Origin of the contact force-dependent response of triboelectric nanogenerators

Triboelectric nanogenerators (TENGs) have attracted significant interest as the alternative source of renewable energy. Their performance is believed to depend on the contact force, but its origin is yet to be established. Herein, we show that the origin lies in the real contact area Ar, probed with novel experiments specifically designed for this purpose. The open circuit voltage Voc, short circuit current Isc and Ar for a TENG, having two nominally flat tribo-contact surfaces, were found to increase with contact force/pressure. The Ar is notably small at low pressures (0.25% at 16 kPa) that are typically experienced in wearable applications. However, it increases 328 fold to as much as 82% when it saturates beyond about 1.12 MPa pressure - achievable for impact with ocean waves. Critically, Voc and Isc saturate at the same contact pressure as Ar suggesting that electrical output follows the evolution of the Ar. Assuming that tribo-charges can only transfer across the interface at areas of real contact, it follows that an increasing Arwith contact pressure should produce a corresponding increase in the electrical output. These results underline the importance of accounting for real contact area in TENG design to boost their performance, the distinction between real and nominal contact area in tribo-charge density definition, and the possibility of using TENGs as a self-powered pressure/load sensors. Crucially, the results indicate that the large contact pressures, readily available in applications such as road-tyre contact and wave energy, alone could be enough to boost the performance, thus avoiding the need for costly surface engineering to increase Ar.