The quest for improved energy density in both conventional batteries and supercapacitors sometimes takes especially interesting paths. In research supported by the Air Force Office of Scientific Research, a team at UCLA worked with other institutions (Mississippi State University, University of Nevada, Central South University) and looked to the structure of tree leaves.
Both leaves and electrodes have a common goal: Maximize their surface area-to-mass ratio to better attract and retain source material. In the case of plants, the leaves seek to capture carbon dioxide needed for photosynthesis; for the supercap, it’s grabbing ions in the electrolyte solution that retain energy until needed later.
The researchers maintain that their design yields an increase in capacitance of 30% for a given mass compared to standard supercaps while offering 10 times more power. Equally important, they say the cells did well by retaining 95% of their capacitance after 10,000 charge/discharge cycles—an area where many “improved” designs fall short.
The ideas, process, and results are detailed in their very lengthy paper “Bioinspired leaves-on-branchlet hybrid carbon nanostructure for supercapacitors” published in Nature, along with additional extensive Supplementary Information. Their “recreation” of nature’s branch-and-leaves approach is based on a pair of carbon-atom nanostructures. For branches, they used arrays of hollow cylinders of carbon nanotubes (CNTs) with a 20- to 30-nm diameter. In contrast, the leaves are sharp-edged and built of graphene petals (GPs)—ultra-thin sheets of carbon up to 100 nm wide that are arranged on the nanotube stems (Fig. 1).
1. The branch-and-leaves design is made up of arrays of hollow, cylindrical carbon nanotubes (the “branches”) and sharp-edged petal-like structures (the “leaves”) made of graphene. (Source: UCLA)
Finally, they made these structures into tunnel-shaped arrays. Ions that transport the stored energy can flow through these arrays with much less “resistance” between the electrolyte and the surface. The resultant structure allows the design to deliver more energy, and do so with more efficiency, than if the electrode surfaces were flat (Fig. 2).
2. Structural characterization of CNT/GP micro-conduits showing a schematic illustration of CNT/GP micro-conduits in a leaves-on-branchlet nanostructure on carbon cloth (CC) substrates for high-performance supercapacitor electrodes (note that the yellow shaded areas in the schematic indicate the selected areas to be magnified) (a); uniform coverage of CNT micro-conduits on carbon fibers at low magnification (c); and a high-resolution TEM image of a petal emerging from a nanotube (h). (Source: UCLA)
Results show that areal capacitance was 2.35 F/cm2 and about 500 F/gm (based on active material mass). They speculate that the sharp graphene-petal edges in the hybrid structure may increase charge storage and ease rapid access of electrolyte ions to the electrodes.
The team noted that there are problems generally associated with CNT-based array electrodes, including poor nanotube bonding to substrates, low tube-to-tube charge-transfer efficiency, and easy destruction of the tube orientation. These weaknesses have resulted in poor mechanical robustness, high internal resistance, and poor cyclic stability. However, they maintain that their approach largely overcomes these well-known problems.
Further, they noted, “The micro-conduits with hollow channels were designed, to the best of our knowledge for the first time, to increase accessible electrode surface area to the electrolyte and to facilitate fast diffusion of ions during charge/discharge,” which is an impressive claim, of course. They added that the graphene petals greatly increased the mechanical robustness of carbon-nanotube micro-conduits, helping to preserve the structural orientation during operation.
3. Shown are galvanostatic charge/discharge curves at high current densities from 40 to 100 mA/cm (a), and areal capacitances and capacitance retention as a function of current density calculated from the galvanostatic charge/discharge curves (b). (Source: UCLA)
The researchers presented extensive results across multiple parameters and perspectives. Among these results were graphs showing the charge/discharge curves as well as the areal capacitances and capacitance retention as a function current-related factors (Fig. 3).
The electrode also performs well in acidic conditions and high temperatures, both environments in which supercapacitors could be used if their performance was stable.