Energy Harvesting

Accidental Discovery Leads to Triboelectric Nanogenerators

University of Alberta researchers have developed a new way to produce electrical power; power that could be used charge small devices used to monitor pipelines and medical implants, to wearing electronics. The discovery sets a new world standard in devices called triboelectric nanogenerators; devices that convert mechanical energy from the environment into electricity for powering small devices.

This accidental discovery resulted when PhD student, Jun Liu, was conducting research unrelated to these tiny generators, using an atomic force microscope (AFM). AFM provides atomic level images using a tiny cantilever to ‘feel' an object, just like running your finger over an object. Liu forgot to press a button to apply electricity to the sample—but he still saw a current coming from the material and didn't know why.

One theory was that it was an anomaly or a technical problem, or interference. Liu eventually pinned the cause on friction from the microscope's probe on the material—like shuffling across a carpet then touching someone and giving them a shock.

New way to generate actual electrical current

It turns out that the mechanical energy of the microscope's cantilever moving across a surface can generate a flow of electricity. Instead of releasing the energy in one burst, the UAlberta team generated a steady current. "Other researchers are trying to generate power at the prototype stage but performance is limited by the current density they're getting—that is the problem we solved," said Liu.

Unlimited potential application

So far, what others have been able to do is to generate very high voltages, but not the current. Jun has discovered a new way to get continuous flow of high current. The discovery means that nano generators have the potential to harvest power electrical devices based on nanoscale movement and vibration: an engine, traffic on a roadway—even a heartbeat.

This could lead to technology with applications in everything from sensors to monitor structures such as bridges or pipelines, engine performance or even wearable electronics. Liu believes the applications are limited only by imagination.

The direct conversion of mechanical energy into electricity by nanomaterial-based devices offers potential for green energy harvesting. A conventional triboelectric nanogenerator converts frictional energy into electricity by producing ac triboelectricity. However, this approach is limited by low current density and the need for rectification.

In a paper titled, "Direct-current triboelectricity generation by a sliding Schottky nanocontact on MoS2 multilayers," the authors show that continuous dc with a maximum density of 106 A m−2 can be directly generated by a sliding Schottky nanocontact without the application of an external voltage. We demonstrate this by sliding a conductive-atomic force microscope tip on a thin film of molybdenum disulfide (MoS2).

Finite element simulation reveals that the anomalously high current density can be attributed to the non-equilibrium carrier transport phenomenon enhanced by the strong local electrical field (105−106 V m−2) at the conductive nanoscale tip.

The authors hypothesize that the charge transport may be induced by electronic excitation under friction, and the nanoscale current−voltage spectra analysis indicates that the rectifying Schottky barrier at the tip-sample interface plays a critical role in efficient dc energy harvesting.

This concept is scalable when combined with microfabricated or contact surface modified electrodes, which makes it promising for efficient dc triboelectricity generation.

University of Alberta
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