SEM of curved graphene sheets (scale bar- 10 µm). Credit: Nano Letters, C. Liu, Zhenning Yu, David Neff, Aruna Zhamu and Bor Z. Jang.
A recent paper by a group of researchers working in the U.S. and China discusses the creation of a supercapacitor whose electrodes are prepared from curvy, single-layer graphene sheets, a method that yields remarkable energy densities. Their work appears in a recent issue of Nano Letters.
A major problem with current supercapacitors in electric vehicle applications is their low energy density compared to batteries. A good supercapacitor might have an energy density of 10 Wh/kg compared to 170 Wh/kg for good lithium-ion batteries or even 35 Wh/kg for a lead-acid battery. On the other hand, the batteries have long recharging times.
The researchers, who are connected with Nanotek Instruments, Angstrom Materials (a spin-off of Nanotek) and the Department of Materials Science and Engineering at the Dalian University of Technology, basically use an approach based on electrical double-layer capacitance that leverages the intrinsic high surface area and capacitance of graphene plus the higher voltages that are possible through the use of ionic liquid electrolytes. Heretofore, EDL capacitors typically have used activated carbon as a high surface area electrode material.
Ordinary single graphene sheets obviously have surfaces that are easy to put in contact with an electrolyte. The problem is that when one is dealing with a bunch of ordinary graphene sheets, they tend to restack themselves. When this occurs, the intergraphene pore sizes greatly limit the accessibility to the electroyte.
This group apparently has gotten around the restacking problem by finding a way – few details are provided – of giving the graphene sheets curves. In brief, they say they use a modified Hummers method to form graphene oxide. Then:
“The suspension was injected into a forced convention oven in which a stream of compressed air was introduced to produce a fluidized-bed situation. Upon removal of the solvent or liquid, we obtained the desired curved graphene sheets.”
This morphology causes the sheets to resist stacking during packing and compression into an electrode structure. They say their method maintains a pore size in the range of 2–25 nm.
So, by making coin-sized capacitor cells using the curved graphene and 1-ethyl-3-methlyimidazolium tetraflouroborate (EMIMBF4), they were able to achieve energy densitites of 85.6 Wh/kg at room temperature and 136 WH/kg at 80°C, measured at a current density of 1 A/g.
These energy densities are comparable to nickel metal hydride batteries — with an important difference: They can be charged or discharged rapidly.