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Changsheng Wu1 Jie Wang1 Yunlong Zi1 Zhong Lin Wang1

1, Georgia Institute of Technology, Atlanta, Georgia, United States

With its light weight, low cost and high efficiency even at low operation frequency, the triboelectric nanogenerator (TENG) is considered a potential solution for self-powered sensor networks and large-scale renewable blue energy. As an energy harvester, its output power density and efficiency are dictated by the triboelectric charge density. Plenty of efforts have been devoted to improving the power output by maximizing the surface charge density. Here we first demonstrated that with the improved soft-contact and fragmental structure, the triboelectric charge density can be increased from 50 to 120 μC/m2 in air when compared to a conventional TENG with only hard contact. However, due to high-voltage air breakdown, most of the enhanced surface charge density brought by material/surface optimization or external ion injection is not retainable or usable for electricity generation during continuous operation of contact-separation mode TENGs. We experimentally validated the existence of the air breakdown effect in a contact-separation mode TENG with a low threshold surface charge density of ~40–50 μC/m2 under the high impedance external load, and conducted the theoretical study of the maximized effective energy output as limited by air breakdown under different air pressures and gas compositions. Subsequently, we applied high vacuum (~10-6 torr) to eliminate the limitation of air breakdown, which boosted the charge density to 660 μC/m2. With the coupling of surface polarization from triboelectrification and hysteretic dielectric polarization from a ferroelectric material, the charge density can further jump to 1003 μC/m2, which elevates the maximum output power density of a conventional TENG from 0.75 to 50 W/m2 even at a low-motion frequency of about 2 Hz, the normal frequency of human walking and ocean waves. These findings open more possibilities for TENGs both as highly efficient mechanical energy harvesters for large-scale energy sources such ocean waves, and as self-powered modules integratable with devices beyond wearable electronics and sensors. Our findings may also give new insights into long-lasting debates over the mechanism of triboelectrification and its kinetics.
Refs:
[1] J. Wang, C. Wu (co-first author), Y. Dai, Z.L. Wang, et al. Achieving ultrahigh triboelectric charge density for efficient energy harvesting. Nat. Commun. 8, 88 (2017)
[2] Y. Zi, C. Wu (co-first author), W. Ding, Z.L. Wang, Maximized effective energy output of contact-separation-triggered triboelectric nanogenerators as limited by air breakdown, Adv. Funct. Mater. 27, 1700049 (2017)

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