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Holey graphene framework (HGF) before and after mechanical compression, note flexibility of compressed HGF film (inset).Researchers at the California NanoSystems Institute at UCLA have used a special type of graphene material, called holey graphene framework because of its three-dimensional perforated structure, to advance the energy density (amount of energy stored and available for use) of an electrochemical capacitor to levels equal to those of lead acid batteries, setting the stage for a watershed in mobile energy storage due to its combined high power density (time rate of energy used) and high energy density.

Electrochemical capacitors (ECs, or supercapacitors) are an important technology in the future of energy storage and mobile power supplies, but to this point have been limited by low energy density. They typically have superior power density and cycle life (number of complete charge/discharge cycles that an energy source is able to support before its capacity falls below 80% of original) to traditional batteries, but have had energy density of at least one order of magnitude below traditional batteries. The main component of an EC is its electrode material, the efficiency of which is responsible for the EC’s overall performance.

Recent research has focused on new electrode materials that are able to increase energy density without sacrificing power density or cycle life. The characteristics needed for a high-performance EC electrode are high electrical conductivity, high ion-accessible surface area, high ionic transport rate, and high electrochemical stability. Current state-of-the-art ECs mostly use porous activated carbon electrodes with energy density much lower than lead acid batteries (4-5 watt hours/kilogram vs. 25-35 watt hours/kilogram, or 5-7 watt hours/liter vs. 50-90 watt hours/liter, respectively).

In their study published online July 31, 2014 in Nature Communications, CNSI researchers led by Dr. Xiangfeng Duan, Professor of Chemistry and Biochemistry, used a holey graphene framework as the electrode material to create an EC with unprecedented performance. The new EC electrode consists of a highly interconnected three-dimensional holey graphene network with superior electrical conductivity, exceptional mechanical flexibility and unique hierarchical porosity, ensuring efficient electron and ion transport and enabling the highest gravimetric energy densities of 127 watt hours/kilogram and volumetric energy density of 90 watt hours/liter. Furthermore, the team have shown that a fully package EC exhibits unprecedented energy densities of 35 watt hours/kilogram or 49 watt hours/liter, about 5-10 times higher than those of the current commercial supercapacitors and on par with acid batteries.

Initial solvated 3D hierarchical porous structure of holey graphene framework.“The holey graphene EC bridges the energy density gap between traditional capacitors and batteries, yet with vastly higher power density,” Duan says, “and it creates exciting opportunities for mobile power supplies for many applications from cell phones to electric vehicles.”

The California NanoSystems Institute (CNSI) is an integrated research facility with locations at UCLA and UC Santa Barbara. Its mission is to encourage University collaboration with industry and enable rapid commercialization of discoveries in nanoscience and nanotechnology. CNSI members, who are also faculty members of UCLA and UCSB, are a multidisciplinary team of preeminent world scientists from the life and physical sciences, engineering, and medicine. The work conducted at CNSI represents world-class expertise in three targeted areas of nanosystems-related research: energy/environment, health/medicine, and information technology.

Release Date: August 8, 2014
Source: California NanoSystems Institute  

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