Triboelectric effect, TENGs and Energy Harvesting

What was cutting-edge technology just a decade ago is now increasingly being left behind on the wayside, as we accelerate faster than ever before, towards a world with practically no boundaries between the physical and the virtual. Technologies like Artificial Intelligence, Machine Learning, and Energy Harvesting have inexorably altered the way our devices operate and interact with us. The triboelectric effect is one such physical property that is being used to harvest energy that can power our wearables. 

What is the triboelectric effect?

The triboelectric effect is a type of contact electrification phenomenon wherein certain specific materials become electrically charged when they come in contact with a different type of material and are then separated. So, in essence, triboelectricity is static electricity that is produced due to friction produced between the surfaces of materials such as amber or wool. 

If you’ve ever noticed light household objects sticking to your clothing, or wondered why your brand new car attracts annoying quantities of dust, it is the triboelectric effect at work. As far back as 600 BC, the Greeks observed that amber, when rubbed vigorously, would pull bits of paper and other light objects towards itself. 

The nature of this phenomenon is actually not completely understood. What we do know is that the triboelectric effect is brought about by an imbalance of charges on the surfaces of these materials, which then results in an electrical discharge. 

The nature and magnitude of this phenomenon depend on a wide variety of factors, including the materials employed and several other environmental conditions. 

Back to the basics

To better understand the triboelectric effect and how it works, we need to understand the basics of electrical charge. Let’s go back to high school physics for a minute - We know that all matter consists of atoms, which are electrically neutral - i.e. they don’t have a positive or negative charge at rest. This is because, the nucleus of the atom contains protons, which are positively charged, and neutrons (which have no electrical charge), while the electrons that revolve around the nucleus are negatively charged. Under normal circumstances, the number of protons and electrons in an atom are the same - this means that at rest, all atoms are electrically neutral. 

While the nucleus is very tightly held in place by, well, nuclear forces, electrons can be exchanged between atoms. Some substances are more likely to lose electrons than others. 

Triboelectric series

As mentioned previously, materials vary in their tendency to hold on to or give up electrons. The triboelectric series is a list of all materials arranged in order of their ability to hold on to or lose electrons. 

This list of materials is known as the triboelectric series. During a triboelectric interaction between two materials, one of them invariably gains electrons, becoming negatively charged, whereas the other invariably loses electrons. The position of these substances with respect to one another on the triboelectric series defines which one will lose electrons. 

Materials high up on the triboelectric series have a strong tendency to acquire a positive charge whereas those at the bottom of the list, readily give up electrons, thereby acquiring a negative charge. Say, if asbestos, which is placed reasonably high in the series were to be rubbed against Teflon, which is placed significantly lower down the series, the asbestos would become positively charged whereas the Teflon, having lost electrons to the asbestos would become negatively charged. 

The amount of charge transferred between two materials corresponds directly to how far away they are from each other on the triboelectric series. Materials in the middle of the series typically don’t have a tendency to either lose or accept electrons. 

What are TENGs?

Today, in the light of our rapidly evolving IoT landscape, the triboelectric effect is more relevant than ever. We have made striking breakthroughs in our quest to unearth newer and greener ways of powering IoT devices such as wearables and control units. Triboelectric nanogenerators or TENGs are one of the most promising recent technologies in this area. 

But before we get into that, let’s start off by understanding what nanogenerators are. A nanogenerator is a device that converts small-scale mechanical or thermal energy, brought about by physical processes, into usable electrical energy. There are three main types of nanogenerators - piezoelectric, pyroelectric, and triboelectric. 

For the sake of this post, let’s dial in and focus on triboelectric nanogenerators - in essence, triboelectric nanogenerators (TENGs) work by converting mechanical agitation into electricity, by harnessing the triboelectric effect and electrostatic induction.

TENGs represent the absolute cutting-edge of alternative energy technology - It was only back in 2012, that the first triboelectric nanogenerator was developed by Professor Zhong Li Wang et al of the Georgia University of Technology. 

In the few years since their advent, TENGs have shown remarkable promise as a technology for the future. In just the 12 months following January 2012 when they were first presented, the output power density of TENGs saw an increase of close to five orders of magnitude! 

The area power density of TENGs was shown to reach a whopping 313 W/m² with a conversion efficiency in the vicinity of 60%. 

Energy harvesting using TENGs

In addition to their staggering output performance figures, TENGs have a number of major advantages as an energy harvesting technology - they are incredibly inexpensive to manufacture and have been demonstrated to be reliable and robust. Moreover, because TENGs are generally made using organic materials, they are eco-friendly, and therefore, they are extremely relevant in the context of the global push towards more sustainable energy technologies. 

Triboelectric nanogenerators can enable us to harvest all sorts of wasted mechanical energy that is readily available to us across various walks of everyday life - walking, rotating tires, vibration, wind, flowing water, human movements, etc. 

TENGs can also alternatively be used as self-powered sensors which can detect changes arising from mechanical agitation. This is hugely promising and is likely going to be used in future touch and smart-skin technologies. 

The scope for these nanogenerators to be used as an energy harvesting technology is massive - in addition to playing around with the choice of materials used, scientists are now experimenting with the microstructure of the materials used, in order to improve surface contact areas and therefore, the triboelectrification. Altering the morphology of the so-called nanopatterns - e.g. pyramidal, square or hemispherical - can potentially result in dramatically improved output performance!

One for the future

Although we have recognized and understood the triboelectric effect for quite some time now, it was only recently that their incredible potential as an energy technology was discovered. 

For the longest time, this phenomenon was seen as a deleterious nuisance to be countered and worked against. NASA would famously delay planned launches fearing adverse effects due to the triboelectric effect if certain types of clouds were prevalent on a given day. All manner of tools and equipment have been routinely coated with antistatic agents to avoid the build-up of static charge. 

Now, however, we see this phenomenon in a completely different light. The development of TENGs has not only opened up a possibility to use triboelectricity in a useful way, but it has thrown open the doors to hundreds of possibilities to power our IoT future. The possibilities are truly endless - in the near future, your shoes could power your wearable electronics; flexible nanogenerators embedded into fabric can make the much-awaited era of functional smart clothing a reality! 

One thing is for certain - TENGs are already poised to play a pivotal role in our never-ending quest to find newer and cleaner energy sources to power our burgeoning consumer energy demand.