Editorial Feature

Lightweight, Paper-Based Nanogenerators to Store Energy from Body Movements

The need for a sustainable mobile energy source has been increasing with the rapid development of mobile and portable electronics1. The most commonly used approach for powering these types of electronics includes batteries and capacitors, both of which have a limited life time, which limits its use as a sustainable energy source for powering electronics1.

Nanogenerators, which utilize a technology that converts mechanical and/or thermal energy generated by a small-scale physical change into electricity, has been proposed as an alternative sustainable source of energy1.

Unlike traditional generators, nanogenerators function based on either of the three traditional approaches that can include piezoelectric, triboelectric and pyroelectric effects. While piezoelectric and triboelectric nanogenerators (TENG) convert mechanical energy into electricity, pyroelectric nanogenerators convert thermal energy from a time-dependent temperature fluctuation into electricity.

Due to the high energy-conversion efficiency and the ability to generate a high output, TENG generators have attracted a substantial amount of attention from researchers1. The fundamental working mechanism of TENG generators involves the coupling of triboelectrification and electrostatic induction.

Triboelectrification is a process of the creation of triboelectric charges that are produced as a result of the physical contact between at least one pair of triboelectric layers, each of which are made up of different materials having different electron affinities1.

When the balance between the electrostatic charge distribution on the electrodes is disturbed by the relative motion of the two layers created by the applied external mechanical force, a potential difference is created between the electrodes which allows for the flow of free electrons to run through external circuits. These free electrons move back to their original electrostatic equilibrium once the triboelectric layers move back to their native position1.

When TENG generators encounter periodical mechanical motions, such as human walking, vibration or ocean waves, a pulsed alternating current (AC) output, which has a variable frequency and irregular amplitude, is supplied to the external circuit. An energy storage device is therefore required to store this pulsed AC energy that is harvested by the nanogenerators, and to also serve as a regulated and manageable output source1.

This led to the development of self charging power units (SCPU), where the energy storage unit is charged by the nanogenerators through a full-wave bridge rectifier1. However, the saturation voltage, or the highest voltage achieved, is lower in the energy storage unit compared to the open-circuit voltage of the TENG, which results in a low energy conversion efficiency of the unit1.

Most traditional TENG’s are heavy due to their high acrylic content, while also requiring several hours to charge small electronic devices such as watches, hearing aids and heart monitors2.

Researchers from the Beijing Institute of Nanoenergy and Nanosystems, in collaboration with Georgia Institute of Technology and National Taiwan University of Science and Technology’s Department of Chemical Engineering, have recently developed an ultra lightweight cut paper-based SCPU (PC-SCPU) for self-powered portable electronic devices.

The PC-SCPU developed by Zhong Lin Wang’s team has a paper-based TENG, which is combined with a super capacitor (SC) in order for it to simultaneously harvest energy from body movements and store the generated energy2.

This device is comprised of a substrate and an assembled cut paper architecture that forms its ultralight rhombic shape2. The four outer sides of the device are coated with gold, graphite-coated sand paper, which makes up the energy-storing supercapacitor element, while the inner surface is made of paper and coated in gold and fluorinated ethylene propylene film to serve as the TENG element2.

Pressing and releasing the PC-SCPU over for a few minutes was found to generate electricity that was enough to charge the device to 1 volt, which is sufficient to power a remote control, temperature sensor or wearables, such as a watch2,3.

The PC-SCPU developed by Wang’s team was also found to have a mass/volume charge output of 82 nC g-1/75 nC cm-3 while traditional acrylic-based TENGs will have a mass/volume charge output of 5.7 nC g-1/5.8 nC cm-3.3 This paper-based ultra lightweight and portable SCPU could potentially offer several practical and medical applications in the near future3.

References

  1. Zi, Yunlong, Jie Wang, Sihong Wang, Shengming Li, Zhen Wen, Hengyu Guo, and Zhong Lin Wang. "Effective Energy Storage from a Triboelectric Nanogenerator." Nature News. Nature Publishing Group, 11 Mar. 2016. Web. https://www.nature.com/articles/ncomms10987.
  2. "Art of Paper-cutting Inspires Self-charging Paper Device." ScienceDaily. ScienceDaily, 12 Apr. 2017. Web. https://www.sciencedaily.com/releases/2017/04/170412091118.htm.
  3. Hengyu Guo, Min-Hsin Yeh, Yunlong Zi, Zhen Wen, Jie Chen, Guanlin Liu, Chenguo Hu, and Zhong Lin Wang. Ultralight Cut-Paper-Based Self-Charging Power Unit for Self-Powered Portable Electronic and Medical Systems. ACS Nano.
  4. Image Credit: Shutterstock.com/forance

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