The emergence of electronic devices/systems with unprecedented functionalities mandatorily requires portable, flexible and sustainable power sources. Energy harvesting technology that can efficiently generate electricity from ambient environmental energy is the prerequisite for the realization of such new power sources. But due to their intrinsic limitation of unstable and uncontrollable electrical output, it is desirable to hybridize them with energy storage devices into single units, which could be capable of sustainably providing a stable output through utilizing the energy in the environment. For the electricity generation, mechanical energy is one of the best choices as energy source owing to its universally-existence in our living environment and human bodies.

A new type of devices—triboelectric nanogenerators (TENGs)—based on contact electrification has been recently invented as an effective and adaptive technology to generate electricity from motions. However, because the device structure and material properties have not been optimized, the output is still insufficient for sustainably driving electronic devices/systems.

Here, we demonstrated a rationally designed arch-shaped TENG as a flexible mechanical energy harvester with extremely-high power output. Through purposely introducing thermal stress on the surface, a flexible substrate was made into a naturally-bent shape, so that a steady gap formed between two triboelectric layers at strain-free conditions. This design facilitates the separation of the opposite tribo-charges, thus maximizes the electrical driving force.

This unique structure, together with the surface modification of tribo-layers, helps to largely enhance the output voltage, and power density to 230 V and 128 kW/m3, respectively, with an efficiency as high as 10~39%. For the first time, it realized the instantaneous driving of tens of regular electronic devices (LEDs), and also the fully charging of a Li-ion battery.

[1] For the development of sustainable power sources, we further hybridized the arch-shaped TENG with a flexible Li-ion-battery into as a single device—a flexible self-charging power unit (SCPU), which allows a battery to be charged directly by ambient mechanical motion. This physical hybridization enables a new operation mode: the “sustainable mode”, in which the environmental mechanical energy is scavenged to charge the battery while the battery keeps driving an external load.

In this mode, the demonstrated SCPU can provide a continuous and sustainable DC current of 2 µA at a stable voltage of 1.55 V for as long as there is mechanical motion/agitation. It can be used to continuously drive a UV sensor for extended period of time. Thus, the SCPU can serve as an independent and sustainable power unit, which will meet the general requirement of almost any electronic device. [2]

[1] Wang, S. H.; Wang, Z. L. et al. Nano Lett. 2013, 13, 2226-2233.

[2] Wang, S. H.; Wang, Z. L. et al. ACS Nano Under review.

Sihong Wang

Georgia Institute of Technology

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