Ultrathin, Rollable, Paper-Based Triboelectric Nanogenerator for Acoustic Energy Harvesting and Self- Powered Sound Recording

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1 Supporting Information Ultrathin, Rollable, Paper-Based Triboelectric Nanogenerator for Acoustic Energy Harvesting and Self- Powered Sound Recording Xing Fan,,,# Jun Chen,,# Jin Yang,,# Peng Bai, Zhaoling Li, and Zhong Lin Wang, * School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta. Georgia , USA. College of Chemistry and Chemical Engineering, Chongqing University, Chongqing , P. R. China. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing , P. R. China. * Address correspondence to zhong.wang@mse.gatech.edu # These authors contributed equally to this work. S1

2 1. The cycle of electricity generation process Figure S1. A schematic illustration to show the cycle of electricity generation process under external acoustic excitation. S2

3 2. Typical electrical output of a paper-based nanogenerator. Figure S2. Electrical measurement results of a paper-based nanogenerator. Open-circuit voltage (V OC ), short-circuit current (I SC ) and charge transfer (Q) of the as-fabricated device at acoustic frequency of 250 Hz with an acoustic pressure of 114 db SPL. S3

4 3. A paper-based triboelectric nanogenerator for recycling the environmental noise to generate power. Figure S3. Demonstration of a paper-based triboelectric nanogenerator anchored on a building glass window for recycling the environmental noise to generate power. S4

5 4. The fabrication process of a rolled-type paper thin triboelectric nanogenerator Figure S4. A sketched diagram to show the fabrication process of a rolled-type paper thin triboelectric nanogenerator. S5

6 5. A transparent, paper-thin TENG for harvesting sound wave energy Figure S5. A transparent, paper-thin TENG for harvesting sound wave energy or sound recording. Electrical measurement results of a transparent nanogenerator with ITO and PTFE membrane as the two contact surfaces. Inset is a photograph of an as-fabricated transparent, paper-thin TENG. S6

7 6. Theoretical analysis of the PTFE membrane vibration A thin membrane with fixed edge has more than one modal vibrations under external acoustic excitations. For different modal vibrations, both the natural frequency and the spatial deformation distribution are different. When the external acoustic frequency equals to the natural frequency of a certain modal vibration, a stable planar standing wave is formed over the membrane, which causes the localized regional resonance of the PTFE membrane. Since it is difficult to give a direct explicit expression of the resonance frequency of a planar standing wave, a simplified string standing-wave model is introduced as an analogue to interpret the frequency response of the paper-based TENG, of which the nature frequency (ω n ) can therefore be expressed explicitly by the following equation. 1,2 ω n =(n/2l)(t/µ) 1/2 (S1) Where, L is the wavelength, µ is the mass density, T is the stretching stress, and n is an integer. For different n, there is a corresponding modal vibration, of which the nature frequency is all close to integral multiples of the fundamental frequency (ω 0 ), ω 0 = (T/µ) 1/2 /2L (S2) As demonstrated in Figure S6, there are more nodal points at higher nature frequency. Because of the limitation of the PTFE membrane s natural deformability, the contact status between the PTFE membrane and the paper electrode will vary with the acoustic frequency under a same acoustic pressure. S7

8 Figure S6. PTFE membrane vibrations under various external acoustic frequencies. (a) The ANSYS software was employed to characterize the membrane vibrations under various sound frequencies. (b) An illustration to interpret the interaction of PTFE membrane with the paper electrode. S8

9 7. Theoretical analysis of the thin-sheet Helmholtz resonator: A case study for the paper based TENG with thicker multihole electrode A thin-sheet Helmholtz resonator with multi-hole absorber is indeed formed for the thicker multihole electrode. In this case, the membrane vibration is affected by both the resonance of the air in the resonator and the pressure difference across the PTFE membrane. A hybrid vibration behavior is observed on our paper based TENG device. As the thickness of the multihole electrode decreases, the frequency response varies from a single-peak type to a multi-peaks type. The former is a typical characterization of the vibration based on Helmholtz resonator, which clearly verify the existence of the thin sheet resonator, of which the resonance frequency (ω H ) could be expressed as follows. 3,4 ω H =(c/2π)(θa/vl) 1/2 (S3) where, c is the speed of sound in air, θ is the void-to-surface ratio of the multihole electrode, A is the apparent area of the multihole electrode, L is the electrode thickness plus 0.8(θA) 0.5 and V is the volume of the gap between two electrodes. The resonator helps to improve the output at the resonance frequency while narrowing the frequency response range as a tradeoff. S9

10 Figure S7. An illustration of the thin-sheet Helmholtz resonator with multi-hole absorber. S10

11 References 1. Goll, E.; Dalhoff, E. J. Modeling the Eardrum as a String with Distributed Force. Acoust. Soc. Am. 2011, 130, Kashy, E.; Johnson, D. A.; McIntyre, J. S.; Wolfe, L. Transverse Standing Waves in a String with Free ends. Am. J. Phys. 1997, 65, Han, F. S.; Seiffert, G.; Zhao, Y. Y.; Gibbs, B. Acoustic Absorption Behaviour of an Open- Celled Aluminium Foam. J. Phys. D: Appl. Phys. 2003, 36, Murray, A. R. J.; Summers, I. R.; Sambles, J. R.; Hibbins, A. P. An Acoustic Double Fishnet Using Helmholtz Resonators. J. Acoust. Soc. Am. 2014, 136, 980. S11