polymers for triboelectric nanogenerators

Chemical modification on contact pair materials is an important means to enhance the output of triboelectric nanogenerators (TENGs) as the electrification performance of contact materials depends critically on their molecular structures. However, limited understanding on the contact electrification mechanism between metal and amorphous polymer leads to the lack of theoretical basis on chemical modification ideas. On the basis of the typical amorphous polymers used in TENGs, polyethylene terephthalate (PET) and polyimide (Kapton), the method of first-principles research on amorphous polymers contact electrification is explored and the charge transfer mechanism between metal and amorphous polymer is studied. It is found that the electron acceptor in PET and Kapton is always the lowest unoccupied molecular orbital (LUMO) under different contact configurations and interface distances, which provides a clear direction for chemical modification on triboelectric materials. On the other hand, the major functional groups gaining electrons in PET and Kapton are found to be the carboxyl (-COOH) and the imide (-C(=O)NC(=O)-), respectively. Moreover, the contributions of double-bonded oxygen atoms in the carboxyl of PET and the imide of Kapton to contact electrification are significantly greater than that of single-bonded oxygen atoms in the carboxyl of PET and the ether of Kapton, respectively. This indicates that the contributions made by the same atomic species in different molecular groups to contact electrification are significantly different, which provides novel ideas for the chemical modification on triboelectric materials.