While the continual progress of efficiencies in solar and wind seem to make these technologies ideal candidates to eventually replace fossil fuels further into the future, one key element stands in the way of their adoption. That element is variability of the power source. While one solution would be to offset these power sources with continuous power sources such as tidal or geothermal, another option is storage.
Currently, two primary methods exist of storing power for later use -- rechargeable batteries and ultracapacitors (other exotic methods have also been proposed). Ultracapacitors are a growing, but not widely known field. Ultracapacitors can be mixed with fuel cells and batteries or used independently to provide power. While expensive, ultracapacitors have numerous advantages over batteries, including higher power capability, longer life, a wider thermal operating range, lighter, more flexible packaging and lower maintenance.
Now a new breakthrough promises even better ultracapacitors. A typical capacitor design features two sheets with an electrolyte between them. Charge is developed and stored on the sheets. The key to the new research is to use graphene as the capacitor sheet material.
Graphene is a unique carbon molecule which is a one-atom-thick planar sheet of sp2-bonded carbon atoms densely packed in a honeycomb-like lattice. The material has exceptional surface area, among other properties. It also is a great conductor, and thus is being explored as a material for transistors.
Rod Ruoff, a University of Texas at Austin mechanical engineering professor and a physical chemist who led the research describes, "Our interest derives from the exceptional properties of these atom-thick and electrically conductive graphene sheets, because in principle all of the surface of this new carbon material can be in contact with the electrolyte. Graphene's surface area of 2630 m2/gram (almost the area of a football field in about 1/500th of a pound of material) means that a greater number of positive or negative ions in the electrolyte can form a layer on the graphene sheets resulting in exceptional levels of stored charge."
Professor Ruoff and his team used a chemically modified graphene sheet, and several widely used commercial electrolytes. The resulting capacitor had a charge stored per weight (called "specific capacitance") rivalling the best available traditional ultracapacitors. And Professor Ruoff is hopeful that the material's storage can be more than doubled with tweaking. He states, "There are reasons to think that the ability to store electrical charge can be about double that of current commercially used materials. We are working to see if that prediction will be borne out in the laboratory."
His current team consists of graduate student Meryl Stoller and postdoctoral fellows Sungjin Park, Yanwu Zhu and Jinho An, all from the Mechanical Engineering Department and the Texas Materials Institute at the university.
Their impressive findings are reported in the forthcoming Oct. 8 edition of Nano Letters.
The U.S. Department of Energy has said advancing storage technologies is one of the most pressing needs of the renewable resource industry. Other fields such as electric cars could also benefit from the research. A graphene ultracapacitor equipped next-generation version of the Chevy Volt could get twice the range on a single charge or cut the battery weight in half. It could even extend the life of laptop batteries.
Resources and funding for the project were provided by the Texas Nanotechnology Research Superiority Initiative, The University of Texas at Austin and a Korea Research Foundation Grant for fellowship support for Dr. Park.