For the first time a piezoelectric nanogenerator that is capable of generating electrical energy from diverse sources — mechanical, acoustic and wind — has been developed. The device, fabricated by a team led by researchers from the Indian Institute of Technology (IIT) Kharagpur, has remarkable energy conversion efficiency of 62%, high output current (over 12 microampere), voltage (about 61.5 volt) and power density (over 9 mW per cubic cm). The high voltage of nanogenerator can be used to light up about 100 commercially available microwatt LEDs.
Vitamin B2 adds stability
The team led by Bhanu Bhusan Khatua from the Materials Science Centre, IIT Kharagpur, used vitamin B2 to stabilise the beta phase of the piezoelectric polymer PVDF (polyvinylidene difluoride) and thereby enhance the piezoelectric performance. The results were published in Nano Energy.
About 80% of PVDF is in alpha phase, which is not piezoelectric in nature. When vitamin B2 is added, it binds to PVDF and causes a change in alignment of the PVDF chain. “This causes the PVDF to change its phase from alpha to predominantly (over 93%) beta, which is highly piezoelectric,” says Prof. Khatua. “The nanogenerator is also completely organic and biocompatible and so can be used for both in vitro and in vivo applications.” The proportion of beta phase in PVDF and hence the output voltage and current increased with increasing amount of vitamin B2 added to it reaching a peak at 5 weight percentage, according to Sumanta Kumar Karan from IIT Kharagpur and first author of the paper. To fabricate the nanogenerator, the researchers made two thin films made by mixing vitamin B2 and PVDF and spin coating the solution. The films with electrodes attached and separated by nylon net are then taped together. The device was found to have good flexibility and durability.
High sensitivity
“The device was found to be highly sensitive to even very small external force such as touching, bending and air-flow,” says Sandip Maiti, co-author of the paper. It also showed high mechanical durability — no change in output performance was seen for about 1,80,000 cycles — and chemical stability for up to 10 weeks, making it suitable for various applications including e-health care monitoring.
When made into a thin film and placed on the wrist, the device was able to convert the biomechanical energy to electrical energy (0.15 volt). Same when it was placed on the throat and the person was speaking (0.5 volt), gargling (1.6 volt) or swallowing (0.29 volt). When pressed by the heel, the device produced about 58 volts, and about 79 volts when placed under a running car. It could convert the pressure applied by water falling from a tap into electricity. When placed on speakers, the device was able to harvest sound energy. Similarly, when placed on the lid of a table fan it could convert wind pressure into electricity.“We want to use this device to convert heartbeat to power a pace maker by using a rechargeable battery. We are also trying to explore large-scale application by placing the device on dancing floor, entrances to high throughput passages and storing the electrical energy in capacitors to run small devices,” says Prof. Khatua.
“We are highly optimistic of commercialising the technology in the near future.”
