Understanding contact electrification via direct covalent bond cleavage of polymer chains for ultrahigh electrostatic charge density

Abstract

The ancient and pervasively observed phenomenon of contact electrification (CE) is generally recognized to involve the transfer of electrons, ions, and materials between surfaces. However, compared to the mechanisms of electron and ion transfer, the understanding of how material transfer specifically contributes to this process remains less thoroughly developed. Herein a triboelectric material with an adhesive surface, namely a viscoelastic polyacrylate adhesive, was used to amplify the CE effect for better mechanism investigation to achieve a higher electrostatic charge density, which is widely accepted as the paramount factor influencing the electricity generation performance of mechanical energy harvesters that operate on the principle of electrostatic induction. A direct covalent bond cleavage mode of polymer chains was inspired and proposed based on the visible fiber drawing phenomenon during the CE process. The formation of mechaoradicals can be well explained via homolytic bond cleavage and generation of electrostatic charges by heterolytic bond cleavage within polymer chains, respectively. They were theoretically and energetically plausible based on systematic analysis of combining entangled polymer chain dynamics and the energy minimization principle with the assistance of polar substances (such as water), and experimentally demonstrated by adjusting the influential factors of relative humidity and interfacial adhesion force. A record charge density exceeding 90 nC cm⁻² was achieved using polytetrafluoroethylene to CE with adhesive surfaces, which is much higher than those generated by CE under ambient conditions in the reported literature. As a proof-of-concept demonstration, adhesive surface-enabled biomechanical energy harvesters with unique frequency-insensitive and high-performance characteristics were further developed to sustainably power a wearable tracking insole system without the anxiety of battery exhaustion and the burden of carrying additional accessories.