Micro-supercapacitors made from fallen leaves

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Schematic illustration of the production of femtosecond laser-induced graphene. (Image: Korea Advanced Institute of Science and Technology)

Researchers have developed green flexible micro-supercapacitors from fallen leaves irradiated with a femtosecond laser.

The work could help address ongoing challenges in facilitating wearable electronics, smart houses, and the Internet of Things.

Micro-supercapacitors have drawn a great deal of interest for their high electrical power density, long lifetimes, and short charging times.

However, they are typically produced from non-biodegradable synthetic polymers, resulting in excessive electronic waste. Moreover, the complex multi-step process required to fabricate them increases their production cost. 

In Advanced Functional materials, researchers from the The Korea Advanced Institute of Science and Technology (KAIST) have recently described a new one-step method of fabricating highly conductive, intrinsically flexible, green microelectrodes from naturally fallen leaves. The porous 3D graphene-inorganic-hybrid microelectrodes are produced in ambient air using femtosecond direct laser writing lithography without any additional materials. Taking this strategy further, the team also devised a method for producing flexible micro-supercapacitors.

The researchers show how the new technique can be used to quickly and easily produce graphene electrodes at a low price, and validated their performance by producing micro-supercapacitors that could power an LED and an electronic watch that could function as a thermometer, hygrometer, and timer. 

These results open up the possibility of the mass production of flexible, green, graphene-based devices that could be applied in wearable electronics, smart houses, and the Internet of Things.

In the abstract of the paper, the researchers summarised the new method: ‘Hierarchically porous graphene is patterned on different types of leaves via a facile, mask-less, scalable, and one-step laser writing. Leaves consist of biominerals, which decompose into inorganic crystals that serve as nucleation sites for the growth of 3D mesoporous few-layer graphene. The femtosecond laser-induced graphene microelectrodes formed on leaves have lower sheet resistance (23.3Ωsq−1) than their synthetic polymer counterparts and exhibit an outstanding areal capacitance (34.68mFcm−2 at 5mVs−1) and capacitance retention (≈99 per cent after 50,000 charge/discharge cycles). 

‘Leaves create forest biomass that comes in unmanageable quantities, so using them for next-generation energy storage devices makes it possible for us to reuse waste resources, thereby establishing a virtuous cycle,’ said Professor Young-Jin Kim, of the KAIST Department of Mechanical Engineering.

This research was sponsored by the South Korean Ministry of Agriculture, Food and Rural Affairs, the Korea Forest Service, and the Korea Institute of Energy Research.

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