Energy Focus
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Energy Focus Graphene nanosheet supercapacitor achieved high-frequency ac circuit-filtering capacity
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upercapacitors, also called electric double-layer capacitors (DLC) are effective charge storage devices, and have applications ranging from electric vehicles to computer memory power backup. A major limitation of current DLCs is their inability to filter voltage ripple in rectified 120 Hz alternating current (ac) line power. The resistor-capacitor (RC) time constant, which is a measure of how fast the capacitor can be charged, is typically ~1 s for present capacitors, a value far too long for effective filtering when the period is ~8.3 ms. This limitation is caused by the highly porous nature of present electrodes, which behaves like a resistor rather than a capacitor at 120 Hz. Recently J. R. Miller of JME, Inc. and Case Western Reserve University; R.A. Outlaw of College of William and Mary, and B.C. Halloway from the Defense Advanced Research Project Agency have developed a graphene nanosheet DLC that achieved an RC time constant of 200 μs, along with other performance improvements. As reported in September 24th issue of Science (DOI: 10.1126/science.1194372; p. 1637), the researchers synthesized ver-
responsible for the change in current. The relaxation time is also strongly dependant on the magnetic field, which can further split the spin states and increase their lifetimes. Significantly, it is possible to observe small differences in spin relaxation times for adatoms in slightly different surface
tically oriented graphene nanosheets on heated nickel (Ni) substrates using radio frequency, plasma-enhanced chemical vapor deposition. These graphene nanosheets were ~600 nm high and
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