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A droplet-based electricity generator with high instantaneous power density, Xu, Zheng, etc., Wang, Zeng, Wang, Nature, 2020

 

Novelty/impact/significance:

A novel water droplet-based electricity generator with significantly high instantaneous power density, several orders of magnitude higher over conventional equivalent ones, is developed. A new operating mechanism through the ingenious device design is presented.

The device is simple in architecture, easy in fabrication, and pliable to be assembled into diverse platforms, thus showing great potential to harvest energy from a variety of water sources such as tap water, raindrops, and sea water.

Scientific question:

How to achieve a high density of electrical power generation from water, as traditional hydraulic power generators are bulky and inefficient while droplet based triboelectric nanogenerators generate very low peak power densities?

Key of how:

By an ingenious design consisting of a PTFE film plus an aluminum (Al) electrode on an indium tin oxide (ITO) substrate, the spreading of an impinged water droplet bridges the originally disconnected components into a closed-loop electrical system. This transforms the conventional interfacial effect, which limits the peak power density generation, into a bulk effect, thus substantially enhancing the instantaneous power density.

Major points:

1. Conventional impinging water droplet-based electricity generation originates from the triboelectric electric effect, a natural interfacial phenomenon: impinging droplet contact causes more negative charges on the contacting surface, thus disturbing previous electrostatic equilibrium; more positive charges need to move to the under layer for re-equilibrium, generating an electricity current between the ground and the under layer.

The present droplet-based electricity generator (DEG) has the same electricity origin but works in a different mechanism through a simple, creative design: a PTFE layer (generation and storage of charges), an Al foil as electrode, and ITO (opposite charge), which enable the droplets to spread and bridge the originally disconnected components into a closed-loop, electrical system, the bulk effect.

2. Continuous droplet imping produces a saturated/stable amount of surface charge, which serves as the basis for electricity generation for the present DEG in comparison with conventional DEG (without the Al electrode).

Individual present DEG produces open-circuit output voltage (143.5 V), short-circuit current (270.0 µA), and instantaneous peak power density (50.1 W m^-2) that are 295 times, 2600 times, and three orders of magnitude, respectively, higher than that of the conventional DEG.    400 commercial light-emitting diodes (LEDs) could be powered to instantaneously light up by the present DEG device (four droplets released from a height of 15.0 cm).

3. It is deduced that the present DEG operates in a different mechanism: the sharp increase in electricity current (originates from the contact of the spreading droplet with the Al electrode) is a result of directional and rapid transfer of charge from the ITO electrode to the Al electrode, by ruling out the effects of droplets and close examination of the time-dependent evolution of the output current with respect to droplet-Al-ITO interaction.  The present DEG also shows reversibility in electricity generation.

4. It is suggested that the electricity generation is exquisitely regulated by the interaction between the impinging droplet and the configuration of the DEG rather than just the interfacial contact electrification, by measuring the transferred charge (ITO to Al) as a function of the Weber number.

5. Analyzing the present DEG by a circuit model, the instantaneous peak output voltage occurs when the spreading droplet is in contact with the Al electrode, and the predicted peak voltage is consistent with the experimentally measured value.

6. Molecular-dynamics simulations, predicting a concerted separation of the mobile positive and negative charges towards the water/PTFE interface and water/Al interface, reveals the charging process of the two capacitors at the two interfaces at the molecular level.

7. The present DEG device also shows exceptional stability under environments with high relative humidity.  In addition to tap water, the present DEG can also harvest hydrodynamic energy from raindrops and seawater, as it is amenable to be assembled into diverse platforms for efficient energy acquisition.

 

This is an interesting and exciting work. Here is the link of fulltext: https://www.nature.com/articles/s41586-020-1985-6

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