Producing wearable electronics that uses a portable nanogenerator which generates electric power when pressure or twist is applied got a shot in the arm, thanks to research carried out by Pune researchers. The nanogenerator, which was fabricated by them, produced 14 volts when thumb pressure was applied. The results were published recently in the journal Advanced Materials & Interfaces.
To demonstrate the potential of the nanogenerator to power small electronic devices, pressure equivalent to thumb pressure was continuously exerted on the nanogenerator for 20 minutes by using a vibration producing motor. About 28 micro watt per square cm power and 14 volt that was generated was stored in a capacitor and used for charging a mobile phone.
Currently, there is considerable research emphasis to develop flexible or wearable devices. Such devices should be portable, lightweight, shock resistant, and inexpensive. And the devices should ideally be powered by harvesting easily available mechanical or vibration energy, making battery or related wiring redundant. Piezoelectric materials, which can generate electrical power locally through stress or flexing, are a great proposition in this regard.
To produce the nanogenerator, researchers from Pune’s Indian Institute of Science Education and Research (IISER) and the National Chemical Laboratory electrospun a piezoelectric polymer [P(VDF-TrFE)] directly onto a flexible, conducting carbon cloth. The carbon cloth was produced by the researchers by heating a piece of cotton cloth at 800 degree C for several hours in an inert atmosphere.
To improve the piezovoltage of the polymer fibres, the researchers coated the fibres with a stronger, inorganic ferroelectric material (BaTiO3) paste. “The nanoparticles from the coating helps fill the gaps between the polymer nanofibres and increase the piezoelectric property,” says Prof. Satishchandra Ogale from the Department of Physics and Centre for Energy Science, IISER Pune and the corresponding author of the paper. In addition, the ferroelectric material was also incorporated into the polymer to further enhance the piezoelectric property. This was done right when the polymer was electrospun.
The amount of BaTiO3 fibre incorporated into the polymer had to be optimised at 5 per cent. When the fibre density was less inside the polymer the density of interfaces (where the separation of positive and negative charges takes place) formed between the fibre and the polymer was also less. But flexibility was reduced when too much was added and it also led to more internal charging resulting in electrical short.
The coated polymer was covered by another piece of flexible carbon cloth before the device was sealed. The carbon cloth on either side of the device acted as two electrodes. The carbon cloth too contributes to the enhanced piezovoltage generated by the nanogenerator through its peculiar morphology as a substrate.
“The cloth has a surface microstructure which produces good bonding between the cloth (electrode) and the active layer. The bonding will be poor in the case of a metal layer,” says Prof. Ogale. “Due to the roughness of the cloth surface, when you press or flex the device the applied force is transmitted along different directions of the piezoelectric active layer. And this improves the piezoelectric property of the nanogenerator.” If the electrode were a flat metallic surface then the force applied would be transmitted in only one direction.
“When thumb pressure was applied on the polymer alone 2-3 volt was produced. In the case of the polymer with BaTiO3 coating the piezovoltage generated was 7-8 volt. But 14 volt was produced when BaTiO3 was incorporated into the polymer and also coated on the fibre surface,” says Dipti Dhakras from NCL and the first author of the paper.
“The voltage of 14 volt with a current of several microamperes is the highest power output reported for wearable type of nanogenerator using conducting cloth as the electrode,” notes the paper.