Maximizing the output power density enhancement of solid polymer electrolyte based-triboelectric nanogenerators via contact electrification-induced ionic polarization

Triboelectric nanogenerators (TENGs) have long been considered as a promising solution to meet the increasing energy demands caused by technological advancements. A notable recent approach to improve the output power of TENGs is by utilizing solid polymer electrolytes (SPEs) as the contact layers. However, the enhancement mechanism brought by the ionic species doped in SPEs was not clearly explained. Herein, polyvinyl alcohol (PVA) was subsequently doped with different ionic species to form PVA-SPEs, which were then paired with silicone rubber (SR) in contact-separation mode TENGs. The effect of different ionic species was investigated using two general classes of PVA-SPEs, i.e. cation-SPEs and anion-SPEs. Among the various PVA-SPEs, PVA-CaCl2 and PVA-NaNO3 provided the largest enhancements in electrical outputs for cation- and anion-SPEs respectively, recording averages of 95% and 90% increments. The addition of ionic species creates an internal ionic polarization within the PVA-SPEs, thus increasing their dielectric constants and ultimately the electrical outputs. Upon contact electrification (CE), the surface charges and electrostatically induced charges on the electrode generates an external electric field across the PVA-SPEs, hence inducing an internal ionic polarization field due to ionic migration. Due to the presence of the internal ionic polarization field, the difference in the highest electrons' energies (Delta E) between PVA-SPE and SR increases, which consequently enhances the electron transfer during CE. The enhancement mechanism is elucidated by a modified overlapped electron cloud (OEC) model. This model is supported by Kelvin Probe Force Microscopy (KPFM) study in which the potential differences after CE of PVASPEs are much higher compared to pristine PVA. A maximum power density of 11.3 W/m2 was recorded for PVACaCl2/SR, which is a 180% increase over that of pristine PVA/SR. This work demonstrates a reliable and straightforward method to enhance the electrical power output of TENGs. Moreover, an in-depth understanding on the enhancement mechanism by an internal ionic polarization field is provided, hence unfolding a new paradigm for polymer-based TENGs.