A nanotechnology approach to generating electricity from water evaporation

(Nanowerk Spotlight) Water evaporation is a fundamental force of nature that biological systems use to convert energy generated from the evaporation of water confined within nanoscale compartments into muscle-like mechanical work in response to changes in environmental humidity.

Man-made energy-harvesting devices using this principle are scarce and researchers are only beginning to exploit this principle for the generation of renewable energy (see: "Renewable energy from evaporating water").

In new work, nanotechnology researchers now have shown that evaporation from the surface of a variety of nanostructured carbon materials can be used to generate electricity: the evaporation driven water flow in nanoporous carbon film converts ambient thermal energy into electricity via the water molecules' interaction with the carbon material.

"We have demonstrated that porous carbon film can be utilized to spontaneously harvest and convert low-grade heat energy into electricity," Jun Zhou, a professor at Huazhong University of Science and Technology and Deputy Director, Wuhan National Laboratory for Optoelectronics, tells Nanowerk.

Jun Zhou, Wanlin Guo and their collaborators published their findings in Nature Nanotechnology ("Water-evaporation-induced electricity with nanostructured carbon materials").

The researchers fabricated their device from a sheet of carbon black and two electrodes made from multi-walled carbon nanotubes. When inserted into deionized water, an open-circuit voltage between the two electrodes is generated.

Evaporation-induced electricity in carbon black (CB). a, Schematic of device and experimental set-up for measuring evaporation-induced voltage. The device consists of a 1.0 × 2.5 cm 2 CB sheet and two 2-mm-wide MWCNT electrodes on a quartz substrate. The device was inserted into deionized water for voltage measurements. b, Typical HR-TEM image of CB (purple circle in a). Scale bar, 5 nm. c, Schematic of the water evaporating from and induced water flow in CB (purple oval in b). (© Nature Publishing Group) (click on image to enlarge)

Video 1: Fabrication process and testing of the device. (Video: Nature Publishing Group)

Video 2: Four devices in series connexion powering a liquid crystal display. (Video: Nature Publishing Group)

"In our experimental setup we have shown that water evaporation from a centimeter-sized carbon black sheet, with a thickness of a few micrometers, can generate a sustained voltage comparable to a standard AA battery," notes Zhou. "In comparison with previously reported methods to harvest energy from complicated environments, such as vibration, solar, wind, and ocean energy, the electricity generation occurs in ambient environment, converting ambient thermal energy directly into electricity."

The next stage of the team's investigation will involve two aspects: 1) developing a device with better mechanical properties and higher electricity energy output; and 2), intensive investigation of the interaction between water molecules and carbon interface, to further explore the underlying mechanism and other potentially unknown phenomena.