Effect of humidity on tribological properties and electrification performance of sliding-mode triboelectric nanogenerator

For vertical contact-separation-mode triboelectric nanogenerator (TENG), the hydrophobic surface is often used to improve the electrical output under high humidity condition. However, for sliding-mode TENG, the triboelectric effect is accompanied with the friction and wear of material, and whether the hydrophobic surface is beneficial to the durability as well as the stability of electrical output has been really ambiguous. In the paper, the pillar arrays with different pitches are fabricated through lithography, deep reactive ion etching and replication techniques to get the hydrophobic surface with different static contact angle. Based on the test platform with humidity control system, the effects of pillar pitch and normal force on the coefficient of friction, mass loss, and open-circuit voltage are investigated experimentally at different relative humidity condition. The results show that the effect of humidity on electrical output for sliding-mode TENG is different from that for vertical contact-separation-mode TENG. For a certain patterned surface of sliding-mode TENG, the coefficient of friction, short-circuit current and open-circuit voltage increase with the increasing relative humidity at first and diminish afterwards. The maximum values of them are achieved at 50% relative humidity. The mass loss diminishes with the increment of relative humidity. For surfaces with different pillar pitch, the coefficient of friction and open-circuit voltage decrease with the increasing pillar pitch, while the mass loss increases. Moreover, the decrease in pillar pitch, in which the contact angle of surface is increased, can improve the durability as well as the stability of output of sliding-mode TENG. In addition, although the greater normal force can enhance the stability of open-circuit voltage for sliding-mode TENG under humidity condition, the contact angle of surface will be reduced more obviously in the process of wear, speeding up the failure of hydrophobic surface. This study reveals the effect of hydrophobic surface on the durability as well as the stability of output for sliding-mode TENG and provides reference for texture design at different humidity conditions.