A team of researchers from the National University of Singapore’s College of Design and Engineering has developed a new moisture-driven electricity generation (MEG) device made using a thin layer of fabric – about 0.3 millimetres in thickness – sea salt, carbon ink and a special water-absorbing gel.
MEG devices rely upon the ability of different materials to generate electricity from the interaction with moisture in the air. They have been receiving growing interest due to their potential use in a wide range of applications, including self-powered devices such as wearable electronics and information storage devices.
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Among the key challenges for current MEG technologies are water saturation of the device when exposed to ambient humidity and unsatisfactory electrical performance – in particular, the inability to produce sufficient energy to power electrical devices or to sustain output for long enough to be viable.
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 that contains 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.
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The NUS team’s MEG device consists of a thin layer of a commercially available fabric made of wood pulp and polyester that has been coated with carbon nanoparticles. One region of the fabric, known as the wet region, is coated with a hygroscopic ionic hydrogel, used to harvest moisture from the air. Made using sea salt, the special water-absorbing gel can absorb more than six times its original weight.
‘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.
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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 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 and 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,’ Tan explained.
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The team’s device also possesses high flexibility, capable of withstand stress from twisting, rolling and bending – characteristics demonstrated by the researchers by folding the fabric into an origami crane, which didn’t affect the device’s overall electrical performance.
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 energy source for powering mobile electronics directly through ambient humidity.
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‘After water absorption, one piece of power-generating fabric that is 1.5 by two centimetres in size can provide up to 0.7 volts of electricity for more than 150 hours under a constant environment,’ said another member of the research team, Zhang Yaoxin.
The NUS team has also successfully demonstrated the scalability of its new device, connecting three pieces of the power-generating fabric together and placing them into a 3D-printed case the size of a standard AA battery. The voltage of the assembled device reached as high as 1.96 volts, enough to power some small electronic devices.
The scalability of the NUS invention, the convenience of obtaining commercially available raw materials and the low fabrication cost of about nine pence per metre 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 commercialisation,’ said Tan.
The researchers have filed a patent for the technology and are planning to explore potential commercialisation strategies for real-world applications.
The research has been published in Advanced Materials.