Gradient doping improves triboelectric nano-generators
Author: EIS Release Date: Aug 16, 2022
Graduated charge-confinement could be the key to much improved triboelectric nano-generators (TENGs), according to the Gwangju Institute of Science and Technology and Chonnam National University in Korea, which have been working on suitable structures.
GISTT triboelectric principle
Triboelectric generators produce power through electrostatic induction when dissimilar insulating materials are brought together and parted (right, and far below).
Two of the things that push back against more efficient triboelectic generators, according to the GIST and CNU research team, are wasteful charge recombination with the rear electrodes used for energy extraction, and charge accumulation leading to charge repulsion on the front surfaces of the materials when they are pre-charged in manufacture.
“In the design of high-performance TENGs, it is critical to transport the charge on the surface to a deep position while reducing charge recombination,” said GIST professor Chanho Pak.
What was needed was mechanism to pull charge from the front surface into the body of the triboelectric material, and a way to stopped this charge getting all the way to the rear electrode.
GISTT triboelectric improvement
The triboelectric materials are chosen for their ability to naturally hold a high positive or negative charge when presented with a high field during manufacture
A gradient of charge-confinement was the answer to both issues, created by layers of carbon nanospheres with different surface areas (diagram left). “The layers transport as well as confine the charges,” said Pak.
Each of the two facing materials has a copper back electrode, then a layer of polymer with 300nm diameter spheres with a high ‘external specific’ surface area, 120m2/g, to strongly confine charge and prevent carriers from reaching the electrode.
The gradient comes from the two subsequent layers, of 40m2/g (700nm) spheres then 20m2/g (1μm) spheres, finally topped by the triboelectric front surface.
To get opposing potentials, one of the structures has a Nylon 11 front face and its carbon spheres are held in a Nylon 11 matrix. For the other, the front face is PTFE and the spheres are held in a polyvinylidene matrix.
“As a result of this gradient arrangement, the injected charges [during manufacture pre-charge] could drift towards the electrode but were confined just before reaching it,” according to the Institute. “With the addition of charge-confining layers, the researchers improved the output voltage and current of the TENG by 40 [15.2V to 600V] and seven times, respectively”.
15.2V is for a TENG without without the carbon gradients and without pre-testing charge injection – the without-carbon material can be persuaded to accept 200V of pre-charge, GIST researcher Jong Gyeong Kim told Electronics Weekly.
In the 600V carbon-and-pre-charge case, the bias charge was stable through 1,000 triboelectric cycles. “Since triboelectric materials are greatly affected by the surrounding environment and humidity, it is inevitable that the output will gradually decrease with time without constant triboelectric charging,” said Kim. “The advanced transport and confinement processes are maximised by the residual charge remaining in the carbon layer after charge injection, enabling the long-term use of TENG. The process of continuous friction or periodic triboelectric charging is one way to maintain the difference in the electrical potential, or store the TENG under favourable conditions for triboelectric charging”, in a dry environment, for example.