NUS team creates device that generates electricity from air moisture

According to NUS, moisture-driven electricity generation (MEG) has received interest due to its potential in applications including wearable electronics, electronic skin sensors, and information storage devices.

Key challenges of current MEG technologies include water saturation when exposed to ambient humidity and unsatisfactory electrical performance.

To overcome these challenges, a research team led by Assistant Professor Tan Swee Ching from the Department of Materials Science and Engineering devised a novel MEG device containing two regions of different properties to continuously maintain a difference in water content across the regions to generate electricity and allow for electrical output for hundreds of hours.

Findings from the team at NUS’ College of Design and Engineering (CDE) were published in Advanced Materials in May 2022.

The MEG device consists of a thin layer of fabric (about 0.3mm thick) coated with carbon nanoparticles. In their study, the team used a commercially available fabric made of wood pulp and polyester.

The wet region of the fabric is coated with a hygroscopic ionic hydrogel. Made using sea salt, the water-absorbing gel can absorb over 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,” Asst Prof Tan said in a statement.

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. Cations are absorbed by the carbon nanoparticles, causing changes to the surface of the fabric and generating an electric field across it. These changes to the surface also give the fabric the ability to store electricity for later use.

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

Using a unique design of wet-dry regions, NUS researchers were able to maintain high water content in the wet region and low water content in the dry region, which sustains 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,” said Asst Prof Tan.

The team’s patent-pending MEG device also demonstrated high flexibility and was able to withstand stress from twisting, rolling, and bending.

The NUS team has also demonstrated the device’s scalability in generating electricity for different applications. They 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.

The scalability of the NUS invention, the convenience of obtaining commercially available raw materials as well as the low fabrication cost of about S$0.15 per metre square make the MEG device suitable for mass production, the team said.