Materials for Triboelectric Energy Harvesting: Current Situation and Some Thoughts

The triboelectric effect is as old as the hills, and the first reported effects of electrification date back to Thales of Millet in 600 BC . However, it was only in the 17th century that experimental evidence of triboelectricity was found, and in the next century that electrostatic generators began to appear, their benefits not being exploited until much later. Triboelectric generators for energy harvesting are a very recent development, not even 15 years old. The advent of these generators and their popularity stem from the work of Prof. Zhong Lin Wang and his team at the Georgia Institute of Technology. They develop triboelectric nanogenerators (TENGs). Paying attention with the ‘nanogenerator’ term, which should not be taken to mean a nanometer-sized generator, but a generator that generally includes nanomaterials within it. More generally today, TENG is the established term for triboelectric generators dedicated to energy harvesting, even if nanomaterials are not present. While the basic concept of triboelectricity is well established, with positive and negative charges separating when two materials rub or make contact, many questions remain. This separation of charges is not necessarily two-sided (i.e. negative charges on one material and positive charges on the other), and research has shown the co-presence of positive and negative charges on the same surface after contact. Generally, electron transfer is involved, but sometimes also ion transfer, as well as material transfer when surfaces come into contact: in some situations, it’s a point of debate. Likewise, the development of nanocomposites raises another question regarding the ranking of these new materials in the classification of the triboelectric series. Some studies show that preconditioning of materials can promote or conversely reduce triboelectricity: is there a sound strategy for implementing this route? As far as the basic operating modes for developing TENGs are concerned, there are several approaches: vertical contact-separation, sliding, free-standing, single electrode… Should one mode be favored over another in terms of expected performance if application requirements do not dictate it? How is performance in terms of energy harvested or produced truly calculated or estimated, and how can performance be easily compared between TENGs?<br/>Through an abundant panorama of works from the literature on TENGs and their applications, we will attempt to shed light on some of these questions, and to highlight the advantages, difficulties and cautions to be taken in the announced promises in the actual use of these generators for energy harvesting. We will also present our research work on TENGs through the development of a test bench dedicated to the precise characterization of these materials in sliding mode, as well as a consolidated theoretical study enabling us to evaluate performance on resistive or capacitive loads. This theoretical study also led to a proposal for optimal TENG operation, which we then validated experimentally.