Self-charging device built of sea salt produces electricity from air moisture

This new invention is highly scalable since its raw materials are commercially available and easy to access.
Loukia Papadopoulos
Asst Prof Tan Swee Ching (centre), together with Dr Zhang Yaoxin (left) and Mr Qu Hao (right), developed a self-charging fabric that generates electricity from air moisture.
Asst Prof Tan Swee Ching (centre), together with Dr Zhang Yaoxin (left) and Mr Qu Hao (right), developed a self-charging fabric that generates electricity from air moisture.

NUS 

A team of researchers from the National University of Singapore’s (NUS) College of Design and Engineering (CDE) has developed a self-charging electricity generation (MEG) device that generates electricity from air moisture, according to a press release by the institution.

Made of readily accessible sea salt

The novel device is comprised of a 0.3-millimeter thick layer of fabric and contains only sea salt, carbon ink, and a special water-absorbing gel.

The new MEG technology is said to overcome the problems often associated with this kind of machines, including water saturation of the device when exposed to ambient humidity leading to unsatisfactory electrical performance.

To achieve this, a research team led by Assistant Professor Tan Swee Ching from the Department of Materials Science and Engineering under CDE engineered a novel MEG device that is equipped with two regions of different properties to perpetually maintain a difference in water content in order to allow for electrical output for hundreds of hours.

The device includes a wet region coated with a special substance made using sea salt. This special water-absorbing gel can absorb more than six times its original weight, and it is used to harvest moisture from the air. This region is complemented by a dry region which creates the right conditions for energy to be produced.

“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,” explained Tan.

The way the MEG device works is that electricity is generated when the ions of the sea salt are separated as water is absorbed in the wet region, 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 use later.

By combining wet and dry regions, the researchers were able to maintain high water content in the wet region and low water content in the dry region that could sustain electrical output even when the wet region is saturated with water. They found the device to work even after being left in an open humid environment for 30 days.

Electric performance improved

“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,” added Tan.

The new NUS invention is highly scalable since its raw materials are commercially available and easy to access. It also comes with a very low fabrication cost of about US$0.15 per meter square. All this means the MEG device is 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 commercialisation," concluded Tan.

The results were published in the print version of scientific journal Advanced Materials.

Abstract:

The interactions between moisture and materials give rise to the possibility of moisture-driven energy generation (MEG). Current MEG materials and devices only establish this interaction during water sorption in specific configurations, and conversion is eventually ceased by saturated water uptake. This paper reports an asymmetric hygroscopic structure (AHS) that simultaneously achieves energy harvesting and storage from moisture absorption. The AHS is constructed by the asymmetric deposition of a hygroscopic ionic hydrogel over a layer of functionalized carbon. Water absorbed from the air creates wet-dry asymmetry across the AHS and hence an in-plane electric field. The asymmetry can be perpetually maintained even after saturated water absorption. The absorbed water triggers the spontaneous development of an electrical double layer (EDL) over the carbon surface, which is termed a hygro-ionic process, accounting for the capacitive properties of the AHS. A peak power density of 70 µW cm-3 was realized after geometry optimization. The AHS shows the ability to be recharged either by itself owing to a self-regeneration effect or via external electrical means, which allows it to serve as an energy storage device. In addition to insights into moisture-material interaction, AHSs further shows potential for electronics powering in assembled devices.