There has been a plethora of interesting research into self-powered energy harvesting devices. In a paper published in Applied Clay Science, a study led by Xuemei Wang has explored mica-based triboelectric nanogenerators for this purpose.
Mica mineral. Study: Mica-based triboelectric nanogenerators for energy harvesting. Image Credit: Minaev/Shutterstock.com
The Need for Smart, Self-Powered Devices
As major research is ongoing into the field of smart, wearable technologies, there is a need for self-powered devices. Materials used for flexible devices must be able to withstand significant levels of mechanical deformation such as folding.
Therefore, materials must be durable, light, flexible, and display superior electrical properties to generate a charge to power devices without bulky components. Components that can harvest mechanical energy from the wearer are of particular interest to researchers in the field of wearable technology. Recently, there has been much focus on materials that take advantage of what is known as the triboelectric effect.
The Triboelectric Effect
Humans have known about the triboelectric effect for millennia. It is a form of contact electrification that occurs when certain materials become electrically charged when they separate from other materials they were in contact with. Most static electricity is triboelectric in nature. Another example of the triboelectric effect is when a pen is rubbed against a sleeve. The induced charge causes the pen to be able to attract small pieces of paper when it is held over them.
The triboelectric effect is highly unpredictable, and therefore it is only possible to make broad generalizations of it. The charge produced differs in polarity and strength due to factors including temperature, the type of materials in contact with each other, and the surface roughness of the materials.
Triboelectric nanogenerators have recently been explored to take advantage of the triboelectric effect. These nanogenerators can convert mechanical energy into electricity, removing the need for conventional batteries that need to be replaced or recharged periodically.
Their use could provide an innovative approach for self-powered devices such as wearable sensors, medical devices, and smart telecommunications devices. They have the added advantage of being thin and flexible, making them ideal for devices that undergo a high degree of mechanical deformation.
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Numerous studies have been carried out in recent years into triboelectric nanogenerators, exploring several materials for their use as friction layers. Considerable progress has been made in tuning the output performance of these devices. Materials that can be used for friction layers include paper, polymers, hydrogels, textiles, fluids, and cement.
Considerations for material choices include cost, elevated levels of power generation, and stability. Elasticity and flexibility must additionally be considered for wearable applications.
Using Mica as a Triboelectric Material
Recently, mica has emerged as one of the most attractive inorganic materials for triboelectric nanogenerators. Mica is an abundant raw clay mineral made of layered silicate. A property of mica that makes it a target for this purpose is the layered silicate’s low friction coefficient. This means that mica can be utilized as a triboelectric layer for energy harvesting without having to be chemically treated or having to use special manufacturing processes.
However, mica’s triboelectric performance characteristics and influencing factors have not been extensively studied thus far. Mica is known to have a propensity for a positive charge from previous studies on the triboelectricity of the material. To study the material’s potential, the team behind the study built a triboelectric nanogenerator using mica as the positive triboelectric material and polytetrafluorethylene film as the negative triboelectric layer.
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The open-circuit voltage achieved was 27.3 V. The system also achieved a short-circuit current of 1.68 µA. The maximum output power density achieved by the nanogenerator was 62.82 mW/m2. This was mainly due to the mica’s smooth, flat surface and triboelectric properties. The research investigated the effects of the thickness of applied mica, the presence of air bubbles defects, applied forces, and working frequency on the performance of the novel triboelectric nanogenerator.
The team tested the nanogenerator by installing it in the sole of a shoe. Walking over a plastic floor generated a charge which powered four LEDs by harvesting the mechanical energy generated by the wearer. Interestingly, the generator required no storage components.
The work in the study has laid the groundwork for the widespread uptake of mica-based nanogenerator devices that are low-cost, stable, and environmentally friendly.
The wearable technology industry is providing products for numerous markets including biotechnology, the mining industry, and the military. Applications of these technologies include medical monitoring devices, implantable devices, head-mounted displays, fitness trackers, smart sensors, wearable communication devices, and many more besides.
Research such as the work in the paper published in Applied Clay Science demonstrates an innovative approach to self-powered devices that could prove revolutionary.
Wang, X et al. (2021) Mica-based triboelectric nanogenerators for energy harvesting [online] Applied Clay Science 215 | sciencedirect.com. Available at:https://www.sciencedirect.com/science/article/abs/pii/S0169131721003549