Researchers have created a self-charging, ultra-thin device that generates electricity from air moisture.

Imagine being able to generate electricity by harnessing moisture in the air around you with just everyday items like sea salt and a piece of fabric, or even powering everyday electronics with a non-toxic battery that is as thin as paper.

Researchers developed the new moisture-driven electricity generation (MEG) device made of a thin layer of fabric—about 0.3 millimeters (about 0.0118 inches) in thickness—sea salt, carbon ink, and a special water-absorbing gel.

The concept of MEG devices is built upon the ability of different materials to generate electricity from the interaction with moisture in the air. This area has been receiving growing interest due to its potential for a wide range of real-world applications, including self-powered devices such as wearable electronics like health monitors, electronic skin sensors, and information storage devices.

Key challenges of current MEG technologies include water saturation of the device when exposed to ambient humidity and unsatisfactory electrical performance. Thus, the electricity generated by conventional MEG devices is insufficient to power electrical devices and is also not sustainable.

To overcome these challenges, a research team led by  Tan Swee Ching, assistant professor from the materials science and engineering department at the National University of Singapore’s College of Design and Engineering (CDE), devised a new MEG device containing two regions of different properties to perpetually maintain a difference in water content across the regions to generate electricity and allow for electrical output for hundreds of hours.

A paper on the work appears in the journal Advanced Materials.

The new moisture-driven electricity generation device capitalizes on the difference in moisture content of the wet and dry regions of the carbon-coated fabric to create an electric current. Sea salt is used as a moisture absorbent for the wet region.

The researchers’ MEG device consists of a thin layer of fabric which was coated with carbon nanoparticles. In their study, the team used a commercially available fabric made of wood pulp and polyester.

One region of the fabric is coated with a hygroscopic ionic hydrogel, and this region is known as the wet region. Made using sea salt, the special water-absorbing gel can absorb more than six times its original weight, and it is used to harvest moisture from the air.

“Sea salt was chosen as the water-absorbing compound due to its non-toxic properties and its potential to provide a sustainable option for desalination plants to dispose of the generated sea salt and brine,” says Tan.

The other end of the fabric is the dry region which does not contain a hygroscopic ionic hydrogel layer. This is to ensure that this region is kept dry and water is confined to the wet region.

Once the MEG device is assembled, electricity is generated when the ions of sea salt are separated as water is absorbed in the wet region. Free ions with a positive charge (cations) are absorbed by the carbon nanoparticles which are negatively charged. This causes changes to the surface of the fabric, generating an electric field across it. These changes to the surface also give the fabric the ability to store electricity for use later.

Using a unique design of wet-dry regions, the researchers were able to maintain high water content in the wet region and low water content in the dry region. This will sustain electrical output even when the wet region is saturated with water. After being left in an open humid environment for 30 days, water was still maintained in the wet region demonstrating the effectiveness of the device in sustaining electrical output.

“With this unique asymmetric structure, the electric performance of our MEG device is significantly improved in comparison with prior MEG technologies, thus making it possible to power many common electronic devices, such as health monitors and wearable electronics,” explains Tan.

The team’s MEG device also demonstrated high flexibility and was able to withstand stress from twisting, rolling, and bending. Interestingly, to show its outstanding flexibility, the researchers folded the fabric into an origami crane which did not affect the overall electrical performance of the device.

The MEG device has immediate applications due to its ease of scalability and commercially available raw materials. One of the most immediate applications is for use as a portable power source for mobile powering electronics directly by ambient humidity.

“After water absorption, one piece of power-generating fabric that is 1.5 by 2 centimeters in size can provide up to 0.7 volts (V) of electricity for over 150 hours under a constant environment,” says research team member Zhang Yaoxin.

The researchers have also successfully demonstrated the scalability of the new device in generating electricity for different applications. The team connected three pieces of the power-generating fabric together and placed them into a 3D printed case that was the size of a standard AA battery. The voltage of the assembled device was tested to reach as high as 1.96V—higher than a commercial AA battery of about 1.5V—which is enough to power small electronic devices such as an alarm clock.

