Credit: Image courtesy of Zhong Lin Wang

Please tell us about your scientific background and research interests.

I received my bachelor’s and master’s degrees in physics from Xidian University, and my PhD in physics from Arizona State University. My thesis was on electron energy loss spectroscopy (EELS) of supported small particles and surfaces. I then undertook postdoctoral work from 1987 to 1989 at SUNY Stony Brook and Cambridge University, where I researched plasmon excitation using EELS. Then, I worked as a researcher at Oak Ridge National Lab and National Institute of Standards and Technology from 1990 to 1994, where I studied high-Tc superconductors, carbon materials, surface structure analysis and electron inelastic scattering theory. I joined Georgia Tech. as an Associate Professor in 1995, later as Professor in 1998. I pioneered nanogenerator technology since 2006, and coined the piezotronics and piezo-phototronics fields in 2007. I now work on distributed energy, self-powered sensors and large-scale blue energy.

What work led to your first demonstration of a TENG?

Early work in my group was on nanomaterials for converting mechanical energy (e.g., vibrations) into electricity via the piezoelectric effect. In 2006 we demonstrated a piezoelectric nanogenerator effect based on deflecting zinc oxide nanowires in an atomic force microscope [https://www.science.org/doi/10.1126/science.1124005]. This work proved that nanomaterials could be used for small-scale energy harvesting, but further work was needed to increase energy generation due to limitations in the properties of the nanowires, including small piezoelectric coefficients. We therefore started to explore other effects that might lead to improved power output, and we focused our efforts on triboelectricity which, at the time, was considered impractical for energy generation. This culminated in our 2012 paper [https://www.sciencedirect.com/science/article/abs/pii/S2211285512000481?via%3Dihub], which demonstrated that contact friction between dissimilar materials (a polymer and a metal) generated sufficient surface charge to drive electron flow. The power output was enough to power devices, which could be fabricated from low-cost, flexible materials. This was the first report of a working TENG.

Subsequent work from our group looked at establishing a theoretical basis for TENGs, and self-powered devices. It should be pointed out that the range of materials that can be used in TENGs (polymers, metal, fabric etc.), including low-cost options, and sufficient power output makes them extremely attractive for self-powered devices.

Is more basic research on TENGs needed or does the community already have a strong understanding of their operation?

The TENG community has established the core working principles, but deeper scientific understanding is needed to optimize their performance. For example, we have good understanding of electrostatic induction mechanisms, theoretical models for voltage/current output (under idealized conditions), how to pair materials in devices, and device architecture for common TENG operating modes (such as sliding, contact-separation, and freestanding). However, charge transfer mechanisms at the contact interface remain to be clarified, such as whether electron transfer, ion transfer or material adhesion dominate. There are also fundamental questions to be asked regarding nanoscale changes at the contact interface, including changes in chemistry, which in-situ testing might help to answer, such as with Kelvin probe microscopy and synchrotron X-rays. Furthermore, first-principles calculations could predict triboelectric properties of novel materials.

Beyond TENG operating principles, we also need to understand the basic mechanisms at play for TENGs used in specific applications and environments. For example, working devices will have to undergo millions of cycles, requiring understanding nanoscale wear mechanisms. Electrical effects such as parasitic capacitance, air breakdown, and impedance mismatch all reduce efficiency, which new dielectric materials or charge-trapping strategies might help to address. There is also the possibility to integrate TENGs with other physical effects for multifunctional devices, such as piezoelectric, pyroelectric, or photovoltaic effects.

While TENGs have outstanding emerging applications, the field somewhat resembles solar cell research in the 2000s – applications are proven, but fundamental discoveries could revolutionize performance.

Which applications of TENGs have been most successful so far, and which upcoming applications are they most promising for?

Working TENGs have been demonstrated for a range of applications, typically those that require low-power and intermittent operation. For example, self-powered sensors that leverage physical movement, such as wearable sensors that harness energy from body movement, and industrial monitoring, where vibration of machinery can power the sensor. Consumer electronics have also benefitted from TENGs, such as keyboards that are powered by striking the keys. TENGs have also been demonstrated for environmental monitoring, such as in agriculture, and for biomedical applications, such as pacemakers powered by lung movement. In all cases, these applications benefit from the ability of TENGs to recover enough energy from physical movement to power a useful application, as well as being low-cost.

Looking beyond this, I believe TENGs hold significant potential for ‘blue energy’ harvesting i.e. harnessing the motion of ocean waves – pilot projects are already underway in China and Europe to assess power output. There is also a European-funded project to embed TENGs in roads, harnessing tire friction to power street lamps, while other smart city applications include powering smart windows from vibration within a building. Looking further afield, NASA are considering TENGs for satellites and rovers, which might harvest energy from dust storms. In short, we are only scratching the surface of applications that might be self-powered by TENGs.

There are several start-up companies that we are collaborating with to bring some of the above examples to commercial fruition, such as Nanotech Energy and PowerBooster, which we hope to see before 2030. Following these more niche examples, we are working towards grid-scale blue energy and smart infrastructure beyond 2030.

What technical challenges do TENGs face in terms of commercialization?

TENGs face a number of challenges, which will need to be solved for them to find widespread use. Firstly, since TENGs rely on the contact between materials, durability and resistance to frictional wear is a major hurdle, especially in high-impact applications (such as car tires or blue energy). We are exploring various techniques such as improved materials, coupling operation modes, and lubrication oil.

Next, TENGs excel in high-voltage, low-current scenarios. This could be addressed by using proper power management techniques, and combining with energy storage devices to form a self-charging power unit. An additional challenge is that TENGs are often powered by non-uniform mechanical input, such as human traffic, making their output intermittent. Combining TENGs with other energy devices, such as solar cells or batteries, might improve power output uniformity. Related to power output, real-world usage presents further difficulties – charge dissipation occurs under high humidity, and materials deform at high temperatures. Device packaging, encapsulation and hydrophobic coatings are possible solutions for this.

There are also no universal standards for measuring output, such as charge density and energy conversion rates. This makes it hard to compare results from different labs, which is particularly an issue for industrial users who are considering applying TENGs. IEEE and IEC are developing TENG testing protocols.

What are you most excited about?

TENGs can play a key role in the push towards energy democratization. By harnessing mundane physical actions, they allow electronics to be self-powered, removing the need to rely on centralized power grids. This ‘ambient energy scavenging’ closely aligns with a number of global trends, such as sustainability, personalized healthcare, and the Internet of Things. TENGs are already viable for low-power, distributed sensing, but their true disruption lies in future large-scale energy harvesting and human-machine synergy.

This interview was conducted by John Plummer, Chief Editor of Communications Materials.