Bench Talk

(Source: Mopic – stock.adobe.com)

In modern-day technologies, there is a push for devices to harness the environment to power them and move away from bulkier power sources. This is where nanogenerators step in. A number of smaller-scale devices are trialing nanogenerators, including implantable medical devices, wearable medical devices, remote monitoring technologies, IoT technologies, and even self-powered air disinfection systems. Though this is not an exhaustive list, these are just a few of the more developed areas that are looking toward self-charging capabilities, as well as some of the more recent developments to come out of the field.

The field of energy harvesting using nanogenerators is still in relative infancy commercially speaking; however, this does not deter the interest and fundamental research in these systems. This is because they can power small, remote devices when other energy storage/energy harvesting technologies are neither feasible nor suitable because of their size or power connection requirements. Nanogenerators could unlock the ability for many small devices to self-charge and open up remote applications that might not otherwise be possible. Though several different nanogenerators are being researched and developed, we’re going to look at one of the more promising and better-developed nanogenerators known as the triboelectric nanogenerator (TENG).

Nanogenerators harvest some form of external stimuli and use this stimulus to generate an electrical charge that can power a small device—or in some, but not widely developed, cases they can be used and integrated in greater numbers to power larger electronic systems. Triboelectric nanogenerators (TENGs) are small-scale, lightweight devices that harvest motion from their surroundings and convert it into an electrical output. In short, TENGs change mechanical energy into electrical energy through a contact-induced electrification mechanism.

TENGs rely on two fundamental mechanisms to change mechanical motion into an electrical output. These mechanisms are contact electrification and electrostatic charge induction. For a TENG to convert the energy from mechanical to electrical, the active triboelectric material in the device must first become electrically charged through the interaction with an external stimulus. This contact electrification mechanism generates frictional forces, which subsequently produce electrical charges on the surface of the TENG. The TENG then undergoes an electrostatic charge induction phase, where the surface charges redistribute across the materials within the TENG. This redistribution of charges generates an electrical current that can then power any attached small-scale devices.

The process of generating and transferring electrical charges across the TENG relies on a number of different small-scale physical and chemical factors—including material deformations, fracturing, heat generation, and electron and ion transfer. Because many factors affect whether a material will exhibit good triboelectric properties, there is no definitive association between the triboelectric effect and fundamental material properties. This means that no exact theory predicts the presence and magnitude of triboelectricity a material could exhibit. Consequently, designers often employ the best-guess approach for determining potentially suitable materials by ranking them according to their ability to lose or gain electrons when in contact with each other.

Given that there are only best-guess approaches for determining whether a material will be triboelectric based on its ability to lose or gain electrons, 2D materials soon became a candidate of interest because of their inherent thinness. The thinness of 2D materials—in some cases one atomic layer thickness, in other cases, a few atomic layers of thickness—means that the surfaces are much more active than bulkier materials, so there is a greater potential for electrons to interact more easily with other surfaces at these nanoscales. Additionally, once you get into the realm of the nanoscale—especially thin surfaces such as 2D materials—quantum effects start to be observed that help facilitate the movement of electrons in ways that are not possible with bulkier materials, i.e., quantum tunneling.

Several different 2D materials have been tested for TENGs. Some of these approaches involved using the 2D materials on their own, while other devices have used 2D materials in conjunction with polymers or polymer composites. Most 2D materials are used as the negative side of a triboelectric junction. This is because 2D materials would rather gain electrons from most materials, leading to a negative charging polarity. It’s also possible to heavily modify this process via doping—something that has been used (and been successful) in applied 2D material research—as it has been shown to improve the electron capturing ability of many 2D materials when placed in a TENG.

Beyond the ability of 2D materials to capture electrons and provide a triboelectric response to friction and motion in their local environment, there are other benefits of using 2D materials over other materials—including other nanomaterials. These are namely the high degree of flexibility, mechanical strength, durability, and transparency that you get with 2D materials—which you don’t even get with some other nanomaterials—meaning that they can undertake a great deal of mechanical stress and and offer l longevity. This is a key property as some of the mechanical motion and friction sometimes involve bending (depending on the application), as well as frictional forces, so the triboelectric materials need to be able to withstand bending and other mechanical stresses.

The trials of the different 2D materials for TENGs include materials tailored toward the application in question. For example, sometimes graphene-based materials are more suitable for external medical devices because they have more flexibility than some of the other 2D materials and will conform better to a patient’s skin. So far, a range of applications, from air disinfection to implantable pacemakers, to wearable monitoring devices, remote sensors, and even the powering of LED TVs have been possible using TENGs as the power source.

The 2D materials used range from graphene and its derivatives, to MXenes, and transition metal dichalcogenides (TMDCs). Out of these, much of the research has gone into graphene materials. but the most popular so far have been TMDCs, with molybdenum disulfide (MoS₂) standing out among the other TMDCs.

While graphene is favored for some applications, molybdenum disulfide is seen as one of the most promising options overall because it exhibits a quantum confinement effect that acts as a charge-trapping agent. Beyond this, its energy level is ideal for transferring electrons and it has a large surface area. Currently, as a general triboelectric material for TENGs, it exhibits the highest output voltage and current among all 2D materials, but the uses of molybdenum disulfide and other 2D materials within TENGs is still governed by the application demands—as the electrical output created is not always the only driving factor for some applications (for example, flexibility or biocompatibility may also be driving factors as well). So, many different 2D materials can support the specific application requirements where TENGs are being trialed and developed, with the material choices and application scope set to expand in the coming years.

2D materials bestow more benefits for TENGs compared to bulkier materials and even other nanomaterials. The inherent thinness of 2D materials provides a more active surface for a more efficient movement of electrons and triboelectric response. Their flexibility and mechanical durability offer an option for creating TENGs that have longevity. Though there is much interest currently in creating TENGs for powering small-scale and remote devices, studies have shown that it is also possible to use TENGs to power larger electronic devices including TVs.