The scalability of the invention, the convenience of obtaining commercially available raw materials as well as the low fabrication cost of about 0.15 Singapore dollars (0.11 USD) per meter square make the MEG device suitable for mass production.

“Our device shows excellent scalability at a low fabrication cost. Compared to other MEG structures and devices, our invention is simpler and easier for scaling-up integrations and connections. We believe it holds vast promise for commercialization,” says Tan.

The researchers have filed a patent for the technology and are planning to explore potential commercialization strategies for real-world applications.

Source: National University of Singapore

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Tiny fibers in fabric could turn sun’s heat into energy

Invisibly small carbon nanotubes aligned as fibers and sewn into fabrics become a thermoelectric generator that can turn heat from the sun or other sources into energy, researchers report.

The researchers worked to make custom nanotube fibers and test their potential for large-scale applications.

Their small-scale experiments led to a fiber-enhanced, flexible cotton fabric that turned heat into enough energy to power an LED. With further development, the researchers say such materials could become building blocks for fiber and textile electronics and energy harvesting.

The same nanotube fibers could also be used as heat sinks to actively cool sensitive electronics with high efficiency.

Heat to energy

The effect seems simple: If one side of a thermoelectric material is hotter than the other, it produces energy. The heat can come from the sun or other devices like the hotplates used in the fabric experiment. Conversely, adding energy can prompt the material to cool the hotter side.

Until now, no macroscopic assemblies of nanomaterials have displayed the necessary “giant power factor,” about 14 milliwatts per meter kelvin squared, that the researchers measured in carbon nanotube fibers.

“The power factor tells you how much power density you can get out of a material upon certain temperature difference and temperature gradient,” says lead author Natsumi Komatsu, a graduate student at Rice University.

She notes a material’s power factor is a combined effect from its electrical conductivity and what’s known as the Seebeck coefficient, a measure of its ability to translate thermal differences into electricity.

“The ultrahigh electrical conductivity of this fiber was one of the key attributes,” Komatsu says.

The source of this superpower also relates to tuning the nanotubes’ inherent Fermi energy, a property that determines electrochemical potential. The researchers were able to control the Fermi energy by chemically doping the nanotubes made into fibers by the Rice lab of coauthor and chemical and biomolecular engineer Matteo Pasquali, allowing them to tune the fibers’ electronic properties.

Don’t waste carbon, fix it

While the fibers they tested were cut into centimeter lengths, Komatsu says there’s no reason devices can’t make use of the excellent nanotube fibers from the Pasquali lab that are spooled in continuous lengths.

“No matter where you measure them, they have the same very high electrical conductivity,” she says. “The piece I measured was small only because my setup isn’t capable of measuring 50 meters of fiber.”

“Carbon nanotube fibers have been on a steady growth path and are proving advantageous in more and more applications,” Pasquali says. “Rather than wasting carbon by burning it into carbon dioxide, we can fix it as useful materials that have further environmental benefits in electricity generation and transportation.”

Whether the new research leads to a solar panel you can throw in the washing machine remains to be seen, but physicist Junichiro Kono, a professor in engineering, a professor of electrical and computer engineering, of physics and astronomy, and of materials science and nanoengineering at Rice, agrees the technology has great and varied potential.

“Nanotubes have been around for 30 years, and scientifically, a lot is known,” he says. “But in order to make real-world devices, we need macroscopically ordered or crystalline assemblies. Those are the types of nanotube samples that Matteo’s group and my group can make, and there are many, many possibilities for applications.”

The paper appears in Nature Communications. Additional coauthors are from Tokyo Metropolitan University and Rice.

The Department of Energy Basic Energy Science program, the National Science Foundation, the Robert A. Welch Foundation, the Japan Society for the Promotion of Science, the US Air Force, and the Department of Defense supported the research.

Source: Rice University

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