The first energy-harvesting, triboelectric fabric that is both waterproof and capable of converting energy from multiple sources – such as wind, rain and human movement – has been developed by researchers in Taiwan and the US. The development could lead to myriad applications in wearable technology, self-powered sensors and ambient energy harvesting.
Triboelectric energy is generated when certain materials are rubbed together. Friction causes electrons to be transferred from one material to the other – creating an electrical potential when the surfaces are separated. Familiar examples include the electrical shock one can experience after walking on a carpet or a balloon sticking to a wall after it has been rubbed on someone’s hair.
Because it converts kinetic energy to electrical energy, there is a great deal of interest in using the effect to harvest energy from both human movement and ambient motion such as wind or rainfall. A number of small-scale triboelectric “nanogenerators” have been developed and are particularly well-suited to harvesting energy from irregular, low-frequency motion.
Difficult to waterproof
Although these devices are low-cost and reliable, they do have some limitations. They tend to be designed to harvest energy from one specific type of motion, for example. They are also unable to function in wet or excessively humid conditions because the presence of water tends to inhibit the triboelectric effect. Devices can be waterproofed, but this is difficult to implement and is not long lasting.
Now, Ying-Chih Lai and colleagues at the National Chung Hsing University and the Georgia Institute of Technology have created a new nanogenerator design made out of layers of waterproof, high-triboelectric effect fabric. The material is as flexible as fabrics used in conventional clothing and its elastic features enable it to collect energy from multiple sources. As a result, the nanogenerator can harvest energy from tiny impacts – such as gusts of wind or individual raindrops – as well as from the motion of a person wearing a garment incorporating the technology.
The nanogenerator uses a “contact-separation” mode of energy gathering, whereby the triboelectric effect is the result of interactions between two active fabrics that make up the device. This design, Lai explains, provides a higher electrical output than other modes. Unlike previous fabric nanogenerators, the conducting fabric in the new design is made by weaving together silver fibres and lyocell rayon.
“Multifunctional yet nimble”
“The multifunctional yet nimble fabric nanogenerator design can not only address the long‐lasting challenge of waterproof, adaptive, deformable, and universal energy devices for locally accessible energy, but also bring a new class of wearable energy and smart fabric articles,” says Lai.
In a paper in Advanced Science, the researchers describe a variety of practical demonstrations of their new fabric, illustrating its potential for use in flags, tents, roof coverings, shoe soles, umbrellas and raincoats.
“The nanogenerators on the umbrella and raincoat can harvest water drops’ impact energy, transforming this into electricity to light up tens of light‐emitting diodes,” notes Lai. He adds that the fabric could be used to develop self-powered, illuminated rain gear to help prevent traffic accidents on rainy days. Under 125 mL/s of rainfall, the nanogenerator charged a 1 μF capacitor up to around 9 V in 5 min. Repeated washings did not diminish the harvester’s performance.
The researchers also demonstrate a rudimentary wearable, wireless interface for controlling an audio player. Wrapped around the user’s arm, the controller has icons that can be pressed to start and pause playback, change tracks and adjust the player’s volume.
“Constructing such triboelectric nanogenerators with properties including wearability, flexibility and water resistance will have paramount importance in the creation of portable, durable and autonomous electronic systems that are practically applicable for future technologies,” says Ishara Dharmasena, an engineer from the University of Surrey, who was not involved in this study.
Maxwell model optimizes motion energy harvesters
James Chen – an engineer from the State University of New York at Buffalo – agrees, noting that the new material will help to reduce the size of the largest component in current wearable electronics, which is the battery.
“The most amazing part was that [Lai and colleagues] were able to combine multiple functions with one material,” Chen adds. “That can be read as the first crack for theoreticians like myself to explore why this material is so special.”
The team is now developing commercial applications for its nanogenerator fabrics.