Researchers in Singapore have developed a low cost rechargeable fabric that provides higher electrical output than a conventional AA battery from moisture in the air.
The team at the National University of Singapore’s (NUS) College of Design and Engineering (CDE) developed the moisture-driven electricity generation (MEG) device made of a thin layer of fabric that is about 0.3mm thick. It is built from layers of sea salt, carbon ink, and a special water-absorbing hydrogel.
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The MEG builds upon the ability of different materials to generate electricity from the interaction with moisture in the air for wearable electronics like health monitors, electronic skin sensors, and information storage devices.
Key challenges include water saturation of the device when exposed to ambient humidity and unsatisfactory electrical performance with insufficient to power electrical devices.
The device developed by Assistant Professor Tan Swee Ching from the Department of Materials Science and Engineering under CDE uses two regions of different properties to maintain a difference in water content across the regions to generate electricity and allow for electrical output for hundreds of hours.
The difference in moisture content of the wet and dry regions of the carbon-coated fabric to create an electric current. A thin layer of fabric was coated with carbon nanoparticles with one region coated with a hygroscopic ionic hydrogel made using sea salt. This 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,” said Tan.
Current 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.
“After water absorption, one piece of power-generating fabric that is 1.5 by 2 centimetres in size can provide up to 0.7 volts (V) of electricity for over 150 hours under a constant environment,” said research team member Dr Zhang Yaoxin.
The design is also scalable with three pieces of the fabric connected together and placed them into a 3D printed case 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 material is also cost effective with a fabrication cost of about S$0.15 per metre square.
The unique design of the wet-dry regions maintains a high water content in the wet region and low water content in the dry region. This 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,” he said.
The MEG device was able to withstand stress from twisting, rolling, and bending, and the team could fold the fabric into an origami crane which did not affect the overall electrical performance of the device.
“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,” shared Asst Prof Tan.
The researchers have filed a patent for the technology and are planning to explore potential commercialisation strategies for real-world applications.
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