Graphene-Based Electrodes for Solar Cells

By Benedette Cuffari, M.Sc.Apr 5 2017

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Image Credits: petrmalinak/shutterstock.com

Ferns are an ancient family of plants that date back over 360 million years ago to the Mesozoic era. This leafy plant is typically found to grow in warm and damp areas of the Earth, where they can typically be found beneath a forest canopy, as twining vines, or floating on the surface of a pond¹.

Ferns are a “vascular plant,” which describes the well-developed internal vein structure that is responsible for maintaining an adequate flow of water and nutrients within the plants. Due to its vascular structure, fern leaves exhibit an exceptional ability to store energy for future use in biological processes, such as photosynthesis, which makes this storage possible as a result of the water transport present on the vein density.

The intricate structure of this plant, particularly of the Polystichum munitum species, recently inspired a group of researchers at RMIT University in Melbourne, Australia to adapt this design to a solar-powered application. The newly developed design of this energy storage unit integrates laser scribed graphene micro-supercapacitors (LSG-MSCs) with interdigited electrodes². LSG-MSCs are created as a result of a direct laser reduction of graphite oxide films to graphene, which subsequently produces a much more robust material that enhances the electrical conductivity of the capacitor³. The bioinspired electrodes applied to the LSG-MSCs resemble the space filling curves of fractals that are present in the fern leaves.  

The size of the pores present in any type of electrical device has a direct correlation with the successful generation of an electric current. As smaller pore sizes allow for a higher energy density to be exuded, larger pore sizes instead contribute to the higher power densities of a film. As currents are successfully generated from the film, there is a direct influence of the current to the active surface area of the film. With this in mind, the RMIT team of researchers utilized three different fractal shapes in order to manipulate the pore formation that can improve energy storage. Of these space filling shapes considered in this study include Hilbert fractals, Peano fractals and Sierpinski fractals, of which the Hilbert fractals exhibited the highest dimension of 1.73, which is close to that found in the Fern leaves².

Solar cells were encapsulated with a glass substrate in order to successfully integrate the thin-film graphene LSG-MSCs onto the device. Solar cell performance was analyzed before and after this encapsulation in order to determine exactly how efficient the addition of these graphene electrodes were to the device a as whole². What these researchers found was that the new electrodes increased the ratio of active surface area to volume while simultaneously reducing the electrolyte ionic path. With a measured energy density of approximately 10^-1Whcm within the device, researchers estimated that their prototype exhibited a storage capacity that is 30 times greater than what is available on the market today².

The immense storage capability of this device brings a new perspective into the reality that solar electricity options can one day become a cost-effective platform for all. One of the greatest challenges present in the solar industry is the development of storage units that maintain the original amounts of stored energy to be used for future use, such as during seasonal changes where energy may be more difficult to be readily collected. As the quest to discover new and renewable sources of energy is becoming of increasing interest to researchers around the world, a system that integrates this graphene-based electrode could ensure the place of solar electricity as becoming a major source of energy in the future.

References

1. Wagner, Warren H., and Warren F. Walker. "Fern." Encyclopædia Britannica. Encyclopædia Britannica, Inc., 21 May 2010. Web. https://www.britannica.com/plant/fern.
2. Litty V. Thekkekara, Min Gu. Bioinspired fractal electrodes for solar energy storages. Scientific Reports, 2017; 7; 45585.
3. El-Kady, Maher F., Veronica Strong, Sergey Dubin, and Richard B. Kaner. "Laser Scribing of High-Performance and Flexible Graphene-Based Electrochemical Capacitors." Science. American Association for the Advancement of Science, 16 Mar. 2012. Web. http://science.sciencemag.org/content/335/6074/1326.

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