Surfactant Self-assembly Enhances Tribopositivity of Stretchable Ionic Conductors for Wearable Energy Harvesting and Motion Sensing

Boosting stretchability and electric output is critical for high-performance wearable triboelectric nanogenerators (TENG). Herein, for the first time, we propose a new approach for tuning the composition of surface functional groups through surfactant self-assembly to improve the tribopositivity, where the assembly increases the transferred charge density and the relative permittivity of water polyurethane (WPU). Incorporating bis(trifluoromethanesulfonyl)imide (TFSI⁻) and alkali metal ions into a mixture of WPU and the surfactant forms a stretchable film that simultaneously functions as positive tribolayer and electrode, preventing the conventional detachment of tribolayer and electrode in long term usage. Further the introduction of surfactant and alkali metal ions in WPU forms a strong crosslinking network. Resultantly, the conductivity of the crosslinked film reaches 3.3 × 10⁻³ mS cm⁻¹ while the elongation at break reaches 362%, which are 4.8 × 10⁴-fold and 1.3-fold higher than pristine WPU, respectively. Moreover, the surfactant self-assembly impedes the adverse impact of the fluorine-containing groups on tribopositivity. The self-assembled and LiTFSI-doped WPU-based TENG generates an open circuit voltage of 120 V, a short circuit current of 7.5 µA and a transfer charge of 62 nC, representing a 4-fold increase compared to LiTFSI-doped WPU-based TENG. Consequently, the charge density reaches 155 µC m⁻², being the highest recorded for WPU-based and stretchable ionic conductive TENG. This work introduces a novel approach for boosting the output charge density while avoiding the adverse effect of ionic salt in solid conductors through a universal surfactant self-assembly strategy, which can be extended to other materials, such as polyvinyl alcohol. Further, the device was used to monitor and harvest the kinetic energy of human body motion.