Energy harvesting through spontaneous electrification of leaves

Contact electrification is a ubiquitous phenomenon responsible for the charging of materials after contact with each other; solids and fluids alike can be subjected to this effect. Despite being known since Ancient Greece, most advancements in understanding and harnessing the power of contact electrification were done in the past 120 years. Contact electrification is responsible for many casualties in the industrial context and has important consequences in natural phenomena although new connections are still being formed. Contact charging is at the base of many technological applications such as laser printing, surface functionalization, pesticide dispensing etc. Among the most recent applications is harvesting of irregular, low frequency environmental energy. Triboelectric nanogenerators (TENGs) are devices that exploit triboelectrification and electrostatic induction to convert the energy of, e.g., wind or raindrops into electricity. Many different designs have been proposed according to the specific application, including soft, transparent and flexible devices that can be installed on plants’ leaves and used to convert the wind-induced fluttering of leaves into electricity that can be used to power small electronic devices, e.g., sensors. These artificial leaves give the opportunity to harvest point-of-use energy in remote contexts for environmental and agricultural monitoring, directly from the largest biointerface on Earth. This PhD dissertation describes the results obtained in the past few years in the study of contact electrification of leaves when hit by raindrops, a novel area of research that has been little investigated despite the potential implications for plants. Researching this subject has required developing custom tools and protocols to manage the biological surfaces. In this context, electric signals produced by an arthropod walking on a leaf were recorded, analyzed and reported for the first time, opening to the possibility of using contact electrification as a tool to study plant-insect interactions. At the same time, the increased understanding of liquid-solid charging has been exploited to introduce a rain-energy harvesting functionality in the artificial leaf by adding a new structure on the original device. The increased robustness of the energy harvesting device allows to expand its reliability in powering small electronics. The artificial leaf can also act as a self-powered sensor by calibrating the output voltage to the wind speed that originally produced it; by coupling the device with a custom circuit and software, it can become a real-time monitoring tool able to send digital notifications on a custom platform when a preset condition is met. Preliminary results on the detailed analysis of droplet electrification and long-term powering of an ion delivery device are also discussed. The research reported here gives new insight on the phenomenon of electrification on leaves when in contact both with artificial surfaces and natural bodies as well as new opportunities for the development of innovative and sustainable technologies for energy harvesting and environmental sensing.