This Small Business Innovation Research Phase I Project will develop high capacitance, low resistance graphene electrodes for supercapacitors to achieve high power and energy densities. One of the major technical barriers to high performance supercapacitors is low specific capacitance of the currently used activated carbon electrodes. This barrier is primarily due to poor access to the activated carbon surface area by an electrolyte: a substantial fraction of the surface area is in the form of micropores (< 2nm diameter) and consequently inaccessible to ion migration and therefore, unable to contribute to energy storage. This difficulty can be ameliorated by replacing microporous activated carbon with graphene, a one-atom-thick, conductive allotrope of carbon with a rare combination of extremely high specific surface area, remarkable thermal/electrical conductivity, an open microstructure, and good thermal stability. The small business, in collaboration with the University of North Carolina, will exploit the newly discovered route to graphene to produce mechanically stable, flexible, mesoporous electrodes for supercapacitors, electrodes that will achieve higher energy storage capacity and enhanced performance.
The broader/commercial impact of this project lies in the many applications that can be enabled by supercapacitors. It is expected that the demand for supercapacitors in consumer electronics will increase from $122 million in 2008 to over $550 million in 2014. And, in large storage applications such as wind turbines and hybrid electric vehicles (HEVs), the market value of supercapacitors is expected to expand from $86 million last year, to over $320 million in five years. In the critical technologies of HEVs, supercapacitors will enable improved gas mileage via two primary functions: leveling the dynamic power loads, and recovering available energy from regenerative breaking. The U.S. government has a strong interest in advancing HEVs to lessen dependence on foreign oil and address the global climate challenge. In the private sector, leading automotive companies are developing HEVs as an alternative to internal combustion engine-powered vehicles. Graphene-based supercapacitors offer the potential for attaining the requisite high energy/power density for high pulse power in HEV application.
Supercapacitors store electrical energy at the interface between a liquid electrolyte and a solid, porous electrode. They are an intermediate power source, bridging the power/energy gap between batteries and conventional dielectric capacitors. Long cycle lifetimes, rapid charge/discharge rates, and maintenance-free usage, makes supercapacitors ideal for applications ranging from hybrid electric vehicles to portable electronics. In supercapacitors, the magnitude of the energy density is proportional to the interfacial area of the electrode material available for charge storage. A higher available surface area translates into higher energy density, which in turn, is the driving force behind the search for high-surface-area, porous electrode materials with an optimal pore size. Activated carbon has been widely used in the electrodes of commercial capacitors. However, one of the major technical barriers to widespread adoption of supercapacitors is the low energy storage capacity of activated carbon electrodes. This limitation results from poor accessibility of its otherwise high surface area by the organic electrolyte, typically tetraethylammonium tetrafluoroborate in acetonitrile or propylene carbonate. In this NSF-SBIR Phase I & IB Project, Allotropica Technologies, in collaboration with the University of North Carolina, has developed a new solution-based process to produce mesoporous, mechanically-exfoliated graphene to achieve higher energy density supercapacitors. Mechanically-exfoliated graphene has a unique morphology and the requisite physical properties (surface area, mesoporous structure) that is more favorable than activated carbon. Mechanically stable graphene electrodes were prepared with an ink-coating technique and their performance (the energy storage capacity and stability) was evaluated in button-cell capacitors. The graphene electrodes exhibited an excellent energy storage capacity, a high purity level, and a fast charge/discharge capability, features that are superior to the industry benchmark, activated carbon electrodes. The specific capacitance of our graphene electrodes is ~ 120F/g, which is 30% higher than that of Norit® activated carbon electrodes. Initial tests show a persistent stability of the graphene electrodes after more than 1500 charge/discharge cycles. In addition to the supercapacitor project, Allotropica Technologies is working with industrial entities and academic institutions to explore graphene applications in other areas including fuel cells, batteries, hydrogen storage, field emission devices, and high-strength polymer composites.
