Nanogenerator

Energy harvesting from the environment has a very long history, dating back to early devices such as watermills, windmills and later hydroelectric plants.

[9] Very early on it was recognized that these could use energy sources such as from walking in shoes,[10] and could have important medical applications,[4] be used for in vivo MEMS devices[11] or be used to power wearable computing.

It is generally used to indicate kinetic energy harvesting devices utilizing nano-scaled piezoelectric material, like in thin-film bulk acoustic resonators.

[20][21] The working principle of the nanogenerator will be explained in two different cases: the force exerted perpendicular to and parallel to the axis of the nanowire.

As the force is removed, the piezoelectric effect diminishes, and the electrons will be flowing back to the top in order to neutralize the positive potential at the tip.

[24] Depending on the configuration of the piezoelectric nanostructure, the nanogenerator can be categorized into 3 types: VING, LING, and NEG.

Since the counter electrode is not in full contact with the tips of the piezoelectric nanowire, its motion in-plane or out-of-plane caused by the external vibration induces the deformation of the piezoelectric nanostructure, leading to the generation of the electrical potential distribution inside each individual nanowire.

Zhong Lin Wang's group has generated counter electrodes composed of ZnO nanorods.

Sang-Woo Kim's group at Sungkyunkwan University (SKKU) and Jae-Young Choi's group at Samsung Advanced Institute of Technology (SAIT) introduced a bowl-shaped transparent counter electrode by combining anodized aluminum and electroplating technology.

NEG was introduced by Momeni et al.[28] A fabric-like geometrical configuration has been suggested where a piezoelectric nanowire is grown vertically on the two microfibers in their radial direction, and they are twined to form a nanogenerator.

[29] One of the microfibers is coated with the metal to form a Schottky contact, serving as the counter electrode for VINGs.

[31] Zhong Lin Wang of the Georgia Institute of Technology introduced p-type ZnO nanowires.

Liwei Lin of the University of California, Berkeley, has suggested that PVDF can also be applied to form a nanogenerator.

Zhong Lin Wang's group has also generated an alternating current voltage of up to 100 mV from the flexible SWG attached to a device for running hamster.

Ramakrishna Podila's group at Clemson University also demonstrated the first truly wireless triboelectric nanogenerators,[45] which were able to charge energy storage devices (e.g., batteries and capacitors) without the need for any external amplification or boosters.

The periodic change in the potential difference induced by the cycled separation and re-contact of the opposite triboelectric charges on the inner surfaces of the two sheets.

Subsequently, when the two sheets are pressed towards each other again, the triboelectric-charge-induced potential difference will begin to decrease to zero, so that the transferred charges will flow back through the external load to generate another current pulse in the opposite direction.

[49] With triboelectrification from sliding, a periodic change in the contact area between two surfaces leads to a lateral separation of the charge centers, which creates a voltage driving the flow of electrons in the external load.

The mechanism of in-plane charge separation can work in either one-directional sliding between two plates[50] or in rotation mode.

In 2013, Zhonglin Wang's group reported a rotary triboelectric nanogenerator for harvesting wind energy.

Researchers have designed an all-weather droplet-based triboelectric nanogenerator that relies on the contact electrification effect between liquid and solid to generate electricity.

[59] The term "self-powered sensors" can refer to a system that powers all the electronics responsible for measuring detectable movement.

[64] Due to thermal fluctuations at room temperature, the electric dipoles will randomly oscillate within a degree from their respective aligning axes.

If the nanogenerator is cooled, the electric dipoles oscillate within a smaller degree of spread angle due to the lower thermal activity.

The thermal deformation can induce a piezoelectric potential difference across the material, which can drive the electrons to flow in the external circuit.

Working principle of nanogenerator where an individual nanowire is subjected to the force exerted perpendicular to the growing direction of nanowire. (a) An AFT tip is swept through the tip of the nanowire. Only negatively charged portion will allow the current to flow through the interface. (b) The nanowire is integrated with the counter electrode with AFT tip-like grating. As of (a), the electrons are transported from the compressed portion of nanowire to the counter electrode because of Schottky contact.
Working principle of nanogenerator where an individual nanowire is subjected to the force exerted parallel to the growing direction of nanowire
A schematic view of a typical Vertical nanowire Integrated Nanogenerator, (a) with full contact, and (b) with partial contact.
A schematic view of a typical Lateral nanowire Integrated Nanogenerator
A schematic view of a typical Nanocomposite Electrical Generator
A summary on the progress made in the output power density of triboelectric nanogenerators within 12 months.
Vertical contact-separation mode of a triboelectric nanogenerator
Lateral sliding mode of triboelectric nanogenerator
Single-electrode mode of triboelectric nanogenerator
An encoder circuit with a graphic of a belt pulley system
Belt-pulley system powers the encoder circuit by converting friction into electrical energy .
The mechanism of the pyroelectric nanogenerator based on a composite structure of pyroelectric nanowries. It is on negative electric dipoles under (a) room temperature, (b) heated, and (c) cooled conditions. The angles marked in the diagrams represent the degrees to which the dipole would oscillate as driven by statistical thermal fluctuations.