The majority of us have experienced the jolt of static electricity when touching a metallic object after donning a sweater or walking on carpet. This is the result of a charge buildup that occurs when two dissimilar materials (such as our body and the fabric) come into contact.
In 2012, scientists from the United States and China utilized this phenomenon, known as the “triboelectric effect,” to construct a triboelectric nanogenerator (TENG) that converts unused mechanical energy into usable electrical energy. Their device consisted of two triboelectric polymer films with metallic electrodes that, when brought together and separated, resulted in charge separation and the production of an electric voltage adequate for powering small electronic devices.
By injecting charges onto the surface of triboelectric films, efforts have been made to increase the power output of TENGs, which are viewed as potential sustainable energy harvesters. However, charge recombination in the electrode and charge repulsion on the material’s surface prevent them from attaining high surface charge densities.
A recent study led by Professor Chanho Pak of the Gwangju Institute of Science and Technology (GIST) in South Korea developed a charge-confinement layer that manages the transfer of injected charges between the triboelectric film and the electrode to increase the charge density on the triboelectric film’s surface. This article was made available online on March 16, 2022, and it appeared in Volume 6 Issue 5 of Small Methods on May 18, 2022.
“In the design of high-performance TENGs, it is essential to transport surface charge to a deep position while minimizing charge recombination,” explains Professor Pak. Electrospun mesoporous carbon spheres were combined with layers of polyvinylidene fluoride (PVDF) and nylon to create the layers. The carbon spheres were arranged in ascending order of their specific surface areas to create a gradient charge-confinement layer. As a result of this gradient configuration, the injected charges could drift toward the electrode, but were contained just before reaching it. Prof. Pak explains, “The layers transport as well as confine the charges.”
By transporting the charges away from the surface, the layers prevent injected charges from accumulating and repelling each other on the triboelectric material’s surface, allowing the material to store more charge. Furthermore, confining the charges close to the electrodes prevents charge loss due to recombination, resulting in a triboelectric surface with a greater charge density.
With the addition of charge-confining layers, the output voltage and current of the TENG were increased by a factor of 40 and 7, respectively. In addition, by combining a cylindrical TENG with an electromagnetic generator, they significantly increased the output current by 1300 times. “With these promising results, TENGs may one day be able to serve as sustainable energy harvesters and power future wearable devices,” says Prof. Pak.