Turkish researchers develop biocompatible and eco-friendly TENG
Triboelectric nanogenerators (TENGs) have attracted a lot of interest in recent years, and for clear reasons. TENGs can be used to harvest mechanical energy in a wide range of applications and at large and small scales; their design is straightforward; and they can be constructed from a wide range of robust, low-cost and lightweight materials. They also show a lot of potential for use in self-powered wearable devices, which capture biomechanical energy from a person’s daily activities (e.g., walking), and converts it into electrical power. But for TENGs to be truly wearable, they should be made from biocompatible materials that can successfully operate while in use on the body.
This has led a growing number of researchers to look to chitosan, a polysaccharide derived from chitin, which, thanks to its presence in the exoskeleton of crustaceans and insects, is the world’s second-most abundant natural polymer (after cellulose). Chitosan has already found widespread application in the food industry and in biomedical research because it can easily be formed into a film. It is also a tribopositive material, which means that when used in a TENG, it donates electrons.
A group of Turkish researchers has proposed a way to enhance the electric output power of chitosan TENGs, and it involves the use of three naturally-occurring, abundant, biocompatible minerals – bentonite, sepiolite, and kaolin. Writing in Nano Energy [DOI: 10.1016/j.nanoen.2023.108354] the researchers start by reporting on the roughness and surface potential of a series of chitosan/clay composite films; each with a different percentage weight by volume (wt%).
SEM images showed that the addition of clay altered the surface morphology of the film dramatically. The root-mean-square (RRMS) of surface roughness for pure chitosan films was 3.38 nm, compared to an RRMS of 157 nm (1 wt% bentonite), 530 nm (3 wt% sepiolite), and 127 nm (1 wt% kaolin). Kelvin probe force microscopy (KPFM) was carried out on the films to determine their surface potential. The chitosan/sepiolite films were shows to have a higher electron-donating ability than the other films, with the highest surface potential measured for the 3 wt% sepiolite composite.
The team next used these composite films to make a series of chitosan/clay-based TENGs. In all cases, the negative triboelectric layer was made from a commercially-available silicone. The nanogenerator’s active layers were cast directly onto aluminium foil substrates which acted as electrodes. Two pieces of plexiglass combined with a series of springs and pins allowed for the TENG to be tested in vertical contact-separation mode, at a known frequency and with a fixed applied force. Under a 10 MΩ load resistance, bentonite and kaolin-based TENGs show their best electrical performance at 1 wt% addition, while sepiolite delivers its highest electrical outputs at 3 wt%. And when tested at high load resistance (66.4 MΩ), chitosan-based TENGs with 3 wt% sepiolite, 1 wt% bentonite, and 1 wt% kaolin demonstrated open circuit voltages of 863, 996, and 963 V, respectively.
In addition, the maximum peak power densities (measured at 1.1 MΩ load resistance) for chitosan-based TENGs with 3 wt% sepiolite, 1 wt% bentonite, and 1 wt% kaolin were 20.4 W/m2, 26.5 W/m2, and 22.8 W/m2, respectively. The authors say that these are “significantly higher power densities in TENGs based on chitosan and clay compared to those reported in previous literature.” The TENGs were retested 12 months later, and no drop in performance was measured.
They conclude that their composite films “…have a high potential for use in self-powered portable and smart systems and offer promising avenues for technological development based on natural clays.”
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Adem Yar, Abdulkerim Okbaz, Serife Parlayici. “A biocompatible, eco-friendly, and high-performance triboelectric nanogenerator based on sepiolite, bentonite, and kaolin decorated chitosan composite film,” Nano Energy 110 (2023) 108354. DOI: 10.1016/j.nanoen.2023.108354